Enantiomeric substituents determine the chirality of luminescent conjugated polythiophenes

Article abstract of DOI:10.1021/ma048859e

Chiral isomers of 3-substituted polythiophenes with amino acid functiontionalized side chains are compared. The polymers show pH-dependent absorption, emission, and circular dichroism spectra in buffered aqueous solution. At pH equal to pi of the amino ac

Full text of DOI:10.1021/ma048859e

6316
Macromolecules 2004, 37, 6316-6321
Enantiomeric Substituents Determine the Chirality of Luminescent
Conjugated Polythiophenes
K. P eter R. Nilsson ,*,† J oh a n D. M. Olsson ,‡ P eter Kon r a d sson ,‡ a n d Olle In ga n a1s†
Department of Physics and Measurement Technology, Biology, and Chemistry, Linko¨ping University,
SE-581 83 Linko¨ping, Sweden
Received J une 10, 2004
ABSTRACT: Chiral isomers of 3-substituted polythiophenes with amino acid functiontionalized side chains
are compared. The polymers show pH-dependent absorption, emission, and circular dichroism spectra in
buffered aqueous solution. At pH equal to pI of the amino acid, the backbones adopt nonplanar helical
conformations, and the polymer chains are separated from each other. Increasing pH leads to more planar
conformations of the backbones and an aggregation of the polymer chains occurs. A lower pH will also
lead to more planar conformation of the backbones, but aggregation of the polymer chains appears to be
absent. The nonplanar to planar transition of the polymer backbone and the separation/aggregation of
different polymer chains is not affected by stereochemistry of the zwitterionic side chain. The two isomers
have almost identical pH-dependent absorption and emission spectra. However, the chirality of the
zwitterionic side chain is reflected in the conformation of the polymer backbone, giving rise to a right-
handed and left-handed helical form of polythiophene chains since the induced circular dichroism patterns
of the two polymers are mirror images.
Sch em e 1a
In tr od u ction
Chiral conjugated polymers (CPs) with a well-defined
structure are of interest, due to their potential for being
used in optoelectronic devices, biosensors, and as arti-
ficial enzymes. Especially, polythiophenes (PTs)1-8 with
an optically active substituent in the 3-position have
been studied for these purposes. Chiral polythiophenes
normally exhibit optical activity in the π-π* transition
region, derived from the main-chain chirality when the
polymer chains are forming supramolecular, π-stacked
self-assembled aggregates in a poor solvent or at low
temperature, whereas they show no activity in the UV-
vis region in a good solvent or at high temperatures.5,9-12
Recent studies,13-16 using optically inactive CPs that
become chiral upon addition of a chiral guest, show that
chirality introduction can also be a result of an acid-
base complexation between the CP and the chiral guest,
forming a one-handed helical structure with a preferred
twist reflecting the stereochemistry of the chiral
guest.13-16
a
(i) TsCl, pyridine, CHCl3, 86 %; (ii) N-t-Boc-D-Ser, K2CO3,
DMF, 35 °C, 57 %; (iii) CH2Cl2/TFA (1:1), quant.; (iv) FeCl3,
TBA-trifluoromethanesulfonate, CHCl3, 15 °C, 61 %; and (v)
Andersson et al.4
Natural biopolymers, such as proteins and DNA,
frequently have one-handed helical conformations that
contribute to the three-dimensional ordered structure
and the specific function of the biopolymer. The prefer-
able twists of the helical conformations are caused by
homochirality of their components (D-sugars and L-
amino acids). Normally, a polypeptide made from L-R-
amino acid residues forms a right-handed helix, and the
exclusive one-handed helical conformation is of great
importance, as the stereochemistry of biomolecules are
widely used to create biospecific interactions and the
specificity of enzymatic reactions.
ena due to different electrostatic interactions and
hydrogen-bonding patterns within the polymer chain
and between adjacent polymer chains. At a pH equal
to the pI of the amino acid, the backbone adopts a
nonplanar right-handed helical conformation, and the
polymer chains are separated from each other. Increas-
ing pH leads to a more planar conformation of the
backbone, and an aggregation of the polymer chains
occurs. A lower pH will also lead to a more planar
conformation of the backbone, but aggregation of the
polymer chains appears to be absent. The polymer
adopts a right-handed helical form, so apparently the
nature of the L-amino acid is reflected in the helical
conformation of the polymer backbone. Consequently,
it would be of great interest to synthesize the polymer
using the D-form of serine to see if the twist of the helix
is reversed.
