Multifunctional dielectric/optical response with broadband white light emission in a hybrid stannic halide crystal

Abstract

The integration of switchable dielectrics and luminescent properties has significant application potential in emerging sensing and optoelectronic devices. Despite great progress in various colours, such as green, red, or yellow lights, the dielectric switching response equipped with intrinsic broadband white-light emission in a single material has been a very rare and ongoing challenge. Herein, we present an organic–inorganic hybrid crystal (TMPA)₂SnCl₆ (TMPA = 3-methoxypropylamine), which simultaneously exhibits a dual-channel electric/optical response with switchable dielectric biostability and broadband white light emission. This is a rare phenomenon among the reported switchable dielectric materials. Additionally, its analogue (TMPA)₂SnBr₆ displays a thermochromic phenomenon related to temperature-dependent [SnBr₆] octahedral distortion. This study would provide constructive insights for exploring the integration of dual-channel optical/electrical response characteristics in a single material at the molecular level.

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Introduction

The integration of multiple physical properties into a single material for multifunctional applications has long been a pursued objective for better application selectivity, freedom and possible superimposed coupling effects.^1–6 As stimulus-responsive materials, switchable dielectrics exhibit a reversible transition between low and high states in dielectric permittivity, holding great promise for use in temperature-control switches and sensing devices.^7–13 Accompanied by thermally driven structural phase transitions, the electric polarization response of switchable dielectrics under an electric field changes reversibly, reflecting the high contrast switching of the dielectric constant.^14–19 This switching response of the electrical channel, in combination with other functions, enables the exploration of more possibilities, including fascinating physical phenomena and exotic effects.^20–23 In particular, luminescent dielectric switches have aroused great interest because they offer great potential in novel optoelectronic applications with switching fluorescence imaging, multi-level control and sensing.^24–27 Among them, white-light-emitting materials, as an attractive solution for the disadvantages of traditional incandescent and fluorescent lighting sources, are highly desired and have rarely been reported so far.²⁸ Inorganic materials have been the mainstream of research and applications of switchable dielectrics with various functions owing to their excellent performance and chemical stability with widespread recognition.²⁹ The preparation of luminescent dielectric switches in inorganic materials often involves doping additional elements, such as rare earth metals, into multi-component products with possible troubles of high-temperature sintering and phase separation.^30,31 The implementation of luminescent dielectric switches in a single homogeneous material has always been an ongoing scientific task, while one with white-light is more challenging.

In molecular systems, organic–inorganic hybrid materials (OIHMs) possess a vast chemical diversity, structural adaptability, and easy preparation that has garnered increasing attention as a beneficial supplement to inorganic materials.^32–35 The presence of organic components in the construction of hybrid materials renders them susceptible to order–disorder phase transitions under thermal stimuli, resulting in dielectric anomalies. Simultaneously, the inorganic metal fraction induces many other fascinating physicochemical properties, such as photoluminescence.^36–38 For example, our group has reported a dielectric responsive OIHMs (pyrrolidinium)MnX₃ (X = Cl, Br), exhibiting a strong red-light emission attributed to the inorganic anion octahedron of [MnX₆].^39,40 Subsequently, many dielectric multifunctional materials emitting various colors, such as red, green, yellow, blue, and others, were explored, significantly enriching the development of fluorescent dielectric multifunctional materials.^38,41–45 Luo et al. also reported white light dielectric responsive lead-based materials, which have a warm white light a color temperature of 4423 K and have high color rendering index (CRI) of up to 86.²⁸ Despite these great advances, the single material simultaneously equipped with a dual channel optical/electrical response of dielectric switching response and intrinsic broadband white-light emission has been very rare and an ongoing challenge.

Herein, we successfully achieved the integration of the dielectric response and luminescent properties in a single material (TMPA)₂SnCl₆ (TMPA = 3-methoxypropylamine). With temperature-dependent structural phase transition, it shows a clear dielectric switching response with good reversibility between low and high states. The phase transition has a large entropy change of 69.3 J mol⁻¹ K⁻¹, which is higher than that of the vast majority of reported dielectric switching responsive materials, showing potential in storing thermal energy and cooling. More remarkably, (TMPA)₂SnCl₆ possesses luminescent properties for which it emits fascinating broadband white light with a long lifetime of 6.382 μs. Its analogue (TMPA)₂SnBr₆ shows obvious thermochromic properties in combination with dielectric switching. This work may offer a fresh perspective on the effective design and tuning integration of the multifunctional properties of white light dielectric multifunctional materials.

