Organic crystals and optical films

Abstract

The present invention relates to crystalline solids comprising organic molecules and to the use of the crystalline materials in optical films and electro-optical devices.

Description

[0001] Organic Crystals and Optical Films

[0002] The present invention relates to crystalline solids comprising organic molecules and to the use of the crystalline materials in optical films and electro-optical devices.

[0003] Optical substrates, films and elements may be based on materials such as glass and inorganic crystals including glassy and crystalline ceramic materials, plastics or films based on reactive mesogens.

[0004] WO 2021 / 173503 A1 , WO 2023 / 003909 A1, US 2023 / 0193505 A1 and US 2024 / 0151993 A1 describe methods of forming organic solid crystal films or layers made of organic molecules. These solid crystal films can have a high refractive index and allow a reduction in size and weight of optical elements and devices such as lenses, waveplates, gratings and waveguides. Such light-weight and compact optical elements and devices may be useful for example in near-eye displays for augmented reality and virtual reality.

[0005] There is still a need in the art to provide organic solid crystals, in particular also in larger sizes and from various feedstocks.

[0006] An object of the present invention is therefore to provide solid crystals from organic molecules, in particular larger contiguous organic solid crystals, using various precursors. It is a further object to provide organic molecular crystals, in particular optically transparent crystals, with favourable optical quality and characteristics which are particularly useful in optical and electro-optical applications.

[0007] Further objects of the present invention are immediately evident to the person skilled in the art from the following detailed description.

[0008] The objects are solved by the subject-matter defined in the independent claims, while preferred embodiments are set forth in the respective dependent claims and are further described below.

[0009] The present invention in particular provides the following items including main aspects, preferred embodiments and particular features, which respectively alone and in combination contribute to solving the above object and eventually provide additional advantages. A first aspect of the present invention provides a crystalline material, wherein the material is a solid comprising an organic compound and having a density of less than 1.75 g cm-3, wherein the material has a refractive index of 1.8 or more.

[0010] Compared to inorganic glasses and ceramics, the crystalline materials as provided are light-weight crystals, in particular having a density of less than 1.75 g cm-3, preferably less than 1.50 g cm-3, more preferably less than 1.25 g cm-3, in particular measured at 22°C.

[0011] The solid crystalline material containing the crystallized organic compound can generally be obtained by solution growth or alternatively by melt growth or by deposition processes.

[0012] For example, a facile and fast process for producing the solid crystalline materials made of organic compounds, and in particular organic single crystals, can be obtained by using crystal growth from solution, in particular using precursor solutions containing organic molecules as the feedstock.

[0013] It has surprisingly been found that solid crystals and in particular larger contiguous organic solid crystals can be provided using organic molecules as precursors. Different precursors as described herein may be used to adjust and set the physical and optical characteristics of the crystalline materials as needed or desired in the optical and electro-optical applications, thus giving benefits also in terms of an improved tunability.

[0014] In particular, large and light-weight crystals and in particular visibly transparent crystals with favourable optical quality and performance, for example materials with a high refractive index, high optical anisotropy and / or high optical clarity, can be obtained.

[0015] It has been found that the crystalline materials according to the invention can have a low weight, preferably and favourably in combination with a high refractive index, thus allowing a reduction in size and weight of optical elements and devices. These materials are therefore especially useful as optical substrates or in optical elements or as components of electro-optical devices, for example lenses, waveplates, gratings and waveguides.

[0016] The materials preferably and favourably are transparent solids, in particular single crystals, comprising crystallized organic molecules which give ease of processing such as cutting and polishing. Therefore, a further aspect provides an optical film which comprises the crystalline material according to the invention, wherein in particular the film has first and second dimensions of 10 mm or more.

[0017] A film herein may include thin films as well as bulk films or crystals which may be freestanding or disposed over a substrate. In case a substrate is used, the substrate preferably is optically transparent.

[0018] It has been found that the optical films can have favourable optical quality and performance, for example a high refractive index, high optical anisotropy and / or high optical clarity. Preferably, the optical films are visibly transparent and colourless single crystals.

[0019] The optical films, in particular after processing such as cutting and polishing, can favourably exhibit minimal surface roughness. In addition, favourable hardness, in particular favourable scratch hardness, indentation hardness and rebound hardness, can be obtained.

[0020] Another aspect provides optical elements and respectively electro-optical devices containing the crystalline materials or the optical films according to the invention.

[0021] A further aspect of the invention relates to the use of the organic compounds as described herein below for the preparation of organic solid crystals, in particular single crystals.

[0022] Without limiting the present invention thereby, in the following the invention is illustrated by the detailed description of the aspects, embodiments and particular features, and particular embodiments are described in more detail.

[0023] The crystalline materials according to the invention are materials which comprise one or more crystals, and which preferably are composed of one or more crystals. The materials thus contain crystals or are in the form of crystals. The materials are in particular solids having or comprising highly ordered microscopic structures forming periodic arrangements or crystal lattices giving solid crystal materials.

[0024] In principle the materials may be polycrystalline, semicrystalline or monocrystalline solids. In some cases the materials may be substantially crystalline but may include amorphous regions. Preferably, the crystalline material is a monocrystalline solid or a polycrystalline solid, more preferably a monocrystalline solid.

[0025] The crystalline material preferably is a single crystal. In particular, the preferably obtained single crystals are macroscopic single crystals which may generally be identified by their geometrical shape exhibiting flat faces with specific, characteristic orientations or crystallographic axes.

