Chiral amphiphilic self-assembled α,α′-linked quinque-, sexi-, and septithiophenes: Synthesis, stability and odd-even effects

Article abstract of DOI:10.1021/ja0607234

The synthesis, characterization, and self-assembly in butanol of a series of well-defined α,α′-linked quinqui-, sexi-, and septithiophenes substituted, via ester links at their termini, by chiral oligo(ethylene oxide) chains carrying an α, β, δ, and ε met

Full text of DOI:10.1021/ja0607234

Published on Web 04/05/2006
Chiral Amphiphilic Self-Assembled r,r′-Linked Quinque-,
Sexi-, and Septithiophenes: Synthesis, Stability and
Odd-Even Effects
Oliver Henze,† W. James Feast,*,† Fabrice Gardebien,§ Pascal Jonkheijm,‡
Roberto Lazzaroni,§ Philippe Lecle`re,‡,§ E. W. Meijer,*,‡ and
Albertus P. H. J. Schenning*,‡
Contribution from the IRC in Polymer Science and Technology, Durham UniVersity, South Road,
Durham DH1 3LE, UK, Laboratory of Macromolecular and Organic Chemistry, EindhoVen
UniVersity of Technology, P.O. Box 513, 5600 MB EindhoVen, The Netherlands, and SerVice de
Chimie des Mate´riaux NouVeaux, UniVersite´ de Mons-Hainaut, B-7000 Mons, Belgium
Received February 8, 2006; E-mail: W.J.Feast@durham.ac.uk; E.W.Meijer@tue.nl; A.P.H.J.Schenning@tue.nl
Abstract: The synthesis, characterization, and self-assembly in butanol of a series of well-defined R,R′-
linked quinqui-, sexi-, and septithiophenes substituted, via ester links at their termini, by chiral oligo(ethylene
oxide) chains carrying an R, â, δ, and ꢀ methyl, respectively, are reported. Studies of the self-assembly of
these molecules using UV/visible absorption, luminescence, and circular dichroism spectroscopies reveal,
for the sexithiophene case, that the magnitude of the observed Cotton effect in the aggregates diminishes
progressively as the chiral substituent is moved away from the thiophene segment. The stability of the
assemblies increases with the length of the oligothiophene and as the substituent chiral unit is moved
away from the aromatic core, being greatest for the unsubstituted case. The sign of the Cotton effect
alternates in an “odd/even” manner as the position of the chiral substituent is moved along the oligo-
(ethylene oxide) chain and on going from the quinquethiophene to the septithiophene having the same
side chain. Atomic force microscopy on materials deposited from solution on an aluminum or glass surface
and optical measurements show that capsules are formed from the oligothiophenes with H-type packing of
the aromatic segments.
Introduction
phenes as a function of oligomer length and position of chiral
substituent in the side chain. Previously, we have studied
Chiroptical techniques, such as circular dichroism (CD), are
commonly used to probe the secondary and tertiary structures
in biopolymers1 and have recently become a highly sensitive
tool for investigating the degree of helical order and stability
in π-conjugated polymer assemblies.2 Chiroptical properties of
these assemblies are associated with an exciton coupled CD
effect,1 and model compounds have been used to investigate
the angle between the π-conjugated segments in the helical
aggregates.3 Aggregation-induced Cotton effects have been
reported for chiral π-conjugated polymers and their related
oligomers.2 The helicity and stability of such assemblies are
important for any putative application since the details of
organization will influence the macroscopic properties of the
final supramolecular materials.2
polythiophenes substituted with chiral side chains and found
that the helical packing is dependent on both the absolute
configuration of the stereocenter and its distance from the rigid
core.4 Now, we have synthesized R,R′-linked oligothiophenes
with five, six, or seven thiophene rings, with penta(ethyleneox-
ide) substituents attached via ester links at the terminal
R-positions (Chart 1) and carrying a stereocenter at four different
locations on the substituent.5 The self-assembly process shows
odd-even effects, as revealed by the sign of the Cotton effect
in the aggregates, with respect to both the position of the chiral
substituent and the length of the oligomer sequence. Odd-even
effects have initially been reported for liquid crystals6 and
afterward for polymers having mesogenic substituents.7 Recently
such effects have been found for a variety of self-assembled
We report here a systematic study of the stability and chiral
aggregation in butanol of a set of R,R′-substituted oligothio-
(4) Lermo, E. R.; Langeveld-Voss, B. M. W.; Janssen, R. A. J.; Meijer, E. W.
