Research ArticleMATERIALS SCIENCE

Controlling information duration on rewritable luminescent paper based on hybrid antimony (III) chloride/small-molecule absorbates

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Science Advances  25 Nov 2020:
Vol. 6, no. 48, eabc2181
DOI: 10.1126/sciadv.abc2181

Abstract

Controlling the duration that information lasts on paper so that it disappears as desired is crucial for information security. However, this area is rarely studied. Here, we report [TEMA]2SbCl5 (1, TEMA+ = methyltriethylammonium), [TEA]2SbCl5 (2, TEA+ = tetraethylammonium), [TEBA]2SbCl5 (3, TEBA+ = benzyltriethylammonium), and [Ph4P]2SbCl5 (4, Ph4P+ = tetraphenylphosphonium) with structure-dependent reversible photoluminescent switching induced by the absorption and thermal release of small guest molecules including H2O, methanol, and ethylene glycol. Comparing the structural disorder levels, bond lengths, and luminescent Stokes shifts of the compounds aided in understanding their selective absorption behavior. Our results indicated that the information duration on the rewritable paper coated with the title compounds is easily tuned by changing the cation of the compounds, the type of guest molecules, and laser heating power. Our study opens previously unidentified avenues for information security and extends the potential applications of rewritable paper.

INTRODUCTION

Recently, research on rewritable paper has attracted growing attention because the annual consumption of paper has reached billions of tons (1, 2). Rewritable paper is developed from stimuli-responsive materials, on which the color or photoluminescent (PL) color can be reversibly changed under stimuli such as heat, light, and moisture (37). Among the stimuli-responsive materials, PL materials are attractive, since their application suits anti-counterfeiting and information encryption (3, 4). Regarding rewritable paper, a crucial performance indicator is the duration of the written content (810). In applications such as anti-counterfeiting labeling and secure information delivery, it is necessary that the information remain readable according to the desired expiration requirements (11). Moreover, the fidelity of the information visualization can additionally be used as a time stamp, and a missing anti-counterfeiting label signals that the document either is counterfeit or has expired. Different items will have different shelf-life requirements. Therefore, it is essential to find a simple way of tuning the information duration on the rewritable paper. However, reported rewritable papers usually have fixed information duration, and the factors influencing these times have not been studied (1). In addition, a notable number of rewritable PL research papers are based on light-responsive PL materials (9, 1214). These materials always require an initial light mask. Therefore, it is essential to develop stimuli-responsive PL materials designed for the mask- and ink-free writing process and an easily tuned information duration for the written content.

Recent evidence indicates that the loss/uptake of small molecules such as H2O in stimuli-responsive materials can result in reversible changes in both the absorption spectra and the PL emission (15, 16). However, these studies used either porous metal-organic frameworks or complex organic molecules as clathrate materials (10, 1517). As a result of the complex structures, most of these studies faced challenges with synthesis and purification. Recently, there has been a growing interest in inorganic-organic hybrid metal halides (IOMHs) based on their perceived advantages, including low cost, simple synthesis and purification procedures, and, notably, chemical flexibility (1822). These materials have a general chemical formula of AmBXn (A = organic cation, B = metal ion, and X = halide anion). Different materials can be obtained simply by changing A, B, and/or X with possible applications of photovoltaics (23), piezochromism (24), luminescence (25), phosphorescence (26), and thermochromism (4, 27, 28). The compositions and ionic structural characteristics dictate relatively weak interactions between cations and anions in IOMHs, resulting in highly resilient structures and chemical reactivity (19). For instance, the chemisorption of iodine in nonporous two-dimensional (2D) IOMHs was reported (29), illustrating the absorption/desorption of guest molecules by IOMH structures. Considering the potential spectroscopic effects of the guest molecules on the PL properties of IOMHs, the absorption/desorption processes would result in reversible changes in the PL spectra for the IOMHs.

