Excitation wavelength dependent S₁-state decay dynamics of 2-aminopyridine and 3-aminopyridine

The decay dynamics of 2-aminopyridine and 3-aminopyridine excited to the S-1 state is investigated using femtosecond time-resolved photoelectron imaging. The lifetime of the S-1 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-1, the decay to the ground state (S-0) via internal conversion through a conical intersection plays an increasingly important role and becomes dominant for vibrational states well above the S-1 state origin. The comparison between 2-aminopyridine and 3-aminopyridine suggests that the intramolecular hydrogen bonding between a hydrogen atom of the NH2 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-1/S-0 conical intersection, and therefore slows down the S-1 to S-0 internal conversion.