Previous studies4,17,18 of a polythiophene, with a free
L-amino acid side chain, poly(3-[(S)-5-amino-5-carboxyl-
3-oxapentyl]-2,5-thiophenylene hydrochloride), L-POWT
(Scheme 1), have shown pH-dependent optical phenom-
* Corresponding author. E-mail: petni@ifm.liu.se.
† Department of Physics.
In this article, we report the synthesis and the optical
properties of a polythiophene with a free D-amino acid
‡ Department of Chemistry.
10.1021/ma048859e CCC: $27.50 © 2004 American Chemical Society
Published on Web 07/27/2004

Macromolecules, Vol. 37, No. 17, 2004
Chirality of Luminescent Conjugated Polythiophenes 6317
Ma tr ix-Assisted La ser Desor p tion /Ion iza tion Tim e-of-
F ligh t Ma ss Sp ectr oscop y (MALDI-TOF -MS). A total of
0.5 µL of an aqueous solution of the polymer and 0.5 µL of
R-cyano-4-hydroxy-trans-cinnamic acid (CHCA) in 0.1% TFA/
acetonitrile (1:1) were mixed and evaporated on a target plate.
The spectra were recorded in linear positive mode with a
Voyager-DE STR Biochemistry Workstation.
Sp ectr oscop ic Exp er im en ts. A stock solution containing
5.0 mg of polymer/mL (L-POWT or D-POWT) in deionized
water was prepared and placed on a rocking table for 1 h. The
following buffer solutions were prepared:
side chain, poly (3-[(R)-5-amino-5-carboxyl-3-oxapentyl]-
2,5-thiophenylene hydrochloride), D-POWT (see Scheme
1), and the comparison of the chiral isomers.
Exp er im en ta l P r oced u r es
Gen er a l Meth od s. Normal workup means drying the
organic phase with MgSO4 (s) or Na2SO4 (s), filtering, and
evaporation of the solvent in vacuo at ∼45 °C. All dry solvents
were collected onto 4 Å predried molecular sieves (Merck).
Thin-layer chromatography (TLC) was carried out on 0.25 mm
precoated silica gel plates (Merck silica gel 60 F254) detected
by UV-abs. (254 nm) and/or by charring with PAA-dip (ethanol/
sulfuric acid/p-anisaldehyde/acetic acid 90:3:2:1) followed by
heating to ∼250 °C. FC means flash column chromatography
using Silica gel (MERCK 60 (0.040-0.063 mm)). 1H and 13C
NMR spectra were performed on a Varian Mercury 300 MHz
instrument at 25 °C. Chemical shifts are given in ppm relative
20 mM sodium acetate (pH 4.0), 20 mM Mes (pH 5.9), and
20 mM sodium carbonate (pH 10.0). All the chemicals used
were of analytical grade.
For the absorption, emission, and circular dichroism (CD)
measurements, 15 µL of the stock solution was diluted with
one of the buffer solutions to a final concentration of 37.5 µg
of polymer/mL solvent, and the sample was placed on a rocking
table for 1 h before the spectrum was recorded. The procedure
was repeated for all the buffer system and deionized water.
Optical spectra were recorded on a Perkin-Elmer Lambda 9
UV/vis/NIR spectrophotometer for UV/vis, a Hitachi F4500
Fluorescence Spectrophotometer for fluorescence, and an I. S.
