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• Multi-stimuli responsive materials with a diversity coupling of multifarious properties have been used in electronic materials, biological fields widely [1]. These materials respond to various stimuli such as light, temperature, pH and pressure in intrinsic crystal structure and external physical properties. Such applications require rapid and reversible changes of physical properties to provide the means to store or sense as well as read information [2]. From a structural point of view, flexible materials can be transformed with temperature change and the implementation of mechanical strain, which is accompanied by changes in physical properties such as dielectric and pyroelectric [3–6]. Illumination as physical stimuli offers an ideal opportunity for remote control with high fatigue resistance [7–9]. A useful system possesses a modulation in one of these responses, while it is rare to identify a system with two or more useful responses being simultaneously modulated. Therefore, a material simultaneously controlled and coupled by two or more responses possessing different optical, stress and thermodynamic properties is of technological importance for non-volatile optical or electrical information storage [10–12]. A successful approach towards such materials is exploring and designing materials with multiple responses in a photo-responsive material. Photo-responsive molecules are characterized by reversible transformation between two isomers with different crystal structures induced by photoirradiation [13]. Photoisomerization can be observed in many typical organic compounds having photoisomerizable chromophores, such as spiropyran, azobenzene, furylfulgide, and salicylideneanilines [14–17]. Recently, Xiong et al. reported multiple single-component organic crystals with being simultaneously controlled by three physical channels by the modification of organic azobenzene [18]. However, the exploration of such kind of multi-stimuli responsive materials is limited only in the pure organic system. In this sense, to explore new multi-response material systems, especially the organic-inorganic hybrid ones, is of particular interest and importance for future multichannel data storage, optoelectronics application.

  As a well-known kind of photo-responsive coordination unit, the pentacyanidonitrosylmetallate ion, has been used to construct numerous inorganic photo-responsive materials. From a structural perspective, nitroprusside ion has a similar octahedral geometry of a [M(CN)₆]ⁿ⁻ unit, which can be assembled into multiple-response material with flexible cation. For instance, Ohkoshi et al. recently reported a photo-switchable inorganic crystal, Cs_1.1Fe_0.95[Mo(CN)₅(NO)]·4H₂O, exhibiting spontaneous electrical polarization, second-harmonic generation (SHG), and superionic conductivity [19]. They also prepared a photo-responsive non-linear optics (NLO) crystal using a pentacyanidonitrosylferrate-based cyanide-bridged bimetal assembly [20]. We have previously obtained two nitroprusside-based hybrid crystals, (Me₂NH₂)[NaFe(CN)₅(NO)] being the first photo-responsive crystalline ferroelectric material and (Me₂NH₂)[KFe(CN)₅(NO)] exhibiting a rare "negative-positive" switching of uniaxial thermal expansion. It is interesting to introduce thermal and mechanical responses into nitroprusside systems by means of molecular assembly [21,22]. We previously found two room-temperature polymorphs could be long-term stable yet easily interconvertible, at a pressure of 4.8 MPa by switching hydrogen bonds via collective reorientation of organic cations in some chiral molecular perovskites [23]. In this work, by assembling nitroprusside anion with chiral organic cation, R-3-hydroxypyrrolidinium (R3HP⁺), we successfully obtained a new chiral hybrid crystal, (R3HP)₂[Fe(CN)₅(NO)] (1), in which the thermal, mechanical and optical multi-stimuli responses can be coupled and controlled. We found that, 1 undergoes an irreversible phase transition with space-group change of C2 (phase Ⅰ) - P2₁22₁ (phase Ⅱ) at 393 K upon heating, while the structure can not return to the original phase Ⅰ upon cooling but transforms into P2₁2₁2₁ (phase Ⅲ). The rearrangement of nitroprusside anion as well as the switch of Fe–C–N···H–N and Fe–C–N···H–O hydrogen bonds (H-bonds) are synergistically involved in the irreversible transitions. It is interesting to note that phase Ⅲ can be converted to the phase Ⅰ after applying a pressure of 14 MPa. In addition, based on the photo-responsive nitroso group in nitroprusside ion, irradiation of the N–bound nitrosyl ligand (ground state) leads to two different configurations isonitrosyl O–bound (metastable state Ⅰ) and side-on nitrosyl conformation (metastable state Ⅱ). A system possesses reversible switches in thermal, mechanical and photology multi-stimuli responsive, suggesting their great potential in multichannel data storage, optoelectronics, and related applications compatible with all-organic electronics and human tissues.

