Cyclization reaction of a photochromic diarylethene derivative, 1,2-bis(2-methyl-3-benzothienyl)perfluorocyclopentene (BT), in nonpolar alkane solutions with different viscosity was investigated by means of femtosecond−microsecond transient absorption spectroscopy and a time-correlated single-photon counting method. Transient absorption measurements revealed that a ring closure rapidly occurred with a time constant of 450 fs. In addition to this rapid cyclization, transient species with longer lifetimes (ca. 150 ps and ca. 1 μs) were observed. The faster time constant of 150 ps was independent of the solvent viscosity and was assigned to the fluorescence lifetime of a conformer with molecular geometry unfavorable for the ring closure. The longer component was strongly quenched in the solution purged with O2 and was attributed to the triplet state of the open-ring form. Steady-state measurement and nanosecond transient absorption spectroscopy revealed that the cyclization process did not occur via the triplet state of BT. These results indicate that only the rapid reaction taking place in subpicosecond time region was responsible for the cyclization process. The key factors regulating the cyclization reaction of diarylethene derivatives were discussed on the basis of the solvent viscosity dependence, by comparing the present results with those obtained for other diarylethene derivatives.

Temperature dependencies of cyclization and cycloreversion processes of a photochromic diarylethene derivative, 1,2-bis(2-methyl-3-benzothienyl)perfluorocyclopentene (BT), were investigated by steady-state spectroscopy and femtosecond laser photolysis methods. Steady-state measurements revealed that the cyclization reaction quantum yield and the fraction of the conformer with C 2v symmetry favorable for the cyclization (antiparallel, AP conformer) were independent of temperature in the range of 253−343 K. These results indicated that the cyclization reaction of the AP conformer in the open-ring isomer in the excited state had no apparent temperature dependence and suggested that the fate of the excited AP conformer in the open-ring isomer, such as cyclization or deactivation to the ground state, was determined at the conical intersection. On the other hand, the cycloreversion reaction was dependent on the temperature; the reaction quantum yield increased together with a decrease in the lifetime of the excited state of the closed-ring isomer with increasing temperature. On the basis of the adiabatic energy surface for the reaction profiles, it was deduced that the rapid deactivation into the ground state took place in the S 1 state in competition with the activated pathways leading to the conical intersection where the cycloreversion occurred.

Cycloreversion processes of three photochromic diarylethene derivatives with extremely low one-photon reaction yields (5.0 x 10(-5) to 1.5 x 10(-2)) were investigated by means of femtosecond and picosecond laser photolysis methods. Femtosecond visible laser photolysis revealed that the excited state of the closed form in these three derivatives decayed into the ground state with 0.7-8 ps time constants and with low cycloreversion yields that were consistent with those obtained by steady-state light irradiation. On the other hand, the cycloreversion reaction was drastically enhanced by picosecond 532 nm laser excitation for all of the three derivatives. From excitation intensity effects of the reaction yield and dynamic behavior, it was found that the successive two-photon absorption process leading to higher excited states opened an efficient cycloreversion channel, with reaction yields of 0.3-0.5. These results are discussed from the viewpoint of the one-photon inerasable but two-photon erasable photochromic system.

Solvent polarity dependence of photochromic reactions such as cyclization and cycloreversion of a photochromic diarylethene derivative, 1,2-bis­(2-methyl-3-benzothienyl)­perfluorocyclopentene, was investigated by steady-state spectroscopic and femtosecond laser photolysis methods. For the cyclization reaction, it was revealed that the quantum yield decreased with an increase in solvent polarities, mainly due to the decrease in the fraction of the conformer with C 2 symmetry favorable for the cyclization. This result indicated that the branching ratio for the cyclization and the deactivation to the open-ring isomer at the conical intersection was almost independent of the solvent polarity. On the other hand, it was found for the cycloreversion process that the closed-ring isomer in the S1 state rapidly deactivated into the ground state in competition with the activated process leading to the conical intersection providing a pathway toward both open- and closed-ring minima in the ground state. The cycloreversion reaction quantum yield also decreasing with an increase in the solvent polarity was attributed to larger increase of the direct deactivation into the ground state from the excited state minimum of the closed-ring isomer.