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DOI: 10.1002/ijch.201900147
Generation and Electron-Transfer Reactivity of the Long-Lived
Photoexcited State of a Manganese(IV)-Oxo-ScandiumNitrate
ComplexNamita Sharma,[a] Yong-Min Lee,*[a, b] Wonwoo Nam,*[a, c]
and Shunichi Fukuzumi*[a, d]
Abstract: Photoexcitation of a manganese(IV)-oxo-scandiumnitrate
complex ([(Bn-TPEN)MnIV(O)]2+ � Sc(NO3)3) in asolvent mixture of
trifluoroethanol and acetonitrile (v/v=1 :1) resulted in generation
of the long-lived photoexcitedstate, which is detected by
nanosecond laser transientabsorption measurements. The transient
absorption maxi-mum (λmax) of the 2E excited state of
[(Bn-TPEN)MnIV(O)]2+� Sc(NO3)3 is observed at 620 nm with lifetimes
of 7.1 μs.The λmax value is blue-shifted and the lifetime
becomeslonger as compared with the previously reported values
of
λmax (640 nm) and lifetime (6.4 μs) of the 2E excited state
ofthe 1 :2 complex between [(Bn-TPEN)MnIV(O)]2+ and Sc(OTf)3
([(Bn-TPEN)Mn
IV(O)]2+ � (Sc(OTf)3)2). The electron-transfer reactivity of the
2E excited states of [(Bn-TPEN)MnIV
(O)]2+ � Sc(NO3)3 was similar to that of [(Bn-TPEN)MnIV
(O)]2+ � (Sc(OTf)3)2. The long lifetime and the high
reactivityof the 2E excited state of [(Bn-TPEN)MnIV(O)]2+ �
Sc(NO3)3provide an excellent photooxidant for oxidation of
com-pounds, which would otherwise be impossible to beoxidized.
Keywords: manganese(IV)-oxo complex · scandium ion ·
photoexcited state · electron transfer
Introduction
Calcium ion (Ca2+) is indispensable for the function
andstructural assembly of the oxygen evolving complex (OEC)
ofphotosystem II (PSII), in which Ca2+ ion acts as a cofactor
foroxygen evolution from water although Ca2+ ion is
redox-inactive.[1–5] Ca2+ ion is proposed to act as a Lewis
acid,modulating the redox reactivity of an Mn(V)-oxo and
Mn(III)-peroxo intermediates, which are involved in the
wateroxidation in the OEC in PSII.[6,7] However, the actual role
ofCa2+ ion in the catalytic oxidation of water in the OEC has yetto
be well clarified. Binding of redox-inactive metal ionsincluding
Ca2+ ion to the oxo moiety of high-valentmanganese(IV)-oxo
complexes is reported to result inenhancement of the redox
reactivity.[8–16] Photoexcitation of aMnIV-oxo complex binding two
scandium ions, [(Bn-TPEN)MnIV(O)]2+ � (Sc(OTf)3)2
(Bn-TPEN=N-benzyl-N,N’,N’-tris(2-pyridylmethyl)-1,2-diaminoethane)[12]
is also reported toresult in the formation of a long-lived
photoexcited state witha lifetime of 6.4 μs, which exhibits high
redox reactivity,capable of hydroxylating benzene with water to
producephenol.[17] Generation of such long-lived photoexcited state
ofa manganese(IV)-oxo complex provides an excellent photo-oxidant
for oxidation of organic substrates. However, there hasbeen no
report on a long-lived photoexcited state of amanganese(IV)-oxo
complex other than that of a 1 :2 complexbetween
[(Bn-TPEN)MnIV(O)]2+ and Sc(OTf)3)2 (i. e., [(Bn-TPEN)MnIV(O)]2+ �
(Sc(OTf)3)2).[18]
We report herein a long-lived photoexcited state of
[(Bn-TPEN)MnIV(O)]2+ that forms a 1 :1 complex with Sc(NO3)3,which
is found to exhibit a similar lifetime to the photoexcited
state of [(Bn-TPEN)MnIV(O)]2+� (Sc(OTf)3)2. The
electron-transfer reactivity of the long-lived photoexcited state
of [(Bn-TPEN)MnIV(O)]2+ � Sc(NO3)3 was examined by
laser-inducedtransient absorption measurements and compared with
that of[(Bn-TPEN)MnIV(O)]2+ � (Sc(OTf)3)2.