  Red block-shaped crystal of 1 were obtained through a slow evaporation of a solution filtrated from a mixture of Ag₂[Fe(CN)₅(NO)] and (R3HP)Cl (Fig. S1 in Supporting information). The phase purity of 1 was verified by powder X-ray diffraction (PXRD) pattern (Fig. S2 in Supporting information). Thermogravimetric analysis showed that 1 could be thermally stable up to 453 K (Fig. S3 in Supporting information). To detect the possible phase transitions for 1, we carried out differential scanning calorimetry (DSC) measurement. As shown in Fig. 1, the DSC curve shows a pair of reversible heat anomalies at 389/377 K (T₁) in the first heating/cooling round. For convenience, the three solid phases of 1 are marked as the phase Ⅰ (T < T₁), Ⅱ (T > T₁), Ⅲ (T < T₁, after the first heating process). The total enthalpy change (ΔH) of the first round in the heating process is estimated to be 6.01 J/mol, while it is estimated to be 1.00 J/mol in the corresponding cooling process. In the second heating/cooling round, the original wide heat anomaly in the heating mode was divided into two sharp anomalies of 369 and 389 K. Meanwhile, the heat anomaly in the cooling mode also maintained the original state (Fig. S4 in Supporting information). Six heating and cooling rounds were further carried out, verifying the cyclic stability of phase transitions for 1 (Fig. S6 in Supporting information).

  Figure 1

  

  Figure 1.  DSC curves for 1 during the heating-cooling mode.

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  To gain a deeper understanding on the phase-transition mechanism, we measured the crystal structures of 1 at 293, 393, and 298 K (after the first heating process) using single-crystal X-ray diffraction for phases Ⅰ, Ⅱ, and Ⅲ, respectively (Fig. 2 and Table S1 in Supporting information). At phase Ⅰ (Fig. 2a), 1 belongs to the chiral monoclinic space group C2 (No. 5, the point group 2) with β angle of 90.143°. Oscilloscope traces of SHG signals for 1 and potassium dihydrogen phosphate (KDP) are shown in Fig. S7 (Supporting information), which verified that 1 is a noncentrosymmetric structure at 293 K. The asymmetric unit consists of four R3HP⁺ cations and two [Fe(CN)₅(NO)]²⁻ inorganic anions. Upon heating to 393 K (phase Ⅱ), 1 belongs to the orthorhombic space group P2₁22₁ (the nonstandard setting of P2₁2₁2; No. 18, the point group 222). As shown in Fig. 2b, R3HP⁺ cations being two-fold disordered. Two C–N bonds are also being two-fold disordered, as the ratios the N and O for the two atoms bonded by the double bond are 0.5:0.5 in inorganic part [Fe(CN)₅(NO)]²⁻. The asymmetric unit consists of one [Fe(CN)₅(NO)]²⁻ moiety and one R3HP⁺ cation. When cooling the sample to room temperature, the resulted phase Ⅲ belongs to space group P2₁2₁2₁ (No. 19, the point group 222), in which the disordered inorganic part [Fe(CN)₅(NO)]²⁻ transform into ordered, while R3HP⁺ cations can not be re-frozen to the original ordered phase (Fig. 2c). Cambridge Crystallographic Data Centre (CCDC) numbers 2311110, 2311111 and 2311112 contain the crystallographic data for 1. The two polymorphs, i.e., phase Ⅰ and Ⅲ, in room temperature differ in the disorder of the cation, which is similar to the previously observation in (R/S-hpip)[CdCl₃] [24].

  Figure 2

  

  Figure 2.  Thermo-induced irreversible structural phase transition in 1. (a) Crystal structure of 1 in the phase Ⅰ (293 K). (b) Crystal structure of 1 in the phase Ⅱ (393 K). (c) Crystal structure of 1 in the phase Ⅲ (cool down to 298 K). For clarity, H atoms are omitted. Symmetric codes: A) 1 + x, 3 + y, 1 + z.

