The invention is directed to a color-tunable emitter material comprising a crystal containing complexes of luminescent biomolecule-based ligands and metal ions and to a method for preparing the color-tunable emitter material. Furthermore, the invention relates to a color-tuned material comprising the abovementioned material and a liquid. Ultimately, a LED-based device comprising either or both of the color- tunable material and the color-tuned materials is part of the present invention.

Recent studies provide evidence that the global energy expenditure on lighting is constantly increasing. The development of more efficient lighting concepts is thus a very promising field of research from an ecological point of view.

One approach in order save energy expenditures on lighting consists in the provision of an appropriate luminescent material. This material needs to combine different properties. It is required to have an excellent photoluminescence quantum yield and a high spectral and environmental stability. Moreover, it should not be tailor-made for a specific use but should rather be eligible for various applications.

Up to now, nature itself has provided some successful luminescent systems.

Fireflies and their analogous beetles (click beetles and railroad worms) for example employ a luminescent system based on luciferase catalyzed reactions. Luciferase triggers the reaction of the dehydrated form of luciferin with ATP and molecular oxygen in the presence of magnesium ions. The reaction products are oxyluciferin and a photon, accompanied by CO2 _, AMP and pyrophosphate (PPi). The quantum yield (the ratio of the chemical energy which is converted into light energy) which is associated with this reaction is astonishingly high and amounts to 41.0 ± 7.4%. Apart from that, it is known that the emitted photon shows variations in its energy and wavelength. Depending on the microenvironment around the center of bioluminescence, the emission is red or blue shifted.

Bioluminescence, i.e., the phenomenon of biological generation of visible light by deexcitation of chemically produced excited states, is also encountered in a number of other insects and organisms besides fireflies, click beetles and railroad worms, for example in certain species of bacteria, jellyfish, mushrooms and squids. The quantum yield of the bioluminescent reaction in fireflies and their analogous beetles is, however, unprecedented.

The disadvantage of the naturally occurring luminescent systems is their reliance on enzymes and co-factors. In vivo , luciferase, ATP and the other co-factors are already present. In vitro , appropriate concentrations of luciferase are necessary in order to obtain a high-yield luminescent reaction of luciferin or one of its derivatives. This is especially true because it has been found that luciferases provide an active-site microenvironment which promotes emissive decay rather than other deactivating processes (cf. Viviani et al., Cell. Mol. Life Sci. 59, 2002, pp. 1833-1850).

Taken together, a transfer of the naturally occurring luminescent system to the technical applications of everyday life is complex.

Moreover, the preparation of the naturally occurring luminescent molecules is demanding. The dehydrated form of luciferin, for instance, has been reported to show marked instabilities and is complicated to purify and crystallize.

In view of the above, there is a need for a novel luminescent material which can outscore the known bioluminescent systems in terms of quantum yield and which can simultaneously retain its original structure and does not require the presence of an enzyme. Apart from satisfying this need, it is an objective of the present invention to provide a luminescent material which can easily be tuned with respect to the color of the emitted light, and which is appropriate for use in LED-based devices. Another objective is the development of a material which facilitates the understanding of bioluminescent systems and their manipulation.

The above need and objectives are fulfilled by the color-tunable emitter material according to claim 1, by the color-tuned emitter material according to claim 11, by the LED-based device according to claim 13 and by the method according to claim 15. The color-tunable emitter material comprises a crystal which contains complexes of luminescent biomolecule-based ligands and metal ions, wherein the crystal has a plurality of pores.

In the context of the present invention, an “emitter material” is a material that emits light after the absorption of photons, i.e., after irradiation/ excitation with light of an appropriate wavelength. The emitted light is preferably visible light which can be perceived by the human eye. Preferably, the emitted light has a wavelength in the range of from 380 to 740 nm.

Moreover, an emitter material is “color-tunable” within the sense of the current invention, if the wavelength/s of the emission peak/s or the full width at half maximum (FWHM) of the emission peak/s can be varied and/or controlled, for example by changing the microenvironment around the emission center/s. Color- tunability of an emitter material can be verified by a comparison of emission spectra that have been obtained from one and the same emitter material but under different conditions. If the emitter material is color-tunable there is a detectable difference between at least some of the emission spectra.

A “crystal” is a solid with a three-dimensional order on the level of atomic dimensions. Whether a solid is a crystal may be determined with the help of diffraction techniques such as PXRD (powder X-ray diffraction).

The “complexes” are molecular entities in which two components, e.g. the luminescent biomolecule-based ligands and the metal ions, are loosely associated to each other by coordination bonds. In other words, the bonding between the components is weaker than in molecular entities with exclusively covalent bonds. Moreover, a complex of luminescent biomolecule-based ligands and metal ions comprises at least two biomolecule-based ligands and at least two metal ions, preferably three biomolecule-based ligands and two metal ions.

The “pores” in the crystal are void spaces. These void spaces are preferably accessible from the surface of the crystal and are preferably large enough to accommodate solvent molecules. A color-tunable emitter material with the abovementioned features has a very high photoluminescence quantum yield (PLQY). The presence of a complex of luminescent biomolecule-based ligands and metal ions, i.e., the existence of coordinate bonds between them, accounts for a high rigidity of the molecular entity and for a low percentage of non-radiative transitions, such as rotational and vibrational transitions, into the deexcited state after irradiation with light.

