Truss bridge-like anhydrous stacking in hybrid crystals triggers ultra-high stability and robust birefringence

Abstract

The pivotal light-modulation function of birefringent crystals has driven wide explorations in organic–inorganic hybrid crystals with significant optical anisotropy, but taking into account both thermal stability and birefringence in this type of material is a challenge. Herein, according to the planarization strategy of dicarboxylic molecules, the terephthalic acid (TPA) is listed as a feasible functional unit to construct birefringent crystals for the first time, and its three alkali metal salts, namely TPANa, TPAK and TPARb, have been successfully constructed. Notably, due to the stable truss bridge-like anhydrous framework constructed by strong ionic bond interactions, TPANa and TPAK exhibit extremely high thermal stability with decomposition temperatures exceeding 580 °C, one of the highest values among the reported organic–inorganic hybrid birefringent crystals. In addition, these materials present large birefringence ranging from 0.37 to 0.54 at 546 nm and a wide transparent range extending deep into the ultraviolet region. Theoretical calculations not only elucidate the synergistic regulatory mechanism through which the coplanarity, lattice plane orientation and density of π-conjugated TPA units govern the birefringence of this family of crystalline materials but also clarify the intrinsic factors underlying the modulation of their microstructural characteristics and optical properties by counter-cation size.