Triplet Mediated C-N Dissociation versus Internal Conversion in Electronically Excited N-Methylpyrrole.

Abstract

The photochemical and photophysical pathways operative in N-methylpyrrole, after excitation in the near part of its ultraviolet absorption spectrum, have been investigated by the combination of time-resolved total ion yield and photoelectron spectroscopies with high-level ab initio calculations. The results collected are remarkably different from the observations made for pyrrole and other aromatic systems, whose dynamics is dictated by the presence of πσ* excitations on X-H (X: N, O, S, ...) bonds. The presence of a barrier along the C-N dissociation coordinate that can not be tunneled triggers two alternative decay mechanisms for the S1 A″ πσ* state. While at low vibrational content the C-N dissociation occurs on the surface of a lower (3)ππ* state reached after efficient intersystem crossing, at higher excitation energies, the A″ πσ* directly internally converts to the ground state through a ring-twisted S1/S0 conical intersection. The findings explain previous observations on the molecule and may be relevant for more complex systems containing similar C-N bonds, such as the DNA nucleotides.