Synthesis and properties of an aggregating heterocyclic helicene

Article abstract of DOI:10.1021/ja011706b

Heterohelicene 10 is synthesized in six steps from 3,3′-bithienyl. Because the number of steps is small, because the yield is 95% in the last (the reaction of a bis-enol ether with 1,4-benzoquinone - a six-step one pot procedure that constructs the helicene skeleton), and because chromatography is not required to purify any of the products in the synthesis, significant amounts are easily prepared. To convert 10 into enantiopure 3, a helicenebisquinone surrounded by four dodecyloxy groups, requires only a precedented three-step sequence. Enantiopure helicene 3, either without solvent or in dodecane (but not in chloroform) aggregates into columnar structures whose optical properties differ markedly from those of the monomer but resemble those shown previously only by aggregates of 1. Evidence of aggregation in the pure material includes optical microscopic observation of long fibrous structures and X-ray diffraction and combined transmission electron microscopic and electron diffraction analyses showing the molecules within the fibers to be organized in columnar arrays. The circular dichroism spectra, specific rotations, and fluorescent emission spectra of the aggregated structures are all distinctive, and, as reported elsewhere, the second harmonic response is very large. The linear polarizations of the monomers' and aggregates' fluorescent emissions differ greatly. The circular polarization of the aggregates' fluorescent emission, after excitation by unpolarized light, is large.

Full text of DOI:10.1021/ja011706b

J. Am. Chem. Soc. 2001, 123, 11899-11907
11899
Synthesis and Properties of an Aggregating Heterocyclic Helicene
Karen E. S. Phillips,† Thomas J. Katz,*,† Steffen Jockusch,† Andrew J. Lovinger,‡ and
Nicholas J. Turro†
Contribution from the Department of Chemistry, Columbia UniVersity, New York, New York 10027, and
Bell Laboratories, Lucent Technologies, Murray Hill, New Jersey 07974
ReceiVed July 12, 2001
Abstract: Heterohelicene 10 is synthesized in six steps from 3,3′-bithienyl. Because the number of steps is
small, because the yield is 95% in the last (the reaction of a bis-enol ether with 1,4-benzoquinonesa six-step
one pot procedure that constructs the helicene skeleton), and because chromatography is not required to purify
any of the products in the synthesis, significant amounts are easily prepared. To convert 10 into enantiopure
3, a helicenebisquinone surrounded by four dodecyloxy groups, requires only a precedented three-step sequence.
Enantiopure helicene 3, either without solvent or in dodecane (but not in chloroform) aggregates into columnar
structures whose optical properties differ markedly from those of the monomer but resemble those shown
previously only by aggregates of 1. Evidence of aggregation in the pure material includes optical microscopic
observation of long fibrous structures and X-ray diffraction and combined transmission electron microscopic
and electron diffraction analyses showing the molecules within the fibers to be organized in columnar arrays.
The circular dichroism spectra, specific rotations, and fluorescent emission spectra of the aggregated structures
are all distinctive, and, as reported elsewhere, the second harmonic response is very large. The linear polarizations
of the monomers’ and aggregates’ fluorescent emissions differ greatly. The circular polarization of the aggregates’
fluorescent emission, after excitation by unpolarized light, is large.
Introduction
chirality is inverted, can be quasi-phase matched by using
interdigitated layers of the enantiomers to generate the second
harmonics.8
Helicene 1 is among few helical conjugated molecules that
self-assemble into long corkscrew-shaped columns (structure
2).1 Others are an ester of the hydroquinone formed by reducing
one of the quinone rings in 1,2 a phthalocyanine that is fused to
four helicene structures,3 a chiral m-phenylene ethynylene
oligomer,4 and a pyridine-pyridazine oligomer.5 Enantiopure
1, when dissolved in appropriate hydrocarbon solvents or when
cooled from the melt, assembles into columnar structures.1 In
the pure material, these columns are hexagonally packed into
very long micrometer-wide lamellar fibers that are easily seen
under an optical microscope.6 The columnar assemblies also
appear to be the source of unusual optical properties. For
example, enantiopure 1 when photoirradiated generates signifi-
cant second harmonics,7 which, because the major components
of the second-order susceptibility tensor change sign when the
We consider here whether there are helicenes, specifically
derivatives of structure 3, that are easier to prepare than 1, but
that exhibit properties just as uncommon. The reasons for
considering structure 3 are the following: replacing benzenes
by thiophenes has beneficial effects on the nonlinear optical
properties of previously known achiral conjugated structures,9
the Friedel-Crafts procedure should transform the heteroaro-
matic nucleus of the benzo[1,2-b:4,3-b′]-dithiophene into its
diacetyl derivative 4,10,11 and the wealth of electrons in the
conjugated systems of double bonds, oxygen, and sulfur atoms
† Columbia University.
‡ Lucent Technologies.
(1) Nuckolls, C.; Katz, T. J.; Katz, G.; Collings, P. J.; Castellanos, L. J.
Am. Chem. Soc. 1999, 121, 79, and references therein.
(2) Nuckolls, C.; Katz, T. J. J. Am. Chem. Soc. 1998, 120, 9541.
(3) Fox, J. M.; Katz, T. J.; Van Elshocht, S.; Verbiest, T.; Kauranen,
M.; Persoons, A.; Thongpanchang, T.; Krauss, T.; Brus, L. J. Am. Chem.
Soc. 1999, 121, 3453.
(4) Brunsveld, L.; Prince, R. B.; Meijer, E. W.; Moore, J. S. Org. Lett.
2000, 2, 1525.
(5) Cuccia, L. A.; Lehn, J.-M.; Homo, J.-C.; Schmutz, M. Angew. Chem.,
Int. Ed. Engl. 2000, 39, 233.
(6) (a) Lovinger, A. J.; Nuckolls, C.; Katz, T. J. J. Am. Chem. Soc. 1998,
120, 264. (b) Nuckolls, C.; Katz, T. J.; Castellanos, L. J. Am. Chem. Soc.
1996, 118, 3767.
(7) (a) Verbiest, T.; Van Elshocht, S.; Kauranen, M.; Hellemans, L.;
Snauwaert, J.; Nuckolls, C.; Katz, T. J.; Persoons, A. Science 1998, 282,
913. (b) Van Elshocht, S.; Verbiest, T.; de Schaetzen, G.; Hellemans, L.;
Phillips, K. E. S.; Nuckolls, C.; Katz, T. J.; Persoons, A. Chem. Phys. Lett.
2000, 323, 340. (c) Verbiest, T.; Van Elshocht, S.; Persoons, A.; Nuckolls,
C.; Phillips, K. E. S.; Katz, T. J. Langmuir 2001, 17, 4685.
(8) Busson, B.; Kauranen, M.; Nuckolls, C.; Katz, T. J.; Persoons, A.
Phys. ReV. Lett. 2000, 84, 79.
(9) (a) Jen, A. K.-Y.; Rao, V. P.; Wong, K. Y.; Drost, K. J. J. Chem.
Soc., Chem. Commun. 1993, 90. (b) Varanasi, P. R.; Jen, A. K.-Y.;
Chandrasekhar, J.; Namboothiri, I. N. N.; Rathna, A. J. Am. Chem. Soc.
1996, 118, 12443, and referencs therein. (c) Kim, O.-K.; Fort, A.; Barzoukas,
M.; Blanchard-Desce, M.; Lehn, J.-M. J. Mater. Chem. 1999, 9, 2227.
(10) The parent benzo[1,2-b:4,3-b′]dithiophene undergoes acylation at
the two positions adjacent to the sulfur atoms. See: (a) Gronowitz, S.;
Dahlgren, T. Chem. Scr. 1977, 12, 97. However, benzo[b]thiophene acylates
mainly at the 3-position. Substituents significantly affect the orientations
of such reactions. See: (b) Katritzky, A. R.; Taylor, R. AdV. Heterocycl.
Chem. 1990, 47, Chapter 8, and references therein.
10.1021/ja011706b CCC: $20.00 © 2001 American Chemical Society
Published on Web 11/06/2001

11900 J. Am. Chem. Soc., Vol. 123, No. 48, 2001
Phillips et al.