Apart from that, the plurality of pores offers an opportunity to modify the microenvironment around the emission center by introducing atoms or small molecules and to tune the color of the emitted light according to the needs of the desired application.

Besides that, the color-tunable emitter material proved to be stable in aqueous phases. The coordination forces in the complexes and the intermolecular forces between neighboring complexes, such as hydrogen bonds, are strong enough to prevent dissolution or disintegration of the crystal in water-rich phases. Only a suspension of the color-tunable emitter material can be obtained upon addition of water. This is another reason which makes the material an ideal candidate for LED- based devices which are expected to have a reasonable lifetime.

In a preferred embodiment, the pores occupy at least 5% of the unit cell volume of the crystal. More preferably, the pores occupy at least 8%, even more preferably at least 11%, in particular at least 13% of the unit cell volume of the crystal. It has proven to be beneficial if the pores are present in a percentage from 5% to 40% or even more preferably in a percentage from 8% to 25% of the unit cell volume. Assuming a constant pore volume distribution, a lower limit of 5% ensures that a sufficient number of atoms or small molecules may enter the crystal. An upper limit of 40% prevents that the crystal becomes too fragile. The percentage of pores is determined by a mix of experimental and computational methods: Firstly, a sample of the crystal is measured by single-crystal X-ray diffraction and an estimate for the unit cell volume is derived from the diffractogram. Subsequently, the estimate for the unit cell volume is refined and computationally optimized by calculating the energy minima of the atomic positions in the structure and assuming a completely solvent-free crystal structure. In a next step, the pores’ volume is evaluated by computing the volume that is available to be filled with a water molecule.

In a further preferred embodiment, at least some of the pores extend between surface sides of the crystal, preferably from a first surface side to a second surface side opposite to the first surface side. This means that the majority of the pores does not consist in superficial cavities. Rather, some of the pores are configured as 1D- channel-like void spaces. This facilitates the exchange of the solvent that may be accommodated in the pores and accounts for a comfortable color-tunability of the material.

The majority of pores preferably consists of mesopores. It may even be that all the pores are mesopores. In this context, “mesopores” are pores having a diameter or an equivalent spherical diameter which is at least 0.4 nm. Preferably, the mesopores are pores having a diameter or an equivalent spherical diameter in a range from 0.4 nm to 20 nm, more preferably from 0.4 to 1.5 nm, even more preferably from 0.4 to 1.0 nm.

The complex is made of luminescent biomolecule-based ligands and metal ions, i.e. of at least two luminescent biomolecule-based ligands and at least two metal ions, preferably of three luminescent biomolecule-based ligands and two metal ions.

The metal ions are selected from the group consisting of magnesium, aluminum, zinc, cadmium, scandium, titanium, vanadium, chromium, manganese, iron, cobalt, nickel, copper, yttrium, zirconium, niobium molybdenum, technetium, ruthenium, rhodium, palladium, silver, mercury and combinations thereof, preferably from the group consisting of manganese, zinc, zirconium, copper and combinations thereof, more preferably from the group consisting of copper namely Cu ⁺ , Cu2 ⁺ and combinations thereof. In a particularly preferred embodiment of the invention, one of the metal ions is Cu ⁺ and the other metal ion is Cu ²⁺ . As can be inferred from the list of preferable metal ions, the color-tunable material does not rely on the use of rare metal ions. This is advantageous in two respects: On the one hand it saves the resources of our planet, on the other hand it helps to avoid high material/production costs. Depending on the redox potential of the metal ion precursor, the luminescent biomolecules, from which the luminescent biomolecule-based ligands are obtained, may or may not be oxidized. If the redox potential is small, the luminescent biomolecules is not oxidized and the structure of the luminescent biomolecule-based ligands is the same as the structure of the luminescent biomolecules that occur in nature, such as compounds belonging to the class of luciferins and derivatives thereof.

If the redox potential of the metal is high, the luminescent biomolecules are oxidized. In this case, the luminescent biomolecules that occur in nature represent the ligand precursors. It may then be said that the luminescent biomolecule-based ligands are obtained from ligand precursors, wherein the ligand precursors are selected from the group consisting of compounds belonging to the class of luciferins and derivatives thereof.

For example, the luminescent biomolecule-based ligands may be obtained from ligand precursors which are selected from the group consisting of compounds (I)-(VI) and derivatives from compounds (I) - (VI):

(I) (P)

More preferably, the luminescent biomolecule-based ligands are obtained from ligand precursors which are selected from the group consisting of compounds (I)-(VI) in a reaction sequence which does not involve more than 3 reaction steps, in particular not more than 2 reaction steps. Most preferably, the luminescent biomolecule-based ligands are obtained from ligand precursors which are selected from the group consisting of compounds (I)-(VI) in a single reaction step.

