Method for preparing organic crystals

Abstract

The present invention relates to methods for preparing organic crystals based on precursor solutions containing organic molecules, to crystalline solids obtained from the methods and to the use of the crystalline materials in optical films and electro-optical devices.

Description

[0001] Method for Preparing Organic Crystals

[0002] The present invention relates to methods for preparing organic crystals based on precursor solutions containing organic molecules, to crystalline solids obtained from the methods 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 and US 2023 / 0193505 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 neareye displays for augmented reality and virtual reality.

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

[0006] An object of the present invention is therefore to provide a method for preparing solid crystals from organic molecules, in particular larger contiguous organic solid crystals, in a fast and efficient manner which is amenable to using various precursors. It is a further object to provide organic molecular crystals obtained from the method, 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.

[0010] A first aspect of the present invention provides a method for the preparation of a crystalline material, wherein in a first step at least one organic compound is provided in a solution and in a subsequent second step a crystal is grown from the at least one organic compound provided in the solution.

[0011] The method thus provides a solid crystalline material containing the crystallized organic compound.

[0012] It has surprisingly been found that a facile and fast process for producing 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] In another aspect crystalline materials are provided which are obtainable from or respectively obtained by carrying out the method according to the invention.

[0014] It has surprisingly been found that by using the method described herein solid crystals and in particular larger contiguous organic solid crystals can be grown in a fast and efficient manner using organic molecules as precursors. In particular, the method can be especially useful 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 large and light-weight crystals and in particular optically transparent crystals with favourable optical quality and performance, for example materials with a high refractive index and / or high optical anisotropy, can be obtained.

[0015] Therefore in another aspect crystalline materials are provided, wherein the materials are transparent solids comprising crystallized organic molecules and 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, and an absorption edge of 2.85 eV or more, preferably 3.25 eV or more.

[0016] 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.

[0017] Another aspect provides optical elements and respectively electro-optical devices containing the crystalline materials according to the invention.

[0018] 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.

[0019] 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.

[0020] 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.

[0021] Preferably, the crystalline material is a monocrystalline solid or a polycrystalline solid, more preferably a monocrystalline solid.

[0022] The crystalline material as prepared according to the invention 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.

[0023] 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 or additives.

[0024] It is however preferred that the crystals, in particular the single crystals, as obtained according to the invention are free of defects. The presence or absence of defects may be determined by visual inspection or more preferably by X-ray diffraction, in particular X-ray topography, or (micro) X-ray fluorescence.

[0025] 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.

[0026] 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.

[0027] 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.

[0028] 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.

[0029] Preferably the organic compound is a crystallizable molecular compound comprising one or more rings selected from aromatic, heteroaromatic, alicyclic and heterocyclic groups.

[0030] 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.

[0031] 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.

[0032] 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.

[0033] 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.

[0034] 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.

[0035] 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.

[0036] 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,

[0037] 1.2.4-oxadiazole, 1,2,5-oxadiazole, 1 ,3,4-oxadiazole, 1,2,3-thiadiazole, 1,2,4-thiadiazole,

[0038] 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.

[0039] 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.

[0040] 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-.

[0041] 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,

[0042] 1.3-dithiane, piperidine, 7-membered groups, such as cycloheptane, and fused groups, such as tetrahydronaphthalene, decahydronaphthalene, indane, bicyclo[1.1.1]pentane-

[0043] 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.

[0044] 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.

[0045] 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. 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.

[0046] More preferred substituents, for example, F, Cl, CN, NO2, CH3, C2H5, OCH3, OC2H5, COCH3, COC2H5, COOCH3, COOC2H5, CF3, OCF3, OCHF2, OC2F5, furthermore phenyl.

[0047] 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.

[0048] 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

[0049] In an embodiment the organic compounds are calamitic compounds, in particular rodshaped or rod-like organic molecules.

[0050] In another embodiment the organic compounds are discotic compounds, in particular discshaped or disc-liked organic molecules.

[0051] 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.

[0052] Particularly preferred organic compounds as used according to the present invention are selected from the group of compounds having the following formulae

[0053] In the method according to the invention the organic compound is provided in a solution.

[0054] 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.

[0055] In this respect, the organic compound is the solute that in the method is grown into the organic crystal.

[0056] 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.

[0057] 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.

[0058] In an embodiment the solvent to provide the solution is selected from the group of nonpolar solvents, polar aprotic solvents and polar protic solvents.

[0059] 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.

[0060] Polar protic solvents include alcohols such as methanol, ethanol, isopropanol and n-butanol.

