Steering the conformation and chiroptical properties of poly(dithienopyrrole)s substituted with chiral OPV side chains

Article abstract of DOI:10.1021/ma902762z

This manuscript discusses the conformation and chiroptical properties of poly(dithienopyrrole)s (PDTPs), substituted with oligo(phenylenevinylene) (OPV) side chains and the influence of the substitution of the OPV moiety on these features. The OPV side ch

Full text of DOI:10.1021/ma902762z

Macromolecules 2010, 43, 2157–2168 2157
DOI: 10.1021/ma902762z
Steering the Conformation and Chiroptical Properties of
Poly(dithienopyrrole)s Substituted with Chiral OPV Side Chains.
Wouter Vanormelingen,†,‡ Lesley Pandey,‡,§ Mark Van der Auweraer,‡,§ Thierry Verbiest,†,‡
and Guy Koeckelberghs*,†,‡
†Laboratory of Molecular Electronics and Photonics, ‡Institute for Nanoscale Physics and Chemistry, and
§Laboratory of Photochemistry and Spectroscopy, Katholieke Universiteit Leuven, Celestijnenlaan 200F,
B-3001 Heverlee, Belgium
Received December 15, 2009; Revised Manuscript Received January 18, 2010
ABSTRACT: This manuscript discusses the conformation and chiroptical properties of poly(dithieno-
pyrrole)s (PDTPs), substituted with oligo(phenylenevinylene) (OPV) side chains and the influence of the
substitution of the OPV moiety on these features. The OPV side chains were equipped with gallic acid moieties
in order to promote the formation of a helical conformation in poor solvents. The polymers were prepared by
Stille-couplings and characterized by GPC and NMR, UV-vis, CD, and emission spectroscopy. It was found
that OPV-PDTPs, solely equipped with (chiral) alkyl groups at the terminal gallic acid group, show a very
strong tendency to adopt a helical conformation, but no resolution of the mixture of helices and therefore no
chiral expression. Additional substitution of the OPV allows for a discrimination of the mixture of helical
senses. In this way, the OPV side chains can be chirally organized by the helical PDTP backbone. Substitution
of the OPV in R-position, however, sterically excludes the possibility to adopt a helical conformation, but
results in a lamellar supramolecular structure in poor solvents. The macromolecular behavior is explained in
terms of space confinement and steric hindrance in the respective structures.
Introduction
way, the gallic acid substituents are chirally organized by the one-
handed helical PDTP backbone. Thisis, for instance, expressed in
Conjugated polymers have been extensively studied and imple-
mented as active materials in, for instance, transistors, organic
light-emitting diodes, and solar cells.1 Although most conjugated
polymers are equipped with substituents, the optical and electro-
nic properties of the material typically originate from the con-
jugated backbone. The substituents, usually aliphatic groups, can
indeed ensure a good solubility and even promote the formation
of a particular supramolecular, for instance lamellar, structure,
but they normally do not contribute to the polymer’s optical or
electronic properties.
the circular dichroism (CD) spectra, which also show a contri-
bution originating from this group.
It is clear that the presence of this gallic acid moiety results in (i)
a one-handed helical conformation of the backbone, which, on its
turn, can organize the substituent; and (ii) a contribution of the
substituent to the optical and electronic properties of the poly-
mer. Nevertheless, the contribution of the side chain remains
limited in the sense that it does not absorb in the visible, but UV
region. However, the contribution of the substituent (in the
absorption range of the polymer) could be significantly enhanced
if side chains with much larger conjugation lengths, such as
oligo(phenylenevinylene) (OPV), are employed. Furthermore,
the absorption of the side chain can be tuned in such way that
it allows an efficient energy transfer to the polymer backbone.
Moreover, this approach also provides a possibility to chirally
align conjugated oligomers by the helical backbone.
This manuscript reports on the synthesis and chiroptical
properties of PDTPs, equipped with OPVs. The OPVs are
functionalized by gallic acid moieties in order to promote a
possible formation of a helical conformation in poor solvents,
but differ in the amount of solubilizing alkoxy-groups. The
influence of the substitution on the OPV side chain on the
conformation and chiroptical properties of the polymers is
studied. Figure 1 shows structures of polymers Pa-Pc.
Poly(dithieno[3,2-b:20,30-d]pyrrole) (PDTP),2 are a particu-
larly interesting class of conjugated polymers, as they show high
conjugation lengths, can easily be oxidized and remain oxidized
for prolonged periods of time. Substitution on the N atom with
sufficiently bulky alkyl groups renders them soluble.2e-h More-
over, the nature of the substituent also plays a crucial role in the
macromolecular and supramolecular structure of the polymers in
poor solvents. If (branched) alkyl groups are used, the polymer
strands stack.2g In contrast, if a tri(alkyl)-substituted gallic acid
moiety is employed, the combination of space confinement,
π-stacking and van der Waals interactions favors a helical
conformation.2h This difference becomes even more clear if chiral
substituents are employed. In the former case, aggregates, com-
posed of chirally stacked (planar) polymer chains are obtained,
giving rise to chiral effects in the absorption band of the polymer
backbone. In the latter case, the chirality of the substituents,
together with proper space confinement and the rigidity of
the benzene moiety, results in an efficient discrimination of the
two enantiomeric helices3 and a one-handed helical backbone,
decorated with the gallic acid substituents, is obtained. In this
Experimental Section
Materials and Instrumentation. All starting materials were
purchased from Acros Organics, Fluka, Merck, or Aldrich and
were used as received. N,N-Dimethylformamide (DMF), toluene,
and THF were dried by distillation over calcium hydride, molecular
sieves, and Na/K, respectively. Gel permeation chromatography
*Corresonding author. E-mail: guy.koeckelberghs@chem.kuleuven.be.
r 2010 American Chemical Society
Published on Web 02/04/2010
pubs.acs.org/Macromolecules

2158 Macromolecules, Vol. 43, No. 5, 2010
Vanormelingen et al.
[R]2D0=þ5.0 deg mL g-1 dm-1 (c=1.6 in THF).
3
3
3
1H NMR (CDCl3): δ = 6.84 (s, 2H), 4.53 (s, 4H), 3.80
(m, 4H), 1.90 (m, 2H), 1.60 (m, 2H), 1.29 (m, 2H), 1.06 (d,
6H), 0.97 (t, 6H).
13C NMR (CDCl3): δ = 150.8, 127.5, 114.4, 73.6, 50.8,
35.0, 29.9, 26.3, 16.8, 11.5
MS: m/z=911 (2 ꢀ M þ Kþ)
Synthesis of (þ)-2,5-Di((S)-2-methylbutoxy)-1,4-di(diethoxy-
phosphonylmethyl)benzene (3b). A mixture of 2b (14.0 g,
32.1 mmol) in triethylphosphite (100 mL) was refluxed under
an argon atmosphere for 3 h. After removal of ethyl bromide
and triethylphosphite under reduced pressure, a colorless solid
was obtained, which was used without further purification.
Yield: 16.9 g (96%).
Mp: 62-65 °C.
[R]2D0=þ6.9 deg mL g-1 dm-1 (c=6.1 in THF).
