Ultrafast Processes in Bimetallic Dyads with Extended Aromatic Bridges. Energy and Electron Transfer Pathways in Tetrapyridophenazine-bridged Complexes

The energy and electron transfer processes taking place in binuclear polypyridine complexes of-ruthenium 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, each-chromophore 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, higher-energy) and MLCT0 (with promoted electron mainly localized on the pyrazine-like portion of tpphz, lower-energy). In the homodinuclear complexes Ru(II)-Ru(II) and Os(II)-Os(II), MLCT1 f 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) f Ru(II)-*Os(II) energy-transfer 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 f MLCT0 relaxation-in the osmium chromophore, (b) in CH3CN, MLCT1 f MLCT0 relaxation in the ruthenium chromophore-followed by osmium-to-ruthenium metal-to-metal electron transfer. In the mixed-valence Ru(II)-Os(III)-species, the *Ru(II)-Os(III) f Ru(III)-Os(II) electron transfer quenching is found to proceed by two-consecutive 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 show-that the tpphz bridge plays an active mechanistic role in these systems, efficiently mediating the transfer-processes with its electronic levels.