Realizing efficient emission and triple‐mode photoluminescence switching in air‐stable tin(IV)‐based metal halides via antimony doping and rational structural modulation

Recently, many lead‐free metal halides with diverse structures and highly efficient emission have been reported. However, their poor stability and single‐mode emission color severely limit their applications. Herein, three homologous Sb3+‐doped zero‐dimensional (0D) air‐stable Sn(IV)‐based metal halides with different crystal structures were developed by inserting a single organic ligand into SnCl4 lattice, which brings different optical properties. Under photoexcitation, (C25H22P)SnCl5@Sb·CH4O (Sb3+−1) does not emit light, (C25H22P)2SnCl6@Sb‐α (Sb3+−2α) shines bright yellow emission with a photoluminescence quantum yield (PLQY) of 92%, and (C25H22P)2SnCl6@Sb‐β (Sb3+−2β) exhibits intense red emission with a PLQY of 78%. The above three compounds show quite different optical properties should be due to their different crystal structures and the lattice distortions. Particularly, Sb3+−1 can be successfully converted into Sb3+−2α under the treatment of C25H22PCl solution, accompanied by a transition from nonemission to efficient yellow emission, serving as a “turn‐on” photoluminescence (PL) switching. Parallelly, a reversible structure conversion between Sb3+−2α and Sb3+−2β was witnessed after dichloromethane or volatilization treatment, accompanied by yellow and red emission switching. Thereby, a triple‐mode tunable PL switching of off–onI–onII can be constructed in Sb3+‐doped Sn(IV)‐based compounds. Finally, we demonstrated the as‐synthesized compounds in fluorescent anticounterfeiting, information encryption, and optical logic gates.


INTRODUCTION
[3][4] Generally, the optical response output of luminescent materials can be precisely controlled by reasonably controlling the input of these external stimuli, thus achieving multimode dynamic reversible luminescence, and has very broad applications in fluorescence anticounterfeiting, information security, data storage, logic gates, and so on.To date, a variety of luminescent materials including organic dye, [5] liquid crystal, [6] carbon dot, [7] and rare earth compound [8] have been processed into various multimode dynamic luminescent materials.However, these traditional luminescent materials severely limit their further application due to some unavoidable technical bottlenecks, such as a complex synthesis process, low luminous efficiency, and poor color contrast.Lead halide perovskite (LHP), as an important functional material, has become a potential candidate material for multimode dynamic luminescence due to its easy synthesis, high luminous efficiency, and tunable emission color. [9][12][13] Although LHPs show great potential in multimode dynamic luminescence, the intrinsic toxicity of LHPs severely limits their further application.
[16][17] Among many lead-free metal halides, Sn(II)-based metal halides become potential candidates to replace LHPs because Sn(II) and Pb(II) belong to the same group IVA, which makes them have similar ionic radius, coordination configuration, and crystal structure. [18,19]However, the presence of high-energy (HE) 5s orbitals in Sn 2+ makes it is susceptible to oxidation to +4 valence and is accompanied by the formation of a high defect density, leading to rapid nonradiative relaxation and thus a dramatic decrease in luminescence efficiency. [20]herefore, the use of high-valence Sn(IV) instead of Pb(II) is considered an effective strategy to address the toxicity and poor stability of LHPs.
23][24] Previous results have shown that the 4d 10 5s • outer-shell electron configuration of Sn 4+ provides the corresponding compounds with a rigid crystal structure and stereochemical inactive, and thus Sn(IV)-based metal halides typically exhibit excellent environmental and thermal stability.However, influenced by the low radiation recombination rate and small exciton binding energy, Sn(IV)-based metal halides generally exhibit low luminous efficiency. [25]Metal ion doping is an effective strategy to modulate the electronic properties of the host matrix and improve the optical properties.Since Sb 3+ ions have unique stereochemical activity, they are introduced as dopants into Sn(IV)-based metal halides to modulate the 5s • outer-shell electron environment of Sn 4+ or improve its ability to passivate defects, thus enhancing the luminous efficiency of Sn(IV)-based metal halides, for example, Cs 2 SnCl 6 :Sb 3+ (orange-red emission, photoluminescence quantum yield [PLQY]∼37%), [26] Rb 2 SnCl 6 :Sb 3+ (orange-red emission, PLQY∼15.2%), [27]Sb 3+ -doped (C 8 H 22 N 2 Cl) 2 SnCl 6 (red emission, PLQY∼41.76%), [24](C 10 H 16 N 2 )SnCl 6 :Sb 3+ (red emission, PLQY∼77%). [23]The metal halide polyhedron MCl 6 (M = Sn, Sb) clusters in the Sb 3+ -doped Sn(IV)-based metal halides reported above are surrounded and completely isolated by A-site cations, forming a typical 0D "host-guest" structure.In such a 0D structure, the isolated polyhedron grants no conception of interoctahedral tilting, and thus the photophysical properties are mainly determined by lattice distortion in the isolated metal halide clusters. [28]Apparently, the above studies show that the optical properties of Sb 3+doped Sn(IV)-based compounds can indeed be effectively improved by aimlessly adjusting the species of A-site cations in the Sn(IV)-based metal halides, but their single steadystate fluorescence emission seriously limits their applications.Therefore, it is urgent to develop a new structural control strategy to improve the luminous efficiency and realize its multimode dynamic luminescence through reasonable structural design in Sb 3+ -doped Sn(IV)-based metal halides.
