Photochromic solution and preparation method thereof

Abstract

The invention provides a photochromic solution and a preparation method thereof, the photochromic solution comprises a negative photochromic compound, a non-photochromic dye, polycaprolactone and an organic solvent, and a proper concentration range is screened. The invention also provides a preparation method of the photochromic solution, which comprises the following steps: firstly dissolving polycaprolactone in an organic solvent to obtain a polycaprolactone solution, and then dissolving the negative photochromic compound and the non-photochromic dye in the polycaprolactone solution. The photochromic solution does not depend on a complex electronic control system, only depends on photochemical response and an environment self-adaptive mechanism to change color, is simple in structure, and reduces the use difficulty and cost.

Description

Technical Field

[0001] The present invention relates to the technical field of photochromic materials, in particular to a photochromic solution and a preparation method thereof. Background Art

[0002] Photochromic materials have long been a hot research topic in materials science and engineering. These materials, capable of undergoing reversible color changes under illumination, hold enormous potential for applications in a variety of key areas, including dynamic camouflage, information encryption, and smart displays. Currently, with technological advancements, demand is growing for photochromic materials that can adapt to diverse light sources, particularly those used in common lighting environments such as red, green, and yellow.

[0003] Traditional photochromic materials mostly rely on electronic drive systems to achieve color changes. For example, electrochromic materials require an external electric field to change their optical properties, while electroluminescent materials require electrical energy to excite them. While these existing solutions achieve photochromic functionality to a certain extent, they fail to effectively address the complexity, high energy consumption, and slow response speed associated with traditional electronically driven color-changing systems.

[0004] Because traditional photochromic materials rely on electronic signals to drive their color changes, their system architecture is complex and difficult to integrate. They involve multiple functional modules, including cameras, signal processing units, sensors, power supplies, and color-changing materials. This compromises the material's flexibility, lightweightness, and wearability, reducing its practicality. They are also highly dependent on electronic devices, which are susceptible to external environmental factors (such as moisture, high temperatures, and electromagnetic interference), resulting in device damage and compromising the stability and reliability of the overall color-changing system. Long-term use also presents challenges with power management, durability, and environmental adaptability. Their high energy consumption and reliance on external energy input to maintain color changes limit their ability to operate autonomously for extended periods, limiting their use in remote, unmanned applications such as military camouflage and field exploration. Their slow response speeds also hinder the rapid color changes of chameleons in nature, while traditional artificial color-changing systems exhibit a lag in color transitions. For example, liquid crystal and electrochromic materials require seconds to minutes to change color, while thermochromic materials take even longer to respond, making them less adaptable to rapidly changing environmental demands. The high cost limits large-scale application. Traditional electronically driven color-changing systems require high-precision visual sensing equipment, signal processing units, and specific manufacturing processes and materials. They also have high energy consumption and high maintenance costs, making it difficult to achieve low-cost industrial production.

[0005] For example, in 2021, Seung Hwan Ko's team proposed an artificial color-changing skin based on thermochromic liquid crystal (TLC) and vertically stacked silver nanowire (Ag NW) heating layers. This solution integrates multiple patterned AgNW heating layers under the TLC layer, and uses the Joule heating effect to precisely control the liquid crystal temperature, thereby achieving multi-layer controllable color change. In combination with a sensing and feedback system, the device can autonomously sense the ambient color and match it. However, this system relies on an external power supply and sensor network, which increases the complexity and cost of actual deployment. In addition, the TLC color change response speed is relatively slow, and it is greatly affected by external temperature fluctuations, requiring precise control of temperature changes. Summary of the Invention

[0006] In response to the above-mentioned problems in the prior art, the present invention provides an adaptive photochromic solution and a preparation method thereof to solve a series of problems existing in photochromic materials. The present invention selects special donor-acceptor Stenhouse adducts (DASAs) as negative photochromic compounds, which undergo linear-cyclic isomerization under visible light irradiation and reversely convert back to a linear state under the action of heat to achieve adaptive color adjustment. Negative photochromic compounds include compounds having structures of formula (I) and formula (II), wherein the structure of formula (I) is

[0007]

[0008] The structure of formula (II) is

[0009]

[0010] The solution of the present invention does not rely on a complex electronic control system, but only relies on photochemical response and environmental adaptive mechanisms to change color, has a simple structure, and reduces the difficulty and cost of use.