[0026] In principle, the crystals may exhibit defects, in particular crystallographic defects such as point defects, line defects, planar defects and bulk defects, and / or contain impurities, dopants or additives.

[0027] It is however preferred that the crystals, in particular the single crystals, as obtained according to the invention are free of defects.

[0028] The presence or absence of defects may be determined by visual inspection, for example using crossed polarizers, or more preferably by X-ray diffraction, in particular X-ray topography, or (micro) X-ray fluorescence.

[0029] The solid crystal material according to the invention contains an organic compound, in particular organic molecules as crystal molecules. The crystalline material thus is an organic crystalline material. Preferably, the solid organic crystal materials are obtained in the form of organic single crystals.

[0030] In an embodiment the solid crystal is produced by growing a crystal of an organic compound, wherein in particular the organic compound is dissolved in a solvent to provide a precursor solution.

[0031] Organic compounds generally are carbon-based substances. The term organic compound herein preferably denotes a compound which contains at least one carbon to hydrogen bond. The organic compounds herein preferably are organic molecules which in particular are uncharged and do not include ions.

[0032] The organic compounds preferably are not soluble in water or are substantially not soluble in water, in particular not soluble in water or aqueous solutions. It is particularly preferred that the organic compounds are soluble in water in amounts of 0.1 g or less per 100 mL of water, preferably determined at 20°C.

[0033] Preferably the organic compound is a crystallizable molecular compound comprising one or more rings selected from aromatic, heteroaromatic, alicyclic and heterocyclic groups, more preferably one or more rings selected from aromatic and heteroaromatic groups.

[0034] The term ring herein refers to a cyclic group having a closed ring structure, i.e. a closed ring of atoms. Herein rings also include annulated, condensed or fused rings, in particular edge-to-edge fused rings, wherein rings are fused if they share two or more atoms.

[0035] The aromatic, heteroaromatic, alicyclic or heterocyclic groups each preferably have 4 to 25 ring atoms and may also contain fused rings and may be unsubstituted, monosubstituted or polysubstituted.

[0036] In an embodiment the organic compound comprises a ring structure and optionally one, two or more polar or non-polar terminal groups, wherein the ring structure preferably comprises aromatic hydrocarbon, heteroaromatic hydrocarbon and / or polycyclic aromatic hydrocarbon moieties.

[0037] Aryl and heteroaryl groups can be monocyclic or polycyclic, i.e. they can contain one ring, such as, for example, phenyl, or two or more rings, which may also be fused, such as, for example, naphthyl, or covalently bonded, such as, for example, biphenyl, or contain a combination of fused and linked rings. Heteroaryl groups contain one or more heteroatoms, preferably selected from O, N, S and Se. A ring system of this type may also contain individual non-conjugated units, as is the case, for example, in the fluorene basic structure.

[0038] Particular preference is given to mono-, bi- or polycyclic aryl groups having 6 to 50 C atoms and mono-, bi- or polycyclic heteroaryl groups having 5 to 50 C atoms, which optionally contain fused rings and are optionally substituted. Preference is furthermore given to 5-, 6- or 7-membered aryl and heteroaryl groups, in which one or more CH groups may be replaced by N, S or O in such a way that O atoms and / or S atoms are not linked directly to one another. Preferred aryl groups are derived, for example, from the parent structures benzene, biphenyl, terphenyl, [1,1':3',1"]terphenyl, tolane, naphthalene, anthracene, binaphthyl, phenanthrene, pyrene, dihydropyrene, chrysene, perylene, tetracene, pentacene, benzopyrene, fluorene, indene, indenofluorene, spirobifluorene, etc.

[0039] Preferred heteroaryl groups are, for example, 5-membered rings, such as pyrrole, pyrazole, imidazole, 1 ,2,3-triazole, 1 ,2,4-triazole, tetrazole, furan, thiophene, selenophene, oxazole, isoxazole, 1,2-thiazole, 1,3-thiazole, 1 ,2,3-oxadiazole,

[0040] 1.2.4-oxadiazole, 1,2,5-oxadiazole, 1 ,3,4-oxadiazole, 1 ,2,3-thiadiazole, 1,2,4-thiadiazole,

[0041] 1.2.5-thiadiazole, 1,3,4-thiadiazole, 6-membered rings, such as pyridine, pyridazine, pyrimidine, pyrazine, 1 ,3,5-triazine, 1 ,2,4-triazine, 1 ,2,3-triazine, 1,2,4,5-tetrazine, 1, 2,3,4- tetrazine, 1,2, 3, 5- tetrazine, or condensed groups, such as indole, isoindole, indolizine, indazole, benzimidazole, benzotriazole, purine, naphthimidazole, phenanthrimidazole, pyridimidazole, pyrazinimidazole, quinoxalinimidazole, benzoxazole, naphthoxazole, anthroxazole, phenanthroxazole, isoxazole, benzothiazole, benzofuran, isobenzofuran, dibenzofuran, quinoline, isoquinoline, pteridine, benzo-5,6-quinoline, benzo-6,7-quinoline, benzo-7,8-quinoline, benzoisoquinoline, acridine, phenothiazine, phenoxazine, benzopyridazine, benzopyrimidine, quinoxaline, phenazine, naphthyridine, azacarbazole, benzocarboline, phenanthridine, phenanthroline, thieno[2,3b]thiophene, thieno[3,2b]thiophene, dithienothiophene, dihydrothieno [3,4-b]-1 ,4-dioxin, isobenzothiophene, dibenzothiophene, benzothiadiazothiophene, or combinations of these groups. The heteroaryl groups may also be substituted by alkyl, alkoxy, thioalkyl, fluorine, fluoroalkyl or further aryl or heteroaryl groups.

