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

Abstract

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.

Photochemical Fate: The First Step Determines Efficiency of H2 Formation with a Supramolecular Photocatalyst

“…[2] The Franck-Condon point must be designed in such a way that already the initial electron-transfer step proceeds directionally towards the catalytically active center. This points out the importance of specific "Franck-Condon design" to be considered along with conventional parameters such as the electron-storage capabilities of the bridging ligand, [21] rapid electron and energy transport within the supramolecular system, [13,22,23] and the stability of the catalytically active charge-separated state.…”

Photoinduced Processes within Compact Dyads Based on Triphenylpyridinium‐Functionalized Bipyridyl Complexes of Ruthenium(II)

“…These planar fully conjugated and structurally made-in-onepiece species actually exhibit electronic properties of bipartite molecules comprising a chromophoric phen subunit (part of P) and a phenazine-like electron-acceptor unit (A) in the typical case of dppz. [55][56][57][58][59] Within one of these systems, a long-lived CS state of about 1.3 ms could be obtained. [16] Also, very recently, Fukuzumi and co-workers [60] succeeded in reaching hundreds of ms timescale for CS lifetimes within a dyad based on weakly coupled Zn-porphyrin (P/D) and C 60 (A) as well as in a purely organic bipartite molecule build up of mesityl (P/D) and methyl-acridinium (A) moieties.…”

Tuning of photocatalytic activity by creating a tridentate coordination sphere for palladium

“…As typically found for hetero bimetallic ruthenium tpphz complexes, the second metal centre induces a MLCT phen → MLCT pz relaxation, which leads to a dominant population of a low-energy "dark state" located on the phenazine part, and so causes the decrease of the emission intensity and the bathochromic shift of the emission maximum. 31,44,45 Electrochemical properties Cyclic voltammetry was used to obtain the redox potentials of Ru(bmptpphz) and Ru(tpphz) in acetonitrile ( Table 2). Analogously to Ru(tpphz), Ru(bmptpphz) shows the metal based oxidation wave at 0.82 V and multiple waves assignable to ligand based reductions, which are attributed to the reduction of the phenazine part (−1.44 V) and the ruthenium bound phen part of tpphz (−2.27 V), as well as to the first and second reduction of the terminal tbbpy ligands (−1.90, −2.10, and −2.44, −2.90 V).…”