Exploration of nonlinear optical enhancement in acceptor–π–donor indacenodithiophene based derivatives via structural variations: a DFT approach

Abstract

Herein, a series of indacenodithiophene-based derivatives (TNPD1–TNPD6) were designed having D–π–A architecture via end capped acceptor modulation of a reference molecule (TNPR) to investigate nonlinear optical (NLO) behavior. Quantum chemical calculations were accomplished to examine electronic, structural and optical properties utilizing a density functional theory (DFT) approach at M06 functional with 6-311G(d,p) basis set. For this, natural bond orbitals (NBOs), density of states (DOS), frontier molecular orbitals (FMOs), transition density matrix (TDM) and non-linear optical (NLO) analyses were performed for TNPR and TNPD1–TNPD6. The structural modifications revealed a significant electronic contribution in tuning the HOMOs and LUMOs of the derivatives with lowered energy gaps and wider absorption spectra. FMOs findings revealed that compound TNPD5 was found with the lowest energy gap (1.692 eV) and with the highest softness (0.591 eV⁻¹) among the derivatives. Furthermore, a UV-Vis study also disclosed that maximum absorption (λ_max = 852.242 nm) was exhibited by TNPD5 in chloroform solvent. All the derivatives exhibited significant NLO results; in particular, TNPD5 showed the highest first hyper-polarizability (β_tot = 4.653 × 10⁻²⁷ esu) and second hyper-polarizability (γ_tot = 9.472 × 10⁻³² esu). These DFT findings revealed that the end-capped substituents play a key role in enhancing the NLO response of these push–pull chromophores and the studied derivatives can be utilized as efficient NLO materials.