Needle-shaped single crystal, molded body, resin composite, and nonlinear optical element, and method for producing needle-shaped single crystal

Abstract

Provide is an organic nonlinear optical material having large nonlinearity even at a crystal level, and enabling second harmonic generation in an ultraviolet light region. This needle-shaped single crystal has a structural unit represented by formula (1) and is used as an organic nonlinear optical material. In formula (1), n represents an integer of 3 or more, Ar represents any of a 1,4-phenylene group, a 2,6-naphthylene group, and a 4,4'-biphenylene group, at least some of the hydrogen atoms in the Ar structure may be substituted by halogen atoms, and X represents an oxygen atom or a sulfur atom.

Description

Acicular single crystal, molded article, resin composite, nonlinear optical element, and method for producing acicular single crystal

[0001] The present disclosure relates to an acicular single crystal, a molded article, a resin composite, a nonlinear optical element, and a method for producing an acicular single crystal.

[0002] With the advancement of the advanced information society, many attempts have been made to apply optical technology to the transmission, processing and recording of information. Under such circumstances, materials exhibiting nonlinear optical effects (nonlinear optical materials) have attracted attention in the fields of optoelectronics and photonics. The nonlinear optical effect is a phenomenon that exhibits a nonlinear relationship between the generated electric polarization and the applied electric field when a strong electric field (optical electric field) is applied to a substance. A nonlinear optical material refers to a material that remarkably exhibits such nonlinearity. As nonlinear optical materials utilizing secondary nonlinear response, materials that generate second harmonics and materials that exhibit the Pockels effect (primary electro-optic effect) which causes refractive index change proportional to the primary order of electric field are known; in particular, application of the latter to electro-optic (EO) optical modulation elements and photorefractive elements has been studied. Furthermore, they are expected to exhibit piezoelectricity and pyroelectricity, and are expected to be applied in various fields.

[0003] Conventionally, inorganic materials and semiconductor materials such as potassium dihydrogen phosphate (KDP), lithium niobate (LiNbO ３ 3), gallium arsenide (GaAs) have been mainly developed. Compared with these materials, organic materials excellent in nonlinear optical performance and having extremely fast optical response speed were discovered, and since then, development of organic nonlinear optical elements using organic materials has been actively carried out. Examples of such organic nonlinear optical materials include p-nitroaniline (pNA), 2-methyl-4-nitroaniline (MNA) (Non-Patent Document 1), 4-(N,N-dimethylamino)-3-acetamidonitrobenzene (DAN) (Non-Patent Document 2), and the like.

[0004] ACS Symposium Series, 233, 1 (1983); Appl. Phys. Lett. 51 (19), 1484 (1987); "New Organic Nonlinear Optical Materials I" published by CMC Co., Ltd., pp. 76-81 (1991)

[0005] However, compounds like pNA, while exhibiting significant nonlinearity at the molecular level, exhibit centrosymmetricity in their crystal structure due to strong intermolecular dipole-dipole interactions, thus preventing the emergence of second harmonics (SHG). Furthermore, while MNA and DAN form non-centrosymmetric crystals and possess significant nonlinearity, their ultraviolet-visible absorption edges are located at 530 nm for MNA and 480 nm for DAN. When using them for wavelength conversion of semiconductor laser light (λ = 700 nm to 1000 nm) via transmission, problems arise such as reduced conversion efficiency and compound degradation due to reabsorption of the second harmonic by the compound itself.

[0006] Compounds intended for generating second harmonics at shorter wavelengths include 3,5-dimethyl-1-(4-nitrophenyl)pyrazole (DMNP) (Non-Patent Literature 3). However, DMNP has an absorption edge of 402 nm, making it suitable for use as a material for blue light conversion, but not for ultraviolet light conversion.

[0007] This invention has been made in view of the above problems, and aims to provide an organic nonlinear optical material that has high nonlinearity even at the crystal level and enables the generation of second harmonics in the ultraviolet light region.

[0008] The present inventors have found that needle-shaped single crystals (whiskers) of specific structures, such as polyoxybenzoate, are effective as organic nonlinear optical materials to address the above-mentioned problems, and have completed this disclosure. Accordingly, the aspects of this disclosure are as follows.

[0009] [1] A needle-shaped single crystal having a structural unit represented by the following equation (1), which is used as an organic nonlinear optical material.

