An organic–inorganic hybrid photoluminescent ferroelastic with high phase transition temperature

Abstract

Organic–inorganic hybrid (OIH) multifunctional materials have been widely studied in recent years due to their applications in information processing, optoelectronic devices, etc. However, it is still challenging to successfully trigger both ferroelasticity and luminescence properties in OIH materials. Herein, we report an OIH luminescent compound [TMIm][MnCl₄] (TMIm = 1,1,3,3-tetramethylimidazolidinium), which experiences a high-temperature ferroelastic phase transition from the mmm point group to the 2/m point group at 443/429 K. The evolution of ferroelastic domains can be clearly observed during the heating and cooling processes. Moreover, [TMIm][MnCl₄] exhibits strong green emission near 520 nm with a photoluminescence quantum yield (PLQY) of 27% and a lifetime of 1.21 ms at room temperature. Distinct luminescence responses in ferroelastic and paraelastic phases are also found through variable temperature fluorescence spectroscopy. This work provides a beneficial supplement for the discovery of OIH multifunctional materials.

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Introduction

Ferroic materials including ferroelectrics, ferroelastics, and ferromagnetics have important and extensive research value.^1–6 As a vital part of ferroic materials, ferroelastics have two or more different orientation states of spontaneous strain, which can be reversed by external mechanical stimuli.^7,8 This characteristic of ferroelastics is called ferroelasticity. Besides, ferroelastics can also exhibit shape memory effects^9,10 and superelasticity,¹¹ and some of the ferroelastic domain walls have superconductivity,¹² making them ideal candidate materials for energy conversion devices, information processors, mechanical switches, actuators, and other electronic devices.^13–17

Materials with multiple functionalities will lead to new physical phenomena and applications. Integrating ferroelasticity and photoluminescence (PL) plays an important role in new optoelectronic applications, information encryption, and multifunctional information reading.^18–23 In recent years, a few ferroelastics have been revealed to present fascinating luminescence properties. For instance, after Sb³⁺ doping, (TAMAC)₂In_1−xCl₅:xSb³⁺ [TAMAC = (CH₃)₃NCH₂CHCH₂] exhibits thermally reversible fluorescence with information encryption.²⁴ {Mn[(i-Pr)₃PO](dca)₂} [dca = N(CN)₂⁻, (i-Pr₃PO) = triisopropylphosphine oxide] is not only a ferroelastic complex, but also a ductile material with blue PL.²⁵ However, the phase transition temperatures of discovered PL ferroelastics are generally not very high.

Organic–inorganic hybrid (OIH) materials have broad application prospects in various fields due to their superior structural tunability, low acoustic impedance, and good biocompatibility.^26–29 OIH materials integrate the advantageous properties of inorganic crystals with those of organic molecules. The inorganic component correlates with high carrier mobilities,³⁰ semiconductivity,^31–33 and PL.^34–38 The organic component enables hybrids to undergo an order–disorder type phase transition, which may generate ferroelasticity.^39–44 Therefore, OIH materials have great advantages in constructing new and excellent molecular luminescent ferroelastics. However, the lack of viable strategies to induce ferroelastic phase transitions in OIH materials has resulted in a limited number of luminescent ferroelastics within this class, particularly those exhibiting high transition temperatures.

Here, by choosing manganese chloride to construct inorganic frameworks and a readily rotatable cyclic amine [TMIm] (TMIm = 1,1,3,3-tetramethylimidazolidinium) as the organic constituent, we successfully synthesized an OIH photoluminescent ferroelastic, [TMIm][MnCl₄], which undergoes an mmmF2/m-type ferroelastic phase transition at 443 K. Its phase transition temperature exceeds that of a large number of ferroelastics, including ferrocenium tetrachloroferrate (407.7 K),⁴⁵ (R-CTA)₂CuCl₄ (417 K), (S-CTA)₂CuCl₄ (420 K),¹⁵etc. Moreover, the obvious ferroelastic domain evolution of [TMIm][MnCl₄] can be observed under orthogonally polarized light during continuous heating and cooling, demonstrating robust temperature-controlled ferroelastic domain switching. Noteworthily, [TMIm][MnCl₄] emits strong green luminescence with a photoluminescence quantum yield (PLQY) of 27% and a lifetime of 1.21 ms under UV light at room temperature. The PL intensity of [TMIm][MnCl₄] decreases significantly when the temperature rises above T_c, showing good responses to the ferroelastic phase transition. This work is a supplement to the family of OIH photoluminescent ferroelastics and hopes to stimulate further studies on functional coupling and integration of the chemically and structurally diverse family.

Experimental section

Sample preparation

The reagents used in this experiment are all analytically pure and do not require further purification. [TMIm][2Cl] was synthesized by the reaction of tetramethylethylenediamine with dichloromethane. Then, [TMIm][MnCl₄] was prepared by slowly evaporating a mixed solution of [TMIm][2Cl] (0.402 g, 2.0 mmol), MnCl₂·4H₂O (0.398 g, 2.0 mmol) and methanol (60 mL) at 313 K. The powder X-ray diffraction (PXRD) patterns of powder and thin film samples prove that [TMIm][MnCl₄] has good phase purity and crystallinity (Fig. S1 and S2, ESI†).