[0011] In a dark environment, DASA molecules are in a linear state, and the solution uniformly absorbs light in the visible light region, appearing black. When irradiated by light of a specific wavelength (such as red, green, or yellow), DASA molecules undergo a linear-to-cyclic isomerization reaction. This reaction changes the conjugated structure of the molecule, resulting in a significant decrease in the absorption of light of a specific wavelength, thereby creating an absorption "gap" in the absorption spectrum of the solution. The reaction processes of the compounds of formula (I) and formula (II) are shown in reaction formulas a and b, respectively.

[0012] Reaction formula a is:

[0013]

[0014] Reaction formula b is:

[0015]

[0016] According to the principle of complementary colors, the color of the solution will appear to be consistent with the color of the incident light, achieving an adaptive photochromic effect. Figure 1 The principle of adaptive photochromism based on the principle of complementary colors and negative photochromism is demonstrated. The core of the method is to use the principle of complementary colors and the characteristics of negative photochromic compounds (DASAs) to achieve adaptive switching of material colors to match the ambient light. Complementary colors refer to colors that are opposite to each other on the color wheel. When two complementary colors are mixed in appropriate proportions, white light can be produced. For example, red and cyan, green and magenta, and blue and yellow are complementary colors to each other. In the design of adaptive photochromism (SAP) materials, by selecting suitable DASAs and fixed-phase organic dyes, the material can produce a specific absorption gap under the irradiation of light of different wavelengths, reflecting or transmitting a color consistent with the incident light, thereby achieving color matching. See the attached figure. Figure 2 DASAs undergo linear-cyclic isomerization under visible light irradiation, resulting in the disappearance of the absorption band at a specific wavelength, while the color is restored in the dark mainly through thermal processes. In the dark, the absorption spectra of DASAs and stationary phase organic dyes in the SAP material are evenly distributed throughout the visible light region, forming a uniform absorption band, corresponding to black. When irradiated with light of a specific wavelength, the absorption of DASAs at that wavelength decreases, forming an absorption gap, and the color of the material switches to the same color as the incident light, achieving color matching with the ambient light.

[0017] Taking green light (520nm) as an example, when green light shines on the solution, the DASAs molecules absorb the green light energy and undergo an isomerization reaction, which reduces the absorption of green light and changes the color of the solution from black to green. Similarly, under red light (660nm) or yellow light (590nm), the solution color will change to red or yellow, respectively. When the light source is removed, under the action of heat, the DASAs molecules will gradually return to their initial linear state, and the color of the solution will gradually return to black. This process is highly reversible.

[0018] The present invention combines specific negative photochromic compounds (DASAs) with non-photochromic dyes to achieve adaptive color-changing properties. The negative photochromic compound (Formula (I)) employed in the present invention has an absorption peak in the 500-600nm range and absorbs green light. The negative photochromic compound (Formula (II)) has an absorption peak in the 600-700nm range and absorbs red light. These compounds work synergistically with Formula (I), enabling the solution to cover a wider spectral range.

[0019] The structure of formula (I) is

[0020]

[0021] The structure of formula (II) is

[0022]

[0023] The role of non-photochromic dyes is to provide basic optical properties for the solution and to cooperate with DASAs to achieve a uniform black initial state.

[0024] In a first aspect of the present invention, an adaptive photochromic solution is proposed, characterized in that the photochromic solution comprises negative photochromic compounds (DASAs), non-photochromic dyes, polycaprolactone and an organic solvent, the negative photochromic compounds include compounds having structures of formula (I) and formula (II), and the non-photochromic dyes include compounds having structures of formula (III) and formula (IV).