[0042] The non-aromatic alicyclic and heterocyclic groups encompass both saturated rings, i.e. those containing exclusively single bonds, and also partially unsaturated rings, i.e. those which may also contain multiple bonds. Heterocyclic rings contain one or more heteroatoms, preferably selected from Si, O, N, S and Se.

[0043] The non-aromatic alicyclic and heterocyclic groups can be monocyclic, i.e. contain only one ring, such as, for example, cyclohexane, or polycyclic, i.e. contain a plurality of rings, such as, for example, decahydronaphthalene or bicyclooctane. Particular preference is given to saturated groups. Preference is furthermore given to mono-, bi- or tricyclic groups having 3 to 25 C atoms, which optionally contain fused rings and are optionally substituted. Preference is furthermore given to 5-, 6-, 7- or 8-membered carbocyclic groups, in which, in addition, one or more C atoms may be replaced by Si and / or one or more CH groups may be replaced by N and / or one or more non-adjacent CH2 groups may be replaced by -O- and / or -S-.

[0044] Preferred alicyclic and heterocyclic groups are, for example, 5-membered groups, such as cyclopentane, tetrahydrofuran, tetrahydrothiofuran, pyrrolidine, 6-membered groups, such as cyclohexane, silinane, cyclohexene, tetrahydropyran, tetrahydrothiopyran, 1 ,3-dioxane,

[0045] 1.3-dithiane, piperidine, 7-membered groups, such as cycloheptane, and fused groups, such as tetrahydronaphthalene, decahydronaphthalene, indane, bicyclo[1.1.1]pentane-

[0046] 1.3-diyl, bicyclo[2.2.2]octane-1 ,4-diyl, spiro[3.3]heptane-2,6-diyl, octahydro-4, 7- methanoindane-2,5-diyl.

[0047] The aryl, heteroaryl, carbon and hydrocarbon radicals optionally have one or more substituents, which are preferably selected from the group comprising silyl, sulfo, sulfonyl, formyl, amine, imine, nitrile, mercapto, nitro, halogen, C1-12 alkyl, C6-12 aryl, C1-12 alkoxy, hydroxyl, or combinations of these groups.

[0048] Preferred substituents are, for example, solubility-promoting groups, such as alkyl or alkoxy, electron-withdrawing groups, such as fluorine, nitro or nitrile, or bulky groups, such as, for example, t-butyl or optionally substituted aryl groups.

[0049] Preferred substituents are F, Cl, Br, I, -CN, -NO2 , -NCO, -NCS, -OCN, -SCN, -C(=O)N(RZ)2, -C(=O)Y1, -C(=O)RZ, -N(RZ)2, in which Rzis unsubstituted or substituted alkyl, preferably having up to 30 C atoms, and Y1denotes halogen, optionally substituted silyl or aryl having 6 to 40, preferably 6 to 20, C atoms, and straight-chain or branched alkyl, alkoxy, alkylcarbonyl, alkoxycarbonyl, alkylcarbonyloxy or alkoxycarbonyloxy having 1 to 25 C atoms, in which one or more H atoms may optionally be replaced by F or Cl.

[0050] More preferred substituents, for example, F, Cl, CN, NO2, CH3, C2H5, OCH3, OC2H5, COCH3, COC2H5, COOCH3, COOC2H5, CF3, OCF3, OCHF2, OC2F5, furthermore phenyl.

[0051] In an embodiment substituents are selected from F, Cl, CN, SCN, SF5 and straight-chain or branched, in each case optionally fluorinated, alkyl, alkoxy, alkylcarbonyl, alkoxycarbonyl, alkylcarbonyloxy and alkoxycarbonyloxy having 1 to 12 C atoms, preferably from F and straight-chain or branched, in each case optionally fluorinated, alkyl or alkoxy having 1 to 7 C atoms. In a preferred embodiment the organic compounds, in particular the organic molecules, as provided in the solution contain one, two, three, four or five rings, preferably two, three or four rings, which may be respectively and independently of one another be connected by single bonds or by bridging or linking groups such as -COO-, -CH=CH- or -C=C-, wherein additionally the compounds may optionally have end groups such as methyl, ethyl, propyl, butyl, pentyl, hexyl, heptyl, cyclopropyl, cyclobutyl, cyclopentyl, methoxy, ethoxy, propoxy, butoxy, pentoxy, -CH=CH-CH3, -CH2-CH=CH-CH3 or -CH2-CH2-CH=CH-CH3, and wherein the rings may preferably be selected from the following ring structures

[0052] In an embodiment the organic compounds are calamitic compounds, in particular rodshaped or rod-like organic molecules.