Scheme 1a
bithienyl, 5. Although this bithienyl had previously been
transformed into benzo[1,2-b:4,3-b′]dithiophene-4,5-dione (6),18
an alternative way to effect the transformation was considered,
that is a Friedel-Crafts acylation with oxalyl chloride. We
thought this procedure could be beneficial because it requires
only one step, whereas the previous one requires three-
bromination,18a conversion of the resulting 2,2′-dibromo-3,3′-
bithienyl into the corresponding dialdehyde (n-butyllithium and
then DMF),18b and condensation and oxidation through reflux
with ethanolic sodium cyanide in air18bsand in the last step
gives a yield of only 50%. The precedents for installing the
diketone bridge in one step include preparations of a number
of substituted benzils,19 a phenanthrene-9,10-dione,20 and dixo-
benzo[1,2-b:4,3-b′]dipyrroles21 from aromatic compounds, ox-
alyl chloride, and aluminum chloride.
a Reagents and conditions: (a) oxalyl chloride, 1,2-dichloroethane,
reflux, 68% yield; (b) Zn, Ac2O, CH2Cl2, 83% yield; (c) Cs2CO3,
C12H25I, CH3CN, 94% yield; (d) AcCl, AlCl3, 1,2-dichloroethane, 83%
yield.
of this diacetyl compound’s bis-enol ethers should, as in other
electron-rich heteroaromatics,12 make the Diels-Alder reactions
with 1,4-benzoquinone and the subsequent oxidations of the
Diels-Alder adduct take place easily.1,13,14 The experiments
described below demonstrate that a derivative of 3 can indeed
be prepared easily and in high yield, that its enantiomers can
be resolved by a procedure whose underlying theory is
understandable and whose applicability has proven widely
useful,15 that enantiopure 3 aggregates both in solution and as
the pure material, and that it gives rise to significantly circularly
polarized fluoresence. In a separate paper, it is also shown that
3 is better than 1 in generating second harmonics.16
The facts are that when the procedures used before (employ-
ing dichloromethane as the solvent at -85 °C,19b carbon
disulfide at 0 °C,19a,20 or 1,2-dichloroethane plus pyridine at
-20 to -30 °C18d) or related procedures using other Lewis
acids22 were applied to 3,3′-bithienyl, they failed to produce
diketone 6 in more than 15% yield, and they required much
effort to separate even this small amount from tarry byprod-
ucts.23,24 However, although thiophenes, unlike pyrroles21,25 and
anisoles,26 seem not to have been found to undergo Friedel-
Crafts acylations without the aid of Lewis acids,27 it turns out
that the diketone can be installed in one step and 50% yield
simply by refluxing 3,3′-bithienyl with oxalyl chloride in 1,2-
dichloroethane. The time required to complete the reaction,
7-10 days, is long, but the product precipitates as it forms,
and the only steps required to obtain it pure are to filter it, wash
it with hexanes, and triturate it with warm ethanol. In this way,
18 g of benzo[1,2-b:4,3-b′]dithiophene-4,5-quinone (6) were
made at one time.
To convert dithienoquinone 6 into dodecyloxy-substituted
benzodithiophene 8, zinc in the presence of acetic anhydride
was used to convert 6 into 7,3,28 and 1-iodododecane and cesium
carbonate in acetonitrile was used to replace 7’s acetate ester
Results
Synthesis. 2,7-Diacetyl-4,5-didodecyloxybenzo[1,2-b:4,3-b′]-
dithiophene (4) was synthesized in five steps from commercially
available 3-bromothiophene (Scheme 1). Thus, following Ja-
yasuria and Kagan,17 the bromothiophene was combined with
its Grignard reagent in the presence of Ni(dppp)Cl2, giving 3,3′-
(18) (a) Kellogg, R. M.; Schaap, A. P.; Wynberg, H. J. Org. Chem. 1969,
34, 343. (b) Wynberg, H.; Sinnige, H. J. M. Recl. TraV. Chim. Pays-Bas.
1969, 88, 1244.
(19) (a) Mohr, B.; Enkelmann, V.; Wegner, G. J. Org. Chem. 1994, 59,
635. (b) Abser, N.; McCabe, R. W.; Parri, O. L.; Price, A. H. J. Chem.
Res. (S) 1994, 252. (c) Tu¨zu¨n, C. Ogliaruso, M.; Becker, E. I. Organic
Syntheses; Wiley: New York, 1973; Collect. Vol. IV, p 111. (d) Shirinyan,
V. Z.; Kosterina, N. V.; Kolotaev, A. V.; Belen_kii, L. I.; Krayushkin, M.
M. Chem. Heterocycl. Compd. (N. Y.) 2000, 36, 219.
(20) Mohr, B.; Enkelmann, V.; Wegner, G. Mol. Cryst. Liq. Cryst. 1996,
281, 215.
(21) Carter, P.; Fitzjohn, S.; Halazy, S.; Magnus, P. J. Am. Chem. Soc.
1987, 109, 2711.
(11) Alternatives would make use of benzo[1,2-b:4,3-b′]dithiophene
lithiating at the positions adjacent to the sulfur atoms. See ref 8a and the
following: (a) Groen, M. B.; Schadenberg, H.; Wynberg, H. J. Org. Chem.
1971, 36, 2797. (b) Lehman, P. G.; Wynberg, H. Aust. J. Chem. 1974, 27,
315. (c) Yoshida, S.; Fuji, M.; Aso, Y.; Otsubo, T.; Ogura, F. J. Org. Chem.
1994, 59, 3077. (d) Larsen, J.; Bechgaard, K. Acta Chem. Scand. 1996, 50,
71.
(12) Dreher, S. D.; Weix, D. J.; Katz, T. J. J. Org. Chem. 1999, 64,
3671.
(22) Conditions tried were AlCl3 in CH2Cl2 at 0, -10, -15, and -78
°C; in CS2 at 0, -15, and -78 °C; SnCl4 in CH2Cl2 at 0 and -78 °C; in
1,2-dichloroethane at 60 °C and reflux; BF3‚OEt2 in CH2Cl2 and in 1,2-
dichloroethane at 0 °C; TiCl4 in 1,2-dichloroethane at 25 °C; FeCl3 in
CH2Cl2 at -10 and 25 °C and at reflux.
(23) Exposure to AlCl3 causes thiophenes to polymerize (Kovacic, P.;
McFarland, K. N. J. Polym. Sci., Polym. Chem. Ed. 1979, 17, 1963).
(24) Oxalyl chloride plus aluminum chloride in methylene chloride
combines with benzenes at temperatures higher than -85 °C to give
arylcarbonyl chlorides and diaryl ketones. [(a) Neubert, M. E.; Fishel, D.
L. Mol. Cryst. Liq. Cryst. 1979, 53, 101. (b) Neubert, M. E.; Fishel, D. L.
Organic Syntheses; Wiley: New York, 1990; Collect. Vol. VII, p 420.]
However, in 1,2-dichloroethane-pyridine, it combines with 2,5-dimethylth-
iophene at -20 to -30 °C to give a diketone.19d
(25) (a) Nenitzescu, C. D.; Necsoiu, I.; Zalman, M. Comun. Acad. Repub.
Pop. Rom. 1958, 8, 659; Chem. Abstr. 1959, 53, 17092c. (b) Archibald, J.
L.; Freed, M. E. J. Heterocycl. Chem. 1967, 4, 335. (c) Treibs, A.; Kreuzer,
F.-H. Justus Liebigs Ann. Chem. 1969, 721, 105.
(26) Pearson, D. E.; Buehler, C. A. Synthesis 1972, 533, and references
therein.
(13) (a) Katz, T. J.; Liu, L.; Willmore, N. D.; Fox, J. M.; Rheingold, A.
L.; Shi, S.; Nuckolls, C.; Rickman, B. H. J. Am. Chem. Soc. 1997, 119,
10054. (b) Fox, J. M.; Goldberg, N. R.; Katz, T. J. J. Org. Chem. 1998, 63,
7456. (c) Paruch, K.; Katz, T. J.; Incarvito, C.; Lam, K.-C.; Rhatigan, B.;
Rheingold, A. L. J. Org. Chem. 2000, 65, 7602. (d) Paruch, K.; Vyklick,
L.; Katz, T. J.; Incarvito, C. D.; Rheingold, A. L. J. Org. Chem. 2000, 65,
8774.
(14) (a) Davies, W.; Porter, Q. N. J. Chem. Soc. 1957, 4958. (b) Ghosh,
A.; Maiti, S. B.; Chatterjee, A.; Raychaudhuri, S. R. Indian J. Chem., Sect.
B: Org. Chem. Incl. Med. Chem. 1989, 28B, 724. (c) Sasaki, T.; Ishibashi,
Y.; Ohno. Heterocycles 1983, 20, 1933. (d) Sasaki, T.; Ishibashi, Y.; Ohno.
J. Chem. Res. (S) 1984 218. (e) Abarca, B.; Ballesteros, R.; Enriquez, E.;
Jones, G. Tetrahedron 1985, 41, 2435.