Experimental section

Synthetic procedures

Without further purification, all chemical reagents used herein are purchased directly from commercial suppliers. 3-Methoxypropylamine (TMPA, 98%, Meryer-Shanghai), Tin(IV) chloride (SnCl₄, 99%, Meryer-Shanghai), Tin(IV) bromide (SnBr₄, 98%, Meryer-Shanghai), Polymethylmethacrylate ((C₅H₈O₂)_n, AR, Meryer-Shanghai), Dichloromethane (CH₂Cl₂, 99.5%, Meryer-Shanghai), Ethanol (CH₃CH₂OH, 99%, Meryer-Shanghai) hydrochloric acid (HCl, AR, GuoYao-Jinghua) and Hydrobromic acid (HBr, AR, Meryer-Shanghai).

Synthesis of (TMPA)₂SnCl₆

Prepare a cup of a mixed solution of concentrated hydrochloric acid and water (1 : 4) with a total of 20 mL. Then, 3-methoxypropylamine (6 mmol, 0.535 g) and Tin(IV) chloride (3 mmol, 0.781 g) were added to the prepared solution and stirred for 30 minutes. By slow evaporation at room temperature, colourless and transparent block crystals of (TMPA)₂SnCl₆ were obtained after three days.

Synthesis of (TMPA)₂SnBr₆

Prepare one cup of a mixed solution of hydrobromic acid and ethanol (1 : 1) with a total of 20 mL. Then, 3-Methoxypropylamine (6 mmol, 0.535 g) and Tin(IV) bromide (3 mmol, 1.314 g) were added to the prepared solution and stirred for 30 minutes. By slow evaporation at room temperature, pale yellow and transparent block crystals of (TMPA)₂SnBr₆ were obtained after one week.

Fabricate a white LED lamp

First, a cup of dichloromethane solution was prepared with 10 mL. Subsequently, polymethylmethacrylate (PMMA, 1 g) was added to the prepared solution and sonicated for 2 hours to complete dissolution. Then, (TMPA)₂SnCl₆ (0.5 g) was added to the mixed solution and stirred for 30 minutes. Finally, the mixed solution was directly coated with a commercial ultraviolet LED lamp (365 nm) and evaporated in the air atmosphere for 24 hours to obtain a white LED lamp.

Measurement methods

The crystal structure and characteristics of this paper were determined by X-ray single crystal diffraction (SC-XRD), the differential scanning calorimetry (DSC) instrument, dielectric measurement, thermogravimetric analysis, powder X-ray diffraction, UV-Vis measurements and fluorescence spectrofluorometer. All the details are presented in the ESI.†

Results and discussion

Thermodynamic analysis of phase transition behaviours

The compounds (TMPA)₂SnCl₆ and (TMPA)₂SnBr₆ are obtained through evaporative crystallization (Fig. 1a). The simulated crystal morphology of (TMPA)₂SnCl₆ is basically consistent with the actual one (Fig. 1b). To investigate the phase transition behaviors of the two compounds, differential scanning calorimetry (DSC) detection was performed. For (TMPA)₂SnCl₆, Fig. 1c illustrates a pair of reversible endothermic/exothermic peaks at 391.3/328.9 K during the heating/cooling process with a wide thermal hysteresis of 62.4 K, characterizing first-order phase transitions. Utilizing DSC data, the entropy variation (ΔS) is determined to be 75.45 J mol⁻¹ K⁻¹ using the formula ΔS = ΔH/T, as detailed in the ESI.† This material with a significant change in entropy holds the potential to serve as a promising candidate for energy storage.^46,47 For (TMPA)₂SnBr₆, the reversible phase transition temperatures are 374.7/361.3 K, and the value of ΔS is calculated to be 49.90 J mol⁻¹ K⁻¹ (Fig. S1, ESI†). Surprisingly, (TMPA)₂SnBr₆ exhibits a switchable thermochromic response, which may be attributed to the distortion of inorganic skeleton octahedra due to the Jahn–Teller effect (Fig. S2 and S3, ESI†).^26,48 Furthermore, thermogravimetric analysis (TGA) was conducted to demonstrate the thermal stability of both compounds. The TGA curves reveal that the decomposition temperatures for (TMPA)₂SnCl₆ and (TMPA)₂SnBr₆ are 516 K and 505 K, respectively (Fig. S4, ESI†), surpassing their phase transition temperatures. For convenience, the phases below and above the phase transition temperature in the heating run were labeled as the low-temperature phase (LTP) and high-temperature phase (HTP), respectively.