Chem. Commun. 1999, 791.
(5) The properties of T6âS and T6 have been reported earlier: (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) Schenning, A. P. H. J.; Kilbinger,
A. F. M.; Biscarini, F.; Cavallini, M.; Cooper, H. J.; Derrick, P. J.; Feast,
W. J.; Lazzaroni, R.; Leclere, Ph.; McDonell, L. A.; Meijer, E. W.; Meskers,
S. C. J. J. Am. Chem. Soc. 2002, 124, 1269.
(6) (a) Gray, G. W.; McDonell, D. G. Mol. Cryst. Liq. Cryst. 1977, 34, 211.
(b) Goodby, J. W.; Chin, E.; Leslie, T. M.; Geary, J. M.; Patel, J. S. J. Am.
Chem. Soc. 1986, 108, 4729. (c) Goodby, J. W., Gray, G. W., Spiess, H.-
W., Vill, V., Eds.; Handbook of Liquid Crystals; Wiley-VCH: Weinheim,
1998.
† Durham University.
‡ Eindhoven University of Technology.
§ Universite´ de Mons-Hainaut.
(1) Circular Dichroic Spectroscopy; Harada, N., Nakanishi, K., Eds.; University
Science Books: Mill Valley, CA, USA; Oxford University Press: Oxford,
1983.
(2) Hoeben, F. J. M.; Jonkheijm, P. Meijer, E. W.; Schenning, A. P. H. J.
Chem. ReV. 2005, 105, 1491.
(3) Langeveld-Voss, B. M. W.; Beljonne, D.; Shuai, Z.; Janssen, R. A. J.;
Meskers, S. C. J.; Meijer, E. W.; Bre´das, J. L. AdV. Mater. 1998, 10, 1343.
9
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J. AM. CHEM. SOC. 2006, 128, 5923-5929
5923

A R T I C L E S
Henze et al.
Chart 1. Chiral Oligothiophenes Used in This Study; for Codes See Text
systems;8 however, for π-conjugated systems odd-even effects
have been seldom observed.4,9 The stability of the assembly
increases with the length of the oligothiophene sequence and
as the chiral unit is moved away from the aromatic core, being
largest for the unsubstituted case. Under the conditions adopted
in this study the oligomers primarily form hollow capsules in
solution,10 presumably being present as self-assembled chiral
domains in the shell of the macroscopic capsules. In earlier
studies of these systems, rodlike helical assemblies and flat
“creˆpes” have been generated on a silicon surface by slow
evaporation of THF solutions;11 it is becoming clear that the
exact course of self-assembly in these systems is a sensitive
function of the experimental protocol adopted (solvent, substrate,
rate of formation, temperature, and deposition method).
terminal R-positions. A simple code has been adopted for iden-
tifying the chiral molecules discussed, consisting of an Arabic
numeral followed by a Greek letter plus either S or R. All the
chiral molecules considered have a structural motif (Figure 1)
Figure 1. Structural motif for S-chiral molecules.
in which the unit M is identified by a number and the sub-
stituents R1-R4 are always one methyl and three hydrogen
atoms. When the methyl substituent is at R1 it is identified by
the letter R, at R2 by â, at R3 by δ, and at R4 by ꢀ; R or S
identifies the chirality at the methyl substituted carbon. The
target molecules were the substituted R,R′-linked quinque-, sexi-,
and septithiophenes, and these are identified as T5, T6, and T7,
respectively.