[0010]

[0011] In formula (1) above, n represents an integer of 3 or more, Ar represents one of a 1,4-phenylene group, a 2,6-naphthylene group, and a 4,4'-biphenylene group, and at least some of the hydrogen atoms in the structure of Ar may be substituted with halogen atoms, and X represents an oxygen atom or a sulfur atom.

[0012] [2] The needle-shaped single crystal described in [1], wherein X in formula (1) above is a sulfur atom.

[0013] [3] The needle-shaped single crystal according to [1] or [2], wherein at least a portion of the hydrogen atoms in the Ar structure in formula (1) are substituted with halogen atoms, and the halogen atoms are chlorine atoms or fluorine atoms.

[0014] [4] An acicular single crystal according to any one of [1] to [3], wherein the length in the long side direction is 3.0 μm or more and 50 μm or less, the length in the short side direction is 0.1 μm or more and 5.0 μm or less, and the aspect ratio is 5 or more.

[0015] [5] A molded body made of a needle-shaped single crystal as described in any of [1] to [4].

[0016] [6] A resin composite comprising a needle-shaped single crystal as described in any of [1] to [4] and a resin.

[0017] [7] A nonlinear optical element comprising a needle-shaped single crystal as described in any of [1] to [4].

[0018] [8] A nonlinear optical element comprising an acicular single crystal as described in any of [1] to [4], wherein the light absorption edge is 400 nm or less.

[0019] [9] A method for producing needle-shaped single crystals according to any one of [1] to [4] by flow synthesis, comprising: preparing a solution containing a raw material monomer and a solvent; raising the temperature of the solution to a predetermined temperature and holding it at the predetermined temperature for a predetermined time in a state of suppressed or no stirring; the holding of the solution is performed by the solution, which is kept at the predetermined temperature, flowing through a flow channel; and separating the needle-shaped single crystals from the suspension obtained after holding for the predetermined time.

[0020]

[10] A method for producing needle-shaped single crystals according to any one of [1] to [4], comprising: preparing a solution containing a raw material monomer and a solvent; raising the temperature of the solution to a predetermined temperature and holding it at the predetermined temperature for a predetermined time in a state of suppressed or no stirring; one or both of the raising of temperature and the holding at the predetermined temperature being performed by microwave irradiation; and separating the needle-shaped single crystals from the suspension obtained after holding for the predetermined time.

[0021]

[11] A method for producing a needle-shaped single crystal according to any one of [1] to [4] by flow synthesis, comprising: preparing a solution containing a raw material monomer and a solvent; raising the temperature of the solution to a predetermined temperature and holding it at the predetermined temperature for a predetermined time in a state of suppressed stirring or without stirring; the holding of the solution is carried out by the flow of the solution maintained at the predetermined temperature through a flow channel; one or both of the raising of temperature and the holding at the predetermined temperature are carried out by microwave irradiation; and separating the needle-shaped single crystal from the suspension obtained after holding for the predetermined time.

[0022]

[12] The manufacturing method according to any one of [9] to

[11] , wherein the predetermined temperature is 200°C or more and 380°C or less.

[0023]

[13] The manufacturing method according to any one of [9] to

[11] , wherein the predetermined time is 0.1 hours or more and 20 hours or less.

[0024] According to this disclosure, it is possible to provide an organic nonlinear optical material that exhibits high nonlinearity even at the crystalline level and enables the generation of second harmonics in the ultraviolet region.

[0025] This is a micrograph of a needle-shaped single crystal according to an embodiment of the present disclosure. This is a micrograph of a needle-shaped single crystal according to an embodiment of the present disclosure. This is a micrograph of a needle-shaped single crystal according to an embodiment of the present disclosure. This is a micrograph of a needle-shaped single crystal according to an embodiment of the present disclosure. This is a micrograph of a needle-shaped single crystal according to an embodiment of the present disclosure. This is a schematic diagram showing an example of the manufacturing method according to the present disclosure. This is a graph showing the SHG signal intensity of the organic nonlinear optical material according to an embodiment of the present disclosure. This is a chart showing the absorption spectra of the organic nonlinear optical material and solvent according to an embodiment of the present disclosure. This is a chart showing the absorption spectrum of the solvent used in Figure 9.

[0026] The embodiments of this disclosure are described below. This disclosure is not limited to the embodiments described below.

