Regulation of afterglow and self-trapped exciton emission in indium-based organic metal halides via metal ion doping for multilevel anti-counterfeiting†

Hongbo Qi^a, Jing Li^a, Hailong Yu^a, Jing Zhang^a, Chen Chen^a, Qiuju Han^b and Wenzhi Wu*^a
aSchool of Electronic Engineering, Heilongjiang University, Harbin, Heilongjiang 150080, China. E-mail: wuwenzhi@hlju.edu.cn
^bSchool of Arts and Sciences, Northeast Agricultural University, Harbin, Heilongjiang 150030, China

First published on 12th June 2025

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1. Introduction

Zero-dimensional hybrid metal halides (0D HMHs) attract significant attention in materials science and optoelectronics due to their unique structures and excellent optical properties, which hold promising applications in areas such as photovoltaic cells,¹ scintillators,^2,3 light-emitting diodes (LEDs),^4,5 photodetectors,⁶ information encryption,^7,8 and lasers.⁹ Unlike inorganic metal halides,^10,11 hybrid metal halides are organic–inorganic composite materials that typically consist of inorganic metal halide ions and organic cations.¹² The structure of these materials is often composed of alternating organic cations and inorganic halide networks, where the metal halide anion units are completely isolated by the organic cations.^13,14 This unique arrangement allows them to exhibit the advantageous properties of both organic and inorganic materials.

Organic cations serve as independent luminous centers, providing emissions that differ from those of the inorganic metal halide units, which impart distinctive optical properties to the materials, including photoluminescence (PL), phosphorescence, and thermally activated delayed fluorescence.^15–17 In particular, organic molecules with room temperature phosphorescence (RTP) properties, such as diphenyl sulfone (SF),¹⁸ triphenylamine (TPA),¹⁹ 4-phenylbenzylamine (PBA),²⁰ tetraphenylene (TEP),²¹ triphenylphosphine (TPP),²² and triphenylsulfonium (TPS),²³ combine with inorganic units to form a rigid structure. This structure effectively reduces non-radiative relaxation induced by molecular vibrations and rotations, improves phosphorescence efficiency and extends lifetime.^24–26 This attracts widespread attention in fields such as biomedical imaging,^27,28 sensors,²⁹ and anti-counterfeiting encryption technologies.^30,31

Metal ion doping is a well-recognized strategy for tuning the bandgap of materials. In particular, incorporating ions with an ns² electronic configuration, such as Bi³⁺ and Sb³⁺, can enhance the stability of the material, improve luminescent properties, and introduce new photoluminescent characteristics, such as high quantum yield, substantial Stokes shift, and new luminous centers.^32–34 Unlike traditional dopants that involve d- and f-electrons, ns² dopants leverage the s-electrons, which allow for the simultaneous modulation of the optical absorption and emission characteristics of the materials. However, the large free volume of organic cations in the mixture often leads to a loss of the persistent luminescence associated with these organic cations when combined with luminescent metal halide polyhedra.^8,35,36 To address this challenge, we propose a strategy that involves the incorporation of In³⁺ ions into organic phosphorescent molecules. This approach aims to enhance the afterglow of the organic compounds while simultaneously introducing ns² ion doping to dynamically regulate the persistent luminescence and STE emissions. However, the complexity of the energy level structure and the various energy transfer pathways introduced by doping complicate the understanding of the mechanisms behind afterglow and STE emissions. This complexity necessitates a detailed investigation into the energy transfer mechanisms and interactions between the doped ions and the luminescent centers to unravel the emission characteristics and optimize the properties for luminescent applications.

In this study, In³⁺ ions are incorporated into the organic phosphorescent molecule triphenyl sulfide, resulting in the synthesis of the 0D HMH (Ph₃S)₂InCl₅. Additionally, the introduction of Bi³⁺ and Sb³⁺ ions with lone pair electrons establishes energy transfer pathways from singlet and triplet states to STE states, enabling controllable modulation of PL and phosphorescence emissions. Compared to the triphenyl sulfide organic molecule, (Ph₃S)₂InCl₅ exhibits significantly enhanced afterglow. However, after doping with Bi³⁺, the persistent luminescence is notably reduced, and it completely disappears upon doping with Sb³⁺, revealing only STE emission. Temperature-dependent PL spectra, TRPL spectra, and Raman spectra are employed to elucidate the STE recombination process and the electron–phonon coupling dynamics. Finally, a dynamic persistent phosphorescence–photoluminescence switchable dual-mode encryption system is achieved through time-resolved multi-level encryption technology.

