Triplet Mediated C−N Dissociation versus Internal Conversion in Electronically Excited N‑Methylpyrrole

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 3ππ* 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 nucleotidesThis work was funded by the “Ayudas para apoyar las actividades de grupos de investigación del sistema universitario vasco” program from the Basque Government, Grant 2014SGR1202 from the Agència de Gestiód’Ajuts Universitaris i de Recerca (AGAUR) from Catalonia (Spain), CTQ2016-69363-P from the Spanish Ministerio de Economıá y Competitividad (MINECO) and UNGI10-4E-801 from the Ministerio de Ciencia e Innovación (MICINN) and the European Fund for Regional Development, and the Xarxa de Refereǹ cia en Quiḿ ica Teor̀ ica i Computacional de Catalunya (AGAUR