Helically annelated and cross-conjugated oligothiophenes: Asymmetric synthesis, resolution, and characterization of a carbon-sulfur [7]helicene

Article abstract of DOI:10.1021/ja0462530

The synthesis and characterization of a novel oligothiophene, in which the thiophene rings are annelated into a [7]helicene with cross-conjugated π-system, are described. Such [7]helicenes may be viewed as fragments of the unprecedented carbon-sulfur (C

Full text of DOI:10.1021/ja0462530

Published on Web 11/02/2004
Helically Annelated and Cross-Conjugated Oligothiophenes:
Asymmetric Synthesis, Resolution, and Characterization of a
Carbon-Sulfur [7]Helicene
Andrzej Rajca,*,† Makoto Miyasaka,† Maren Pink,‡ Hua Wang,† and Suchada Rajca†
Contribution from the Department of Chemistry, UniVersity of Nebraska,
Lincoln, Nebraska 68588-0304, and IUMSC, Department of Chemistry,
Indiana UniVersity, Bloomington, Indiana 47405
Received June 24, 2004; E-mail: arajca1@unl.edu
Abstract: The synthesis and characterization of a novel oligothiophene, in which the thiophene rings are
annelated into a [7]helicene with cross-conjugated π-system, are described. Such [7]helicenes may be
viewed as fragments of the unprecedented carbon-sulfur (C2S)n helix, possessing sulfur-rich molecular
periphery. Racemic synthesis of [7]helicene is based upon iterative alternation of two steps: C-C bond
homocouplings between the â-positions of thiophenes and annelation between the R-positions of thiophenes.
Asymmetric synthesis is carried out using (-)-sparteine-mediated annelation of the axially chiral
bis(aryllithium) with electrophilic sulfur equivalent. Alternatively, enantiomers of the [7]helicene are obtained
via resolution using menthol-based chiral siloxanes. Racemic [7]helicene and four other macrocyclic products
of the annelation are characterized by X-ray crystallography. One of the solvent polymorphs of the [7]-
helicene possesses π-stacked columns of opposite enantiomers and multiple short intermolecular contacts,
including both homochiral and heterochiral short S‚‚‚S contacts, suggesting an effective intermolecular
electronic coupling in two-dimensions. The [7]helicene is configurationally stable at room temperature and
racemizes at 199 °C with a half-time of about 11 h. Selected physicochemical studies (UV-vis absorption,
CD, optical rotation, and cyclic voltammetry) of the [7]helicene are described.
Introduction
Oligothiophenes are among the most studied π-systems with
favorable electrical and optical properties for a wide range of
new technologies.1-8 Current efforts have been focused on
R-oligothiophenes, such as R-sexithiophene (Figure 1), in which
thiophene rings are connected at R-positions, forming a linearly
extended, conjugated π-system.1-10 However, analogous â-oli-
gothiophenes have received little attention,11-13 in part due to
† University of Nebraska.
‡ Indiana University.
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Figure 1. Oligothiophenes: R-sexithiophene and annelated carbon-sulfur
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J. AM. CHEM. SOC. 2004, 126, 15211-15222
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A R T I C L E S
Rajca et al.
delocalization. Notably, thiophene-based annelations of linear
â-oligothiophenes give helical carbon-sulfur (C2S)n oligomers
for large n, possessing moderate curvature characteristic of
helicenes (Figure 1).15-17 Such esthetically pleasing structures
are also associated with cross-conjugated π-systems for the
carbon-carbon frameworks.18-21 In contrast, analogous carbon-
sulfur oligomers derived from R-oligothiophenes are planar,
quasi-linear, with conjugated π-systems (thienoacenes).22-26
In these sulfur-rich annelated oligomers, all sulfur atoms are
positioned at the molecular periphery, facilitating multiple short
intermolecular S‚‚‚S contacts, which may increase the effective
dimensionality of the electronic structure, leading to enhanced
transport properties. The relevant examples are improved
mobility in organic field effect transistors,27,28 formation of
neutral organic metals,29 organic conductor-to-semiconductor
phase transformations,30 and increased critical temperatures in
organic superconductors.31 Furthermore, cross-conjugated π-sys-
tems have attracted recent attention because of their unusually
robust optical properties; however, the studies of their electronic
properties are in the early stages.18-21
extended [n]helicenes.17,32-34 In this aspect, solution-based
(rather than photochemical) synthetic methodologies for non-
racemic [n]helicenes and applications of [n]helicenes as organic
materials have become topics of current interest.35-44
Following the preliminary communication of racemic syn-
thesis of [7]helicene 1 (Figure 1),45 we now report the improved
racemic synthesis, asymmetric synthesis, and resolution of
carbon-sulfur [7]helicenes.14,46-48 Also, crystal structures,
determinations of barrier for racemization, and chirooptical and
cyclic voltammetric studies for 1 and related compounds are
described.
Results and Discussion
Racemic Synthesis of [7]Helicene 1. The synthesis of [7]-
helicene rac-1 consists of two iterations (Scheme 1). Each
iteration consists of two key steps, connection of thiophene
derivatives at their â-positions and annelation to form a new
thiophene ring.45
In the first iteration, 4,4′-dibromo-3,3′-bithienyl (2) is obtained
from 3,4-dibromothiophene, using optimized conditions for the
Li/Br exchange, followed by oxidation of the resultant 3-bromo-
4-thienyllithium with CuCl2.49 Selective 5,5′-dilithiation (R,R′-
An additional motivation to develop syntheses of such
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Synthesis of Cross-Conjugated Oligothiophenes
A R T I C L E S
Scheme 1. Synthesis of [7]Helicene rac-1a
a Conditions: (i) n-BuLi (1.05 equiv), ether/hexane, -78 °C for 2 h, then -50 °C for 15-30 min; (ii) CuCl2 (1.3-10 equiv), -78 °C to room temperature
for 2 d; (iii) LDA (2.3 equiv), ether/hexane, 0 °C for 2 h; (iv) TMSCl; (v) (PhSO2)2S (1.0 equiv), ether, -78 °C for 3 h, then room temperature for 12+ h;
(vi) TFA, CHCl3; (vii) LDA (2.3 equiv), ether/hexane or toluene/hexane, 0 °C for 2 h.
dilithiation) of 2 with LDA, followed by a TMS-quench,
provides 3. Dilithiation of the two TMS-unprotected 2,2′-R-
positions in 3 with LDA, followed by the reaction with bis-
(phenylsulfonyl) sulfide, gives annelated product 4.50 The
primary side product is polymer 8. In addition, small amounts
of macrocyclic side products are obtained, such as dimer 5,
chiral dimer 6, and trimer 7; the overall yield of the three
macrocycles (when isolated as a mixture) is 4-9%.51
In the second iteration, one of the TMS groups in 4 is
removed to give 9. The Li/Br exchange on dithienothiophene 9
is followed by oxidation of the intermediate aryllithium, to
provide bis(dithienothiophene) 11. Selectivity for the single Li/
Br exchange at the less hindered 4′-position of dithienothiophene
9 is further verified by the protonation and deuteration of the
intermediate aryllithium, giving 4-bromo-5-trimethylsilyl-
dithieno[2,3-b:3′,2′-d]thiophene (9-H) and its 4′-D isotopomer
(9-D), respectively (Supporting Information). In the final step,
bis(dithienothiophene) 11 is annelated to give [7]helicene rac-
1. Two side products, dimer 12 and polymer 13, are isolated
(Scheme 1).
