Photoinduced Electron Transfer in 9-Substituted 10-Methylacridinium Ions

Article abstract of DOI:10.1002/chem.201604527

A series of 9-substituted 10-methylacridinium ions (Acr⁺-R) in which an electron-donor moiety (R) is directly linked with an electron-acceptor moiety (Acr⁺) at the 9-position was synthesized, and the photodynamics was fully investigated to determine the rate constants of photoinduced electron transfer (ET) and back electron transfer. The driving forces of photoinduced electron transfer and back electron transfer were determined by means of electrochemical and photophysical measurements. The dependence of the ET rate constants on driving force was well analyzed in the light of the Marcus theory of ET. The quantum yields of formation of the triplet ET states vary significantly, depending on the interaction between the donor (R) and acceptor (Acr⁺) moieties. Among the Acr⁺-R examined, the 9-mesityl-10-methylacridinium ion (Acr⁺-Mes) exhibits the best performance in terms of the lifetime of the triplet ET state and the quantum yield. Photoexcitation of Acr⁺-Mes results in formation of the triplet ET state [³(Acr^.-Mes^.+)], which has a long lifetime, a high energy (2.37 eV), and a high quantum yield (>75 %) in acetonitrile. The triplet ET state exhibits both the oxidizing and reducing activity of the Mes^.+and Acr^.moieties, respectively.

Full text of DOI:10.1002/chem.201604527

A Journal of
Accepted Article
Title: Photoinduced Electron Transfer in 9-Substituted 10-
Methylacridinium Ions
Authors: Takeshi Tsudaka, Hiroaki Kotani, Kei Ohkubo, Tatsuo
Nakagawa, Nikolai V. Tkachenko, Helge Lemmetyinen, and
Shunichi Fukuzumi
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: Chem. Eur. J. 10.1002/chem.201604527
Link to VoR: http://dx.doi.org/10.1002/chem.201604527
Supported by

Chemistry - A European Journal
10.1002/chem.201604527
DOI: 10.1002/chem.201xxxxxx
█
Organophotocatalyst
Photoinduced Electron Transfer in 9-Substituted 10-Methylacridinium
Ions
[
a]
[b]
[a,c,d]
[e]
Takeshi Tsudaka, Hiroaki Kotani, Kei Ohkubo,*
Tatsuo Nakagawa, Nikolai V.
[
f]
[f]
[c,g]
Tkachenko,* Helge Lemmetyinen,* and Shunichi Fukuzumi*
Abstract: A series of 9-substituted 10-methylacridinium ions of the triplet ET states vary significantly depending on the
+
+
(
Acr –R), in which an electron donor moiety (R) is directly interaction between the donor (R) and acceptor (Acr )
+
+
linked with an electron acceptor moiety (Acr ) at the 9- moieties. Among Acr –R examined, 9-mesityl-10-methyl-
+
position, has been synthesized and the photodynamics has acridinium ion (Acr –Mes) exhibits the best performance in
been fully investigated to determine the rate constants of terms of the lifetime of the triplet ET state and the quantum
both the photoinduced electron transfer and back electron yield. The photoexcitation of 9-mesityl-10-methylacridinium
+
transfer. The driving forces of photoinduced electron transfer ion (Acr –Mes) results in formation of the triplet ET state
3
•
•+
and back electron transfer have been determined by the [ (Acr –Mes )], which has a long lifetime, a high energy (2.37
electrochemical and photophysical measurements. The eV) and a high quantum yield (>75%) in acetonitrile. The
driving force dependence of the electron-transfer (ET) rate triplet ET state delivers both the oxidizing and reducing
•
+
•
constants has been well analyzed in light of the Marcus activity of the Mes and Acr moieties, respectively.
theory of electron transfer. The quantum yields of formation
Introduction
terminal electron acceptor via electron mediators, which are well-
organized in a protein matrix, to attain a long lifetime of the final
[
1-3]
The natural photosynthetic reaction center utilizes sequential
multi-step electron transfer from the excited chromophore to the
charge-separated (CS) state as long as 1 second.
Extensive
efforts have been devoted to develop artificial systems mimicking
[
4–11]
the photosynthetic reaction center.
The best molecule
mimicking multi-step electron transfer processes in the
photosynthetic reaction center so far reported is a ferrocene-
[
a]
Prof. Dr. K. Ohkubo, T. Tsudaka
3
meso,meso-linked porphyrin trimer-fullerene pentad (Fc-(ZnP) -
Department of Material and Life Science
Graduate School of Engineering, Osaka University and
SENTAN, Japan Science and Technology Agency (JST)
Suita, Osaka 565-0871 (Japan)
C
60) where the C60 and the ferrocene (Fc) are tethered at both the
[
12]
3
ends of (ZnP) (Ree = 46.9 Å). The lifetime of the final CS state
(
0.53 s at 163 K) has been attained without lowering the CS
[
b]
Prof. Dr. H. Kotani
[
12]
efficiency (Φ = 0.83). However, a significant amount of energy
Department of Chemistry, Graduate School of Pure and Applied
Sciences, University of Tsukuba, 1-1-1 Tennoudai, Tsukuba, Ibaraki
is lost during the multi-step electron transfer processes to reach
[
1]
3
05-8571 (Japan)
the final CS state (0.50 eV). In the photosynthesis, two–step
photoexcitation, so called “Z-scheme” is thereby required to
recover the energy loss via the multi-step electron transfer
processes and to gain the high oxidizing power to oxidize water
[
[
c]
d]
Prof. Dr. K. Ohkubo, Prof. Dr. S. Fukuzumi
Department of Chemistry and Nano Science,
Ewha Womans University, Seoul 120-750 (Korea)
E-mail: fukuzumi@chem.eng.osaka-u.ac.jp
Prof. Dr. K. Ohkubo,
+
as well as the high reducing power to reduce NAD
Division of Applied Chemistry, Graduate School of Engineering,
Osaka University, Suita, Osaka 565-0871 (Japan)
Tel: (+81) 6-6879-4131
E-mail: ookubo@chem.eng.osaka-u.ac.jp
Dr. T. Nakagawa.
[1,13]
coenzyme.
It is highly desired to design simple molecular dyads which are
capable of fast charge separation but retain slow charge
[
14]
[
[
e]
f]
recombination. Theoretically it is possible to obtain such an
electron donor-acceptor dyad based on the classic Marcus theory
Unisoku Co., Ltd, SENTAN, Japan Science and Technology Agency
(
JST), Hirakata, Osaka 573-0131 (Japan)
[
15]
of electron transfer [Equation (1)]. Herein V is the electronic
Prof. Dr. N. V. Tkachenko, Prof. Dr. H. Lemmtyinen
Institute of Materials Chemistry, Tampere University of Technology,
P.O. Box 541, FIN-33101 Tampere, Finland, E-mail:
nikolai.tkachenko@tut.fi, helge.lemmetyinen@tut.fi
Prof. S. Dr. Fukuzumi
! !
4
π!
Δ!!" + ! !
!
!!"
!= !
! !exp −
!!!!!!! (1)
!
ℎ !! !
4!! !
!
!
[
g]
Faculty of Science and Technology, Meijo University
SENTAN, Japan Science and Technology Agency (JST)
Nagoya, Aichi 468-8502 (Japan)
coupling matrix element, h is the Planck constant, and T is the
absolute temperature. According to Equation (1), logarithm of the
ET rate constant (log kET) is related parabolically to the ET driving
force (negative ET free energy change) between electron donors
Supporting information for this article is available on the WWW
under http://www.chemeurj.org/ or from the author.
1
This article is protected by copyright. All rights reserved.

Chemistry - A European Journal
10.1002/chem.201604527
DOI: 10.1002/chem.201xxxxxx
and acceptors (–ΔGET), and the ET reorganization energy (λ), that
is, the energy required to structurally reorganize the donor,
acceptor and their solvation spheres upon ET. It is now well
recognized that the parabolic driving force dependence of log kET
provides the theoretical basis for understanding ET processes in
photosynthesis the log kET value increases with increasing the ET
driving force (–ΔGET). When the magnitude of the driving force
becomes the same as the reorganization energy (–ΔGET = λ), the
reaction rate reaches a maximum and is basically controlled by
the magnitude of electronic coupling (V) between the donor and
acceptor moiety [Equation (1)]. Upon passing this thermodynamic
electron reduction potential (Ered vs. SCE = –0.43 V) than 0.07
[
23]
V.
Thus, a variety of photoredox catalytic reactions including
oxygenation, cycloadditions, atom transfer cyclizations, C–H
functionalizations, alkene hydrofunctionalizations and bond
cleavage reactions have been made possible by utilizing the high
•
+
•
oxidizing and reducing ability of the Mes and Acr moiety of the
[
20-27]
ET state, respectively.
The triplet locally excited state of
+
Acr –Mes would never be able to be utilized for variety of
photoredox catalytic reactions due to the lack of the oxidizing or
reducing ability. However, the systematic photodynamics
including the intersystem crossing from the singlet ET state to the
+
maximum, the highly exothermic region of the parabola (–ΔGET
>
triplet ET state of Acr –Mes and other derivatives have yet to be
λ) is entered, in which an additional increase of the driving force
results in an actual slow-down of the reaction rate, due to an
increasingly poor vibrational overlap of the product and reactant
wave functions. This highly exergonic range is generally referred
examined. The oxidizing and reducing ability of the
+
photogenerated species of Acr –Mes (singlet and triplet ET or
locally excited triplet) has yet to be fully clarified, either.
We report herein the synthesis and photodynamics of a series
[
15]
+
to as the Marcus inverted region. In such a case, the magnitude
of the reorganization energy is the key parameter to control the
ET process. The smaller the reorganization energy, the faster is
the forward photoinduced CS process, but the CR process
becomes slower when the driving force for back electron transfer
of 9-substituted 10-methylacridinium ions (Acr –R, R = donor
moieties such as alkylbenzene, naphthalene, and anthracene
derivatives), in which R is directly linked with an electron acceptor
+
moiety (Acr ) at the 9-position. The redox and photophysical
properties were scrutinized including the intersystem crossing to
analyze the driving force dependence of the rate constants of
photoinduced electron transfer and back electron transfer in light
of the Marcus theory of electron transfer. In order to examine the
nanosecond time range in a wide spectral range, we have applied
(
–ΔGET) is larger than the ET reorganization energy (λ). Thus, the
CS lifetimes can be finely controlled by the λ value, which is
determined by the choice of the donor and acceptor pair, the type
of linkage between the donor and acceptor molecules and also
the solvent.
a
newly developed randomly-interleaved-pulse-train (RIPT)
[
28]
We reported the design and synthesis of a simple electron
donor-acceptor linked molecule with a small λ value and a high-
lying triplet excited state. Acridinium ion is the best candidate for
such a purpose, since the λ value for the electron self-exchange
between the acridinium ion and the corresponding one-electron
reduced radical is the smallest (0.3 eV) among the redox active
method in this study. The surprisingly long lifetime of the triplet
+
ET state of Acr –Mes, which exhibited the best performance
+
among Acr –R is well rationalized based on the driving force
dependence of the rate constants of electron transfer and also by
the subtle difference in the interaction between the donor and
acceptor moieties. The oxidizing and reducing ability of the triplet
[
16]
+
organic compounds. An electron donor moiety (mesityl group)
is directly connected at the 9-position of the acridinium ion to yield
ET state of Acr –Mes is well analyzed in light of the Marcus
theory of electron transfer.
