Solvent-assisted excited state proton transfer and photoacidity of 2-hydroxypyridine. A nonadiabatic dynamics study

Spectroscopic characterization of the complexes of 2-(2'-pyridyl)-benzimidazole and (H2O)1,2, (CH3OH)1,2, and (NH3)1,2 isolated in the gas phase

The hydrogen-bonded docking preferences of small solvent molecules on 2-(2'-pyridyl)-benzimidazole (PBI) were studied experimentally aided by computational findings. The PBI-S1,2 complexes (S = H2O, CH3OH, and NH3) were produced in a supersonically jet-cooled molecular beam and probed using resonant two-photon ionization and laser-induced fluorescence spectroscopy, with multiple isomers confirmed by UV-UV hole-burning spectroscopy. Two distinct isomers of PBI-H2O and PBI-(H2O)2 complexes were identified, while PBI-CH3OH and PBI-NH3 each formed a single 1 : 1 and 1 : 2 complex. Computational results with experimental findings revealed PBI-S-a as the most stable structure, with a solvent molecule forming a hydrogen-bonded bridge between imidazolyl-NH (NIH) and pyridyl-N (NP) at site-a. The site-a isomers exhibit higher S1 state stability compared to the S0 state, resulting in red-shifted S0 → S1 band origin for PBI-S-a and a further red-shift for the PBI-(S)2-aa isomers. In contrast, the PBI-S-b isomer, with a hydrogen bond between imidazolyl-N (NI) and pyridyl-CH (CPH) at site-b opposite to site-a, showed a blue-shifted band origin transition. A unique PBI-(H2O)2-ab isomer was detected with solvent molecules bound at both sites a and b, displaying a smaller red-shift in the band origin transition than the aa-isomer. The energy barrier for solvent-to-chromophore proton transfer varies with isomeric configuration. PBI-H2O-b isomers show significantly higher barriers (>800 cm-1), while PBI-(H2O)-aa has a slightly increased barrier (>436 cm-1) compared to the PBI-H2O-a (420 ± 10 cm-1) isomer. This study explores the potential landscape of PBI, enhancing our understanding of stabilization effects, spectral shifts, and their impact on chromophore excited-state dynamics in various environments.

Preparation of Tautomer-Pure Molecular Beams by Electrostatic Deflection

Tautomers are ubiquitous throughout chemistry and typically considered inseparable in solution. Yet (bio)chemical activity is highly tautomer-specific, with common examples being the amino and nucleic acids. While tautomers exist in an equilibrium in solution, in the cold environment of a molecular beam the barrier to tautomerization is typically much too high for interconversion, and tautomers can be considered separate species. Here we demonstrate the first separation of tautomers within a molecular beam and the production of tautomerically pure gas-phase samples. We show this for the 2-pyridone/2-hydroxypyridine system, an important structural motif in both uracil and cytosine. Spatial separation of the tautomers is achieved via electrostatic deflection in strong inhomogeneous fields. We furthermore collect tautomer-resolved photoelectron spectra using femtosecond multiphoton ionization. This paves the way for studying the structure-function-dynamic relationship on the level of individual tautomers, using approaches that typically lack the resolution to do so, such as ultrafast dynamics experiments.