[0025] In the photochromic solution, the molar concentration of the compound represented by formula (I) is 25-65 μM; the molar concentration of the compound represented by formula (II) is 10-25 μM; the molar concentration of the compound represented by formula (III) is 20-64 μM; the molar concentration of the compound represented by formula (IV) is 10-28 μM; wherein,

[0026] The structure of formula (I) is

[0027]

[0028] The structure of formula (II) is

[0029]

[0030] The structure of formula (III) is

[0031]

[0032] The structure of formula (IV) is

[0033]

[0034] The organic solvent is selected from one or more of dichloromethane, tetrahydrofuran, acetone and acetonitrile, preferably a combination of dichloromethane and tetrahydrofuran.

[0035] In the photochromic solution, the molar concentration of the compound represented by formula (I) is preferably 38 μM, and the molar concentration of the compound represented by formula (II) is preferably 15 μM; the molar concentration of the compound represented by formula (III) is preferably 36 μM, and the molar concentration of the compound represented by formula (IV) is preferably 16 μM.

[0036] A second aspect of the present invention provides a method for preparing a photochromic solution, characterized in that it comprises the following steps:

[0037] (1) dissolving polycaprolactone in an organic solvent to obtain a polycaprolactone solution, wherein the polycaprolactone concentration in the solution is 0.05-0.2 g / mL;

[0038] (2) dissolving the negative photochromic compound and the non-photochromic dye in the solution prepared in step (1), and mixing them to obtain the photochromic solution;

[0039] The negative photochromic compound includes compounds having structures of formula (I) and formula (II), and the non-photochromic dye includes compounds having structures of formula (III) and formula (IV);

[0040] In the photochromic solution, the molar concentration of the compound represented by formula (I) is 25-65 μM; the molar concentration of the compound represented by formula (II) is 10-25 μM; the molar concentration of the compound represented by formula (III) is 20-64 μM; the molar concentration of the compound represented by formula (IV) is 10-28 μM; wherein,

[0041] The structure of formula (I) is

[0042]

[0043] The structure of formula (II) is

[0044]

[0045] The structure of formula (III) is

[0046]

[0047] The structure of formula (IV) is

[0048]

[0049] The organic solvent is selected from one or more of dichloromethane, tetrahydrofuran, acetone and acetonitrile, preferably dichloromethane and tetrahydrofuran.

[0050] The concentration of the polycaprolactone solution in step (1) is preferably 0.1 g / mL.

[0051] Preferably, in the photochromic solution prepared by the present invention, the molar concentration of the compound represented by formula (I) is 38 μM, the molar concentration of the compound represented by formula (II) is 15 μM; the molar concentration of the compound represented by formula (III) is 36 μM, and the molar concentration of the compound represented by formula (IV) is 16 μM.

[0052] The above technical features can be combined in various suitable ways or replaced by equivalent technical features, as long as the purpose of the present invention can be achieved.

[0053] This invention can be used for dynamic camouflage. The adaptive photochromic solution can be applied to military camouflage to create products such as camouflage nets and camouflage coatings. These products can adaptively adjust their color based on changes in ambient light, blending military targets with the background and enhancing camouflage effectiveness. For example, in a jungle environment, the camouflage net can appear green as the reflected light from the surrounding vegetation changes; in a desert environment, it can change to yellow, effectively evading enemy detection.

[0054] This invention can be used for intelligent displays. Utilizing the solution's color-changing properties, it can be applied to information encryption and display technology. By controlling the display and concealment of displayed content using specific wavelengths of light, it enables encrypted storage and transmission of information. For example, in displaying confidential documents, the document's contents can only be displayed under specific red light; under normal light, the document appears black and cannot be easily identified.

[0055] This invention can be used for environmental monitoring. By responding to light of specific wavelengths through color changes, it can be used to monitor light environments. The adaptive photochromic solution can be made into a sensor to monitor changes in the wavelength and intensity of ambient light in real time. For example, in monitoring plant growth environments, the sensor can monitor plant growth and pest and disease conditions based on changes in reflected light from the plant.

