Stimulus-responsive multifunctions in a zinc(II) sulfate complex: photochromism, photoswitching nonlinear optical properties, amine detection and visual film application

Abstract

Stimulus-responsive functional materials show broad applications in the fields of sensing, optical switching and energy conversion devices. Herein, a one-dimensional chain compound constructed from the electron-deficient rigid polypyridine ligand 2,4,6-tri(4-pyridyl)-1,3,5-triazine and electron-rich sulfate was successfully synthesized with reversibly electron transferred photochromic and photoluminescent behaviors. Second-harmonic generation (SHG) active behavior studies showed that the compound exhibits photomodulation of nonlinear optical properties with an impressive high SHG-switching contrast (ca. 6 times) before and after light irradiation. Furthermore, this compound exhibited selectively vapochromic behavior in response to different amines, allowing its application in a visual recognition detector for specific amines. Based on the photochromic and photoluminescent properties, applications in inkless printing and information encryption were successfully demonstrated. This work shows a facile way to obtain sulfate-based compounds with stimulus-responsive multifunctions, expanding the repertoire of multifaceted chromic materials for advanced visual chromic sensors.

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Introduction

Stimulus-responsive functional materials constitute a fascinating class of materials that possess the remarkable ability to react to external stimuli, directly eliciting changes in physical or chemical properties.^1–4 These materials have garnered immense interest due to their promising potential applications in innovative fields such as smart windows, advanced sensing technologies, and intelligent glass systems.^5–9 Over the past few decades, researchers have diligently sought out versatile materials with a variety of structures, functionalities, and applications. Coordination polymers (CPs) have stood out as formidable contenders in the design of stimuli-responsive materials, showcasing their immense potential for applications spanning medicinal,^10–12 inorganic chemistry,^13,14 biochemistry,^15,16 materials science,^17,18 and electrochemistry.^19,20 Therein, due to their unique electron-deficient properties, electron transferred CPs constructed by viologen derivatives have garnered widespread application in the fields of magnetism,^21,22 luminescence,^23–25 electrical conductivity,^26,27 sensing, and catalysis.^28,29 In particular, they exhibit fascinating reactive properties to external stimuli such as light,^26,30,31 heat,³² electricity,³³ humidity, and organic amines.^34,35 Currently, most viologen-based CPs are constrained to responding solely to a single stimulus, and their sluggish and unstable response significantly hinder their practical application in industrial manufacturing and daily life. Hence, it is imperative to devise and construct multi-stimulus responsive functional materials to unlock and expand their potential applications.

In the design of viologen-based CPs with electron transfer properties, electron-rich oxalates, phosphates, organophosphines, and other oxygenates are usually chosen as electron donors (EDs), and viologen derivatives, featuring rigid pyridine/imidazole components, act as electron acceptors (EAs).^21,36,37 The assembly of diverse EDs and EAs with metal ions presents an opportunity to create functional materials endowed with intriguing photoresponsive properties. The sulfate group, as a typical ED, has garnered significant attention due to its distinctive tetrahedral geometry, substantial optical band gap, facile crystallization, and pronounced proclivity for fostering a diverse array of structural configurations.^38–40 Combining large π-conjugated units and tetrahedral active motifs could allow the preparation of nonlinear optical (NLO) materials with photomodulated properties.⁴¹ The integration of the aforementioned active units facilitates the development of multiple properties mediated by electron transfer, empowering a single compound with the potential to embody multifunctionality.

