From Absorption Shifts to Frontier Orbitals: A Comprehensive Optical Study of a Cyano-Functionalized Liquid Crystal

The nematic liquid crystal 4′-(hexyloxy)-4-biphenylcarbonitrile (HOBC) was examined to elucidate its solvent-dependent optical, solvatochromic, and electronic properties through combined experimental and theoretical approaches. Solvatochromic behavior was characterized by analyzing UV-Vis absorption and fluorescence spectra across a range of solvents, with solvent–solute interactions interpreted using the Kamlet–Aboud–Taft (KAT) and Catalán (CAT) linear solvation energy relationship models. Prominent π–π* and n→π* transitions exhibited bathochromic shifts in polar solvents, indicating the presence of intramolecular charge transfer (ICT) mechanisms. Ground and excited state dipole moments were calculated via multiple solvatochromic shift models, revealing notable changes in dipole strength upon electronic excitation. Quantum chemical calculations based on DFT/B3LYP/6-311 + + G(d,p) provided insight into the HOMO–LUMO energy levels, electrostatic potential distribution, solvent-accessible surface, and global reactivity descriptors. The optical band gap (Eg), calculated using the Tauc method, varied between 3.06–3.79 eV depending on solvent polarity, supporting the insulating nature of the compound. Refractive index values, determined via five empirical equations, ranged from 1.4 to 2.7. Fluorescence emission behavior pointed to a twisted intramolecular charge transfer (TICT) process. Collectively, these findings highlight the strong sensitivity of HOBC to its chemical environment, making it a promising material for solvent-sensitive optical and optoelectronic applications.