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

Abstract

The hydrogen-bonded docking preferences of small solvent molecules on 2-(2′-pyridyl)-benzimidazole (PBI) were studied experimentally aided by computational findings. The PBI–S_1,2 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 (N_IH) and pyridyl-N (N_P) 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 (N_I) and pyridyl-CH (C_PH) 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.