[0027] [Acicular Single Crystal] The acicular single crystal used as an organic nonlinear optical material according to this embodiment has a structural unit represented by the following formula (1). Such an acicular single crystal is obtained by crystallizing a polymer consisting of monomers having the structural unit represented by the following formula (1). Since the acicular single crystal having the structural unit represented by the following formula (1) has an absorption edge on the short wavelength side, it is possible to generate second harmonics in the ultraviolet light region and is therefore preferably used as an organic nonlinear optical material. Organic nonlinear optical materials are useful in the fields of optoelectronics and photonics, and can be applied to optical modulators, optical switches, optical integrated circuits, optical computers, optical memories, wavelength conversion elements, hologram elements, etc., which are useful in fields such as optical information communication, optical information processing, and imaging using light.

[0028]

[0029] In formula (1) above, n represents an integer of 3 or more, Ar represents one of a 1,4-phenylene group, a 2,6-naphthylene group, and a 4,4'-biphenylene group, and at least some of the hydrogen atoms in the structure of Ar may be substituted with halogen atoms, and X represents an oxygen atom or a sulfur atom.

[0030] Specific examples of monomers having the structural unit shown in formula (1) above include 4-hydroxybenzoic acid, 2-hydroxy-6-naphthoic acid, 4-hydroxy-4'-biphenylcarboxylic acid, 4-mercaptobenzoic acid, 2-fluoro-4-hydroxybenzoic acid, 3-fluoro-4-hydroxybenzoic acid, 2,6-difluoro-4-hydroxybenzoic acid, 2,3-difluoro-4-hydroxybenzoic acid, 2,3,5,6-tetrafluoro-4-hydroxybenzoic acid, 2-chloro-4-hydroxybenzoic acid, 3-chloro-4-hydroxybenzoic acid, and 3-methyl-4-hydroxybenzoic acid.

[0031] The needle-shaped single crystal having the structural unit represented by formula (1) above is a polymer of the monomer. The polymer of the monomer may be partially copolymerized, but it is preferably a homopolymer of the monomer (for example, poly(4-hydroxybenzoic acid), poly(2-hydroxy-6-naphthoic acid), poly(4-hydroxy-4'-biphenylcarboxylic acid), poly(4-mercaptobenzoic acid), poly(2-fluoro-4-hydroxybenzoic acid), poly(3-fluoro-4-hydroxybenzoic acid), poly(2,6-difluoro-4-hydroxybenzoic acid), poly(2,3-difluoro-4-hydroxybenzoic acid), poly(2,3,5,6-tetrafluoro-4-hydroxybenzoic acid), poly(2-chloro-4-hydroxybenzoic acid), poly(3-chloro-4-hydroxybenzoic acid), poly(3-methyl-4-hydroxybenzoic acid), etc.).

[0032] In formula (1) above, X is preferably a sulfur atom. Furthermore, at least some of the hydrogen atoms in the Ar structure in formula (1) above are substituted with halogen atoms, and these halogen atoms are preferably chlorine atoms or fluorine atoms. This results in a needle-shaped single crystal with a higher nonlinear optical effect.

[0033] In the above formula (1), n ​​is preferably 10 or more and 2000 or less, more preferably 20 or more and 1500 or less, and even more preferably 50 or more and 1000 or less, from the viewpoint of obtaining a desirable mechanical strength.

[0034] The specific gravity of the needle-shaped single crystals in this disclosure is preferably 1.3 or more and less than 2.0. This reduces the weight of the resin composite containing the needle-shaped single crystals, thereby reducing energy consumption during product transportation or use. The above specific gravity may also be 1.5 or more and less than 1.7.

[0035] The length in the longitudinal direction of the needle-shaped single crystal of this disclosure is preferably 3.0 μm or more, more preferably 10 μm or more, and even more preferably 15 μm or more. The length in the longitudinal direction is preferably 50 μm or less. The length (diameter) in the short direction of the needle-shaped single crystal is preferably 0.1 μm or more and 2.0 μm or less, more preferably 0.3 μm or more and 1.5 μm or less, and even more preferably 0.5 μm or more and 1.0 μm or less.

[0036] The aspect ratio of the needle-shaped single crystal of this disclosure is preferably 5 or more, more preferably 10 or more, and even more preferably 15 or more. The above aspect ratio is preferably 500 or less, more preferably 200 or less, and even more preferably 100 or less.

[0037] [Method for producing needle-shaped single crystals] Needle-shaped single crystals according to this embodiment are obtained, for example, by preparing a solution containing raw material monomers and a solvent, raising the temperature of the solution to a predetermined temperature, and holding it at the predetermined temperature for a predetermined time in a state of reduced or no stirring to allow polymerization, and then separating the needle-shaped single crystals from the resulting suspension.

