A Versatile Tetraphenylethene Derivative Bearing Excitation Wavelength Dependent Emission, Multistate Mechanochromism, Reversible Photochromism, and Circularly Polarized Luminescence and Its Applications in Multimodal Anticounterfeiting

Abstract

Open AccessCCS ChemistryRESEARCH ARTICLES2 Sep 2022A Versatile Tetraphenylethene Derivative Bearing Excitation Wavelength Dependent Emission, Multistate Mechanochromism, Reversible Photochromism, and Circularly Polarized Luminescence and Its Applications in Multimodal Anticounterfeiting Qing Xia, Weiwei Xie, Tingchao He, Hongyan Zhang, Zujin Zhao, Guangxi Huang, Bing Shi Li and Ben Zhong Tang Qing Xia Key Laboratory of New Lithium-Ion Batteries and Mesoporous Materials, College of Chemistry and Environmental Engineering, Shenzhen University, Shenzhen, Guangdong 518055 Google Scholar More articles by this author , Weiwei Xie Key Laboratory of New Lithium-Ion Batteries and Mesoporous Materials, College of Chemistry and Environmental Engineering, Shenzhen University, Shenzhen, Guangdong 518055 Google Scholar More articles by this author , Tingchao He College of Physics and Optoelectronic Engineering, Shenzhen University, Shenzhen, Guangdong 518060 Google Scholar More articles by this author , Hongyan Zhang Key Laboratory of New Lithium-Ion Batteries and Mesoporous Materials, College of Chemistry and Environmental Engineering, Shenzhen University, Shenzhen, Guangdong 518055 Google Scholar More articles by this author , Zujin Zhao State Key Laboratory of Luminescent Materials and Devices, Guangdong Provincial Key Laboratory of Luminescence from Molecular Aggregates, SCUT-HKUST Joint Research Institute, South China University of Technology, Guangzhou, Guangdong 510641 Google Scholar More articles by this author , Guangxi Huang *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] Key Laboratory of New Lithium-Ion Batteries and Mesoporous Materials, College of Chemistry and Environmental Engineering, Shenzhen University, Shenzhen, Guangdong 518055 Google Scholar More articles by this author , Bing Shi Li *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] Key Laboratory of New Lithium-Ion Batteries and Mesoporous Materials, College of Chemistry and Environmental Engineering, Shenzhen University, Shenzhen, Guangdong 518055 Google Scholar More articles by this author and Ben Zhong Tang *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] School of Science and Engineering, Shenzhen Institute of Aggregate Science and Technology, The Chinese University of Hong Kong, Shenzhen, Guangdong 518172 Google Scholar More articles by this author https://doi.org/10.31635/ccschem.022.202202173 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Multimodal anticounterfeiting has become increasingly challenging in modern society to guarantee information security and the safety of property. In this study, a versatile cholesterol-containing tetraphenylethene derivative is shown to have multiple optical properties, including stimuli-responsive fluorescence, reversible photochromism, excitation wavelength dependent luminescence, and circularly polarized luminescence. After the application of diverse processing methods (writing, screen painting, drawing, and pyrography), we found that this molecule can serve as an anticounterfeiting toolbox to provide rich anticounterfeiting effects through the synergistical use of multiple optical properties. This work offers important insight for designing novel small organic molecules for advanced multimodal anticounterfeiting technology. Download figure Download PowerPoint Introduction Anticounterfeiting has become a pivotal issue in modern commercial activities. Easily forged banknotes, certifications, authorized documents, and product labels facilitate commercial crimes and cause serious economic losses every year as well. To suppress counterfeiting, anticounterfeiting methods have been upgraded from the use of original watermarks on special paper, ink, printing techniques, digital technology, and so on. Luminescence, as a basic attribute of substances, is a natural choice to realize the anticounterfeiting function due to its convenience in the chemical anticounterfeiting field. Thus, patterns with fluorescence,1–3 upconversion luminescence,4 phosphorescence,5–7 or circularly polarized luminescence (CPL)8–10 under light excitation have been widely used in many circumstances. However, simple static patterns cannot secure anticounterfeiting efficiently because they can be easily duplicated by using compounds with similar luminescent color or quantum yields. In this respect, patterns with dynamic responses of photophysical properties upon external stimuli have a much higher security