A. J obin-Yvon CD6 (5 mm quartz cell) for CD.
1
to TMS in CDCl3 (δ 0.00) for H and 13C or CD3OD (δ 3.31) for
1H and CD3OD (δ 49.0) for 13C NMR. Optical rotations were
recorded at room temperature with
a Perkin-Elmer 141
polarimeter. IR spectra were recorded as KBr pellets on a
Perkin-Elmer SPECTRUM 1000 FTIR spectrometer.
Syn t h esis of (R)-2-ter t-Bu t oxyca r b on yla m in o-3-(2-
th iop h en -3-yl-eth oxy)-p r op ion ic Acid (1). 3-Thiopheneeth-
anol (0.75 g, 5.85 mmol) was dissolved in CHCl3 (30 mL) and
cooled to 0 °C. Pyridine (1.5 mL, 18.64 mmol) and p-toluene-
sulfonyl chloride (2.78 g, 14.58 mmol) were added. After 24 h,
the reaction was quenched by adding H2O (6 mL) and was
diluted with Et2O (40 mL). The organic layer was washed with
2 M HCl (2 × 15 mL), sat. NaHCO3 (aq) (2 × 15 mL), and
H2O (2 × 15 mL) and was subjected to normal workup. FC
(toluene) and recrystallization from EtOAc/hexane afforded
2-(3-thienyl)ethanol tosylate (1.42 g, 5.03 mmol, 86%) as white
crystals (Rf ) 0.74, toluene/EtOAc 4:1). 1H NMR was in
accordance with those previously reported.19 N-t-Boc-D-Ser
(1.85 g, 9.02 mmol) and tosylated product (1.42 g, 5.03 mmol)
were dissolved in dry DMF (100 mL) under N2 atmosphere.
The solution was heated to 35 °C, and K2CO3 (2.42 g, 17.51
mmol) was added. After 26 h, the mixture was poured over
cold 2 M HCl (aq) (150 mL) and washed with Et2O (3 × 50
mL). The organic layer was washed with 1 M HCl (aq) (2 ×
50 mL) and H2O (2 × 50 mL), subjected to normal workup,
and purified by FC (toluene/EtOAc 4:1) to give 1 (0.90 g, 2.87
mmol, 57%) as a colorless syrup. Rf ) 0.57 (toluene/EtOAc 1:1);
[R]D ) +6.1 (c 2.0, CHCl3) (for the S-enantiomer [R]D ) -6.1
(c 2.0, CHCl3)).
Resu lts a n d Discu ssion
3-Thiophene ethanol was converted to corresponding
tosylate as described earlier20 and then displaced by a
protected unnatural amino acid, N-t-Boc-D-Ser. The Boc
group was removed in CH2Cl2/TFA to readily enable the
ammonium salt for polymerization. There are many
well-known methods to polymerize thiophenes, which
have been reviewed by, for example, McCullough.21
Because of the functional groups of the amino acid
substituted thiophene monomer, the polymerization was
performed by a method reported by Sugimoto et al.
using chemical oxidation with anhydrous ferric chloride
in chloroform22,23 (see Scheme 1). The water-soluble
polymer was then precipitated by adding acetone and
concentrated hydrochloric acid.
Lately, MALDI-TOF-MS has been developed as a
powerful tool when analyzing synthetic polymers, both
for chain-length studies and for end-group analysis of
different polymers. Different techniques considering the
use of matrix, cationization agents, and the matrix/
analyte ratio have also been investigated, regarding the
use of MALDI-TOF-MS as a tool for the determina-
tion of the chain-length distribution in conjugated
polymers.24-26 The length of the polythiophene backbone
of the conjugated polymers described herein was further
elucidated by MALDI-TOF-MS.
