Star-shaped pyrrole-fused tetrathiafulvalene oligomers: Synthesis and redox, self-assembling, and conductive properties

Article abstract of DOI:10.1021/ol2014279

A series of star-shaped pyrrole-fused tetrathiafulvalene (TTF) oligomers 1-3 was synthesized via an S_NAr reaction of fluorinated benzenes with the pyrrolyl sodium salts. Electrochemical and chemical oxidations of 1-3 revealed that a radical cat

Full text of DOI:10.1021/ol2014279

ORGANIC
LETTERS
2011
Vol. 13, No. 15
3896–3899
Star-Shaped Pyrrole-Fused
Tetrathiafulvalene Oligomers: Synthesis
and Redox, Self-Assembling, and
Conductive Properties
Masayoshi Takase,* Naofumi Yoshida, Tohru Nishinaga, and Masahiko Iyoda*
Department of Chemistry, Graduate School of Science and Engineering, Tokyo
Metropolitan University, Hachioji, Tokyo 192-0397, Japan
mtakase@tmu.ac.jp; iyoda@tmu.ac.jp
Received May 26, 2011
ABSTRACT
A series of star-shaped pyrrole-fused tetrathiafulvalene (TTF) oligomers 1ꢀ3 was synthesized via an SNAr reaction of fluorinated benzenes with
the pyrrolyl sodium salts. Electrochemical and chemical oxidations of 1ꢀ3 revealed that a radical cation moiety on each TTF unit was successfully
accumulated in all oligomers. Self-assembled structures of neutral and oxidized species were characterized by SEM and XRD, and their
conductive properties of the iodine-doped 1ꢀ3 as well as an intermolecular mixed-valence ion radical salt were investigated.
Tetrathiafulvalene (TTF) and its analogues have been
intensively studied because of their unique π-donor
properties.1 After the discovery of the high electric conduc-
tivity of the charge transfer (CT) complex of TTF and
tetracyanoquinodimethane (TCNQ),2 many TTF analogues
have been synthesized from the viewpoint of materials and
physical sciences. In general, however, conductive molecular
complexes should be studied in a single crystal state, which
would be problematic in processing materials for applica-
tions. To overcome this problem, a supramolecular system
has currently been applied to this area, which facilitates the
development of electroactive soft materials. Inspired by
Jørgensen and Bechgaad’s first example of TTF fibers,3 other
supramolecular structures consisting of TTF derivatives have
been constructed by utilizing mainly hydrogen-bonding in-
teractions, together with weaker van der Waals, πꢀπ stack-
ing, and S S interactions.4 Among them, Kato et al.
3 3 3
demonstrated that doping of the fibers by iodine vapor
resulted in the formation of charge-transfer states exhibiting
(3) Jørgensen, M.; Bechgaard, K. J. Org. Chem. 1994, 59, 5877.
(4) (a) Kitahara, T.; Shirakawa, M.; Kawano, S.; Beginn, U.; Fujita,
N.; Shinkai, S. J. Am. Chem. Soc. 2005, 127, 14980. (b) Wang, C.; Zhang,
D.; Zhu, D. J. Am. Chem. Soc. 2005, 127, 16372. (c) Kitamura, T.;
Nakaso, S.; Mizoshita, N.; Tochigi, Y.; Shimomura, T.; Moriyama, M.;
Ito, K.; Kato, T. J. Am. Chem. Soc. 2005, 127, 14769. (d) Puigmartı-Luis,
ꢀ
J.; del Pino, A. P.; Laukhina, E.; Esquena, J.; Laukhin, V.; Rovira, C.;
Vidal-Gancedo, J.; Kanaras, A. G.; Nichols, R. J.; Brust, M.;
Amabilino, D. B. Angew. Chem., Int. Ed. 2008, 47, 1861. (e) Akutagawa,
T.; Ohta, T.; Hasegawa, T.; Nakamura, T.; Christensen, C. A.; Becher, J.
Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 5028. (f) Gall, T. L.; Pearson., C.;
Bryce, M. R.; Petty, M. C.; Dahlgaad, H.; Becher, J. Eur. J. Org. Chem.
2003, 3562.
(5) (a) Kobayashi, Y.; Hasegawa, M.; Enozawa, H.; Iyoda, M. Chem.
Lett. 2007, 36, 720. (b) Ahn, S.; Kim, Y.; Beak, S.; Ishimoto, S.;
Enozawa, H.; Isomura, E.; Hasegawa, M.; Iyoda, M.; Park, Y. J. Mater.
