Spectral characteristics of the methylated derivatives of 2-(2′-aminophenyl)benzimidazole:: effects of solvents

Absorption. fluorescence and fluorescence excitation spectra of 2-(2'-N,N-dimethylaminophenyl)-N-methylbenzimidazole (TMAPBI). 2-( 2'-N,N-dimethylamino phenyl)benzimidazole (2-DMAPBI) and 2-(2'-N-methylaminophenyl)-N-methyl benzimidazole (2-N(Me)(>NMe)DMAPBI), have been studied in different solvents. Absorption spectra and the semi-empirical quantum mechanical calculations establish the presence of two rotamers in each molecule. Besides the short wavelength (SW) fluorescence hand, a long wavelength (LW) emission is observed as a tail to the SW emission band in each molecule. Band maxima of both the emissions are red shifted and the fluorescence quantum yield increased with the increase in the polarity and hydrogen bonding nature of the solvents. Single exponential decay observed in the SW emission in non-polar solvents suggests that the equilibrium is established between the two rotamers of each molecule. Biexponential decay in the SW emission in polar solvents indicates that the equilibrium is not established between the two rotamers in the SI state. These results are substantiated by the semi-empirical quantum mechanical calculations, The SW emission is assigned to rotamer I and the tail part of the emission is assigned to rotamer IV. (C) 2001 Elsevier Science B.V. All rights reserved.