Understanding Solvation: Comparison of Reichardt's Solvatochromic Probe and Related Molecular Core Structures

The compound 2,6-diphenyl-4-(2,4,6-triphenylpyridinium-1-Aphenolate, p-RB, shows distinct colors in different solvents (solvatochromism). The compound 4-(pyridinium-1-yl)phenolate, p-CB, represents the part of p-RB which is responsible for this phenomenon. We compared the solvatochromism of both compounds and also the structurally related 2-(pyridinium-1-yl)phenolate, o-CB, and ( 2,4-dim ethyl-6-( 2,4,6-triphenyl-N-pyri dinium-1-yl)phenolate, o-RB. In pure solvents, plots of the empirical solvent polarity parameter [ET(probe), kcal/mol] of the different probes correlate linearly with slopes close to unity. That is, these probes are similarly sensitive to specific and nonspecific interactions with the solvents. The solvatochromism of p-CB and o-CB was studied, for the first time, in binary mixtures of water with dimethyl sulfoxide (DMSO) and 1-propanol (1-PrOH). The dependence of E-T(probe) on mixture composition was nonideal due to preferential solvation of the probe by one component of the binary solvent mixture. We treated our solvatochromic data using a solvent-exchange model that considers formation of the complex solvents [HOH center dot center dot center dot O=S(CH3)(2)] and [HOH center dot center dot center dot O(H)-C3H7]. The model applies satisfactorily to our data and shows the importance to solvation of hydrogen-bonding and hydrophobic interactions. The preferential solvation of (more hydrophobic) p-RB is more pronounced than that of p-CB or o-CB. The solvent complex [OH2 center dot center dot center dot O(H)-C3H7] is more efficient than [OH2 center dot center dot center dot O=S(CH3)(2)] because of more possibilities of hydrogen bonding.