Influence of substituents in the phenyl ring on photophysical properties of 3-[2-(phenyl)benzoxazol-5-yl]alanine derivatives

High molar absorption coefficient values and fluorescence quantum yields are characteristic for all compounds studied. Moreover, in the case of derivatives possessing electron-donor substituents in position 2 of the phenyl ring the fluorescence quantum yield increases with the solvent polarity whereas the opposite effect is observed for derivatives substituted by electron-acceptor groups. Also, the position of the absorption and emission band depends on the solvent polarity and is shifted to longer wavelengths in more polar solvents as a result of a charge transfer from a substituent to the benzoxazole moiety. In the case of derivatives substituted by 2-hydroxyphenyl, the intramolecular proton transfer process is observed. However, the character of the group in position 4 of the phenyl ring influences on it. An electron-acceptor substituent facilitates the proton transfer whereas the opposite is true for an electron-donor group.