Exploration of second and third order nonlinear optical properties for theoretical framework of organic D–π–D–π–A type compounds

Organic compounds expressed excellent nonlinear optical (NLO) performance which can effectively be utilized in numerous fields like optical fibers, optical communications, and optical modulation. In this study, eight new compounds abbreviated as HCPBD1–HCPBD8 were designed by structural alterations at donor and acceptor parts using HCPBR (hexyl-2-cyano-3-[4-(pyren-1-yl) benzen-1-yl]) as reference molecule for promising nonlinear optical responses. For this purpose, natural bonding orbital (NBO), absorption spectra, frontier molecular orbitals (FMOs) and nonlinear optical (NLO) computations of HCPBR and HCPBD1–HCPBD8 were employed via M06 level of theory using 6-31G(d,p) basis set in dichloromethane solvent. All the designed molecules (HCPBD1–HCPBD8) showed smaller HOMO–LUMO energy band gap in comparison to HCPBR. Furthermore, these derivatives expressed larger softness magnitudes than HCPBR which indicated derivatives were more polarizable than parent molecule. Additionally, HCPBD1–HCPBD8 displayed red shift than HCPBR, particularly, HCPBD8 exhibited highest λ_max as 1003.75 nm followed by low transition energy. Accompanying with, NBO computations revealed that prolonged hyper-conjugation and strong internal molecular interaction play key role in their stabilization as well as support to their NLO responses. Consequently, linear polarizability 〈α〉 and NLO responses such as first hyperpolarizability (β) and second-order hyperpolarizability 〈γ〉 values of HCPBD1–HCPBD8 were higher than HCPBR. Interestingly, HCPBD8 contained highest values 1576.05 (a.u.), 293,462.15 (a.u.) and 91.57 × 10⁶ (a.u.) of 〈α〉, β_total and 〈γ〉, respectively. This study showed that structural tailoring with various donor and acceptor units plays a crucial role to obtain alluring NLO material for optoelectronic applications.