Marcus theory and its application tochemical reactions

Literature seminar

M1, Takeshi Inoue, 2023/7/20

Contents• Introduction
Ø Theory and experiment• Application to chemical reaction
Ø Catalytic oxidation of benzene to phenol•

Summary2

Contents• Introduction
Ø Theory and experiment• Application to chemical reaction
Ø Catalytic oxidation of benzene to phenol•

Summary3

Kinetics of thermal chemical reactions[Reaction coordinate diagram]
EnergyEnergyR

Potential energy
surfaceP𝐸!
Δ𝐺 #Reaction coordinate

Reaction coordinate(Arrhenius equation)𝐸!
𝑘 = 𝐴 $ 𝑒𝑥𝑝 −𝑘" 𝑇• Intuitive speculation
More exergonic
Reaction proceeds faster
More endergonic
Reaction proceeds slower (?)

4animal cryptochromes.
Recently23, we have combined ab initio quantum chemistry andclassical all-atom molecular dynamics (MD) simulations to investigate the primary photoreaction of plant cryptochrome that triggers
radical pair formation, denoted in Fig. 1 as electron transfer I. We[Electron transfer in the cell]demonstrated that a radical pair [FAD N2 1 W400(H)N1] involving

Single electron transfer
Article Recommendations[Photoredox catalysts]

red
nes
uct
cal
−H
toby
cal
FlavinO

N Shown
Meis theFigure 1 | Electron and proton transfers in cryptochrome.
HN
flavin cofactor, the tryptophan triad (W400, W377, W324) and the D396
N
N
MeO
residue forming the active site of cryptochrome-1 from Arabidopsis
Rthaliana. Electron (red) and proton (green) transfer processes that likely
govern cryptochrome activation are presented by arrows. The three
N1electron transfers
W400 Rflavin,, W377
R W400 N1, and W324 R W377
cells
chemoluminescence
SET is also the important process in photosynthesis, solar···are labeled I, II, and III, respectively. Electron transfer following flavin→ Marcus theory describes kinetics of theseelectron transfer.
photo-excitation leads to a radical pair state of cryptochrome: transfer I to
[FAD N2 1 W400N1Nicewicz] with the averaged
pair separationof 7.4Å, sequential
et al.N1Chem.
Rev. 2022, 122, 1925.
N2transfer I 1 II to [FAD 1 W377 ] with pair separation of 12.2 Å,rmations involving Nitrogened Radicals
rmations involving Phosphorus-,

1980

electron (latter twolarger dis
In the
W400N1electron
Arabidoptransfer I
the transf
takes placstabilized
questionfer II canin cryptoenergetica

Results
Model focryptochthat surro
In particu
aspartic a
Fig. 1, sta
flavin pr
deprotontransfer,tion. D39
state andthe prot5
structure

Inner-sphere and outer-sphere machanism
ACS Catalysis

Review

3. ]Potential-energy surfaces[Outer-spherefor electron transfers
follow two
llow two possible regimes:
nonadiabatic mechanismand Figure
adiabatic.
mechanism[Inner-sphere] possible regimes:

adiabatic
ling termn by thegure 3).
another,• Adiabatic process• Non-adiabatic process−13
theBoltzmann
two possible
regimes,
and adiabatic
nuclears).Among
kB is theconstant,ℏADisnonadiabatic• vibration,
HDA > 0 ∼ 10
• H
=0nuclear v
electron
it is temperature.
the value of the electronic coupling term
the reducthe reduced Planck constant,
T is transfers,
the absolute
originallydeveloped
explain
thestates,
outer-spherebetweenreactant(R) ofandproduct
given by the
As a coAs a Marcus
corollary theoryof eq 1, was
one
can
plot
a function
logto
kET
with (P)(non-adiabatic)
electrontransfer.
that diﬀerentiates
them (Figure 3).
resonance
integral
V,30,37deﬁnes
ΔG0, thu
ΔG0, thus
producing a concave
curve.
This curvethree
6
Paixaõ
et al. ACS. 2016, 6, 1389.
an electrontransferredif frommolecule
to Catal
another,domains:log kETis increases
ΔG0onedomains: (i) ΔG0 > − λ,AswhereReorganization energy[Self-exchange reactions (with isotopic labelling)][*FeII(H2O)6]2++[*FeIII(H2O)6]3+[FeIII(H2O)6]3++[FeII(H2O)6]2+∗; Isotopic labelling[Reorganization of structure][Reorganization of solvents]

EnergyFe3+ + Fe2+
SET

Fe2+ + Fe3+Fe2+

Structural
relaxation

+

Fe3+

Fe3+

Fe3+ + Fe2+

+

Fe2+

solventδ+δ-

Reorganization energy (𝝀); Energy needed for a vertical electron transition
without changes in the nuclear configuration.

