D-π-A Molecular Probe to Unveil the Role of Solute-Solvent Hydrogen Bonding in Solvatochromism, Location Specific Preferential Solvation and Synergistic Effect in Binary Mixtures

An azo dye 4-(p-Nitrophenylazo) phenol (NPAP) characterized with a donor-pi-acceptor (D-pi-A) framework has been used as a molecular probe to study solvatochromic behavior in neat and binary solvent mixtures comprised of chloroform (a hydrogen bond donor; HBD solvent) and a hydrogen bond acceptor (HBA) solvent. The specificity of the probe lies in its inherent characteristics of having a HBD group and a HBA group at two different ends of the conjugated pi-network. UV-Vis spectroscopic technique is used for the purpose. The probe demonstrates a very expressive reversal in solvatochromic behavior, with the solvatochromic switch at E-T(30) of similar to 45 kcal mol(-1). Multiparametric treatment of the solvatochromic data using SEV technique displays excellent sensitivity of the probe towards HBA basicity, dipolarity and polarizability of the medium with a smaller but significant contribution from HBD acidity and hydrophobicity/apolarity. The event of preferential solvation in the chloroform-HBA mixture is explained on the basis of specific solute-solvent interaction through HBD and HBA ability of the solvent components. HBA solvents preferentially solvate the phenolic site while HBD solvents preferentially solvate the nitrobenzene group through complementary hydrogen bonding, which results in an increase in local concentration at specific sites resulting in synergistic preferential solvation. The proposition of site selective PS through molecular recognition is also supported by theoretical studies.