+
[17]
9
-mesityl-10-methylacridinium ion (Acr –Mes),
in which the
solvent reorganization of ET is minimized because of the short
ꢀExperimentalꢀSection
[
16]
linkage between the donor and acceptor moieties.
The
+
Materials: Benzonitrile (PhCN) used as a solvent were distilled by
photoexcitation of Acr –Mes affords the long-lived electron-
transfer (ET) state with a virtually infinite lifetime of the electron-
[
29]
P
2
O
5
in vacuo.
3
Acetonitrile (MeCN) and chloroform (CHCl ) were
•
•+
used as received. Naphthalene and 1-bromonaphthalene used as
electron donors were obtained commercially and used as received. 9-
transfer state (Acr –Mes ) at 77 K, a high energy (2.37 eV) and a
[
17]
high quantum yield close to unity (98%). Because the charge is
not separated in electron transfer from the Mes moiety to the
+
Phenylacridine, 10-methylacridinium ion (AcrH ) and 7,7,8,8-
+
tetracyanoquinodimethane (TCNQ) were purchased from Tokyo
singlet excited state of the Acr moiety, we denotes the resulting
2
Chemical Industry Co., Ltd. Deuterated [ H
3
]acetonitrile (CD
3
CN) was
state as the ET state. The long-lived ET state was questioned by
Benniston et al. who reported that the triplet energy (1.96 eV) of
obtained from EURI SO–TOP, CEA, France and used as received. p-
+
+
[18]
Benzoquinone supplied by Aldrich, was purified by vacuum
the Acr moiety of Acr –Mes was lower than the ET state. Such
[
30]
+
+
sublimation. 1-Benzyl-1,4-dihydronicotinamide (BNAH),
1-benzyl-
a triplet excited state of the Acr moiety of Acr –Mes would never
be able to oxidize electron donors which have the one-electron
oxidation potentials higher than the one-electron reduction
[
31]
1
,4-dihydronicorin amide dimer [(BNA)
2
]
and 1,1-dihexyl-4,4'-
2
+
–
[32,33]
dipyridinium diperchlorate [HV (ClO
4
)
2
]
were prepared
+
+
according to the literature.
potential of the triplet excited state of the Acr moiety of Acr –Mes
*
[19]
(Ered vs. SCE = 1.53 V). By the same token, the triplet excited
+
+
+
Syntheses
of
Acr –R:
9-Substituted
10-methylacridinium
state of the Acr moiety of Acr –Mes would never be able to
reduce electron acceptors which have the one-electron reduction
potentials lower than the one-electron oxidation potential of the
+
–
4
perchlorates (Acr –R ClO ) were prepared by the reaction of 10-
methylacridone in dichloromethane with the corresponding Grignard
reagents (RMgBr), then addition of sodium hydroxide for the
hydrolysis and perchloric acid for the neutralization, and purified by
recrystallization from ethanol-diethyl ether. 9-(1-Naphthyl)-10-
+
*
triplet locally excited state of Acr –Mes (Eox vs. SCE = 0.07 V),
[
19]
+
either. However, the photoexcitation of Acr –Mes with electron
donors and/or acceptors resulted in electron-transfer oxidation of
a variety of electron donors which have much higher one-electron
+
1
methylacridinium (Acr –1NA) perchlorate: H NMR (CD
3
CN, 300
[
20-22]
MHz) δ (ppm) 8.64 (d, J = 9.0 Hz, 2H), 8.28-8.38 (m, 3H), 8.15 (d, J =
oxidation potentials than 1.53 V,
and also electron-transfer
9
.0 Hz, 1H), 7.72-7.80 (m, 5H), 7.60 (d, J = 9.0 Hz, 2H), 7.32 (t, J =
reduction of hexyl viologen which has the much lower one-
2
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Chemistry - A European Journal
10.1002/chem.201604527
DOI: 10.1002/chem.201xxxxxx
[
36]
9
.0 Hz, 1H), 6.94 (d, J = 9.0 Hz, 1H), 4.88 (s, 3H). Anal. Calcd for
V.
All electrochemical measurements were carried out under an
C
24
H
18ClNO
4
･0.5(H
2
O): C, 67.21; H, 4.47; N, 3.27. Found: C, 67.51; H,
atmospheric pressure of argon.
+
4
.32; N, 3.30. 9-(2-Naphthyl)-10-methylacridinium (Acr –2NA)
1
perchlorate: H NMR (CD
3
CN, 300 MHz) δ (ppm) 8.62 (d, J = 9.0 Hz,
H), 8.35-8.41 (m, 2H), 8.24 (d, J = 9.0 Hz, 1H), 8.03-8.13 (m, 5H),
.71-7.84 (m, 4H), 7.60 (d, J = 9.0 Hz, 1H), 4.85 (s, 3H). Anal. Calcd
Time-resolved absorption
spectral
measurements:
2
7
Femtosecond to picosecond time-resolved absorption spectra and
fluorescence lifetime measurements were collected using a pump-
[
37]
for C24
H
18ClNO
4
･0.25 (H
2
O): C, 67.93; H, 4.39; N, 3.30. Found: C,
probe technique as described elsewhere. The femtosecond pulses
6
7.81; H, 4.32; N, 3.34. 9-[2-(6-Methoxynaphthyl)]-10-methyl
of the Ti:sapphire generator were amplified by using a multipass
+
1
acridinium (Acr –2NA(6-OMe)) perchlorate: H NMR (CD
3
CN, 300
amplifier (CDP-Avesta, Moscow, Russia) pumped by a second
MHz) δ (ppm) 8.63 (d, J = 9.0 Hz, 2H), 8.40 (t, J = 9.0 Hz, 2H), 8.14
d, J = 9.0 Hz, 3H), 7.98 (t, J = 9.0 Hz, 2H), 7.85 (t, J = 9.0 Hz, 2H),
harmonic of the Nd:YAG Q-switched laser (model LF114, Solar TII,
Minsk, Belorussia). The amplified pulses were used to generate
second harmonic (420 nm) for sample excitation (pump beam) and
white continuum for time-resolved spectrum detection (probe beam).
The samples were placed into 1 mm rotating cuvettes, and averaging
of 100 pulsed at 10 Hz repetition rate was used to improve signal-to-
noise ratio. Typical response time of the instrument was 150 fs
(fwhm). A global multi-exponential fitting procedure was applied to
process the data. The procedure takes into account the instrument
time response function and the group velocity dispersion of the white
continuum, allowing calculation of the decay time constants and
dispersion-compensated transient absorption spectra.
(
7
3
3
.57 (t, J = 9.0 Hz, 2H), 7.36 (d, J = 9.0 Hz, 1H), 4.86 (s, 3H), 4.02 (s,
H). Anal. Calcd for C25 ･0.5 (H O): C, 65.43; H, 4.61; N,
.05. Found: C, 65.56; H, 4.46; N, 3.04. 9-[1-(2-Methylnaphthyl)]-10-
H
20ClNO
5
2
+
1
methylacridinium (Acr –1NA(2-Me)) perchlorate: H NMR (CD
3
CN,
3
8
7
1
00 MHz) δ (ppm) 8.67 (d, J = 9.0 Hz, 2H), 8.37 (t, J = 9.0 Hz, 2H),
.20 (d, J = 9.0 Hz, 1H), 8.07 (d, J = 9.0 Hz, 1H), 7.66-7.75 (m, 5H),
.49 (t, J = 9.0 Hz, 1H), 7.22 (t, J = 9.0 Hz, 1H), 6.68 (d, J = 9.0 Hz,
4 2
H), 4.89 (s, 3H), 1.96 (s, 3H). Anal. Calcd for C25H20ClNO ･0.2(H O):
C, 68.63; H, 4.70; N, 3.20. Found: C, 68.66; H, 4.73; N, 3.10. 9-(9-
+
1
Anthryl)-10-methylacridinium (Acr –An) perchlorate: H NMR (CD
3
CN,
00 MHz) δ (ppm) 8.99 (s, 1H), 8.70 (d, J = 9.0 Hz, 2H), 8.28-8.39 (m,
H), 7.46-7.64 (m, 6H), 7.30 (t, J = 9.0 Hz, 2H), 6.97 (d, J = 9.0 Hz,
H), 4.95 (s, 3H). Anal. Calcd for C28 ･0.5(H O): C, 70.22; H,
.42; N, 2.92. Found: C, 70.17; H, 4.34; N, 2.90. 9-(4-Tolyl)-10-
3
4
2
4
Time-resolved fluorescence spectra for sub-picosecond range
were measured by a Photon Technology International GL-3300 with a
Photon Technology International GL-302, nitrogen laser/pumped dye
H
20ClNO
4
2
laser system, equipped with
a four channel digital delay/pulse
+
1
methylacridinium (Acr –Tol) perchlorate: H NMR (CD
δ (ppm) 8.59 (d, J = 9.0 Hz, 2H), 8.37 (t, J = 9.0 Hz, 2H), 8.05 (d, J =
.0 Hz, 2H), 7.86 (t, J = 9.0 Hz, 2H), 7.57 (d, J = 9.0 Hz, 2H), 7.42 (d,
J = 9.0 Hz, 2H), 4.81 (s, 3H), 2.56 (s, 3H). Anal. Calcd for
: C, 65.71; H, 4.73; N, 3.65. Found: C, 65.55; H, 4.76; N,
3
CN, 300 MHz)
generator (Stanford Research System Inc. DG535) and a motor driver
(Photon Technology International MD-5020). Excitation wavelength
was 430 nm using dimethyl-POPOP (Exciton Co., USA) as a laser
dye.
9
C
21
H
18ClNO
4
Sub-nanosecond laser-induced transient absorption spectra were
collected by a customized measuring system based on the recently
+
1
3
.64. 9-(2,4-Xylyl)-10-methylacridinium (Acr –Xyl) perchlorate:
H
[
28]
NMR (CD
3
CN, 300 MHz) δ (ppm) 8.59 (d, J = 9.0 Hz, 2H), 8.34-8.40
proposed RIPT method by Nakagawa et al. The pump source is a
passively Q-switched microchip laser, (PowerChip PNV-M02510,
Teem Photonics, 1 kHz, 350 ps, 355 nm, 25 µJ), and the probe
source is a supercontinuum radiation source (SC-450, Fianium, 20
MHz, 50–100 ps, 450–2000 nm). The monochromatized probe pulses
with a pre-dispersive monochromator (MD200, Unisoku) which is
asynchronous with a pump pulse were irradiated on the sample and
the beams transmitted through the sample and its reference beam
were detected by InGaAs-photodiodes (G10899-01K, 400-1600 nm,
Hamamatsu). A post-dispersive monochromator (CM110, Spectral
Products) were positioned before the detector to minimize
fluorescence signals from the sample. Si PIN photodiodes (S5972,
Hamamatsu) were used to pick up the pump pulse and the probe
pulses and to evaluate delay-times between them in a shot-by-shot
manner. All photodiodes’ outputs are recorded with a digitizing
oscilloscope (HDO8038, Teledyne Lecroy) then transferred to a PC to
construct the TA temporal profile. Time resolution of the system was
estimated at 400 ps from the 10-90% rise time.