3
3
3
1H NMR (CDCl3): δ = 6.92 (s, 2H), 4.01 (m, 8H),
3.73 (m, 4H), 3.23 (d, J = 20.1 Hz, 4H), 1.85 (m, 2H),
1.57 (m, 2H), 1.30 (m, 2H), 1.23 (t, 12H), 1.02 (d, 6H),
0.94 (t, 6H).
Figure 1. Structure of the polymers.
Synthesis of 5a. Under argon atmosphere, a solution of 3a
(4.16 g, 11.0 mmol) in dry THF (10 mL) was added to a
suspension of NaH (0.260 g, 11.0 mmol) in dry THF (20 mL).
The mixture was refluxed for 5 min, after which a solution of
4a (5.74 g, 10.0 mmol) in dry THF (10 mL) was added. The
reaction mixture was refluxed for an additional 2 h and was
allowed to cool down. An aqueous HCl solution (1 M,
20 mL) and CH2Cl2 were added and the aqueous layer was
extracted with CH2Cl2. The combined organic layers were
washed with an aqueous NaHCO3 solution and H2O and
dried over MgSO4. After removal of the organic solvents
under reduced pressure, the crude product was purified by
column chromatography (SiO2, CH2Cl2 and CH2Cl2/MeOH
95/5), resulting in a colorless oil.
(GPC) measurements were done in tetrahydrofuran (THF) as
eluent toward polystyrene standards with a Shimadzu 10A appa-
ratus equipped with a tunable absorbance detector and a differ-
ential refractometer. 1H and 13C nuclear magnetic resonance
(NMR) measurements were carried out with a Bruker Avance
300 MHz. UV-vis spectra were measured on a Varian Cary 400 or
a Perkin-Elmer Lambda 40 UV/vis spectrophotometer; the CD
spectra were recorded with and a JASCO 62 DS apparatus. The
DSC measurements were performed on a Perkin-Elmer DSC 7
apparatus. Optical rotation was done on a Polaar 20; the concen-
tration (in g/100 mL) and solvent used are given in parentheses. PL
emission spectra were taken on a Fluorolog-3 FL3-22 spectrometer
from Horiba Yobin Yvon. PL quantum efficiencies (ΦPL) were
determined using relative measurements with fluorescence refer-
ences. The absorbance of the solutions used to determine the
fluorescence quantum yields and excitation spectra did not exceed
0.1 at the excitation wavelength. All solutions were degassed prior
to the emission experiments. The fluorescence decay times were
determined using a single photon timing setup described in detail
elsewhere.4 The frequency doubled output (488 nm, 4.09 MHz,
1.2 ps fwhm) of a mode-locked Ti:sapphire laser (Tsunami, Spectra
Physics) was used as vertically polarized excitation light. The
fluorescence was via a polarizer (magic angle) and focused on a
monochromator (Sciencetech 9030) and detected with a micro-
channel plate (R3809U, Hamamatsu). Data acquisition was done
using a time correlated single photon card (TCSPC PC card, SPC
630, Becker & Hickl GmbH, Germany). The experimental instru-
ment response function was on the order of 50 ps. Decays were
collected in 4096 channels. The decays were analyzed globally with
a time-resolved fluorescence analysis (TRFA) software, based on
iterative reconvolution and a nonlinear least-squares approach.5
The fitting was judged by the value of the reduced χ2 parameter and
also by the appearance of the residuals and the autocorrelation
function of the residuals.
Yield: 2.55 g (32%)
[R]2D0=-4.33 deg mL g-1 dm-1 (c=4.2 in THF).
3
3
3
1H NMR (CD2Cl2): δ=7.49 (d, J=8.2 Hz, 2H), 7.31 (dd,
J=8.2 Hz, 1.8 Hz, 2H), 7.05 (s, 2H), 6.77 (s, 2H), 4.05 (m,
10H), 3.13 (d, J=21.9 Hz, 2H), 1.85 (m, 6H), 1.60 (m, 6H),
1.31 (m, 18H), 1.17 (t, 6H), 0.88 (d, 6H), 0.84 (d, 3H), 0.83 (d,
18H).
13C NMR (CDCl3): δ=153.7, 138.4, 136.4, 132.9, 131.6,
131.4, 130.5, 129.1, 127.4, 126.7, 105.1, 71.9, 67.5, 62.4, 39.7,
37.8, 36.9, 34.7, 32.8, 30.2, 28.4, 25.2, 23.0, 22.8, 19.8, 16.6.
MS: m/z=822.2 (Mþ þ Naþ).
Synthesis of 5b. The same procedure was followed as
described for the synthesis of 5a, starting from 3b (5.00 g,
9.10 mmol), NaH (0.218 g, 9.10 mmol), and 4b (4.46 g,
9.10 mmol). The crude product was purified by column
chromatography (SiO2, hexane/EtOAc 1/1) and isolated as
a pale yellow, green fluorescent solid.
Yield: 1.91 g (24%).
Mp: 28-31 °C.
1b,6 3a,7 4a,8 4b,9 6,10 and 711 were synthesized according to
literature procedures.
[R]2D0=þ2.6 deg mL g-1 dm-1 (c=1.1 in THF).
3
3
3
1H NMR (CD2Cl2): δ=7.36 (d, J=16.4 Hz, 1H), 7.07
(s, 1H), 7.01 (d, J=16.4 Hz, 1H), 6.92 (d, J=2.7 Hz, 1H), 6.72
(s, 2H), 3.91 (m, 14H), 3.21 (d, J=22.0 Hz, 2H), 1.76 (m, 10
H), 1.48 (m, 6H), 1.32 (m, 26H), 1.24 (t, 6H), 1.07 (d, 3H),
1.05 (d, 3H), 0.97 (t, 3H), 0.97 (t, 3H), 0.88 (t, 9H).
Synthesis
Synthesis of the OPV-DTP, 9a and 9b. Synthesis of (þ)-
2,5-Di(bromomethyl)-1,4-di((S)-2-methylbutoxy)benzene (2b).
Paraformaldehyde (4.07 g, 136 mmol) was added to a solution
of HBr (45 mL, 33% in AcOH) and 1b (11.3 g, 45.2 mmol) in
AcOH (200 mL). After being stirred at 60 °C for 1 h, the
reaction mixture was allowed to cool down and subsequently
filtered. The precipitate was washed with H2O, an aqueous
NaHCO3 solution, H2O, and MeOH and dried under reduced
pressure, yielding white crystals.
13C NMR (CD2Cl2): δ=153.7, 151.5, 151.4, 150.8, 138.3,
133.6, 128.8, 126.2, 126.1, 122.8, 121.2, 121.1, 116.5, 74.7,
74.1, 73.7, 69.2, 62.2, 62.1, 35.6, 35.5, 32.4, 32.3, 30.8, 30.0,
29.9, 29.8, 26.8, 26.6, 23.1, 16.9, 16.8, 16.7, 16.6, 14.3, 11.7.
MS: m/z=1796 (2 ꢀ M þ Naþ).
Synthesis of N-(4-Formylfenyl)dithieno[3,2-b:20,30-d]-
pyrrole (8). A mixture of 7 (3.44 g, 10.6 mmol), NaOt-
Bu (2.43 g, 25.4 mmol), Pd2dba3 (0.242 g, 0.265 mmol)
Yield: 14.0 g (71%).