𝝀 = 𝝀𝒊𝒏𝒏𝒆𝒓 (structure) +𝝀𝒐𝒖𝒕𝒆𝒓 (solvent)7

Marcus theory[Libbyʼs model][Marcusʼs model]

Δ𝐺 ‡

λ

λ

λ
= Δ𝐺 ‡
4
RP• Requires a great deal of energy.
→ Contradictory to the fact that thisreaction proceeds even in the dark.

RP• Electron transfer occurs at theintersection of the two potential
energy surface.

8Effects of reorganization energy[Self exchange reactions][*FeII(H2O)6]2+ + [FeIII(H2O)6]3+[*FeIII(H2O)6]3+ + [FeII(H2O)6]2+
λ[*RuII(bpy)3]2+ + [RuIII(bpy)3]3+[*RuIII(bpy)3]3+ + [RuII(bpy)3]2+

λ= Δ𝐺 ‡
4
R

P

Parameters[FeIII/II(OH2)6]3+/2+[RuIII/II(bpy)3]3+/2+

radii (+2), Å

3.52

5.9

radii (+3), Å

3.39

5.9

𝝀𝒊𝒏𝒏𝒆𝒓 (eV)

1.25

0

𝝀𝒐𝒖𝒕𝒆𝒓 (eV)

1.16

0.67

Δ𝑮𝟎 (eV)00

Δ𝑮‡ (eV)

0.60

0.27

𝐤 𝐄𝐓 (s-1)

𝟓𝟎

𝟒. 𝟗×𝟏𝟎𝟖

The reorganization energy has a signiﬁcant eﬀect on these electron transfers.

9Marcus inverted region(i)Normal region−𝜟𝑮𝒐 < 𝝀,R(ii) Barrierless (Top region)

𝜟𝑮‡ > 𝟎−𝜟𝑮𝒐 = 𝝀,𝜟𝑮‡ = 𝟎(iii) Inverted region−𝜟𝑮𝒐 > 𝝀,𝜟𝑮‡ > 𝟎

P

Δ𝐺 ‡
−Δ𝐺 #

Δ𝐺 ‡
−Δ𝐺 # = λ−Δ𝐺 ##Δ𝐺 + λ
Δ𝐺 =
4λ
‡

,

Bell-shaped 𝑣 − Δ𝐺 # graph,
𝐻/0

Δ𝐺 # + λ ,𝑘-. =$$ exp −
ħ4λ𝑘" 𝑇
4πλ𝑘" 𝑇
2π

10

Experimental observation of inverted region
logkET
Marcus theory

Diffusion limit
Observed rate

Highly exergonic, diﬃcult to achieve.

λΔG0• Inverted region canʼt be observed to the extent that 𝒌𝑬𝑻 is bigger than 𝒌𝒅𝒊𝒇 .

• Observation of intramolecular single electron transfer.
• Observation of back electron transfer in radical ion pair.
• Suppress the diffusion with highly viscous solvent.

11

First observation of Marcus inverted region• Intramolecular electron transfer
Intramolecularelectron transfer

A

A

Acceptor group (A)−Δ𝑮𝟎 (eV)𝒌𝒊𝒏𝒕𝒓𝒂 (s-1)

4-biphenyl

0

5.6×10-2-naphthyl

0.05(1.5 ± 0.5)×10.

9-phenanthryl

0.16(1.25 ± 0.2)×10/

1-pyrenyl

0.52(1.5 ± 0.5)×100

Hexahydronaphthoquinon-2-yl

1.23> 2×100

2-naphthoquinonyl

1.93(3.8 ± 1)×101

2-benzoquinonyl

2.10(2.5 ± 0.3)×101

5-chlorobenzoquinon-5-yl

2.29(1.7 ± 0.2)×101

5,6-dichlorobenzoquinon-2-yl

2.40(7 ± 3)×10/
Solvent ; 2-Me-THF

Millar et al. J. Am. Chem. Soc. 1984, 106, 3047.

