Cycloreversion Reaction of a Fulgide Derivative in Higher Excited States Attained by Femtosecond Two-Photon Pulsed Excitation

The mechanism of the cycloreversion reaction of a fulgide derivative in the higher excited states (Sn states) pumped by the stepwise two-photon excitation was investigated by femtosecond transient absorption spectroscopy. The first photon excitation of the ground state by the visible light led to the production of the Franck–Condon 1B state, which underwent internal conversion to the 2A state with 100 fs time constant and the structural reorganization accompanied with the vibrational cooling with 1.5 ps time constant. After these relaxation processes, the 2A state underwent the cycloreversion reaction (11% reaction yield) with competitive nonradiative as well as radiative deactivation processes into the ground state with a time constant of 10 ps. Under the double-pulse pump condition, whereas the excitation of the 1B state induced no remarkable cycloreversion reaction, the excitation of the 2A state resulted in the marked enhancement of the cycloreversion reaction with a reaction quantum yield of ca. 80% ...