Article
Macromolecules, Vol. 43, No. 5, 2010 2159
(dba = dibenzylidene acetone), and DPPF (0.588 g,
1.06 mmol) (DPPF = diphenylphosphinoferrocene) in dry
toluene (50 mL) was purged with argon for 15 min, after
which a solution of 6 (1.84 g, 11.0 mmol) in dry toluene
(10 mL) was added. After being refluxed for 19 h, the
reaction mixture was allowed to cool down and H2O was
added. The aqueous layer was extracted with Et2O twice and
the solvents of the combined organic layers were removed
under reduced pressure. The residue was dissolved in THF
(100 mL) and an aqueous HCl solution (1.0 M, 50 mL) was
added. This mixture was stirred vigorously at room tem-
perature for 1.5 h. Next, Et2O (150 mL) was added and the
organic layer was washed with aqueous NaHCO3 and NaCl
solutions. Purification of the crude product by column
chromatography (SiO2, CH2Cl2/hexane 8/2) yielded a yellow
solid.
(16.4 g, 109 mmol) was added, and the mixture was stirred at
55 °C for one night. Water was carefully added and the
aqueous layer was extracted with hexane (2 ꢀ 100 mL).
The combined organic layers were dried over MgSO4, and
the solvents were removed under reduced pressure. The
crude product was purified by column chromatography
(SiO2, CH2Cl2), yielding a pale yellow liquid.
Yield: 4.20 g (31%)
[R]2D0= þ14.8 deg mL g-1 dm-1 (c=1.5 in THF).
3
3
3
1H NMR (CD2Cl2): δ=10.49 (s, 1H), 7.31 (d, J=2.7 Hz,
1H), 7.12 (dd, J=2.7 Hz, J=9.1 Hz), 6.92 (d, J=9.2 Hz, 1H),
3.81 (m, 4H), 1.88 (m, 2H), 1.57 (m, 2H), 1.28 (m, 2H), 1.04
(d, 3H), 1.00 (d, 3H), 0.96 (t, 3H), 0.94 (t, 3H).
13C NMR (CD2Cl2): δ=189.5, 156.7, 153.5, 125.5, 123.9,
114.6, 111.1, 74.1, 73.8, 35.2, 35.1, 26.4, 16.7, 16.7, 11.4.
MS: m/z=279 (Mþ),209(Mþ - C5H10),138(Mþ - C10H21).
Synthesis of (þ)-2,5-Di((S)-2-methylbutoxy)-4-nitrobenz-
aldehyde (13c). A solution of 12c (5.40 g, 19.7 mmol) and
Yield: 1.03 g (34%).
Mp: 186-190 °C.
1H NMR (CDCl3): δ=10.05 (s, 1H), 8.06 (d, J=8.2 Hz,
2H), 7.77 (d, J=8.2 Hz, 2H), 7.24 (s, 4H).
CuSO4 5H2O (17.0 mg, 70.0 μmol) in acetic acid anhydride
3
(50 mL) was stirred at 45 °C for 48 h. After cooling the
reaction mixture to 10 °C, an aqueous HNO3 solution
(1.90 mL, 70%) was added dropwise while keeping the
temperature in the range of 10-15 °C. After being stirred
for an additional hour at this temperature, the reaction
mixture was extracted with Et2O (2 ꢀ 50 mL) and the
combined organic layers were washed with an aqueous
solution of NaHCO3 and H2O. After removal of the solvent
under reduced pressure, the residue was dissolved in THF
(50 mL) and an aqueous NaOH (1 M, 30 mL) was added. The
reaction mixture was stirred vigorously at room temperature
for 2 h and then extracted with Et2O (2 ꢀ 50 mL). The
combined organic layers were washed with aqueous HCl
(1 M), a NaHCO3 solution and H2O. Purification of the
crude product by column chromatography (SiO2, EtOAc/
hexane 1/9) yielded a yellow solid.
Synthesis of 9a. A solution of 5a (5.70 g, 7.10 mmol) in dry,
argon purged THF (15 mL) was added to a suspension of
NaH (0.170 g, 7.10 mmol) and 8 (1.84 g, 6.50 mmol) in dry,
argon purged THF (10 mL). The reaction mixture was
refluxed for 1 h, and after the mixture had cooled down,
CH2Cl2 (30 mL) and an aqueous HCl solution (1 M, 20 mL)
were added. The organic and aqueous layers were separated
and the latter was again extracted with CH2Cl2 (30 mL). The
combined organic layers were dried over MgSO4, and after
removal of the solvents, purification by column chromato-
graphy (SiO2, CH2Cl2) yielded a yellow solid.
Yield: 1.02 g (55%).
Mp: 76-77 °C
[R]2D0=-6.03 deg mL g-1 dm-1 (c=3.3 in THF).
3
3
3
1H NMR (CD2Cl2): δ=7.70 (d, J=8.2 Hz, 2H), 7.61 (d, J=
8.2 Hz, 2H), 7.54 (s, 4H), 7.22 (s, 4H), 7.19 (s, 2H), 7.08 (d, J=
16.4 Hz, 1H), 7.02 (d, J=16.4 Hz, 1H), 6.75 (s, 2H), 3.96 (m,
6H), 1.71 (m, 12H), 1.26 (m, 18H), 0.97 (d, 6H), 0.93 (d, 3H),
0.88 (d, 18H).
Yield: 4.30 g (68%).
Mp: 64-66 °C
[R]2D0=þ9.8 deg mL g-1 dm-1 (c=6.7 in THF).
3
3
3
1H NMR (CDCl3): δ=10.50 (s, 1H), 7.51 (s, 1H), 7.40
(s, 1H), 3.92 (m, 4H), 1.92 (m, 2H), 1.56 (m, 2H), 1.31
(m, 2H), 1.06 (d, 3H), 1.02 (d, 3H), 0.94 (m, 6H).
13C NMR (CDCl3): δ=188.2, 154.6, 146.0, 143.7, 127.6,
113.5, 109.9, 74.9, 74.3, 34.7, 34.6, 26.1, 25.9, 16.5, 16.4, 11.3.
MS: m/z=324 (Mþ), 254(Mþ - C5H10), 184(Mþ - C10H20)
Synthesis of 14c. A solution of 5b (0.531 g, 0.600 mmol) in
dry THF (10 mL) was added to a suspension of NaH (14.4
mg, 0.600 mmol) and 13c (0.177 g, 0.546 mmol) in dry THF
(5 mL). After 2 h of reflux, an aqueous HCl solution (1 M)
was added. The aqueous layer was extracted with Et2O (2 ꢀ
30 mL) and the combined organic layers were washed with an
aqueous NaHCO3 solution and H2O. After drying over
MgSO4, the organic solvents were removed and purification
of the crude product with column chromatography (SiO2,
CH2Cl2/hexane 1/1) yielded an orange solid.
13C NMR (CD2Cl2): δ=153.7, 144.1, 139.2, 138.4, 137.3,
136.7, 135.4, 132.8, 129.2, 128.7, 128.0, 127.6, 127.4, 127.2,
127.0, 123.9, 122.8, 117.3, 112.7, 105.1, 72.0, 67.6, 39.7, 37.7,
36.8, 30.2, 28.3, 25.1, 22.8, 19.7.
