Field of invention

The present invention relates to cyclopropenium compounds, and in particular to cyclopropenium compounds which are capable of forming a liquid crystal mesophase.

Background art

Liquid crystals are used extensively in a wide range of applications such as displays, solar cells, optoelectronics, molecular sensors and detectors. In the liquid crystal phase, molecules are arranged to have a degree of positional and/or orientational order whilst not exhibiting the physical properties of a solid material. This ordering of molecules in a liquid crystal phase can result in the material exhibiting particular electronic, optical or magnetic properties. Many liquid crystals are thermotropic, exhibiting phase transitions between crystalline (i.e. solid) and liquid crystal phases as temperature is changed. Most thermotropic liquid crystals will also exhibit an isotropic (i.e. liquid) phase at higher temperatures.

Discotic liquid crystals are a subset of thermotropic liquid crystals which are capable of forming mesophases with molecules aligned or stacked in columns (the columnar phase). Molecules which exhibit this behaviour typically have certain common structural features, notably a rigid aromatic core surrounded by flexible chains such as alkyl groups. Self- assembly into columns is facilitated by p-p interactions between adjacent aromatic cores. These interactions can also increase charge carrier mobility along the axis of the columns. Consequently some discotic liquid crystals exhibit anisotropic charge transfer in the dimension aligned with the column axis.

Charge transfer can be enhanced by including ions within the liquid crystal phase. This may be achieved through ion doping (by adding free ions to the system) or by including ionic moieties within the structure of the mesogenic molecules themselves. Various ionic columnar/discotic liquid crystals have been synthesised including anilinium salts (T. Kato et al, Mol. Syst. Des. Eng., 2019, 4, 342), phosphonium salts (K. V. Axenov, S. Laschat, Materials, 2011, 4, 206-259) and imidazolium salts (T. Fukushima et al, Chem. Commun., 2005, 101-103. Many of these molecules exhibit enhanced conductivity compared to non ionic liquid crystals and some also exhibit highly anisotropic charge transfer. However these properties are typically only observed in the liquid crystal mesophase, with conductivity decreasing significantly below the crystal-crystal phase transition temperature or as molecules rearrange into a different configuration in response to temperature changes. As a result of this, such liquid crystals may only be operable in electronic devices at higher temperatures and over narrow temperature ranges.

Summary of the Invention

The present invention seeks to provide an ionic compound which is capable of forming a columnar liquid crystal mesophase and which retains conductivity through the liquid crystal - crystal phase transition. Viewed from a first aspect the present invention provides a cyclopropenium compound having a structure according to Formula (I): Formula (I) wherein: each of Ar ¹ , Ar ² and Ar ³ is an aryl group having one or more substituents; each of X ¹ , X ² and X ³ is independently selected from a group consisting of a carbon-carbon bond (i.e. a carbon atom of the cyclopropenium is bonded directly to the respective Ar group with no intermediate atoms) or an acyclic hetero atomic bridging group; and A is an anion.

By selecting each of X ¹ , X ² and X ³ from a group consisting of a carbon-carbon bond or an acyclic heteroatomic bridging group, the positive charge of the cyclopropenium can advantageously be stabilised across a wider delocalised aromatic system encompassing one or more of the aryl groups and lone pairs on a bridging heteroatom (where present).

Each of X ¹ , X ² and X ³ may be different. Two of X ¹ , X ² and X ³ may be the same.

Preferably X ¹ , X ² and X ³ are the same. Preferably the acyclic heteroatomic bridging group is an epoxy group, an epithio group or an amino group. Particularly preferably the acyclic heteroatomic bridging group is an amino group.

In a preferred embodiment, each of X ¹ , X ² and/or X ³ is a tertiary amine. Preferably the tertiary amine includes an alkyl group (eg a Ci- ₆ -alkyl group). Particularly preferably the tertiary amine includes a methyl, ethyl or propyl group.

Each of Ar ¹ , Ar ² and Ar ³ may be independently selected from a group consisting of substituted phenyl, napthyl, anthracyl, furyl, pyrrolyl, thienyl, benzofuryl, isobenzofuryl, indolyl, isoindolyl, benzothienyl, imidazolyl, benzimidazolyl, purinyl, pyridyl, quinolyl, or isoquinolyl groups. Preferably each of Ar ¹ , Ar ² and Ar ³ is a substituted phenyl, anthracyl, napthyl, furyl, pyrrolyl or thienyl group. Particularly preferably each of Ar ¹ , Ar ² and Ar ³ is a substituted phenyl group.

Each aryl group may be different. Two of the aryl groups may be the same. Preferably each of the aryl groups are the same.

The substituents on the aryl groups may be independently selected from a group consisting of alkyls, alkoxyls, oxyalkyls, alkenyls, carbonyls, carbonates, esters, amides, carbamates and organophosphates. A preferred substituent is alkoxyl (eg Cs-is-alkoxyl). Preferably each alkoxyl substituent is a Cs-n-alkoxyl substituent.

The substituents on the aryl groups may be independently selected from a group consisting -NR ¹ C(=0)R, -C(=0)NR ₂ , -NR ¹ C(=0)0R, -0C(=0)NR ₂ or -0-P(=0)(0R) ₂ (wherein R and R ¹ which may be the same or different are alkyl groups (eg Ci-is-alkyl groups)).