(
m, 2H), 7.81-7.91 (m, 4H), 7.33-7.41 (m, 2H), 7.16 (d, 1H), 4.82 (s,
3
4
H), 2.50 (s, 3H), 1.83 (s, 3H). Anal. Calcd for C22H20ClNO : C, 66.42;
H, 5.07; N, 3.52. Found: C, 66.17; H, 4.96; N, 3.59. 9-Mesityl-10-
+
[17] 1
methylacridinium (Acr –Mes) perchlorate:
MHz) δ (ppm) 8.59 (d, J = 9.0 Hz, 2H), 8.37 (t, J = 9.0 Hz, 2H), 7.83 (s,
H), 7.22 (s, 2H), 4.80 (s, 3H), 2.46 (s, 3H), 1.68 (s, 6H). Anal. Calcd
･0.15(H O): C, 66.63; H, 5.42; N, 3.38. Found: C,
6.44; H, 5.22; N, 3.49. 9-Phenyl-10-methylacridinium (Acr –Ph)
3
H NMR (CD CN, 300
4
for C23
H
22ClNO
4
2
+
6
1
perchlorate: H NMR (CD
3
CN, 300 MHz) δ (ppm) 7.5-8.6 (m, 13H),
+
4
1
.83 (s, 3H). 9-Durenyl-10-methylacridinium (Acr –Dur) perchlorate:
H NMR (CD
.42 (m, 2H), 7.81 (d, J = 9.0 Hz, 4H), 7.34 (s, 1H), 4.81 (s, 3H), 2.33
s, 6H), 1.57 (s, 6H). Anal. Calcd for C24 ･0.3(H O): C, 66.83;
H, 5.75; N, 3.25. Found: C, 66.83; H, 5.48; N, 3.18.
3
CN, 300 MHz) δ (ppm) 8.61 (d, J = 9.0 Hz, 2H), 8.34-
8
(
H
24ClNO
4
2
Electrochemical
measurements were performed at 298
electrochemical analyzer in deaerated MeCN containing 0.1
Bu NClO (TBAP) as supporting electrolyte. A conventional three-
measurements: Cyclic voltammetry (CV)
K
on an ALS 630B
M
For nanosecond laser flash photolysis experiments, deaerated
+
4
4
MeCN solutions of Acr –R were excited by an Nd:YAG laser
electrode cell was used with a platinum working electrode (surface
(Continuum, SLII-10, 4-6 ns fwhm) at λ = 355 nm with the power of 10
mJ per pulse. The transient absorption measurements in the visible
and near-IR region were performed using a continuous xenon lamp
(150 W) as a probe light and a photomultiplier (Hamamatsu R2949;
350–800 nm) and an InGaAs-PIN photodiode (Hamamatsu G5125–
10; 800-1200 nm) as a detector, respectively. The output from the
photodiodes and a photomultiplier was recorded with a digitizing
oscilloscope (Tektronix, TDS3032, 300 MHz).
2
area of 0.3 mm ) and a platinum wire as the counter electrode. The Pt
working electrode (BAS) was routinely polished with a BAS polishing
alumina suspension and rinsed with acetone before use. The second
[
35]
harmonic AC voltammetry (SHACV) measurements of 9-substituted
0-methylacridinium ions were performed on BAS 100B
electrochemical analyzer. The measured potentials were recorded
with respect to the Ag/AgNO (0.01 M) reference electrode. The redox
potentials (vs. Ag/Ag ) are converted to those vs. SCE by adding 0.29
1
a
3
+
3
This article is protected by copyright. All rights reserved.

Chemistry - A European Journal
10.1002/chem.201604527
DOI: 10.1002/chem.201xxxxxx
Quantum yields determination. The quantum yields of triplet ET
position to minimize the solvent reorganization of electron transfer
+
+
state (ΦTET) of Acr –R were determined from the comparison with the
(ET). The absorption and fluorescence spectra of Acr –R in
absorption band at 500 nm due to the triplet ET state observed at 1
acetonitrile (MeCN) are superpositions of the spectra of the
+
µs after laser excitation at 355 nm and that of Acr –Mes (ΦTET
=
component chromophores making up these molecules (see
[
17]
[42]
0
1
.98). The strong fullerene triplet-triplet absorption (λ = 750 nm, ε =
Figure S1 in Supporting information (SI)).
The absorption
4
–1
–1
[38]
.8 × 10 M cm in benzene; Φ
T
= 0.98) served as a reference to
maxima and absorption coefficients due to acridinium ion moiety
are virtually the same irrespective of the difference in the R
moiety (see Table 1).
+
confirm the quantum yields of Acr –R. The minimum molar extinction
+
coefficients of the triplet ET state of Acr –R at 500 nm were ε = 3.9 ×
3
–1
–1
3
–1
–1
1
1
0
M
cm in MeCN and ε = 4.1 × 10
M
cm in PhCN were
The chemical shifts of H NMR spectra of the acridinium ion
•
+
determined from the absorbance at 500 nm due to Acr radical
moiety in Acr –R are also the same irrespective of the difference
+
produced
by
the
ET
reduction
of
Acr –1NA
by
in the R moiety (see Experimental Section). Indeed, the dihedral
angle of the X-ray crystal structure of 9-mesityl-10-
[
16]
tetramethylsemiquinone radical anion. It should be noted that the
+
+
determination of the quantum yields of the triplet ET state of Acr –R
methylacridinium ion (Acr –Mes) made by aromatic ring planes
•
[17]
using the ε value of Acr –1NA affords the maximum values, because
was found to be approximately perpendicular.
The calculated
•
+
+
absorbance due to R is somewhat overlapped with absorbance at
structure of Acr –Mes agreed with the X-ray crystal structure,
which exhibits that the dihedral angle is also perpendicular
•
5
00 nm due to the Acr moiety in the triplet ET state.
+
+
[17]
between the Mes and Acr moieties of Acr –Mes.
In such a
Spectral measurements.
A
standard actinometer (potassium
case, the orbital interaction between the donor and acceptor
[
39]
+
ferrioxalate) was used for the quantum yield determination of the
moieties is minimized. In contrast, the dihedral angle of Acr –Ph
2
+
photochemical reactions of BNAH with hexyl viologen (HV ) in the
is not perpendicular (68˚) according to the reported crystal
+
[43,44]
presence Acr –R. Typically, a square quartz cuvette (10 mm i.d.)
structure.
The dihedral angles of other 9-subsituted
3
+
which contained a deaerated MeCN solution (3.0 cm ) of Acr –R (2.0
acridinium ions were determined by DFT method with Gaussian
98 (B3LYP/6-31G(d) basis set) as listed in Table 1. The
–
4
–4
2+
–3
×
10 M), BNAH (2.0 × 10 M), and HV (1.0 × 10 M) was
+
irradiated with monochromatized light of λ = 430 nm from a Shimadzu
orientation between the R and Acr moieties is mostly orthogonal
RF-5300PC fluorescence spectrophotometer. Under the conditions of
except for R = Tol, 2-NA and 2-NA(6-OMe). This indicates that
+
actinometry experiments, the actinometer and Acr –R absorbed
introduction of a substituent at the ortho-position is essential to
+
essentially all of the incident light of λ = 430 nm. The light intensity of
minimize the interaction between the R and Acr moieties (see
–
9
monochromatized light of λ = 430 nm was determined as 2.98 × 10
Chart 1).
–
1
+
einstein s . The photochemical reaction was monitored using a
The one-electron redox potentials of Acr –R were determined
Hewlett Packard 8453 diode-array spectrophotometer. The quantum
by cyclic voltammetry measurements. The one-electron reduction
•
+
+
yields in the absence of oxygen [Φ(HV )] were determined from
process of the Acr moiety was observed as a well-defined
•
+
4
–1
–1 [40]
+
+
increase in absorbance of HV at 605 nm (ε = 1.0 × 10 M cm ).
reversible wave at –0.57 V for Acr –Mes, –0.55 V for Acr –1NA,
+
and –0.52 V for Acr –An in MeCN (see Figure S2 in SI). The one-
+
ESR measurements. The ESR spectra were taken on a JEOL X-
band spectrometer (JES-RE1XE) with a quartz ESR tube (1.2 mm
i.d.). The g values and the zero-field splitting parameters (D and E)
electron reduction potentials of Acr –R (Ered) thus determined are
listed in Table 2. On the other hand, the one-electron oxidation
+
potentials (Eox) of Acr –R were determined by the second-
2
+
•
•
were calibrated using an Mn marker. The Acr –Mes and Acr –Dur
harmonic AC voltammetry (SHACV) method, since the one-
electron oxidation process was irreversible in the CV
measurements (see Figure S3 in SI). The Eox values are also
listed in Table 2. The Eox values agree with those of the
+
were generated by the electron-transfer reduction of Acr –Mes (1.0 ×
–
4
+
–4
1
0
M) and Acr –Dur (1.0 × 10 M) with tetramethylsemiquinone
–
4
radical anion (1.0 × 10 M) generated by comproportionation of
tetramethyl-p-benzoquinone and tetramethyl-p-hydroquinone with
tetra-n-butylammonium hydroxide. The solution containing the radical
was transferred to an ESR tube under an atmospheric pressure of Ar.
The hyperfine coupling constants were determined by computer
simulation using a Calleo ESR Version 1.2 program coded by Calleo
Scientific on a personal computer.
[
17]
corresponding aromatic donor compounds. The one-electron
R
Me
Me
R =
N
Me
Me
+
+
+
1
: Acr –Ph
2: Acr –Tol
3: Acr –Xyl
Theoretical
calculations were performed on
Geometry optimizations were carried out using the Becke3LYP
calculations.
Density-functional
theory
(DFT)
Me
a
COMPAQ DS20E computer.
Me
Me
Me
Me
[
41]
Me
Me
functional and 6-31G(d) basis set
with the unrestricted Hartree-
Fock (UHF) formalism and as implemented in the Gaussian 98
program.
+
+
+
+
4
: Acr –Mes
5: Acr –Dur
6: Acr –1NA
7: Acr –2NA
OMe
ResultsꢀandꢀDiscussionꢀ
Me
+
+
Spectral and redox properties of Acr –R: A series of 9-
+
+
+
8
: Acr –1NA(2-Me)
9: Acr –An
10: Acr –2NA(6-OMe)
substituted 10-methylacridinium ions (Acr –R) synthesized in this
study are shown in Chart 1. An electron donor moiety (R) is
+
directly linked with an electron acceptor moiety (Acr ) at the 9-
Chart 1. 9-Substituted acridinium ion used in this study.