MS: m/z=928 (Mþ).
Synthesis of 9b. The same procedure as descibed for 9a was
followed, starting from 5b (1.77 g, 2.00 mmol), NaH (48.0
mg, 2.00 mmol) and 8 (0.514 g, 1.82 mmol). The product was
isolated as yellow solid.
Yield: 1.27 g (69%).
Mp: 107-109 °C
1H NMR (CD2Cl2): δ=7.71 (d, J=9.1 Hz, 2H), 7.61 (d, J=
9.1 Hz, 2H), 7.55 (d, J=16.4 Hz, 1H), 7.41 (d, J=16.4 Hz,
1H), 7.25 (s, 4H), 7.24 (d, J=16.4 Hz, 1H), 7.18 (s, 1H), 7.15
(s, 1H), 7.08 (d, J=16.4 Hz, 1H), 6.75 (s, 2H), 4.01 (t, 4H),
3.91 (m, 6H), 1.99 (m, 2H), 1.82 (m, 4H), 1.68 (m, 4H), 1.37
(m, 32H), 1.24 (d, 3H), 1.12 (d, 3H), 1.02 (t, 6H), 0.89 (t, 9H).
13C NMR (CD2Cl2): δ=153.6, 151.6, 151.3, 144.2, 139.0,
138.2, 136.1, 133.4, 129.0, 127.9, 127.7, 127.3, 126.6, 123.8,
122.9, 122.5, 117.2, 112.7, 110.8, 110.1, 104.8, 74.7, 74.4,
73.7, 69.2, 35.5, 35.4, 32.2, 30.6, 29.9, 29.8, 29.7, 29.6, 26.7,
26.4, 23.0, 17.0, 16.9, 14.2, 11.7, 11.5.
Yield: 2.47 g (80%).
Mp: 75-78 °C
[R]2D0=þ1.0 deg mL g-1 dm-1 (c=24.3 in THF).
3
3
3
1H NMR (CD2Cl2): δ=7.66 (d, J=17.4 Hz, 1H), 7.48 (d,
J=17.4 Hz, 1H), 7.45 (s, 1H), 7.39 (d, J=16.4 Hz, 1H), 7.32
(s, 1H), 7.18 (s, 1H), 7.14 (s, 1H), 7.07 (d, J=16.4 Hz, 1H),
6.74 (s, 2H), 4.00 (t, 6H), 3.89 (m, 8H), 1.97 (m, 4H), 1.82 (m,
4H), 1.66 (m, 6H), 1.36 (m, 34H), 1.04 (m, 24H), 0.88 (t, 9H).
13C NMR (CD2Cl2): δ=153.6, 151.8, 151.3, 149.8, 147.9,
138.3, 137.9, 134.2, 133.3, 129.5, 128.2, 127.3, 126.2, 122.3,
121.2, 112.0, 110.5, 110.3, 109.7, 104.9, 75.0, 74.6, 74.5, 73.7,
69.2, 35.4, 35.1, 32.2, 30.7, 29.9, 29.8, 29.7, 29.6, 26.7, 26.6,
26.4, 26.2, 23.0, 16.9, 16.8, 16.5, 14.2, 11.6, 11.5, 11.4.
MS: m/z=1016 (Mþ)
Synthesis of the OPV-DTP 9c. Synthesis of (þ)-2,5-
Di((S)-2-methoxy)benzaldehyde (12c). Under nitrogen atmos-
phere, a solution of 2,5-dihydroxybenzaldehyde 10 in dry
DMF (20 mL) was added dropwise to a suspension of
NaH (2.38 g, 99.0 mmol) in dry DMF (80 mL). Next, 11

2160 Macromolecules, Vol. 43, No. 5, 2010
Vanormelingen et al.
MS: m/z=1079 (M þ Naþ).
[R]2D0=-5.12 deg mL g-1 dm-1 (c=3.3 in THF).
3 3 3
Synthesis of 15c. Tin(II) chloride (2.16 g, 11.4 mmol) was
added to a solution of 14c (1.50 g, 1.42 mmol) in EtOAc/
EtOH 1/1 (20 mL). This mixture was refluxed for 45 min,
after which another portion of tin(II) chloride (0.216 g,
1.14 mmol) was added. The reaction mixture was refluxed
for an additional h and then poured in an aqueous NaOH
solution (1 M, 50 mL). The aqueous layer was extracted with
CH2Cl2 (2 ꢀ 100 mL) and the combined organic layers were
washed with an aqueous NaHCO3 solution and water.
The solvents were removed under reduced pressure and the
residue was dissolved in Et2O. A few drops of HCl (37%)
were added, resulting in precipitation of the product as its
ammonium salt, which was filtered off and washed with
Et2O. An aqueous solution of NaOH (1 M) and Et2O was
added and the aqueous layer was extracted with Et2O. After
drying the combined organic layers over MgSO4, the sol-
vents were removed, yielding a yellow solid.
1H NMR (CD2Cl2): δ = 7.69 (d, J = 8.2 Hz, 2H), 7.54
(s, 4H), 7.50 (d, J=8.2 Hz, 2H), 7.24 (s, 2H), 7.19 (s, 2H), 7.08
(d, J=16.4 Hz, 1H), 7.01 (d, J=16.4 Hz, 1H), 6.75 (s, 2H),
4.01 (m, 6H), 1.71 (m, 12H), 1.24 (m, 18H), 0.97 (d, 6H), 0.92
(d, 3H), 0.87 (d, 18H).
13C NMR (CD2Cl2): δ=153.6, 140.8, 138.4, 138.1, 137.4,
136.5, 136.2, 132.7, 129.2, 128.1, 127.3, 127.2, 127.0, 123.0,
117.1, 116.0, 110.7, 105.0, 72.0, 67.6, 39.7, 39.6, 37.9, 36.8,
30.2, 30.1, 28.3, 25.1, 22.8, 22.7, 19.8, 19.7.
MS: m/z=1086 (Mþ).
Synthesis of 16b. The same procedure as described for
compound 16a was followed, starting from 9b (0.810 g, 0.800
mmol) and NBS (0.320 g, 1.80 mmol) in CHCl3 (8 mL). The
product was isolated as a yellow solid.
Yield: 0.721 (77%)
Mp: 120-124 °C
[R]2D0=þ0.6 deg mL g-1 dm-1 (c=5.1 in THF).
3
3
3
z1H NMR (CD2Cl2): δ=7.69 (d, J=8.2 Hz, 2H), 7.56 (d,
J=16.4 Hz, 1H), 7.50 (d, J=8.2 Hz, 2H), 7.41 (d, J=16.4 Hz,
1H), 7.25 (s, 2H), 7.23 (d, J=16.4 Hz, 1H), 7.17 (s, 1H), 7.15
(s, 1H), 7.08 (d, J=16.4 Hz, 1H), 6.75 (s, 2H), 4.02 (t, 4H),
3.91 (m, 6H), 2.00 (m, 2H), 1.81 (m, 4H), 1.78 (m, 4H), 1.68
(m, 4H), 1.37 (m, 32H), 1.14 (d, 3H), 1.12 (d, 3H), 1.02 (t,
6H), 0.89 (t, 9H).
