Protonation dependent photoinduced intramolecular charge transfer in 2-(p-dimethylaminostyryl)benzazoles

The absorption and fluorescence spectra of 2-(p-dimethylaminostyryl)benzoxazole, DMASBO, and its benzothiazole analogue, DMASBT, have been studied in different solvents and at various acid concentrations. Two types of monocation are formed simultaneously in all solvents at relatively lower acid concentrations. The neutral DMASBO and DMASBT as well as the monocations formed by protonation of the -N = atom exhibit highly solvatochromic intramolecular charge transfer (ICT) emission. This indicated an increase in the dipole moment of the S-1 state (mu(e)). The mu(e) values, which were calculated from the solvatochromic shifts and the ground state dipole moments, came out to be 14.3 and 12.6 D for DMASBT and DMASBO, respectively. For their azolium cations mu(e) are 9.0 and 8.6 D. on the other hand, the monocations formed by protonation of the -NMe2 group as well as the dications formed at relatively higher acid concentration, showed insignificant spectral shift on changing the solvent polarity. Also, the effect of the heteroatom and the protonation site on the ICT process is discussed.