Fluorenylporphyrins functionalized by electrochromic ruthenium units as redox-triggered fluorescence switches

Two dyads containing tris-and tetrakis-meso-fluorenyl-substituted porphyrin and ethynylruthenium units, 1 and 2, were investigated by emission spectro-electrochemical (SEC) methods for their potential use as fluorescence switches. The ruthenium group as a potential electron donor and the porphyrin as a potential electron acceptor are connected by a phenylene bridge in 1 and by a fluorenylene bridge in 2. The new fluorenyl-linked dyad 2 was probed by UV-visible, near-infrared (NIR) and infrared (IR) absorption SEC methods, and the properties interpreted with the aid of hybrid-DFT computations, for comparison with reported data for 1. The porphyrin-based fluorescence of 1 decreased in intensity upon oxidation to 1 + and decreased further on oxidising 1 + to 1 2+. Negligible change in the fluorescence intensity of 2 was observed upon oxidation to 2 + but the intensity decreased upon subsequent oxidation of 2 + to 2 2+. These findings contrast with data reported for some other porphyrins appended with redox-active ruthenium or iron units, where fluorescence intensities increase upon oxidation of the peripheral metal centers, but they match data reported more recently for closely related arrays. A rationale for these apparently contrasting observations is proposed. A c c e p t e d m a n u s c r i p t