12

Photocatalyst withlong-lived charge-separated state
Figure 1. (a) Schematic diagram of photoinduced electron transfer of an electron donor−acceptor (D−A) dyad. (b) Driving fo[Fukuzumiʼs catalyst]

kET with diﬀerent λ values (for the meaning of λ, see text).

[Artificial photosynthesis]𝑘-. = 2.4×10788 𝑠 78𝑘"-. = 98 𝑠 78

PhCN, 298K (intramolecular ET)

Lemmetyinen et al. J. Am. Chem. Soc. 2004, 126, 1600.

• Rigid frameworks such as Mes-Acr+ andfullerene minimize the reorganizationenergy.

(a) 5(b) 1-4
(c) 6
(a)

2014
Chem.
Res
, 47, 1455.
Figure 2. Electron donor−acceptor dyads of Suenobu
porphyrins, 2etandal.
6; Acc.
chlorines,
3, 4, and
5;.anda bacteriochlorin
1 with the CS
16−19
13Marcus plots for (a) 5, (b) 1−4, and (c) 6.

Contents• Introduction
Ø Theory and experiment• Application to chemical reaction
Ø Catalytic oxidation of benzene to phenol•

Summary

14

Synthesis of phenol from benzene[Conventional method (Cumene process)]
O2
H+

O

Δ

H+

[Inorganic photocatalyst]
TiO2 or ZnOhν

O2 or H2O

Low yield. (~5%)
High energy is required.

OH

OH

Byproducts
OH

OH
HO

O
,

O

,

CO2

Low yield by overoxidationof phenol.

Selective photocatalytic synthesis of phenol from benzene is highly valuable.
Fukuzumi et al. J. Am. Chem. Soc. 2013, 135, 5368.

15

In

containing benzene (30 mM) and H2O (0.5 M) using a xenon
lamp (500 W) with a color glass ﬁlter (λ = 390−600 nm) to
produce phenol and DDQH2 (eq 1), as shown in Figure 1. The
yield of phenol was 99%, with 99% conversion (>99%
selectivity).
[Photooxygenation by DDQ]

40

Single electron oxidation by excited DDQ
20
0
1

2

[GC-MS]

3
Time, min

4

5

(b)

DDQ
1.3 eqOH

100

Blue; Reaction w/ H216O
Red; Reaction w/ H218O

quant

Intensity

80

MeCN, H2O (0.5M)
r.t., 390~600 nm

16OH

60

18OH

40

The source of oxygen
atom is H2O.

20
0
90

92

94
96
m/z

98

100

Figure S2. (a) GC-MS chart of reaction mixture after photoirradiation and (b) GC-MS
spectra of phenol produced by photooxygenation of benzene with DDQ in nitrogensaturated MeCN at 298K: [benzene] = 10 mM; [DDQ] = 5.0 mM; [H2O] = 1.0 M[(blue) H216O; (b) H218O (98%, red)].

Eox=2.48 V
O
Cl

CN

Cl

CN
O

1

Figure 1. (a) H NMR spectral changes in the photochemical
oxygenation of benzene (30 mM) with DDQ (40 mM) and D2O (0.5
M) in deaerated CD3CN (0.6 mL) at 298 K. (b) Reaction time
proﬁles of benzene and phenol.

With the addition of TBN, which oxidizes DDQH2 to DDQ
in the presence of O (Figure S1 in the Supporting

Ered=0.51 V vs SCE

S3

*

O
Cl

CN

Cl

CN

H 2O

OH

O

+

+

3DDQ*

DDQ

DDQH2

Ered≒+3.18 V

Fukuzumi et al. J. Am. Chem. Soc. 2013, 135, 5368.
König et al. Chem. Eur. J. 2017, 23, 18161.

