Research ArticleMATERIALS SCIENCE

On-demand regulation of photochromic behavior through various counterions for high-level security printing

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Science Advances  17 Apr 2020:
Vol. 6, no. 16, eaaz2386
DOI: 10.1126/sciadv.aaz2386

Abstract

Materials exhibiting reversible changes in optical properties upon light irradiation have shown great potential in diverse optoelectronic areas. In particular, the modulation of photochromic behavior on demand for such materials is of fundamental importance, but it remains a formidable challenge. Here, we report a facile and effective strategy to engineer controllable photochromic properties by varying the counterions in a series of zinc complexes consisting of a spirolactam-based photochromic ligand. Colorability and coloration rate can be finely tuned by conveniently changing their counterions. Through utilization of the reversible feature of the metal-ligand coordination bond between Zn2+ and the spirolactam-based ligand, dynamic manipulation of photochromic behavior was achieved. Furthermore, we demonstrated the practical applications of the tunable photochromic properties for these complexes by creating photochromic films and developing multilevel security printing. These findings show opportunities for the development of smart materials with dynamically controllable responsive behavior in advanced optoelectronic applications.

INTRODUCTION

Photochromic molecules have emerged as critically notable materials for advanced photonic applications, including fluorescence imaging (13), smart lenses (48), optical data storage (914), and anticounterfeiting (15). Current efforts in this field are focused on developing new photochromic molecules and controlling the photochromic behavior of these materials to meet the criteria for different optoelectronic applications. For instance, materials that respond to red or near-infrared light in the bio-optical window (700 to 900 nm) have been exploited for biological applications (16). To meet the criteria for rewritable information recording, photochromic materials with superior fatigue resistance and reduced bleaching rate have been developed (12). However, less attention has been paid on the modulation of colorability (the colorability or photocolorability refers to the ratio between absorbances of the photostationary state and the initial state, A/A0) and coloration rates of photochromic materials. Photochromic molecule with fast response to light stimulus is desirable for application in smart lenses (17). Besides, in data security protection area, the reduced sensitivity to photoirradiation is favorable to keep the information safe (18). In particular, the ability to tune the coloration rate on demand can significantly improve the security level of the recorded texts, because different information would appear using various irradiation times. To date, progress has been made in the regulation of colorability and coloration rates by incorporation of electron donor or acceptor substitutions in photochromic molecules (17, 18). Nevertheless, the prepared photochromic materials generally have constant colorability and coloration rates under the same light irradiation condition (1925). Besides, the regulation of photochromic behavior often requires complex chemical synthesis and tedious purification processes, making it inconvenient and inefficient. Therefore, the achievement of more accessible control of photochromic behavior is highly desirable.

It has been reported that spirolactam-containing dyes show photoinduced ring-opening reactions that result in notable changes in absorption or photoluminescence (PL) properties (2629). Nevertheless, the stabilities of ring-open isomers in this class of molecules are extremely low because the negatively charged nitrogen atoms would destabilize the ring-open form (30). Metal complexation to spirolactam-based ligands can reduce the negative charge density of the nitrogen atom via inductive effects and thus substantially stabilize the ring-open isomers (31, 32). Therefore, the variations of the charge density around the spiro-C-N moiety would have remarkable influences on their photochromic behavior. Considering that the counterions have direct effect on the charge density of metal center (33, 34), we envisioned that changing the counterions around the metal center might be an effective strategy to control the photochromic behavior of spirolactam-containing complexes. Besides, the reversible metal-ligand coordination between metal ions and spirolactam-containing components makes the dynamic manipulation of colorability and coloration rates possible, which can promote the tunable photochromic behavior from static to dynamic.

