Large electronic coupling in a homoconjugated donoracceptor system involving carbon-bridged oligo(p-phenylenevinylene) and triazine

Article abstract of DOI:10.1246/cl.140022

We have synthesized a new homoconjugated donoracceptor (DA) system featuring a rigid planar carbon-bridged oligo(pphenylenevinylene) that shows efficient intramolecular charge separation (CS, 98%). The charge recombination rate is 100- times slower than that of CS, and located in the Marcus inverted region. The electronic coupling was estimated to be 1.0 103 cm ^-1, which is one of the largest values among reported DA interactions.

Full text of DOI:10.1246/cl.140022

Received: January 13, 2014 | Accepted: January 21, 2014 | Web Released: January 28, 2014
CL-140022
Large Electronic Coupling in a Homoconjugated Donor-Acceptor System
Involving Carbon-bridged Oligo(p-phenylenevinylene) and Triazine
Junpei Sukegawa,1 Hayato Tsuji,*1,2 and Eiichi Nakamura*1
1Department of Chemistry, School of Science, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-0033
2JST-PRESTO, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012
(E-mail: tsuji@chem.s.u-tokyo.ac.jp, nakamura@chem.s.u-tokyo.ac.jp)
We have synthesized a new homoconjugated donor-acceptor
(D-A) system featuring a rigid planar carbon-bridged oligo(p-
phenylenevinylene) that shows efficient intramolecular charge
separation (CS, 98%). The charge recombination rate is 100-
times slower than that of CS, and located in the Marcus inverted
region. The electronic coupling was estimated to be 1.0 ©
103 cm¹1, which is one of the largest values among reported
D-A interactions.
obtained by the SNAr reaction of Grignard reagent with cyanuric
chloride (1) in moderate yield. The Miyaura-Ishiyama-Hartwig
borylation of the dibromide 3 provided the corresponding
boronic ester 4. The Suzuki-Miyaura cross coupling between 2
and 4 afforded the ketone 5 in good yield. The dialkyne 6 was
treated with lithium naphthalenide (LiNaph) and the resulting
dilithio intermediate 6¤ was trapped by 5 to produce the diol 7.
Finally, an intramolecular Friedel-Crafts reaction using an
excess of BF3¢OEt2 yielded COPV2-(TRZ)4 as a pale yellow
solid.
Development of donor-acceptor (D-A) systems that show
efficient charge separation (CS) and charge recombination (CR)
in the Marcus inverted region is required for artificial photo-
synthesis.1 The rate of charge transfer is primarily determined
by the D-A electronic coupling (V), which depends on the type
of conjugation or interaction between D and A, such as π
conjugation,2 Si-Si σ conjugation,3 cross-conjugation,4 and π-π
stacking.5 However, D-A systems electronically coupled via
homoconjugation have received less attention.6,7 Here we show
that the carbon-bridged oligo(p-phenylenevinylene) (COPV)8
possessing homoconjugated triazine (TRZ) substituents
COPV2-(TRZ)4 undergoes an efficient CS (98%) and CR,
which is 100-times slower than the CS in a polar solvent
(Figure 1). The electronic coupling was estimated to be as large
as 1.0 © 103 cm¹1, which is one of the largest values among
conjugations and interactions. The rigid and planar skeleton of
COPVs is responsible for the good electron-donating ability, as
we have previously demonstrated in an application in which
COPVs served as efficient photosensitizers in dye-sensitized
solar cells.9 Triazines, which are commonly used for electron-
accepting and -transporting materials,10 were employed because
of the relatively rigid planar skeleton, and the thermal and
chemical stabilities.
Cyclic voltammetry showed a reversible oxidation wave at
0.69 V (vs. Fc+/Fc) and reduction wave at ¹2.23 V (vs. Fc+/Fc)
in tetrahydrofuran (THF), which can be assigned to the one-
electron oxidation of the COPV2 and the four-electron reduction
of the TRZ moieties, respectively (Figure 2), indicating that the
COPV and the TRZ moieties are independent of each other.