Although we have high expectations, most of the reported low-dimensional lead-free metal halides rely on small shifts of emission wavelengths with low-contrast to achieve color adjustable PL switches, and there are few to achieve PL off-on switch at room temperature (RT), let alone multimode dynamic luminescence.Therefore, achieving multiple luminescence transformations in Sb 3+ -doped Sn(IV)-based systems remains an enormous challenge.Based on above considerations, we here proposed a crystalengineering to change the coordination structure and the degree of lattice distortion of Sn(IV)-based metal halides by inserting a single organic ligand into SnCl 4 lattice, thus achieving efficient emission and triple-mode tunable PL switching in air-stable Sb 3+ -doped Sn(IV)-based metal halides.More specifically, we synthesized three homologous Sb 3+ -doped 0D Sn(IV)-based metal halides with different crystal structures, namely, (C 25 H 22 P)SnCl 5 @Sb⋅CH 4 O (Sb 3+ −1), (C 25 H 22 P) 2 SnCl 6 @Sb-α (Sb 3+ −2α), and (C 25 H 22 P) 2 SnCl 6 @Sb-β (Sb 3+ −2β), by accurately controlling the ratio of precursors and employing various chemical synthesis techniques.Upon photoexcitation, Sb 3+ −1 does not emit light, Sb 3+ −2α emits bright yellow emission with a PLQY of 92%, and Sb 3+ −2β exhibits intense red emission with a PLQY of 78%.These three as-synthesized compounds exhibit completely different optical properties due to their different coordination structures and lattice distortions.Particularly, a unique triple-mode dynamic luminescence switching of the as-synthesized three Sb 3+ -doped samples was realized under the treatment of different external stimuli, that is, the nonemission Sb 3+ −1 can be successfully converted into the yellow emission Sb 3+ −2α under the treatment of C 25 H 22 PCl solution, and further undergo reversible structural transformation to red emission Sb 3+ −2β after dichloromethane or volatilization treatment.Hence, a triple-mode tunable PL switching of off-on I -on II can be constructed in Sb 3+ -doped Sn(IV)-based metal halides.Given the fascinating optical properties, ultra-high structural and optical stability of the as-synthesized compounds, we demonstrated the application of the related compounds in solid-state lighting (SSL), fluorescent anticounterfeiting, information encryption, and optical logic gates.Thus, our work not only deepens the understanding of the effect of local coordination structures and lattice distortion on the photophysical mechanism of Sb 3+ -doped Sn(IV)-based compounds, but also provides a novel theoretical foundation for acquiring multimode dynamic luminescence of 0D metal halides through rational structural modulation.each Sn 4+ coordinates with five chlorine atoms and one oxygen atom, thus forming a distorted octahedral structure of [SnCl 5 O] (Figures 1A and B).Unlike 1, the asymmetric units in 2α and 2β contain two organic ligands of C 25 H 22 P + and one [SnCl 6 ] 2− species.Moreover, 2α and 2β exhibit the six coordinated structure, that is, each tin atom coordinates with six chlorine atoms to form [SnCl 6 ] 2− octahedron (Figures 1C-F).Interestingly, 2α and 2β adopt the same monoclinic system, but they exhibit different space groups of P2 1 /n and P2 1 /c, respectively, which makes them have different crystal structure parameters (Tables S2 and S3) and corresponding photophysical properties.In parallel, the discrete inorganic polyhedrons in the as-synthesized compounds are encircled by the large organic ligands, which makes them all can be viewed as 0D crystals.Figures 1G-I depict the powder X-ray diffraction (PXRD) patterns of compound 1, 2α, and 2β, which exhibits similar characteristics to the simulated SCXRD results, illustrating the reliability of SCXRD results.The shortest distances of Sn-Sn in the as-synthesized compounds are 11.39 Å for 1, 11.04 Å for 2α, and 10.02 Å for 2β (Figure S1), which indicates that the absence Sn-Sn interaction in the as-synthesized compounds due to its large Sn-Sn distance.More detailed crystallographic data of the as-synthesized compounds are given in Tables S1-S3.Generally, Sn(IV)-based metal halides exhibit poor luminescent properties (Figure S2). [29,30]Hence, in order to enhance the PL intensity of the as-synthesized compounds, the Sb 3+ ions with 5s 2 electronic configuration were introduced into the host matrix as a new luminescence center, and we synthesized Sb 3+ -doped Sn(IV)-based metal halides, that is, Sb 3+ -doped (C 25 H 22 P)SnCl 5 ⋅CH 4 O (Sb 3+ −1), Sb 3+doped (C 25 H 22 P) 2 SnCl 6 -α (Sb 3+ −2α), and Sb 3+ -doped (C 25 H 22 P) 2 SnCl 6 -β (Sb 3+ −2β).The crystal structures of Sb 3+ -doped samples were depicted in Figure S3, and Sb 3+ ions tend to partially occupy the Sn 4+ site for their similar ionic radius.Figures S4-S6 show the scanning electron microscope (SEM) images of the as-synthesized 15%Sb 3+doped samples, and the corresponding energy dispersive spectrometer (EDS) results show that the dopant of Sb 3+ has entered the host lattice, but the actual doping concentration of Sb 3+ is lower than the feed content (Tables S4-S6).The element mapping results suggest that the dopant is evenly distributed in the as-synthesized Sb 3+ -doped samples, which further demonstrates the reliability of photophysical mechanism analysis.Figure S7 shows the X-ray photoelectron spectroscopy (XPS) spectra of the as-synthesized Sb 3+ -doped samples, the characteristic bands of Sn and Sb can be observed clearly, which demonstrates that the dopant Sb 3+ has indeed entered the host matrix.The binding energies of Sn 3d 5/2 and 3d 3/2 are located at 487.4 and 494.7 eV, respectively, which demonstrates Sn 4+ in the as-synthesized Sn(IV)-based compounds. [14]In parallel, the binding energies centered at 539.1 and 531.5 eV can be observed in the high-resolution XPS spectra of Sb 3d, corresponding to the Sb 3+ 3d 3/2 and 3d 5/2 , respectively. [31,32]The PXRD of Sb 3+ -doped samples were also measured, and similar PXRD patterns with the pristine one were witnessed, which suggests the phase purity of the Sb 3+ -doped compounds (Figure S8).Moreover, the diffraction peak of Sb 3+ -doped Sn(IV)based compounds shifts to a low angle in the enlarged view of PXRD, which should be caused by the lattice expansion due to the ion radius of Sb 3+ (r = 0.76 Å, CN = 6) is larger than Sn 4+ (r = 0.69 Å, CN = 6).Subsequently, the thermal stability of the as-synthesized compounds was measured  S9c), which suggests the remarkable thermal stability of the corresponding compounds.

