Excited-State Proton and Charge Transfer in Protonated Amino and Methylated Derivatives of 2-(2′-Hydroxyphenyl)benzimidazole

We studied the excited-state behavior of a family of mono- and diprotonated derivatives of 2-phenylbenzimidazole in different solvents, using steady-state and time-resolved fluorescence spectroscopy. The species investigated were 2-(4'-amino-2'-hydroxyphenyl)benzimidazole (<bold>1</bold>), the diethylamino analogue 2-(4'-N,N-diethylamino-2'-hydroxyphenyl)benzimidazole (<bold>2</bold>) and its N-methylated derivative 1-methyl-2-(4'-N,N-diethylamino-2'-hydroxyphenyl)benzimidazole (<bold>3</bold>). The O-methoxy derivatives of <bold>2</bold> and <bold>3</bold> (<bold>2-OMe</bold> and <bold>3-OMe</bold>), and the simpler models 2-phenylbenzimidazole (<bold>4</bold>) and its 4'-amino (<bold>5</bold>) and 4'-dimethylamino (<bold>6</bold>) derivatives were also studied. We found that the dications of <bold>1</bold>, <bold>2</bold>, and <bold>3</bold> (protonated at the benzimidazole N3 and at the amino group) were strong photoacids, which were deprotonated at the hydroxyl group upon excitation in aqueous solution (totally for <bold>2</bold> and <bold>3</bold>) to give a tautomer of the ground-state monocation. In contrast, no photodissociation was observed for the monocations of these species. Instead, some of the monocations studied behaved as molecular rotors, for which electronic excitation led to a twisted intramolecular charge transfer (TICT) state. The monocations of <bold>2</bold>, <bold>3</bold>, <bold>2-OMe</bold>, <bold>3-OMe</bold>, and <bold>6</bold>, protonated at the benzimidazole N3, experienced a polarity- and viscosity-dependent radiationless deactivation associated with a large-amplitude rotational motion. We propose that this process is connected to an intramolecular charge transfer from the dimethylaminophenyl or diethylaminophenyl moiety (donor) to the protonated benzimidazole group (acceptor) of the excited monocation, which yields a twisted charge-transfer species. No fluorescence from this species was detected except for <bold>3</bold> and <bold>3-OMe</bold> in low-viscosity solvents.