The interplay of photochromism and fluorescence was studied by attaching anthracene as chromophore to dithienylperfluorocyclopentene (1,2-bis[5-anthryl-2-methylthien-3-yl]perfluorocyclopentene, Ac-BMTFP). The blue fluorescence of the open isomer of Ac-BMTFP is suppressed by the ring-closure reaction. The spectroscopic properties and the reaction dynamics of this compound were characterized by measurements of the fluorescence yield and decay dynamics, and the quantum yields of the photochromic ring-closure and ring-opening reactions, as well as the spectra and time evolution of reaction intermediates. The data are analyzed in terms of a model potential and single-electron density matrices, which are calculated using the collective electronic oscillator (CEO) approach and the INDO/S semiempirical Hamiltonian. For the ringopening reaction, single-exponential decays with a time constant of 8 ( 0.5 ps were determined for the photoinduced bleaching and absorption transients. In contrast, because of the presence of reacting and nonreacting conformers, the dynamics measured for ring closure are more complex. Both conformers of the open isomer undergo a fast electronic-conformational relaxation on a time scale of ≈0.9 ps after excitation of the S 1 or S 3 state. Nonreacting conformers fluoresce with a distribution of lifetimes ranging from less than 100 ps to more than 400 ps. Reacting conformers reach a precursor state with a lifetime of 10 ps from which the ring-closure reaction takes place. The rates of the ring-opening and ring-closure reactions are determined as 9.5 × 10 9 and 6 × 10 10 s -1 , respectively. Rather than being a drawback, the presence of different conformers in the sample is argued to be a requirement for applications relying on efficient switching of the fluorescence.

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.