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...
This article presents an extensively curated rich set of gas phase spectroscopic data of 2,7-diazaindole in the ground and excited states. Single vibronic level fluorescence spectroscopy (SVLF) was performed to determine the ground state vibrations of the molecule, which depicted a large Franck-Condon activity of the bands beyond 2600 cm-1. For the excited state, laser-induced fluorescence (LIF) and resonant two-colour two-photon ionization spectroscopy (R2PI) were performed. The band origin 〖(0〗_0^0) for S1←S0 transition appeared at 33910±1 cm-1 . The Frank-Condon active vibrational modes in the spectra were seen till 〖(0〗_0^0)+ 1600 cm-1 region, which suggested the similar ground and excited state geometries. The lower energy asymmetric vibrational modes at 207, 252 and 358 cm-1, observed in the excited state were absent in the SVLF spectrum. IR-UV double Hole Burning spectroscopy confirmed the absence of any other isomeric species in the molecular beam. Ionization potential (I.P) of the molecule was found to be 8.9310.001 eV, recorded using photoionization efficiency spectroscopy. The above value is significantly higher than the related azaindole derivatives. The ground and excited state N-H stretching frequencies of the molecule were determined using fluorescence-dip infrared spectra (FDIR) and resonant ion-dip infrared spectroscopy (IDIR), obtained at 3523 and 3467 cm-1, respectively. The lower value of NH in the electronic excited state implies the higher acidity of the group compared to the ground state. Moreover, to understand the excited state properties of the molecule, a comparative analysis of the experimental LIF/2C-R2PI spectra was done against Franck-Condon simulated spectra at three different levels of theories. The vibrational frequencies calculated at B3LYP-D4/def2-TZVPP showed the most accurate prediction on comparison with the experimentally detected symmetric modes in the ground state. However, in the excited state, the low energy asymmetric modes were correctly determined at B3LYP/def-SVP level of theory. This is most probably due to the distortion observed at the pyrazolyl ring leading to the appearance of asymmetric vibrational modes. However, all the three methods have shown nearly similar correlation with the experimental frequencies in the excited state, which was evident from their similar scaling factors.
Excited state hydrogen (ESHT) and proton (ESPT) transfer pathways in the solvent clusters of 6-azaindole 6AI-S3,4 and 2,6-diazaindole 26DAI-S3,4 (S=H2O, NH3) were computationally explored to understand the fate of photo-excited biomolecules. The ESHT energy barriers in (H2O)3 complexes (39.6-41.3 kJmol-1) were decreased in (H2O)4 complexes (23.1-20.2 kJmol-1). Lengthening the solvent chain reduced the barrier because of the relaxed transition states geometries with reduced angular strains. Replacing the water molecule with ammonia drastically decreased the energy barriers to 21.4-21.3 kJmol-1 in (NH3)3 complexes and 8.1-9.5 kJ mol-1 in (NH3)4 complexes. The transition state was identified as Ha atom attached to the first solvent molecule. The formation of stronger hydrogen bonds in (NH3)3,4 complexes resulted in facile ESHT reaction than that in the (H2O)3,4 complexes. The ESPT energy barriers, in 6AI-S3,4 and 26DAI-S3,4 are found to range between 40-73 kJmol-1. The above values were significantly higher than that of the ESHT processes and hence are considered a minor channel in the process. The energetics of ESHT and ESPT explored in this study would be of great importance to study the photochemistry of N-rich biomolecules in the presence of various protic environments.
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 43110 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.
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