Synthesis, chiroptical behavior, and sensing of carboxylic acid functionalized poly(phenylene ethynylene-alt-bithiophene)s

Article abstract of DOI:10.1021/ma101816z

Several poly(phenylene ethynylene-alt-bithiophene)s with (chiral) nonfunctionalized substituents were synthesized with a variable phenylene ethynylene (PE) spacer length (up to 4 repeating units). The chiroptical behavior was evaluated with UV-vis and cir

Full text of DOI:10.1021/ma101816z

7412 Macromolecules 2010, 43, 7412–7423
DOI: 10.1021/ma101816z
Synthesis, Chiroptical Behavior, and Sensing of Carboxylic Acid
Functionalized Poly(phenylene ethynylene-alt-bithiophene)s
Steven Vandeleene, Michiel Verswyvel, Thierry Verbiest, and Guy Koeckelberghs*
Laboratory of Molecular Electronics and Photonics, Katholieke Universiteit Leuven, Celestijnenlaan 200F,
3001 Heverlee (Leuven), Belgium
Received August 9, 2010
ABSTRACT: Several poly(phenylene ethynylene-alt-bithiophene)s with (chiral) nonfunctionalized substit-
uents were synthesized with a variable phenylene ethynylene (PE) spacer length (up to 4 repeating units).
The chiroptical behavior was evaluated with UV-vis and circular dichroism (CD) spectroscopy, revealing a
highly solvent-sensitive aggregate formation. Based on this high sensitivity, both chiral and achiral carboxylic
acid functionalized analogues were prepared, of which the length of the spacer connecting the carboxylic acid
to the polymer backbone was varied. A combination of UV-vis, CD, and emission spectroscopy showed a
clear affinity of chiral amines toward the functionalized polymers both in solution and in film. However, a
different supramolecular behavior of the polymers was observed depending on the length of the carboxylic
acid functionalized side chain.
Introduction
The first part of the manuscript deals with the influence of the
length of the OPE part on the chiroptical properties of the
The optical, electrical, and electronic properties of conjugated
polymers not only depend on their molecular structure but also
are to a large extent governed by their conformation and
supramolecular structure. A nice example of this phenomenon
is the solvatochromism of poly(phenylene ethynylene)s.1 Addi-
tion of nonsolvent to a solution of this polymer results in a red-
shift and narrowing of the absorption and fluorescence band,
which originates from a planarization and stacking of the poly-
mer chains. In general, the conformation and supramolecular
structure can be manipulated by the solvent quality,2 the use of
additives,3 temperature,4 etc.
In the case of chiral polymers, the aggregates can be composed
of chirally, instead of parallel, stacked coplanar polymer chains,
which give rise to chiroptical effects such as circular dichroism.1b,5
Chirality can easily be implemented by chiral substituents, but
also by the use of chiral solvents, media, additives, etc. The
binding of a chiral additive can result in an enantioselective and
possibly even in a diastereoselective response, as the chiral
additive invokes (chiral) aggregation and hence alters the chir-
optical properties of the material.
Thismanuscript describes the synthesisand chiroptical proper-
ties of alternating copolymers of oligo(phenylene ethynylene)s
(OPE) and 3,30-dialkoxybithiophene (BT) units. The choice of
these building blocks is motivated by the fact that their corre-
sponding homopolymers (poly(phenylene ethynylene)s1 and
HH-TT coupled poly(3-alkoxythiophene)s6) are known to adopt
a lamellar supramolecular structure in which chirality is strongly
expressed. Moreover, the BT units, being electron-rich, can easily
be oxidized, allowing the introduction of radical cations confined
within the OPE parts. The alternating, regular nature of the
copolymer (in contrast to a random copolymer) should allow the
formation of a targeted supramolecular behavior. Finally, by the
use of two different monomers, one monomer can be equipped
with chiral side-chains, while the second monomer can bear a
functional group.
polymers (P1-4, Figure 1). It is investigated whether the targeted
supramolecular behavior is indeed present in these alternating
copolymers. In the second part, a carboxylic acid functional
group is introduced in the OPE part (P5b-8b, Figure 1) and the
influence of an additive on the chiroptical properties is investi-
gated in detail.
Experimental Section
Reagents and Instrumentation. All reagents were purchased
from Aldrich Chemical Co., Acros Organics, Merck, and Fluka.
Reagent grade solvents were dried and purified by distillation.
Gel permeation chromatography (GPC) measurements were
done with a Shimadzu 10A apparatus with a tunable absorbance
detector and a differential refractometer in tetrahydrofuran
(THF) as eluent toward polystyrene standards. 1H nuclear
magnetic resonance (NMR) measurements were carried out
with a Bruker Avance 300 MHz. The IR spectra were obtained
with a Bruker ALPHA spectrometer (ATR). UV-vis and CD
spectra were recorded with a Varian Cary 400 and a JASCO 62
DS apparatus, respectively. The DSC measurements were per-
formed on a Perkin-Elmer DSC 7 apparatus. The fluorescence
measurements were done on a PTI Photon Technology Inter-
national apparatus. The samples were excited near the absorp-
tion wavelength. Films for UV-vis and CD experiments were
prepared by spin coating from THF solutions (1200 rpm, 20 s).
Compounds 1,7 9,3g 20,8 21,9 22,6 and 236 were synthesized
according to literature procedures.
Monomer Synthesis. Synthesis of 2. 1 (12.6 mmol, 5.91 g),
Pd(PPh3)2Cl2 (0.630 mmol, 0.442 g), and PPh3 (0.630 mmol,
0.164 g) were added to a mixture of dry THF (40 mL) and dry
diisopropylamine (50 mL) under argon atmosphere. The solu-
tion was heated to 40 °C, followed by the dropwise addition of
propargyl alcohol (14.2 mmol, 0.791 g) and a solution of CuI
(1.89 mmol, 0.356 g) in a mixture of dry THF (1.5 mL) and dry
diisopropylamine (6.5 mL). The reaction was followed by TLC,
and after completion, petroleum ether was added and the
mixture was washed with water and filtered. The organic layer
*Corresponding author. E-mail: guy.koeckelberghs@chem.kuleuven.be.
pubs.acs.org/Macromolecules
Published on Web 09/02/2010
r 2010 American Chemical Society

Article
Macromolecules, Vol. 43, No. 18, 2010 7413
1.67-1.65 (m, 8 H), 1.34-1.23 (m, 24 H), 0.88 (d, 12 H). 13C
NMR (CDCl3): δ = 143.3, 142.8, 142.3, 132.9, 132.8, 123.4,
122.9, 122.5, 104.3, 99.5, 93.3, 92.6, 84.5, 54.5, 54.1, 53.7, 53.4,
53.0, 51.9, 34.5, 34.3, 32.3, 32.2, 31.1, 30.9, 29.7, 29.6, 23.1, 14.4.
IR: ν (cm-1) = 3404, 2954, 2920, 2852, 1496, 1457, 860, 836.
MS: m/z = 592 (Mþ).
Synthesis of 6. A suspension of MnO2 (59.7 mmol, 5.19 g) and
KOH (29.3 mmol, 1.64 g) in diethyl ether (60 mL) was brought
under argon atmosphere and shielded from light. A solution of 5
(3.76 mmol, 2.23 g) in diethyl ether (10 mL) was added, and the
reaction was monitored by TLC. After completion, the suspen-
sion was filtered and the organic layer was washed with brine
and dried over MgSO4. The crude product was purified by
column chromatography (eluent: dichloromethane), yielding a
red oil. Yield: 1.31 g (61%). 1H NMR (CD2Cl2): δ = 7.35 (s, 4
H), 3.36 (s, 2 H), 2.84-2.74 (m, 8 H), 1.68-1.66 (m, 8 H),
1.35-1.28 (m, 24 H), 0.89 (d, 12 H). 13C NMR (CDCl3): δ =
143.6, 142.7, 133.7, 133.1, 124.0, 122.2, 93.5, 83.0, 82.2, 34.8,
34.6, 32.6, 32.4, 31.5, 31.3, 30.0, 29.8, 23.4, 14.6. IR: ν (cm-1) =
2953, 2920, 2852, 1500, 1460, 865, 839. MS: m/z = 958 (((M -
C6H13)2 þ Na)þ).