Each aryl group may have multiple substituents which may be the same or different. The maximum number of substituents will be dictated by the number of suitable substitution sites on the aryl group. Preferably each aryl group has one, two or three substituents. Each of the three aryl groups may have the same or a different number of substituents. Each of the three aryl groups may have the same or different substituents.

Where each of the aryl groups are the same or where two of the aryl groups are the same, each aryl group may have the same or different substituents in the same or different positions.

Where the aryl group is phenyl, the phenyl group may be substituted in the ortho-, meta- and/or para- positions with respect to the X group. The phenyl group may have one or two substituents in the ortho- (2- or 6-) positions, one or two substituents in the meta- (3- or 5-) positions, and/or one substituent in the para- (4-) position. Preferably where the aryl group is phenyl, the phenyl group has a total of between 1 and 3 substituents. Particularly preferably where the aryl group is phenyl, the phenyl group has a total of three substituents with one in each of the 2-, 3- and 4- (ortho-, meta- and para-) positions.

Preferably R = Cnthn+i and R ¹ = Cmthm+i and a value for n and/or n+mis greater than or equal to 4 for at least one substituent on each aryl group. Particularly preferably a value for n and/or n+m is between 8 and 18 for at least one substituent on each aryl group. More preferably a value for n and/or n+mis 8, 10, 12, 14 or 16 for at least one substituent on each aryl group. Where there are multiple substituents on an aryl group the value for n and/or n+mis preferably the same for each substituent. Substituents with an alkyl chain length of four or more arranged around the aromatic core (the cyclopropenium and surrounding aryl groups) give the compound a dendritic structure. The relatively hydrophobic alkyl groups (coupled with p-p interactions between aromatic cores) assist the self-assembly of the cyclopropenium compound into columnar structures.

Each of Ar ¹ , Ar ² and Ar ³ may be different. Two of Ar ¹ , Ar ² and Ar ³ may be the same. Preferably Ar ¹ , Ar ² and Ar ³ are the same.

Preferably one or more of Ar ¹ , Ar ² and Ar ³ is a phenyl group with one or more alkoxyl (eg Cs-is-alkoxyl) substituents. Particularly preferably each of Ar ¹ , Ar ² and Ar ³ is a phenyl group with one or more alkoxyl (eg Cs-is-alkoxyl) substituents. More preferably Ar ¹ , Ar ² and Ar ³ are alkoxyl derivatives of pyrogallol (i.e. 1,2,3-trialkoxybenzene) or catechol (i.e. 1,2-dialkoxybenzene) with alkoxyl (eg Cs-is-alkoxyl) substituents in the 2-, 3-, and 4- positions with respect to the X group.

A may be a multiply charged anion or a monoanion. Where A is multiply charged,

Formula (I) may include multiple cyclopropenium cations to balance the negative charge of the anion. For example where A is a dianion there may be two cyclopropenium cations per anion. Alternatively or additionally the cyclopropenium cation may include one or more additional positively charged groups as part of the X or Ar groups. Preferably A is a monoanion. Particularly preferably A is a small (e.g. monatomic) anion such as a halide. A small anion is less disruptive to cyclopropenium intermolecular packing and therefore is less likely to disrupt the formation of columnar structures. More preferably A is a chloride or bromide anion.

Viewed from a second aspect the present invention provides a method for producing a cyclopropenium compound having a structure according to Formula (I) wherein each of X ¹ , X ² and X ³ is a carbon-carbon bond, the method comprising: adding tetrahalocyclopropene and a Lewis acid to a first solvent to form a mixture; subjecting the mixture to a first temperature for a first time period; adding a first substituted aryl to the mixture; subjecting the mixture to a second temperature for a second time period; adding a second substituted aryl to the mixture; and subjecting the mixture to a third temperature for a third time period.

During the first time period a cyclopropenium cation is formed. Friedel-Crafts substitution of the substituted aryls onto the cyclopropenium occur during the second and third time periods.

Between about 0.8 and about 1.5 molar equivalents of the Lewis acid may be added to the solvent, relative to the amount of tetrahalocyclopropene added. Preferably between about 0.9 and about 1 molar equivalent of the Lewis acid is added to the solvent relative to the amount of tetrahalocyclopropene added. Between about 0.8 and about 5 molar equivalents of the first substituted aryl may be added to the mixture. Preferably between about 1 and about 3 molar equivalents of the first substituted aryl are added to the mixture. Even more preferably about 2 molar equivalents of the first substituted aryl are added to the mixture. These molar equivalents are relative to the amount of tetrahalocyclopropene added.

Between about 0.8 and about 5 molar equivalents of the second substituted aryl may be added to the mixture. Preferably between about 1 and about 3 molar equivalents of the second substituted aryl are added to the mixture. Even more preferably about 2 molar equivalents of the second substituted aryl are added to the mixture. These molar equivalents are relative to the amount of tetrahalocyclopropene added.

The method may further comprise adding a third substituted aryl to the mixture. The third substituted aryl may be added together with either the first or the second substituted aryl. Alternatively the method may further comprise adding a third substituted aryl to the mixture after the third time period; and subjecting the mixture to a fourth temperature for a fourth time period.

The first and second or first, second and third substituted aryls may be the same or may be different. Preferably all the substituted aryls are the same.