4
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Chemistry - A European Journal
10.1002/chem.201604527
DOI: 10.1002/chem.201xxxxxx
+
[17]
Table
1
Absorption maxima (λmax), absorbance coefficients (ε),
(AcrH : τ = 37 ns in MeCN) was significantly reduced when it
+
fluorescence maxima (λfl) and dihedral angles between acridinium ion
and the donor moieties of 9-substituted 10-methylacridinium ions.
was replaced by Acr –1NA (τ = 310 fs) due to the quenching of
the singlet excited state of Acr by 1NA moiety (Table 2). The
+
–
3
+
1
0
ε,
dihedral
fluorescence decay curve of Acr –1NA at 500 nm was well-fitted
+
–1
–1 [a]
[b]
Acr –R
λ
max, nm
M
cm
λ
fl, nm
angle
by a single-exponential decay (Figure 1a). Photoexcitation of
+
+
Acr –1NA in MeCN with femtosecond laser at 420 nm affords a
Acr –Tol
360, 425
360, 423
359, 422
359, 422
360, 424
359, 428
360, 423
359, 447
19.3, 7.50
17.0, 6.50
15.8, 5.60
14.3, 4.70
15.5, 5.10
14.0, 5.90
18.2, 6.10
15.0, 7.40
501
500
501
495
62.3˚
88.0˚
89.7˚
89.5˚
89.9˚
61.9˚
89.6˚
58.0˚
89.0˚
+
transient absorption spectrum with maxima at 530 and 700 nm,
Acr –Xyl
[
46,47]
which are assigned to acridinyl radical
and naphthalene
respectively (Figure 1b). Thus, the transient
absorption spectrum in Figure 1b can be assigned to the singlet
+
Acr –Mes
[48]
radical cation,
+
Acr –Dur
+
1
•
•+
Acr –1NA
501,590
500, 594
497
ET state [ (Acr –1NA )]. The rate constant of formation of the
+
12
–1
Acr –2NA
singlet ET state (3.4 × 10 s ) agrees with the fluorescence
+
12
–1
+
decay rate constant of Acr –1NA (3.2 × 10 s ) in Figure 1a.
Such agreement of the rate constant of formation of the singlet
ET state with the fluorescence decay rate constant was also
Acr –1NA(2-Me)
+
[c]
Acr –An
n.d.
+
[c]
Acr –2NA(6-OMe) 365, 387, 422 21.6, 10.1, 6.50
n.d.
+
[17]
confirmed for Acr –Mes in MeCN at 298 K (Figure S4 in SI).
+
The time profile of the singlet ET state of Acr –Mes was
examined by sub-nanosecond laser-induced transient absorption
[
a] In MeCN at 298 K. [b] Determined by DFT calculation (B3LYP/6-31G(d)
basis set). [c] Not determined.
[
28]
measurements (see Experimental Section) as shown in Figure
2. The decay of the absorption band at 500 nm was accompanied
by appearance of the NIR absorption around 1000 nm. The
appearance of the NIR absorption was previously reported to
result from the formation of the π-dimer radical cation between
(a)
(b)
4
3
2
1
00
00
00
00
0.08
0.06
0.04
0.02
0
0.06
NA•+
Acr•
0
.04
.02
0
s
b
A
+
+
[49]
Δ
the ET state of Acr –Mes and the ground state Acr –Mes.
0
(a)
(b)
0
–
0.5
0
0.5
1.0
1.5
500
600
700
800
Time, ps
Wavelength, nm
Figure 1. (a) Fluorescence decay time profile (●) and absorption time profile
+
Acr –Mes
+
–4
(
◯) of Acr –1NA (1.0 x 10 M) in MeCN after laser excitation at 420 nm at 298
+
K. (b) Transient absorption spectrum of Acr –1NA in deaerated MeCN taken at
1
.3 ps after femtosecond laser excitation at 420 nm.
+
redox potentials of Acr –R were also determined in chloroform
3
+
Acr –Mes
(
CHCl
3
) and benzonitrile (PhCN) and the values are listed in
Table 2.
+
Formation of ET states of Acr –R. The fluorescence lifetimes
+
–4
+
Figure 2. (a) Transient absorption spectra of Acr -Mes (5.0 × 10 M) in
deaerated MeCN at 298 K taken 5 ns (●) and 50 ns (◯) after laser excitation at
355 nm. (b) Time profiles of the absorbance decay at 500 nm (●) and the rise
(
τ) of Acr –R were measured by using femtosecond laser
excitation at 420 nm (see Experimental Section). The
fluorescence lifetime of the unlinked 10-methylacridinium ion
+
–3
at 1000 nm (◯) of Acr –Mes (2.0 × 10 M) in deaerated MeCN at 298 K after
laser excitation at 355 nm.
Table 2. One-electron oxidation and reduction potentials and fluorescence lifetimes of 9-substituted 10-methylacridinium ions.
in MeCN
in PhCN
in CHCl
3
τ, ps
+
E
ox vs.
E
red vs.
E
ox vs.
E
red vs.
E
ox vs.
Ered vs.
SCE, V
Acr –R
MeCN
PhCN
CHCl
3
SCE, V
SCE, V
SCE, V
SCE, V
SCE, V
+
[a]
a
[b]
b
Acr –Ph
–––
–––
–0.55
2.27
2.06
2.08
1.88
1.84
1.78
1.85
1.77
1.34
1.45
–0.60
–0.62
–0.61
–0.49
–0.62
–0.57
–0.60
–0.58
–0.57
–0.61
–––
–––
–0.47
1500
3100
1500
200
4.2
2200
+
Acr –Tol
–0.56
–0.56
–0.57
–0.62
–0.55
–0.56
–0.54
–0.52
–0.57
–0.46
–0.46
–0.46
–0.48
–0.46
–0.46
–0.44
–0.40
–0.48
310
80
1200
120
0.12
5.3
+
Acr –Xyl
2.15
2.06
1.84
1.72
1.87
1.37
1.25
1.25
2.24
2.20
2.12
1.88
1.87
1.82
1.53
1.52
+
Acr –Mes
4.2
+
Acr –Dur
0.86
0.31
0.70
0.22
0.34
0.18
3.7
+
Acr –1NA
3.6
5.4
+
Acr –2NA
3.7
7.9
+
Acr –1NA(2-Me)
0.38
0.30
0.25
1.0
+
Acr –An
0.26
1.9
+
Acr –2NA(6-OMe)
[
a] Taken from ref. [45]. [b] Taken from ref. [46].
5
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Chemistry - A European Journal
10.1002/chem.201604527
DOI: 10.1002/chem.201xxxxxx
+
However, the rate of appearance of the NIR band at 1000 nm
obeyed first-order-kinetics and the first-order rate constant
500 nm due to the ET states of Acr –R are shown in Figure 4a.
The decay of the ET state occurs via intermolecular back electron
+
3
•
•+
remained constant with increasing concentration of Acr –Mes
transfer between two
intramolecular back electron transfer from the Acr moiety to the
(Acr –R ) molecules rather than
•
(
Figure S5 in SI). Thus, the observed spectral change in Figure 2
•
+
results from the intramolecular process, which is assigned to be
intersystem crossing (ISC) from the singlet ET state to the triplet
ET state. The absorption maximum remained the same at 500 nm
but the absorbance decreased by ISC, whereas the NIR
absorption band appeared by ISC. The similar NIR absorption
band was also observed by ISC form the singlet ET state to the
R
moiety, because the intermolecular back electron transfer is
spin-allowed, whereas the intramolecular back electron transfer is
•
•+
spin-forbidden. The ET state (Acr –R ) population obeys second-
order kinetics rather than first-order kinetics as indicated by the
•
•+ –1
linear plot of [Acr –R ] vs. time in Figure 4b, where the
•
•+
concentration of Acr –R was determined using the ε value at 500
+
•
3
–1
–1
triplet ET state of Acr –1NA (Figure S6 in SI). The increase in the
nm of Acr –1NA (ε = 3.9 × 10 M cm in MeCN and ε = 4.1 ×
3 –1 –1 •
10 M cm in PhCN). In such a case, the concentration of Acr –
NIR band in Figure 2 (and Figure S6 in SI) may result from the D
1
[
50-52]
•+
←
D
0
dipole-forbidden transition,
which becomes slightly
R may be overestimated due to the overlap of the absorbance of
•
allowed in the triplet ET state, whereas it is forbidden in the
singlet ET state. Due to operation of the Pauli principle, the
integrated repulsion between electrons of like spin in the triplet is
lower than that between electrons of opposite spin in the singlet.
the Acr moiety at 500 nm. Thus, the second- order decay rate
constants determined form the slopes of the plots in Figure 4b are
regarded as the minimum values, which are already close to the
diffusion-limited values as listed in Table 3. In addition, the
+
Thus, the unpaired electron in the donor radical cation of the
energy diagram and photodynamics of Acr –R (R = Mes and
•
triplet ET state may be slightly more delocalized in the Acr moiety, 1NA) in MeCN were summarized in Scheme 1.
making the forbidden transition in the NIR region to be observable.
The rate constant of the intersystem crossing of the singlet ET
Table 3. Rate constants of intermolecular back electron transfer (kBET
of 9-substituted 10-methylacridinium ions
)
+
state of Acr –Mes was determined from the slope of the first-order
8
–1
plot (Figure S5 in SI) to be 1.3 × 10 s at 298 K. Similarly the
–1 –1
s
k
BET, M
rate constant of the intersystem crossing of the singlet ET state of
+
Acr –R
+
8
–1
in MeCN
in PhCN
Acr –1NA was determined to be 2.5 × 10 s at 298 K (Figure S6
+
9
9
in SI).
Acr –Mes
9.3 × 10
3.0 × 10
4.6 × 10
6.9 × 10
7.9 × 10
+
The transient absorption spectra of Acr –An are shown in
+
10
9
9
9
Acr –1NA
1.7 × 10
[a]
Figure 3a, where the absorption due to the anthracene radical
+
Acr –1NA(2-Me)
+
(a)
(b)
Acr –2NA
[a]
0
.10
.08
0.10
3
.8 ps
59 ps
10 ps
[
a] Not determined.
0
2
7
20 nm
1Acr+*–R
0
.06
4.2 ps
1(
•
•+
Acr –Mes )
2.63 eV
8
–1
0
.05
2.72 eV
1.3 × 10 s
3
•
•+
580 nm
(Acr –Mes )
0
.04
.02
0
9
.3 × 10 M s–1
9
–1
3
10 fs
0
1(
Acr –1NA )
2.27 eV
•
•+
8
–1
2
.5 × 10 s
3
•
•+
hν
(Acr –1NA )
0
5
50
600
650
700
750
800
0
200
400
600
800
–4
1000
Wavelength, nm
Time, ps
+
Figure 3. (a) Transient absorption spectra of Acr –An (1.0 × 10 M) in
1
.7 × 1010 M–1 s–1
deaerated MeCN at 298 K taken after femtosecond laser excitation at 420 nm.
•
•+
(
b) Decay time profiles of Acr –An at 580 and 720 nm.
+
Acr –R (R = Mes and 1NA)
cation is clearly detected at 720 nm, which agrees with the
[
48,53]
+
absorption maximum of anthracene radical cation.
In this
Scheme 1. The energy diagram and photodynamics of Acr –R (R =
case, the transient absorption at 720 nm as well as at 580 nm
decays to zero in the observed time range up to 1 ns. This
indicates that the singlet ET state decays to the ground state with
a lifetime of 90 ps prior to the intersystem crossing to the triplet
Mes and 1NA)
3
•
•+
The decay lifetime of (Acr –Mes ) due to intramolecular back
electron transfer in MeCN at 298 K were determined to be 2.0 s
+
3
*
+
ET state. The decay to the locally excited triplet state (Acr – An )
is energetically unfavorable, since the triplet energy of anthracene
by incorporating Acr –Mes into nanosized mesoporous silica-
alumina in order to prohibit the intermolecular back electron
[
54]
+
[55]
3
•
•+
moiety (1.84 eV)
1.77 eV). The fast decay of the singlet ET state [ (Acr –An )] to
the ground state results from the much smaller driving force as
is higher than that of ET state of Acr –An
transfer.
intramolecular back electron transfer in PhCN at 298 K were also
And the rate constant of (Acr –Mes ) due to
1
•
•+
(
–
1
–1
determined to be 98 s by the linear plot of ln(kBET/T) vs. T
1
•
•+
[17]
compared with the case of (Acr –Mes ) (vide infra).
measured by laser flash photolysis and ESR methods.
