A combined spectroscopic and computational investigation on the solvent-to-chromophore excited-state proton transfer in the 2,2′-pyridylbenzimidazole–methanol complex

Abstract

This article demonstrates experimental proof of excited state ‘solvent-to-chromophore’ proton transfer (ESPT) in the isolated gas phase PBI (2,2′-pyridylbenzimidazole)–CH₃OH complex, aided by computational calculations. The binary complexes of PBI with CH₃OH/CH₃OD were produced in a supersonic jet-cooled molecular beam and the energy barrier of the photo-excited process was determined using resonant two-colour two-photon ionization spectroscopy (R2PI). The ESPT process in the PBI–CH₃OH complex was confirmed by the disappearance of the Franck–Condon active vibrational transitions above 0⁰₀ + 390 cm⁻¹. In the PBI–CH₃OD complex, the reappearance of the Franck–Condon transitions till 0⁰₀ + 800 cm⁻¹ confirmed the elevation of the ESPT barrier upon isotopic substitution due to the lowering of the zero-point vibrational energy. The ESPT energy barrier in PBI–CH₃OH was bracketed as 410 ± 20 cm⁻¹ (4.91 ± 0.23 kJ mol⁻¹) by comparing the spectra of PBI–CH₃OH and PBI–CH₃OD. The solvent-to-chromophore proton transfer was confirmed based on the significantly decreased quantum tunnelling of the solvent proton in the PBI–CH₃OD complex. The computational investigation resulted in an energy barrier of 6.0 kJ mol⁻¹ for the ESPT reaction in the PBI–CH₃OH complex, showing excellent agreement with the experimental value. Overall, the excited state reaction progressed through an intersection of ππ* and nπ* states before being deactivated to the ground state via internal conversion. The present investigation reveals a novel reaction pathway for the deactivation mechanism of the photo-excited N-containing biomolecules in the presence of protic-solvents.