Each aryl may be independently selected from a group consisting of substituted benzene, naphthalene, anthracene, furan, pyrrole, thiophene, benzofuran, isobenzofuran, indole, isoindole, benzothiophene, imidazole, benzimidazole, purine, pyridine, quinoline, or isoquinoline. Preferably each aryl is a substituted benzene, anthracene, naphthalene, furan, pyrrole or thiophene. Particularly preferably each aryl is a substituted benzene.

Each aryl has one or more substituents. The substituents may be independently selected from a group consisting of alkyls, alkoxyls, oxyalkyls, alkenyls, carbonyls, carbonates, esters, amides, carbamates and organophosphates. The substituents may be independently selected from a group consisting of -R, -OR, -R ¹ - -C(=0)NR ₂ , -NR ¹ C(=0)0R, -0C(=0)NR ₂ or -0-P(=0)(0R) ₂ ; where R and R ¹ are alkyl groups.

Each aryl may have multiple substituents. The maximum number of substituents will be dictated by the number of suitable substitution sites on the aryl. The substituents on the same aryl may be the same or different. Preferably each aryl has 1-3 substituents. Each of the aryls may have the same or a different number of substituents. Each of the aryls may have the same or different substituents.

Where each of the aryls are the same or where two of the aryls are the same, each aryl may have the same or different substituents in the same or different positions.

Where an aryl is a substituted benzene, the benzene may have up to five substituents. Preferably the benzene has between one and three substituents. Preferably the benzene has three substituents on adjacent carbon atoms (i.e. the benzene is 1-, 2-, 3- substituted).

Preferably R = C _n H _2n+i and R ¹ = C _m H _2m+i and a value for n and/or n+mis greater than or equal to 4 for at least one substituent on each aryl. Particularly preferably a value for n and/or n+mis between 8 and 18 for at least one substituent on each aryl, or more preferably a value for n and/or n+m 8, 10, 12, 14 or 16 for at least one substituent on each aryl. Where there are multiple substituents on an aryl the value for n and/or n+mis preferably the same for each substituent.

Preferably each aryl is a 1,2,3-trialkoxybenzenes with three alkoxyl substituents n is preferably between 8 and 12 inclusive for each alkoxyl substituent.

The solvent is preferably a non-polar or a polar aprotic solvent. Preferably the solvent is benzene or a chlorinated solvent such as chloroform, dichloromethane, 1,1-dichloroethane or 1,2-dichloroethane. Particularly preferably the solvent is selected from a group consisting of benzene, chloroform or dichloromethane. The tetrahalocyclopropene is preferably tetrachlorocyclopropene and the cyclopropenium halide is cyclopropenium chloride.

The Lewis acid may be any Lewis acid. The Lewis acid is preferably iron(III) chloride, silver trifluoroacetate or aluminium chloride. Particularly preferably the Lewis acid is aluminium chloride.

Optimal values for the first, second and third temperatures and times will depend on the reagents and solvent used. The first temperature may be greater than or equal to 20 °C. Preferably the first temperature is between 50 °C and 100 °C. Even more preferably the first temperature is between 70 °C and 90 °C, eg about 80 °C.

The second temperature may be less than or equal to 60 °C. Preferably the second temperature is between -10 °C and 50 °C. Even more preferably the second temperature is between -5 °C and 5 °C, eg about 0 °C.

The third temperature may be greater than or equal to 20 °C. Preferably the third temperature is between 25 °C and 75 °C. Even more preferably the third temperature is between 40 °C and 60 °C, eg about 50 °C.

The first time period may be 5 hours or shorter. Preferably the first time period is shorter than 3 hours. Even more preferably the first time period is about 1 hour.

The second time period may be 5 hours or shorter. Preferably the second time period is shorter than 3 hours. Even more preferably the second time period is about 1 hour.

The third time period may be 5 hours or longer. Preferably the third time period is between 10 hours and 20 hours. Even more preferably the third time period is between 12 and 18 hours, eg about 14 hours. The method may further comprise exchanging the halide by reacting the cyclopropenium halide with a noble metal salt. The noble metal salt is preferably a silver salt. For example where the cyclopropenium halide is cyclopropenium chloride, the chloride may be exchanged for a non-halide anion such as hexafluorophosphate, tetrafluoroborate or (Tf) ₂ N by reacting the cyclopropenium halide with silver hexafluorophosphate, silver tetrafluoroborate or silver bis(trifluoromethane sulfonimide) respectively.

The halide may be exchanged by mixing together the cyclopropenium halide and noble metal salt in a solvent. The solvent is preferably nitromethane or a mixture of dichloromethane and acetonitrile.

Alternatively the method may further comprise exchanging the halide with an alternative anion A by reacting the cyclopropenium halide with water in the presence of sodium acetate to form a base (e.g. a cyclopropanol derivative). This addition of hydroxide breaks the aromaticity of the cyclopropenium ring. The base can then be reacted with an acid (HA) to regenerate water and a cyclopropenium salt with the alternative anion A . For example where the cyclopropenium halide is cyclopropenium chloride, the chloride may be exchanged for another halide such as bromide or iodide by reacting the cyclopropenium halide with water in the presence of sodium acetate, and then reacting the resulting base with hydrobromic acid or hydroiodic acid respectively. The anion exchange reaction may be carried out in any suitable solvent such as dichloromethane.

The alternative anion may be another halide (such as fluoride, bromide, iodide etc) or a larger anion such as hexafluorophosphate, tetrafluoroborate or (Tf) ₂ N . The acid may be a hydrogen halide (such as hydrofluoric acid, hydrobromic acid, hydroiodic acid etc) or an inorganic acid such as hexafluorophosphoric acid, tetrafluoroboric acid or bistriflimidic acid.