[0061] 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, benzonitrile, N,N-dimethylformamide and dimethyl sulfoxide. In an embodiment the use of acetone is preferred. In another embodiment the use of tetrahydrofuran is preferred.

[0062] The solvent used in the 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.

[0063] The formation or growing of the crystal may occur via different mechanisms of crystallization or solidification.

[0064] The crystal may in particular be grown by spontaneous nucleation, by using a seed crystal or by reactive crystallization.

[0065] 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. 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.

[0066] 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.

[0067] 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.

[0068] Crystal growth 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 and the boiling point of the used solvent.

[0069] The method thus allows to obtain crystals of organic material by crystal growth from solution at low or relatively low temperatures.

[0070] In an embodiment the crystal growth is performed at ambient or atmospheric pressure.

[0071] It is preferred that the growth solution is stirred.

[0072] 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.

[0073] 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.

[0074] Seed crystals may for example be obtained from larger crystals by slicing and polishing, or from growing small crystals in suitable solvents, preferably using several small flasks placed in a programmable thermal chamber.

[0075] 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.

[0076] 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.

[0077] In the 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.

[0078] In a particular embodiment two or more crystals are grown in a single container at the same time. For example, two, three, four or more crystals, in particular bulk crystals, may be grown in the same crystallizer in parallel. The size and the configuration of the growth vessel may be suitably set depending on the number of crystals to be grown at the same time. Preferably, in this embodiment seed crystals are used, where the number of provided seed crystals corresponds to the number of bulk crystals to be grown.

[0079] In a preferred embodiment of the 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. In a particular embodiment the organic compound is provided in the solution in an amount ranging from 10% by weight to 60% by weight. Providing the compound in a comparatively high concentration can favourably contribute to obtaining a fast and effective process.

[0080] The method can favourably give relatively high crystal growth rates. Preferably, the growing of the crystal from the organic compound occurs at a rate of at least 1 mm / day, more preferably at least 2 mm / day, even more preferably at least 5 mm / day, and in particular at least 6 mm / day.

[0081] 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.

[0082] 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.

[0083] In a particularly preferred embodiment the organic compound, the solvent, preferably the organic solvent, and / or the solution are purified before growing the crystal.

[0084] Surprisingly the method according to the invention can give a crystal growth process to produce large organic crystals at comparatively high growth rates.

[0085] The grown crystal may be extracted or removed from the solution or respectively from the growth vessel or growth container with ease.

[0086] In an embodiment after removal from solution the crystals may be dried using a lint-free cloth. The crystals may be stored under ambient conditions. In this respect, it is found that the crystals favourably and preferably are hydrophobic.

[0087] In an embodiment the growing of the crystal from the organic compound is performed using a restriction, in particular using a restriction which modifies the crystal morphology.

[0088] In a particular embodiment in the method restricted growth of the crystal is carried out, wherein preferably the crystal structure and the crystal morphology are influenced or modified. The restriction or limitation in the growth may be based on a provided mechanical or physical limitation, where in particular the crystal grows into the mechanical restriction. Alternatively or in addition, the crystal may be grown in a restricted way by using additives such as a surfactant. The restricted growth may influence and alter crystallization and crystal morphology, in particular by restricting or suppressing the occurrence of natural crystal facets. For example, by providing mechanical boundaries or restrictions made from e.g. metals or other solvent-resistant materials the natural morphology of the grown bulk crystal can be suppressed and instead a different crystal morphology may be obtained.

[0089] In a preferred embodiment two or more crystals are grown at the same time using restricted growth in a single crystallizer.

[0090] Subsequent to obtaining the grown crystal, in particular the single crystal, a further step of cutting the crystal may be carried out. In this step one or more crystals 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.

[0091] In an embodiment the grown crystal, in particular the grown single crystal, before the cutting has a first dimension and a second dimension of at least 10 mm, wherein preferably a third dimension is at least 1 mm, more preferably at least 5 mm.

[0092] The crystal obtained after the cutting preferably is a single crystal.

[0093] By carrying out the method according to the present invention, in particular by subjecting the organic compound in solution to a crystallization process, favourable crystalline organic materials can be obtained, wherein the materials in particular are solids comprising crystallized organic molecules.

[0094] In a preferred embodiment the crystallized organic molecules comprise one or more rings selected from aromatic, heteroaromatic, alicyclic and heterocyclic groups.

[0095] 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. The obtained 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.

[0096] The obtained 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.

[0097] 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.

[0098] 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.