Optimization of the reaction conditions for the two key steps
(connection and annelation) is carried out, as described in the
following paragraphs.
In the connection steps, 2 and 11 are obtained via the C-C
homocoupling reactions between the 3-thienyllithium deriva-
tives. Slight modification of the conditions for the Li/Br
exchange at -78 °C, that is, by allowing the reaction mixture
to attain -50 °C for 15-30 min, prior to the addition of CuCl2
at -78 °C, improves the isolated yields of 2 and 11 from 40%
to 68% and 30% to 60%, respectively.52-54
In the annelation steps, the efficient dilithiations of 3 and 11
are important for the yield of the target products 4 and 1,
(52) Typical conditions for the formation of 4-bromo-3-thienyllithium or
3-thienyllithium employ BuLi and the corresponding bromothiophene in
ether at -70 °C, to affect the Li/Br exchange at the â-position of the
thiophene. It has been reported that allowing 3-thienyllithiums to reach
room temperature for 5 min (or longer) leads either to the migration of
lithium to the more acidic R-position or to the opening of thiophene ring:
(a) Gronowitz, S. Thienyl-Lithium Derivatives and Their Ring-Opening
Reactions. In Organic Sulfur Chemistry; Stirling, C. J. M., Ed.; Butter-
worths: London, 1975; pp 203-228. (b) Gronovitz, S.; Frejd, T. Acta Chem.
Scand. 1970, 24, 2656-2658.
(53) Bis(dithienothiophene)11 is obtained in 40% yield from a relatively large-
scale reaction.
(54) In CC-homocoupling reactions, using Cu(II) halides, good stirring, which
is more difficult with large amounts of relatively heavy Cu(II) halide, was
found to be essential for good yields: Rajca, A.; Wang, H.; Bolshov, P.;
Rajca, S. Tetrahedron 2001, 57, 3725-3735.
(49) When 4-bromo-3-thienyllithium was prepared at -70 °C, via addition of
ethereal solution of 3,4-dibromothiophene to n-BuLi in ether, isolation of
2 in 52% yield was reported: (a) Gronovitz, S. Acta Chem. Scand. 1961,
15, 1393-1395. (b) Gronowitz, S.; Moses, P.; Hornfeldt, A.-B. Ark. Kemi
1961, 17, 237-247.
(50) The previously reported yield for 4 was 65% (ref 45, based upon three
reactions), and yields as high as 73% were obtained; however, after more
than 40 reactions carried out by several students, the isolated yields average
about 40%.
(51) For chiral dimer 6, HPLC with chiral column shows two partially resolved
peaks of equal intensity (Figure 3s, Supporting Information).
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Rajca et al.
Figure 2. 1H NMR (500 MHz, Et2O/hexane with 3% of benzene-d6) spectra for products of lithiation of 11 with LDA (∼1 equiv). Plots A, B, C, and D
show spectra for consecutive additions of (-)-sparteine, that is, samples with 0, 1.5, ∼10, and ∼80 equiv, respectively, as described in the Experimental
Section. In plot D, the left inset plot corresponds to the set of two singlets at δ ≈ 7.34 ppm for unreacted 11; the right inset plot shows the analogous set
of two singlets in the reference sample of 11 with ∼80 equiv of (-)-sparteine. Both inset plots have the spectral width of 0.013 ppm. Unknown impurities
are labeled with asterisks.
respectively.55 Reactions of 3 and 11 with LDA in Et2O/hexane
(with 3-10% of benzene-d6) are monitored with 1H NMR
spectroscopy with double solvent suppression, using conditions
similar to those in the preparative reactions.
For 11 treated with ∼1 equiv of LDA, 1H NMR spectra show
a relatively slow formation of R-monolithiated 11-Li1 and R,R′-
dilithiated 11-Li2 (Figure 2A).56 In particular, R-monolithiated
11-Li1 possesses one resonance (δ ≈ 7.079 ppm) in the aromatic
region and two nearly coincident resonances in the TMS-region
(δ ≈ 0.160 and 0.155 ppm). Addition of ∼2.3 equiv of LDA
provides a much simpler 1H NMR spectrum, in which the only
singlet at δ ≈ 0.094 ppm (with a shoulder) is assigned to the
methyl groups of the TMS group in R,R′-dilithiated 11-Li2
(Figure 3A). Typically, the dilithiation is completed after 2 h
at 0 °C.57
For 3 treated with <1 equiv of LDA, the 1H NMR spectrum
1
in the aromatic region shows two H resonances at 7.439 and
7.247 ppm, which are assigned to 3 and 2-monolithiated
intermediate 3-Li1 (Supporting Information), respectively. With
2.3 equiv of LDA, the expected 2,2′-dilithiated intermediate
3-Li2 (Supporting Information) is formed; that is, no 1H
resonances are detected in the aromatic region, and a new
resonance at 0.283 ppm appears in the TMS region.
(56) For example, after 30 min at 0 °C, and then 10 min at 300 K, conversion
of 11 is less than 50% complete; after an additional 70 min at 300 K, the
reaction progress is barely detectable by 1H NMR integration (<5%).
(55) The replacement of LDA with a more bulky lithium tetramethylpiperide
in the lithiation of 3 did not improve yields of 4.
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Synthesis of Cross-Conjugated Oligothiophenes
A R T I C L E S
Scheme 2. Asymmetric Synthesis of [7]Helicene (-)-1
In summary, the dilithiated intermediates, 3-Li2 and 11-Li2,
are formed nearly quantitatively and are moderately persistent.58
These results indicate that the dilithiations are not limiting
factors in the annelation steps. Also, the efficient formation of
the dilithiated intermediates is reflected in moderate molecular
weights for polymers 8 (Mw ≈ 4.4-13 kDa) and 13 (Mw ≈ 8
kDa), for which predominant connectivity is likely derived from
3-Li2 and 11-Li2, respectively.
