Excited-State Behavior of 2-(4'-Pyridyl)benzimidazole in Aqueous Solution: Proton-Transfer Processes and Dual Fluorescence

Abstract

The excited-state acid-base behavior of 2-(4'-pyridyl)benzimidazole in aqueous solution has been studied over a wide range of acidity by UV absorption and fluorescence spectroscopy. The species detected in the lowest electronically excited singlet state turned out to be identical to those of the ground state: the anion A, the neutral molecule N, the monocation C protonated at the benzimidazole N3, the monocation T protonated at the pyridyl nitrogen, and the dication D. Nevertheless, the strong increase in the basicity of the pyridyl nitrogen atom in the excited state induces different excited-state proton-transfer processes. The dual fluorescence of 4PBI at neutral pH is explained by the protonation of N* to give T*. On the basis of the steady-state data together with the fluorescence decay measurements at several acidities, a mechanism is proposed to explain the acid-base behavior of 4PBI in the first excited singlet state.

Spectral characteristics of 2-(4′-amino-3-pyridyl)benzimidazole: Effects of solvent and acid or base concentration

“…(i) first protonation occurs at pyridine QN-atom, which becomes more basic on excitation to S 1 state as compared to BI QN-atom and thus will hold on to the proton. (ii) DC1 formed at pHo4 cannot decompose to only MC1 as suggested by Rodriguez et al [3][4][5][6] at this acid concentration. (iii) In general, charge transfer emission band is more sensitive to solvent polarity [29][30][31].…”

“…In case of BIs, hardly any change is observed with spectral characteristics of MC formed by the protonation of BI QN-atom [29][30][31], whereas red shift is noticed when pyridine QN-atom is protonated [2][3][4][5][6] if p-p* is the lowest energy transition.The fluorescence spectrum of the protonated species of BI is largely red shifted and in case of charge transfer is the lowest energy transition [29][30][31], but this emission is sensitive to solvent polarity. Lastly protonation of -NH 2 group always leads to blue shifts in the spectral characteristics of neutral species [44][45][46][47].…”

“…The acidic or basic character of a molecule becomes further challenging if there is more than one basic character. Rodriguez-Prieto et al [3][4][5][6] have studied extensively the acid-base character of different isomers of pyridyl benzimidazoles. In case of 2-(2 0 -pyridyl)-and 2-(3 0 -pyridyl)benzimidazoles (2-PyBI, , the first site of protonation is BIQN-atom (MC1) in S 0 state, followed by pyridyl nitrogen atom.…”

Conformations, vibrational spectra and force field of 1-methyl-2-(2′-pyridyl)benzimidazole: experimental data and density functional theory investigation in comparison with 2-(2′-pyridyl)benzimidazole

“…Derivatives of the 2-(2 0 -pyridyl)benzimidazole (2PBI) have recently received much attention [1][2][3][4][5][6][7] due to the protontransfer reactions in the exited states of these compounds. Sulfur-containing derivatives of 2PBI can serve as a very effective protection of various metal nanoparticles [8,9].…”

“…Me2PBI-A and Me2PBI-B are rotamers; structure C is the second tautomeric form. It is known that 2PBI molecule exists in the similar conformations [12], but according to the investigation of absorption spectra in a wide variety of solvents [2][3][4], 2PBI-C form is absent in the ground state.…”