Solvent-Dependent Ground- and Excited-State Tautomerism in 2-(6‘-Hydroxy-2‘-pyridyl)benzimidazole

Abstract

The ground- and excited-state tautomerism of 2-(6‘-hydroxy-2‘-pyridyl)benzimidazole (1) and 1-methyl-2-(6‘-hydroxy-2‘-pyridyl)benzimidazole (2) in various solvents was investigated by means of UV−vis absorption spectroscopy, steady-state and time-resolved fluorescence spectroscopy, and quantum-mechanical ab initio calculations. A solvent-dependent tautomeric equilibrium was observed for both compounds in the ground state between the lactim or normal form and the lactam tautomer resulting from a proton translocation between the hydroxyl group and the pyridine nitrogen. Here, we report evidences for a solvent-dependent switching in the nature of the excited-state proton transfer (ESPT) reactions undergone by 1 and 2. In the aprotic solvent acetonitrile, no significant ESPT takes place. The protic solvents ethanol and water facilitate the proton transfer through bridges of solvent molecules, but each solvent catalyzes specifically a different ESPT process. In aqueous solution, the excited lactim species of 1 and 2 undergo a proton transfer from the hydroxyl group to the pyridine nitrogen, favoring the lactam tautomer in the first excited singlet state. In ethanol the ESPT takes place for 1 from the benzimidazole NH to the pyridine nitrogen, originating a new tautomer not observed in the ground state. Compound 2, without a NH group, does not tautomerize in the excited state in ethanol.

A theoretical investigation about the excited state behavior for 2‐(6'‐hydroxy‐2'‐pyridyl)benzimidazole: The water‐assisted excited state proton transfer process

“…Based on the optimized S 0 ‐state 2HPB, we calculated the absorption and fluorescence spectra (see Figure ). It can be seen clearly that our calculated absorption and emission peaks are located at around 317 and 389 nm, respectively, which are in good agreement with previous experimental results (316 and 370 nm), respectively. That is to say, the theoretical level we used in this work was reasonable.…”

“…Moreover, based on the optimized S 0 state 2HPB–H 2 O and S 1 state 2HPB–H 2 O and 2HPB–H 2 O‐PT, their absorption and fluorescence spectra were calculated (shown in Figure ). It can be clearly seen that the maximum absorption of 2HPB–H 2 O is located at 317 nm, which is consistent with the experimental one at 339 nm . The emission peak of 2HPB–H 2 O is at ~396 nm, which is also close to the experimental maximum at 417 nm .…”

“…It can be clearly seen that the maximum absorption of 2HPB–H 2 O is located at 317 nm, which is consistent with the experimental one at 339 nm . The emission peak of 2HPB–H 2 O is at ~396 nm, which is also close to the experimental maximum at 417 nm . Compared to the maximum absorption peak, the red shift of ~80 nm can be ascribed to the normal Stokes shift.…”

Early events associated with the excited state proton transfer in 2-(2′-pyridyl)benzimidazole

“…However, the pK a ‫ء‬ for the second protonation of 2PBI is 3.19 as estimated by the Forster cycle method. 7 So, C ‫ء‬ is expected to get protonated in the pH range studied to form D ‫ء‬ . This excited state protonation process is expected to be ultrafast and should contribute to the depletion of the C ‫ء‬ emission.…”

“…[1][2][3][4][5] Excited state proton transfer ͑ESPT͒ in hydrogen-bonded systems such as benzimidazoles, in particular, has attracted a lot of attention. 6,7 The ESPT of the cationic form of 2-͑2Ј-pyridyl͒benzimidazole ͑2PBI͒ is manifested in a distinct dual emission in its aqueous solutions in the pH range of 3.5-0.5 ͑Scheme 1͒. 8 The inclusion of 2PBI in cyclodextrins hinders the solvent-mediated ESPT to some extent due to shielding of the fluorophore from water.…”