Tetrathiafulvalenylallene: A new class of donor molecules having strong chiroptical properties in neutral and doped states

Article abstract of DOI:10.1021/ol201857f

A chiral tetrathiafulvalene (TTF) dimer bridged by an allene framework (1) was synthesized. An X-ray analysis of 1 revealed an effective conjugation between TTF and the allene backbone. Allene 1 was resolved into both enantiomers, which showed strong chir

Full text of DOI:10.1021/ol201857f

ORGANIC
LETTERS
2011
Vol. 13, No. 17
4688–4691
Tetrathiafulvalenylallene: A New Class of
Donor Molecules Having Strong Chiroptical
Properties in Neutral and Doped States
Masashi Hasegawa,* Yasuto Sone, Seiya Iwata, Hideyo Matsuzawa,
and Yasuhiro Mazaki*
Department of Chemistry, School of Science, Kitasato University, Sagamihara,
Kanagawa 252-0373, Japan
masasi.h@kitasato-u.ac.jp; mazaki@kitasato-u.ac.jp
Received July 14, 2011
ABSTRACT
A chiral tetrathiafulvalene (TTF) dimer bridged by an allene framework (1) was synthesized. An X-ray analysis of 1 revealed an effective
conjugation between TTF and the allene backbone. Allene 1 was resolved into both enantiomers, which showed strong chiroptical electrochromic
properties. Absolute configuration of the allene was validated by theoretical study of the electronic circular dichroism (ECD) spectrum. ECD
spectra of cationic species 12þ and 14þ exhibited intense Cotton Effects over the visible region.
The design and synthesis of redox-mediated chiroptical
organic materials have attracted considerable attention
because of their potential applications for photonic devices
related to polarized light, including optical switches, mem-
ory, and optical modulators that have chiroptical pro-
perties.1 A wealth of redox systems with asymmetric units
based on diverse platforms have been prepared thus far.1ꢀ4
In most of these chiral systems, polymeric or oligomeric
donor molecules having chiral alkyl groups are employed.
The chiral side chain often gives rise to a helical structure,
and the redox results in conformational changes with
chiroptical switching.2 Contrastingly, the installation of redox
active moieties into a chiral framework is a simple and
practical route to design functional molecules exhibiting
chiroptical electrochromic properties because a strong and
ascertainable chiral exciton coupling of the chromophore can
be anticipated during their redox process without large con-
formational changes. However, the applicable chiral back-
bone is limited to a 1,10-binaphthyl skeleton.3,4
An allene framework is so rigid that the axial chirality
induces a well-defined chiral relationship between the terminal
(5) Modern Allene Chemistry; Krause, N., Hashimi, A. J., Eds; Wiley-
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2010, 1, 841–848. (c) Goto, H. J. Mater. Chem. 2009, 19, 4914–4921.
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r
10.1021/ol201857f
Published on Web 08/05/2011
2011 American Chemical Society

chromophores; hence, chiroptical effects can be generated.5ꢀ7
We have here designed 1,3-bis(tetrathiafulvalenyl)allene deri-
vative 1 as a new class of electron-donor molecule that is
expected to show strong chiroptical properties.
which is a normal chemical shift for the nonpolarized
allene.5ꢀ7 The IR (1953 cmꢀ1 (νCdCdC)) spectrum also
supported the linear structure of the allene moiety of 1.
To date, chirality has been introduced into tetrathiafulva-
lene (TTF)8 to observe new phenomena, such as chiral
conductors and chiroptical molecular switches,9,10 but there
are only a few studies on TTF involved in axial chirality,
which can be expected to have pronounced chiroptical
effects.4 Previously, binaphthalene molecules linked by two
and four TTF units through long alkyl chains were prepared,
and their chiral molecular switching properties were
investigated.4 In these systems, although the chiral properties
were tunable by the addition of oxidants, only small chirop-
tical effects at high energy region were observed at any
oxidation stage, owing to the small chiroptical interactions
between their chromophores. Through the direct connection
of TTF moieties to an allene backbone with axial chirality, we
assume that 1 might exhibit considerably larger chiroptical
electrochromic properties over various energy ranges. In this
paper, we report the synthesis, structures, optical resolution of
the allene, and significant chiroptical electrochromic proper-
ties during oxidation.