level. Materials suitable for dynamic patterns include mechanochromic, photochromic, and excitation wavelength dependent (Ex-De) luminescent materials. Organic mechanochromic materials are characterized by tunable luminescent color or intensity in the solid state upon application of mechanical forces, such as pressing, grinding, crushing, or rubbing.11–14 Though many mechanochromic luminogens have been reported, most of them exhibit only two luminescent states. High contrast and completely reversible multicolored/multistate mechanochromic materials are still limited in the scope of their application.15–17 Photochromic materials show reversible photoinduced structural transformations between different states with distinct appearance or luminescent color.18–24 However, most reported molecular systems are still far from meeting the requirements for rapidity and convenience in anticounterfeiting applications. They suffer from slow responsiveness, complicated synthetic routes, and irreversible transitions in the solid state. Thus, fast-responding, self-recovering, photochromic systems with simple molecular structures are in high demand.16,25–27 Ex-De luminescent materials have attracted particular interest because their tunable emission color can be realized by merely adjusting the excitation wavelength, which has both facile and noninvasive features. Ex-De behavior mainly occurs in nanoparticles (nanocrystals and carbon/silica dots) and metal complexes due to their heterogeneous aggregated states.28–35 Purely organic molecules with obvious Ex-De fluorescence or phosphorescence have rarely been observed.36–40 In practical counterfeiting applications, synergistically integrating multiple modes into a single sample is preferred for the advantages of effectively increasing the counterfeiting threshold and decreasing cost.8,9,41–44 We envision that the integration of multiple dynamic controllable photophysical properties will produce a more powerful weapon against counterfeiting.15,45,46 However, this integration remains a paramount challenge to design ideal materials because it not only requires novel design principles but also easy processability with high stability to be useful in a wide variety of applications. Our group has reported a tetraphenylethene (TPE) derivative with multistate mechanochromism and photochromism.16 Therein, we reported that its isomer with the cholesterol group connecting to the metaposition of TPE ( 1 in Figure 1) gave additional intriguing Ex-De fluorescence and CPL properties in polymethyl methacrylate (PMMA) composite film. The four properties serving as anticounterfeiting tools work well, both independently and cooperatively with each other, to successively achieve multilevel optical anticounterfeiting. To the best of our knowledge, this is the first example of organic molecules showing such multiple anticounterfeiting effects in the solid state. Figure 1 | Multimodal anticounterfeiting of TPE derivative 1. Download figure Download PowerPoint Experimental Section 1H and 13C NMR spectra were recorded on a VNMRS 400 NMR spectrometer (Varian, Palo Alto, CA, USA). High-resolution mass spectra were recorded using an Autoflex III mass spectrometer (matrix-assisted laser desorption ionization time-of-flight mass spectrometry [MALDI-TOF-MS], Bruker, Karlsruhe, Germany). Element analysis was performed using a vario EL cube elemental analyzer (Elementar, Hanau, Germany). UV–vis spectra were recorded using a UV-2600 spectrometer (Shimadzu, Kyoto, Japan). CD spectra were recorded using a J-1500 spectropolarimeter (JASCO, Toyko, Japan). Fluorescence and excitation spectra were recorded using a F-7000 fluorescence spectrometer (Hitachi, Toyko, Japan). CPL spectra were recorded using a CPL-300 instrument (JASCO, Toyko, Japan) at room temperature. Absolute quantum efficiency was measured on an integrating sphere (FS5, Edinburgh, United Kingdom). Fluorescence lifetime was measured on a compact fluorescence lifetime spectrometer (HORIBA FluoroCube, Glasgow, United Kingdom). Sample heating was performed on a microcomputer temp-controlled heating board (JF-966A, JFTOOLS, Dongguan, China). Differential scanning calorimetry (DSC) measurements were carried out on a NETZSCH DSC 200F3 instrument at a heating rate and a cooling rate of 10 °C min−1 in nitrogen. Thermogravimetric analysis was performed on a NETZSCH SA409PC thermogravimeter. Powder X-ray diffraction (PXRD) patterns were carried out in the reflection mode at room temperature using a 2.2 kW Empyrean X-ray Diffraction System (PANalytical, Almelo, The Netherlands). Microphotographs were taken using BX53F fluorescence microscopes (Olympus, Toyko, Japan). Photos and videos were taken