Squarylium liquid crystal material, synthesis method and application thereof

Abstract

The application discloses a squarylium material with liquid crystal properties and a preparation method thereof. First, the squarylium liquid crystal material has a symmetrical dendritic chemical structure, the formed liquid crystal material has a very high extinction coefficient in the near infrared region, and has a low liquid crystal temperature zone (< 80 DEG C), good chemical stability and thermal stability. Secondly, the preparation method of the squarylium liquid crystal material has excellent expandability, and the kind of squarylium liquid crystal material can be greatly widened by controlling the chemical structure variable, and the preparation method also has the characteristics of economy, green, simple and efficient. Finally, the squarylium liquid crystal material combines the excellent optical properties of squarylium and the advantages of large-area anisotropic liquid crystal material, and the uniform orientation thin film material has excellent absorption anisotropy and thermochromic characteristics in the near infrared region, and has great application potential in the fields of heat sensing and display devices.

Description

Technical Field

[0001] This invention belongs to the field of organic dyes, specifically relating to a squaric acid cyanine liquid crystal material, its synthesis method, and its application in the field of thermochromic sensing. Background Technology

[0002] Near-infrared light accounts for over 50% of sunlight and is the primary source of heat for the Earth's surface. Current research on smart windows mainly relies on electrically driven inorganic materials such as silicates, which require high voltage and consume additional energy. Nature Sustainability 2024, DOI: 10.1038 / s41893-024-01349-z). Smart windows based on near-infrared dye liquid crystal materials require no electricity; they can achieve intelligent adjustment in multiple modes using sunlight and heat generated by irradiation, offering significant advantages in energy conservation and environmental protection. Squamousine has a characteristic four-membered ring, exhibiting a donor-acceptor-donor structure with intramolecular charge separation, resulting in an extremely high extinction coefficient in the near-infrared region. Therefore, squamousine has significant research and potential application value in the field of smart windows based on near-infrared dye liquid crystal materials. However, common bulk squamousine materials have limited optical properties due to their fixed aggregate structure, making dynamic and rapid regulation impossible, thus failing to meet the requirement of smart windows to respond quickly to external light and heat stimuli. By utilizing the special material state of "liquid crystal," which combines order and fluidity, and through the rational design of the squamousine molecular structure to become a liquid crystal material, it can respond rapidly to external fields such as heat and light, thereby achieving the above functional requirements.

[0003] Thermotropic liquid crystal has high strength, high modulus and other excellent mechanical properties, good self-reinforcing effect, small linear expansion coefficient, excellent heat resistance, low melt viscosity, good flowability and other characteristics, and is widely used in various modern science and technology and electronic equipment. The squarylium cyanine with thermotropic liquid crystal property is extremely rare, and so far only one example reported by Frank Wurthner exists, but its high temperature zone (above 150℃) and single phase structure (nematic phase) greatly limit the application of the squarylium cyanine liquid crystal material (Advanced Optical Materials, 2016, 4 (8), 1186-1189.). The reasons why the squarylium cyanine liquid crystal material is difficult to achieve mainly include two points: the lack of squarylium cyanine liquid crystal material design strategy and the difficulty in synthesis and preparation. Squarylium cyanine has a zwitterionic structure, and there is strong electrostatic interaction, including quadrupole / dipole interaction, so it has a high melting point (100 ~ 200 ℃), which is not conducive to the existence of liquid crystal phase state in the near normal temperature range and practical application. Therefore, how to realize the squarylium cyanine material with liquid crystal state through reasonable molecular structure design is a technical problem. In addition, due to the peculiar zwitterionic structure of squarylium cyanine, the solubility is poor after condensation, and it can only be dissolved in a few solvents, which greatly limits the performance of other chemical reactions, and the conjugated structure of squarylium cyanine, that is, the chromophore of squarylium cyanine, is easily destroyed by the generated electron / free radical intermediates in the chemical reaction and bleached, which greatly limits the chemical modification based on squarylium cyanine molecules and hinders the development of squarylium cyanine liquid crystal materials.

