Halogen bonds regulating structures and optical properties of hybrid iodobismuthate perovskites.

Successive structural transformations were observed in a methanolic solution containing 4-iodo-1-methylpyridin-1-ium iodide (IPyMe·I) and bismuth iodide (BiI). When kept in the solution, the amorphous solid (P_1) obtained immediately on mixing would transform to needle crystals (C_1) in hours, which would convert to prismatic crystals (C_2) in around 2 days. In the presence of hydroiodic acid, the hydrothermal reaction of IPyMe·I and BiI also gave rise to C_2, and crystals of C_2 in this solution would transform to a third crystalline product C_3 in 3 days. X-ray single crystal diffraction experiments show C_1 containing one-dimensional {BiI} chains, C_2 as a binuclear BiI structure, and C_3 consisting of a monomeric BiI unit, all with IPyMe as counter cations. Halogen bonds exist between IPyMe and the iodobismuthate, which may play key roles in the structural transformation. By introducing halogen bonding, the hybrids demonstrate excellent water-resistance. A thermal-induced reversible colour change from yellow to dark red occurred from 100 K to 450 K for all three hybrids, in which lattice expansion over the temperature range may be a reason for the thermochromism. The bandgaps derived from the UV-vis diffusion reflectance for the three complexes were 1.80 eV for C_1, 1.84 eV for C_2 and 2.00 eV for C_3. DTF computations followed by electron density topological analysis were applied to explain the structure-optical property relationship for complexes of diverse iodobismuthate types but the same counter cation. It was found that the nature of the Bi-I bonds rather than the dimensionality of the inorganic iodobismuthates is mainly responsible for the light absorption of the materials.