Ketocyanine dyes as sensors for proticity and pH of a medium

Photophysical properties of two structurally similar ketocyanine dyes have been studied in aprotic+protic (dioxane+water, tetrahydrofuran+water) and protic+protic (methanol+water) solvent mixtures. The observed photophysical properties of these dyes show different trends with an increase in solvent polarity (in the E(T)(30) scale) in aprotic and protic media. The H-bonding at the carbonyl centres of the dyes in the excited state presumably plays a role in modifying the emitting state and hence the photophysical properties show similar trend in a protic solvent or in a mixed aprotic+protic solvent at higher percentage of the later. Thus the dyes are supposed to act as useful probes for studying H-bonding ability of a medium. Absorption and fluorescence spectral characteristics of these dyes have been found to get modified in the presence of strong acid. The presence of H(+) ions persists the dyes to exist in equilibrium with the protonated forms having different absorption and fluorescence characteristics. Reversible change of the equilibrium with pH of the medium makes the dyes, particularly the symmetric one, good probes for indicating the pH of the medium.