with a Nikon D7100 camera. Results and Discussion Compound 1 was synthesized through a one-step reaction between the TPE moiety and the cholesterol moiety with high productivity ( Supporting Information Scheme S1 and Figures S18–S20).47 Molecule 1 had high thermal and photo stability, verified by thermal gravity analysis and NMR spectra ( Supporting Information Figures S1 and S21), respectively. The dilute tetrahydrofuran solution of compound 1 showed an absorption peak at 308 nm ( Supporting Information Figure S2), and it exhibited a typical aggregation-induced emission property upon the addition of poor solvents ( Supporting Information Figure S3). Similar to its isomer,16 molecule 1 also displayed excellent multistate mechanochromic properties (Figure 2). The photophysical properties of 1 in different states are summarized in Supporting Information Table S1 and Figure S4. The as-prepared powder 1o emitted sky-blue fluorescence (λem = 463 nm) under 365 nm UV light irradiation (denoted as @365, and this denotation applies to all specific excitation wavelengths in this paper). After grinding, both the appearance color and the fluorescence color showed negligible change ( 1g: λem = 460 nm, Φf = 10.5%, τ = 3.13 [email protected]). Powder 1g changed into deep-blue emissive 1f-dh (λem = 451 nm, Φf = 5.5%, τ = 1.70 [email protected]) after exposing it to the vapor of the dichloromethane and the hexane mixture (v/v = 1/1) for about 3 min. When 1g was respectively fumed with hexane for 1 min or annealed at 130 °C for 15 s, dim 1f-h (λem = 460 nm, Φf = 2.6%, τ = 1.32 [email protected]) or 1h (λem = 460 nm, Φf = 2.4%, τ = 1.50 [email protected]) formed, in sharp contrast to the bright 1g ( Supporting Information Figure S5). Interconversion between sky-blue emissive 1g and dim-emissive 1h was repeated many times through a heating/grinding process without any deterioration ( Supporting Information Figure S6a). Meanwhile, interconversions among other states ( 1g and 1f-h, 1g and 1f-dh, 1h and 1f-dh) were also reversible ( Supporting Information Figure S6b–d). Therefore, the mechanochromism of compound 1 was totally reversible among three fluorescence states (sky blue, deep blue, and dim). Such examples with the multiple reversible mechanochromic properties have rarely been reported in previous research.15,16 Needle-like polycrystal 1c (λem = 460 nm, Φf = 1.7%, τ = 0.64 [email protected]) was obtained by slow evaporation of the solution of 1 ( Supporting Information Figure S7). The fluorescence of 1c @365 was even dimmer than other states of 1. When heated to about 190 °C, the powder of 1 melted into transparent liquid. After cooling for several seconds, its solidified sample was annealed at 130 °C for 5s to give the white agglomerated state 1m (λem = 454 nm, Φf = 2.73%, τ = 1.73 [email protected]). Figure 2 | (a) The molecular structure and photographs of 1 in different solid states: the original powder state 1o, ground powder 1g, heated powder 1h, fumed powder 1f-h (with hexane), fumed powder 1f-dh (with dichloromethane and hexane), agglomerated state 1m, and polycrystal 1c. Treatments: I, grinding; II, annealing at 130 °C for 15 s; III, fuming with dichloromethane and hexane vapor for 15 s; IV, fuming with dichloromethane and hexane vapor for 3 min; V, fuming with hexane vapor for 1 min; VI, heating to molten state and cooling, then annealing at 130 °C for 5 s; VII, crystallizing from acetone/H2O solution. Photos were taken under available UV lamps (@365/@302/@254), respectively. (b) PXRD patterns of all the corresponding states of 1. Download figure Download PowerPoint Besides its mechanochromic properties, an unexpected Ex-De property of 1h, 1f-h, 1m, and 1c was discovered when the UV irradiation light was changed from 365 to 254 nm, as shown in Figure 2 and Supporting Information Figure S5; 1h, 1f-h, and 1c emitted greenish fluorescence @254, which was much brighter than the dim blueish fluorescence @365; while 1m showed a slightly enhanced emission with its fluorescence color transitioning from bluish into greenish. The combination of multiple emissive states and different excitation light unambiguously endows 1 with multichannel response that is more powerful than with other reported molecules.40 To better decipher the mechanochromic property, PXRD and DSC were then performed. As shown in Figure 2b, the almost flat diffraction signals demonstrated that 1o and 1g were amorphous powder while other states of 1 were in the crystalline state with distinct crystallinity. The diffraction patterns of 1h, 1f-h, 1m, and 1c had a certain degree of similarity at 2θ = 10–20°, especially 1h and 1f-h. Sample 1f-dh only exhibited several broad reflection bands, revealing