Here, we propose a novel strategy to achieve tuning of the visualized information duration on rewritable PL paper. We describe tuning the structure of various IOMHs by changing the organic cations, thereby tuning their absorption and desorption behavior for different guest molecules and hence the spectral characteristics of the complexes. One effect was the demonstration of variable durations for the written material visualized by PL, which relies on the absorption/desorption rate of guest molecules and the effects on the PL spectra. We therefore synthesized a series of Sb-IOMHs with a general formula of A2SbCl5 and specifically characterized [TEMA]2SbCl5 (1, TEMA+ = methyltriethylammonium), [TEA]2SbCl5 (2, TEA+ = tetraethylammonium) (30), [TEBA]2SbCl5 (3, TEBA+ = benzyltriethylammonium) (31), and [Ph4P]2SbCl5 (4, Ph4P+ = tetraphenylphosphonium) (32). The compounds all exhibited bright luminescence in the solid state under dry conditions. Notably, the reversible absorption/desorption of a series of small molecules, including H2O, methanol, and ethylene glycol (EG), was observed. The four compounds each exhibited different absorption and desorption behavior for the small-molecule series. In addition, the absorption process resulted in strong quenching of the intrinsic PL of the compounds. The reversible spectroscopic changes, i.e., the regeneration of strongly quenched luminescence, were triggered by directly heating the compounds. By coating the compounds on paper or polymethyl methacrylate (PMMA) films, a series of rewritable luminescent objects with tunable visible features were fabricated, and the writing process could be performed by direct laser writing, which is mask-free and ink-free. We additionally found that the laser intensity affects the process itself and the persistence time of the written content. Therefore, the duration of the written content was easily tuned by changing either the organic cation of the IOMHs, the guest molecule, or the laser intensity.

RESULTS

Crystal structures and PL properties of the materials

The commonality of the crystal structures is that they have 0D ionic structures with a similar anion of [SbCl5]2− and the neighboring cations surrounding the anions. By changing the organic cations, the packing of the ions is adjusted accordingly (Fig. 1). The crystallographic asymmetric unit of 1 consists of four cations and two anions. The crystallographic asymmetric units of both 2 and 3 have eight cations and four isolated [SbCl5]2− anions. The crystallographic asymmetric unit of 4 has two cations and one [SbCl5]2− anion.

Fig. 1 Crystal structures of the compounds and their PL properties.

(A) Unit cell of 1 viewed along the c axis. (B) Unit cell of 2 viewed along the c axis. (C) Unit cell of 3 viewed along the a axis. (D) Unit cell of 4 viewed along the a axis. (E) PL spectra of the compounds. (F) Commission internationale de l’éclairage (CIE) coordinates of the emission colors. a.u., arbitrary units.

The presence of disordered structures indicates the different structural variance in the compounds with “free space” for the atom and guest molecular movement. In 1, the position of the methyl group of the cation is split between four positions with equal occupancy, making TEMA+ look similar to TEA+. In 2, there are two crystallographically inequivalent TEA+ ions in the crystal structure: One is ordered, and the other is disordered (30). Each methylene (CH2) of the disordered TEA+ is equally split between two positions, as shown in fig. S1. Regarding the anions, the position of the axial chlorine atoms in 1 and 4 is equally split between two opposite positions, making the anionic unit in 1 and 4 appear as six coordinates. Proceeding from 1 to 3, the increasing cation volume and the increased steric hindrance result in the decrease of the disorder level, implying that the free space for the atom movement or the cation/anion rotation is shrinking. However, in 4, although the volume of the cation is larger, sufficient space is left for the anion because of the planarity of the benzene rings.