The
3
•
•+
much longer lifetime of (Acr –Mes ) due to intramolecular back
electron transfer in PhCN than that in MeCN results from the
larger driving force of the back electron transfer in PhCN than that
in MeCN (see the Eox and Ered values in PhCN vs. MeCN in Table
2).
+
Quantum yields of formation of the ET states of Acr –R. The
+
transient absorption due to the ET state of Acr –Mes decays in
the time range up to 3 µs when the residual absorption still
remains. The decay time profiles of the transient absorbance at
6
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Chemistry - A European Journal
10.1002/chem.201604527
DOI: 10.1002/chem.201xxxxxx
The initial absorbance at 500 nm is significantly different
depending on the type of R in Figure 4a. The absorbance at 500
+
nm due to the triplet ET state of Acr –Dur is certainly not zero in
the time range up to 80 µs. This indicates that there is a long-
•
•+
lived component even in the case of Acr –Dur , which affords the
smallest absorbance at 500 nm in Figure 4a.
+
The quantum yields of the triplet ET states (ΦTET) of Acr –R
were determined from the initial absorbance at 500 nm due to the
triplet ET state in Figure 4a (Table 4). The ΦTET values vary
significantly depending on the substituted donor moieties in the
+
•
•+
series of Acr –R. The ΦTET value of Acr –Mes is the largest
98%), which is close to unity. It should be noted again, however,
the ΦTET value is the maximum value determined by using the
(
•
3
–1
–1
ε value at 500 nm of Acr –1NA (ε = 3.9 × 10 M cm in MeCN
3
–1
–1
and ε = 4.1 × 10 M cm in PhCN).
(
a)
(b)
0.12
0.08
0.04
0
8.0
6.0
+
Acr –Mes
98%
36%
+
Acr –1NA
(D)
+
Acr –1NA(2-Me) 23%
+
Acr –2NA
–Dur
10%
4%
+
Acr
Figure 5. Driving force (–ΔGET or –ΔGBET) dependence of the logarithm of
(C)
+
intramolecular ET and BET rate constants (log kET or log kBET) in Acr –R in
4
2
.0
.0
0
(B)
3
MeCN (●), PhCN (■) and CHCl (▲) at 298 K.
(A)
expected to be very different for reactions involving the singlet
and triplet ET states. A better approximation can be achieved by
using semi-quantum theory or using Equation (2),
0
20
40
Time, µs
60
80
0
20
40
60
80
Time, µs
Figure 4. (a) Decay time profiles at 500 nm and quantum yields of formation of
ET state of Acr –R (5.0 × 10 M) in PhCN at 298 K after laser excitation at 355
+
–5
3/2
2
∞
i
#
2
&
(
'
2π
V
−S
S
(ΔGET
+ λ +iE
o v
)
q
ET
k
=
e
exp −
%
(2)
nm determined by the comparative method. (b) Second-order plots of the decay
∑
+
+
+
of the transient absorption spectra of (A) Acr –Mes, (B) Acr –1NA, (C) Acr –
h
λ k T i=0
i!
%
$
4λ
o
k
B
T
(
o
B
+
1
NA(2-Me) and (D) Acr –2NA. MeCN at 298 K after laser excitation at 355 nm.
Driving force dependence of intramolecular photoinduced ET
where E
vibrational coupling, and λ
energy in this case. The internal reorganization energy can be
calculated as λ = E /S. The fit of the data involving the singlet ET
states only is shown in Figure 5 by the green dashed line, and it
v
is the vibrational frequency, S is the electronic-
+
and BET of Acr –R. The driving force of photoinduced ET (–
ο
is the outer sphere reorganization
1
+*
ΔGET) from the 1NA to Acr in MeCN is determined to be 0.45
eV from the one-electron reduction potential (Ered vs. SCE = –
ι
v
1
*
+
0
.55 V) and the excited state energy ( ΔE = 2.72 eV) of the Acr
–
1 [56]
moiety and the one-electron oxidation potential of the 1NA moiety
yields V = 130 cm ,
The corresponding internal reorganization energy is λ
and the total reorganization energy λ = λ + λ = 0.8 eV. The
λ
ο
= 0.48 eV, E
v
= 0.17 eV, and S = 1.9.
+
(Eox vs. SCE = 1.72 V) in Acr –1NA (see Experimental Section).
ι
= 0.32 eV
+
Similarly the –ΔGET values of Acr –R were determined from the
ο
ι
1
*
E
ox, Ered and ΔE values. The rate constants of photoinduced ET
semi-quantum theory predicts somewhat lower reorganization
energy, essentially lower electronic coupling, and gives much
better data approximation, but the rates of the triplet ET reactions
–1 –1
are determined from the difference between τ and τ
0
, where τ
+
+
and τ
(
0
are fluorescence lifetimes of Acr –R in Table 2 and AcrH
[
46]
37 ns in MeCN, 35 ns in PhCN and 30 ns in CHCl
3
).
The
(
two points in the right-bottom corner of the plot) lay completely
driving force dependence of log kET is shown in Figure 5. The
outside this dependence. The latter is expected since the triplet
+
same plot shows also rate constants obtained for BET for Acr –
ET reaction must have much lower V. Actually all other
+
2
NA(6-OMe) and Acr –An in which case the driving force was
parameters describing ET dependence (E
v
, λ , and S) are
ο
evaluated as differences between Eox and Ered. This part of plot
associated with nuclear subsystem and should be insensitive to
the multiplicity at least to the first approximation. Assuming that
only V differs the reactions involving singlet and triplet ET states,
the dotted curve was generated in Figure 5. This curve predicts
presents ET reactions with the same multiplicity. As mentioned
+
above, the slowest BET was observed for Acr –Mes, and was
attributed to the triplet ET state for which electronic coupling may
differ gradually form that of the singlet BET state. Also the driving
force may be expected to be somewhat smaller for the triplet BET
compared to that of the singlet one. All the points including the
T
the electronic coupling for the triplet ET reactions to be V =
–
1
0
.07 cm , or much smaller than that for the singlet ET, as
expected.
Over all the λ value corresponds to that of the intermolecular
+
BET in Acr –Mes form a bell-shape dependence expected for
1
+*
Marcus-type photoinduced ET and BET processes. If modeled by
photoinduced ET from various alkylbenzenes to AcrH in MeCN
[
15]
–1
[
16]
Equation (1), the values λ = 0.92 eV and V = 330 cm can be
obtained. However, it is known that the classic Marcus theory
describes poorly so-called inverted region (–ΔGET > 1 eV, in this
case), the obtained value of electronic coupling is to large to treat
vibrational motion classically, and, more importantly, V is
(
0.88 eV). In the normal region of the Marcus parabola (–ΔGET
λ), the kET value increases with increasing the –ΔGET value.
<
+
8
–1
Thus, the kET value of Acr –Ph (4.8 × 10 s ) in the Marcus
+
normal region is much smaller than the kET value of Acr –2NA(6-
1
2
–1
OMe) (5.6 × 10 s ) at the top region (–ΔG ≈ λ) (see Figure S7
ET
7
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Chemistry - A European Journal
10.1002/chem.201604527
DOI: 10.1002/chem.201xxxxxx
in SI). On the other hand, the rate constant of intramolecular back
Since the quantum yield of formation of the triplet excited state
1
•
•+
+
electron transfer (BET) in the case of (Acr –An ) (Figure 3) fits
well the Marcus semi-quantum curve in the inverted region.
of Acr –Mes is unity, exhibiting the highest photocatalytic
2
+
efficiency for the one-electron reduction of HV by BNAH (vide
3
•
•+
•
Whereas the BET rates for (Acr –Mes ), which were determined
supra), the oxidizing and reducing abilities of the ET state (Acr –
+
•+
+
by using Acr –Mes incorporated into nanosized mesoporous
Mes ) are examined in detail. The triplet ET state of Acr –Mes is
expected to act as a strong electron acceptor (Ered vs. SCE = 2.06
V in MeCN) as well as a strong electron donor (Eox vs. SCE = –
0.57 V in MeCN), which can oxidize and reduce the various
substrates, respectively. We also examined the electron-transfer
[
55]
silica-alumina in MeCN, can be used to compare the electronic
couplings for the ET reactions with the same and changing
multiplicities.
It should be noted that the second-order rate constants of
intermolecular back electron transfer in Table 3 cannot be plotted
in Figure 5. The predominant intermolecular back electron
transfer as compared with the slow intramolecular back electron
transfer may result from the much larger solvent reorganization
energy for the intermolecular back electron transfer as observed
•
+
oxidation of various aromatic compounds by the Mes moiety.
•
•+
Since the one-electron reduction potential of Acr –Mes (Ered vs.
SCE = 2.06 V) is more positive than that of the one-electron
[
59]
oxidation potential of naphthalene (Eox vs. SCE = 1.80 V),
•
+
1
•
electron transfer from naphthalene to the Mes moiety of (Acr –
[
57]
•+
for solvent-separated radical ion pair.
Mes ) is energetically feasible. The rate constant of electron
•
•+
•+
1
•
•+
Oxidizing and reducing ability of Acr –Mes . We have
transfer from naphthalene to the Mes moiety of (Acr –Mes )
was determined from the decay of absorbance at 500 nm in the
previously reported the photocatalytic one-electron reduction of
2
+
3
•
•+
hexyl viologen (HV ) by an NADH analogue, 1-benzyl-1,4-
dihydronicotin amide (BNAH) to produce two equivalents of the
time scale of intersystem crossing to (Acr –Mes ) by
1
0
–1 –1
femtosecond laser excitation to be 1.0 × 10
Figure S8 (SI). The rate constant of electron transfer from
M
s as shown in
•
+
one-electron reduced species (HV ) and one-equivalent of the
+
[40]
•+
3
•
•+
two-electron oxidized species (BNA ).
The quantum yields of
naphthalene to the Mes moiety of (Acr –Mes ) was also
determined by nanosecond laser excitation of a deaerated MeCN
•
+
•+
formation of HV [Φ(HV )] in Table 4 were determined by using
Acr –R as a photocatalyst (see Experimental Section).
+
[58]
+
The
solution of Acr –Mes containing naphthalene at 355 nm, which
•
+
largest Φ(HV ) value is obtained in the case of the
resulted in the formation of naphthalene radical cation (λmax = 690
2
+
[38]
•
photoreduction of HV
by 1-benzyl-1,4-dihydronicotinamide
nm), whereas the transient absorption due to Acr –Mes (λmax =
+
[17,49]
[
(BNA)
formation of the triplet ET state (98%).