RESULTS AND DISCUSSIONS
The RT optical properties of the as-synthesized compounds were investigated.Figure S10 shows the absorption spectra of the pristine and Sb 3+ -doped samples.Compared with pure Sn(IV)-based compounds, the introduction of Sb 3+ makes a distinct red-shift of the absorption bandgap and significantly enhances the absorption strength of the low-energy (LE) band of the host matrix, which suggests that Sb 3+ has a great influence on the optical properties for Sn(IV)-based compounds.Similar to 1, Sb 3+ −1 also does not emit light under 365 nm irradiation at RT (Figure S11).In stark contrast, Sb 3+ −2α and Sb 3+ −2β exhibit bright yellow and red emission under ultraviolet (UV) excitation, respectively (Figures 2A and B).Figures 2C and D show the RT photoluminescence (PL) and PL excitation (PLE) spectra of Sb 3+ −2α and Sb 3+ −2β.Upon 370 nm excitation, both Sb 3+ -doped compounds exhibit a single LE broadband emission centered at 580 (Sb 3+ −2α) and 637 nm (Sb 3+ −2β), respectively.Moreover, their broad emission bands show a full width at half-maximum (FWHM) of 184 (Sb 3+ −2α) and 172 nm (Sb 3+ −2β) with a large Stokes shift of 232 and 267 nm, which indicates a negligible self-absorption in the as-synthesized Sb 3+ -doped Sn(IV)based compounds.Particularly, both Sb 3+ -doped samples show a dual-emission band, that is, an additional narrow HE emission band at 445 and 490 nm was observed upon 310 nm irradiation.Figure S12 shows the PL spectra with different doped concentrations (5-25%), which shows an identical spectral profile in their respective Sb 3+ -doped Sn(IV)-based compounds (Sb 3+ −2α and Sb 3+ −2β), but the PL intensity increases first and then decreases, which should be due to the concentration quenching effect. [33]When the content of dopants is 15% for Sb 3+ −2α and 20% for Sb 3+ −2β, the corresponding Sb 3+ -doped compounds exhibit the strongest PL intensity with a PLQY of 92% and 78% (Figure S13), respectively, which further ensures the as-synthesized Sb 3+doped samples have enormous application potential in SSL.Subsequently, the PLE spectra of Sb 3+ -doped Sn(IV)-based metal halides were measured at RT. Clearly, the sharp PLE bands at 314 and 313 nm can be observed when monitored at HE emission bands of Sb 3+ −2α and Sb 3+ −2β, and the broad PLE bands at 348 and 370 nm can be seen when monitored at LE emission bands (Figures 2C and D).The significant difference in PLE spectra in their respective Sb 3+ -doped compounds, which indicates that the dual-emission band in Sb 3+ −2α and Sb 3+ −2β stems from the different excited states.This finding is further supported by the PL spectra under various excitation wavelengths, which exhibit distinct PL properties (Figures S14 and S15).In general, Sb(III) possesses five energy levels, namely, ground state (GS) 1 S 0 and excited state 1 P 1 and 3 P n (n = 0, 1, 2).[36][37] Combined with Sb 3+ -based metal halides and Sb 3+ -doped lead-free metal halides, the observed HE and LE PLE bands in Sb 3+ -doped Sn(IV)-based compounds can be assigned to 1 S 0 → 1 P 1 and 1 S 0 → 3 P 1 transitions of Sb 3+ , respectively. [37]The RT PL decay curves of Sb 3+ -doped Sn(IV)-based compounds were measured under 310 nm laser excitation.As shown in Figures 2E and F, the LE bands exhibit a long decay lifetime (microsecond level), vastly longer than the short decay lifetime (nanosecond level) of HE bands, which further highlights their different PL properties.In addition, the PL intensities of Sb 3+ −2α and Sb 3+ −2β under different excitation powers (excited by 405 nm laser) were measured at RT, and a linear variation can be observed (Figure S16), which ruled out the possibility of emission caused by permanent defects. [38]ecently, many low-dimensional metal halides with efficient emission have been reported, and the observed broadband emission can be attributed to self-trapped exciton (STE) emission.Generally, the STEs can be viewed as "excitedstate defects", and strong electron-phonon coupling and lattice distortion are the dominant reasons for the formation of STEs in low-dimensional metal halides with soft lattice characteristics. [39,40]Classically, the stereochemical activity Sb 3+ ions with 5s 2 lone pair, their corresponding Sb 3+based metal halides and Sb 3+ -doped metal halides generally exhibit bright STE emission out of lattice distortion and strong electron-phonon coupling. [31]Moreover, the singlet STE generally shows a shorter decay lifetime than the triplet STE, hence the observed HE and LE emission bands in Sb 3+ −2α and Sb 3+ −2β come from singlet and triplet STEs, respectively. [36,38]s we know, the structure of the material is related to its physical properties, and the optical properties of leadfree compounds are closely related to the distortion of metal halide polyhedron.Therefore, in order to further understand the different photophysical properties of the as-synthesized Sb 3+ -doped compounds, the deformation parameters (Δd) and the asymmetric coordination environment (σ 2 ) of inorganic species were calculated (Detailed calculation processes were given in the Supporting Information.), and the obtained results show that Sb 3+ −1 (Δd = 9.64 × 10 −5 , σ 2 = 17.256) is more distorted than Sb 3+ −2α (Δd = 2.61 × 10 −5 , σ 2 = 0.5468) and Sb 3+ −2β (Δd = 6.33 × 10 −5 , σ 2 = 1.394).