[0099] 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 measured using a UV / Vis / NIR spectrometer to record transmission spectra.

[0100] 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.

[0101] It is preferred that the crystalline materials are not soluble in water.

[0102] Preferably, the melting point of the crystalline material, in particular the monocrystalline bulk solid, is at least 70°C, more preferably at least 85°C, even more preferably at least 100°C, still more preferably at least 115°C, and in particular at least 120°C.

[0103] In addition to being relatively light-weight and of high optical quality, the obtained crystalline solids 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. 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, still more preferably 55 mm or more, and in particular 60 mm or more.

[0104] In a preferred embodiment the crystalline material, in particular the monocrystalline solid, 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 at least 50 mm.

[0105] The obtained crystal may also be cut into predefined sizes and shapes.

[0106] By carrying out the method organic materials with a high refractive index and / or high optical anisotropy can be provided.

[0107] Preferably, the crystalline material and in particular the monocrystalline solid has a refractive index of 1.5 or more, in particular determined at a wavelength of 470 nm.

[0108] 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 1.5 at 470 nm, more preferably at least 1.8 at 470 nm, even more preferably at least 2.0 at 470 nm, and in particular at least 2.1 at 470 nm.

[0109] 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 n3.

[0110] In a preferred embodiment the crystalline material 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, and in particular of 0.50 or more, in particular determined at 470 nm.

[0111] The optical properties 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 particularly preferred embodiment the crystalline material is biaxial, preferably monoclinic or triclinic.

[0112] In another preferred embodiment the crystalline material is orthorhombic.

[0113] In another preferred embodiment the crystalline material is uniaxial.

[0114] In an alternative embodiment the crystalline material is optically isotropic.

[0115] The obtained crystalline material may be subjected to further process steps and in particular further processing steps.

[0116] 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.

[0117] The material may thus for example be used as or respectively in gratings, waveguides, lenses or multilayer films.

[0118] In the following drawings are described which further illustrate the present invention.

[0119] Brief description of the drawings

[0120] The drawings show:

[0121] Fig. 1 Image of an organic crystal obtained by the method according to the invention,

[0122] Fig. 2 Singe crystal bulk X-ray diffraction data obtained from an organic crystal prepared by the method according to the invention,

[0123] Fig. 3 Plot of the absorption coefficient of an organic crystal in dependence of wavelength and energy,

[0124] Fig. 4 Image of an organic crystal with hexagonal shape obtained by restricted growth, and

[0125] Fig. 5 Image of an organic crystal with cuboid shape obtained by restricted growth. Fig. 1 shows an image of an organic crystal grown using the method as described herein, in particular using the organic compound described in Example 8, which is placed on graph paper. The solid crystal is transparent and colourless and exhibits birefringent properties.

[0126] Fig. 2 shows X-ray diffraction data, in particular detector images, from a single crystal which is grown using the method according to the invention, in particular using the organic compound described in Example 1. 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.

[0127] Fig. 3 shows the absorption coefficient of an organic crystal grown using the method as described herein, in particular using the organic compound described in Example 8, in dependence of wavelength and respectively energy.

[0128] Fig. 4 shows an image of an organic crystal obtained by restricted growth, in particular using the organic compound described in Example 11, which is placed on graph paper and shown next to a tape measure. The solid crystal is transparent and colourless.

[0129] Fig. 5 shows an image of an organic crystal obtained by restricted growth, in particular using the organic compound described in Example 12, which is placed on graph paper. In this restricted growth, the crystal is grown into sidebars and an upper platform of the growth vessel, where by means of the restriction a cuboid crystal instead of a crystal with a hexagonal prism morphology is obtained. The solid crystal is transparent, colourless and exhibits birefringent properties.

[0130] 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.

[0131] 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.

[0132] 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 of3.55eV. The solid has a melting point of 88°C.

[0133] X-ray diffraction measurements are performed in transmission on the bulk crystal. The space group of the crystal is determined to be P2i / c.

[0134] 31% by weight of the compound of formula is dissolved in acetone at 50°C and slowly cooled down to room temperature. A planar single crystal in the form of a 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 60 mm in length and 15 mm in width and the height of the crystal is 1 mm.

[0135] The obtained crystal is a transparent and colourless solid having an absorption edge of 3.89 eV.

[0136] Example 3

[0137] 50% by weight of the compound of formula is dissolved in acetone at 50°C. An isometric single crystal in the form of a truncated octahedron is grown in 1 day, wherein the growth of the crystal is obtained from spontaneous nucleation. The maximum extension of the crystal surface is 26 mm in length and 23 mm in width and the height of the crystal is 10 mm.