Experimental Section
Materials. Commercially available chemicals were used with-out
further purification unless otherwise indicated.
Benzenederivatives, such as benzene, toluene, ethylbenzene,
m-xylene,mesitylene, 1,2,4,5-tetramethylbenzene (durene),
naphthaleneand scandium(III) triflate (Sc(OTf)3), and scandium(III)
nitrate
[a] N. Sharma, Y.-M. Lee, W. Nam, S. FukuzumiDepartment of
Chemistry and Nano Science, Ewha WomansUniversity, Seoul 03760,
KoreaE-mail: [email protected]
[email protected]@ewha.ac.kr
[b] Y.-M. LeeResearch Institute for Basic Sciences, Ewha Womans
University,Seoul 03760, Korea
[c] W. NamState Key Laboratory for Oxo Synthesis and Selective
Oxidation,Suzhou Research Institute of LICP, Lanzhou Institute of
ChemicalPhysics (LICP), Chinese Academy of Sciences, Lanzhou
730000,China
[d] S. FukuzumiFaculty of Science and Engineering, Meijo
University, Nagoya,Aichi 468-8502, Japan
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(Sc(NO3)3) were purchased from Aldrich Chemical Co. andTokyo
Chemical Industry Co., Ltd. and used as received.Solvents were
dried according to published procedures anddistilled under Argon
prior to use.[19] Iodosylbenzene (PhIO)was prepared by literature
method.[20] MnII(CF3SO3)2 ·2CH3CNwas prepared by literature
method.[21]
N-benzyl-N,N’,N’-tris(2-pyridylmethyl)-1,2-diaminoethane (Bn-TPEN)
ligand and[(Bn-TPEN)MnII]2+ were synthesized according to the
liter-ature methods.[11,12] [(Bn-TPEN)MnIV(O)]2+ was generated
bythe reaction of [(Bn-TPEN)MnII]2+ with PhIO.[11,12] C60 used asa
reference compound for determination of the quantum yieldof the
excited state of [(Bn-TPEN)MnIV(O)]2+ � Sc(NO3)3 waspurchased from
Tokyo Chemical Industry Co., Ltd. and usedas received.
Instrumentation. Nanosecond time-resolved transient ab-sorption
measurements were performed using the laser systemprovided by
UNISOKU Co., Ltd. A mixture solution(CF3CH2OH/CH3CN v/v=1 :1) in a
quartz cell (1.0 cm ×1.0 cm) was excited by a Nd:YAG laser
(Continuum SLII-10,4–6 ns fwhm, λex = 355 nm, 80 mJ pulse� 1, 10
Hz). The ratesof electron transfer were monitored by continuous
exposure toa xenon lamp for visible region as a probe light and
aphotomultiplier tube (Hamamatsu 2949) as a detector. UV-visspectra
were recorded on Hewlett Packard 8453 diode arrayspectrophotometer
equipped with a UNISOKU ScientificInstruments Cryostat USP-203 A.
X-band electron paramag-netic resonance (EPR) spectra were taken at
77 K using aJEOL X-band spectrometer (JES-FA100). The
experimentalparameters for EPR measurements by JES-FA100 were
asfollows: microwave frequency=9.028 GHz, microwavepower=1.0 mW,
modulation amplitude=1.0 mT, modulationfrequency=100 kHz and time
constant=0.03 s.
