Ultrafast excited-state dynamics in a prototype of the peptide bond: Internal conversion of the isolated N,N-dimethylformamide

Abstract

A combination of femtosecond time-resolved photoelectron imaging technique and time-resolved mass spectroscopy technique are implemented to investigate the electronic excited-state dynamics in N,N-dimethylformamide (NNDMF). The ultrafast internal conversion (IC) of ${S}_{1}$ ($n$\ensuremath{\pi}${}^{*}$) and ${S}_{2}$ (\ensuremath{\pi}\ensuremath{\pi}${}^{*}$) excited states of NNDMF are observed in this experiment. The molecule is excited to the lowest-lying \ensuremath{\pi}\ensuremath{\pi}${}^{*}$ state (${S}_{2}$ state) following absorption of two 400-nm photons. It is found that the population of the ${S}_{2}$ (\ensuremath{\pi}\ensuremath{\pi}${}^{*}$) state undergoes ultrafast IC to the highly vibrationally excited ${S}_{1}$ ($n$\ensuremath{\pi}${}^{*}$) state within 99 fs by very fast C-N stretching, while the nonradiative deactivation of the ${S}_{1}$ ($n$\ensuremath{\pi}${}^{*}$) state occurs in 2.4 ps, and it is to a large extent due to the C-N bond cleavage from the ${S}_{1}$ potential energy surface, which would be able to efficiently compete with the IC of ${S}_{1}$ \ensuremath{\rightarrow} ${S}_{0}$ through ${S}_{1}$/${S}_{0}$ conical intersections.