(15) (a) Thongpanchang, T.; Paruch, K.; Katz, T. J.; Rheingold, A. L.;
Lam, K.-C.; Liable-Sands, L. J. Org. Chem. 2000, 65, 1850. (b) Paruch,
K.; Katz, T. J.; Incarvito, C.; Lam, K.-C.; Rhatigan, B.; Rheingold, A. L.
J. Org. Chem. 2000, 65, 7602.
(16) Verbiest, T.; Van Elshocht, S.; Persoons, A.; Nuckolls, C.; Phillips,
K. E. S.; Katz, T. J. Langmuir 2001, 17, 4685.
(27) 3,3′-Bithienyl is acylated by a Vilsmeier reagent (Nenajdenko, V.
G.; Baraznenok, I. L.; Balenkova, E. S. J. Org. Chem. 1998, 63, 6132).
(17) Jayasuria, N.; Kagan, J. Heterocycles 1986, 24, 2261.

Aggregating Heterocyclic Helicene
J. Am. Chem. Soc., Vol. 123, No. 48, 2001 11901
Scheme 2a
a TIPS ) i-Pr3Si. Reagents and conditions: (a) TIPSOTf, Et N,
3
CH2Cl2, 100% yield; (b) p-benzoquinone, heptanes, reflux, 95% yield.
Scheme 3a
Figure 1. (a) CD spectra (ordinate on the left) and (b) UV-vis
absorption spectra (ordinate on the right) of solutions of (+)-3 in
dodecane whose concentrations are as follows: (--) 2 × 10-5, (- -)
2 × 10-3, and (s) 2 × 10-2 M.
a Reagents and conditions: (a) CsF, C12H25I, DMF, 93% yield; (b)
Zn, Et3N, (1S)-(-)-camphanoyl chloride, DMAP, 1,2-dichloroethane,
90% yield (-)-diastereomer and 70% yield (+)-diastereomer; (c) (i)
BuLi, Et2O and (ii) chloranil, 88% yield (-)-enantiomer, 85% yield
(+)-enantiomer.
reductively acylated by means of zinc in the presence of (1S)-
(-)-camphanoyl chloride.1,13b,31 Cleaving the camphanate moi-
eties from the helicene skeletons with the aid of methyllithium
and oxidizing with chloranil1,13a then gave the pure enantiomers
of 3. As expected,15a,b the enantiomer derived from the tetra-
camphanate with the lower Rf (the more polar tetracamphanate)
is dextrorotatory. Its UV and CD spectra are similar to those of
similar [6]- and [7]carbohelicenebisquinones13b,32 but are shifted
ca. 50 nm to longer wavelengths. It is on the basis of these
optical properties and the Rfs of the tetracamphanates that
dextrorotatory 3 is assigned the (P)-stereochemistry.
CD and UV-Visible Absorption Spectra of 3. The specific
rotations of solutions of enantiopure 3 in dodecane change
appreciably when their concentrations are increased from 2 ×
10-5 to 2 × 10-2 M. [R]D is 2800 at the former concentration,
10 400 at the latter, and 5000 at the intermediate concentration
of 2 × 10-3 M. The CD spectra of these solutions also change
markedly. Figure 1 displays some of these spectra,33 as well as
the UV-visible absorption spectra of the same solutions. For
example, the circular dichroism at 281 nm changes from ∆ꢀ )
+146 to ∆ꢀ ) -37 and that at 346 nm from ∆ꢀ ) +44 to ∆ꢀ
) -62. When the solvent is chloroform instead of dodecane,
the changes due to high concentration disappear. (The CD
spectra are in the Supporting Information. The specific rotation
of a 2 × 10-3 M solution in CHCl3 is 3100.) When the solvent
is dodecane and the concentration is increased further, or when
there is no solvent, the changes, illustrated in Figure 2, become
more pronounced. The figure displays the CD spectra of a 0.05
M solution in dodecane and of a film formed by evaporating
the solvent from a nonane solution, and it compares them to
the spectrum of the most concentrated solution in Figure 1. This
last solution, like others of 3 in dodecane that are more
functions with dodecyloxy groups.3 Friedel-Crafts acylation
with acetyl chloride and aluminum chloride then gave diacetyl
derivative 4, which required only trituration with cold methanol
to be obtained pure. The yield was 83%. The NMR spectra show
that the molecules of this product have a plane of symmetry,
but to verify the structure, an alternative synthesis was carried
out. Benzodithiophene 8 was combined with n-butyllithium, then
with dimethylacetamide, and finally with aqueous acid. The
resulting diacetyl derivative was identical to the one from the
Friedel-Crafts procedure. The yield was impractically low,
approximately 7%, and to purify the product required preparative
TLC. However, since thiophenes and benzodithiophenes are
known to lithiate R to their sulfur atoms,11 the experiment shows
that the acetyl groups in 4 are in the thiophenes’ R positions.
To obtain helicenebisquinone 10 (Scheme 2), enol ether 9
was first prepared by combining 4 with triisopropylsilyl triflate
and triethylamine in dichloromethane.29 The yield was quantita-
tive, and purification required only trituration with methanol.
The amount of 9 prepared at one time was 22 g. When combined
with 1,4-benzoquinone, it gave helicene 10. The yield was
extraordinary. Although the conVersion of 9 to 10 is comprised
of six transformations (two Diels-Alder reactions and four
dehydrogenations), 25 g of 10 were obtained in a single
experiment in 95% yield. Chromatography is not required to
purify any of the products in the entire synthesis.
The silyl groups were then replaced by aliphatic chains, and
the enantiomers were resolved (Scheme 3). Cesium fluoride and
1-iodododecane in DMF29a,30 gave, in 93% yield, the racemic
tetradodecyloxyhelicenebisquinone 3, which could be purified
simply by precipitating it by adding ethanol and methanol to
its solutions in dichloromethane.
In accord with analogies and a theory described previously,18
chromatography on silica gel separated the diastereomeric
tetracamphanate esters that formed when the bis-quinone was
(29) In related syntheses, silyl enol ethers give higher yields than alkyl
enol ethers. (a) Fox, J. M.; Goldberg, N. R.; Katz, T. J. J. Org. Chem.
1998, 63, 7456. (b) See footnote 7 in ref 13c.
(30) Paruch, K.; Vyklick, L.; Katz, T. J.; Incarvito, C. D.; Rheingold,
A. L. J. Org. Chem. 2000, 65, 8774.
(31) See footnote 28 in ref 13b.
(32) The assignment of absolute stereochemistry to the [7]helicenebis-
quinones in ref 13b was later confirmed (in ref 13c) by X-ray crystal-
lographic analysis.
(33) The CD spectrum of a 2 × 10-4 M solution in dodecane is much
more similar to the spectrum of a solution that is 10 times more dilute than
to that of a solution that is 10 times more concentrated (see the Supporting
Information).
(28) Katz, T. J.; Liu, L.; Willmore, N. D.; Fox, J. M.; Rheingold, A. L.;
Shi, S.; Nuckolls, C.; Rickman, B. H. J. Am. Chem. Soc. 1997, 119, 10054,
and ref 19 therein.

11902 J. Am. Chem. Soc., Vol. 123, No. 48, 2001
Phillips et al.
and increases in intensity when compared to the emission at
440 nm. It is attributed to aggregates. Also shown in Figure 3
is that, in accord with these assignments and the absence, noted
above, of circular dichroism characteristic of aggregates when
chloroform is the solvent, no peak at ca. 620 nm is seen in the
fluorescence from chloroform solutions of (+)-3, even when
their concentration is high (2 × 10-3 M). Only a peak at ca.
440 nm is seen.
The assignments of the fluorescence peaks at 440 nm to
monomers and at 620 nm to aggregates is supported by
experiments that show how rotationally mobile the fluorescing
species are. The exciting light for these experiments is linearly
polarized. The light from fluorophores that are unable to rotate
before they emit should be polarized parallel to the exciting
light. The magnitude of the polarization (P) should be the
maximum possible, 0.5.35 The polarization of the light from
fluorophores that do rotate before they emit should be less.
Indeed, the polarization of the emission at 440 nm from a 2 ×
10-5 M dodecane solution of (+)-3 is low (P ) 0.02 ( 0.01),
Figure 2. CD spectra of (+)-3: (- -) as a 2 × 10-2 M solution in
dodecane, (--) as a 0.05 M solution in dodecane, and (s) as a drop-
cast film. To facilitate comparison with Figure 1, the latter two spectra
are the negatives of the spectra of the material analyzed, the (-)-
enantiomer. The circular dichroisms of the film are arbitrarily scaled
so they are similar to those of the concentrated solution.
while that of the emission at 620 nm from a 1 × 10-3
M
dodecane solution of (+)-3 is very high (P ) 0.39 ( 0.01).
