Femtosecond transient absorption studies on photochromism of dithienylethene derivates

The kinetics of the photochromic ring-opening reaction of benzoyl-phenyl-ethenyl substituted 1,2-bis[2-methyl-thien-3-yl]perfluorocyclopentene(bpe-BMTFP) (bpe-BMTFP) in solution was investigated using fs transient absorption spectroscopy. While resonant excitation of the S-0-S-1 transition of the closed isomer is performed with a 100 fs pump pulse, the decay of the S-1 state as well as the recovery of the S-0 state were monitored using a white-light continuum probe pulse. Within the first 500 fs after excitation bpe-BMTFP is observed to undergo fast structural relaxations into two minima of the S-1 potential-energy surface which differ significantly in their lifetimes tau(L) and tau(dis). While the long lifetime of about tau(dis) = 9 ps is assigned to the potential minimum reached by the disrotatory twist motion, the other minimum with tau(L) = 1.9 ps is ascribed to the prestate of the ring-opening process. (C) 1998 Elsevier Science B.V. All rights reserved.