The structural resilience of the compounds could also be estimated using their different PL properties. Mostly, low-dimensional IOMHs exhibit self-trapped exciton (STE) emission, resulting in large Stokes shift and broad emission spectra (24, 3337). The STE properties in IOMHs are closely associated with the structural distortion in their excited states (3739). Recent evidence indicates that the distortion levels of the individual anion part of IOMHs can be calculated by the formula shown belowΔd=(15)Σ[dndd]2where d is the average Sb─Cl bond length and dn values are the five individual Sb─Cl bond lengths. The ∆d values of the four compounds are 6.664 × 10−3, 1.607 × 10−3, 1.664 × 10−3, and 4.45 × 10−3, respectively. As for the PL properties, the emission peaks of the four compounds are respectively located at 636, 626, 584, and 650 nm, as shown in Fig. 1E. The Stokes shifts in emission of the compounds are 280, 241, 224, and 275 nm, respectively, and the order of Stokes shifts is usually consistent with the calculated ∆d values of the compounds, with 2 and 3 as exceptions. We additionally found that the structural distortion level of [SbCl5]2− could be predicted simply by the bond length between the Sb atom and the axial Cl atom, which is more consistent with the order of Stokes shifts. The bond length can also be affected by the steric bulk of the organic cation (33). Increasing the steric bulk restricts the free space of Cl, increasing the bond length (32, 35, 40). A shorter bond length allows a greater increase in the bond length in excited states, resulting in a larger Stokes shifts (41). As listed in tables S1 to S4, the longest Sb─Cl bond exists in 3 (2.376 Å), while the shortest occurs in 1 (2.091 Å). As the volume of the branched-chain quaternary ammonium ions increases from 1 to 3, the distortion levels and Stokes shifts decrease accordingly. Regarding 4, the short bond length (2.197 Å) and large ∆d result in a notable Stokes shift. Furthermore, the large free space in 1 and 4 increases the vibronic options of the atoms, leading to energy dissipation and a decrease in PL quantum efficiency (PLQY). The PLQYs of the compounds were 46, 96, 99, and 87%, respectively.

The emission colors of the compounds are shown in Fig. 1F, and the CIE coordinates of 1, 2, 3, and 4 emissions are (0.590, 0.405), (0.560, 0.436), (0.501, 0.491), and (0.612, 0.386), respectively. The absorption spectra in fig. S2A show that the PL of the four compounds mainly originates from the 1S03P1 transitions in Sb3+ in [SbCl5]2− (4, 31, 36). For the compounds with nitrogen-containing cations (1, 2, and 3), the absorption band at approximately 300 nm is attributed to the 1S01P1 transitions in Sb3+ in [SbCl5]2− (31), and the absorption intensity decreases with the increasing chain length. For excitation at 309 nm, 1 shows dual-band emissions at 483 and 636 nm (fig. S2B), which is also present in 2 and 3 (31); however, the ratio of Ihigh energy emission/Ilow energy emission decreases with the increasing chain length.

Reversible photoluminescence switching

Partially based on the free space in the crystal structures, the compounds show different absorption and thermal release behavior for small guest molecules such as H2O, MeOH, and EG. The intrinsic PL of the compounds was strongly quenched by absorption of the small molecules. The PL of 1 and 2 was quenched in 1 hour in a high-humidity environment [relative humidity (RH) ≈ 100%], and the rate of absorption was 12 (fig. S3A). The total water uptake of 1 was >2 (mass uptake was 118.7 versus 114.9%, respectively), and weight changes of 5.9 and 2.9% were measured for 3 and 4, respectively, as shown in Fig. 2B. The origin of the weight change can be divided into three parts: the water absorbed into the structure of the compounds, the water adsorbed on the surface of the powder, and the water adsorbed by the compounds due to their deliquescence. The deliquescence of crystalline material mostly starts from forming its water-containing structure, implying the relatively strong interaction between water molecules and the material (42). The interaction would help the material in capturing the water molecules into it. On the basis of these data, we observed some selectivity for the absorption of water by the materials studied. Selectivity depends on the free space in the crystal structures, steric effects, and polarity of the organic cations. As mentioned earlier, the free space of the crystal structures of the title compounds is estimated by their structural disorder levels and the PL Stokes shifts and is ranked in the order of 14 > 2 > 3. However, we observed that the H2O absorption in 4 is far less than that in 1. This could be attributed to the high-symmetry structure of Ph4P+, which reduces its polarity, and the benzene rings, which increases the steric effect. After that, MeOH molecules with a lower polarity index of 6.6 (43) and a larger molecular volume were studied by treating the available powders (for more details, see Materials and Methods). The polarity index number shows the ability of the solvent to interact with solutes (43). Consequently, the PL of 1, 2, and 4 was completely quenched in 2 hours, and the rate of absorption was 142 (fig. S3B), with a weight gain of 141.4% for 2, 128.8% for 4, 120.9% for 1, and 101.2% for 3 (Fig. 2B). When the guest molecule was EG, with a similar polarity index number (6.9) but much larger molecular volume than MeOH, only the PL of 1 was quenched after 12 hours of exposure, with a weight change of 22.7% (Fig. 2B and fig. S3C).