Kinetic Measurements. All the reactions were run in a1.0 cm
quartz cuvette and followed by monitoring transientabsorption
spectral changes (excited at 355 nm) of the reactionsolutions of
[(Bn-TPEN)MnIV(O)]2+ � Sc(NO3)3 (0.50 mM) inthe presence of benzene
(100–500 mM) in CF3CH2OH/CH3CN(v/v=1 :1) at 298 K. The same
procedure was used forspectral measurements for oxidation of other
benzene deriva-tives. Second-order rate constants were determined
underpseudo-first-order conditions (i. e.,
[substrate]/[[(Bn-TPEN)MnIV(O)]2+ � Sc(NO3)3] >10) by fitting
the changes in transientabsorbance for the decay of peaks due to
the excited state of[(Bn-TPEN)MnIV(O)]2+ � Sc(NO3)3 in the
oxidation reactionsof substrates (0.10–500 mM) in CF3CH2OH/CH3CN
(v/v=1 :1) at 298 K. Electron transfer reactions of substrates,
suchas benzene, toluene, ethylbenzene, m-xylene, mesitylene
anddurene, to the 2E excited state of [(Bn-TPEN)MnIV(O)]2+ �
Sc(NO3)3 were monitored by the decay of the transientabsorption
bands at 620 nm.
Results and Discussion
Binding of Scandium Nitrate to an Mn(IV)-Oxo Complex.Scandium
ion-bound [(Bn-TPEN)MnIV(O)]2+ complex was
generated by addition of Sc(OTf)3 to a
trifluoroethanol/actonitrile (TFE/MeCN; v/v=1 :1) mixture solution
of [(Bn-TPEN)MnIV(O)]2+ as reported previously.[11,12] Addition of
upto two equiv. of Sc(OTf)3 resulted in the blue shift of
theabsorption band of [(Bn-TPEN)MnIV(O)]2+ (λmax=1020 nm)with an
isosbestic point at 900 nm to the absorption band atλmax=740
nm.[12] Further addition of Sc(OTf)3 resulted in amore blue shift
to the absorption band at 690 nm.[12] No furtherspectral change was
observed by addition of more than nineequiv. of Sc(OTf)3. Such
stepwise spectral change indicatesbinding of one and two Sc(OTf)3
molecules to [(Bn-TPEN)MnIV(O)]2+ to produce the 1 :1 (i. e.,
[(Bn-TPEN)MnIV(O)]2+ � Sc(OTf)3) and 1 :2 (i. e.,
[(Bn-TPEN)MnIV(O)]2+ � (Sc(OTf)3)2)complexes, respectively.[12]
Addition of three equiv. of tetrabutylammonium nitrate(TBANO3,
1.5 mM) to a TFE/MeCN (v/v=1 :1) mixturesolution of
[(Bn-TPEN)MnIV(O)]2+ � (Sc(OTf)3)2 results in redshift of the
absorption band from 690 nm to 720 nm as shownin Figure 1, which is
similar to that due to the 1 :1 [(Bn-TPEN)MnIV(O)]2+ � Sc(OTf)3
complex. The conversion from1 :2 [(Bn-TPEN)MnIV(O)]2+ � (Sc(OTf)3)2
complex to the 1 :1[(Bn-TPEN)MnIV(O)]2+ � Sc(NO3)3 complex by
addition ofthree equiv. of NO3� [Equation (1)] results from the
strongerbinding of NO3� than that of OTf� because of the
strongernucleophilicity of NO3� than that of OTf� .[22]
½ðBnTPENÞMnIVðOÞ�2þ� ðScðOTfÞ3Þ2 þ 3NO3� !
½ðBnTPENÞMnIVðOÞ�2þ� ScðNO3Þ3 þ ScðOTfÞ3 þ 3OTf�(1)
The formation of the 1 :1 [(Bn-TPEN)MnIV(O)]2+ � Sc(NO3)3complex
was confirmed independently by addition of Sc
Figure 1. UV-vis spectral changes showing the conversion from
[(Bn-TPEN)MnIV(O)]2+ � (Sc(OTf)3)2 (0.50 mM, black line) to
[(Bn-TPEN)MnIV(O)]2+ � Sc(NO3)3 (0.50 mM, red line) by addition of
tetrabuty-lammonium nitrate (TBANO3, 0.50 mM each addition) to a
TFE/MeCN (v/v=1 :1) solution of [(Bn-TPEN)MnIV(O)]2+ �
(Sc(OTf)3)2(0.50 mM) at 273 K.
