Switching dynamics of the photochromic 1,1-dicyano-2-(4-cyanophenyl)-1,8a-dihydroazulene probed by sub-30 fs spectroscopy

We report the first time resolved investigation of the ring opening dynamics of the 1,1-dicyano-2-(4-cyanophenyl)-1,8a-dihydroazulene (CN-DHA) towards its vinylheptafulvene (CN-VHF) isomer. The kinetics are measured by sub-30 fs transient absorption spectroscopy for numerous probe wavelengths from 485 to 690 nm. The ring opening takes place within 1.2 ps on the CN-DHA-VHF S₁ potential energy surface. It is followed by the internal conversion from CN-VHF-S₁ to CN-VHF-S₀ in 13 ps. We observe coherent oscillations of low frequency modes (150, 190, 330 and 500 cm⁻¹) that are closely associated with the skeleton motions driving the CN-DHA structural changes immediately after the 30 fs UV excitation.