Doublet−Quartet Intersystem Crossing and Electron Transfer of Cobalt(II) Moieties in [LRuIII(L−L)CoIIL]5+ (L = 2,2‘:6‘,2‘ ‘-Terpyridine and L−L = A Bridging Ligand)

Abstract

Return electron transfer (RET) and intersystem crossing (ISC) of [(tpy)RuIII(tpy-ph-tpy)2,4CoII(tpy)]5+ (tpy = 2,2‘:6‘,2‘ ‘-terpyridine and tpy-ph-tpy = 1,4-bis[2,2‘:6‘,2‘ ‘-terpyridine-4‘-yl]benzene) and [(tpy)RuII(tpy-tpy)CoIII(tpy)]5+ (tpy-tpy = 6‘,6‘ ‘-bis(2-pyridyl)-2,2‘:4‘,4‘ ‘:2‘ ‘,2‘ ‘‘-quarter-pyridyne) produced on the subpicosecond laser excitation have been investigated in a wide temperature range. A biexponential recovery process of [(tpy)RuII(tpy-ph-tpy)CoIII(tpy)]5+ and [CoIII(t)] = [2CoII(0)](1 − [fα exp(−kαt) + fβ exp(−kβt)]) occurs immediately after the production of [2RuIII(tpy)(tpy-pph-tpy)2CoII(tpy)]5+ in the excited-state electron-transfer quenching of [(tpy)RuII(tpy-ph-tpy)CoIII(tpy)]5+. Four parameters of biexponential recovery, kα, fα, kβ, and fβ in a range of 180−300 K, are interpreted in terms of RET and ISC reactions among three states: [(tpy)2RuIII(tpy-ph-tpy)2CoII(tpy)]5+, [(tpy)2RuIII(tpy-tpy)4CoII(tpy)]5+, and [(tpy)RuII(tpy-ph-tpy)CoIII(tpy)]5+. RET of [(tpy)2RuIII(tpy-ph-tpy)2CoII(tpy)]5+ regenerating [(tpy)RuII(tpy-ph-tpy)CoIII(tpy)]5+ with kRET(D) of ∼4 × 1010 s-1 competes with doublet−quartet ISC with kQD of 0.20 × 1010 s-1 at of 0.044 eV and 2.8 × 1010 s-1 at of −0.01 eV. Meanwhile, the rate constant of the quartet−doublet ISC, kDQ (∼1.6 × 1010 s-1at of −0.01 eV), is much larger than RET of [(tpy)2RuIII(tpy-ph-tpy)4CoII(tpy)]5+. Dependences of kQD on give rise to a rough estimation of reorganizational free energy (0.03 eV). As for [(tpy)RuIItpy-tpy)CoIII(tpy)]5+, kRET(D), kDQ, and kQD are evaluated to be 2.3 × 1011 s-1, 1.5 × 1011 s-1 and 1.5 × 1011 s-1 at of −0.01 eV.