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

Claudio Saracini, Deesha D. Malik, Muniyandi Sankaralingam, Yong Min Lee, Wonwoo Nam, Shunichi Fukuzumi

Research output: Contribution to journalArticlepeer-review

8 Scopus citations

Abstract

Photodynamics and electron-transfer reactivity of an 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 ]·3(H 2 O) (2) in a 1:1 acetonitrile/acetone solvent mixture at 5 °C, were investigated using a combination of femtosecond and nanosecond laser absorption spectroscopy. Visible light photoexcitation of 1 (λ 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 ∗ (λ max = 580 nm), with rate constant k c = 7(2) × 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) × 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 •+ ). 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 π-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).

Original languageEnglish
Pages (from-to)10945-10952
Number of pages8
JournalInorganic Chemistry
Volume57
Issue number17
DOIs
StatePublished - 4 Sep 2018

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