The involvement of triplet receiver states in the ultrafast excited state processes of small esters

The photoinduced processes of methyl formate and methyl acetate have been probed by femtosecond time-resolved mass spectrometry and photoelectron spectroscopy experiments supported by quantum chemical calculations. Upon excitation to a vibrationally hot S₁ state both molecules were found to relax away from the Franck-Condon geometry faster than the cross-correlation (≈170 fs). During relaxation of the S₁ surface intersystem crossing to the triplet manifold is possible via the T₂ state which enables an El-Sayed allowed transition. The time-resolved photoelectron spectra show indications of a triplet state formed on an ultrafast timescale. We suggest that the ultrafast timescale of the intersystem crossing process is possible due to the shape of the potential energy surface directing and restricting the dynamics along the carbonyl group de-planarization coordinate, where an El-Sayed allowed intersystem crossing is possible at all times, and where the S₁ and T₂ states are nearly iso-energetic all the way along the reaction coordinate.