In the first experiment, the mixture containing R-monolithi-
ated 11-Li1, R,R′-dilithiated 11-Li2, unreacted 11, and unreacted
LDA in Et2O/hexane/benzene-d6, which is obtained via the
reaction of 11 with ∼1 equiv of LDA (the sample shown in
Figure 2A), is further treated with (-)-sparteine (1.5, ∼10, and
∼80 equiv per LDA). The initial addition of (-)-sparteine (1.5
equiv) leads to the relatively fast increase in concentration of
1
11-Li1 with the corresponding decrease of H integration for
Asymmetric Synthesis of [7]Helicene (-)-1. (-)-Sparteine
has been shown to be an effective additive for induction of
enantiomeric excess (ee) in reactions involving organometallics,
and especially organolithiums.59,60 Because the final annelation
leading to [7]helicene 1 involves axially chiral R,R′-dilithiated
11-Li2, its complexation with (-)-sparteine, followed by the
reaction with bis(phenylsulfonyl)sulfide, may provide non-
racemic 1.
Treatment of bis(dithienothiophene) 11 with LDA (∼2.0 or
∼2.3 equiv) and (-)-sparteine (3.5 equiv) in ether/hexane at 0
°C is followed by the addition of bis(phenylsulfonyl)sulfide at
-78 °C; [7]helicene (-)-1 is isolated in 20-37% yields and
ee’s of 19-47% (Scheme 2).61
11 and 11-Li2 (Figure 2B). Moreover, resonances for 11-Li1 in
the aromatic region appear as two singlets at δ ≈ 7.142 and
7.105 ppm with the relative integrations of 1:0.8 (Figure 2B);
both singlets are downfield-shifted, as compared to the singlet
at δ ≈ 7.079 ppm for 11-Li1 in the absence of (-)-sparteine in
Figure 2A.
The subsequent additions of (-)-sparteine (to a total of ∼10
and ∼80 equiv) result in slight downfield shifts relative to the
benzene-d5 multiplet for all resonances, especially in the
aromatic region. However, the relative integration of 1:0.8 for
the two singlets for 11-Li1 is unchanged (Figure 2C and D). In
1
the TMS-region, the H resonances for 11-Li1 are interpreted
Nonracemic (-)-1 with high ee’s and enantiopure (-)-1 are
obtained by fractional recrystallizations. The absolute config-
uration of such nonracemic (-)-1 was determined as (M) (left-
handed helix) by vibrational circular dichroism study.46 This
implies that R,R′-dilithiated 11-Li2 with axially chiral (R)
configuration is primarily converted to (M)-(-)-1 (Scheme 2).
Such (-)-sparteine-mediated stereoinduction involving axial (R)-
configuration has another precedence in the Cu(II) oxidation
of 2,2′-dilithiobiaryls to derivatives of o-tetraphenylene.54
Notably, the higher yields of 1 are associated with the lower
ee’s and vice versa.
The effect of addition of (-)-sparteine on R-monolithiated
11-Li1 and R,R′-dilithiated 11-Li2, and the reaction of 11 with
LDA, is monitored using 1H NMR spectroscopy, as illustrated
by two experiments (Figures 2 and 3).
(57) Also, about 10 min at room temperature is required prior to acquisition of
the NMR spectrum.
(58) (a) For both 3-Li2 and 11-Li2 in Et2O/hexane/benzene-d6, no new resonances
are found in 1H NMR spectra over 4 h at ambient temperature. Dilithiated
intermediate 11-Li2 is studied over a longer period; after 16+ h at room
temperature, new 1H resonances are detectable. Upon quenching with water
and usual workup, 1H NMR spectra and FAB MS indicate 11 and unknown
side products, although not simple products of migration of TMS. Migration
of TMS is a common reaction for R-lithiated thiophenes: for example, ref
14. (b) Studies of polymers 8 and 13 and their derivatives will be reported
elsewhere.
(59) (a) Beak, P.; Basu, A.; Gallagher, D. J.; Park, Y. S.; Thayumanavan, S.
Acc. Chem. Res. 1996, 29, 552-560. (b) Weisenburger, G. A.; Faibish, N.
C.; Pippel, D. J.; Beak, P. J. Am. Chem. Soc. 1999, 121, 9522-9530.
(60) (a) Hoppe, D.; Hense, T. Angew. Chem., Int. Ed. Engl. 1997, 36, 2283-
2316. (b) Seppi, M.; Kalkofen, R.; Reupohl, J.; Fro¨hlich, R.; Hoppe, D.
Angew. Chem., Int. Ed. 2004, 43, 1423-1427.
(61) (a) The replacement of (-)-sparteine with [R-(R*,R*)]-(+)-bis(R-methyl-
benzyl)amine gives 1 with negligible ee (0-5% range by NMR). (b) The
replacement of ether with hexane, toluene, or tetrahydrofuran for the
reactions using (-)-sparteine leads to negligible ee < 5% for 1.
Figure 3. 1H NMR (500 MHz, Et2O/hexane with 3% of benzene-d6) spectra
for products of lithiation of 11 with LDA (∼2.3 equiv). Plots A, B, and C
show spectra for consecutive additions of (-)-sparteine, that is, samples
with 0, 3.5, and ∼10 equiv, respectively, as described in the Experimental
Section. The broad resonance at δ ≈ 0.27 ppm is assigned to excess of
LDA.
9
J. AM. CHEM. SOC. VOL. 126, NO. 46, 2004 15215

A R T I C L E S
Rajca et al.
Scheme 3. Resolution of [7]Helicene 14a
a Conditions: (i) TFA, CHCl3, room temperature overnight; (ii) LDA (2.3 equiv), ether/hexane/benzene-d6, 0 °C for 2 h, then briefly room temperature;
(iii) (-)-MenthSiCl (2.5 equiv) in ether at -25 °C, then 0 °C for 0.5 h, and then room temperature overnight.
as two sets of peaks with unequal integrations: a downfield
broad singlet (with a shoulder) and two upfield singlets (Figure
2B, C, and D). These results are compatible with the diaster-
eomeric ratio of about 1:0.8 for the observed complexes of 11-
Li1 with (-)-sparteine.62
Li2 are configurationally stable, complete conversion of 11 to
11-Li2 would annul any kinetic resolution during the reaction
of 11 with LDA in the presence of (-)-sparteine. However,
further studies would be needed to quantify contributions to ee
from the kinetic resolution versus the reaction of the diaster-
eomeric complexes of 11-Li2/(-)-sparteine with bis(phenylsul-
fonyl)sulfide.
Notably, the resonances for 11 in the aromatic region appear
as two unequal singlets at δ ≈ 7.343 and 7.341 ppm for the
sample with ∼80 equiv of (-)-sparteine in Figure 2D. Although
accurate integration of such closely spaced singlets is problem-
atic, their relative peak heights are clearly different, analogous
to those for 11-Li1. The reference sample of 11 with ∼80 equiv
of (-)-sparteine in the identical solvent mixture shows two
singlets at the similar chemical shifts, but their peak heights
are equal (right inset in Figure 2D). These results indicate that
11 is configurationally stable on the laboratory time scale (at
least 1 h at 300 K); furthermore, the observed diastereomeric
ratio for 11-Li1 may arise from the partial kinetic resolution,
primarily caused by the accelerated reaction of 11 with LDA
upon the initial addition of (-)-sparteine.