2. Results and discussion

2.1 Basic optical and structural characterization

The synthesis of (Ph₃S)₂InCl₅ crystals occurs via the solvent evaporation method by using triphenylsulfonium and InCl₃ in DMF solution. The structure observed along the a-axis is depicted in Fig. 1(a). In this structure, one In atom coordinates with five Cl atoms, forming a distorted [InCl₅]²⁻ pyramidal structure. The Ph₃S⁺ cations isolate the [InCl₅]²⁻ pentagonal units, thereby constituting a zero-dimensional organic–inorganic hybrid material. The crystal structure adopts the C2 space group of the monoclinic crystal system, and the unit parameters are a = 15.819 Å, b = 14.712 Å, c = 9.508 Å, α = 90°, β = 126.42°, and γ = 90°. The crystal data and structural refinement of (Ph₃S)₂InCl₅ are shown in Table S1.† The morphology and distribution of elements are characterized using scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS) (Fig. 1(b)). The elements S, In, and Cl show uniform distribution, with the ratio of S, In, and Cl being 3.4:1.8:7.7 (Table S1†), which is close to 2:1:5, corresponding to the stoichiometry of (Ph₃S)₂InCl₅. Given the similar ionic radii of In³⁺ (CN = 5, 0.71 Å), Sb³⁺ (CN = 5, 0.80 Å) and Bi³⁺ (CN = 5, 0.96 Å),³⁷ indium-based hybrids serve as the matrix to modify its PL properties by co-doping with ns² electrons from Bi³⁺ and Sb³⁺. By adding BiCl₃ and SbCl₃ separately to the DMF solution of Ph₃SCl and InCl₃, (Ph₃S)₂In_1−xBi_xCl₅ and (Ph₃S)₂In_1−xSb_xCl₅ crystals (x = 0–0.2) are obtained.

The SEM and EDS characterization are shown in Fig. S1,† where Bi and Sb elements are uniformly distributed, demonstrating that Bi³⁺/Sb³⁺ ions are successfully incorporated homogeneously into the (Ph₃S)₂InCl₅ lattice. The Bi³⁺/Sb³⁺ ions partially replace In³⁺ to form [BiCl₅]²⁻ and [SbCl₅]²⁻ pentagonal units. The doping concentrations of Bi³⁺/Sb³⁺ are detailed in Tables S2 and S3.† The precursor containing 1 mmol [Ph₃S]⁺, 0.45 mmol In³⁺, and 0.05 mmol Bi³⁺/Sb³⁺ yields (Ph₃S)₂In_0.9Bi_0.1Cl₅ and (Ph₃S)₂In_0.9Sb_0.1Cl₅ with doping amounts of approximately 12.5% and 10.5% (Bi/[In + Bi] × 100%, Sb/[In + Sb] × 100%), which are consistent with the actual feed ratio. Subsequently, X-ray photoelectron spectroscopy (XPS) further confirms the successful doping of Bi³⁺/Sb³⁺, as shown in Fig. 1(d). The XPS spectrum exhibits two peaks at 445.1 eV and 452.9 eV corresponding to the 3d characteristic peaks of In, while the peak at 284.8 eV corresponds to the 1s characteristic peak of C. The high-resolution XPS spectra of Cl, S, Bi, O, and Sb are illustrated in Fig. 1(e, f) and S2.† After doping with Bi³⁺/Sb³⁺, the peaks of O 1s, C 1s, S 2p, and Cl 2p exhibit certain positive shifts. This is attributed to the weaker electronegativity of In³⁺ ions being replaced by the stronger electronegativity of Bi³⁺/Sb³⁺ ions, leading to positive shifts in binding energy. Furthermore, the successful synthesis of the crystals can also be verified using powder X-ray diffraction (PXRD). As shown in Fig. 1(c) and S3,† the PXRD of the measured samples exhibits a high degree of correspondence with the simulated single-crystal X-ray diffraction (SCXRD), and there are no additional diffraction peaks, indicating the successful synthesis of (Ph₃S)₂InCl₅ without the formation of impurities. As the concentration of Bi³⁺/Sb³⁺ increases, the XRD peaks of the Sb³⁺ doped samples shift towards lower diffraction angles. This shift occurs because the ionic radius of Bi³⁺ (0.96 Å) is larger than that of Sb³⁺ (0.80 Å) and In³⁺ (0.71 Å), resulting in lattice expansion, which confirms that Bi³⁺/Sb³⁺ successfully replaces a portion of In³⁺. Additionally, the XRD peaks of the doped samples show a high degree of consistency with the XRD peaks of the matrix and the simulated XRD peaks (Fig. 1(c)), indicating that no phase transition occurs with the doping of Bi³⁺/Sb³⁺.

