Excited-state deactivation via solvent-to-chromophore proton transfer in an isolated 1 : 1 molecular complex: experimental validation by measuring the energy barrier and kinetic isotope effect

Abstract

We have experimentally demonstrated conclusive evidence of solvent-to-chromophore excited-state proton transfer (ESPT) as a deactivation mechanism in a binary complex isolated in the gas phase. This was achieved by determining the energy barrier of the ESPT processes, qualitatively analysing the quantum tunnelling rates and evaluating the kinetic isotope effect. The 1 : 1 complexes of 2,2′-pyridylbenzimidazole (PBI) with H₂O, D₂O and NH₃, produced in supersonic jet-cooled molecular beam, were characterised spectroscopically. The vibrational frequencies of the complexes in the S₁ electronic state were recorded using a resonant two-colour two-photon ionization method coupled to a time-of-flight mass spectrometer set-up. In PBI–H₂O, the ESPT energy barrier of 431 ± 10 cm⁻¹ was measured using UV-UV hole-burning spectroscopy. The exact reaction pathway was experimentally determined via the isotopic substitution of the tunnelling-proton (in PBI–D₂O) and by increasing the width of the proton-transfer barrier (in PBI–NH₃). In both cases, the energy barriers were significantly increased to >1030 cm⁻¹ in PBI–D₂O and to >868 cm⁻¹ in PBI–NH₃. The heavy atom in PBI–D₂O decreased the zero-point energy in the S₁ state significantly, resulting in elevation of the energy barrier. Secondly, the solvent-to-chromophore proton tunnelling was found to decrease drastically upon deuterium substitution. In the PBI–NH₃ complex, the solvent molecule formed preferential hydrogen bonding with the acidic (PBI)N–H group. This led to the formation of weak hydrogen bonding between ammonia and the pyridyl-N atom, thus increasing the proton-transfer barrier width (H₂N–H⋯N_pyridyl(PBI)). The above resulted in an increased barrier height and a decreased quantum tunnelling rate in the excited state. The experimental investigation, aided by computational investigations, demonstrated conclusive evidence of a novel deactivation channel for an electronically excited biologically relevant system. The variation observed for the energy barrier and the quantum tunnelling rate by substituting NH₃ in place of H₂O can be directly correlated with the drastically different photochemical and photophysical reactions of biomolecules under various microenvironments.