16

Catalytic cycle[Proposed mechanism][Reaction with TBN]
DDQ

t-BuONO (TBN)

0.3 eq , Chemical
0.05 eq
Journal of the American
SocietyO2-saturated MeCN, H2O (0.5 M),
390~600 nm

Communication

OH

TBN

DDQ (3DDQ*) was determined from the decay of the T−T
y. 93%
absorption band (Figure 3b) to be 2.4 μs. The transientabsorption spectrum of an MeCN solution containing benzene
and DDQ exhibited the formation of benzene π-dimer radical
cation (λmax = 900 nm)13,24 and DDQ•− (λmax = 595 nm),18
where benzene π-dimer radical cation was formed by thereaction of benzene radical cation with the large excess ofneutral benzene (Figure 3a, red line). Thus, the photooxygenation of benzene to phenol is initiated by intermolecular
photoinduced electron transfer from benzene to 3DDQ*,
because the free energy change for electron transfer is negative(ΔGet = −0.70 eV), as determined from the one-electronoxidation potential of benzene (Eox = 2.48 V vs SCE)19 and theone-electron reduction potential of 3DDQ* (Ered = 3.18 V vsCatalytic cyclewith TBN;
SCE).25 The decay rate constant(kobs) of 3DDQ*
increased
Hu et al. Adv. Synth. Catal. 2011, 353, 3031.
with increasing concentration of benzene (Figure 3b). The
1
Figure 2. (a) H NMR spectral changes in the photochemicalelectron-transfer rate constant of 3DDQ* with benzene (ket)oxygenation of benzene (30 mM) with DDQ (9.0 mM), D2O (0.5 M),
was determined from the slope of the plot of kobs versus [C6H6]
and TBN (1.5 mM) in O2-saturated CD3CN (0.6 mL) at 298 K. (b)(Figure3b inset)to be(5.3 ± 0.3)
× 109 M−1 s−1.
Phenolwasobtained
in
93%
yield
without
overoxidation.
•
Reaction time proﬁles of benzene and phenol.
DDQ•− and the benzene π-dimer radical cation disappeared
via back electron transfer to the ground state in dry MeCN.
17

rate
rateconstantconstantfor
forthethereaction
reactionof
benzeneradical
radicalcation
cationororthe
theπ-π2 ofbenzene
resolvedtransientabsorption
experiments.
The triplet−me-resolved
transientabsorption
experiments.
The
triplet−
On
the
basis
of
above-mentioned
results,thecatalytic
water.
water.
dimer
dimer
radicalcationwith
withHH2O
wasdetermined
determined
from
the
slope
slope
On
theradicalbasiscationof above-mentioned
results,from
thethecatalytic
2Owas
tipletabsorption
spectrumofDDQ
wasobserved
at
630
nm
in
absorption
spectrum ofofofoxygenation
DDQ
was observed
atwas
630
nmmechanism
in byby mechanism
is summarizedin [HScheme1.beBenzeneThe
Themechanism
mechanism
oxygenation
ofofbenzene
benzenewasexaminedexamined
ofofkkobsobsversus
versus[H2O]
tobekScheme
kHH2O2O==radical
(5.5
(5.5
±cation0.3)
0.3)××10
107 7MM−1−1cations−1
s−1. .
is summarizedin
1. ±Benzene
radical
2O]to
ated
MeCN
by
nanosecond
laser
ﬂash
photolysis(λ=exeaerated time-resolved
MeCN
by nanosecond
laser ﬂash
photolysis(λextriplet−=time-resolved
transienttransient absorption
absorption
experiments.
experiments.
The
The
triplet−On
On thethe basis
basis ofof above-mentioned
above-mentioned results,
results, thethe catalytic
catalyticnm) (Figure 3a). The lifetime of the triplet excited state of
𝑘𝑒𝑡 = 𝟓. 𝟑 ± 𝟎. 𝟑 ×𝟏𝟎𝟗 𝑀34𝑠 34
Schemetriplettripletabsorption
absorption
spectrumofof]ofthe
DDQ
DDQ
was
wasobserved
observedstate
atat630630
nminin1
30 nm)[Transient(Figure3a). absorption
The spectrum
lifetime
tripletexcitedofnm
mechanismmechanism
isissummarizedsummarizedininScheme
Scheme1.1.Benzene
Benzeneradical
radicalcation
cationScheme1deaerateddeaeratedMeCN
MeCNbybynanosecond
nanosecondlaser
laserﬂash
ﬂashphotolysis
photolysis(λ(λexex==
430
430nm)nm)(Figure(Figure3a).
3a).The
Thelifetime
lifetimeofofthe
thetriplettripletDDQ
excitedexcitedstate
stateofof
Black;
SchemeScheme11