[0038] As the raw material monomer, monomers obtained by acetoxyling the hydroxyl group of a monomer having the structural unit shown in formula (1) above, or by thioacetylating the mercapto group, can be used.

[0039] The solvent is not particularly limited as long as it can dissolve the raw material monomer and allows the polymer to be obtained in an acicular shape. The solvent is preferably liquid at room temperature, capable of dissolving the raw material monomer at approximately 100°C to 200°C, and has a boiling point of 300°C or higher. Specific examples include liquid paraffin, alkylbenzene, hydrogenated terphenyl, triethylbiphenyl, and dibenzyltoluene. Dibenzyltoluene is preferred from the viewpoints of repeated usability, stability and safety of the solvent.

[0040] The solution is obtained by mixing the raw material monomer with a solvent and completely dissolving the raw material monomer while stirring. The concentration of the raw material monomer in the solution is not particularly limited as long as the polymer can be obtained in an acicular shape, and may be a concentration depending on the type of the raw material monomer. For example, the concentration of the raw material monomer is preferably 0.1 to 5% by mass, more preferably 1 to 3% by mass.

[0041] The polymerization reaction is carried out by heating the solution to a predetermined temperature at which acicular single crystals precipitate, and holding the solution for a predetermined time in a state where stirring is suppressed or no stirring is performed. The predetermined temperature can be set according to the raw material monomer, and is preferably 200°C or higher and 380°C or lower. The predetermined time is preferably 0.1 hour or more and 20 hours or less. From the viewpoint of obtaining acicular single crystals having high mechanical strength, the predetermined time is preferably 3 hours or more.

[0042] For the above polymerization reaction, an esterification catalyst can also be used as long as it does not decompose at the predetermined temperature. As the esterification catalyst, dibutyltin oxide, sodium acetate, potassium acetate, N-methylimidazole, N,N-dimethylaminopyridine and the like are suitably used.

[0043] In order to improve the molecular weight (degree of polymerization) of the acicular single crystals obtained by the polymerization reaction, heat treatment may be performed after the polymerization reaction in the presence of an inert gas such as nitrogen gas or argon gas. The temperature of the heat treatment can be higher than the temperature of the polymerization reaction, for example, 300°C or higher and lower than 400°C. The duration of the heat treatment can be, for example, several hours (2 hours or more and 6 hours or less).

[0044] The method for carrying out the above polymerization reaction is not particularly limited, and examples thereof include flow synthesis, batch synthesis and the like. From the viewpoint of improving the productivity of acicular single crystals, flow synthesis is preferable.

[0045] An example of a production system for continuously producing acicular single crystals by flow synthesis will be described below with reference to FIG. 7. As shown in FIG. 7, the production system 1 includes a raw material storage unit 10, a liquid feeding unit 20, a polymerization reaction unit 30, a pressure adjustment unit 40, and a storage unit 50. Each of these components is provided on a single line L. Flow rate regulating valves V1 and V2 for regulating the flow rate of the solution may be provided on the line L. The configuration of the production system 1 other than the above is arbitrary.

[0046] The raw material storage unit 10 is a container that stores the above solution. The above solution may be heated in advance in the raw material storage unit 10 to a temperature lower than the reaction temperature (for example, about 180° C.). The raw material storage unit 10 may be provided with a mechanism for mixing a raw material monomer with a solvent and stirring the mixture. The liquid feeding unit 20 feeds the solution stored in the raw material storage unit 10 toward the polymerization reaction unit 30 at the subsequent stage. The liquid feeding unit 20 is, for example, a pump device such as a positive displacement pump, a centrifugal pump, an electromagnetic pump, a micropump, or a diaphragm pump.

[0047] The polymerization reaction unit 30 includes a flow path constituted by a tube, a column, or the like, and a heating mechanism that heats the solution flowing through the flow path. A microreactor may be used as the polymerization reaction unit 30. By the above heating mechanism, the solution flowing through the flow path is heated to the above predetermined temperature at which acicular single crystals precipitate. The length of the flow path and the flow rate of the solution flowing through the flow path are set such that the time for the solution to flow through the flow path is the above predetermined time.