its different molecular packing from that of 1h, 1f-h, 1m, or 1c. DSC thermograms of all the states of 1 displayed an endothermic peak at 185–187 °C, attributed to the melting point ( Supporting Information Figure S8). A cold crystallization peak was observed for 1o (101.3 °C), 1g (99.4 °C), and 1f-dh (103.6 °C), corresponding to their transformation into a more stable crystalline state 1h, as verified by its almost flat DSC curve below the melting point. Similar to 1h, DSC curves of 1f-h and 1m also revealed their thermal stability, in accordance with their high crystallinity. The fluorescence colors of 1h and 1f-h displayed chromatic aberration @254, @302, and @365 (Figure 2a), which also corresponded well with the subtle difference of their PXRD patterns and DSC curves, indicating that their molecular packings were not exactly the same. Systematical investigation of the Ex-De property of 1 was then conducted (Figure 3 and Supporting Information Figures S9 and S10). The corresponding fluorescence color changes of 1h, 1f-h, 1m, or 1c in response to different excitation wavelengths are illustrated in the Commission International de l’Eclairage (CIE) coordinate diagram in Figure 3c and Supporting Information Figure S11. For 1o, 1g, or 1f-dh, their emission peak did not obviously change with excitation light ( Supporting Information Figure S9) while the fluorescence spectra of 1h, 1f-h, 1m, and 1c exhibited dual emission characteristics (∼455 and 495 nm, Figure 3a). The emission bands were clearly dependent on the excitation wavelength, suggesting that the emissions originated from more than one excited state. Their excitation spectra can be classified into three types (Figure 3b and Supporting Information Figure S10): 1g/ 1f-dh bearing two excitation peaks at 270 and 365 nm with comparable intensity; 1h/ 1f-h/ 1c exhibiting two excitation peaks at 270 and 335 nm with the later showing much weaker intensity; the 1m between the two former types. Different excitation spectra of molecule 1 are ascribed to its diverse molecular packing states upon corresponded treatments. What is unique about this Ex-De system is that the higher energy excitations result in enhanced emissions at longer wavelengths, contrary to the bathochromic shift phenomena of other Ex-De TPE derivatives.40 For example, the maximum emission peak of 1f-h transitioned from 495 to 455 nm with a change in the excitation wavelength from 240 to 360 nm, accompanied by an obvious hypsochromic shift from green (CIEx,y = (0.168, 0.330)) to sky blue (CIEx,y = (0.154, 0.179)) ( Supporting Information Figure S11). Consistent with Ex-De fluorescence, the quantum yields and lifetimes of 1h, 1f-h, and 1c were also largely enhanced by varying the excitation wavelength from 360 to 260 nm ( Supporting Information Table S1), resulting in strong visual contrast (Figure 3c). While 1m had comparable quantum yields (3.4%@260, 2.7%@360), which agreed well with the small variation in fluorescence color/CIE coordinate ((0.166, 0.272)@240, (0.167, 0.206)@360). Figure 3 | (a) Fluorescence spectra, (b) excitation spectra, and (c) CIE coordinate diagram of 1h, 1f-h, 1m, and 1c under different excitation wavelengths. The inset images in panel (c) showed the fluorescence photographs @254/@302/@365. Download figure Download PowerPoint It is noteworthy that when cholesterol pendant was connected to the para position of TPE, Ex-De fluorescence was absent, but three-state mechanochromism remained ( Supporting Information Figure S12).16 Molecule 1 and its isomer lack π–π interactions and strong intermolecular interactions, so we conjectured that molecular conformation played an important role in the Ex-De fluorescence property. Compared to the stretched conformation of its isomer, molecule 1 had a relatively twisted conformation, which is probably detrimental to the formation of perfect crystal packing. Moreover, the Ex-De fluorescence of ground TPE derivatives had even been reported by Zhang et al.,40 who suggested that it derived from the coverage of multiple-layer amorphous molecules on an amount of small microcrystals. Therefore, the possible mechanism of Ex-De fluorescence can be described as the following (Figure 4). Polycrystal 1c showed an almost unique excitation peak and very weak emission @365, from which it can be deduced that green emission has tight ties to the crystalline state. From this it logically follows that unavailable ideal crystal only displays green emission @254 and is dark @365. Quick annealing ( 1h, 1m), fuming ( 1f-h), or even slow crystallization ( 1c) could not induce molecules to completely arrange into a single ordered state, leading to the coexistence of crystalline and amorphous