Excited-State Proton Transfer in <i>syn</i>-2-(2′-Pyridyl)pyrrole Occurs on the Nanosecond Time Scale in the Gas Phase

Abstract

Microwave and UV excitation spectra of 2-(2′-pyridyl)pyrrole (2PP) have been recorded at high resolution in the gas phase. Analyses of these data show that the <i>syn</i> conformer of 2PP is a planar molecule in both the ground (S₀) and first excited (S₁) electronic states, and that the S₁ state undergoes a relatively slow excited-state proton transfer (ESPT) reaction when excited by light, as measured by the homogeneous line broadening that is observed in its UV spectrum. Apparently, excitation of the S₁ state moves electronic charge from the pyrrole ring to the pyridine ring, but the simultaneous transfer of the proton is inhibited by an unfavorably oriented dipole under solvent-free conditions. The rate of the ESPT reaction is enhanced by more than an order of magnitude with simultaneous excitation of a 144 cm^–1 in-plane vibrational mode