Synthesis of 7. The same procedure as described for the
synthesis of 2 was used, starting from 1 (4.75 mmol, 2.22 g).
The pure product was purified by column chromatography
(eluent: n-hexane) and isolated as a gray solid. Yield: 2.00 g
(85%). mp: 104.5 °C. 1H NMR (CD2Cl2): δ = 7.39 (s, 4 H), 7.34
(s, 2 H), 7.30 (s, 2 H), 2.84-2.71 (m, 16 H), 1.71-1.67 (m, 16 H),
1.51-1.27 (m, 48 H), 0.88 (d, 24 H), 0.27 (s, 18 H). 13C NMR
(CD2Cl2): δ = 142.9, 142.1, 141.9, 132.5, 123.1, 122.9, 122.5,
104.1, 99.1, 93.1, 34.3, 31.9, 31.8, 30.8, 29.4, 22.8, 14.3, 0.14. IR:
ν (cm-1) = 2955, 2919, 2852, 2149, 1499, 1458, 839. MS: m/z =
1215 (((M - C45H65)2 þ Na)þ).
Figure 1. Structure of the polymers.
was washed with a HCl solution (1.00 M), a saturated NaHCO3
solution, and brine, and finally dried over MgSO4. The solvents
were removed under reduced pressure, and the crude product was
purified by column chromatography (eluent: dichloromethane).
The product was isolated as a brown oil and was used in the next
Synthesis of 8. The same procedure as described for the
synthesis of 5 was used, starting from 7 (1.20 mmol, 1.49 g). A
gray solid was obtained after the purification of the crude
product by crystallization from cold methanol. Yield: 1.16 g
(88%). mp: 68 °C. 1H NMR (CD2Cl2): δ = 7.38 (s, 4 H), 7.35 (s,
2 H), 7.34 (s, 2 H), 3.36 (s, 2 H), 2.83-2.72 (m, 16 H), 1.70-1.60
(m, 16 H), 1.45-1.24 (m, 48 H), 0.88 (d, 24 H). 13C NMR
(CD2Cl2): δ = 142.9, 142.1, 141.9, 133.2, 132.6, 132.5, 123.5,
123.4, 123.3, 122.9, 122.8, 121.5, 93.2, 92.9, 82.6, 81.6, 34.4, 34.3,
32.0, 31.8, 30.8, 30.6, 29.4, 29.3, 22.8, 14.2. IR: ν (cm-1) = 2953,
2919, 2851, 1500, 1457, 892. MS: m/z = 1099.8 (Mþ).
Synthesis of 10. A suspension of NaH (55.0 mmol, 1.32 g) in
dry DMF (50.0 mL) was purged with argon, and a solution of 9
(25.0 mmol, 9.05 g) in dry DMF (50.0 mL) was dropwise added.
The reaction mixture was stirred under argon at room tempera-
ture for 30 min. Then, a solution of methyl bromoacetate (60.0
mmol, 9.18 g) in DMF (10.0 mL) was added and the reaction
mixture was stirred overnight at 40 °C. Afterward, water (10
mL) was added and a white precipitate was formed which was
filtered off. The precipitate was thoroughly washed with water
and diethyl ether, dried under vacuum, and isolated as a white
solid. Yield: 8.10 g (64%). mp: 195.8 °C. 1H NMR (DMSO-d6):
δ = 7.36 (s, 2H), 4.91 (s, 4H), 3.74 (s, 6H). 13C NMR (CDCl3):
δ = 169.8, 152.8, 123.5, 87.3, 67.1, 52.8. IR: ν (cm-1) = 2962,
2906, 1711, 1482, 1442, 1423, 1079, 848, 818. MS: m/z = 1035
((2M þ Na)þ).
1
step without further purification. Yield: 4.96 g (94%). H NMR
(CD2Cl2): δ = 7.24 (s, 1 H), 7.23 (s, 1 H), 4.50 (d, 2 H), 2.71-2.65
(m, 4 H), 1.72 (t, 1 H), 1.59-1.52 (m, 4 H), 1.41-1.21 (m, 12 H),
0.87 (m, 6 H), 0.25 (s, 9 H).
Synthesis of 3. 2 (10.8 mmol, 4.30 g) was dissolved in a mixture
of methanol (60 mL) and THF (45 mL) under argon atmosphere. A
NaOH solution (5.00 M, 12.0 mL) was added, and the reaction was
monitored in TLC. After completion, diethyl ether (120 mL) and
water were added, and the compound was extracted with diethyl
ether. The collected organic layers were washed with brine and dried
over MgSO4. After removal of the solvent under reduced pressure,
the product was purified by column chromatography (eluent:
dichloromethane). The product was isolated as a brown oil. Yield:
3.84 g (100%). 1H NMR (CDCl3): δ = 7.28 (s, 1 H), 7.24 (s, 1 H),
4.49 (d, 2 H), 3.33 (s, 1 H), 2.72-2.65 (m, 4 H), 1.65-1.52 (m, 4 H),
1.38-1.20 (m, 12 H), 0.87 (m, 6 H).
Synthesis of 4. The same procedure as described for the
synthesis of 2 was used, starting from 1 (0.876 mmol, 861 mg).
The pure product was purified by column chromatography (eluent:
dichloromethane) and isolated as a brown oil. Yield: 3.30 g (60%).
1H NMR (CD2Cl2): δ = 7.32 (s, 1 H), 7.31 (s, 1 H), 7.28 (s, 2 H),
4.50 (d, 2 H), 2.80-2.69 (m, 8 H), 1.73 (t, 1 H), 1.67-1.57 (m, 8 H),
1.48-1.28 (m, 24 H), 0.88 (d, 12 H), 0.25 (s, 9 H).
Synthesis of 5. A solution of 4 (4.08 mmol, 2.72 g) in THF (240
mL) was brought under argon atmosphere and shielded from
light. Next, a tetrabutylammonium fluoride solution (1.00 M,
12.6 mL) in THF was added, and the reaction was stirred at
room temperature. After 2 h, diethyl ether (100 mL) and water
(100 mL) were added. The aqueous layer was extracted with
diethyl ether, and the collected organic fractions were washed
with brine. Afterward, the solution was dried over MgSO4 and
the solvents were removed under reduced pressure. Purification
by column chromatography (eluent: dichloromethane) yielded a
light red oil. Yield: 2.23 g (92%). 1H NMR (CDCl3): δ = 7.34 (s,
3 H), 7.29 (s, 1 H), 4.52 (s, 2 H), 3.37 (s, 1 H), 2.82-2.70 (m, 8 H),
Synthesis of 11. A suspension of NaH (55.0 mmol, 1.32 g) in
dry DMF (50 mL) was purged with argon, and a solution of 9
(25.0 mmol, 9.05 g) in dry DMF (50 mL) was dropwise added.