The optical properties of (Ph₃S)₂InCl₅ are revealed through absorption spectra, PL spectra, PLE spectra, and TRPL spectra. The absorption spectra of (Ph₃S)₂In_0.9Bi_0.1Cl₅ and (Ph₃S)₂In_0.9Sb_0.1Cl₅ at x = 0, 0.05, 0.1, 0.15, and 0.2 are shown in Fig. S4.† The high-energy absorption peak at 277 nm is a combination of the π–π* transition of the phenyl group and the absorption of [InCl₅]²⁻.^38–40 The prominent absorption peak at 335 nm corresponds to the transition from Bi^{3+ 1}S₀ to ³P₁, while the absorption peak at 301 nm corresponds to the transition from Sb^{3+ 1}S₀ to ¹P₁, and the absorption peak at 331 nm is due to the transition from Sb^{3+ 1}S₀ to ³P₁. Additionally, as the doping concentration increases, the corresponding absorption peaks gradually intensify. These UV-visible absorption results indicate the presence of multiple electronic absorption transitions. The band gaps are calculated using the Tauc plot. As shown in Fig. S5,† Bi³⁺/Sb³⁺ doping leads to a narrowing of the band gap. With increasing doping concentration, the band gap of (Ph₃S)₂In_1−xBi_xCl₅ (x = 0, 0.05, 0.1, 0.15, 0.2) decreases from 4.15 eV to 3.54 eV, while the band gap of (Ph₃S)₂In_1−xSb_xCl₅ (x = 0, 0.05, 0.1, 0.15, 0.2) decreases from 4.15 eV to 3.87 eV. This reduction is attributed to the increased lattice distortion caused by the higher doping concentration, resulting in a decrease in the band gap. The PLE and PL spectra of (Ph₃S)₂InCl₅ are shown in Fig. S6(a).† It can be observed that the excitation and emission spectra of (Ph₃S)₂InCl₅ are generally consistent with those of TPS-Cl. The TRPL spectrum of (Ph₃S)₂InCl₅ under 375 nm excitation monitored at 440 nm (prompt PL peak) is presented in Fig. S6(b),† revealing a PL lifetime of 0.78 ns, which is similar to that of TPS-Cl.²³ This further indicates that [InCl₅]²⁻ does not directly participate in the luminescence process, and the emission of (Ph₃S)₂InCl₅ originates from the organic Ph₃S⁺ cation. The Ex–Em scans of TPS-Cl and (Ph₃S)₂InCl₅, which are shown in Fig. S9(a) and (b),† indicate that they have consistent luminescent centers, and it can be observed that they exhibit excitation wavelength dependence, which suggests the presence of multiple emission centers in this organic emitter. The similar wavelength-dependent PL spectra further confirm that these PL emissions arise from the relaxation of the same excited state. After doping, as shown in Fig. S9(c)† and 4(d), (Ph₃S)₂In_1−xBi_xCl₅ exhibits a blue broad emission at 490 nm, while (Ph₃S)₂In_1−xSb_xCl₅ displays a yellow broad emission at 610 nm. As illustrated in Fig. S7 and S8,† the PL intensity increases with the increasing doping concentration of Bi³⁺/Sb³⁺, reaching a maximum at x = 0.1. For x > 0.1, the PL intensity decreases with increasing doping concentration, attributed to PL quenching from excessive doping. Therefore, in subsequent tests, the sample with x = 0.1 was selected for further study, which resulted in the measurement of the PLQY of (Ph₃S)₂In_0.9Bi_0.1Cl₅ of 32.23%. Surprisingly, we found that (Ph₃S)₂In_0.9Sb_0.1Cl₅ exhibited superior optical performance, with a PLQY that reached as high as 89.72% (Fig. 4(c) and S10†). This lays a solid foundation for its future applications in the optoelectronic field, such as in light-emitting diodes (LEDs), laser diodes (LDs), and photodetectors.^41,42

The crystals of TPS-Cl, (Ph₃S)₂InCl₅, (Ph₃S)₂In_0.9Bi_0.1Cl₅, and (Ph₃S)₂In_0.9Sb_0.1Cl₅ exhibit distinct colors under natural light, ultraviolet irradiation (365 nm), and ultraviolet quenching. Under natural light, all crystals appear colorless. Under a 365 nm UV lamp, TPS-Cl, In–Cl, and In–Bi–Cl emit blue luminescence, while In–Sb–Cl emits bright orange-yellow light. When the UV lamp is turned off, a notable yellow-green afterglow is observed in the TPS-Cl, In–Cl, and In–Bi–Cl crystals. Additionally, the afterglow duration of the In–Cl crystal is the longest, approximately 1.5 seconds, while the afterglow duration of the TPS-Cl crystal is about 0.7 seconds, and the In–Bi–Cl crystal exhibits the shortest afterglow duration of approximately 0.5 seconds. No afterglow emission is observed in the In–Sb–Cl crystal. This indicates that the doping of Bi³⁺ and Sb³⁺ plays a crucial role in the afterglow of the crystals. Furthermore, the formation of [InCl₅]²⁻ significantly promotes the afterglow of [Ph₃S]⁺.

