Low Quantum Yields of Relaxed Electron Transfer Products of Moderately Coupled Ruthenium(II)−Cobalt(III) Compounds on the Subpicosecond Laser Excitation

Abstract

Photoinduced electron-transfer reactions of [(tpy)RuII(tpy-tpy)CoIII(tpy)]5+ (tpy = 2,2‘:6‘,2‘ ‘-terpyridine and tpy-tpy = 6‘,6‘ ‘-bis(2-pyridyl)-2,2‘:4‘,4‘ ‘:2‘ ‘,2‘ ‘‘-quarterpyridyne), [(tpy)RuII(tpy-ph-tpy)CoIII(tpy)]5+ (tpy-ph-tpy = 1,4-bis[2,2‘:6‘,2‘ ‘-terpyridine-4‘-yl]benzene), and [(bpy)2RuII(tpphz)CoIII(bpy)2]5+ (bpy = 2,2‘-bipyridine, and tpphz = tetrapyrido[3,2-a:2‘,3‘-c:3‘ ‘,2‘ ‘-h::2‘ ‘‘,3‘ ‘‘-j]phenazine) were studied in the range 140−298 K by means of subpicosecond transient absorption spectroscopy. 3MLCT(Ru) of [(tpy)RuII(L−L)CoIII(tpy)]5+ (L−L: tpy-tpy and tpy-ph-tpy) underwent intramolecular electron-transfer (EET) reaction to form [2RuIII(tpy)(L−L)2CoII(tpy)]5+ in a shorter time than 10 ps. Low quantum yields of the EET products (0.53 for tpy-tpy and 0.41 for tpy-ph-tpy in butyronitrile at 298 K) measured before the fast return electron-transfer were found as in the intermolecular electron-transfer quenching of 3MLCT(Ru) by [Co(tpy)2]3+. The quantum yield of [(tpy)RuIII(tpy-tpy)CoII(tpy)]5+ (Φ) decreases to 0.38 in a slowly reorganizing solvent of propylene carbonate and that of [(tpy)RuII(tpy-ph-tpy)CoIII(tpy)]5+ to 0.21 at 180 K. The reductions in the EET yields, 1 − Φ, can be ascribed to a fast transition of the nonrelaxed EET product to the lowest triplet d−d* state of [CoIII(tpy)2] moiety during the solvent reorganization. A tunneling transition of the nonrelaxed EET products to the lowest lying d−d excited-state of [CoIII(tpy)2] moiety takes place as a hole transfer, HT, from the dπ-orbital of Ru(III) to that of Co(II) with a configuration of dπ6dσ*. An electronic coupling of dπ(Ru)−dπ(Co) estimated from the intensity of inter-valence transition of [(tpy)RuIII(L−L)RuII(tpy)]5+ (Collin, J.-P.; Laine, P.; Launay, J.-P.; Sauvage, J.-P.; Sour, A. J. Chem. Soc., Chem. Commun. 1993, 434) is large enough to carry out the HT. The weak coupling of dπ(Ru)−dπ(Co) in the case of [(bpy)2Ru(tpphz)Co(bpy)2]5+ attenuates the HT rate to bring about a high yield of EET (0.8). The possibility that energy-transfer of 3MLCT(Ru) to the Co(III) moiety via an intermediate super-exchange coupling between dπ(Ru) and dπ(Co) occurs in competition with the EET of [(tpy)RuIII(tpy-ph-tpy)CoII(tpy)]5+ is also pursued.