Yield: 1.04 g (71%).
Mp: 96-98 °C.
[R]2D0=þ5.7 deg mL g-1 dm-1 (c=8.6 in THF).
3
3
3
1H NMR (CD2Cl2): δ=7.46 (d, J=16.4 Hz, 1H), 7.38 (d,
J=16.4 Hz, 1H), 7.27 (d, J=16.4 Hz, 1H), 7.16 (s, 1H), 7.10
(s, 1H), 7.07 (s, 1H), 7.04 (d, J=16.4 Hz, 1H), 6.73 (s, 2H),
6.32 (s, 1H), 3.87 (m, 16H), 1.90 (m, 8H), 1.65 (m, 6H), 1.38
(m, 34H), 1.03 (m, 24H), 0.87 (m, 9H).
13C NMR (CD2Cl2): δ=153.6, 151.6, 151.3, 140.9, 138.2,
138.0, 136.9, 133.4, 129.1, 128.0, 127.4, 126.4, 124.3, 123.1,
122.4, 117.1, 116.0, 110.8, 110.7, 110.0, 104.8, 74.6, 74.4,
73.7, 69.2, 35.5, 35.4, 32.2, 30.7, 30.0, 29.9, 29.8, 29.7, 26.8,
26.7, 26.5, 23.0, 17.0, 16.9, 14.2, 11.7, 11.7.
13C NMR (CD2Cl2): δ=153.6, 152.1, 151.5, 151.0, 141.4,
138.0, 133.6, 128.5, 128.2, 125.8, 123.5, 122.7, 118.4, 116.6,
110.5, 109.5, 109.0, 104.7, 100.7, 74.7, 74.6, 74.4, 74.0, 73.7,
69.2, 35.5, 35.2, 32.3, 32.2, 30.7, 29.9, 29.8, 29.7, 26.7, 26.6,
26.5, 23.0, 16.9, 16.7, 14.2, 11.6, 11.4.
MS: m/z=1173 (Mþ), 1013 (Mþ - 2 ꢀ Br).
MS: m/z=1026 (Mþ).
Synthesis of 16c. The same procedure as described
for compound 16a was followed, starting from 9c (0.596 g,
0.500 mmol) and NBS (0.196 g, 1.10 mmol) in CHCl3
(10 mL). Instead of stirring the mixture for 1 h at room
temperature, this reaction mixture was kept at 0 °C for
45 min. A yellow solid was obtained.
Synthesis of 9c. The same procedure as described for 8 was
followed, starting from 7 (0.304 g, 0.94 mmol), NaOtBu
(0.217 g, 2.26 mmol), Pd2dba3 (21.5 mg, 23.5 μmol), DPPF
(52.0 mg, 94.0 μmol), and 15c (0.960 g, 0.940 mmol) in dry,
argon purged, toluene. The crude product was purified by
column chromatography (SiO2, CH2Cl2/petroleum ether),
yielding a yellow solid.
Yield: 0.511 (76%)
Mp: 106-108 °C
[R]2D0=-1.08 deg mL g-1 dm-1 (c=3.7 in THF).
Yield: 0.378 g (34%)
Mp: 53-55 °C
3
3
3
1H NMR (CD2Cl2): δ=7.57 (s, 2H), 7.41 (d, J=16.4 Hz,
1H), 7.39 (s, 1H), 7.22 (s, 1H), 7.15 (s, 1H), 7.08 (d, J=16.4
Hz, 1H), 7.01 (s, 2H), 6.96 (s, 1H), 6.75 (s, 2H), 4.02 (t, 4H),
3.80 (m, 10H), 1.99 (m, 3H), 1.67 (m, 11H), 1.37 (m, 29H),
1.05 (m, 18H), 0.89 (t, 9H), 0.78 (m, 6H).
[R]2D0=þ0.7 deg mL g-1 dm-1 (c=18.0 in THF).
3
3
3
1H NMR (CD2Cl2): δ=7.58 (s, 2H), 7.41 (s, 1H), 7.41 (d,
J=16.4 Hz, 1H), 7.22 (s, 1H), 7.15 (s, 1H), 7.14 (d, J=5.5 Hz,
2H), 7.08 (d, J=16.4 Hz, 1H), 7.04 (s, 1H), 6.98 (d, J=5.5 Hz,
2H), 6.75 (s, 2H), 4.02 (t, 4H), 3.82 (m, 10H), 1.98 (m, 3H),
1.73 (m, 10H), 1.37 (m, 35H), 1.05 (m, 18H), 0.89 (t, 9H), 0.75
(t, 3H), 0.75 (d, 3H).
13C NMR (CD2Cl2): δ=153.7, 151.6, 151.5, 151.2, 147.8,
142.3, 138.4, 133.6, 129.2, 128.0, 127.5, 127.2, 127.1, 124.3,
122.6, 122.3, 116.8, 116.2, 111.7, 110.5, 110.4, 110.0, 105.1,
74.8, 73.8, 69.3, 35.7, 35.5, 35.2, 32.5, 32.4, 30.9, 30.1, 30.0,
29.9, 26.9, 26.8, 26.4, 23.2, 17.2, 17.0, 16.7, 14.5, 11.9, 11.4.
MS: m/z=1368 (Mþ þ Naþ).
13C NMR (CD2Cl2): δ=153.7, 151.6, 151.5, 151.3, 147.9,
145.7, 138.3, 133.5, 129.1, 128.4, 127.3, 127.2, 123.8, 123.0,
122.6, 122.5, 116.3, 113.5, 111.9, 111.8, 110.4, 105.0, 74.9,
74.7, 74.6, 73.8, 69.3, 35.6, 35.4, 35.1, 32.4, 32.3, 30.8, 30.0,
29.8, 26.8, 26.7, 26.6, 26.3, 23.1, 17.1, 17.0, 16.9, 16.6, 14.3,
11.7, 11.3.
Synthesis of 17a. The same procedure as described for
compound 16a was followed, starting from 9a (0.300 g, 0.320
mmol) and N-iodosuccinimide (NIS) (0.158 g, 0.704 mmol)
in CHCl3 (3 mL). The crude product was purified by column
chromatography (SiO2, CH2Cl2/hexane 1/1), yielding a
yellow solid.
MS: m/z=1212 (M þ Naþ þ 2H)
Synthesis of the Monomers. Synthesis of 16a. N-Bromo-
succinimide (NBS) (0.430 g, 2.42 mmol) was added to an ice
cooled solution of 9a (1.00 g, 1.10 mmol) in CHCl3 (5 mL),
shielded from light. After stirring the mixture for 15 min at
0 °C and for 1 h at room temperature, an aqueous NaHSO3
solution was added. The aqueous layer was extracted twice
with CH2Cl2. The combined organic layers were washed with
an aqueous NaHCO3 solution and H2O and dried over
MgSO4. Purification by column chromatography (SiO2,
CH2Cl2/hexane 1/1) yielded a yellow solid.
Yield: 0.294 (78%).
Mp: 96-98 °C
[R]2D0=-5.16 deg mL g-1 dm-1 (c=8.9 in THF).