The reaction mixture was stirred at room temperature for 30
min. Then, a solution of methyl 6-bromohexanoate (60.0 mmol,
12.5 g) in dry DMF (10 mL) was added, and the reaction was
stirred overnight at 40 °C. After reaction, dichloromethane (20
mL) was added and the organic layer was washed respectively
with a saturated NaCl solution and water. The organic phase
was dried over MgSO4 and concentrated. The crude product
was purified by column chromatography (eluent: n-hexane/
ethyl acetate 5/5) and recrystallized from n-hexane to remove

7414 Macromolecules, Vol. 43, No. 18, 2010
Vandeleene et al.
residual traces of methyl 6-bromohexanoate. The pure product
was isolated as a white crystalline powder. Yield: 13.91 g (90%).
mp: 96.0 °C. 1H NMR (CDCl3): δ = 7.17 (s, 2H), 3.94 (t, 4H), 3.68
diluted HCl solution(0.05M), saturatedNaClsolution, andwater,
respectively. The organic layer was dried over MgSO4 and concen-
trated in vacuo. The crude product was purified by column
chromatography (eluent: n-hexane/ethyl acetate 9/1) and isolated
(s, 6H), 2.37 (t, 4H), 1.82 (m, 4H), 1.70 (m, 4H), 1.53 (m, 4H). 13
C
1
NMR (CDCl3): δ = 174.4, 153.1, 123.1, 86.6, 70.3, 51.9, 34.3, 29.1,
26.0, 24.9. IR: ν (cm-1) = 2937, 2875, 1719, 1488, 1456, 1421, 1066,
859. MS: m/z = 641 ((M þ Na)þ).
as a yellow solid. Yield: 0.86 g (20%). mp: 83.6 °C. H NMR
(CDCl3): δ = 7.34 (s, 2H), 7.29 (s, 2H), 6.96 (s, 2H), 4.72 (s, 4H),
3.81 (s, 6H), 2.79 (t, 4H), 2.72 (t, 4H), 1.63 (m, 8H), 1.31 (m, 24H),
0.88 (m, 12H), 0.26 (s, 18H). 13C NMR (CDCl3): δ = 169.3, 153.5,
143.1, 142.6, 132.8, 132.7, 123.0, 122.8, 118.2, 115.2, 104.3, 99.4,
95.3, 89.9, 67.2, 52.6, 34.5, 34.3, 32.1, 30.9, 29.6, 29.5, 22.9, 14.4,
0.32. IR: ν(cm-1) = 2954, 2923, 2853, 2145, 1766, 1504, 1417, 1087,
838. MS: m/z = 1989 ((2M þ Na)þ).
Synthesis of 12. 10 (20.0 mmol, 10.1 g) and Pd(PPh3)2Cl2
(0.150 mmol, 105 mg) were dissolved in a mixture of dry ethyl
acetate (50 mL) and dry triethylamine (12.5 mL) and purged
with argon. The reaction mixture was heated at 40 °C, and
trimethylsilyl acetylene (46.0 mmol, 4.52 g) was added. Next,
CuI (4.00 mmol, 762 mg) dissolved in a mixture of dry ethyl acetate
(8 mL) and dry triethylamine (2 mL) was added, and the reaction
mixture was stirred for 15 min at 40 °C. Afterward, dichloro-
methane (20 mL) was added and the organic phase was washed
withaHClsolution(0.05M), asaturatedNaClsolution, andwater,
respectively. The organic phase was isolated and dried over MgSO4,
and after concentration in vacuo, the crude product was purified by
column chromatography (eluent: n-hexane/ethyl acetate 8/2). The
obtained product was isolated as a white solid. Yield: 4.32 g (48%).
mp: 162.0 °C 1H NMR (CD2Cl2): δ = 6.86 (s, 2H), 4.65 (s, 4H),
3.78 (s, 6H), 0.24 (s, 18H). 13C NMR (CDCl3): δ = 169.3, 153.9,
119.5, 115.2, 102.0, 100.2, 67.4, 52.6, 0.19. IR: ν (cm-1) = 2955,
2913, 2157, 1769, 1530, 1437, 1407, 1089, 883. MS: m/z = 916
((2MþNa)þ).
Synthesis of 17. The same procedure as described for the
synthesis of 13 was used, starting from 16 (0.876 mmol, 861 mg).
Pure product was obtained after column chromatography
(eluent: n-hexane/ethyl acetate 9/1) and isolated as a brown
1
oil. Yield: 0.50 g (69%). H NMR (CDCl3): δ = 7.37 (s, 2H),
7.32 (s, 2H), 6.97 (s, 2H), 4.73 (s, 4H), 3.81 (s, 6H), 3.31 (s, 2H),
2.81 (t, 4H), 2.74 (t, 4H), 1.66 (m, 8H), 1.31 (m, 24H), 0.88 (m,
12H). 13C NMR (CDCl3): δ = 169.0, 153.3, 142.8, 142.4, 133.1,
132.5, 123.0, 121.9, 117.8, 114.9, 95.0, 89.8, 82.6, 81.9, 66.8, 66.0,
52.24, 34.1, 34.0, 32.1, 31.9, 30.7, 29.9, 29.6, 29.4, 22.8, 15.4,
14.2. MS: m/z = 862 ((M þ Na)þ).
Synthesis of 18. The same procedure as described for the
synthesis of 16 was used, starting from 15 (5.00 mmol, 2.07 g).
Pure product was obtained after column chromatography
(eluent: n-hexane/ethyl acetate 9/1) and isolated as a yellow
solid. Yield: 3.00 g (55%). mp: 78.3 °C. 1H NMR (CDCl3): δ =
7.31 (s, 2H), 7.29 (s, 2H), 6.96 (s, 2H), 4.02 (t, 4H), 3.64 (s, 6H),
2.80 (t, 4H), 2.72 (t, 4H), 2.32 (t, 4H), 1.87 (m, 4H), 1.67 (m,
12H), 1.31 (m, 28H), 0.87 (m, 12H), 0.26 (s, 18H). 13C NMR
(CDCl3): δ = 174.3, 153.7, 143.0, 142.3, 132.8, 132.6, 123.2,
122.7, 116.9, 114.4, 104.4, 99.3, 94.3, 90.8, 69.4, 51.8, 34.5, 34.3,
32.1, 30.9, 29.6, 29.5, 29.4, 25.9, 25.0, 22.9, 14.4, 0.33. IR: ν
(cm-1) = 2950, 2924, 2855, 2147, 1737, 1506, 1433, 1026, 838.
MS: m/z = 1120 ((M þ Na)þ).
Synthesis of 13. 12 (9.96 mmol, 4.33 g) was dissolved in THF
(250 mL), brought under inert atmosphere, and protected from
ambient light. The mixture was ice-cooled, and a solution of
tetrabutylammonium fluoride trihydrate (12.5 mmol, 3.97 g) in
THF (50 mL) was slowly added. The reaction mixture was stirred
for 5 min at 0 °C, followed by addition of water (250 mL) and ethyl
acetate (150 mL). The organic phase was isolated and concentrated
by rotatory evaporation, and the crude product was purified by
recrystallization from a n-hexane/ethyl acetate mixture (8/2). The
pure product was filtered off, dried under vacuum, and isolated as a
white solid. Yield: 1.77 g (60%). mp: 162.4 °C. 1H NMR (CDCl3):
δ = 7.03 (s, 2H), 4.88 (s, 4H), 4.44 (s, 2H), 3.70 (s, 6H). 13C NMR
(CDCl3): δ = 169.9, 153.6, 118.2, 113.4, 87.7, 80.3, 66.2, 52.8. IR:
ν (cm-1) = 2954, 2911, 1761, 1500, 1404, 1084, 851. MS: m/z =
303 (Mþ).
Synthesis of 14. The same procedure as described for the
synthesis of 12 was used, starting from 10 (20.0 mmol, 12.4 g).