[0004] In summary, the existing squarylium cyanine liquid crystal material is rare in kind, and the design strategy for liquid crystal is lacking and the synthesis and preparation are difficult, which becomes a technical problem to be solved in this field. SUMMARY

[0005] In order to solve the above problems, the present application provides a new squarylium cyanine liquid crystal material and its synthesis path, to overcome the defects of the existing squarylium cyanine liquid crystal material design and preparation difficulty, and proposes the application of the squarylium cyanine liquid crystal material in the field of thermochromic sensing.

[0006] The first object of the present application is to provide a squarylium cyanine liquid crystal material with high extinction coefficient, good photo-thermal stability and low liquid crystal temperature zone, which has the structure shown in general formula (I):

[0007] ,

[0008] wherein R is selected from a saturated alkyl chain, an alkyl chain with a double bond, an alkyl chain with a triple bond or an oligomeric ethylene glycol chain.

[0009] The second object of the present application is to provide a squarylium cyanine liquid crystal preparation method with excellent expandability, that is, to condense the compound with the structure shown in general formula (II) with squarylium to obtain a squarylium cyanine liquid crystal material with the structure shown in general formula (I),

[0010] , ,

[0011] wherein R is selected from a saturated alkyl chain, an alkyl chain with a double bond, an alkyl chain with a triple bond, or an oligoethylene glycol chain.

[0012] The condensation reaction is carried out under anhydrous and anaerobic conditions at 120-140°C, the compound of the structure of general formula (II) is dissolved in a mixed solvent of toluene and n-butanol, and the reaction is carried out in a Dean-Stark reaction device for 6-12 hours. After the reaction is completed, methanol is added for precipitation and filtration, and the solid is obtained by suction filtration. The solid is dissolved and separated by column chromatography, and then recrystallized in a dichloromethane and methanol solution to obtain a dark purple or blue solid, which is the squarylium cyanine liquid crystal material of general formula (I).

[0013] Preferably, the molar ratio of the compound of the structure of general formula (II) to squaric acid is (2-3): 1.

[0014] Further, the compound of the structure of general formula (II) is prepared according to the following steps: under anhydrous and anaerobic conditions at 70-90°C, the compound of the structure of general formula (III) is mixed with Dowex (50WX8-100-200(H)) resin in a mixed solvent of tetrahydrofuran and methanol and refluxed for 12 hours. After the reaction is completed, filtration and concentration are carried out to obtain the compound of the structure of general formula (II),

[0015] .

[0016] Further, the compound of the structure of general formula (III) is prepared according to the following steps: under anhydrous and anaerobic conditions at room temperature, the compound of the structure of general formula (IV), 3, 4, 5-substituted-benzoic acid derivative, 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride (EDCI), and 4-dimethylaminopyridine (DMAP) are dissolved and stirred in anhydrous dichloromethane solution, and the reaction is stirred for 10-14 hours. After the reaction is completed, water washing, drying, and concentration are carried out to obtain a white powder solid, which is the compound of the structure of general formula (III),

[0017] .

[0018] wherein the structure of the 3, 4, 5-substituted-benzoic acid derivative is ;

[0019] Further, the compound of the structure of general formula (IV) is synthesized by using m-aminophenol as a precursor, including the following steps:

[0020] Step a, under anhydrous and anaerobic conditions at 80~100℃, mix m-aminophenol and methyl acrylate, and add glacial acetic acid and sodium bromide as catalyst, and react for 10~14 hours; after the reaction is completed, the reaction solution is neutralized with sodium bicarbonate, extracted and separated, and concentrated, and the product is separated by column chromatography to obtain colorless oily liquid product A, with a structural formula of ;

[0021] Step b, under anhydrous and anaerobic conditions at -10~10 ℃, gradually add 3,4-dihydro-pyran (DHP) to a dichloromethane solution of compound A and a catalyst pyridinium p-toluenesulfonate (PPTS), after one day of reaction, extract and separate with sodium bicarbonate solution, concentrate, and separate by column chromatography to obtain colorless liquid product, i.e. compound B, with a structural formula of ;

[0022] Step c, under anhydrous and anaerobic conditions at -10~10 ℃, gradually add lithium aluminum hydride in tetrahydrofuran to a tetrahydrofuran solution of compound B, after the addition is completed, transfer to room temperature and react for two hours, after the reaction is completed, quench with water, filter to remove water, and concentrate, and the obtained crude product is subjected to rapid column chromatography to obtain yellow oily product, i.e. compound C, i.e. a compound with the structure shown in general formula (IV).