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  Polycrystalline samples of 1 were studied by variable-temperature PXRD (Fig. 3a). Below T₁, there was a good match between the experimental patterns and the simulated pattern from the single-crystal structure at 293 K. With increasing temperature from 303 K to 403 K, the reflections at 14.24° disappeared and new reflections at 15.19° appeared. Meanwhile the appearance of new continuous reflections at 20.35° and 20.53° proved the existence of phase transition. With decreasing temperature from 403 K to 313 K, new reflections at 18.16° appeared, continuous reflections at 20.35° and 20.53° merged into one reflection, which is different from the original synthetic phase. The PXRD patterns at 298, 403 and 313 K evidenced the irreversible phase transition between phase Ⅰ and phase Ⅱ, which is clearly in agreement with DSC measurements.

  Figure 3

  

  Figure 3.  (a) The variable-temperature experimental and simulated PXRD patterns of 1. (b) The comparison of PXRD patterns between 1 at 298 K (phase Ⅰ) and 1 (phase Ⅲ) under static pressure of 14 MPa after 38 H at 298 K. (c) Schematic diagrams of the detailed transformation of the four phases. (d) An inter conversion of two room-temperature by switching inter-cationic H-bonds N–H···N and O–H···N.

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  It is worth emphasizing that phase Ⅲ could be stable for a long time at ambient temperature, which is different from the commonly-observed spontaneous transitions from the kinetically favoured disordered structures to a thermo-dynamically favoured ordered structure. In addition, the phase Ⅲ cannot return to the original synthetic phase Ⅰ by cooling before the low temperature of 233 K (Fig. S8 in Supporting information). Based on previous research [25,26], two room-temperature long-term stable crystalline polymorphs can be switched via the induction of pressure. Since the phase Ⅰ is denser thus thermodynamically stable below T_c, it is expected that the dynamic locking of the less dense and thermodynamically metastable phase Ⅲ could be overcome by pressing. A variable-temperature PXRD and pressure measurement was carried out to confirm the transition from phase Ⅲ to phase Ⅰ. As shown in Fig. 3b, it is distinct that phase Ⅲ can convert to phase Ⅰ under static pressure of 14 MPa for 38 h (Fig. 3c). The phenomenon that adscititious pressure as an external energy that causes a structural phase transition in crystals was observed in reported compounds (CH₃CH₂NH₃)[Cu(HCOO)₃] and (R-C₅H₁₂NO)[CdCl₃] [22,27].

  To understand the mechanism of irreversible phase transitions, we analyze the difference between phase Ⅰ and phase Ⅲ in detail. As shown in Fig. 3d, except the cyano group opposite the nitroso-group, all the other four cyano groups interact with the R3HP⁺ cations by N–H···N H-bonds in phase Ⅰ. However, these cyano groups in phase Ⅲ not only interact with R3HP⁺ by N–H···N H-bonds, but also with hydroxyl group by O–H···N H-bonds. In other words, the O–H···N H-bonds probably lock the metastable phase Ⅲ. Furthermore, the disordered R3HP⁺ cations with larger volumes prop up the inorganic skeleton. Hence, the distance between the iron atoms in the adjacent nitroprusside anion changes from 8.40 Å in phase Ⅰ to 8.62 Å in phase Ⅲ. The calculated density decreases from 1.444 g/cm³ in phase Ⅲ to 1.437 g/cm³ in phase Ⅰ. H-bonds in three phases are shown in Table S2 (Supporting information). In short, the structure analyses suggested that the rearrangement of nitroprusside anion as well as the switch of N–H···N and O–H···N H-bonds are synergistically involved in the irreversible transition.