Herein, we successfully synthesized a stimulus-responsively functional compound [Zn(HSO₄)₂ (TPT)] (1), featuring an infinite chain architecture via the self-assembly of ZnSO₄·7H₂O and 2,4,6-tri(4-pyridyl)-1,3,5-triazine (TPT). Upon irradiation with a 300 W xenon lamp, the compound swiftly undergoes photochromism from yellow to gray. This coloration reversibly fades upon heating or in the dark. Comprehensive characterizations were conducted using powder X-ray diffraction (PXRD), infrared (IR), UV-visible (UV-vis) spectroscopy, photoluminescence measurements, electron paramagnetic resonance (EPR) spectra and X-ray photoelectron spectroscopy (XPS), all of which corroborated the photochromic nature attributed to the photogeneration of radical species from sulfate groups to TPT units. Our investigation into the second-harmonic generation (SHG) efficiency of 1 revealed its remarkable SHG-active nature, further showcasing its intensity's capacity for photomodulation. Notably, after light irradiation, it exhibits an impressive high SHG switching contrast of ca. 6 times, underscoring its potential for advanced optical applications. Moreover, the compound also demonstrates a selective affinity for various volatile amines, accompanied by a striking color change. The underlying mechanism of this colorimetric response has been confirmed through a meticulous spectral analysis, attributed to electron transfer from the volatile amines to the TPT component. Capitalizing on its remarkable multi-stimulus responsiveness and photoluminescence properties, we have successfully showcased the versatility of this synthesized compound in diverse applications, including volatile amine detection, inkless printing, and information encryption.

Experimental

Materials and physical measurements

2,4,6-Tri(4-pyridyl)-1,3,5-triazine (TPT) was purchased from Chemsoon. 2-Hydroxyacetic acid was purchased from Macklin, ZnSO₄·7H₂O was purchased from Acmec, N,N-dimethylacetamide (DMA) and H₃PO₄ were purchased from Sinopharm Chemical Reagent Co., Ltd. All chemicals were reagent grade and used as purchased without further purification. Elemental analyses (for C, H and N) were performed on a PerkinElmer 240C analyzer (PerkinElmer, USA). IR spectra of 1, 1a, 1-NH₃, 1-MA, 1-DEA, 1-TEA were performed using a IRTracer-100 spectrometer. Through a Rigaku standard MiniFlex600 diffractometer, powder X-ray diffraction (PXRD) spectra were performed. Simulation of the PXRD curve was carried out by the single-crystal data and diffraction-crystal module of the mercury (Hg) program with the free method supported on the Internet at https://www.iucr.org. The luminescence data were analyzed by an F-4700 fluorescence spectrometer. Time-dependent UV-vis spectroscopy was recorded on a Puxi Tu-1901 spectrophotometer using BaSO₄ as a reference. Room temperature EPR spectroscopy was recorded on a CIQTEK EPR200-Plus with continuous-wave X band frequency. The SHG response was measured using the Q-switched Nd:YAG laser (1064 nm) on powder samples. Thermogravimetric (TG) analyses were measured under a N₂ atmosphere on a TG-DTA 8121 analyzer. X-ray photoelectron spectra (XPS) were measured on PHI5000 Versaprobe III XPS at room temperature. For the irradiation apparatus, a perfect light PLS-SXE 300 Xe lamp (320–780 nm, 300 W) was used to prepare colored sample 1a.

Synthesis of [Zn(HSO₄)₂(TPT)] (1)

A mixture of ZnSO₄·7H₂O (0.09 g), 2-hydroxyacetic acid (0.30 g), 2,4,6-tris(4-pyridyl)-1,3,5-triazine (TPT) (0.03 g), and N,N-dimethylacetamide (DMA) (0.2 mL) was sealed in a Teflon-lined stainless steel autoclave. After stirring evenly, 0.15 mL H₃PO₄ was added to adjust the pH conditions. After a 7-day reaction in an oven at 120 °C, the autoclave was taken out and naturally cooled to room temperature. The product was washed repeatedly with anhydrous ethanol and filtered and dried at room temperature to obtain yellow block crystals. Elemental analysis (%): calcd for C₁₈H₁₄N₆O₈S₂Zn (571.84): C, 37.80; H, 2.47; N, 14.70. Found: C, 37.72; H, 2.56; N, 14.75. IR (cm⁻¹): 3079.10(w), 2975.37(w), 1630.21(w), 1516.97(w), 1394.70(s), 1271.62(w), 1091.92(w), 1045.14(w), 984.61(w), 875.29(w), 794.89(w), 506.85(w), 421.79(w).