The configurational stability of bis(dithienothiophene) 11 may
be discussed in terms of its hindered biaryl structure. Because
dithienothiophene 4 is approximately planar, with the nearly
parallel C-Br bond axes,45 bis(dithienothiophene) 11 may be
viewed as an analogue of 1,1′-binaphthtyl; the Br-substitution
in 11, occupying the “backside” positions, would correspond
to the 8,8′-disubstitution in 1,1′-binaphthyl. Notably, the
configurational stabilities of 1,1′-binaphthyl-8,8′-dicarboxylic
acid and its derivatives are overall only marginally better than
that of 1,1′-binaphthyl.64 Therefore, the thoroughly studied half-
time for racemization of 1,1′-binaphthyl, which is about 10 h
at 25 °C, may provide a rough estimate for the time scale for
racemization of 11.64,65 Analogously to 2,2′-dilithio-1,1′-bi-
naphthyl, R,R′-dilithiated 11-Li2 should be configurationally
stable as well.66,67
In another experiment, the solutions of 11-Li2 in Et2O/hexane/
benzene-d6, which are obtained via the reaction of 11 with 2.3
equiv of LDA (the sample shown in Figure 3A), are further
treated with (-)-sparteine (3.5 and ∼10 equiv). Although the
greater degree of lithiation of 11 by the excess amount of LDA
leads to significant line broadening, the corresponding 1H NMR
spectra in Figure 3B and C clearly show formation of a new,
upfield-shifted resonance at δ ≈ 0.13 ppm (broad singlet with
shoulders), with the concomitant decrease of the δ ≈ 0.094 ppm
resonance. This is consistent with the formation of the new
species, which is probably a diastereomeric complex of axially
chiral 11-Li2 with (-)-sparteine.62,63
Resolution of [7]Helicene 14. Because the (-)-sparteine-
mediated synthesis gives only a relatively low ee of (-)-1,68
a
resolution-based approach to nonracemic carbon-sulfur [7]-
helicene is developed. Our approach adopts menthol-derivatized
chlorosiloxane, (-)-MenthSiCl (Scheme 3), which has been
recently reported as a chiral silylation reagent for the determi-
(62) For complexes of (-)-sparteine with organolithiums, characterized by X-ray
crystallography, see: (a) Papasergio, R. I.; Skelton, B. W.; Twiss, P.; White,
A. H.; Raston, C. L. J. Chem. Soc., Dalton Trans. 1990, 1161-1172. (b)
Hoppe, I.; Marsch, M.; Harms, K.; Boche, G.; Hoppe, D. Angew. Chem.,
Int. Ed. Engl. 1995, 34, 2158-2160.
The 1H NMR spectroscopic monitoring included the reaction
mixtures for which dilithiations of 11 are carried out with LDA
in the presence of (-)-sparteine (3.5 equiv). For several reaction
(63) The presence of (-)-sparteine (3.5 or 10 equiv) has no detectable effect
on the persistence of 11-Li2.
(64) Cooke, A. S.; Harris, M. M. J. Chem. Soc. B 1963, 2365-2373.
(65) Colter, A. K.; Clemens, L. M. J. Phys. Chem. 1964, 68, 651-654.
(66) 2,2′-Dilithio-1,1′-binaphthyl: Brown, K. J.; Berry, M. S.; Waterman, K.
C.; Lingenfelter, D.; Murdoch, J. R. J. Am. Chem. Soc. 1984, 106, 4717-
4723.
1
mixtures, such as those shown in Figure 3C, the H NMR
monitoring is followed by the addition of bis(phenylsulfonyl)-
sulfide (Experimental Section). 1H NMR analyses of the
corresponding crude mixtures suggest that the addition of bis-
(phenylsulfonyl)sulfide at low temperature (-78 °C) and
incomplete dilithiation of 11 (presence of 11-Li1) are associated
with higher ee’s. Indeed, if axially chiral 11, 11-Li1, and 11-
(67) Cu(II)-mediated oxidation of 2,2′-dilithio-1,1′-binaphthyl at -78 °C to room
temperature gives the corresponding dimer without detectable racemiza-
tion: Rajca, A.; Safronov, A.; Rajca, S.; Wongsriratanakul, J. J. Am. Chem.
Soc. 2000, 122, 3351-3357.
(68) The enantiomer of (-)-sparteine is not readily available: ref 60a; Smith,
B. T.; Wendt, J. A.; Aube´, J. Org. Lett. 2002, 4, 2577-2579.
9
15216 J. AM. CHEM. SOC. VOL. 126, NO. 46, 2004

Synthesis of Cross-Conjugated Oligothiophenes
A R T I C L E S
Chart 1.a
The structures of the macrocyclic dimers 5 and 12, and trimer
7 include one solvent molecule per formula unit (chloroform
for 5 and 7, and benzene for 12). Chiral dimer 6 crystallizes
without inclusion of solvent; however, the crystal is pseudo-
merohedrally twinned with a twin domain ratio of 70:30
(Supporting Information).
The dimers and the trimer possess two and three biaryl
moieties, respectively. These sterically hindered biaryls cor-
respond to bithienyl and bis(dithienothiophene) moieties in 5-7
and 12, respectively; each of such biaryls has a chiral axis. In
each of the approximately C2h-symmetric dimers 5 and 12, the
biaryl moieties have opposite configurations (axial R vs axial
S); for both dimers, the biaryls adopt cisoid conformations, with
the corresponding torsional angles of less than 90° (Table 2s,
Supporting Information). The dithienothiophene moieties in
dimer 12 are slightly more curved as compared to the
dithienothiophene 4; for example, the torsion angles along the
inner C-C-C-C backbone of the dithienothiophene moiety
(such as the C2-C3-C5-C7 torsion angle in dimer 12) are in
the 11.7-13.6° range in dimer 12, as compared to 7.1° in 4.
The structure of chiral dimer 6 shows that the molecule has the
approximate D2 point group; one of the three approximate C2-
axes relates the homochiral bithienyl moieties with transoid
conformations (Figure 4 and Table 2s in the Supporting
Information). In trimer 7, two of the bithienyl moieties possess
the same relative configuration; these two homochiral moieties
adopt cisoid conformations, while the third moiety has a transoid
conformation. Thus, the molecule has chiral conformation,
somewhat distorted from the C2 point group. The other possible
diastereomers of 7 and 12, in which all biaryl moieties are
axially homochiral (analogous to 6), have not been isolated.