To reveal the relationship between afterglow and In³⁺, Bi³⁺, and Sb³⁺, the photophysical properties of TPS-Cl, In–Cl, In–Bi–Cl, and In–Sb–Cl crystals were studied. Their prompt and delayed (obtained 10 ms after excitation) PLE spectra, as well as prompt and delayed (obtained 10 ms after excitation) PL spectra under 300 nm excitation, are shown in Fig. 3. As illustrated in Fig. 3(d), the luminescent center of TPS-Cl is located at 440 nm and exhibits a long tail. The afterglow emission center is at 515 nm, which explains the yellow-green afterglow observed in Fig. 2. The luminescent center of In–Cl, which is consistent with that of TPS-Cl as previously mentioned, indicates that their emissions originate from the organic [Ph₃S]⁺ cation. Additionally, the afterglow emission of In–Cl aligns with that of TPS-Cl, which further supports this connection. The measured afterglow quantum yields of TPS-Cl, In–Cl and In–Bi–Cl are 1.8%, 2.5% and 0.8%, respectively.

However, the phosphorescence PLE signals in Fig. 3(a) and (b) show that the signal for TPS-Cl approaches zero in the 375–400 nm range, while In–Cl still exhibits a strong phosphorescence PLE signal. This indicates the presence of energy transfer from [InCl₅]²⁻ to the organic [Ph₃S]⁺ cation. In other words, light in the 375–400 nm range cannot directly excite the afterglow of TPS-Cl but can excite [InCl₅]²⁻, which then transfers energy to the triplet state of [Ph₃S]⁺. The enhancement of the persistent luminescence observed in In–Cl, as shown in Fig. 2, is thus elucidated. The luminescent center of In–Bi–Cl is located at 490 nm, which corresponds to the typical blue emission of Bi³⁺. The PL of In–Bi–Cl is fundamentally consistent with that of TPS-Cl (Fig. 3(g)), indicating that the persistent emission from In–Bi–Cl still originates from the triplet state of [Ph₃S]⁺. As illustrated in Fig. 3(c), the phosphorescence PLE signal of In–Bi–Cl exhibits a distinct excitation peak in the range of 280–320 nm, which emanates from [Ph₃S]⁺. Additionally, the PLE signal in the same range also exhibits a prominent excitation peak from [Ph₃S]⁺, suggesting the presence of energy transfer from [Ph₃S]⁺ to [BiCl₅]²⁻. In Fig. 4(b), the PLE signal of In–Bi–Cl overlaps partially with the persistent luminescence signal of TPS-Cl (indicated by the red dashed area), which further confirms the energy transfer from [Ph₃S]⁺ to [BiCl₅]²⁻. This also explains the reason for the decay of the persistent luminescence observed for In–Bi–Cl in Fig. 2. Under 300 nm excitation, In–Sb–Cl exhibits a dual-peak emission at 490 nm and 610 nm, as shown in Fig. S11(b),† and it is noted that no PL signal is detected after the excitation ceases. The PLE spectrum of In–Sb–Cl (Fig. S11(a)†) shows a prominent [Ph₃S]⁺ PLE signal in the 280–320 nm range, indicating energy transfer from [Ph₃S]⁺ to [SbCl₅]²⁻. In Fig. S18,† the PLE signal of In–Sb–Cl detected at 610 nm partially overlaps with the PL signal of In–Cl (indicated by the red dashed area), which further confirms the energy transfer from [Ph₃S]⁺ to the triplet states of [SbCl₅]²⁻. Additionally, no phosphorescence PLE signal is detectable for In–Sb–Cl, consistent with the earlier observation of the absence of a persistent luminescence signal.

