wherein the wavy lines indicate the points of attachment to M, R ¹ and R ² each independently represent one or several substituents selected in the group consisting of H, halogen, C1-C6 alkyl, C6-C10 aryl, OR ⁹ and NR ¹⁰ R ¹¹ , R ³ to R ⁶ each independently represent a substituent selected in the group consisting of H, halogen, C1-C6 alkyl, C6-C10 aryl, OR ¹² and NR ¹³ R ¹⁴ , R ⁷ and R ⁸ each independently represent one or several substituents selected in the group consisting of H, halogen, C1-C6 alkyl, C6-C10 aryl, OR ¹⁵ and NR ¹⁶ R ¹⁷ , R ⁹ to R ¹¹ are each independently selected in the group consisting of H and C1-C6 alkyl, and R ¹² to R ¹⁷ are each independently selected in the group consisting of H, C1-C6 alkyl and CO-(C ₁ -C ₆ alkyl), LIG2 is a bidentate ligand having the following formula (a) or (b): wherein the wavy lines indicate the points of attachment to M, LIG ₃ is a bidentate ligand having the following formula (c) or (d): wherein the wavy lines indicate the points of attachment to M, each represents a single or a double bond, provided that each cycle A, b, C and D is a heteroaromatic cycle, T ₁ is NR _a1 (e.g. NH) or CR _a1 , T ₂ is NR _a2 (e.g. NH) or CR _a2 , T ₃ is NR _a3 (e.g. NH) or CR _a3 , T ₄ is NR _a4 (e.g. NH) or CR _a4 , T ₇ is NR _a7 (e.g. NH) or CR _a7 , T ₈ is NR _a8 (e.g. NH) or CR _a8 , T ₉ is NR _a9 (e.g. NH) or CR _a9 and T ₁₀ is NR _a10 (e.g. NH) or CR _a10 , provided that when T ₁ is NR _a1 , then T ₂ is CR _a2 , when T ₃ is NR _a3 , then T ₄ is CR _a4 , when T ₇ is NR _a7 , then T ₈ is CR _a8 and when T ₉ is NR _a9 , then T ₁₀ is CR _a10 , Z ₁ is N or CR _b1 , Z ₂ is N or CR _b2 , Z ₃ is N or CR _b3 , Z ₄ is N or CR _b4 , Z ₅ is N or CR _b5 , Z ₆ is N or CRb6, Z9 is N or CRb9, Z10 is N or CRb10, Z11 is N or CRb11, Z12 is N or CRb12, Z13 is N or CRb13 and Z ₁₄ is N or CR _b14 , provided that at least two of Z ₁ to Z ₃ and at least two of Z ₄ to Z ₆ and at least two of Z9 to Z11 and at least two of Z12 to Z14 are not N, Ra1 to Ra12 and Rb1 to Rb16 each independently represent H, halogen, optionally substituted C ₁ -C ₆ alkyl, optionally substituted C ₂ -C ₆ alkenyl, optionally substituted C ₂ - C6 alkynyl, optionally substituted carbocycle, optionally substituted aryl, optionally substituted heteroaryl, optionally substituted heterocycle, CN, NO2, N3, COR ¹⁸ , OR ¹⁹ or NR ²⁰ R ²¹ , or Z3 and Z4 in formula (b) are linked together so that LIG2 represents: Z11 and Z12 are linked in formula (d) together so that LIG3 represents: , wherein R ^x and R ^y each independently represent one or several substituents selected in the group consisting of H, halogen, optionally substituted C1-C6 alkyl, optionally substituted C2-C6 alkenyl, optionally substituted C2-C6 alkynyl, optionally substituted carbocycle, optionally substituted aryl, optionally substituted heteroaryl, optionally substituted heterocycle, CN, NO2, N3, COR ¹⁸ , OR ¹⁹ and NR ²⁰ R ²¹ , R ¹⁸ is selected in the group consisting of H, optionally substituted C1-C6 alkyl, OR ²² and NR ²³ R ²⁴ , R ¹⁹ to R ²⁴ are each independently selected in the group consisting of H, optionally substituted C1-C6 alkyl and optionally substituted CO-(C1-C6 alkyl), X ^m- is a pharmaceutically acceptable anion, preferably selected in the group consisting of PF6-, Cl-, Br-, I-, BF4-, (C1-C6 alkyl)-C(O)O-, (C1-C6 haloalkyl)-C(O)O-, (C1-C6 alkyl)-SO3-, (C1-C6-haloalkyl)-SO3-, SO4 ^2- and PO4 ^3- , m and n are independently 1, 2, 3 or 4, wherein n is 1 when M is rhenium, n is 2 when M is ruthenium or osmium, n is 3 when M is rhodium or iridium and n is 4 when M is platinum, and y1 is 1, 2 or 3, y2 and y3 are independently 0, 1 or 2, provided that y1+y2+y3 is 3. In a second aspect, the present invention relates to a pharmaceutical composition comprising at least one compound of formula (I) or a pharmaceutically acceptable salt and/or solvate thereof and at least one pharmaceutically acceptable excipient. In a third aspect, the present invention relates to a compound of formula (I) according to the invention or a pharmaceutically acceptable salt and/or solvate thereof, or a pharmaceutical composition according to the present invention for use as a drug. The present invention also relates to the use of a compound of formula (I) according to the invention or a pharmaceutically acceptable salt and/or solvate thereof, or a pharmaceutical composition according to the present invention as a drug or for the manufacture of a drug. In a fourth aspect, the present invention relates to a compound of formula (I) according to the invention or a pharmaceutically acceptable salt and/or solvate thereof, or a pharmaceutical composition according to the present invention for use as a photosensitizer agent in photodynamic therapy. The present invention therefore also relates to the use of a compound of formula (I) or a pharmaceutically acceptable salt and/or solvate thereof or a pharmaceutical composition as described herein for the manufacture of a drug intended to be used as a photosensitizer agent in photodynamic therapy. The present invention also relates to the use of a compound of formula (I) or a pharmaceutically acceptable salt and/or solvate thereof or a pharmaceutical composition as described herein as a photosensitizer agent in photodynamic therapy. The present invention also concerns a method of treatment by photodynamic therapy comprising administering to an animal, in particular a mammal such as a human, in need thereof an effective amount of a compound of formula (I) or a pharmaceutically acceptable salt and/or solvate thereof as a photosensitizer agent. In a fifth aspect, the present invention concerns methods of preparation of compounds of formula (I) according to the invention. Preferably, said compound is not:

which is described in Humphrey, M.G. et al., 2007. Preferably, said compound of formula (I) is also not:

described in Leidner et al., 1987. Preferably, said compound of formula (I) is also not:

described in Zuniga César et al., 2014. Preferably said compound of formula (I) is also not:

described in Francisco Gajardo et al., 2011. Preferably said compound of formula (I) is also not: Preferably, said compound is also not:

described in Ayman, A. et al., 2000. Definitions The term “stereoisomers” used in this invention refers to configurational stereoisomers and more particularly to optical isomers. In the present invention, the optical isomers result in particular from the different position in space of the three bidentate ligands of the metal. The metal of the complex thus represents a chiral or asymmetric center. Optical isomers that are not mirror images of one another are thus designated as “diastereoisomers”, and optical isomers, which are non-superimposable mirror images are designated as “enantiomers”. An equimolar mixture of two enantiomers of a chiral compound is designated as a racemic mixture or racemate. For the purpose of the invention, the term “pharmaceutically acceptable” is intended to mean what is useful to the preparation of a pharmaceutical composition, and what is generally safe and non-toxic, for a pharmaceutical use. The term “pharmaceutically acceptable salt and/or solvate” is intended to mean, in the framework of the present invention, a salt and/or solvate of a compound which is pharmaceutically acceptable, as defined above, and which possesses the pharmacological activity of the corresponding compound. The pharmaceutically acceptable salts comprise: (1) acid addition salts formed with inorganic acids such as hydrochloric, hydrobromic, sulfuric, nitric and phosphoric acid and the like; or formed with organic acids such as acetic, benzenesulfonic, fumaric, glucoheptonic, gluconic, glutamic, glycolic, hydroxynaphtoic, 2-hydroxyethanesulfonic, lactic, maleic, malic, mandelic, methanesulfonic, muconic, 2-naphtalenesulfonic, propionic, succinic, dibenzoyl-L25 tartaric, tartaric, p-toluenesulfonic, trimethylacetic, and trifluoroacetic acid and the like, and (2) base addition salts formed when an acid proton present in the compound is either replaced by a metal ion, such as an alkali metal ion, an alkaline-earth metal ion, or an aluminium ion; or coordinated with an organic or inorganic base. Acceptable organic bases comprise diethanolamine, ethanolamine, N-methylglucamine, triethanolamine, tromethamine and the like. Acceptable inorganic bases comprise aluminium hydroxide, calcium hydroxide, potassium hydroxide, sodium carbonate and sodium hydroxide. Acceptable solvates for the therapeutic use of the compounds of the present invention include conventional solvates such as those formed during the last step of the preparation of the compounds of the invention due to the presence of solvents. As an example, mention may be made of solvates due to the presence of water (these solvates are also called hydrates) or ethanol. The term “halogen”, as used in the present invention, refers to a fluorine, bromine, chlorine or iodine atom. The term “C1-C6 alkyl”, as used in the present invention, refers to a straight or branched monovalent saturated hydrocarbon chain containing from 1 to 6 carbon atoms including, but not limited to, methyl, ethyl, n-propyl, iso-propyl, n-butyl, iso-butyl, sec- butyl, t-butyl, n-pentyl, n-hexyl, and the like. The term “C2-C6 alkenyl”, as used in the present invention, refers to a straight or branched monovalent unsaturated hydrocarbon chain containing from 2 to 6 carbon atoms and comprising at least one double bond including, but not limited to, ethenyl, propenyl, butenyl, pentenyl, hexenyl and the like. The term “C2-C6 alkynyl”, as used in the present invention, refers to a straight or branched monovalent unsaturated hydrocarbon chain containing from 2 to 6 carbon atoms and comprising at least one triple bond including, but not limited to, ethynyl, propynyl, propynyl, butynyl, pentynyl, hexynyl and the like. The term "C ₁ -C ₆ haloalkyl" refers to a C ₁ -C ₆ alkyl chain as defined above wherein one or more hydrogen atoms are replaced by a halogen atom selected from fluorine, chlorine, bromine or iodine, preferably a fluorine atom. For example, it is a CF ₃ group. The term “carbocycle” refers to a non-aromatic hydrocarbon ring, saturated or unsaturated, typically comprising from 3 to 20 carbons and comprising one or more fused or bridged ring(s). For example, it is a saturated hydrocarbon cycle, especially a C ₃ -C ₇ cycloalkyl. In particular, it is an unsaturated hydrocarbon cycle, especially a C ₃ -C ₈ cycloalkene or cycloalkyne including, but not limited to, cyclopropene, cyclobutene, cyclopentene, cyclohexene, 1,4-cyclohexadiene, cycloheptene, cycloheptyne, cyclooctene, cyclooctyne and the like. The term “C ₃ -C ₇ cycloalkyl” refers to a saturated hydrocarbon ring comprising from 3 to 7 carbons, including cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl and cycloheptyl. The term “heterocycle” as used in the present invention refers to a non-aromatic, saturated or unsaturated monocycle or polycycle (comprising fused, bridged or spiro rings) comprising preferably 5 to 10, notably 5 or 6, atoms in the ring(s), in which the atoms of the ring(s) consist of carbon atoms and one or more, advantageously 1 to 4, and more advantageously 1 or 2, heteroatoms, such as a nitrogen, oxygen or sulphur atom, the remainder being carbon atoms. In particular, it can be an unsaturated ring, such as an unsaturated 5 or 6-membered monocycle. Preferably it comprises 1 or 2 nitrogen, in particular one. A heterocycle can be notably piperidinyl, piperizinyl, pyrrolidinyl, pyrazolidinyl, imidazolidinyl, azepanyl, thiazolidinyl, isothiazolidinyl, oxazocanyl, thiazepanyl, benzimidazolonyl. The term "aryl" refers to an aromatic hydrocarbon group preferably comprising from 6 to 12 carbon atoms and comprising one or more fused rings, such as, for example, a phenyl or naphthyl group. Advantageously, it is a phenyl group. The term “heteroaryl”, as used in the present invention, refers to an aromatic group comprising one or several, notably one or two, fused hydrocarbon cycles in which one or several, notably one to four, advantageously one or two, carbon atoms each have been replaced with a heteroatom selected from a sulfur atom, an oxygen atom and a nitrogen atom, preferably selected from an oxygen atom and a nitrogen atom. It can be a furyl, thienyl, pyrrolyl, pyridyl, oxazolyl, isoxazolyl, thiazolyle, isothiazolyl, imidazolyl, pyrazolyl, oxadiazolyl, thiadiazolyl, triazolyl, tetrazolyl, pyridazinyl, pyrimidinyl, pyrazinyl, triazinyl, quinolyl, isoquinolyl, quinoxalyl or indyl. In the context of the present invention, “unsaturated” means that the hydrocarbon chain may contain one or more unsaturation(s), i.e. a double bond C=C or a triple bond C≡C, advantageously one. In the context of the present invention, "optionally substituted" means that the group in question is optionally substituted with one or more substituents which may be selected in particular from halogen, C ₁ -C ₆ alkyl, C ₁ -C ₆ haloalkyl, C ₂ -C ₆ alkenyl, C ₂ -C ₆ alkynyl, aryl, N ₃ , oxo, NR ^a R ^b , COR ^c , CO ₂ R ^d , CONR ^e R ^f , OR ^g , CN and NO ₂ wherein R ^a to R ^g are, independently of one another, H, C ₁ -C ₆ alkyl, C ₁ -C ₆ haloalkyl or aryl, preferably H or C ₁ -C ₆ alkyl. In the context of the present invention, when a group is “optionally substituted”, it is preferably optionally substituted with one or more substituents selected from halogen, C1-C6 alkyl, aryl, NR ^a R ^b and OR ^g , R ^a , R ^b and R ^g being preferably H or C ₁ -C ₆ alkyl. The term “pharmaceutical composition” is meant in the framework of the present invention a composition having preventive and curative properties towards disease such as cancer, in particular lung cancer, bladder cancer, oesophageal cancer, colon cancer, stomach cancer, liver cancer, skin cancer, ovarian cancer, pancreatic cancer, head and neck cancer, or brain cancer; bacterial infection, such as sinusitis, diabetic feet, burned wounds; fungal infection, such as mycoses; viral infection such as herpes; and skin disorders, such as acne, port wine stains. In particular, the pharmaceutical composition in the context of the present invention has curative properties in photodynamic therapy, i.e. in combination with molecular oxygen and light irradiation. The term “photodynamic therapy” (PDT) refers to a non-invasive medical therapy which involves light and a photosensitizing chemical substance, called a photosensitizer (PS) used in conjunction with molecular oxygen to elicit cell death. The PDT is notably intended to treat a disease selected from cancer, bacterial infection, fungal infection, viral infection and skin disorders. A photosensitizer becomes highly toxic upon light irradiation, notably at wavelengths comprised between 450 nm and 900 nm. During photodynamic therapy, the PS is administered either systemically or locally. The diseased area is then exposed to light. Upon light irradiation, the PS is able to create reactive oxygen species (ROS), such as singlet oxygen ( ¹ O ₂ ) or other radicals. Due to their high reactivity, these species can cause oxidative stress and damage in different surrounding cellular compartments (i.e. membrane, nucleus, endoplasmic reticulum, lysosome, mitochondria) leading to cell death. Detailed description Compound of formula (I) The compounds according to the present invention can be in the form of a stereoisomer or a mixture of stereoisomers, such as a mixture of enantiomers, notably a racemic mixture, provided that LIG ₁ is a (E,E’)-4-4’-bisstyryl-2,2’-bipyridine derivative. When y1 is different from 3, LIG ₂ and LIG ₃ are preferably different from LIG1. According to a particular embodiment, y1 is 1, 2 or 3, y2 is 2, 1 or 0 and y3 is 0. In other terms, compound of formula (I) advantageously corresponds to the following formulas (I-A), (I-B) or (I-C): (I-A) (I-B) (I-C) Compound of formula (I) is preferably (I-A) or (I-B), preferably (I-A). In other terms, y1 is advantageously 1 or 2, preferably 1, y2 is 2 or 1 respectively, preferably 2, and y3 is 0. Preferably, y1 is 1, y2 is 2 and y3 is 0. According to another particular embodiment, y1 is 2, y2 is 1 and y3 is 0. It can be also a compound of following formula (I-D) when y1 = y2 = y3 = 1. According to a preferred embodiment, M is selected among ruthenium, osmium and iridium, preferably M is ruthenium or osmium, notably ruthenium. ^ LIG ₁ LIG1 is a bidentate ligand having the following formula: Advantageously, R ¹ and R ² each independently represent one or several substituents selected in the group consisting of H, halogen such as fluorine, OR ⁹ and NR ¹⁰ R ¹¹ , wherein R ⁹ to R ¹¹ are preferably each independently H or C1-C6 alkyl, such as methyl. Preferably, R ¹ and R ² each independently represent one or several substituents, notably one substituent, selected in the group consisting of halogen, such as fluorine, OR ⁹ and NR ¹⁰ R ¹¹ , wherein R ⁹ to R ¹¹ are as defined above. More preferably, R ¹ and R ² each independently represent one or several substituents, notably one substituent, selected from OR ⁹ and NR ¹⁰ R ¹¹ , in particular OR ⁹ , R ⁹ to R ¹¹ being preferably H or C1-C6 alkyl, in particular C1-C6 alkyl, such as methyl. Advantageously, R ¹ and R ² each represent one substituent as defined above, and preferably OR ⁹ . According to this embodiment, R ¹ and R ² are in ortho, meta or para position of the phenyl group, notably in para position. According to a preferred embodiment, R ¹ and R ² are identical. Advantageously, R ³ to R ⁶ each independently represent a substituent selected in the group consisting of H, C1-C6 alkyl and C6-C10 aryl, preferably H and C1-C6 alkyl, such as methyl. More preferably, R ³ and R ⁶ are identical to each other and R ⁴ and R ⁵ are identical to each other. According to a particular embodiment, R ³ and R ⁶ are C ₁ -C ₆ alkyl, in particular methyl and R ⁴ and R ⁵ are both hydrogen. According to another particular embodiment, R ³ and R ⁶ are both hydrogen and R ⁴ and R ⁵ are C ₁ -C ₆ alkyl, in particular methyl. Even more preferably R ³ to R ⁶ are hydrogen. Advantageously, R ⁷ and R ⁸ each independently represent one or several substituents, preferably one, selected in the group consisting of H and C ₁ -C ₆ alkyl, such as methyl. Typically, R ⁷ and R ⁸ are identical. Preferably R ⁷ and R ⁸ are hydrogen. In a preferred embodiment, LIG ₁ is of formula (LIG1-A): (LIG1-A), wherein R ¹ and R ² are as defined above and preferably R ¹ and R ² each represent one OR ⁹ group. Advantageously, when R ¹ and R ² are both NR ¹⁰ R ¹¹ in LIG ₁ , y1 is 1 or 2 and LIG ₂ and LIG ₃ are different from LIG ₁ . ^ LIG ₂ In a particular embodiment, LIG ₂ is a bidentate ligand of the following formula (a): In particular, LIG2 of formula (a) may correspond to the following ligands: Advantageously, LIG2 of formula (a) corresponds to formula (a-1), notably (a-1’). Preferably, when LIG2 is of formula (a), in particular of formula (a-1), notably (a-1’), Ra1 to R _a6 each independently represent H, halogen, optionally substituted C ₁ -C ₆ alkyl, optionally substituted aryl, OR ¹⁹ or NR ²⁰ R ²¹ , with R ¹⁹ to R ²¹ being preferably each independently selected in the group consisting of H and C1-C6 alkyl. More preferably, R _a1 to R _a6 each independently represent H, halogen, C ₁ -C ₆ alkyl, aryl, OR ¹⁹ or NR ²⁰ R ²¹ , with R ¹⁹ to R ²¹ being as defined above such as H or C1-C6 alkyl. Even more preferably, R _a1 to R _a6 each independently represent H or C ₁ -C ₆ alkyl. Typically, R _a1 to R _a6 are identical. In a preferred embodiment, R _a1 to R _a6 are H. In a preferred embodiment, LIG2 is a bidentate ligand of the following formula (b): In particular, LIG2 of formula (b) may correspond to the following ligands: Preferably, LIG2 of formula (b) corresponds to formula (b-1), (b-2), (b-3), (b-4) or (b-5), more preferably to formula (b-1) or (b-5), even more preferably to formula (b-1). According to the previous embodiments, when LIG2 is of formula (b), Rb1 to Rb8 each independently represent advantageously H, halogen, optionally substituted C1-C6 alkyl, optionally substituted aryl, OR ¹⁹ or NR ²⁰ R ²¹ , with R ¹⁹ to R ²¹ being preferably each independently selected in the group consisting of H and C1-C6 alkyl. Preferably, Rb1 to Rb8 each independently represent H, halogen, C1-C6 alkyl or aryl, more preferably H or aryl. Rb1 to Rb8 are typically identical. Even more preferably, Rb1 to Rb8 are H. In the particular embodiment when LIG2 is of formula (b-5) or (b-6), in particular of formula (b-5), Rb1, Rb2, Rb5 to Rb8 are as defined above and R ^x represents advantageously one or several substituents selected in the group consisting of H, halogen, optionally substituted C1-C6 alkyl, optionally substituted aryl, OR ¹⁹ and NR ²⁰ R ²¹ , with R ¹⁹ to R ²¹ being preferably each independently selected in the group consisting of H and C1-C6 alkyl. Preferably, R ^x represents one or several substituents selected in the group consisting of H, C ₁ -C ₆ alkyl and aryl, such as H or aryl, more preferably H. Even more preferably, when LIG2 is of formula (b-5), LIG2 represents: . In a particularly preferred embodiment, LIG ₂ is of formula (b-1) with R _b1 to R _b8 being H. ^ LIG3 In a particular embodiment, LIG3 is a bidentate ligand of the following formula (c): , In particular, LIG3 of formula (c) may correspond to the following ligands:

in particular Advantageously, LIG3 of formula (c) corresponds to formula (c-1), notably (c-1’). Preferably, when LIG3 is of formula (c), in particular of formula (c-1), notably (c-1’), Ra7 to Ra12 each independently represent H, halogen, optionally substituted C1-C6 alkyl, optionally substituted aryl, OR ¹⁹ or NR ²⁰ R ²¹ , with R ¹⁹ to R ²¹ being preferably each independently selected in the group consisting of H and C1-C6 alkyl. More preferably, Ra7 to Ra12 each independently represent H, halogen, C1-C6 alkyl, aryl, OR ¹⁹ or NR ²⁰ R ²¹ , with R ¹⁹ to R ²¹ being as defined above. Even more preferably, Ra7 to Ra12 each independently represent H or C1-C6 alkyl. Typically, Ra7 to Ra12 are identical. In a preferred embodiment, Ra7 to Ra12 are H. In a preferred embodiment, LIG3 is a bidentate ligand of the following formula (d): In particular, LIG3 of formula (d) may correspond to the following ligands: Preferably, LIG3 of formula (d) corresponds to formula (d-1), (d-2), (d-3), (d-4) or (d-5), more preferably to formula (d-1) or (d-5), even more preferably to formula (d-1). According to the previous embodiments, when LIG3 is of formula (d), Rb9 to Rb16 each independently represent advantageously H, halogen, optionally substituted C ₁ -C ₆ alkyl, optionally substituted aryl, OR ¹⁹ or NR ²⁰ R ²¹ , with R ¹⁹ to R ²¹ being preferably each independently selected in the group consisting of H and C ₁ -C ₆ alkyl. Preferably, R _b9 to R _b16 each independently represent H, halogen C ₁ -C ₆ alkyl or aryl, more preferably H or aryl. R _b9 to R _b16 are typically identical. Even more preferably, R _b9 to R _b16 are H. In the particular embodiment when LIG ₃ is of formula (d-5) or (d-6), in particular of formula (d-5), R _b9 , R _b10 , R _b13 to R _b16 are as defined above and R ^y represents advantageously one or several substituents selected in the group consisting of H, halogen, optionally substituted C ₁ -C ₆ alkyl, optionally substituted aryl, OR ¹⁹ and NR ²⁰ R ²¹ , with R ¹⁹ to R ²¹ being preferably each independently selected in the group consisting of H and C ₁ -C ₆ alkyl. Preferably, R ^y represents one or several substituents selected in the group consisting of H, C ₁ -C ₆ alkyl and aryl, more preferably H. Even more preferably, when LIG ₃ is of formula (d-5), LIG ₃ represents: According to a preferred embodiment, compound of formula (I) corresponds to the following formula: and R ² are as defined above, typically R ¹ and R ² each independently represent OR ⁹ , R ⁹ being preferably H or C ₁ -C ₆ alkyl, such as methyl, and M is as defined above, preferably ruthenium or osmium, typically ruthenium. According to another preferred embodiment, compound of formula (I) corresponds to the following formula:

, i.e. y1 is 2, y2 is 1 and ¹ y3 is 0, R and R ² are as defined above, typically R ¹ and R ² each independently represent OR ⁹ or NR ¹⁰ R ¹¹ , in particular OR ⁹ , R ⁹ to R ¹¹ being preferably H or C1-C6 alkyl, such as methyl, and M is as defined above, preferably ruthenium or osmium, typically ruthenium. According to another preferred embodiment, compound of formula (I) may be one of the following formulas: ,

preferably i.e. y1 is 1, y2 is 2, y3 is 0 R ¹ and R ² are as defined above, typically R ¹ and R ² each independently represent OR ⁹ or NR ¹⁰ R ¹¹ , in particular OR ⁹ , R ⁹ to R ¹¹ being preferably H or C ₁ -C ₆ alkyl, such as methyl, and M is as defined above, preferably ruthenium or osmium, typically ruthenium. According to a particular embodiment, the present invention relates to the following compounds of formula (I):

, ,

, ,

Pharmaceutical composition The present invention also relates to a pharmaceutical composition comprising at least one pharmaceutically acceptable excipient and at least one compound of formula (I) as described above or a pharmaceutically acceptable salt and/or solvate thereof. The pharmaceutical compositions of the invention can be intended to oral or parenteral (e.g. subcutaneous, intramuscular, intravenous) administration, preferably oral or intravenous administration. The active ingredient can be administered in unit forms for administration, mixed with conventional pharmaceutical carriers, to animals, preferably mammals including humans. For oral administration, the pharmaceutical composition can be in a solid or liquid (solution or suspension) form. A solid composition can be in the form of tablets, gelatin capsules, powders, granules and the like. In tablets, the active ingredient can be mixed with pharmaceutical vehicle(s) such as gelatin, starch, lactose, magnesium stearate, talc, gum arabic and the like before being compressed. The tablets may be further coated, notably with sucrose or with other suitable materials, or they may be treated in such a way that they have a prolonged or delayed activity. In powders or granules, the active ingredient can be mixed or granulated with dispersing agents, wetting agents or suspending agents and with flavor correctors or sweeteners. In gelatin capsules, the active ingredient can be introduced into soft or hard gelatin capsules in the form of a powder or granules such as mentioned previously or in the form of a liquid composition such as mentioned below. A liquid composition can contain the active ingredient together with a sweetener, a taste enhancer or a suitable coloring agent in a solvent such as water. The liquid composition can also be obtained by suspending or dissolving a powder or granules, as mentioned above, in a liquid such as water, juice, milk, etc. It can be for example a syrup or an elixir. For parenteral administration, the composition can be in the form of an aqueous suspension or solution which may contain suspending agents and/or wetting agents. The composition is advantageously sterile. It can be in the form of an isotonic solution (in particular in comparison to blood). The compounds of the invention can be used in a pharmaceutical composition at a dose ranging from 0.01 mg to 1000 mg a day, administered in only one dose once a day or in several doses along the day, for example twice a day in equal doses. The daily administered dose is advantageously comprised between 5 mg and 500 mg, and more advantageously between 10 mg and 200 mg. However, it can be necessary to use doses out of these ranges, which could be noticed by the person skilled in the art. According to a particular embodiment, the compound of formula (I) or a pharmaceutically acceptable salt and/or solvate thereof, or the pharmaceutical composition according to the present invention is encapsulated within a polymeric matrix, such as 1,2-distearoyl-sn-glycero-3-phosphoethanolamine-N- [biotin(polyethylene glycol)-2000] ammonium salt (DSPE-PEG2000-biotin). Such an encapsulation can be performed according to methods well-known from the skilled person, in particular according to methods described in Gasser, G. et al., Angew. Chem. Int. Ed, 2019 and in Gasser, G. et al. Inorg Chem.2019. The encapsulation is particularly useful for controlled, targeted and/or extended release of compound of formula (I) in the body of the subject in need thereof. Treatment The compound of formula (I) or a pharmaceutically acceptable salt and/or solvate thereof, or the pharmaceutical composition according to the present invention is useful as a drug. According to a preferred embodiment, the compound of formula (I), or a pharmaceutically acceptable salt and/or solvate thereof, or the pharmaceutical composition according to the present invention is useful as a photosensitizer agent in photodynamic therapy. It is particularly intended to treat by photodynamic therapy a disease selected from cancer, such as lung cancer, bladder cancer, oesophageal cancer, colon cancer, stomach cancer, liver cancer, skin cancer, ovarian cancer, pancreatic cancer, head and neck cancer, or brain cancer; bacterial infection, such as sinusitis, diabetic feet, burned wounds; fungal infection, such as mycoses; viral infection such as herpes; and skin disorders, such as acne, port wine stains. Methods of preparation of a compound of formula (I-A), (I-B), (I-C) or (I-D) The present invention relates also to a first method of preparation of a compound of the following formula (I-A) or (I-D): (I-A) (I-D) or a pharmaceutically acceptable salt and/or solvate thereof, and thus more particularly to a compound of formula (I) or a pharmaceutically acceptable salt and/or solvate thereof, where y1 = 1 and thus y2 +y3 = 2, wherein LIG1 and LIG2 are as defined above and M is ruthenium or osmium, preferably ruthenium, said method comprising the following steps: (a) reacting a compound of the following formula (II) in which LIG ₂ , LIG ₃ , y2 and y3 are as defined above, R ²⁵ and R ²⁶ each independently represent halogen, OR ²⁷ or S(O)R ²⁸ R ²⁹ , R ²⁷ is H or C ₁ -C ₆ alkyl, R ²⁸ and R ²⁹ are each independently a (C ₁ -C ₆ )alkyl, preferably a methyl, with a compound of formula (III) in which R ¹ to R ⁸ are as defined above, then (b) reacting the product resulting from step (a) with a salt A ^m+ X ^m- , wherein X ^m- is as defined above and A ^m+ is a counter cation. Step (a) In the compound of formula (II), R ²⁵ and R ²⁶ are preferably identical and/or both represent a halogen, such as Cl. Compound of formula (II) can be obtained using suitable ligands according to methods described in the literature, in particular in Meyer, T. et al., 1978 and in McCusker, C.E and McCusker, J.K, 2011. For example, when R ²⁵ = R ²⁶ = halogen such as Cl and preferably M = Ru, compound of formula (II) can be prepared by reacting LIG2 and/or LIG3 with M(R ²⁵ )3 such as Ru(III)Cl3, notably in the presence of LiCl. Compound of formula (II) with R ²⁵ = R ²⁶ = halogen such as Cl and preferably M = Ru can also be prepared by reacting LIG2 and/or LIG3 with M(R ^25’ )4(R ²⁵ )2 (with R ^25’ being OR ²⁷ or S(O)R ²⁸ R ²⁹ ) such as Ru(II)Cl2DMSO4. Compound of formula (II) advantageously corresponds to the following compounds (II- A), (II-B) or (II-C): Compound of formula (II-A), also called Ru(bipy) ₂ Cl ₂ , compound of formula (II-B), also called Ru(phen) ₂ Cl ₂ or dichlorobis(1,10-phenantroline)ruthenium(II), and compound of formula (II-C), also called Ru(4,7-diphenyl-1,10-phenantroline) ₂ Cl ₂ or dichlorobis(4,7- diphenyl-1,10-phenantroline)ruthenium(II), are synthesized as previously published using the respective ligands (McCusker, C.E. and McCusker, J.K., 2011). Compound of formula (II) preferably corresponds to formula (II-A). Compound of formula (III) can be obtained using suitable substituents R ¹ to R ⁸ according to methods described in the literature (Meyer, T.J. et al., 1987, Wu, Q. et al., 1995). According to a preferred embodiment, compound of formula (III) corresponds to the following compound (III-A):