[0053] In a particular embodiment the organic compounds, in particular the organic molecules, as provided in the solution contain one or more groups selected from the following structures wherein

[0054] R denotes H, halogen, CN, or an alkyl or alkenyl radical having up to 15 C atoms which is unsubstituted, monosubstituted by CN or CF3 or at least monosubstituted by halogen, where, in addition, one or more CH2 groups in these radicals may be replaced

[0055] -O-, -S-, -C=C-, -CF2O-, -OCF2-, -OC-O- or -O-CO- in such a way that O atoms are not linked directly to one another,

[0056] R1to R4independently of one another denote H, methyl or ethyl, and wherein each aryl ring may optionally be mono- or polysubstituted, preferably be mono- or polysubstituted by L, wherein L denotes on each occurrence, identically or differently, OH, F, Cl, CN, SCN, SF5 or straight-chain or branched, in each case optionally fluorinated, alkyl, alkoxy, alkylcarbonyl, alkoxycarbonyl, alkylcarbonyloxy or alkoxycarbonyloxy having 1 to 12 C atoms.

[0057] In another embodiment the organic compounds are discotic compounds, in particular discshaped or disc-liked organic molecules.

[0058] In a preferred embodiment the organic compound comprises a mesogenic group, wherein the mesogenic group comprises one or more rings as described herein which may optionally be linked by bridge groups, and wherein the compound may further comprise one or more linking groups, one or more spacer groups and / or one or more terminal groups. In this respect, an overview of terms and definitions used in connection with mesogenic groups and compounds is given e.g. in C. Tschierske et al., Angewandte Chemie, 2004, 116, pp. 6340-6368. In a particular embodiment the organic compound is a mesogenic compound. In this respect, the molecular structure in particular exhibits anisotropy, and preferably the compound has a comparatively high degree of anisotropy and in particular a comparatively high nematic order parameter, for example 0.5 or more, preferably 0.6 or more, and in particular 0.7 or more.

[0059] In a preferred embodiment the organic compound comprises highly polarizable moieties, in particular including TT-conjugated structures and / or aromatic and heteroaromatic groups.

[0060] In a particularly preferred embodiment the organic compound is selected from pyrimidine-, tolane-, terphenyl- and naphthalene-based compounds. It is further preferred that the compounds comprise cyano, fluorine and / or oxygen-based groups. In certain embodiments it is preferred that the compounds do not contain chlorine atoms and do not contain -NCS groups.

[0061] In a preferred embodiment the organic compound is not a macromolecule or respectively is not a polymer. Additionally, the organic compound preferably is not an oligomer.

[0062] Preferably, the organic compound is not a polymerizable compound, in particular is not a polymerizable monomer and particularly is not a polymerizable mesogenic compound or respectively is not a reactive mesogen.

[0063] In an embodiment the molecular weight of the organic compound is below 750 g mol’1, preferably below 600 g mol’1, and in particular below 500 g mol’1.

[0064] In a preferred embodiment the organic compound is a mesogenic compound having a terminal -CN group.

[0065] In an embodiment the organic crystal, in particular the organic single crystal, may contain more than one organic compound. In this case it is preferred that the organic crystal comprises two or more organic compounds and in particular contains two organic compounds. According to this embodiment the crystalline material preferably is a cocrystal or respectively a single mixed crystal.

[0066] Particularly preferred organic compounds as used according to the present invention are shown in the Examples below. In an embodiment the organic crystal may be grown using a solution process. In this method the organic compound is provided in a solution.

[0067] The term solution generally refers to a homogeneous mixture consisting of a solute dissolved in a solvent, wherein the solvent-solute mixture consists of a single phase and the solute and the solvent are uniformly distributed at a molecular level and the solvent acts as the dissolving medium.

[0068] In this respect, the organic compound is the solute that in the method is grown into the organic crystal.

[0069] The solution, in particular the solution which the organic crystal is grown from, is thus formed by dissolving the organic compound, in particular the organic molecules, in a solvent. The solvent preferably is an organic solvent. Organic solvents generally are carbon-based substances, in particular compounds that have a carbon-based molecular structure, capable of dissolving one or more other substances.

[0070] The organic solvents are preferably selected to give sufficient solubility for the organic compound to thus be effective in dissolving the organic compound. Furthermore, the solvents preferably have a sufficiently high boiling point and an adequate volatility to facilitate the crystal growth at the desired temperatures, which are typically substantially below the boiling point of the solvent. In addition, it is preferred that the solvent is inert and has no chemical reaction with the dissolved organic compound.

[0071] In an embodiment the solvent to provide the solution is selected from the group of nonpolar solvents, polar aprotic solvents and polar protic solvents.

[0072] Nonpolar solvents generally have low dielectric constants and are not miscible with water. Nonpolar solvents include for example aliphatic solvents or nonpolar hydrocarbon solvents, in particular alkanes, nonpolar ether solvents, nonpolar chlorocarbon solvents such as tetrachloromethane, and aromatic solvents such as toluene. In an embodiment the use of toluene is preferred.

[0073] Polar protic solvents include alcohols such as methanol, ethanol, isopropanol and n-butanol. Polar aprotic solvents include ketones such as acetone and esters such as ethyl acetate. Further examples of polar aprotic solvents, including moderately polar solvents, are tetrahydrofuran, dichloromethane, acetonitrile, N,N-dimethylformamide and dimethyl sulfoxide. In an embodiment the use of acetone is preferred. In another embodiment the use of tetrahydrofuran is preferred.

[0074] The solvent used in the solution growth method may also be a solvent mixture, i.e. a mixture of compounds. For example, in an embodiment it is preferred to use as the solvent a mixture of acetone and ethanol.