In this work, crystal violet lactone salicylaldehyde hydrazine (CVLSH)–based zinc complexes were selected as an example to demonstrate the controllable photochromic behavior. By changing the counterions of the zinc complexes (CVLSH-Zn-X, X = CH3COO, CF3SO3, NO3, Cl, or Br), the colorability and coloration rates of these photochromic molecules can be finely controlled on demand. Based on the reversible interaction and dissociation of the metal-ligand coordination bonds between Zn2+ and CVLSH ligands, the possibility of dynamic control of photochromic behavior has been demonstrated. Furthermore, smart photochromic films were prepared, and multilevel security printing was successfully achieved by taking advantage of the controllable photochromic properties of these zinc complexes. Our strategy has paved the way for constructing light-responsive materials whose photochromic behavior can be controlled on demand.

RESULTS

Synthesis and characterization

CVLSH was synthesized in two steps: First, crystal violet lactone (CVL) hydrazide was prepared with high yield by refluxing an ethanol solution of CVL and N2H4 for 2 hours. Next, a reaction between 2-hydroxy-4-methoxybenzaldehyde and CVL hydrazide was completed according to previous literature (28). CVLSH-Zn-X was prepared by the addition of 5 eq zinc salt to CVLSH in CH2Cl2. According to the Benesi-Hildebrand equation of [1/1(AA0)], the absorbance of CVLSH changes at 414 nm upon addition of different zinc salts and the results were recorded (fig. S1). The results indicate that the stoichiometric ratio between zinc salts and CVLSH is 1:1. 1H NMR spectroscopy, 13C NMR spectroscopy, and matrix-assisted laser desorption/ionization–time-of-flight (MALDI-TOF) mass spectrometry were used to characterize the obtained products.

Photochromic behavior

First, the photochromic behavior of CVLSH and the prepared zinc complexes was studied. The zinc complexes, not CVLSH alone, exhibited reversible photochromic behavior in CH2Cl2 solution. For example, the colorless solution of CVLSH-Zn-Br gradually turned to deep blue upon continuous 365-nm irradiation, as shown in Fig. 1. The absorption bands of CVLSH-Zn-Br were below 430 nm, and a maximum absorption peak centered at 607 nm emerged after the solution was exposed to 365-nm irradiation (Fig. 1B). In addition, common light sources and even sunlight could lead to the formation of the ring-open state. The absorption peak gradually disappeared in the absence of a light source, and it immediately decreased when the complex was heated. Next, the investigations on the effects of different solvents on the photochromic reaction of CVLSH-Zn-Br were performed. CVLSH-Zn-Br exhibited no photochromic property in high-polar solvents such as CH3CN, N,N′-dimethylformamide, and dimethyl sulfoxide. In contrast, the photochromism was valid in solvents with low polarity, including toluene, CH2Cl2, and CHCl3 (fig. S2). In addition to Zn2+, other metal ions were also used to form complex with CVLSH to investigate their influences on the photochromic properties. As shown in fig. S3, when Cu2+, Co2+, Mn2+, Sn2+, and Ni2+ were added to the solution of CVLSH in CH2Cl2, marked changes in absorption spectra can be observed after UV irradiation.

Fig. 1 Photochromic property of CVLSH-Zn-X.

(A) Reversible photo-induced ring-opening and ring-closing reaction process for CVLSH-Zn-X. (B) Visible color changes (inset) and the corresponding absorption spectral changes of CVLSH-Zn-Br in CH2Cl2 (1 × 10−5 M). λex = 365 nm.

It was demonstrated that ultraviolet (UV) irradiation promoted isomerization of the salicylaldehyde group from the enol form to the keto form (Fig. 1A) (35), which resulted in an open-ringed isomer of the CVLSH component because the charge separation was preferred in the excited state. Zinc ion complexation to the CVLSH moiety can reduce the negative charge density of nitrogen atom via inductive effects, and thus, it substantially stabilized the open-ringed isomer. Considering that the different counterions have distinct effects on the charge density of the metal center, this may lead to different photochromic properties. Therefore, the influences of various counterions on photochromic properties of CVLSH-Zn-X (X = Br, Cl, NO3, CF3SO3, and CH3COO) were investigated. Figure 2D shows the photocolorability (A/A0) of these CVLSH-Zn-X complexes. The results reveal that the photocolorability of those complexes is counterion dependent. The zinc complexes show remarkable increase in photochromic activity with the decrease in basicity of different counter anions. The photocolorability for CVLSH-Zn-Br is as high as 112.3, while the colorability of CVLSH-Zn-CH3COO is about 1.0.