[0138] The obtained crystal has a density, determined at 22°C, of 1.29 g cm-3.

[0139] X-ray diffraction measurements are performed in transmission on the bulk crystal. The space group of the crystal is determined to be P2i / c.

[0140] Example 4

[0141] 43% by weight of the compound of formula is dissolved in toluene at 50°C. An acicular and slightly isometric single crystal in the form of a hexagonal prism is grown in 4 days, wherein the growth of the crystal is obtained from spontaneous nucleation. The maximum extension of the crystal surface is 31 mm in length and 7 mm in width and the height of the crystal is 3 mm.

[0142] Example 5

[0143] 55% by weight of the compound of formula is dissolved in acetone at 50°C. An isometric single crystal in the form of a truncated octahedron is grown in 4 days, wherein the growth of the crystal is obtained from spontaneous nucleation. The maximum extension of the crystal surface is 35 mm in length and 35 mm in width and the height of the crystal is 15 mm. The obtained crystal has a density, determined at 22°C, of 1.31 g cm-3.

[0144] X-ray diffraction measurements are performed in transmission on the bulk crystal. The space group of the crystal is determined to be P1.

[0145] Example 6

[0146] 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.

[0147] 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.

[0148] The grown crystal is cut into four slices using a diamond-coated cutting thread.

[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] Example 7

[0151] 35% by weight of the compound of formula is dissolved in a mixture of acetone and ethanol (50:50) and 200 pL of desalinated water at 50°C. A planar single crystal in the form of a cuboid is grown in 4 days, wherein the growth of the crystal is obtained from reactive crystallization. The maximum extension of the crystal surface is 22 mm in length and 17 mm in width and the height of the crystal is 1 mm.

[0152] The obtained crystal is a transparent and colourless solid having a density, determined at 22°C, of 1.40 g cm-3and an absorption edge of 3.4 eV.

[0153] X-ray diffraction measurements are performed in transmission on the bulk crystal. The space group of the crystal is determined to be 12 / a.

[0154] Example 8

[0155] 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.

[0156] 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.

[0157] X-ray diffraction measurements are performed in transmission on the bulk crystal. The space group of the crystal is determined to be P2i / c.

[0158] 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.

[0159] 50% 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 truncated octahedron is grown in 4 days, wherein the crystal is obtained by initially adding a small seed crystal to the solution. The maximum extension of the crystal surface is 42 mm in length and 29 mm in width and the height of the crystal is 7 mm.

[0160] The obtained crystal is a transparent and colourless solid having a density, determined at 22°C, of 1.29 g cm-3and an absorption edge of 3.69 eV.

[0161] X-ray diffraction measurements are performed in transmission on the bulk crystal. The space group of the crystal is determined to be P2i / c.

[0162] Example 10

[0163] 40% by weight of the compound of formula is dissolved in toluene at 50°C. An isometric and slightly planar single crystal in the form of a dodecahedron is grown in 2.7 days, wherein the crystal is obtained by initially adding a small seed crystal to the solution and cooling the solution from a starting temperature of 33°C over the time to a final temperature of 28.5°C. The maximum extension of the crystal surface is 15 mm in length and 12 mm in width and the height of the crystal is 5 mm.

[0164] 46% by weight of the compound of formula is dissolved in toluene at 50°C. An isometric single crystal in the form of a hexagon is grown in 21 days, wherein the crystal is obtained by initially adding a small naturally grown seed crystal to the solution and cooling the solution from a starting temperature of 40.1 °C over the time to a final temperature of 29.5°C and by keeping a stirring rotation speed of 3007s. A hexagonal crystal shape is obtained by restricted growth. The maximum extension of the crystal surface is 65 mm in length and 59 mm in width and the height of the crystal is 61 mm. The weight of the crystal is 183 g.

[0165] The obtained crystal is a transparent and colourless solid having a density, determined at 22°C, of 1.28g cm-3and an absorption edge of 3.75 eV. The solid has a melting point of 85°C.

[0166] X-ray diffraction measurements are performed in transmission on the bulk crystal. The space group of the crystal is determined to be P2i / n.

[0167] Example 12

[0168] 40% by weight of the compound of formula is dissolved in toluene at 50°C. A planar single crystal in the form of a cuboid is grown in 29 days, wherein the crystal is obtained by initially adding a small naturally grown seed crystal to the solution and cooling the solution from a starting temperature of 36.8°C over the time to a final temperature of 28.7°C and by keeping a stirring rotation speed of 3007s. The rectangular shape of the crystal is obtained by restricted growth. The maximum extension of the crystal surface is 64 mm in length and 16 mm in width and the height of the crystal is 63 mm. The weight of the crystal is 74 g.

