DFT study of the effect of substitution and of the alkyl chain length on optical properties and polarizabilities of CnF2(COX) (X = O, S, Se)

Abstract

Structures, optical properties and circular dichroism of chiral smectic liquid crystals, specifically denoted as C_nF₂(COO), C_nF₂(COS), and C_nF₂(CO Se) with n ranging from 7 to 12, were investigated using density functional theory (DFT). Based on the optimized molecular geometries of these compounds, we report the UV–Vis absorption spectra, optical properties, circular dichroism, and involving ππ* and nπ*electronic transitions. Our analysis delves into the influence of the alkyl chain length and substituents on optical properties, circular dichroism, as well as the highest occupied molecular orbital (HOMO), lowest unoccupied molecular orbital (LUMO), and the energy gap. Notably, the substituents lead to shifts in the UV–Vis absorption spectra. In addition, we explore the changes in polarizability associated with the alkyl chain length of these different substitutions. The interaction between an external electric field and liquid crystals is reconsidered, by studying its effect on polarizabilities and on the dipole moment. We note significant changes in the dipole moment and polarizability occurring at an electric field of about 4.5  V/nm for C₇F₂(COO), 5 V/nm for C₇F₂(COS) and 5.5 V/nm for C₇F₂(COSe).