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 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 NH 2 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.