Chem. 2010, 20, 10817. (c) Isomura, E.; Nishinaga, T.; Iyoda, M.
Supramolecular Chem. 2011, 23, 304.
€
(1) For selected reviews, see: (a) Adam, M.; Mullen, K. Adv. Mater.
1994, 6, 439. (b) Bryce, M. R. J. Mater. Chem. 1995, 5, 1481. (c) Otsubo,
T.; Aso, Y.; Takimiya, K. Adv. Mater. 1996, 8, 203. (d) Bryce, M. R.
J. Mater. Chem. 2000, 10, 589. (e) Nielsen, M. B.; Lomholt, C.; Becher, J.
Chem. Soc. Rev. 2000, 29, 153. (f) Segura, J. L.; Martın, N. Angew.
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Chem., Int. Ed. 2001, 40, 1372. (g) Canevet, D.; Salle, M.; Zhang, G.;
Zhang, D.; Zhu, D. Chem. Commun. 2009, 2245. (h) Hasegawa, M.;
Iyoda, M. Chem. Soc. Rev. 2010, 39, 2420.
(2) Wudl, F.; Smith, G.; Hufnagel, E. J. Chem. Soc., Chem. Commun.
1970, 1453.
r
10.1021/ol2014279
Published on Web 07/06/2011
2011 American Chemical Society

semiconductivity of ca. 10ꢀ5 Scmꢀ1.4c Although other simple
TTF derivatives possessing polar functional groups like
amide and ester were reported to form gels and fibrous
structures with high conductivity upon iodine doping,5 the
synthesis and self-assembly of more sophisticated TTF oli-
gomers with planar cyclic6 and star-shaped7 structures have
also been reported. In these planar π-systems, a number of
The SNAr reaction of fluorinated benzenes with the pyrrolyl
sodium salts of 4 gave 1ꢀ3 in moderate yields (25ꢀ64%,
Scheme 1). Characterization of 1ꢀ3 was perfomed using 1H
NMR, 13C NMR, LDI-TOF MS, and elemental analyses.
For example, in the 1H NMR spectra, R-protons of pyrroles
were observed at δ 5.93 (1a), 6.41 (2a), and 6.89 ppm (3a),
respectively. The signals of 1a and 2a at higher fields are
ascribable to the ring currents of adjacent pyrrolyl moieties as
predicted by theoretical calculations (Figure S7, Supporting
Information). Moreover, the exact structure and conforma-
tion as well as self-assembly of 3a were revealed by X-ray
single-crystal structure analysis (Figure S6, Supporting
Information). Although 3a forms a dimer in the unit cell,
two of the three TTF units are bent simply to fill an empty
space, and the other one stacks with a distance of ca. 3.7 A in
contrast to the expected stacked structures with cooperative
TTF units work to enhance cooperative S S and πꢀπ
3 3 3
interactions and thus make an efficient conduction path
through the stacked supramolecular architectures.
With these approaches in mind to make electroactive
self-assembled structures, we have designed and synthe-
sized a series of star-shaped pyrrole-fused TTF oligomers
1ꢀ3, where TTFs are introduced in a less conjugated
manner that still maintains their rigid structures compared
with previously reported star-shaped8a or dendritic8b,9
TTF oligomers. On the basis of this molecular design, it
is expected that star-shaped structures of TTFs enhance
the intermolecular interactions to form one-dimensional
columnar assemblies, and less intramolecular conjugation
between TTF units contributes to the accumulation of
radical cation moiety on TTF oligomers.9
S
S and πꢀπ interactions. The torsion angles between the
3 3 3
mean planes of the pyrrole and central benzene ring are 11°,
18°, and 31° and 7°, 11°, and 32° for two independent
structures, indicating the conformational flexibility of the
pyrroleꢀbenzene linkage.
Scheme 1. Synthesis of Star-Shaped TTF Oligomers 1ꢀ3
Figure 1. Cyclic voltammograms of 1aꢀ3a (0.1 mM) in benzo-
nitrile with 0.1 M n-Bu4NPF6 as the supporting electrolyte, Ag/
AgNO3 as the reference electrode, glassy carbon as the working
electrode, Pt wire as the counter electrode, and a scan rate of
100 mV/s. Values are half-wave potentials.
Synthesis of star-shaped TTF oligomers 1ꢀ3 is based
on the nucleophilic aromatic substitution (SNAr) of
fluorinated benzenes (Scheme 1).10 Pyrrole-fused
TTFs 4a,b were chosen as key units11 because pyrroleꢀ
benzene conjugation can be adjustable in terms of torsion an-
gles during assembly and on electrochemical/chemical stimuli.
