Ring closure reaction dynamics of diarylethene derivatives in solution

Photochromic ring closure reaction dynamics of 1,2-bis(2-methylbenzo[b] thiophen-3-yl)hexafluoro cyclopentene and its derivatives in solution has been studied by femtosecond time-resolved fluorescence. Time-resolved spontaneous fluorescence of the open isomer reveals a fast component of around 1 ps and a slow component on the order of 100 ps. Fluorescence time profiles, reaction quantum yields, and relative populations of the parallel (C_s symmetry) and antiparallel (C₂ symmetry) conformations indicate that both time components are attributable mostly to the C₂ conformer that undergoes the ring closure reaction. The fast component is assigned to the direct ring closure reaction, and the slow component is assigned to the reaction through conformation change. Time constants of the slow component for the derivatives are inversely proportional to the reaction quantum yields, suggesting that the rate of the conformational dynamics is comparable to the rate of other population relaxation processes. The relative amplitude and exact time constant of the fast component depend on the detection wavelength displaying a higher relative amplitude with shorter time constant at longer wavelengths. The results allow us to propose a conformational inhomogeneity model, in which a broad distribution of conformations of the open isomers in the ground state is projected into two minima in the excited electronic potential surface to lead to the slow and the fast reaction pathways.