[43] Hence, the Sb 3+ −1 with the maximum distortion degree exhibits negligible emission, while Sb 3+ −2α and Sb 3+ −2β with moderate lattice distortion exhibit an efficient emission.[46] In our systems, the average Sb-Sb distance of Sb 3+ −2α is greater than Sb 3+ −2β, which further illustrates the former has higher PLQY.Moreover, the Stokes shifts of Sb 3+ −2α and Sb 3+ −2β are 232 and 267 nm, respectively, which echoes well with the calculated deformation parameters of the corresponding compounds, that is greater lattice distortion will bring larger Stokes shift. [47]hen, we performed density functional theory (DFT) to investigate the electronic properties of three as-synthesized Sn(IV)-based metal halides and the corresponding Sb 3+doped compounds, and the results are given in Figure 3.For compound 1, it exhibits an indirect bandgap characteristic (Figure 3A), which suggests that the generated photogenerated carriers in compound 1 will mainly recombine by nonradiative pathway, making it difficult to observe PL emission in this compound.Meanwhile, the compounds 2α and 2β exhibit a direct bandgap characteristic (Figures 3E and I), but they show a large bandgap, thus making it difficult for them to observe visible light emission under photoexcitation at RT. Upon Sb 3+ doping, there is a significant change in the band structure of all doped samples, the typical feature is that all the doped samples exhibit obvious direct bandgap characteristics (Figures 3C, G, and K), and their bandgap values have significantly decreased, which is consistent with the extended absorption range of the doped samples (Figures S10d-S10f).The calculated bandgap values of the doped samples are much smaller than the experimental values (Figures S10g-S10i), which can be attributed to the acknowledged Perdew-Burke-Ernzerhof (PBE) calculation error. [34,48]Moreover, the valence and conduction bands of all the pure and doped compounds show negligible dispersive, which indicates that all samples exhibit an intense quantum confinement effect.The flat bands in the valence band maximum (VBM) and conduction band minimum (CBM) illustrate that the electronic interactions between adjacent inorganic metal halide species can be almost ignored, which echoes well with the large M-M (M = Sn 4+ , Sb 3+ ) distance of the as-synthesized samples (Figure S1).In the density of states (DOS) of pure compound 1 (Figure 3B), its VBM is mainly dominated by Cl-p orbital and organic component, while the CBM mainly consists of Sn-s and Cl-p states, hence there is a charge transferred between organic component and inorganic metal halide in compound 1.After Sb 3+ doping, there is a significant change in the DOS of Sb 3+ −1, and the VBM is constituted by Cl-p and Sb-s states, while CBM is composed of Cl-p and Sn-s orbitals (Figure 3D).Particularly, O atoms in methanol also play an important role in the CBM, thus generating energy level defects from which electrons tend to escape thermally, grinding out the possibility of observing PL in exciton transition under photoexcitation.In the DOS of 2α and 2β (Figures 3F and J), the VBM is mainly formed by Cl-p state, and the CBM is composed of Clp and Sn-s characters.The organic counterpart of C 25 H 22 P + plays a negligible role in the VBM and CBM, thus the photophysical processes of 2α and 2β should be determined by the isolated inorganic [SnCl 6 ] 2− species.In the doped system, there is a new impurity level in the VBM of Sb 3+ −2α and Sb 3+ −2β, which is constituted by Cl-p and Sb-s orbitals (Figures 3H and L).However, the CBM is almost unaffected by Sb 3+ because the Sb-p is much higher than the Sn-s character, and the doping of Sb 3+ will cause the downshift of Cl-p and Sn-s characters, but has no effect on VBM.Hence, the doping of Sb 3+ will change the energy level and appear a new charge transfer band, which further shortens the forbidden band and offers the possibility of exciton transition under photoexcitation, thus finally boosting the luminous efficiency in Sb 3+ −2α and Sb 3+ −2β. [49,50]o gain a deeper understanding of the luminescence mechanism, the PL spectra of the as-synthesized compounds were measured at various temperatures.For compounds 1 and Sb 3+ −1, no emission signal was detected even at 80 K. On the contrary, the spectral signals of other compounds were detected at low temperatures.Figures S17a and S18a show the temperature-dependent PL spectra of 2α and 2β, and there are dual-emission bands in the whole measured temperature windows.Moreover, the low-temperature PLE and time-resolved PL (TRPL) were given in Figures S17b, c and S18b, c.Hence, 2α and 2β show a large Stokes shift, a broad FWHM and a long decay lifetime (microsecond).To explore the origin of these dual-emission bands observed in 2α and 2β, we also measured the low-temperature PL and TRPL of organic hydrochloride counterpart of C 25 H 22 PCl (Figures S19 and S20), and the results show that it shows a weak emission band at 430 nm and a short decay lifetime