Exploring the ultrafast dynamics of a diarylethene derivative using sub-10 fs laser pulses.

A diarylethene derivative, 1,2-bis(2,4-dimethyl-5-phenyl-3-thienyl)perfluorocyclopentene (DMP), is a photoswitch molecule utilizing a reversible aromatic ring-opening reaction. The quantum yield of the ring-opening reaction is however remarkably low. We investigate the origin of this behaviour by means of ultrafast transient absorption spectroscopy utilizing sub-10 fs pulses, which is an invaluable tool for simultaneously studying both the electronic and the vibrational molecular dynamics. Namely, a noncollinear optical parametric amplifier (NOPA) generating sub-10 fs pulses in the spectral range 605-750 nm is employed. The transient absorption signal is modulated by several vibrational modes, which are compared with experimental and computational Raman spectra and then assigned to the ground or excited electronic state. We observe that the most pronounced vibrational mode - the ethylenic stretching mode at a frequency of 1501 cm-1 - exhibits instantaneous frequency and amplitude modulation. The observed modulations occur due to weak coupling with another 1431 cm-1 stretching mode mediated by a vibrational mode of low frequency, i.e. around 60 cm-1. Fast internal conversion S1 → S0 originates in a relaxation through a conical intersection (found by density-functional theory computations), facilitated by the two aforementioned stretching modes.