Fig. 2 Effect of different counterions on photochromic properties of CVLSH-Zn-X.

(A) Schematic illustration of the effect of different counterions on the coloration rates of CVLSH-Zn-X. (B) Changes in absorbance at maximum absorption wavelengths of CVLSH-Zn-X under the same conditions. (C) Calculated coloration rates of complexes with different counterions according to the first-order reaction kinetics. (D) Photochromic properties and kinetic parameters of CVLSH-Zn-X.

In addition, photochemical first-order rate constants (kUV = rate constant of UV-induced ring opening, kUVT = thermal backward reaction following ring opening) were determined by monitoring the variations in absorbance at the maximum absorption wavelengths and then fitting with a monoexponential function. The kinetic parameters were significantly different from the complexes containing various counterions (Fig. 2). A linear relationship was found in Fig. 2C, which shows the calculated ln[(AAt)/(AA0)] values as a function of irradiation time for all photochromic complexes. The calculated reaction rate constants (kUV) of these photochromic complexes during ring-opening reactions are in the order of CVLSH-Zn-Br (0.01887 s−1) > CVLSH-Zn-Cl (0.01093 s−1) > CVLSH-Zn-NO3 (0.00875 s−1) > CVLSH-Zn-CF3SO3 (0.00208 s−1) > CVLSH-Zn-CH3COO (approximately 0). The changes associated with photoisomerization of the zinc complexes were related to the nature of counterions, suggesting an increase in the energy required to stabilize the open ring form as the increase of basicity of counterions. The color switching performance of CVLSH-Zn-Br at different concentrations in CH2Cl2 was also studied. As shown in fig. S4, the differences in kUV of CVLSH-Zn-Br at different concentrations (from 1 × 10−6 M to 2 × 10−5 M) during ring-opening reactions are not notable, suggesting that the color switching for CVLSH-Zn-Br is concentration independent. In addition, with the power density enhancement of the UV irradiation (from 4.08 to 10.88 mW/cm2), the rate constants of all complexes subsequently increased following the same order, indicating the insignificant effect of irradiation power on the order of rate constants for those complexes (fig. S5). Conversely, Fig. 2D exhibited that the thermal relaxation constants (kUVT) at room temperature are noticeably decreased along the order of Br → Cl → NO3 → CF3SO3 → CH3COO (figs. S6 and S7).

The observed trends with counterion basicity likely reflect inductive effects. The counterion effects on the kinetic and thermodynamic parameters of spirolactam ring opening and closing are generally interpreted in terms of electronic effects (31, 32). Because the ring-open state has a remarkable charge separation character, it is reasonable to believe that inductive effects of the metal center would result in the stabilization of the open-ringed isomer (26). Stabilization of the ring-open state was therefore supported by the withdrawal of electron density from the spiro-C-N moiety of the molecule. Metal complexation was expected to decrease the negative charge density of the spiro-C-N moiety through inductive effects. As the basicity of the counter anions increased, the acidity of the zinc center decreased, resulting in the increase of bonding constants between zinc salts and the CVLSH derivative from 2.58 × 102 to 7.08 × 104 (fig. S1). An increase in inductive effects was expected to result in a stabilization of the open-ringed state via the decreased electron density on the spiro-moiety.