Of the above compounds, compound (I) is commonly known as firefly luciferin, compound (II) as 3 -hydroxy -hispidin (from fungi), compound (III) as coelentrazine (from marine organisms), compound (IV) as latia luciferin (from bacteria), compound (V) as vargulin or cypridinluciferin and compound (VI) as dinoflagellate luciferin. The ligand precursors are preferably compounds comprising at least one heterocycle. Additionally, or alternatively the ligand precursors may comprise at least one of the group consisting of an urea functionality, an aldehyde functionality, a ketone functionality, an ester functionality and a carboxyl group. Selecting the luminescent biomolecule-based ligands according to these rules brings about an intrinsically high PLQY. Apart from that, the heteroatoms in the heterocycle are ideal coordination sites for the metal ions and support the formation of the complexes.

It is particularly preferred if the luminescent biomolecule-based ligands are obtained from ligand precursors comprising at least one carboxyl group. During the complex formation the carboxyl group may be deprotonated, and the oxygen anion of the deprotonated carboxyl group may act as a favorable electron donor for the metal ion.

The luminescent biomolecule-based ligands may be selected from selected from the group consisting of the conjugate bases of the compounds according to the following formula (G), wherein X ¹ and X ² are independently selected from the group consisting of sulfur, selenium, tellurium and amino; wherein R ¹ and R ² are substituents independently selected from the group consisting of hydrogen, hydroxyl, amino, alkylamino, alkoxy, cyano or thiol or wherein R ¹ and R ² are bridged to form an extended unsubstituted or substituted aromatic system; and wherein m is 0 or an integer from 1 to 3.

It is expected that a crystalline material produced from a metal salt and a conjugate base of a compound according to formula (G) wherein m is an integer from 1 to 3 has a higher porosity than a crystal produced from a metal salt and a conjugate base of a compound according to formula (G) wherein m is 0. Selecting m to be in the range of 1-3 is thus particularly preferable, if a high degree of porosity is desired. More preferably, the luminescent biomolecule-based ligands may be selected from the group consisting of the conjugate bases of the compounds according to one of the following formulae (I'a) - (I'c):

Of those, the compound according to formula (I'a) is known as dehydroluciferin.

In this context, the term “ligand precursor” refers to a molecule which only slightly deviates from the luminescent biomolecule-based ligand. For example, the oxidation state of one or more atoms in the ligand precursor may be increased or decreased by 1 in comparison to the luminescent biomolecule-based ligand. Alternatively or additionally, the ligand precursor may be the conjugate base or acid of the luminescent biomolecule- based ligand.

The term “derivatives” refers to compounds which have a compound according to one of formulae (I)-(VI) as parent compound, but differ from the parent compound in the bond order, in one or more atoms and/or groups of atoms, and combinations thereof. The derivative can differ from the parent compound, for example, in the presence of absence of one or more substituents, which may include one or more atoms, functional groups, or substructures. In general, the derivative can be imagined to be formed, at least theoretically, from the parent compound via chemical and/or physical processes. A compound (Ic) as shown below is, for example, a derivative of the compound according to formula (I) since it only differs from the compound of formula (I) by the substitution of sulfur against selenium. Likewise, the compound (lb) as shown below qualifies as a derivative of the compound according to formula (I) since it only differs by an extended aromatic system.

The compounds according to formulae (I'b) and (I'c) are the conjugate bases of the compound according to formulae (lb) and (Ic).

The luminescent biomolecule-based ligands which are obtained from the derivatives from compounds (I) - (VI) are preferred because the sites which coordinate the metal ions are distributed in such a way that unique constraints are imposed on the structure of the crystallized complex. Pores with a favorable shape are obtained. Additionally, the derivatives from compounds (I) - (VI) have an excellent electron density distribution. In particular, they have n-electrons which are delocalized (which can be confirmed by the concept of the mesomeric effect).

In a particularly preferred embodiment, the color-tunable emitter material of the current invention comprises a crystal containing complexes of the formula M ₂ L ₃ , wherein the crystal has a plurality of pores. In this formula M represents the metal ions and L represents the luminescent biomolecule-based ligands. Preferably, copper ions, namely Cu ⁺ and Cu ²⁺ , are included as the metal ions M. Further preferably, luciferin, i.e. the compound according to formula (I), is the ligand precursor from which the luminescent biomolecule-based ligands L are obtained, in particular by oxidation to dehydroluciferin (I'a), by deprotonation of the carboxyl groups and by coordination to the metal ions. This complex is also termed CU ₂ L _3.

In the particularly preferred embodiment above, both copper ions form coordinate bonds with the nitrogen atoms in each of the thiazole rings and each of the carboxylate oxygens of the three luminescent biomolecule-based ligands. Additionally, the copper ion with the single charge, Cu ⁺ , bridges the thiazole and benzothiazole moieties of at least one luminescent biomolecule-based ligand L by simultaneous sulfur and nitrogen coordination. The hydroxyl groups of the ligand precursor are unaffected during complex formation, i.e. the 6’-OH terminals are present in the luminescent biomolecule-based ligands L and the complexes of formula CU2L3.

The present invention further provides a color-tuned emitter material comprising the color-tunable emitter material as described above and a liquid.

