Influence of substituents on the nitrogen atom of 3-[2-(4-aminophenyl)benzoxazol-5-yl]alanine derivatives on their photophysical properties - solvatochromic studies

Abstract

Spectral and photophysical properties of three derivatives of 3-[2-(4-aminophenyl)benzoxazol-5-yl]alanine were studied in 36 solvents. The amino group was substituted by methyl and/or phenyl in various combinations (N,N-dimethyl, N-phenyl, N-methyl-N-phenyl). It has been found that the type of substituents on the nitrogen atom determines the spectral and photophysical properties of the compounds studied. The change of the dipole moment in going from the ground to the excited state as well as the dependence of the spectral and photophysical properties of compounds on the solvent polarity parameter E^N_T were analyzed. Additionally, spectral and photophysical properties of the compounds studied were analyzed applying multi-linear correlation using the three-parameter solvents scales of Kamlet–Taft and Catalán. Moreover, the new four-parameter Catalán solvent scale, which takes into account polarizability, dipolarity, acidity, and basicity of the solvents, was also applied giving a better fit than the three-parameter solvent scales. The correlation analysis reveals that the position of the UV/Vis absorption band depends primarily on the change of polarizability of the environment of the dye, although the solvent dipolarity cannot be neglected. However, the position of the fluorescence band and photophysical properties such as the fluorescence rate constant depend mostly on the solvent dipolarity.