For all four macrocycles, conformational mobility in solution
is expected to be highly restricted, especially due to the highly
substituted biaryl moieties. This is consistent with the lack of
broadened resonances in the 1H and 13C NMR spectra for 5-7
and 12; furthermore, the molecular symmetry in solution on
the NMR time scale corresponds to the approximate molecular
symmetry found by crystallography. Thus, the NMR spectra at
room temperature show only one type of thiophene moiety for
the C2h-symmetric and D2-symmetric dimers, and three non-
equivalent thiophene moieties for the C2-symmetric trimer.
Crystal Packing. Because [7]helicene 1 and the macrocyclic
side products may be viewed as molecules with sulfur-rich
peripheries (e.g., Figure 1), a plethora of short intermolecular
S‚‚‚S (and of S to other atoms) contacts are expected, especially
in polymorphs of [7]helicene 1. Both the feasibility of π-stacking
and the stereochemistry in 1 are of interest in context of the
curvature of the helical π-system.72,73 (Individual planes of
thiophenes in 1, which are nearly planar,45,74 will be used as
reference planes.) Both short intermolecular contacts (especially,
S‚‚‚S) and π-stacking are known to facilitate high carrier
mobilities in organic semiconductors.27,28,75,76
a (-)-MenthSi is defined in Scheme 3.
nation of the absolute configuration of alcohols by NMR
spectroscopy.69
Starting with rac-1, both TMS groups are removed using TFA
in chloroform, to give rac-14; the isolated yields are not nearly
quantitative due to the poor solubility of rac-14.70 LDA-mediated
1
dilithiation of rac-14 at both R-positions is monitored by H
NMR spectroscopy (with double solvent suppression), until no
1H resonance can be detected in the aromatic region. The
resulting suspension of dilithiated rac-14 is quenched with chiral
chlorosiloxane (-)-MenthSiCl. The diastereomeric mixture of
disiloxanes (-)-15 (Rf ) 0.31) and (+)-15 (Rf ) 0.27) is
separated by column or preparative thin-layer chromatography
on silica. The removal of the siloxanes using TFA in chloroform
gives [7]helicenes (-)-14 and (+)-14 (Scheme 3).
The presence of two chiral auxiliaries, such as in (-)-15 and
(+)-15, is essential for good chromatographic separation of
diastereomers. The corresponding side products (-)-17 and (+)-
17 (Chart 1), in which only one R-position is functionalized
with the chiral auxiliary, have coinciding Rf-values by TLC.
Probably, this precludes the effective resolution based upon the
somewhat more soluble [7]helicene rac-16 (Chart 1), which may
be readily obtained from rac-1 via selective removal of a single
TMS group.
Crystal Structures. Structures for [7]helicene rac-1 and
selected macrocyclic products of the annelation steps are
determined using X-ray crystallography (Table 1, Figure 4, and
Figure 5).
For the [7]helicenes, only the racemic crystals, including two
solvent polymorphs for rac-1, are studied; although nonracemic
[7]helicenes 1 and 14 are crystalline, attempts to obtain data of
adequate quality, including an attempt to measure microcrystals
with synchrotron radiation for 1, were not successful.
In both solvent polymorphs of rac-1, [7]helicene possesses
an approximate C2 point group. Also, its overall helix structure
is nearly identical in both solvent polymorphs; the helix of the
chloroform-containing polymorph is similar to that of phenylene-
based [6]helicene, as discussed in detail in the preliminary
communication.45,71 However, the local structures for both
solvent polymorphs of [7]helicene, as characterized by selected
intramolecular distances and angles, are somewhat different; for
example, the intramolecular Br‚‚‚Br distances are 3.90 and 3.72
Å for the chloroform-containing and solvent-free polymorphs,
respectively.
(72) For substituent effects on chirooptical properties and analyses of solid-
state structures of [6]helicenes, see: Wachsmann, C.; Weber, E.; Czugler,
M.; Seichter, W. Eur. J. Org. Chem. 2003, 2863-2876.
(73) Curtis, M. D.; Cao, J.; Kampf, J. W. J. Am. Chem. Soc. 2004, 126, 4318-
(69) Chiral silylation reagents for the determination of absolute configuration
by NMR spectroscopy: Weibel, D. B.; Walker, T. R.; Schroeder, F. C.;
Meinwald, J. Org. Lett. 2000, 2, 2381-2383.
(70) Nonracemic 14 is much more soluble in typical organic solvents as
compared to rac-14.
(71) Fits of the (C2S)7C2 thiophene-only and the (C2S)7C2Br2Si2 frameworks
have weighed rms deviations of 0.145 and 0.192 Å for the atoms,
respectively.
4328.
(74) For individual thiophene rings in rac-1 (CHCl3), mean deviations of the
least-squares planes between 0.01 and 0.04 Å are calculated (ref 45).
Selected mean deviations in solvent-free polymorph rac-1 are as follows:
0.02-0.03 Å (thiophenes containing S2, S3, and S6) and 0.08 Å (thiophene
containing S5).
(75) Pappenfus, T. M.; Chesterfield, R. J.; Frisbie, C. D.; Mann, K. R.; Casado,
J.; Raff, J. D.; Miller, L. L. J. Am. Chem. Soc. 2002, 124, 4184-4185.
9
J. AM. CHEM. SOC. VOL. 126, NO. 46, 2004 15217

A R T I C L E S
Rajca et al.
Table 1. Summary of the X-ray Crystallographic Data
property
rac-1 (CHCl3)a
rac-1
5
6
7
12
molecular
formula
C23H19Br2Cl3S7Si2
C22H18Br2S7Si2
C29H37Br4Cl3S6Si4
C28H36Br4S6Si4
C43H55Br6Cl3S9Si6
C50H42Br4S14Si4
formula weight
temperature, K
wavelength, Å
crystal/color
crystal system
space group
a, Å
842.15
173(2)
0.71073
colorless
trigonal
R-3
36.319(2)
36.319(2)
12.8012(8)
90
90
120
14 623.8(16)
18
1.721
722.78
130(2)
0.71073
colorless
orthorhombic
Pbca
12.471(3)
13.846(3)
31.152(7)
90
90
90
5379(2)
8
1.785
1116.30
173(2)
0.71073
colorless
orthorhombic
Pbcn
19.699(2)
13.0798(15)
17.734(2)
90
90
90
4569.3(9)
4
1.623
996.93
112(2)
0.71073
colorless
orthorhombic
Pbca
18.6063(6)
23.7325(8)
18.6074(6)
90
90
90
8216.5(5)
8
1.612
1614.76
109(2)
0.71073
colorless
monoclinic
P21/n
9.7758(2)
26.2874(6)
26.0709(6)
90
100.202(1)
90
6593.8(3)
4
1.627
1523.68
120(2)
0.71073
colorless
triclinic
P-1
8.1993(3)
10.5684(4)
18.3955(7)
91.3400(10)
92.7710(10)
110.6170(10)
1488.81(10)
1
b, Å
c, Å
R, deg
â, deg
γ, deg
volume, Å3
Z
Dcalcd/g cm-3
reflns collected
unique reflns
R1 (I > 2σ(I))
wR2 (I > 2σ(I))
GOF
1.699
50 542
7463
0.0370
0.0857
0.965
61 201
6240
0.0678
0.1379
1.310
26 840
4052
0.0488
0.1113
1.020
203 965
13 053
0.0303
0.0676
1.031
71 707
19 254
0.0326
0.0663
1.010
23 666
8712
0.0295
0.0698
1.029
a Polymorph rac-1 (CHCl3), ref 45.