The liquid may be a water-rich liquid. However, the liquid may also comprise or consist of small organic solvent molecules. Preferably the liquid is selected from the group of water, methanol, ethanol, acetonitrile, 2-propanol, chloroform, dimethylformamide (DMF), DMSO, N-methyl-2-pyrrolidone (NMP), p-xylene, benzene and combinations of these. In each case the color-tuned emitter material is a suspension, since the color-tunable emitter material is hardly soluble. Nevertheless, depending on the composition of the liquid, the color-tuned material emits visible light of a different wavelength upon excitation.

If the particularly preferred embodiment of the color-tunable emitter material (the one comprising crystals containing CU2L3 complexes) is used, the liquid is preferably selected from a mixture of water and ethanol. More preferably the liquid is composed of 20-30 wt.-% water and 70-80 wt.-% ethanol. In this case, the color- tuned emitter material emits nearly pure white light with CIE coordinates (CIEx,CIEy according to Commission Internationale de l'Eclairage) in the range of (0.27,0.35) to (0.29, 0.33). Moreover, a PLQY of between 55 and 60% may be achieved. This is amongst the highest PLQY reported for single species white light emitters. In a further preferred embodiment, the color-tuned emitter material is coated with a surfactant, preferably with cetrimonium bromide (CTAB). This coating is advantageous if the color-tuned emitter material is intended to be applied in LED- based devices.

In another aspect the current invention provides a LED-based device, in particular an organic light emitting diode (OLED) device, which comprises an emitting layer (also denoted emission layer, EML) and wherein the emitting layer comprises the color-tunable emitter material and/or the color-tuned emitter material as described before.

The emitting layer of such a LED-based device further preferably comprises a host material. The host material may be selected from the group consisting of 4,4'-bis(N- carbazoyl)-l,l'biphenyl (CBP), BaTiCL -doped poly(m ethyl methacrylate) (PMMA), Bis[2-(diphenylphosphino)phenyl] ether oxide (DPEPO), l,3,5-Tris(carbazol-9- yl)benzene (TCB), l,3-Di(9H-carbazol-9-yl)benzene (mCP), 9-(4-tert- Butylphenyl)-3,6-bis(triphenylsilyl)-9H-carbazole (CzSi), 4,7-Diphenyl-l,10- phenanthroline (BPhen), Poly(9-vinylcarbazole) (PVK), Polyvinylpyrrolidone (PVP), 2,9-Dimethyl-4,7-diphenyl-l,10-phenanthroline (BCP), and combinations thereof.

A preferable setup of the LED-based device comprises an emitting layer (EML) which is sandwiched between a low work-function metal cathode and a transparent anode (e.g. fluoride-tin-oxide anode, aluminum zinc oxide anode, indium tin oxide anode), wherein the emitting layer essentially consists of a host material which is doped with the color-tunable emitter material and/or the color-tuned emitter material according to the present invention. Further layers such as electron transport layers (ETL), hole transport layer (HTL), electron injection layers (EIL), hole injection layers (HIL) may be present between the emitting layer and each of the electrodes. In such a setup, electrons and holes which are injected into the EML preferably recombine and give rise to electroluminescent light emission. For those color- tunable emitter materials and/or color-tuned emitter materials which are not capable of electroluminescence, the LED-based device further comprises a source for backlight illumination in order to lift the material into an excited state and generate emission.

Furthermore, the current invention provides a method for producing the color- tunable emitter material as described above. This method comprises the steps of providing a ligand precursor and reacting the ligand precursor with a metal salt in a solvent mixture of water (H2O) and organic solvents, preferably in a solvent mixture comprising or consisting of water, dimethylformamide (DMF) and ethanol (EtOH), in order to obtain crystals and separating the obtained crystals from the solvent mixture.

During the reaction, the ligand preferably reduces at least a part of the metal salt while simultaneously being oxidized. The solvent mixture which is selected for the reaction preferably has a DMF : EtOH ratio in the range of 2.5 : 1 to 3.5 : 1, a DMF : H2O ratio in the range of 2.5 : 1 to 3.5 : 1 and a EtOH : H2O ratio in the range of 0.5 : 1 to 1 : 0.5.

Preferably, the reaction of the ligand precursor with a metal salt is carried out at an elevated temperature. The reaction may, for example, take place at a temperature of from 60-130°C for at least 1 hour, preferably from 85-115°C for at least 5 hours.

The nature of the present invention will become more clearly apparent and better understood from the following examples and accompanying drawings in which:

Figure 1 shows a reaction scheme leading to a compound of formula (lb).

Figure 2 shows three diagrams (a), (b) and (c). Diagram (a) contains two photoluminescence emission spectra, one of an as-synthesised CU2L3 crystal, formed on a glass substrate (broken line) and one of the CU2L3 crystal suspended in a liquid containing 25% H2O (solid line). Diagram (b) contains the corresponding CIE coordinates which are located at CIEx = 0.33 and CIEy = 0.39. Diagram (c) shows the emission spectra of CU2L3 and LH2 after excitation with light of a wavelength of 350 nm. Figure 3 shows a CIE-diagram with the computed CIE coordinates of the complexes which have been prepared in example 1 and in the further examples in their respective protonated and deprotonated form (the protonation referring to the 6’ -OH terminal group).