The method may further comprise one or more purification steps.

Viewed from a third aspect the present invention provides a method for producing a cyclopropenium compound having a structure according to Formula (I) wherein each of X ¹ , X ² and X ³ is an acyclic heteroatomic bridging group, the method comprising: adding pentachlorocyclopropane to a solvent to form a solution; subjecting the solution to a first temperature; adding a base to the solution; adding a first substituted aryl to the solution; stirring the solution at a first temperature for a first time period; then stirring the solution at a second temperature for a second time period.

The acyclic heteroatomic bridging group may be an epoxy group, an epithio group or an amino group. Where the X group is an amino group, the first substituted aryl is a substituted aryl alkyl amine. Where the X group is an epoxy group, the first substituted aryl is a substituted aryl alkoxide. Where the X group is an epithio group, the first substituted aryl is an aryl thioether. Preferably the X group is an amino group and the first substituted aryl is a substituted aryl alkyl amine.

Preferably the base is N,N-Diisopropylethylamine or triethylamine. Particularly preferably the base is triethylamine.

The method may further comprise adding a second substituted aryl to the solvent at the same time as the first substituted aryl, or to the reaction mixture before or after the first time period. The method may further comprise adding a third substituted aryl to the solvent at the same time as the first and/or second substituted aryl, or to the reaction mixture before or after the second time period. Where the third substituted aryl is added to the reaction mixture after the second time period, the method may further comprise stirring the reaction mixture for a third time period at a third temperature after adding the third substituted aryl. The second and third substituted aryls may be substituted aryl alkyl amines, aryl alkoxides, secondary a-aryl amides or aryl thioethers depending on the desired X group.

Each substituted aryl may be independently selected from a group consisting of substituted benzene, napthalene, anthracene, furan, pyrrole, thiophene, benzofuran, isobenzofuran, indole, isoindole, benzothiophene, imidazole, benzimidazole, purine, pyridine, quinoline, or isoquinoline. Preferably each aryl is a substituted benzene, pyridine, napthalene or quinoline. Particularly preferably each aryl is a substituted benzene derivative selected from a group consisting of substituted N-alkylaniline, alkoxybenzene, benzamide or alkylthiobenzene depending on the desired X group. Particularly preferably the X group is an amine and each aryl is a substituted N-alkylaniline.

Each aryl has one or more substituents. The substituents may be independently selected from a group consisting of alkyls, alkoxyls, oxyalkyls, alkenyls, carbonyls, carbonates, esters, amides, carbamates and organophosphates.

The substituents may be independently selected from a group consisting of -R, -OR, -R ¹ - -C(=0)NR ₂ , -NR ¹ C(=0)0R, -0C(=0)NR ₂ or -0-P(=0)(0R) ₂ ; where R and R ¹ are alkyl groups.

Each aryl may have multiple substituents. The maximum number of substituents will be dictated by the number of suitable substitution sites on the aryl. The substituents on the same aryl may be the same or different. Preferably each aryl has 1-3 substituents. Each aryl may have the same or a different number of substituents. Each aryl may have the same or different substituents.

Where each of the aryls are the same or where two of the aryls are the same, each aryl may have the same or different substituents in the same or different positions.

Where an aryl is a substituted benzene derivative, the benzene derivative may have one, two or three substituents. Preferably the benzene has three substituents on adjacent carbon atoms. The three substituents are preferably in the two meta- positions and the para- position with respect to the amine, amide, alkoxyl or sulfanyl.

Preferably R = C _n H _2n+i and R ¹ = C _m H _2m+i and a value for n and/or n+mis greater than or equal to 4 for at least one substituent on each aryl. Particularly preferably a value for n and/or n+mis between 8 and 18 for at least one substituent on each aryl, or more preferably a value for n and/or n+m 8, 10, 12, 14 or 16 for at least one substituent on each aryl. Where there are multiple substituents on an aryl the value for n and/or n+mis preferably the same for each substituent.

Preferably there is no second or third substituted aryls and the first substituted aryl is a substituted N-alkylaniline with three alkoxyl substituents. Even more preferably the first substituted aryl is a substituted N-methylaniline with three alkoxyl substituents n is preferably between 8 and 12 inclusive for each alkoxyl substituent.

The solvent is preferably a non-polar or a polar aprotic solvent. Preferably the solvent is a chlorinated solvent such as chloroform, dichloromethane, 1,1-dichloroethane or 1,2- dichloroethane. Particularly preferably the solvent is chloroform or dichloromethane.

Optimal values for the first and second temperatures and times will depend on the reagents and solvent used. The first temperature may be less than or equal to 20 °C. Preferably the first temperature is between -10 °C and 20 °C. Even more preferably the first temperature is between -5 °C and 5 °C, eg about 0 °C.

The second temperature may be greater than or equal to 10 °C. Preferably the second temperature is between 10 °C and 30 °C. Even more preferably the second temperature is ambient room temperature (i.e. about 20 °C).

The first time period may be 5 hours or shorter. Preferably the first time period is shorter than 3 hours. Even more preferably the first time period is about 1 hour.

The second time period may be 5 hours or longer. Preferably the second time period is between 15 hours and 30 hours. Even more preferably the second time period is between 20 and 25 hours, eg about 24 hours.