Scheme 1. Synthesis of 1
Single crystals of racemic 1 were obtained by the slow
diffusion of methanol into a toluene solution at 4 °C, and
X-ray analysiswas carried out atꢀ100 °C (Figure1).12 The
allene skeleton was almost linear (C1ꢀC2ꢀC3 = 178.17°),
and the two TTF moieties were connected orthogonally to
each other through the central allene. Each TTF adopted a
slight boat conformation and dimerized with the same part
of the other enantiomer in the lattice. The shortest inter-
molecular distance between the atoms and the least-
squared plane within the stacked TTFs was found to be
3.39ꢀ3.64 A, which suggested a significant contact be-
Novel symmetrical allene 1 was synthesized using palla-
dium-catalyzed SN20 substitution of propargyl acetate as
the key reaction (Scheme 1).6a Propargylic alcohol 6 was
easily prepared from 4,5-bis(methylthio)-tetrathiafulva-
lene 2 in four steps. When 6 was treated with acetic
anhydride (Ac2O) in the presence of N,N-dimethyl-4-ami-
nopyridine (DMAP), the corresponding propargylic acet-
ate 7 was formed in situ, though the acetate 7 did not prove
satisfactorily stable for isolation under ambient condi-
tions. Sequential palladium-catalyzed substitution of 7
with PhZnCl in one pot proceeded to give the expected
allene 1 in a 23% yield from 6. The structure of 1 was
tween two intermolecular TTFs through S S and πꢀπ
3 3 3
interactions. Furthermore, dihedral angles of the TTF
groups against the central allene were 5.6 and 18.0°,
respectively, whereas those of the two phenyl groups were
much larger: 69.2 and 45.8°, respectively. It was thus clear
that the two TTFs were conjugated to the central allene
more effectively than the phenyl groups.
1
characterized by H NMR, 13C NMR, DI-MS, IR, and
elemental analysis. The chemical shift of the central carbon
of 1 in the 13C NMR spectrum was observed at 209.2 ppm,
(8) For reviews on tetrathiafulvalene see: (a) TTF Chemistry:
Fundamental and Applications of Tetrathiafulvalene; Yamada, J., Sugimoto,
T., Eds; Kodansha-Springer: Tokyo, 2004. (b) Iyoda, M.; Hasegawa, M.;
Miyake, Y. Chem. Rev. 2004, 104, 5085–5113. (c) Segura, J. L.; Martın,
´
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N. Angew. Chem., Int. Ed. 2001, 40, 1372–1409. (d) Canevet, D.; Salle,
M.; Zhang, G.; Zhu, D. Chem. Commun. 2009, 2245–2269.
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(9) (a) Saad, A.; Jeannin, O.; Fourmigue, M. Tetrahedron 2011, 67,
3820–3829. (b) Danila, I.; Riobe, F.; Piron, F.; Puigmartı-Luis, J.;
´
Wallis, J. D.; Linares, M.; Agren, H.; Beljonne, D.; Amabilino, D. B.;
Avarvari, N. J. Am. Chem. Soc. 2011, 133, 8344–8353. (c) Wallis, J. D.;
Griffiths, J.-P. J. Mater. Chem. 2005, 15, 347–365. (d) Matsumiya, S.;
Izuoka, A.; Sugawara, T.; Taruishi, T.; Kawada, Y.; Tokumoto, M.
ꢀ
ꢀ
Bull. Chem. Soc. Jpn. 1993, 66, 1949–1954. (e) Rethore, C.; Avarvari, N.;
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Canadell, E.; Auban-Senzier, P.; Fourmigue, M. J. Am. Chem. Soc.