Fig. 2 Absorption/desorption behavior of the compounds (1 to 4).

(A) Schematic diagram of the absorption and subsequent thermal release of small guest molecules by the compounds. (B) PL switching and weight changes of the compounds in the absorption and thermal release process (under 365-nm UV light illumination).

The excess adsorbed weight of the guest molecules in all of the materials studied declines again after heat-treating the non-emissive powders (Fig. 2B) or placing them in a dry environment, and the original PL spectrum is regenerated (fig S3). The thermogravimetric (TGA) curves clearly show no weight loss <473 K for all the pristine compounds (fig. S4A), indicating that the weight loss of the non-emission powders can be ascribed to the loss of absorbates. We additionally found that the non-emissive powders start to lose absorbates from room temperature (RT), as shown in figs. S3 and S4 (B and C). The low-temperature desorption processes suggest that the restriction of the compounds on the small guest molecules is not strong. As the environment changes, the guest molecules would escape from the structure, and the original PL spectrum of the compounds would be regenerated. The moisture-dependent PL changes of 1 and 2 were studied further by locating the powders in a series of environments with different RHs (for more details, see Materials and Methods). The RH threshold for PL quenching and regeneration was 33 to 43% for 1 and 59 to 70% for 2, as shown in fig. S5. Concerning the PL quenching mechanism, as the small molecules are absorbed into the crystal structures, a dissipation mechanism for the excited-state energy is provided by the O—H vibration and rotation of the guest molecules, resulting in the quenching of PL. The regeneration of the PL is caused by the release of the molecules.

To understand the effects of small-molecule absorption into the actual solid-state structures of the compounds, we recorded the powder x-ray diffraction (PXRD) patterns of the gas-treated powders. Here, we used 1-H2O and H2O@1 as short designations for the H2O gas–treated powder of 1 and the complex generated from loading H2O molecular into the structure of 1, respectively. For example, after treating 1 with H2O gases, the composition of 1-H2O could be pristine 1, pure H2O@1, or a mixture of them. As shown in Fig. 3 (A and B), the peaks marked by the yellow star in the PXRD spectra suggested similar structures for gas-treated 1 and 2, regardless of the kind of gas used. This indicates that the absorption/desorption processes are nondestructive. Notably, the peaks marked by the yellow stars indicate that the solid-state structures of H2O@2 and MeOH@2 are similar to H2O@1, MeOH@1, and EG@1. The arrangements of the cations and anions in 2-HT (structure of 2 at a temperature higher than 353 K) (30) and 1 are similar to each other, as shown in Fig. 1B and fig. S6. The insertion of gas molecules into the solid-state structures causes an expansion of their crystal lattice, which is similar to the effect of increasing temperature. As a result, the ions in 1 and 2 twist to adapt to the inserted molecules, yielding five similar structures. In addition, the peaks marked by the yellow triangle and yellow star in the PXRD spectrum of 2-H2O show that the powder is a mixture of 2 and H2O@2. The dry environment present during the PXRD measurements explains the in situ structural transformation of some solvated powders. The PXRD of EG-treated 2 (Fig. 3C) shows that EG molecules were only slightly absorbed into the structure of 2. The peaks marked by the yellow stars indicate a structure similar to the MeOH@2 generated. This is consistent with the weight change shown in Fig. 2B. A similar phenomenon was observed in 4-EG. Compared with H2O absorption, the absorption of MeOH by 4 results in a structural transformation, as shown in the PXRD spectrum (Fig. 3D). This phenomenon is similar to findings from a previous study (32), in which the rapid crystallization of 4 from its N,N′-dimethylformamide (DMF) solution yielded a DMF-containing structure. The peaks in the high-angle area (18° to 20°) of as-synthesized 4 are a little different from the simulated PXRD spectrum of 4. This phenomenon can also be found in a previous study (32). A possible reason for the peaks’ shift is the crystal lattice distortion caused by the absorption/desorption of solvent molecular from the crystal structure. Concerning 3, no obvious structural transformation was observed after gas treatment, but with a shift of some of the PXRD peaks due to the slight gas adsorption. For all the gas-treated powders, heating at 393 K for 1 hour restores their pristine structures, as shown by the PXRD spectra and the restoration of the original PL spectra (Figs. 2B and 3).