3
3
3
1H NMR (CDCl3): δ = 7.69 (d, J = 8.2 Hz, 2H), 7.54
(s, 4H), 7.50 (d, J=8.2 Hz, 2H), 7.38 (s, 2H), 7.18 (s, 2H), 7.08
(d, J=16.4 Hz, 1H), 7.01 (d, J=16.4 Hz, 1H), 6.75 (s, 2H),
3.02 (m, 6H), 1.70 (m, 12H), 1.27 (m, 18H), 0.97 (d, 6H), 0.92
(d, 3H), 0.87 (d, 18H).
Yield: 0.857 (72%)
Mp: 84-85 °C
13C NMR (CD2Cl2): δ=153.6, 142.9, 138.5, 138.2, 137.3,
136.5, 136.0, 132.8, 129.2, 129.1, 128.1, 127.3, 127.2, 127.0,

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Macromolecules, Vol. 43, No. 5, 2010 2161
122.8, 121.9, 121.6, 105.1, 72.0, 71.8, 67.6, 39.7, 37.9, 37.7,
36.8, 30.2, 30.1, 28.3, 25.1, 22.9, 22.8, 19.8.
Soxhlet extraction with acetone, hexane, and THF. The
polymer (=THF fraction) was isolated as a black solid.
Yield: 81.5 mg (25%).
MS: m/z=1180 (Mþ).
Synthesis of 18a. A solution of n-BuLi in hexane (2.5 M,
0.300 mL, 0.755 mmol) was added to an ice cooled, with
argon purged solution of 16a (0.400 g, 0.37 mmol) in dry
Et2O (8 mL). After the mixture was stirred for 15 min at 0 °C,
Bu3SnCl (0.240 mL, 0.869 mmol) was added. The reaction
mixture was allowed to slowly reach room temperature and
was stirred at room temperature for one night. The solvent
was removed under reduced pressure and hexane was added
to the residue. After filtration and removal of the organic
solvents in vacuo, a yellow, viscous oil was obtained.
Yield: 0.535 g (96%)
Synthesis of Pc. The same procedure as described for Pa
was used, starting from a solution of 18c (0.299 g, 0.169
mmol) and 16c (0.227 g, 0.169 mmol) in dry, argon purged
toluene (13 mL) and a solution of CuO (13.4 mg, 0.169
mmol) and Pd(PPh3)4 (8.4 μmol, 9.7 mg) in dry, argon
purged DMF (13 mL). The polymer is fractionated by
Soxhlet extraction with acetone, 1,4-dioxane under reduced
pressure and THF. The polymer (=THF fraction) was
isolated as a black solid.
Yield: 43.8 mg (11%).
[R]2D0=þ8.46 deg mL g-1 dm-1 (c=0.4 in THF).
Results and Discussion
3
3
3
1H NMR (CDCl3): δ=7.71 (d, J=8.2 Hz, 2H), 7.64 (d, J=
8.2 Hz, 2H), 7.55 (s, 4H), 7.20 (s, 4H), 7.08 (d, J=16.4 Hz,
1H), 7.02 (d, J=16.4 Hz, 1H), 6.75 (s, 2H), 4.02 (m, 6H), 1.71
(m, 24H), 1.23 (m, 46H), 0.89 (m, 47H).
Synthesis of the Dithienopyrroles. The synthesis of the
OPV-functionalized DTP-building blocks for the prepara-
tion of Pa and Pb is depicted in Scheme 1. The key reaction is
the Wittig-Horner coupling between the phosphonate 5a,b
and the benzaldehyde-functionalized DTP 8. 1H NMR
spectroscopy confirmed the E-configuration of the double
bond. The synthesis of 5a,b is straightforward and starts with
a double bromomethylation of the alkylated hydroquinone
1b and subsequent Arbuzov reaction, yielding the bis-
(phosphonate) 2b. A Wittig-Horner reaction using equi-
molar amounts of the aldehyde 4a,b and bis(phosphonate)s
3a,b resulted in the targeted 5a,b. 8, in its turn, was prepared
by a Buchwald-Hartwig reaction of the acetal-protected
p-aminobenzaldehyde and 7 and subsequent deprotection of
the aldehyde functionality.
When employing a similar procedure to prepare 9c, we
experienced great difficulties with the Buchwald-Hartwig
reaction of 7 and the corresponding dialkoxy-derivative of 6
(i.e., 2-(4-amino-2,5-dialkoxyphenyl)-1,3-dioxalane), which
could be attributed to the instability of the latter compound.
Therefore, an alternative strategy was used (Scheme 2), in
which the formation of such very electron-rich aniline deri-
vative was avoided. After alkylation of 10, 12c was nitrated,
and this aldehyde was coupled to the phosphonate 5b,
building the OPV moiety. Then, the nitro functionality
was reduced to the amine using SnCl2. Finally, the OPV-
functionalized DTP was formed by a Buchwald-Hartwig
reaction.
Polymer Synthesis and Characterization. The polymers
were prepared using a Stille-coupling, since, compared with
other polymerization methodologies, such as chemical or
electrochemical oxidation, this method produces PDTPs
with the highest molar mass and allows optimal control of
the molecular structure.2g It was preferred to use diiodated
DTPs as dihalogenated monomers, but, unfortunately, reac-
tion with N-iodosuccinimide (NIS) and 9, was only success-
ful for 9a (Scheme 3). Because the dibromation of 9 (with
N-bromosuccinimide (NBS)) could, in contrast, readily be
performed on 9a-c, Pa was prepared using diiodated DTP
(17a), while Pb-c were synthesized from the dibromated DTP
monomers (16b-c). The distannylated monomers were pre-
pared from 16a-c in a two-step process, consisting of a
Li-Br exchange reaction using n-BuLi and quenching the
dilithium salt with Bu3SnCl. The polymerization was per-
formed using Pd[PPh3]4 as a catalyst in the presence of
CuO.12 After polymerization and precipitation, the polymers
were washed with several solvents (see Experimental Section)
using a Soxhlet extraction apparatus in order to remove
byproduct and oligomers; typically, the THF-soluble frac-
tion, being the highest-molar mass fraction, was used for
further experiments.
MS: m/z = 1218 (Mþ - SnC12H24), 929 (Mþ - 2 ꢀ
SnC12H24).
Synthesis of 18b. The same procedure as described for 18a
was used, starting from 16b (0.300 g, 0.256 mmol) in dry Et2O
(12 mL), n-BuLi (0.215 mL, 2.5 M in hexane, 0.538 mmol)
and Bu3SnCl (0.153 mL, 0.560 mmol). The product was
isolated as a yellow, viscous oil.
Yield: 0.394 g (97%)
1H NMR (CDCl3): δ=7.71 (d, J=8.2 Hz, 2H), 7.63 (d, J=
8.2 Hz, 2H), 7.55 (d, J=16.4 Hz, 1H), 7.41 (d, J=16.4 Hz,
1H), 7.25 (d, J=16.4 Hz, 1H), 7.21 (s, 2H), 7.18 (s, 1H), 7.15
(s, 1H), 7.08 (d, J=16.4 Hz, 1H), 6.75 (s, 2H), 4.02 (t, 4H),
3.91 (m, 6H), 2.00 (m, 2H), 1.82 (m, 4H), 1.59 (m, 18H), 1.32
(m, 46H), 1.08 (m, 12H), 0.89 (m, 39H).
