Preferential solvation of 4-(4-nitrophenylazo) aniline in alcohol-dioxane binary solvent mixture

Electronic spectroscopy is a suitable method for studying solvation. It has been observed that maximum energy of electronic transition of various solutes depends to a great extent on the local environment around the solute. In binary solvent, the composition of liquid mixture around the solute (solvation shell) is different from that of the bulk. The inhomogenecity created by the solute molecule in the solvation shell is known as preferential solvation. Owing to the applicability of various azo dyes and the importance to study solute-solvent interaction, herein we have made an attempt to study the preferential solvation of 4-(4-nitrophenylazo) aniline (NPA) in binary solvent mixtures of alcohol with dioxane. With increase in solvent polarity, the maximum spectroscopic absorption of NPA experience a red shift (bathochromic shift) indicating a positive solvatochromism. These facts can be explained in terms of the greater stabilization of the excited state compared to the ground state by greater solvating power of polar solvents through specific (hydrogen bonding) and nonspecific (dipole-dipole) interaction, which reduces the energy difference between ground and excited state intern reducing the maximum energy of absorption. In all the case, the molecule of solute is preferentially solvated by alcohols over dioxane due to (i) hydrogen bonding (ii) dipole- dipole interaction and (iii) apolar/hydrophobic interaction In some cases, synergism is observed due to higher solvent-solvent interaction than solute-solvent interaction. The balancing of both polar and apolar characetrisitcs in the solvent cage show significant solvatochromism