(BNA)
bond cleavage of [(BNA)]
compared with the deprotonation of BNAH to produce BNA in
2
] with Acr –Mes (174%) as is the case of the ΦTET value of
500 nm)
remains the same as shown in Figure 6a. Transient
[
17]
•+
The larger Φ(HV ) of
(
a)
(b)
[
2
] than those of BNAH result from the much faster C-C
•
+
•
+
2
to produce BNA and BNA as
+
NA
NA
•
+
•
10
5.0
2.5
1.3
0
3
+
Acr –Mes
[
31]
competition with the back electron transfer. It should be noted
•
+
that the maximum Φ(HV ) value should be twice of the ΦTET
value, because two equivalents of HV is produced by the
•
+
2
+
[31]
photoreduction of HV by BNAH and (BNA)
2
.
As mentioned
+
NA
above, the ΦTET value of 98% estimated using the ε value at 500
•
nm of Acr –1NA is the maximum value, because of the overlap of
•
+
absorbance due to the R moiety in the triplet ET state. The
•
+
maximum Φ(HV ) value of 174% provides the minimum ΦTET
value of 87%. Thus, the actual ΦTET value is between 87 and 98%.
(c)
•
+
A parallel relationship between ΦTET and Φ(HV ) in Table 4
2
+
indicates that the photocatalytic reduction of HV was catalyzed
+
by the long-lived triplet ET states of Acr –R.
+
Table 4 Quantum yields of the triplet ET State (ΦTET) of Acr –R and
•
+
•+
quantum yields [Φ(HV )] of formation of HV in the photocatalytic
2
+
2
reduction of HV by BNAH and (BNA) .
[
a]
+
[b]
+ [c]
Acridinium
Φ
TET , %
Φ(HV˙ ) , %
Φ(HV˙ ) , %
+
Acr –Tol
5
8
74
100
2
6
95
174
6
+
Acr –Xyl
55
98
4
+
Acr –Mes
–
4
+
+
Figure 6. (a) Transient absorption spectra of Acr -Mes (1.0 × 10 M) in
Acr –Dur
–1
the presence of NA (2.0 × 10 M) in deaerated MeCN at 298 K taken 2 µs
+
Acr –1NA
36
10
23
[d]
[d]
24
8
28
8
after laser excitation at 355 nm. (b) Time profiles of the absorbance rise at
690 nm. (c) Rise rate constant versus concentrations of naphthalene.
+
Acr –2NA
+
Acr –1NA(2-Mes)
8
17
[d]
[d]
absorption spectra of radical cation species of 9,10-
dimethylanthracene, 9-methylanthracene, anthracene, fluorene,
2-methoxynaphthalene, and 1-bromonaphthalene were also
observed by electron transfer from these aromatic donors to the
+
Acr –An
[d]
[d]
+
Acr –2NA(6-OMe)
–
4
2+
[
a] Determined from Figure 4a. [b] [BNAH] = 2.0 × 10 M; [HV ] = 1.0 ×
•+
•
•+
–
3
+
–4
–4
2+
Mes moiety of Acr –Mes in MeCN when the electron transfer is
1
1
0
0
M; [Acr –R] = 2.0 × 10 M. [c] [(BNA)
2
] = 4.0 × 10 M; [HV ] = 1.0 ×
[
60]
–
3
+
–4
thermodynamically feasible (see Figure S9 in SI).
M; [Acr –R] = 2.0 × 10 M. [d] Too small to be determined accurately.
8
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Chemistry - A European Journal
10.1002/chem.201604527
DOI: 10.1002/chem.201xxxxxx
+
Photoirradiation of an MeCN solution of Acr –Mes containing
SI). We also examined the electron-transfer reduction of TCNQ
2
+
•+
•
3
•
•+
HV results in formation of hexyl viologen radical cation (HV ;
by the Acr moiety of (Acr –Mes ) (Figure S11 in SI). The rate
constants of the electron-transfer reduction of electron acceptors
[
40]
λ
max = 605 nm) with a concomitant decrease in the absorption
•
•
3
•
•+
band due to the Acr moiety as shown in Figure 7. Electron
by the Acr moiety of (Acr –Mes ) are listed in Table 5 together
with the driving force of electron transfer. As shown in Table 5,
•
3
•
•+
2+
transfer from the Acr moiety of (Acr –Mes ) to HV (Ered vs.
[
38]
3
•
•+
[63]
SCE = –0.42 V) is energetically feasible (–ΔGET = 0.15 eV).
When p-benzoquinone (Q; Ered vs. SCE = –0.50 V)
(Acr –Mes ) acts as both an electron donor and acceptor. The
driving force dependence of the rate constants of electron-
[
61,62]
is
•–
2
+
3
•
•+
employed instead of HV , p-benzosemiquinone radical anion (Q
transfer reactions of (Acr –Mes ) with electron donors and
acceptors is shown in Figure 8. The solid line is drawn by using
the Marcus equation for intermolecular outer-sphere electron-
transfer reactions [Equation (3)],
[
61,62]
;
λ
max = 400 nm)
is also produced by the electron transfer
•
3
•
•+
from the Acr moiety of (Acr –Mes ) to Q (see Figure S10 in
(a)
(b)
2
1
/ket = 1/kdiff + 1/{Zexp[–(λ/4)(1 + ΔGet/λ) /k
B
T]}
(3)
3
+
Acr –Mes
1
1
–1 –1
s , λ is
where Z is the collision frequency taken as 1 × 10
M
the reorganization energy of electron transfer, k is the Boltzmann
B
+
HV
[
15,64]
constant, and T is the absolute temperature.
The λ value is
determined to be 1.0 eV as the best fit value of Equation (3) and
this value is larger than the value obtained for intramolecular
2
50
1
00
7
3
+
6
photoinduced ET and BET of Acr –R (Figure 5), because a larger
3
solvent reorganization is required for intermolecular ET as
compared with intramolecular ET. The driving force dependence
of log ket for intermolecular electron transfer from electron donors
(c)
•
+
•
to the Mes moiety and also from the Acr moiety in the triplet ET
3
•
•+
state [ (Acr –Mes )] to electron acceptors in MeCN clearly
indicates that the photogenerated species is the triplet ET state,
which is capable of not only oxidizing electron donors but also
reducing electron acceptors as long as the one-electron oxidation
potentials of electron donors are lower (less positive) than the
+
one-electron oxidation potential of Acr –Mes and the one-electron
reduction potentials of electron acceptors are higher (more
+
positive) than the one-electron reduction potential of Acr –Mes
(
note than the redox potentials are somewhat different depending
+
on the solvent in Table 2). The locally excited triplet state of Acr –
Mes would never be able to oxidize electron donors with the
oxidation potentials higher than 1.53 V (vs. SCE) and to reduce
electron acceptors lower than 0.07 V (vs. SCE) as discussed in
Introduction.
–
4
+
Figure 7. (a) Transient absorption spectra of Acr -Mes (1.0 × 10 M) in
2
+
–4
the presence of HV (5.0 × 10 M) in deaerated MeCN at 298 K taken 2
µs (●) and 20 µs (◯) after laser excitation at 355 nm. (b) Time profiles of
the absorbance rise at 600 nm. (c) Rise rate constant versus
2
+
concentrations of HV
.
Table 5 One-electron oxidation of electron donors and reduction
potentials of electron acceptors and rate constants of electron transfer
+
from electron donors to the triplet ET state of Acr -Mes and from the
+
triplet ET state of Acr -Mes to electron acceptors.
–
1
–1
8
electron donor
-bromonaphthalene
naphthalene
E
ox vs. SCE, V –ΔG , eV
k
et, M
s
et
1
1.94
1.80
1.52
1.45
1.19
1.11
0.12
0.26
0.54
0.61
0.87
0.95
1.9 × 10
7.4 × 10
1.9 × 10
4.1 × 10
7.5 × 10
8
9
9
9
[
a]
2
-methoxynaphthalene
[
a]
fluorene
[
a]
anthracene
[
a]
10
9
-methylanthracene
1.3 × 10
1.4 × 10
1
0
9
,10-
1.05
1.01
[
a]
dimethylanthracene
electron acceptor
–
1
–1
7
E
red vs. SCE, V –ΔG , eV
k
et, M
s
et
p-benzoquinone
–0.55
–0.42
0.17
0.07
0.15
0.74
3.0 × 10
4.1 × 10
2.7 × 10
Figure 8. Driving force dependences of log ket for electron transfer from
2
+
8
9
•
+
•
•
HV
electron donors (●) to Mes and from Acr in the triplet ET state of Acr –
•
+
Mes to electron acceptors (■) in MeCN at 298 K and the fit of the curve
TCNQ
based on the Marcus theory of electron transfer [Equation (3)].
[
a] Taken from ref. [60a].
9
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Chemistry - A European Journal
10.1002/chem.201604527
DOI: 10.1002/chem.201xxxxxx
1
•
3
•
•+
8
–
The ket value of electron transfer from naphthalene to (Acr –
transfer from the same naphthalene to (Acr –Mes ) (7.4 × 10 M
1 –1
•
+
10
–1 –1
Mes ) (1.0 × 10
M
s ) is ten times larger than that of electron
Me
s ). Such a difference in the ket value results from the higher
energy of the singlet ET state as compared with the triplet ET
state. The singlet-triplet energy gap may be smaller than 0.1 eV
+
+
because the singlet excite state of the Acr moiety of Acr –Mes
(2.73 eV) is only by 0.1 eV higher than the energy of the triplet ET
state (2.63 eV in PhCN). If the energy of the singlet ET state is
higher than 2.73 eV, electron transfer from the Mes moiety to the
Me
g = 2.0025 2 1 H
Me
H
H
H
H
•
3
H
N
4
9
+
H
H
singlet excited state of the Acr moiety would be endergonic.
(
a)
Me
Origin of the large difference in the ET quantum yields of
+
Acr –Mes
Exp.
+
Acr –R. Large quantum yields (ΦTET) of formation of the long-
+
lived triplet ET states of Acr –R in Table 4 are obtained when the
orientation of the R and Acr+ moieties is nearly orthogonal (see
Table 1). Thus, introduction of a substituent at the ortho-position
seems essential to minimize the interaction between the R and
5
G
+
Acr moieties and to attain large quantum yields of the long-lived
+
ET states. However, the triplet ET quantum yield of Acr –Dur
a (H1) = 3.29 G
+
(
4%) is significantly lower than that of Acr –Mes (98%), although
a (H2) = 0.67 G
a (H3) = 3.66 G
a (N9) = 2.52 G
a (N-Me) = 2.07 G
º
both dihedral angles are nearly 90 . The Eox values of the Mes
and Dur moieties are also virtually the same. In order to examine
the difference in the orbital interaction between R = Mes and Dur,
(
b)
Sim.
•
•
the one-electron reduced radicals, Acr –Mes and Acr –Dur, were
+
+
produced by the one-electron reduction of Acr –Mes and Acr –
Dur by tetramethylsemiquinone radical anion and the ESR
5
G
[
16]
ΔHmsl = 0.36 G
spectra are measured as shown in Figure 9.
splitting constants (hfc) and the maximum slope line widths
ΔHmsl) were determined by computer simulation of the ESR
The hyperfine
(
spectra. The hfc values thus determined are given in Figure 9.
