Solvatochromism of 3-[2-(4-diphenylaminophenyl)benzoxazol-5-yl]alanine methyl ester. A new fluorescence probe.

The photophysical properties of 3-[2-(4-diphenylaminophenyl)benzoxazol-5-yl]alanine methyl ester (1b) and its Boc derivative (1a) were studied in a series of solvents. Its UV-Vis absorption spectra are less sensitive to the solvent polarity than the corresponding fluorescence spectra which show pronounced solvatochromic effect leading to large Stokes shifts. Using an efficient solvatochromic method, based on the molecular-microscopic empirical solvent polarity parameter E(T)(N), a large change of the dipole moment on excitation has been found. From an analysis of the solvatochromic behaviour of the UV-Vis absorption and fluorescence spectra in terms of bulk solvent polarity functions, f(epsilon(r),n) and g(n), a large excited-state dipole moment (mu(e) = 11D), almost perpendicular to the smaller ground-state dipole moment, was observed. This demonstrates the formation of an intramolecular charge-transfer excited state. Large changes of the fluorescence quantum yields as well as the fluorescence lifetimes with an increase of a solvent polarity cause that the new non-proteinogenic amino acid, 3-[2-(4-diphenylaminophenyl)benzoxazol-5-yl]-alanine methyl ester, is a new useful fluorescence probe for biophysical studies of peptides and proteins.