[0056] The photochromic solution and preparation method provided by the present invention have at least the following beneficial effects compared with the prior art: (1) No electronic device drive is required, thus avoiding the problem of electronic devices being affected by external environmental factors, enhancing the stability and reliability of the color-changing system, reducing power consumption, achieving long-term autonomous operation, and expanding its application range in remote, long-term unattended scenarios. (2) It has a fast response speed and can quickly adapt to changes in ambient light, meeting the application requirements of dynamic camouflage, intelligent display, and other applications with high requirements for fast response. (3) The preparation process is relatively simple, low-cost, and easy to achieve industrial large-scale production, promoting the widespread application of adaptive photochromic materials, and providing more efficient and practical solutions for fields such as dynamic camouflage, information encryption, and intelligent display. BRIEF DESCRIPTION OF THE DRAWINGS

[0057] The present invention will be described in more detail below based on embodiments and with reference to the accompanying drawings, wherein:

[0058] Figure 1 Schematic diagram of the principle of realizing adaptive photochromism based on the complementary color principle and negative photochromism of the present invention;

[0059] Figure 2 Molecular selection for adaptive photochromic solutions;

[0060] Figure 3 UV-visible absorption spectra of the adaptive photochromic solution under 520 nm green light irradiation at different light source distances;

[0061] Figure 4 UV-visible absorption spectra of the adaptive photochromic solution under 590 nm green light irradiation at different light source distances;

[0062] Figure 5 UV-visible absorption spectra of the adaptive photochromic solution under 620 nm green light irradiation at different light source distances;

[0063] Figure 6 UV-visible absorption spectra of the adaptive photochromic solution under 660 nm green light irradiation at different light source distances;

[0064] Figure 7 Photos of the adaptive photochromic solution under different light irradiation. DETAILED DESCRIPTION

[0065] The present invention is demonstrated by the following examples, but the scope of protection of the present invention is not limited to the examples.

[0066] Example 1 Preparation of negative photochromic compounds (DASAs) Formula (I)

[0067] 2,2-dimethyl-1,3-dioxane-4,6-dione (1.44 g, 10 mmol) was dissolved in 30 mL of distilled water under stirring. After slowly adding 2-furaldehyde (0.96 g, 10 mmol), the solution was heated to 40 ° C and maintained for 2 h. The yellow solid formed was collected by vacuum filtration and then washed with distilled water. The solid was dissolved in 30 mL of dichloromethane (DCM) and washed twice with 30 mL of saturated sodium bisulfite aqueous solution and 30 mL of saturated sodium chloride aqueous solution. The organic layer was dried over sodium sulfate and purified by column chromatography to obtain 2.11 g of yellow product. The yellow product (1.11 g, 5 mmol) was further dissolved in 50 mL of DCM, followed by the slow addition of methylaniline (0.535 g, 5 mmol). The reaction was stirred at 40 ° C and monitored by thin layer chromatography (TLC). The mixture was concentrated by rotary evaporation and further purified by column chromatography to obtain 0.89 g of the compound of formula (I) as a dark purple solid (yield: 50%).

[0068]

[0069] Example 2 Preparation of negative photochromic compounds (DASAs) Formula (II)

[0070] 1-Phenyl-3-(trifluoromethyl)-1H-pyrazol-5(4H)-one (2.28 g, 10 mmol) and 2-furaldehyde (0.96 g, 10 mmol) were dissolved in 30 mL of dichloromethane (DCM) and stirred at 40° C. for 2 h. The mixture was concentrated by rotary evaporation, washed with water, and further purified by column chromatography to give 2.45 g of the intermediate product as a dark red solid (yield: 80%).

[0071] The intermediate product (1.53 g, 5 mmol) was dissolved in methanol at 20°C, followed by the slow addition of 5-methoxy-2,3-dihydro-1H-indole (0.745 g, 5 mmol). A green crystalline solid slowly precipitated from the dark blue solution, which was filtered and washed several times with cold methanol. The solid was collected and dried overnight to yield 1.47 g of compound of formula (II) (yield: 70%).

