Crystal structure and thermochromic behavior of the quasi-0D lead-free organic-inorganic hybrid compounds (C7H9NF)8M4I16 (M = Bi, Sb)

The quasi 0D lead-free thermochromic organic-inorganic hybrid single crystals (C7H9NF)8M4I16 (M = Bi, Sb) were synthesized by a conventional solvothermal reaction method. The crystal structures were analyzed using single crystal X-ray diffraction (SXRD). Interestingly, as the temperature increases, the (C7H9NF)8M4I16 (M = Bi, Sb) crystals exhibit a peculiar thermochromic phenomenon. In order to explore the phenomenon of thermochromism, temperature dependent ultraviolet−visible-near-infrared (UV–vis–NIR) absorption spectra were measured. The bandgaps of (C7H9NF)8Sb4I16 and (C7H9NF)8Bi4I16 at room temperature are 2.34 eV and 2.1 eV, respectively, and they gradually decrease as the temperature increases. Combined with ab initio molecular dynamics (AIMD), the reduction of the bandgap at higher temperature and the occurrence of thermochromism could be caused by the stronger electronic activity (thermal excitation). The thermochromic characteristics of these low-dimensional organic-inorganic hybrid metal halides will provide alternative materials for smart windows temperature sensors, visual thermometers, and daylight control glasses and beyond.