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(NO3)3 to a TFE/MeCN (v/v=1 :1) solution of [(Bn-TPEN)MnIV(O)]2+
(Figure 2a). Addition of large excess Sc(NO3)3resulted in no
further change of the absorption spectrum. Thisindicates that
[(Bn-TPEN)MnIV(O)]2+ forms only the 1 :1complex with Sc(NO3)3
[Equation (2)].
½ðBn-TPENÞMnIVðOÞ�2þ þ ScðNO3Þ3K! �
½ðBn-TPENÞMnIVðOÞ�2þ� ScðNO3Þ3(2)
The formation constant of [(Bn-TPEN)MnIV(O)]2+ � Sc(NO3)3(K) is
given by Equation (3),
K ¼ ½1�=½½MnIVðOÞ�2þ�½ScðNO3Þ3� (3)
where [1]= [[(Bn-TPEN)MnIV(O)]2+ � Sc(NO3)3] and K is
theformation constant of 1. Equation (3) can be rewritten
byEquation (4), where [MnIV(O)]0 and [Sc(NO3)3]0 are the
initialconcentrations of [(Bn-TPEN)MnIV(O)]2+ and Sc(NO3)3,
re-spectively.
1=K ¼ ð½MnIVðOÞ�0� ½1�Þð½ScðNO3Þ3�0 � ½1�Þ=½1�
¼ ð½MnIVðOÞ�0=½1�� 1Þð½ScðNO3Þ3�0� ½1�Þ(4)
Equation (5) is derived from Equation (4), where α=
[1]/[MnIV(O)]0.
ða� 1� 1Þ� 1 ¼ Kð½ScðNO3Þ3�0� ½1�Þ (5)
A plot of (α� 1� 1)� 1 vs. [Sc(NO3)3]0� [1] is shown in Figure
2b,which exhibits a linear correlation, confirming the 1 :1complex
formation in Equation (5). The K value wasdetermined from the slope
of the linear correlation (α� 1� 1)� 1vs. [Sc(NO3)3]0� [1] to be
1.9×103 M� 1 at 298 K.
The cold-spray ionization time-of-flight mass (CSI-MS)spectrum
of [(Bn-TPEN)MnIV(O)]2+ � Sc(NO3)3 confirmedbinding of Sc(NO3)3 to
[(Bn-TPEN)MnIV(O)]2+, exhibiting ionpeaks at m/z=787.4 and 874.4,
which shift to 789.4 and 876.4when PhI18O was used to generate
[(Bn-TPEN)MnIV(18O)]2+(Figure 3). An X-band EPR spectrum of
[(Bn-TPEN)MnIV(O)]2+ � Sc(NO3)3 exhibits a signal that is
characteristic of S=3/2 MnIV as the case of [(Bn-TPEN)MnIV(O)]2+
(Figure 4).[12]
Figure 2. (a) UV-vis spectral changes observed in the formation
of[(Bn-TPEN)MnIV(O)]2+� Sc(NO3)3 upon addition of Sc(NO3)3 (0–4.0
mM) to a TFE/MeCN (v/v=1 :1) solution of [(Bn-TPEN)MnIV
(O)]2+ at 273 K. Inset shows the plot of concentration of
[(Bn-TPEN)MnIV(O)]2+ � Sc(NO3)3 produced upon addition of Sc(NO3)3
to [(Bn-TPEN)MnIV(O)]2+ in TFE/MeCN (v/v=1 :1) at 273 K vs.
initialconcentration of Sc(NO3)3. (b) Plot of (α� 1� 1)� 1 vs.