€
2005, 127, 5748–5749. (f) Rikken, G. L. J. A.; Folling, J.; Myder, P. Phys.
Figure 1. X-ray structure of 1, (a) top view and (b) side view.
Hydrogen atoms are omitted for clarity.
Rev. Lett. 2001, 87, 236602–236605.
(10) Gomar-Nadal, E.; Jaume, V.; Rovira, C.; Amabilino, D. B. Adv.
Mater. 2005, 17, 2095–2098.
Org. Lett., Vol. 13, No. 17, 2011
4689

The cyclic voltammogram (CV) of 1 (1.0 ꢁ 10ꢀ3 M in
PhCN) showed two reversible redox waves assigned to
TTF/TTF•þ and TTF•þ/TTF2þ at 0.06 and 0.40 V (vs Fc/
Fcþ), which were similar to those of 2 (0.03 and 0.35 V
under similar conditions).13 Because the allene 1 possesses
a good donor ability, chemical oxidation affords the
oxidized TTF species (Scheme 2). In fact, treatment of 1
with 2 and 6 equiv. of Fe(ClO4)3 in CH2Cl2-MeCN (v/v =
4:1) gave 12þ and 14þ, which were comparable to 2•þ and
22þ, respectively (Figure 2). During the sequential addition
of Fe(ClO4)3 up to 2 equiv to 1, only one phase changed
while the same isosbestic points were observed.11 This
behavior clearly indicated that there was nointramolecular
Since strong chiroptical effects between the TTF moi-
eties of optically active 1 were anticipated, we carried out
optical resolution of racemic 1 by chiral HPLC (DAICEL
chiralpak IA-3). Elution of rac-1 with CHCl3-hexane-
EtOH solution (v/v = 10:40:0.2) afforded optically pure
24
(þ)-1 and (ꢀ)-1, whose optical rotations ([R]D in
CH2Cl2) were þ760 and ꢀ760°, respectively.11 The elec-
tronic circular dichroism (ECD) spectra of the allenes (þ)-
1 and (ꢀ)-1 were measured in CH2Cl2 solution and are
depicted in Figure 3. Both of the ECD spectra showed
strong Cotton Effects over their entire absorption range,
and they possessed distinctive bisignate bands at 280 and
335 nm together with shoulder band tails up to approxi-
mately 540 nm. Because the absorption spectrum of 1
extended into the long wavelength region up to 550 nm,
the observed Cotton Effects in the visible region were
associated with the transition of the TTF moieties.
electronic interaction between the two TTF units in 12þ
.
Scheme 2. Chemical Oxidation of 1
To assign the absolute configuration of the optically
active allene and to study the origin of the observed Cotton
Effects, the electronictransition energiesand therotational
strengths were calculated with time-dependent (TD) DFT
calculations after geometry optimization at the B3LYP/6-
31G(d,p) level.15 The optimized molecular geometry of 1,
withC2 symmetry, wasclosetothe structure obtainedfrom
X-ray analysis. Asshown inFigure 3, TD-DFT calculation
ofthe optimized structure of (S)-1 provideda reproduction
of the features of the qualitative ECD spectrum experi-
mentally obtained from the (þ)-allene isomer. Therefore,
we assumed that the (þ)-1 obtained from the chiral HPLC
separation should have the (S) configuration. The simu-
lated spectrum of (R)-1 was also attributed to a mirror
spectrum opposite to (S)-1, and hence the (ꢀ)-allene
isomer should have the (R) configuration.11 The TD-
DFT calculation of (S)-1 revealed that the positive rota-
tional strengths at lower energy (S1, S2, and S3 in Figure 3)
were related to the transition of the TTF moieties. Thus,
the direct connection of two TTFs to the allene induced
strong chiroptical properties, as evidenced in the ECD
spectra.