[0075] The formation or growing of the crystal may occur via different mechanisms of crystallization or solidification.

[0076] The crystal may in particular be grown by spontaneous nucleation, by using a seed crystal or by reactive crystallization.

[0077] In principle the initial formation of the crystalline material from solution may occur by spontaneous nucleation, i.e. primary nucleation, or by secondary nucleation which may be induced by the presence of a heterogeneous source such as seeds or impurities.

[0078] For crystallization to occur spontaneously generally the crystalline state must be thermodynamically favourable. In this respect, the solvent as well as the temperature and the concentration of the organic compound in particular may influence the characteristics of the crystallization.

[0079] The solvent together with the temperature has an influence on the solubility of the organic compound, where generally a decrease of solubility is observed with decreasing temperature, i.e. upon cooling. Similarly, solvent evaporation has an influence on the concentration of the organic compound and thus solubility. For example, when a saturated solution is provided, cooling of the solution and / or evaporation, in particular slow evaporation, of the solvent may induce a supersaturated solution which may be followed by spontaneous nucleation and crystal growth.

[0080] In this respect, for a given organic compound and a given solvent, an equilibrium concentration and the temperature dependence of the solubility may be determined. Based thereon the saturation point or saturation temperature may be set as suitable, and a solution with the desired saturation temperature and in turn a saturated solution and in particular a supersaturated solvent as desired can be prepared.

[0081] Crystal growth using the solution growth method preferably occurs at room temperature or temperatures around room temperature, in particular at 20°C, and also at elevated temperatures, for example at temperatures in the range from 30°C to 65°C, preferably from 45°C to 60°C and in particular from 50°C to 55°C, however typically at temperatures significantly below the melting point of the organic compound. This method thus allows to obtain crystals of organic material by crystal growth from solution at low or relatively low temperatures.

[0082] In an embodiment the crystal growth is performed at ambient or atmospheric pressure.

[0083] It is preferred that the growth solution is stirred.

[0084] In some cases solutions may be cooled down or concentrated up to give conditions where the solution is less thermodynamically stable than the crystal but still no crystals might form for hours or even days. In such cases the use of a seed crystal may be favourable to promote growth and to avoid slow natural crystal growth. In this respect, a seed crystal is a small piece of single crystal or polycrystalline material from which a large crystal of typically the same material is grown. Seed crystals may also be referred to as growth crystals.

[0085] Therefore, in an embodiment where the crystal is grown by using a seed crystal a preformed nucleus or generally a growth crystal is added to the solution.

[0086] Crystal growth times may also be reduced or minimized by using reactive crystallization, where in one embodiment an additive, preferably water, more preferably a small amount of water, may be added to the solution, wherein the additive, in particular water, acts as a reagent which reduces the nucleation barrier of the organic compound, in particular the water-insoluble organic compound.

[0087] In embodiments where the solution comprises acetone as a solvent, it is preferred to add water as the additive. In other embodiments, e.g. in cases where the precursor solution contains toluene, it can be preferred to use acetone as the additive. In the solution growth method suitable and conventional equipment may be used such as suitable vessels or containers, preferably crystallizers which may include a growth tank and a lid, e.g. Pyrex crystallizers, as well as stirrers, heaters, coolers, water baths and thermocouples.

[0088] In a preferred embodiment of the solution growth method the organic compound is provided in the solution in an amount of 10% by weight or more, preferably 15% by weight or more, more preferably 20% by weight or more, even more preferably 30% by weight or more and in particular 40% by weight or more.

[0089] Providing the compound in a comparatively high concentration can favourably contribute to obtaining a fast and effective process.

[0090] The method can favourably give relatively high crystal growth rates. Preferably, the growing of the crystal, in particular a defect-free crystal, from the organic compound occurs at a rate of at least 1 mm / day, more preferably 3 mm / day and in particular 5 mm / day.

[0091] It is preferred that the organic compound and the solvent as provided in the method have high purity. Thus in an embodiment the organic compound and / or the solvent, preferably both, are purified before the provision of the solution, for example by using filtration, chromatography, distillation or recrystallization techniques.

[0092] Furthermore, it is preferred that the solution used for growing the crystal is subjected to a purification step, in particular a filtration step, preceding the crystallization.

[0093] In a particularly preferred embodiment the organic compound, the solvent, preferably the organic solvent, and / or the solution are purified before growing the crystal.

[0094] Surprisingly the method can give a crystal growth process to produce large organic crystals at comparatively high growth rates.

[0095] The grown crystal may be extracted or removed from the solution or respectively from the growth vessel or growth container with ease.

[0096] In an embodiment after removal from solution the crystals may be dried using a lint-free cloth. The crystals according to the invention may be stored under ambient conditions. In this respect, it is found that the crystals favourably and preferably are hydrophobic.

[0097] Preferably and favourably the crystalline materials are water-insoluble.

[0098] The organic crystals can give ease of processing. In an embodiment the provided crystal, in particular the single crystal, is therefore subjected to a step of cutting. In this step one or more crystals or one or more optical films may be produced which conform to a desired or predetermined shape and / or size. Methods for cutting can include, for example, mechanical cutting, slicing, dicing, or sawing, in particular sawing with a wire saw, e.g. a diamond wire saw. In this step e.g. one or more smaller slices or discs may be separated from the bulk crystal.