To understand the differences in the photochromic materials during ring-closing and ring-opening isomerization reactions triggered by light irradiation, computational approaches were used to further study the ring-opening reaction. Analysis of the calculated activation energy for different photochromic complexes provided a qualitative view of the ring-opening reaction process. As shown in Fig. 3 and fig. S8, although CVLSH-Zn-X required a similar activation energy (from 0.4 to 2.7 kcal/mol) for C─N bond cleavage, the required activation energy for H transfer process (a crucial step to stabilize the ring-open state) is distinctly different (Fig. 3B). The results showed that CVLSH-Zn-Br and CVLSH-Zn-Cl required the lower activation energy (50.6 and 49.6 kcal/mol) for H transfer process. When the basicity of counterions increased, activation energy was substantially enhanced from 49.6 to 58.0 kcal/mol (36). These calculated results are close to the experimentally obtained photochromic rate changes in the solution, which further demonstrated that counterions play a dominant role in the control of photochromic behavior.

Fig. 3 Theoretical calculation of the open loop process.

(A) The qualitative view of the ring-opening reaction process of CVLSH-Zn-X. (B) The calculated activation energy (Ea and Eb) of the ring-opening reaction. All structures were optimized with density functional theory functional B3lYP-D3 and 6-31+G (d, p) basis set by Gaussian 09. The free energies of the products and reactants were calculated by the intrinsic reaction coordinate calculations.

Dynamic control of coloration rate

Demonstrating that altering the counterions of CVLSH-Zn-X is an effective way to control the rate of ring-opening reaction, efforts were concentrated on achieving dynamic manipulation of the photochromic rates. Figure 4A illustrates the mechanism for dynamic manipulation of coloration rates for CVLSH-Zn-X, which is on the basis of the reversible coordination bonds between Zn2+ and CVLSH derivatives. The fluoride ion had strong binding affinity with the zinc ion, and it had been demonstrated that the coordination bonds between Zn2+ and CVLSH derivatives would be dissociated upon addition of fluoride ions (37). Therefore, the CVLSH can coordinate with other zinc salts to form new photochromic complexes with distinct coloration rates (Fig. 4B). For example, CVLSH-Zn-CF3SO3 first had a coloration rate of 0.00216 s−1 under UV irradiation, but ZnF2 precipitate was formed when it was treated with tetrabutylammonium fluoride. Subsequently, ZnCl2 was used to coordinate CVSLH ligand and form CVLSH-Zn-Cl, with a distinct coloration rate (0.01122 s−1) (Fig. 4B). The obtained reaction rate constants are similar to the results from Fig. 2C, indicating that free anions of former CVLSH-Zn-X have insignificant effects on the coloration rates of new ones.

Fig. 4 Dynamically controlling coloration rates.

(A) The dynamic regulation of coloration rates for CVLSH-Zn-X. (B) Calculated coloration rate of CVLSH-Zn-X during the dynamic manipulation process. (C) Schematic illustration of multilevel information encryption based on different coloration rates of CVLSH-Zn-Br, CVLSH-Zn-NO3, and CVLSH-Zn-CF3SO3. (D) Dynamically tuning the coloration rates of the binary codes for security data recording.

The remarkable tunability in photochromic rates of CVLSH-Zn-X complexes suggested great potential in multilevel information encryption. As a proof of concept, a two-color microarray information storage pattern was constructed according to ASCII binary code, with colorless and blue color representing “0” and “1,” respectively. Initially, CVLSH with different zinc salts were colorless in the cuvettes; thus, the “00000000” is translated into null character according to the ASCII binary codes. Next, when these cuvettes were exposed to the UV light for 10 s, the colors of the cells filled with CVLSH-Zn-Br were rapidly changed from colorless to blue, while the colors of the other cuvettes remained constant. Therefore, the information was decoded into “AAA” as shown in Fig. 4D. Then, these cuvettes were irradiated for another 50 s, CVLSH-Zn-NO3 was then altered from its closed ring to open ring form, and thus, the information changed to “IAM” based on the binary codes displayed in Fig. 4D. When the irradiation times were extended to 200 s, the new information of “KAO” was decrypted. Different information was decrypted upon various UV irradiation times for these microarray patterns; thus, only an authorized individual who knew the correct encryption algorithm and exact irradiation time was able to access the confidential information. These microarray patterns could be treated with the fluoride ion to dissociate the metal-ligand coordination bond between Zn2+ and CVLSH. Then, different zinc salts can be used to coordinate with CVLSH again to generate different photochromic complexes with distinct coloration rates. As a result, the coloration rates of those binary codes can be significantly altered and the binary code patterns can be changed under different UV irradiation times. Therefore, because of these remarkably dynamic tunable photochromic properties, the security of this binary coding system has been promoted to a higher level.

