Noncovalent chalcogen bonds and disulfide conformational change in the cystamine-based hybrid perovskite [H₃N(CH₂) ₂SS(CH₂)₂NH₃]Pb^III ₄

The cystamine-based hybrid perovskite, α-[NH₃(CH ₂)₂S-S(CH₂)₂NH₃]PbI ₄ (1a), can be transformed into its polymorph, β-[NH ₃(CH₂)₂S-S(CH₂)₂NH ₃]PbI₄ (1b), by heat activation (T = 150 °C). The crystal structures have been characterised by single-crystal X-ray diffraction, whereas the phase transition was followed by both solid-state ¹H,¹³C cross-polarisation magic-angle spinning (CPMAS) NMR spectroscopy and thermodiffractometry techniques. At 150 °C, compound 1a is transformed into 1b, and, remarkably, the β phase (1b) can be nearly retained down to room temperature, which means that both polymorphs 1a and 1b can coexist over a large temperature range. The structure of 1b has been solved, and it was found that cystamine molecules are disordered over two positions: the two related components with opposite helical conformations. Solid-state ¹H,¹³C CPMAS NMR spectroscopic measurements show a significant broadening of the NMR spectroscopic line associated with two disordered carbon atoms when cooling 1b from 160 to 50 °C, thereby revealing the presence of exchange between these related atoms, and this favours a molecular dynamical disorder. Disulfide bridges of cystamine molecules are engaged in weak interactions with neighbours, either another cystamine molecule in 1a (SS···SS interactions), or iodine atoms in 1b (SS···I interactions). To evaluate the donating and accepting abilities of the disulfide bridge, and their impact on such weak interactions, a detailed partition of the interaction energy of ten dimer models has been calculated and revealed that the main contribution to the intermolecular bonding comes from the dispersion forces. The conformational change of the cystamine (H₂cys) dication and its influence on the solid-state transition from α- to β-(H₂cys)PbI₄ were investigated by means of variable-temperature X-ray powder diffraction and NMR spectroscopy, coupled with quantum chemistry. Noncovalent interactions that involve the sulfur atoms lead to a large hysteresis of more than 100 °C.