Measurement of rate constants

3 DDQ＊

Red; DDQ+benzene[Decay time proﬁle]at 630nm

e 3. (a) Transient absorption spectra of DDQ (1.2 mM) in thece (black) and presence (red) of benzene (1.0 M) in deaerated

𝑘5!6 =
at 900nm
𝟓. 𝟓 ± 𝟎. 𝟑 ×𝟏𝟎𝟕
O
Cl
formed by photoinduced electron transfer
reactswithCNwater to 𝑀34𝑠 34
-1
k ’
•−
yield the OH-adduct radical.13 On theother hand,
DDQ
can
Cl
CN
O
react with the
OH-adduct
radical to form
phenoland
DDQH
formedby byphotoinduced
electrontransfer
reacts
withwaterwatertototo
2. with
OH
formedformedbyphotoinduced
photoinduced
electronelectrontransfertransfer
reactsreactswithwater
13 with TBN and O•−
DDQH2 is yield
knowntheto OH-adductbe oxidized byradical.
reaction
•−
•−
2
other
hand,
DDQ
can
yield
yieldthetheOH-adduct
OH-adduct
radical.
radical.1313On
On
Onthe
the
theotherotherhand,
hand,
DDQ
DDQ
can
can
21
via NO2 toreact
regenerate
DDQ.
withwith
thetheOH-adduct
radicalphenolandDDQH
DDQH
.
react
react
with
the
OH-adduct
OH-adduct
radicalradicalto
totoform
form
formphenol
phenoland
and
DDQH
2.2.2
The quantum
yields(Φ)
forformationofphenol
under
:
𝟏𝟎
34
34
𝑘89 by=𝟏.reaction
𝟐 ± 𝟎.with
𝟐
×𝟏𝟎
𝑠O2O
DDQH
totobeoxidized
with
TBN𝑀
and
DDQH
DDQH
known
tobebeoxidizedoxidized
bybyreaction
reaction
with
TBN
TBN
and
and
O
2 is2known
2isisknown
22
photoirradiation withmonochromatizedlight
2121 (λ = 420 nm)
21
via
viaNO
NO
toregenerate
regenerate
DDQ.
DDQ.
via NO
to2 2toregenerate
DDQ.
were determined by2 GC−MS
from the phenol peak using anThequantumquantumyieldsyields(Φ)(Φ)forfor
forformation
formationformationat
ofofof phenol
phenolunder
TheThequantumyields(Φ)
under
Oxidationofon
phenoloccursrateunder
actinometer. The
dependenceof Φ
the concentrationof aphenol
photoirradiation
photoirradiation
with
with
monochromatizedmonochromatizedlight
light (λ(λ(λ ===420420nm)
photoirradiation
monochromatizedlight
420nm)
nm)
greaterthanthatbenzene.
H2O is shownin Figure
4. withThevalueof Φofincreases
with
18
were
were
determined
determined
bybyGC−MS
GC−MSfromfromfromthethethephenol
phenolpeak
peakusing
using
determined
phenolpeakusingananan
increasing wereconcentrationof HbyO GC−MS
until it reaches
a constant
et

ket’

ntrast to the case of benzene, where the transienttion band at 900 nm due to benzene π-dimerBack
radical electron transfer•−
and that at 595 nm due to DDQ were clearly observede 3a), no transient absorption due to phenol π-dimer
cation[Transientor DDQ•− wasobserved in] the case of phenol[Oxidation of benzene][Oxidation of phenol]
absorption
e of fast back electron transfer (Figure 5). Such
Figure 6. Electronic charges on the carbon atoms in (a) ﬂuoro- (b)DDQchloro-, and (c) bromobenzene radical cations calculated at the density
functional theory at the B3LYP/6-31+G(d,p) level with natural
population analysis.