of 3.12 ns, which is different from the emission characteristics of 2α and 2β.Hence, we can exclude the effect of organic counterpart on the optical properties of 2α and 2β, and the observed dualemission band should stem from the radiation recombination of dual-STEs in [SnCl 6 ] 2− species.Interestingly, the emission intensity of HE band in 2α and 2β decreases gradually with the increase of temperature, while the emission intensity of LE band in 2α and 2β increases first and then decreases.The occurrence of this abnormal phenomenon should be due to the existence of energy barriers (ΔΕ 2α = kT = 15.5 meV and T = 180 K; ΔΕ 2β = 12.1 meV and T = 140 K) between these two self-trapped states, as previously observed in other lead-free metal halides. [51,52]Hence, the excited carrier with kinetic energy larger than the ΔΕ can overstep the barrier and enter STE2 state, thereby enhancing the LE emission intensity with the rise of temperature.However, the emission intensity decreases gradually with the further increasing of temperature, which should be due to the thermal quenching effect and phonon scattering, [53] and the more detailed pho-tophysical processes of 2α and 2β are given in Figures S17d  and S18d, respectively.
Subsequently, the emission mechanism of Sb 3+ −2α and Sb 3+ −2β was also investigated via temperature-dependent emission spectra under 310 nm excitation, respectively (Figure S21).Upon Sb 3+ doping, the emission intensity of 2α and 2β was enhanced, and the singlet and triplet emission bands of Sb 3+ −2α and Sb 3+ −2β can be observed clearly in the whole temperature window.We have measured the low-temperature differential scanning calorimetry of the assynthesized Sb 3+ −1, Sb 3+ −2α, and Sb 3+ −2β (Figure S22), and the results show that no phase transition is found, which excludes the effect of phase transition on the temperaturedependent luminescent properties of the as-synthesized title compounds.As shown in Figures 4A and C, the FWHM of these compounds become wider at high temperature, which should be attributed to the enhanced electron-phonon interaction in this soft lattice structure. [54]As the temperature increases, the emission intensity of Sb 3+ −2α and Sb 3+ −2β initially increases, and then decreases gradually as the temperature continues to rise to 300 K.This interesting phenomenon has also been found in pure compounds of 2α and 2β, which should be attributed that there is an energy barrier (Δ Sb 3+ −2 = 10.34 meV and T = 120 K; Δ Sb 3+ −2 = 8.61 meV and T = 100 K) between the 1 P 1 state and singlet state (ES1), and the excited carrier with kinetic energy larger than the ΔΕ can eclipse the barrier and enter the self-trapped state, thus yielding a strongest PL intensity.Moreover, the energy barrier between 3 P 1 state and triplet state (ES2) for Sb 3+ −2α and Sb 3+ −2β should be less than 6.91 meV because the emission intensity under 365 nm excitation decreases gradually with the increase of temperature from 80 to 300 K, and the thermal quenching effect and phonon scattering should be the dominant reason for this phe-nomenon (Figure S23). [53]Parallelly, the emission intensity of LE emission band decreases faster than the HE emission band with the increase of temperature (Figures S21a and  S21c), which should be attributed to the thermal-enhanced energy transfer from singlet STEs to triplet STEs. [24,33]ubsequently, the exciton binding energy (E a ) of Sb 3+ −2α and Sb 3+ −2β was calculated according to the Arrhenius-type equation: [55] where I(T) and I 0 are the integrated emission intensity at different temperatures (T) and 0 K, respectively, and k B represents the Boltzmann constant.Here, the calculated E a for the HE and LE emission bands of Sb 3+ −2α are 140.63 and 67.02 meV (Figure S24), and the calculated E a for the HE and LE emission bands of Sb 3+ −2β are 100.89and 80.77 meV (Figure S25), respectively.Clearly, all the calculated E a values for both compounds are greater than the thermal ionization energy at RT (26 meV), which illustrates the excitons in compounds Sb 3+ −2α and Sb 3+ −2β can overcome the thermal quenching and yield highly efficient STE emission. [51]s another key physical parameter, the magnitude of Huang-Rhys factor (S) can represent the strength of electron-phonon coupling, which can be calculated using Equation 2: [56] FWHM = 2.36 where the Huang-Rhys factor (S) and ℏω phonon are the electron-phonon coupling strength and phonon energy, respectively.As shown in Figures 4B and D an intense electron-phonon interaction in both compounds, which facilitates the formation of STEs. [57,58]Particularly, the calculated S values are comparable with other lead-free metal halides reported previously, but much smaller than some compounds with weak PL (Table S7). [51]Therefore, the moderate electron-phonon coupling for Sb 3+ −2α and Sb 3+ −2β can inhibit the nonradiative recombination and further ensure bright STEs emission.Moreover, we also note the E a and S values of the dual-emission bands are different in both compounds, which should be assigned to the respective emission characteristics of the singlet and triplet STEs in Sb 3+ −2α and Sb 3+ −2β.
