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 S0−S2 transition of the open isomer with pump pulses at 288 nm, while 657 and 410 nm pulses were used to excite the S1 and S2 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 S1 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 S1 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.