Smart photochromic films

In addition, the advantage of this approach for controlling the photochromic behavior of these zinc complexes in smart photochromic films has been demonstrated. The introduction of counterions with strong basicity resulted in a remarkable decrease in the photocolorability. Therefore, smart photochromic films with various light transmittances were easily obtained on demand by using different zinc salts to coordinate with CVLSH. The photochromic properties of the photochromic films were similar to those in solution as exhibited in Fig. 1. As shown in Fig. 5, with the exposure of the fabricated smart photochromic films to solar simulator light (350 to 1100 nm, AM 1.5G), the polymer film containing photochromic materials with different counterions evidently varied from colorless to a light blue or even a deep blue. The light transmittances of the fabricated photochromic films were determined to be 77, 58, 45, and 20%, corresponding to the CVLSH-Zn-CF3SO3, CVLSH-Zn-NO3, CVLSH-Zn-Cl, and CVLSH-Zn-Br complexes, respectively. This feature of on-demand control of transmittances of photochromic films is attractive for application in smart glasses or windows.

Fig. 5 Smart photochromic films.

(A) Light transmittance of glass pieces coated with CVLSH-Zn-X under light irradiation for 3 min. (B) The poly(methyl methacrylate) (PMMA) films vary from pale yellow to light blue or deep blue under solar simulator light irradiation.

In addition, rewritable information recording experiments have been carried out on the paper coated with polyethylene glycol–polypropylene glycol–polyethylene glycol (PEG-PPG-PEG) film containing CVLSH-Zn-Br. As a demonstration, we printed patterns in the film by UV light irradiation through a preproduced photomask. After 365-nm UV irradiation for 2 min, the exposed areas turned to blue, while the unexposed regions remained colorless, replicating images from the photomask to the film. As shown in fig. S9, a dinosaur with high legibility was achieved on the paper. The prints can be erased completely by heating at 70°C in air for 1 min. Then, various complicated patterns can also be printed on the paper with good resolution, as shown in the examples in fig. S9.

Security paper

By using the zinc salts to manipulate photochromic properties of CVLSH-Zn-X, security paper with practical usage has been developed in this work. As illustrated in Fig. 6, the proposed security paper consisted of three layers. First, PEG-PPG-PEG is coated on parchment paper as the passivation layer. Second, CVLSH as the data recording medium was coated onto the first layer. Last, PEG-PPG-PEG was coated on the top as the protection layer. Next, the fabricated paper was used to record confidential information with a commercially available inkjet printer and zinc salt aqueous solutions as ink. Initially, the printed texts were invisible to the naked eye under ambient light. Upon 365-nm UV irradiation for 1 min, the highly legible concealed information clearly appeared. The information was hidden again rapidly by heating the paper at 70°C for 1 min; alternatively, the information gradually disappeared after 1 hour under ambient conditions. A detailed variation of the reflective UV-visible spectrum after the completion of the encryption and decryption processes was recorded (fig. S10). As shown in Fig. 6C, these cycles can be repeated at least 15 times without notable loss in the contrast, suggesting the potential of the proposed security paper for information encryption and decryption.

Fig. 6 Multilevel security printing on paper substrate.

(A) Schematic illustration of four-layer structure used to create the security paper based on CVLSH. (B) Photograph of a paragraph of text printed on security paper using a commercially available inkjet printer with a cartridge filled with ZnBr2 aqueous solution as the ink. The information can be clearly seen after UV irradiation for 1 min. Scale bars, 3 cm. (C) A plot of the reflectivity at 610 nm versus the number of cycles as the security paper is cycled through UV irradiation and heating. (D) Multilevel security information printing was realized by using various zinc salts as the inks, distinct information can be read by exposing the security paper under UV irradiation for different times. Photo Credit: Yaxin Yu, Nanjing University of Posts and Telecommunications.