in which R ¹ and R ² are as defined above. Preferably R ¹ and R ² are identical. The inventors have also developed a novel synthesis of a compound of formula (III-A) wherein R ¹ and R ² are identical to avoid tedious purifications, multistep synthesis and overall low yields of the prior art (Meyer, T.J. et al., 1987, Wu, Q. et al., 1995). Thus, the present invention relates also to a method of preparation of a compound of formula (III-A) wherein R ¹ and R ² are identical comprising the step of reacting one equivalent of 4,4’-dimethyl-2,2’-bipyridine (n°CAS 1134-35-6) in the presence of a strong base with a compound of following formula (IV): which R ¹ , identical to R ² , is as defined above. Such a method has the advantage to be carried out in one step and in mild conditions. Advantageously, the strong base is non nucleophilic. The strong base is preferably selected in the group consisting of lithium diisopropylamide (LDA), sodium bis(trimethyl)silylamide (NaHMDS), potassium bis(trimethyl)silylamide (KHMDs), sodium hydride (NaH), potassium hydride (KH) and potassium tert-butoxide ( ^t BuOK). More preferably, the strong base is potassium tert-butoxide. Advantageously, the reaction is carried out with 2 to 3 equivalents, preferably 2 to 2.5 of a compound of formula (IV) compared to 4,4’-dimethyl-2,2’-bipyridine. The reaction is typically carried out with 3 to 5 equivalents of the strong base, preferably with 4 equivalents, compared to 4,4’-dimethyl-2,2’-bipyridine. The reaction is preferably carried out in a polar solvent, in particular an aprotic polar solvent such as dimethylformamide (DMF), dimethylsulfoxide (DMSO), pyridine, acetonitrile, ethyl acetate, acetone, butanone and mixtures thereof. Preferably, the solvent is DMF. The reaction is preferably carried out under inert atmosphere such as nitrogen (N2) or argon (Ar) atmosphere. Step (a) corresponds to a ligand exchange wherein substituents R ²⁵ and R ²⁶ are replaced by the bidentate ligand LIG1 as described above in compound of formula (I-A). Eventually, additional steps of protection/deprotection and/or of functionalization well-known from the skilled person in the art may occur between steps (a) and (b) to afford compound of formula (I-A) with the suitable substituents as described above. The reaction is preferably carried out with 1 to 1.5, more preferably 1 to 1.2 equivalent of compound of formula (III) compared to compound of formula (II). The reaction is preferably carried out in a polar solvent, preferably selected among water, alcohols, such as methanol, ethanol, propanol, butanol, and mixtures thereof. Preferably, the solvent is an alcohol, in particular ethanol. The reaction is preferably carried out under inert atmosphere such as nitrogen (N ₂ ) or argon (Ar) atmosphere. The reaction is preferably carried out at a temperature corresponding to the boiling temperature of the solvent. Step (b) X ^m- is a pharmaceutically acceptable anion, preferably selected in the group consisting of PF ₆ -, Cl-, Br-, I-, BF ₄ -, (C ₁ -C ₆ alkyl)-C(O)O-, (C ₁ -C ₆ haloalkyl)-C(O)O-, (C ₁ -C ₆ -alkyl)-SO ₃ , (C ₁ -C ₆ -haloalkyl)-SO ₃ -, SO ₄ ^2- and PO ₄ ^3- , in particular PF ₆ -, Cl-, Br-, BF ₄ -, CH ₃ C(O)O-, CF ₃ C(O)O- and CF ₃ SO ₃ -, more preferably X ^m- is PF ₆ -. A ^m+ is a counter cation preferably selected among (N ⁺ R’R’’R’’’R’’’’)m, (H ⁺ )m, (Na ⁺ )m, (K ⁺ )m and (Li ⁺ )m, wherein m is 1, 2 or 3 and R’, R’’, R’’’ and R’’’’ are H or C ₁ -C ₆ alkyl. Preferably (N ⁺ R’R’’R’’’R’’’’) _m corresponds to (NH4 ⁺ )m or (NBu4 ⁺ )m. The salt A ^m+ X ^m- is thus preferably selected among the salts, but not limited to, NH4PF6, NBu4PF6, KCl, KBr, LiCl, LiBr, HBF4, NaOC(O)CH3, KOC(O)CH3, NH4OCOCH3, Na2SO4, H3PO4. Preferably, the salt used in step (b) is NH4PF6. The present invention also relates to a second method of preparation of a compound of the following formula (I-B): (I-B) or a pharmaceutically acceptable salt and/or solvate thereof, wherein LIG1 and LIG2 are as defined above and M is ruthenium or osmium, preferably ruthenium, said method comprising the following steps: (c) reacting of a compound of formula (III) in which R ¹ to R ⁸ are as defined above, with of a compound of the following formula (V): in which R ²⁵ and R ²⁶ are as defined above provided that R ²⁵ and R ²⁶ are not identical, then (d) reacting the compound resulting from step (c) with a compound of formula (VI) or (VII): in which , T ₁ to T ₄ , Z ₁ to Z ₆ , R _a5 , R _a6 , R _b7 and R _b8 are as defined above, then (e) reacting the product resulting from step (d) with a salt A ^m+ X ^m- , wherein X ^m- is as defined above and A ^m+ is a counter cation. Step (c) Compound of formula (III) can be obtained as mentioned previously. In the compound of formula (V), R ²⁵ and R ²⁶ are preferably selected in the group consisting of halogen, such as Cl, and S(O)R ²⁸ R ²⁹ , such as S(O)(CH ₃ ) ₂ . Advantageously, R ²⁵ is a better leaving group than R ²⁶ , so that R ²⁵ can represent for example S(O)R ²⁸ R ²⁹ , such as S(O)(CH ₃ ) ₂ , and R ²⁶ can represent halogen, such as Cl. Compound of formula (VI) advantageously corresponds to Ru(DMSO)4Cl2 (n°CAS : 67-68-5). Ru(DMSO)4Cl2 can also be synthesized as previously published (Bratsos, I. and Alessio, E., 2010). Step (c) corresponds to a ligand exchange wherein the four substituents R ²⁵ are replaced by two bidentate ligands LIG1 as described above in compound of formula (I- B). Eventually, additional steps of protection/deprotection and/or of functionalization well-known from the skilled person in the art may occur between steps (c) and (d) to afford at the end of step (e) compound of formula (I-B) with the suitable substituents as described above. The reaction is preferably carried out with compound of formula (III) in excess, preferably with 1.5 to 3 equivalents, more preferably with 2 equivalents, compared to compound of formula (V). The reaction is preferably carried out in a polar solvent, in particular an aprotic polar solvent such as dimethylformamide (DMF), dimethylsulfoxide (DMSO), pyridine, acetonitrile, ethyl acetate, acetone, butanone and mixtures thereof. Preferably, the solvent is DMF. The reaction is preferably carried out under inert atmosphere such as nitrogen (N2) or argon (Ar) atmosphere. The reaction is preferably carried out at a temperature corresponding to the boiling temperature of the solvent. Step (d) The intermediate resulting from step (c) is typically of the following formula: , in which LIG ₁ and R ²⁶ are as described above. Preferably, LIG ₁ is of formula (LIG ₁ -A): (LIG1-A), with R ¹ and R ² being as described above, advantageously being identical. In this intermediate, R ²⁶ preferably represents a halogen, such as Cl. Step (d) corresponds to a ligand exchange wherein the two substituents R ²⁶ are replaced by a bidentate ligands LIG2 as described above. Preferably, the compound resulting from step (c) reacts with 1 to 1.5 equivalent of compound of formula (VI) or (VII), more preferably with 1.2 equivalent compared to compound of formula (V). Eventually, additional steps of protection/deprotection and/or of functionalization well-known from the skilled person in the art may occur between steps (d) and (e) to afford at the end of step (e) compound of formula (I-B) with the suitable substituents as described above. The reaction is preferably carried out in a polar solvent, preferably selected among water, alcohols, such as methanol, ethanol, propanol, butanol, and mixtures thereof. Preferably, the solvent is an alcohol, in particular ethanol. The reaction is preferably carried out under inert atmosphere such as nitrogen (N2) or argon (Ar) atmosphere. The reaction is preferably carried out at a temperature corresponding to the boiling temperature of the solvent. Step (e) Step (e) involves the same embodiments as those described for step (b) above. The present invention also relates to a third method of preparation of a compound of the following formula (I-B): or a pharmaceutically acceptable salt and/or solvate thereof, wherein LIG1 and LIG2 are as defined above and M is ruthenium or osmium, preferably ruthenium, said method corresponding to the first method described above for preparing a compound of formula (I-A), in which LIG1 and LIG2 are switched. The present invention relates to a fourth method of preparation of a compound of the following formula (I-C): or a pharmaceutically acceptable salt and/or solvate thereof, wherein LIG1 is as defined above and M is ruthenium or osmium, preferably ruthenium, said method comprising the following steps: (f) reacting one equivalent of a compound of formula (V) in which R ²⁵ and R ²⁶ are as defined above provided that R ²⁵ and R ²⁶ are not identical, with a compound of formula (III) in which R ¹ to R ⁸ are as defined above, then (g) reacting the product resulting from step (a) with a salt A ^m+ X ^m- , wherein X ^m- is as defined above and A ^m+ is a counter cation. Step (f) Compound of formula (III) can be obtained as mentioned above. In the compound of formula (V), R ²⁵ and R ²⁶ are preferably selected in the group consisting of halogen, such as Cl, and S(O)R ²⁸ R ²⁹ , such as S(O)(CH3)2. Advantageously, R ²⁵ is a better leaving group than R ²⁶ , so that R ²⁵ can represent for example S(O)R ²⁸ R ²⁹ , such as S(O)(CH3)2, and R ²⁶ can represent halogen, such as Cl. Compound of formula (VI) advantageously corresponds to Ru(DMSO)4Cl2 (n°CAS : 67-68-5). Step (f) corresponds to a ligand exchange wherein the six substituents R ²⁵ and R ²⁶ are replaced by three bidentate ligands LIG1 as described above in compound of formula (I-C). Eventually, additional steps of protection/deprotection and/or of functionalization well-known from the skilled person in the art may occur between steps (f) and (g) to afford at the end of step (e) compound of formula (I-C) with the suitable substituents as described above. The reaction is preferably carried out with 3 to 6 equivalents, more preferably with four equivalents of compound of formula (III), compared to compound of formula (V). The reaction is preferably carried out in a protic polar solvent, preferably selected among water, alcohols, such as methanol, ethanol, propanol, butanol, and mixtures thereof. In another embodiment, the reaction can be carried out in an aprotic polar solvent such as dimethylformamide (DMF), dimethylsulfoxide (DMSO), pyridine, acetonitrile, ethyl acetate, acetone, butanone and mixtures thereof. Preferably, the solvent is an alcohol, in particular ethanol, or DMF. The reaction is preferably carried out under inert atmosphere such as nitrogen (N ₂ ) or argon (Ar) atmosphere. The reaction is preferably carried out at a temperature corresponding to the boiling temperature of the solvent. The above described methods of preparation of compounds of formula (I), i.e. compounds of formulas (I-A), (I-B), (I-C) and (I-D), can be applied for others metals than ruthenium or osmium selected from rhenium, rhodium, iridium and platinum. The skilled person knows the suitable reagents to be used, notably in place of reagents of formula (II) and (V). According to the previous embodiments, in methods of preparation of a compound of formula (I), i.e. compounds of formulas (I-A), (I-B), (I-C) and (I-D), a compound obtained at the end of a reacting step can be separated from the reaction medium by methods well known to the person skilled in the art, such as by extraction, evaporation of the solvent or by precipitation or crystallisation (followed by filtration). Said compound can be also purified if necessary by methods well known to the person skilled in the art, such as by recrystallisation, by distillation, by chromatography on a column of silica gel or by high performance liquid chromatography (HPLC). Description of the figures Figure 1: Absorption spectra in CH3CN of compounds a) 1-4, b) 5-8, c) 9-11. Figure 2: One- (OPM, λex = 458 nm, λem = 600 – 750 nm) and two-photon (TPM, λex = 800 nm, λ = 600 - 750 nm) excited Z-stack images in HeLa MCTS after incubation of compound 7 after 12 h (20 μM, 2% DMSO, v%). a) Z-axis images scanning from the top to the bottom of an intact spheroid. b) 3D z-stack of an intact spheroid. Figures 3-5: Tumour growth inhibition assay. Change of the volume in HeLa MCTS in correlation to the time of the treatment. The MCTS were treated with compounds 1-3 (20 μM, 2% DMSO, v%) for Figure 3, compounds 4-7 (20 μM, 2% DMSO, v%) for Figure 4 and H2TPP (20 μM, 2% DMSO, v%) or cisplatin (10 μM Pt-10 and 30 μM Pt-30) for Figure 5. The MCTS were a) strictly kept in the dark, b) exposed to 1P irradiation (500 nm, 10 J/cm ² ), c) exposed to 2P irradiation (800 nm, 10 J/cm ² with a section interval of 5 μm) on day 3. The error bars correspond to the standard deviation of the three replicates. Figure 6: Representative image of a viability assay in HeLa MCTS. MCTS were treated with compounds 1-7 (20 μM, 2% DMSO, v%) in the dark for three days. After this time, MCTS were kept in the dark, exposed to 1P irradiation (500 nm, 10 J/cm ² ) or to 2P irradiation (800 nm, 10 J/cm ² , section interval of 5 μm). After two days, the cell viability was assessed by measurement of the fluorescence of calcein (λ _ex = 495 nm, λ = 515 nm), which is generated in living cells from calcein AM. Figure 7: PDT in vivo. The mice were randomly allocated into six different treatments: (i) 7-injected with 2P irradiation (7 + TP), (ii) 7-injected with 1P irradiation (7 + OP), (iii) physiological saline and 2P irradiation (TP), (iv) physiological saline and 1P light irradiation (TP), (v) 7-injected only (7) and (vi) physiological saline injected only (control). (A) In vivo tumour growth inhibition and (B) body weight curves for different treated mice. (C) Representative photographs of SW620/AD300 tumors in mice with different treatments. Figure 8: Absorption spectra in CH3CN of compounds a) 12-14, b) 15-17, c) 18-20. The present invention is illustrated by the following examples. Examples 1. Synthesis The synthesis of the complexes is presented. Structures

