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The absorption and fluorescence spectra in different solvents and at various pH have indicated that monocation and dication of 2-(p-aminophenyl)benzimidazole (PBNH2) are formed by protonating first the tertiary amine nitrogen atom and then the amino group, both in S0 and S1 whereas the monoanion in S0 is obtained by deprotonating the imino group but in S1 it is formed by deprotonating the amino group. Dianion at H–15 in S1 is obtained by deprotonating the amino and then imino group.
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Mishra et al. (1985) studied this question.
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