Solvatochromic behavior of lawsone investigated through experimental and DFT computational studies

This study evaluates the solvatochromic behavior and photophysical properties of lawsone henna in different solvents with varying polarities using UV-Vis/fluorescence spectroscopy, density functional theory (DFT), and time-dependent density functional theory (TD-DFT) calculations (CAM-B3LYP/6–31 + G(d, p)). Experimental results indicated three characteristic absorption bands at approximately 270, 336, and 412 nm. A significant bathochromic shift in emission and a notable variation in Stokes shift ( \\(\\:\\varDelta\\:\\nu\\:\\) ) are observed, from 2.528 eV in nonpolar solvents (cyclohexane) to 3.118 eV in polar protic solvents (ethanol). DFT calculations show that the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels shifted from − 8.852 to − 2.342 eV in the gas phase to − 8.577 and − 2.093 eV in acetonitrile, with the HOMO-LUMO gap decreasing from 6.503 eV in n -hexane to 6.40 eV in water. Significantly, the dipole moment decreased from 4.734 D (ground state) to 4.359 D (excited state) in water, confirming negative solvatochromism. These results, which are supported by a fluorescence emission energy range of 1.837–1.847 eV, indicate that Lawsone’s optical response is controlled by a competition between dielectric interactions and specific solute-solvent hydrogen bonding.