Materials All chemicals were obtained from commercial sources and used without further purification. Solvents were dried over molecular sieves if necessary. The Ru(II) precursors Ru(DMSO)4Cl2 and Ru(bipy)2Cl2, Ru(phen)2Cl2 and Ru(bphen)2Cl2 were synthesised as previously reported using the respective ligand (Bratsos, I. and Alessio, E., 2010, Meyer, T.J. et al., 1987 and Wu, Q. et al, 1995). Instrumentation and methods ¹ H and ¹³ C NMR spectra were recorded on a Bruker 400 MHz or 500 MHz NMR spectrometer. Chemical shifts (δ) are reported in parts per million (ppm) referenced to tetramethylsilane (δ 0.00) ppm using the residual proton solvent peaks as internal standards. Coupling constants (J) are reported in Hertz (Hz) and the multiplicity is abbreviated as follows: s (singulet), d (doublet), dd (doublet of doublet), t (triplet), m (multiplet). ESI-MS experiments were carried out using a LTQ-Orbitrap XL from Thermo Scientific (Thermo Fisher Scientific) and operated in positive ionization mode, with a spray voltage at 3.6 kV. No Sheath and auxiliary gas were used. Applied voltages were 40 and 100 V for the ion transfer capillary and the tube lens, respectively. The ion transfer capillary was held at 275°C. Detection was achieved in the Orbitrap with a resolution set to 100,000 (at m/z 400) and a m/z range between 150-2000 in profile mode. Spectrum was analyzed using the acquisition software XCalibur 2.1 (Thermo Fisher Scientific). The automatic gain control (AGC) allowed accumulation of up to 2.10 ⁵ ions for FTMS scans, Maximum injection time was set to 300 ms and 1 µscan was acquired.10 µL was injected using a Thermo Finnigan Surveyor HPLC system (Thermo Fisher Scientific) with a continuous infusion of methanol at 100 µL.min ^-1 . Elemental microanalyses were performed on a Thermo Flash 2000 elemental analyser. The absorption of the samples has been measured with a SpectraMax M2 Spectrometer (Molecular Devices). For analytic and preparative HPLC the following system has been used: 2 x Agilent G13611260 Prep Pump system with Agilent G7115A 1260 DAD WR Detector equipped with an Agilent Pursuit XRs 5C18 (Analytic: 100Å, C18 5 μm 250 x 4.6 mm, Preparative: 100Å, C18 5 μm 250 x 300 mm) Column and an Agilent G1364B 1260-FC fraction collector. The solvents (HPLC grade) were millipore water (0.1% TFA, solvent A) and acetonitrile (0.1% TFA, solvent B). Inductive coupled plasma mass spectrometry (ICP-MS) experiments were carried out on an iCAP RQ ICP-MS instrument (Thermo Fisher). Synthesis of the Ligands (E,E’)-4,4´-Bisstyryl-2,2´-bipyridine The synthesis of 4,4´-Bisstyryl-2,2´-bipyridine is already published (Meyer, T.J. et al., 1987) but in this study another synthetic route was employed. 4,4´-Dimethyl-2,2´- bipyridine (1000 mg, 5.43 mmol, 1.0 equiv.) was dissolved in dry DMF under nitrogen atmosphere and Benzaldehyde (1.2 mL, 11.84 mmol, 2.2 equiv.) was added to the solution. Afterwards potassium tert-butoxide (2436 mg, 21.72 mmol, 4.0 equiv.) was added slowly. The colour of the solution turned to green and the mixture was stirred for 24 h. After that the mixture was poured into H2O (400 mL) and the suspension cooled down to 5 °C. The crude product which precipitated, was filtered and washed with Methanol. The product was purified by recrystallization from boiling acetic acid. The obtained solid was dissolved in Dichloromethane and the mixture was washed with a 5% LiCl aqueous solution, brine and H ₂ O. The solvent was removed and the product was isolated by recrystallization from boiling acetic acid. 1292 mg of (E,E’)-4,4´- Bisstyryl-2,2´-bipyridine (3.58 mmol, 66 %) were yielded as a beige solid. 4,4´-Bis[p-(N,N-dimethylamino)styryl]-2,2´-bipyridine The synthesis of 4,4´-Bis[p-(N,N-dimethylamino)styryl]-2,2´-bipyridine is already published (Wu, Q. et al) but in this study another synthetic route was employed.4,4´- Dimethyl-2,2´-bipyridine (1000 mg, 5.43 mmol, 1.0 equiv.) was dissolved in dry DMF (100 mL) under nitrogen atmosphere and potassium tert-butoxide (2437 mg, 21.72 mmol, 4.0 equiv.) was added slowly. After 1.5 h of stirring, 4- (Dimethylamino)benzaldehyde (1701 mg, 11.40 mmol, 2.1 equiv.) was added to the reaction mixture. The colour of the solution turned to yellow and the mixture was heated at 90 °C for 19 h. After that the mixture was poured into H2O (400 mL) and the suspension cooled down to 5 °C. The crude product which precipitated, was filtered and washed with H2O and Et2O. The product was isolated by recrystallization from DCM/Pentane.1541 mg of (E,E’)-4,4´-Bis[p-(N,N-dimethylamino)styryl]-2,2´-bipyri dine (3.45 mmol, 64 %) were yielded as a yellow solid. (E,E’)-4,4´-Bis[p-methoxystyryl]-2,2´-bipyridine The synthesis of (E,E’)-4,4´-Bis[p-methoxystyryl]-2,2´-bipyridine is already published (Wu, Q. et al) but in this study another synthetic route was employed.4,4´-Dimethyl- 2,2´-bipyridine (532 mg, 2.89 mmol, 1.0 equiv.) was dissolved in dry DMF (25 mL) under nitrogen atmosphere and 4-Methoxybenzaldehyde (0.88 mL, 7.22 mmol, 2.5 equiv.) was added to the solution. Afterwards potassium tert-butoxide (1360 mg, 12.13 mmol, 4.2 equiv.) was added slowly. The colour of the solution turned to green and the mixture was stirred for 24 h. After that the mixture which turned bright was poured into H2O (200 mL) and the suspension cooled down to 5 °C. The crude product which precipitated, was filtered and washed with Methanol. The product was purified by recrystallization from boiling acetic acid. The obtained solid was dissolved in Dichloromethane and the mixture was washed with a 5% LiCl aqueous solution, brine and H ₂ O. The solvent was removed and the product was isolated by recrystallization from boiling acetic acid. 925 mg of (E,E’)-4,4´-Bis[p-methoxystyryl]-2,2´-bipyridine (2.20 mmol, 76 %) were yielded as a beige solid. (E,E’)-4,4´-Bis[m-methoxystyryl]-2,2´-bipyridine (E,E’)-4,4´-Bis[m-methoxystyryl]-2,2´-bipyridine was synthesized according to the literature (Aranyos, V. et al., 2001). (E,E’)-4,4´-Bis[o-methoxystyryl]-2,2´-bipyridine (E,E’)-4,4´-Bis[o-methoxystyryl]-2,2´-bipyridine was synthesized according to the literature (Sinha, S., 2015) (E,E’)-4,4´-Bis[p-fluorostyryl]-2,2´-bipyridine 4,4´-Dimethyl-2,2´-bipyridine (1.00 g, 5.43 mmol, 1.0 equiv.) was dissolved in dry DMF (50 mL) under nitrogen atmosphere and 4-fluorobenzaldehyde (1.46 mL, 13.57 mmol, 2.5 equiv.) was added to the solution. Afterwards potassium tert-butoxide (2.44 g, 21.74 mmol, 4.0 equiv.) was added slowly. The colour of the solution turned to green and the mixture was stirred for 24 h. After that the mixture which turned bright was poured into H2O (500 mL) and the suspension cooled down to 5 °C. The crude product which precipitated, was filtered, and washed with methanol. The product was purified by recrystallization from boiling acetic acid. The obtained solid was dissolved in Dichloromethane and the mixture was washed with a 5% LiCl aqueous solution, brine, and H2O. The solvent was removed to yield (E,E’)-4,4´-Bis[p-fluorostyryl]-2,2´- bipyridine as a beige solid (1.38 g, 3.48 mmol, 64 %). ¹ H-NMR (400 MHz, CD2Cl2): δ 8.75 (dd, J = 5.0, 0.7 Hz, 2H), 8.68 (dd, J = 1.7, 0.7 Hz, 2H), 7.69 (dd, J = 8.7, 5.3 Hz, 4H), 7.55 (d, J = 16.5 Hz, 2H), 7.52 (dd, J = 5.0, 1.7 Hz, 2H), 7.22 (dd, J = 8.7, 5.3 Hz, 4H), 7.20 (d, J = 16.5 Hz, 2H); ESI-HRMS (pos. detection mode): calcd for C26H19N2F2, 397.1516; found, 397.1513. (E,E’)-4,4´-Bis[p-hydroxystyryl]-2,2´-bipyridine (E,E’)-4,4´-Bis[p-hydroxystyryl]-2,2´-bipyridine was synthesized according to the literature (Balasubramanian K. K. et al., 1995). (E,E’)-4,4´-Bis[p-nitrostyryl]-2,2´-bipyridine (E,E’)-4,4´-Bis[p-nitrostyryl]-2,2´-bipyridine was synthesized according to the literature (Balasubramanian K. K. et al., 1995). (E,E’)-4,4´-Bis[p-aminostyryl]-2,2´-bipyridine (E,E’)-4,4´-Bis[p-aminostyryl]-2,2´-bipyridine was synthesized according to the literature (Gajardo, F. et al., 2011). Tetraethyl 2,2'-bipyridine-4,4'-bisphosphonate Tetraethyl-2,2'-bipyridine-4,4'-bisphosphonate was synthesized according to the literature (Woo, S.J., 2019). 4,4'-bis((E)-2-(4-methoxyphenyl)prop-1-en-1-yl)-2,2'-bipyrid ine Under nitrogen, tetraethyl-2,2'-bipyridine-4,4'-bisphosphonate (200 mg, 0.44 mmol, 1.0 equiv.) was placed in a flame-dried flask. Anhydrous THF (10 mL) was added and the solution was cooled down to 0 °C. KHMDS (2.6 mL, 1.30 mmol, 3.0 equiv., 0.5 mol/L solution in toluene) was added and the mixture was allowed to warm up to room temperature. The mixture was heated at 60 °C for 1 h. The mixture was then cooled down and 4-acetanisole (330 mg, 2.20 mmol, 5.0 equiv.) dissolved in anhydrous THF (5 mL) was added. The mixture was heated at 60 °C for 15 h. The mixture was cooled down and distilled water (35 mL) was added. The suspension was centrifuged, the solid was washed with hot ethanol and dried under vacuum to yield a beige powder (65 mg, 0.14 mmol, 32%). ¹ H-NMR (400 MHz, CDCl3): δ 8.66 (d, J = 5.1 Hz, 2H), 8.39 (s, 2H), 7.49 (d, J = 8.8 Hz, 4H), 7.28 (d, J = 4.4 Hz, 2H), 6.93 (d, J = 8.8 Hz, 4H), 6.79 (s, 2H), 3.85 (s, 6H), 2.35 (d, J = 1.32, 6H). ¹³ C-NMR (CDCl3, 125 MHz): δ = 159.6, 156.4, 149.2, 147.3, 141.1, 135.8, 127.4, 124.1, 123.8, 121.5, 114.0, 55.5, 18.0. ESI-HRMS (pos. detection mode): calcd for C30H29N2O2 m/z [M+H] ⁺ 449.2229; found: 449.2223. Synthesis of the Ruthenium Complexes [Ru(E,E’)-4,4´-Bisstyryl-2,2´-bipyridine)3][PF6]2 (1) (E,E’)-4,4´-Bisstyryl-2,2´-bipyridine (400 mg, 1.11 mmol, 4.0 equiv.) and Ru(DMSO) ₄ Cl ₂ (134 mg, 0.28 mmol, 1.0 equiv.) were suspended in dry Ethanol (150 mL) under nitrogen atmosphere and the mixture was refluxed for 24 h. Then the solution was cooled down and undissolved residue was removed via filtration. To the residual solution a sat. aqueous solution of NH4PF6 was added. The crude product, which precipitated as a PF ₆ salt was collected by centrifugation and washed with Ethanol, H ₂ O and Et ₂ O. The crude product was dissolved in Dichloromethane and washed with a 5% LiCl aqueous solution, brine and H ₂ O. After drying, 323 mg of 1 (0.22 mmol, 79 %) were yielded as a red solid. ¹ H-NMR (CD ₃ CN, 500 MHz): δ = 8.76 (d, ³ J = 1.8 Hz, 6H), 7.78 (d, ³ J = 16.4 Hz, 6H), 7.76 (d, ³ J = 5.9 Hz, 6H), 7.71-7.69 (m, 12H), 7.51 (dd, ^3,4 J = 5.9, 1.8 Hz, 6H), 7.49-7.46 (m, 12H), 7.44-7.40 (m, 6H), 7.33 (d, ³ J = 16.4 Hz, 6H). ¹³ C-NMR (CD ₃ CN, 125 MHz): δ = 158.2, 152.4, 147.6, 137.3, 136.8, 130.6, 130.1, 128.4, 125.4, 125.1, 121.7. ESI-HRMS (pos. detection mode): calcd for C78H60N6Ru m/z [M] ²⁺ 591.1956; found: 591.1978. Elemental analysis calcd for C78H60F12N6P2Ru+4*H2O (%): C 60.66, H 4.44, N 5.44; found: C 60.62, H 4.43, N 5.86. [Ru(E,E’)-4,4´-Bisstyryl-2,2´- bipyridine) ₃ ][Cl] ₂ : The counter ion PF ₆ was exchanged to Cl by elution with MeOH from the ion exchange resin Amberlite IRA-410. Elemental analysis calcd for C78H60Cl2N6Ru (%): C 74.75, H 4.83, N 6.71; found: C 74.36, H 4.51, N 6.37. [Ru((E,E’)-4,4´-Bis[p-(N,N-dimethylamino)styryl]-2,2´-bi pyridine)3][PF6]2 (2) (E,E’)-4,4´-Bis[p-(N,N-dimethylamino)styryl]-2,2´-bipyri dine (338 mg, 0.76 mmol, 4.0 equiv.), Ru(DMSO)4Cl2 (92 mg, 0.19 mmol, 1.0 equiv.) and LiCl (401 mg, 9.46 mmol, 50.0 equiv.) were dissolved in dry DMF (50 mL) under nitrogen atmosphere. The mixture was refluxed for 48 h. The solution was then cooled down and a sat. aqueous solution of NH4PF6 was added. The crude product, which precipitated as a PF6 salt was collected by centrifugation and washed with Ethanol, H2O and Et2O. The residue was dissolved in Dichloromethane and washed with a 5% LiCl aqueous solution, brine and H2O. The solvent was removed under reduced pressure and the crude product recrystallized from DCM/Pentane. The product was isolated via fractionated precipitation from CH ₃ CN by adding dropwise Et ₂ O.86 mg of 2 (0.05 mmol, 26 %) were yielded as a black solid. ¹ H-NMR (CD3CN, 500 MHz): δ = 8.62 (d, ³ J = 1.9 Hz, 6H), 7.66 (d, ³ J = 16.2 Hz, 6H), 7.65 (d, ³ J = 6.1 Hz, 6H), 7.55-7.52 (m, 12H), 7.38 (dd, ^3,3 J = 6.1, 1.9 Hz, 6H), 7.03 (d, ³ J = 16.2 Hz, 6H), 6.81-6.78 (m, 12H), 3.01 (s, 36H). ¹³ C-NMR (CD ₃ CN, 125 MHz): δ = 159.4, 158.1, 152.6, 148.3, 137.7, 130.0, 124.5, 124.4, 120.6, 119.7, 113.2, 40.4. ESI-HRMS (pos. detection mode): calcd for C90H90N12Ru m/z [M] ²⁺ 720.3222; found: 720.3247. Elemental analysis calcd for C90H90F12N12P2Ru (%): C 62.46, H 5.24, N 9.71; found: C 62.54, H 5.17, N 9.79. [Ru((E,E’)-4,4´-Bis[p-(N,N- dimethylamino)styryl]-2,2´-bipyridine) ₃ ][Cl] ₂ : The counter ion PF ₆ was exchanged to Cl by elution with MeOH from the ion exchange resin Amberlite IRA-410. Elemental analysis calcd for C90H90Cl2N12Ru (%): C 71.51, H 6.00, N 11.12; found: C 71.19, H 5.93, N 10.84. [Ru((E,E’)-4,4´-Bis[p-methoxystyryl]-2,2´-bipyridine) ₃ ][PF ₆ ] ₂ (3) (E,E’)-4,4´-Bis[p-methoxystyryl]-2,2´-bipyridine (286 mg, 0.68 mmol, 4.0 equiv.) and Ru(DMSO) ₄ Cl ₂ (82 mg, 0.17 mmol, 1.0 equiv.) were suspended dry EtOH (50 mL) under nitrogen atmosphere. The mixture was refluxed for 15 h. The solution was then cooled down and undissolved solid was removed by filtration. A sat. aqueous solution of NH4PF6 was added and the crude product, which precipitated as a PF6 salt was collected by filtration. The solid was washed with H2O and Et2O. The residue was purified via fractionated precipitation from CH3CN by adding dropwise Et2O. The collected product was dissolved in Dichloromethane and washed with a 5% LiCl aqueous solution, brine and H2O. After drying, 218 mg of 3 (0.13 mmol, 76 %) were yielded as a black solid. ¹ H-NMR (CD3CN, 500 MHz): δ = 8.79 (d, ⁴ J = 1.7 Hz, 6H), 7.79 (d, ³ J = 16.4 Hz, 6H), 7.71 (d, ³ J = 6.0 Hz, 6H), 7.64 (d, ³ J = 8.9 Hz, 12H), 7.44 (d, ^3,4 J = 6.0, 1.7 Hz, 6H), 7.17 (d, ³ J = 16.4 Hz, 6H), 7.00 (d, ³ J = 8.9 Hz, 12H), 3.83 (s, 18H). ¹³ C- NMR (CD3CN, 125 MHz): δ = 162.0, 158.2, 152.2, 148.0, 137.0, 130.1, 129.5, 125.0, 122.7, 121.3, 115.5, 56.2. ESI-HRMS (pos. detection mode): calcd for C84H72N6O6Ru m/z [M] ²⁺ 681.2290; found: 681.2273. Elemental analysis calcd for C84H72F12N6O6P2Ru (%): C 61.05, H 4.39, N 5.09; found: C 61.17, H 4.44, N 5.21. [Ru((E,E’)-4,4´-Bis[p-methoxystyryl]-2,2´-bipyridine) ₃ ][Cl] ₂ : The counter ion PF ₆ was exchanged to Cl by elution with MeOH from the ion exchange resin Amberlite IRA-410. Elemental analysis calcd for C84H72Cl2N6O6Ru (%): C 70.38, H 5.06, N 5.86; found: C 70.62, H 5.28, N 5.57. [Ru(bipy)((E,E’)-4,4´-Bis[p-(N,N-dimethylamino)styryl]-2, 2´-bipyridine) ₂ ][PF ₆ ] ₂ (4) (E,E’)-4,4´-Bis[p-(N,N-dimethylamino)styryl]-2,2´-bipyri dine (220 mg, 0.49 mmol, 2.0 equiv.), Ru(DMSO) ₄ Cl ₂ (119 mg, 0.25 mmol, 1.0 equiv.) and LiCl (1044 mg, 24.63 mmol, 100 equiv.) were suspended in dry DMF (30 mL) under nitrogen atmosphere. The mixture was refluxed for 4 h. The solution was then cooled down and H ₂ O was added. The crude product, which precipitated was collected by filtration and washed with H ₂ O and Et ₂ O. The formation of [Ru((E,E’)-4,4´-Bis[p-(N,N- dimethylamino)styryl]-2,2´-bipyridine) ₂ Cl ₂ ] was analysed via HPLC. [Ru((E,E’)-4,4´- Bis[p-(N,N-dimethylamino)styryl]-2,2´-bipyridine) ₂ Cl ₂ ] and 2,2´-Bipyrdine (47 mg, 0.3 mmol, 1.2 equiv.) were suspended in dry Ethanol (50 mL) under nitrogen atmosphere. The mixture was refluxed for 7 h. The solution was then cooled down and undissolved solid was removed by filtration. A sat. aqueous solution of NH4PF6 was added and the crude product, which precipitated as a PF ₆ salt was collected by filtration. The solid was washed with H2O and Et2O. The residue was purified via preparative HPLC as a TFA salt. The solvents were millipore water (0.1% TFA, solvent A) and acetonitrile (solvent B). The following HPLC gradient has been used: 0-3 minutes: isocratic 50% A (50% B); 3- 17 minutes: linear gradient from 50% A (50% B) to 0% A (100% B); 17-23 minutes: isocratic 0% A (100% B). The flow rate was 20 mL/min and the chromatogram was detected at 250nm, 350nm, 450nm. The collected product was dissolved in CH3CN and a sat. aqueous solution of NH4PF6 was added. The product, which precipitated as a PF6 salt was collected by filtration and washed with H2O, Et2O and Pentane. 89 mg of 4 (0.06 mmol, 24 %) were yielded as a dark red solid. ¹ H-NMR (CD3CN, 400 MHz): δ = 8.61 (s, 4H), 8.50 (d, J = 8.2 Hz, 2H), 8.04 (td, J = 8.0, 1.5 Hz, 2H), 7.86 (ddd, J = 5.7, 1.4, 0.6 Hz, 2H), 7.66 (dd, J = 16.2, 1.9 Hz, 4H), 7.64 (d, J = 6.3 Hz, 2H), 7.56-7.50 (m, 10H), 7.43- 7.34 (m, 6H), 7.02 (dd, J = 16.2 Hz, 4H), 6.81-6.77 (m, 8H), 3.02 (s, 12H), 3.01 (s, 12H). ¹³ C-NMR (CD ₃ CN, 100 MHz): δ = 158.1, 158.0, 152.7, 152.6, 151.9, 151.9, 148.5, 138.3, 137.9, 130.0, 128.4, 125.1, 124.4, 120.7,119.6, 113.2, 40.4. ESI-HRMS (pos. detection mode): calcd for C70H68N10Ru m/z [M] ²⁺ 575.2330; found: 575.2347. Elemental analysis calcd for C70H68F12N10P2Ru (%): C 58.37, H 4.76, N 9.72; found: C 58.19, H 4.62, N 9.72. [Ru(bipy)((E,E’)-4,4´-Bis[p-methoxystyryl]-2,2´-bipyridi ne) ₂ ][PF ₆ ] ₂ (5) (E,E’)-4,4´-Bis[p-methoxystyryl]-2,2´-bipyridine (490 mg, 1.17 mmol, 2.0 equiv.), Ru(DMSO) ₄ Cl ₂ (282 mg, 0.58 mmol, 1.0 equiv.) and LiCl (2470 mg, 58.26 mmol, 100 equiv.) were suspended in dry DMF (75 mL) under nitrogen atmosphere. The mixture was refluxed for 6 h. The solution was then cooled down and purged into H ₂ O. The crude product, which precipitated was collected by filtration and washed with H ₂ O and Et ₂ O. The formation of [Ru((E,E’)-4,4´-Bis[p-methoxystyryl]-2,2´-bipyridine) ₂ Cl ₂ ] was analysed via HPLC. [Ru((E,E’)-4,4´-Bis[p-methoxystyryl]-2,2´-bipyridine) ₂ Cl ₂ ] and 2,2´-Bipyrdine (109 mg, 0.70 mmol, 1.2 equiv.) were suspended in dry Ethanol (100 mL) under nitrogen atmosphere. The mixture was refluxed for 6 h. The solution was then cooled down and undissolved solid was removed by filtration. A sat. aqueous solution of NH ₄ PF ₆ was added and the crude product, which precipitated as a PF ₆ salt was collected by centrifugation. The solid was washed with H2O and Et2O. The residue was purified via preparative HPLC as a TFA salt. The solvents were millipore water (0.1% TFA, solvent A) and acetonitrile (solvent B). The following HPLC gradient has been used: 0-3 minutes: isocratic 50% A (50% B); 3- 17 minutes: linear gradient from 50% A (50% B) to 0% A (100% B); 17-23 minutes: isocratic 0% A (100% B). The flow rate was 20 mL/min and the chromatogram was detected at 250nm, 350nm, 450nm. The collected product was dissolved in CH3CN and a sat. aqueous solution of NH4PF6 was added. The product, which precipitated as a PF6 salt was collected by filtration and washed with H2O, Et2O and Hexane.248 mg of 5 (0.18 mmol, 31 %) were yielded as a dark red solid. ¹ H-NMR (CD3CN, 400 MHz): δ = 8.72 (d, J = 1.6 Hz, 4H), 8.51 (d, J = 8.2 Hz, 2H), 8.06 (td, J = 8.0, 1.3 Hz, 2H), 7.85 (dd, J = 5.6, 1.1 Hz, 2H), 7.75-7.68 (m, 6H), 7.67-7.61 (m, 8H), 7.60 (d, J = 5.9 Hz, 2H), 7.46-7.39 (m, 6H), 7.17 (dd, J = 16.4, 2.1 Hz, 4H), 7.05-6.99 (m, 8H), 3.85 (s, 6H), 3.84 (s, 6H). ¹³ C-NMR (CD ₃ CN, 100 MHz): δ = 162.0, 158.1, 152.6, 152.2, 148.1, 138.6, 137.1, 132.8, 130.1, 129.5, 128.5, 125.2, 125.0, 122.7, 121.3, 115.5, 56.2. ESI-HRMS (pos. detection mode): calcd for C66H56N6O4Ru m/z [M] ²⁺ 549.1698; found: 549.1707. Elemental analysis calcd for C66H56F12N6O4P2Ru+1Hexane (%): C 57.96, H 4.44, N 5.87; found: C 58.43, H 3.94, N 5.79. [Ru(bipy) ₂ ((E,E’)-4,4´-Bis[p-(N,N-dimethylamino)styryl]-2,2´ -bipyridine)][PF ₆ ] ₂ (6) Ru(bipy) ₂ Cl ₂ (350 mg, 0.72 mmol, 1.0 equiv.) and (E,E’)-4,4´-Bis[p-(N,N- dimethylamino)styryl]-2,2´-bipyridine (388 mg, 0.87 mmol, 1.2 equiv.) were suspended in dry Ethanol (50 mL) under nitrogen atmosphere and the mixture was refluxed for 6 h. Then the solution was cooled down and a sat. aqueous solution of NH ₄ PF ₆ was added. The crude product, which precipitated as a PF ₆ salt was collected by filtration and washed with H ₂ O and Et ₂ O. The product was isolated via fractionated precipitation from CH ₃ CN by adding dropwise Et ₂ O. 449 mg of 6 (0.39 mmol, 54 %) were yielded as a dark red solid. ¹ H-NMR (CD ₃ CN, 500 MHz): δ = 8.62 (d, J = 1.7 Hz, 2H), 8.50 (d, J = 8.2 Hz, 4H), 8.07-8.02 (m, 4H), 7.87-7.84 (m, 2H), 7.75-7.72 (m, 2H), 7.67 (d, J = 16.3 Hz, 2H), 7.57-7.52 (m, 4H), 7.52-7.49 (d, J = 6.0 Hz, 2H), 7.44-7.34 (m, 6H), 7.02 (d, J = 16.3 Hz, 2H), 6.81-6.76 (m, 4H), 3.01 (s, 12H). ¹³ C-NMR (CD ₃ CN, 125 MHz): δ = 158.1, 158.0, 158.0, 152.7, 152.7, 152.6, 151.9, 148.8, 138.6, 138.0, 130.0, 128.5, 128.5, 125.2, 124.4, 124.3, 120.8, 119.5, 113.1, 40.4. ESI-HRMS (pos. detection mode): calcd for C50H46N8Ru m/z [M] ²⁺ 430.1439; found: 430.1441. Elemental analysis calcd for C50H46F12N8P2Ru (%): C 52.22, H 4.03, N 9.74; found: C 51.97, H 4.04, N 9.71. [Ru(bipy)2((E,E’)-4,4´-Bis[p-(N,N-methoxy)styryl]-2,2´-b ipyridine)][PF6]2 (7) Ru(bipy)2Cl2 (432 mg, 0.89 mmol, 1.0 equiv.) and (E,E’)-4,4´-Bis[p-(N,N- methoxy)styryl]-2,2´-bipyridine (450 mg, 1.07 mmol, 1.2 equiv.) were suspended in dry Ethanol (100 mL) under nitrogen atmosphere and the mixture was refluxed for 6 h. Then the solution was cooled down and a sat. aqueous solution of NH4PF6 was added. The crude product, which precipitated as a PF6 salt was collected by filtration and washed with H ₂ O and Et ₂ O. The product was isolated via fractionated precipitation from CH3CN by adding dropwise Et2O.358 mg of 7 (0.32 mmol, 36 %) were yielded as a dark red solid. ¹ H-NMR (CD ₃ CN, 500 MHz): δ = 8.71 (d, J = 1.4 Hz, 2H), 8.51 (dd, J = 8.2, 0.7 Hz, 4H), 8.06 (td, J = 8.0, 1.5 Hz, 4H), 7.86-7.84 (m, 2H), 7.76-7.73 (m, 2H), 7.72 (d, J = 16.4 Hz, 2H), 7.66-7.62 (m, 4H), 7.59 (d, J = 6.0 Hz, 2H), 7.45-7.38 (m, 6H), 7.16 (d, J = 16.4 Hz, 2H), 7.03-6.98 (m, 4H), 3.83 (s, 6H). ¹³ C-NMR (CD ₃ CN, 125 MHz): δ = 162.0, 158.1, 158.0, 152.7, 152.6, 152.2, 148.2, 138.7, 138.7, 137.1, 130.1, 129.5, 128.6, 128.5, 125.2, 125.0, 122.6, 121.4, 115.5, 56.2. ESI-HRMS (pos. detection mode): calcd for C48H40N6O2Ru m/z [M] ²⁺ 417.1123; found: 417.1126. Elemental analysis calcd for C48H40F12N6O2P2Ru (%): C 51.30, H 3.59, N 7.48; found: C 51.23, H 3.48, N 7.61. [Ru(bipy) ₂ ((E,E’)-4,4´-Bis[p-(N,N-methoxy)styryl]-2,2´-bipyr idine)][Cl] ₂ : The counter ion PF ₆ was exchanged to Cl by elution with MeOH from the ion exchange resin Amberlite IRA-410. Elemental