π-conjugated multidonor/acceptor arrays of fullerene-cobaltadithiolene- tetrathiafulvalene: From synthesis and structure to electronic interactions

Article abstract of DOI:10.1021/ja902312q

The synthesis, structure, photoelectrochemical behavior, and nonlinear optical (NLO) properties of a symmetric acceptor-acceptor-donor-acceptor- acceptor array, C₆₀-Co-TTF-Co-C₆₀, have been described. The precursors, namely, cobalt d

Full text of DOI:10.1021/ja902312q

Published on Web 08/11/2009
π-Conjugated Multidonor/Acceptor Arrays of
Fullerene-Cobaltadithiolene-Tetrathiafulvalene: From
Synthesis and Structure to Electronic Interactions
Yutaka Matsuo,*,†,‡ Masashi Maruyama,‡ S. Shankara Gayathri,§ Tomoya Uchida,|
Dirk M. Guldi,*,§ Hideo Kishida,*,| Arao Nakamura,| and Eiichi Nakamura*,†,‡
Nakamura Functional Carbon Cluster Project, ERATO, Japan Science and Technology Agency,
Hongo, Bunkyo-ku, Tokyo 113-0033, Japan, Department of Chemistry, The UniVersity of Tokyo,
Hongo, Bunkyo-ku, Tokyo 113-0033, Japan, Department of Chemistry and Pharmacy and
Interdisciplinary Center for Molecular Materials, Friedrich-Alexander-UniVersita¨t,
Erlangen-Nu¨rnberg, Germany, and Department of Applied Physics, Nagoya UniVersity,
Furo-cho, Chikusa-ku, Nagoya, 464-8603 Japan
Received March 24, 2009; Revised Manuscript Received July 15, 2009; E-mail:
matsuo@chem.s.u-tokyo.ac.jp; dirk.guldi@chemie.uni.erlangen.de; kishida@nuap.nagoya-u.ac.jp;
nakamura@chem.s.u-tokyo.ac.jp
Abstract: The synthesis, structure, photoelectrochemical behavior, and nonlinear optical (NLO) properties
of a symmetric acceptor-acceptor-donor-acceptor-acceptor array, C60-Co-TTF-Co-C60, have been
described. The precursors, namely, cobalt dicarbonyl complexes Co(C60Ar5)(CO)2 were synthesized from
the penta(organo)[60]fullerenes, C60Ar5H, as starting materials. In the next step, two cobalt-fullerene
complexes were connected to a tetrathiafulvalene (TTF) tetrathiolate bridge to obtain the C60-Co-TTF-Co-
C60 array. In addition, the monomeric compounds, Co(C60Ar5)(S2C2R2) (R ) CO2Me and CN) and
Co(C60Ar5)(S2C2S2 C ) CS2C2R2) were synthesized as references. The C60-Co-TTF-Co-C60 array exhibits
very strong transitions in the near-infrared region (λmax ) 1,100 nm, ε ) 30 000 M-1 ·cm-1) due to a ligand-
to-metal-charge-transfer (LMCT) transition and six reversible electron transfer processes. In the crystal, a
fullerene/TTF-layered packing structure is evident. Femtosecond flash photolysis revealed that photoex-
citation of the array results in a charge separated state involving the strongly interacting cobaltadithiolene
and TTF constituents which electronically relax via a resonance effect that extends all throughout the
acceptor parts of the C60-Co-TTF-Co-C60 array. The third-order NLO measurement of the array gave the
magnitude of the third-order nonlinear susceptibility, |ꢀ(3)|, values to be 9.28 × 10-12 esu, suggesting
the π-conjugation of donors and acceptors in the array.
Introduction
tional donor-spacer-acceptor hybrids/conjugates by virtue of
their high charge mobility. It is mainly the larger mobility rate
Connecting and integrating several electron-donor and
-acceptor moieties into π-conjugated donor/acceptor arrays
have received tremendous attention owing to their unique
applications in organic conductive materials,1 nonlinear optical
(NLO) materials,2 near-infrared dyes,3 molecular wires,4 and
so on. π-Conjugated donor/acceptor arrays differ from conven-
that leads to faster response times when interacting with, for
example, light. In comparison to ordinary π-electron conjugated
systems such as π-conjugated wires and ribbons,5 the π-con-
jugated donor/acceptor arrays may undergo charge separation
under light or even in the dark via charge transfer from the
donor to the acceptor. Such interactions impart unique physical
features that range, for instance, from narrow HOMO-LUMO
gaps to high polarizabilities. In particular, the former has recently
attracted increased interests concerning photoelectric conver-
sion.6
† Nakamura Functional Carbon Cluster Project.
‡ The University of Tokyo.
§ Friedrich-Alexander-Universita¨t.
| Nagoya University.
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Chem. Soc. 1996, 118, 10389–10399.
Attempts have been made to obtain a wide range of
π-conjugated donor/acceptor molecules and polymers.7 Alter-
natively, donors and acceptors are assembled in the crystal lattice
through molecular association,8 which is known as crystal
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9
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A R T I C L E S
Matsuo et al.
and molecular functional device14 areas, the present motif with
multiple electron donor-acceptor arrays will open myriad
opportunities in these fields.