Also significant are the extent to which this emission is
circularly polarized and the comparison of the dissymmetry
factors for emission and absorption between the ground- and
first-excited electronic states.35,36 As Figure 4 shows, when
unpolarized light excites 1 × 10-3 M solutions of (+)- or (-)-
3, the magnitudes of the emissions by the aggregates of the
two enantiomers, between 600 and 700 nm, are measured to be
essentially the same. Their signs differ. The dissymmetry factor,
glum ) 2(IL - IR)/(IL + IR), where IL and IR are the intensities
of left- and right-circularly polarized emissions, is high (|glum
|
) 0.01). It is also similar to the dissymmetry factor measured
for absorption of light35 at this same solution’s longest
500-550nm
wavelength peak (gabs
) ∆ꢀ/ꢀ ) 0.01).
Polarized Light Microscopy and Differential Scanning
Calorimetry. Samples of (+)-3 are seen by polarized light
microscopy to be birefringent below 245 °C and, when cooled
from the isotropic melt to 232 °C, to crystallize into thin long
uniaxial and negatively birefringent fibers,6a displayed in Figure
5. Differential scanning calorimetric analyses (DSC, displayed
in the Supporting Information) show sharp transitions at 245
°C (∆H ) 48.0 kJ/mol) in the first heating cycle and two
transitions, at 232 and at 211 °C, in the first cooling cycle. As
expected, the helicene appears to racemize significantly at these
high temperatures. Thus, in subsequent cycles, two peaks appear
in the heating phase, and peaks in both the heating and cooling
phases become broader and shift to lower temperature. In still
later cycles, peaks merge so there is only one on heating (at
220 °C) and one on cooling (at 208 °C). These last thermograms
are essentially the same as those of racemic 3, which shows a
phase transition at 221.4 °C (∆H ) 44.6 kJ/mol) on heating
and at 206.3 °C (∆H ) -42.1 kJ/mol) on cooling. Like the
enantiopure material, racemic 3 between crossed polarizers is
seen to be birefringent at temperatures below 220 °C. However,
unlike the enantiopure material, when cooled from this tem-
perature, it does not form visible fibers.
Figure 3. Fluorescence spectra of solutions of (+)-3 in dodecane and
in chloroform after excitation at 325 nm: (-2-) 2 × 10-6
M (in dodecane); (--) 2 × 10-5 M (in dodecane); (- -) 2 × 10-4
(in dodecane); (- -) 2 × 10-2 M (in dodecane); (-O-) 2 × 10-3
(in CHCl3). The intensities are normalized to the peak at 440 nm.
M
M
concentrated than 2 × 10-3 M, is noticeably viscous. The 0.05
M solution is a gel. The spectrum of the film in Figure 2 is the
average of twelve spectra (all very similar) measured as the
sample in a circular holder was rotated through successive 30°
angles, a procedure that should eliminate effects of linear
birefringence and linear dichroism.34 Lowering the temperature
of dodecane solutions affects the CD spectra in much the same
way as increasing their concentration. The Supporting Informa-
tion shows this for a 1 × 10-4 M solution whose temperature
varies between -5 and +20 °C.
Fluorescence Emissions and Their Linear and Circular
Polarization. Figure 3 shows the spectra of fluorescence emis-
sion from solutions at various concentrations of (+)-3 in
dodecane. At the lowest concentration (2 × 10-6 M), monomers
are likely to predominate, and the observed fluorescence, cen-
tered at ca. 440 nm, is attributed to them. As the concentration
is increased, another emission, centered at ca. 620 nm, appears
(34) (a) Kuball, H.-G.; Scho¨nhofer, A. In Circular Dichroism: Principles
and Applications; Nakanishi, K., Berova, N., Woody, R., Eds.; VCH: New
York, 1994; Chapter 4. (b) Scho¨nhofer, A.; Kuball, H.-G. Chem. Phys. 1987,
115, 159. (c) Norde´n, B. Acta Chem. Scand. 1972, 26, 1763. (d) Tunis-
Schneider, M. J. B.; Maestre, M. F. J. Mol. Biol. 1970, 52, 521. (e) Cornell,
D. G. J. Colloid Interface Sci. 1979, 70, 167.
(35) Kliger, D. S.; Lewis, J. W.; Randall, C. E. Polarized Light in Optics
and Spectroscopy; Academic Press: San Diego, CA, 1990.
(36) (a) Riehl, J. P.; Richardson, F. S. Chem. ReV. 1986, 86, 1. (b)
Brittain, H. G. Chirality 1996, 8, 357.

Aggregating Heterocyclic Helicene
J. Am. Chem. Soc., Vol. 123, No. 48, 2001 11903
Figure 6. Transmission electron micrograph of fibers of (+)-3.
Figure 4. Spectra of (a) total luminescence and (b) circularly polarized
luminescence from solutions at 23 °C of (+)- and (-)-3 in dodecane
(1 × 10-3 M) after excitation with unpolarized light (λex ) 325 nm).
IL and IR are, respectively, the luminescence intensities of the left and
right circularly polarized emission.
Figure 7. Electron diffraction pattern from a thin region of fibers of
(+)-3 tilted about the axis of the substrate by 30°.
Figure 5. Fibers between crossed polarizers of (+)-3 that had been
cooled from the isotropic melt to 232 °C. The magnification is 160×.
Transmission Electron Microscopy and Electron Diffrac-
tion. The transmission electron microscopy (TEM) images in
Figure 6 shows that the fibrous crystals of (+)-3 adopt a lamellar
morphology similar to that of nonracemic 1.25 The electron
diffraction patterns from these crystals are consistent with the
hypothesis that the molecules are stacked in columns that are
packed into a hexagonal lattice. When thin areas of the fibrous
crystals are tilted by 30° about the axis of the fibers, intense
reflections, otherwise not seen, become apparent (Figure 7) at
3.4, 1.7, and 1.13 nm, transverse to the direction of the fibers’
long axes. These are assigned to the first-, second- and third-
order reflections from {110}/(020) planes in the hexagonal
lattice that the tilt brings parallel to the electron beam. When
Figure 8. Electron diffraction from thick pack of fibers of (+)-3. The
electron beam is perpendicular to the sample plane.
fairly thick regions of the sample are examined, reflections are
also seen without tilting, at spacings of 1.99 and 0.99 nm (Figure
8), corresponding to successive orders of reflection from the
(200) planes. In thicker samples, there is an additional reflection,
at 0.395 nm, (Figure 9) on a line parallel to the long axis of the
fibers.

11904 J. Am. Chem. Soc., Vol. 123, No. 48, 2001
Phillips et al.
In addition, the two peaks in the fluoresence emission spectra
in Figure 3 show that molecules of enantiopure 3 dissolved in
dodecane aggregate.41 Thus, the emission at ca. 440 nm appears
to be from monomers of 3, and that at ca. 620 nm from its
aggregates. When the concentration is as small as 2 × 10-6
M
or when the solvent is chloroform, the only emission observed
is from the monomer. However, when the concentration in
dodecane is 2 × 10-5 M, emission from the aggregates is also
seen, and as the concentration is increased further, its intensity,
compared to emission from the monomers, increases (as should
aggregation). The assignments of the emission at 620 nm to
aggregates and of the emission at 440 nm to monomers are
supported by the fluorescence depolarization measurements.
Thus, the polarization of the fluorescence from the aggregates
(P ) 0.39 ( 0.01) is close to the maximum value (P ) 0.5) for
molecules that do not rotate appreciably in the interval between
the time they are excited and the time they emit light,35 while
the polarization of the fluorescence from the monomers is
essentially nil (P ) 0.02 ( 0.01). The implication is that the
species emitting at 620 nm rotate more slowly in solution than
those emitting at 440 nm, which is reasonable since the former
are larger. Moreover, the wavelength of the emission attributed
to the aggregates is, as it should be, higher than that attributed
to the monomer.41
Chiral aggegates, which should be more dissymmetric than
their monomers,42 are known to emit circularly polarized
fluorescence,39a,b and accordingly it is not surprising that the
fluoresence from aggregates of enantiopure 3 is circularly
polarized.43 However, it is interesting that the dissymmetry
factors of the emission, and absorption from the solutions are
in fact very high. Although considerably smaller than the very
large values reported for absorption and emission by films of
some chiral poly(fluorene)s44 and absorption by some films of
chiral poly(p-phenylene)s,45 they are slightly higher than those
reported for absorption by solutions and films of some poly-
(p-phenyleneethynylene)s,39e comparable to those for absorption
and emission by solutions and films of some chiral poly-
(phenylenevinylene)s,46 and 0.5-4 times as large as those of
the absorptions and emissions from aggregated chiral sexithio-
phenes39a and polythiophenes.39b That the dissymmetry found
for the circular polarization of the emission is not an artifact
caused by the anisotropy of the samples47 is shown by the
essentially perfect inversion of the sign of the polarization,
displayed in Figure 4b, when enantiomers are interchanged. The
similarity of the dissymmetry factors of the aggregates’ lumi-
Figure 9. Pattern of reflected electrons from a thick region of fibrous
(+)-3 showing a meridional reflection (at 0.395 nm) corresponding to
the spacing between aromatic cores.