[0023] The third object of the present application is to provide a liquid crystal film with anisotropic absorption and thermochromic properties, which can be applied to the field of thermochromic sensing as a visual temperature sensing material.

[0024] Compared with the prior art, the present application has the following beneficial effects:

[0025] 1. The squarylium liquid crystal molecules designed in the present application have good chemical stability and thermal stability by introducing alkyl chains and connecting segments to improve molecular activity and reduce melting point, and the liquid crystal temperature range is lower than 80 ℃, completely reaching the liquid crystal phase interval that can be used at ambient temperature, such as compound II with hexagonal columnar liquid crystal phase structure with an alkyl chain length of 12~16, which enriches the squarylium liquid crystal phase structure and material types, and the aggregates exhibit different spectral absorption properties under the regulation of temperature, thereby having different colors.

[0026] 2. The squarylium liquid crystal preparation method proposed in the present application adopts the strategy of modifying the precursor with liquid crystal functional groups before condensation with squarylium, and various squarylium liquid crystal molecules with different symmetries and different molecular shapes can be designed by controlling the structure variables of the precursor, greatly enriching the types of squarylium materials, and the preparation method has the characteristics of simple reaction steps, mild reaction conditions, high yield and strong expandability.

[0027] 3. The squaric acid cyanine liquid crystal film proposed in this invention can achieve a macroscopically uniform alignment film through shearing. It has anisotropic absorption characteristics in the red region. Furthermore, because the liquid crystal aggregate changes with temperature along with the phase structure, different absorption curves are generated at different temperatures, resulting in significant color differences. It has thermochromic properties and can therefore be used as a visual thermal sensing material in the field of thermochromic sensing. Attached Figure Description

[0028] Figure 1 The synthesis route of the squaric acid cyanine liquid crystal material is shown.

[0029] Figure 2 For compound II ( n = 12) Time difference indicates scanning calorimetry curve.

[0030] Figure 3 For compound II ( n = 12) Polarizing microscope optical texture, a) showing a typical fan-shaped texture of hexagonal prism phase, b) is the texture diagram after shearing.

[0031] Figure 4 For compound II ( n Small-angle scattering results (= 12).

[0032] Figure 5 For compound II ( n = 12) UV-Vis absorption and fluorescence spectra.

[0033] Figure 6 For compound II ( n = 12) Temperature scanning UV-Vis absorption curve of liquid crystal film.

[0034] Figure 7 For compound II ( n = 12) Anisotropic liquid crystal film and anisotropic ultraviolet-visible light absorption curves, a) is the linear dichroism of ultraviolet-visible light absorption of the alignment film, b), c), and d) are three texture images of the alignment film rotated counterclockwise under a polarizing microscope, and the double arrow in the upper right corner indicates the alignment direction of the film.

[0035] Figure 8 For compound II ( n = 12) Schematic diagram of the thermochromic properties of liquid crystal film, a) is the film color at 80 ℃, b) is the film color at 72 ℃. Detailed Implementation

[0036] In the following, the technical solutions in the embodiments of the present application will be described clearly and completely with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments of the present application. Based on the embodiments in the present application, all the other embodiments obtained by a person of ordinary skill in the art without creative work should belong to the protection scope of the present application.