Scheme 1
Result and Discussion
Synthesis and Structural Characterization of the Fullerene-
Cobaltadithiolene-TTF Arrays. The synthesis was started with
preparing a new cobalt dicarbonyl complexes of penta(aryl)[60]-
fullerenes (Scheme 1). Treatment of C60Ar5H15 with KH in THF
generated a potassium complex K(thf)n(C60Ar5),16 which was
reacted with a cobalt iodide complex17 derived from Co2(CO)8
and I2 in THF to afford the dicarbonyl complexes, Co(C60Ar5)-
(CO)2 (1a, Ar ) 4-nBuC6H4; 1b, Ar ) 4-nBuC6H4) in 61% and
38% yield, respectively. Structural characterization of this
starting material was done by single crystal X-ray analysis of
1b.18 We next synthesized the cobaltadithiolene complexes,
since the cobaltadithiolenes generally represent a light-absorbing,
redox-active organometallic π-system. The reaction of 1a with
excess amounts of electron-deficient alkyne and S8 in toluene
at 110 °C produced the cobaltadithiolene fullerene complex 2a
in 61% yield. Alternatively, 1a was reacted with 4,5-dicyano-
1,3-dithiol-2-one (10 equiv) in xylene at 140 °C to obtain 2b
in 74% yield. Then, we selected TTF as a photophysically and
electrochemically active organic bridge to expand the π-con-
jugated molecular array. Carbonyldithio-bis(methoxyl carbon-
yl)tetrathiafulvalene (1.2 equiv) was treated with 1a under the
same conditions to produce C60-Co-TTF complex 3 in 78%
yield. Finally, C60-Co-TTF-Co-C60 complex 4 was obtained
in 59% yield by the reaction of 1a with bis(carbonyldithio)tetrathi-
afulvalene (2 equiv) under similar conditions.
All the cobalt compounds (1-4) were stable in air, and were
either purified by silica gel column chromatography or by HPLC
(i.e., equipped with a Buckyprep column). It is likely that the
stability of 4 originates from protecting effects of the
penta(aryl)[60]fullerenes,19 while the core structure TTF tetrathi-
olato bismetal complexes are known to be air-sensitive in
general.20
The molecular structure of C60-Co-TTF-Co-C60 (4) was
unambiguously determined by X-ray crystallographic analyses
(Figure 1a). We obtained two polymorphic crystals from
different solvent systems. Dark green single crystals of
4·(CS2)·(C6H6) and 4·(PhCl) were successfully obtained by
slow diffusion of diethyl ether or ethanol into benzene/CS2 (1/
1) or chlorobenzene solution of 4. Molecular length of a long
axis with van der Waals radii was 22.8 Å. A crystal packing
view of 4·(CS2)·(C6H6) (Figure 1b) revealed a lamellar struc-
engineering.9 Herein, we report the two-step synthesis, structure,
and photophysical functions of a symmetric acceptor-accep-
tor-donor-donor-acceptor-acceptor array (Scheme 1), which
combines two fullerenes, two cobaltadithiolenes, and one
tetrathiafulvalene (TTF) to afford C60-Co-TTF-Co-C60 (4).
The compound 4 represents a multiredox dπ-conjugated
donor-acceptor array, which differs from previously reported
donor-fullerene dyads for its compact and rigid one-dimen-
sional structure.10-12 Electrochemical and time-resolved pho-
tophysical investigations revealed its multielectron redox be-
havior, near-infrared light absorption, and resonant electronic
interaction within the symmetric molecule. In addition, NLO
studies were performed, since the one-dimensional confinement
of the electron is of general interest in the area of optical devices.
In fact, our results corroborate the π-conjugation between the
donors and acceptors in the array. Given the growing interest
in the design of molecules that are applicable as photoelec-
tronically active materials, in both organic electronic device13
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(Figure S1).
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π-Conjugated Multi-Donor/Acceptor Arrays
A R T I C L E S
occurring reduction of the two C60 (-1.68 V) (Figure 2c,d).27
Relative to penta(organo)[60]fullerenes that show reduction
potentials at ca. -1.4 V,28 the reduction of the fullerene moiety
in 4 is negatively shifted by ca. 0.3 V due to electronic
interactions with the anion of the cobaltadithiolene moieties.
Overall, a remarkably small energy gap of 1.19 eV evolves,
when considering the lowest oxidation (0.12 V) and lowest
reduction (-1.07 V) potentials of the TTF and cobaltadithiolene,
respectively. Such a value is in excellent agreement with the
optically determined band gap of 1.13 eV.29 These LMCT
features are solvent independent, that is, no appreciable shifts
evolve when the solvent polarity is increased gradually from
toluene to benzonitrile. The presence of the butyl phenyl groups
is likely to be responsible for this trend, since they shield the
TTF/cobaltadithiolene couple from the surrounding solvent. A
similar picture evolves for 2b (Vide infra) and 3 (see Supporting
Information). For 2b, oxidation is limited to a dithiolene centered
process (1.05 V), while sequential reduction of cobaltadithiolene
and C60 transpire at -0.61 and -1.49 V. Again, considering
the lowest oxidation (1.05 V) and lowest reduction (-0.61 V)
potentials, we derive an energy gap of 1.67 eV, which matches
the optical band gap of 1.82 eV. An independent proof for the
LMCT character came from the selective reduction of the metal
by means of radiolytical and spectroelectrochemical means. To
this end, differential absorption spectra reveal bleach at 1110
(4), 934 (3), and 680 nm (2b), which mirror images the ground
state absorption. Implicit is that the reduced metal lacks the
susceptibility to accept electrons from TTF in the ground state.