Measurement methods

Detailed measurement methods are presented in the ESI,† including PXRD, variable-temperature single-crystal diffraction, thermogravimetry analysis (TGA), differential scanning calorimetry (DSC), dielectric measurements, ferroelastic measurements, and variable temperature photoluminescence spectroscopy (VT-PL).

Results and discussion

Variable-temperature crystal structure analyses

Variable-temperature single-crystal X-ray diffraction technology is commonly used to determine the crystal structure and to further explore the mechanism of phase transitions. We collected the crystal data at 293 K (room-temperature phase, RTP) and 453 K (high-temperature phase, HTP). In the RTP, [TMIm][MnCl₄] crystallizes in the monoclinic space group P2₁/c belonging to the centrosymmetric point group 2/m. The unit cell parameters are a = 11.1678(6) Å, b = 9.3158(5) Å, c = 13.5838(6) Å, Z = 4, β = 91.710(4)° and V = 1412.59(12) ų (Table S1, ESI†). The asymmetric unit cell is a constituent of one independent [TMIm] cation and one ordered [MnCl₄] anion (Fig. 1a). The Mn1 atom coordinates with four Cl atoms to form a tetrahedral [MnCl₄] framework. The lengths of the four Mn1–Cl bonds are not equal and vary from 2.3578(10) Å to 2.3642(10) Å, showing that the [MnCl₄] anion is a slightly distorted tetrahedron in the RTP. The corresponding six nonequivalent Cl–Mn1–Cl bond angles range from 105.6(4) Å to 111.38(4) Å (Table S2, ESI†). Organic cations are located in the voids around the [MnCl₄] anion framework. Anions and cations arrange alternatively, forming a zero-dimensional packing structure (Fig. 1c).

Fig. 1 Crystal structure of [TMIm][MnCl₄] at (a) 293 K and (b) 453 K; (c) perspective views of the structure along the c-axis of [TMIm][MnCl₄] at (c) 293 K and (d) 453 K. Purple lines indicate mirror planes perpendicular to the a-axis and b-axis. The hydrogen atoms of the [TMIm] cations have been omitted for clarity.

In the HTP, [TMIm][MnCl₄] crystallizes in the orthorhombic space group Pmmn (point group mmm) with unit cell parameters of a = 9.4512(9) Å, b = 11.3124(12) Å, c = 6.8298(7) Å, Z = 2, α = β = γ = 90° and V = 730.21(13) ų. The correlation of unit cell parameters between the RTP and HTP is a^RTP ≈ b^HTP, b^RTP ≈ a^HTP, and c^RTP ≈ 2c^HTP, and the unit cell volume in the HTP is reduced by about half compared with that in the RTP. The asymmetric unit cell in the RTP also changes to contain half of a [TMIm] cation and half of a [MnCl₄] anion (Fig. 1b). Interestingly, [TMIm] cations and [MnCl₄] anions maintain the same arrangement as in the RTP, but all of them lie at special symmetrical positions of the mirror planes perpendicular to the a-axis and b-axis with Mn atoms located on both of the mirror planes (Fig. 1d). Besides, the anions and cations are all reoriented and present a highly disordered state. Consequently, only four different Mn1–Cl bonds are formed, with distances ranging from 2.271(18) Å to 2.405(8) Å (Table S3, ESI†). Through variable temperature structure analysis, it is revealed that [TMIm][MnCl₄] experiences an order–disorder phase transition from the HTP to the RTP. The order–disorder transition of [TMIm] anions and [MnCl₄] cations is the origin of the phase transition. According to Aizu rotation, this phase transition between the point group 2/m and the point group mmm belongs to mmmF2/m among the 94 species of paraelastic–ferroelastic phase transition.⁷

Thermal properties

TGA of [TMIm][MnCl₄] was first carried out in the temperature range of 300 K to 900 K. Fig. S3 ESI† shows that it has good thermal stability below 527 K. The structural phase transition of crystals is generally accompanied by heat absorption and release. Therefore, differential scanning calorimetry (DSC) measurements were utilized to examine the phase transition of [TMIm][MnCl₄]. As shown in Fig. 2a, DSC curves present a pair of obvious endothermic and exothermic peaks at 443 K and 429 K, far below its thermal decomposition temperature (527 K), indicating the existence of a reversible phase transition. The phase transition temperature (T_c) of [TMIm][MnCl₄] has reached 443 K, exceeding those of many OIH ferroelastics, such as (TAMAC)₂InCl₅ (333 K),²⁴ ferrocenium tetrachloroferrate (407.7 K),⁴⁵ (R-CTA)₂CuCl₄ (417 K) and (S-CTA)₂CuCl₄ (420 K).¹⁵

Fig. 2 (a) DSC curves and (b) the variation of ε′ at various frequencies during the heating process.