cation is localized on C1 (+0.46) and C4 (+0.22). A watermolecule
can be added at C1 to form phenol via
DDQ +dehalogenation. In the case of chloro- and bromobenzene
radical cation, the positive charges on C1 OH
are signiﬁcantly+
smaller than in ﬂuorobenzene radical
Thus, addition of
DDQcation.
−
OH at the para position results in the formation of phalophenol selectively.
DDQ ++5. Transient absorption spectra of DDQ with benzene (1.0 M,
In conclusion,DDQ
DDQ
has been shown to act as an eﬃcientBlack;
DDQ+benzene
and phenol (1.0 M, red) in deaerated MeCN taken 3.0 ns after
photocatalyst for the one-step oxygenation of benzene toRed;
DDQ+phenol
cond laser excitation at 393 nm.
phenolunder visible-light irradiation witha highest
quantum
InvertedregionMarcustop
regionyield of 45%. The radical intermediates in the photocatalyticof benzene with H2O were successfully detected bysting results are well-rationalized by the diﬀerence inBET <oxygenation
Dissociation of
BET ≫ Dissociation oflaser ﬂash photolysis to clarify the photocatalytic mechanism.
ving forces for back electron transfer. The driving force
radical ion pairradical ion pairelectron transfer from DDQ•− to benzene radical cation(additionof H
ASSOCIATED
CONTENT2O)eV) is much larger than that from DDQ•− to phenol
S Supporting Information*
cation (1.09 eV). Because the driving force in the case ofUV−vis proceedand GC-MS
data anddetails. Thise radical cation is so large,
the back electronoftransfer
Overoxidation
phenol doesnʼtbecauseof experimental
very fast BET
27material is available free of charge via the Internet at http://
ccur in the Marcus inverted region, where the back

OH

■

19

(ﬂuoro-, chloro-, and bromobenzene), photooxygenation also
(ﬂuoro-,occurred
chloro-,with
andDDQ
bromobenzene),
photooxygenation
also
under similar reaction
conditions to yieldcorresponding
phenols(thereaction
GC dataconditions
are shown intoFigure
occurredthewith
DDQ under
similaryield
S5). The substrateselectivities
arethe corresponding
phenolsconversions(the GC anddataproductare shownin Figuresummarized
Table 1.
S5). The
substrateinconversions
and product selectivities are

Photooxygenation of halogenated benzene[o, p-selective oxygenation]summarized in Table 1.

[Quantum yield]
DDQ
t-BuONO (TBN)
0.3 eq ,0.05 eq

OH

Table 1. Reactant Conversions
and Product Selectivities in
DDQ
1.3 eqX
X
Photooxygenation of Halogenated Benzene
Derivatives

Table 1. Reactant Conversions
and Product Selectivities in
MeCN, H O (0.5M)
OH % (selectivity, %)substrate (conversion,r.t.,%)390~600 nm product: yield,
Photooxygenation of Halogenated Benzene
Derivatives

O2-saturated MeCN, H2O (0.5 M),
390~600
nm quantum yield for formation of phenol (Φ) vs
Figure 4. Plotof the2phenol: 14 (32)substrate (conversion, %)product:yield, % (selectivity,
p-ﬂuorophenol:
24 (55) %)Journal of the [HAmericanChemical Society y. 93%
Communication
2O].
o-ﬂuorophenol:
ﬂuorobenzene (44)phenol:14 (32) 5.7 (13)Figure 4. Plot of the quantum yield for formation of phenol (Φ) vs reaction would
occurchlorobenzene
despite the(34)diﬀusion-controlled
photophenol:
0 (0)much easier than that of benzene, because the one-electron
p-ﬂuorophenol: 24 (55)
[H2O].
26 induced transfer.
p-chlorophenol: 28 (82)oxidation potential of phenol (Eox = 1.60 V vs SCE) is smaller
o-ﬂuorophenol:
benzenes5.76.1(13)(18)
than that for benzene. Furthermore, as evidenced byWhen
nano- benzene was replaced by halogenatedo-chlorophenol:
chlorobenzene(34)
phenol:0(0)much easier thansecond
that andof benzene,
becausethe
one-electron(14) photooxygenation
phenol: 0also(0)(ﬂuoro-, chloro-, andbromobenzene
bromobenzene),
femtosecond laser ﬂash photolysis
measurements
26
p-chlorophenol:
p-bromophenol:(80)oxidation potentialof phenol(EoxS4),= 1.60
V vs SCE)electronis smaller
occurred
to yield2811(82)(FiguresS3 andphotoinduced
transfer
fromwith DDQ under similar reaction conditions
3o-bromophenol:(20)o-chlorophenol:
the corresponding
phenols (the GC data are shownin Figure6.12.8(18)phenol toFurthermore,
DDQ* occurred
rate constantofthan that for benzene.
as eﬃciently
evidencedwith
bya nano−1

