Experimental and Theoretical Exploration of Ultrafast Excited State Double Proton Transfer in 2,5-Bis(2-benzimidazolyl)hydroquinone

Abstract

The electronic properties and excited-state intramolecular double proton transfer of 2,5-bis(2-benzimidazolyl)hydroquinone (bis-HBI) in a nonpolar solvent were investigated using a combined experimental and theoretical approach. Bis-HBI was successfully synthesized and its characterization was confirmed through 1H NMR and FT-IR. Three distinct emission peaks of bis-HBI were observed at 484, 597, and 730 nm, which were theoretically assigned to the di-enol (EE), mono-keto (EK), and di-keto (KK) species, respectively. The emission peaks at longer wavelengths (597 and 730 nm) are attributed to tautomerization upon photoexcitation and are assigned to the mono-keto and di-keto species, which result from multiple proton transfers. These species exhibit kinetically and thermodynamically favorable behaviors. On-the-fly dynamics simulations reveal that the double proton transfer process occurs ultrafast, within 433 fs. Additionally, both backward and forward proton transfers are observed during the first and second proton transfers, indicating tautomeric equilibria between the three species—EE, EK, and KK—on the excited-state surface. This is consistent with the potential energy surface along the proton transfer coordinate.