Results and Discussion
Synthesis. A series of chirally substituted (ethylene oxide)s
were prepared and used to derivatize oligothiophenes at their
Schemes 1 and 2 show the route adopted for the synthesis of
the chiral alcohols 11RS, 11âS, 11δS, and 11ꢀS starting from
3S and 4S. The latter molecules are made from ethyl (S)-(-)-
lactate (purity of 98%) and methyl (R)-(+)-lactate (98% pure
with an ee of >96%), respectively.11 The ee value of ethyl (S)-
(-)-lactate was determined by gas chromatography (GC) on a
capillary column with a permethylated â-cyclodextrin stationary
phase yielding an ee > 99.5%.12 Part of this synthetic route is
based on earlier work that also showed the fidelity of keeping
the enantiomeric excess as high as that of the starting materials.13
The chiral alcohol 11âR is not shown in the schemes but was
synthesized in an analogous way to that described previously
(7) (a) Ramos, E.; Bosch, J.; Serrano, J.-L.; Sierra, T.; Veciana, J. J. Am. Chem.
Soc. 1996, 108, 4729. (b) Amabilino, B.; Ramos, E.; Serrano, J. L.; Veciana,
J. AdV. Mater. 1998, 10, 1001. (c) Amabilino, B.; Ramos, E.; Serrano,
J.-L.; Sierra, T.; Veciana, J. J. Am. Chem. Soc. 1998, 120, 9126.
(8) For example: (a) Aoki, K.; Kudo, M.; Tamaoki, N. Org. Lett. 2004, 6,
4009. (b) Itoh, T.; Shichi, T.; Yui, T.; Takagi, K. Langmuir 2005, 21, 3217.
(c) Ishi-I, T.; Iguchi, R.; Snip, E.; Ikeda, M.; Shinkai, S. Langmuir 2001,
17, 5825. (d) Li, Y.; Li, P.; Wang, J.; Wang, Y.; Yan, H.; Thomas, R. K.
Langmuir 2005, 21, 6703. (e) Sumiyoshi, Y.; Nishimura, K.; Nakano, M.;
Handa, T.; Miwa, Y.; Tomioka, K. J. Am. Chem. Soc. 2003, 125, 12137.
(9) An odd-even effect concerning the position of chiral center has been
observed in the circular dichroism spectrum of polyfluorene films: Oda,
M.; Nothofer, H.-G.; Scherf, U.; Sunjic, V.; Richter, D.; Regenstein, W.;
Neher, D. Macromolecules 2002, 35, 6792.
(10) Shklyarevskiy, I. O.; Jonkheijm, P.; Christianen, P.; Schenning, A. P. H.
J.; Meijer, E. W.; Henze, O.; Kilbinger, A. F. M.; Feast, W. J.; de Guerzo,
A.; Desvergne, J.; Maan, J. C. J. Am. Chem. Soc. 2005, 127, 1112.
(11) Leclere, Ph.; Surin, M.; Lazzaroni, R.; Kilbinger, A. F. M.; Henze, O.;
Jonkheijm, P.; Biscarini, F.; Cavallini, M.; Feast, W. J.; Meijer, E. W.;
Schenning, A. P. H. J. J. Mater. Chem. 2004, 14, 1959.
(12) Janssen, H. M.; Peeters, E.; v. Zundert, M. F.; v. Genderen, M. H. P.;
Meijer, E. W. Macromolecules 1997, 30, 8113.
