Observation of Proton Transfer in 2-Aminopyridine Dimer by Electron and Mass Spectroscopy

A photoinitiated intermolecular electron−proton transfer reaction in 2-aminopyridine dimer was investigated by femtosecond pump−probe electron−ion coincidence spectroscopy and accompanying theory. Excited-state population dynamics were observed in real time by time-resolved mass spectroscopy, and the respective excited-state character of locally excited and proton/hydrogen transfer states was identified in coincident electron spectra. Two reaction channels for an ultrafast (sub-50 fs) and a slower (∼75 ps) proton/hydrogen transfer were observed and indicate that vibrational energy redistribution may lead to efficient population trapping in the excited state. Spectroscopic evidence of an unexpected hydrogen-transfer reaction in photoexcited aminopyridine monomer is also presented.