X-ray Diffraction. The structure of (+)-3 was also analyzed
by X-ray diffraction. Samples were prepared on a glass substrate
by evaporating the solvent from solutions in nonane. The
resulting films were heated to 242 °C, cooled to 220 °C at 10
°C/min, and then cooled further to room temperature. The
diffraction pattern of X-rays reflected from the surface, shown
in the Supporting Information, is dominated by a peak at 3.45
nm, at the same position as the major peak in the electron
diffraction pattern in Figure 7, and there are small second- and
third-order peaks at 1.75 and 1.14 nm, respectively. There is a
broad peak centered at ca. 0.4 nm, ascribed to diffraction by
mobile side chains. No sharp peak is seen that is the analogue
of one in the electron diffraction pattern in Figure 9 that was
ascribed to the reflection from planes of stacked aromatic rings.
Discussion
Enantiopure 1, either dissolved in dodecane or as the pure
material, assembles into columnar aggregates.1,6a,b,7a,37 The col-
umns assemble into hexagonal arrays, forming long fibers that
are easily seen under an optical microscope.6b The aggregates,
both in solution and pure, exhibit circular dichroisms and spe-
cific rotations distinctly different from those of the monomer.1,6b,37
Enantiopure 3 appears to exhibit essentially similar properties.
A number of observations described above imply that mole-
cules of enantiopure 3 self-assemble when solutions in dodecane
or nonane are concentrated or cooled. These observations are
the following: the changes recorded in Figures 1 and 2 that
occur in the CD spectra;1,6b,37 the changes that occur in the
specific rotations;1,6b the absence of changes in the CD spectra
and specific rotations when the solvent is chloroform rather than
dodecane (because in chloroform the molecules do not ag-
gregate);1,3,38 the changes in the CD spectra that occur when
the temperature is changed (Figure 3);4,39 and the increased
viscosity noted above when the solutions are concentrated.40
(39) (a) Kilbinger, A. F. M.; Schenning, A. P. H. J.; Goldoni, F.; Feast,
W. J.; Meijer, E. W. J. Am. Chem. Soc. 2000, 122, 1820. (b) Langeveld-
Voss, B. M. W.; Janssen, R. A. J.; Christiaans, M. P. T.; Meskers, S. C. J.;
Dekkers, H. P. J. M.; Meijer, E. W. J. Am. Chem. Soc. 1996, 118, 4908.
(d) Nelson, J. C.; Saven, J. G.; Moore, J. S.; Wolynes, P. G. Science 1997,
277, 1793. (d) De Rossi, U.; Dahne, S.; Meskers, S. C. J.; Dekkers, H. P.
J. M. Angew. Chem., Int. Ed. Engl. 1996, 35, 760. (e) Fiesel, R.; Halkyard,
C. E.; Rampey, M. E.; Kloppenburg, L.; Studer-Martinez, S. L.; Scherf,
U.; Bunz, U. H. F. Macromol. Rapid Commun. 1999, 20, 107.
(40) Reference 1 and references therein.
(41) Fluorescence spectra showing two peaks, from monomers at lower
wavelengths and from aggregates at higher wavelengths, are reported for
phenylene ethynylene oligomers in: (a) Prince, R. B.; Saven, J. G.; Wolynes,
P. G.; Moore, J. S. J. Am. Chem. Soc. 1999, 121, 3114. (b) Lahiri, S.;
Thompson, J. L.; Moore, J. S. J. Am. Chem. Soc. 2000, 122, 11315. (c)
Reference 39e and references therein. They are reported for poly-
(phenylbenzodithiazole) in (d) Moldowan, J. M.; Dahl, J.; Huizinga, B. J.;
Fago, F. J.; Hickey, L. J.; Peakman, T. M.; Taylor, D. W. Science 1994,
265, 768, and for chiral polythiophenes in the following references: for
R,ω-disubstituted sexithiophenes in ref 39a and for poly{3,4-di[(S)-2-
methylbutoxy]thiophene} in ref 39b.
(37) Nuckolls, C.; Katz, T. J.; Verbiest, T.; Van Elshocht, S.; Kauranen,
M.; Kuball, H.-G.; Kiesewalter, S.; Lovinger, A. J.; Persoons, A. J. Am.
Chem. Soc. 1998, 120, 8656.
(38) (a) Palmans, A. R. A.; Vekemans, J. A. J. M.; Havinga, E. E.; Meijer,
E. W. Angew. Chem., Int. Ed. Engl. 1997, 36, 2648. For similar changes
brought about by 1,2-dichloroethane, see: (b) Markovitsi, D.; Bengs, H.;
Ringsdorf, H. J. Chem. Soc., Faraday Trans. 1992, 88, 1275. (c) Gallivan,
J. P.; Schuster, G. B. J. Org. Chem. 1995, 60, 2423. For chloroform
unwinding m-phenylene ethylene oligomers from the helical conformation
they adopt in hydrocarbon solutions, see ref 4 and references therein.
(42) Inai, Y.; Sisido, M.; Imanishi, Y. J. Phys. Chem. 1990, 94, 8365.
(43) (a) Dekkers, H. P. J. M. In Circular Dichroism; Nakanishi, K.,
Berova, N., Woddy, R. W., Eds.; VCH: New York, 1994; Chapter 6. (b)
Riehl, J. P.; Richardson, F. S. Chem. ReV. 1986, 86, 1.

Aggregating Heterocyclic Helicene
J. Am. Chem. Soc., Vol. 123, No. 48, 2001 11905
550-750nm
nescence and absorption at longest wavelength (glum
Moreover, the nonlinear optical properties of 3 are similar to
or superior to those of 1, previously the only material to exhibit
a significant spontaneous54 second harmonic response and to
generate quasi phase-matched second harmonics from layers
of enantiomers.8
) gabs500-550nm ) 0.01) implies that the ground and excited states
have similar geometries.48
Like enantiopure 1, enantiopure 3 when cooled from the melt
crystallizes into long fibers (Figure 5). The enthalpy changes
accompanying the melting of both materials are similar (48 kJ/
mol for enantiopure 3, 45 kJ/mol for enantiopure 1)1 and
significantly greater than those accompanying the clearing of
columnar discotic mesophases (1-20 kJ/mol).49 The implication
may be that the fibrous materials are plastic crystals.
TEM images show fibers of enantiopure 3 (Figure 6) and 16a
to have similar lamellar morphologies. That the underlying
organization is hexagonal is proven by the 30° tilt about the
fiber axes required for strong {110}/(020) reflections to be seen
and by the observation of the (200) reflections when the sample
is not tilted. The 3.40-3.45 nm distance between reflection
planes measured both by electron and X-ray diffraction is
consistent with a structure for enantiopure 3 that consists of
hexagonally arrayed columns of helicenes, each (3.40-3.45)
Experimental Section
THF and Et2O were distilled from Na/benzophenone, CH2Cl2,
(ClCH2)2, and Et3N from CaH2. 1,4-Benzoquinone (Aldrich, 98%) was
purified by slurrying it in CH2Cl2 with 4 times its weight of basic
alumina, filtering through Celite, removing the solvent, and drying under
a vacuum. Zn dust (Aldrich, <10 µm, 98%) was activated prior to
use.55 The following were used without further purification: AlCl3
(99%), acetyl chloride (99%), acetic anhydride (99%), CH3CN (anhy-
drous, 99.8%), Cs2CO3 (99%), CsF (99%), DMAP (99%), DMF
(anhydrous, 99.8%), and 1-iodododecane (98%)sall from Aldrich,
n-BuLi (2.6M in hexanes) and heptane (99%) from Acros, and
triisopropylsilyl triflate (98%) from GFS. 3,3′-Bithienyl was synthesized
according to ref 17. Glassware was flame-dried under vacuum and
cooled under N2. Unless otherwise specified, reactions were run under
N2, and reaction mixtures were stirred magnetically.
x
× (2/ 3) ) 3.9-4.0 nm in diameter. This diameter is
Benzo[1,2-b:4,3-b′]dithiophene-4,5-quinone (6). Oxalyl chloride
(6.0 mL, 69 mmol) was added to a flask containing 3,3′-bithienyl (20.0
g, 120 mmol) dissolved in 1,2-dichloroethane (350 mL), and the
mixture, under a CaCl2 drying tube open to the atmosphere, was
refluxed for 5 days. More oxalyl chloride (6.0 mL, 69 mmol) was added,
and reflux was continued for another 5 days, after which time the
mixture was cooled to 25 °C and left to stand overnight. It was filtered,
and the filter cake was triturated thoroughly with hexanes and warm
ethanol. Obtained were 18 g of red solid 6 (68% yield); mp (from EtOH)
240 °C (with decomposition); lit.14b (from n-propanol) 240 °C.17b 1H
NMR (CDCl3, 500 MHz): δ 7.82 (d, 2H, 5 Hz), 7.29 (d, 2H, 5 Hz).