[Sc(NO3)3]0� [1],(where α= [1]/[MnIV(O)]0 and 1=
[[(Bn-TPEN)MnIV(O)]2+ � Sc(NO3)3]) to determine the formation
constant of [(Bn-TPEN)Mn
IV
(O)]2+ � Sc(NO3)3 by addition of Sc(NO3)3 (0–4.0 mM) to
[(Bn-TPEN)MnIV(O)]2+ (0.50 mM) in TFE/MeCN (v/v=1 :1) at 273 K.
Figure 3. CSI-MS spectrum of [(Bn-TPEN)MnIV(O)]2+ � Sc(NO3)3
inTFE/MeCN (v/v=1 :1) at 298 K. The peaks at m/z=787.4 and
874.4correspond to [MnIV(16O)(Bn-TPEN)(Sc(NO3)3)(NO3)]
+ (calcd. m/z=787.5) and
[MnIV(16O)(Bn-TPEN)(Sc(NO3)3)(CF3SO3)]
+ (calcd.m/z=874.5), respectively. Insets show the isotope
distributionpatterns of [(Bn-TPEN)MnIV(16O)]2+ � Sc(NO3)3 (black
lines) and[(Bn-TPEN)MnIV(18O)]2+ � Sc(NO3)3 (red lines),
respectively.
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It was recently reported that Sc(OTf)3 underwent rapidhydrolysis
by residual water in MeCN (ca. 0.5–3 mM H2O) togenerate HOTf under
the conditions used for catalyticoxidations with H2O2 and a Mn(II)
complex [Mn2(μ-O)3(tmtacn)2](PF6)2
(tmtacn=N,N’,N’’-trimethyl-1,4,7-triazacyclo-nonane).[23] It should
be noted that [(Bn-TPEN)MnIV(O)]2+�(Sc(OTf)3)2 (λmax=690 nm) is
clearly different from a HOTf-bound complex, [(Bn-TPEN)MnIV(O)]2+ �
(HOTf)2 (λmax=560 nm) in TFE/MeCN (v/v=1 :1).[24] In addition, a
CSI-MSspectrum of [(Bn-TPEN)MnIV(O)]2+ � Sc(NO3)3 (Figure 3)clearly
indicates the binding of Sc(NO3)3 rather than HNO3.
Reactivity of Photoexcited State of [(Bn-TPEN)MnIV(O)]2+ �
Sc(NO3)3. Although [(Bn-TPEN)MnIV(O)]2+ exhib-ited no long-lived
transient absorption, nanosecond laserexcitation of a deaerated
TFE/MeCN (v/v=1 :1) of [(Bn-TPEN)MnIV(O)]2+ � Sc(NO3)3 resulted in
the formation of thelong-lived excited state, which has an
absorption band atλmax=620 nm (Figure 5a). From the decay time
profile ofabsorbance at 620 nm, the lifetime of the photoexcited
state of[(Bn-TPEN)MnIV(O)]2+ � Sc(NO3)3 at 298 K was determinedto
be τ=7.1 μs (Figure 5b), which is similar to the lifetime(τ=6.4 μs)
due to the doublet 2E photoexcited state of [(Bn-TPEN)MnIV(O)]2+ �
(Sc(OTf)3)2 reported previously.[17] Thislong-lived excited state
results from the doublet 2E photo-excited state because of the spin
forbidden decay to the quartetground state as reported for
Mn4+-doped compounds.[25]
The 2E excited state was produced via an extremely
rapidintersystem crossing (ISC) process from the quartet 4E
excitedstate. The quantum yield (Φ) of the 2E excited state of
[(Bn-TPEN)MnIV(O)]2+ � Sc(NO3)3 was determined to be 0.96 usingthe
triplet excited state of C60 (3C60*: λmax=750 nm, ɛ=1.8×104 M� 1
cm� 1 in benzene; �T=0.98)[32,33] as a reference andnaphthalene
radical cation (λmax=685 nm, ɛ=6.8×103 M� 1 cm� 1)[34,35] as the
product of electron transfer formnaphthalene to the 2E excited
state of [(Bn-TPEN)MnIV(O)]2+� Sc(NO3)3 (vide infra, see Figure 6
and ExperimentalSection). First, the initial concentration of
naphthalene radical
cation produced by electron transfer from naphthalene(1.0 mM) to
the 2E excited state of [(Bn-TPEN)MnIV(O)]2+� Sc(NO3)3 at 1.0 μs
after the laser excitation was determinedto be