An aqueous solution of the polymer was mixed with
a CHCA in 0.1% TFA/acetonitrile (1:1) as matrix. This
matrix has earlier shown low grade of fragmentation
for oligo- and polythiophenes.26 Other matrixes known
for a low grade of fragmentation (e.g., dithranol and
dihydroxybenzoic acid (DHB)) were also tried, but the
best spectra were achieved using CHCA as matrix. The
MALDI-TOF-MS spectra of the product from polymer-
ization with FeCl3 in CHCl3 showed polymers with a
molecular weight mainly between 2600 and 3900 fol-
lowing ion series corresponding to [213n]+, [213n + 35]+,
or [213n + 70]+ (see Figure 1). Recently, McCarley et
al. reported that polymers made by FeCl3 polymeriza-
tion and characterized by MALDI-TOF-MS adduct
chlorine atoms at the R-carbon ends of the polymer
backbone when ionized by MALDI-TOF-MS.27 So the
series [213n + 35]+ and [213n + 70]+ corresponds to
serinesubstituted thiophene (ST) units and chlorine
IR νmax cm-1: 778, 1060, 1164, 1367, 1506, 1716, 2976, 3436.
13C NMR (CDCl3) δ: 28.2 (3C), 29.3, 55.7, 63.2, 65.2, 80.1,
121.7, 125.7, 128.1, 137.5, 155.7, 170.8.
1H NMR (CDCl3) δ: 1.44 (s, 9H), 2.99 (t, 2H, J ) 6.9 Hz,),
3.82 (dd, 1H, J ) 3.6, 11.1 Hz), 3.89 (dd, 1H, J ) 3.9, 11.1
Hz), 4.37 (m, 3H), 6.96 (dd, 1 H, J ) 1.2, 4.8 Hz), 7.04 (m,
1H), 7.26 (dd, 1H, J ) 3.0, 4.8 Hz).
Syn t h esis of P oly(3-[(R)-5-a m in o-5-ca r b oxyl-3-oxa -
p en tyl]-2,5-th iop h en ylen e Hyd r och lor id e) (2). Compound
1 (107 mg, 0.34 mmol) was dissolved in CH2Cl2/TFA (1:1, 4
mL). The reaction was quenched after 4 h by adding MeOH
(10 mL) and coconcentrated with toluene (3 × 10 mL). The
ammonium salt and TBA-trifluoromethanesulfonate (0.164 g,
0.42 mmol) were dissolved in dry CHCl3 (2 mL), and the
solution was cooled to 13 °C. A slurry of anhydrous FeCl3 (0.56
g, 3.45 mmol) in dry CHCl3 (1 mL) was added dropwise to this
solution under Ar atmosphere. After 26 h, the reaction was
quenched with H2O (4 mL) and diluted with CHCl3 (3 mL).
The organic layer was washed with H2O (3 × 3 mL). The
aqueous solution was diluted with acetone (30 mL), and conc.
HCl (2 mL) was added. After 2 h, the mixture was centrifuged
(4 min/2500 rpm). The red salt was washed with acetone (2 ×
35 mL), dissolved in H2O (2 mL), and precipitated from
acetone/concentrated HCl (25:1, 26 mL). The washing proce-
dure was repeated twice to give 2 (52 mg, 0.207 mmol, 61%)
1
as a red-orange powder. The H NMR in CD3OD and the IR
were identical with those earlier published for the S-enanti-
omer of the polymer.4

6318 Nilsson et al.
Macromolecules, Vol. 37, No. 17, 2004
the polymer side chains. Interestingly, the alteration of
the stereochemistry of the side-chain is not influencing
the absorption properties of the polymer backbone, as
the pH induced conformational changes of the polymer
backbone are the same for the two polymers. The
absorption maxima from L-POWT and D-POWT are
slightly different, but this is probably due to the
difference in chain-length distributions between the two
polymers. The altered stereochemistry of the chiral
center on the zwitterionic side chain does not affect the
absorption properties of the polymer backbone.
The pH-induced conformational changes of the poly-
mer chains will also alter the emission spectra for the
polymer solutions (see Figure 2). The polymers emit
light with a longer wavelength, as the net charge of the
polymer side chains becomes more negative. At pH 10,
where almost all the amino groups are neutral and all
the carboxyl groups are negatively charged, the poly-
mers emit light with a wavelength around 605 nm.