Figure 4. Structures of 1 and macrocyclic dimers 5 and 6. The top and side views of the ORTEP plots at 50% probability levels are shown for each
molecule. The molecule of chloroform, included in the crystal of 5, is omitted for clarity.
The chloroform-containing polymorph, that is, rac-1 (CHCl3),
crystallizes in the R-3 space group with 18 molecules per unit
cell. The crystal packing may be described in terms of the C3-
symmetric “propellers”, consisting of three homochiral [7]-
helicenes (Figure 6). The propellers form columns along the
crystallographic c-axis; the nearest-neighbor propellers, which
contain [7]helicenes of opposite handedness, are staggered via
an improper rotation (S6-axis). The [7]helicenes within the
column of propellers are canted with respect to each other; that
is, π-stacking is absent. However, close proximity between the
sufur-rich edges and the π-systems of [7]helicenes causes a
plethora of short intermolecular contacts involving sulfur atoms,
both homochiral (e.g., S3‚‚‚S3 ) 3.520 Å) and heterochiral (e.g.,
S3‚‚‚S3 ) 3.330 Å, S3‚‚‚S4 ) 3.492 Å, S1‚‚‚S2 ) 3.564 Å,
S1‚‚‚C2 ) 3.451 Å, S1‚‚‚C3 ) 3.486 Å, S4‚‚‚C6 ) 3.472 Å).
For each pair of the nearest-neighbor heterochiral [7]helicenes,
belonging to different columns, the terminal thiophene rings
are π-stacked. However, the degree of π-overlap is rather small,
involving only sulfur atoms with short contact, S7‚‚‚S7 ) 3.726
(76) High-performance field-effect transistors based upon single crystals of
rubrene: Sundar, V. C.; Zaumseil, J.; Podzorov, V.; Menard, E.; Willett,
R. L.; Someya, T.; Gershenson, M. E.; Rogers, J. A. Science 2004, 303,
1644-1646.
9
15218 J. AM. CHEM. SOC. VOL. 126, NO. 46, 2004

Synthesis of Cross-Conjugated Oligothiophenes
A R T I C L E S
Figure 5. Structures of macrocyclic trimer 7 and dimer 12. The top and side views of the ORTEP plots at 50% probability levels are shown for each
molecule. The molecules of chloroform and benzene, included in the crystals of 7 and 12, respectively, are omitted for clarity.
Figure 6. Unit cell showing molecular packing of rac-1 (CHCl3) as viewed
down the crystallographic c-axis (top plot). The disordered molecules of
chloroform are omitted for clarity. Handedness of the highlighted [7]-
helicenes is color-coded; the [7]helicenes belonging to two distinct columns
of propellers are shown in wire frame and ball-and-stick.
Å. (Also, two S‚‚‚Br short contacts of about 3.73 Å are present.)
Disordered molecules of chloroform fill the voids between the
columns.
Figure 7. Unit cells showing molecular packing of solvent-free rac-1 as
viewed down the crystallographic a-axis (top cell) and as viewed from a
selected direction (bottom cell). The enantiomers in two π-stacks (out of
four for each cell) are color-coded in blue and magenta. Each of the two
π-stacks is shown either in stick or in stick-and-ball. Intercepted blue lines
correspond to short intermolecular contacts.
The solvent-free polymorph, rac-1, crystallizes in the space
group Pbca with 8 molecules per unit cell. The molecules pack
in “π-stacked” columns extending along the crystallographic
a-axis (Figure 7).
Along the crystallographic c-axis, the TMS-groups from the
neighboring columns are interlocked, forming a lamellar-like
structure. Along the crystallographic b-axis, multiple short
contacts are found between the columns, both between the same
enantiomers (e.g., S2‚‚‚S5 ) 3.551 Å, S3‚‚‚S6 ) 3.562 Å,
S3‚‚‚C12 ) 3.459 Å) and opposite enantiomers (S6‚‚‚C8 )
3.436 Å and S7‚‚‚C6 ) 3.436 Å). These contacts arise from
the approximately perpendicular (herringbone-like) relative
orientations of the molecules in the adjacent columns; that is,
9
J. AM. CHEM. SOC. VOL. 126, NO. 46, 2004 15219

A R T I C L E S
Rajca et al.
Figure 9. Electronic absorption UV-vis spectra for [7]helicene rac-1,
dithienothiophene 4, and macrocycles 5-7 in dichloromethane at room
temperature.
Figure 8. Selected π-stacks of solvent-free rac-1 spanning 1.5 unit cells
along the a-axis as viewed normal and parallel to the mean planes of the
highlighted thiophene rings (shown in dark magenta color): plots A (S2-
C3-C4-C5-C6) and plots B (S5-C9-C10-C11-C12). Short contacts,
S5‚‚‚C3 ) 3.442 Å and S5‚‚‚C4 ) 3.413 Å, are indicated with red
intercepted lines.
rings. The shortest atom-to-plane distances for the atoms
projecting inside the S3-, S4-, and S6-thiophene rings are as
follows: S3-thiophene (3.794 Å for C12), S4-thiophene (3.872
Å for C14), and S6-thiophene (3.752 Å for C5). Although these
distances are relatively long, three atoms projecting near the
S4-thiophene ring have rather short atom-to-plane distances:
C15 (3.431 Å) and C16 (3.315 Å), and S2 (3.249 Å).
The π-stacking within the columns and multiple short contacts
between the columns (especially S‚‚‚S) may result in an effective
electronic coupling in two dimensions in solvent-free polymorph
rac-1.
Molecular packing of the sulfur-rich macrocycles 5, 6, 7, and
12 does not lead to any short S‚‚‚S contacts. Only dimers 6
and 12 possess intermolecular short contacts involving atoms
other than hydrogen; that is, 4 short Br‚‚‚Br contacts for 6 and
12 are in the 3.438-3.497 Å range and 3.576 Å, respectively.