[0037] It should be noted that the terms "first", "second" and the like in the description and claims of the present application and the above-mentioned drawings are used to distinguish similar objects, and do not necessarily indicate a specific order or sequence. It should be understood that the data thus used can be interchanged under appropriate circumstances, so that the embodiments of the present application described herein can be implemented in an order other than that illustrated or described herein. In addition, the terms "include" and "have" and any variations thereof are intended to cover non-exclusive inclusion, for example, a process, method, system, product or device that includes a series of steps or units does not necessarily have to be limited to those steps or units clearly listed, but can include other steps or units not clearly listed or inherent to these processes, methods, products or devices.

[0038] The present application will be described in further detail below with reference to the drawings:

[0039] The above benzoic acid derivatives can be purchased directly or commissioned from Shanghai Xinhua Chemical Technology Co., Ltd. / Shanghai Bide Pharmaceutical Technology Co., Ltd. / Shanghai Dibo Biological Technology Co., Ltd. There are also corresponding reports on their mature synthesis routes (Angewandte Chemie International Edition, 59 (25), 10143-10150; Journal of the American Chemical Society, 2011, 133 (7), 2163-2169.). The remaining reagents are commercial reagents and do not require further treatment.

[0040] The squarylium liquid crystal molecule has the following structural formula:

[0041]

[0042] Among them, R is a functional group of different lengths. When R is an alkyl chain, its structural formula is shown as (II). When R is an alkyl chain with a double bond / triple bond, it is shown as compound (III / IV). When R is an oligoethylene glycol chain, it is shown as compound (V),

[0043] .

[0044] R in the above structural formulan indicates the length of the segment, n The value of n is 8-18.

[0045] The synthesis route of squarylium liquid crystal molecules is shown in Figure 1 The present application is further specifically described by specific embodiments.

[0046] Example 1

[0047] 1. The precursor compound C is prepared according to the following method

[0048] (1) Synthesis of compound A

[0049] In a reaction bottle, 10.75 g of methyl acrylate (124.66 mmol), 2.27 g of m-aminophenol (20.77 mmol), 0.51 g of sodium bromide (5.02 mmol) and 3 ml of glacial acetic acid were added and refluxed at 95°C for 12 hours under the protection of argon. After the reaction was completed, 30 ml of sodium carbonate aqueous solution was added to neutralize the acetic acid and extracted with ethyl acetate for 3 times. After the organic layer was separated, anhydrous sodium sulfate was used for water removal, and after filtration and rotary evaporation, the crude product was obtained. Then the crude product was separated by column chromatography, and the eluent was petroleum ether / ethyl acetate (2 / 1). Finally, 4.59 g of colorless oily liquid product A was obtained with a yield of 95%. The nuclear magnetic resonance data are shown in reference ( Anal. Chem. 2018, 90 (15), 8807-8815.).

[0050] (2) Synthesis of compound B

[0051] The product 4.59 g of A (16.32 mmol) and PPTS (pyridinium p-toluenesulfonate) (0.41 g, 1.63 mmol) were dissolved in 60 ml of anhydrous dichloromethane, and 3,4-dihydro-pyran (3 ml, 32.69 mmol) was gradually added dropwise under the protection of argon and 0 o C and argon. The dropwise addition time was 30 minutes. Then the reaction solution was stirred at room temperature for two days, after the reaction was completed, the reaction solution was neutralized with 20 ml of saturated sodium bicarbonate solution and separated by liquid-liquid extraction, the organic layer was separated and dried with anhydrous magnesium sulfate, and after filtration and solvent evaporation, the crude product was quickly separated by column chromatography, and the eluent was ethyl acetate / petroleum ether (1 / 4 containing 1% triethylamine). Finally, 3.58 g of colorless viscous liquid product B was obtained with a yield of 60%. The nuclear magnetic resonance data are as follows: 1 HNMR (600 MHz, CDCl3) δ 7.09 (t, J= 8.2 Hz, 1H), 6.53 – 6.21 (m, 3H), 5.35 (t, J = 3.3 Hz, 1H), 3.94 – 3.82 (m, 1H), 3.66 – 3.52 (m, 11H), 2.61 – 2.53 (m,4H), 2.02 – 1.91 (m, 1H), 1.85 – 1.77 (m, 2H), 1.68 – 1.52 (m, 3H). 13 C NMR (151 MHz, CDCl3) δ 172.44, 158.54, 147.98, 130.07, 106.23, 104.70, 101.33,96.42, 62.12, 51.64, 46.93, 32.22, 30.46, 25.24, 19.00.