9
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Chiral Self-Assembled
R
,
R′-Linked Oligothiophenes
A R T I C L E S
Scheme 1. Synthesis of the S-Enantiomers of the Oligo(ethylene
oxide) 9RS, 9âS, 11RS, and 11âS
coupling reaction in the last step of the synthesis. The red crude
products obtained were readily soluble in chloroform and were
purified by repeated reprecipitation and size exclusion chro-
matography using BioBeads.14
All the key compounds, T5âS, T6, T6RS, T6âS, T6δS,
T6ꢀS, T7âS, T5âR, T6âR, and T7âR, used in the subsequent
1
studies were characterized via correct elemental and H NMR
and 13C NMR analyses and appropriate spectroscopic proper-
ties.14 Also the sequences used are known to yield nonracemized
products.13 However, using MALDI-TOF MS and high-tem-
perature GPC, we established that there were trace amounts of
impurities in some compounds.14 Thus, some had traces of the
compound in question with one EO unit less than required,
which arises from the difficulty in obtaining a perfect separation
of the homologues 9RS/11δS and 9âS/11ꢀS (see Scheme 2),
and some had additional thiophene residues in the aromatic core,
arising from the well-documented multiple coupling side
reactions associated with Stille coupling.15 The estimated purities
of the samples are all well above 96%. The trace impurities did
not influence the self-assembly behavior of interest here since
a self-consistent argument describes the whole set of compounds
irrespective of whether trace impurities were detected; neverthe-
less it seems proper to record their existence since it may be
relevant in future extensions of this work.
Scheme 2. Synthesis of the S-Enantiomers of the Oligo(ethylene
oxide) 11δS and 11ꢀS
Self-Assembly in Solution. The UV/vis absorption spectrum
of T5âS in tetrahydrofuran (THF) shows a maximum at λ )
433 nm. For all T6 derivatives, this maximum is located at λ
) 435 nm, while, for T7âS, it occurs at λ ) 447 nm. In the
fluorescence spectra, maxima are found at λ ) 511, 530, and
548 nm for T5âS, all T6 derivatives, and T7âS, respectively.
The position of the maximum depends on the conjugation length,
and the shape of the spectra are similar to those reported earlier
for molecularly dissolved R,R′-terminally disubstituted oligo-
thiophenes.16 The self-assembly of the oligothiophenes was
studied in butanol. A solution of T5âS in butanol at 300 K
shows a UV/vis spectrum similar to that recorded in THF, and
the absorption maximum is located at λ ) 438 nm (Figure 2a).
Upon cooling to 213 K, this band shifts to the blue by 63 nm,
λ ) 375 nm, indicating H-type aggregation. For the T6
derivatives, the shift upon cooling depends on the position of
the stereocenter. The smallest blue shift is observed for T6RS
(37 nm), while T6âS shifts by 52 nm, T6δS shifts by 62 nm,
and T6ꢀS shifts by 65 nm (Figure 2c). The largest shift (69
nm) was observed for T6a indicating that the sexithiophenes,
lacking the methyl group of the stereocenter, show a stronger
exciton coupling probably as a result of a better packing (vide
infra). For T7âS in butanol, a similar blue shift from λ ) 450
nm at 343 K to λ ) 410 nm at 273 K is found (Figure 2d).
Hence, it is now clear why absorption data reported in the
literature for similar aggregated oligothiophenes can change
from derivative to derivative depending on a very subtle
difference in the side chain.17
for the alcohol 11âS.5,14 Base-catalyzed reactions of the tosylated
tetraethylene oxide 2 with the monoprotected chiral glycol
derivatives 3S and 4S followed by cleavage of the THP or
benzyl protecting group gave the alcohols 11RS and 11âS,
respectively. The shorter molecules 9RS and 9âS were obtained
using the tosylated tri(ethylene glycol) 1 and the analogous
sequence of reactions (Scheme 1).
Reaction of 9RS and 9âS with the tosylated achiral benzyl
protected glycol 12 and cleavage of the benzyl protecting group
gave the alcohols 11δS and 11ꢀS (Scheme 2).
The synthesis route for the target molecules T5âS, T6RS,
T6âS, T6δS, T6ꢀS, and T7âS is shown in Scheme 3. The
respective enantiomeric compounds T5âR, T6âR, and T7âR
are not shown but were synthesized in an analogous way.14
Reaction of the chiral alcohols 11RS, 11âS, 11δS, and 11ꢀS
with 2-bromothiophene-5-carbonyl chloride 13 using pyridine
as base gave the esters 14RS, 14âS, 14δS, and 14ꢀS, respec-
tively. Following the strategy published in our previous work,5
the required oligothiophene units were formed by a Stille cross-
(15) (a) Stille, J. K. Angew. Chem., Int. Ed. Engl. 1986, 25, 508. (b) Espinet,
P.; Echavarren, A. M. Angew. Chem., Int. Ed. 2004, 43, 4704.