In the electronic spectrum of 12þ, the absorption max-
imum was found at 790 nm, which was typically assigned
to an absorption band derived from an electron transition
to the SOMO in TTF•þ, while the absorption maxima of
2•þ under similar conditions was observed at 744 nm.14
Such significant red-shift of 12þ was due to the effective
conjugation of the TTF units and the central allene
moieties. Similarly for the tetracation 14þ, the absorption
maximum of 652 nm, which is typically assigned to the
HOMOꢀLUMO transition in TTF2þ moieties, was also
red-shifted compared with the corresponding peak in 22þ
(λmax = 622 nm).14 In addition, neutral 1 was recovered
without significant decomposition when these cationic
species were treated with excess H2NNH2 H2O.
3
Although the internal rotation barrier of the allenic
CdCdC axis is generally large (ΔGq = 46 kcal/mol for
(12) Crystal data for 1: C31H24S12 1.5(C7H8), Mw = 918.92, red
3
block, triclinic, space group P1 (#2), a = 9.9898(7), b = 14.5484(10), c =
15.5784(10) A, R = 93.462(1), β = 94.603(1), γ = 101.437(1)°, V =
2205.2(3) A3, Z = 2, Dc = 1.384 gcmꢀ3, 173 K, μ = 0.624 mmꢀ1, 12 441
reflections measured, 9472 unique (Rint = 0.0131), final R indices [I >
2σ(I)]: R1 = 0.0394, wR2 = 0.1046, GOF = 1.035.
(13) The CV analysis was carried out in PhCN solution with 0.1 M
nBu4NClO4 as a supporting electrolyte at 23 °C using Pt working and
counter electrodes.þThe potentials were measured against Ag/Agþ and
converted to Fc/Fc .
(11) See Supporting Information.
(14) (a) Nakamura, K.-I.; Takashima, T.; Shirahata, T.; Hino, S.;
Hasegawa, M.; Mazaki, Y.; Misaki, Y. Org. Lett. 2011, 13, 3122–3125.
(b) Iyoda, M.; Hasegawa, M.; Kuwatani, Y.; Nishikawa, H.; Fukami,
K.; Nagase, S.; Yamamoto, G. Chem. Lett. 2001, 1146–1147.
(15) Frisch, M. J.; et al. Gaussian 09, Revision B.01; Gaussian Inc.:
Pittsburgh, PA, 2009.
(16) (a) Roth, W. R.; Ruf., G.; Ford, P. W. Chem. Ber. 1974, 107, 48–
52. (b) Dynamic Stereochemistry of Chiral Compounds; Wolf, C., Ed.;
RSC Publishing: Cambridge, 2008.
Figure 2. Electronic spectra and the colors of 1 (solid gray line),
12þ (dash line), and 14þ (solid black line) in CH2Cl2-MeCN
(v/v = 4:1) solution.
(17) (a) Klett, M. W.; Johnson, R. P. J. Am. Chem. Soc. 1985, 107,
ꢀ
3971–3980. (b) Alonso-Gomez, J. L.; Schanen, P.; Rivera-Fuentes, P.;
Seiler, P.; Diederich, F. Chem.;Eur. J. 2008, 14, 10564–10568.
4690
Org. Lett., Vol. 13, No. 17, 2011

1,3-dialkylallenes),16 some allenes having electron donating
groups are susceptible to photoracemization in a solution.16b,17
In our system, the optically pure allene 1 was gradually
racemized in solution at rt without chemical decomposition.
The racemization of 1 in solution showed a good relationship
in the first-order rate plot.11 The half-lives of (þ)-1 in CH2Cl2
and CH2Cl2-MeCN (v/v = 4:1) solution (5.1 ꢁ 10ꢀ5 M),
obtained from the analysis of the ellipticity changes at 281
nm, were 56 and 76 min, respectively. Whereas the ellipticity
decreased during daylight, no racemization was observed in
solution in the dark or stored in amber glassware.18
exciton-type signal shape, unlike the ECD spectrum of the
dication, a very strong exciton coupling whose transition
moments were localized at the terminal TTF2þ moieties
was induced by the axial chirality.
