Excitation wavelength dependent S 1 -state decay dynamics of 2-aminopyridine and 3-aminopyridine

The decay dynamics of 2-aminopyridine and 3-aminopyridine excited to the S state is investigated using femtosecond time-resolved photoelectron imaging. The lifetime of the S state for both molecules shows a rapid decrease with the increase of the vibrational energy. It is shown that, besides intersystem crossing to the lower-lying triplet state of T , the decay to the ground state (S ) internal conversion through a conical intersection plays an increasingly important role and becomes dominant for vibrational states well above the S state origin. The comparison between 2-aminopyridine and 3-aminopyridine suggests that the intramolecular hydrogen bonding between a hydrogen atom of the NH group and the heterocyclic nitrogen atom in 2-aminopyridine effectively hinders the ring deformation at lower vibrational states which is required for the wavepacket to reach the S /S conical intersection, and therefore slows down the S to S internal conversion.