Synthesis of 1a

A perfect light PLS-SXE 300 Xe lamp (320–780 nm, 300 W) was equipped to prepare the colored samples 1a. Elemental analysis (%): calcd for C₁₈H₁₄N₆O₈S₂Zn (571.88): C, 37.83; H, 2.44; N, 14.68. Found: C, 37.71; H, 2.58; N, 14.77. IR (cm⁻¹): 3096.26(w), 2924.24(w), 1630.20(w), 1516.97(s), 1375.83(s), 1304.44(w), 1096.84(s), 974.58(m), 870.36(w), 794.87(m) 511.78(m), 422.33(w).

Synthesis of 1@PBAT hybrid film

The powder sample 1 (0.02 g) and PBAT were mixed homogeneously at a ratio of 1 : 100 in 10 mL of trichloromethane and stirred overnight at room temperature. The solution was then poured into a Petri dish and slowly evaporated for one day to form a 1@PBAT hybrid film (PBAT = polybutylene adipate-terephthalate).

X-ray crystallography

The single-crystal X-ray diffraction data of 1 and 1a were collected on a XtaLAB Synergy R, DW system, HyPix at 293(2) K with Mo-Kα radiation and refined on F² by full-matrix least-squares methods using Olex and SHELXTL 2018. All non-hydrogen atoms were anisotropically refined and all H atoms were localized in their calculated positions. Detailed crystallographic data for 1 and 1a are summarized in Table S1 (ESI†), the continuous shape measure (CShM) analyses of geometries for compounds 1 and 1a are listed in Tables S6 and S7 (ESI†), the selected bond lengths and angles are listed in Tables S2 and S3 (ESI†), and the hydrogen bond lengths and angles are listed in Tables S4 and S5 (ESI†). Full crystallographic data have been deposited with the CCDC 2368746 for 1 and 2385349† for 1a.

Results and discussion

Single crystal X-ray diffraction showed that compound 1 crystallizes in the polar Cc space group of the monoclinic crystal system (Table S1, ESI†). As shown in Fig. 1a, the asymmetric unit contains one Zn²⁺ ion, two HSO₄⁻, and one neutral TPT molecule. The metal center follows the tetra-coordination pattern of [ZnO₃N], with three oxygen atoms from HSO₄⁻, and the nitrogen atom from the TPT ligand. The Zn–O bond lengths of compound 1 are in the range of 1.869(8)–1.932(6) Å, the Zn–N bond length is 2.040(6) Å, with the Zn central bond angles in the range of 105.5(4)–113.6(4)° (Table S2, ESI†). Continuous shape measurement analysis (CShM) indicated that the geometry of the Zn center is close to a tetrahedron (CShM = 0.127, Table S6, ESI†). Two neighboring Zn²⁺ ions are bridged by HSO₄⁻ to form a one-dimensional (1D) inorganic chain (Fig. 1b), TPT ligands distribute on both sides of the 1D chains, and the neighboring inorganic chains are connected by hydrogen bonding interactions to form a two-dimensional (2D) layered structure (Fig. 1c). The layer-to-layer TPT ligands are arranged parallel along the c-axis and form a three-dimensional (3D) supramolecular structure via π–π stacking between aromatic rings (Fig. 1d and Fig. S1, ESI†). Such unique arrangement may serve an instrumental role in photochromism and vapochromism.

Fig. 1 (a) Asymmetric unit of 1. (b) 1D chain structure of 1. (c) 2D layered skeleton formed by HSO₄⁻ chains through hydrogen bonding interactions. (d) 3D supramolecular structure of 1 (all hydrogen atoms are omitted for clarity).