The oxidation peak was anodically shifted compared with that
of COPV2-(OctPh)4 (0.53 V), while the reduction peak was
essentially the same as that of TRZ (¹2.18 V).11 The origin of
the anodic shift of the COPV2 moiety can be ascribed to the
orbital interaction between HOMOs of the COPV2 and TRZ
nBu
nBu
BrMg
(3 equiv)
nBu
Cl
N
N
N
N
N
Cl
Cl
N
N
N
THF, 50 °C
N
nBu
Cl
[Pd(PPh3)4]
K2CO3
nBu
1
2 (43%)
O
Toluene
THF–H2O
reflux
Bpin
Br
Br
N
N
[PdCl2(dppf)]
N
nBu
B2pin2, KOAc
1,4-dioxane, 80°C
O
O
O
The synthesis of COPV2-(TRZ)4 was achieved by a method
nBu
reported previously8 (Scheme 1). The diaryl triazine 2 was
Bpin =
B
Bpin
O
3
4 (83%)
5 (80%)
nBu
C8H17
nBu
C8H17
Ph Ph
hv
N
N
Ph
Ph
Li
Ph
Ph
N
N
MeO
LiNaph (4 equiv)
THF, rt
N
nBu
nBu
N
Ph Ph
electron
donating
OMe
Ph
Ph Ph
C8H17
C8H17
Ph
Ph
homo-
conjugation
Li
Ph
COPV2–(OctPh)4
6
6'
+
N
Ph Ph
N
Ar
Ar
nBu
nBu
N
Ph
Ph
OH
Ar
Ar
Ph
Ph
N
N
N
N
•
e–
5 (2 equiv)
BF3 OEt2
N
N
electron
accepting
CCl4, rt
Ph
Ph
Ar
Ar
HO
Ar
Ph
Ph
Ar
COPV2–(TRZ)4
nBu
nBu
TRZ
7 (84%)
COPV2–(TRZ)4 (89%)
Figure 1. Molecular structures of COPV2-(OctPh)4, TRZ, and
COPV2-(TRZ)4.
Scheme 1. Synthesis of COPV2-(TRZ)4.
Chem. Lett. 2014, 43, 699–701 | doi:10.1246/cl.140022
© 2014 The Chemical Society of Japan | 699

104
103
102
101
100
15
10
5
PhMe
PhCN
0
-5
-10
-15
-20
0
20
40
60
80
-2.5 -2 -1.5 -1 -0.5
0
0.5
1
Time / ns
Potential / V (vs. Fc+/Fc)
Figure 4. Fluorescence lifetime measurements of COPV2-
(TRZ)4 in PhMe and PhCN upon excitation at 405 nm.
Figure 2. Cyclic voltammogram of COPV2-(TRZ)4 in THF
(0.1 M Bu4NPF6, 100 mV s scan speed).
¹1
(a) PhMe
(b) PhCN
(a)
(b)
COPV2•+–(TRZ•–)(TRZ)3
2.5
2.0
1.5
1.0
0.5
0
1.0
0.8
0.6
0.4
0.2
0
PhMe
PhCN
COPV2–(TRZ)4
COPV2–(OctPh)4
2.98 eV
1(COPV2)*–(TRZ)4
1(COPV2)*–(TRZ)4
COPV2•+–(TRZ•–)(TRZ)3
CS
2.88 eV
k
CS = 5.3 109 s–1
2.85 eV
kF = 1.0 108 s–1
kCT = 3.2 107 s–1
kF = 4.2 108 s–1
= 0.02
= 0.34
= 0.84
knr = 8.2 107 s–1
nr1 = 1.6 107 s–1
knr2 = 6.2 107 s–1
k
250 300 350 400 450 500
Wavelength / nm
450
500
550
600
650
COPV2–(TRZ)4
COPV2–(TRZ)4
Wavelength / nm
Figure 5. Energy diagram in (a) PhMe and (b) PhCN upon
excitation at 405 nm.
Figure 3. Steady-state photophysical properties. (a) UV-vis
absorption spectra of COPV2-(TRZ)4 in PhCN and COPV2-
(OctPh)4 in CH2Cl2. (b) Fluorescence spectra of COPV2-(TRZ)4
excited at 400 nm.
(95%). The fast component can be assigned as the fluorescence
from the singlet excited state of COPV2 moiety quenched by
CS, while the slow component is caused by the CT emission.