Figure 4. ECD spectra of (S)-12þ (dash red line), (S)-14þ (solid
red line), (R)-12þ (dash blue line), and (R)-14þ (solid blue line) in
CH2Cl2-MeCN (v/v = 4:1) solution. A low S/N ratio on the
spectra at lower energy was an instrumental artifact.
In summary, we have designed and synthesized a novel
chiral allene (1) that has TTF moieties as electron donors,
and we have succeeded in the optical resolution of 1 into its
enantiomers, whose absolute configurations were vali-
dated by TD-DFT calculations. As expected, both of the
optically pure allenes exhibited strong chiroptical proper-
ties. The chemical oxidation of these optically pure allenes
resulted in different Cotton Effects in their ECD spectra
depending on the oxidation stage of the TTF units. These
results indicated that a wide range of intensely chiroptical
electrochromic properties could be realized by controlling
the oxidation of the TTF moieties. We believe that the
present redox-active allene provides a new class of chir-
optical electrochromic materials.
Figure 3. ECD spectra of (S)-(þ)-1 (solid black line), (R)-(ꢀ)-1
(dash line) in CH2Cl2, the simulated ECD spectrum of (S)-1
(gray line), and the selected rotational strength (gray bar) at TD-
DFT (B3LYP/6-31G(d,p)) calculated for 64 states.
Because the photoracemization is slow enough in CH2Cl2-
MeCN solution, we carried out the chemical oxidation of
optically pure allene 1 with Fe(ClO4)3.18 As shown in the
ECD spectra of 12þ (Figure 4), strong Cotton Effects were
observed over almost the whole visible region. For example,
CD spectrum of (S)-12þ displayed a board positive band at
low energy with a maximum at approximately 800 nm, a
bisignate signal in the 330ꢀ620 nm region, and a strong
negative sign with maximum at 293 nm. Meanwhile, the
ECD spectrum of (R)-12þ was its mirror image. Since
the longest wavelength absorption of 12þ was assigned to
the electron transition of the TTF•þ moiety, the observed
negative/positive sign in the ECD spectrum was a result of the
strong chiroptical nature of the allene. In particular, each
optically pure 12þ notably exhibited only negative/positive
molar ellipticity over the range 445ꢀ1000 nm.
Acknowledgment. We are grateful to Prof. Kazumasa
Kajiyama (Kitasato University) and Mr. Ken-ichiro
Miyazawa (Daicel Chemical Industries, Ltd.) for helpful
advice on optical resolution, the Interdisciplinary Laboratory
for Nanoscale Science and Technology in NIMS for mea-
surements of ECD spectra, Dr. Yoshihiro Miyake (University
of the Tokyo) for helpful discussion, and Prof. Mao Minoura
(Kitasato University) for valuable advice on X-ray crystal-
lographic analysis. This work was partially financial sup-
ported by a Daicel Chemical Industry Award in The Society
of Synthetic Organic Chemistry, Japan, and by Kitasato
University Research Grant for young researchers.
The ECD spectra of tetracation 14þ were also found to be
quite different from those of 12þ (Figure 4). A pair of very
intense Cotton Effects (λex = 704 and 600 nm) in the region
of the HOMOꢀLUMO transition (λmax = 652 nm) was
observed together with relatively weak bisignate signals at
high energy with maxima at 378 and 330 nm (Figure 4).
Since the intense couplet at low energy possessed a typical
Supporting Information Available. Detailed experimen-
tal procedures and characterization data for new com-
pounds, HPLC chromatograms, photoracemization
details for 1, DFT molecular coordinates of 1, and full
reference of 15. This material is available free of charge
via the Internet at http://pubs.acs.org.
(18) The manipulation for the measurements of ECD treated under
the dark. The chemical oxidation was carried out in amber glassware. As
for the racemization under light, further studies of the kinetics and
mechanism using an arc lamp are currently underway.
Org. Lett., Vol. 13, No. 17, 2011
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