When the pH decreases and the net charge of the
polymers become close to neutral, light with a shorter
wavelength is emitted, and at pH 5.9 (pI for serine), the
peak is shifted to approximately 550 nm. Earlier stud-
ies17,18 of POWT have shown an analogous trend in the
photoluminescence (PL) of intra- and interchain phe-
nomena in the polymer. As the chains were separated,
the PL maximum was blue-shifted by approximately 105
nm, as compared to emission in the solid state using
dense packing of the polymer chains. This observation
indicates that the two distinct separated peaks in the
emission spectra of POWT originate from an intrachain
event (550 nm) and an interchain event (605 nm). At
pH 10, a mixture of these two states is present in the
solution. Aggregation of the polymer chains seems to
be encouraged at higher pH where the polymer back-
bone adopts a more planar conformation, and this could
also been seen in the sample used, as the polymer
precipitate at an alkaline pH. A solution of POWT in
the 20 mM Mes buffer (pH 5.9) remains stable for a
couple of months, but in the 20 mM carbonate buffer
(pH 10), the polymer precipitates after a couple of days.
Apparently, the deprotonation of the amino groups is
important for the polymer chains to form aggregates.
A deprotonated amino group will be able to function as
a hydrogen bond acceptor, suggesting that hydrogen
bonding is important for directing aggregation. The
intensity of the fluorescence for the aggregated phase
of polythiophenes derivatives, as compared with the
fluorescence for the single chain, has previously been
shown to be weaker by approximately 1 order of
magnitude,10,12 in agreement with the results in this
study. The earlier studies of thin films of POWT also
showed an analogous trend, as the PL quantum ef-
ficiency increased from 4 to 16% in the absence of the
interchain event.17
F igu r e 1. MALDI-TOF-MS spectra for poly(3-[(S)-5-amino-
5-carboxyl-3-oxapentyl]-2,5-thiophenylene hydrochloride) (top)
and poly(3-[(R)-5-amino-5-carboxyl-3-oxapentyl]-2,5-thiophen-
ylene hydrochloride) (bottom) recorded in linear positive mode.
atoms as end groups, [STn + Cl]+ and [STn + 2 Cl]+,
where n is the number of ST in the backbone. The
covalently bound chlorine atoms originate from the
oxidizing agent FeCl3, which are known to give impuri-
ties of iron or chlorine,28 or originate from the hydro-
chloride form of the polymer. The spectrum corresponds
to polymers containing between 13 and 19 ST units in
the backbone. Even though traces of small peaks down
to n ) 11 and up to n ) 22 can be found, the backbone
length can be approximated to n ) 16 ×b1 3. The
D-POWT and L-POWT show similar length distribution.
The absorption spectra for L-POWT and D-POWT in
deionized water and different buffer solutions are shown
in Figure 2. The polymers undergo a pH induced yellow
to orange color (a shift of the absorption maximum from
405 to 450) change, indicating that deprotonation and
protonation of the amino and carboxyl groups have an
influence on the coil-to-rod (nonplanar to planar) con-
formation transition of the polymer backbones. The
nonplanar conformations are most abundant in deion-
ized water and in the pH 5.9 buffer solutions. As the
side chains become charged, either positive or negative,
the polymer chains adopt a more planar conformation,
observed as a red shift. The polymers are probably
adopting the rod-shaped conformation due to electro-
static repulsion forces between the polymer side
chains29,30 or from hydrogen bonding31 and hydrophobic
assembly between nearby polymer chains, as these
molecular forces are highly influenced by the charge of
Surprisingly, in the nonplanar-to-planar conforma-
tional transition of the backbone at pH 4, where the net
charge of the polymer chains becomes more positive,
light with a slightly longer wavelength is emitted, but
the intensity of the fluorescence is not decreasing in the
same way as for the solution with pH 10. These results
indicate that the emitted light is due to an intrachain
event. Consequently, the positive net charge of the
polymer seems to favor a more rod shape conformation
of the polymer chain, but aggregation of the polymer
chains is most likely absent. The rod shape form of the
polymer chains at acidic pH could be induced by

Macromolecules, Vol. 37, No. 17, 2004
Chirality of Luminescent Conjugated Polythiophenes 6319
F igu r e 2. Absorption spectra (left) and emission spectra (right) of L-POWT (top) and D-POWT (bottom) after 30 min of incubation
in deionized water (black line), pH 4.0 (green line), pH 5.9 (red line), and pH 10 (blue line). All the emission spectra were recorded
with excitation at 400 nm.