[0052] (3) Synthesis of compound C

[0053] Lithium aluminum hydride (0.74 g, 19.50 mmol) was dissolved in 30 ml of anhydrous tetrahydrofuran and placed in a round-bottom flask. Simultaneously, product B was dissolved in 30 ml of anhydrous tetrahydrofuran and placed in a 0. o Add C dropwise over half an hour under argon protection. The reaction was stirred for 2 hours until complete, then quenched with 10 ml of water in an ice-water bath, producing a large amount of gray, gel-like insoluble matter, which was filtered. Anhydrous magnesium sulfate was added to the filtrate for drying, followed by filtration. Finally, the crude product was concentrated under reduced pressure. The crude product was rapidly separated by column chromatography using dichloromethane / methanol (50 / 1) as the eluent, ultimately yielding 2.73 g of a yellow, viscous liquid, product C, in 90% yield. NMR data: 1 H NMR (600 MHz, CDCl3) δ 7.10 (t, J= 8.2 Hz, 1H), 6.52 – 6.32 (m, 3H), 5.40 (t, J = 3.3 Hz, 1H), 3.98 – 3.88 (m,1H), 3.71 – 3.65 (m, 4H), 3.62 – 3.55 (m, 1H), 3.48 – 3.32 (m, 4H), 2.06 –1.94 (m, 1H), 1.88 – 1.77 (m, 6H), 1.71 – 1.54 (m, 3H). 13C NMR (151 MHz, CDCl3) δ 158.19, 149.39, 129.80, 106.54, 104.30, 101.26, 96.12, 62.08, 60.32, 48.04, 30.37, 29.97, 25.15, 18.78.。

[0054] 2. Liquid crystal group modification of precursor compound C by introducing different liquid crystal functional groups

[0055] (4) Synthesis of compound II-D

[0056] Into a reaction flask, 670 mg of compound F, i.e. 3, 4, 5-tri-dodecyloxy-benzoic acid (0.99 mmol), 140 mg of product E (0.45 mmol), 0.19 g of EDCI (0.99 mmol) and 55 mg of DMAP (0.45 mmol) were sequentially dissolved in 20 ml of dichloromethane, and stirred at room temperature for 16 hours. After the reaction was completed, 30 ml of dichloromethane was added, and extracted with 20 ml of ultrapure water three times. The organic layer was obtained, dried with anhydrous magnesium sulfate, then filtered, and rotary evaporated to obtain a crude product. The crude product was separated by column chromatography, and eluted with ethyl acetate / petroleum ether (1 / 40, containing 1% triethylamine) to finally obtain 0.42 g of white powder solid D with a yield of 60%. NMR: 1 H NMR (600 MHz, CDCl3) δ 7.26 (s, 4H), 7.07 (t, J = 8.1 Hz, 1H), 6.46 – 6.35 (m, 3H), 5.35 (t, J = 3.1 Hz,1H), 4.35 (t, J = 6.3 Hz, 4H), 4.01 (q, J = 6.7 Hz, 12H), 3.93 – 3.86 (m, 1H),3.61 – 3.53 (m, 1H), 3.48 (t, J = 7.1 Hz, 4H), 2.06 (dd, J = 23.7, 17.1 Hz, 4H), 1.97 – 1.88 (m, 1H), 1.86 – 1.71 (m, 14H), 1.66 – 1.52 (m, 3H), 1.52 – 1.41 (m, 12H), 1.39 – 1.18 (m, 96H), 0.88 (t, J= 6.9 Hz, 18H). 13 C NMR (151 MHz, CDCl3) δ 166.32, 158.49, 152.83, 148.96, 142.44, 129.89, 124.63, 107.90, 106.29, 104.23, 101.40, 96.40, 73.47, 69.14, 62.75, 62.08, 47.99, 31.91, 30.38, 30.32, 29.73, 29.71, 29.69, 29.67, 29.64, 29.62, 29.56, 29.41, 29.37, 29.35, 29.31, 26.76, 26.09, 26.05, 25.19, 22.67, 18.89, 14.09..