Figure 4 shows the emission spectra of the product obtained in example 1 in suspensions of different solvents.

Figure 5 includes two diagrams (a) and (b). Diagram (a) shows the dependence of photoluminescence spectrum (normalised to the 550 nm peak) on the concentration of EEO in the pores of CU ₂ L _3. It can be noted that the emission of CU ₂ L ₃ in 25% water is broad with a FWHM of 182.1 nm and has peaks at 450 and 550 nm in a 0.75:1.00 intensity ratio. The PLQY has been determined to be 57 %. Diagram (b) shows the corresponding emission colors. As the ratio of water is increased, the emission coordinates of the crystals follow the white region of the CIE map from cool to warm (left to right). CIE coordinates of CU2L3 synthesised in 25% H20 corresponds to (0.29, 0.33). The reported CIE coordinates together with the PLQY is unique: It is the highest quantum yield of an emitter so close to the equal energy white point in the CIE map.

Figure 6 provides three experimentally obtained PXRD spectra of the crystalline CU2L3 complexes. Spectrum 6 is after soaking in aqueous solution (a mixture of water and ethanol with a FbCFEtOH ratio being 0.25:0.75) overnight, centrifuging and drying. Spectrum 7 is after storing it three months in air. Spectrum 8 is as synthesized without soaking and without storage. Spectrum 9 is a calculated PXRD spectrum. By comparison of spectra 6 and 7 with spectrum 8 it can be concluded that the complex is stable in air and water.

Figure 7 is a comparison of the FTIR spectra of the crystalline product obtained in example 1 before and after soaking in water (overnight). The lighter curve belongs to the material which had been subjected to soaking.

Figure 8 shows two diagrams. The upper diagram (a) shows computed absorption spectra (ref. no. 1 and 2) and emission spectra (ref. no. 3-5) of the complex according to example 1 ( ref. no. 1, 4, 5) and of dehydroluciferin L ( ref. no. 2 and 3), which has been prepared as a reference. Protonated (ref. no. 5) and deprotonated (ref. no. 4) emission states (0-) have been considered for the complex. A width of 0.3 eV was used in the spectra. The lower diagram b shows experimental data, namely UV- Vis absorption (via diffuse reflectance) spectra and photoluminescence emission spectra. Of the two UV-Vis absorption spectra (the ones which are assigned to the left y-axis with F(R) units), the spectrum with the broken line belongs to dehydroluciferin L which has been prepared as a reference material. The spectrum with the solid line belongs to the complex CU ₂ L ₃ which has been obtained in example 1. From the comparison of the spectra, it can be concluded that there is a redshift in the peak absorption of the ligand L once it is present in its complexed form. Of the two photoluminescence emission spectra (the ones which are assigned to the right y-axis with intensity in arbitrary units) the spectrum with the solid line again belongs to the complex CU ₂ L ₃ which has been obtained in example 1. The spectrum with the broken lines belongs to the complex CU ₂ L ₃ in its fully deprotonated state (with O in the 6’ -position of the ligands).

Figure 9 illustrates the molecular orbitals associated with the electronic transition corresponding to the absorption peak of L computed at 340 nm, and with the three main excitations contributing to the absorption band of CU2L3 complex computed at 390, 380 and 370 nm.

Figure 10 shows computed emission spectra (solid lines) of modified CU ₂ L ₃ complexes. The emission spectrum shown in figure 10 (a) is the spectrum obtained from the modified CU ₂ L ₃ complex with the Se-substituted L ligand according to formula (Fc). The emission shown in figure 10 (b) is the spectrum obtained from the modified CU ₂ L ₃ complex with the naphthalene variant of the L ligand according to formula (Fb). Computed emission spectra of the CU2L3 with the unmodified ligand L (i.e. L being dehydroluciferin) are also shown for comparison (dotted lines).

Figure 11 with the two graphs in (a) and (b) largely corresponds to figure 5 (a) and (b). Only the results of two more measurements have been added. The photoluminescence spectrum 10 and point 10 in the CIE map are at 5% EtOH and the photoluminescence spectrum 20 and point 20 in the CIE map are at 10% EtOH. STARTING MATERIALS

All of the starting chemicals are commercially available and were used without any further purification. For example, D-cysteine was purchased from Fluorochem. 2- Cyano-6-hydroxybenzothiazole and potassium carbonate were purchased from Acros. CU(N0 ₃ ) _2' 2.5H ₂ 0 (98%) was purchased from Alfa Aeser.

ANALYSIS METHODS

Diffuse reflectance spectra of CU ₂ L ₃ and its precursor were obtained experimentally. To that end, carefully-ground samples of the respective materials were tightly packed into cells with parallel quartz windows and reflectance was measured with a PerkinElmer LAMBDA 1050 spectrometer equipped with a praying mantis. A scattering coefficient F(R) was filtered out mathematically by treating the spectra with the Kubelka-Munk function. This coefficient F(R) contains information about the geometric irregularities of the inhomogeneous samples.