analysis calcd for C48H40Cl2N6O2Ru (%): C 63.70, H 4.46, N 9.29; found: C 63.51, H 4.30, N 9.11. [Ru(phen) ₂ ((E,E’)-4,4´-Bis[p-(N,N-dimethylamino)styryl]-2,2´ -bipyridine)][PF ₆ ] ₂ (8) Ru(phen)2Cl2 (455 mg, 0.86 mmol, 1.0 equiv.) and (E,E’)-4,4´-Bis[p-(N,N- dimethylamino)styryl]-2,2´-bipyridine (458 mg, 1.03 mmol, 1.2 equiv.) were suspended in dry Ethanol (50 mL) under nitrogen atmosphere and the mixture was refluxed for 19 h. Then the solution was cooled down, undissolved residue was removed via filtration and washed with Ethanol. To the residual solution a sat. aqueous solution of NH4PF6 was added. The crude product, which precipitated as a PF6 salt was collected by filtration and washed with water and Et2O. The solid was dissolved in Dichloromethane and washed with a 5% LiCl aqueous solution, brine and water. The solvent was removed under reduced pressure and the product was purified via fractionated precipitation from Acetonitrile by adding dropwise Et2O. The obtained solid was separated by filtration and was washed with H2O, Et2O and Pentane.427 mg of 1 (0.36 mmol, 41 %) were yielded as a red solid. ¹ H-NMR (CD3CN, 400 MHz): δ = 8.68- 8.63 (m, 4H), 8.54 (dd, J = 8.3, 1.4 Hz, 2H), 8.34-8.31 (m, 2H), 8.29-8.14 (m, 6H), 7.89 (dd, J = 5.4, 1.4 Hz, 2H), 7.81 (dd, J = 8.3, 5.1 Hz, 2H), 7.64 (d, J = 16.4 Hz, 2H), 7.57-7.49 (m, 4H), 7.46 (d, J = 6.1 Hz, 2H), 7.24 (dd, J = 6.0, 1.7 Hz, 2H), 7.00 (d, J = 16.4 Hz, 2H), 6.80-6.74 (m, 4H), 3.00 (s, 12H). ¹³ C-NMR (CD3CN, 100 MHz): δ = 158.3, 153.7, 153.6, 152.7, 152.4, 148.9, 148.7, 137.9, 137.6, 137.5, 132.0, 130.0, 129.0, 127.0, 126.8, 124.3, 124.2, 120.7, 119.5, 113.7, 113.1, 40.4. HR-MS (pos. detection mode): calcd for C54H46N8Ru m/z [M] ²⁺ 454.1439; found: 454.1455. Elemental analysis calcd for C ₅₄ H ₄₆ F ₁₂ N ₈ P ₂ Ru (%): C 54.21, H 3.91, N 9.24; found: C 54.14, H 3.87, N 9.35. [Ru(phen) ₂ ((E,E’)-4,4´-Bis[p-methoxystyryl]-2,2´-bipyridine) ][PF ₆ ] ₂ (9) Ru(phen) ₂ Cl ₂ (443 mg, 0.83 mmol, 1.0 equiv.) and (E,E’)-4,4´-Bis[p-methoxystyryl]-2,2´- bipyridine (420 mg, 0.99 mmol, 1.2 equiv.) were suspended in dry Ethanol (50 mL) under nitrogen atmosphere and the mixture was refluxed for 24 h. Then the solution was cooled down and undissolved residue was removed via filtration. To the residual solution a sat. aqueous solution of NH ₄ PF ₆ was added. The crude product, which precipitated as a PF ₆ salt was collected by filtration and washed with water and Et ₂ O. The product was isolated by column chromatography on silica gel with an Acetonitrile/aq. KNO ₃ (0.4 M) solution (10:1). The fractions containing the product were united and the solvent was removed under reduced pressure. The residue was dissolved in Acetonitrile and undissolved KNO ₃ was removed by filtration. The solvent was removed again and the product was dissolved in H2O (50 mL). Upon addition of NH4PF6 the product precipitated as a PF6 salt. The solid was obtained by filtration and was washed three-times with H2O and Et2O. 672 mg of 2 (0.57 mmol, 69 %) were yielded as a red solid. ¹ H-NMR (CD3CN, 400 MHz): δ = 8.70 (s, 2H), 8.65 (d, J = 8.2 Hz, 2H), 8.55 (d, J = 8.2 Hz, 2H), 8.32-8.22 (m, 6H), 7.89 (d, J = 4.7 Hz, 2H), 7.81 (dd, J = 8.2, 5.2 Hz, 2H), 7.68 (d, J = 16.2 Hz, 2H), 7.63-7-53 (m, 8H), 7.30 (d, J = 5.5 Hz, 2H), 7.13 (d, J = 16.2 Hz, 2H), 7.01 (d, J = 8.0 Hz, 4H), 3.83 (s, 6H). ¹³ C-NMR (CD3CN, 100 MHz): δ = 162.0, 158.4, 153.7, 153.6, 152.7, 148.9, 148.6, 148.1, 137.7, 137.6, 137.0, 132.0, 130.0, 129.4, 129.0, 127.0, 126.8, 124.7, 122.6, 121.2, 115.5, 56.1. HR-MS (pos. detection mode): calcd for C52H40N6O2Ru m/z [M] ²⁺ 441.1123; found: 441.1131. Elemental analysis calcd for C52H40F12N6O2P2Ru (%): C 53.29, H 3.44, N 7.17; found: C 53.18, H 3.35, N 7.26. [Ru(4,7-Diphenyl-1,10-phenanthroline)2((E,E’)-4,4´-Bis[p- (N,N-dimethylamino)styryl]- 2,2´-bipyridine)][PF ₆ ] ₂ (10) Ru(4,7-Diphenyl-1,10-phenanthroline) ₂ Cl ₂ (335 mg, 0.40 mmol, 1.0 equiv.) and (E,E’)- 4,4´-Bis[p-(N,N-dimethylamino)styryl]-2,2´-bipyridine (215 mg, 0.48 mmol, 1.2 equiv.) were suspended in dry Ethanol (100 mL) under nitrogen atmosphere and the mixture was refluxed for 24 h. Then the solution was cooled down and undissolved residue was removed via filtration. The residual solution was diluted with a mixture of Ethanol and water and a sat. aqueous solution of NH ₄ PF ₆ was added. The crude product, which precipitated as a PF ₆ salt was collected by centrifugation and washed with water and Et ₂ O. The solid was dissolved in Acetonitrile and undissolved residue was removed via filtration. The solvent was removed under reduced pressure and the obtained solid was dissolved in Dichloromethane and washed with a 5% LiCl aqueous solution, brine and water. The solvent was removed under reduced pressure. The product was isolated via fractionated precipitation from Methanol by adding dropwise Et ₂ O. After drying, 289 mg of 3 (0.19 mmol, 48 %) were yielded as a red solid. ¹ H-NMR (CD ₃ CN, 400 MHz): δ = 8.73 (d, J = 1.5 Hz, 2H), 8.42 (d, J = 5.5 Hz, 2H), 8.26 – 8.16 (m, 4H), 8.12 (d, J = 5.5 Hz, 2H), 7.78 (d, J = 5.5 Hz, 2H), 7.70 (d, J = 16.3 Hz, 2H), 7.67 – 7.57 (m, 24H), 7.54 (d, J = 8.9 Hz, 4H), 7.34 (dd, J = 6.1, 1.7 Hz, 2H), 7.05 (d, J = 16.3 Hz, 2H), 6.78 (d, J = 9.0 Hz, 4H), 3.01 (s, 12H). ¹³ C-NMR (CD3CN, 100 MHz): δ = 158.3, 153.1, 152.7, 152.4, 149.9, 149.8, 149.5, 149.4, 148.8, 138.0, 136.8, 136.7, 130.8, 130.8, 130.6, 130.1, 130.1, 130.0, 129.9, 127.2, 127.0, 127.0, 127.0, 124.3, 120.8, 119.5, 113.1, 40.4. HR-MS (pos. detection mode): calcd for C78H62N8Ru m/z [M] ²⁺ 606.2065; found: 606.2078. Elemental analysis calcd for C78H62N8RuP2F12 + 1.5 MeOH (%): C 61.59, H 4.42, N 7.23; found: C 61.73, H 4.48, N 6.88. [Ru(4,7-Diphenyl-1,10-phenanthroline)2((E,E’)-4,4´-Bis[p- methoxystyryl]-2,2´- bipyridine)][PF6]2 (11) Ru(4,7-Diphenyl-1,10-phenanthroline)2Cl2 (300 mg, 0.36 mmol, 1.0 equiv.) and (E,E’)- 4,4´-Bis[p-methoxystyryl]-2,2´-bipyridine (181 mg, 0.43 mmol, 1.2 equiv.) were suspended in dry Ethanol (50 mL) under nitrogen atmosphere and the mixture was refluxed for 24 h. Then the solution was cooled down and undissolved residue was removed via filtration. The residual solution was diluted with a mixture of Ethanol and water and a sat. aqueous solution of NH ₄ PF ₆ was added. The crude product, which precipitated as a PF6 salt was collected by filtration and washed with water and Et2O. The solid was dissolved in Dichloromethane and washed with a 5% LiCl aqueous solution, brine and water. The solvent was removed under reduced pressure. After drying, 395 mg of 4 (0.27 mmol, 74 %) were yielded as a red solid. ¹ H-NMR (CD ₃ CN, 400 MHz): δ = 8.78 (s, 2H), 8.41 (d, J = 5.5 Hz, 2H), 8.21 (s, 4H), 8.13 (d, J = 5.5 Hz, 2H), 7.78 (d, J = 5.5 Hz, 2H), 7.75 (d, J = 2.3 Hz, 2H), 7.72 (d, J = 8.0 Hz, 2H), 7.66-7.58 (m, 26H), 7.41 (d, J = 6.1 Hz, 2H), 7.19 (d, J = 16.4 Hz, 2H), 7.02 (d, J = 8.7 Hz, 4H), 3.84 (s, 6H). ¹³ C- NMR (CD ₃ CN, 100 MHz): δ = 162.0, 158.4, 153.1, 152.8, 150.0, 149.9, 149.5, 149.3, 148.2, 137.1, 136.8, 136.7, 130.8, 130.8, 130.7, 130.1, 130.1, 130.1, 130.0, 129.9, 129.4, 127.2, 127.0, 124.9, 122.7, 121.3, 115.5, 56.2. HR-MS (pos. detection mode): calcd for C ₇₆ H ₅₆ N ₆ O ₂ Ru m/z [M] ²⁺ 593.1749; found: 593.1768. Elemental analysis calcd for C ₇₆ H ₅₆ F ₁₂ N ₆ O ₂ P ₂ Ru + 4*H ₂ O (%): C 58.95, H 4.17, N 5.43; found: C 58.58, H 3.94, N 5.72. [Ru(1,10-phenanthroline)3][PF6]2 ([Ru(phen)3][PF6]2) (Comparative example) [Ru(phen) ₃ ][PF ₆ ] ₂ was synthesized as previously published (Zuloaga, F. and Kasha, M., 1968) using RuCl2(DMSO)4 precursor. Purity of the sample was assessed by HPLC and elemental analysis. Elemental analysis calcd for C36H24F12N6P2Ru (%): C 46.41, H 2.60, N 9.02; found: C 46.34, H 2.54, N 8.83. [Ru(bpy)2((E,E’)-4,4´-Bis[m-methoxystyryl]-2,2´-bipyridi ne)][PF6]2 (12) Under nitrogen, Ru(bpy)2Cl2 (200 mg, 0.41 mmol, 1.0 equiv.) and (E,E’)-4,4´-Bis[m- methoxystyryl]-2,2´-bipyridine (208 mg, 0.50 mmol, 1.2 equiv.) were suspended in dry ethanol (50 mL) and the mixture was refluxed for 6 h. Then the solution was cooled down and a sat. aqueous solution of NH4PF6 was added. The crude product, which precipitated as a PF6 salt was collected by filtration and washed with water and diethyl ether. The product was purified by silica gel chromatography using acetonitrile/potassium nitrate 0.3 M in water (9:1 v/v) as eluent. The fractions containing the product were collected and the solvent evaporated. The residue was dissolved in acetonitrile and filtered. The filtrate was concentrated to dryness under vacuum. The residue was dissolved in ethanol and a sat. aqueous solution of NH ₄ PF ₆ was added. The precipitate was filtered, washed with water and diethyl ether, and dried under vacuum to yield 12 (390 mg, 0.35 mmol, 85%) as a dark red solid. ¹ H-NMR (400 MHz,CD ₃ CN): δ 8.72 (d, J = 1.9 Hz, 2H), 8.52 (d, J = 8.2 Hz, 4H), 8.07 (td, J = 7.9, 1.5, 4H), 7.85 (d, J = 5.6 Hz, 2H), 7.78-7.61 (m, 6H), 7.54-7.09 (m, 14H), 6.95 (dd, J = 7.3, 1.9, 2H), 3.84 (s, 6H). ¹³ C-NMR (CDCl ₃ , 101 MHz): δ = 161.2, 158.2, 158.0, 152.7, 152.4, 147.6, 138.8, 138.3, 137.2, 131.1, 128.6, 125.4, 125.3, 125.2, 121.7, 121.3, 116.4, 113.2, 56.0. ESI-HRMS (pos. detection mode): calcd for C ₄₈ H ₄₀ N ₆ O ₂ Ru m/z [M] ²⁺ 417.1128; found: 417.1126. Elemental analysis calcd for C ₄₈ H ₄₀ F ₁₂ N ₆ O ₂ P ₂ Ru (%): C 51.30, H 3.59, N 7.48; found: C 51.06, H 3.61, N 7.38. [Ru(bpy)2((E,E’)-4,4´-Bis[o-methoxystyryl]-2,2´-bipyridi ne)][PF6]2 (13) Under nitrogen, Ru(bpy)2Cl2 (150 mg, 0.31 mmol, 1.0 equiv.) and (E,E’)-4,4´-Bis[o- methoxystyryl]-2,2´-bipyridine (156 mg, 0.37 mmol, 1.2 equiv.) were suspended in dry ethanol (50 mL) and the mixture was refluxed for 6 h. Then the solution was cooled down and a sat. aqueous solution of NH ₄ PF ₆ was added. The crude product, which precipitated as a PF ₆ salt was collected by filtration and washed with water and diethyl ether. The product was purified by silica gel chromatography using acetonitrile/potassium nitrate 0.3 M in water (9:1 v/v) as eluent. The fractions containing the product were collected and the solvent evaporated. The residue was dissolved in acetonitrile and filtered. The filtrate was concentrated to dryness under vacuum. The residue was dissolved in ethanol and a sat. aqueous solution of NH4PF6 was added. The precipitate was filtered, washed with water and diethyl ether, and dried under vacuum to yield 13 (225 mg, 0.20 mmol, 65%) as a dark red solid. ¹ H-NMR (400 MHz,CD3CN): δ 8.71 (d, J = 1.9 Hz, 2H), 8.59-8.33 (m, 4H), 8.07 (m, 4H), 7.95 (d, J = 16.5 Hz, 2H), 7.84 (d, J = 4.3 Hz, 2H), 7.74 (ddd, J = 5.6, 1.5, 0.8 Hz, 2H), 7.68 (dd, J = 7.7, 1.7 Hz, 2H), 7.61 (d, J = 6.0 Hz, 2H), 7.42 (m, 10H), 7.06 (m, 4H), 3.94 (s, 6H). ¹³ C- NMR (CDCl3, 101 MHz): δ = 158.9, 158.2, 158.0, 152.6, 152.2, 148.3, 138.7, 132.7, 132.0, 129.0, 128.5, 125.8, 125.4, 125.2, 125.0, 121.9, 121.8, 112.6, 56.3. ESI-HRMS (pos. detection mode): calcd for C48H40N6O2Ru m/z [M] ²⁺ 417.1128; found: 417.1126. Elemental analysis calcd for C ₄₈ H ₄₀ F ₁₂ N ₆ O ₂ P ₂ Ru+H ₂ O (%): C 50.49, H 3.71, N 7.36; found: C 50.27, H 3.41, N 7.66. [Ru(bpy) ₂ ((E,E’)-4,4´-Bis[p-fluorostyryl]-2,2´-bipyridine)] [PF ₆ ] ₂ (14) Under nitrogen, Ru(bpy) ₂ Cl ₂ (242 mg, 0.50 mmol, 1.0 equiv.) and (E,E’)-4,4´-Bis[p- fluorostyryl]-2,2´-bipyridine (238 mg, 0.60 mmol, 1.2 equiv.) were suspended in dry ethanol (50 mL) and the mixture was refluxed for 6 h. Then the solution was cooled down and a sat. aqueous solution of NH ₄ PF ₆ was added. The crude product, which precipitated as a PF ₆ salt was collected by filtration and washed with water and diethyl ether. The product was purified by silica gel chromatography using acetonitrile/potassium nitrate 0.3 M in water (9:1 v/v) as eluent. The fractions containing the product were collected and the solvent evaporated. The residue was dissolved in acetonitrile and filtered. The filtrate was concentrated to dryness under vacuum. The residue was dissolved in ethanol and a sat. aqueous solution of NH ₄ PF ₆ was added. The precipitate was filtered, washed with water and diethyl ether, and dried under vacuum to yield 14 (273 mg, 0.29 mmol, 57%) as a dark red solid. ¹ H- NMR (CD3CN, 400 MHz): δ = 8.72 (d, J = 1.9 Hz, 2H), 8.53 (d, J = 8.2 Hz, 4H), 8.07 (t, J = 7.9 Hz, 4H), 7.85 (d, J = 5.6 Hz, 2H), 7.80 - 7.68 (m, 8H), 7.65 (d, J = 6.0 Hz, 2H), 7.52- 7.36 (m, 6H), 7.26 (d, J = 16.5 Hz, 2H), 7.19 (d, J = 8.8 Hz, 4H). ¹³ C-NMR (CD3CN, 101 MHz): δ = 164.3 (d, ¹ J C-F = 248 Hz), 158.2, 158.0, 158.0, 152.7, 152.4, 147.6, 138.8, 136.0, 133.4 (d, ⁴ JC-F = 3 Hz), 130.5 (d, ³ JC-F = 8 Hz), 128.6, 125.2, 125.0, 121.8, 116.9 (d, ² JC-F = 22 Hz). ¹⁹ F NMR (376 MHz, CD3CN) δ = -72.77 (d, J = 706.7 Hz), -112.83. ESI-HRMS (pos. detection mode): calcd for C46H34N6F2Ru m/z [M] ²⁺ 405.0928; found: 405.0927. Elemental analysis calcd for C46H34F14N6O2P2Ru+H2O (%): C 49.43, H 3.25, N 7.52; found: C 49.76, H 3.02, N 7.47. [Ru(bpy)2((E,E’)-4,4´-Bis[p-hydroxystyryl]-2,2´-bipyridi ne)][PF6]2 (15) Under nitrogen, Ru(bpy)2Cl2 (150 mg, 0.31 mmol, 1.0 equiv.) and (E,E’)-4,4´-Bis[p- hydroxystyryl]-2,2´-bipyridine (145 mg, 0.37 mmol, 1.2 equiv.) were suspended in ethylene glycol (5 mL) and the mixture was heated at 130 °C for 24 h. Then the solution was cooled down and a sat. aqueous solution of NH ₄ PF ₆ was added. The crude product, which precipitated as a PF ₆ salt was collected by filtration and washed with water and diethyl ether and dried under vacuum to yield 15 (297 mg, 0.27 mmol, 87%) as a dark red solid. ¹ H-NMR (CD ₃ CN, 400 MHz): δ = 8.66 (d, J = 1.9 Hz, 2H), 8.50 (d, J = 8.1 Hz, 4H), 8.05 (td, J = 8.0, 1.5 Hz, 4H), 7.84 (d, J = 5.1 Hz, 2H), 7.74 (d, J = 5.0 Hz, 2H), 7.68 (d, J = 16.4 Hz, 2H), 7.61 - 7.51 (m, 6H), 7.46 - 7.35 (m, 8H), 7.12 (d, J = 16.5 Hz, 2H), 6.90 (d, J = 8.6 Hz, 4H). ¹³ C-NMR (CD ₃ CN, 101 MHz): δ = 159.5, 158.01, 157.95, 152.6, 152.1, 148.2, 138.6, 137.3, 130.2, 128.7, 128.5, 125.1, 124.9, 122.1, 121.2, 116.9. ESI-HRMS (pos. detection mode): calcd for C ₄₆ H ₃₆ N ₆ O ₂ Ru m/z [M] ²⁺ 403.0972; found: 403.0970. Elemental analysis calcd for C ₄₆ H ₃₆ F ₁₂ N ₆ O ₂ P ₂ Ru+H ₂ O (%): C 49.60, H 3.44, N 7.55; found: C 49.46, H 3.35, N 7.31. [Os(bpy) ₂ ((E,E’)-4,4´-Bis[p-methoxystyryl]-2,2´-bipyridine) ][PF ₆ ] ₂ (16) Under nitrogen, Os(bpy) ₂ Cl ₂ (100 mg, 0.17 mmol, 1.0 equiv.) and (E,E’)-4,4´-Bis[p- methoxystyryl]-2,2´-bipyridine (84 mg, 0.20 mmol, 1.2 equiv.) were suspended in ethylene glycol (5 mL). The mixture was degassed by nitrogen bubbling for 15 min and heated at 130 °C for 24 h. Then the solution was cooled down and a sat. aqueous solution of NH4PF6 was added. The crude product, which precipitated as a PF6 salt was collected by filtration and washed with water and diethyl ether and dried under vacuum to yield 20 (189 mg, 0.16 mmol, 94%) as a dark brown solid. ¹ H-NMR (CD3CN, 400 MHz): δ = 8.64 (d, J = 2.0 Hz, 2H), 8.49 (d, J = 8.0 Hz, 4H), 7.86 (ddt, J = 9.7, 8.2, 1.7 Hz, 4H), 7.74 (d, J = 5.4 Hz, 2H), 7.71 - 7.59 (m, 8H), 7.50 (d, J = 6.1 Hz, 2H), 7.38 - 7.26 (m, 6H), 7.16 (d, J = 16.3 Hz, 2H), 7.02 (d, J = 8.8 Hz, 4H), 3.84 (s, 6H). ¹³ C-NMR (CD3CN, 101 MHz): δ = 162.0, 159.97, 159.92, 159.87, 151.8, 151.7, 151.3, 147.5, 138.0, 137.2, 130.1, 129.3, 129.0, 125.4, 125.2, 122.2, 121.4, 115.4. ESI-HRMS (pos. detection mode): calcd for C48H40N6O2Os m/z [M] ²⁺ 462.1414; found: 462.1408. Elemental analysis calcd for C48H40F12N6O2OsP2+H2O (%): C 46.83, H 3.44, N 6.83; found: C 46.92, H 3.29, N 6.34. [Ru(bpy) ₂ ((E,E’)-4,4´-Bis[p-aminostyryl]-2,2´-bipyridine)][ PF ₆ ] ₂ (17) Compound 17 was synthesized following a procedure described in the literature (Storrier, G.D. and Colbran, S.B., 1997). Elemental analysis calcd for C ₄₆ H ₃₈ F ₁₂ N ₈ P ₂ Ru+3H ₂ O (%): C 48.13, H 3.86, N 9.76; found: C 48.09, H 3.37, N 10.05. [Ru(bpy) ₂ (4,4'-(E,E’)-2-(4-methoxyphenyl)prop-1-en-1-yl)-2,2' -bipyridine)][PF ₆ ] ₂ (18) Under nitrogen, Ru(bpy) ₂ Cl ₂ (63 mg, 0.13 mmol, 1.0 equiv.) and 4,4'-bis((E)-2-(4- methoxyphenyl)prop-1-en-1-yl)-2,2'-bipyridine (65 mg, 0.15 mmol, 1.1 equiv.) were suspended in dry ethanol and the mixture was refluxed for 15 h. Then the solution was cooled down and a sat. aqueous solution of NH ₄ PF ₆ was added. The crude product, which precipitated as a PF ₆ salt was collected by filtration and washed with water and diethyl ether. The product was purified by silica gel chromatography using acetonitrile/potassium nitrate 0.3 M in water (9:1 v/v) as eluent. The fractions containing the product were collected and the solvent evaporated. The residue was dissolved in acetonitrile and filtered. The filtrate was concentrated to dryness under vacuum. The residue was dissolved in ethanol and a sat. aqueous solution of NH ₄ PF ₆ was added. The precipitate was filtered, washed with water and diethyl ether, and dried under vacuum to yield 18 (124 mg, 0.11 mmol, 85%) as a dark red solid. ¹ H- NMR (CD3CN, 400 MHz): δ = 8.55 - 8.41 (m, 6H), 8.07 (tdd, J = 7.8, 4.7, 1.5 Hz, 4H), 7.91 - 7.82 (m, 2H), 7.79 - 7.73 (m, 2H), 7.66 - 7.53 (m, 6H), 7.50 - 7.35 (m, 6H), 7.06 - 6.95 (m, 4H), 6.95 - 6.87 (m, 2H), 3.94 - 3.74 (m, 6H), 2.41-2.24 (m, 6H). ¹³ C NMR (101 MHz, CD3CN) δ = 161.10, 160.60, 157.94, 157.69, 152.66, 152.60, 151.83, 148.61, 145.49, 138.61, 135.58, 129.86, 128.45, 127.64, 125.14, 124.79, 122.58, 114.86, 56.00, 18.11. ESI-HRMS (pos. detection mode): calcd for C50H44O2N6Ru m/z [M] ²⁺ 431.1285; found: 431.1282. Elemental analysis calcd for C50H44F12N6O2P2Ru+2H2O (%): C 50.55, H 4.07, N 7.07; found: C 50.73, H 3.79, N 7.21. [Ru(bpz)2((E,E’)-4,4´-Bis[p-methoxystyryl]-2,2´-bipyridi ne)][PF6]2 (19) Under nitrogen, Ru(bpz)2Cl2 (100 mg, 0.21 mmol, 1.0 equiv.) and (E,E’)-4,4´-Bis[p- methoxystyryl]-2,2´-bipyridine (104 mg, 0.25 mmol, 1.2 equiv.) were suspended in dry ethylene glycol (5 mL) and the mixture was heated at 130 °C for 24 h. Then the solution was cooled down and a sat. aqueous solution of NH4PF6 was added. The crude product, which precipitated as a PF ₆ salt was collected by filtration and washed with water and diethyl ether. The product was purified by silica gel chromatography using acetonitrile/potassium nitrate 0.3 M in water (9:1 v/v) as eluent. The fractions containing the product were collected and the solvent evaporated. The residue was dissolved in acetonitrile and filtered. The filtrate was concentrated to dryness under vacuum. The residue was dissolved in ethanol and a sat. aqueous solution of NH ₄ PF ₆ was added. The precipitate was filtered, washed with water and diethyl ether, and dried under vacuum to yield 19 (43 mg, 0.04 mmol, 19%) as a dark red solid. ¹ H- NMR (CD ₃ CN, 400 MHz): δ = 9.76 (t, J = 1.5 Hz, 4H), 8.71 (d, J = 1.9 Hz, 2H), 8.61 (dd, J = 9.8, 3.2 Hz, 4H), 7.90 (dd, J = 3.3, 1.2 Hz, 2H), 7.87 (dd, J = 3.2, 1.2 Hz, 2H), 7.76 (d, J = 16.4 Hz, 2H), 7.66 (d, J = 8.9 Hz, 4H), 7.54 (d, J = 6.0 Hz, 2H), 7.48 (dd, J = 6.1, 1.8 Hz, 2H), 7.19 (d, J = 16.3 Hz, 2H), 7.03 (d, J = 8.8 Hz, 4H), 3.85 (s, 6H). ¹³ C-NMR (CD ₃ CN, 101 MHz): δ = 162.2, 157.2, 152.6, 151.7, 149.7, 149.34, 149.30, 147.9, 147.3, 146.1, 138.0, 130.2, 129.2, 125.3, 122.2, 121.6, 115.5, 56.1. ESI-HRMS (pos. detection mode): calcd for C44H36N10O2Ru m/z [M] ²⁺ 419.1033; found: 419.1030. Elemental analysis calcd for C ₄₄ H ₃₆ F ₁₂ N ₁₀ O ₂ P ₂ Ru+4H ₂ O (%): C 44.04, H 3.70, N 11.67; found: C 44.11, H 2.95, N 11.60. [Ru(4,7-Diphenyl-1,10-phenanthroline)3][PF6]2 ([Ru(bphen)3][PF6]2) (Comparative example) [Ru(bphen)3][PF6]2 was synthesized as previously published (Crosby, G.A. and Watts, R.J., 1971) using RuCl2dmso4 precursor. Purity of the sample was assessed by HPLC and elemental analysis. Elemental analysis calcd for C72H48F12N6P2Ru (%): C 62.30, H 3.49, N 6.05; found: C 62.28, H 3.44, N 5.92. 2. Photophysical properties Photophysical measurements were performed to evaluate the potential of the complexes as photosensitizers. Spectroscopic measurements The absorption of the samples was measured with a SpectraMax M2 Spectrometer (Molecular Devices). The emission was measured by irradiation of the sample in fluorescence quartz cuvettes (width 1 cm) using a NT342B Nd-YAG pumped optical parametric oscillator (Ekspla) at 355 nm. Luminescence was focused and collected at right angle to the excitation pathway and directed to a Princeton Instruments Acton SP-2300i monochromator. As a detector, a XPI-Max 4 CCD camera (Princeton Instruments) has been used. Luminescence quantum yield measurements For the determination of the luminescence quantum yield, the samples were prepared in an CH ₃ CN solution with an absorbance of 0.1 at 355 nm. This solution was irradiated in fluorescence quartz cuvettes (width 1 cm) using a NT342B Nd-YAG pumped optical parametric oscillator (Ekspla) at 355 nm. The emission signal was focused and collected at right angle to the excitation pathway and directed to a Princeton Instruments Acton SP-2300i monochromator. As a detector a XPI-Max 4 CCD camera (Princeton Instruments) has been used. The luminescence quantum yields were determined by comparison with the reference [Ru(bipy) ₃ ]Cl ₂ in CH ₃ CN (Φ _em =5.9%) (Nakamaru, K., 1982) applying the following formula: Φ _{em, sample} = Φ _{em, reference} * (F _reference / F _sample ) * (I _sample / I _reference ) * (n _sample / n _reference )2 F = 1 – 10 ^-A Φem = luminescence quantum yield, F = fraction of light absorbed, I = integrated emission intensities, n = refractive index, A = absorbance of the sample at irradiation wavelength. Lifetime measurements For the determination of the lifetimes, the samples were prepared in an air saturated and in a degassed CH3CN solution with an absorbance of 0.2 at 355 nm. This solution was irradiated in fluorescence quartz cuvettes (width 1 cm) using a NT342B Nd-YAG pumped optical parametric oscillator (Ekspla) at 355 nm. The emission signal was focused and collected at right angle to the excitation pathway and directed to a Princeton Instruments Acton SP-2300i monochromator. As a detector a R928 photomultiplier tube (Hamamatsu) has been used. Table 1. Spectroscopic properties of characterised compounds 1-19 in CH ₃ CN at room temperature. λ _abs = Absorption maximum, = Emission maximum, Φ _em = Luminenscene Quantum Yield, τ = Lifetime.

