Electronic Spectrum of 5,6-Dimethyl-2-aminobenzimidazole:Effect of Solvents & pH on Spectral Behaviour

Abstract

Effect of solvents on the electronic spectrum of 5.6-dimethyl-2-aminobenzimidazole indicates that the amino group acts as a proton donor to proton acceptor and proton acceptor to proton donor solvents in <em>S</em>₀ state but it acts as a proton donor in the <em>S</em>₁ state. The absorption and fluorescence spectra indicate that DMAB and its dication appear to behave like arylamine and arylammonium ion respectively in <em>S</em>₀ and <em>S</em>₁ states. The above studies along with the infrared spectra have shown that the monocation (protonated at tertiary nitrogen) of DMAB has cyclic amidine structure in <em>S</em>₀ and <em>S</em>₁ states. The studies at different <em>p</em>H values also support the above results and the values of <em>pK</em>_a have been explained on the basis of relative contributions of increase in acidic and basic character of respective species.