Fig. 1 (a) The synthesis of (TMPA)₂SnCl₆. (b) (TMPA)₂SnCl₆ crystal simulation diagram. (c) DSC curve of (TMPA)₂SnCl₆ in heating–cooling runs.

Structural and intermolecular interaction analyses

The mechanism of reversible phase transitions is revealed through variable-temperature single-crystal X-ray diffraction. At LTP, both (TMPA)₂SnCl₆ and (TMPA)₂SnBr₆ are crystallized in space group Pbca in orthorhombic point group mmm, showing a zero-dimensional structure, where the cations are in the cavities of the octahedron (Table S1 and Fig. S5, ESI†). As shown in Table S1 (ESI†), the cell parameters of the two compounds are similar, while (TMPA)₂SnBr₆ exhibits a larger cell volume than (TMPA)₂SnCl₆. The asymmetric units of (TMPA)₂SnX₆ (X = Cl, Br) comprise half of an inorganic octahedron [SnX₆] and an organic amine cation (Fig. S6, ESI†). For inorganic skeleton, it is composed of a well-organized arrangement of octahedrons, coordinated by one Sn ion and six Cl ions, in which the bond length of Sn–Cl is between 2.4200 and 2.4566 Å and the bond angle of Cl–Sn–Cl varies from 88.24 to 91.75° (Table S2, ESI†). At HTP, (TMPA)₂SnBr₆ belonged to the Pnma space group in the orthorhombic mmm point group, while the detailed structural information for (TMPA)₂SnCl₆ could not be obtained via single-crystal X-ray diffraction owing to poor crystal quality at high temperatures (Table S1, ESI†). Hence, variable-temperature powder X-ray diffraction was conducted to investigate the structural alterations of (TMPA)₂SnCl₆ (Fig. S7a, ESI†). As depicted in Fig. S7a (ESI†), it is evident that the symmetry of (TMPA)₂SnCl₆ gradually increases and that the diffraction peaks undergo significant changes as the temperature increases, providing evidence for a structural transition occurring in (TMPA)₂SnCl₆. Using Pawley refinement, it was subsequently determined that the space group of (TMPA)₂SnCl₆ at HTP is likely Pcmn, which is a non-standard space group derived from the Pnma space group (Fig. S7b and c, ESI†). Coincidentally, it is identical to that of (TMPA)₂SnBr₆ at HTP. In fact, it is reasonable that (TMPA)₂SnCl₆ and (TMPA)₂SnBr₆ are in the same space group after undergoing phase transition because of the similar stacking models of the two compounds at LTP.

Therefore, analysing the structural transition of (TMPA)₂SnBr₆ from low to high temperatures can provide insights into the corresponding transformation in (TMPA)₂SnCl₆. (TMPA)₂SnBr₆ displays two distinct types of hydrogen bonding, namely intramolecular hydrogen bonding within the amine itself and intermolecular hydrogen bonding between the amine and bromine atoms in the inorganic framework (Fig. S5 and S8, ESI†). For intramolecular hydrogen bonding, as shown in Fig. S8 (ESI†), the distance at HTP is 2.372 Å, which is larger than that at LTP (2.033 Å), indicating a reduction in the intramolecular hydrogen bonding force after the phase transition. Similarly, in the case of intermolecular hydrogen bonding, the N–H⋯Br hydrogen bonding distance ranges from 2.71 to 2.88 Å in the LTP and between 2.72 and 3.08 Å in the HTP, implying a significant decrease in intermolecular hydrogen bonding force during phase transition (Tables S5 and S6, ESI†). According to the single-crystal X-ray diffraction data, the cations undergo an order–disorder transformation, resulting in the weakening of the hydrogen bond strength, which in turn enhances the likelihood of atomic vibration at high temperatures (Fig. 2a–c and Fig. S1, ESI†).

Fig. 2 (a) Molecular structure of (TMPA)₂SnX₆ (X = Cl, Br) at 300 K. (b) Molecular structure of (TMPA)₂SnX₆ (X = Cl, Br) at 380 K. (c) Diagram of ordered and disordered about TMPA. (d) Packing structures of (TMPA)₂SnCl₆ at 300 K. (e) Packing structures of (TMPA)₂SnBr₆ at 300 K.

