SOLVATOCHROMISM AND PROTOTROPISM OF 2-(3'-HYDROXYPHENYL)BENZOXAZOLE AND 2-(4'-HYDROXYPHENYL)BENZOXAZOLE IN THE EXCITED SINGLET-STATE

The absorption and fluorescence spectral properties of 2-(3'-hydroxyphenyl)benzoxazole (m-HPBO) and 2-(4'-hydroxyphenyl)benzoxazole (p-HPBO) were studied in a series of organic solvents and in aqueous solutions with H}/pH/H0 in the range 16.0 to -10.4. The Stokes shifts of m-HPBO and p-HPBO, correlated with various polarity scales, suggest that the former molecule is more polar and has a larger dipole moment than the latter in the S1 state. Neutral-zwitterion and monocation-zwitterion equilibria exist in the S1 state of the molecules. The excited state proton transfer reactions of m-HPBO were studied kinetically. The pK(a) values of the different prototropic equilibria in the S0 and S1 states were determined.