Conformation and orientation of alkyl chains in the layered organic-inorganic hybrids: (C_nH_2n+1NH₃)₂PbI₄(n=12,16,1 8)

Abstract

Layered organic-inorganic hybrids based on perovskite-derived alkylammonium lead halides have been demonstrated as important new materials in the construction of molecular electronic devices. Typical of this class of materials are the single-perovskite slab lead iodides of the general formula (C_nH_2n+1NH₃)₂PbI₄. While for small n, these compounds are amenable to single-crystal structure determination, the increasing degree of disorder in the long chain (n=12,14... ) compounds makes such an analysis difficult. In this study, we use powder X-ray diffraction, and vibrational and ¹³C NMR spectroscopies to establish the conformation, orientation and organization of hydrocarbon chains in the series of layered alkylammonium lead iodides (C_nH_2n+1NH₃)₂PbI₄ (n=12,16,18). We find that the alkyl chains adopt a tilted bilayer arrangement, while the structure of the inorganic layer remains invariant with respect to the value of n. Conformation-sensitive methylene stretching modes in the infrared and Raman spectra, as well as the ¹³C NMR spectra indicate that bonds in the methylene chain are in trans configuration. The skeletal modes of the alkyl chain in the Raman spectra establish that there is a high degree of all-trans conformational registry for the values of n studied here. From the orientation dependence of the infrared spectra of crystals of (C_nH_2n+1NH₃)₂PbI₄ (n=12,16), we find that the molecular axis of the all-trans alkyl chains are tilted away from the interlayer normal by an angle of 55°. This value of this tilt angle is consistent with the dependence of the c lattice expansion as a function of n, as determined from powder X-ray diffraction.