Ring-Opening and -Closure Reaction Dynamics of a Photochromic Dithienylethene Derivative

The influence of a bulky, strongly coupled substituent (benzoyl−phenyl−ethenyl, BPE) on the dynamics of the photoinduced ring-closure and ring-opening reactions of 1,2-bis[[2−methyl−thien−3−yl]]perfluorocyclopentene (BMTFP) in solution was investigated. UV/vis absorption spectroscopy in combination with chemical actinometry was employed to determine the reaction quantum yields. The relaxation and reaction dynamics were studied by transient absorption spectroscopy, exciting the S 0 −S 2 transition of the open isomer with pump pulses at 288 nm, while 657 and 410 nm pulses were used to excite the S 1 and S 2 states of the closed isomer, respectively. Transient absorption spectra in the range of 350−950 nm were recorded using temporally delayed, white-light continuum probe pulses. After structural relaxation in the S 1 excited state, the ring-closure reaction takes place with high quantum yield through a conical intersection. This conical intersection acts also as relaxation funnel for the S 1 excited state of the closed isomer, from which it is separated by an energy barrier. The branching in the conical intersection favors the closed isomer, so that the ring-opening reaction yield is small (10 -3 −10 -4 ). This yield was found to vary by a factor of 6 for different isomers obtained by rotation around the ethylene bond of the substituent.