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

• Published in: Journal of the American Chemical Society Vol. 125; no. 2; pp. 483 - 491
• Main Authors: Chiorboli, Claudio, Rodgers, Michael A. J, Scandola, Franco
• Format: Journal Article
• Language: English
• Published: Washington, DC American Chemical Society 15.01.2003
• Subjects: Condensed matter: electronic structure, electrical, magnetic, and optical properties; Exact sciences and technology; Optical properties and condensed-matter spectroscopy and other interactions of matter with particles and radiation; Other solid inorganic materials; Photoluminescence; Physics; Cationic complex; Nitrogen heterocycle; Organic ligand; Experimental study; Intramolecular energy transfer; Intramolecular charge transfer; Transients; Transition elements Complexes; Osmium Ruthenium Complexes; Complexes Mixed; Heteronuclear complex; Luminescence quenching; Dinuclear complex; Luminescence spectrometry; Platinoid Complexes; Ultraviolet visible spectrometry
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/ja0284916

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 → 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) 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 → MLCT0 relaxation in the osmium chromophore, (b) in CH3CN, MLCT1 → 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) → 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.