[0057] (5) Synthesis of compound II-E

[0058] In a round bottom flask, 0.42 g of D and 0.1 g of Dowex (50WX8-100-200(H)) were added, followed by a mixed solvent of methanol and tetrahydrofuran (methanol / tetrahydrofuran = 30 ml / 10 ml), followed by refluxing at 80°C under argon protection and vigorous stirring for 12 hours. After the reaction was completed, the Dowex resin was filtered off, and concentrated under reduced pressure to obtain 400 mg of pure product E in the form of white powder, with a yield of 96%. NMR: 1 H NMR (600 MHz, CDCl3) δ 7.27(s, 4H), 7.06 – 6.97 (m, 1H), 6.36 – 6.10 (m, 3H), 4.34 (t, J = 6.2 Hz, 4H),4.06 – 3.97 (m, 12H), 3.46 (t, J = 7.1 Hz, 4H), 2.12 – 2.00 (m, 4H), 1.87 –1.70 (m, 12H), 1.51 – 1.43 (m, 12H), 1.40 – 1.18 (m, 96H), 0.88 (t, J = 7.0 Hz,18H). 13C NMR (151 MHz, CDCl3) δ 166.40, 157.11, 152.85, 149.17, 142.48,130.16, 124.55, 107.92, 104.92, 103.54, 99.47, 73.50, 69.16, 62.75, 47.88,31.90, 30.31, 29.72, 29.71, 29.69, 29.67, 29.64, 29.62, 29.56, 29.41, 29.37,29.35, 29.31, 26.70, 26.09, 26.04, 22.66, 14.08.。

[0059] 3. Synthesis of dendritic squarylium liquid crystal molecules by condensation reaction

[0060] (6) Synthesis of compound II (n = 12)

[0061] In a Dean-Stark apparatus, 400 mg of E (0.26 mmol) and 15 mg of squaric acid (0.13 mmol) were added to a mixed solvent of 18 ml of toluene and 6 ml of n-butanol. The reaction was carried out at a reaction temperature of 140 °C under the protection of argon gas, and was stirred vigorously. The reaction solution changed from colorless to yellow in a few minutes, then to green, and finally to blue with a red color. The reaction was stopped after 12 hours, cooled to room temperature, and 10 ml of methanol was added, immediately a purple solid was precipitated, the precipitated solid was collected and washed with methanol three times, and the solid was filtered and dried, and then the product was purified by column chromatography (methanol / dichloromethane = 1 / 500 ~ 1 / 300) to obtain 205 mg of a blue-purple solid, the yield was 50%. NMR: 1 H NMR (600 MHz, Chloroform-d) δ 7.90 (d, J =9.1 Hz, 2H), 7.25 (s, 8H), 6.41 (d, J = 9.1 Hz, 2H), 6.22 (s, 2H), 4.36 (t, J= 6.1 Hz, 8H), 4.02 (q, J = 6.3 Hz, 24H), 3.71 – 3.49 (m, 8H), 2.22 – 2.09(m, 8H), 1.84 – 1.72 (m, 24H), 1.50 – 1.44 (m, 24H), 1.35 – 1.22 (m, 192H),0.87 (td, J = 7.0, 4.2 Hz, 36H). 13C NMR (151 MHz, CDCl3) δ 182.58, 174.21,166.28, 165.00, 156.23, 152.94, 142.70, 132.78, 124.14, 110.51, 107.91, 107.42, 98.91, 73.53, 69.22, 62.08, 48.49, 31.93, 31.92, 30.35, 29.75, 29.73, 29.71, 29.69, 29.67, 29.66, 29.59, 29.44, 29.39, 29.37, 29.33, 27.11, 26.13, 26.06, 22.68, 14.11.。

[0062] The synthesis route of compounds III, IV, V is only different from the synthesis method of compound II in this embodiment in that 3,4,5-benzoic acid derivatives F with different liquid crystal functional groups are used to introduce different liquid crystal functional groups R into the squarylium liquid crystal molecules through different 3,4,5-benzoic acid derivatives F.