(16) (a) Ba¨uerle, P. Electronic Materials: The oligomer Approach; Mu¨llen, K.,
Wegner, G., Eds.; VCH: Weinheim, 1998. (b) Katz, H. E.; Dodabalapur,
A.; Torsi, L.; Elder, D. Chem. Mater. 1995, 7, 2238. (c) Katz, H. E. J.
Mater. Chem. 1997, 7, 369. (d) Katz, H. E.; Laquindanum, J. G.; Lovinger,
A. J. Chem. Mater. 1998, 10, 633. (e) Garnier, F.; Yassar, A.; Hajlaoui,
R.; Horowitz, G.; Deloffre, F.; Servet, B.; Ries, S.; Alnot, P. J. Am. Chem.
Soc. 1993, 115, 8716. (f) Parakka, J. P.; Cava, M. P. Tetrahedron 1995,
51, 2229.
(13) (a) Mori, K. Tetrahedron 1976, 32, 1101. (b) Perkins, M. V.; Kitching,
W.; Ko¨ning, W. A.; Drew, R. A. I. J. Chem. Soc., Perkin Trans. 1 1990,
2501. (c) Cowie, J. M. G.; Hunter, H. W. Makromol. Chem. 1990, 191,
1393. (d) Chiellini, E.; Galli, G.; Carrozzino, S. Macromolecules 1990,
23, 2160. (e) Brunsveld, L.; Zhang, H.; Glasbeek, M.; Vekemans, J. A. J.
M.; Meijer, E. W. J. Am. Chem. Soc. 2000, 122, 6175.
(14) See Supporting Information.
(17) Kawano, S.-i.; Fujita, N.; Shinkai, S. Chem.sEur. J. 2005, 11, 4735.
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Scheme 3. Synthesis of the S-Enantiomers of the Oligothiophenes T5âS, T6RS, T6âS, T6δS, T6ꢀS, and T7âSa
a Where for RS R1 ) CH3, R2, R3, R4 ) H; for âS R2 ) CH3, R1, R3, R4 ) H; for δS R3 ) CH3, R1, R2, R4 ) H; and for ꢀS R4 ) CH3, R1, R2,
R3 ) H. For T5âS m ) 3; for T6RS, T6âS, T6δS, and T6ꢀS m ) 4; and for T7âS m ) 5.
Figure 2. Temperature-dependent UV/vis and emission spectra in butanol (2.6 × 10-5 M) for (a) T5âS, (b) T6RS, and (d) T7âS. UV/vis spectra at 283
K are plotted for all T6 derivatives in (c). The direction of the arrows indicates an increase in temperature (see text).
The fluorescence intensity of solutions of T5âS in butanol
at 213 K was reduced when compared to that found at 300 K,
and the maximum was red shifted to λ ) 579 nm, typical for
H-aggregates (Figure 2a). All T6 derivatives show an emission
maximum at λ ) 610 nm below 300 K, while above 370 K, a
much more intense emission is found having a maximum at λ
) 508 nm (Figure 2b). In the case of T7âS, a similar red shift
and quenching of the emission are observed on going from λ
) 540 nm at 343 K to λ ) 625 nm at 273 K (Figure 2d).
Odd-Even Effects in Chiral Packing. For T5âS, a bisignate
Cotton effect is found at 213 K, while at 300 K no Cotton effect
exists. The Cotton effect shows a positive sign at lower energy
(λ[+] ) 407 nm) and a negative Cotton effect at higher energy
(λ[-] ) 347 nm) revealing a right-handed helical packing (Figure
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Chiral Self-Assembled
R
,
R′-Linked Oligothiophenes
A R T I C L E S
Figure 3. (a) CD spectra of T5âS, T6âS, and T7âS in butanol (2.6 × 10-5 M) at 283 K. (b) CD spectra for all chiral T6 derivatives (8 × 10-5 M) at
283 K.
3). The zero-crossing at the absorption maximum of the
chromophore absorption indicates exciton coupling as a result
of chiral aggregation.1 The sign of the Cotton effect of T5âS is
opposite to that observed for the corresponding T6âS aggregates
(λ[-] ) 419 nm and λ[+] ) 375 nm, left-handed helicity) but
similar to that found for the aggregates of T7âS (λ[+] ) 425
nm and λ[-] ) 394 nm, right-handed helicity) (Figure 3a).