In the next stage, transient absorption measurements30 were
performed for 2b, 3, and 4 to examine the electronic interactions
between the redox- and the photoactive components.31,32 In 4,
commencing with the 388 nm laser excitation, where the
fullerene moiety contributes more than 95% to the overall
absorption, the instantaneous formation of a new transient (i.e.,
1.5 ps) is registered (Figure 3, upper panel). In the visible range
of the transient spectrum, fine structured features are seen with
a minimum at 610 nm and maxima at 460 and 540 nm. In the
near-infrared range, the new features include a broad transient
bleach at around 1060 nm and a broad transient maximum at
1315 nm. Considering the perfect match with the spectroelec-
trochemically generated spectra (see Figures S11 and S12), we
infer the prompt formation of a charge separated state involving
the strongly interacting cobaltadithiolene and TTF constituents.33,34
Figure 1. X-ray crystal structure of 4·(CS2)·(C6H6). Solvent molecules
are omitted for clarity. (a) Ball and stick drawing. Location of the carbon
atoms in the butyl chains were optimized with geometrical restraints on
the SHELX97 program.23 (b) CPK drawing of the crystal packing. The
butyl phenyl groups are omitted for clarity.
ture21 containing fullerene and TTF in alternative layer, where
fullerene parts are closely packed through van der Waals
interactions within the same layer.22 The interlayer distance was
19.4 Å.
Ground and Excited State Features. First, the ground state
features of 2b, 3, and 4 are considered. They are all dominated
by broad absorptions centering at 684 nm (1.81 eV) for 2b
(Figure 2b), 934 nm (1.33 eV) for 3 (Figure S6), and 1100 nm
(1.13 eV) for 4 (Figure 2a) with extinction coefficients in the
range of 10 000, 15 000, and 30 000 M-1 cm-1, respectively.24
Notable is the red-shift of the absorption maximum when
contrasting 2b, 3, and 4 with the corresponding cyclopentadienyl
cobaltadithiolene, CpCoS2C2(CN)2 (λmax ) 559 nm).25 Implicit
are strong electronic interactions between the different con-
stituents (i.e., C60,26 cobaltadithiolene, and TTF), that is, partial
redistribution of charge density between the electron donating
(i.e., TTF) and electron accepting (i.e., cobaltadithiolene)
constituents. There is no ground state charge separation as
suggested by the fact that 4 is NMR active. This is corroborated
by the lack of appreciable fluorescence due to the two
penta(organo)[60]fullerene moieties (, 10-5), which are ex-
pected to show stronger fluorescence (quantum yield of 2.2 ×
10-3). The long-wavelength absorptions are assigned to LMCT
bands that are centered at the cobaltadithiolene part, whose
absorption bands are known to shift in the presence of extended
π-systems.25
Further support for this notion came from the cyclic volta-
mmetric measurements. In the anodic range, array 4 gives rise
to two reversible one-electron oxidation processes that are
centered at TTF (0.12 and 0.42 V) and that correspond to the
formation of the radical cation and dication, respectively. In
the cathodic range, two reversible one-electron reduction
processes are associated with the step-by-step reduction of the
two cobaltadithiolenes (-1.07 and -1.30 V), while a reversible
two-electron reduction process relates to the simultaneously
(27) Electrochemical data for 2a, 2b, and 3 are stored in Supporting
Information (Figures S7-S10).
(28) Kuninobu, Y.; Matsuo, Y.; Toganoh, M.; Sawamura, M.; Nakamura,
E. Organometallics 2004, 23, 3259–3266.
(29) A delocalization of charges involving C60 was ruled out by adding
acetonitrile that is known to protonate the one-electron reduced C60.
(30) (a) Guldi, D. M.; Rahman, G. M. A.; Marczak, R.; Matsuo, Y.;
Yamanaka, M.; Nakamura, E. J. Am. Chem. Soc. 2006, 128, 9420–
9427. (b) Matsuo, Y.; Matsuo, K.; Nanao, T.; Marczak, R.; Gayathri,
S. S.; Guldi, D. M.; Nakamura, E. Chem. Asian J. 2008, 3, 841–848.
(c) Marczak, R.; Wielopolski, M.; Gayathri, S. S.; Guldi, D. M.;
Matsuo, Y.; Matsuo, K.; Tahara, K.; Nakamura, E. J. Am. Chem. Soc.
2008, 130, 16207–16215.
(21) Zhong, Y.-W.; Matsuo, Y.; Nakamura, E. J. Am. Chem. Soc. 2007,
129, 3052–3053.
(22) Crystal packing of 4·(PhCl) showed a layer-by-layer structure, where
molecules form two different sheets. Figures are stored in Supporting
Information (Figures S2, S3).