Barrier for Racemization of 1. At 199 °C, enantiomerially
enriched [7]helicene 1 racemizes in the solid state with a half-
life of 11 ( 1 h. This corresponds to a free energy barrier for
racemization of 39.0 ( 0.2 kcal/mol at 199 °C (Supporting
Information, including Figure 1s). Analogously, the half-life is
about 5 h at 203 °C. The propensity of 1 for racemization is
similar to that of all-phenylene [6]helicene.77 This is consistent
with the similar helical structures for 1 and all-phenylene [6]-
helicene.45,78
the sulfur-containing edges of [7]helicenes interact with the
π-systems of the neighboring [7]helicenes.
In each column, the nearest “π-stacked” neighbors have the
opposite handedness. Even in this herringbone-like structure,
some π-overlap is retained within the column, as illustrated by
the selected views of the three “π-stacked” neighbors (Figure
8). Because of the helical distortions of the π-system of 1, the
point-to-plane distances between selected atoms and the mean
planes of the nearly planar thiophene rings are primarily
employed to estimate the degree of π-stacking between the
neighboring molecules in the column.74
The most significant π-overlap is associated with the intra-
stack short contacts, such as S5‚‚‚C3 ) 3.442 Å and S5‚‚‚C4
) 3.413 Å (Figure 8). In the projection on the mean plane of
the S2-C3-C4-C5-C6 thiophene (S2-thiophene), S5 is
projected inside of the S2-thiophene ring with the atom-to-plane
distance of 3.342 Å (Figure 8A). Two other atoms, C10 and
C12, which are projected very near the S2-thiophene ring, have
atom-to-plane distances of 3.744 and 3.724 Å, respectively
(Figure 8A). In the projection on the mean plane of the S5-
C9-C10-C11-C12 thiophene (S5-thiophene), C3 and C4 are
projected somewhat outside of the S5-thiophene ring but with
very short atom-to-plane distances of 3.110 and 3.190 Å,
respectively (Figure 8B). The only atom (C6) projecting inside
the S5-thiophene ring has an atom-to-plane distance of 3.759
Å (Figure 8B). Because the interplanar angle between the S2-
and S5-thiophene rings of the stacked molecules is <90°, the
corresponding π-orbitals are pointing toward each other. In
conjunction with the relatively short atom-to-plane distances,
this leads to a pronounced π-overlap between the parts of the
S2- and S5-thiophenes for each pair of neighboring molecules
within the column.
UV-Vis Spectroscopy, CD Spectroscopy, and Cyclic
Voltammetry. Electronic absorption UV-vis spectra for se-
lected compounds in dichloromethane are shown in Figure 9.
The spectrum for rac-1 with λmax ) 266 nm (with shoulder
at 274 nm) is significantly blue-shifted, as compared to λmax
)
390 nm for R-quaterthiophene, considering that both π-systems
have an equal number of C-C double bonds.25,79 However, the
spectra of [7]helicene rac-1 and dithienothiophene 4 are
significantly different, with respect to both a much greater
(77) Racemization of [6]helicene: (a) Martin, R. H.; Marchant, M. J. Tetrahedron
1974, 30, 347-349. (b) Janke, R. H.; Haufe, G.; Wu¨rthwein, E.-U.; Borkent,
J. H. J. Am. Chem. Soc. 1996, 118, 6031-6035.
(78) Helical structure of [6]helicene: Navaza, I.; Tsoucaris, G.; Le Bas, G.;
Navaza, A.; de Rango, C. Bull. Soc. Chim. Belg. 1979, 88, 863-870.
Additional π-overlap within the column may be estimated
using projections on the mean planes of the other thiophene
9
15220 J. AM. CHEM. SOC. VOL. 126, NO. 46, 2004

Synthesis of Cross-Conjugated Oligothiophenes
A R T I C L E S
Figure 11. Main plots: cyclic voltammetry for dithienothiophene 4 (top)
and [7]helicene 1 (bottom) in dichloromethane (tetrabutylammonium
hexafluorophosphate as supporting electrolyte) at room temperature, using
200 mV/s scan rates; potentials are given vs SCE using ferrocene (0.46 V)
as the reference.83 Inset plots: HOMOs for dithienothiophene 10 and [7]-
helicene 14 at the semiempirical AM1 level of theory.
Table 2. Summary of Chirooptical Data for [7]Helicenes (-)-1,
(-)-14, and (-)-15 in Cyclohexane
λ
max/nm
λmax/nm
∆ꢀmax/L mol-1 cm-
1
1
rt
compound
(
ꢀmax/L mol-1 cm-
)
(
)
[
R]D
[Φ]
(-)-1
(-)-14
(-)-15
256 (4.7 × 104)
246 (2.8 × 104)
260 (4.2 × 104)
285 (-117)
283 (-61)
290 (-110)
-1019
-1260
-730
-7370
-7280
-9140
bathochromic shifts, as compared to those for nearly planar
dithienothiophene 4. Chiral dimer 6 also possesses the most
unique conformation, including the smallest torsional angles
along the C-S-C-C linkages (40-48°) and the bithienyl
moieties with transoid conformations (Table 2s, Supporting
Information). However, the torsional angles associated with the
bithienyl moieties in 5-7 are too close to 90°, with the smallest
torsions of 62-73° for two of the bithienyl moieties in 7, to
have a significant contribution to the bathochromic shifts (Table
2s, Supporting Information). This suggests that the bis(R-
thienyl)sulfur chromophore, involving conjugation through
sulfur atom, affects absorption at the longest wavelength,
especially in 6 with the relatively small torsions along the C-S-
C-C linkages.81
UV-vis spectra for [7]helicenes (-)-14, (-)-1, and (-)-15
in cyclohexane show an intense, long-wavelength, structured
band with λmax ) 246, 256, and 260 nm, respectively (Figure
10, Table 2). The corresponding CD spectra possess both
spectral envelopes and absorbances analogous to typical con-
jugated helicenes.17 Based upon the assignment of absolute
configuration of (-)-1 as (M),46 its negative sign of the longest
Figure 10. Top plot: UV-vis electronic absorption spectra for [7]helicenes
(-)-1, (-)-14, and (-)-15 in cyclohexane. Bottom plot: CD spectra for
(-)-1, (-)-14, (+)-14, (+)-15, and (-)-15 in cyclohexane at room
temperature.
integrated absorbance and the bathochromic shift in rac-1, as
compared to those in 4.45 These spectral differences for formally
cross-conjugated homologues may also be affected by helical
distortion in rac-1. In addition, the involvement of sulfur atoms
in conjugation is feasible;80 for example, the HOMOs for both
dithienothiophene 10 and [7]helicene 14 have significant atomic
coefficients at the sulfur atoms (Figure 11). Macrocycles 5-7
may provide further insight on the role of sulfur atoms in
conjugation.
