Computational design of halogen-substituted cinnoline benzimidazole derivatives as high-performance nonlinear optical materials: A DFT study on structure-property relationships

The development of high-performance nonlinear optical (NLO) materials is critical for advancing photonic and optoelectronic technologies. This study presents a systematic theoretical investigation of a novel series of halogen-substituted cinnoline benzimidazole derivatives ( III-H , III-o-Cl , III-m-Cl , III-p-Cl , III-o-Br , III-m-Br , III-p-Br ) using density functional theory (DFT) at the B3LYP/6-311G(d,p) level of theory. The molecular geometries were optimized and confirmed as energy minima through vibrational frequency analysis, ensuring structural stability. Electronic structure analysis revealed that halogen substitution significantly reduces the HOMO-LUMO energy gap, with III-p-Br exhibiting the smallest gap (3.422 eV), indicating enhanced charge transfer compared to the parent molecule III-H (3.566 eV). Molecular electrostatic potential (MEP) maps demonstrated increased electron density polarization near halogen atoms, facilitating intramolecular charge transfer (ICT). The first hyperpolarizability (β total ) values for halogenated derivatives surpassed those of the unsubstituted analog, with III-p-Br (5.54 × 10 −30 esu) exhibiting the highest NLO response, approaching about ten times that of the standard urea (0.60 × 10 −30 esu). Polarizability (α ave ) calculations further confirmed the superior optical activity of halogenated systems, with III-p-Br (37.34 × 10 −24 esu) exhibiting a polarizability nine times that of urea. Time-dependent DFT (TD-DFT) simulations of UV–Vis spectra revealed a red shift in absorption maxima for halogenated derivatives, with III-o-Br (380.18 nm) displaying the most pronounced shift, indicating extended conjugation and improved light-harvesting capabilities.