4,5-Diacetoxybenzo[1,2-b:4,3-b′]dithiophene (7). Dry CH2Cl2 (300
mL) was added by cannula to a flask containing 6 (5.6 g, 25 mmol)
and Zn dust (16.6 g, 250 mmol). Acetic anhydride (24 mL, 250 mmol)
and triethylamine (53 mL, 380 mmol) were sequentially syringed in.
The mixture was stirred for 4 h at 25 °C and then filtered through
Celite. The filtrate was washed with water, 1 M HCl, saturated aqueous
NaHCO3, and again with water. The organic layer was dried (Na2SO4),
filtered, treated with activated charcoal, and filtered again through
Celite. Removal of solvent under reduced pressure and further drying
under high vacuum yielded 6.34 g of 7 (83% yield), an off-white solid;
approximately that expected (4.3 nm) for two extended dode-
cyloxy side chains (ca. 2 × 1.73 nm) plus the diameter of the
helicene core (ca. 0.8 nm), reduced by the side chains inter-
digitating.50 The negative birefringence is consistent with the
planes of the aromatic rings being approximately perpendicular
to the long axes of the fibers,51 as is the observation of electron
diffraction, but not X-ray diffraction, by planes separated by
3.95 Å, the likely separation of these rings.52
Thus the morphology of pure 3 and the aggregating properties
of 3 in solution are similar to the morphology and aggregating
properties of 1. Moreover, as reported elsewhere, the nonlinear
optical properties are also similar.7a,53 The second-order non-
linear optical response of Langmuir-Blodgett films of 3 is large
because the material is chiral.7a,53 And because it aggegates into
columnar structures, the response varies sinusoidally as the
sample, especially one that has been heated to the clearing
temperature and cooled, is rotated about an axis perpendicular
to the surface.7b,53 However, films of higher symmetry and
higher optical quality can be formed by spin-coating both 1 and
3, and the second-order susceptibility of these films is higher
for 3 than for 1.7c
1
mp (from CH2Cl2/MeOH) 194 °C. IR (CCl4): 1787 cm-1. H NMR
(CDCl3, 400 MHz): δ 7.62 (d, 2H, 5.4 Hz), 7.51 (d, 2H, 5.4 Hz), 2.41
ppm (s, 6H). 13C NMR (CDCl3, 75 MHz): δ 167.5, 134.0, 133.6,
131.5,127.1, 122.2, 20.4 ppm. Anal. Calcd for C14H10O4S2: C, 54.89;
H, 3.29. Found: C, 54.58; H, 3.31.
Conclusions
4,5-Didodecyloxybenzo[1,2-b:4,3-b′]dithiophene (8). A flask con-
taining 7 (6.34 g, 20.7 mmol) and Cs2CO3 (16 g, 103 mmol) was fitted
with a reflux condenser. Acetonitrile (300 mL) was added through a
cannula and 1-iodododecane (26 mL, 103 mmol) was syringed in. The
mixture was stirred and refluxed for 3 days, after which it was cooled
to 25 °C. The solvent was removed under reduced pressure, and the
remaining solids were partitioned between CH2Cl2 and water. The
organic layer was washed with 1 M HCl and with water, dried
(Na2SO4), and filtered. The solvent was removed, and the product was
triturated thoroughly with cold ethanol and dried under high vacuum,
yielding 11.1 g (94% yield) of 8, an off-white, waxy solid; mp (from
The experiments show that helicenebisquinone 3 is easy to
prepare in large amounts. Enantiopure, just like 1, it self-
assembles into columnar aggregates both when dissolved in
saturated hydrocarbon solvents and in the absence of solvent.
(44) Dissymmetry factors greater than 0.15 and up to 0.25 for absorption
and emission from extensively annealed films are reported in the following.
The details are not yet available. (a) Oda, M.; Meskers, S. C. J.; Nothofer,
H. G.; Scherf, U.; Neher, D. Synth. Met. 2000, 111-112, 575. (b) Oda,
M.; Nothofer, H. G.; Lieser, G.; Scherf, U.; Meskers, S. C. J.; Neher, D.
AdV. Mater. 2000, 12, 362.
(45) Fiesel, R.; Neher, D.; Scherf, U. Synth. Met. 1999, 102, 1437.
(46) Peeters, E.; Christiaans, P. T.; Janssen, R. A. J.; Schoo, H. F. M.;
Dekkers, H. P. J. M.; Meijer, E. W. J. Am. Chem. Soc. 1997, 119, 9909.
(47) Dekkers, H. P. J. M.. In Circular Dichroism: Principles and
Applications; Nakanishi, K., Berova, N., Woody, R., Eds.; VCH: New York,
1994; Chapter 6, p 134.
(48) Brittain, H. G. In Molecular Luminescence Spectroscopy: Methods
and Applications, Part 1; Schulman, S. G., Ed.; Wiley: New York, 1985;
Chapter 6.
(49) Reference 2 and footnote 15 therein.
(50) See ref 6a and footnotes 19 and 20 therein.
1
CH2Cl2/MeOH) 39-40 °C. H NMR (CDCl3, 400 MHz): δ 7.62 (d,
2H, 5.4 Hz), 7.45 (d, 2H, 5.4 Hz), 4.25 (t, 4H, 6.6 Hz), 1.84 (quint,
4H, 7.5 Hz), 1.54 (quint, 4H, 7.6 Hz), 1.20-1.40 (m, 32H), 0.87 ppm
(t, 6H, 6.8 Hz). 13C NMR (CDCl3, 75 MHz): δ 142.3, 133.6, 131.2,
125.6, 122.1, 73.7, 31.9, 30.4, 29.7-29.4 (m), 26.1, 22.7, 14.1 ppm.
2,7-Diacetyl-4,5-didodecyloxybenzo[1,2-b:4,3-b′]dithiophene (4).
Acetyl chloride (2.63 mL, 37 mmol) was syringed into a cold (0 °C)
suspension of AlCl3 (4.9 g, 35 mmol) in 100 mL of 1,2-dichloroethane.
The mixture was stirred at 0 °C for 45 min, during which time it turned
(51) See ref 6a and footnotes 23-26 therein.
(52) Reference 2 and the references in footnotes 14, 15b, and 23 therein.
(53) Sioncke, S.; Van Elshocht, S.; Verbiest, T.; Persoons, A.; Mauranen,
M.; Phillips, K. E. S.; Katz, T. J. J. Chem. Phys. 2000, 113, 7578.
(54) That is, a response in the absence of an external orienting force.
(55) Shriner, R. L.; Neumann, F. W. Organic Syntheses; Wiley: New
York, 1955; Collect. Vol. III, p 73.

11906 J. Am. Chem. Soc., Vol. 123, No. 48, 2001
Phillips et al.
clear. A solution of 8 (9.0 g, 16 mmol) in 1,2-dichloroethane (100 mL),
also at 0 °C, was then added by cannula, and 25 mL of 1,2-
dichloroethane was used to complete the transfer. The resulting mixture
was stirred at 0 °C for 5 h, allowed to warm to 25 °C, quenched by the
addition of water (250 mL), and transferred to a separatory funnel.
The organic layer was washed with two portions of water, dried
(Na2SO4), and filtered. The solvent was then removed under reduced
pressure. The remaining solid was triturated with cold methanol and
then dried under high vacuum. This gave 8.6 g (83% yield) of 4, a
lemon-yellow solid; mp (from CH2Cl2/MeOH) 69 °C. IR (CCl4): 1669
69.7, 32.0, 30.4, 29.7-29.4 (m), 29.0, 26.1, 22.7, 14.1 ppm. Anal. Calcd
for C74H106O8S2: C, 74.83; H, 9.00. Found: C, 74.76; H, 9.11.
