Solvatoalignment index and solvent-dependent behavior of asymmetric discotic liquid crystals: effect of polarity and alkyl chain length

Discotic liquid crystals (DLCs) are emerging as versatile functional materials with significant potential for optical and sensing applications. Their photophysical behavior is highly sensitive not only to solvent polarity but also to molecular asymmetry and environmental factors such as viscosity and hydrogen-bonding ability. In this study, we systematically investigated the solvatochromic behavior of four structurally asymmetric DLC derivatives bearing diverse alkyl side chains across a wide range of solvents differing in polarity, viscosity, and hydrogen-bonding strength. Steady-state UV-Vis absorption and fluorescence spectroscopy revealed that solvent polarity alone cannot fully account for the observed spectral variations. Instead, the Kamlet-Abboud-Taft (KAT) model particularly the dipolarity/polarizability parameter provided a more accurate interpretation of solvent effects. Dipole moment calculations further demonstrated that even subtle structural modifications can markedly influence molecular polarity and photophysical responses. To address the dynamic aspects of solute-solvent interactions, we introduce the Solvatoalignment Index (SAI), a composite descriptor that integrates solvent polarity, viscosity, and the solute's dipole moment. This index effectively predicts solvent-induced spectral shifts and offers a more holistic perspective on solvent-solute interactions in anisotropic systems. Overall, our findings underscore the importance of considering multiple solvent parameters alongside molecular asymmetry for the rational design of next-generation DLC-based materials with tunable optoelectronic properties.