The UV-visible absorption spectra of CU ₂ L ₃ and its precursors could then be calculated from the scattering coefficient F(R). This mathematical procedure is the common approach in order to obtain absorption spectra for rather insoluble powders such as the inventive material.

Steady state photoluminescence spectra and absolute quantum yields were measured using a Horiba Fluorolog-3 equipped with an integrating sphere. Reported quantum yield value is an average of five independent measurements.

EXAMPLE 1

An exemplary color-tunable emitter material has been prepared according to the following two steps (a) and (b).

(a) Synthesis of a Precursor Ligand, D-Luciferin (LH2)

D-Cysteine (1.444 g, 11.92 mmol) and 2-cyano-6-hydroxybenzothiazole (2.000 g, 11.35 mmol) were suspended in MeOH-FfO 2:1 mixture (80 mL) in a 100 mL Schlenk flask. After addition of potassium carbonate (1.6448 g, 11.92 mmol) to the mixture the color changed to bright yellow-green and the resulting solution was stirred under N2 overnight. Methanol was removed in vacuo and the remaining aqueous solution was acidified with 1M HC1 solution to maximize the amount of precipitate, which was extracted with EtOAc (3x50 mL). The combined organic phase was dried over Na2SC>4 and evaporated to obtain D-luciferin (LH2) as a pale- yellow solid (2.630 g, 96%). ¾ NMR (400 MHz, Methanol-^) d 7.89 (d, J = 8.9 Hz, 1H), 7.33 (d, J= 2.4 Hz, 1H), 7.06 (dd, J= 8.9, 2.4 Hz, 1H), 5.38 (t, J= 9.1 Hz, 1H), 3.75 (m, 2H). ¹³ C NMR (101 MHz, MeOD) d 173.34, 167.64, 159.00, 158.50, 148.14, 139.11, 125.91, 118.18, 107.32, 79.47, 35.84. ¹ ¾ (400.13 MHz) and ¹³ C (100.62 MHz) NMR spectra were recorded on a Bruker Avance II 400 spectrometer at 298 K.

(b) Synthesis of a Crystal containing complexes CU ₂ L ₃

CU(N0 ₃ ) _2' 2.5H ₂ 0 (20 mg, 0.068 mmol) and of LH2 (20 mg, 0.071 mmol) were introduced into borosilicate glass scintillation vials, dissolved in a DMF-H ₂ 0-EtOH 3: 1:1 mixture (6 mL) and then sealed, heated to 100°C and kept at this temperature for 16 hours. Reddish-brown crystals of CU2L3 were isolated by centrifugation with a yield of -70% (15 mg). The purity of the product was confirmed by elemental analysis and PXRD of the solid crystals.

Without being bound to theory it is assumed that the LH2 precursor reduces some of the Cu ¹¹ to Cu ¹ , while simultaneously being oxidized to its planar aromatic 2-(6'- hydroxy-2'-benzothiazolyl)-thiazole-4-carboxylic acid (dehydroluciferin, L) form (Fig. la). The oxidation of LH2 to form L removes the chirality of the ligand precursor. CU2L3 thus crystallizes in the triclinic centrosymmetric space group RΪ with one metal complex present in the asymmetric unit. Each metal complex of CU2L3 is composed of three L molecules acting as ligands linked by two Cu ions. Both Cu ions display distorted tetrahedral coordination geometries and form coordinate bonds with the N atoms of all three thiazole rings, as well as with the O atoms of all the ligand terminal carboxylate groups. Cu in the oxidation state +1 additionally bridges the thiazole and benzothiazole fragments of the central ligand via the S and N atoms. FURTHER EXAMPLES

Further color-tunable emitter materials can be prepared by exchanging LH2 against a luciferin derivative and reacting it with a copper hydrate salt according to the protocol in step (b) above.

For example, a compound according to formula (Ic) or (lb) may be used in place of LH _2.

The synthesis of a compound according to formula (Ic) may be carried out analogous to the synthesis described in Conley et. al., A selenium analogue of firefly D- luciferin with red-shifted bioluminescence emission , Angew. Chem. Int. Ed. 2012;51(14) : 3350—3353. doi: 10.1002/anie.201105653.

The production of a compound according to formula (lb) would be feasible by proceeding according to the reaction scheme which is shown in Fig. 1.

REFERENCE MATERIAL

Synthesis of dehydroluciferin

D-luciferin (115 mg, 0.41 mmol) was dissolved in 250 mL of aqueous NaOH (1M) and the resulting solution was heated to 100°C. Air was bubbled through the solution for 6 h with vigorous stirring, then the reaction mixture was cooled, acidified with 1M HC1 and extracted with EtOAc (3x25 mL). The product was subjected to column chromatography (DCM-MeOH, 4:1), resulting in dark violet solid (47 mg, 41%). 1H NMR (400 MHz, DMSO-d6) 10.17 (s, 1H), 8.67 (s, 1H), 7.91 (d, J = 8.8 Hz, 1H), 7.46 (d, J = 2.4 Hz, 1H), 7.04 (dd, J = 8.8, 2.4 Hz, 1H). 13C NMR (DMSO-d6): 5=162.1, 161.8, 157.5, 157.0, 148.8, 146.8, 137.2, 131.6, 124.7, 117.6, 107.5. ESI MS m/z 276.9768 (calcd 276.9742 [M-H]-).