Pure product was obtained after column chromatography
(eluent: n-hexane/ethyl acetate 7/3) and isolated as a white solid.
Yield: 7.95 g (71%). mp: 95.5 °C. 1H NMR (CDCl3): δ = 6.88 (s,
2H), 3.95 (t, 4H), 3.67 (s, 6H), 1.81 (m, 4H), 1.71 (m, 4H), 1.52
(m, 4H), 0.25 (s, 18H). 13C NMR (CDCl3): δ = 174.4, 154.2,
117.5, 114.2, 101.2, 100.6, 69.3, 51.9, 34.4, 29.3, 25.9, 25.0, 0.34.
IR: ν (cm-1) = 2942, 2902, 2156, 1739, 1495, 1467, 1035, 868.
MS: m/z = 582 ((M þ Na)þ).
Synthesis of 19. The same procedure as described for the
synthesis of 17 was used, starting from 18 (5.00 mmol, 2.07 g). Pure
product was obtained after column chromatography (eluent: n-
hexane/ethyl acetate 9/1) and isolated as a yellow solid. Yield: 1.60 g
(74%). mp: 91.3 °C. 1H NMR (CDCl3): δ = 7.34 (s, 2H), 7.33 (s,
2H), 6.98 (s, 2H), 4.03 (t, 4H), 3.64 (s, 6H), 3.30 (s, 2H), 2.81 (t, 4H),
2.74 (t, 4H), 2.32 (t, 4H), 1.87 (m, 4H), 1.69 (m, 12H), 1.31 (m,
28H), 0.88 (m, 12H). 13C NMR (CDCl3): δ = 174.3, 153.7, 143.1,
142.4, 133.3, 132.6, 123.6, 121.7, 116.9, 114.4, 94.1, 90.8, 82.8, 81.8,
69.4, 51.8, 34.3, 34.2, 34.1, 32.1, 32.0, 30.8, 30.0, 29.4, 29.3, 25.9,
25.0, 22.9, 14.4. IR: ν (cm-1) = 2949, 2925, 2850, 1726, 1504, 1485,
1457, 1014, 844. MS: m/z = 1925 ((2M þ Na)þ).
General Synthesis of P1-4. A solution of 22 (0.800 mmol,
0.509 g) and Pd(PPh3)4 (0.0160 mmol, 18.5 mg) in a mixture of
piperidine (1.5 mL) and THF (3.5 mL) was purged with argon.
The reaction mixture was heated at 60 °C, and 24 (0.800 mmol,
0.236 g), dissolved in THF (0.5 mL), and CuI (36.0 μmol, 63.9
mg), dissolved in a mixture of piperidine/THF (3/7, 0.5 mL),
were added respectively. The reaction mixture was stirred over-
night, and afterward, CHCl3 was added. The organic layer was
washed with a HCl solution (0.500 M) and water and dried with
Na2SO4. After concentration by rotatory evaporation, the
polymer solution was precipitated in methanol, filtered off,
and dried. Further purification was accomplished by Soxhlet
extractions with methanol, n-pentane, n-hexane, and CHCl3,
respectively. The chloroform-soluble fraction was concentrated
and precipitated in methanol. Finally, the polymer was filtered
off and dried. P1: Yield = 499 mg (81%). 1H NMR (CD2Cl2):
δ = 7.34 (s, 2H), 7.05 (s, 2H), 4.20 (s, 4H), 2.78 (s, 4H), 1.94-1.69
(m, 8H), 1.51-1.19 (m, 28H), 0.85 (m, 24H). IR: ν (cm-1) = 2951,
2922, 2855, 2187, 1525, 1457, 1409, 1060, 806. P2: Yield = 536 mg
(72%). 1H NMR (CD2Cl2): δ = 7.36 (s, 4H), 7.05 (s, 2H), 4.20 (s,
4H), 2.83 (s, 8H), 1.94-1.69 (m, 8H), 1.51-1.19 (m, 44H), 0.86 (m,
30H). IR: ν(cm-1) = 2952, 2922, 2854, 2184, 1524, 1456, 1411, 893,
Synthesis of 15. The same procedure as described for the
synthesis of 13 was used, starting from 14 (14.2 mmol, 7.95 g).
Pure product was obtained after column chromatography
(eluent: n-hexane/ethyl acetate 65/35) and isolated as a white
solid. Yield: 4.31 g (73%). mp: 92.7 °C. 1H NMR (CDCl3): δ =
6.94 (s, 2H), 3.97 (t, 4H), 3.67 (s, 6H), 2.34 (s, 2H), 2.35 (t, 4H),
1.82 (m, 4H), 1.74 (m, 4H), 1.52 (m, 4H). 13C NMR (CDCl3):
δ = 174.4, 154.2, 118.0, 113.6, 82.9, 79.9, 69.6, 51.9, 34.3, 29.1,
25.9, 24.9. IR: ν (cm-1) = 2947, 2869, 1720, 1499, 1467, 1436,
1039, 862. MS: m/z = 438 ((M þ Na)þ).
Synthesis of 16. 13 (4.50 mmol, 1.36 g) and Pd(PPh3)2Cl2
(0.225 mmol, 158 mg) were dissolved in a mixture of ethyl
acetate (100 mL) and triethylamine (25 mL) and purged with
argon. The mixture was heated at 60 °C, followed by the
addition of 1 (9.10 mmol, 4.26 g) and CuI (0.90 mmol, 0.171
g), both dissolved in a mixture of ethyl acetate (8 mL) and
triethylamine (2 mL). The reaction mixture was stirred for
30 min, and afterward, water (20 mL) and dichloromethane
(50 mL) were added. The organic phase was washed with a

Article
Macromolecules, Vol. 43, No. 18, 2010 7415
Scheme 1. Synthesis of Monomers 6 and 8a
a (i) propargyl alcohol, Pd(PPh3)2Cl2, CuI, THF/iPr2NH; (ii) NaOH (aq), THF; (iii) 1, Pd(PPh3)2Cl2, CuI, THF/iPr2NH; (iv) TBAF, THF; (v) MnO2,
KOH, Et2O.
Scheme 2. Synthesis of Monomers 17 and 19a
a (i) NaH, methyl bromoacetate (10), or methyl 6-bromohexanoate (11); (ii) trimethylsilylacetylene, Pd(PPh3)2Cl2, CuI, Et3N/ethyl acetate;
(iii) TBAF, THF; (iv) 1, Pd(PPh3)2Cl2, CuI, Et3N/EtOAc.
807. P3: Yield = 165 mg (79%). 1H NMR (CDCl3): δ = 7.38 (s,
6H), 7.05 (s, 2H), 4.21 (s, 4H), 2.83 (s, 12H), 1.94-1.71 (m, 8H),
1.55-1.20 (m, 60H), 0.87 (m, 36H). IR: ν (cm-1) = 2952, 2922,
2954, 2182, 1524, 1456, 1411, 894, 807. P4: Yield = 276 mg (22%).
1H NMR (CDCl3): δ = 7.39 (s, 8H), 7.05 (s, 2H), 4.20 (s, 4H), 2.83
(s, 16H), 1.94-1.71 (m, 8H), 1.35-1.18 (m, 76H), 0.88 (m, 42H).
IR: ν (cm-1) = 2953, 2921, 2853, 2183, 1525, 1456, 1411, 893, 805.