Resolution of 3. Preparation of 11. Dichloroethane (250 mL) was
added to a flask containing (()-3 (7.6 g, 6.4 mmol), Zn (21 g, 320
mmol), (1S)-(-)-camphanoyl chloride (11 g, 51 mmol), and DMAP
(1.6 g, 13 mmol). Et3N was syringed in, and the mixture was stirred at
60 °C overnight. After it had cooled to 25 °C, it was filtered through
Celite, and the filtrate was diluted with CH2Cl2. It was then washed
with 1 M HCl and then saturated aqueous NaHCO3 and dried
(Na2SO4). Evaporation of the solvent under vacuum and column
chromatography on silica gel using 25-30% EtOAc in hexanes yielded
the two diastereomeric esters, 11a (less polar) and 11b (more polar) in
90 and 70% yields, respectively. Both are pale yellow solids.
1
cm-1. H NMR (CDCl3, 400 MHz): δ 8.23 (s, 2H), 4.27 (t, 4H, 6.6
Hz), 2.71 ppm (s, 6H). 13C NMR (CDCl3, 75 MHz): δ 191.6, 144.2,
144.0, 138.7, 131.9, 126.9, 77.0, 74.0, 31.9, 30.4, 29.7-29.4 (m), 29.0,
26.1, 22.7, 14.1 ppm. Anal. Calcd for C38H58O4S2: C, 70.98; H, 9.09.
Found: C, 70.79; H, 9.20.
(a) Properties of 11a. Mp (from CH2Cl2-MeOH) 141 °C. IR
1
(CCl4): 1798, 1753 cm-1. H NMR (CDCl3, 500 MHz): δ 7.27 (s,
2H), 7.01 (d, 2H, 8.3 Hz), 6.09 (d, 2H, 8.3 Hz), 4.45 (m, 4H),
4.29-4.23 (m, 4H), 2.73 (m, 2H), 2.40 (m, 2H), 2.10-1.87 (m, 12H),
1.63-1.57 (m, 92H), 1.04 (m, 2H), 0.90-0.87 (m, 18H), 0.45-0.38
(m, 8H), 0.34 ppm (s, 6H). 13C NMR (CDCl3, 75 MHz): δ 177.9,
177.0, 166.3, 165.1, 153.4, 143.1, 142.7, 142.5, 134.3, 133.3, 130.3,
128.8, 127.2, 120.8, 118.1, 113.9, 96.7, 91.3, 89.1, 74.0, 68.9, 55.1,
54.6, 54.3, 54.0, 32.0, 31.2, 30.5, 29.7-28.9 (m), 28.5, 26.2 (d), 22.7,
17.2-17.1 (d), 16.0-15.8 (d), 14.1, 9.8, 9.5 ppm. Anal. Calcd for
Triisopropylsilyl Enol Ether 9. A solution of 4 (14.6 g, 22.7 mmol)-
in CH2Cl2 (800 mL) contained in a round-bottomed flask was cooled
in an ice bath. Triethylamine (32 mL, 227 mmol) and triisopropylsilyl
triflate (13.4 mL, 49.5 mmol) were sequentially syringed in. The mixture
was stirred at 0 °C for 10 min and then allowed to warm to 25 °C
during 1 h, after which it was transferred to a separatory funnel and
washed with 10% aqueous NaOH and saturated aqueous NaHCO3. The
organic layer was dried (Na2SO4) and filtered, and the solvent was
removed under reduced pressure. The remaining yellowish-brown oil
was triturated repeatedly with methanol until the wash was colorless.
Drying under high vacuum gave 22.0 g (100% yield) of 9, an amber
oil. 1H NMR (CDCl3, 400 MHz): δ 7.69 (s, 2H), 4.91 (d, 2H, 2.2 Hz),
4.46 (d, 2H, 2.2 Hz), 4.23 (t, 4H, 6.5 Hz), 1.83 (quint, 4H, 7.9 Hz),
1.54 (quint, 4H, 7.7 Hz), 1.38-1.68 (m, 74H), 0.88 ppm (t, 6H, 6.8
Hz). 13C NMR (CDCl3, 75 MHz): δ 151.6, 142.4, 142.1, 133.0, 131.6,
118.8, 91.1, 73.7, 32.0, 30.5, 29.7-29.4 (m), 26.2, 22.7, 18.1, 14.2,
12.8 ppm.
C114H158O20S2: C, 71.59; H, 8.33. Found: C, 71.37; H, 8.24.
(b) Properties of 11b. Mp (from CH2Cl2-MeOH) 110 °C. IR
1
(CCl4): 1801, 1751 cm-1. H NMR (CDCl3, 500 MHz): δ 7.16 (s,
2H), 7.01 (d, 2H, 8.3 Hz), 6.15 (d, 2H, 8.3 Hz), 4.49 (m, 2H),
4.36-4.25 (m, 6H), 2.73 (m, 2H), 2.42 (m, 2H), 2.10-1.82 (m, 14H),
1.00-0.80 (m, 24H); 13C NMR (CDCl3, 75 MHz) δ 177.6, 176.8, 166.6,
165.0, 153.0, 143.6, 143.0, 142.7, 135.3, 132.3, 130.2, 128.5, 127.0,
121.4, 118.7, 115.1, 96.5, 90.9, 90.1, 74.2, 69.1, 54.9, 54.4, 54.2, 53.9,
31.9, 31.3, 30.5, 29.7-28.8 (m), 27.6, 26.5, 26.1, 22.7, 17.1-17.0 (δ),
16.3-16.2 (δ), 14.1, 9.7, 9.5. Anal. Calcd. for C114H158O20S2: C, 71.59;
H, 8.33. Found: C, 71.31; H, 8.37.
Thiohelicenebisquinone 10. A mixture of 9 (21.5 g, 22.5 mmol),
1,4-benzoquinone (36.5 g, 337 mmol), and heptane (200 mL) was
refluxed for 30 h, during which time it turned deep brownish-red. After
it had cooled to 25 °C, the solvent was removed under reduced pressure.
The remaining solids at a pressure of ca. 20 mmHg were heated to
100 °C, which sublimed away most of the residual 1,4-benzoquinone
and hydroquinone. The product was then dissolved in hexanes, and
insoluble materials were filtered. After the solvent had been evaporated,
the remaining solid was dissolved in a small volume of CH2Cl2, and a
portion, precipitated by the addition of excess methanol, was filtered.
The filter cake was dissolved in CH2Cl2, which was subsequently
evaporated, leaving the deep burgundy 10, which was dried under high
vacuum. The yield was 25.0 g, 95%; mp (from CH2Cl2-MeOH) 108
Conversion of 11 into Enantiopure 3. Distilled Et2O (200 mL)
was added to 11b (3.1 g, 1.6 mmol), and the resulting solution was
cooled to 0 °C. MeLi (23 mL, 1.4 M) was then added slowly from a
syringe, and the mixture was stirred at 0 °C for 3 h and then allowed
to warm to 25 °C during the course of 1 h. A deoxygenated solution
of saturated aqueous NH4Cl, added through a cannula, was stirred
vigorously with the reaction mixture for 30 min. After extraction with
CH2Cl2, washing with 1 M HCl and water, drying (Na2SO4), and
filtering, the CH2Cl2 solution was stirred at 25 °C for 20 min with
chloranil (1.2 g, 4.9 mmol). The solvent was then evaporated, and the
crude solid product was filtered through a plug of silica gel, eluting
first with 25% hexanes in CH2Cl2 and then with 20% hexanes in
CH2Cl2. After the solvent had been evaporated, the remaining solid
was dissolved in a small volume of CH2Cl2, to which excess MeOH
was then added to precipitate the pure burgundy (P)-(+)-3, the yield
of which, after drying, was 1.6 g (85%). The levorotatory enantiomer,
(M)-3, was obtained by the same procedure starting from 11a. The
yield was 1.7 g (88%)
(a) Properties of (M)-(-)-3. Mp (from CH2Cl2-MeOH) 245 °C. IR
(CCl4): 1663 cm-1. 1H NMR (CDCl3, 400 MHz): δ 7.49 (s, 2H), 6.75
(d, 2H, 10.1 Hz), 6.51 (d, 2H, 10.1 Hz), 4.4-4.29 (m, 8H), 2.00-1.91
(m, 8H), 1.61-1.55 (m, 8H), 1.49-1.20 (m, br, 64H), 0.90-0.87 ppm
(m, 12H). 13C NMR (CDCl3, 75 MHz): δ 185.5, 183.9, 157.6, 144.0,
139.3, 137.3, 136.8, 136.3, 135.6, 132.1, 128.7, 124.7, 103.2, 74.2,
69.7, 32.0, 30.4, 29.7-29.4 (m) 29.0, 26.1, 22.7, 14.1 ppm. Anal. Calcd
for C74H106O8S2: C, 74.83; H, 9.00. Found: C, 74.89; H, 8.85.
