Enhanced Electron-Transfer Reactivity of a Long-Lived Photoexcited State of a Cobalt-Oxygen Complex

Abstract

Photodynamics and electron-transfer reactivity of an r excited state derived from an earth-abundant mononuclear cobalt-oxygen complex ground state, [(TAML)Co-IV(O)](2-) (1; H(4)TAML = 3,4,8,9-tetrahydro-3,3,6,6,9,9-hexamethyl-1H-1,4,8,11-benzotetraazo-cyclotridecane-2,5,7,10-(6H, 11H)tetrone), prepared by electron-transfer oxidation of Li[(TAML)Co-III]center dot 3(H2O) (2) in a 1:1 acetonitrile/acetone solvent mixture at 5 degrees C, were investigated using a combination of femtosecond and nanosecond laser absorption spectroscopy. Visible light photoexcitation of 1 lambda(exc) = 393 nm) resulted in generation of the excited state S-2* (lifetime: 1.4(4) ps), detected 2 ps after laser irradiation by femtosecond laser spectroscopy. The initially formed excited state S-2* converted to a lower-lying excited state, S-1* (lambda(max) = 580 nm), with rate constant k(c) = 7(2) X 10(11) s(-1) (S-2* -> S-1*). S-1* exhibited a 0.6(1) ns lifetime and converted to the initial ground state 1 with rate constant k(d) = 1.7(3) X 10(9) s(-1) (S-1* -> 1). The same excited state dynamics was observed when 1 was generated by electron-transfer oxidation of 2 using different one-electron oxidants such as Cu(OTf)(2) (OTf (-) = triflate anion), [Fe(bpy)(3)](3+) (bpy = 2,2'-bipyridine), and tris(4-bromophenyl)ammoniumyl radical cation (TBPA(center dot+)). The electron-transfer reactivity of S-1* was probed by nanosecond laser photoexcitation of 1 in the presence of a series of electron donors with different one-electron oxidation potentials (E-ox vs SCE): benzene (2.35 V), toluene (2.20 V), m- xylene (2.02 V), and anisole (1.67 V). The excited state S-1* engaged in electron-transfer reactions with m-xylene and anisole to generate pi-dimer radical cations of m-xylene and anisole, respectively, observed by nanosecond laser transient absorption spectroscopy, whereas no reactivity was observed toward benzene and toluene. Such differential electron-transfer reactivity depending on the E-ox values of electron donors allowed the estimation of the one-electron reduction potential of S-1* (E-red*) as 2.1(1) V vs SCE, which is much higher than that of the ground state (E-red = 0.86 V vs SCE).