EXPERIMENTAL CHARACTERIZATION OF EXAMPLE MATERIALS

Single-crystal X-ray diffraction Diffraction data of the product obtained in example 1 were collected on a BRUKER D8 VENTURE four-circle diffractometer equipped with a Mo Ka microfocus sealed X-ray tube and a Photon 100 2D CMOS detector. The crystal structure was solved with SHELXT and refined with SHELXL programs integrated within the 01ex2 Crystallography Software program suite (developed by the Durham University). All non-H atoms were found upon solution and refined anisotropically. Aromatic H atoms were introduced based on the molecular geometry with the AFIX 43 command, while those belonging to the hydroxy groups were found from the Fourier-difference maps and refined either independently (H3) or using AFIX 83 or AFIX 147 commands (H6 and H9). No additional H atoms in proximity to N or S atoms were found from the Fourier-difference maps. Several restraints were imposed on the atomic displacement parameters of the non-H atoms. The contribution of the disordered solvent molecules found in the structural voids to the measured structure factors was quantified using the solvent masking procedure implemented in 01ex2 equivalent to PLATON SQUEEZE. Structure-derived pore volume was calculated from the hypothetical solvent-free structure with Mercury (a program developed at the Cambridge Crystallographic Data Centre), more specifically by use of the 'voids'-feature in Mercury. Doing so, the probe radius was set to 1.2 A, which corresponds to the approximate molecular radius of water. Moreover, a grid spacing of 0.7 A was applied and the “Contact Surface” calculation scheme was adopted. This calculation scheme maps the volume that can be occupied by the full probe (including its radius) and outputs the volume that could be filled by solvent or guest molecules of the preset radius.

Powder X-ray diffraction

Powder X-ray diffraction (PXRD) data of the product obtained in example 1 were collected on a Bruker D8 Advance diffractometer at ambient temperature using monochromated Cu Ka radiation (l= 1.5418 A), with a 2Q step of 0.02° and a 2Q range of ~2 to 70°. Simulated PXRD patterns were generated from the corresponding crystal structures using Mercury 3.0. X-ray photoelectron spectroscopy (XPS) was performed using a PHI 5000 Versaprobe-II instrument from Physical Electronics. Samples of the product obtain in example 1 were deposited on an insulating double sided, vacuum compatible, taped, and charge neutralization was applied during the XPS measurements. The binding energy scale was normalized based on the position of the C-C bond of the Cls photoelectron peak located at 284.6 eV. Peak fitting of the Cu 2p3/2 photoelectron peak, along with the kinetic energy of the Cu L3M4 _, 5M4,5 peak, were used to evaluate the chemical state of copper.

Results are shown in table 1 below.

Table 1. Crystal data and structure refinement for CU2L3.

Identification code CU2L3

Empirical formula C33H16CU2N6O9S6

Formula weight 959.96

Temperature/K 127.93

Crystal system triclinic

Space group Pi at A 10.6952(5) b/A 13.6637(7) c/A 15.4144(8) oJ° 114.8850(10) b/° 98.494(2) g/° 100.936(2)

Volume/ A ³ 1939.77(17) Z 2

P _calc g/cm ³ 1.644 //mm ^-1 1.480 (000) 964.0

Crystal size/mm ³ 0.225 x 0.05 x 0.04

Radiation MoKa (l = 0.71073)

2Q range for data collection/ ⁰ 4.358 to 52.898

Index ranges -13 < h < 13, -17 < k < 17, -19 < / <

Reflections collected 57298

Independent reflections 7962 [R _nt = 0.0460, R _{s gma} = 0.0331] Data/restraints/parameters 7962/331/543 Goodness-of-fit on F ² 1.069

Final R indexes [/>2s(7)] Ri = 0.0431, WR ₂ = 0.0907 Final R indexes [all data] Ri = 0.0567, WR ₂ = 0.0949

Largest diff. peak/hole / e A ³ 0.76/-0.74

Optical characterization

Optical characterization was carried out on crystals obtained in example 1, both in the solid state and in dilute suspensions. For measurements in suspension, the CU2L3 crystals were crushed and suspended in ethanol/water solvent mixtures with varying ratios of water. For solid measurements, crystals were grown on glass pieces placed inside the reaction vials, and the optical properties of the obtained films were measured. PXRD patterns were measured after all measurements and stability of the material could be confirmed. Similarity between the emission spectra of CU2L3 films grown on glass substrate and CU2L3 crystals dispersed in a dilute suspension (25 wt.- % water and 75 wt.-% ethanol as suspension medium) can be seen in diagram (a) of Fig. 2. The CIE-coordinates obtained from CU2L3 films grown on glass substrate and CU2L3 crystals dispersed in a dilute suspension are almost identical and correspond to CIEx ~ 0.315 and CIEy ~ 0.37 (cf. diagram (b) of Fig. 2). Figure 2c shows how the emission spectrum of CU2L3 crystals dispersed in a dilute suspension compares to the emission spectrum of LFL in the same suspension.

