Cooperative supramolecular polymers with anthracene‒endoperoxide photo-switching for fluorescent anti-counterfeiting

Innovative technologies are highly pursued for the detection and avoidance of counterfeiting in modern information society. Herein, we report the construction of photo-responsive supramolecular polymers toward fluorescent anti-counterfeit applications, by taking advantage of multicycle anthracene‒endoperoxide switching properties. Due to σ-metalation effect, photo-oxygenation of anthracene to endoperoxide is proceeded under the mild visible light irradiation conditions, while the backward conversion occurs spontaneously at room temperature. Supramolecular polymers are formed with cooperative nucleation‒elongation mechanism, which facilitate fluorescence resonance energy transfer process via two-component co-assembly strategy. Fluorescence resonance energy transfer efficiency is delicately regulated by either light-triggered anthracene‒endoperoxide conversion or vapor-induced monomer–polymer transition, leading to high-contrast fluorescent changes among three different states. On this basis, dual-mode anti-counterfeiting patterns have been successfully fabricated via inkjet printing techniques. Hence, cooperative supramolecular polymerization of photo-fluorochromic molecules represents an efficient approach toward high-performance anti-counterfeit materials with enhanced security reliability, fast response, and ease of operation.

C ounterfeiting, widely spread in banknotes, diplomas, tax stamps, and certificates, has become a serious and socialthreatening problem 1 . It is in keen pursuit to develop innovative anti-counterfeiting technologies, which avoids the forgeries and ensures security reliability for the genuine documents [2][3][4][5] . In this respect, photo-fluorochromic compounds are regarded as an intriguing choice, because of their reversible emission outputs in response to light [6][7][8] . Generally, inkjet printable anti-counterfeiting materials are prepared by direct mixing of photo-fluorochromic compounds with the polymer matrix 9,10 . However, their limited inclusiveness potentially leads to the decrease in ink's quality. In addition, most of the reported anticounterfeiting inks are operated in a single-mode fashion [11][12][13][14][15][16][17] , which can still be easily replicated. Hence, it is highly desirable to fabricate anti-counterfeiting inks with bright fluorescence and multi-mode responsiveness.
Based on these considerations, we sought to develop photofluorochromic supramolecular polymers toward anti-counterfeiting applications. The molecular design relies on anthracene-endoperoxide interconversion, which displays on/off switching for the emission signals [37][38][39][40][41] . Notably, harsh reaction conditions are required for the traditional anthracene-endoperoxide photoswitching (365 nm UV light irradiation for oxygenation process, while 254 nm UV light or thermolysis for deoxygenation reaction) 37 . Photo-degradation side-products are prone to form during the processes, which hamper the practical application in fluorescent anti-counterfeiting inks. To overcome this issue, herein anthracene-based monomer 1 (Fig. 1) has been designed, with the attachment of Pt(PEt 3 ) 2 moiety on both ends of 9,10-diacetylide anthracene. Due to σ-metalation effect [42][43][44][45] , photo-oxygenation of 1 can be rapidly proceeded via the less destructive visible light source, while the backward conversion occurs at room temperature in a spontaneous manner. These mild conditions facilitate the quantitative anthracene-endoperoxide switching for multiple cycles. Additionally, two amide units are connected to the rod anthracene core of 1. Ascribed to the synergistic participation of hydrogen bonding and π-π stacking interactions, supramolecular polymerization of 1 is expected to feature distinctive nucleation and elongation events [46][47][48][49][50][51][52] . It thus mimics the cooperative process adopted by biological selfassembled systems 53,54 , leading to macroscopic fluorescent variations before and after light irradiation. On this basis, a twocomponent supramolecular system 1/2 is established, which gives rise to fluorescence resonance energy transfer (FRET) from donor 1 to acceptor 2 (Fig. 1). The FRET efficiency can be precisely regulated, by either light-triggered anthracene-endoperoxide or vaportriggered monomer-polymer transitions. Accordingly, dual-mode anti-counterfeiting pattern ( Fig. 1) in response to the orthogonal stimuli is achieved, which guarantees higher security reliability.

