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

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.

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.

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⋯Npyridyl(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.

Accurate measurement of sequential Ar desorption energies from the dispersion-dominated Ar1-3 complexes of aromatic molecules.

We present experimental determination of the energies associated with the gradual desorption of Ar atoms from the aromatic molecular surface. Non-covalently bound 2,2'-pyridylbenzimidazole-Ar1-3 complexes were produced in the gas phase and characterized using resonant two-photon ionization (R2PI) spectroscopy. The single Ar desorption from the PBI-Ar, PBI-Ar2 and PBI-Ar3 complexes were measured as 581 ± 18, 656 ± 30 and 537 ± 31 cm-1, respectively. The energies were bracketed between the last observed band in the respective R2PI spectra and the disappeared intramolecular modes of PBI. The Arn dissociation energies in the S1 state were measured as 581 ± 18, 1237 ± 48 and 1774 ± 79 cm-1, respectively, for n = 1, 2 and 3. The calculated dissociation energies of the respective complexes, obtained using three computational methods, show excellent agreement with the experimental data. The ground state dissociation energies were estimated by subtracting the Δν shift of the origin band, and the respective values are 541 ± 18, 1160 ± 48 and 1634 ± 79 cm-1. Overall, the calculated values resulted in scaling factors ranging from 0.956 to 1.017, which depict the predictive power of the methods to determine dispersion energies. The current investigation describes a unique methodology to accurately determine the dissociation and desorption energies of Ar atoms from the surfaces of bio-relevant aromatic molecules.