To investigate the exciton dynamics in these materials, a series of time-resolved photoluminescence (TRPL) spectroscopy measurements were conducted. The persistent luminescence lifetime of TPS-Cl was measured at 515 nm, yielding a value of 150.38 ms, corresponding to the triplet emission of [Ph₃S]⁺. In contrast, for the In–Cl associated delayed emission, the lifetime is extended to 191.85 ms when monitored at 515 nm, which is attributed to triplet energy transfer from the [InCl₅]²⁻ anion to the [Ph₃S]⁺ cation. However, in the case of In–Bi–Cl, energy transfer from the organic [Ph₃S]⁺ cation to the [BiCl₅]²⁻ anion resulted in partial quenching of the persistent luminescence, reducing the lifetime to 88.00 ms. The observed tri-exponential decay behavior in the afterglow arises from a combination of thermally activated delayed fluorescence (TADF), phosphorescence, and the heterogeneous microenvironment within the solid sample. Detailed results are summarized in Table S4.†^43,44 The long lifetime component (τ₃) values of 220.97 ms, 352.97 ms, and 170.23 ms for TPS-Cl, In–Cl, and In–Bi–Cl, respectively, confirm the above energy transfer process. In the triple-exponential fitting analysis, the medium lifetime component (τ₂) exhibits the highest amplitude contribution (a₂), which corresponds to the radiative complexation of the triplet exciton of the organic cation [Ph₃S]⁺. The a₂ value of In–Bi–Cl decreases after Bi-doping, but is still dominant, which suggests that the radiative complexation pathway has not been completely quenched. For In–Sb–Cl, which exhibits a dual emission peak, TRPL spectroscopy was performed on both peaks. As shown in Fig. S13,† a lifetime of 19.62 ns was detected at 490 nm, corresponding to the singlet state emission of [SbCl₅]²⁻. These findings highlight the complex interactions and energy transfer processes within these hybrid materials, providing insights into their exciton dynamics and the effects of different dopants on luminescence properties. At 610 nm, the measured lifetime is as shown in Fig. S14,† yielding a value of 5.32 μs, which corresponds to the triplet STE emission of [SbCl₅]²⁻.^45–47 The strong electron–phonon interactions within [SbCl₅]²⁻ cause the excited electrons and holes to induce elastic distortions in the surrounding lattice. Under the influence of the Jahn–Teller effect, this results in a broad PL emission with a high quantum yield.^48,49 To further analyze the TRPL lifetimes of In–Bi–Cl and In–Sb–Cl crystals, a 405 nm picosecond laser was focused using a 10× objective to measure the PL dynamics at different positions of a single crystal. The PL images excited at various locations—top (P1), left (P2), center (P3), right (P4), and bottom (P5)—are shown in Fig. S14 and S16,† where the blue emission of In–Bi–Cl and the yellow emission of In–Sb–Cl are clearly observed. The corresponding PL decay curves, illustrated in Fig. S15 and S17,† show that the PL dynamics at the center and edges of the crystal are essentially similar, suggesting that the crystal is relatively uniform in structure and properties. For In–Sb–Cl, an excitation–emission (Ex–Em) scan was conducted, as shown in Fig. 4(d). Two distinct luminescent centers corresponding to the singlet and triplet emissions of [SbCl₅]²⁻ were observed. When excited within the range of 250–340 nm, both singlet and triplet emissions were detected, resulting in a white light emission. In contrast, excitation in the range of 340–400 nm led to yellow-white light emission dominated by triplet emission. These findings illustrate the complex interplay of energy transfer and electron–phonon coupling in these materials, providing valuable insights into their photoluminescent properties and potential applications in optoelectronic devices.

The system regulates the luminescence mechanism through energy transfer between metal halides and organic cations. In TPS-Cl, the radiative transition from S₁ to S₀ produces fluorescence at 440 nm, while intersystem crossing (ISC) to the T₁ state results in afterglow luminescence at 515 nm. In the In³⁺ system, Dexter energy transfer transmits energy from [InCl₅]²⁻ to the organic T₁ state, enhancing the afterglow luminescence. After doping with Bi³⁺, the S₁ state of [BiCl₅]²⁻ acts as an intermediate level that captures electrons from the organic S₁ state (ET₁), competing with the radiative transition of organic T₁ luminescence, which leads to a reduction in persistent luminescence. The Sb³⁺ system exhibits a unique dual-channel: the S₁ state generates fluorescence at 490 nm, while the T₁ state, through Jahn–Teller distortion, forms a low-energy STE state (610 nm). Through multipath energy transfer, excitons from the organic S₁/T₁ states are trapped in the STE state, ultimately resulting in STE emission via exciton localization, which completely quenches organic persistent luminescence.