[0099] In an embodiment the crystal, in particular the single crystal and especially the optical film, has a first dimension and a second dimension of at least 10 mm. Preferably, a third dimension is at least 0.1 mm, more preferably at least 1 mm, and even more preferably at least 5 mm.

[0100] The crystal and / or the optical film obtained after the cutting preferably is a single crystal.

[0101] In embodiments or applications where smaller dimensions are desirable, the crystal and / or the optical film may have first and second dimensions in the range from for example 50 pm to 5 mm, preferably from 100 pm to 2 mm.

[0102] In embodiments or applications where especially thin layers are desirable, the crystal and / or the optical film may have a third dimension in the range from for example 10 nm to 100 pm, preferably from 50 nm to 50 pm.

[0103] In respect to further process or processing steps, in an embodiment the crystal or respectively the optical film can be subjected to a coating step.

[0104] Additionally or alternatively, the crystal or respectively the optical film can in a particular embodiment be subjected to an annealing step.

[0105] In the present invention favourable crystalline organic materials can be obtained, wherein the materials in particular are solids comprising crystallized organic molecules. In a preferred embodiment the crystallized organic molecules comprise one or more rings selected from aromatic, heteroaromatic, alicyclic and heterocyclic groups, preferably selected from aromatic and heteroaromatic groups.

[0106] In comparison to e.g. glass, calcite and other inorganic crystals, or ceramic materials, light-weight crystals can be obtained which in addition can be monocrystalline and exhibit high optical quality and which may be obtained in different crystal shapes using a wide range of materials, wherein the properties may be tailored to specific applications.

[0107] The crystalline materials, in particular the single crystals, can favourably have a density of less than 1.75 g cm-3, preferably less than 1.50 g cm-3, more preferably less than 1.25 g cm-3, and in particular less than 1.15 g cm-3, in particular determined at 22°C.

[0108] The crystalline materials, in particular the single crystals, can favourably be transparent, in particular transparent and colourless. In particular, it is preferred that light in the visible spectrum, which herein is taken to mean the wavelength range from 380 nm to 780 nm, is not absorbed or is only absorbed to a negligible extent by the crystalline materials. This means that the organic crystals preferably have low or even no discernible absorption in the visible spectrum of light.

[0109] In an embodiment the crystalline materials, in particular the single crystals, have an absorption edge of 2.85 eV or more, preferably 3.25 eV or more.

[0110] The term absorption edge refers to a discontinuity in the absorption spectrum of the crystalline substance, where in particular the wavelength or the energy of the absorbed photon corresponds to an electronic transition.

[0111] In an embodiment the term transparent solid herein refers to a solid which has a transmission of visible light, in particular at 589 nm, of at least 40 %, preferably of at least 50%, more preferably of at least 60%, preferably measured using a UV / Vis / NIR spectrometer to record transmission spectra.

[0112] In an embodiment the absorption coefficients of the crystals at 589 nm, preferably determined from the transmission measurements, are preferably below 10 cm-1, and in particular in the range from 1 cm-1to 5 cm-1or even lower. In an embodiment the internal transmittance of the material taking into account radiation loss by absorption is at least 80% over the visible spectrum of light, more preferably at least 90% and in particular at least 95%.

[0113] It is preferred that the crystalline materials are not soluble in water. It is particularly preferred that the crystalline materials are soluble in water in amounts of 0.1 g or less per 100 mL of water, preferably determined at 20°C.

[0114] Preferably, the melting point of the crystalline material, in particular the monocrystalline bulk solid, is at least 80°C, more preferably at least 100°C, even more preferably at least 120°C and in particular at least 140°C.

[0115] The melting point is the temperature at which the solid phase changes to the liquid phase. The melting point may for example be determined using differential scanning calorimetry or by optically observing the phase change.

[0116] In addition to being relatively light-weight and of high optical quality, the obtained crystalline solids, in particular the optical films, can favourably have a first dimension and a second dimension each of at least 10 mm, preferably at least 25 mm, more preferably at least 40 mm and in particular at least 50 mm.

[0117] It is preferred that in at least one plane the size of the crystal in two orthogonal directions has an extent of 15 mm or more, more preferably 30 mm or more, even more preferably 45 mm or more and in particular 60 mm or more.

[0118] In a preferred embodiment the crystalline material, in particular the monocrystalline solid and / or the optical film, additionally has a third dimension, which preferably is orthogonal to the first and second dimensions or respectively directions, with a length of at least 1 mm, preferably at least 5 mm, more preferably at least 10 mm, even more preferably at least 25 mm and in particular 50 mm.

[0119] The crystal and the film may also be cut into predefined sizes and shapes.

[0120] According to the invention the organic crystalline materials have a comparatively high refractive index. The crystalline material and in particular the monocrystalline solid has a refractive index of 1.8 or more, in particular determined at a wavelength of 589 nm, further particular at a temperature of 20°C.

[0121] In particular, it is preferred that the crystalline material and in particular the monocrystalline solid has a refractive index along at least one principal axis of at least 1.8 at 589 nm, more preferably at least 2.0 at 589 nm, even more preferably at least 2.2 at 589 nm, even more preferably at least 2.3 at 589 nm, even more preferably at least 2.4 at 589 nm, and in particular at least 2.5 at 589 nm.

[0122] This refractive index particularly refers to the maximum refractive index observed of the refractive indices along the spatial or optical axes of the material.

