DUAL FLUORESCENCE OF 2-(4'-(N,N-DIMETHYLAMINO)PHENYL)BENZOTHIAZOLE AND ITS BENZIMIDAZOLE ANALOG - EFFECT OF SOLVENT AND PH ON ELECTRONIC-SPECTRA

The absorption and fluorescence spectra of 2-(4'-(N,N-dimethylamino)phenyl)benzothiazole (DMAPBT) and 2-(4'-(N,N-dimethylamino)phenyl)benzimidazole (DMAPBI) have been recorded in solvents of different polarity and hydrogen-bonding nature. In all the solvents, a dual fluorescence is observed for both the compounds. The normal Stokes-shifted emission (B) originates from a locally excited pi* electronic state, and the large Stokes-shifted band (A) is due to emission from a twisted intramolecular charge-transfer (TICT) state. The dual fluorescence observed, even in nonpolar solvents, indicates that the energy of the TICT state is always lower than that of the locally excited state. Two kinds of monocations, one protonated at the tertiary nitrogen atom and the other protonated at the amino group, are formed simultaneously in the case of DMAPBT. The former is more stable in nonpolar solvents and the latter one in polar solvents. On the other hand, only one kind of monocation is formed in DMAPBI. Fluorimetric titrations have indicated that the radiative lifetimes of the respective conjugate acid-base pairs are shorter than the reciprocal rates of the protonation/deprotonation reactions.