α- to β-[C6H4(NH3)2]2Bi2I10 reversible solid-state transition, thermochromic and optical studies in the p-phenylenediamine-based iodobismuthate(III) material

α-[C6H4(NH3)2]2Bi2I10, which is a new material containing low-dimensional iodobismuthate anions, was synthesized and through its single crystal X-ray diffraction measurements, was proven to crystallize at room temperature in the centrosymmetric space group P21/c. It consists of a p-phenylenediammonium dication and a discrete (0-D) anion built up of edge-sharing bioctahedron. Due to the hydrogen bonds and the interatomic distances (Bi–I, I⋯I and π–π) changes, α-phase was transformed into the corresponding centrosymmetric β-phase, β-[C6H4(NH3)2]2Bi2I10, through a single-crystal to single-crystal transformation occurring upon cooling to −28/−26 °C. Below the transition temperature, β-[C6H4(NH3)2]2Bi2I10 crystallizes in the monoclinic system, centrosymmetric space group P21/n. Besides, the optical transmission measurements on α-[C6H4(NH3)2]2Bi2I10 thin films have revealed two absorption bands at 2.47 and 3.01 eV. Finally, two room temperature photoluminescence emissions attributed to excitons radiative recombinations confined within the bioctahedra Bi2I104−, were observed in the red spectral range at 1.9 and 2.05 eV energy.