[0123] In principle, the refractive indices along three principal axes may be the same or different, wherein the relations of the refractive indices in particular may be ni=n2=n3, ni=n2 na, n n2=n3, ni=ns n2 and n n2 ns

[0124] In an embodiment the refractive index may vary along different spatial or optical axes of the material.

[0125] In a preferred embodiment the crystalline material thus exhibits an optical anisotropy or birefringence (An), preferably of 0.05 or more, more preferably of 0.10 or more, even more preferably of 0.25 or more, still more preferably of 0.40 or more, and in particular of 0.50 or more, in particular determined at 589 nm.

[0126] The optical properties, including the refractive index, may preferably be determined by a goniometer-spectrometer method using Snell's law, ellipsometric methods, UV / Vis / NIR measurements, polarimetry or other spectrophotometric methods. In a particular embodiment three axis ellipsometry data are recorded.

[0127] In a particularly preferred embodiment the crystalline material is biaxial, preferably orthorhombic, monoclinic or triclinic. In an embodiment the crystalline material is monoclinic or triclinic. In another embodiment the crystalline material is orthorhombic.

[0128] In another preferred embodiment the crystalline material is uniaxial, in particular trigonal, tetragonal or hexagonal. In an alternative embodiment the crystalline material is optically isotropic.

[0129] The provided crystalline material may be subjected to further process steps and in particular further processing steps.

[0130] For example, the material may be transformed or respectively integrated into optical films and optical elements or further integrated or included in optical and electro-optical components and optical and electro-optical devices.

[0131] The material may thus for example be used as or respectively in gratings, waveguides, lenses or multilayer films.

[0132] In this respect, the optical films can exhibit minimal surface roughness and favourable hardness.

[0133] In an embodiment the optical film has a surface roughness, in particular a surface roughness Ra, of 10 nm or less, preferably of 5 nm or less and in particular of 1 nm or less. The surface roughness may for example be determined using a profilometer or laser scanning.

[0134] In an embodiment the optical film has a hardness, in particular on the Mohs scale of hardness, of 3 or more. The hardness may also for example be determined using a hardness tester.

[0135] In the following drawings are described which further illustrate the present invention.

[0136] Brief description of the drawings

[0137] The drawings show:

[0138] Fig. 1 Image of an organic crystal obtained by solution growth,

[0139] Fig. 2 Singe crystal bulk X-ray diffraction data obtained from an organic crystal, and

[0140] Fig. 3 Plot of the absorption coefficient of an organic crystal in dependence of wavelength and energy. Fig. 1 shows an image of an organic crystal grown using solution growth, in particular using the organic compound described in Example 1 , which is placed on graph paper. The solid crystal is transparent and colourless and exhibits birefringent properties.

[0141] Fig. 2 shows X-ray diffraction data, in particular detector images, from a single crystal, in particular a crystal comprising the organic compound described in Example 2. The X-ray diffraction measurements are obtained from the bulk crystal measured in transmission under ambient conditions using a diffractometer with the theta angle of 0° (image on the left) and of 45° (image on the right). The diffraction patterns are evaluated and assigned to the (100) plane.

[0142] Fig. 3 shows the absorption coefficient of an organic crystal, in particular a crystal comprising the organic compound described in Example 1, in dependence of wavelength and respectively energy.

[0143] The following examples are merely illustrative of the present invention and they should not be considered as limiting the scope of the invention in any way. The examples and modifications or other equivalents thereof will become apparent to those skilled in the art in the light of the present disclosure.

[0144] Example 1

[0145] 35% by weight of the compound of formula is dissolved in toluene at 50°C. A planar and slightly isometric single crystal in the form of a dodecahedron is grown in 5 days, wherein the crystal is obtained by initially adding a small seed crystal to the solution. The maximum extension of the crystal surface is 27 mm in length and 21 mm in width and the height of the crystal is 3 mm. The obtained crystal is a transparent and colourless solid having a density, determined at 22°C, of 1.32 g cm-3and an absorption edge of 3.62 eV. The solid has a melting point of 94°C.

[0146] The crystal has a refractive index of 1.99 at a wavelength of 589 nm, determined from reflection measurements under ambient conditions with a UV / Vis / NIR spectrometer and using the Fresnel equation.

[0147] 34% by weight of the compound of formula is dissolved at 50°C in acetone in an Erlenmeyer flask and subsequently cooled to room temperature. A planar single crystal in the form of a distorted prism is grown in 2 days, wherein the growth of the crystal is obtained from spontaneous nucleation. The maximum extension of the crystal surface is 45 mm in length and 28 mm in width and the height of the crystal is 300 pm.

[0148] The obtained crystal is a transparent and colourless solid having a density, determined at 22°C, of 1.39 g cm-3and an absorption edge of 3.55 eV. The solid has a melting point of 88°C.

[0149] X-ray diffraction measurements are performed in transmission on the bulk crystal. The space group of the crystal is determined to be P2i / c.

[0150] 20% by weight of the compound of formula is dissolved in tetrahydrofuran and a single crystal is grown, wherein the crystal is obtained by initially adding a small seed crystal to the solution. An isometric and slightly planar single crystal in the form of a dodecahedron is obtained.

[0151] The obtained crystal is a transparent and colourless solid having a density, determined at 22°C, of 1.34 g cm-3. The solid has a melting point of 127°C. The space group of the crystal is determined to be P21 / n. The crystal has a maximum refractive index of 2.19 at a wavelength of 589 nm. A second refractive index of 1.55 and a third refractive index of 1.59 are determined.