The method may further comprise exchanging the chloride by reacting the cyclopropenium chloride with a noble metal salt. The noble metal salt is preferably a silver salt. For example the chloride may be exchanged for a non-halide anion such as hexafluorophosphate, tetrafluoroborate or (TfkN by reacting the cyclopropenium chloride with silver hexafluorophosphate, silver tetrafluoroborate or silver bis(trifluoromethane sulfonimide) respectively.

The chloride may be exchanged by mixing together the cyclopropenium chloride and noble metal salt in a solvent at a fourth temperature for a fourth time period. The solvent is preferably a mixture of dichloromethane and acetonitrile. The fourth temperature is preferably greater than 10 °C, such as ambient temperature (i.e. about 20 °C). The fourth time period is preferably less than 1 hour, and more preferably less than 30 minutes, such as about 15 minutes.

Viewed from a fourth aspect the present invention provides a liquid crystal material comprising a cyclopropenium compound according to Formula I.

Viewed from a fifth aspect the present invention provides the use of a cyclopropenium compound according to Formula I or a liquid crystal material as hereinbefore defined in an optoelectronic device (eg solar cells, photoresistors, photodiodes, phototransistors, charge- coupled imaging devices, LEDs, OLEDs and consumer electronic devices). A cyclopropenium compound according to Formula I or a liquid crystal material as hereinbefore defined may also be used as an electrolyte in a battery.

Specific implementation of the present invention will now be described, by way of example only, and with reference to the accompanying drawings in which:

Figure 1 shows a general formula (Formula (I)) of a cyclopropenium compound according to the present invention.

Figure 2 is a graphical representation of the self-assembly of the cyclopropenium compounds of the present invention into columnar structures.

Figure 3 illustrates two different types of columnar structures formed by the cyclopropenium compounds of the present invention. Figure 4 is a reaction scheme for the preparation of a cyclopropenium compound according to a first embodiment.

Figure 5 is a reaction scheme for exchanging the counter ion of the cyclopropenium compound of Figure 4.

Figure 6 is a reaction scheme for the preparation of an aminocyclopropenium compound according to a second embodiment.

Figure 7 is a reaction scheme for exchanging the counter ion of the aminocyclopropenium compound of Figure 6.

Figure 8 is a graph illustrating the phase behaviour of several cyclopropenium compounds. Figure 9 shows a wide-angle X-ray scattering profile, diffraction image and electron density map of compound DC 10. Cl at 52 °C showing the formation of a hexagonal columnar liquid crystal phase.

Figure 10 shows a wide-angle X-ray scattering profile, diffraction image and electron density map of compound DC16.C1 at 50 °C showing the formation of a rectangular columnar liquid crystal phase.

Figure 11 is a graph showing the temperature dependence of conductivity for several cyclopropenium compounds.

Figure 12 is a graph showing the trends in real permittivity over frequency at a range of temperatures for compound DC 12.0.

Figure 13 is a graph showing conductivity against inverse temperature for both parallel (“guard ring”) and perpendicular (“in plane”) measurements of DC10.C1 at a frequency of 10 ⁴ Hz.

Figure 14 shows XRD spectra at room temperature (25 °C) of DC10.C1 and DC18.C1 Figure 15 shows FTIR spectra at room temperature (25 °C) of DC10.C1 and DC18.C1 Figure 16 is an ESP map of the DC1 cation.

Figure 17 shows (a) front and back views of an ESP map of the DC8 cation; (b) an ESP map of the trimethyloctylammonium cation; and (c) an ESP map of the methyloctylimidazolium cation.

Figure 18 shows an XPS spectrum of DC 10. Cl in the energy region of the Cl 2p signal Preparation of cyclopropenium compounds

Example 1: Compound DC12.C1

Compound DC12.C1 is a compound according to Formula (I) (as shown in Figure 1) where each of X ^{1 3} is a carbon-carbon bond, each of Ar ^1-3 is 2,3,4-tris n-dodecyloxyphenyl and A is chloride.

The reaction scheme for the synthesis of compound DC 12. Cl is shown in Figure 4, where R = C12H25.

Tetrachlorocyclopropene (0.14 g, 0.82 mmol) and AICI ₃ (0.10 g, 0.75 mmol) were added to an oven dried microwave vial under an inert atmosphere (N2) in CHCI ₃ (0.1 mL) to form a mixture. The mixture was then sonicated for 5 minutes before being heated to 80 °C for 1 hour. The mixture was then cooled to 0 °C and 1,2,3-Tri n-dodecyloxybenzene (0.9 g, 1.5 mmol) dissolved in CHCI ₃ was added to the vial and left to stir for 1 hour. The mixture was then heated to 50 °C and further 1,2,3-Tri n-dodecyloxybenzene (0.9 g, 1.5 mmol) dissolved in CHCI ₃ was added to the mixture. The mixture was then stirred for 14 hours at 50 °C. The solvent was removed under reduced pressure, and the mixture was then purified by column chromatography (Teledyne Isco Combiflash Rf-i- system, 12 g S1O2, CthCb/MeOH gradient elution). The solvent was then removed under reduced pressure and the resultant material was recrystallised in MeCN to yield compound DC12.C1 (tris(l,2,3-tri(n-dodecyloxy)phenyl)cyclopropenium chloride) as a yellow solid (0.51 g, 0.35 mmol, 47% yield).