PXRD experiments were carried out to verify the purity of 1 (Fig. S2, ESI†). As the electron-rich sulfate chains in combination with extremely electron-deficient TPT may generate interesting stimulus-responsive behaviors, we firstly probed the photochromic behavior of 1 at ambient conditions. As shown in Fig. 2a, powder samples changed from yellow to gray within 6 min under 300 W xenon lamp irradiation, yielding a colored variant designated as 1a. The UV-vis spectra were measured to record the time-dependent coloration process. After irradiation, broad absorption peaks centered at 634 nm appeared on the spectra and the intensity gradually increased with the extension of the irradiation time, which should be attributed to the photogeneration of TPT^• radicals.^21,26 These characteristic peaks completely disappeared when heating the gray samples at 120 °C for 1 h or keeping them in the dark for a few days, revealing a reversible decoloration behavior. The anti-fatigue property of 1 was characterized by measuring the absorption intensity at 634 nm in the UV-vis spectra with alternating irradiation and heating treatments, and the overall gradually decreasing intensity illustrates that the reversibility of photochromism could be stabilized over 10 cycles of alternating treatments (Fig. 2b). To understand the mechanism of photochromism, we collected single-crystal data for 1a (Table S1, ESI†). Compound 1a exhibits a Zn–O bond length ranging from 1.857(9) to 1.933(10) Å, a Zn–N bond length of 2.032(8) Å, and a Zn center bond angle ranging from 105.8(4) to 113.3(4)° (Table S3, ESI†). The CShM indicates that the geometry of the Zn center is close to tetrahedral (CShM = 0.102, Table S7, ESI†). These observations indicate that no obvious structural changes occurred during the coloration process. Furthermore, the PXRD (Fig. S2, ESI†) and IR spectra (Fig. S3, ESI†) revealed no changes after irradiation, thereby ruling out the occurrence of photoinduced dissociation or/and photoisomerization during irradiation. To further confirm the generated radicals during the coloring process, time-dependent EPR spectra were measured using powder samples at ambient temperature (Fig. 2c). Before irradiation, the weak free radical signal could be attributed to the photosensitivity of the samples. With increase of the irradiation time, a sharp EPR signal appeared at g = 2.0006 and reached the maximum after irradiation for 20 min, confirming the generation of radicals.

Fig. 2 (a) Time-dependent UV-vis spectra of a solid powder sample of 1 upon irradiation under ambient conditions. Inset: Photos of the powder sample upon irradiation. (b) The switching of the photoinduced coloration and decoloration process in 10 cycles for 1. (c) Time-dependent EPR spectra of the solid powder sample of 1 upon irradiation under ambient conditions. (d) In situ fluorescence spectra of 1 at ambient temperature with different irradiation times.

After decolorization, the EPR signal disappeared with the vanishing of the radicals during decolorization, which was also observed in other viologen-based families.^21,42 XPS provides a reliable way to explore the electron transfer pathways. As shown in Fig. S4–S7 (ESI†), the core-level spectra of both O 1s and N 1s before irradiation showed characteristic absorption bands. In detail, the O 1s can be fitted as O 1s A and O 1s B, which correspond to the sulfate group and the metal oxide (Zn–O) of inorganic chains, respectively. The binding energy of O 1s A and O 1s B respectively increased from 532.17 and 530.97 eV to 532.47 and 531.17 eV after irradiation, indicating that O atoms of the sulfate lose electrons in the coloration process. The N 1s can be fitted as N 1s A and N 1s B, correlating to the Zn–N bonds and C=N/C–N, respectively. After irradiation, the binding energy of N 1s A and N 1s B, respectively, decreased from 400.79 eV and 398.87 eV to 400.26 eV and 398.81 eV, suggesting that N atoms of the TPT component gain electrons during the coloration process. The core-level spectral characteristic absorption bands of Zn 2p, S 2p and C 1s remained invariant before and after coloration. In addition, the photo-induced electron transfer pathway can also be elucidated by structural analysis (Fig. S8–S9, ESI†). The sulfate layered structure can be regarded as an electron-rich layer, and the electron-deficient TPT columns are arranged in an orderly manner between the layers. This column-layer structure is highly conducive to the generation of electron transfer photochromism.^21,37 The columns and layers are rich in C–H⋯O weak hydrogen bonding interactions and close N⋯O distance can also further promote electron transfer (Table S4, ESI†). Therefore, the photochromic behavior of 1 is ascribed to the electron transfer from sulfate to the TPT component, accompanied with the generation of radicals. Not only that, 1 exhibited light-induced fluorescence changes. The fluorescence change process with irradiation time was monitored by the in situ fluorescence spectrum. When the powder sample of 1 was excited by light at 385 nm, a striking broad emission peak centered at 490 nm was observed on the spectrum, and the fluorescence intensity decreased by 55.90% with the increase of the irradiation time (Fig. 2d). This fluorescence quenching phenomenon was due to the photogeneration of radicals attributed to the electron transfer from sulphates to TPT ligands. After decolorization, the fluorescence intensity recovered to the initial state, demonstrating the switchable photoluminescence of 1.