0.0078/6.80×103=1.15×10� 6 M (Figures 6a and 6b).Under the present
experimental conditions, no π-dimer radicacation of naphthalene was
produced in the presence ofnaphthalene (1.0 mM) in TFE/MeCN (v/v=1
:1).[35,36] Becausethe quenching ratio of the 2E excited state of
[(Bn-TPEN)MnIV(O)]2+ � Sc(NO3)3 by naphthalene (1.0 mM) was
determinedfrom the quenching constant (ketτ=5.5×109×7.1×10�
6=3.9×104 M� 1) to be 3.9×104×1.0×10� 3/(1+3.9×104×1.0×10�
3)=0.975, the concentration of the 2E excited state of
[(Bn-TPEN)MnIV(O)]2+ � Sc(NO3)3 was determined to be 1.15×10�
6/0.975=1.18×10� 6 M. On the other hand, the concen-tration of
3C60* produced at 1.0 μs after laser excitation of C60(0.20 mM) at
355 nm in benzene was determined to be 0.0308/1.80×104=1.71×10� 6 M
(Figure 6d). Because the absorbanceof C60 at 355 nm is 0.80 (Figure
6c), the incident light intensity
Figure 4. EPR spectrum of [(Bn-TPEN)MnIV(O)]2+ � Sc(NO3)3(1.0
mM) in TFE/MeCN (v/v=1 :1) recorded at 5 K.
Figure 5. (a) Transient absorption spectral changes of the 2E
excitedstate of [(Bn-TPEN)MnIV(O)]2+ � Sc(NO3)3 (0.50 mM) upon
nano-second laser excitation at 355 nm in TFE/MeCN (v/v=1 :1) at
298 K.(b) Time profile of absorbance monitored at 620 nm.
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was determined to be 1.71×10� 6/(1–10� 0.80)/0.98=2.08×10�
6einstein dm� 3. The incident light intensity absorbed by
[(Bn-TPEN)MnIV(O)]2+ � Sc(NO3)3 at 355 nm was determined to
be2.08×10� 6× (1–10� 0.39)=1.23×10� 6 einstein dm� 3, becausethe
absorbance of [(Bn-TPEN)MnIV(O)]2+ � Sc(NO3)3 at355 nm was 0.39
(Figure 6c). Finally, the quantum yield (Φ)of the 2E excited state
of [(Bn-TPEN)MnIV(O)]2+ � Sc(NO3)3was determined to be 1.18×10�
6/1.23×10� 6=0.96�0.10, inwhich the experimental error is �10%. It
was confirmed thatvirtually the same Φ value of the 2E excited
state of [(Bn-TPEN)MnIV(O)]2+ � Sc(NO3)3 was obtained using a
largerconcentration of naphthalene (2.0 mM).
When benzene was added to a deaerated TFE/MeCN (v/v=1 :1)
solution of [(Bn-TPEN)MnIV(O)]2+ � Sc(NO3)3, thedecay of absorbance
at 620 nm due to the 2E excited state of[(Bn-TPEN)MnIV(O)]2+ �
Sc(NO3)3 was accelerated by elec-tron transfer from benzene to the
2E excited state of [(Bn-TPEN)MnIV(O)]2+ � Sc(NO3)3 and the decay
rate of absorbanceat 620 nm increased with increasing concentration
of benzene.The first-order decay rate constant increased linearly
withincreasing concentration of benzene (Figure 7a). The
rateconstant of electron transfer from benzene to the 2E
excited
state of [(Bn-TPEN)MnIV(O)]2+ � Sc(NO3)3 was determinedfrom the
slope of the linear plot of kobs vs. concentration ofbenzene to be
(1.0�0.1)×106 M� 1 s� 1 at 298 K. The first-orderdecay rate
constants also increased linearly with increasingconcentration of
toluene (Figure 7b). The rate constant ofelectron transfer from
toluene to the 2E excited state of [(Bn-TPEN)MnIV(O)]2+ � Sc(NO3)3
was determined from the slopeof the linear plot of kobs vs.
concentration of toluene to be(5.1�0.4)×107 M� 1 s� 1 at 298 K.