(b) Properties of (P)-(+)-3. The mp (from CH2Cl2-MeOH), IR
spectrum (CCl4), 1H NMR spectrum (CDCl3, 400 MHz), and 13C NMR
spectrum (CDCl3, 75 MHz) was identical to that of the (-)-enantiomer.
Anal. Calcd for C74H106O8S2: C, 74.83; H, 9.00. Found: C, 74.49; H,
9.11.
1
°C. H NMR (CDCl3, 400 MHz): δ 7.47 (s, 2H), 6.74 (d, 2H, 10.1
Hz), 6.52 (d, 2H, 10.1 Hz), 4.47-4.42 (m, 2H), 4.31-4.27 (m) 1.94-
1.87 (quint, 4H, 6.5 Hz), 1.64-1.19 (m, 78H), 0.88 ppm (t, 6H, 6.8
Hz). 13C NMR (CDCl3, 75 MHz) :δ 185.3, 184.0, 155.1, 143.9, 139.5,
138.9, 137.3, 137.2, 136.3, 132.0, 129.0, 124.9, 110.1, 74.2, 31.9, 30.4,
29.7-29.4 (m) 26.2, 22.7, 18.0, 14.1, 12.9 ppm. Anal. Calcd for
C68H98O8S2Si2: C, 70.18; H, 8.49. Found: C, 69.98; H, 8.31.
Preparation of Racemic 3. DMF (150 mL) was added through a
cannula to a flask containing 10 (3.00 g, 2.58 mmol) and CsF (1.96 g,
12.9 mmol). The resulting black solution was stirred and heated for 90
min in an oil bath at 65 °C. 1-Iodododecane (4.5 mL, 18.0 mmol) was
syringed into the mixture, which was then heated for an additional 3
h. The mixture was cooled to 25 °C and partitioned between water
and diethyl ether. The organic extract was washed twice more with
water, dried (Na2SO4), and filtered. The solvent was removed under
reduced pressure, and the product was precipitated from a small volume
of CH2Cl2 by adding ethanol and evaporating. The ethanolic suspension
was then filtered, and the filter cake was washed with cold ethanol.
The yield of deep burgundy solid (()-3, after it had been dried under
high vacuum, was 2.86 g (93%); mp (from CH2Cl2-MeOH) 220 °C.
CD and UV-Visible Absorption Spectra and Specific Rotations.
The dodecane used in these and other experiments was either purchased
in olefin-free form from Fluka or was freed of olefin by shaking it
with concentrated H2SO4, washing, drying over basic alumina and
1
IR (CCl4): 1663 cm-1. H NMR (CDCl3, 400 MHz): δ 7.49 (s, 2H),
6.75 (d, 2H, 10.1 Hz), 6.51 (d, 2H, 10.1 Hz), 4.4-4.29 (m, 8H), 2.00-
1.91 (m, 8H), 1.61-1.55 (m, 8H), 1.49-1.20 (m, br, 64H), 0.90-0.87
ppm (m, 12H). 13C NMR (CDCl3, 75 MHz): δ 185.5, 183.9, 157.6,
144.0, 139.3, 137.3, 136.8, 135.6, 132.1, 128.7, 124.7, 103.2, 74.2,
distilling. The cells used to measure the spectra of 2 × 10-5, 2 × 10-4
,
2 × 10-3, and 2 × 10-2 M solutions were quartz and had path lengths

Aggregating Heterocyclic Helicene
J. Am. Chem. Soc., Vol. 123, No. 48, 2001 11907
of 1.0, 0.10, 0.010, and 0.0010 cm, respectively. The spectra of solutions
more concentrated than 2 × 10-2 M were also measured using the
0.0010 cm cell. The optical densities of the solutions at all reported
wavlengths was e1.4. A solvent-free sample for CD analysis was
prepared by drop-casting (-)-3 from a solution in nonane onto a quartz
disk. After most of the solvent had evaporated at 25 °C, any remaining
traces were driven off by heating the film to 170 °C.
temperature. They were obliquely shadowed with Pt, which was evap-
orated at an angle of tan-1 0.5 to the surface, and then coated with a
thin layer of amorphous carbon.57 The mica substrate was then cut into
small squares, and the films were floated off, transferred to copper
grids, and dried. The JEOL transmission electron microscope used to
study the samples was operated at 100 keV and equipped with a rotating
tilt stage. The samples were scanned in the diffraction mode under
low-dose conditions, and the diffraction patterns were recorded when-
ever a suitable region was found. The morphology of the region was
then photographed using bright-field TEM.
Fluorescence Emission and Circularly Polarized Luminescence.
A Fluorolog-1680, 0.22 m double spectrometer (SPEX) was used to
record the steady state fluorescence emission and excitation spectra.
This same instrument was used to measure circular polarized fluores-
cence after a photoelastic modulator (PEM-90, Hinds Instruments) had
been placed between the sample cell and the emission monochromator.
The photoelastic modulator, operated in the quarter wave retardation
mode at a frequency of 50 kHz, transformed circularly polarized light
emitted by the sample into linearly polarized light, which was analyzed
with a Glan-Thompson Polarizer (Oriel). The photocurrent of a R928
photomultiplier (Hamamatzu) was amplified with a SR560 preamplifier
(Stanford Research Systems) and processed with a SR530 lock-in-
amplifier (Stanford Research Systems) operating at 50 kHz. To measure
the total emission intensity, the excitation light was chopped at 300
Hz, and the fluorescence was analyzed with the lock-in-amplifier
operating at 300 Hz. Similar setups were reported previously.56 To avoid
artifacts that might result if the exciting light were polarized, a scrambler
(SPEX) was placed between the light source and the sample. A
FluoroMax-2 spectrometer (SPEX) with Glan Thompson polarizers was
used to make the linear fluorescence polarization measurements.
Polarized Light Microscopy and Differential Scanning Calorim-
etry. The optical microscope was a Leica DMRXP polarizing micro-
scope equipped with a Wild Leitz MPS46 Photoautomat along with a
Linkam LTS 350 hot stage and a Linkam TP 92 controller. Differential
scanning calorimetry was measured using a Perkin-Elmer DSC 7 with
a Perkin-Elmer Pyris thermal analysis data station.
Acknowledgment. We thank the NSF for support (Research
Grant CHE00-94723 and a graduate fellowship to K.E.S.P.), to
Evan Y. Lau for help with the early work on the synthesis, and
to Dr. Holger Eichhorn and Professor Timothy M. Swager for
help (at MIT) with the polarized light microscopy and DSC
measurements. The work was supported in part by the MRSEC
Program of the National Science Foundation under Award
Number DMR-98-09687 and by NSF Grant CHE98-12676 to
N.J.T..
1
Supporting Information Available: Figures showing H
NMR, 13C NMR, and IR spectra of (+)-3, (-)-3, (()-3, 4, 7,
8, 10, (+)-11, and (-)-11, comparison of the CD spectra of
(-)-3 in CHCl3 and in dodecane, CD spectra of (-)-3 in
dodecane at -5, 0, +10, and +20 °C, DSC thermograms of
(+)- and racemic 3, and the X-ray diffractogram from (+)-3
(PDF). This material is available free of charge via the Internet
at http://pubs.acs.org.
JA011706B
Transmission Electron Microscopy and Electron Diffraction.
Samples were prepared by casting films from a solution in nonane onto
a freshly cleaved mica substrate. The nonane was evaporated by heating
the samples to 160 °C. The resulting thin films were heated further to
242 °C, then cooled to 220 °C at 10 °C/min, and returned to room
(56) (a) Luk, K. C.; Richardson, F. S. J. Am. Chem. Soc. 1973, 96, 2006.
(b) Steinberg, I. Z.; Gafni, A. ReV. Sci. Instrum. 1972, 43, 409. (c)
Rexwinkel, R. B.; Schakel, P.; Meskers, S. C. J.; Dekkers, H. P. J. M.
Appl. Spectrosc. 1993, 47, 731.
(57) Grubb, D. T. J. Mater. Sci. 1974, 9, 1715.