Optically controlled molecular switching of an indolobenzoxazine-type photochromic compound

Photochromic forward (oxazine ring-opening) and backward (oxazine ring-closing) switching dynamics of an indolobenzoxazine compound were studied by femtosecond pump-repump-probe technique. A UV pulse was used to excite the ring-closed form of the photochromic compound, causing a C-O bond cleavage and the formation of a spectrally red-shifted isomer within a time scale of ca. 100 ps. A successive, temporally delayed near-IR pulse, resonant to the red-most absorption maximum of the ring-opened form, was used to reexcite the molecular system, causing a fast photoinduced oxazine ring closure, thereby "short-circuiting" the normally nanosecond lasting photocycle and returning ∼6% of the molecules to the main molecular ground state. Two possible models, involving the S1 excited state of the terminal photoproduct and its hot ground state, are introduced to explain the pre- and post-reexcitation spectral development and the photoinduced switching back mechanics.