Multi‐Component Collaborative Step‐by‐Step Coloring Strategy to Achieve High‐Performance Light‐Responsive Color‐Switching

Abstract Light‐responsive color‐switching materials (LCMs) are long‐lasting hot fields. However, non‐ideal comprehensive performance (such as color contrast and retention time cannot be combined, unsatisfactory repeatability, and non‐automated coloring mode) significantly hinder their development toward high‐end products. Herein, the development of LCMs that exhibit long retention time, good color contrast, repeatability, and the property of automatic coloring is reported. The realization of this goal stems from the adoption of a bio‐inspired multi‐component collaborative step‐by‐step coloring strategy. Under this strategy, a conventional one‐step photochromic process is divided into a “light+heat” controlled multi‐step process for the fabrication of the desired LCMs. The obtained LCMs can effectively resist the long‐troubled ambient‐light interference and avoid its inherent yellow background, thereby achieving the longest retention time and good repeatability. Multiple colors are generated and ultra‐fast imaging compatible with the laser‐printing technology is also realized. The application potential of the materials in short‐term reusable identity cards, absorptive readers, billboards, and shelf labels is demonstrated. The results reported herein can potentially help in developing and designing various high‐performance, switchable materials that can be used for the production of high‐end products.

5 PC spectrophotometer. 1 H NMR (400, 500, 600 MHz) and 13 C NMR (101,126,151 MHz) spectra were recorded on a Bruker AVANCE400 (AVANCE500 or AVANCE600) at room temperature. LC-HRMS analysis was performed on an Agilent 1290-micro TOF-Q II mass spectrometer (electrospray ionization (ESI) source). CIE L*, a*, b* was measured by X-rite spectrodensitometer. Microscale colors of the VLCM were imaged in transmission, reflective and fluorescence modes using Leica DM4000 M microscope. Scanning electron microscopy (SEM) images were taken using field-emission scanning electronic microscopy (FE-SEM; SU8020, HITACHI) and field-emission environment scanning electronic microscopy (Quattro, ThermoFisher Scientific) and energy dispersive spectroscopy (EDS) images were taken using energy dispersive spectrometer (XFLASH 6/60, Bruker). Photographs were captured by using the Nikon D7100 camera and enlarged image was obtained by using microscope. The melting points were taken using an SGW X-4B microscopy melting point apparatus (Shanghai, China).
Density Functional Theory (DFT) Calculation. All structures were optimized with DFT functional B3LYP and 6-31G (d, p) basis set by Gaussian 09 S1 . The energy barriers were calculated and solvent effects with the polarizable continuum model (PCM) was considered. Computed structures were illustrated using CYLVIEW drawings.
Preparation of compounds 1-4. Compounds 1-4 were prepared according to the literature S2 . To a solution of Rhodamine B (0.67 g, 1.4 mmol) in ClCH2CH2Cl (10 mL) at room temperature, POCl3 (1.03 mL, 11.2 mmol) was added dropwise. The mixture was kept stirring at room temperature for 15 min, heated to 85 o C for 6 h and then cooled to room temperature and concentrated under vacuum to give a salt. The salt was dissolved in dry CH3CN (10 mL), then the solution was added dropwise to a solution of desired NH2-R (2.8 mmol) in dry CH3CN (5 mL) containing Et3N (10 mL). The mixture was stirred at room temperature overnight. The mixture was concentrated under vacuum.