The redox behaviors of 1ꢀ3 were first investigated us-
ing cyclic voltammetry (CV) in benzonitrile (0.1 mM)
(Figure 1). Although tetrasubstituted 2a exhibits typical
two reversible oxidation waves with half-wave potentials
(9) For examples of multiple TTF radical cations on oligomeric
TTFs, see: (a) Devonport, W.; Bryce, M. R.; Marshallsay, G. J.; Moore,
A. J.; Goldenberg, L. M. J. Mater. Chem. 1998, 8, 1361. (b) Wang, C.;
Bryce, M. R.; Batsanov, A. S.; Goldenberg, L. M.; Howard, J. A. K.
J. Mater. Chem. 1997, 7, 1189.
(10) (a) Biemans, H. A. M.; Zhang, C.; Smith, P.; Kooijman, H.;
Smeets, W. J. J.; Spek, A. L.; Meijer, E. W. J. Org. Chem. 1996, 61, 9012.
(b) Takase, M.; Enkelmann, V.; Sebastiani, D.; Baumgarten, M.;
(6) (a) Enozawa, H.; Hasegawa, M.; Takamatsu, D.; Fukui, K.;
Iyoda, M. Org. Lett. 2006, 8, 1917. (b) Hara, K.; Hasegawa, M.;
Kuwatani, Y.; Enozawa, H.; Iyoda, M. Heterocycles 2010, 80, 909. (c)
Anderson, A. S.; Kilsa, K.; Hassenkam, T.; Gisselbrecht, J.-P.; Boudon,
C.; Gross, M.; Nielsen, M. B.; Diederich, F. Chem.;Eur. J. 2006, 12,
8451.
€
(7) (a) Hasegawa, M.; Takano, J.; Enozawa, H.; Kuwatani, Y.;
Iyoda, M. Tetrahedron Lett. 2004, 4109. (b) Hasegawa, M.; Enozawa,
H.; Kawabata, Y.; Iyoda, M. J. Am. Chem. Soc. 2007, 129, 3072.
(8) (a) Christian, C. A.; Bryce, M. R.; Batsanov, A. S.; Becher, J.
Chem. Commun. 2000, 331. (b) Christian, C. A.; Goldenberg, L. M.;
Bryce, M. R.; Batsanov, A. S.; Becher, J. Chem. Commun. 1998, 509.
Mullen, K. Angew. Chem., Int. Ed. 2007, 46, 5524. (c) Dutta, T.; Woody,
K. B.; Watson, M. D. J. Am. Chem. Soc. 2008, 130, 452.
(11) (a) Jeppesen, J. O.; Takimiya, K.; Jensen, F.; Becher, J. Org.
Lett. 1999, 8, 1291. (b) Jeppesen, J. O.; Takimiya, K.; Jensen, F.;
Brimert, T.; Nielsen, K.; Thorup, N.; Becher, J. J. Org. Chem. 2000,
65, 5794.
Org. Lett., Vol. 13, No. 15, 2011
3897

(Eox) at 0.044 and 0.35 V (vs Fc/Fcþ), hexasubstituted
1a and trisubstituted 3a show broad and split first
peaks, respectively, with Eox1 at ca. 0.097 V (1a) and ꢀ
0.086 and 0.020 V (3a), following second peaks with
2
Eox at 0.37 V (1a) and 0.45 V (3a). Judging from the
result of 2a, which possesses two sets of ortho-substi-
tuted TTFs, it is suggested that there is no intramole-
cular charge delocalization between the adjacent TTF
units. Therefore, the splitting of the first oxidation waves in
1a and 3a is considered to be caused by intermolecular
interactions.12
To further investigate these phenomena, chemical oxi-
dation of 1aꢀ3a was performed with Fe(ClO4)3 in a
mixture of CH2Cl2 and CH3CN (2:1, v/v). Figure 2 shows
the absorption spectral changes of 1aꢀ3a upon the addi-
tion of the oxidant. When Fe(ClO4)3 was added incremen-
tally up to 1 equiv with respect to each TTF unit, the
absorption spectra changed dramatically (blue to green
spectra in Figure 2). For tetrasubstituted 2a, the changes
were observed with several isosbestic points, indicating
that each TTF unit was oxidized from the neutral to the
radical cation (TTF•þ) in a stepwise fashion (Figure 2b). In
contrast, for hexa- and trisubstituted 1a and 3a, no iso-
sbestic points were observed (Figure 2a,c). For 3a, a new
broad peak around 1850 nm appeared in the presence of
1.5 equiv of the oxidant, which is attributed to the forma-
tion of mixed-valence complexes. These results were con-
sistent with the peak splitting of the CV.7,8,13 On the other
hand, the similar appearance of a new peak around the
NIR region was not observed for 1a even at a high
concentration. Because such a phenomena was not ob-