(31) In compound 2b, the cobaltadithiolene moiety acts as an electron donor.
Oxidation occurs at the cobaltadithiolene, while reduction can occur
at both cobaltadithiolene and penta(organo)[60] fullerene moieties; see
Supporting Information.
(32) Takayama, C.; Kajitani, M.; Sugiyama, T.; Sugimori, A. J. Organomet.
Chem. 1998, 563, 161–171.
(23) Mu¨ller, P., Ed.; Crystal Structure Refinement, A Crystallographer’s
Guide to SHELXL, Oxford University Press: Oxford, 2006.
(24) Absorption spectra and data for 2a and 3 are stored in Supporting
Information (Figures S4-S6).
(25) Akiyama, T.; Watanabe, Y.; Miyasaka, A.; Komai, T.; Ushijima, H.;
Kajitani, M.; Shimizu, K.; Sugimori, A. Bull. Chem. Soc. Jpn. 1992,
65, 1047–1051.
(33) Excitation with 388 and 1100 nm light led to the same observation.
(34) Notable are, however, the differences seen relative to the separate
constituents, that is, one-electron oxidized TTF and one-electron
reduced cobaltadithiolene with marked features limited to the visible
range.
(26) Iikura, H.; Mori, S.; Sawamura, M.; Nakamura, E. J. Org. Chem. 1997,
62, 7912–7913.
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Matsuo et al.
Figure 2. Photophysical and electrochemical properties of 2b and 4. (a) UV-vis-NIR spectrum of 4 in toluene. (b) UV-vis-NIR spectrum of 2b in toluene.
(c) Oxidation waves on the cyclic voltammogram of 4 in CH2Cl2 containing nBu4NClO4 as supporting electrolyte. (d) Reduction waves on the cyclic
voltammogram of 4 in THF containing the same supporting electrolyte.
a resonance effect that extends all throughout the acceptor parts
of the array on a time scale of up to 40 ps in THF (Figure 3,
lower panel). For example, the electrons may undergo equilibra-
tion between the two acceptor terminal, namely, cobaltadithi-
olene/C60Ar5.35 Such a mechanism has been proposed in the
past, although it has never been confirmed spectroscopically.36
Although there is no doubt about the primary electron acceptors,
namely, the cobaltadithiolenes, the C60 part acts only as
secondary electron acceptors (Vide infra), charge equilibration
may as well be anticipated to occur between the primary and
secondary electron acceptors.
On a time scale of up to 300 ps, we observed the decay of
the relaxed/equilibrated charge separated state. The latter decay
is accompanied by a long-lived state, a triplet excited state that
is located predominantly on the fullerene part.37 Characteristics
include a transient maximum at 640 nm and an excited state
lifetime of about 3 to 4 µs (see Figure S13). The mechanism
by which the triplet excited state is formed remains unclear at
present; possible pathways include charge recombination or
intersystem crossing from a localized singlet excited state.
In 3, the presence of just one acceptor terminal impacts the
dominating LMCT absorptions, which are now shifted to 925
nm in THF. Still, upon excitation, the differential absorption
changes reveal strong changes in the visible and near-infrared
regions. Prevailing is the minimum at 925 and the maximum at
1230 nm (Figure 4, upper panel). These features are formed
instantaneously (i.e., <2 ps). In other words, 3 and 4 give rise
to a similar spectral pattern, which prompts to the unique
combination of C60 and cobaltadithiolene as well as TTF. Unlike
Figure 3. Time-resolved photophysical data for 4. (Upper panel) Dif-
to what has been seen for 4, the excited state of 3 lacks any
notable shifts during the relaxation step. Kinetic confirmation
ferential absorption spectra (visible and near-infrared) obtained upon
femtosecond flash photolysis (388 nm - 200 nJ) of 4 (10-6 M) in THF with
several time delays between 0 and 200 ps at room temperature; see legend
for details about the time progression. (Lower panel) Time-absorption
profiles of the spectra at 620 and 1150 nm.
(35) Note that the penta(organo)[60]fullerene radical anion exhibits absorp-
tion maximum at 1,100 nm. See. ref. 15a The value is smaller than
absorption maximum for the TTF cation of 4 (1260 nm). This is the
origin of the blue-shift in the relaxation process.
(36) Sa´nchez, L.; Sierra, M.; Mart´ın, N.; Guldi, D. M.; Wienk, M. W.;
Janssen, R. A. J. Org. Lett. 2005, 7, 1691–1694.
Interestingly, with time, the near-infrared features shift to the
blue, that is, to 1020 and 1260 nm, respectively. Less obvious
are the changes in the visible part, that is, a blue-shift from 540
to 525 nm. The time absorption profiles taken at 620 and 1150
nm (see Figure 3) further corroborate this step. A likely rationale
implies electronic relaxation of the charge separated state with
(37) When this triplet state of 4 is compared with those of the references
(C60Ph5H and Ru(C60Ph5)Cp), the absorption maximum (660 nm) is
identical. Lifetime (3∼4 ms in toluene) is shorter than that of C60Ph5H
(14 ms in benzonitrile), but longer than Ru(C60Ph5)Cp (1.5 ms in
benzonitrile). See ref 30a.