General test method for the prontonation kinetics of 1-4 with PAH-F. Unless otherwise
specified, all the kinetic data for the prontonation of 1-4 by PAH-F was obtained according to the following method: PAH-F in MeOH was firstly added to the MeOH solution of 1-4. The mixture of PAH-F and 1-4 was then irradiated by a blue LED light till the fully conversion of PAH-F to SP-F (set as time = 0, the initial time of kinetics test), which could be visualized as the mixture turned to be transparent from yellow. The mixture was kept irradiating, illustrating that the photoacid was in its SP-F form during the measurement. While its spectra was measured at set time intervals and the kinetics data was obtained by measuring the maximum absorption changes of the protonated 1-4 with time.
General test method for the prontonation kinetics of 1-4 with CH3SO3H. CH3SO3H in MeOH was added to the MeOH solution of 1-4 at time = 0 s (the initial time of kinetics test). The kinetics data was obtained by measuring the maximum absorption changes of the protonated 1-4 with time. Fig. S1 X-ray single crystal diffraction of PAH-F.
The protonation of 1-4 can be described below (Eqn. (1)) according to the reported work S2 : At the early stage of the reaction, the rate controlling step is the second stepring-opening step, due to the high concentration (10 equivalent to Rh-B derivatives) of hydrogen ions in the solution. And we assume that k2 >> k-2, thus the rate of the protonation process can be described as below (Eqn. (2)): If we assume the first reaction step is in rapid equilibrium, then: Then the rate of the protonation process is: Defines the constant k to give Eqn. (6): Integral the above eqn. (6) to obtain: Therefore, the data collected at the early stage can be fitted well into a first order reaction where C0 and Ct are the initial concentration of closed form of 1-4 and the concentration at time t of the protonated forms (Fig. S3).

Note S2 Ea(s) of 1-4.
To test the temperature-dependent protonation rates of 1-4 induced by PAH-F for further evaluating their apparent activation energy (Ea) respectively during this process, continuous light irradiation as well as heating are needed simultaneously while monitoring the UV-vis spectra. As the used light for irradiation would influence the spectral measurement, sampling gradually over time and testing rather than in situ kinetics was adopted. Under this operation condition, the protonation rates of 2-4 with fast protonation rates, especially at higher temperatures, cannot be acquired.
Considering the limitation of the experimental operating conditions and the protonation rates of 1 induced by CH3SO3H and PAH-F respectively are very similar ( Fig. 1e and S4, Table S2) as well as similar acidity of PAH-F with CH3SO3H, CH3SO3H was used for replacing PAH-F to evaluate the Ea(s) for the protonation process of 1-4. In this case, even though the obtained Ea(s) may be not exactly the same by using CH3SO3H and PAH-F, their values should be in the same order.
Based on the obtained rate constants in Table S2, the energy barrier can be calculated from Arrhenius equation:

Ea k
Thus, the energy barrier is obtained by plotted in the form lnk versus the reciprocal of temperature with the slope of the curves are -Ea / R (Fig. S6).

Note S3 Reversibility of 1 & PAH-F under dark or sole heating condition.
Because of the relaxation process of SP-F form to PAH-F taking back the free proton in the solution in the dark, continuous light irradiation is needed to ensure 1 to be maximized pronated. Otherwise, SP-F and 1 will compete for proton in dark conditions, which will lower the pronation of 1. For a deep understanding, the mixture of PAH-F and 1 was first irradiated till the fully conversion of PAH-F to SP-F (set as time = 0), which could be visualized as the mixture turned to be transparent from yellow. Then the mixture was put in dark and meanwhile the absorption at 554 nm was measured with time ( Fig. S10 solid lines). As a comparison, the absorption at 554 nm of the transparent mixture kept on irradiating was measured at set intervals ( Fig. S10 dot lines). For the former situation, large amount of proton released during the irradiation, so for a period of time after the light is off, there is enough proton for 1 to be protonated. As shown in Fig. S10, the absorption at 554 nm increased first and the rates increased with temperature. With time going on, the absorption at 554 nm decreased to almost 0. It is inferred that SP-F took the proton from protonated 1 and recovered to PAH-F, which further indicating that the proton transfer is reversible as it will turn to original in dark. While for the latter situation, photoacid existed in the SP-F form all the time, so 1 kept protonated until it reached equilibrium and the time to equilibrium is positively correlated with temperature. Besides, it could be calculated that up to 20% of 1 could be protonated without continuous light irradiation.