Similarly, the rate constantsof electron transfer (ket) from other
benzene derivatives suchas ethylbenzene, m-xylene, mesitylene, and
durene were alsodetermined from the slopes of the linear plots of
kobs vs.concentrations of benzene derivatives as listed in Table
1.
The dependence of the logarithm of the rate constants ofelectron
transfer (log ket) from benzene derivatives to the 2Eexcited state
of [(Bn-TPEN)MnIV(O)]2+ � Sc(NO3)3 on the one-electron oxidation
potentials of benzene derivatives (Eox) isshown in Figure 8, where
log ket exhibits typical dependenceof the rate constant for
photoinduced electron transfer; the logket value increases with a
decrease in Eox to reach a diffusionlimited value, as expressed by
the Marcus equation of electrontransfer [Equation (6)],[26–28]
Figure 6. (a) Transient absorption spectral change of the 2E
excited state of [(Bn-TPEN)MnIV(O)]2+ � Sc(NO3)3 (0.50 mM,) in the
presence ofnaphthalene (2.0 mM, red closed circles) observed at 1.0
μs after nanosecond laser excitation at 355 nm in TFE/MeCN (v/v=1
:1) at 298 K.(b) Time profile of absorbance monitored at 685 nm due
to naphthalene radical cation produced by electron transfer from
naphthalene to the2E excited state of [(Bn-TPEN)MnIV(O)]2+ �
Sc(NO3)3 observed at 1.0 μs after nanosecond laser excitation at
355 nm in TFE/MeCN (v/v=1 :1)at 298 K. (c) UV-vis absorption
spectra of C60 (0.20 mM, black line), and [(Bn-TPEN)Mn
IV(O)]2+ � Sc(NO3)3 (0.20 mM, red line) withabsorbance at 355
nm. (d) Time profile of absorbance monitored at 750 nm due to the
triplet excited state of fullerene (3C60*) at 1.0 μs
afternanosecond laser excitation at 355 nm in argon-saturated
benzene at 298 K.
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1=ket ¼ 1=kdiff þ 1=ðZexp½ð� l=4Þð1þ DGet=lÞ2=ðkBTÞ�Þ(6)
where λ is the reorganization energy of electron transfer, kdiff
isthe diffusion rate constant, Z is the collision frequency,
whichis taken as 1011 M� 1 s� 1, kB is the Boltzmann constant, T is
theabsolute temperature, and kdiff=1.0×1010 M� 1 s� 1.[26–28]
TheGibbs energy change of electron transfer (ΔGet) is given
byEquation (7),
DGet ¼ eðEox� EredÞ (7)
where e is the elementary charge, Ered and Eox are the
one-electron reduction potential of an electron acceptor and
theone-electron oxidation potential of an electron donors. Thebest
fit line in Figure 8 gives the Ered value of the 2E excitedstate of
[(Bn-TPEN)MnIV(O)]2+ � Sc(NO3)3 ([(Bn-TPEN)MnIV(O)]2+ �
Sc(NO3)3*)=2.1(1) V with λ=0.53(4) eV. The Ered
Figure 7. Plots of kobs vs. concentration of electron donors
[(a) benzene (100–400 mM), (b) toluene (5.0–20 mM), (c)
ethylbenzene (1.0–4.0 mM), (d) m-xylene (0.1–0.4 mM), (e)
mesitylene (0.1–0.4 mM), and (f) durene (0.1–0.4 mM)] for electron
transfer from electron donors tothe 2E excited state of
(Bn-TPEN)MnIV(O)]2+ � Sc(NO3)3 (0.50 mM) in TFE/MeCN (v/v=1 :1) at
298 K. The concentration of the
2E excitedstate of [(Bn-TPEN)MnIV(O)]2+ � Sc(NO3)3 (0.50 mM) was
determined to be 1.18×10
� 3 mM from the concentration of naphthalene radicalcation
produced by electron transfer from naphthalene to the 2E excited
state of [(Bn-TPEN)MnIV(O)]2+ � Sc(NO3)3. The smallest
substrateconcentration used is 0.10 mM, which is much larger than
the concentration of the 2E excited state of [(Bn-TPEN)MnIV(O)]2+ �
Sc(NO3)3, whenthe pseudo-first-order conditions are fulfilled.