electrostatic repulsion forces or by hydrogen bonding
within the polymer chains. At acidic pH, the protonated
carboxyl group will be able to function as a hydrogen
bond donor, and this might encourage another type of
hydrogen-bonding pattern, which will prevent aggrega-
tion but induce a more rod shape conformation of the
polymer backbone. The polymer does not precipitate in
solutions with acidic pH, in agreement with this as-
sumption.
The different chirality of the polymer side chains does
not seem to influence the emission properties from the
polymer chains. Although there are minor differences
between the emission spectra, an analogous pH-depend-
ent trend is seen for the two isomers. Hence, the altered
stereochemistry of the zwitterionic side chain is not
reflected as an alteration of the nonplanar/planar
transition of the polymer backbone or aggregation/
separation of the polymer chains.
As previously reported4,18 the polymers exhibit a split-
type induced circular dichroism (ICD) in the π-π*
transition region. The two major CD peaks observed in
water, interpreted as due to syn conformers, disap-
peared in methanol where anti conformations domi-
nate.4 The CD spectra of L-POWT in different buffer
solutions are shown in Figure 3. The red shifts in
absorption, induced by different buffer systems, are
accompanied with a decrease in the ICD, indicating that
F igu r e 3. Circular dichroism (CD) spectra of L-POWT (top)
chirality induction may not be derived from π-stacked
chiral aggregation of the polymer.8-12 Instead, the ICDs
is a result of main-chain chirality, such as a predomi-
nantly one-handed helical structure induced by the
zwitterionic group of the polymer side chain.13-16,18 The
CD spectra show that near the zwitterionic point, the
and D-POWT (bottom) after 30 min of incubation in deionized
water (black line), pH 4.0 (red line), pH 5.9 (green line), and
pH 10 (blue line).
polymer chain will adopt the nonplanar syn conforma-
tion with a high helicity. In alkaline or acidic environ-

6320 Nilsson et al.
Macromolecules, Vol. 37, No. 17, 2004
F igu r e 4. Newman projection of the zwitterionic side chain and schematic drawing of proposed backbone conformations of L-POWT
(left) and D-POWT (right) at different pH.
ment, when the side chains become more charged,
negatively or positively, the helicity is reduced. Hence,
the polymer chain adopts a more rod-like conformation,
in agreement with the results previously seen in the
absorption measurement. In water, the thiophene rings
will be shielded from the solvent, and the polymer
chains will probably form a micelle-like structure with
the polar side chains pointing outward to the solvent.
The helical structure is most likely stabilized by hydro-
phobic interactions, electrostatic interactions, or hydro-
gen bonding between the polymer side chains. As the
magnitude of the Cotton effect is highest near the
isoelectric point for serine and decreases with an
alteration of the pH, the hydrogen bonded ion pair
complex seems to favor the helicity of the polymer
chains.
Previous studies of a stereoregular poly((4-carboxy-
phenyl)acetylene) forming acid-base complex with
amines and amino alcohols in DMSO13 indicate that R-
and S-enantiomers induce split type ICD of mirror
images. Interestingly, the S-form of serine used in the
synthesis of L-POWT4 gives a similar ICD pattern
(negative/positive) as the primary S-amines in this
earlier study.13 The shape and sign of the ICD pattern
are characteristic of a right-handed helical form of
polythiophene first observed by Meier.10,11 Normally, a
polypeptide made from L-R-amino acid residues forms
a right-handed helix, so apparently the nature of the
amino acid is reflected in the helical conformation of
the polymer backbone.
is a mirror image as compared with L-POWT, indicating
that the altered chirality of the polymer side chain is
influencing the helical twist of the polymer backbone.