Fig. 3 PXRD spectra of the as-synthesized compounds and gas-treated compounds before and after thermal release.

PXRD spectra of compound 1–related powders (A), compound 2–related powders (B), compound 3–related powders (C), and compound 4–related powders (D).

Application in rewritable PL paper

The small-molecule absorption/desorption behavior of materials 1 to 4 and the reversible effects on the PL spectra make these materials suitable for rewritable PL paper. By coating materials 1 to 4 on paper and treatment with certain small molecules such as H2O, MeOH, and EG, we found that the initial PL was quenched, and images could be written by spatially resolved heating. In contrast with the other rewritable paper techniques based on chemical species or light-triggered color-changing materials, the heating could be accomplished by a laser, providing an ink- and mask-free process. The schematic diagram of the writing process is shown in Fig. 4A. Note that there is an RH threshold when considering the effect of the external environment on the PL properties of the compounds (figs. S3 and S5). If the RH of the environment is higher than the threshold, then the PL of the written contents would be quenched gradually. In contrast, the PL of the non-emissive part on the paper would be restored in a low-humidity environment. The PL restoring time and quenching time can both be used to calculate the duration of information, and the duration was easily tuned by choosing both the cation of the IOMH and the guest molecules.

Fig. 4 Image retention in rewritable paper.

(A) Schematic diagram of the laser writing process. The images of 2@paper-H2O (B), 2@paper-MeOH (C), and 4@paper-MeOH (D) during writing and rewriting (under 365-nm UV light illumination).

As a proof-of-concept experiment (for more details, see Materials and Methods), pieces of filter paper were coated with polycrystals of the title compounds as described. After that, we annealed the coated paper at 353 K for 2 hours. No notable color change was observed by eye on the coated filter paper after the coating and heating process. The samples emitted visible light under ultraviolet (UV) excitation. To simplify the designation, we use 2@paper-H2O to indicate paper that was coated with 2 and then exposed to a H2O gas–filled environment until the PL was completely quenched. After that, an image and text were written on the paper using a laser engraving machine. The image and letters can be seen under only UV light, as shown in Fig. 4B. However, the writing of the PL image cannot be achieved on samples of 1@paper-H2O, 1-MeOH, and 1-EG. This is because the PL recovery threshold laser power of 1 is higher than the carbonization laser power of the filter paper. Once the power of the laser was enhanced beyond the carbonization laser power, the paper was charred. In contrast, 2@paper-H2O, 2@paper-MeOH, and 4@paper-MeOH could be written by careful selection of the laser power, as shown in Fig. 4. The information duration of 2@paper-H2O, 2@paper-MeOH, and 4@paper-MeOH was measured by eye as 3 days, 5 days, and 3 months, respectively (from October to December in Chongqing; RH ≈ 70%). However, in a practical application, the lifetime of the written contents on paper could be defined as the time when the PL intensity decreases to 5% (or other reasonable value) of the initial value. The PL papers could be regenerated by heating, gas treatment, and then rewriting, and the process can be repeated at least 10 times.