Since 1 crystallizes in the polar Cc space group of the monoclinic crystal system, it was meaningful to investigate its SHG property. SHG response was measured using a pulsed laser at the wavelength of 1064 nm. As shown in Fig. 3a, the SHG intensity of the pristine samples was approximately 1.04 times that of KH₂PO₄ (KDP). The direction of the dipole moments was calculated by a bond-valence approach (Table S8, ESI†),⁴³ the polarized b-component of one unit cell completely cancels out, while the polarized a- and c-components are effective. And the net dipole moment lies on the ac-plane. Structurally, viewing along the c-axis, all TPT components in 1 are ordered in a parallel manner with the same direction through hydrogen-bonding interactions, leading to the highly polar structure of 1 (Fig. 3b). Interestingly, the SHG intensity of 1a turned to be very weak after 20 min of coloration, which decreased by 81.81% compared with the initial value. Meanwhile, the yellow sample turned gray-green. The PXRD and IR spectra of the samples were tested to explore if the crystals were damaged after 1064 nm pulsed laser irradiation. As shown in Fig. S10 (ESI†), the unchanged curves indicate that the reduction in SHG intensity was not due to continuous laser radiation. Similar to previously reported CPs constructed from viologen-based derivatives,^44,45 the coloration process altered the charge distribution and averaged the electron density between EDs and EAs, causing a reduction of the permanent dipole moment, which may be a critical factor leading to high SHG-switching contrast. In addition, the self-absorption effect cannot be neglected. During the coloration process, the absorption peak near 634 nm is obviously enhanced, indicating the significant enhancement of the self-absorption effect after coloration. Therefore, we can believe that the high SHG-switching contrast of 1 is attributed to the apparent discrepancy in the permanent dipole moment and self-absorption effects before and after coloration. After decolorization, the SHG intensity was close to that of the original sample, showing the photomodulation of the NLO properties in 1.

Fig. 3 (a) SHG intensity of 1 at ambient temperature with different irradiation times. (b) Polarization direction of TPT groups in 1.

Industrially manufactured organic amines are volatile hazardous substances to animal and human health, and detection of colorless organic amines is still challenging. The unifying trait of these amines is their electron-rich nature, rendering them susceptible to electron transfer via intermolecular interactions with electron-deficient compounds such as viologen-based derivatives.^46,47 The capability of amine vapor detection in the dark environment of 1 was characterized by employing common volatile amines in laboratories, i.e. ammonia (NH₃), methylamine (MA), diethylamine (DEA), and triethylamine (TEA). As shown in Fig. S11 (ESI†), a miniature glass vial containing 1 is positioned inside a larger glass vial, with the amine solution strategically placed between the two glass vials. This configuration boasts the advantage of creating a hermetically sealed environment rich in amine, while simultaneously averting any direct contact between 1 and the amine solution. When exposed to different vapors, 1 exhibited various color changes. In detail, 1 underwent color changes from yellow to gray-white (noted as 1-NH₃) in 5 min upon exposure to NH₃ (Fig. 4a), and from yellow to gray-green (noted as 1-MA) in 30 min upon exposure to MA, respectively, while no color changes appeared in DEA and TEA (noted as 1-DEA, 1-TEA, respectively) even after prolonging the exposure time. The time-dependent UV-vis spectra of the original and amine-fumigated samples were recorded to elucidate the underlying mechanism at ambient temperature (Fig. 4b). The absorption bands of 1-NH₃ and 1-MA occurred in the range of 600–800 nm, similar to the absorption curves of photochromism in Fig. 2a, suggesting that free radicals were generated during the fumigation of the different amines. EPR spectra of amine-fumigated samples were measured and sharp signals of radicals at g = 2.0026 and 2.0018 were observed for 1-NH₃ and 1-MA, respectively (Fig. 4c), while the signals for 1-DEA and 1-TEA were consistent with the original samples.