[0072]

[0073] Example 3 Preparation of non-photochromic dye compound of formula (III)

[0074] First, 780 mg (5 mmol) of solid barbituric acid was weighed and added to a 50 mL round-bottom flask. 30 mL of deionized water was then added and ultrasonicated for 10 minutes to fully dissolve it. Then, 415 μL of furfural was slowly added dropwise to the solution using a pipette and stirred at 30°C for 2 hours. The reaction system turned yellow and solid precipitated. The reaction was further monitored by TLC. After the reaction was completed, suction was filtered using a Buchner funnel. The yellow solid obtained by suction was transferred to a beaker and dissolved in 40 mL of dichloromethane. 40 mL of saturated NaHSO3 solution and 40 mL of deionized water were added to the solution for extraction, resulting in a bright yellow solution. Solid anhydrous magnesium sulfate (MgSO4) was added to the solution to remove water molecules from the solution. The solution was then filtered again and purified by column chromatography using dichloromethane as the eluent. The solvent was removed by vacuum concentration to obtain a light yellow powder, which was the compound of formula (III) with a yield of 85%.

[0075]

[0076] Example 4 Preparation of non-photochromic dye compound of formula (IV)

[0077] 1.44 g of Michaelis' acid and 1.49 g (10 mmol) of p-dimethylaminobenzaldehyde were dissolved in 25 mL of ethanol. The solution was then heated at 90°C with vigorous stirring for 4 h. The red precipitate was collected by filtration and dried overnight to yield 2.59 g of a red solid, which is the compound of formula (IV) (yield: 90%).

[0078]

[0079] Example 5 Preparation of negative photochromic solution

[0080] Polycaprolactone is dissolved in an organic solvent to obtain a polycaprolactone solution, wherein the concentration of polycaprolactone in the solution is 0.1 g / mL.

[0081] The compound of formula (I), the compound of formula (II), the compound of formula (III), and the compound of formula (IV) were weighed and dissolved in the solution prepared in step (1) to prepare a negative photochromic solution. In the prepared photochromic solution, the molar concentration of the compound of formula (I) was 38 μM, the molar concentration of the compound of formula (II) was 15 μM; the molar concentration of the compound of formula (III) was 36 μM, and the molar concentration of the compound of formula (IV) was 16 μM. During the dissolution process, a magnetic stirrer was used to continuously stir at a speed of 500-600 rpm for 30 minutes to ensure that the components were fully dispersed and evenly mixed.

[0082] Example 6-10 Screening of the concentration of the compound of formula (I) in the negative photochromic solution

[0083] The photochromic solutions of Examples 6-10 were prepared according to the method described in Example 5. In the negative photochromic solutions prepared in Examples 6-10, the molar concentrations of the compounds of Formula (I) to Formula (IV) in the solutions are shown in Table 1 below.

[0084] Table 1 Concentrations of compounds of formula (I) to (IV) in negative photochromic solutions

[0085] Example 6 Example 7 Example 8 Example 9 Example 10 Formula (I) 19 μM 25 μM 38μM 65μM 76μM Formula (II) 15 μM 15 μM 15 μM <![CDATA[15 μ M]]> <![CDATA[15 μ M]]> Formula (III) 36μM 36μM 36μM <![CDATA[36 μ M]]> <![CDATA[36 μ M]]> Formula (IV) 16μM 16μM 16μM <![CDATA[16 μ M]]> <![CDATA[16 μ M]]>

[0086] Examples 11-15 Screening of the concentration of the compound of formula (II) in the negative photochromic solution

[0087] The photochromic solutions of Examples 11-15 were prepared according to the method described in Example 5. In the negative photochromic solutions prepared in Examples 11-15, the molar concentrations of the compounds of Formula (I) to Formula (IV) in the solutions were as shown in Table 2 below.

[0088] Table 2 Concentrations of compounds of formula (I) to (IV) in negative photochromic solutions

[0089] Example 11 Example 12 Example 13 Example 14 Example 15 Formula (I) 38μM 38μM 38μM 38μM 38μM Formula (II) 7.5 μM 10 μM 15 μM 25 μM 30 μM Formula (III) 36μM 36μM 36μM 36μM 36μM Formula (IV) 16μM 16μM 16μM 16μM 16μM

[0090] Examples 16-20 Screening of the concentration of the compound of formula (III) in the negative photochromic solution

[0091] The photochromic solutions of Examples 16-20 were prepared according to the method described in Example 5. In the negative photochromic solutions prepared in Examples 16-20, the molar concentrations of the compounds of Formula (I) to Formula (IV) in the solutions are shown in Table 3 below.

