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.

A combined experimental and computational study on the deactivation of a photo-excited 2,2'-pyridylbenzimidazole-water complex via excited-state proton transfer.

In this report, we present solvent assisted excited-state proton transfer coupled to the deactivation of a photo-excited 2,2'-pyridylbenzimidazole bound to a single water molecule. Experimentally, the mass-selected 1 : 1 complex was probed using two-colour resonant two-photon ionization (2C-R2PI) and UV-UV hole-burning (HB) spectroscopy in a supersonically jet-cooled molecular beam. Computationally, three structural isomers were identified as the normal, the tautomer and the proton transfer product of the PBI-H2O complex in the excited S1 state using B3LYP-D4/def2-TZVPP and ADC(2) (MP2)/cc-pVDZ levels of theory. The most stable form in the ground state, i.e., the normal form, was identified using the excitation spectrum in the 30 544 to 30 936 cm-1 region. The 2C-R2PI spectrum showed a sudden break-off above the 000 + 392 cm-1 region, even though the Frack-Condon activity of the S1 ← S0 transition was measured beyond 000 + 1000 cm-1 in the HB spectrum. The intensity of the bands associated with the excited state intermolecular vibrational modes near the break-off region was found to be drastically decreased, which indicates efficient quantum mechanical tunnelling along the hydrogen transfer coordinate. The sudden disappearance of the intermolecular vibrational modes in the spectrum revealed the existence of a deactivation channel in the PBI-H2O complex near 392-450 cm-1 above the 000 transition. The computational investigation predicted that the deactivation of the excited-state occurred via the intersection between the S1 and S0 states, which was associated with the proton transfer from the H2O to the PBI molecule along the O(3)-H(4)→N(5) coordinate. The highest energy structure was identified as the point of intersection between the nπ* (S2) and ππ* (S1) states. The associated barrier height was experimentally determined to be 392-450 cm-1, which showed a reasonable agreement with the calculated excited-state proton transfer barrier. Competing reaction channels such as dissociation and tautomerization were found to be highly energetically inaccessible.

Solvent assisted excited-state deactivation pathways in isolated 2,7-diazaindole-S1-3 (S = Water and Ammonia) complexes.

The role of solvent molecules in the deactivation of photo-excited 2,7-diazaindole (DAI) - (H2O)1-3 and DAI - (NH3)1-3 complexes were computationally investigated. An excited-state proton transfer (ESPT) path from the solvent to the DAI molecule was followed using the TD-DFT-D4 (B3LYP) level of theory. The computed potential energy profile of ESPT process has shown intersection between ππ* and nπ* states facilitated via relative stabilization of the nπ* state with decreasing N(7)-Hb bond length. The ESPT process, starting from the DAI-Sn (ππ*) state, crosses through a barrier ranging from 27 to 36 kJmol-1 for water complexes and 26-30 kJmol-1 for ammonia complexes. The energy of the excited state was rapidly decreased with a shorter N(7)-Hb bond length. Subsequently, a significant trend of finding a second intersection between the ground and the excited state was observed for all the complexes. The results firmly suggested a significant deactivation channel of excited azaindole derivatives. In the present system, two competing channels, ESPT and ESHT, were found to be energetically accessible. The energy barriers associated with the ESPT barriers for DAI-(H2O)1-3 complexes are similar to the ESHT barrier, depicting equal dominance of both processes. The increased basicity of the N(7) atom in the excited state resulted a facile ESPT process from the water to N(7) of the DAI molecule. However, DAI-(NH3)1-3 complexes show clear preference for ESHT over ESPT process owing to its higher gas-phase pKa value making it a poor proton donor. The above systems can be used as a model to computationally and experimentally investigate the competing radiative and deactivation pathways of photo-excited solvated complexes of N-H-bearing bio-relevant molecules via proton and hydrogen transfer reactions.

A combined spectroscopic and computational investigation on dispersion-controlled docking of Ar atoms on 2-(2'-pyridyl)benzimidazole.

The dispersion-controlled docking of inert Ar atoms on the face of polycyclic 2-(2'-pyridyl)-benzimidazole (PBI) was studied experimentally aided by computational findings. The PBI-Arn (n = 1-3) complexes were produced in a supersonically jet-cooled molecular beam and probed using resonant two-photon ionization coupled with a time-of-flight mass spectrometric detection scheme and laser-induced fluorescence spectroscopy. The ground state vibrational frequencies were obtained from single vibronic level fluorescence spectroscopy. The formation of multiple isomers was verified using UV-UV hole-burning spectroscopy. The geometries of PBI-Arn (n = 1-3) complexes were derived by mutual agreement between experimental findings and computational results such as vibrational frequencies in the ground and excited electronic states, Franck-Condon factors and spectral shift of the S1← S0 transitions. All the above analyses provided good agreement between the experimental and simulated spectrum with the most stable PBI-Arn (n = 1-3) clusters. The highest intermolecular interaction between PBI and Ar was obtained with the Ar atom positioned above the imidazolyl ring. A second Ar atom was preferably docking on the other side of the imidazolyl ring than the pyridyl ring. The subsequent addition of the third Ar atom preferred the position above the pyridyl ring. The current investigation can be useful to investigate the preferential docking of dispersion-controlled interacting partners in multifunctional aromatic side chains present in biological systems.

T and V-shaped donor-acceptor-donor molecules involving pyridoquinoxaline: large Stokes shift, environment-sensitive tunable emission and temperature-induced fluorochromism.

The intramolecular charge transfer-driven emission properties of T and V-shaped donor-acceptor-donor molecules involving a new acceptor core of pyridoquinoxaline were demonstrated. The T-shaped molecule exhibits a large Stokes shift, red emission in the solid state and remarkable viscosity and temperature-dependent tunable fluorescence including a thermally-induced single-component near white-light emission.