regions. Residual amorphous molecules showed blueish emission, which mixed with the green emission in the crystalline state to form Ex-De fluorescence characteristic. The ratio of crystalline and amorphous regions decided the fluorescence chromatic aberration among 1h, 1f-h, 1m, and 1c under different excitation wavelength. Figure 4 | Schematic illustration of the Ex-De mechanism of 1. The amorphous state can be excited @254 and @365 with blueish emission while the ideal crystal can only be excited @254 with greenish fluorescence. State 1f-h, 1h, or 1m with the coexistence of crystalline and amorphous regions showed combinational greenish and blueish fluorescence. Download figure Download PowerPoint Among all the solid states of 1, only 1m exhibited apparent photochromic property. The white appearance of 1m turned into red after 365 nm UV irradiation (∼10 mW/cm2) for a short time (∼3 s), as shown in Figure 5. When the UV irradiation ceased, the red appearance of 1m began to decolorize and return to its white color within 1 min (Figure 5a). UV–vis reflection spectroscopy of 1m was then measured to clarify its photochromic property. A strong absorption band peaked at 500 nm in the visible light region that emerged upon irradiation (Figure 5b), suggesting increased conjugation of the molecules, which is in accordance with the red color of 1m. Once the UV light source was turned off, the new absorption band gradually decreased, and the absorption spectrum recovered to its original state. As shown in Figure 5c, the colorization-decolorization cycle of 1m was regulated 10 times, and the reflectance at 500 nm remained stable in a certain range without any apparent fatigue. Similar to its isomer, the photochromism of 1m can be attributed to photocyclization of the stilbene unit.16,48 Figure 5 | (a) Time-dependent photochromic bleaching process of 1m in 60 s at room temperature after UV irradiation stopped. (b) Changes in the UV–vis reflection spectra of 1m upon UV irradiation. (c) Fatigue resistance of 1m upon irradiation with 365 nm UV light (5 s) and standing in ambient light (2 min) alternatively. The reflection was measured at 500 nm just for qualitative analysis on account of the delayed collection of spectra. Download figure Download PowerPoint Inspired by multistate mechanochromic, Ex-De, and photochromic properties, anticounterfeiting applications of compound 1 were explored through multiple processing methods including writing, screen painting, drawing, and pyrography. Powder of compound 1 could be directly ground by a glass rod or pestle to form various numbers, letters, or characters. 1g was written on a weighing paper as a string of numbers: 128 e 980 and then converted into blue 1f-dh through solvent fuming (Figure 6a). When the upper part of numbers was annealed at 130 °C for 1 min, the meaningless numbers changed into the meaningful sentence ILOVEYOU @365. The formation of heated state 1h at the upper part led to the disappearance of fluorescence and solved the mystery. Under daylight, the phase transformation of the powders could be disguised. 302 or 254 nm UV light can excite cyan or green fluorescence of 1h and recover the numbers, respectively. This interesting puzzle displayed the facile transformation among different fluorescence states. Figure 6 | Illustration of the application potential of anticounterfeiting toolbox 1 through different processing methods including (a) writing, (b) screen painting, (c) drawing, and (d) pyrography, which displayed its dual-channel multistate mechanochromism, Ex-De fluorescence and photochromism properties. Download figure Download PowerPoint Compared to crude writing, screen printing is more widely used for low-cost printing of more delicate patterns. Molecule 1 can be used in this application. The screen-printing ink was prepared by dispersing the powder in vacuum grease. As shown in Figure 6b, patterns were painted on a piece of weighing paper by screen printing. The school badge and two-dimensional barcode of the WeChat Subscription of our institution, Shenzhen University, was fabricated by using 1g and 1f-h ink, respectively. Under daylight, the white appearance and background made the quick response (QR) code difficult to see. Under the 365 nm UV-lamp excitation, the QR code was too dark to be recognized by WeChat scanning. However, when the UV lamp was switched to 254 or 302 nm, the bright-green or dark-green fluorescence of the QR code was well irradiated in the dark and could be identified immediately by scanning the QR code, thus transferring it to the homepage of the subscription of Shenzhen University, as shown in Supporting Information Video S1. For the 1g-prepared school badge, the