[0048] The pressure adjustment unit 40 adjusts the pressure of the solution flowing through the polymerization reaction unit 30. The pressure adjustment unit 40 is, for example, a back pressure regulator that keeps the pressure on the primary side constant. By using the pressure adjustment unit 40 in combination with the flow rate regulating valves V1 and V2, the flow rate and pressure of the solution flowing through the polymerization reaction unit 30 can be controlled within a predetermined range.

[0049] The storage section 50 is a container for storing a suspension containing needle-shaped single crystals obtained by the polymerization reaction in the polymerization reaction section 30. Needle-shaped single crystals are obtained by separating them from the suspension by methods such as filtration.

[0050] A manufacturing system for continuously producing needle-shaped single crystals by flow synthesis is not limited to the configuration of manufacturing system 1 described above. For example, instead of the polymerization reaction section 30 having a flow path composed of tubes, columns, etc., a method of connecting multiple reaction vessels and extending the residence time of the solution may be adopted.

[0051] A method for producing needle-shaped single crystals by batch synthesis involves, for example, adding the above solution to a reaction vessel, raising the temperature of the solution in the reaction vessel to a predetermined temperature, stopping the heating after a predetermined time to obtain a suspension containing needle-shaped single crystals, and separating the needle-shaped single crystals by methods such as filtration.

[0052] In the above flow synthesis and batch synthesis, the heating of the solution and its maintenance at a predetermined temperature are preferably performed by microwave irradiation from the viewpoint of improving production efficiency. Microwave irradiation can be performed using a known microwave irradiation device.

[0053] In the polymerization reaction described above, the state of suppressed or no stirring means that no external force, such as stirring force, is applied to the solution in order to precipitate needle-shaped single crystals, or that the solution is held in a state where stirring is suppressed. For example, when the polymerization reaction is carried out by batch synthesis, it means that the solution is not stirred in the reaction vessel during the reaction. Alternatively, when the polymerization reaction is carried out by flow synthesis, it means that the flow rate of the solution in the flow channel of the polymerization reaction section 30 is kept low enough so as not to hinder the precipitation of needle-shaped single crystals.

[0054] [Nonlinear Optical Element] The nonlinear optical element according to this embodiment includes the above-described needle-shaped single crystal. The needle-shaped single crystal has a non-centrosymmetric crystal structure, and nonlinear optical effects such as second harmonic (SHG) and shift current can be obtained. The needle-shaped single crystal is a one-dimensional (linear) crystal. In a one-dimensional crystal, the motion of electrons is restricted to one dimension. Therefore, compared to a three-dimensional crystal, significant absorption can occur in a specific energy region in a one-dimensional crystal, and it is predicted that the density of states at the band edge will be high. Furthermore, it has been theoretically analyzed that the needle-shaped single crystal is a material in which the difference between the dipole moments of the ground state and the excited state is very large. Therefore, the needle-shaped single crystal may be a material with a large shift current.

[0055] In the design of conventional nonlinear optical materials, methods to enhance nonlinear optical effects include setting the absorption wavelength to a longer wavelength and reducing the band gap. However, theoretical calculations predict that the above-mentioned acicular single crystal will exhibit high nonlinear optical effects despite having a short absorption wavelength. Therefore, it becomes possible to broaden the range of wavelengths that can be converted in, for example, second harmonic (SHG) generation.

[0056] In this embodiment, the nonlinear optical element preferably has an optical absorption edge of 400 nm or less. More preferably, the optical absorption edge is 350 nm or less, and even more preferably 300 nm or less.

[0057] (Molded body) The nonlinear optical element may consist only of the above-mentioned needle-shaped single crystals, and may be a molded body formed by the entanglement of the above-mentioned needle-shaped single crystals. The method of molding is not particularly limited, and examples include a method of entangling the needle-shaped single crystals by applying pressure.

[0058] (Resin Composite) The nonlinear optical element may be a resin composite comprising the above-mentioned needle-shaped single crystal and a resin. The resin is not particularly limited as long as it has a relatively low absorption rate of incident and outgoing light to the nonlinear optical element. Specific examples of the resin include acrylic resin (PMMA).

[0059] The resin composite according to this embodiment may contain additives other than those mentioned above, as necessary, within a range that does not hinder the incidence or emission of light to the nonlinear optical element. Specific examples of additives include coupling agents, plasticizers, stabilizers, flame retardants, antioxidants, foaming agents, antibacterial agents, and lubricants.

[0060] The method for obtaining a resin composite containing at least needle-shaped single crystals and a resin is not particularly limited, but one method is to mix raw materials containing at least needle-shaped single crystals and a resin and then mold them.