The assignment of the hfc values was made based on
comparison of the observed hfc values with those predicted by
DFT calculation (see Experimental Section). There is no
Me
Me
Me
g = 2.0025
Me
H
H
•
1
2
3
appreciable difference in the hfc values between Acr –Mes and
H
H
H
H
•
•
Acr –Dur and no spin is delocalized on the donor moieties as
expected by the orthogonal orientation between the donor and
acceptor moieties. This shows sharp contrast with the case of
N
4
9
H
H
(c)
Me
+
•
Acr –Ph in which the unpaired spin is delocalized on the Ph
Acr –Dur
[16]
Exp.
moiety,
as expected from the non-orthogonal orientation
+
[43,44]
between the Ph and Acr moieties (the dihedral angle is 68˚).
In contrast with the case of the one-electron reduced species
+
+
5
G
of Acr –Mes and Acr –Dur, which exhibited no appreciable
difference in the hfc values, the calculated hfc values of hydrogen
atoms of 2- and 6-methyl groups of the one-electron oxidized
+
+
a (H1) = 3.43 G
a (H2) = 0.66 G
a (H3) = 3.73 G
a (N9) = 2.48 G
a (N-Me) = 1.96 G
(a) Acr –Mes
(b) Acr –Dur
(d)
Sim.
5
G
ΔHmsl = 0.26 G
•
Figure 9. (a) ESR spectrum of Acr –Mes in deaerated MeCN at 298 K and
(
b) the computer simulated spectrum with the hfc and ΔHmsl values. (c)
•
ESR spectrum of Acr –Dur in deaerated MeCN at 298 K and (d) the
+
+
computer simulated spectrum with the hfc and ΔHmsl values.
Figure 10. HOMO orbitals of (a) Acr –Mes and (b) Acr –Dur calculated by
a DFT method at B3LYP/6-31G(d) level.
1
0
This article is protected by copyright. All rights reserved.

Chemistry - A European Journal
10.1002/chem.201604527
DOI: 10.1002/chem.201xxxxxx
+
•+
+
•+
species are quite different between Acr –Mes and Acr –Dur :
[9] S. Fukuzumi, K. Saito, K. Ohkubo, T. Khoury, Y. Kashiwagi, M. A.
Absalom, S. Gadde, F. D’Souza, Y. Araki, O. Ito, M. Crossley, Chem.
Commun. 2011, 47, 7980.
+
•+
+
the hfc value of Acr –Dur is 11.8 G, whereas those of Acr –
•
+
[65]
Mes is 0 G.
Such a drastic difference results from the
+
[10] a) T. Kojima, T. Honda, K. Ohkubo, M. Shiro, T. Kusukawa, T. Fukuda, N.
Kobayashi, S. Fukuzumi, S. Angew. Chem., Int. Ed. 2008, 47, 6712; b) S.
Fukuzumi, T. Honda, K. Ohkubo, T. Kojima, Dalton Trans. 2009, 3880; c)
T. Kojima, K. Hanabusa, K. Ohkubo, M. Shiro, S. Fukuzumi, Chem.–Eur. J.
disappearance in the unpaired spin at the ortho-position in Acr –
Mes (see the spin distributions in Figure 10). The large spin
distribution on 2- and 6-methyl groups of the orbital interaction
•
+
•
•
•+
between the Dur and Acr moieties of Acr –Dur is expected to
2
010, 16, 3646.
+
•+
the durene moiety of Acr –Dur
results from the σ*-π*
[11] S. Fukuzumi, In Functional Organic Materials (Eds.; T. J. J. Müller, U. H. F.
Bunz), Wiley-VCH, 2007, 465.
hyperconjugation. In such a case, the interaction between the
•
+
•
•
•+
[
12] H. Imahori, Y. Sekiguchi, Y. Kashiwagi, T. Sato, Y. Araki, O. Ito, H.
Yamada, S. Fukuzumi, Chem.–Eur. J. 2004, 10, 3184.
Dur and Acr moieties of Acr –Dur would be much larger than
•
+
•
•
•+
that between the Mes and Acr moieties of Acr –Mes , despite
the orthogonal orientation between the donor and acceptor
moieties in both cases, resulting in the fast back electron transfer
in the singlet ET state in competition with the intersystem
crossing to afford the long lived triplet ET state.
[
[
[
13] J.-R. Shen, Annu. Rev. Plant Biol. 2015, 66, 23.
14] A. Harriman, Angew. Chem., Int. Ed. 2004, 43, 4985.
15] a) R. A. Marcus, Angew. Chem., Int. Ed. Engl. 1993, 32, 1111; b) R. A.
Marcus, N. Sutin, Biochim. Biophys. Acta 1985, 811, 265.
16] S. Fukuzumi, K. Ohkubo, T. Suenobu, K. Kato, M. Fujitsuka, O. Ito, J. Am.
Chem. Soc. 2001, 123, 8459.
[
Conclusion
[17] S. Fukuzumi, H. Kotani, K. Ohkubo, S. Ogo, N. V. Tkachenko, H.
Lemmetyinen, J. Am. Chem. Soc. 2004, 126, 1600.
[
18] a) A. C. Benniston, K. J. Elliott, R. W. Harrington, W. Clegg, Eur. J. Org.
Chem. 2009, 253; b) A. C. Benniston, A. Harriman, P. Li, J. P. Rostron, H.
J. van Ramesdonk, M. M. Groeneveld, H. Zhang, J. W. Verhoeven, J. Am.
Chem. Soc. 2005, 127, 16054; c) A. C. Benniston, A. Harriman, P. Li, J. P.
Rostron. J. W. Verhoeven, Chem. Commun. 2005, 2701; d) A. C.
Benniston, A. Harriman. J. W. Verhoeven, Phys. Chem. Chem. Phys.
2008, 10, 5156.
In conclusion, we have obtained a bell-shaped driving force
dependence of rates of intramolecular photoinduced ET including
the BET rate constants in 9-substituted 10-methylacridinium ions
+
+
(
Acr –R). The ET states of Acr –R are extremely long-lived
except for R = An because of the small reorganization energy of
electron transfer and the high energy of the triplet ET sates which
are lower than the locally excited triplet states. In particular, the
[19] K. Ohkubo, H. Kotani, S. Fukuzumi, Chem. Commun. 2005, 4520.
+
quantum yield of the extremely long-lived ET state of Acr –Mes is
[20] a) S. Fukuzumi, K. Ohkubo, T. Suenobu, Acc. Chem. Res. 2014, 47,
+
1
455; b) S. Fukuzumi, K. Ohkubo, Chem. Sci. 2013, 4, 561.
the highest among Acr –R, being close to unity, because of weak
[
[
[
21] a) N. A. Romero, D. A. Nicewicz, Chem. Rev. 2016, 116, 10075; b) K. A.
Margrey, D. A. Nicewicz, Acc. Chem. Res. 2016, 49, 1997.
orbital interaction between the donor and acceptor moiety in the
+
ET state. This makes Acr –Mes the most efficient organic
22] a) N. A. Romero, K. A. Margrey, N. E. Tay, D. A. Nicewicz, Science 2015,
photocatalyst available for construction of energy conversion
systems and synthetic applications.
3
49, 1326; b) C. L. Cavanaugh, D. A. Nicewicz, Org. Lett. 2015, 17, 6082.
23] a) N. A. Romero, D. A. Nicewicz, J. Am. Chem. Soc. 2014, 136, 17024; b)
D. J. Wilger, J. M. Grandjean, T. Lammert, D. A. Nicewicz, Nat. Chem.
Acknowledgmentsꢀ
This work was supported by Grants-in-Aid (no. 16H02268 to S.F.,
2
014, 6, 720; c) T. M. Nguyen, N. Manohar, D. A. Nicewicz, Angew.
Chem., Int. Ed. 2014, 53, 6198; d) D. A. Nicewicz, D. S. Hamilton, Synlett
014, 25, 1191; e) A. J. Perkowski, D. A. Nicewicz, J. Am. Chem.
2
1
6K13964, 26620154 and 26288037 to K.O.) from the Ministry of
Soc. 2013, 135, 10334; f) T. M. Nguyen, D. A. Nicewicz, J. Am. Chem.
Soc. 2013, 135, 9588; g) D. J. Wilger, N. J. Gesmundo, D. A. Nicewicz,
Chem. Sci. 2013, 4, 3160; h) D. S. Hamilton, D. A. Nicewicz, J. Am. Chem.
Soc. 2012, 134, 18577; i) D. A. Nicewicz, T. M. Nguyen, ACS Catal.
Education, Culture, Sports, Science and Technology (MEXT);
SENTAN projects from JST, Japan (to S.F.).
Keywords: photoredox catalyst • charge separation • transient
2
014, 4, 355.
24] S. Fukuzumi, A. Ito, T. Suenobu, K. Ohkubo, J. Phys. Chem. C 2014, 118,
4188.
absorption spectroscopy • donor-acceptor system
[
[
2
25] a) N. Xu, P. Li, Z. Xie, L. Wang, Chem.–Eur. J. 2016, 22, 2236; b) A. J.
Perkowski, C. L. Cruz, D. A. Nicewicz, J. Am. Chem. Soc. 2015, 137,
[
[
1] The Photosynthetic Reaction Center, J. Deisenhofer, J. R. Norris, Eds.,
Academic Press, San Diego, 1993.
1
5684; c) P. D. Morse, D. A. Nicewicz, Chem. Sci. 2015, 6, 270; d) Y. Gu,
K. Ellis-Guardiola, P. Srivastava, J. C. Lewis, ChemBioChem 2015, 16,
880.
2] a) Anoxygenic Photosynthetic Bacteria (Eds.: R. E. Blankenship, M. T.
Madigan, C. E. Bauer), Kluwer Academic Publisher, Dordecht, 1995; b)
Molecular Level Artificial Photosynthetic Materials, G. J. Meyer, Wiley,
New York, 1997.
1
[
26] a) C. Cassani, G. Bergonzini, C.-J. Wallentin, Org. Lett. 2014, 16, 4228; b)
T. Chinzei, K. Miyazawa, Y. Yasu, T. Koike, M. Akita, RSC Adv. 2015, 5,
[
3] a) R. A. Wheeler, Introduction to the Molecular Bioenergetics of Electron,
Proton, and Energy Transfer: ACS Symposium Series 2004, 883, 1; b) W.
Leibl, P. Mathis, Electron Transfer in Photosynthesis. Series on
Photoconversion of Solar Energy, 2004, 2, 117.
2
4
3
1297; c) M. A. Zeller, M. Riener, D. A. Nicewicz, Org. Lett. 2014, 16,
810; d) J. Grandjean, D. A. Nicewicz, Angew. Chem., Int. Ed. 2013, 52,
967.
[
[
27] T. Hering, A. U. Mayer, B. König, J. Org. Chem. 2016, 81, 6927.
[
[
4] a) B. D. Sherman, M. D. Vaughn, J. J. Bergkamp, D. Gust, A. L. Moore, T.
A. Moore, Photosynth. Res. 2014, 120, 5; b) D. Gust, T. A. Moore, A. L.
Moore, Acc. Chem. Res. 2009, 42, 1890.
28] T. Nakagawa, K. Okamoto, H. Hanada, R. Katoh, Opt. Lett. 2016, 41,
1
498.
[
[
[
29] W. L. F. Armarego, C. L. L. Chai, Purification of Laboratory Chemicals, 6th
ed., Butterworth-Heinemann, Amsterdam, 2009.