The absolute photoluminescence quantum yields (Φ) in
toluene and benzonitrile were determined to be 0.84 and
0.36, respectively. The lower Φ in the polar solvent supports
the electron transfer. In toluene, the singlet excited state
1(COPV2)*-(TRZ)4 is quenched by either fluorescence (kF)
or nonradiative decay (knr) at rate constants of 4.2 © 108 and
8.2 © 107 s¹1, respectively, according to the equations kF =
Φ/¸ and Φ = kF/(kF + knr) (Figure 5a). In benzonitrile, CS
(kCS) occurs to form the charge-separated state COPV2•+-
moieties via homoconjugation, which is symmetrically allowed
(see Supporting Information).12 The driving forces for CS
(¹¦G0CS) and CR (¹¦G0CR) in benzonitrile (PhCN, ¾S = 25.2)
were estimated to be 0.03 and 2.85 eV, respectively, using
Weller’s model.13 Note that ¹¦G0CS in a less-polar solvent, such
as toluene (PhMe, ¾S = 2.38), was negative (i.e., ¹0.10 eV).
Thus, CS is thermodynamically feasible only in highly polar
solvents.
The steady-state UV-vis absorption spectrum of COPV2-
(TRZ)4 shows a very small red shift of the COPV2 moiety
(6 nm) compared with the parent COPV2, suggesting only small
electronic coupling between the COPV2 and TRZ moieties
in the ground state (Figure 3a). Steady-state fluorescence spectra
in toluene (upon exciting at both 300 and 400 nm) showed
emission from the COPV2 moiety (Figure 3b), while that in
benzonitrile showed an additional tail over 500 nm (Figure 3b).
This band was assigned to charge-transfer (CT) emission.
Excitation spectra indicated that the energy transfer from the
TRZ to the COPV2 moieties (see Supporting Information)12
took place effectively through homoconjugation.14 This antenna
effect indicates an advantage of COPV structures as efficient
light-harvesting materials.
¹1
(TRZ•¹)(TRZ)3 at a rate constant of 5.3 © 109 s based on the
equation kCS = 1/¸ ¹ 1/¸0 (Figure 5b). The quantum yield of
CS is estimated to be as high as 98%, which can be attributable
to the 4-fold acceptors.15 The availability of multiacceptors is an
advantage of COPVs for efficient CS. The charge-separated state
decays via CT emission (kCT) or a nonradiative process (knr2) at
rate constants of 3.2 © 107 and 6.2 © 107 s¹1, respectively. The
nonradiative process is composed of CR via tunneling mecha-
nisms and some other relaxation pathways. Thus, the CR rate is
approximately 100 times slower than the CS rate, which indi-
cates the CR process is located in the Marcus inverted region.16
The electronic coupling (V) can be obtained by the
The fluorescence lifetime (¸) of COPV2-(TRZ)4 markedly
depends on the polarity of the solvent, as depicted in Figure 4.
The emission excited at 405 nm in toluene decays monoexpo-
nentially with a lifetime of 1.97 ns, which is similar to that for
COPV2-(OctPh)4 (¸0 = 1.81 ns), suggesting that no fluores-
cence quenching by CS occurs. In contrast the emission in
benzonitrile decays biexponentially in 0.17 (5%) and 10.8 ns
following equation.17
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
1:39 ꢁ 105kCT
V ꢀ
¼ 1:0 ꢁ 103 cmꢂ1
ð1Þ
vmaxn2R2
where n is the refractive index of the solvent (1.53), R is the
center-to-center D-A distance (9.15 ¡), and ¯max is the max-
imum absorption wavenumber (2.27 © 104 cm¹1). The V value
700 | Chem. Lett. 2014, 43, 699–701 | doi:10.1246/cl.140022
© 2014 The Chemical Society of Japan

is quite large compared with other D-A systems, such as
contacted radical-ion pairs of zinc porphyrin-fullerene dyads
(100 cm¹1).18,19
7
a) P. Maslak, A. Chopra, C. R. Moylan, R. Wortmann, S.
Lebus, A. L. Rheingold, G. P. A. Yap, J. Am. Chem. Soc.
1996, 118, 1471. b) S. M. King, S. I. Hintschich, D. Dai, C.
Rothe, A. P. Monkman, J. Phys. Chem. C 2007, 111, 18759.
c) L. Zhu, C. Zhong, C. Liu, Z. Liu, J. Qin, C. Yang,
ChemPhysChem 2013, 14, 982. d) M.-C. Yoon, S. Lee, S.
Tokuji, H. Yorimitsu, A. Osuka, D. Kim, Chem. Sci. 2013, 4,
1756. e) T. P. I. Saragi, T. Spehr, A. Siebert, T. Fuhrmann-
Lieker, J. Salbeck, Chem. Rev. 2007, 107, 1011.
a) X. Zhu, C. Mitsui, H. Tsuji, E. Nakamura, J. Am. Chem.
