Unveiling electronic and remarkable non-linear optical properties of boron–nitrogen carbazole-based compounds <i>via</i> modification of π-linker and donor units: a DFT study

Boron-nitrogen carbazole (BNCz) based aromatic chromophores have been considered as promising materials in non-linear optical domains due to their distinctive electronic and charge-transfer capabilities. This study presents the electronic and non-linear optical properties of BNCz-based compounds (BTNC-BNPZ). Additionally, a new series of four BNCz-based compounds (BNTP-BNTO) with D-π-A framework was designed by modifying the donors and π-spacer. Structural optimization and optoelectronic properties of BNCz based compounds were determined using density functional theory/time-dependent density functional theory (DFT/TD-DFT) calculations at M06/6-311G(d,p) level. The optimized structures were used to perform frontier molecular orbitals (FMOs), density of states (DOS), transition density matrix (TDM), UV-Visible and non-linear optical (NLO) analyses of examined compounds. The red-shifted absorption spectrum (412.854-566.138 nm) combined with a suitable energy gap (2.784-3.774 eV) facilitates significant charge migration from HOMO to LUMO. The global reactivity descriptors revealed notable softness and significant chemical reactivity in all above-mentioned chromophores. Among all the studied compounds, BNPZ displayed the narrowest band gap (2.784 eV), the highest absorption peak (566.138 nm), and lowest excitation energy (2.190 eV), highlighting its remarkable electronic characteristics. Furthermore, DOS visualizations and TDM heat maps support the FMO findings, confirming the presence of charge densities in a chromophore. All the compounds showed an increased exciton dissociation rate due to their lower exciton binding energy values (<i>E</i> _b = 0.771-0.480 eV). Moreover, NBO analysis revealed that enhanced hyperconjugation and strong intramolecular interactions played a crucial role in stabilizing the studied compounds. Among all the derivatives, BNTP exhibited the highest <i>β</i> _tot (74.0 × 10^-30 esu) and <i>γ</i> _tot (81.1 × 10^-35 esu) values, suggesting its promising potential as an NLO material.