[0063] Effect Example 1

[0064] The following is the characterization of the squarylium liquid crystal properties:

[0065] (1) Differential scanning calorimetry test: TA differential scanning calorimeter was used to characterize the phase transition temperature and phase transition enthalpy value of the liquid crystal, 2 ~ 5 mg of solid sample prepared by Example 1 was placed in an aluminum crucible and tested with a reference disc, nitrogen was purged, and the heating and cooling rate was 10 K / min.

[0066] (2) Polarizing microscope observation: the polarizing texture of the liquid crystal thin film sample was observed using an Olympus polarizing microscope, the liquid crystal sample was uniformly dispersed between the glass slide and the cover glass, and was placed on the Linkam hot stage for heating, and the optical texture change caused by the phase transition of the sample was observed in situ.

[0067] (3) Structure analysis: Anton Paar small-angle X-ray scattering instrument was used for characterization, in which the sample was placed in a quartz capillary with an inner diameter of 1 mm, and the scattering signal of the sample was obtained by in-situ heating test through the built-in hot stage accessory of the instrument, and then the phase structure, including symmetry and phase structure parameters, was determined by peak position calibration.

[0068] Effect Example 2

[0069] The following is the spectral test for squarylium liquid crystal:

[0070] (1) UV-Vis absorption spectrum test: UV-Vis spectrophotometer (UV-3600plus, Shimadzu) was used to test the solution or thin film sample. The test range of square acid cyanine solution was 300 - 900 nm, the sample was dichloromethane solution with concentration of 1 x 10 -5 M -1 , and the test instrument was cuvette; the test range of liquid crystal thin film material was 250 - 1000 nm, and the thin film sample was heated in situ in a custom perforated hot stage for temperature scanning test; the liquid crystal thin film polarization absorption test required additional addition of 400 - 700 nm polarizer in the chamber of the test instrument, and the absorption value at different angles was tested by rotating the polarizer. The above tests all had reference samples, and the baseline elimination was required to eliminate experimental error.

[0071] (2) Fluorescence spectrum test: Edinburgh FLS1000 fluorescence spectrometer was used to test the fluorescence, the concentration of the test sample was 1 x 10 -5 M -1 dichloromethane solution, the maximum excitation wavelength was scanned by full wavelength scanning, and then the maximum excitation wavelength was set as fixed excitation wavelength, the wavelength range was 600 - 1000 nm, the above tests all had reference samples, and the baseline elimination was required to eliminate experimental error.

[0072] Figure 2 is the differential scanning thermodynamic curve of the material of example 1 at the first cooling and the second heating, wherein the smaller phase transition peak represents the transition peak of molten liquid to liquid crystal, and the larger transition peak represents the transition peak of liquid phase-crystal and the transition peak of liquid crystal-crystal.

[0073] Figure 3 is the optical texture under polarizing microscope of the material of example 1, (a) presents typical fan-shaped texture of hexagonal columnar phase; (b) texture diagram after shearing, indicating the fluidity of liquid crystal.

[0074] Figure 4 is the small-angle scattering result of the material of example 1, which is determined as hexagonal columnar phase structure by calibration.

[0075] Figure 5 is the UV-Vis absorption curve and fluorescence emission curve of the material of example 1 in dichloromethane solution (1 x 10 -5 M -1 ), the maximum absorption peak is 642 nm, the excitation wavelength is 514 nm, the maximum emission is 667 nm, the Stokes shift is 25 nm, and the extinction coefficient is 3.3 x 10 5 M -1 cm -1 .

[0076] Figure 6 is the temperature scanning UV-Vis absorption spectrum of the liquid crystal thin film of the material of Example 1 as the temperature decreases (82 - 70 °C), which reflects the change of the molecular packing mode by testing the change of the UV-Vis absorption curve in the liquid-liquid crystal-crystal state.

