Proton sponge lead halides containing 1D polyoctahedral chains

Abstract

Hybrid one-dimensional lead halides, containing the protonated 1,8-bis(dimethylamino)naphthalene moiety (C₁₄H₁₉N₂, monoprotonated “proton sponge”), were prepared by simple one-pot methods and investigated in terms of crystal structure, morphology, thermal stability and electronic properties. The as-precipitated (C₁₄H₁₉N₂)PbBr₃ and (C₁₄H₁₉N₂)PbI₃ species are isostructural and crystallize in the orthorhombic Pbca space group, resulting in 1D crystal phases with ([PbX₃]⁻)_∞ chains (built by face-sharing [PbX₆] octahedra; X = Br, I), among which the (C₁₄H₁₉N₂)⁺ cations are inserted. The two compounds display complete miscibility in the solid state: both (C₁₄H₁₉N₂)PbI₂Br and (C₁₄H₁₉N₂)PbIBr₂ crystal phases were successfully synthetized and do not show segregation effects. TGA and DSC measurements showed that, while all these materials are stable up to 250 °C, the iodine-rich (C₁₄H₁₉N₂)PbI₃ and (C₁₄H₁₉N₂)PbI₂Br species undergo an irreversible phase transition around 220 °C to a high-temperature phase, with a smaller molar volume and an apparent symmetry lowering (to orthorhombic Pca2₁). In these two cases, variable-temperature X-ray diffraction measurements showed a substantial crystal-to-crystal transformation, without intermediate melting or amorphization. The thermal expansion coefficients of (C₁₄H₁₉N₂)PbI₃ and (C₁₄H₁₉N₂)PbBr₃ are highly anisotropic and on the order of 10⁻⁴ K⁻¹ along the b crystallographic axis. Microscopic and spectroscopic measurements were performed to complement the structural analysis of the samples: band gaps in the near UV range (3.3 and 3.8 eV for (C₁₄H₁₉N₂)PbI₃ and (C₁₄H₁₉N₂)PbBr₃, respectively) were obtained.