Intramolecular charge transfer and photoisomerization of 2-(p-dimethylaminostyryl)benzoxazole:

2-(p-Dimethylaminostyryl)benzoxazole (DMASBO) was prepared and its absorption and fluorescence spectra were measured in various solvents. The absorption spectra are less sensitive to the solvent polarity while the fluorescence spectra are highly solvatochromic leading to a large Stokes shift. An analysis of the solvatochromic behaviour of the fluorescence spectra in terms of Δf(ε,n) revealed that the emission occurs from a highly polar excited state. The large dipole moment change together with the strongly red-shifted fluorescence, as the solvent polarity is increased, demonstrate the formation of an intramolecular charge-transfer state (ICT). The pKa-values of nitrogen protonation in both ground and excited states were estimated in aqueous solutions. The quantum yields of fluorescence and trans ⇌ cis photoisomerization were determined in different solvents. Although a low φf-value was found in all the investigated solvents (except in viscous media) the trans/cis isomerization occurs with substantial quantum yield. The role of the ICT state in the trans/cis isomerization and fluorescence quenching was also discussed.