  In addition to the thermal and mechanical responses, we expect the compound to have photoisomeristic properties based on nitroprusside anion photo-responsive groups. Schematic view of photo induced linkage isomers GS, SI in coordination unit [Fe(CN)₅(NO)]²⁻ is shown in Fig. 4a. The photo-induced linkage isomerism of phase Ⅰ and phase Ⅲ were investigated with variable-temperature IR spectra. As shown in Figs. 4b and c, new peaks appear at 1801 cm⁻¹ for phase Ⅰ and 1803 cm⁻¹ for phase Ⅲ after 532 nm photo irradiation (800-mW power Ti: sapphire laser) for 30 min at 78 K. The peak at 1947/1945 cm⁻¹ and the newly appeared peak at 1801/1803 cm⁻¹ were assigned as the dominant stretching vibration of NO bonds, which is confirmed by DFT calculations for phase Ⅰ with an ordered structure (Fig. S9 in Supporting information). These new absorption bands, which are close to the typical peak of 1832 cm⁻¹ in sodium nitroprusside, indicate that a linkage isomerism (SI) occurs as a consequence of metal-to-ligand charge transfer of 3d *(NO) after irradiation. It is worth noting that NO vibration from 1801 cm⁻¹ in as-grown phase divided into two groups at 1798 cm⁻¹ (shoulder), 1803 cm⁻¹ and 1805 cm⁻¹ (shoulder). In phase Ⅰ, the completely ordered structural incorporation of the [Fe(CN)₅(NO)]²⁻ anion and organic cations provide large enough free space for the NO-ligand rotation from Fe–NO to Fe–ON. While in phase Ⅲ, the NO in [Fe(CN)₅(NO)]²⁻ anion is distorted by the surrounding disordered organic cations near the NO ligand. A similar phenomenon of peak-splitting was observed in trans-[RuCl(pyridine)₄(NO)](PF₆)₂·0·5H₂O [28]. The above analysis proves that 1 has an optical response behavior similar to sodium nitroprusside.

  Figure 4

  

  Figure 4.  FTIR NO stretching bands. (a) Schematic view of light-induced linkage isomers GS, SI in [Fe(CN)₅(NO)]²⁻. FTIR of 1 in (b) phase Ⅰ and (c) phase Ⅲ. The FTIR before irradiation at 78 K (black), The FTIR after irradiation at 78 K (red) and at the indicated temperatures after irradiation.

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  In summary, we have presented a new hybrid salt, i.e., 1, which exhibits thermal, mechanical and optical multiple responses. In detail, 1 undergoes thermally irreversible phase transitions from phase Ⅰ (C2) to phase Ⅱ (P2₁22₁) upon heating to above 389 K and then phase Ⅲ upon cooling back to room temperature, while the reverse transition from phase Ⅲ to phase Ⅰ can only be induced by applying mechanical pressure. More importantly, 1 reveals different configurations caused by light in 532 nm irradiation arising from photo-induced structural transformation of the anionic framework. In short, the thermal motion characteristics of organic cations, the photo-responsive characteristics of anionic skeleton and the pressure characteristics benefiting from H-bonds are simultaneously integrated into 1. This coupling mechanism of multi-stimuli responsive materials was rarely observed in organic-inorganic hybrid crystals, which opens up a new way to achieve multichannel control in hybrid crystals with great application prospects.

  Declaration of competing interest

  The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

  Acknowledgments

  This work was supported by the National Natural Science Foundation of China (Nos. 22071273 and 21821003), Fundamental Research Funds for the Central Universities, Sun Yat-sen University (No. 23lgzy001). The National Supercomputing Center in Guangzhou (NSCC-GZ, Tianhe-2) is highly appreciated for providing computation sources.

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2024.109507.

  
  
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  Figure 1  DSC curves for 1 during the heating-cooling mode.

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  Figure 2  Thermo-induced irreversible structural phase transition in 1. (a) Crystal structure of 1 in the phase Ⅰ (293 K). (b) Crystal structure of 1 in the phase Ⅱ (393 K). (c) Crystal structure of 1 in the phase Ⅲ (cool down to 298 K). For clarity, H atoms are omitted. Symmetric codes: A) 1 + x, 3 + y, 1 + z.

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  Figure 3  (a) The variable-temperature experimental and simulated PXRD patterns of 1. (b) The comparison of PXRD patterns between 1 at 298 K (phase Ⅰ) and 1 (phase Ⅲ) under static pressure of 14 MPa after 38 H at 298 K. (c) Schematic diagrams of the detailed transformation of the four phases. (d) An inter conversion of two room-temperature by switching inter-cationic H-bonds N–H···N and O–H···N.

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  Figure 4  FTIR NO stretching bands. (a) Schematic view of light-induced linkage isomers GS, SI in [Fe(CN)₅(NO)]²⁻. FTIR of 1 in (b) phase Ⅰ and (c) phase Ⅲ. The FTIR before irradiation at 78 K (black), The FTIR after irradiation at 78 K (red) and at the indicated temperatures after irradiation.

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