Results
Spectroscopic characterization of 1. As a first step, spectroscopic experiments were performed for the designed monomer 1. Specifically, it displays three main UV-Vis absorption bands in CHCl 3 (Fig. 2a). According to the previous literatures [55][56][57] , the intense vibronic band at 250-310 nm, together with the moderately intense band at 400-525 nm (λ max = 454 and 484 nm for the two absorption maxima), are assigned to 1 L b and 1 L a bands of anthracene unit, respectively. In addition, the broad and intense band locating at 310-395 nm (λ max = 331 nm) belongs to π-π* transition of platinum acetylide (-C≡C-Pt-C≡C-) segment, which is absent for the counterpart compound 4 (see the structure in Fig. 2a, inset). Meanwhile, 1 shows the 1 L a emission band centered at 512 nm, along with a vibronic peak at 542 nm  Fig. 2b). As compared to 4, 1 displays ca. 50 nm bathochromic shifts for both 1 L a absorption and emission bands (Fig. 2a, b). Such phenomena are primarily ascribed to the overlapping between Pt d-orbitals and anthracene p-orbitals, resulting in the enhancement of π-electron delocalization 45,55-57 .
Supramolecular polymerization behaviors of 1. As widely documented, monomeric state dominates for π-conjugated molecules in dilute CHCl 3 solution, while self-aggregation is more preferred in aliphatic media 58 . For 1 in methylcyclohexane (MCH), the 1 L a absorbance bands are centered at 451 and 480 nm at 298 K (Fig. 3a). It displays three isosbestic points (374, 460, and 487 nm) upon varying the temperature, suggesting the reversible transition between self-assembled and monomeric states (Fig. 3a). The strong aggregation tendency of 1 in aliphatic media can be further reflected by 1 H NMR measurements. Briefly, broadened and unresolved aromatic resonances are present in d 12 -cyclohexane, which are in sharp contrast to the well-defined peaks observed in d-chloroform ( Supplementary Fig. 2a). Moreover, the two amide units are prone to form intermolecular hydrogen bonds at room temperature, as evidenced by the upfield shifting of amide protons upon elevating the temperature ( Supplementary Fig. 2b). Circular dichroism (CD) spectroscopy is then utilized to probe the regularity of the supramolecular assemblies 59 . To induce the helical bias for 1 at the supramolecular level, optically active (S)-3,7-dimethyloctyl groups are introduced to the peripheral side chains (Fig. 1). At 298 K, no CD signals are detected for 1 in Meanwhile, no linear dichroism is detected for 1 ( Supplementary  Fig. 5), suggesting the measured CD signal is real to reflect the supramolecular chirality 60 . Accordingly, it supports the formation of long-range-ordered supramolecular polymers in apolar MCH medium, accompanying with the chirality transfer from the alkyl periphery to the inner anthracene unit 59 .
Supramolecular polymerization mechanism of 1 is further elucidated via temperature-dependent CD experiments. Specifically, CD signals completely lose upon elevating temperature to 322 K, and fully restore upon cooling (Fig. 3b). At the melting rate of 40 K h −1 , no heating-cooling hysteresis can be observed, revealing that the self-assembly process is under thermodynamic control ( Supplementary Fig. 3) 18 . When monitoring the CD intensity at 486 nm versus temperature, a non-sigmoidal melting curve is obtained for 1 (Fig. 3c). Hence, similar to the biological self-assembled polymers such as tobacco mosaic virus and cytoskeleton proteins 53,54 , 1 follows cooperative nucleation-elongation mechanism. For the counterpart compound 3 (see the structure in Fig. 1) with the replacement of anthracene unit on 1 by the benzene group, it also shows cooperative supramolecular polymerization behavior ( Fig. 3c and Supplementary Fig. 4). Meijer-Schenning-van-der-Schoot mathematical model (Supplementary Equations 1 and 2) is employed to acquire the thermodynamic parameters for both self-assembly processes 46,47 . As shown in Table 1, monomer 1 features the higher T e , h e and lower K a values than those of 3, indicating the formation of more stable polymers with the higher cooperativity 47 . Considering that the structures between 1 and 3 only differ in the inner π-surface, it unambiguously supports that, in addition to intermolecular hydrogen bonds, π-π stacking interactions also participate in the supramolecular polymerization process.