The Raman spectra of Sb 3+ −1, Sb 3+ −2α, and Sb 3+ −2β crystals were measured at RT.In Figure S26, the Raman band in 50-500 cm −1 is the vibration of the inorganic lattice.Clearly, a strong model at 94 cm −1 can be witnessed in the Raman spectra, which can be attributed to the vibration of Sb-Cl, [59] suggesting that Sb 3+ has been inserted into the lattice of Sb 3+ −1, Sb 3+ −2α, and Sb 3+ −2β.Moreover, Sb 3+ −1 exhibits the strongest Raman intensity among them, suggesting that this compound has the softest lattice structure, and therefore brings a large structure distortion, which is coincident with the calculated Δd and σ 2 of inorganic species as discussed above.Moreover, the calculated phonon energies of LE and HE emission bands of Sb 3+ −2α and Sb 3+ −2β are coincident with the Raman bands, which further illustrate the reliability of the fitting results of FWHM as a function of temperature.
Based on the abovementioned discussion, the photophysical processes of Sb 3+ −2α and Sb 3+ −2β are illustrated in Figure 4E, and the observed PL stems from [SbCl 6 ] 3− octahedrons.Under photo-absorption, the electrons in the GS rapidly transition to excited state.Subsequently, the generated free excitons are self-trapped quickly to form STEs out of the intense electron-phonon coupling.Under the HE photoabsorption (e.g., 310 nm), the electrons will be excited to the HE excited state of 1 P 1 , and then they will enter singlet STE state rapidly.Under this circumstance, a portion of STEs relax to GS and yield a narrow HE emission.Parallelly, the remaining STEs are inclined to undergo intersystem crossing from the singlet to triplet STEs state, and a broad LE emission band can be obtained.However, the electrons can only be excited to the LE excited state of 3 P 1 under the LE photoexcitation (e.g., 370 nm), and the excited electrons can only directly transfer to the triplet STE state due to the large energy barrier between 3 P 1 state and singlet STE state, [24] so only a single broadband emission can be seen, which derives from the triplet STEs emission.More importantly, Sb 3+ −2α and Sb 3+ −2β exhibit various photophysical properties, which should be due to the different crystal structures of above two compounds.
Stability is an essential parameter of samples in practical application.Apart from the compelling optical performance, Sb 3+ −2α and Sb 3+ −2β also exhibit outstanding stabilities.When Sb 3+ −2α and Sb 3+ −2β were stored in atmosphere for 6 months, the PL intensity remained at a high level (Figures S27a and S27c) and the PXRD results did not reveal any structural decomposition (Figures S27b and S27d), which demonstrates the remarkable chemical stability of assynthesized compounds.The praiseworthy photostability of Sb 3+ −2α and Sb 3+ −2β was also demonstrated, and the emission intensity was almost unchanged when the samples were irradiated for 4 h under 365 nm UV lamp (Figure S28).Moreover, we are also surprised to find that Sb 3+ −2α and Sb 3+ −2β have high antiwater stability, which can be demonstrated by the PXRD and XPS results, as shown in Figures S29 and S30.After water treatment of Sb 3+ −2α and Sb 3+ −2β, the amorphous tin oxide layer formed on the surface of the corresponding compounds can protect them from further degradation, [22,60] which is the dominant reason for their high antiwater stability.Particularly, Sb 3+ −2α and Sb 3+ −2β also exhibit excellent optical properties, which illustrates the Sb 3+ -doped Sn(IV)-based metal halides have great application potential in SSL, and we have demonstrated Sb 3+ −2α in high-performance white light-emitting diode (WLED, Figure S31).
In further experiment, we unexpectedly found that the yellow-emitting Sb 3+ −2α can quickly change into red emission upon dichloromethane (CH 2 Cl 2 ) treatment, and can return to yellow emission after being placed in the air environment for a brief period of time.This reversible PL switching characteristic of Sb 3+ −2α enables us to explore its intrinsic nature.At first, we measured the PXRD patterns of Sb 3+ −2α after CH 2 Cl 2 treatment.As shown in Figure 5A, we found that the measured PXRD results exhibit the Sb 3+ −2α and Sb 3+ −2β mixed phases, which indicates that Sb 3+ −2α can be induced to Sb 3+ −2β by CH 2 Cl 2 .The reason for seeing the mixed phase in the PXRD results is that Sb 3+ −2α can quickly convert to Sb 3+ −2β in the air, especially when it is irradiated by X-rays, which will accelerate this process even faster.As another evidence, we measured the PL spectra of Sb 3+ −2α, Sb 3+ −2β and Sb 3+ −2α after CH 2 Cl 2 treatment and volatilization.As shown in Figure 5B, Sb 3+ −2α after CH 2 Cl 2 treatment exhibits a similar PL spectral profile with Sb 3+ −2β, and its spectral profile can be further recovered after the volatilization of CH 2 Cl 2 , which further confirms that Sb 3+ −2α does become Sb 3+ −2β after CH 2 Cl 2 treatment.Subsequently, Sb 3+ −2α and Sb 3+ −2β were soaked in various solvents (Figure S32), including (1) ethanol, (2) n-hexane, (3) acetone, (4) methanol, (5) toluene, (6) carbon tetrachloride, (7) trichloromethane, (8) dichloromethane.The final experimental results show that the emission color of Sb 3+ −2β remains unchanged after different solvent treatments, which indicates its stable phase structure.In contrast, only when Sb 3+ −2α is immersed in CH 2 Cl 2 , the luminous color can be changed from yellow to red emission, which indicates that Sb 3+ −2α exhibits significant selective fluorescence conversion characteristics for CH 2 Cl 2 .Furthermore, this reversible PL switching shows remarkable repeatability with almost no decrease in the PL intensity in the case of 10 consecutive cycles (Figure S33).Hence, this unique PL switching characteristic was first reported in Sn(IV)-based metal halides, and its low toxicity, excellent stability and high luminous efficiency provide a great opportunity for its application in the next-generation intelligent materials.