Synthesis of 18c. The same procedure as described for 18a
was used, starting from 16c (0.300 g, 0.223 mmol) in dry
Et2O, n-BuLi (0.187 mL, 2.5 M in hexane, 0.468 mmol), and
Bu3SnCl (0.140 mL, 0.515 mmol). The crude product
was dissolved in CH2Cl2 (2 mL), precipitated in cold MeOH
(-78 °C) and filtered cold. At room temperature, the product
is present as a dark yellow, viscous oil.
Yield: 0.299 g (76%)
1H NMR (CDCl3): δ=7.58 (s, 2H), 7.42 (s, 1H), 7.41 (d,
J=16.4 Hz, 1H), 7.22 (s, 1H), 7.15 (s, 1H), 7.08 (d, J=16.4
Hz, 1H), 7.05 (s, 1H), 6.96 (s, 2H), 6.75 (s, 2H), 4.02 (t, 4H),
3.83 (m, 10H), 1.98 (m, 3H), 1.87 (m, 4H), 1.60 (m, 27H), 1.33
(m, 44H), 1.06 (m, 29H), 0.89 (m, 28H), 0.77 (m, 6H).
Synthesis of the Polymers. Synthesis of Pa. A solution of
18a (0.301 g, 0.200 mmol) and 17a (0.236 g, 0.200 mmol) in
dry, argon purged toluene (15 mL) was added to a sus-
pension of CuO (0.200 mmol, 16.0 mg) and Pd(PPh3)4
(10.0 μmol, 11.8 mg) in dry, argon purged DMF (15 mL).
After the mixture was purged for 30 min, it was refluxed for
13 h. The polymer was precipitated in MeOH, filtered off,
and fractionated by Soxhlet extraction with subsequently
MeOH, 1,4-dioxane under reduced pressure, and THF. The
THF fraction, being the higher molecular-mass fraction, was
concentrated in vacuo and added dropwise to MeOH. The
precipitate was filtered off and dried under reduced pressure,
resulting in a black solid.
Yield: 0.317 g (86%).
Synthesis of Pb. The same procedure as described for Pa
was used, starting from a solution of 18b (0.254 g, 0.160
mmol) and 16b (0.188 g, 0.160 mmol) in dry, argon purged
toluene (12 mL) and a solution of CuO (12.7 mg, 0.160
mmol) and Pd(PPh3)4 (8.0 μmol, 9.2 mg) in dry, argon
purged DMF (12 mL). The polymer was fractionated by

2162 Macromolecules, Vol. 43, No. 5, 2010
Vanormelingen et al.
Scheme 1. Synthesis of the OPV-DTP 9a,ba
a Conditions: (i) HBr (33% in AcOH), p-formaldehyde, (ii), P(OEt)3, (iii) NaH, and (iv) (1) Pd2dba3, DPPF, NaOtBu, (2) H3Oþ.
Scheme 2. Synthesis of the DTP 9ca
a Conditions: (i) NaH, DMF; (ii) (1) Ac2O, CuSO4, (2) HNO3 (70%); (3) NaOH, H2O, (iii) NaH; (iv) SnCl2; (v) Pd2dba3, DPPF, NaOtBu.
The molar mass of the polymers was determined by GPC in
THF toward polystyrene standards. The number-averaged
molar mass (M n) amounts to 17, 9.8, and 18 kg/mol for
Pa, Pb, and Pc, respectively. The polydispersities were rather
low for a polycondensation (1.5, 1.7, and 1.2), which can be
attributed to the intensive fractionation. DSC experiments
revealed no melting, nor glass transition, nor degradation
below 250 °C. Finally, the H NMR spectrum of the poly-
mers (Supporting Information) showed no defects, nor end-
groups, indicative for a regular structure and a relatively high
_
1

Article
Macromolecules, Vol. 43, No. 5, 2010 2163
Scheme 3. Synthesis of the DTP 9ca
a Conditions: (i) NBS; (ii) NIS; (iii) (1) n-BuLi, (2) Bu3SnCl; (iv) Pd[PPh3]4, CuO.
degree of polymerization, which is in line with the choice of
polymerization reaction.
coils;can be obtained if a dichlorobenzene solution is
heated (Figure 3). It is clear that Pa adopts a coil-like
conformation in hot dichlorobenzene, but that it folds into
a helical conformation or stacks upon cooling and further
decreasing the solvent quality. Intermediate spectra are
obtained in o-dichlorobenzene at room temperature. The
fact that a clear isosbestic point is observed suggests that the
transition is a one-step process. In order to discriminate
between the two possible transitions (coil-helix or coil-
aggregate), the concentration dependence of the UV-vis
spectrum in o-dichlorobenzene was investigated (Figure S1,
see Supporting Information), which demonstrates that the
transition occurs intramolecularly, as is also the case for
Pgal. On the other hand, two differences in the conforma-
tional behavior of Pgal and Pa can be observed. First, it can
be concluded that Pa shows a far stronger tendency to fold
into a helical structure than Pgal as, in conditions in which
Pgal solely adopts a coil-like state (THF, chloroform, di-
chlorobenzene, ..., at room temperature), Pa is already
partially present as helices. This can be attributed by the
increased π-interactions of the OPV side chain compared
with the gallic acid substituent, which favors the helical
conformation. A similar behavior has already been observed
for OPV-functionalized poly(ureidophthalimide)s.13 A sec-
ond difference can be observed from the CD spectra in poor
solvents: while the chiral substituent in Pgal can discriminate
between both helical senses, resulting in Cotton effects, no
UV-Vis and CD Spectroscopy. The UV-vis spectra of Pa,
Pb, and Pc in a good (THF) and poor solvent (THF/hexane
or THF/Et2O mixtures) and the corresponding CD spectra
in a poor solvent are displayed in Figure 2b,c. No (or very
small) Cotton effects are observed in a good solvent. For
comparison, also the spectra of Pgal (Figure 2a) are incor-
porated. As previously indicated,2g the spectra of Pgal are
composed of three transitions: one near 270 nm with corre-
sponding bisignate Cotton effects in poor solvent, arising
from the gallic acid substituent, one near 350 nm with
corresponding monosignate Cotton effect in poor solvent,
originating from a localized transition on the polymer back-
bone and the delocalized π-π* transition of the polymer
backbone around 600 nm, which is CD silent.
Naturally, as the conjugation length of the side chain in
Pa-Pc is increased compared with Pgal, also its λmax and
extinction coefficient increase. If the spectrum of Pa is
considered, the absorption near 370 nm is in fact a super-
position of the OPV absorption and the localized backbone
transition. Apart from the shifted side chain absorption, also
the delocalized transition of the backbone differs from that
of Pgal in THF. Decreasing the solvent quality (Figure 2b)
further shifts the spectra. However, a spectrum wherein the
low-energy absorption band resembles that of Pgal in a
THF;in which the polymer is present as (unordered)

2164 Macromolecules, Vol. 43, No. 5, 2010
Vanormelingen et al.