Remarkably, 1 and 3 also display distinct gelation behaviors. In particular, when the monomer concentration of 1 exceeds 8 mM in MCH, it tends to form yellow transparent gels at room temperature (Inset of Fig. 3b). Depending on transmission electron microscopy (TEM) measurements, long ribbon-like fibers are observed for 1, with several microns in length and around 100 nm in width (Fig. 3d). These one-dimensional fibers are entangled to form fibrous networks, which lay the basis for the formation of supramolecular gels. In sharp contrast, neither gelation (even increasing the concentration to 80 mM) nor entangled long fibers can be visualized for 3 ( Supplementary  Figs. 6-7). Overall, minor modification on the monomeric structure brings about huge differences in supramolecular polymerization and macroscopic gelation behaviors.
Anthracene-endoperoxide photo-switching of 1. We then turned to investigate the photo-responsiveness of 1. 460 nm LED lamp is employed, considering that 1 absorbs light in this region. Upon photo-irradiation of 1 in CHCl 3 , both 1 L a absorption (400-525 nm) and emission (475-650 nm) bands decrease rapidly in their intensities, and level off within 20 s (Fig. 4a and Supplementary Fig. 8). Two isosbestic points (318 and 354 nm) are observed in UV-Vis spectra, revealing the conversion from 1 to its photo-chemical product (Supplementary Fig. 8). The process can be further demonstrated by means of 1 H NMR experiments (Fig. 5a, b), which show the disappearance of anthracene resonances (H a 8.74 ppm, H b 7.44 ppm), together with the emergence of a new set of resonances in the upfield region (H a , 7.83 ppm, and H b , 7.34 ppm).
The exact photo-chemical product is then elucidated. When N 2 is purged into 1, both UV-Vis spectroscopic and 1 H NMR signals hardly change (Supplementary Fig. 9). Accordingly, it suggests the participation of O 2 during photo-conversion process. Furthermore, light irradiation is performed for the mixture of 1 and 2,2,6,6-tetramethylpiperidine (TEMP). In electron paramagnetic resonance spectroscopy, three narrow lines are observed with equal intensity (g value = 2.006) 61,62 , validating the formation of paramagnetic nitroxide radical, TEMPO ( Supplementary  Fig. 10). We rationalized that, due to Pt(II) σ-metalation effect,  For endoperoxide-to-anthracene transition, deep UV light (254 nm) or thermolysis is commonly required for the conventional 9,10-disubstituted anthracenes 37 . In stark contrast, 1-O 2 converts to 1 at room temperature in a spontaneous manner. The conclusion is verified by the reappearance of anthracene 1 H NMR resonances (Fig. 5c), together with the complete restore of 1 L a and 1 L b spectroscopic bands ( Supplementary Fig. 11). Half-life time (t 1/2 ) for the deoxygenation process is determined to be 15.9 min by UV-Vis measurements (c = 2.00 × 10 −4 M for 1-O 2 at 293 K), which can be further modulated upon varying the temperature (3.70 min at 308 K, 8.60 min at 298 K, and 61.0 min at 288 K, see Supplementary Fig. 12). We rationalized that, when comparing to the anthracene unit in 1, the bulky endoperoxide ring in 1-O 2 imposes larger hindrance to the neighboring two Pt(PEt 3 ) 2 units 28 . To release the steric stain, it could undergo spontaneous endoperoxide-to-anthracene transition for 1-O 2 even at room temperature. With the successive light irradiation and roomtemperature standing, 1 and 1-O 2 can be reversibly switched for multiple cycles, which are confirmed by the vanishing/reappearance of both absorbance and emission bands (Inset of Fig. 4a, and Supplementary Fig. 11).
We also performed anthracene-endoperoxide photo-switching for the control compound 5 (see the structure in Fig. 6a, inset). Only 60% conversion takes place for the photo-oxygenation process, with the requirement of substantially longer irradiation time (800 s for 5 versus 20 s for 1 at 2.00 × 10 −4 M, 298 K, see Fig. 6a and Supplementary Fig. 14). For the deoxygenation process (Fig. 6b), the t 1/2 value for 5-O 2 is determined to be 26 min, which is also slower than that of 1-O 2 (8.6 min). Hence, it proves that Pt(II) σ-metalation of anthracene is essential for the rapid and quantitative anthracene-endoperoxide switching.
