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 <i>S</i>₁ 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