A positive and negative ICD pattern has previously been
seen combining a stereoregular poly((4-carboxyphenyl)-
acetylene) with primary R-amines,13 supporting that the
D-form of serine will induce a left-handed helical form
of the polythiophene backbone. Likewise, polypeptides
made from D-R-amino acids form left-handed helices.
A closer look at the CD spectra for L- and D-POWT
shows a minor peak at approximately 280 nm, which is
associated with the absorption of the thiophene ring.
Interestingly, this peak has also opposite signs for the
polymers, indicating that the chirality of the thiophene
backbone is different for the two polymers and that the
altered chirality of the side chain is reflected in the
conformation of the polymer backbone. These peaks are
also decreasing at alkaline and acidic pH, suggesting
that the backbone becomes more planar in these envi-
ronments. We also find that by mixing equal amounts
of D- and L-POWT, we create racemic mixtures with
no CD signal (within the error of solution mixing error).
Also, we find that by mixing at different ratios, the CD
signal is proportional to the difference between the
concentration of D- and L-POWT in the solution, thus
faithfully reflecting the stoichiometry. It is tempting to
draw even stronger conclusions from these observations.
It has been suggested that only in aggregation is
chirality created for some classes of polythiophenes.13-16
This should lead to the observation of vanishing chiral-
ity in dilute solutions, which is not observed here, where
signals are remaining at dilutions of 550 µM polymer
(on a monomer basis), and no aggregation of the polymer
is observed. Also, if the D- and L-POWT are added in
the same solution, these molecules are chemically
The CD spectra of D-POWT in different buffer solu-
tions are shown in Figure 3. The intensity of the ICD
signals follows the same analogous pH-dependent trend
as seen for the L-POWT, with the highest helicity at
pH 5.9. But the shape and the sign of the ICD pattern

Macromolecules, Vol. 37, No. 17, 2004
Chirality of Luminescent Conjugated Polythiophenes 6321
(6) Bouman, M. M.; Meijer, E. W. Adv. Mater. 1995, 7, 385.
(7) Bidan, G.; Guillerez, S.; Sorokin, V. Adv. Mater. 1996, 8, 157.
(8) Andreani, F.; Angiolini, L.; Caretta, D.; Salatelli, E. J . Mater.
Chem. 1998, 8, 1109.
(9) Langeveld-Voss, B. M. W.; Bouman, M. M.; Christiaans, M.
P. T.; J anssen, R. A. J .; Meijer, E. W. Polym. Prepr. 1996,
37, 499.
(10) Langeveld-Voss, B. M. W.; J anssen, 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.
identical but different in the character of the chiral
center. We do not see that the excess of one or the other
of the two forms causes the other form to convert into
the opposite chirality. This also argues against aggrega-
tion as the necessary condition for chirality. Here,
chirality of the optical transitions on the main chain is
due to the chiral geometry induced by chiral substitu-
ents.
(11) Langeveld-Voss, B. M. W.; Christiaans, M. P. T.; J anssen,
R. A. J .; Meijer, E. W. Macromolecules 1998, 31, 6702.
(12) Langeveld-Voss, B. M. W.; J anssen, R. A. J .; Meijer, E. W. J .
Mol. Struct. 2000, 521, 285.
(13) Yashima, E.; Matsushima, T.; Okamoto, Y. J . Am. Chem. Soc.
1997, 119, 6345.
(14) Yashima, E.; Maeda, K.; Okamoto, Y. Nature 1999, 399, 449.
(15) Maeda, K.; Okada, S.; Yashima, E.; Okamoto, Y. J . Polym.
Sci., Part A: Polym. Chem. 2001, 39, 3180.
(16) Ishikawa, M.; Maeda, K.; Yashima, E. J . Am. Chem. Soc.
2002, 124, 7448.
(17) Berggren, M.; Bergman, P.; Fagerstro¨m, J .; Ingana¨s, O.;
Andersson, M.; Weman, H.; Granstro¨m, M.; Stafstro¨m, S.;
Wennerstro¨m, O.; Hjertberg, T. Chem. Phys. Lett. 1999, 304,
84.