Photo-responsiveness of supramolecular polymers from 1. For 1, anthracene-endoperoxide interconversion occurs not only in the monomeric state (CHCl 3 solution), but also in the supramolecular polymeric state (MCH solution). Notably, the kinetics are different between the two states. In particular, the observed rate constant (k obs ) for photo-oxygenation is 0.0128 s −1 in MCH, which is slower than that in CHCl 3 (k obs = 0.71 s −1 , see Fig. 6a and Supplementary Fig. 17). Similarity, for the backward conversion from 1-O 2 to 1, t 1/2 values vary from 15.9 min in CHCl 3 to 34.0 min in MCH ( Fig. 6b and Supplementary Fig. 18). The retarded conversion rates in supramolecular polymeric state are probably ascribed to the shielding of photo-fluorochromic units from 1 O 2 . Another reason for the different kinetics originates from the varied dipolarity/polarizability for the two solvents 63 .
Photo-responsiveness of 1 gives rise to the macroscopic changes of the resulting supramolecular polymers. In detail, the bisignate CD signals of 1 in MCH totally disappear upon 460 nm light irradiation, denoting the loss of supramolecular chirality (Fig. 7a). Besides, supramolecular gels of 1 (10 mM in MCH) completely collapse within 4 min, accompanying with the weakening of green emission (Inset of Fig. 6b, and Supplementary  Fig. 19). Upon standing at room temperature, highly emissive supramolecular gels reform, suggesting the complete deoxygenation of 1-O 2 .
We then sought to exploit anti-counterfeiting applications of 1, by taking full advantage of its fluorescent on-off switching properties. Specifically, the supramolecular polymeric solution of 1 is filled in the customized cartridge of an inkjet printer. A monochromic QR code is printed on a non-fluorescent paper, which is invisible under natural light and can be decoded under UV light by a smartphone (Fig. 8a and Supplementary Fig. 21). When the printed paper is exposed to 460 nm light, the QR code is completely erased within 2 min. The initial state restores after standing at room temperature for 10 min, which is accelerated by heat treatment (3 min upon heating at 40°C). Thanks to the quantitative anthracene-endoperoxide photo-switching of 1, the  inkjet-printed QR pattern can be read and hidden for multiple cycles (Fig. 8b).
Dual-mode anti-counterfeiting inks from copolymers 1/2. On this basis, FRET is introduced to the supramolecular polymeric system via a two-component co-assembly strategy (mixing monomers 1 and 2 together, see Fig. 1), which represents an effective route for the fabrication of multi-mode fluorescent anticounterfeiting inks. For the efficient FRET process between 1 and 2, the prerequisite is their close distance and appropriate spectral overlap. Depending on the spectroscopic measurements, 2 features absorbance at 425-575 nm 52 , which overlaps well with the emission band of 1 (spectral overlap integral: J = 9.47 × 10 11 M −1 cm −1 nm 4 , see Supplementary Figs. [22][23]. Hence, monomers 1-2 are capable of serving as FRET donor-acceptor pairs. The FRET efficiency is then evaluated, by progressive addition of 2 into the supramolecular polymeric solution of 1 (molar ratios of 1/2: from 500: 1 to 10:1). As shown in Fig. 9a, the emission signal of 1 (λ = 505 nm) decreases in its intensity, while that of 2 centered at 560 nm gradually increases, with an iso-emissive point at 541 nm. During the titration processes, the emission colors change from green to yellow (Inset of Fig. 9a, and Supplementary   Fig. 24). For a 10:1 mixture of 1 and 2 in MCH, Φ ET (energy transfer efficiency) and k ET (energy transfer rate constant) are determined to be 80% and 2.40 × 10 9 s −1 , respectively. Notably, neither emission spectra nor color changes can be observed for the 1/2 mixture in CHCl 3 ( Supplementary Fig. 25), suggesting that FRET occurs only in the co-assembled state.
