Noncovalent Chalcogen Bonds and Disulfide Conformational Change in the Cystamine-Based Hybrid Perovskite [H3N(CH2)2SS(CH2)2NH3]PbIII4
Résumé
The cystamine-based hybrid perovskite, alpha-(NH3(CH2)2SS(CH2)2NH3)PbI4 (1a), can be transformed into its polymorph, beta-(NH3(CH2)2S-S(CH2)2NH3)PbI4 (1b), by heat activation (T = 150°C). The crystal structures are characterized by single crystal X-Ray diffraction while the phase transition has been followed by both solid state 1H-13C CPMAS NMR and thermodiffractometry techniques. At 150°C, 1a is transformed into 1b, and, remarkably, the beta-phase (1b) can be nearly retained down to room temperature, which means that both polymorphs 1a and 1b can co-exist in a large temperature range. The structure of 1b has been solved, and it is found that cystamine molecules are disordered over two positions, the two related components having opposite helical conformations. Solid state 1H-13C CPMAS NMR measurements show a significant broadening of the NMR line associated to two disordered carbons when cooling 1b from 160°C to 50°C, revealing the presence of exchange between these related atoms, and this is in favour of a molecular dynamical disorder. Disulfide bridges of cystamine molecules are engaged in weak interactions with neighbours, either another cystamine molecules in 1a (SS***SS interactions), or iodine atoms in 1b (SS***I interactions). In order to evaluate the donating and accepting abilities of the disulfide bridge, and their impact on such weak interactions, detailed partition of the interaction energy of 10 dimer models has been calculated and revealed that the main contribution to the intermolecular bonding comes from the dispersion forces.