Fig. 4 (a) Photographs of color changes after amine vapor treatment. (b) UV-vis and (c) EPR spectra after amine vapor treatment. (d) Photographs of color changes of the filter paper coated with 1 exposed to NH₃ and MA, respectively.

XPS results provided more direct evidence of the electron transfer mechanism. As shown in Fig. 5a, the N 1s spectra showed two peaks at 400.79 and 398.87 eV prior to amine vapor treatment. The two peaks migrated toward the lower binding energy of 398.90 and 398.55 eV for 1-NH₃, and 398.99 eV and 398.53 eV for 1-MA, respectively. Both O 1s, S 2p and Zn 2p showed no significant changes in core-level spectra before and after fumigation with different amines (Fig. S12 and S13a, ESI†). This suggests that electron transfer happens from volatile amine to TPT. Furthermore, the core-level spectra of C 1s varied dramatically before and after amine treatment (Fig. S13b, ESI†), implying structural transformation during fumigation. After amine vapor treatment, the crystal edges blurred and distorted, and the original crystal morphology could hardly be maintained, so it is impossible to obtain its single-crystal structure by a single-crystal X-ray diffractometer. However, the PXRD patterns and IR spectra of 1-NH₃ and 1-MA changed and confirmed that structural transformation occurred during volatile amine fumigation (Fig. 5b and Fig. S14, ESI†). The PXRD analysis revealed the disappearance of some peaks or the appearance of new peaks of 1-NH₃ and 1-MA, indicating that the crystal structure had changed. Additional absorption peaks in the IR spectra at 3300 cm⁻¹, 3000 cm⁻¹, and 1000 cm⁻¹ appeared in 1-NH₃ and 1-MA compared to the original samples, which was ascribed to alkyl ν(N–H), ν(C–H), and ν(C–N) absorption, respectively. For 1-DEA and 1-TEA, no significant changes were observed in the PXRD patterns and IR spectra. The adsorption of volatile amines by 1 was also validated by thermogravimetric analysis (TGA). 1 could be stabilized until 300 °C under a N₂ atmosphere (Fig. S15, ESI†), whereas the weights of 1-NH₃ and 1-MA decreased continuously with increasing temperatures. From 30 to 130 °C, the weight loss for 1-NH₃ was 0.93 wt%, and a weight loss of 4.74 wt% from 30 to 100 °C was observed in 1-MA, respectively, which could be attributable to the release of adsorbed volatile amines (Fig. S16, ESI†). The skeleton after the removal of volatile amines could also be stabilized until 300 °C. According to the comprehensive study, we can conclude that the TPT ligands receive lone pair electrons from the volatile amine, resulting in radical generation in the dark environment. For larger DEA and TEA, the electron transfer process encounters significant hindrance due to the spatial site resistance and spatial constraints, making effective adsorption of amine molecules more difficult. Based on the distinct chromogenic properties of 1 towards various amines, we developed filter papers impregnated with 1, specifically tailored for the detection of NH₃ and MA. As shown in Fig. 4d, the filter paper showed gray and gray-green color as visible to the naked eye under the fumigation of NH₃ and MA, respectively, proving potential application in volatile amine detection.

Fig. 5 (a) XPS spectra of N 1s in original samples and different amine treatments. (b) PXRD patterns of original, 1-NH₃, 1-MA, 1-DEA and 1-TEA.