Dielectric properties

We further verified the phase transition of [TMIm][MnCl₄] by measuring the temperature-dependent real part (ε′) of complex dielectric permittivity at various frequencies. Fig. 2b shows the variation of the ε′ value during the heating process at several frequencies. With the temperature increasing, the ε′ value gradually increases and an abnormal peak appears near T_c at several frequencies. In addition, the variation of ε′ at 100 kHz during the heating and cooling processes is demonstrated in Fig. S4, ESI.† The obvious dielectric anomalies during the heating and cooling processes once again confirmed the existence of a phase transition, which is also consistent with DSC results.

Ferroelastic domains

From the above analysis results, it can be seen that [TMIm][MnCl₄] undergoes an mmmF2/m type paraelastic–ferroelastic phase transition. Polarized light microscopy can help us explore the microscopic domain structure of ferroelastics. As shown in Fig. 3a, a thin film of [TMIm][MnCl₄] was observed in situ under orthogonally polarized light and many bright and dark alternating stripe domains can be clearly observed at 298 K. At the same time, no such stripe pattern was observed in its morphology (Fig. 3d). Then, upon heating this film to 453 K (above T_c), all the stripe domains disappeared rapidly (Fig. 3b). When the film was cooled to below 430 K, the stripe domains appeared again (Fig. 3c). The disappearance and reappearance of the stripe domains during the heating and cooling cycles are in accordance with the paraelastic–ferroelastic phase transition, thereby indicating good reversibility in the ferroelastic phase transition.

Fig. 3 Ferroelastic domain evolution during the heating and cooling processes (a–c) and its corresponding morphology images (d–f).

Optical properties

It is worth noting that [TMIm][MnCl₄] emits strong green luminescence under 365 nm light in the RTP. We first recorded the excitation and emission spectra of [TMIm][MnCl₄] at 298 K. As depicted in Fig. 4a, [TMIm][MnCl₄] has two strong excitation peaks at 360 nm and 450 nm and a strong emission peak at around 520 nm, which is attributed to the ⁴T₁(G) → ⁶A₁(S) transition of the Mn²⁺ ion in the four-coordinated environment and corresponds to its green emission. The PL emission is unshifted at various excitation wavelengths from 360 nm to 465 nm, indicating its intrinsic nature. It shows stronger emission intensity under 360 nm and 450 nm excitation than other excitation sources, consistent with its excitation spectra. The PLQY of [TMIm][MnCl₄] is found to be 27% with a lifetime of 1.21 ms at room temperature (Fig. S5, ESI† and Fig. 4b), comparable to other luminescent manganese halides.^18,46,47 To explore the potential influence of the ferroelastic phase transition on its optical properties, VT-PL measurements are conducted. It was found that the PL intensity of [TMIm][MnCl₄] gradually decreased with increasing temperature, accompanied by a negligible shift of the peak center and a quenching speed of about 3.0 × 10⁵ counts per kelvin (Fig. 4c and d). Notably, the PL intensity showed a faster quenching with a speed of 4.3 × 10⁶ counts per kelvin when the temperature exceeded T_c, which is more than one magnitude larger than that of the ferroelastic phase. The temperature dependence of PL intensities illustrated a clear turning point of the quenching speed near T_c. Moreover, the variation of the peak center and the full width at half maximum with temperature demonstrates an obvious turning point of the rate of increase near T_c (Fig. S6 and S7, ESI†). The distinct PL responses in ferroelastic and paraelastic phases imply the coupling of ferroelasticity and PL, which may provide new functionality in luminescent ferroelastics.

Fig. 4 (a) Excitation and emission spectra of [TMIm][MnCl₄] at 298 K. (b) The decay curve of [TMIm][MnCl₄]. (c) VT-PL spectra of [TMIm][MnCl₄]. (d) The integrated PL intensity as a function of temperature of [TMIm][MnCl₄] during the heating process. “FEP” and “PEP” refer to the ferroelastic phase and the paraelastic phase, respectively.

Conclusions

In summary, an OIH luminescent ferroelastic [TMIm][MnCl₄] with a high phase transition temperature has been designed and synthesized by a simple evaporation solvent method. This material undergoes a reversible order–disorder ferroelastic phase transition at 443 K/429 K, accompanied by the evolution of ferroelastic domains during the heating and cooling processes under a polarized light microscope. In addition, the PL emission of [TMIm][MnCl₄] has a significant response to the ferroelastic phase transition, with distinct PL quenching behaviours in the ferroelastic phase and paraelastic phase. This work demonstrates the coexistence and potential coupling of ferroelasticity and PL properties in an OIH, which would stimulate further studies on functional coupling and integration of the chemically and structurally diverse family.

Author contributions

W.-L. Yang conceived the study and wrote the manuscript. W.-L. Yang, X. Yan, M. Wang, and H. Yuan performed the general characterization studies, and X.-J. Song guided this work. Y. Qin carried out the characterization of optical properties. X.-J. Song, Y. Qin, and Y.-Y. Tang revised the manuscript.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (22203040 and 22375082).

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Footnote

† Electronic supplementary information (ESI) available: Fig. S1–S7 and Tables S1–S3. CCDC 2370180 and 2370182. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4qi01780c

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