Alkyl Chain Alloying and the Optical Band Gap of the Layered Lead Halide Perovskites: The Role of Interlayer Chain–Chain Interactions

The two-dimensional layered lead halide perovskites that consist of lead halide layers separated by an organic moiety offer multiple pathways to modulate optical properties─substitution of the lead, the halide, or the organic moiety. Here, we have investigated the variation of the optical band gap with chain length of the linear alkyl chain lead bromide ((C_nH_2n+1NH₃)₂PbBr₄ ((C_n)₂PbBr₄), where n is the number of carbon atoms) perovskites and also by alkyl chain alloying forming the (C_nH_2n+1NH₃)(C_mH_2m+1NH₃)PbBr₄ ((C_n)(C_m)PbBr₄, n ≠ m) series of perovskites. The interlayer spacing of the alloyed and unalloyed perovskites showed a linear variation with alkyl chain lengths with similar slopes. The conformations of the alkyl chains, too, are similar, adopting a planar all-trans conformation; this was established from the presence of progression bands, arising from the coupling of all-trans methylene units, in the vibrational spectra of both the alloyed and unalloyed perovskites. The experimental optical band gaps of the (C_n)₂PbBr₄ and (C_n)(C_m)PbBr₄ perovskites exhibit a sharp jump when the total number of carbons increases from 20 to 24. Below and above this jump, there is no significant change in the band gap with chain length, indicating the absence of electronic coupling between the inorganic layers. Electronic density functional theory-based calculations, with the inclusion of spin–orbit coupling, can reproduce the experimental trend. The sharp jump in the optical band gap arises because the inorganic layer is distorted by a change in the Pb–Br–Pb octahedral tilt angle with increase in alkyl chain length. This distortion occurs to accommodate a change in the tilt angle of the all-trans alkyl chains above a critical chain length, which is required to maintain the interdigitation length and thus optimize favorable interactions between chains on opposing layers.