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Figure 4. Time-resolved photophysical data for 3. (Upper panel) Dif-
ferential absorption spectra (visible and near-infrared) obtained upon
femtosecond flash photolysis (388 nm - 200 nJ) of 3 (10-6 M) in THF with
several time delays between 0 and 50 ps at room temperature; see legend
for details about the time progression. (Lower panel) Time-absorption
profiles of the spectra shown above at 500, 620, 925, and 1045 nm.
Figure 5. Time-resolved photophysical data for 2b. (Upper panel)
Differential absorption spectra (visible and near-infrared) obtained upon
femtosecond flash photolysis (388 nm - 200 nJ) of 2b (10-6 M) in THF
with several time delays between 0 and 50 ps at room temperature; see
legend for details about the time progression. (Lower panel) Time-absorption
profiles of the spectra shown above at 640, 740, and 950 nm.
ceptibility, |ꢀ(3)|, for cast films on SiO2 substrates were evaluated
by the third-harmonic generation (THG). The photon energy
of the excitation laser was tuned as one-third of the absorption
peak energy of each compound, where the three-photon
resonance was expected. The obtained |ꢀ(3)| values are 9.28 ×
10-12 esu for 4 at 0.376 eV (3300 nm) and 4.73 × 10-12 esu
for 3 at 0.422 eV (2934 nm). The 2-fold enhancement of |ꢀ(3)|
values observed in 4 can be ascribed to the symmetric charge-
transfer (CT) excitation in the molecule.
for the relaxation came from the time absorption profiles at 500,
620, 925, and 1045 nm. Important is that the relaxation is much
faster (i.e., ∼10 ps; Figure 4, lower panel) than what has been
detected for 4. Considering the lack of two acceptor terminals
in 3, this leaves the resonance between the primary and
secondary electron acceptor as the only viable mechanism.
Commencing with this relaxation, we note the decay of the
relaxed/equilibrated charge separated state, which depends
strongly on the solvent polarity: toluene - 43 ps; THF - 36
ps; benzonitrile - 65 ps. Beyond this time, a 640 nm maximum
was detected in the femtosecond and complementary nanosecond
experiments. In 3, like in 4, the lifetime of this triplet excited
state is affected by the presence of molecular oxygen to generate
correspondingly singlet oxygen.
Finally, changes associated with 2b shall be discussed, which
lacks TTF. The differential absorption changes are mainly
limited to the visible region, where a maximum at 550 nm and
a minimum at 680 nm (Figure 5, upper panel) are formed within
2.0 ps following the excitation. The minimum correlates with
the maximum of the LMCT ground state absorption at 680 nm.
On the other hand, much weaker are the characteristics in the
near-infrared region with maxima at 960 and 1080 nm. Like
what has been seen for 3, no spectral shifts were noted to take
place during the 10 ps relaxation. The relaxed excited state in
2b decays quickly with solvent independent kinetics (i.e., 21
ps; Figure 5, lower panel).
The observed |ꢀ(3)| values were compared with three-photon
resonant |ꢀ(3)| of other organic CT conjugated materials. Third-
order optical nonlinearity of donor-acceptor type CT conjugated
polymers has been reported.2 Poly(thiophene-alt-quinoxaline)
(PThQx), which is known to have strong CT character between
donor and acceptor molecules,1 showed enhanced optical
nonlinearity.2a When comparing different systems, one should
pay attention to the difference in the density of π-electrons.
|ꢀ(3)| values are proportional to the density of π-electrons, defined
as π-electron numbers on a molecule divided by the molecular
volume, which depends on the molecular structures including
nonconjugated side groups. To extract the optical nonlinearity
inherent to π-conjugated systems, we adopted the figure of merit
defined as |ꢀ(3)|/R (R: absorption coefficient). Since both |ꢀ(3)|
and R are proportional to the density of π-electrons, the
difference in the density of π-electrons is canceled in |ꢀ(3)|/R.
|ꢀ(3)|/R of 4 and 3 is 3.29 × 10-16 and 1.66 × 10-16 esu cm,
respectively, and that of PThQx evaluated by the three-photon
resonant THG process reaches 4.26 × 10-16 esu cm, while that
of homopolymer consisting of a single component with no CT
Nonlinear Optical Properties of the Array. The third-order
NLO properties of the array 4 and the reference 3 were
investigated and the magnitudes of third-order nonlinear sus-
9
J. AM. CHEM. SOC. VOL. 131, NO. 35, 2009 12647

A R T I C L E S
Matsuo et al.
unreacted KH residue remained, and then the reaction mixture was
warmed up to 60 °C. After 15 min stirring, the mixture was diluted
with toluene and filtered through a pad of silica gel. The filtrate
was reprecipitated with MeOH. Obtained crude mixture was puri-
fied with preparative HPLC (Nacalai Tesque, Buckyprep, toluene/
iPrOH ) 5/5) to afford 1a (0.204 g, 0.136 mmol, 63%) as orange
character, regioregular poly(3-hexylthiophene) (RR-P3HT), is
2.41 × 10-16 esu cm. A comparison with these polymers reveals
that the compound 4 has larger |ꢀ(3)|/R than a non-CT polymer
RR-P3HT, but a smaller value than a CT conjugated polymer
PThQx. Considering that one-dimensional confinement enhances
the optical nonlinearity in the polymer materials, the observed
optical nonlinearity in 4 is quite large and the CT in the excited
states largely enhances the optical nonlinearity.