(18) Nilsson, K. P. R.; Andersson, M. R.; Ingana¨s, O. J . Phys.:
Condens. Matter 2002, 14, 10011.
(19) Noyce, D. S.; Castenson, R. L. J . Am. Chem. Soc. 1973, 95,
1247.
(20) Shevchenko, N. E.; Nenajdenko, V. G.; Balenkova, E. S.
Synthesis 2003, 8, 1191.
(21) McCullough, R. D. Adv. Mater. 1998, 10, 93.
(22) Sugimoto, R.; Takeda, S.; Gu, H. B.; Yoshino, K. Chem.
Express 1986, 1, 635.
Con clu sion s
By comparing two isomers of a conjugated poly-
thiophene with a free amino acid side chain, we have
shown that the nonplanar to planar transition of the
polymer backbone and the separation/aggregation of
different polymer chains is not affected by stereochem-
istry of the zwitterionic side chain. The two isomers
have almost identical pH-dependent absorption and
emission spectra. The chirality of the zwitterionic side
chain is reflected in the conformation of the polymer
backbone, giving rise to a right-handed and left-handed
helical form of polythiophenes. The ICD patterns of the
two polymers are mirror images. A schematic drawing
of the different polymer chain conformations are sum-
marized in Figure 4. Our future efforts in this system
will be to determine the different types of interactions
forcing the polymers to show this extraordinary behav-
ior and how this can be used in different types of
biosensor and bioelectronic systems. It would be of great
interest to combine the different isomers with natural
chiral biomolecules to see if the difference in chirality
will cause a different interaction with the biomolecule
of interest. Another tantalizing possibility is to use the
isomers as chiral construction elements for the assembly
of bioelectronic devices.
(23) Yoshino, K.; Hayashi, S.; Sugimoto, R. J pn. J . Appl. Phys.
1984, 23, L899.
(24) Liu, J .; Loewe, R. S.; McCullough, R. D. Macromolecules 1999,
32, 5777.
(25) Puglisi, C.; Samperi, F.; Alicata, R.; Montaudo, G. Macro-
molecules 2002, 35, 3000.
(26) J ayakannan, M.; van Dongen, J . L. J .; J anssen, R. A. J .
Macromolecules 2001, 34, 5386.
(27) McCarley, T. D.; Noble, C. O.; DuBois, C. J .; McCarley, R. L.
Macromolecules 2001, 34, 7999.
(28) Abdou, M. S. A.; Lu, X.; Xie, Z. W.; Orfino, F.; Deen, M. J .;
Holdcroft, S. Chem. Mater. 1995, 7, 631.
(29) Fa¨ıd, K.; Leclerc, M. J . Am. Chem. Soc. 1998, 120, 5274.
(30) Kim, B.; Chen, L.; Gong, J .; Osada, Y. Macromolecules 1999,
32, 3964.
(31) McCullough, R. D.; Ewbank, P. C.; Loewe, R. S. J . Am. Chem.
Soc. 1997, 119, 633.
Refer en ces a n d Notes
(1) Lemaire, M.; Delabouglise, D.; Garreau, R.; Guy, A.; Roncali,
J . J . Chem. Soc., Chem. Commun 1988, 658.
(2) Kotkar, D.; J oshi, V.; Ghosh, P. K. J . Chem. Soc., Chem.
Commun. 1988, 917.
(3) Roncali, J .; Garreau, R.; Delabouglise, F.; Garnier, F.; Le-
maire, M. Synth. Met. 1989, 28, 341.
(4) Andersson, M.; Ekeblad, P. O.; Hjertberg, T.; Wennerstro¨m,
O.; Ingana¨s, O. Polymer Commun. 1991, 32, 546.
(5) Bouman, M. M.; Havinga, E. E.; J anssen, R. A. J .; Meijer, E.
W. Mol. Cryst. Liq. Cryst. 1994, 256, 439.
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