Remarkably, the behavior is different from T4, T5, T6 or-
ganogelators bearing similar cholesteric side chains that did not
show an alternation of the sign of the Cotton effect.17 As
expected, the sign of the Cotton effect is reversed for aggregates
of T6âR compared to T6âS (Figure 3b) having a stereocenter
with the opposite configuration. The sign of the Cotton effect
also depends on the position of the stereocenter in the substituent
(Figure 3b). For odd-numbered positions, i.e. T6RS and T6ꢀS,
the low energy side has a positive sign, while the even
numbered, i.e., T6âS and T6δS, have a negative sign. Interest-
ingly, the magnitude of the Cotton effect decreases upon moving
the stereocenter away from the chromophore, as seen when
comparing T6ꢀS with T6RS and T6δS with T6âS. In addition,
the intensity of the Cotton effect increases upon extending the
conjugation length, and remarkably, the sign also depends on
the number of thiophene rings; compare T5âS, T6âS, and T7âS
(Figure 3a). The observed odd-even effects in the optical
activity of the sexithiophene assemblies are similar to those
reported earlier for â-substituted polythiophenes4 and are in
agreement with the established ROD-REL rules for cholesteric
liquid crystals and helical polymers.6,7
Stability of Aggregates. When the UV/vis maxima for all
T6 derivatives at the same concentration are plotted versus
temperature, the phase-transition curves shift to higher temper-
atures (Figure 4) as the stereocenter is moved further away from
the thiophene segment. The Tm of the thiophene stacks, as
recorded by cooling a sample at a rate of 10 K/h, is located at
288 K (T6RS), 316 K (T6âS), 323 K (T6δS), 328 K (T6ꢀS),
and 330 K (T6) showing that the achiral stacks are the most
stable. The enthalpy gain computed from the slope of the curves
(Figure 4a) is lowest for T6RS (-11 kJ/mol) and highest for
T6 (-28 kJ/mol). The differences in the transition curves reflect
a better packing of the T6 segments when the steric hindrance
of the stereocenter in the side chains is removed. This behavior
indicates that the preferred orientation is similar to that found
for oligothiophenes in the solid state in which the angle between
the long axis of adjacent oligothiophenes is close to zero.16 In
the case of methyl substitution, this group sterically hinders an
angle of 0°. As a result, both the position of the stereocenter
on the side chain and the conjugation length of oligothiophenes
determine the stability of the stacks (Figure 4b). Thus, the
observation that more π-π interactions enhance the stability
of the stacks, as indicated by the change of the Cotton effect
intensity in aggregates obtained upon cooling solutions of T5âS,
T6âS, and T7âS, is similar to earlier conclusions from studies
of a series of oligothiophene organogelators17 and stacked oligo-
(p-phenylenevinylene)s.18
Molecular Modeling. We have modeled the assembly of the
oligothiophenes (starting from T7âS), with a combination of
density functional theory calculations (for the internal structure
of the molecules) and force-field molecular mechanics and
molecular dynamics (MD) simulations (for the structure of
stacks). To take into account both the lateral and the in-stack
intermolecular interactions, the model system studied consisted
of two parallel columns of eight molecules of T7âS. MD
simulations14 reveal two stable structures corresponding to right-
and left-handed helices. The molecules within a stack are not
in perfect registry (i.e., they are slightly shifted and rotated)
with respect to each other, while the planes of the T7 segments
remain parallel to each other. As a result, the molecular
organization within the stacks, seen from the side of a single
column in Figure 5, clearly shows a helical character. The mean
distance between the π-systems of adjacent molecules is 3.7 (
0.5 Å, which is typical of π-stacked oligothiophenes.16 The
difference in the energy calculated for the left- and right-handed
assemblies is 200 kJ/mol in favor of the right-handed assembly.