ﬂuorobenzene (44)−1

(1.2 ± 0.2)laser
× 10 ﬂash
M photolysis
s , as determinedfrom theS5).
slopesin substrate
The
conversions
bromobenzene(14) and product selectivities
phenol: 0 (0)aresecond and femtosecond
measurements
Selective
photooxygenation
wasobserved forandinsetsphotoinducedof Figures S3 andS4. This transfer
value is larger
than
ket for in Table 1.
summarized
p-bromophenol:
11 chloro(80)(Figures S3 andtheS4),
electronfrombromobenzene. In contrast, photooxygenation of ﬂuorobenbenzene [(5.3 ± 0.3) × 109 M−1 s−1]. In such a case, the facto-bromophenol: 2.8 (20)phenol to 3DDQ*
occurredeﬃciently
with
a
rate
constantofzeneConversions
gave phenol together
with theﬂuorophenols.
that
oxygenation of phenol to hydroquinone
Table 1. Reactant
and ProductSelectivitiesin The product10 no−1photocatalytic−1(1.2 ± 0.2) × 10occurred
M may
s , asdeterminedfromtheslopesinselectivity
depends onBenzene
the electronic
charge of the radical cation
result from the much slower reaction with
H2O
Photooxygenation
ofphotooxygenation
HalogenatedDerivatives
Selectivewasobserved
for chloroandthe insets of Figures
S3 and
S4.fastThis
is larger
thanketinfra).
forspecies (Figure6). The positive
charge in ﬂuorobenzene
radicalcomparedwithbackvalue
electrontransfer(vide
Electronic
chargesofradicalcation[]9bromobenzene.
photooxygenation(conversion, %) In contrast,
product: yield,% (selectivity, %) of ﬂuorobenbenzene [(5.3 ± 0.3)
10backM−1electrons−1]. In
such ina case,the offact
The ×fasttransferthe casephenolsubstratewaszene gave (44)phenol together phenol:with theﬂuorophenols. The productﬂuorobenzene
14 (32)
conﬁrmed
byyield
femtosecond
laserofﬂash
photolysis
thatno 4.photocatalytic
oxygenationof phenol
to hydroquinone
FigurePlot of thequantumfor formation
phenol(Φ) vs measurements.
p-ﬂuorophenol:charge
24 (55) of the radical cation
In contrast
the caseof benzene,
whereHthetransientselectivity depends on the electronic[H2O]. may result
occurredfrom theto muchslowerreaction with
2O
• Highabsorption
quantum
theto benzene π-dimer radical
o-ﬂuorophenol:
bandyield
at transfer
900 fornm (videdue
species (Figure 6). The positive
charge5.7in(13)ﬂuorobenzene radical
compared with fastback electron
infra).
•−
chlorobenzene (34)
phenol: 0 (0)thatphenol
at 595because
nm(𝜱duetheto DDQmucheasierthancationthat andof ofbenzene,one-electron
formation=
𝟎.
𝟒𝟓).were clearly observed
The fast
back
electrontransfer
in
the
case
26of phenol was
p-chlorophenol: 28 (82)
(Figure3a), (Enoox transientabsorption
to phenol π-dimer
oxidation potentialof phenol= 1.60 V vs
SCE) isduesmaller•−
conﬁrmedbyfemtosecond
laser
ﬂash
photolysis
measurements.
o-chlorophenol: 6.1 (18)radical cation
or DDQas evidenced
was observed
the case of phenol
than that for benzene.
Furthermore,by innanoIn contrast to because
the caseof ofwheretransfer
the transientbromobenzene (14)phenol: 0 (0)fast benzene,
back electron(Figure 5). Such
1020

Applications to other reactions[Synthesis of protected aniline]

Substrate scope

König et al. Chem. Eur. J. 2017, 23, 18161.

[Oxidation of chloride ion]

Reaction mechanism

Fukuzumi et al. Chem. Asian J. 2016, 11, 996.

