Proton sponge lead halides containing 1D polyoctahedral chains

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](-))(infinity) 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 degrees C, the iodine-rich (C14H19N2)PbI3 and (C14H19N2)PbI2Br species undergo an irreversible phase transition around 220 degrees C to a high-temperature phase, with a smaller molar volume and an apparent symmetry lowering (to orthorhombic Pca2(1)). 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.