We then mixed the title compounds with a polymer such as PMMA to produce a rewritable “PL paper” to address the challenge of the lower heat resistance of filter paper and the problem of poor adhesion between reagents and the filter paper. The PMMA-based paper can also be written on by laser, and the duration of the written content is easily tuned by laser power. Hence, the circle shown in Fig. 5 was fabricated by the laser writing process, in which the laser power was increasing gradually in a clockwise direction. The duration of the information on PMMA increased with increased laser power. This is because more dehydration is accomplished under a higher power laser exposure. Once the PMMA paper was treated under ambient conditions (January, Chongqing; RH ≈ 50%), the PL of the circle on 1@PMMA-H2O was quenched because of the moisture. However, the PL on 2@PMMA-H2O slowly regenerated over time owing to the lower PL quenching threshold than 1 (33 to 43% for 1 and 59 to 70% for 2; fig. S5). Consequently, if the paper is applied in a low-humidity environment, then the lifetime of the written contents could be defined as the time when the PL intensity of the non-emission part increases to 95% (or other reasonable value) of the initial value.

Fig. 5 Image retention in rewritable doped polymer films.

The images of 1@PMMA-H2O (top) and 2@PMMA-H2O (bottom) over time (under 365-nm UV light illumination).

DISCUSSION

The time-stamped paper could be applied in fields where the recorded information should disappear with time, such as for waybills and secret information delivery. In this way, people’s privacy and secret information could be better protected. For example, most waybills contain private information about the sender and the receiver, creating a risk of privacy leakage. By using the limited duration writing, ideally, the private information would disappear after the package is successfully delivered. Another possible application of the coated paper described is temporary access certification, where the certification would become invalid with time along with the disappearance of the container’s information. Another application for time-dependent PL changes on the paper is as time indicators on commodities with a fixed shelf life. The shelf lives of most products are fixed, and environmental factors such as temperature and humidity on location are ignored. Therefore, using materials that respond to humidity and temperature as time indicators can better show whether the product has expired. Last, on the basis of the fluorescent properties of the material, indicators are additionally used in anti-counterfeiting labels. Notably, the title compounds may be toxic to humans, limiting their application in food and children’s product industry. However, the toxicity of the compounds could be reduced by replacing Sb3+ with lower-toxicity metal ions (4447).

In conclusion, the structure-dependent absorption and thermal release of small molecules (such as H2O, MeOH, and EG) from IOMHs and reversible PL switching were reported. Comparing the structural disorder levels, bond lengths, and PL Stokes shifts of the compounds aided in understanding the selective absorption behavior of the compounds. The reversible absorption/desorption of small molecules into the solid state was confirmed by analyzing the PXRD spectra and the weight gain of the compounds during the absorption and release process. Similarly, the process was studied by TGA. The regeneration of the photoluminescence (removing the absorbate) was achieved by heating the non-emissive powder. The rewritable PL paper was prepared by coating the title compounds on paper or doping them into polymeric materials such as PMMA. Words and images could be written on the “paper” through laser heating, which is mask- and ink-free. This is to be compared with the light- or chemical species–triggered PL switching reported in the literature. Notably, our report shows that the duration of the written content on the rewritable paper is easily tuned by changing the cation of the IOMHs, guest molecules, laser power, or substrate materials. The combination of applications such as anti-counterfeiting, information encryption, time stamp indicators, etc. for the rewritable paper creates a new avenue in research for rewritable paper.

MATERIALS AND METHODS

Materials

SbCl3, PMMA, [TEMA]Cl, [TMA]Cl, [TEBA]Cl, and [PPh4]Cl were procured from Aladdin and used without further purification.

Characterization

All the characterizations were performed at RT. Photo credit for all the figures: Zeping Wang, Chongqing University. UV–visible spectra were obtained by using BaSO4 as a standard (100% reflectance) with a Shimadzu UV3600. PXRD was performed on a PANalytical X’Pert Powder instrument. Simulated PXRD patterns through x-ray single-crystal structures were generated by using the Mercury program. TGA analysis was performed on a Mettler Toledo TGA/DSC1/1600LF in a N2 atmosphere from 25° to 800°C. Single-crystal x-ray diffraction data were collected on a SuperNova charge-coupled device diffractometer with graphite-monochromated MoKα (0.71073 Å) at 293 K for 1 and 298 K for 3. Emission and excitation spectra were recorded on an Agilent Cary Eclipse. The quantum yield measurements were performed on an FLS1000 produced by Edinburgh Instruments.