Example 2: Compound DC10.C1

Compound DC10.C1 is a compound according to Formula (I) (as shown in Figure 1) where each of X ^{1 3} is a carbon-carbon bond, each of Ar ^{1 3} is 2,3,4-tris n-decyloxyphenyl and A is chloride.

The reaction scheme for the synthesis of compound DC10.C1 is shown in Figure 4, where R = C10H21. Compound DC10.C1 was prepared according to the same procedure as compound DC12.C1 described above, using 1,2,3-Tri n-decyloxybenzene (0.8 g 1.5 mmol) in both instances rather than 1,2,3-Tri n-dodecyloxybenzene.

Compound DC 16. Cl is a compound according to Formula (I) (as shown in Figure 1) where each of X ^{1 3} is a carbon-carbon bond, each of Ar ^1-3 is 2,3,4-tris n-cetyloxyphenyl and A is chloride.

The reaction scheme for the synthesis of compound DC16.C1 is shown in Figure 4, where R = C16H33. Compound DC16.C1 was prepared according to the same procedure as compound DC12.C1 described above except 1,2,3-Tri n-cetyloxybenzene (1.2 g 1.5 mmol) was used in both instances rather than 1,2,3-Tri n-dodecyloxybenzene, and benzene was used as the solvent rather than CHCI3.

Compound DC12.Br is a compound according to Formula (I) (as shown in Figure 1) where each of X ^{1 3} is a carbon-carbon bond, each of Ar ^{1 3} is 2,3,4-tris n-dodecyloxyphenyl and A is bromide.

Tris(l,2,3-tri(n-dodecyloxy)phenyl)cyclopropenium chloride (i.e. compound DC 12. Cl) (0.41 g, 0.20 mmol) was dissolved in CH2CI2 (50 mL), and then washed with aqueous NaOAc solution (3 x 50 mL, 0.365 M). The organic solution was then washed with aqueous hydrobromic acid (15 mL, 1M). The solvent was removed under reduced pressure, and the product was dissolved in acetonitrile (15 mL) under reflux. The solution was filtered, and then cooled to -8 °C to precipitate out the product, which was collected by filtration and dried under vacuum to give the product as a colourless solid (50 mg, 0.025 mmol, 12% yield).

Compound AC12.C1 is a compound according to Formula (I) (as shown in Figure 1) where each of X ^{1 3} is an alkylamino (methylamino) group, each of Ar ^{1 3} is 2,3,4-tris n- dodecyloxyphenyl and A is chloride. The reaction scheme for the synthesis of compound AC12.C1 is shown in Figure 6, where R = C12H25.

Pentachlorocyclopropane (30 mg, 0.14 mmol) was added to CH2CI2 (20 mL) to form a mixture. The mixture was then cooled to 0 °C. 3,4,5-Tris(n-dodecyl-l-oxy)methylaniline (0.35 g, 0.48 mmol) and Et3N (0.1 mL, 0.87 mmol) were added to the mixture in succession. The mixture was then stirred for 1 hour at 0 °C and then warmed to room temperature (20 °C) and stirred for a further 24 hours. The reaction mixture was then washed with H2O (3 x 10 mL), dried with CaCh, and filtered. The solvent was removed under reduced pressure and the crude mixture was purified by column chromatography (Teledyne Isco Combiflash Rf+ system, 12 g S1O2, CEbCb/MeOH gradient elution) to yield compound AC12.C1 (tris(methyl-l,2,3-tris(n-dodecyl-l- oxy)phenyl)aminocyclopropenium chloride) as a yellow solid (0.21 g, 0.10 mmol, 75% yield).

Example 6: Compound AC 12.1

Compound AC12.I is a compound according to Formula (I) (as shown in Figure 1) where each of X ^{1 3} is an afkylamino (methylamino) group, each of Ar ^{1 3} is 2,3,4-tris n- dodecyloxyphenyl and A is iodide.

Tris(methyl-l,2,3-tris(n-dodecyl-l-oxy)phenyl)aminocyclop ropenium chloride (i.e. compound AC12.C1) (100 mg, 0.05 mmol) was dissolved in CH2CI2 (10 mL). The solution was washed with Nal solution (10 mL, 2.5 mM solution in H2O) three times. The solution was dried with MgSCL, filtered and the solvent was then removed under reduced pressure to give the product as a yellow liquid (83 mg, 0.04 mmol, 80% yield).

Characterisation of Example Compounds

Example compounds 1-6 were investigated by polarised optical microscopy and differential scanning calorimetry. Example compounds 1-5 showed birefringent character under a polarised microscope. Differential scanning calorimetry analysis showed liquid crystal phase transition temperatures (T _k ) in the range of 40 °C - 70 °C for all the example compounds. Example compounds transitioned to an isotropic liquid in the 65 °C - 90 °C range. Results of the differential scanning calorimetry tests are summarised in Table 1.

