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

Abstract

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 (1), the diethylamino analogue 2-(4'-N,N-diethylamino-2'-hydroxyphenyl)benzimidazole (2) and its N-methylated derivative 1-methyl-2-(4'-N,N-diethylamino-2'-hydroxyphenyl)benzimidazole (3). The O-methoxy derivatives of 2 and 3 (2-OMe and 3-OMe), and the simpler models 2-phenylbenzimidazole (4) and its 4'-amino (5) and 4'-dimethylamino (6) derivatives were also studied. We found that the dications of 1, 2, and 3 (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 2 and 3) 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 2, 3, 2-OMe, 3-OMe, and 6, 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 3 and 3-OMe in low-viscosity solvents.

Light-Driven Switching between Intramolecular Proton-Transfer and Charge-Transfer States

“…46,75−85 It has also been reported that the fluorescence yield decreases significantly upon substitution of the electron acceptor and donor. 46,77,79,80 DHP has strong electron donor and electron acceptor; therefore, its fluorescence decreases significantly. The relatively large quantum yield of the methoxy derivative supports this hypothesis.…”

ESIPT‐Active Pyrene‐imidazole Fluorophores: GSIPT, Dual Solid‐ and Solution‐State Emission plus Counter‐Intuitive Crystal Packing**

“…The ESIPT-active compounds have been known to undergo deprotonation to yield phenolates and exhibit low energy absorption. [35,36] In order to rule out such possibility, the absorption of 3 has been measured in the presence of a drop of acetic acid. (Figure S5).…”

Spectral and Theoretical Studies of Benzimidazole and 2-Phenyl Substituted Benzimidazoles

“…These maximum are similar to 2-(m-methylphenyl)benzimidazole, 2-(p-methylphenyl)benzimidazole, 2-(m-methoxyphenyl)benzimidazole, 2-(pmethoxyphenyl)benzimidazole. But the 2-(o-hydroxyphenyl)benzimidazole molecule is observed the specific property in the ground and excited states to describe the keto-enol tautomeric equilibrium through the excited state intramolecular proton transfer [58][59][60][61][62][63][64][65], which property already given in introduction part and theoretical study also done for ketoenol tautomer in solvent effect. The absorption maximum of 2-(o-hydroxyphenyl)benzimidazole was observed at 335, 318 and 295 nm and fluorescence maximum at 355 and 465 nm in DMSO [32].…”