Element analysis of C, H, and N was carried out on an Elementar Analysensysteme GmbH Unicube instrument. C14H36N2SbCl5 (1) calculated: C, 31.64%; H, 6.83%; N, 5.27%; found: C, 31.22%; H, 6.75%; N, 5.16%. C16H40N2SbCl5 (2) calculated: C, 34.35%; H, 7.21%; N, 5.01%; found: C, 34.23%; H, 6.93%; N, 4.97%. C26H44N2SbCl5 (3) calculated: C, 45.68%; H, 6.49%; N, 4.10%; found: C, 45.34%; H, 6.47%; N, 4.22%. C48H40P2SbCl5 (4) calculated: C, 58.96%;H, 4.12%; found: C, 58.68; H, 3.85%.

Crystallography

Structures 1 and 3 were determined by single-crystal x-ray diffraction and solved by direct methods and refined by full-matrix least-squares on F2 by using the programs SHELX-2014 (48) and OLEX 2 (49).

Crystal data for compound 1: C14H36N2Cl5Sb1, M = 531.45, tetragonal, I4/mmm, a = 10.0282(5), c = 12.3775(17) Å, V = 1244.7(2) Å3, T = 293(2) K, Z = 2, μ(Mo-Kα) = 1.711 mm−1, F(000) = 540, crystal size = 0.3 × 0.2 × 0.2 mm, 457 independent reflections (Rint = 0.034). Final R1 = 0.0585 for 326 reflections with I > 2σ(I), and wR2 = 0.1856 for all data.

Crystal data for compound 3: C26H44Cl5N2Sb, M = 683.64, orthorhombic, P212121, a = 9.3316(3), b = 13.7575(4) Å, c = 24.4943(8) Å, V = 3144.57(17) Å3, T = 298(2) K, Z = 4, μ(Mo-Kα) = 1.711 mm–1, F(000) = 1400, crystal size = 0.24 × 0.23 × 0.23 mm, 10,894 independent reflections (Rint = 0.023). Final R1 = 0.0395 for 5912 reflections with I > 2σ(I), and wR2 = 0.0664 for all data.

The Cambridge Crystallographic Data Centre (CCDC) 1993074 (1; 293 K) and 1993075 (3; 298 K) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre. The crystal data for compounds 2 and 4 could be found in (30) and (32), respectively.

Synthesis of the compounds

Compound 1 was prepared by mixing 1 mmol SbCl3 and 2 mmol [TEMA]Cl in 10 ml of DMF at RT, followed by the addition of 5 ml of diethyl ether into the solution. After that, the crystals were filtered and heated at 373 K for 3 hours. Block-like colorless crystals with orange light emission were formed with a yield of near 100%. Block-like crystals of 2, 3, and 4 were respectively formed in the same manner by changing the reactants to [TMA]Cl, [TEBA]Cl, and [PPh4]Cl, respectively.

Fabrication of PL paper

The slurry of 2 was prepared by adding 100 mg of 2 as a powder into 2 ml of DMF. After that, the slurry was transferred to a filter paper (2 cm × 2 cm) through the brush coating method. After transferring the paper to an oven, followed by heating at 393 K for 1 min, the PL paper (2@paper) was generated. The same method was used for the fabrication of 4@paper.

Synthesis of PL film

We first prepared a saturated acetone solution of PMMA. Then, 100 mg of either 1 or 2 was added to 10 ml of MeOH to form a clear solution. The MeOH solution was then mixed with 10 ml of the saturated acetone solution of PMMA. The mixture was then transferred to a watch glass and sealed with a parafilm, with several holes in the film. The watch glass was left standing for 1 week to produce a non-emissive, doped PMMA film. The PL in the film was restored through heating at 353 K for 3 hours.