Furthermore, higher security information protection was realized by using different zinc salts as the inks. For example, ZnBr2, Zn(NO3)2, and Zn(CF3SO3)2 in aqueous solutions were used as the inks for security information recording. As shown in Fig. 6D, the recorded information cannot be seen under natural visible light. However, when the paper was exposed to the UV light for 10 s, the Chinese characters of “Nanjing” were clearly revealed. After irradiation for more than 30 s, two other Chinese characters of “University” have appeared; thus, the decrypted information changed to “Nanjing University.” Next, when the UV irradiation time was extended to 200 s, new Chinese characters of “Posts and Telecommunications” can be observed by the naked eye, and thus, the revealed information has become “Nanjing University of Posts and Telecommunications.” Different information was read upon the UV irradiation of this security paper using various irradiation times. Thus, only an authorized individual who knows the correct encryption algorithm and exact irradiation times can access the confidential information, which significantly improved the level of security for the concealed information.

DISCUSSION

In summary, an effective strategy to achieve controllable photochromic behavior of spirolactam-containing zinc complexes by changing counterions was presented. Important insights into the relationship between counter anions and spirolactam-containing zinc complexes with their photochromic properties have been obtained, which allowed for the controllable photocolorability and coloration rates. The photocolorability was increased and the reaction rate was accelerated by decreasing the basicity of the counterions. Moreover, after gaining insights into the reversible metal-ligand coordination bonds between Zn2+ and CVLSH, dynamic manipulation of photochromic properties was achieved. On the basis of controllable photochromic behavior, smart glass with tunable transmittance, rewritable information recording, and multilevel security printing have been successfully achieved.

Overall, this effective strategy of controlling photochromic behavior for spirolactam-containing zinc complexes by changing their counterions is promising for a wide range of optoelectronic applications, such as data recording, information encryption and decryption, smart windows, and so on. It is believed that by changing the spirolactam-containing component from CVL to fluorane and rhodamine derivatives, multicolor species with controllable photochromic behavior could be prepared. Moreover, it is expected that this design principle can be extended to various stimuli-responsive materials to achieve controllable response behavior for advanced photonic applications.

MATERIALS AND METHODS

Materials

All starting materials and reagents were purchased from commercial suppliers and used without further purification unless otherwise stated. All solvents were purified before use. The solvent is carefully dried and distilled from a suitable desiccant before use.

Measurements

1H NMR (400 MHz) and 13C NMR (100 MHz) spectra were recorded on a Bruker ACF400 spectrometer at 298 K using deuterated solvents. The 1H NMR chemical shifts are reported relative to tetramethylsilane (TMS) (0.00 ppm) or deuterated solvents (7.26 ppm for CDCl3). The 13C NMR chemical shifts are reported relative to TMS (0.00 ppm) or deuterated solvents (77.0 ppm for CDCl3). Mass spectra were recorded on a Bruker autoflex MALDI-TOF MS. The UV-visible absorption spectra were obtained with a Shimadzu UV-3600 UV-VIS-NIR spectrophotometer. Reflection spectroscopy was measured with a Shimadzu UV-2600 UV-VIS-NIR. The x axis shows the light wavelength, and the y axis represents reflectivity. The transmission spectrum was measured with LAMBDA 35 UV-VIS-NIR. The x axis shows the light wavelength, and the y axis represents transmittance. PL spectra were measured with a HITACHI F-7000 fluorescence spectrophotometer.