No interconversion occurs between the two structures during
the 800 ps of the simulation at 300 K. This is in agreement
with the experimental results: aggregates of T7âS display a
positive Cotton coupling (Figure 3a), which is the signature of
right-handed helicity.1 The methyl groups at the chiral centers
uniformly point upward (downward) in the groups on the left
(right) side of the stack. The steric hindrance of the methyl
groups at the chiral centers probably induces the longitudinal
shift and rotation of the molecules in the stacks, giving rise to
the helical structure. The angle measured between the long axis
of neighbor molecules in the stack is 7 ( 1° and is similar for
both left- and right-handed assemblies. Based on this angle, a
full turn would require about 50 molecules, which would
(18) Jonkheijm, P.; Hoeben, F. J. M.; Kleppinger, R.; van Herrikhuyzen, J.;
Schenning, A. P. H. J.; Meijer, E. W. J. Am. Chem. Soc. 2003, 125, 15941.
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Figure 4. (a) Phase transition curves of T6 derivatives at 2.6 × 10-5 M in butanol based on UV/vis data. (b) Phase transition curves of T5âS (0), T6âS
(O), and T7âS (4) at 2.6 × 10-5 M in butanol based on CD data. (c) Melting temperature Tm versus number of thiophene rings and substitution pattern.
Figure 5. Side view of a column of T7âS molecules. The hydrogen atoms are omitted, for the sake of clarity. The carbon atoms of the methyl groups at
the chiral centers are represented as green balls. In the top molecule, the T7 segment is oriented parallel (a) or perpendicular (b) to the page; views c, d, and
e correspond to successive rotations by 15° along the vertical axis, starting from b.
correspond to a helical pitch of around 18 nm. Analogous results
were obtained for the other enantiomer T7âR. However, in this
case the left-handed helix is the most stable structure.
Two series of simulations were also carried out for two
adjacent columns of T6âR molecules and T5âS molecules,
respectively. In those cases, also distances between the cofacial
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R
,
R′-Linked Oligothiophenes
A R T I C L E S
Figure 6. Images of drop cast solution of T6âS (butanol). (a) SEM image on an Al surface (scale bar represent 100 nm). TM-AFM images on glass, (b)
height, 5 × 5 µm2 scale 25 nm; (c) 1.8 × 1.8 µm2 scale 15 nm.
Conclusions
molecules is found to be 3.7 ( 0.5 Å for both, while the angle
measured between the main axis of neighbor molecules within
a stack is 2 ( 1° and 4 ( 1° for T6âR and T5âS, respectively.
For the T5âS molecular stack, the right-handed helix and, for
the T6âS molecular stack, the left-handed helix were obtained.
These data are in full agreement with the experimental data,
providing additional evidence for the odd-even effect. Note
should be made that in the modeling of the T6âS assemblies
three interconverting structures were observed: a right-handed
helix equilibrated via a straight column with a left-handed helix,
with the latter having the highest stability. The smaller calculated
energy difference between supramolecular assemblies formed
by T6 in comparison with T5 and T7 could be due to the
difference in point group.19,20 Even-numbered oligothiophenes
possess a quasi-inversion center resulting in C2h symmetry, while
odd-numbered oligothiophenes have a C2 axis in the molecular
plane and perpendicular to the long molecular axis (C2V). This
implies that for odd-numbered oligothiophenes the transition
dipole moment is perfectly aligned with the long molecular axis,
while for even oligomers there is an offset resulting in a different
packing of odd- and even-numbered thiophenes. Finally, another
interesting value that can be derived from the simulations is
the average intermolecular interaction energy per molecule, ETn.
The ET5, ET6, and ET7 energies are calculated to be 661, 741,
and 795 kJ/mol, respectively,21 showing that the molecular sim-
ulations are consistent with the increasing values of the melting
temperature measured for the T5, T6, and T7 aggregates.
