Molecular modeling of indole-based materials with efficient electron withdrawing moieties to enhance optical nonlinearity: Quantum chemical investigation

Herein, a series of seven indole-based donor-π-acceptor (D-π-A) chromophores ( 3aiR and 3ai2 - 3ai8 ) was designed via strategic modification of the terminal acceptor moieties. Density functional theory (DFT) and time-dependent DFT (TD/DFT) calculations were executed at M06/6-311G(d,p) level. To investigate the optoelectronic properties of 3aiR and 3ai2 - 3ai8, various analyses were conducted, including UV-Visible, frontier molecular orbitals (FMO), density of states (DOS), transition density matrix (TDM), natural bond orbitals (NBOs), and nonlinear optical (NLO) properties. Among all the designed derivatives, 3ai6 exhibited the narrowest band gap (2.497 eV), the most significant redshift (478.907 nm), and the highest chemical reactivity, as indicated by its maximum softness value of 0.401 eV⁻ 1 . Moreover, DOS and TDM analyses further supported the charge delocalization observed in FMO analysis. The NBO analysis revealed significant hyperconjugative interactions and enhanced intramolecular charge transfer (ICT), contributing to the stability of the designed compounds. While all derivatives demonstrated promising NLO properties, 3ai6 exhibited the highest static first-order ( β total = 2.89×10 -28 cm⁴ statvolt⁻ 1 ) and second-order ( γ total = 2.32×10 -33 cm 5 statvolt⁻ 1 ) hyperpolarizability values. The dynamic first-order hyperpolarizability β(−ω;ω,0) EOPE, shows enhanced values at 532 nm , with 3ai5 exhibiting the maximum β total of 599 × 10⁻ 2 ⁸ statvolt⁻ 1 cm⁴. Similarly, the second harmonic generation (SHG) (−2ω;ω,ω) is more pronounced at this shorter wavelength. In contrast, the second-order hyperpolarizability γ(−ω;ω,0,0) shows slightly higher values at 532 nm , peaking at -421000 × 10⁻ 33 statvolt⁻ 1 cm 5 for 3ai8 , indicating a frequency-dependent NLO response. These findings provide crucial insights into the structure-property relationships of indole-based materials, facilitating further advancements in the NLO domain.