Modulating the birefringence of two-dimensional hybrid lead bromide perovskites using pyridine derivative cations†

Birefringent crystals for modulating the polarization of light are of technological importance in optical communications. Herein we provide two novel two-dimensional hybrid halide perovskites, [(H₂-dpys)(PbBr₄)] (1) (dpys = di(pyridin-4-yl)sulfane) and [(H-cmpy)₄(Pb₃Br₁₀)] (2) (cmpy = 4-chloro-3-methylpyridine), which can act as birefringent crystals. Remarkably, the crystal structures and the optoelectronic performance of the hybrid lead bromide perovskites are elaborately regulated by the organic cations of pyridine derivatives. Compound 2 constructed from the H-cmpy⁺ cations containing the single pyridyl moiety has a significantly enhanced birefringence (0.315@550 nm) compared to compound 1 (0.192@550 nm) with two pyridyl moieties of H₂-dpys²⁺ cations, and it is larger than those of all commercial birefringent crystals and most of the hybrid metal halide perovskites. The results of the theoretical calculations showed that the highly distorted PbBr₆ octahedra and the delocalized π-conjugation of H-cmpy⁺ cations synergistically contribute to the enhanced birefringence of 2. This work provides a useful strategy for modulating the crystal structure and optoelectronic performance of the hybrid lead halide perovskites.