We have comprehensively studied the chiroptical properties
of a series of chiral self-assembled oligothiophenes. These
studies show that the stability of the assemblies depends on the
π-conjugation length and the position of the stereocenter relative
to the π-conjugated core. Odd-even effects are observed in
the sign of the Cotton effect as the position of the chiral sub-
stituent is moved along the oligo(ethylene oxide) chain. More-
over, the sign alternates on going from the quinquethiophene
to the septithiophene having the same side chain. The stability
of the assemblies is the largest for the achiral sexithiophenes,
which reveals a competition between the packing of the
π-conjugated segments and the chiral ethylene oxide side chains.
It also shows that the perfect packing of oligo- and poly-
thiophenes is found in which the angle between the long axis
of adjacent thiophene segments is zero. Our results give some
initial design rules for the construction of helical assemblies,
which is important for any putative applications.
Acknowledgment. This research was carried out in the
framework of the European Commission Training and Mobility
of Researchers Networks LAMINATE (Contract Number
HPRN-CT-2000-00135). We thank the EPSRC and Durham
University for provision of facilities. This work was carried out
in the framework of the EU-Integrated Project NAIMO (NMP4-
CT-2004-500355). Research in Mons is partly supported by the
InterUniversity Attraction Pole Program (PAI V/3) of the
Belgian Federal Government, the European Commission, the
Government of the Region of Wallonia (Phasing Out -
Hainaut), and the Belgian National Fund for Scientific Research
FNRS/FRFC. Ph.L. is Chercheur Qualifie´ of the “Fonds National
de la Recherche Scientifique” (FNRS-Belgium). The research
in Eindhoven was supported by the Council for Chemical
Sciences of The Netherlands Organization for Scientific Re-
search (CW-NWO).
Shape of Aggregates. The shape of the aggregates of T6âS
in butanol solution was studied by dynamic light scattering
(DLS). Spherical objects were revealed having an average radius
of 55 nm at 293 K, which could be visualized by scanning
electron microscopy (SEM, Figure 6a) and tapping mode atomic
force microscopy (TM-AFM, Figure 6b,c). The shapes of the
aggregates are similar as previously found in 2-propanol
solution.10 Clusters of the capsules were observed when a
concentrated solution was applied to a glass surface (2 × 10-3
M, Figure 6b). The AFM images showed spherical objects 100
nm in diameter, based on the height using low tip forces. Isolated
and much smaller spheres of 20 nm in diameter were observed
after depositing a 2 × 10-5 M solution.
Supporting Information Available: Experimental procedures
for all compounds and techniques and theoretical methodology
used. This material is free of charge via the Internet at
http://pubs.asc.org.
JA0607234
The assemblies are presumed to be constructed from a layer
of T6âS molecules forming capsules, i.e., hollow spheres, simi-
lar to those found for oligo(p-phenylenevinylene)s and oligo-
thiophene-based surfactants, which form vesicles in water.10,22
It is difficult to construct vesicles from helically organized oligo-
thiophenes. Therefore, we believe that self-assembled chiral
domains are present in the shell of the macroscopic capsules.
(19) Meinardi, F.; Cerminara, M.; Borghesi, A.; Spearman, P.; Bongionvanni,
G.; Mura, A.; Tubino, R. Phys. ReV. Lett. 2002, 89, 157403-1
(20) Kouki, F. J. Chem. Phys. 2000, 113, 385.
(21) The values are high for π-π interactions which points to additional
secondary interactions such as side chain association.
(22) (a) Jiang, L.; Hughes, R. C.; Sasaki, D. Y. Chem. Commun. 2004, 1028.
(b) Hoeben, F. J. M.; Shklyarevskiy, I. O.; Pouderoijen, M. J.; Engelkamp,
H.; Schenning, A. P. H. J.; Christianen, P. C. M.; Maan, J. C.; Meijer E.
W. Angew. Chem., Int. Ed. 2006, 45, 1232.
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