Notably, the phase transition temperature of (TMPA)₂SnCl₆ surpasses that of (TMPA)₂SnBr₆, which can be explained by the structural disparities between them. This phenomenon may be attributed to the steric effect within a unit cell. The cell volume of (TMPA)₂SnBr₆ is significantly larger than (TMPA)₂SnCl₆, indicating that the steric effect of (TMPA)₂SnCl₆ is stronger (Fig. 2d and e). For both intramolecular and intermolecular hydrogen bondings, the hydrogen bond distance of (TMPA)₂SnCl₆ is shorter than that of (TMPA)₂SnBr₆ (Fig. S5a and b, ESI†), meaning that organic cations establish a higher energy barrier in (TMPA)₂SnCl₆. Consequently, more energy is required for the steric effect, resulting in a higher phase transition temperature for (TMPA)₂SnCl₆. In addition, the void volume was 1924.6 and 2067 ų, and the volumes of the octahedral cavities for (TMPA)₂SnCl₆ and (TMPA)₂SnBr₆ were calculated to be 19.3 and 23.4 ų, respectively (Table S7, ESI†). The cationic amine in (TMPA)₂SnCl₆ exhibits a greater steric hindrance, which breaks through a higher energy barrier during phase transformation, thereby leading to a higher phase transition temperature for (TMPA)₂SnCl₆. Moreover, powder X-ray diffraction analysis was conducted on (TMPA)₂SnCl₆ and (TMPA)₂SnBr₆ (Fig. S9a and b, ESI†). The high agreement between the experimental values and simulation results provides credible evidence for the phase purity of the crystals. We performed powder PXRD measurement experiments on samples kept in an air atmosphere for 1 week, 2 weeks and 6 months. The positions of the diffraction peaks of the samples stored at different times remained consistent, indicating that the samples had good stability (Fig. S10, ESI†).

Photoluminescence properties

(TMPA)₂SnCl₆ exhibits photoluminescence (PL) and emits white light. As shown in Fig. 3a, both the powder and centimetre-level crystals of (TMPA)₂SnCl₆ display a white appearance when exposed to natural light. Remarkably, (TMPA)₂SnCl₆ exhibits uncommon white luminescence under commercial UV irradiation (365 nm), which is exceedingly rare among organic–inorganic hybrid phase transition materials. The luminescence properties of (TMPA)₂SnCl₆ were further investigated through UV absorption spectroscopy and solid-state PL emission spectroscopy analysis. The ultraviolet absorption spectrum exhibits a tiny absorption peak at approximately 330 nm, signifying that the excitation peak of (TMPA)₂SnCl₆ is close to this wavelength (Fig. 3b). Expectedly, the excitation wavelength of (TMPA)₂SnCl₆ was obtained at 337 nm, and the maximum emission peak of broadband light was observed at 527 nm with a large Stokes shift of 190 nm (Fig. S9, ESI†). Based on the average lifetime obtained from τ_ave = τ₁a₁ + τ₂a₂, the average decay lifetime (τ_ave) of the emitted light was calculated to be 6.382 μs for (TMPA)₂SnCl₆ (Fig. 3c). (TMPA)₂SnCl₆ (0.31, 0.35) is a high-quality pure white light material that closely matches the standard pure white light colour coordinates (0.33, 0.33), as determined by the International Committee on Electronic Analysis (CIE) (Fig. 3d). The broadband white light emission of tin bromide, characterized by a large Stokes shift (190 nm), may be attributed to self-trapped excitons (STEs), which are induced by structural distortions in the octahedral excited state of [SnCl₆]²⁻ (Fig. 3b and Fig. S11, ESI†).^49,50

Fig. 3 (a) Crystal and powder of (TMPA)₂SnCl₆ emit light at 365 nm. (b) Absorption and emission spectra of (TMPA)₂SnCl₆. (c) Luminous lifetime of (TMPA)₂SnCl₆. (d) CIE chromaticity coordinates of (TMPA)₂SnCl₆.

To further clarify the broadband emission mechanism, we measure temperature-dependent emission spectra of (TMPA)₂SnCl₆ at the excitation of 337 nm. It is obvious that the PL intensity of (TMPA)₂SnCl₆ slowly decreases as the temperature increases. An increase in test temperature results in an increase in the nonradiative transitions, thereby weakening the PL (Fig. 4a). The broad emission of the bulk crystal weakened and increased with increasing temperature, indicating that the white-light emission of (TMPA)₂SnCl₆ is attributed to STEs (Fig. 4a). Moreover, the quantum yield of (TMPA)₂SnCl₆ was measured to be 3.88%. (TMPA)₂SnCl₆ is warm white light with a correlated color temperature of (CCT) 5390 K and a high color render index (CRI) of up to 86.7. It is higher than the classical white photoluminescence source (CRI ≈ 65). The CRI of (TMPA)₂SnCl₆ is remarkably higher than that of most tin/lead materials and exhibits excellent color rendering capacity (Fig. 4c and Table S8, ESI†).^28,51–55 To show the practicality of the material, (TMPA)₂SnCl₆ is evenly coated with commercial LED light (365 nm), which emits bright white light (Fig. 4d). Thus, (TMPA)₂SnCl₆ possesses potential as a solid-state broadband white light emitter.