[0061] The method for mixing needle-shaped single crystals with resin can be selected according to the respective materials and applications, and is not particularly limited. For example, dry mixing, wet mixing, and melt mixing can be used. Specifically, methods such as roll mill mixing, kneader mixing, extruder mixing, internal mixer mixing, twin-screw extruder mixing, batch mixer mixing, high-speed mixer mixing, ultrasonic mixing, electrostatic mixing, solution mixing, and mechanical stirring mixing are used.

[0062] The method for molding the resin composition obtained as described above is not particularly limited, but examples include injection molding, extrusion molding, compression molding, injection compression molding, press molding, injection press molding, and transfer molding.

[0063] Next, to clarify the effects of this disclosure, examples will be used. However, this disclosure is not limited to these examples.

[0064] [Synthesis of Needle-Shaped Single Crystals] (Example 1) 1.5 g of 4-acetoxybenzoic acid (Sigma-Aldrich) and 100 ml of liquid paraffin (Sigma-Aldrich) were sequentially added to a 300 ml three-necked flask. The mixture was stirred at 300 rpm with a stirring bar under a nitrogen flow, and the temperature was raised to 180°C using a mantle heater. After confirming complete dissolution, the mixture was stirred and held at 180°C for 10 minutes. After that, stirring was stopped and the temperature was raised to 320°C. After reaching 320°C, the mixture was held for 6 hours, then cooled to room temperature and filtered. The obtained needle-shaped single crystals were washed with hexane and acetone, and dried under vacuum overnight to obtain 1.0 g of the needle-shaped single crystals of Example 1. Microscopic images of the above needle-shaped single crystals were taken using a polarizing microscope (Nikon LV100, Nikon Corporation) (Figure 1). Image analysis of the above microscope photograph revealed that the average fiber length (length in the long side direction) of the needle-shaped single crystal was 18 μm, and the average fiber diameter (length in the single side direction) was 1 μm. The specific gravity of the needle-shaped single crystal was 1.5 g / cm³. ３ That was the case.

[0065] (Example 2) Needle-shaped single crystals of Example 2 were obtained under all the same conditions as in Example 1, except that the holding time after the liquid temperature of the raw material reached 320°C was 3 hours. Microscopic images of the needle-shaped single crystals were taken in the same manner as in Example 1 (Figure 2). The average fiber length (length in the long side direction) of the above needle-shaped single crystals was 16 μm.

[0066] (Example 3) Needle-shaped single crystals of Example 3 were obtained under all the same conditions as in Example 1, except that the holding time after the liquid temperature of the raw material reached 320°C was 20 hours. Microscopic images of the needle-shaped single crystals were taken in the same manner as in Example 1 (Figure 3). The average fiber length (length in the long side direction) of the above needle-shaped single crystals was 14 μm.

[0067] (Example 4) Needle-shaped single crystals of Example 4 were obtained under the same conditions as in Example 1, except that 2-fluoro-4-acetoxybenzoic acid was used instead of 4-acetoxybenzoic acid. Microscopic images of the needle-shaped single crystals were taken in the same manner as in Example 1 (Figure 4). The average fiber length (length in the long side direction) of the above needle-shaped single crystals was 16 μm.

[0068] (Example 5) Needle-shaped single crystals of Example 5 were obtained under all conditions the same as in Example 1, except that 2-fluoro-4-acetoxybenzoic acid was used instead of 4-acetoxybenzoic acid, and the temperature of the raw material was raised to 300°C and maintained at 300°C. Micrographs of the needle-shaped single crystals were taken in the same manner as in Example 1 (Figure 5). The average fiber length (length in the long side direction) of the above needle-shaped single crystals was 16 μm.

[0069] (Example 6) Needle-shaped single crystals of Example 6 were obtained under all conditions the same as in Example 1, except that 4-(acetylthio)benzoic acid was used instead of 4-acetoxybenzoic acid, and the temperature of the raw material liquid was raised to 300°C and maintained at 300°C. Micrographs of the needle-shaped single crystals were taken in the same manner as in Example 1 (Figure 6). The average fiber length (length in the long side direction) of the above needle-shaped single crystals was 16 μm.