5] a) M. Rudolf, S. V. Kirner, D. M. Guldi, Chem. Soc. Rev. 2016, 45, 612; b)
G. Bottari, G. de la Torre, D. M. Guldi, T. Torres, Chem. Rev. 2010, 110,
30] S. Fukuzumi, S. Koumitsu, K. Hironaka, T. Tanaka, J. Am. Chem. Soc.
6
768; c) V. Sgobba, D. M. Guldi, Chem. Soc. Rev. 2009, 38, 165.
1
987, 109, 305.
[
[
[
6] M. R. Wasielewski, Acc. Chem. Res. 2009, 42, 1910.
7] S. Fukuzumi, Phys. Chem. Chem. Phys. 2008, 10, 2283.
8] a) F. D’Souza, O. Ito, Chem. Commun. 2009, 4913; b) F. D’Souza, O. Ito,
Coord. Chem. Rev. 2005, 249, 1410; c) M. E. El-Khouly, S. Fukuzumi, F.
D'Souza, ChemPhysChem 2014, 15, 30.
31] a) S. Fukuzumi, T. Suenobu, M. Patz, T. Hirasaka, S. Itoh, M. Fujitsuka, O.
Ito, J. Am. Chem. Soc. 1998, 120, 8060; b) M. Patz, Y. Kuwahara, T.
Suenobu, S. Fukuzumi, Chem. Lett. 1997, 567.
[
32] S. Fukuzumi, H. Imahori, H. Yamada, M. E. El-Khouly, M. Fujitsuka, O. Ito,
1
1
This article is protected by copyright. All rights reserved.

Chemistry - A European Journal
10.1002/chem.201604527
DOI: 10.1002/chem.201xxxxxx
D. M. Guldi, J. Am. Chem. Soc. 2001, 123, 2571.
donor-acceptor dyads. See: a) M. Bixon, J. Jortner, J. W. Verhoeven, J.
Am. Chem. Soc. 1994, 116, 7349; b) R. M. Williams, M. Koeberg, J. M.
Lawson, Y.-Z. An, Y. Rubin, M. N. Paddon-Row, J. W. Verhoeven, J. Org.
Chem. 1996, 61, 5055; c) S. V. Rosokha, D.-L. Sun, J. K. Kochi, J. Phys.
Chem. A 2002, 106, 2283; d) H. Imahori, N. V. Tkachenko, V. Vehmanen,
K. Tamaki, H. Lemmetyinen, Y. Sakata, S. Fukuzumi, J. Phys. Chem. A
2001, 105, 1750. The relatively small V value in the present case results
[
33] J. Bruinink, C. G. A. Kregting, J. J. Ponjee, J. Electrochem. Soc. 1977,
1
24, 1854.
[
[
34] H. Kotani, K. Ohkubo, S. Fukuzumi, J. Am. Chem. Soc. 2004, 126, 15999.
35] The SHACV method provides a superior approach to directly evaluating
the one-electron redox potentials in the presence of a follow-up chemical
and reaction, relative to the better-known dc and fundamental harmonic ac
methods. See: a) M. R. Wasielewski, R. Breslow, J. Am. Chem. Soc. 1976,
+
+
from little interaction between the R and Acr moieties in Acr –R as
+
9
8, 4222; b) E. M. Arnett, K. Amarnath, N. G. Harvey, J. Cheng, J. Am.
described in the section of spectral and redox properties of Acr –R.
Chem. Soc. 1990, 112, 344.
[57] I. R. Gould, S. Farid, Acc. Chem. Res. 1996, 29, 522.
+
[
[
36] C. K. Mann, K. K. Barnes, Electrochemical Reactions in Non-aqueous
Systems, Mercel Dekker, New York, 1970.
[58] Thermal hydride transfer from BNAH to Acr –R is negligible because
•
+
determination of Φ(HV ) was carried out under low concentrations of
2
+
37] Femtosecond fluorescence decays and time-resolved transient absorption
spectra were measured by an up-conversion and pump-probe methods as
described previously. See: N. V. Tkachenko, L. Rantala, A. Y. Tauber, J.
Helaja, P. H. Hynninen, H. Lemmetyinen, J. Am. Chem. Soc. 1999, 121,
BNAH and HV (see Experimental Section). Slow thermal hydride transfer
has been reported; see a) J. Hajdu, D. S. Sigman, J. Am. Chem. Soc.
1975, 97, 3524; b) J. W. Bunting, V. S. F. Chew, G. Chu, N. P. Fitzgerald,
A. Gunasekara, H. T. P. Oh, Bioorg. Chem. 1984, 12, 141.
[59] a) K. Brüggermann, R. S. Czernuszewicz, J. K. Kochi, J. Phys. Chem.
1992, 96, 4405; b) I. R. Gould, D. Ege, J. E. Moser, S. Farid, J. Am. Chem.
Soc. 1990, 112, 4290.
9
378.
[
[
[
[
38] C. Luo, M. Fujitsuka, A. Watanabe, O. Ito, L. Gan, Y. Huang, C.-H. Haung,
J. Chem. Soc., Faraday Trans. 1998, 94, 527.
39] G. J. Kavarnos, Fundamental of Photoinduced Electron Transfer, Wiley-
VCH, New York, 1993.
[60] a) H. Kotani, K. Ohkubo, S. Fukuzumi, Appl. Catal. B: Environ. 2008, 77,
317; b) K. Ohkubo, K. Mizushima, R. Iwata, S. Fukuzumi, Chem. Sci. 2011,
2, 715
40] S. Fukuzumi, H. Imahori, K. Okamoto, H. Yamada, M. Fujitsuka, O. Ito, D.
M. Guldi, J. Phys. Chem. A 2002, 106, 1903.
[61] S. Fukuzumi, T. Okamoto, K. Ohkubo, J. Phys. Chem. A 2003, 107, 5412.
[62] S. Fukuzumi, K. Ohkubo, T. Okamoto, J. Am. Chem. Soc. 2002, 124,
14147.
41] a) A. D. Becke, J. Chem. Phys. 1993, 98, 5648; b) C. Lee, W. Yang, R. G.
Parr, Phys. Rev. B 1988, 37, 785; c) W. J. Hehre, L. Radom, P. v. R.
Schleyer, J. A. Pople, Ab Initio Molecular Orbital Theory, Wiley, New York,
+
[63] The triplet excited state of the Acr moiety is certainly not involved in the
+
+
1
986.
42] The exciplex emission was observed for R = 1NA and 2NA at 594 nm and
90 nm, respectively.
photoexcitation of Acr –Mes, because the triplet excited state of the Acr
[
[
moiety has no reducing ability.
5
[64] Equation (3) has been applied to many intermolecular electron-transfer
reactions; see: a) A. Apostolopoulou, M. Vlasiou, P. A. Tziouris, C.
Tsiafoulis, A. C. Tsipis, D. Rehder, T. A. Kabanos, A. D. Keramidas, E.
Stathatos, Inorg. Chem. 2015, 54, 3979; b) Y.-M. Lee, H. Kotani, T.
Suenobu, W. Nam, S. Fukuzumi, J. Am. Chem. Soc. 2008, 130, 434; c) J.
Park, Y. Morimoto, Y.-M. Lee, W. Nam, S. Fukuzumi, J. Am. Chem. Soc.
2011, 133, 5236; d) P. Comba, S. Fukuzumi, H. Kotani, S. Wunderlich,
Angew. Chem., Int. Ed. 2010, 49, 2622; e) J. Park, Y. Morimoto, Y.-M. Lee,
Y. You, W. Nam, S. Fukuzumi, Inorg. Chem. 2011, 50, 11612; f) A.
Takahashi, T. Kurahashi, H. Fujii, Inorg. Chem. 2011, 50, 6922; g) K.
Ohkubo, S. Fukuzumi, J. Phys. Chem. A 2005, 109, 1105; h) M. Murakami,
K. Ohkubo, S. Fukuzumi, Chem.–Eur. J. 2010, 16, 7820.
43] K. Goubitz, C. A. Reiss, D. Heijdenrijk, S. A. Jonker, J. W. Verhoeven,
Acta Cryst. 1989, C45, 1348.
[
[
44] C. A. Reiss, K. Goubitz, D. Heijdenrijk, Acta Cryst. 1989, C45, 1350.
45] S. Fukuzumi, K. Ohkubo, Y. Tokuda, T. Suenobu, J. Am. Chem. Soc.
2
000, 122, 4286.
46] K. Ohkubo, K. Suga, K. Morikawa, S. Fukuzumi, J. Am. Chem. Soc. 2003,
25, 12850.
[
[
[
[
[
[
[
1
47] M. Tanaka, K. Yukimoto, K. Ohkubo, S. Fukuzumi, J. Photochem.
Photobiol. A 2008, 197, 206.
48] a) T. Shida, S. Iwata, J. Am. Chem. Soc. 1973, 95, 3473; b) F. Cataldo, S.
Iglesias-Groth, A. Manchado, Spectrochim. Acta A 2010, 77, 998.
49] S. Fukuzumi, H. Kotani, K. Ohkubo, Phys. Chem. Chem. Phys. 2008, 10,
+
•+
[65] The DFT calculations of Acr –R were performed using the B3LYP/6-
5
159.
31G(d) basis set.
50] S. F. Nelsen, M. N. Weaver, D. Yamazaki, K. Komatsu, R. Rathore, T.
Bally, J. Phys. Chem. A 2007, 111, 1667.
51] H. Friha, G. Féraud, T. Troy, C. Falvo, P. Parneix, P. Bréchignac, Z.
Dhaouadi, T. W. Schmidt, T. Pino, J. Phys. Chem. A 2013, 117, 13664.
52] K. Hall, M. Boggio-Pasqua, M. Bearpark, M. Robb, J. Phys. Chem. A 2006,
Received: ((will be filled in by the editorial staff))
Revised: ((will be filled in by the editorial staff))
1
10, 13591.
Published online: ((will be filled in by the editorial staff))
[
[
[
53] K. R. Naqvi, T. B. Melø, Chem. Phys. Lett. 2006, 428, 83.
54] R. S. H. Liu, R. E. Kellogg, J. Am. Chem. Soc. 1969, 91, 250.
55] S. Fukuzumi, K. Doi, A. Itoh, T. Suenobu, K. Ohkubo, Y. Yamada, K. D.
Karlin, Proc. Natl. Acad. Sci. U.S.A. 2012, 109, 15572.
[
56] Larger V values have been reported for photoinduced ET in electron
1
2
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Chemistry - A European Journal
10.1002/chem.201604527
DOI: 10.1002/chem.201xxxxxx
Entry for the Table of Contents (Please choose one layout only)
The photoexcitation of 9-
mesityl-10-
Takeshi Tsudaka,
Hiroaki Kotani, Kei
Ohkubo,* Tatsuo
methylacridinium
ion
in
+
(
Acr -Mes)
results
Nakagawa, Nikolai V.
Tkachenko,* Helge
Lemmetyinen,* and
Shunichi Fukuzumi*
formation of the triplet
electron-transfer state,
which has a long lifetime,
a high energy (2.37 eV)
and a high quantum yield
Page No. – Page No.
(
>75%) in acetonitrile,
providing both the
oxidizing and reducing
Photoinduced
Electron Transfer in
9
-Substituted
10-
•
+
activity of the Mes and
Methylacridinium
Ions
•
Acr moieties, respectively.
1
3
This article is protected by copyright. All rights reserved.