The photochromic and photoluminescent properties in 1 are suitable for inkless printing owing to the swift photoresponse that triggers a distinct color transformation and the reversible coloring and fading processes via irradiation and heating treatment. The potential applications for 1 in inkless printing and information encryption are demonstrated in Fig. 6. The powder sample of 1 and PBAT (poly(butylene adipate-co-terephthalate)) were mixed in trichloromethane at a 1 : 100 ratio and poured into a Petri dish to dry naturally to generate a 1@PBAT hybrid film. The as-prepared 1@PBAT hybrid films exhibited photoluminescent properties under UV light, indicating effective binding of 1 and the polymer. By covering the hollowed-out templates on the 1@PBAT hybrid film and subjecting it to 20 min of xenon lamp irradiation, intricate patterns including a swallow (Fig. 6a) and a crab (Fig. 6b) were “printed” onto the hybrid film. Under UV illumination, these patterns became more vivid and discernible.

Fig. 6 Photographs show the swallow pattern (a) and crab pattern (b) “printed” on 1@PBAT hybrid film. (c) Schematic diagram of the information encryption and decryption process for photoluminescence and photochromism based on 1.

In addition, we devised an anti-counterfeiting encryption system based on a simplified ASCII-binary encryption algorithm (American Standard Code for Information Exchange) using the photochromic and photoluminescent properties of 1. As shown in Fig. 6c, powder samples of yellowish 1 and holmium oxide were homogeneously filled in different points in a 4 × 8 matrix. The points are defined as valid information “1” when they turn to gray-green under xenon light or green luminescence under UV light. Conversely, they are classified as invalid information “0” if their color remains unchanged after irradiation. Under daylight, all points in the matrix were yellow and assigned a value of “0”. After xenon light, 11 dots filled with powders of 1 in the matrix became gray-green, which can be directly deciphered as “ILQD” (which means I love Qingdao) by converting the binary code to ASCII. Alternatively, when the matrix was exposed to UV light, the holmium oxide powders emitted almost no light, while powders of 1 showed a striking green light and the information can also be directly deciphered as “ILQD” through conversion. The present results revealed potential applications of 1 in the fields of inkless printing and information encryption.

Furthermore, we conducted a comparative analysis of compound 1 with other TPT-based and Zn-based complexes. As illustrated in Table S9 (ESI†), most complexes are limited to responding to a single stimulus source with the individual photochromic performance, thereby hindering the practical utilization of numerous materials. Conversely, compound 1 exhibits multiple response functionalities under light and vapor stimuli, presenting a wide array of application prospects and making it superior to most TPT-based stimuli-responsive materials. This study offers valuable insights into the development of TPT-based stimuli-responsive materials that possess multiple response capabilities.

Conclusions

In conclusion, we successfully constructed a stimulus-responsive multifunctional material utilizing electron-deficient TPT and electron-rich sulfate components. This material exhibits distinct photochromism after light irradiation and reversible fading under heating or dark conditions. UV-vis spectra, in situ fluorescence spectra, EPR spectra and XPS measurements confirmed the radical-induced photochromic behavior, accompanied by electron transfer from the sulfate to the electron-deficient TPT component. In addition, the considerable discrepancy in permanent dipole moment and self-absorption effects before and after coloration resulted in an about 6 times higher SHG-switching contrast, confirming a photomodulation of the NLO property for 1. Furthermore, 1 displays a selective vapochromic behavior which is significantly influenced by size and spatial resistance factors. Smaller amines, with reduced spatial resistance, facilitate electron transfer to polypyridine units, fostering stable radical formation. Such characteristic renders 1 highly promising for applications in detecting volatile amines. Based on the photochromic behavior, we realized the application of 1 in inkless printing and information encryption. The present work not only enriches the family of stimulus-responsive multifunctional materials, but also provides a new idea and strategy for exploring potential applications in the field of sensing and inkless printing.

Author contributions

J. X. H. conceived the idea and designed the experiments. S. L. conducted a series of experiments and wrote the original draft. S. K. Y. and Y. X. W. analyzed the experimental data. Y. R. Z. and J. Z. performed XPS, EPR and TGA measurements.

Data availability

The data supporting this article have been included as part of the ESI.†

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (22171155), the Natural Science Foundation of Shandong Province (ZR2022YQ07) and the Taishan Scholar Program (tsqn202306166).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. CCDC 2368746 and 2385349. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4tc04169k

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