Synthesis of CVL salicylaldoxime

Crystalline lactone hydrazide (858 mg, 2 mmol) was dissolved in absolute ethanol (60 ml) and dissolved with stirring. Further, 2-hydroxy-4-methoxybenzaldehyde (456 mg, 3 mmol) was placed in a bottle, heated to 90°C, and refluxed overnight. The solvent was dried under reduced pressure and purified by silica gel column chromatography (ethyl ether:ethyl acetate = 3:1) to afford product (406 mg). The yield of the reaction was 36%. 1H NMR (400 MHz, CDCl3) δ (ppm): 11.13 (s, 1 H), 9.73 (s,1 H), 7.20 to 7.14 (m, 7 H), 6.93 to 6.90 (m, 1 H), 6.66 to 6.64 (d, J = 8.8 Hz, 4 H), 6.47 to 6.44 (m, 1 H), 6.41 to 6.40 (d, J = 2.4 Hz,1 H), 3.80 (s, 3 H), 3.03 (s, 6 H), 2.94 (s, 12 H). 13C NMR (100 MHz, CDCl3) δ (ppm): 165.76, 162.64, 160.71, 156.35, 150.65, 149.87, 137.47, 132.95, 130.56, 129.26, 128.59, 124.27, 117.66, 112.39, 112.11, 106.58, 105.83, 101.38, 75.62, 55.43, 40.84, 40.50. MS (MALDI-TOF) [m/z (mass/charge ratio)]: [M]+ calcd. for C34H37N5O3, 563.70; found, 562.60.

Preparation of security paper

The rewritable paper is prepared in a layer-by-layer manner. First, a PPG-PEG-PPG passivation layer (90 mg/ml; average Mn: ~14,600) dissolved in a CH2Cl2 solution was coated on a parchment substrate and vacuum-dried at 40°C. Second, an image formation layer was constructed by coating a passivation layer with a solution of CVLSH (2 mg/ml) and PPG PEG-PPG (90 mg/ml) in CH2Cl2 and then vacuum drying at 40°C. Last, another layer of PPG-PEG-PPG protective layer was applied to the top using a brush. The manufacture of the encrypted paper can follow the same procedure as described above.

Preparation of photochromic film

The photochromic film is prepared on a transparent glass sheet with a size of 1.5 cm by 1.5 cm. First, polymethyl methacrylate (PMMA; 80 mg/ml) was added to a CVLSH-Zn solution with a concentration of 5 × 10−3 M. Last, the mixture solution is evenly applied onto the glass piece.

Density functional theory calculation

The theoretical calculation uses the Gaussian 09 software (38). The B3lYP-D3 density functional method was used in this work to carry out all the calculations. The 6-31G (d,p) basis set was used for all atoms. Vibrational frequency analyses were performed on the optimized geometries at the same level of theory to characterize the stationary points as local minima (no imaginary frequency) or transition states (one imaginary frequency). In addition, intrinsic reaction coordinate calculations were used to verify that the transition state connects with the appropriate reactant and product.

SUPPLEMENTARY MATERIALS

Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/6/16/eaaz2386/DC1

This is an open-access article distributed under the terms of the Creative Commons Attribution-NonCommercial license, which permits use, distribution, and reproduction in any medium, so long as the resultant use is not for commercial advantage and provided the original work is properly cited.

REFERENCES AND NOTES

Acknowledgments: Funding: We thank the financial support from the National Funds for Distinguished Young Scientists (61825503), the National Natural Science Foundation of China (21701087 and 61775101), NSF of Jiangsu Province of China (BK20160885), the National Program for Support of Top-Notch Young Professionals, and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD, YX030003). Author contributions: Y.M., Q.Z., and W.H. conceived the idea for this work and designed the experiments. Y.X.Y. performed the synthesis work and studies on photochromic properties and security printing. P.F.S. and J.Y.L. contributed to the implementation of the experiments and the measurements. Y.M., Q.Z., and W.H. analyzed the data and wrote the manuscript. S.J.L. revised the manuscript and provided some suggestions. All authors discussed the results and commented on the manuscript at all stages. Competing interests: The authors declare that they have no competing interests. Data and materials availability: All data needed to evaluate the conclusions in the paper are present in the paper and/or the Supplementary Materials. Additional data related to this paper may be requested from the authors.
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