[0152] Example 4

[0153] 20% by weight of the compound of formula is dissolved in toluene and a single crystal is grown, wherein the crystal is obtained by initially adding a small seed crystal to the solution. An isometric single crystal in the form of a truncated dodecahedron is obtained.

[0154] The obtained crystal is a transparent and colourless solid having a density, determined at 22°C, of 1.24 g cm-3. The solid has a melting point of 94°C. The space group of the crystal is determined to be P-1. The crystal has a maximum refractive index of 2.50 at a wavelength of 589 nm. A second refractive index of 1.52 and a third refractive index of 1.67 are determined.

[0155] Example 5

[0156] 10% by weight of the compound of formula is dissolved in toluene and a single crystal is grown, wherein the crystal is obtained by initially adding a small seed crystal to the solution. An isometric single crystal in the form of a hexagonal prism is obtained.

[0157] The obtained crystal is a transparent and colourless solid having a density, determined at 22°C, of 1.32 g cm-3. The solid has a melting point of 160°C. The space group of the crystal is determined to be P-1. The crystal has a maximum refractive index of 2.15 at a wavelength of 589 nm. A second refractive index of 1.47 and a third refractive index of 1.80 are determined.

[0158] Example 6

[0159] 30% by weight of the compound of formula is dissolved in acetone at 50°C. A planar and slightly isometric single crystal in the form of a polyhedron is grown in 3 days, wherein the growth of the crystal is obtained from reactive crystallization by adding a drop of deionized water to the solution. The maximum extension of the crystal surface is 15.5 mm in length and 15 mm in width and the height of the crystal is 3 mm.

[0160] The obtained crystal is a transparent and colourless solid having an absorption edge of 3.33 eV. The solid has a melting point of 86°C. The crystal has a maximum refractive index of 2.10 at a wavelength of 589 nm.

[0161] The grown crystal is cut into four slices using a diamond-coated cutting thread.

[0162] Examples 7 to 190

[0163] The following single crystals of the organic compounds having the formulae as shown in the following table are obtained, in particular by solution growth, having the properties as indicated in the table. In the table Ex. denotes Example, THF denotes tetrahydrofuran, DCM denotes dichloromethane and EtOH denotes ethanol.

[0164]

[0165]

[0166]

[0167] The compounds having the formulae as shown in Examples 1 to 190, in a particular embodiment in particular the compounds having the formulae as shown in Examples 145 to 150, are particularly useful to prepare crystalline materials and optical films, in particular the crystalline materials and optical films according to the invention.

Claims

Claims1. A crystalline material, wherein the material is a solid comprising an organic compound and having a density of less than 1.75 g cm-3, and wherein the material has a refractive index of 1.8 or more.

2. The crystalline material according to claim 1 , wherein the crystalline material is a monocrystalline solid.

3. The crystalline material according to claim 1 or 2, wherein the material is a waterinsoluble transparent material having an absorption edge of 2.85 eV or more, and which in particular is colourless.

4. The crystalline material according to one or more of claims 1 to 3, wherein the material has a melting of point of 80°C or more, preferably 100°C or more, more preferably 120°C or more, and in particular 140°C or more.

5. The crystalline material according to one or more of claims 1 to 4, wherein the refractive index is 2.0 or more, preferably 2.2 or more, more preferably 2.3 or more, even more preferably 2.4 or more, and in particular 2.5 or more, in particular determined at 589 nm.

6. The crystalline material according to one or more of claims 1 to 5, wherein the organic compound is a crystallizable molecular compound comprising one or more rings selected from aromatic, heteroaromatic, alicyclic and heterocyclic groups, preferably one or more rings selected from aromatic and heteroaromatic groups.

7. The crystalline material according to one or more of claims 1 to 6, wherein the material exhibits an optical anisotropy, preferably an optical anisotropy of 0.05 or more, more preferably of 0.10 or more, even more preferably of 0.25 or more, and in particular of 0.50 or more, in particular determined at 589 nm.

8. The crystalline material according to one or more of claims 1 to 7, wherein the crystalline material is biaxial, preferably orthorhombic, monoclinic or triclinic, in particular monoclinic or triclinic.

9. The crystalline material according to one or more of claims 1 to 8, wherein the organic compound is a mesogenic compound.

10. An optical film, comprising the material according to one or more of claims 1 to 9, wherein the film has a first dimension and a second dimension of at least 10 mm.

11. The optical film according to claim 10, wherein the film has a surface roughness of 10 nm or less.

12. The optical film according to claim 10 or 11 , wherein the film has a hardness of 3 or more on the Mohs hardness scale.

13. An optical element or an electro-optical device comprising the crystalline material according to one or more of claims 1 to 9 or the optical film according to one or more of claims 10 to 12.

14. Use of a compound selected from the group of compounds of the formulae as shown in Examples 1 to 190 for the preparation of a crystalline material, in particular the crystalline material according to one or more of claims 1 to 9, or for the preparation of an optical film, in particular the optical film according to one or more of claims 10 to 12.

15. Use of a mesogenic compound having a terminal -CN group for the preparation of a crystalline material, in particular the crystalline material according to one or more of claims 1 to 9, or for the preparation of an optical film, in particular the optical film according to one or more of claims 10 to 12, wherein preferably the compound is selected from the group of compounds of the following formulae

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