Results The photophysical properties of the prepared complexes were systematically investigated to evaluate their potential as PSs (Table 1). One crucial parameter in a PDT treatment is the penetration depth of the light and therefore the used wavelength. Based on this, the one-photon absorption spectra of the compounds were determined in CH3CN (Figures 1 and 8). The compounds have generally a strong absorbance with a large red shift of about 50-70 nm for the symmetric Ru(II) complexes 1-3 in comparison to the prototype complex [Ru(bpy)3] ²⁺ with an MLCT band at 450 nm (Balzani, V. et al., 2007). The comparison between the symmetric 2-3 and asymmetric compounds 4-19 shows that the characteristic absorption profile stays the same while the red shift in absorption as well as the extinction coefficients are increasing with the number of (E,E’)-4,4´-Bisstyryl-2,2´-bipyridine ligands coordinated to the Ru(II) core. Importantly, the compounds have an absorption tail in the therapeutical window (600-900 nm) potentially enabling them for the treatment of deep seated tumors. More particularly, compound 16 absorbs light at up to 720 nm, which is much higher than most metal based PS. Afterwards, the emission in CH3CN has been determined for compounds 1-11 upon excitation at 355 nm. The emission of the compounds was measurable between 550- 900 nm with a maximum between 664-709 nm. Worthy of note, the complexes which show a high red shift of the MLCT transition, have accordingly also the emission maximum at higher wavelengths. This large Stokes shift for all investigated compounds implies minimal inference between excitation and emission. Following this, the luminescence quantum yields were measured upon excitation at 355 nm. The comparison between the values shows luminescence quantum yield between 0.028 - <0.001. Consequently, the luminescence lifetimes were determined in degassed and air saturated CH ₃ CN upon excitation at 355 nm to investigate the influence of oxygen on the excited state. The measured lifetimes of the compounds 1-11 were found to be in the nanosecond range in a degassed solution between 222-542 ns and in an aerated solution between 36-96 ns. Importantly, for all Ru(II) polypyridyl complexes a decrease of the lifetime could be observed in an aerated solution in comparison to a degassed solution. This indicates that the excited state of the complex ( ³ MLCT) is able to interact with a component in the air. 3. Singlet oxygen generation The ability to generate reactive oxygen species upon light exposure was investigated for compounds 1-11. Electron spin resonance (ESR) measurements For verification of the reactive species formed upon light exposure of the compounds, the respective ESR spectra were recorded. The samples with a final concentration of 10 μM were dissolved in CH3CN or PBS containing 20 mM TEMP (2,2,6,6– tetramethylpiperidine) as a ¹ O2 scavenger or 20 mM DMPO (5,5-dimethyl-1-pyrroline N-oxide) as a *OH radical scavenger. Capillary tubes were filled with the solution and sintered by fire. EPR spectra were recorded on a Bruker A300 spectrometer with 1 G field modulation, 100 G scan range and 20 mW microwave power. The samples were measured in exclusion from light and after irradiation for 60 s (450 ± 10 nm, 21.8 mW cm ^-2 ). Singlet oxygen measurements - Direct evaluation The samples were prepared in an air saturated CH3CN or D2O solution with an absorbance of 0.2 at 450 nm. This solution was irradiated in fluorescence quartz cuvettes (width 1 cm) using a mounted M450LP1 LED (Thorlabs) whose irradiation, centered at 450 nm, has been focused with aspheric condenser lenses. The intensity of the irradiation has been varied using a T-Cube LED Driver (Thorlabs) and measured with an optical power and energy meter. The emission signal was focused and collected at right angle to the excitation pathway and directed to a Princeton Instruments Acton SP-2300i monochromator. A longpass glass filter was placed in front of the monochromator entrance slit to cut off light at wavelengths shorter than 850 nm. As a detector an EO-817L IR-sensitive liquid nitrogen cooled germanium diode detector (North Coast Scientific Corp.) has been used. The singlet oxygen luminesce at 1270 nm was measured by recording spectra from 1100 to 1400 nm. For the data analysis, the singlet oxygen luminescence peaks at different irradiation intensities were integrated. The resulting areas were plotted against the percentage of the irradiation intensity and the slope of the linear regression calculated. The absorbance of the sample was corrected with an absorbance correction factor. As reference for the measurement Rose Bengal (Φ = 76%) (Kochevar, I.E. and Redmond, R.W., 2000) was used and the singlet oxygen quantum yields were calculated using the following formula: Φsample = Φreference * (Ssample / Sreference) * (Ireference / Isample) I = I ₀ * (1 – 10 ^-A ) Φ = singlet oxygen quantum yield, S = slope of the linear regression of the plot of the areas of the singlet oxygen luminescence peaks against the irradiation intensity, I = absorbance correction factor, I0 = light intensity of the irradiation source, A = absorbance of the sample at irradiation wavelength. - Indirect evaluation For the measurement in CH3CN: The samples were prepared in an air-saturated CH3CN solution containing the complex with an absorbance of 0.2 at the irradiation wavelength, N,N-dimethyl-4-nitrosoaniline aniline (RNO, 24 µM) and imidazole (12 mM). For the measurement in PBS buffer: The samples were prepared in an air- saturated PBS solution containing the complex with an absorbance of 0.1 at the irradiation wavelength, N,N-dimethyl-4-nitrosoaniline aniline (RNO, 20 µM) and histidine (10 mM). The samples were irradiated on 96 well plates with an Atlas Photonics LUMOS BIO irradiator for different times. The absorbance of the samples was measured during these time intervals with a SpectraMax M2 Microplate Reader (Molecular Devices). The difference in absorbance (A ₀ -A) at 420 nm for the CH ₃ CN solution or at 440 nm a PBS buffer solution was calculated and plotted against the irradiation times. From the plot the slope of the linear regression was calculated as well as the absorbance correction factor determined. The singlet oxygen quantum yields were calculated using the same formulas as used for the direct evaluation. Table 2. Singlet oxygen quantum yields in CH ₃ CN and aqueous solution. Average of three independent measurements. n.d. = not detectable.

