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 3 ). 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 3 also gave rise to C_2 , and crystals of C_2 in this solution would transform to a third crystalline product C_3 in ca. 3 days. X-ray single crystal diffraction experiments show C_1 containing one-dimensional {BiI 4 − } n chains, C_2 as a binuclear Bi 2 I 9 3− structure, and C_3 consisting of a monomeric BiI 6 3− 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. Halogen bonds (XB) induce successive amorphous to single-crystalline transformations in iodobismuthate perovskites, and improve the water-resistance. DFT computations show the Bi-I bond nature determines the semiconductivity of the materials.