Table 1: Phase transitions of cyclopropenium compounds identified by differential scanning calorimetry

Compound Tc / °C Tc / °C TK / °C TK / °C TI / °C TI / °C

(DH / kj (DH / kj (DH / kj (DH / kj (DH / kj (DH / kj mol ¹ ) mol ¹ ) mol ¹ ) mol ¹ ) mol ¹ ) mol ¹ )

Heating Cooling Heating Cooling Heating Cooling

DC12.C1 -3 (24.3) -1 (-24.4) 47 (24.4) 45 (-23.6) 71 (2.9) 71 (2.8)

DC10.C1 -35 (0.9) -38 (-0.9) 40 (12.0) 38 (-12.2) 67 (3.1) 61 (-3.0)

DC16.C1 - - 45 (66.6) 42 (-67.0) 64 (63.2) 58 (-62.6)

DC12.Br - - 57 (68.8) 55 (-95.5) 78 (26.4) 64 (-26.8)

AC12.C1 7 (28.4) 5 (28.4) 60 (0.4) 53 (0.2)

AC 12.1 -20 (17.0) -20 (11.8)

In Table 1, Tc corresponds to crystal-crystal phase transitions, TK corresponds to crystal- liquid crystal phase transitions, and Ti corresponds to liquid crystal-isotropic phase transitions. Table 1 shows the temperatures and transition enthalpies on heating and cooling the samples. Where no phase transition is given, it is either not present for the compound or was not detected in the temperature range used.

Example compounds 1-4 were also investigated to determine the nature of the liquid crystal phases formed. X-ray diffraction (XRD), short-angle X-ray diffraction and wide- angle X-ray diffraction measurements were carried out. The results of these measurements are summarised in Table 2. Figure 8 shows the phase transition temperatures and different phases present between -20 °C and 100 °C for a series of example cyclopropenium compounds. Figure 3 shows the arrangement of columns (when viewed along the column axis) for hexagonal and rectangular columnar liquid crystal mesophases. Table 2: The mesophase and lattice parameters of cyclopropenium compounds from XRD measurements

Compound Measurement Mesophase Lattice Parameter

Temperature / °C /A

DC12.C1 60 Colhex a = 30.2

DC10.C1 52 Colhex a = 28.0

DC16.C1 50 Colrec - c2mm a = 66.3 b = 30.9

DC12.Br 50 Colhe: a = 54.7

As shown in Figure 9, compound DC10.C1 gives a small-angle reflection in the liquid crystal phase at 2Q = 3.6 °. This is assigned as a (10) later reflex. A higher angle diffuse region at around 2Q = 20 ⁰ is also observed. This likely arises from the disordered region of alkyl chains surrounding the core. Figure 9 shows that compound DC 10. Cl exhibits a hexagonal columnar phase, with a lattice parameter (a) of 28 A.

As shown in Figure 10, the scattering profile of compound DC16.C1 shows two close peaks in the 2Q = 2.4 - 3.2 ⁰ region, corresponding to the (20) and (11) layer reflex. The same diffuse region around 2Q = 20 ° is also observed, corresponding to the disordered alkyl chains surrounding the core. Figure 10 shows that compound DC16.C1 exhibits a rectangular columnar phase with a symmetry of c2mm.

As seen in Table 2, compound DC12.Br shows a hexagonal phase similar to DC12.C1, but with a larger lattice parameter.

XRD measurements were also carried out below the TK phase transition temperature for each compound, confirming the formation of the crystalline phase. For examples exhibiting a hexagonal columnar phase, XRD crystal phase spectra show that the small- angle diffraction peak and diffuse regions are still present. This indicates that the hexagonal columnar structure is somewhat retained in the crystal phase. In contrast, examples exhibiting a rectangular columnar phase show significant disruption of the columnar structure upon phase transition to the crystal phase. XRD measurements were taken at room temperature (25 °C) (in the solid phase) for compounds DC10.C1, DC10.Br, DC12.C1 and DC12.Br as well as for compound DC18.C1. For DClO.Cl/Br and DC12.Cl/Br compounds, two peaks could be identified for the packing of the alkyl peaks, showing a mixture of both fluid peaks and hexatically packed peaks. This indicates that the solid phase of these compounds has some degree of fluidity. In comparison, the XRD measurement of the longer alkyl chain of DC 18. Cl does not show any fluid alkyl chains. Figure 14 shows XRD spectra at room temperature (25 °C) of both DC10.C1 and DC18.C1, indicating the absence of fluid chains in DC18.C1. The less ordered structure of the hexagonal columnar phase exhibited in DC10-12.Cl/Br may assist in the fluid chains persisting into the solid phase for the hexagonal columnar materials, with a similar structure to the liquid crystal for both phases. By comparison no fluid chains persist in the solid phase for the compounds which exhibit a rectangular columnar phase (such as DC 18. Cl).

Compounds DC10.C1 and DC18.C1 were also measured by FTIR spectroscopy. FTIR spectra of these two compounds are shown in Figure 15. As shown in Figure 15, the DC 18. Cl spectrum shows a characteristic shift to a slightly lower wavenumber for both the symmetric and asymmetric stretching vibrations at room temperature compared to the DC 10. Cl spectrum (see Table 3). This suggests that the alkyl chains are packed more tightly in the DC 18. Cl solid phase.

Table 3: Wavenumbers of C-H FTIR peaks for compounds DC10.C1 and DC18.C1

Example compounds were also analysed to determine their charge transfer properties. The compounds were added into pre-made ITO coated dielectric glass cells through capillary action at 100 °C. Then, with the use of a dielectric bridge, the compounds were screened over a range of frequencies, temperatures and voltages to measure their dielectric properties. Conductivities were measured over a temperature range of 80 °C to 30 °C for all example compounds in an axis parallel to the axis of homeotropic alignment (i.e. parallel to the column axis). Conductivities for compound DC12.C1 were also measured down to 2 °C and at -10 °C. Figure 11 shows the conductivities of a series of example cyclopropenium compounds over a range of temperatures from -10 °C to 80 °C. The phase for each compound at each temperature is indicated by the shape of the marker. Figure 12 shows the trends in real permittivity over frequency at a range of temperatures for compound DC 12. Cl.

