Near-infrared two-photon absorption and excited state dynamics of a fluorescent diarylethene derivative

Near-infrared two-photon absorption and excited state dynamics of a fluorescent diarylethene (fDAE) derivative were investigated by time-resolved absorption and fluorescence spectroscopies. Prescreening with quantum chemical calculation predicted that a derivative with methylthienyl groups (mt-fDAE) in the closed-ring isomer has a two-photon absorption cross-section larger than 1000 GM, which was experimentally verified by Z-scan measurements and excitation power dependence in transient absorption. Comparison of transient absorption spectra under one-photon and simultaneous two-photon excitation conditions revealed that the closed-ring isomer of mt-fDAE populated into higher excited states deactivates following three pathways on a timescale of ca. 200 fs: (i) the cycloreversion reaction more efficient than that by the one-photon process, (ii) internal conversion into the S₁ state, and (iii) relaxation into a lower state (S₁' state) different from the S₁ state. Time-resolved fluorescence measurements demonstrated that this S₁' state is relaxed to the S₁ state with the large emission probability. These findings obtained in the present work contribute to extension of the ON-OFF switching capability of fDAE to the biological window and application to super-resolution fluorescence imaging in a two-photon manner.