[0092] Table 3 Concentrations of compounds of formula (I) to (IV) in negative photochromic solutions

[0093] Example 16 Example 17 Example 18 Example 19 Example 20 Formula (I) 38μM 38μM 38μM 38μM 38μM Formula (II) 15 μM 15 μM 15 μM 15 μM 15 μM Formula (III) 18 μM 20 μM 36μM 64 μM 72μM Formula (IV) 16μM 16 μM 16μM 16μM 16 μM

[0094] Examples 21-25 Screening of the concentration of the compound of formula (IV) in the negative photochromic solution

[0095] The photochromic solutions of Examples 21-25 were prepared according to the method described in Example 5. In the negative photochromic solutions prepared in Examples 21-25, the molar concentrations of the compounds of Formula (I) to Formula (IV) in the solutions are shown in Table 4 below.

[0096] Table 4 Concentrations of compounds of formula (I) to (IV) in negative photochromic solutions

[0097] Example 21 Example 22 Example 23 Example 24 Example 25 Formula (I) 38μM 38 μM 38 μM 38μM 38μM Formula (II) 15 μM 15 μM 15 μM 15 μM 15 μM Formula (III) 36μM 36μM 36μM 36μM 36μM Formula (IV) 8μM 10 μM 16μM 28 μM 32μM

[0098] Example 22 Color Change Performance Detection of Negative Photochromic Solution

[0099] The negative photochromic solutions prepared in Examples 5-25 were tested under LED light sources at 520 nm (green), 590 nm (yellow), 620 nm (orange), and 660 nm (red). The light intensity was 60 mW / cm², and the distances between the sample and the light source were maintained at 10 cm, 30 cm, 50 cm, and 100 cm. The absorption spectra of the solutions before and after illumination were recorded using a UV-visible spectrophotometer, using a cuvette with a 0.1 cm optical path length and controlling the initial absorbance to 0.6.

[0100] The UV-visible absorption spectra and appearance color of the negative photochromic solution prepared in Example 5 under 520nm, 590nm, 620nm and 660nm green light irradiation are shown in the attached figure. Figure 3-Figure 7 .

[0101] Before irradiation, the solution is initially black and uniformly absorbs light in the visible light region. When irradiated with 520nm, 590nm, and 660nm light, the solution color switches to green, yellow, and red, respectively. Under 520nm light, the absorption spectrum of the solution forms an absorption gap at 500-600nm, see attached. Figure 3 , the color is green, see attached Figure 7 a; Under 590nm light, the absorption gap is located at 550-650nm, see attached Figure 4 , the solution is yellow, see attached Figure 7 b; Under 620nm light, the absorption gap is mainly at 600-700nm, see attached Figure 5 , the solution color turns orange, see attached Figure 7 c. Under 660nm light, the absorption gap is mainly at 600-700nm, see attached Figure 6 , the solution color turns red, see attached Figure 7 d.

[0102] The experimental results of the negative photochromic solutions prepared in Examples 6 to 25 are shown in Table 5 below.

[0103] Table 5 Color change performance test results of photochromic solution

[0104] Example Black state Photochromic properties 6 Black state misalignment good 7 Normal black good 8 Normal black Excellent 9 Normal black good 10 Normal black Limited color change performance 11 Black state misalignment good 12 Normal black good 13 Normal black Excellent 14 Normal black good 15 Normal black Limited color change performance 16 Black state misalignment good 17 Normal black good 18 Normal black Excellent 19 Normal black good 20 Normal black Limited color change performance 21 Black state misalignment good 22 Normal black good 23 Normal black Excellent 24 Normal black good 25 Normal black Limited color change performance

[0105] Among them, the black state misalignment and the color change performance limited state are embodiments that do not meet the requirements of the present invention, and the good and excellent color change performance are both embodiments that meet the requirements of the present invention, among which the excellent is the embodiment with the best color change performance.