1
solid. H NMR (500 MHz, C6D6) δ: 0.83 (t, J ) 7.5 Hz, 15H,
CH3), 1.21 (m, 10 H, CH2), 1.46 (m, 10H, CH2), 2.46 (t, J ) 7.5
Hz, 10H, CH2), 7.09 (d, J ) 8.6 Hz, 10H, Ar), 7.99 (d, J ) 8.6
Hz, 10H, Ar); 13C NMR (125 MHz, C6D6) δ: 14.0 (5C, CH3), 22.5
(5C, CH2), 33.7 (5C, CH2), 35.5 (5C, CH2), 58.2 (5C, C60(sp3)),
109.4 (5C, C60(Cp)), 128.8, 129.1, 139.2, 142.9, 144.2, 145.0, 147.7,
148.7, 149.2, 152.1, 210.8 (2C, CO); Anal. Calcd for C112H65O2Co:
C, 89.58; H, 4.36. Found: C, 89.35; H, 4.57.
Conclusion
Two fullerenes, two cobaltadithiolenes, and one TTF bridge
were assembled into a compact and rigid one-dimensional donor/
acceptor array 4 that shows reversible multielectron redox
behavior, accepting and giving up a total of six electrons, and
near-infrared light absorption at 1100 nm. Electronic interactions
within the array were elucidated by femtosecond flash photolysis
experiments and third-order NLO measurements. The former
time-resolved studies indicated relaxation of the charge separated
state, involving the strongly interacting cobaltadithiolene and
TTF constituents, which is formed initially, via a resonance
effect that extends all throughout the acceptor parts of the array.
The latter optical studies revealed large optical nonlinearity of
the array roughly being in the middle of CT polymers and
conjugated polymers. One-dimensional structure as well as
electrochemical, photophysical, optical properties of 4 will
provide a unique wire motif for molecular electronics.38-40
Crystal engineering that can make fullerene/TTF layered
structures also suggests the use of 4 in organic thin-film
devices41 and crystalline organic electronic devices.42 Potentially
ambipolar nature and light absorption at long wavelength region
with relatively large absorption coefficiency will be of immense
interest in this field.
Co[C60(4-tBuC6H4)5](CO)2 (1b). The procedure described for
1a was performed to obtain orange solid 1b (0.120 g, 0.083 mmol,
38%). 1H NMR (500 MHz, C6D6) δ: 1.25 (s, 45H, tBu), 7.34 (d, J
) 8.6 Hz, 10H, Ar), 8.04 (d, J ) 8.0 Hz, 10H, Ar); 13C NMR (500
MHz, C6D6) δ: 31.3, 34.5, 58.1, 109.2, 125.6, 128.9, 138.7, 144.2,
145.1, 147.7, 148.7, 149.2, 151.1, 152.2, 201.3 (CO); Anal. Calcd
for C112H65O2Co: C, 89.58; H, 4.36. Found: C, 89.35; H, 4.60.
Co[C60(4-nBuC6H4)5][(S2C2)(CO2Me)2] (2a). A solution of 1a
(20.0 mg, 13.3 µmol), elemental sulfur (4.3 mg, 0.13 mmol) and
dimethyl acetylenedicarboxylate (4.2 µL, 1.3 mmol) in toluene (5.0
mL) was heated at 110 °C for 2 days. The resulting dark green
reaction mixture was filtered through a pad of silica gel, and then
the filtrate was reprecipitated with MeOH. Obtained crude mixture
was subjected to silica gel column chromatography (toluene as
eluent). A green band was collected and reprecipitated with MeOH
1
to afford 2a (13.4 mg, 8.1 µmol, 61%) as green solid. H NMR
(500 MHz, C6D6) δ: 0.85 (t, J ) 7.2 Hz, 15H, CH3), 1.21 (m, 10
H, CH2), 1.46 (m, 10H, CH2), 2.42 (t, J ) 7.7 Hz, 10H, CH2), 3.45
(s, 6H, CH3), 7.02 (d, J ) 8.0 Hz, 10H, Ar), 7.87 (d, J ) 8.0 Hz,
10H, Ar); 13C NMR (125 MHz, C6D6) δ: 14.1 (5C, CH3), 22.5
(5C, CH2), 33.7 (5C, CH2), 35.4 (5C, CH2), 58.5 (5C, C60(sp3)),
102.0 (5C, C60(Cp)), 128.8, 129.8, 136.6, 143.3, 144.2, 144.3, 147.9,
148.8, 149.1, 151.5, 165.1, 165.8; UV-vis (solution in CH2Cl2)
max (ε): 640 (0.98 × 104); Anal. Calcd for C116H71O4S2Co: C, 84.34;
Experimental Section
λ
H, 4.33. Found: C, 84.17; H, 4.46.