Soc. 2009, 131, 13596. b) X. Zhu, H. Tsuji, J. T. L.
Navarrete, J. Casado, E. Nakamura, J. Am. Chem. Soc. 2012,
134, 19254.
In summary we have designed and synthesized a new class
of D-A system that features homoconjugative interaction
between the D and the A parts, and found that this system
shows a highly efficient intramolecular CS, and a CR that is
located in the Marcus inverted region. The electronic coupling
via homoconjugation is larger than by other types of conjuga-
tions or interactions. The aryl substituents of COPVs can readily
be modified and thus the energy levels and the frontier orbitals
can be tuned. We are now working on the orbital engineering
to maximize the CS rate and minimize the CR rate by making
use of the symmetry rule for homoconjugation. This study is
a cornerstone for the development of homoconjugated D-A
systems toward artificial photosynthesis.
8
9
X. Zhu, H. Tsuji, A. Yella, A.-S. Chauvin, M. Grätzel, E.
Nakamura, Chem. Commun. 2013, 49, 582.
10 a) Z.-F. An, R.-F. Chen, J. Yin, G.-H. Xie, H.-F. Shi, T.
Tsuboi, W. Huang, Chem.®Eur. J. 2011, 17, 10871. b) H.
Tanaka, K. Shizu, H. Miyazaki, C. Adachi, Chem. Commun.
2012, 48, 11392.
11 L. Zeng, T. Y.-H. Lee, P. B. Merkel, S. H. Chen, J. Mater.
Chem. 2009, 19, 8772.
This work was supported by the Specially Promoted
Research Grant (KAKENHI No. 22000008 to E.N.) and by
JST-PRESTO “New Materials Science and Element Strategy”
(for H.T.).
12 Supporting Information is available electronically on the
CSJ-Journal Web site, http://www.csj.jp/journals/chem-lett/
index.html.
References and Notes
1
2
S. Fukuzumi, Bull. Chem. Soc. Jpn. 2006, 79, 177.
E. A. Weiss, M. J. Ahrens, L. E. Sinks, A. V. Gusev, M. A.
Ratner, M. R. Wasielewski, J. Am. Chem. Soc. 2004, 126,
5577.
13 A. Weller, Z. Phys. Chem. 1982, 133, 93.
14 J. O. Barcina, N. Herrero-García, F. Cucinotta, L. De Cola,
P. Contreras-Carballada, R. M. Williams, A. Guerrero-
Martínez, Chem.®Eur. J. 2010, 16, 6033.
3
a) M. Sasaki, Y. Shibano, H. Tsuji, Y. Araki, K. Tamao, O.
Ito, J. Phys. Chem. A 2007, 111, 2973. b) Y. Kobori, Y.
Shibano, T. Endo, H. Tsuji, H. Murai, K. Tamao, J. Am.
Chem. Soc. 2009, 131, 1624.
15 M. Ogawa, B. Balan, G. Ajayakumar, S. Masaoka, H.-B.
Kraatz, M. Muramatsu, S. Ito, Y. Nagasawa, H. Miyasaka, K.
Sakai, Dalton Trans. 2010, 39, 4421.
4
5
6
A. B. Ricks, G. C. Solomon, M. T. Colvin, A. M. Scott, K.
Chen, M. A. Ratner, M. R. Wasielewski, J. Am. Chem. Soc.
2010, 132, 15427.
A. Molina-Ontoria, M. Wielopolski, J. Gebhardt, A.
Gouloumis, T. Clark, D. M. Guldi, N. Martin, J. Am. Chem.
Soc. 2011, 133, 2370.
16 R. A. Marcus, J. Chem. Phys. 1956, 24, 966.
17 V. Vehmanen, N. V. Tkachenko, H. Imahori, S. Fukuzumi,
H. Lemmetyinen, Spectrochim. Acta, Part A 2001, 57, 2229.
18 A. H. Al-Subi, M. Niemi, N. V. Tkachenko, H. Lemmetyinen,
J. Phys. Chem. A 2012, 116, 9653.
19 J. Sukegawa, C. Schubert, X. Zhu, H. Tsuji, D. M. Guldi, E.
Nakamura, submitted for publication.
a) J. Karpiuk, Phys. Chem. Chem. Phys. 2003, 5, 1078. b) J.
Karpiuk, J. Phys. Chem. A 2004, 108, 11183.
Chem. Lett. 2014, 43, 699–701 | doi:10.1246/cl.140022
© 2014 The Chemical Society of Japan | 701