Influence of central heteroatom and donor or acceptor groups on the linear and nonlinear optical properties of epindolidione derivatives using DFT study

The core E, obtained on replacing one -NH- with -S- demonstrates the lowest energy gap among observed cores, with values of 3.26 eV in B3LYP and 5.63 eV in CAM-B3LYP. This small energy gap between the HOMO and LUMO facilitates enhanced charge transfer. Among cores A, B, and C, N(CH₃)₂-substituted thioepindolidiones (core B) shows the highest vertical excitation, while the N(CH₃)₂-substituted thiazaepindolidione (core E) ranks highest among the cores D, E and F. Core B exhibits the greatest linear polarizability (α), while core E has the highest first-order hyperpolarizability, making them the suitable candidates for nonlinear optical (NLO) applications. Cores A, B, and C, with their symmetric structures, have zero dipole moment, effectively cancelling out each other’s dipoles. Oxoepindolidione shows excellent stability, demonstrated by higher ionization potential (IP), hardness (η), hyperharness (Γ), and energy gap. NO₂ substitution generally provides the best performance across most density functional theory (DFT) descriptors.