(*) comparative examples For identification of the type of ROS produced upon light exposure, electron spin resonance (ESR) spectroscopy in CH3CN as well as in phosphate-buffered saline (PBS) was employed. As a singlet oxygen ( ¹ O2) scavenger 2,2,6,6–tetramethylpiperidine (TEMP) and as a ^• OOH or ^• OH radical scavenger 5,5-dimethyl-1-pyrroline N-oxide (DMPO) was used. While no signal for the formation of a ^• OOH or ^• OH radicals were detected, the formation of ¹ O2 in CH3CN and PBS was confirmed by observation of the characteristic ¹ O2 - induced triplet signal in the ESR spectrum. Following this, the amount of generated ¹ O2 was quantitatively determined by two methods, namely 1) direct measurement of the phosphorescence of ¹ O ₂ and 2) indirect measurement of the variation in absorbance of a ¹ O2 scavenger and monitoring its change in absorbance. The singlet oxygen quantum yields (Table 2) were found to be between 16-82% in CH ₃ CN and 1-11% in an aqueous solution. Therefore, the compounds of the invention 1-11 are able to produce ¹ O2 in an efficient way. Overall, compound 7 was found to have an impressive singlet oxygen production (i.e., CH ₃ CN: 68-77%, aqueous solution: 10-11%). 4. Stability As a crucial parameter in view medicinal applications, the stability of the compounds in a biological environment was investigated as previous investigations have shown that this could be problematic for metal complexes. Stability in human plasma The stability of the complexes 1-11 was evaluated with caffeine as an internal standard, which has already been shown to be suitable for these experiments (Guy, P.A. et al., 2009). The pooled human plasma was obtained from Biowest and caffeine from TCI Chemicals. Stock Solutions of the compounds (20 μM) and caffeine (40 μM) were prepared in DMSO. One aliquot of the solutions was added to 975 μL of human plasma to a total volume of 1000 μL. Final concentrations of the compounds of 0.25 μM and caffeine of 0.5 μM were achieved. The resulting solution was incubated for 48 h at 37 °C with continuous gentle shaking (ca.300 rpm). The reaction was stopped after the incubation time by addition of 3 mL of methanol. The mixture was centrifuged for 60 min at 3000 rpm at 4 °C. The methanolic solution was filtered through a 0.2 μm membrane filter. The solvent was evaporated under reduced pressure and the residue was dissolved in 1:1 (v/v) CH3CN/ H2O 0.1% TFA solution. The solution was filtered through a 0.2 μm membrane filter and analyzed using a HPLC System. The solvents (HPLC grade) were millipore water (0.1% TFA, solvent A) and acetonitrile (solvent B). Method M1: 0-3 minutes: isocratic 50% A (50% B); 3- 17 minutes: linear gradient from 50% A (50% B) to 0% A (100% B); Method M2: 0-3 minutes: isocratic 95% A (5% B); 3- 17 minutes: linear gradient from 95% A (5% B) to 0% A (100% B); 17-23 minutes: isocratic 0% A (100% B). The flow rate was 1 mL/min and the chromatogram was detected at 250nm. Photostability The samples were prepared in an air saturated CH ₃ CN solution. To measure the photostability, the samples were irradiated at 450 nm in 96 well plates with an Atlas Photonics LUMOS BIO irradiator during time intervals from 0-10 min. The absorbance spectrum from 350-700 nm was recorded with a SpectraMax M2 Microplate Reader (Molecular Devices) after each time interval and compared. As a positive control [Ru(bipy) ₃ ]Cl ₂ and as a negative control Protoporphyrin IX has been used. Results To assess the compatibility of the here reported compounds under biological conditions, their stability in human pooled plasma was tested. For this purpose, the compounds were incubated in human plasma at 37 °C for 48 h and after this time extracted from the plasma. The compounds were analysed before and after giving them into the human plasma via HPLC. As an internal standard, caffeine has been used. The comparison of the HPLC chromatograms shows no change before and after the incubation for the compounds 1-11 which indicates the stability of the compounds under biological conditions. Next to the stability in human plasma, the stability upon irradiation was investigated. Importantly, currently approved photosensitizers are associated with a low stability upon irradiation. For this purpose, the compounds were exposed to irradiation at 450 nm and their UV/Vis absorption monitored in constant intervals. [Ru(bipy)3]Cl2 has been used as a positive control and Protoporphyrin IX has been used as a negative control. The results show that the photobleaching effect is correlating with the coordinated ligand. The complexes 1-11 show little to no photobleaching in comparison to the photosensitizer Protoporphyrin IX, which completely changed in the investigated time interval. 5. Dark and (Photo)-cytotoxicity in monolayer cells The compounds 1-19 were tested in various cell lines to determine their ability to act as a photosensitizer. Cell culture The human glioblastoma astrocytoma (U373) cell line was cultured in MEM medium supplemented with 10% FBS, 1% NEAA (non-essential amino-acids) and 1% penicillin/streptomycin. The human cervical carcinoma (HeLa), doxorubicin-resistant human colon adenocarcinoma (SW620/AD300) and the mouse colon carcinoma (CT- 26) cell lines were cultured in DMEM medium supplemented with 10% FBS and 1% penicillin/streptomycin. The human retinal epithelial cells (RPE-1, non-cancerous cells immortalized with hTERT) were cultured in DMEM-F12 medium supplemented with 10% FBS and 1% penicillin/streptomycin. All cell lines were obtained from the American Type Culture Collection (ATCC) and cultured at 37°C and 5% CO _2. Before an experiment, the cells were passaged three times. (Photo-)cytotoxicity on 2D cell monolayers The cytotoxicity of the compounds was assessed by measuring cell viability using a fluorometric resazurin assay. The cultivated cells were seeded in triplicates in 96 well plates with a density of 4000 cells per well in 100 μL of media. After 24 h, the medium was removed and the cells were treated with increasing concentrations of the compound diluted in cell media achieving a total volume of 200 μL. The cells were incubated with the compound for 4 h. After this time, the media was removed and replaced with 200 μL of fresh medium. For the phototoxicity studies, the cells were exposed to light with an Atlas Photonics LUMOS BIO irradiator. Each well was constantly illuminated with either a 480 nm or 540 nm irradiation. During this time, the temperature was maintained constantly at 37 °C. The cells were grown in the incubator for additional 44 h. For the determination of the dark cytotoxicity, the cells were not irradiated and after the medium exchange directly incubated for 44 h. After this time, the medium was replaced with fresh medium containing resazurin with a final concentration of 0.2 mg/mL. After 4 h incubation, the amount of the fluorescent product resorufin was determined upon excitation at 540 nm and measurement its emission at 590 nm using a SpectraMax M2 Microplate Reader (Molecular Devices). The obtained data was analyzed with the GraphPad Prism software. Table 3. IC ₅₀ values in µM in the dark and upon irradiation at 480 nm for compounds 1- 18 in comparison to [Ru(phen)3] ²⁺ (PF6-)2, [Ru(bphen)3] ²⁺ (PF6-)2, cisplatin and Protoporphyrin IX (PpIX) in non-cancerous retinal pigment epithelium (RPE-1) and nm for compounds 1-19 in comparison to [Ru(phen) ₃ ] ²⁺ (PF ₆ -) ₂ , [Ru(bphen) ₃ ] ²⁺ (PF ₆ -) ₂ , cisplatin and Protoporphyrin IX (PpIX) in human cervical carcinoma (HeLa) cells. Compounds 1-11 were also tested at 540 nm. Average of three independent measurements.

