Non-Traditional Excited-State Deactivation in N-Containing Chromophores: A Combined Spectroscopic and Computational Study

We report the determination of the excited-state deactivation energy barrier of a hydrogen-bonded 2-(2'-pyridyl)-benzimidazole (PBI)-H₂O complex isolated in the gas phase. The combined electronic and ion-dip IR spectroscopic investigations revealed the formation of a _PBIN···H-OH hydrogen bonding interaction in the complex. Long-range R2PI and UV-UV hole-burning spectroscopy enabled the determination of the excited-state energy barrier of 893-1067 cm^-1 in the S₁ state. Detailed computational investigations revealed that the reaction proceeds via a proton migration from the solvent along the _PBIN···H-OH → _PBINH⁺···OH^- path in the S₁ excited state. The activation barrier is attributed to the excited ππ*-nπ* intersection along the path. The subsequent S₁ to S₀ internal conversion led to the reverse proton transfer forming the _PBIN···H-OH hydrogen bond in the ground state. Our findings show that N-containing chromophores can efficiently deactivate in the excited state without a protic NH group, via a solvent-catalyzed pathway. This alternative mechanism reshapes our understanding of energy dissipation in asymmetric hydrogen-bonded systems and expands the design strategies for functional chromophores.