Optimizing Core Modifications for High‐Performance D‐A‐D Molecular Systems: a Multi‐Faceted Study on NLO Properties, Solvent Effects, Charge Transfer, and Photovoltaic Efficiency

Abstract Developing materials with strong nonlinear optical (NLO) response is crucial for advancing optoelectronic technologies. This study investigates the electro‐optical behavior of thiadiazole‐based derivatives ( PAH‐P to PAH‐BTZ‐3 ), designed through central core modifications to enhance NLO performance. Quantum chemical calculations were conducted using density functional theory (DFT) with M06‐2X functional and 6‐311G** basis set. Among the studied compounds, PAH‐BTZ‐3 exhibited the highest third‐order NLO polarizability <γ> of 1294.7 × 10 −36 esu, which is ≈168 times greater than the benchmark molecule para ‐nitroaniline ( p ‐NA). Solvent effects demonstrated using the Polarizable Continuum Model (PCM) and Conductor‐like Screening Model (COSMO) showed a ≈2 times increase in <γ> values compared to the gas phase. Frequency‐dependent analysis revealed maximum γ(−ω;ω,0,0) and γ(−2ω;ω,ω,0) values of 3673.8 × 10 −36 esu and 7076 × 10 −36 esu, for PAH‐BTZ‐3 , highlighting resonance reabsorption. Frontier molecular orbital (FMO) analysis displayed a reduction in the crucial orbital energy gap from 5.80 eV to 3.74 eV. UV‐visible spectra confirmed strong intramolecular charge transfer, with a maximum red shift at 552 nm for PAH‐BTZ‐3 . Photovoltaic parameters, like open‐circuit voltage ranges from 1.57 eV to 2.89 eV, indicate the potential of these compounds in solar energy applications.