[63][64][65] However, they generally exhibit single-model or double-model dynamic luminescent characteristics, which severely limit their further application.Developing new external stimulus responsive luminescent materials with triplet-mode and multimode dynamic luminescence switching characteristics is an effective strategy to enhance information security protection because they are rare and difficult to replicate.Inspired by the transformation of the structure and luminescent color from yellow-emitting CsCu 2 I 3 to the blue-emitting Cs 3 Cu 2 I 5 after treatment by CsI ethanol solution, [66] we are surprised to find that nonluminescent Sb 3+ −1 can transform into yellow-emitting Sb 3+ −2α after treatment by a certain concentration of C 25 H 22 PCl methanol solution (0.5 mmol/mL, Figure S34).Combined with the reversible luminescence switching between Sb 3+ −2α and Sb 3+ −2β, thus a triple-mode tunable PL switching of off-on I -on II was constructed in this family of Sb 3+ -doped Sn(IV)-based compounds under dual external stimuli of C 25 H 22 PCl and CH 2 Cl 2 solution, respectively.
As we know, laser inkjet printing has many advantages, such as maskless, high spatial resolution and accuracy, easy processing, and so forth, so it has attracted extensive attention as an effective means to make fluorescent anticounterfeiting applications in recent years. [67]Hence, we can use laser inkjet printing to make various anticounterfeiting patterns in the proof-of-concept experiment based on the excellent solution processability of Sb 3+ −1, and the detailed manufacturing process is shown in Figure S35.First, the commercial A4 paper was soaked in a methanol solution of C 25 H 22 PCl/SnCl 4 (2:1, 0.5 mmol/mL) for 3 s and dried completely.Then, the SbCl 3 methanol solution (0.1 mmol/mL) was loaded into a commercial ink cartridge as ink, and the designed luminescent anticounterfeiting patterns were printed on the processed A4 paper using a commercial HP 1112 laser inkjet printer.When the methanol solvent is completely volatilized, Sb 3+ −1 microcrystals will form on A4 paper.In subsequent experiments, the emblem and QR code patterns of Guangxi University were successfully printed on A4 paper via laser inkjet printing, as shown in Figures 6A  and B. Particularly, the as-fabricated patterns are almost colorless under sunlight and invisible under UV light excitation, which ensures that the real information can be well hidden.After the pattern was treated with C 25 H 22 PCl solution as an external stimulus, the invisible patterns changed into bright yellow emission under 365 nm irradiation, corresponding to the structural transformation from Sb 3+ −1 to Sb 3+ −2α.Subsequently, the yellow patterns can convert into red emission within 30 s after treatment by CH 2 Cl 2 , which is due to the transition from Sb 3+ −2α to Sb 3+ −2β.Moreover, the red emission patterns can recover to the original yellow emission within 120 s due to the volatilization of CH 2 Cl 2 .Thus, a double fluorescent anticounterfeiting can be achieved by reasonably controlling different external stimuli.
In further experiments, we also demonstrated the application of as-synthesized Sb 3+ -doped Sn(IV)-based compounds in multiple information encryption.Initially, the Sb 3+ −2α@PMMA composite was prepared by utilizing polymethyl methacrylate (PMMA) as a protective shell to encapsulate Sb 3+ −2α.As we expected, Sb 3+ −2α@PMMA composite remained yellow emission under 365 nm excitation even under the treatment of CH 2 Cl 2 (Figure S36), which should be attributed to the remarkable organic solvent resistance of PMMA.Hence, two different optical responses can be obtained in Sb 3+ −2α and Sb 3+ −2α@PMMA composite under CH 2 Cl 2 treatment, which enables them to serve as a pair of fluorescence counterparts and be used for advanced information storage.Based on this triplemode tunable PL switching of off-on I -on II characteristics in Sb 3+ -doped Sn(IV)-based metal halides, we constructed a triple-mode information encryption-decryption scheme, in which the red emission signal is specified as the correct encrypted information, while other signals are considered as interference information.As shown in Figure 6C, we designed a combined luminescent anticounterfeiting pattern including flower (A), stem (B), and flowerpot (C), where A, B, and C are made by Sb 3+ −1, Sb 3+ −2α@PMMA, and Sb 3+ −2α, respectively.For single-mode encryption, only a bright yellow emission of B and C parts can be observed without any processing.Parallelly, all the patterns exhibit yel-low emission after treatment by C 25 H 22 PCl solution, which is a double-model information encryption.In particular, the correct information can be obtained when the as-fabricated patterns were treated by the CH 2 Cl 2 , in which parts A and C show red emission upon 365 nm excitation, and a triple-mode information encryption can be obtained.More specifically, only by fully grasping these external stimuli can correct information be decrypted.Therefore, the triple-mode tunable PL switching of off-on I -on II characteristics observed in Sb 3+ -doped 0D Sn(IV)-based metal halides ensures that such materials will show great application potential in advanced optical information encryption-decryption in the near future.