General Synthesis of P5a-8a. Monomer 22 (0.060 mmol, 50.0
mg) and Pd(PPh3)4 (1.49 μmol, 1.73 mg) were dissolved in a
mixture of THF and triethylamine (8/2, 2 mL), and the solution
was purged with argon and protected from ambient light. The
reaction mixture was heated at 40 °C and a solution of monomer
17 (0.060 mmol, 37.9 mg) was added, followed by the addition of
CuI (3.00 μmol, 0.567 mg). The reaction mixture was stirred
overnight, and afterward CHCl3 (2 mL) was added. The organic
phase was washed with a HCl solution (0.05 M), a saturated
NaCl solution, and water, respectively, and dried over Na2SO4.
The polymer solution was concentrated and poured into metha-
nol. The precipitate was filtered off, and further purification was
accomplished by Soxhlet extractions with acetone, n-hexane,
and CHCl3, respectively. The chloroform-soluble fraction was
concentrated and precipitated in methanol. Finally, the polymer
was filtered off and dried. P5a: Yield = 15 mg (10%). 1H NMR
(CDCl3): δ = 7.39 (s, 2H), 7.33 (s, 2H), 7.02 (s, 2H), 6.98 (s, 2H),
4.74 (s, 4H), 4.18 (s, 4H), 3.83 (s, 6H), 2.81 (s, 8H), 1.95-1.36 (m,
54H), 0.88 (m, 30H). IR: ν (cm-1) = 2952, 2923, 2854, 2186,
1763, 1504, 1455, 1435, 1082, 854, 806. P6a: Yield = 34 mg
(22%). 1H NMR (CDCl3): δ = 7.39 (s, 2H), 7.34 (s, 2H), 7.01 (s,
2H), 6.98 (s, 2H), 4.74 (s, 4H), 4.14 (s, 4H), 3.83 (s, 6H), 2.80 (s,
8H), 1.88-1.32 (m, 32H), 0.88 (m, 18H). IR: ν (cm-1) = 2922,
2853, 2182, 1765, 1525, 1454, 1412, 1063, 891. P7a: Yield = 64
mg (43%). 1H NMR (CDCl3): δ = 7.36 (s, 4H), 6.99 (s, 4H), 4.17
(s, 4H), 4.04 (s, 4H), 3.65 (s, 6H), 2.82 (s, 8H), 2.34 (s, 4H),
1.80-1.21 (m, 64H), 0.87 (m, 30H). IR: ν (cm-1) = 2924, 2855,
2342, 2183, 1738, 1456, 1418, 1063, 893. P8a: Yield = 109 mg
(76%). 1H NMR (CDCl3): δ = 7.36 (s, 4H), 6.99 (s, 4H), 4.15 (s,
4H), 4.04 (s, 4H), 3.65 (s, 6H), 2.82 (s, 8H), 2.34 (s, 4H),
1.89-1.25 (m, 68H), 0.88 (m, 18H). IR: ν (cm-1) = 2922,
2854, 2177, 1738, 1524, 1417, 1050, 894.
General Synthesis of P6b-8b. Polymer 8a (29.1 μmol, 40.0 mg)
was dissolved in THF (25 mL) and stirred under argon atmosphere.
An aqueous KOH solution (0.500 M, 0.2 mL) was added dropwise,
and the mixture was stirred until complete conversion was reached
(monitored by IR spectroscopy). Afterward, chloroform was added
and the polymer solution was thoroughly washed with a HCl
solution (0.05 M) and water. The organic layer was isolated and
dried over Na2SO4 and concentrated. Finally, the polymer was
precipitated in ice-cooled methanol, filtered off, and dried. P6b:
Yield = 16 mg (53%). IR: ν (cm-1) = 2922, 2853, 2360, 1714,
1455, 1430, 1051, 886. P7b: Yield = 37 mg (74%). IR: ν (cm-1) =
2921, 2855, 2359, 2335, 1709, 1456, 1050, 859. P8b: Yield = 35 mg
(70%). IR: ν (cm-1) = 2924, 2855, 2360, 2341, 1708, 1414,
1050, 805.
Results and Discussion
Monomer and Polymer Synthesis. The polymers were pre-
pared by a Sonogashira reaction between a dibromo-function-
alized bithiophene and an oligo(para-phenylene ethynlene)
(OPE). The Sonogashira reaction also plays a crucial role in
the synthetic strategy for OPEs. It has however been reported9
that a controlled elongation of OPEs is only possible by selective
protection/deprotection steps of the acetylene moieties.
As already mentioned, both OPEs with and without ester
functionalities were prepared. The synthesis of dimer 6 is
depicted in Scheme 1 and starts from 1. After a Sonogashira
reaction with propargyl alcohol, 2 is formed with two
different protecting groups. The polarity change accompa-
nying the transition of 1 to 2 allows an easy chromatographic
purification. Selective deprotection with NaOH leads to 3,
on which again a Sonogashira reaction is performed with 1,
resulting in 4. The trimethylsilyl group was removed by
reaction of 4 with tetrabutylammonium fluoride (TBAF),
affording 5. The use of TBAF results in a clean deprotection,
while on the other hand deprotection with NaOH leads to
side-products which complicate the purification. Dimer 6
was obtained after removal of the alcoholic protection

7416 Macromolecules, Vol. 43, No. 18, 2010
Vandeleene et al.
group, which was accomplished with NaOH in the presence
of MnO2. Tetramer 8, finally, was synthesized starting from
6 by performing a Sonogashira reaction with 2 equiv of 1. A
double deprotection step on 7 with TBAF yielded 8.
precipitated in methanol, filtered off, and dried, giving
P1-P4 in moderate to high yields.
The synthesis of the ester-functionalized polymers is dis-
played in Scheme 4 and is again based on a Sonogashira
reaction. Using the chiral (22) and achiral (23) bithiophene
monomers, the chiral polymers P5a/P7a and the achiral
polymers P6a/P8a were obtained. The same purification
procedure was performed as described for P1-4, resulting
in P5a-P8a in low to moderate yields. The structure of all
Scheme 2 displays the synthesis of the ester-functionalized
trimers 17 and 19. Both trimers were synthesized based on a
similar synthetic strategy. Starting from 9, deprotonation of
the phenolic groups was accomplished with NaH, which was
followed by a nucleophilic substitution on methyl bromo-
acetate or methyl 6-bromoacetate, leading to 10 and 11,
respectively. Again, a Sonogashira reaction with trimethyl-
silyl acetylene was performed, yielding 12 and 14. The
trimethylsilyl groups were removed by means of TBAF.
Next, both 13 and 15 underwent a Sonogashira reaction
with 2 equiv of 1, resulting in 16 and 18, respectively. A final
deprotection step with TBAF converted 16/18 into 17/19.
Scheme 3 shows the synthesis of P1-4. The polymeriza-
tions were carried out in a mixture of piperidine and THF
(3/7), in which the polymers are readily soluble. Removal of
residual monomers and oligomers was accomplished by
Soxhlet extraction with different solvents (acetone and
n-hexane). The polymers were extracted with chloroform,
1
polymers (P1-4 and P5a-8a) was confirmed by H NMR
analysis.
The ester-functionalized polymers were saponificated with
an aqueous potassium hydroxide solution (Scheme 5). The
deprotection was performed in THF in the presence of small
amounts of water and methanol. It appeared crucial to work
in a diluted medium to avoid polymer degradation. The
saponification was monitored by means of IR spectroscopy.
A clear shift of the carbonyl stretch from 1740 cm-1 (ester) to
1710 cm-1 (carboxylic acid) was observed (Figure S1 of the
Supporting Information). After complete carboxylate for-
mation, chloroform was added, and the polymer solution
was acidified with HCl. The polymers P5b-8b were precipi-
tated in methanol, filtered off, and dried.