A dog pattern with yellow emission is then obtained, by inkjet printing co-assembly 1/2 on the non-fluorescent papers ( Fig. 9c and Supplementary Fig. 30). Upon 460 nm light irradiation, the emission signal is significantly weakened for its intensity (since 1-O 2 is non-emissive, the weak fluorescence is derived from the trace amount of 2), while restored within 3 min at 40°C. Considering that no chemical transformation takes place for 2 during the light irradiation processes (Supplementary Fig. 29), the fluorescent switching of 1/2 is unambiguously attributed to the interconversion between 1 and 1-O 2 , which deactivate/reactivate the FRET effect (Fig. 9b). In the meantime, fuming the dog pattern with CHCl 3 vapor leads to the appearance of green emission, which promptly turns back to yellow upon drying (Fig. 9c). The phenomena are highly plausible, since polar solvent destroys the co-assembled structure and thereby shows the characteristic fluorescent signal of 1. More importantly, the anti-counterfeiting fluorescent inks derived from 1/2 feature sufficient stability and sustainability, since almost no emission decay can be visualized upon placing the printed dog pattern under the ambient conditions for 2 weeks  Fig. 31). Overall, co-assembly 1/2 features three distinct emission signals (yellow, green, and weak fluorescence) upon light/vapor treatment, which is employed for dual-mode anticounterfeiting applications with enhanced security reliability.

Discussion
In summary, we have successfully developed photo-responsive supramolecular polymers on the basis of multicycle anthraceneendoperoxide switching. Due to Pt(II) σ-metalation effect, photooxygenation of 1 to 1-O 2 can be proceeded via the mild visible light irradiation, while the backward conversion occurs spontaneously at room temperature. The fluorescent changes of 1 in response to light can be macroscopically amplified, because of the involvement of nucleation-elongation cooperative supramolecular polymerization process. For the resulting supramolecular copolymers 1/2, FRET efficiency is orthogonally modulated via light and vapor stimuli, which facilitates the fabrication of dualmode fluorescent anti-counterfeiting inks. Hence, supramolecular polymerization of photo-fluorochromic molecules provides an effective strategy toward high-performance anti-counterfeit materials with high security reliability, fast response, and ease of operation under the mild conditions.

Methods
Measurements. 1 H NMR spectra were collected on a Varian Unity INOVA-300 spectrometer with TMS as the internal standard. 13 C NMR spectra were recorded on a Varian Unity INOVA-300 spectrometer at 75 MHz. MALDI-TOF measurements were recorded on a Bruker Autoflex Speed spectrometer with DCTB as the matrix. UV-Vis spectra were recorded on a UV-1800 Shimadzu spectrometer. CD measurements were performed on a Jasco J-1500 circular dichroism spectrometer, equipped with a PFD-425S/15 Peltier-type temperature controller. Solution excitation and steady-state fluorescence emission spectra were recorded on a FluoroMax-4 spectrofluorometer (Horiba Scientific) and analyzed with an Origin (v8.0) integrated software FluoroEssence (v2.2). TEM images were performed on a Tecnai G2 Spirit BioTWIN electron microscope (acceleration voltage: 120 kV). Electron paramagnetic resonance (EPR) measurements were performed on a JEOL JES-FA200 apparatus, with the utilization of 2,2,6,6-tetra-methylpiperidine as the spin trap for 1 O 2 . Fluorescence microscopy images were performed on an inverted fluorescence microscope (Olympus IX81).
Theoretical calculations. All optimized structures were performed on Gaussian 09 software packages. All of the non-metallic elements (C, H, O, N, and P) were described by PBEPBE/6-31G computational method, while Pt atoms were described by LANL2DZ core potential. There are no imagery frequencies for the optimized geometries.
Inkjet printing experiments. Printing tests were performed on an HP inkjet office printer (HP Deskjet 2131 model) with the customized HP803 black ink cartridges. The filled inks (black) from the inkjet cartridge were removed, and the cartridge was washed extensively with ethanol, water, and dried with N 2 blowing. The inks (0.5 mL) of 1 (1 mM) or 1/2 (2.5 mol% of 2, 0.5 mM for 1) were then loaded in the clean black ink cartridge to perform the printing experiments.

Data availability
The data that support the findings of this study are available from the corresponding author on request.