General. All manipulations were carried out under argon
atmosphere using standard Schlenk techniques. Toluene and THF
were used as dried over Na and distilled before use. p-Xylene was
distilled over CaH2 before use. All NMR spectra were recorded on
JEOL ECA-500, and reported in parts per million (ppm, δ scale)
form internal tetramethylsilane (δ 0.00 ppm) or residual protons
Co[C60(4-nBuC6H4)5][S2C2(CN)2] (2b). A solution of 1a (20.0
mg, 13.3 µmol) and 4,5-dicyano-1,3-dithiol-2-one (22.4 mg, 0.133
mmol) in p-xylene (5.0 mL) was heated at 140 °C for 10 h. The
resulting dark green reaction mixture was diluted with toluene (5
mL), and filtered through a pad of silica gel. The filtrate was
reprecipitated with MeOH to give dark yellow-green solid. Obtained
crude mixture was subjected to silica gel column chromatography
(toluene/hexane ) 2/1). A green band was collected and reprecipi-
tated with MeOH to afford dark green solid 2b (15.7 mg, 9.9 µmol,
1
of the deuterated solvent for H NMR (δ 7.15 ppm for C6D6) and
solvent carbon for 13C NMR (δ 128 ppm for C6D6). Elemental
analysis was performed at the University of Tokyo, Department of
Chemistry, Organic Elemental Analysis Laboratory. UV-vis-NIR
spectra were recorded on JASCO V-570.
1
74%). H NMR (500 MHz, C6D6) δ: 0.90 (t, J ) 7.3 Hz, 15H,
CH3), 1.23 (m, 10 H, CH2), 1.49 (m, 10H, CH2), 2.43 (t, J ) 7.7
Hz, 10H, CH2), 7.02 (d, J ) 8.6 Hz, 10H, Ar), 7.75 (d, J ) 8.6
Hz, 10H, Ar); 13C NMR (125 MHz, C6D6) δ: 14.1 (5C, CH3), 22.4
(5C, CH2), 33.8 (5C, CH2), 35.4 (5C, CH2), 58.5 (5C, C60(sp3)),
100.3 (5C, C60(Cp)), 103.2, 129.0, 129.8, 136.1, 143.8, 144.3, 144.3,
145.0, 147.9, 148.9, 149.1, 150.7, 166.6; UV-vis-NIR (solution
in CH2Cl2) λmax (ε): 683 (0.94 × 104); Anal. Calcd for
Co[C60(4-nBuC6H4)5](CO)2 (1a). A suspension of C60(4-
nBuC6H4)5H (0.300 g, 0.22 mmol) and excess amount of KH
(dispersed in oil) in THF (2.0 mL) was warmed up to 60 °C and
stirred for 30 min. To a solution of Co2(CO)8 (0.25 g, 0.73 mmol)
in THF (1.5 mL) in another Schlenk tube was slowly added I2 (0.15
g, 0.59 mmol) to turn green with gas evolution. To the green
solution was added the solution of KC60(4-nBuC6H4)5 slowly as
C
114H65N2S2Co: C, 86.34; H, 4.13; N, 1.77. Found: C, 86.07; H,
4.28; N, 1.54.
(38) Martin, C. A.; Ding, D.; Sørensen, J. K.; Bjørnholm, T.; van
Ruitenbeek, J. M.; van der Zant, H. S. J. J. Am. Chem. Soc. 2008,
130, 13198–13199.
Co[C60(4-nBuC6H4)5][S2C2S2CdCS2C2(CO2Me)2] (3). A solu-
tion of 1a (20.0 mg, 13.3 µmol) and dimethyl 2-(5-oxo-
[1,3]dithiolo[4,5-d][1,3]dithiol-2-ylidene)-1,3-dithiole-4,5-dicar-
boxylate43 (6.5 mg, 15.8 µmol) in p-xylene (10 mL) was heated at
140 °C for 1 h. The resulting red reaction mixture was diluted with
toluene (5 mL), and filtered through a pad of silica gel. The filtrate
was reprecipitated with MeOH to give dark red solid, which was
subjected to silica gel column chromatography (toluene as eluent).
A dark red band was collected and reprecipitated by MeOH to afford
(39) (a) Matsuo, Y.; Tahara, K.; Nakamura, E. J. Am. Chem. Soc. 2006,
128, 7154–7155. (b) Matsuo, Y.; Tahara, K.; Fujita, T.; Nakamura,
E. Angew. Chem., Int. Ed. 2009, in press.
(40) (a) Matsuo, Y.; Kanaizuka, K.; Matsuo, K.; Zhong, Y.-W.; Nakae,
T.; Nakamura, E. J. Am. Chem. Soc. 2008, 130, 5016–5017. (b)
Sakamoto, A.; Matsuo, Y.; Matsuo, K.; Nakamura, E. Chem. Asian J.
2009. in press.
(41) Niinomi, T.; Matsuo, Y.; Hashiguchi, M.; Sato, Y.; Nakamura, E. J.
Mater. Chem. 2009, in press.
(42) Takahashi, T.; Takenobu, T.; Takeya, J.; Iwasa, Y. Appl. Phys. Lett.
2006, 88, 033505.