GPC and DSC Analysis. P1-4 and P5a-P8a show an
excellent solubility in common organic solvents, such as
chloroform and THF. In contrast, P5b-P8b are moderately
soluble in THF, only poorly soluble in chloroform, and
insoluble in most other solvents.
Scheme 3. Synthesis of Polymers P1-4a
The molar mass (Table 1) was measured by GPC in THF
toward polystyrene standards. All polymers, except P4,
show polydispersities between 2 and 3, which are typical
for polycondensation. The higher polydispersity of P4 prob-
ably originates from aggregation during polymerization.
DSC experiments were performed on P1-4 and P5a-8a,
showing for all polymers but P5a a clear glass transition. In
a (i) Pd(PPh3)4, CuI, piperidine/THF.
Scheme 4. Synthesis of Polymers P5a-8aa
a (i) Pd(PPh3)4, CuI, THF/Et3N.
Scheme 5. Synthesis of Polymers P5b-8ba
a (i) KOH, THF/MeOH/H2O.

Article
Table 1. Number Average Molar Mass (M ), Weight Average Molar
Mass (Mw), Polydispersity (D), Degree of Polymerization (X n),
Glass Transition Temperature (Tg), and Melting Temperature (Tm)
of P1-4 and P5a-8a
Macromolecules, Vol. 43, No. 18, 2010 7417
_
the angle by which the stacked polymer chains are rotated
toward each other.10 Therefore, the discrepancy between the
evolution of the UV-vis and CD spectra upon decreasing
the solvent quality points at a change of the angle between
the stacked polymer chains. Initially (i.e., at a low nonsolvent
ratio), this angle is rather large, while at higher nonsolvent
concentrations a more parallel orientation, which gives rise
to bisignate Cotton effects of lower intensity, is obtained.
This explains the drop of the CD intensity at higher non-
solvent concentrations: the increase of the fraction of aggre-
gated polymer chains is overcompensated by their smaller
individual CD. A similar behavior has already been observed
for other chiral (homo)polymers and was attributed to the
fact that the addition of a solvent which poorly interacts with
the conjugated backbone strengthens the intermolecular
π-interactions, favoring a parallel orientation of the polymer
chains.
_
_
n
_
_
Mn
(kg mol-1 a
Mw
(kg mol-1 a
_
)
)
Da X n Tg (°C)b Tm (°C b
3
3
P1
P2
P3
P4
P5a
P6a
P7a
P8a
26.0
24.0
68.0
43.0
20.0
12.6
18.5
38.0
64.0
57.0
179
2.5
2.4
2.6
4.0
2.0
2.6
2.1
2.8
33
23
52
27
15
10
13
28
167
104
158
132
197
150
201
183
174
39.5
32.0
38.1
105
130
148
121
a Determined by GPC toward polystyrene standards in THF.
b Determined by DSC. Heating rate: 40 °C/min.
addition, P1-4 also show clear melting peaks, revealing their
semicrystalline behavior. For the ester-functionalized poly-
mers P5a-8a, no melting peaks could, however, be observed.
Chiroptical Properties: Solvatochromism. Polymers P1-4.
In these polymers, the length of the achiral OPE was varied,
while keeping the chiral bithiophene unit constant. Conse-
quently, a comparison of the chiroptical behavior allows for
the determination of the optimal length of the OPE spacer in
terms of chiral expression, sensitivity toward external stimuli
(e.g., nonsolvent), and synthetic accessibility. In Figure 2, the
UV-vis and CD spectra in various CHCl3/iPrOH mixtures are
displayed. iPrOH and not, for instance, methanol was used as
nonsolvent, since precipitation of the polymers occurred in the
latter solvent. Except for P4, decreasing the solvent quality
results in a red-shift and the occurrence of a sharp band, which
can be attributed to a planarization and stacking of the polymer
chains.1b λmax of P1-4 is intermediate between that of the
respective homopolymers (λmax = 580/610 nm for HH-TT
P3AOTs6 and 400/438 nm for PPEs1b) in both dissolved and
aggregated states and decreases with increasing OPE length,
which nicely reflects the different conjugation lengths of the
bithiophene unit and the OPE parts. The solvatochromism is
accompanied by the emergence of circular dichroism. More in
particular, the CD spectra of P1-3 are composed of a bisignate
Cotton effect in the π-π* transition with a zero-crossing
around 450-480 nm, originating from chiral exciton coupling
of chirally stacked polymer chains,5a together with a mono-
signate Cotton effect (positive for P1 and P3, negative for P2)
corresponding to the sharp, red-shifted band.5d,e The CD
spectrum of P4 is unclear and much less intense. Probably,
already some aggregation occurs in good solvent, which com-
plicates the stacking. The poor solubility and presence of
aggregates also explains its divergent UV-vis spectra.
A closer inspection of the UV-vis and CD spectra
(Figure 2) reveals that the transition from unordered to
chirally stacked polymer chains occurs within a rather small
window of solvent composition. For instance, the major
changes in the UV-vis spectra of P3 are observed from
41.9 to 50.0% iPrOH. Interestingly, the CD spectra are even
more sensitive toward the solvent composition: a change
from 41.3 to 41.9% iPrOH already gives rise to CD of
approximately half of the maximal intensity. Moreover,
the maximal intensity is reached at 45.0% iPrOH;further
addition of nonsolvent results in a decrease of the CD
intensity. In this respect, it should be noted that, on the
one hand, UV-vis spectroscopy probes the planarization
(and aggregation) process; CD, on the other hand, is sensi-
tive for chiral aggregation. As a consequence, the UV-vis
spectra depend on the number of aggregates formed, while
the CD spectra are determined by both the number of
aggregates and their chirality and, more in particular, by
A decrease of the solvent quality results in a decrease of the
emission intensity together with a red-shift (Figure S2). This
is fully in line with a planarization and stacking of the
polymer chains. The residual emission does not originate
from remaining unaggregated polymer chains, as this cannot
account for the red-shift. Therefore, the fluorescence must
arise from the aggregates.
The possibility to selectively oxidize the BT units in the
copolymers was investigated by cyclic voltammetry and
chemical oxidation. A pseudoreversible oxidation at E1/2
≈
0.9 V (scan rate = 100 mV/s), independent of the length of
the OPE, was observed. The fact that oxidation occurs at a
much lower potential than that of poly(phenylene ethylene)s
and irrespective of the length of the OPE, demonstrates that
the BT part is selectively oxidized. The polymer solutions
could also be oxidized by a chemical oxidant (FeCl3), form-
ing new bands near 700 and 1000 nm (Figure S3).
From the chiroptical characterization, it can be concluded
that, in terms of chiral expression, sensitivity toward external
stimuli (e.g., nonsolvent), solubility, and synthetic accessi-
bility, the trimeric OPE gives the best results, and therefore, it
was decided to incorporate the carboxylic acid functions in
ter(phenylene ethynylene)s. Indeed, since we anticipated that
the incorporation of carboxylic acid groups decreases the
solubility (which is also the case, see later), we opted to use a
functionalized OPE composed of only one phenyl group
bearing carboxylic acid functions and at least one alkyl-
functionalized phenyl moiety. This excludes the use of a
monomeric OPE. A dimeric and tetrameric OPE was not
preferred, since the synthesis of such an asymmetric mono-
mer is not straightforward and since it introduces additional
problems of regioregularity in the resulting polymer. In
addition, the absence of the targeted chiroptical behavior
in the related polymer (P4) impeded the use of the tetrameric
OPE. For synthetic convenience, the functional group was
implemented at the central phenyl group of the trimeric OPE.
Polymers P5a-8a. In a next step, an ester/carboxylic acid
function was introduced in the polymers (P5a-8a). In order
to investigate whether the presence of this group affects the
(chiral) organization of the copolymers, the influence of the
solvent quality on the UV-vis and CD spectra of P5a and
P7a was examined (Figure 3).