served for 2a, it is concluded that this mixed-valence
complex is obtained by an intermolecular fashion, which
is completely different from previously reported star-
shaped7,8 and other TTF dimers.13 From these observa-
tions, trisubstituted 3a appears to form more aggregated
structures than other TTF oligomers 1a and 2a. In fact,
when the temperature of the CH2Cl2 and CH3CN (2:1, v/v)
mixed solution of 1a6(•þ), 2a4(•þ), and 3a3(•þ) (0.3 mM) was
reduced, inter TTF bonding (π-dimer formation) was
induced, which was confirmed by absorption and ESR
spectra (Figure S4, Supporting Information). Qualitative
order of the intermolecular interactions, 3a3(•þ) . 1a6(•þ)
> 2a4(•þ), appears to be determined by two factors, i.e., the
torsion angles between the pyrrole and central benzene
ring and the number of TTF units, and the former would
be more effective (the torsion angles estimated by theore-
tical calculations are 34° (3), 45° (2), and 59° (1), Figure S7,
Supporting Information). Again, because tetraradical ca-
tion 24(•þ) forms the π-dimer and possibly higher
Figure 2. Stepwise oxidation of (a) 1a (0.03 mM), (b) 2a (0.05
mM), and (c) 3a (0.02 mM) with incremental addition of Fe-
(ClO4)3 in a mixture of CH2Cl2 and CH3CN (2:1, v/v) at room
temperature. The blue line indicates the neutral absorp-
tion spectra, the green line the multiple TTF radical cations
1a6(•þ)ꢀ3a3(•þ), and the red line the TTF dications 1a12þꢀ3a6þ
.
aggregates only at low temperatures, the above-mentioned
TTF interactions are brought on in an intermolecular
fashion, which is preferable for making electroactive su-
pramolecular structures. Further addition of the oxidant
to 1a6(•þ)ꢀ3a3(•þ) gave spectral changes with isosbestic
points (green to red spectra in Figure 2), which attribute to
a decrease of TTF•þ and an increase of TTF2þ. The above
spectral changes were all confirmed by spectroelectro-
chemical analyses at various applied potentials (Figure
S3, Supporting Information).
(12) One reviewer pointed out the possibility that degenerated mole-
cular orbitals of 1a and 3a attributes to the splitting of the redox waves.
However, as far as we know, similar phenomena could not be observed
in other C3-symmetrical molecules possessing multiple redox-active
components.
(13) (a) Iyoda, M.; Hasegawa, M.; Kuwatani, Y.; Nishikawa, H.;
Fukami, K.; Nagase, S.; Yamamoto, G. Chem. Lett. 2001, 1146. (b)
Hasegawa, M.; Kobayashi, Y.; Hara, K.; Enozawa, H.; Iyoda, M.
Heterocycles 2009, 77, 837. (c) Nakamura, K-i.; Takashima, T.;
Shirahata, T.; Hino, S.; Hasegawa, M.; Mazaki, Y.; Misaki, Y. Org.
Lett. 2011, 13, 3122.
Once the accumulation of the radical cation moiety on
each TTF of 1ꢀ3 were demonstrated, their self-assembled
Org. Lett., Vol. 13, No. 15, 2011
3898

XRD patterns (Figure S5, Supporting Information),
showed a lower conductivity of 2.5 ꢁ 10ꢀ3 S cmꢀ1. Similar
enhancement of the conductivities of the supramolecular
structures were also observed for other TTF oligomers
1ꢀ3 (Table S5, Supporting Information), reflecting the
importance of their molecular-level alignments. Interest-
ingly, an averaged conductivity of three single crystals of
3a after iodine doping was 1.8 ꢁ 10ꢀ2 S cmꢀ1, which is
similar to that of 3a-fiber, even though their XRD patterns
are different (Figure S6, Supporting Information).
Furthermore, not only neutral fibers but also deep
green fibrous materials were also formed when a
ꢀ
CH2Cl2 solution of 3a1.5(•þ) (ClO4
)
was mixed
1.5
3
with an excess amount of hexane (Figure 3c).