Finally, a molecular-based optical AND logic gate was designed based on the unique photophysical properties of the as-synthesized compounds, and the details was given in Figure 6D.In our scheme, Sb 3+ −1 is defined as 1 and nothing is denoted as 0 for input A. Moreover, C 25 H 22 PCl solution is assigned as input B (presence as "1" and absence as "0"), and CH 2 Cl 2 solution is assigned as input C (presence as "1" and absence as "0").Particularly, the emission color is defined as the output signal D, in which the nonemission and red emission are set to "0" and "1", respectively.Obviously, the output D signal of the logic gate will be 1 only if the inputs A, B, and C are both signals 1.0][71] 3 CONCLUSION  3+ -doped Sn(IV)-based compounds.Given these unusual PL characteristics, we demonstrated the as-synthesized compounds in the applications of multiple fluorescent anticounterfeiting, information encryption-decryption, and optical logic gates by reasonably controlling different external stimuli.Hence, our results not only deepen the understanding of the relationship between the structure and optical properties of low-dimensional lead-free metal halides, but also further broaden their applications in the field of information security.

Characterization
The crystal structure of the as-synthesized compounds was characterized using SCXRD (Bruker D8 Venture).The PXRD data were collected using the LabX XRD-6100 instrument.SEM was characterized by Hitachi SU8020 instrument, and the corresponding element content was collected by EDS (Oxford X-Max Aztec).XPS data were collected using the SHIMADZU AXIS Ultra DLD instrument.Thermal analysis was performed using the Mettler-Toledo instrument.
The PL and PLE were measured using a FluoroMax+ spectrometer.The PLQY was determined at RT using the FLS1000 instrument.The emission spectra at various temperatures were recorded using a Fluorolog-3 spectrometer under 310 nm excitation.Raman spectra were acquired using the HR Evolution instrument.Absorption spectra were measured with the LAMBDA1050 instrument, with BaSO 4 serving as the reference sample.Decay lifetimes were determined using the FLS1000 instrument upon 310 nm laser excitation.

Calculation methods
All calculations at DFT are carried out using the Vienna Ab initio simulation package. [72]The generalized gradient approximation of the PBE [73] parameterization with projector-augmented wave [74] method is performed for the exchange and correlation functional.For the elements C, O,

F I G U R E 1
Crystal structure of the as-synthesized Sn(IV)-based metal halides, (A and B) compound 1, (C and D) compound 2α, and (E and F) compound 2β.Experimental and calculated PXRD patterns of (G) compound 1, (H) compound 2α, and (I) compound 2β.

F I G U R E 2
Photophysical properties of Sb 3+ −2α and Sb 3+ −2β at RT. Images of Sb 3+ −2α (A) and Sb 3+ −2β (B) single crystals under sunlight and UV lamp.PLE and PL spectra of Sb 3+ −2α (C) and Sb 3+ −2β (D).PL decay curves monitored at the LE emission band of Sb 3+ −2α (E) and Sb 3+ −2β (F).The insert exhibits the PL lifetime of the HE emission band of Sb 3+ −2α and Sb 3+ −2β.by thermogravimetric analysis, and the initial decomposition temperature of Sb 3+ −1 is 150 • C (Figure S9a), which should be attributed to CH 4 O being stripped from the 1 under high temperature.Meantime, the initial decomposition temperatures of Sb 3+ −2α and Sb 3+ −2β are 210 • C (Figures S9b and

F I G U R E 6
Photographs of printed patterns for anticounterfeiting application, (A) emblem and (B) QR code pattern of Guangxi University.(C) Triplemode information encryption-decryption application based on the combined compounds of Sb 3+ −1, Sb 3+ −2α@PMMA and Sb 3+ −2α.(D) Optical logic gate application based on the as-synthesized Sb 3+ -doped Sn(IV)-based compounds.
Three homologous air-stable 0D Sb 3+ -doped Sn(IV)-based metal halides, namely, Sb 3+ −1, Sb 3+ −2α, and Sb 3+ −2β, were synthesized by inserting a single organic ligand into the inorganic lattice.Interestingly, Sb 3+ −1 does not emit light, Sb 3+ −2α shines bright yellow emission centered at 580 nm with a PLQY of 92%, and Sb 3+ −2β emits an efficient red emission centered at 637 nm with a PLQY of 78%.The detailed photophysical processes of these compounds were investigated via crystal structure and luminescence properties, and found that the lattice distortion is the dominant reason for their different photophysical properties.Interestingly, the nonemitting of Sb 3+ −1 can convert into the yellow-emitting Sb 3+ −2α after treatment with C 25 H 22 PCl solution, and it can further turn into the red-emitting Sb 3+ −2β under CH 2 Cl 2 treatment, thus a triple-mode tunable PL switching of off-on I -on II was realized in Sb