For these experiments, it appeared not to be necessary to
use iPrOH, and hence, methanol was used. For both poly-
mers, chiral aggregation could be induced, but the shape of
the spectra differs. While the spectra of P7a resemble more or
less those of P1-4, the spectra of aggregated P5a lack the
sharp, low-energy band with the corresponding Cotton
effect. Since this band originates from a (chiral) aggregate
of multiple polymer chains and while the broad band with

7418 Macromolecules, Vol. 43, No. 18, 2010
Vandeleene et al.
Figure 2. UV-vis spectra of P1 (c = 11.2 ꢀ 10-3 g/L) (a), P2 (c = 12.0 ꢀ 10-3 g/L) (b), P3 (c = 12.5 ꢀ 10-3 g/L) (e), and P4 (c = 12.5 ꢀ 10-3 g/L)
(f ) and CD spectra of P1 (c), P2 (d), P3 (g), and P4 (h).
bisignate Cotton effects stems from (an ensemble of) chirally
oriented polymer chains, these findings suggest that P5a does
not form an aggregated superstructure under poor solvent
conditions. This might be correlated with the absence of a
melting peak in the DSC experiments. The disappearance of
the formation of such a chiral superstructure in P5a can

Article
Macromolecules, Vol. 43, No. 18, 2010 7419
Figure 3. UV-vis spectra of P5a (c = 13.5 ꢀ 10-3 g/L) (a) and P7a (c = 14.0 ꢀ 10-3 g/L) (b) and CD spectra of P5a (c) and P7a (d).
temptatively be attributed to the presence of the ester
functionality close to the polymer backbone, which might
disrupt the formation of such a regular structure. Indeed,
lamellar aggregation of P5a incorporates the (polar) ester
functionality into the alkyl phase. In the case of P7a, an all-
trans conformation of the longer alkyl spacer locates the
ester function at the periphery of the alkyl phase, which is
energetically more favorable.
Polymers P5b-8b. In a first series of experiments, it was
again verified whether the deprotected polymers P5b-P8b
show the same chiroptical behavior as their protected coun-
terparts. The solvatochromism experiments (Figure S4) of
P5b-P8b again reveal that all polymers show a red-shift but
that only P7b-P8b display the low-energy band originating
from the superstructure composed of aggregated polymer
chains. The quenching of the fluorescence was also more
pronounced for the latter polymers. Quite remarkably, no
Cotton effects are observed for the chiral P7b in a poor
solvent mixture.
In summary, these experiments show that stacking of the
polymer chains can be induced but that in the case of the
shorter alkyl spacers the stacking is complicated and no
chiral resolvation is present. This might be attributed to
the H-bonding of the carboxylic groups, which overwhelms
the possible chiral induction of the chiral substituents.
Chiroptical Properties in Solution: Interaction with Addi-
tives. Influence of the Additive. Apart from poor solvents,
aggregation can also be induced by additives which are able
to interact with the carboxylic acid functionality. Especially
chiral additives are of interest, as they might induce chiral
aggregation of achiral polymers.2,11,12
polymers. Among the additives tested (carboxylic acids,
alcohols, primary amines, and tertiary amines), only primary
amines resulted in aggregation (Figure 4a). In these cases, a
clear red-shift can be observed together with an additional
shoulder at 514 nm. Clearly, smaller (primary) amines, such
as (S)-(þ)-2-butylamine and (R)-(-)-2-amino-1-butanol, in-
duce aggregation at already low base concentrations, while
more bulky amines, such as (L)-(-)-R-methylbenzylamine,
require a higher concentration. Concerning the chiral expres-
sion (Figure 4b), the more bulky primary amines resulted in the
occurrence of (bisignate) Cotton effects, which implies a trans-
fer of chirality from the amine to the polymer chain upon
interaction with the carboxylate functional group. In contrast,
the small chiral amines ((S)-(þ)-2-butylamine and (R)-(-)-2-
amino-1-butanol), which clearly induce aggregation as shown
by UV-vis spectroscopy, are not capable of inducing a twist of
the stacking of the polymer chains. Therefore, the bulkiness of
the chiral amine seems to be crucial to induce chiral aggregation
of the achiral polymer P8b. Emission spectroscopy also con-
firms the aggregation induced by the (primary) amine, as the
fluorescence of the polymer backbone is quenched upon
aggregation (Figure 4c).
In order to verify whether the aggregation indeed origi-
nates from an interaction between the carboxylic acid and
amine, (L)-(-)-R-methylbenzylamine was added to the ester-
functionalized polymer P8a, which did not change the
spectra at all (Figure S5). Therefore, it can be concluded
that the aggregation is induced by this interaction. The
aggregation induced by the addition of primary amines is
likely due to salt formation, followed by H-bonding between
the ammonium and (two) carboxylate groups.
A broad range of (a)chiral additives were screened. There-
fore, the achiral P8b was dissolved in THF and the additive
was added. The samples were allowed to stabilize for 12 h,
since preliminary experiments showed that some additives
did not instantly interact with the functional groups of the
Similar experiments were also performed on the chiral
polymer P7b. As shown in Figure S6, addition of amines
results in aggregation, as eminent from the slight red shift
(10-15 nm) and the occurrence of a very weak, however
typical, shoulder at 514 nm and the quenching of the

7420 Macromolecules, Vol. 43, No. 18, 2010
Vandeleene et al.
Figure 5. Interaction of different benzylamines (c = 9.65 ꢀ 10-2 M)
with P7b in THF (c = 0.0098 g/L): (a) UV-vis spectra; (b) CD spectra.
Finally, the interaction of amines with P6b was investi-
gated (Figures S7). However, no indication of aggregation
was found, as the UV-vis spectra only display a slight red-
shift, but no typical shoulder near 514 nm. Emission spec-
troscopy also only shows a slight quenching of the fluores-
cence but no evidence of a similar aggregate emission band as
observed for P8b. The decrease in emission intensity can be
attributed to clustering of some polymer chains, resulting in
traps. The absence of interactions between amines and the
functional groups of the polymer in P6b can again be corre-
lated with the polymer structure. Indeed, the carboxylic acid
functional groups in P6b are located close to the polymer
backbone. As a consequence, in lamellar aggregates, the short
alkyl spacer locates the interacting functional groups in the
alkyl phase, which is energetically not favorable.
Influence of Amine Concentration. The influence of the
amine concentration on the chiroptical properties of P8b was
investigated by gradual addition of (L)-(-)-R-methylbenzyl-
amine. Prior to the measurements, all samples were allowed
to stabilize for 12 h. The added amounts of amine largely
exceed the number of carboxylic acid groups (1.6 ꢀ 10-5 M)
present in the polymer solutions. This large discrepancy can
be attributed to the very low concentrations and the relative
small difference in acidity of a carboxylic acid and an
ammonium functionality. As a consequence, the deprotona-
tion of the carboxyl acid by 1 equiv of amine does not result
in complete conversion and higher base concentrations are
required for a complete deprotonation.
Figure 4. Interaction of different additives (c = 2.32 ꢀ 10-2 M) with
P8b in THF solution (c = 0.0106 g/L): (a) UV-vis spectra; (b) CD
spectra; (c) emission spectra.
fluorescence (Figure S6). As shown in Figure S4, P7b
aggregates upon addition of a poor solvent, but no Cotton
effects are observed; that is, no chiral aggregation occurs. On
the other hand, addition of benzylamine does induce chiral
aggregation, as shown by the (small) CD signal (Figure 5).