[0077] Figure 7 is the anisotropic polarized light characterization and UV-Vis absorption characteristics of the liquid crystal thin film of the material of Example 1, under the polarizing microscope, the field of view presents the change of bright-dark-bright as the sample is rotated, which indicates that the sample presents large-scale uniform orientation, and the orientation direction is indicated by the arrow direction. By testing the vertical absorption and parallel absorption near 640 nm, the dichroic ratio of the thin film material is measured to be 1.7. Figure 7

[0078] Figure 8 is the color change of the liquid crystal thin film of the material of Example 1 at different temperatures, which changes from blue (a) 80 °C to purple (b) 72 °C, indicating the thermochromic effect of the sample, which is caused by the significant change of the UV-Vis absorption curve of the liquid crystal thin film.

[0079] The above is only to illustrate the technical idea of the present application, and cannot limit the protection scope of the present application. Any modification made according to the technical idea of the present application on the basis of the technical scheme falls within the protection scope of the claims of the present application.​

Claims

1. A squarylium liquid crystal material, characterized by, A squarylium liquid crystal material having a structure represented by general formula (I): wherein R is selected from a saturated alkyl chain having a length of 12-16.

2. A method of preparing the squarylium liquid crystal material of claim 1, characterized by, The compound having a structure represented by general formula (I) is prepared by condensation reaction of a compound having a structure represented by general formula (II) with squaric acid to obtain a squarylium liquid crystal material having a structure represented by general formula (I), 。 3. The method of claim 2, wherein the squarylium liquid crystal material is prepared by the steps of: The condensation reaction is carried out under anhydrous and anaerobic conditions at a temperature of 120-140 ℃ for 6-12 h.

4. The method for preparing the squaricocyanine liquid crystal material according to claim 2, characterized in that, The molar ratio of the compound having a structure represented by general formula (II) to squaric acid is (2-3):

1.

5. The method for preparing the squaric acid cyanine liquid crystal material according to any one of claims 2 to 4, characterized in that, The compound having a structure represented by general formula (II) is prepared by removing a protective group from a compound having a structure represented by general formula (III) in a mixed solvent of tetrahydrofuran and methanol under a protective atmosphere, 。 6. The method of claim 5, wherein the squarylium liquid crystal material is prepared by the following steps of: The protective group removal reaction is carried out under a protective atmosphere at a temperature of 70-80 ℃ for 6-12 h.

7. The method of claim 5, wherein the squarylium liquid crystal material is prepared by the following steps of: The compound having a structure represented by general formula (III) is prepared by esterification reaction of a compound having a structure represented by general formula (IV), a 3, 4, 5-substituted-benzoic acid derivative, 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride and 4-dimethylaminopyridine under a protective atmosphere, 。 8. The method of claim 7, wherein the squarylium liquid crystal material is synthesized by using m-aminophenol as a precursor. The structural formula of the 3, 4, 5 substituted-benzoic acid derivative is .

9. The method for preparing the squaricocyanine liquid crystal material according to claim 7, characterized in that, The compound having a structure represented by general formula (IV) is synthesized by using m-aminophenol as a precursor, comprising the following steps: Step a, under protective atmosphere, mixing m-aminophenol and methyl acrylate, adding acid and catalyst, after reaction, adding base to neutralize the reaction solution, extracting and separating, concentrating, and chromatographic separation to obtain product A, with the structural formula of ; Step b, under a protective atmosphere, a solution of compound A and an organic sulfonate salt in dichloromethane is added with a protective group reagent, after reaction neutralization, extraction separation, concentrated and chromatographic separation to obtain product B, the structural formula is ; c. under a protective atmosphere and at low temperature, a reducing agent is added to a tetrahydrofuran solution of compound B, which is then transferred to room temperature for reaction, and after the reaction is completed, it is quenched, filtered, concentrated, and the obtained crude product is chromatographed to obtain compound C, i.e. the compound having a structure represented by general formula (IV).

10. Use of the squarylium liquid crystal molecule of claim 1 in the field of thermochromic sensing.

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