2.2 Steady-state PL spectra, transient PL spectra, and Raman spectra at different temperatures

The temperature-dependent PL spectra are used to investigate the electron–phonon coupling processes in the excited state. The temperature-dependent PL spectra of (Ph₃S)₂InCl₅ from 80 K to 250 K are provided in the ESI.† The temperature-dependent PL spectra of (Ph₃S)₂In_0.9Bi_0.1Cl₅ are shown in Fig. 5(a), exhibiting strong blue emission at low temperatures. Similar to (Ph₃S)₂InCl₅, the PL intensity of (Ph₃S)₂In_0.9Bi_0.1Cl₅ gradually decreases in the temperature range of 80–150 K. From 150 to 220 K, the PL intensity decreases rapidly. Above 220 K, the intensity remains relatively constant due to the saturation of non-radiative recombination channels. Fitting yields an exciton binding energy of 53.22 meV. This exciton binding energy represents the energy required for the dissociation of excitons into free electrons and holes. The exciton binding energy of (Ph₃S)₂In_0.9Bi_0.1Cl₅ is higher than that of (Ph₃S)₂InCl₅, which is attributed to the presence of [BiCl₅]²⁻, making it more difficult for the electron–hole pairs to dissociate. It is concurrently observed that the emission centers redshift as the temperature increases. From 80 to 240 K, the emission center of (Ph₃S)₂InCl₅ shifts from 437.76 nm to 445.01 nm. From 80 to 300 K, the emission center of (Ph₃S)₂In_0.9Bi_0.1Cl₅ shifts from 466.73 nm to 480.96 nm. This occurs due to the enhanced electron–phonon coupling at elevated temperatures, leading to a reduction in the bandgap, often referred to as the bandgap renormalization effect. In contrast, (Ph₃S)₂In_0.9Sb_0.1Cl₅ exhibits a strong resistance to thermal quenching, maintaining significant emission at 400 K (as shown in Fig. 5(b)). The fitting yields an exciton binding energy of 63.52 meV, with this high exciton binding energy further confirming the presence of the STE state.⁵⁰ The STE state localizes electrons and holes, resulting in enhanced stability of the excitons and an increase in their binding energy. Moreover, as the temperature increases, the full width at half maximum (FWHM) of all three samples broadens, which is attributed to the enhanced electron–phonon interactions at higher temperatures. The electron–phonon interactions can be revealed by the different FWHM values at varying temperatures using eqn (S3).† The specific fitting results are shown in Fig. 3(c), where the Huang–Rhys factor of (Ph₃S)₂In_0.9Bi_0.1Cl₅ is 27.75, with an average phonon energy of 43.71 meV. For (Ph₃S)₂In_0.9Sb_0.1Cl₅, S is 15.62, and the average phonon energy is 35.67 meV. The smaller S value for (Ph₃S)₂In_0.9Sb_0.1Cl₅ indicates weaker electron–phonon coupling in this material. The lower phonon energy is associated with its weaker electron–phonon interactions and a larger interatomic distance, thus resulting in better resistance to thermal quenching.

Temperature-dependent TRPL spectroscopy provides a powerful tool and method for a deeper understanding of the radiative and non-radiative relaxation mechanisms of exciton recombination. As shown in Fig. S20 and S21,† we measured the TRPL spectra of (Ph₃S)₂In_0.9Bi_0.1Cl₅ and (Ph₃S)₂In_0.9Sb_0.1Cl₅ at different temperatures. It can be observed that their PL lifetimes decrease with increasing temperature, a phenomenon that is attributed to the enhanced electron–phonon coupling discussed above, which leads to thermally activated non-radiative recombination processes;⁵¹ additionally, the specific fitting results can be found in the ESI.† By fitting the PL lifetime as a function of temperature using eqn (S4)† (Fig. 5(d)), we obtained the radiative transition lifetimes of (Ph₃S)₂In_0.9Bi_0.1Cl₅ and (Ph₃S)₂In_0.9Sb_0.1Cl₅ to be 9.66 ns and 5.48 μs, respectively, which are close to their respective PL lifetimes. Furthermore, it corroborates the earlier fitting results indicating a smaller Huang–Rhys factor (S) and average phonon energy (ħω) in (Ph₃S)₂In_0.9Sb_0.1Cl₅. We plot the potential energy diagram to study the thermal activation of carrier trapping (Fig. S27†). Here, ΔE represents the energy difference between points E and C in the conduction band, which corresponds to the thermal activation energy. From the fitting in Fig. 5(d), we obtain the thermal activation energies for (Ph₃S)₂In_0.9Bi_0.1Cl₅ and (Ph₃S)₂In_0.9Sb_0.1Cl₅ as 122.05 meV and 403.53 meV, respectively. The energy barrier for (Ph₃S)₂In_0.9Sb_0.1Cl₅ is significantly higher, making it more difficult for the non-radiative transitions caused by phonon thermal vibrations to be activated in (Ph₃S)₂In_0.9Sb_0.1Cl₅.

