| Abstract
| - The energy and electron transfer processes taking place in binuclear polypyridine complexes ofruthenium and osmium based on the tetrapyrido[3,2-a:2‘,3‘-c:3‘ ‘,2‘ ‘-h:2‘ ‘‘-3‘ ‘‘-j]phenazine bridging ligand(tpphz) have been investigated by ultrafast absorption spectroscopy. In the binuclear complexes, eachchromophore is characterized by two spectrally distinguishable metal-to-ligand charge transfer (MLCT)excited states: MLCT1 (with promoted electron mainly localized on the bpy-like portion of tpphz, higherenergy) and MLCT0 (with promoted electron mainly localized on the pyrazine-like portion of tpphz, lowerenergy). In the homodinuclear complexes Ru(II)−Ru(II) and Os(II)−Os(II), MLCT1 → MLCT0 relaxation(intraligand electron transfer) is observed, with strongly solvent-dependent kinetics (ca. 10-10 s in CH2Cl2,ca. 10-12 s in CH3CN). In the heterodinuclear Ru(II)−Os(II) complex, *Ru(II)−Os(II) → Ru(II)−*Os(II) energytransfer takes place by two different sequences of time-resolved processes, depending on the solvent: (a)in CH2Cl2, ruthenium-to-osmium energy transfer at the MLCT1 level followed by MLCT1 → MLCT0 relaxationin the osmium chromophore, (b) in CH3CN, MLCT1 → MLCT0 relaxation in the ruthenium chromophorefollowed by osmium-to-ruthenium metal-to-metal electron transfer. In the mixed-valence Ru(II)−Os(III)species, the *Ru(II)−Os(III) → Ru(III)−Os(II) electron transfer quenching is found to proceed by twoconsecutive steps in CH3CN: intraligand electron transfer followed by ligand-to-metal electron transfer.On a longer time scale, charge recombination leads back to the ground state. Altogether, the results showthat the tpphz bridge plays an active mechanistic role in these systems, efficiently mediating the transferprocesses with its electronic levels.
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