The possibility to induce chiral aggregation by addition of a
primary amine, which breaks the dimerization of the car-
boxylic acid functionalities, supports the hypothesis that this
dimerization is the preventing factor for a chiral stacking of
P7b in poor solvent mixtures.
Next, the chiroptical behavior of this chiral polymer upon
addition of chiral amines is studied (Figure 5). Therefore,
(L)-(-)-R-methylbenzylamine and (D)-(þ)-R-methylbenzyl-
amine were added to P7b. To compensate for the experimental
error, the experiments were repeated five times and averaged.
Clearly, the Cotton effect induced by (D)-(þ)-R-methyl-
benzylamine is significantly larger than that with (L)-(-)-
R-methylbenzylamine. It also differs from the sum of the
Cotton effect obtained by benzylamine, in which the chiral
alkyl groups on the bithiophene invoke the chiral effects, and
the inverse of the CD spectra of (L)-(-)-R-methylbenzyl-
amine, in which the chirality originates from the chiral
amine, demonstrating the diastereomeric interactions.
As shown in Figure 6c, addition of 2.50 ꢀ 10-6 M of
(L)-(-)-R-methylbenzylamine leads to a slight quenching of
the fluorescence. No red-shift, however, of the fluorescence is
observed, nor are any changes in the UV-vis and CD spectra
present (Figure 6a and b). This suggests that initial, partial
deprotonation induces some clustering of polymer chains,
which serve as energy traps, but no aggregation. At 2.50 ꢀ
10-3 M of base added, also some tailing in the UV-vis
spectra can be observed. This is again correlated with the

Article
Macromolecules, Vol. 43, No. 18, 2010 7421
Figure 8. Schematic presentation of the supramolecular behavior upon
interaction with (chiral) amines of (a) P7b-8b and (b) P5b-6b.
clustering, but no aggregation, as the typical aggregation
band at 514 nm and the appearance of a CD signal are still
absent. From a concentration of 25.0 ꢀ 10-3 M of (L)-(-)-R-
methylbenzylamine, the polymer chains start to aggregate,
as is eminent from the appearance of a CD signal. Slight
increases of amine concentration (50.0 ꢀ 10-3 M) result in
the appearance of large CD signals, the aggregation-related
absorption band at 514 nm, and red-shifted emission of
much smaller intensities;all of which are indications of
the formation of aggregates.
Majority Rules. Finally, the influence of the enantiomeric
excess (ee) of (L)-(-)-R-methylbenzylamine on the intensity
of the Cotton effects of P8b was evaluated (majority rules
experiment). If Δε at 514 nm is evaluated as a function of the
ee of the base added (Figure 7), it is clear that a nonlinear
relationship is present, demonstrating cooperativity. For
instance, at ee = 50, the CD signal already equals that in
which pure (L)-(-)-R-methylbenzylamine was added. A
similar behavior has also been observed for poly(thiophene)s
upon the addition of chiral nonsolvent.12 Moreover, this
cooperative behavior also demonstrates that both enantio-
mers randomly interact with the polymer chains. Indeed, if
each polymer chain exclusively interacts with one isomer and
if each aggregate is exclusively composed of polymer chains
interacting with the same enantiomer, the intensity of the CD
signal would linearly scale with the ee, which is clearly not
observed.
Figure 6. Sensitivity analysis of P8b (c = 0.0106 g/L in THF) upon
addition of (L)-(-)-R-methylbenzylamine: (a) UV-vis spectra; (b) CD
spectra; (c) emission spectra.
Supramolecular Behavior. Figure 8 schematically sum-
marizes the supramolecular behavior of P5b-8b upon inter-
action with (chiral) amines. As presented in Figure 8a,
P7b-8b show the tendency to form polymer clusters at low
amine concentrations. This is observed by a slight quenching
of the fluorescence and small changes in the shape of the
UV-vis spectra. At higher amine concentrations, (chiral)
aggregation is induced.
In contrast, P5b-6b only show the formation of polymer
clusters even at a high amine concentration, as can be
concluded from the UV-vis, CD, and fluorescence spectra.
This phenomenon can be understood in terms of the polymer
structure. In a lamellar supramolecular structure, the func-
tional groups of P5b-6b are partially shielded from their
environment, which complicates interactions of multiple
chains with the (chiral) amines, allowing only polymer
clustering. In contrast, the molecular structure of P7b-8b
Figure 7. Influence of the enantiomeric excess of R-methylbenzylamine
(c = 0.1 M) on Δε at 514 nm of P8b (c = 0.011 g/L) in THF. The
straight line corresponds to a noncooperative behavior and is added as a
guide to the eye.

7422 Macromolecules, Vol. 43, No. 18, 2010
Vandeleene et al.
Figure 9. Influence on the circular dichroism upon dipping a film of P7b in a (a) (L)-(-)-R-methylbenzylamine solution (c = 0.031 M in n-hexane) and
a (b) (D)-(-)-R-methylbenzylamine solution (c = 0.031 M in n-hexane).
facilitates aggregate formation, as the functional groups are
located in the periphery of the alkyl phase, allowing a less
hindered interaction between the amines and different poly-
mer chains.
dichroism can be observed. Moreover, diastereoselectivity and
the “majority-rules principle” could be observed. The amine-
induced aggregation is a multistep process, in which, first
(achiral), clustering occurs and, in a subsequent step, chiral
aggregation. Finally, the diastereomeric behavior could be
exploited in film by dipping them in a solution of chiral amine.
Chiroptical Properties in Film. Finally, also the chiroptical
properties of films of P7b were evaluated. Therefore, films
were spin coated from a THF solution (3.1 mg/mL). As P7b
is chiral, the as-prepared film shows a small bisignate Cotton
effect (Figure 9). Next, the films were dipped in solutions of
either (L)-(-)-R-methylbenzylamine or (D)-(þ)-R-methyl-
benzylamine in n-hexane, rinsed with n-hexane (in order to
remove residual amine), and dried. The experiments were
repeated three times in order to verify the reproducibility.
The interaction with (L)-(-)-R-methylbenzylamine results in
an inversion of the CD signal (Figure 9a), while exposure to
(D)-(þ)-R-methylbenzylamine increases the magnitude of
the CD signal (Figure 9b). Importantly, treatment of the
samples with pure n-hexane did not affect the CD signal,
which excludes solvent-induced annealing effects. These
results demonstrate the potential of this polymer as a chiral
sensor. Note that the signals in the CD spectra at λ>600 nm,
at which wavelength the polymer does not absorb, can-
not be due to circular dichroism but probably arise from
scattering.
Acknowledgment. We are grateful to the Katholieke Uni-
versiteit Leuven (GOA and Onderzoeksfonds K.U.Leuven/
Research Fund K.U.Leuven), the Fund for Scientific Research
(FWO-Vlaanderen), and the Air Force Office of Scientific
Research, Air Force Material Command, USAF, for financial
support. G.K. is a postdoctoral fellow of the Fund for Scientific
Research (FWO-Vlaanderen).
Supporting Information Available: IR spectra of the functio-
nalized polymers, showing the conversion of the ester functional
group into a carboxylic acid group. Also, the emission spectra of
P1-4, in both their dissolved and aggregated states. The UV-vis
spectra of P1-4 upon oxidation with FeCl3 are shown. Figures
concerning the functionalized polymers are included, showing the
solvatochromism of P7b and P8b, the interaction of P8a with
(L)-(-)-R-methylbenzylamine, and the interaction of P6b and P7b
1
with different (chiral) amines. Finally, H NMR and 13C NMR
spectra of the new compounds are presented. This material is
available free of charge via the Internet at http://pubs.acs.org.
Conclusion
References and Notes
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