Cycloreversion Reaction of a Diarylethene Derivative at Higher Excited States Attained by Two-Color, Two-Photon Femtosecond Pulsed Excitation.

Two-color, two-pulse femtosecond pulsed excitation was applied to the elucidation of the dynamics and mechanism of cycloreversion reaction of a diarylethene derivative in the higher excited states. Transient absorption spectroscopy under one-photon visible excitation revealed that the 1B state produced by the excitation undergoes the internal conversion into the 2A state with a time constant of 200 fs. Geometrical rearrangement of the 2A state takes place concomitantly with the vibrational cooling with a time constant of 3 ps. The resultant 2A state undergoes the transition into the conical intersection point in competition with nonradiative as well as radiative deactivation into the ground state with a time constant of 12 ps. The second pulse excitation of the 2A state, especially the geometrically relaxed 2A state, led to the significant enhancement of the cycloreversion reaction through the large reaction quantum yield of ca. 50-90% in the higher excited state (Sn state), while the excitation of the 1B state, leading to the Sn' state, did not induce such enhancement. By integrating with the excitation wavelength dependence of the second pump laser pulse, we discussed the chemical reactivity of diarylethene derivatives in terms of the symmetry of the electronic states.