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& Co. KGaA, Weinheim www.ijc.wiley-vch.de 1054
http://www.ijc.wiley-vch.de
-
value of [(Bn-TPEN)MnIV(O)]2+ � Sc(NO3)3* is the same asthat of
[(Bn-TPEN)MnIV(O)]2+ � (Sc(OTf)3)2* (Ered vs. SCE=2.1 V), probably
because a decrease in the Ered value at theground state may be
canceled out by an increase in theexcitation energy of
[(Bn-TPEN)MnIV(O)]2+� Sc(NO3)3 ascompared with that of
[(Bn-TPEN)MnIV(O)]2+ � (Sc(OTf)3)2*.The LMCT energy of
[(Bn-TPEN)MnIV(O)]2+ ([(Bn-TPEN*+)MnIII(O)]2+) is expected to
decrease by increasing the numberof binding molecules of Sc(OTf)3
from one to two, becausethe electron accepting ability of the
MnIV(O) moiety increaseswith an increase in the number of binding
molecules of Sc(OTf)3 from one to two. The λ value of 0.53 eV is
muchsmaller than that of the ground state of MnIV(O)
complexes(λ=2.2–2.3 eV)[12,29] because of the ligand centered
electrontransfer to the ligand-to-metal charge transfer (LMCT)
excitedstate of [(Bn-TPEN)MnIV(O)]2+ � Sc(NO3)3 as compared withthe
metal-centered electron transfer to the ground state of MnIV
(O) complexes. Sc(NO3)3 is known to act as a strong Lewisacid
even in an aqueous solution.[30,31]
Conclusion
Photoexcitation of a MnIV(O) complex binding one
Sc(NO3)3molecule ([(Bn-TPEN)MnIV(O)]2+ � Sc(NO3)3) affords a
long-lived 2E excited state with a lifetime of 7.1 μs at 298 K
viaintersystem crossing from the quartet 4E excited state.
Electrontransfer from benzene and its derivatives to the
long-liveddoublet 2E excited state of [(Bn-TPEN)MnIV(O)]2+ �
Sc(NO3)3occurs and the electron-transfer driving force dependence
ofthe logarithm of the rate constants of electron transfer is
fittedby the Marcus equation for outer-sphere electron transfer
toafford a small reorganization energy of electron transfer (λ=0.53
eV), because of the ligand-centered electron transfer tothe
ligand-to-metal charge transfer (LMCT) excited state
of[(Bn-TPEN)MnIV(O)]2+ � Sc(NO3)3 as compared with
themetal-centered electron transfer to the ground state of
MnIV(O)complexes. This study provides a new photooxidant that has
along-lived photoexcited state of a redox-inactive metal ion-bound
high-valent metal-oxo complex and high oxidizingcapability.
Acknowledgement
This work was supported by NRF of Korea through
CRI(NRF-2012R1A3A2048842 to W.N), GRL (NRF-2010-00353to W.N.),
Basic Science Research Program(2017R1D1A1B03029982 to Y.M.L.
and2017R1D1A1B03032615 to S.F.), and Grants-in-Aid (no.16H02268 to
S.F.) from the Ministry of Education, Culture,Sports, Science and
Technology (MEXT).
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