[0169] Example 13

[0170] 60% by weight of the compound of formula is dissolved in toluene at 40°C. An isometric single crystal in the form of a cuboid is grown in 11 days, wherein the crystal is obtained by initially adding a small seed crystal to the solution and cooling the solution from a starting temperature of 31.2°C over the time to a final temperature of 27.8°C and by keeping a stirring rotation speed of 3007s. The rectangular shape of the crystal is obtained by restricted growth. The maximum extension of the crystal surface is 64 mm in length and 30 mm in width and the height of the crystal is 50 mm. The weight of the crystal is 103 g.

[0171] Example 14

[0172] 15% by weight of the compound of formula is dissolved in benzonitrile at 50°C. An isometric and slightly planar single crystal in the form of a polyhedron is grown in 25 days, wherein the crystal is obtained by initially adding a small naturally grown seed crystal to the solution and cooling the solution from a starting temperature of 46.1 °C over the time to a final temperature of 31.9°C and by keeping a stirring rotation speed of 3007s. The maximum extension of the crystal surface is 65 mm in length and 34 mm in width and the height of the crystal is 23 mm. The weight of the crystal is 37 g.

[0173] The obtained crystal is a transparent and colourless solid having a density, determined at 22°C, of 1.34g cm-3and an absorption edge of 3.38 eV. The solid has a melting point of 127°C.

[0174] X-ray diffraction measurements are performed in transmission on the bulk crystal. The space group of the crystal is determined to be P2i / n.

[0175] The crystal exhibits one high refractive index of 2.2 and two lower refractive indices of 1.6 and 1.5 determined at a wavelength of 470 nm by ellipsometric measurements.

[0176] Example 15

[0177] The crystal growth according to Example 12 is carried out, wherein however two growth platforms are stacked in the growth vessel with a seed crystal mounted on each platform. Two bulk crystals are obtained by the simultaneous growth in the one crystallizer.

Claims

Claims1. A method for the preparation of a crystalline material, comprising(i) providing an organic compound in a solution, and(ii) growing a crystal from the organic compound.

2. The method according to claim 1, wherein the solvent used to provide the solution is an organic solvent, preferably an organic solvent selected from the group of nonpolar solvents, polar aprotic solvents and polar protic solvents, wherein preferably the organic compound, the solvent and / or the solution are purified before growing the crystal.

3. The method according to claim 1 or 2, wherein the crystal is grown by spontaneous nucleation, by using a seed crystal or by reactive crystallization.

4. The method according to one or more of claims 1 to 3, wherein the organic compound is a crystallizable molecular compound comprising one or more rings selected from aromatic, heteroaromatic, alicyclic and heterocyclic groups.

5. The method according to one or more of claims 1 to 4, wherein the organic compound is provided in the solution in an amount of 10% by weight or more.

6. The method according to one or more of claims 1 to 5, wherein the method comprises a further step of cutting the grown crystal to obtain at least one crystal of predetermined shape and size, wherein preferably the at least one crystal is a single crystal, and wherein preferably the grown crystal before the cutting has a first dimension and a second dimension of at least 10 mm.

7. A crystalline material obtainable from or respectively obtained by carrying out the method according to one or more of claims 1 to 6.

8. A crystalline material, wherein the material is a water-insoluble transparent solid comprising crystallized organic molecules and having a density of less than 1.75 g cm-3and an absorption edge of 2.85 eV or more.

9. The crystalline material according to claim 7 or 8, wherein the crystalline material is a monocrystalline solid.

10. The crystalline material according to one or more of claims 7 to 9, wherein the material is a solid having a first dimension and a second dimension of at least 10 mm.

11. The crystalline material according to claim 10, wherein the solid has a third dimension of at least 1 mm.

12. The crystalline material according to one or more of claims 7 to 11, wherein a refractive index is 1.5 or more, wherein preferably the material exhibits an optical anisotropy.

13. The crystalline material according to one or more of claims 7 to 12, wherein the crystalline material is biaxial, preferably monoclinic or triclinic.

14. The crystalline material according to one or more of claims 8 to 13, wherein the organic molecules comprise one or more rings selected from aromatic, heteroaromatic, alicyclic and heterocyclic groups.

15. An optical element or an electro-optical device comprising the crystalline material according to one or more of claims 7 to 14.

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