Excited State Deactivation via Solvent to Chromophore Proton Transfer in Isolated 1:1 Molecular Complex: Experimental Validation by Measuring the Energy Barrier and Kinetic Isotope Effect

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. The above 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 a 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-ofFlight mass spectrometer set-up. In the 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 by isotopic substitution of the tunnelling proton (in PBI-D2O) and increasing the width of the proton transfer barrier (in PBI-NH3). In both cases, the energy barrier was significantly increased to > 1030 cm-1 in the 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 the 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 a preferential hydrogen bonding with the acidic (PBI)N-H group. This led to the formation of a weak hydrogen bonding between the ammonia and the pyridyl-N atom, thus, increasing the proton transfer barrier width (H2NH‧‧‧Npyridyl(PBI)). The above resulted in increased barrier height and decreased quantum tunnelling rate in the excited state. The experimental investigation, aided by computational investigations, demonstrated conclusive evidence of a novel deactivation channel of 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 to the drastically different photochemical and photo-physical reactions of biomolecules under various microenvironments.