Design and Theoretical Exploration of Imidazo[2,1-b] [1,3,4]thiadiazole-Based Donor–π–Acceptor Chromophores for Improved Nonlinear Optical Properties

Abstract

The growing need for future photonic technologies in telecommunications, data processing, and biophotonics has accelerated interest in organic nonlinear optical (NLO) chromophores. Donor–π–Acceptor (D–π–A) molecules are very promising, but predictive design strategies have not yet been explored. Density Functional Theory (DFT) and Time-Dependent DFT (TD-DFT) calculations at the CAM-B3LYP/6-311+G(d,p) level were carried out in this research to probe a parent imidazo[2,1-b][1,3,4] thiadiazole-derived chromophore and six systematically varied derivatives. Electronic, geometric, and optical properties were evaluated to define definite correlations between substituent effects, HOMO–LUMO energy gaps, absorption maxima (λ_max), and first-order hyperpolarizabilities (β_tot).

The results show that electron-withdrawing groups significantly increase hyperpolarizability, with the –CF₃ derivative (A1D1) displaying β_tot of 315 × 10⁻³⁰ esu, almost double that of the reference. Even more remarkable, guanidine substitution in the donor site (A1D5) boosted β_tot to 1250 × 10⁻³⁰ esu—an order-of-magnitude enhancement. These results identify the imidazo[2,1-b] [1,3,4]thiadiazole scaffold as a superior π-bridge for NLO applications, offering rational design principles for maximizing D–π architectures in future experimental syntheses.