Influence of 6’-OH Protonation/Deprotonation on CIE coordinates

CU2L3 crystals are emissive, porous molecular crystals which mimic bioluminescence color-tuning in response to a changing chemical environment within the pores. CU2L3 crystals exhibit stable, tuneable and broad-spectrum emission. The optical properties of CU2L3 are ligand-based and therefore similar to those of the LFL precursor. Namely, the neutral complex emits at 430 nm, and deprotonation of the photoexcited 6 '-OH groups by water molecules present in the pores of CU2L3 crystals results in a redshifted emission peak centered at 550 nm (cf. Fig. 3 showing CIE-coordinates of the complexes, wherein complexes with a protonated 6'-OH group are represented by open circles).

Influence of Solvent on Emission Spectra

The product obtained in example 1 has been dispersed in several non-aqueous solvents, including ethanol, dimethylformamide (DMF), N-methyl-2-pyrrolidone (NMP) and benzene. These dispersions exhibit a 430 nm phenol signature peak in their emission spectra (cf. Fig. 4). This indicates a full protonation of the benzothiazole terminals.

Dispersions of the product obtained in example 1 in pure water and aqueous buffers of pH 9 and 4 show a 550 nm phenolate emission peak, indicating complete deprotonation of the benzothiazole terminals (cf. Fig. 4). Notably, the effect of pH on the protonation state of the phenol terminals is negligible compared to that of H2O, due to the low pKa of these terminals in the excited state, as confirmed by the dominance of the 6'-0- characteristic peak at pH 4.

Moreover, the product obtained in example 1 has been dispersed in ethanol-water mixtures of varying H2O : EtOH content. Photoluminescence spectra after excitation with light of a wavelength of 350 nm are shown in figure 5a. The direction of the arrow in figure 5b shows how the CIE coordinates are shifted with an increasing amount of water.

Stability of the complex

PXRD spectra of the crystalline product obtained in example 1 have been recorded before and after soaking in aqueous solution overnight, centrifuging and drying. These spectra, which are depicted in fig. 6 together with the predicted pattern, confirm that the insoluble complex is stable and retains its crystallinity after being dispersed in water-rich solvent mixtures.

Fourier Transform Infra-red (FTIR) spectra before and after soaking (fig. 7) further confirm the stability of the product obtain in example 1. The more pronounced phenol-OH band around 3200 cm ¹ in the FTIR of the soaked material suggests that water molecules in the solution approach the -OH groups of the complex. The fingerprint region of the spectra is the same before and after soaking, which confirms that the material remains stable.

COMPARISON WITH DFT-CALCULATIONS

Absorption spectra calculations were done with Kohn-Sham DFT and linear response Time-Dependent DFT (TDDFT) using the PBEO functional and 6-31 lg(d) basis set including implicit solvent (ethanol) by means of the CPCM method as implemented in Gaussian 09 code, for the isolated ligand and for the CU ₂ L ₃ complex being all 6’ -OH groups protonated. Analysis of the active excited states allowed determining that Si is the low-lying absorbing state of the ligand corresponding to the HOMO-LUMO excitation. The computed peak of the CU ₂ L ₃ complex (cf. Fig. 8a) is slightly red-shifted with respect to the isolated ligand in agreement with the experiments (cf. Fig. 8b) and corresponds to the sum of the local excitations of the three L ligands (cf. Fig. 9). Low-lying charge transfer states involving transitions from the d-orbitals of Cu to the ligands are found with weak intensities at lower energies.

USE OF THE INVENTIVE MATERIAL AS A GENERIC MODEL

CU ₂ L ₃ can serve as a model for investigating bioluminescence color tuning effects. Based on CU2L3 different factors which have a contribution on the optical properties may be disentangled and separately investigated. This makes a contribution to the design of application-specific luciferases. In CU2L3, the thiazole rings are fixed in a planar aromatic (“enol-like”) structure, and the COOH terminals, which take part in coordination, are unavailable to interact with the pore environment. Rigidity of the packed structure prevents rotation about the C2-C2' bond, eliminating any effects of geometry relaxation on the radiative deexcitation energy. Access of the 6'-OH groups of all three ligands in the asymmetric unit of CU2L3 allows to monitor the impact of the pore environment on the emission wavelength, in the absence of any other structural or chemical modifications.

Key structural similarities between the ligands in CU2L3, crystallized oxyluciferin and AMP-complexed oxyluciferin in the Luciola cruciata binding site affirm the use of CU2L3 as a molecular model. Binding of the thiazole N3 to Cu in all three ligands of CU ₂ L ₃ is comparable to N3-0 binding to a 6'-OH in crystallised oxyluciferin and to a proximate water molecule in AMP-complexed luciferin of the L. cruciata species. N3-Cu bond lengths in 1 ₂ L ₃ range from 1.927 A to 1.983 A, while in both, crystallised oxyluciferin and LH ₂ @L.cruciata the N-0 bonds range from 2.782 A to 2.878 A.