[0070] [Measurement of Nonlinear Optical Effects (SHG)] Needle-shaped single crystals and liquid paraffin according to Examples 1 to 6 were dispersed in a mortar, and films with a thickness of 55 μm were prepared using a Kapton tape mask. The nonlinear optical effects (SHG) were measured using the Boxkar signal (DC voltage: (V)) obtained by an amplifier voltmeter at 600 V. The results are shown in Figure 8.

[0071] The results shown in Figure 8 confirm that the needle-shaped single crystals according to the present invention exhibit high nonlinear optical effects (SHG). In particular, the needle-shaped single crystal in which the hydrogen atom in the Ar structure of formula (1) is substituted with a halogen atom (chlorine atom) (Example 4), and the needle-shaped single crystal in which X in formula (1) is a sulfur atom (Example 6) were found to have even higher nonlinear optical effects (SHG).

[0072] [Optical Absorption Spectrum Measurement] The needle-shaped single crystal according to Example 1 was dispersed in liquid paraffin, and the optical absorption spectrum was measured. The results are shown in Figure 9. In addition, the optical absorption spectrum of liquid paraffin alone was measured as a reference. The results are shown in Figure 10. In Figures 9 and 10, the horizontal axis represents wavelength (nm). The horizontal axis represents transmittance (%). Transmittance (%) is the ratio of transmitted light intensity to incident light intensity expressed as a percentage. From the difference between Figure 9 and Figure 10, it was confirmed that the absorption edge of the needle-shaped single crystal according to Example 1 is around 300 nm.

Claims

1. A needle-shaped single crystal having a structural unit represented by the following equation (1), used as an organic nonlinear optical material. In formula (1) above, n represents an integer of 3 or more, Ar represents one of a 1,4-phenylene group, a 2,6-naphthylene group, and a 4,4'-biphenylene group, and at least some of the hydrogen atoms in the structure of Ar may be substituted with halogen atoms, and X represents an oxygen atom or a sulfur atom.

2. The needle-shaped single crystal according to claim 1, wherein X in formula (1) above is a sulfur atom.

3. The needle-shaped single crystal according to claim 1, wherein at least some of the hydrogen atoms in the Ar structure in formula (1) are substituted with halogen atoms, and the halogen atoms are chlorine atoms or fluorine atoms.

4. The needle-shaped single crystal according to claim 1, wherein the length in the long side direction is 3.0 μm or more and 50 μm or less, the length in the short side direction is 0.1 μm or more and 5.0 μm or less, and the aspect ratio is 5 or more.

5. A molded body made of the needle-shaped single crystal according to claim 1.

6. A resin composite comprising the needle-shaped single crystal described in claim 1 and a resin.

7. A nonlinear optical element comprising the needle-shaped single crystal described in claim 1.

8. A nonlinear optical element comprising the needle-shaped single crystal described in claim 1, wherein the light absorption edge is 400 nm or less.

9. A method for producing needle-shaped single crystals according to claim 1 by flow synthesis, comprising: preparing a solution containing raw material monomers and a solvent; raising the temperature of the solution to a predetermined temperature and holding it at the predetermined temperature for a predetermined time in a state of suppressed or no stirring; holding the solution by flowing the solution, which has been maintained at the predetermined temperature, through a flow channel; and separating the needle-shaped single crystals from the suspension obtained after holding for the predetermined time.

10. A method for producing a needle-shaped single crystal according to claim 1, comprising: preparing a solution containing a raw material monomer and a solvent; raising the temperature of the solution to a predetermined temperature and holding it at the predetermined temperature for a predetermined time in a state of suppressed or no stirring; performing one or both of the raising of temperature and the holding at the predetermined temperature by microwave irradiation; and separating the needle-shaped single crystal from the suspension obtained after holding for the predetermined time.

11. A method for producing a needle-shaped single crystal according to claim 1 by flow synthesis, comprising: preparing a solution containing a raw material monomer and a solvent; raising the temperature of the solution to a predetermined temperature and holding it at the predetermined temperature for a predetermined time in a state of suppressed or no stirring; holding the solution is carried out by the solution, which is kept at the predetermined temperature, flowing through a flow channel; one or both of the raising of temperature and holding at the predetermined temperature are carried out by microwave irradiation; and separating the needle-shaped single crystal from the suspension obtained after holding for the predetermined time.

12. The manufacturing method according to any one of claims 9 to 11, wherein the predetermined temperature is 200°C or more and 380°C or less.

13. The manufacturing method according to any one of claims 9 to 11, wherein the predetermined time is 0.1 hours or more and 20 hours or less.

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