The XRD pattern of the fibers supports that it
is mainly composed of a lamellar structure (d =
13.8 A)14 with a πꢀπ stacking distance of d = 3.3 A
(Figure 3d). Thꢀus, an averaged electric conductivity of
3a1.5(•þ) (ClO4
)
1.5-fiber was evaluated to be 2.9 ꢁ
3
10ꢀ4 S cmꢀ1 without further doping,15 which is a
still-rare example of electroactive supramolecular fi-
bers prepared from organic ion radical salts.7b
In summary, we have demonstrated the synthesis of a
series of star-shaped TTF oligomers 1ꢀ3 via SNAr reac-
tion. The less conjugated but still rigid pyrroleꢀbenzene
linkages of 1ꢀ3 gave no intramolecular interactions be-
tween neutral, oxidized, and neutral/oxidized TTFs, so
thataccumulationofredoxactiveTTF unitswassuccessful
in all star-shaped oligomers. However, possible increment
of the charge carrier density brought on by the accumu-
lated radical cation moieties in the oligomers do not
seem to contribute effectively to the conductivities of
the self-assembled structures. Also, the relationship
between the substitution numbers/positions and conduc-
tivities of the assembled structures remains to be fully
elucidated. Further investigations like a development of
well-defined TTF oligomers are currently underway in our
group.
Figure 3. (a) SEM image and (b) XRD profile (2θ = 3.3° (100),
5.7° (110), 6.6° (200), 8.7° (210), 9.9° (300), 19.7° (600), 25.2°,
and 26.7°) of 3a-fiber and (c) SEM image and (d) XRD profile
(2θ =ꢀ6.4° (100), 12.8° (200), 19.2° (300), and 27.3°) of 3a1.5(•þ)
3
(ClO4 )1.5-fiber.
structures were prepared by mixing an excess amout of
poor solvent such as hexane and MeOH to the CH2Cl2 and
THF solutions, which resulted in the formation of various
types of assemblies. When increasing the number of TTF
substitutions from trimer (3a,b) to hexamer (1a,b) or
increasing the length of the alkyl chains from butyl
(1aꢀ3a) to dodecyl (1bꢀ3b) groups, poor solvents with
high polarity are necessary to form assembled structures,
resulting in the tendency to form spherical morphologies
(Figure S5, Supporting Information). As a representative
example, when hexane was added to a CH2Cl2 solution of
3a (0.21 mg/mL in CH2Cl2/hexane = 1:4, v/v), a yellow
fibrous material was obtained. Scanning electron micro-
scopy (SEM) analysis of 3a-fiber revealed its micrometer-
scale tape structure, whose internal structure was investi-
gated by X-ray diffractometry (XRD) (Figure 3a,b). Thus,
a strong (100) and accompanying (110), (200), (210), (300),
and (600) reflections are indicative of hexagonal columnar
structure (d = 26.8A)14 withadditional reflections at2θ =
25.2° and 26.7° (d = 3.5 A and 3.3 A) possibly correspond-
ing to the neutralꢀneutral and stronger neutralꢀradical
cation (mixed-valence) πꢀπ stackings of TTFs, respec-
tively. Doping of iodine vapor into the compressed pellets
of 3a-fiber produced a black charge transfer complex,
which showed an averaged electric conductivity of 1.9 ꢁ
Acknowledgment. This work was supported by Grants-
in-Aid for Science and Research (Nos. 20850030 and
22245024) from MEXT, Japan, and research grants from
The Mazda Foundation and The Kao Foundation for Arts
and Sciences. We thank Dr. K. Yamashita (Tokyo Metro-
politan University) for help with X-ray crystal structure
analysis.
Supporting Information Available. Experimental pro-
cedures and the characterization data for all new com-
pounds; Figure S3ꢀS7, Table S5, atomic coordination of
the optimized structures of 1ꢀ3 (B3LYP/6-31G(d)), and
X-ray data for 3a (CIF). This material is available free of
charge via the Internet at http://pubs.acs.org.
10ꢀ2 S cmꢀ1 15 On the other hand, the spin-coat film of 3a,
.
consisting of a noncrystalline structure as revealed by
(15) The compressed pellets gave XRD patterns similar to those of
the fibrous materials, and the values of conductivities are averages of
three runs.
(14) According to the theoretical calculation, the molecular diameter
of 3 (methylthio derivative) is ca. 25 A and the length of pyrrole-fused
TTF unit is ca. 12 A. See Figure S7 (Supporting Information).
Org. Lett., Vol. 13, No. 15, 2011
3899