Quantum chemical tailoring of intrinsic donor–acceptor configurations as efficient nonlinear optical materials

Abstract

Laser optics are playing a crucial role in modern hi-tech applications. Nonlinear optical (NLO) materials are key components to modulate laser optics. In the current study, unlike traditional donor–acceptor compounds, a series of D–π–A compounds (1–8) were designed to contain nitrogen and boron atoms as intrinsically electron donor and acceptor combinations, respectively. Among the systematic designing, two experimentally reported compounds were also fitted for comparisons within the series. Quantum chemical techniques were employed to investigate the optoelectronic and NLO properties of the designed compounds. In particular, M06/6-311G* functional was used to explore the NLO response properties like second hyperpolarizabilities  <γ> of the studied compounds. To investigate the optoelectronic response of the aforementioned compounds, several analysis including molecular electrostatic potentials, frontier molecular orbitals (FMOs), density of states and transition density matrix (TDM) were used. According to FMO analysis, compound 8 had the smallest energy gap (1.84 eV) and exhibited the most efficient transfer of charge from the donor to the acceptor. Additionally, the FMO results were validated by DOS pictographs and TDM maps, which corroborated the existence of charge separation states and effective charge transitions. The <γ> amplitudes of designed compounds 1–8 were found to be 40.07 × 10⁻³⁶, 68.33 × 10⁻³⁶, 559.0 × 10⁻³⁶, 369.4 × 10⁻³⁶, 377.3 × 10⁻³⁶, 433.6 × 10⁻³⁶, 398.0 × 10⁻³⁶, and 7161 × 10⁻³⁶ esu, respectively. Among all the compounds, compound 8 showed the largest <γ> amplitude of 7161 × 10⁻³⁶ esu by implementing the dual design with intrinsic (B/N atoms) and external donor–acceptor (p-methoxy and cyano) groups.