All the example materials show similar trends in real and imaginary permittivity against frequency, consistent with a semiconductive material. As shown in Figure 12 (for compound DC 12. Cl) conductance against frequency plateaus in the range of 10 ³ -10 ⁴ Hz. Conductivity measurements were therefore performed in this frequency region for all materials.

As shown in Figure 11, for each of the cyclopropenium compounds the conductivity increases with temperature. This is indicative of a semi-conductive system. Example materials that possess a hexagonal columnar phase retain the same trend of conductivity against temperature when passing through the phase transition temperature (TK) between liquid crystal and crystal phases. The trend is even retained through the crystal-crystal phase transition (shown in Figure 11 for DC12.C1). This is indicative of the retention of the hexagonal columnar structure in the crystal phase.

In contrast, materials which form a rectangular columnar phase show a significant drop in conductivity as the material cools through the liquid-crystal - crystal phase transition temperature (T _K ). This is indicative of significant rearrangement of the columnar structure upon transition to the solid phase.

Compound DC12.Br exhibits conductivity around an order of magnitude greater than the chloride equivalent (DC12.C1). This may be because bromide less strongly coordinates with the cyclopropenium and is therefore more able to undergo charge transport. Conductivity measurements were also carried out perpendicular to the axis of homeotropic alignment (i.e. perpendicular to the column axis) for compound DC10.C1. It was assumed that conductivity in the isotropic phase would be the same in both axes. Perpendicular conductance was therefore compared to parallel conductivity over a range of temperatures above the isotropic-liquid crystal phase transition temperature to calculate a mean conversion factor, which was then applied to conductance to determine conductivity in the axis perpendicular to the axis of homeotropic alignment. Figure 13 shows that the conductivity is very similar in both parallel (“guard ring”) and perpendicular (“in plane”) axes across a majority of the temperature range studied. This is indicative of charge transport being primarily driven by anion diffusion.

To better understand whether charge transport is driven by anion (counter-ion) diffusion, electrostatic potential (ESP) maps were calculated for a series of cations: DC1; DC2; DC4 and DC8 (i.e. cations according to the cation of Formula 1 where each of X ^{1 3} is a carbon- carbon bond and each of Ar ^{1 3} is 2,3,4-tris methoxyphenyl (for DC1); 2,3,4-tris ethoxyphenyl (for DC2); 2,3,4-tris n-butoxyphenyl (for DC4); or 2,3,4-tris n-octoxyphenyl (for DC8)). The ESP map for the DC1 cation is shown in Figure 16 and indicates that the charge is highly delocalised across the surface of the cation, rather than being restricted to the centre. The greatest charge density of the cation is on the methoxy groups rather than on the cyclopropenium core.

The potential at the centre of the cation decreases with increasing alkyl chain length: from a potential of +0.100 in DC1 (as shown in Figure 16) to +0.093 in DC2; +0.089 in DC4 and +0.088 in DC8 (see Figure 17). As shown in Figure 17 for the larger cations (such as DC8) the region with highest potential is on the alkyl regions close to the oxygen atoms, with decreasing charge going further along the alkyl chains. In comparison (as shown in Figure 17) ammonium and imidazolium species with 8-carbon alkyl chains show a much greater degree of localisation of charge on the central cation.

The difference in potential between the centre-most methoxy carbon remains around 0.01 throughout all the cyclopropenium compounds for which ESP maps were calculated. This suggests that it is these methoxy groups that will be most strongly interacting with the counter- ion (anion) rather than the central aromatic regions.

X-ray photoelectron spectroscopy (XPS) measurements were also performed on compound DC 10. Cl at room temperature (25 °C) in the solid phase to investigate the environments of the elements. The XPS spectrum shows a single oxygen peak and two carbon peaks, with the sp ³ and sp ² carbon peaks merged together and the signals for the carbon bonded to the oxygen identifiable separately. The merger of the sp ³ and sp ² peaks supports the ESP maps showing the high degree of charge delocalisation along the alkyl chains and electron donation into the molecular centre.

Figure 18 shows the XPS spectrum of DC 10. Cl at room temperature in the energy region of the Cl 2p signal. Three distinct chloride peaks are visible at 196 eV, 198 eV and 200 eV. The most intense peak at 196 eV shows free chloride which is not bound to the cation. The 198 eV peak shows the presence of chloride ions which are interacting with the cation. The smallest peak at 200 eV shows chloride ions interacting strongly with the cation, either by strong ionic interaction or a covalent interaction. The presence of free chloride ions indicates that the bulk of the material conductivity is likely to be driven by anion diffusion.

The discotic liquid crystals advantageously show conductivity over a range of temperatures in both liquid crystal and isotropic phases. Certain liquid crystals which exhibit a hexagonal columnar phase also retain their conductivity at lower temperatures in the solid phase. Such materials therefore have a much wider operational temperature range than conventional ionic liquid crystal materials as well as a lower operational temperature. Such materials can also take advantage of the properties of both liquid crystal and solid phases and are therefore able to be integrated into solid state devices.