Reversible PL switching

We placed the polycrystalline powders in glass bottles and then placed them in at 100% humidity environment for 2 hours, which caused the PL of 1 and 2 to be completely quenched. The PL of all the samples was restored by heating the powders at 323 K for 1 hour. The same operations were performed on the polycrystalline powders by replacing saturated H2O vapor with saturated MeOH vapor and EG vapor (Fig. 2B and fig. S3).

Moisture-induced PL quenching in 1 and 2

A series environment with different RH values was created by sealing saturated solutions of different salts in different large glass bottles. The salts include LiCl, MgCl2, K2CO3, NaBr, KI, and NaCl, corresponding to the RH values of 11, 33, 43, 59, 70, and 75% at 298 K, respectively. Then, a series of small glass bottles containing pristine 1 or 2 were separately localized in the large bottles with different RH values. The PL of some of the powder would be quenched after 2 hours, as shown in fig. S5.

Laser writing process

1@PMMA-H2O. A piece of 1@PMMA (1 cm × 1 cm) was first treated with H2O vapor (~100% RH, 303 K) until the PL was completely quenched. After that, an image was written on the film using a laser engraving machine. The laser power was set as 4.16, 4.32, and 4.48 W for the third circle at the top left, top right, and bottom, respectively (the top panels in Fig. 5). The film was stored under ambient conditions (January in Chongqing; 50% RH, 288 K). The PL of the top left sector was first quenched, then the top right sector, and, last, the bottom sector.

2@PMMA-H2O. A piece of 2@PMMA (1 cm × 1.2 cm) was first treated with H2O vapor (~100% RH, 303 K) until the PL was completely quenched. After that, an image was written on the film using a laser engraving machine. The laser power was set as 4, 4.16, and 4.32 W for the third circle at the top left, top right, and bottom, respectively (the bottom panels in Fig. 5). The film was stored under ambient conditions (January in Chongqing; 50% RH, 288 K). The PL intensity of the non-emission part of the film would first increase to the intensity level of the top left sector, then the top right sector, and, last, the bottom sector.

2@paper-H2O. The preformed 2@paper was first treated with H2O vapor (~100% RH, 303 K) for 2 hours. After that, an image was written on the paper using a laser engraving machine, and the power was set as 2.88 W. The film was then stored under ambient conditions (October in Chongqing; 70% RH, 298 K).

2@paper-MeOH. The preformed 2@paper was first located in a sealed container with MeOH vapor for 2 hours (303 K). After that, an image was written on the paper with a laser engraving machine, and the power was set as 2.96 W. The film was stored under ambient conditions (October in Chongqing; 70% RH, 298 K).

4@paper-MeOH. The preformed 4@paper was first located in a sealed container with MeOH vapor for 2 hours (303 K). After that, an image was written on the paper with a laser engraving machine, and the power was set as 3.6 W. The film was stored under ambient conditions (October in Chongqing; 70% RH, 298 K).

SUPPLEMENTARY MATERIALS

Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/6/48/eabc2181/DC1

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REFERENCES AND NOTES

Acknowledgments: Funding: This work is supported by the Key Science and Technology Program of Chongqing under grant no. CSTC2017SHMS-ZDYFX0028; the Technology Innovation and Application Project of Chongqing under grant no. CSTC2018JSZX-CYZD0587; the Pre-research project under grant no. L-SD-11; the Pre-Research Project for 13th Five-Year Plan under grant no. 614041404; the Fundamental Research Funds for the Central Universities under grant nos. 2019CDXYDQ0009, 2019CDQYDQ024, 2019CDJGFGD007, 2019CDXYGD0028, and 106112017CDJQJ128836; and the National Key Research and Development Program of China Stem Cell and Translational Research under grant no. 2018YFB2100100. Author contributions: Z.W. and X.C. conceived the project. Z.W. designed and performed most of the experiments. Z.W. and D.X. carried out the syntheses. Z.W., D.X., and F.Z performed the TGA analysis, x-ray single crystal diffraction, and XRD experiments. Z.W analyzed the data and prepared the manuscript. C.P.W., X.C., and J.Y. revised the manuscript. All the authors discussed the results and commented on the manuscript. X.C. and C.P.W. supervised the project. 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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