fluorescent color did not change with the excited wavelength. The discrepancy of fluorescent color and intensity under different UV light certainly endowed the QR with a strong anticounterfeiting effect. Similarly, other states of 1 could also be used to prepare fluorescente patterns through screen printing. To exhibit the practicability of the advanced anticounterfeiting function of 1, complicated patterns with multiple fluorescence states and simple operation methods were then demonstrated. A dichloromethane solution of 1 (∼2 mg·mL−1) was prepared in advance. When the solution was used to draw on the weighing paper with a Chinese brush, the organic solvent evaporated within 5 s and left a sky-blue emissive pattern, which was attributed to its amorphous state (equivalent to 1o or 1g). Starting from the amorphous state, states 1f-dh or 1h were obtained. In the picture in Figure 6c, bamboo trunks were first drawn and fumed with mixed solvent (dichloromethane and hexane) for 30 s, corresponding to the emergence of state 1f-dh. Then the birds and bamboo leaves were drawn, followed by heating the area of the birds on a hot stage for 3 min ( Supporting Information Figure S13). Under 254 nm UV light, the picture showed deep-blue bamboo trunks with sky-blue bamboo leaves, one green bird extending its wings in the sky, and the other resting on a branch (the enlarged picture is shown in Supporting Information Figure S14). However, the two birds almost disappeared upon exposure to 365 nm UV light due to their heated state 1h (the enlarged picture is shown in Supporting Information Figure S15) while the fluorescent color of bamboo trunks and bamboo leaves remained unchanged. Thus, it was clear that 1 exhibited three states with high contrast in fluorescence emission efficiency and colors (green, sky blue, and deep blue) under 365/254 nm UV light. This three-state mechanochromic property under two excitation light channels integrated in one molecule has seldom been reported and will be difficult to mimic. In practical applications, different regions of the pattern can be easily heated or fumed to change its fluorescent state. An even more interesting anticounterfeiting function was then realized through the combination of Ex-De and photochromic properties. Pyrography is the art of producing designs on wood, leather, or other materials by using heated tools or a fine flame. Learning from pyrography, an electric iron was used to shape the pattern (Figure 6d). Powder 1o was first spread on a glass plate and instantaneously melted by an electric iron (200 °C). The desired pattern was generated via a fast melting and cooling process. After retrieving redundant powder, the pattern was reannealed at 130 °C to form 1m. Here three letters, “SZU” (the initials of Shenzhen University), were prepared as an example. Under 365 and 254 nm excitation light, the letters emitted bluish and greenish fluorescence, respectively. When UV light was removed, the letters displayed red color and gradually disappeared to recover to the white appearance within 1 min. The above process was recorded, and the video can be found in Supporting Information Video S2. This dynamic fluorescent and photochromic phenomenon was manipulated repeatedly. Similarly, different logo patterns or signatures can be devised according to specific requirements in the personalized anticounterfeiting field. Due to the presence of a chiral cholesterol unit, molecule 1 was also expected to possess CPL activity in a restricted environment. Composite film of 1 in PMMA (2 wt %) was prepared by evaporating the dichloromethane solution in a petri dish (Figure 7a,d). When the film was irradiated under 365 nm UV light, sky-blue fluorescence with obvious negative CPL signal ranging from 450 to 470 nm was observed (Figure 7b,e), and the corresponding dissymmetry factors (glum) were around −2 × 10−3 ( Supporting Information Figure S16). The transparent film unexpectedly turned orangey after irradiation for 5 s, especially the focused position of UV light (Figure 7c). Meanwhile, the red color of the lateral edge of the film was in sharp contrast to its original transparent state irrespective the thickness of film (Figure 7f and Supporting Information Figure S17). According to our previous paper,16 the photochromic property is anisotropy because different crystal planes display various degrees of color changing. Thus the abnormal discrepancy between the red lateral edge and the orangey focused position probably derived from the anisotropy, which has rarely been noticed in previous anticounterfeiting applications. The combination of CPL and photochromic properties will be more difficult to counterfeit and will produce