Interestingly, the long-wavelength band edges for nonplanar
macrocycles 5-7, in particular for the chiral dimer 6, show
(79) Qualitative UV-vis spectra for cyclic R,â-quaterthiophene (tetra[2,3-
thienylene]) show λmax ≈ 270 nm with a relatively weak and broad shoulder
extending to about 400 nm: Marsella, M. J.; Reid, R. J.; Estassi, S.; Wang,
L.-S. J. Am. Chem. Soc. 2002, 124, 12507-12510.
(80) Oligo(thio-2,5-thienylenes): Nakayama, J.; Katano, N.; Shimura, Y.;
Sugihara, Y.; Ishii, A. J. Org. Chem. 1996, 61, 7608-7610. Katano, N.;
Sugihara, Y.; Ishii, A.; Nakayama, J. Bull. Chem. Soc. Jpn. 1998, 71, 2695-
2700. Crystallographic data for one of the structures in the last reference
were corrected, see: March, R. E.; Kapon, M.; Hu, S.; Herbstein, F. H.
Acta Crystallogr., Sect. B 2002, 58, 62-77.
(81) Colonna, F. P.; Distefano, G.; Reichenbach, G.; Santini, S. Z. Natursforsch.,
A: Phys. Sci. 1975, 30, 1213-1214.
9
J. AM. CHEM. SOC. VOL. 126, NO. 46, 2004 15221

A R T I C L E S
Rajca et al.
wavelength CD band is consistent with all other known
helicenes; that is, the possible cross-conjugation does not
significantly impact the CD spectra of (-)-1 and its derivatives.
As illustrated in Figure 10 and Table 2, silane-, and especially
siloxane-, substituted [7]helicenes (-)-1 and (-)-15 have
relatively enhanced chirooptical properties, as compared to [7]-
helicene (-)-14.72,82 The CD spectra for [7]helicene (-)-1 and
(-)-14 possess similar spectral patterns, although the spectrum
for (-)-1 is much more intense and shows bathochromic shifts
throughout the recorded wavelength range. The chiral (-)-
menthol-based siloxane substituent in (-)-15 induces a some-
what greater bathochromic shift in the longest wavelength CD
band and significantly perturbs spectral shapes of the short
wavelength CD bands, as compared to the spectra of (-)-1 and
(-)-14. The CD spectra for diastereomers (-)-15 and (+)-15
are indistinguishable at λ > 270 nm, while they are significantly
different at the shorter wavelengths, as expected for a perturba-
tion of the [7]helicene chromophore by the relatively weak and
short-wavelength absorbing (-)-menthol-based chromophore.
Finally, the CD spectra for (-)-14 and (+)-14 are mirror images,
as expected for enantiomers.
Cyclic voltammetry of [7]helicene 1 gives two reversible
waves at E1° ) +1.34 and E2° ) +1.82 V, most likely
corresponding to oxidation to the radical cation and dication,
respectively. For 4, only an irreversible redox process with the
anodic peak potential at about +1.8 V is found (Figure 11).
Interestingly, chiral dimer 6 shows a reversible wave at E1° )
+1.59; also, a second, nearly reversible wave at E2° ) +1.89
V is observed (near the electrolyte solution limit), even at a
relatively slow scan rate of 50 mV/s.
those for alkyl-substituted R-sexithiophenes (E1° ) +0.80 and
E2° ) +1.00 V) or polythiophene (E1° ≈ +0.8 V).85
The highest occupied molecular orbitals (HOMOs) for 10 and
14 possess significant atomic contributions from sulfur atoms
(Figure 11). Interestingly, the HOMO for [7]helicene 14, with
very low atomic contributions from six carbon and two sulfur
atoms, is largely nonbonding.
Conclusion
The syntheses of racemic and enantiopure carbon-sulfur [7]-
helicenes, fragments of the (C2S)n helix of annelated thiophenes,
provide a novel class of oligothiophenes. Although [7]helicene
1 and its lower planar homologue are both formally cross-
conjugated, 1 shows increased electron delocalization as evi-
denced by both a significant bathochromic shift in the UV-vis
absorption spectra and its reversible oxidation at relatively low
potentials; both conjugation through sulfur atoms and the helical
distortion may play a role. However, the degree of electron
delocalization in 1 is significantly less as compared to analogous
R-oligothiophenes. Chirooptical properties of carbon-sulfur [7]-
helicenes are similar to those of conjugated helicenes. In the
solid state, π-stacking and multiple intermolecular short S‚‚‚S
contacts for the solvent-free polymorph of [7]helicene 1 suggest
an effective intermolecular electronic coupling in two-dimen-
sions.
Acknowledgment. This research was supported by the
National Science Foundation (CHE-0107241). MS analyses were
carried out at the Nebraska Center for Mass Spectrometry. We
thank Dr. Jun Li and Kristin Fulton for assistance with cyclic
voltammetry and synthesis. We thank Dr. Victor G. Young, Jr.
of the X-ray Crystallographic Laboratory at the Department of
Chemistry, University of Minnesota, for X-ray crystallographic
determination of dimer 5.
The oxidation potentials for [7]helicene 1 may be compared
to those obtained under similar conditions for typical R-oligo-
thiophenes.83 The potentials for 1 are similar to E1° ) +1.35
and E2° ) +1.75 V reported for a derivative of R-terthiophene,
that is, 3′,4′-dibutyl-2,5′′-diphenyl-2,2′:5′,2′′-terthiophene.84 How-
ever, these oxidation potentials are significantly higher than
Supporting Information Available: Experimental section,
including X-ray crystallographic files in CIF format. This
material is available free of charge via the Internet at
http://pubs.acs.org.
(82) An analogous effect of silicon substitution on chirooptical properties is
observed in [7]helicenes, which may be formally derived via the replace-
ment of the center thiophene ring in 1 with a benzene ring. Although for
such helicenes, trimethylsilyl-substitution may be associated with flatter
helix, with lower helical climb, as determined by X-ray crystallography,
the effect of crystal packing on such helical structures is uncertain: ref 47.
For the most studied all-phenylene [6]helicenes, the relationship between
the crystal structure (substituent-induced helical distortion) and optical
rotation could not be established: ref 72.
JA0462530
(84) Graf, D. D.; Duan, R. G.; Campbell, J. P.; Miller, L. L.; Mann, R. R. J.
Am. Chem. Soc. 1997, 119, 5888-5899.
(85) Ba¨uerle, P.; Segelbacher, U.; Gaudl, K.-U.; Huttenlocher, D.; Mehring, M.
Angew. Chem., Int. Ed. Engl. 1993, 32, 76-78.
(83) For dichloromethane with TBAPF6 as supporting electrolyte, potentials are
referenced to ferrocene couple at +0.46 V: Connelly, N. G.; Geiger, W.
E. Chem. ReV. 1996, 96, 877-910.
9
15222 J. AM. CHEM. SOC. VOL. 126, NO. 46, 2004