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

A series of N-(tert-butoxycarbonyl)-3-[2-(phenyl)benzoxazol-5-yl]alanine methyl ester derivatives possessing electron-donor and electron-acceptor substitutents in the phenyl ring were synthesized and their photophysical properties in methanol, acetonitrile and methylcyclohexane were studied by means of absorption and fluorescence spectroscopies as well as theoretical calculations. The structures of compounds studied in the ground and excited state were calculated using the MOPAC 2002 package with COSMO model of the solvent whereas the positions of vertical absorption and emission transitions were calculated using a INDO/S or CNDO/Param method within a SCRF approximation for the solvent but they are not in good agreement with the experimental values. olar 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.