Solvent-dependent photophysical properties of aminophenoxazone dyes as optical probes

Abstract

Photophysical properties of three aminophenoxazone (APO) dyes were investigated in many kinds of pure and mixed solvents to assess the possibility for application as an optical probe. The absorption band underwent a significant red shift which was accompanied by a slight change in the spectral profile when increasing the polarity of the solvent. The wavenumber showed a linear correlation with the solvent polarity/polarizability π ∗ in aprotic solvents, indicating that the APO dyes are only proton-accepting. The results in dioxane-water deviated downward from this correlation, suggesting that the hydrogen-bonding interaction with water molecules enhances the red shift. The fluorescence spectrum also exhibited a marked red shift and a change in spectral profile when increasing the polarity of the solvent. The relationship between the wavenumber and π ∗ was nonlinear in aprotic solvents and dioxane-water. A decrease in the rate constant of nonradiative fluorescence decay ( k nr) abruptly changed to increase when π ∗ increased in dioxane-water. This is presumably because two kinds of nonradiative decay process are involved. The alteration of the chemical structure, i.e., the exchange of ethyl to methyl of the substituent at the 7-amino group and the introduction of a methyl group to the 1 position of the phenoxazone ring caused an increase in the Stokes shift and moved the point of the abrupt change in k nr to a larger π ∗ value.