[0106] Black state misalignment means the solution displays a color other than black. Limited color change performance means the solution contains too much negative photochromic compound, preventing the solution from changing color. Excellent color change performance means red color change times are within 60 seconds, green color change times are within 90 seconds, and yellow color change times are within 35 seconds. Good color change performance means red color change times are between 60-75 seconds, green color change times are between 90-100 seconds, and yellow color change times are between 35-45 seconds.

[0107] Although the present invention is described herein with reference to specific embodiments, it should be understood that these embodiments are merely illustrative of the principles and applications of the invention. It should be understood that many modifications may be made to the illustrative embodiments, and that other arrangements may be devised, without departing from the spirit and scope of the invention as defined by the appended claims. It should be understood that the various dependent claims and features described herein may be combined in ways other than those described in the original claims. It should also be understood that features described in conjunction with individual embodiments may be employed in conjunction with other described embodiments.

Claims

1. A photochromic solution, characterized in that: The photochromic solution comprises a negative photochromic compound, a non-photochromic dye, polycaprolactone and an organic solvent, wherein the negative photochromic compound comprises a compound having structures of formula (I) and formula (II), and the non-photochromic dye comprises a compound having structures of formula (III) and formula (IV); In the photochromic solution, the molar concentration of the compound represented by formula (I) is 25-65 μM; the molar concentration of the compound represented by formula (II) is 10-25 μM; the molar concentration of the compound represented by formula (III) is 20-64 μM; the molar concentration of the compound represented by formula (IV) is 10-28 μM; wherein, The structure of formula (I) is The structure of formula (II) is The structure of formula (III) is The structure of formula (IV) is 2. The photochromic solution according to claim 1, characterized in that The organic solvent is selected from one or more of dichloromethane, tetrahydrofuran, acetone and acetonitrile.

3. The photochromic solution according to claim 2, characterized in that The organic solvent is a combination of dichloromethane and tetrahydrofuran.

4. The photochromic solution according to any one of claims 1 to 3, characterized in that In the photochromic solution, the molar concentration of the compound represented by formula (I) is 38 μM, and the molar concentration of the compound represented by formula (II) is 15 μM; the molar concentration of the compound represented by formula (III) is 36 μM, and the molar concentration of the compound represented by formula (IV) is 16 μM.

5. A method for preparing a photochromic solution, characterized in that: The following steps are involved: (1) dissolving polycaprolactone in an organic solvent to obtain a polycaprolactone solution, wherein the polycaprolactone concentration in the solution is 0.05-0.2 g / mL; (2) dissolving the negative photochromic compound and the non-photochromic dye in the solution prepared in step (1), and mixing them to obtain the photochromic solution; The negative photochromic compound includes compounds having structures of formula (I) and formula (II), and the non-photochromic dye includes compounds having structures of formula (III) and formula (IV); In the photochromic solution, the molar concentration of the compound represented by formula (I) is 25-65 μM; the molar concentration of the compound represented by formula (II) is 10-25 μM; the molar concentration of the compound represented by formula (III) is 20-64 μM; the molar concentration of the compound represented by formula (IV) is 10-28 μM; wherein, The structure of formula (I) is The structure of formula (II) is The structure of formula (III) is The structure of formula (IV) is 6. The method for preparing a photochromic solution according to claim 5, wherein: The organic solvent is selected from one or more of dichloromethane, tetrahydrofuran, acetone and acetonitrile.

7. The method for preparing a photochromic solution according to claim 6, wherein: The organic solvents are dichloromethane and tetrahydrofuran.

8. The method for preparing a photochromic solution according to any one of claims 5 to 7, characterized in that: The concentration of the polycaprolactone solution in step (1) is 0.1 g / mL.

9. The method for preparing a photochromic solution according to any one of claims 5 to 7, characterized in that: In the photochromic solution, the molar concentration of the compound represented by formula (I) is 38 μM, and the molar concentration of the compound represented by formula (II) is 15 μM; The molar concentration of the compound represented by formula (III) is 36 μM, and the molar concentration of the compound represented by formula (IV) is 16 μM.

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