Influence of Electron Donating and Electron Withdrawing Substituents on the Structural Electronic Spectroscopic and Nonlinear Optical Properties of Liquid Crystalline Molecules: A DFT and TD DFT Investigation Based on Published Evidence Through 2025

Abstract

Substituent engineering connects the electronic structure of an organic chromophore with the collective ordering required for liquid-crystal applications. Electron donors and acceptors redistribute charge, modify conjugation and change the response to electric and optical fields. Their effects nevertheless depend on position, molecular conformation, linkage chemistry and condensed-phase organization. This article presents an original critical synthesis of density functional theory and time-dependent density functional theory concepts alongside published evidence available before 2026. It considers aromatic mesogens containing imine, ester and related conjugated units, while identifying nonmesogenic donor–acceptor compounds used only as mechanistic comparisons. Published numerical findings are replotted in original graphs, and qualitative tables distinguish established tendencies from conditional interpretations. The analysis emphasizes the difference between orbital gaps and optical excitations, between molecular hyperpolarizability and bulk nonlinear susceptibility, and between calculated molecular stability and experimentally established mesophase stability. Stronger electronic polarization can improve an individual molecule’s optical response but can also increase absorption, aggregation or orientational cancellation. Useful liquid-crystalline optical materials therefore require coordinated control of electronic asymmetry, molecular shape, transparency and polar organization. No new molecular calculations or experimental measurements are represented as having been performed.