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12648 J. AM. CHEM. SOC. VOL. 131, NO. 35, 2009

π-Conjugated Multi-Donor/Acceptor Arrays
A R T I C L E S
1
3 (19.1 mg, 10.4 µmol, 78%) as dark orange solid. H NMR (500
MHz, C6D6) δ: 0.87 (t, J ) 7.5 Hz, 15H, CH3), 1.22 (m, 10 H,
CH2), 1.48 (m, 10H, CH2), 2.45 (t, J ) 7.5 Hz, 10H, CH2), 3.18 (s,
6H, CH3), 7.10 (d, J ) 8.0 Hz, 10H, Ar), 8.04 (d, J ) 7.1 Hz,
10H, Ar); 13C NMR (125 MHz, C6D6/CS2) δ: 14.2 (5C, CH3), 22.5
(5C, CH2), 33.9 (5C, CH2), 35.5 (5C, CH2), 52.7 (2C, CH3), 58.4
(5C, C60(sp3)), 99.5 (5C, C60(Cp)), 128.8, 130.0, 132.0, 143.2, 144.2,
144.5, 147.8, 148.8, 149.1, 151.9, 159.6, 165.5; UV-vis-NIR
(solution in CH2Cl2) λmax (ε): 934 (1.5 × 104); Anal. Calcd for
C120H71 O4S6Co: C, 78.84; H, 3.91. Found: C, 79.06; H, 4.19.
[Co{C60(4-nBuC6H4)5}(S2C2S2C)]2 (4). A solution of 1a (10 mg,
6.7 µmol) and bis(carbonyldithio)tetrathiafulvalene (5.1 mg, 13
µmol) in p-xylene (2.0 mL) was heated at 140 °C for 4 h. The
resulting dark yellow reaction mixture was diluted with toluene (5
mL), and filtered through a pad of silica gel. The filtrate was
reprecipitated with MeOH to give dark yellow-green solid, which
was purified with preparative HPLC separation (Buckyprep, toluene/
iPrOH ) 8/2) to afford 4 (6.3 mg, 2.0 µmol, 59%) as dark green
Ti:Sapphire laser system (CPA 2001 Laser, Clark-MXR, Inc.).
Nanosecond laser flash photolysis experiments were performed with
337-nm laser pulses from a nitrogen laser (8-ns pulse width) in
front face excitation geometry.
Third Harmonic Generation (THG). The evaluation of |ꢀ(3)|
was performed with the standard Maker’s fringe method,44 where
the reference sample was SiO2.45 The excitation light was generated
by combining a femtosecond regenerative amplifier system, Spitfire
Pro (SpectraPhysics), and an optical parametric amplifier, TOPAS
(Light conversion). By difference frequency generation of signal
and idler lights from TOPAS, we obtained the excitation light of
3300 and 2934 nm. To avoid the effect of THG from ambient air,
we performed the measurements in a vacuum cell.
Acknowledgment. This work was partially supported by
KAKENHI (#18105004 and #18684014) and the Global COE
Program for Chemistry Innovation of the MEXT, Japan. Financial
support from Alexander von Humboldt Stiftung is acknowledged
by S.S.G.
1
solid. H NMR (500 MHz, C6D6) δ: 0.93 (t, J ) 7.5 Hz, 15H,
CH3), 1.29 (m, 10 H, CH2), 1.54 (m, 10H, CH2), 2.51 (t, J ) 7.5
Hz, 10H, CH2), 7.10 (d, J ) 8.1 Hz, 10H, Ar), 8.05 (d, J ) 8.0
Hz, 10H, Ar); 13C NMR (125 MHz, C6D6/CS2) δ: 14.5 (5C, CH3),
22.9 (5C, CH2), 34.0 (5C, CH2), 35.7 (5C, CH2), 58.1 (5C, C60(sp3)),
99.1 (5C, C60(Cp)), 120.0 (2C, sp2), 128.7, 129.8, 137.2, 142.8,
144.1, 144.4, 147.8, 148.7, 149.0, 151.6, 164.9 (4C, sp2); UV-vis-
NIR (solution in CH2Cl2) λmax (ε): 1100 (3.1 × 104), 613 (4.5 ×
103); Anal. Calcd for C226H130S8Co2: C, 84.08; H, 4.08. Found: C,
84.30; H, 4.07.
Flash Photolysis Experiments. Femtosecond transient absorp-
tion studies were performed using the 388 nm laser pulses of 150
fs pulse width and 200 nJ energy, generated by a beta-Barium borate
crystal upon higher order non-linear processes from an amplified
Supporting Information Available: Crystallographic data for
1b and 4 (CIF file), absorption spectra, electrochemical and
photoelectrochemical measurement data. This material is avail-
able free of charge via the Internet at http://pubs.acs.org.
JA902312Q
(44) Kajzar, F.; Messier, J.; Rosilio, C. J. Appl. Phys. 1986, 60, 3040–
3044.
(45) (a) Miller, R. C. Appl. Phys. Lett. 1964, 5, 17–19. (b) Garrett, C. G. B.;
Robinson, F. N. H. IEEE J. Quantum Electron. 1966, QE-2, 328–
329. (c) Buchalter, B.; Meredith, G. R. Appl. Opt. 1982, 21, 3221–
3224.
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