The temperature-dependent PL spectra and the parameters obtained from TRPL spectra fitting for (Ph₃S)₂In_0.9Bi_0.1Cl₅ and (Ph₃S)₂In_0.9Sb_0.1Cl₅ are shown in Table S5.† (Ph₃S)₂In_0.9Sb_0.1Cl₅ exhibits smaller ħω, S and larger radiative lifetime as well as a higher thermal activation energy, indicating that its phonon thermal vibrations have a relatively weak effect on electrons, resulting in more stable PL performance at high temperatures. Considering these results comprehensively, (Ph₃S)₂In_0.9Sb_0.1Cl₅ demonstrates higher thermal activation energy with temperature variations, suggesting a stronger ability to resist thermal quenching in high-temperature environments. This characteristic highlights the potential of (Ph₃S)₂In_0.9Sb_0.1Cl₅ for practical applications, particularly in optoelectronic devices and luminescent materials.

In addition, the thermal vibrations of phonons in the material can also be studied using temperature-dependent Raman spectroscopy. The Raman spectrum at room temperature is shown in Fig. 5(e). With the doping of Bi³⁺ and Sb³⁺, there are essentially no changes in the Raman peaks. The three vibrational bands observed at 96.5 cm⁻¹ (asymmetric stretching, E_2g mode), 140.4 cm⁻¹ (asymmetric stretching, E_g mode), and 251.4 cm⁻¹ (symmetric stretching, A_1g mode) originate from the vibrations of the [InCl₅]²⁻/[BiCl₅]²⁻/[SbCl₅]²⁻ pyramids. The modes 2E_2g and 2E_g correspond to the overtone of the 96.5 cm⁻¹ and 140.4 cm⁻¹ modes. The peak near 1000 cm⁻¹ is attributed to the vibrations of the C–S bond in organic molecules, while the peak at 1575.2 cm⁻¹ is due to the vibrations of the CC bond in the benzene ring. The temperature-dependent Raman spectra of (Ph₃S)₂InCl₅ are shown in Fig. 5(f). From 80 K to 480 K, the peaks of the organic part gradually red-shift. As the temperature increases, the intermolecular interactions may weaken, leading to a decrease in the energy of the vibrational modes of the molecules, which results in a blue shift of the Raman peaks. Moreover, at higher temperatures, thermal expansion effects may cause the molecular chains to expand and disorder, increasing the degrees of freedom of the molecules, which also contributes to the red shift of the Raman peaks. In contrast, the inorganic component exhibits a significant blue shift at high temperatures. This blue shift occurs because the increase in temperature causes lattice expansion and enhanced lattice thermal vibrations, which in turn increase the vibrational frequency of the phonons and strengthen the electron–phonon interactions. Therefore, we present the temperature-dependent frequency changes in Fig. S22.† At high temperatures, temperature-dependent changes in Raman mode frequencies enable the examination of phonon scattering. The temperature coefficients for the E_2g and A_1g modes are −0.039 and −0.008 cm⁻¹ K⁻¹, respectively (eqn (S5)†). The phonon vibrations for the E_2g mode are more pronounced. We use eqn (S6)† to fit the FWHM of the E_2g mode. The fitting results are shown in Fig. S22(d),† yielding a phonon energy at 0 K of 48.01 meV. The phonon lifetimes for the E_2g and A_1g modes are shown in Fig. S22(c) (fitting using eqn (S7)).† As the temperature increases, the thermal vibrations of the phonons are enhanced, which increases the coupling between electrons and phonons. This enhanced interaction enables more rapid interaction between phonons and excited-state electrons, resulting in shorter phonon lifetimes.

To study the stability of the samples, after being placed under ambient conditions (22–28 °C, relative humidity 40–60%) for two months, the samples are characterized by PXRD and PL spectroscopy (excitation wavelength 360 nm). The PXRD patterns of all samples (Fig. S25(a–c)†) are completely consistent with those of fresh samples, with no obvious impurity peaks or peak position shifts, indicating that the crystal structures remain stable in air. In terms of optical stability, the PL intensity of (Ph₃S)₂InCl₅ decreased to 91.1% of the original value, (Ph₃S)₂In_0.9Bi_0.1Cl₅ to 94.6%, and (Ph₃S)₂In_0.9Sb_0.1Cl₅ to 97.6% (Fig. S25(d–f)†). These data fully demonstrate that this series of materials exhibit excellent long-term stability in air, and Sb/Bi doping significantly enhances their stability.