a ⁾ due to solubility limitations the compounds were investigated as chloride salts. Table 4. IC50 values in µM in the dark and upon irradiation at 480 and 540 nm for compounds 1-11 in comparison to [Ru(phen)3] ²⁺ (PF6-)2, [Ru(bphen)3] ²⁺ (PF6-)2, cisplatin and Protoporphyrin IX (PpIX) in mouse colon carcinoma (CT-26) and human glioblastoma astrocytoma (U373) cells. Average of three independent measurements.

b ⁾ due to solubility limitations the compounds were investigated as chloride salts. Results To investigate the toxicity of the compounds, the IC50 values in the dark as well as upon light exposure at 480 (10 min, 3.1 J/cm ² ) and 540 nm (40 min, 9.5 J/cm ² ) were determined. For this as a control of non-cancerous cells retinal pigment epithelium (RPE-1) was chosen and as cancerous cells human cervical carcinoma (HeLa), mouse colon carcinoma (CT-26) and human glioblastoma astrocytoma (U373) were chosen. Importantly for a PS, in all investigated cell lines (Table 3 and 4) the compounds 1-9 and 12-19 were found to be non-toxic in the dark (IC50, dark >100 μM, IC50, dark = 62 μM for compound 13), and the bphen coordinated compounds (10-11) showed little dark cytotoxicity which does not prevent their use as PS. The results show that all compounds are able to act as a PS with PI values from >1.2 - >167. Compounds 7, 9 and 12-15 display a phototoxicity on HeLa cells in the low micromolar range while being non-toxic in the dark. More particularly, the impressive ability of compounds 7 and 12 with PI values of >142.9 and >167 in CT-26 and HeLa cells respectively could have been unveiled. 6. Dark and (Photo)-cytotoxicity in 3D multicellular tumour spheroids The abilities of compounds 1-11 were investigated in 3D multicellular tumour spheroids (MCTS) as this presents a model which is closer to clinically treated tumours. Generation and analysis of 3D multicellular tumor spheroids (MCTS) A suspension of 0.75% agarose in PBS buffer was heated inside a high-pressure autoclave. The hot emulsion was transferred into wells (50 μL per well) of a 96 cell culture well plate. The plates were exposed for 3h to UV irradiation to ensure the sterility and allow the agarose solution to cool down. After this time, the agarose was overlayed with a HeLa cell suspension at a density of 3000 cells per well in 150 μL of media. The MCTS were cultivated and maintained at 37 °C in a cell culture incubator at 37 °C with 5% CO ₂ atmosphere. The culture media was replaced every two days. Within two-three days MCTs were formed from the cell suspension. The formation as well as integrity, diameter and volume of the MCTs was monitored by an Axio Observer Z1 (Carl Zeiss) phase contrast microscope. The volume was calculated using the following formula: V = 4/3πr ³ . The luminescence images along the z-axis were captured by a one- (λ _ex = 458 nm, λ _em = 600 - 750 nm) or two-photon (λ _ex = 800 nm, λ _em = 600 - 750 nm) excitation in the z-stack mode with a an LSM 880 (Carl Zeiss) laser scanning confocal microscope equipped with Argon and a Coherent Chameleon 2-Photon laser and a GaAsP detector. Results After analysis of the effect that the compounds of the invention have on monolayer cells, their ability in 3D multicellular tumour spheroids (MCTS) has been investigated for compounds 1-11. MCTS are an employed tissue model for the assessment of the delivery of drugs as it is closer to clinically treated tumours. Worthy of note, many investigated anticancer agents have failed at the transition of a cancer monolayer cell to in vivo studies. Partly this is been attributed to the failed drug delivery through the penetration of extracellular barriers. It has been shown that small MCTS with diameters of 200 μm are able to simulate intercellular interactions and therefore investigate the drug delivery. Recent studies have shown that the investigation of larger MCTS can also mimics the pathological conditions found in solid tumours as hypoxia in the tumour centre and its proliferation gradients. Consequently, we have chosen to investigate MCTS with diameters of 600-800 μm as an in vivo model. For this purpose, HeLa MCTS were incubated with the compounds 1-11 and the distribution of the compounds analysed via one and two-photon Z-stack imaging microscopy. After incubation for 12 h shows that the compounds 6-11 show a strong luminescence signal at every section depth corresponding with a complete penetration of the compounds in the MCTS. This is illustrated on Figure 2 for compound 7. Similar results have been obtained for the others tested compounds. 3D multicellular tumor spheroids (MCTS) growth inhibition assay MCTS were treated with the corresponding compounds (20 μM 1-7, 20 μM tetraphenylporphyrin H ₂ TPP, 10 μM cisplatin, 30 μM cisplatin, 2% DMSO, v%) by replacing 50% of the media with drug supplemented media in the dark for three days. After this time, the MCTS were exposed to a two-photon irradiation (800 nm, 10 J/cm ² ) with a section interval of 5 μm using a LSM 880 (Carl Zeiss) laser scanning confocal microscope equipped with a Coherent Chameleon 2-Photon laser. The cell culture media was replaced every two days. The integrity and diameter of the MCTs was monitored with an Axio Observer Z1 (Carl Zeiss) phase contrast microscope every 24 h. Results MCTS with a diameter of about 800 μm were incubated with compounds 1-7 (20 μM, 2% DMSO, v%), tetraphenylporphyrin (H ₂ TPP) (20 μM, 2% DMSO, v%), cisplatin (10 μM and 30 μM) for three days strictly in the dark. The MCTS were then exposed to 1P irradiation (500 nm, 10 J/cm ² ) or 2P irradiation (800 nm, 10 J/cm ² , section interval of 5 μm) on day 3. During the whole time period, the shape and volume of the MCTS was constantly monitored. As expected, the volume of the MCTS in the control group which was treated purely with cell media/DMSO as well as of compounds 1-7 and H2TPP (Figures 3, 4 and 5) were increasing in a similar manner, indicating that the investigated compounds do not show any inhibitory effect in the dark. Contrary to this, cisplatin showed a weak effect on the tumours growth at 10 μM, whereas it was significantly decreasing the volume of the MCTS at 30 μM. In comparison, the volume of the MCTS treated with compounds 1-7 and exposed to light irradiation significantly decreased, demonstrating their strong tumour inhibition effect. As expected from previous investigations on cell monolayers presented above, compound 7 had the strongest phototoxic effect. Under identical conditions, the treatment with H2TPP had only slight effect, demonstrating the ability of compound 7 to act as a PDT PS at low drug doses and low light doses. 3D multicellular tumor spheroids (MCTS) viability assay MCTS were treated with the corresponding compounds (20 μM 1-7, 2% DMSO, v%) by replacing 50% of the media with drug supplemented media in the dark for three days. After this time, the MCTS were exposed to a two-photon irradiation (800 nm, 10 J/cm ² ) with a section interval of 5 μm using a LSM 880 (Carl Zeiss) laser scanning confocal microscope equipped with a Coherent Chameleon 2-Photon laser. The cell culture media was replaced every two days. Two days after the irradiation the MCTS viability was tested using a Viability/Cytotoxicity Kit for mammalian cells (Invitrogen). Living cells can be identified from dead cells through the presence of ubiquitous intracellular esterase activity which can be monitored by the enzymatic conversion of the non- fluorescent calcein AM to the intensely fluorescent calcein (λ _ex = 495 nm, = 515 nm). As the spectroscopic properties of the dead cell probe EthD-1 overlaps with the one of the investigated compounds, this probe was not used and only calcein AM as a probe for living cells was used. MCTS were incubated with calcein AM (2 μM) for 30 min and images of the MCTS taken with an Axio Observer Z1 (Carl Zeiss, Germany) inverted fluorescence microscope. Results To further study the effect the complexes on tumour survival, the treated MCTS were stained with calcein AM (Figure 6), which can identify living from dead cells through the presence of ubiquitous intracellular esterase activity, converting the non- fluorescent calcein AM into the intensely fluorescent calcein. As expected from the tumour growth inhibition assay, the MCTS treated with compounds 4-7 in the dark showed a strong green fluorescence signal, indicating that the MCTS are still intact. Contrary to this, light treatment by 1P irradiation (500 nm, 10 J/cm ² ) or 2P irradiation (800 nm, 10 J/cm ² , section interval of 5 μm) had a drastic effect on cell survival in the MCTS. No significant fluorescence signal for compounds 4-7 could be observed, indicating that the MCTS were mostly eradicated. (Photo-)cytotoxicity on 3D multicellular tumor spheroids (MCTS) The cytotoxicity of the compounds in 3D multicellular tumor spheroids (MCTS) was assessed by measurement of the ATP concentration. MCTS were treated with increasing concentrations of the compound (2% DMSO, v%) by replacing 50% of the media with drug supplemented media and incubation for 12 h. After this time, the MCTS were divided in three identical groups. The first group was strictly kept in the dark. The second group was exposed to a one-photon irradiation (500 nm, 10 J/cm ² ) using a LED and the third group was exposed to a two-photon irradiation (800 nm, 10 J/cm ² ) with a section interval of 5 μm using a LSM 880 (Carl Zeiss, Germany) laser scanning confocal microscope equipped with a Coherent Chameleon 2-Photon laser. After the irradiation, all groups were incubated additional 48 h. The ATP concertation was measured using a CellTiter-Glo 3D Cell Viability kit (Promega) by measuring the generated chemiluminescence with an infinite M200 PRO (Tecan) plate reader. The obtained data was analyzed with the GraphPad Prism software. Table 5. IC ₅₀ in μM values in the dark and upon 1-Photon irradiation (500 nm, 10 J/cm ² ) irradiation or 2-Photon irradiation (800 nm, 10 J/cm ² , section interval of 5 μm) for compounds 1-11 in comparison to cisplatin and tetraphenylporphyrin (H ₂ TPP) in HeLa MCTS. Average of three independent measurements. ^a) due to solubility limitations the compounds were investigated as chloride salts. Results To quantify the photodynamic effect that the compounds have on HeLa MCTS, the (photo-)cytotoxicity (Table 5) was determined in the dark as well as upon 1Photon (500 nm, 10 J/cm ² ) and 2Photons irradiation (800 nm, 10 J/cm ² , section interval of 5 μm) by measurement of the ATP concentration of living cells through conversion into chemiluminescence. Importantly, no measurable cytotoxicity in the dark could be observed in HeLa MCTS for all compounds. The compounds 4-6, 8, 10 were found to be phototoxic in the micromolar range with PI values from >1.1 to >11.3. In comparison, compounds 7, 9 and 11 were found with much higher phototoxicity. These results confirm the (photo-)cytotoxicity evaluation on 2D monolayer cells. As a promising compound, 7 was identified. Importantly, no significant toxicity in the dark (IC50, dark > 300 μM) and high phototoxicity in the low micromolar range (IC50, 500 nm > 6.8 ± 0.2 μM, IC50, 800 nm > 1.4 ± 0.2) with a PI value of >44.1 or >214.3, respectively were determined. These results are very promising in comparison to H2TPP which did not have a cytotoxic effect (IC50, dark = IC50, 500 nm = IC50, 800 nm > 100 μM) under identical experimental conditions. The comparison between 1Photon and 2Photons irradiation demonstrates a stronger 2Photons phototoxicity, which is likely attributed to the deeper penetration depth of the longer wavelength as the intensity of the light is declining with tissue penetration. Overall, the highly phototoxic compound 7 in MCTS, which is able to act efficiently upon 1Photon and 2Photon irradiation could be unveiled. 7. In Vivo Evaluation Based on the remarkable properties of compound 7, this compound was investigated inside a mouse model. In vivo experiment 6 weeks age nu/nu female mice were purchased from Charles River. Compound 7 was dissolved into physiological saline at first. Tumor xenograft: 6×10 ⁶ SW620/AD300 cells were subcutaneously (s.c.) injected in the nude mice, the cells were suspension in 150 μL Matrigel (Corning) and saline (1:1, v/v). After a week, the tumor volumes of the mice reached approximately 80 mm ³ . In vivo (Photo-)cytotoxicity 30 SW620/AD300nude tumour-bearing nude mice were randomly separated into 6 groups and five mice for each group. Group 1: injected 7 (2 mg/Kg 50 μL) intravenously and irradiate under 800 nm laser (50 mW, 1 kHz, pulse width 35 fs, 5 s/mm) 1h after injection; Group 2: injected 7 (2 mg/Kg 50 μL) intravenously and irradiate under 500 nm light (10 mW/cm ² , 60 min); Group 3: injected by physiological saline (50 μL) and treated with 800 nm laser (50 mW, 1 kHz, pulse width 35 fs, 5 s/mm) 1h after injection; Group 4: injected by physiological saline (50 μL) and treated with 500 nm light (10 mW/cm ² , 60 min); Group 5: intravenous injected 7 (2 mg/Kg 50 μL); Group 6: injected 50 μL physiological saline. The mice were anesthetized by the injection of 4% chloral hydrate aqueous solution (0.2 mL/20g) before treatment. The tumour volume and body weight were measured and record for each two days. Tumor volume was calculated by the following formula Histological examination After the treatment, the mice were euthanized. The tumour was collected and fixated by 4% paraformaldehyde, and then obtained as paraffin-embedded samples and stained with hematoxylin and eosin (H&E). A Carl Zeiss Axio Imager Z2 microscope was used to observe the tissue structure and cell state of the sections. Results In vivo PDT experiments were prepared on SW620/AD300 (doxorubicin-selected P-gp- overexpressing human colon cancer cell) tumour-bearing nude mice. After the tumours volume reached 80 mm ³ , the mice were randomly separated into six groups and treated (day 1); group 1: injection of 2 mg/Kg 7 intravenously and 2P irradiation at 800 nm, group 2: injection of 2 mg/Kg 7 intravenously and 1P irradiation at 500 nm, group 3 and 4: were injection of physiological saline and treated with 1P and 2P irradiation, group 5: injection of 2 mg/Kg 7 intravenously and group 6 injection of the same volume of physiological saline. Importantly, the animals treated with 7 behave normally, without signs of pain, stress or discomfort. 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