21

Contents• Introduction
Ø Theory and experiment• Application to chemical reaction
Ø Catalytic oxidation of benzene to phenol•

Summary22

Summary[Marcus theory]

Chem Soc Rev

Articlee reorganizabond-length
as describedorce constant,d length is the(5)librium bondthe reaction.
bond-length

Published on 29 November 2017. Downloaded by The University of Tokyo on 7/19/2023 9:56:16 AM.

l deﬁnition oforganic field effect transistors (OFET), re
understanding of the interfacial processes.
ever, the complexity of interfaces, simpler
needed. This is where electron donor–acceon electron donating metalloporphyrins andfullerenes as p- and n-type materials, respec
great asset.
Covalently linking metalloporphyrins anin eliminating diffusion as the rate limiti
transfer processes. The first of such exampl
Gust et al. and Imahori et al.16,17 employing
amide linkers. A closer look at the cha
reveals a significant stabilization. The syntof metalloporphyrin- and fullerene-based
acceptor conjugates followed in the next fe
Another milestone within the context opotential of C60 was the work in 199618 andTo illustrate the impact, which the nature of t
has on electron transfer processes, the work fo
2018with, either, 702.
of metalloporphyrins
C60 or benzo
acceptor. Substantial
differencesin the reduc2019,, 2450.
and benzoquinone hampered, however, concl
A better match was realized when using naph
comparing it to C60. Comparable reductio
electron acceptors, electron donor–acceptor di"nic couplings all result in similar !DGET . A
kinetic difference must correlate with the dienergies of the electron acceptors. In fact, a l v
C60 conjugate of 0.59 eV is significantly smaller
naphthalenediimide conjugate with 1.41 eV.
study has corroborated the superior prop
3-dimensional electron acceptor.
The efficiency of charge collection in OP
separation of charges and a slow recomb
Spatial separation of charges is, for exa
means to suppress their recombination.
correlates, however, with weaker electroni

Fig. 2 Marcus plot of log(kET) as a function of free energy changes for
Figure 2. Types of electron transfer reactions as deﬁned by theelectron transfer reactions.
Clarifiedthe, andrelationshipbetween• reaction
reorganization energy, λ.
(From left to electron transfer rate, Gibbs free energy change, andfree energy, ΔG°
right) Self-exchange reactions between energetically degenerate
reorganization
andpredicted
the inverted
region.
render fullerenes,
in general, and C60, in particular, perfect
donorsand acceptors. “Normal”energy,
electron transfer
reactions,
λ >electron
acceptors
for highly efficient, long lasting charge
ΔG°. “Barrierless” reactions when λ = ΔG°, which corresponds to atransfer processes.
rate maximum. “Inverted” electron transfer when λ < ΔG° which is
Guldi et al. Chem. Soc. Rev.
47
The total reorganization energy is at the heart of the Marcus
predicted to result in a decrease in rate of electron transfer. Note that
Meyer
et al.
J. Chem.
96
theory on electron transfer reactions.
In brief,
the Marcus
theory Educ.
λ is constant across the series.
treats the dependence of the electron transfer rate (k ) on the freeation energyET
ic to thenewphotocatalytic synthesis of phenol][Selectiveenergy changes (!DGET ) of the electron transfer reaction as aical, εop, andelectronic interaction is often associated with new spectroparabola (Fig. 2).11–13 In the resulting Marcus parabola, threeometric D−A
regions are identified. First, in the normal region, where thescopic features, altered reduction potentials, or even expected
35,43
electron transfer is weakly exergonic, the theory predicts an
ceptor, rA, allfrom molecular structures.
This phenomenon is referred toincrease in the rate constant with increasing thermodynamic.27,41
as electronic coupling, HDA.
reaction was achieved using• Selective catalytic
driving force (!DGET o l). Second, at the top of the parabola,
A prediction of quantum mechanics is that wave function
Marcus theory.the driving force matches the reorganization energy (!DGET # l).
amplitudes decay exponentially with distance providing ﬁnite
In this activation-less region, the rate constant is at its maximum
overlap,even
acrosslong
distances.
Suchcoupling
canfacilitate
and it is mostly governed by the magnitude of the electronic(6)
coupling (V) element between the electron donor and acceptor.
long distance nonadiabatic electron transfer, Figure• 3A.ItAncan also be applied
toexergonic
otherconversions.
Third, in the highly
region,usefulwhich starts upongoing
s often muchbeyond the maximum, the rate constant starts to decrease withii because theincreasing driving force (!DGET 4 l). Responsible for this trend
cting species.
is a poor vibrational overlap between the wave functions of thewith electron
product and the reactant. It is often referred to as the inverted
"

"

"

"

23

Appendix[Previous report]

Fukuzumi et al. Angew. Chem. Int. Ed. 2011, 50, 8652.

24