Fig. 4 (a) Temperature-dependent emission spectra of (TMPA)₂SnCl₆ at the excitation of 337 nm. (b) Diagram of PL mechanism in (TMPA)₂SnCl₆. (c) A color rendering index of (TMPA)₂SnCl₆ compared with tin/lead-based materials. (d) Photographs of LEDs (i) commercial UV–LED and (ii) commercial UV-LED lamps illuminate light (365 nm); the same UV-LED coated with (TMPA)₂SnCl₆ in the states of turned off (iii) and on (iv).

Dielectric analysis

The reversible phase transition induced by temperature often leads to significant dielectric anomalies. Temperature-dependent permittivity measurements were performed to further substantiate the reversible phase transitions of the two compounds and to investigate their dielectric characteristics. As illustrated in Fig. 4a, the dielectric response of (TMPA)₂SnCl₆ exhibits a pair of anomalous changes at approximately 390/330 K, with the dielectric constant transitioning from 6 to 10. For (TMPA)₂SnBr₆, the dielectric anomalies occurring near 370 K also demonstrate a phase transition (Fig. S12a, ESI†). Moreover, the dielectric constant exhibits a decreasing trend with increasing frequency, indicating that the dielectric constant of the two compounds is not only temperature sensitive but also frequency dependent (Fig. S12b, ESI†). A dielectric phenomenon refers to the polarization of a dielectric material under the influence of an applied voltage, which can be classified into four types: electronic displacement polarization, ionic displacement polarization, orientational polarization and space charge polarization. At higher frequencies, the electron polarization exhibits an immediate response to changes in the electric field, while the dipole orientation polarization does not exhibit instantaneous matching, thereby causing electric polarization dependence on frequency. A high dielectric constant state at a high temperature means “ON”, and a low dielectric constant state at a low temperature means “OFF” (Fig. 5c). When the temperature of the environment is higher than the phase transition temperature, the dielectric switch connects “ON”. The alarm is automatically turned off when the ambient temperature falls below the phase transition temperature (Fig. 5d). Moreover, the combination of the dielectric switching property and white light emission property of (TMPA)₂SnCl₆ renders it a promising candidate for employment as a luminescent sensor.

Fig. 5 (a) Dielectric constant of (TMPA)₂SnCl₆. (b) Temperature dependence of the real part (ε′) of (TMPA)₂SnCl₆ at different frequencies in the heating run. (c) Scheme of the dielectric switch with a bistable state. (d) Simulated application of the optical/thermal integrated multifunctional switch sensor.

Conclusions

In summary, we report the dual-channel electric/optical response with switchable dielectric biostability and broadband white light emission in an organic–inorganic hybrid crystal (TMPA)₂SnCl₆. Its analogue (TMPA)₂SnBr₆ displays a thermochromic phenomenon that is related to temperature-dependent [SnBr₆] octahedral distortion in combination with a dielectric switching response. Both of them experience cationic order–disorder type phase transitions at 391.3 K and 374.3 K, respectively. The higher phase transition temperature of the former is attributed to the steric effect, which constrains the dynamic motion of cations. Strikingly, (TMPA)₂SnCl₆ emits fascinating broadband white light with a long lifetime of 6.382 us and a high CRI of up to 86.7, which are higher than those of most organic–inorganic hybrid crystals. Our finding provides an attractive case to enrich the white light emitting material and would facilitate the development of a multi-channel electric/optical response toward more possibilities for new optoelectronics.

Author contributions

Q.-F. L., D.-W. F. and Z.-X. Z. conceived and designed the experiments. Q.-F. L. and P.-Z. H. prepared the samples and wrote the paper. H.-F. N. and Y. Z. performed the single crystal measurement and analysis. P.-Z. H., M. Z., Q.-H. Z, contributed to DSC, dielectric measurements. Z.-X. Z and C.-F. W. contributed to the analysis of photoluminescence. D.-W. F., Y. Z. and Z.-X. Z. supervised the project. All the authors discussed the results and commented on the manuscript.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by the Natural Science Foundation of Zhejiang Province (LZ20B010001) and the National Natural Science Foundation of China (grant 21991141).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental section, Fig. S1–S12, Tables S1–S8. CCDC 2174141, 2174142 and 2242964. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d3qm00800b

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