Department of Chemistry and Nano Science , Ewha Womans University , Seoul 03760 , Korea.
Faculty of Science and Engineering, Meijo University, SENTAN, Japan Science and Technology Agency (JST) , Nagoya , Aichi 468-8502 , Japan.
Inorg Chem. 2018 Sep 4;57(17):10945-10952. doi: 10.1021/acs.inorgchem.8b01571. Epub 2018 Aug 22.
Photodynamics and electron-transfer reactivity of an excited state derived from an earth-abundant mononuclear cobalt-oxygen complex ground state, [(TAML)Co(O)] (1; HTAML = 3,4,8,9-tetrahydro-3,3,6,6,9,9-hexamethyl-1 H-1,4,8,11-benzotetraazo-cyclotridecane-2,5,7,10-(6 H, 11 H)tetrone), prepared by electron-transfer oxidation of Li[(TAML)Co]·3(HO) (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 = 393 nm) resulted in generation of the excited state S* (lifetime: 1.4(4) ps), detected 2 ps after laser irradiation by femtosecond laser spectroscopy. The initially formed excited state S* converted to a lower-lying excited state, S* (λ = 580 nm), with rate constant k = 7(2) × 10 s (S* → S*). S* exhibited a 0.6(1) ns lifetime and converted to the initial ground state 1 with rate constant k = 1.7(3) × 10 s (S* → 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) (OTf = triflate anion), [Fe(bpy)] (bpy = 2,2'-bipyridine), and tris(4-bromophenyl)ammoniumyl radical cation (TBPA). The electron-transfer reactivity of S* was probed by nanosecond laser photoexcitation of 1 in the presence of a series of electron donors with different one-electron oxidation potentials ( E vs SCE): benzene (2.35 V), toluene (2.20 V), m-xylene (2.02 V), and anisole (1.67 V). The excited state S* 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 values of electron donors allowed the estimation of the one-electron reduction potential of S* ( E*) as 2.1(1) V vs SCE, which is much higher than that of the ground state ( E = 0.86 V vs SCE).
通过在 5°C 的 1:1 乙腈/丙酮溶剂混合物中电子转移氧化 Li[(TAML)Co]·3(HO) (2) 制备得到的[(TAML)Co(O)] (1; HTAML = 3,4,8,9-四氢-3,3,6,6,9,9-六甲基-1 H-1,4,8,11-苯并四氮杂环十三烷-2,5,7,10-(6 H,11 H)四酮) 的激发态的光动力学和电子转移反应活性，使用飞秒和纳秒激光吸收光谱学的组合进行了研究。通过飞秒激光光谱学，在激光辐照后 2 ps 时检测到 1(λ = 393nm) 的可见光光激发生成激发态 S*(寿命：1.4(4) ps)。最初形成的激发态 S以速率常数 k = 7(2)×10 s( S→S*)转化为较低的激发态 S*(λ = 580nm)。S表现出 0.6(1) ns 的寿命，并以速率常数 k = 1.7(3)×10 s( S→1)转化为初始基态 1。当使用不同的单电子氧化剂，如 Cu(OTf)(OTf = 三氟甲磺酸根阴离子)、[Fe(bpy)] (bpy = 2,2'-联吡啶)和三(4-溴苯基)铵自由基阳离子(TBPA)，通过电子转移氧化 2 生成 1 时，观察到相同的激发态动力学。通过在一系列具有不同单电子氧化电势(E vs SCE)的电子供体存在下，用纳秒激光光激发 1，探测到 S的电子转移反应活性：苯(2.35 V)、甲苯(2.20 V)、间二甲苯(2.02 V)和苯甲醚(1.67 V)。通过纳秒激光瞬态吸收光谱学观察到激发态 S与间二甲苯和苯甲醚发生电子转移反应，分别生成间二甲苯和苯甲醚的π-二聚自由基阳离子，而对苯和甲苯没有反应性。这种取决于电子给体 E 值的差异电子转移反应性允许估计 S*(E*)的单电子还原电势为 2.1(1) V vs SCE，这远高于基态(E = 0.86 V vs SCE)的还原电势。
