Exploration of promising key electronic and nonlinear optical properties of bifluorenylidene based chromophores: a TD-DFT/DFT approach

Herein, D-π-A configured bifluorenylidene-based chromophores (BFTMD1 to BFTMD8) were designed to study their nonlinear optical (NLO) properties. The molecular structures of the designed derivatives were optimized at M06/6-311G(d, p) functional. Comprehensive theoretical analysis such as frontier molecular orbitals (FMOs), density of states (DOS), transition density matrix (TDM), binding energy (E_b), UV-Visible (UV-Vis), natural bond orbitals (NBOs) and nonlinear optical (NLO) properties were carried out at same functional mentioned above. The FMO analysis revealed that all the derivatives exhibit lesser band gap values (1.898 to 2.464 eV) than the reference BFTMR, with BFTMD5 demonstrating the least band gap among all the derivatives at 1.898 eV manifesting highest reactivity and NLO properties. Furthermore, NBO calculations demonstrated strong stabilization and extensive hyperconjugation in the chromophores that contribute to their structural stability. UV-Vis analysis illustrated that all entitled compounds exhibited wider absorption spectra (647.606 to 667.515 nm) in solvent phase, contrary to BFTMR. The TDM and DOS calculations confirmed efficient charge transfer from HOMO to LUMO, showing high charge transmission. In the NLO analyses, all the designed derivatives demonstrated higher NLO performance, with first-order hyperpolarizability (β_total) and second-order hyperpolarizability (γ_total) values significantly surpassing those of the standard p-NA and FHD3. This research demonstrates the potential of chromophores derived from bifluorenylidene in enhancing structural stability and NLO efficiency.