Photoinduced Processes in Dyads and Triads Containing a Ruthenium(II)-Bis(terpyridine) Photosensitizer Covalently Linked to Electron Donor and Acceptor Groups

Five supramolecuiar systems containing the Ru(ttp)22+ photosensitizer (P) covalently linked to an electron acceptor (A), MV2+, and/or an electron donor (D), PTZ or DPAA, have been synthesized; ttp is 4′-p-tolyl-2, 2′:6; 2″-terpyridine, MV2+ is methyl viologen, PTZ is phenotiazine, and DPAA is di-p-anisylamine. In the D-P-A triads the electron donor and acceptor groups are linked in opposite positions with respect to the photosensitizer. The spectroscopic properties (room-temperature absorption spectra, emission spectra and lifetimes in the 90-200 K temperature range, and transient absorption spectra and lifetimes at 150 K) and the (room-temperature) electrochemical behavior of the supramolecuiar systems and of their components have been investigated. At 90 K, where the solvent is frozen, no quenching of the photosensitizer luminescence is observed for all the supramolecuiar systems. At 150 K, where the solvent is fluid, the results obtained were as follows. In the PTZ-Ru(ttp)22+ dyad, neither quenching of the photosensitizer luminescence nor formation of oxidized donor are observed. In the DPAA-Ru(ttp)22+ dyad, luminescence quenching and transient formation of the oxidized donor take place. For the Ru(ttp)22+-MV2+ dyad, transient formation of the reduced acceptor is observed, but the lifetime of the photosensitizer luminescence increases, indicating that charge recombination leads back to the excited photosensitizer. The PTZ-Ru(ttp)22+-MV2+ triad behaves as the Ru(ttp)22+-MV2+ dyad. For the DPAA-Ru(ttp)22+-MV2+ triad, strong luminescence quenching is observed, and transient absorption spectroscopy shows that charge separation is followed by a very fast charge recombination reaction (r < 100 ns). Thermodynamic and kinetic aspects of the photoinduced electron-transfer processes are discussed.