2.3 Multilevel anti-counterfeiting encryption system

Dynamic phosphorescence–photoluminescence switches can be used for multi-level anti-counterfeiting. The compounds (Ph₃S)₂InCl₅, (Ph₃S)₂In_0.9Bi_0.1Cl₅, and (Ph₃S)₂In_0.9Sb_0.1Cl₅ are ground into powders and filled into different positions on a digital plate, as shown in Fig. 6. Under natural light, the powders appear white, showing no visible differences. When illuminated with a 365 nm UV lamp, the samples are excited and display the number 8, defined as a. After turning off the UV lamp for 0.3 seconds, both (Ph₃S)₂InCl₅ and (Ph₃S)₂In_0.9Bi_0.1Cl₅ exhibit green afterglow, while (Ph₃S)₂In_0.9Sb_0.1Cl₅ shows no afterglow, thereby displaying the number 3, designated as b. After turning off the UV lamp for 1.5 seconds, the afterglow duration of (Ph₃S)₂InCl₅ is longer than that of (Ph₃S)₂In_0.9Bi_0.1Cl₅, leading to the disappearance of the afterglow from (Ph₃S)₂In_0.9Bi_0.1Cl₅. As a result, the display shows the number 7, defined as c. Using the defined decryption formula (A × 3 + B)/3 − C, the encrypted result is output as 2. Therefore, through afterglow, quadruple encryption of the numbers is achieved, and this time-dependent characteristic enables dynamic and encrypted information output.

In addition, this study constructs an optical information decryption system with spatiotemporal resolution characteristics by integrating a dynamic phosphorescent–photoluminescent switch with the hexadecimal RGB coding system. As shown in Fig. 6, three samples are integrated into a 9 × 9 matrix, presenting a uniform white camouflage state under natural light. When a 365 nm ultraviolet excitation is applied, the main control unit, (Ph₃S)₂In_0.9Sb_0.1Cl₅, exhibits characteristic orange-yellow emission (logic “1”), while the remaining control units output blue light (logic “0”), thus creating the initial binary information matrix. The system employs a first-column checking mechanism to determine data validity: when the first column value is “1”, true information parsing for that row is triggered, while a first column value of “0” marks that row as pseudo data flow. After the ultraviolet light source is turned off, the blue light signal captured at the 0.3 second timestamp is designated as interference information, while the phosphorescent decay signal extracted over a 1.5 second time window is identified as a valid information carrier. Based on a multi-level decryption algorithm, the binary data stream is grouped vertically into three rows for hexadecimal reconstruction (e.g., 10111010 → BA, 11011010 → DA, 11000001 → C1), ultimately generating color encoding output that conforms to the sRGB standard (e.g., #BADAC1).

3. Conclusions

In summary, we prepared (Ph₃S)₂InCl₅ crystals through solvent evaporation and regulated the afterglow and STE emission by doping with Bi³⁺/Sb³⁺. We investigated the energy transfer among the components through excitation and emission spectra, ultimately demonstrating that the STE state quenches the afterglow emission, while the PLQY of (Ph₃S)₂In_0.9Sb_0.1Cl₅ reaches up to 89.72%. By analyzing temperature-dependent PL, TRPL, and Raman spectra, we explored the electron–phonon coupling processes. Fitting these data allowed us to obtain physical information such as exciton binding energy, the Huang–Rhys factor, average phonon energy, thermally activated energy, radiative transition lifetime, and non-radiative transition lifetime, revealing the strong thermal quenching resistance of (Ph₃S)₂In_0.9Sb_0.1Cl₅. Furthermore, the use of these materials to construct a multi-level anti-counterfeiting and information security encryption platform paves the way for designing high-security, dynamic, encrypted output anti-counterfeiting materials.

Author contributions

Hongbo Qi: data curation, formal analysis, writing – original draft, and preparation. Jing Li: conceptualization and software. Hailong Yu: conceptualization and software. Jing Zhang: software and validation. Chen Chen: software and validation. Qiuju Han: validation. Wenzhi Wu: writing – review & editing, supervision, and funding acquisition.

Conflicts of interest

There are no conflicts to declare.

Data availability

All data supporting the findings of this study are available within the paper and its ESI.† Data will be made available on request from the corresponding author.

Acknowledgements

W. Z. acknowledges the financial support from the Heilongjiang Provincial Key Laboratory of Micro-nano Sensitive Devices and Systems (Heilongjiang University), the Basic Research Project for Outstanding Young Teachers of Heilongjiang Province (YQJH2023128), and the Cultivation Project of Double First-class Initiative Discipline by Heilongjiang Province (LJGXCG2022-061).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5qi00894h

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