TY - JOUR
T1 - Enhanced Electron-Transfer Reactivity of a Long-Lived Photoexcited State of a Cobalt-Oxygen Complex
AU - Saracini, Claudio
AU - Malik, Deesha D.
AU - Sankaralingam, Muniyandi
AU - Lee, Yong Min
AU - Nam, Wonwoo
AU - Fukuzumi, Shunichi
N1 - Publisher Copyright:
Copyright © 2018 American Chemical Society.
PY - 2018/9/4
Y1 - 2018/9/4
N2 -
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).
AB -
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).
UR - http://www.scopus.com/inward/record.url?scp=85053062452&partnerID=8YFLogxK
U2 - 10.1021/acs.inorgchem.8b01571
DO - 10.1021/acs.inorgchem.8b01571
M3 - Article
AN - SCOPUS:85053062452
SN - 0020-1669
VL - 57
SP - 10945
EP - 10952
JO - Inorganic Chemistry
JF - Inorganic Chemistry
IS - 17
ER -