Figure 3. Thermochromism of Pa in o-dichlorobenzene (c = 0.15 g/L).
chiral discrimination can be attributed to the reduced space
filling around the chiral alkyl chains;similar to that in
chirally substituted cis-poly(thiophene)s, which also adopt
a (wide) helical conformation, resulting in insufficient chiral
discrimination.14
In order to increase the chiral discrimination, Pb, in which
the chiral substituents are located closer to the polymer
backbone, was prepared. In this way, the space filling around
the chiral moieties is more demanding and the chirality of the
alkyl groups has an increased ability to discriminate between
both helical senses.
A comparison of the UV-vis spectra of Pa and Pb shows
that the absorption band of the OPV substituent is red-
shifted and that, consequently, this absorption and the
localized absorption of the backbone appear as two separate
bands. From the solvatochromism and the clear presence of
an isosbestic point (Figure 4) it can again be concluded that
two different states are possible, i.e., (unordered) coils and a
helix or aggregates. Although still some residual absorption
of the helix/aggregate is visible in neat THF at room tem-
perature, it is clear that Pb shows a stronger tendency to
adopt a coil conformation than Pa, which can be attributed
to the increased steric hindrance accompanying the intro-
duction of the additional alkoxy groups. Importantly, also
some (small) Cotton effects can be observed in a poor solvent
mixture. Analogously to Pgal, a bisignate Cotton effect is
observed in the absorption band of the substituent. More-
over, the high-energy backbone absorption also shows a
(small) monosignate Cotton effect, as is the case for Pgal.
The presence of such (negative) monosignate Cotton effect,
centered near 350 nm, can be argued from the lower intensity
of the positive lobe of the bisignate Cotton effect compared
with the negative lobe: in that case, the Cotton effect near
350 nm is a superposition of the positive lobe of a bisignate
Cotton effect and a negative monosignate Cotton effect. The
delocalized backbone transition is;similar to that for
Pgal;CD silent.2h
A second step in the chiroptical characterization again
consists of the determination whether a helical conformation
or aggregates are formed in poor solvents. Therefore, the
concentration dependence of the UV-vis and CD spectra
was investigated in solvent conditions in which intermediate
spectra are observed (i. e., THF/Et2O (1/1)) (Supporting
Information, Figure S2). This experiment demonstrates that
also Pb shows a coil-helix transition. As a consequence, it can
be concluded that both the conformational behavior and the
ability of chiral discrimination of Pb resembles those of Pgal.
Furthermore, it also shows a coil-helix transition and the
chiral substituents can discriminate between both helical senses.
Figure 2. (a) Structure of Pgal, (b) UV-vis spectra of Pgal, Pa, Pb, and
Pc in a good solvent, (c) UV-vis spectra of Pgal, Pa, and Pb in a poor
solvent and (d) CD spectra of Pgal, Pa, Pb, and Pc in a poor solvent.
(cPgal = 0.35 g/L, cPa = 0.30 g/L, cPb = 0.30 g/L, and cPc = 0.70 g/L).
Cotton effects are observed for Pa, indicating that a racemic
mixture of both helical enantiomers is formed. This loss of

Article
Macromolecules, Vol. 43, No. 5, 2010 2165
Figure 5. Solvatochromism of Pc in THF/Et2O mixtures: (a) UV-vis
and (b) CD spectra (c = 35 mg/L).
Figure 4. Solvatochromism of Pb in THF/Et2O mixtures: (a) UV-vis
and (b) CD spectra (c = 0.30 g/L).
If, finally, the UV-vis and CD spectra of Pc are examined,
it is obvious that completely different spectra are obtained.
In a good solvent, the UV-vis spectrum is not a super-
position of two different states, instead, it strongly resembles
the spectrum of Pgal in chloroform. The fact that Pc, in
contrast to Pa and Pb, is solely present as unordered coils in
THF can be attributed to the increased steric hindrance,
originating from the additional alkyl groups. In poor
solvents, the UV-vis spectrum, especially the region assign-
able to the backbone transition, undergoes significant
changes, as, for instance, a sharp low-energy band near
660 nm appears. In addition, strong Cotton effects are
observed.
In order to further investigate this, again the dependence
of the UV-vis and CD spectra on the solvent composition
was investigated (Figure 5). Next, the concentration depen-
dence of the spectra was evaluated in a solvent mixture
(THF/Et2O (1/1)) in which intermediate spectra are ob-
served, which revealed that the transition is concentration
dependent (Supporting Information, Figure S3). Therefore,
it must be concluded that the observed transition does not
occur intramolecularly, but intermolecularly, i.e. no helical
conformation is formed, but the polymer strands stacks.
In summary, the UV-vis and CD experiments demon-
strate that Pa and Pb both adopt a helical conformation in a
poor solvent mixture, while Pc instead aggregates. In good
solvent conditions, all polymers tend to adopt an unordered
coil-like conformation. For Pc, this is the case in THF, while
Pa and, to a smaller extent, Pb, are present as a mixture of
both conformations in those conditions. Concerning the
chiral discrimination, chirality is expressed in both Pb and
Pc, but is absent in Pa.
Emission Spectroscopy. In order to confirm the proposed
conformational behavior of the polymers, their emission
spectra and fluorescence decays were measured in good
and poor solvents. In Figure 6, the emission spectra of 9b,c
in THF and Pgal in toluene and THF, which serve as model
compounds for the OPV side chain and the polymer back-
bone in Pb and Pc, respectively, are shown, together with the
emission spectra of Pb and Pc in THF and THF/Et2O (1/9).
Because of the difficulty to force Pa into the nonhelical state,
this polymer was not further studied. While the decays of the
fluorescence of 9b,c at 480 (excitation at 375 nm) could be
analyzed as single exponential decay, the fluorescence decays
of Pgal, Pb, and Pc were generally more complex both for
excitation at 375 nm (OPV-side chains) and 550 nm
(backbone). The fluorescence lifetimes and quantum yields
are summarized in Table 1. Both OPV side chains 9b,c show
similar and intense emission spectra with a maximum at 455
and 465 nm and a shoulder at 481 and 489 nm, respectively.
They display a monoexponential decay with a fluorescence
lifetime of ∼1,3 ns and a fluorescence quantum yield of
∼70%.
The fluorescence spectrum of the polymer backbone Pgal
(excitation at 550 nm) consists in a maximum at 611 nm and a
shoulder at 665 nm, which is slightly more intense in toluene.
The fluorescence quantum yield decreases, however, from
26% in THF to 15% in toluene. The fluorescence decays in
THF and toluene can be analyzed globally from 610 until
665 nm as a sum of two exponentials with decay times of

2166 Macromolecules, Vol. 43, No. 5, 2010
Vanormelingen et al.
Figure 6. Normalized absorbance and emission spectra of (a) 9b-c, (b) Pgal, (c) Pb, and (d) Pc.
Table 1. Fluorescence Lifetimes (τ) and Quantum Yields (Φf) of 9b,
9c, Pb, and Pc in Different Solvents
the polymer adopts two different conformations in THF and
toluene (coil and helix), the nature of the emitting species;
apart from being stacked or not;is hardly distinguishable in
both conformations: the shape of the emission spectra, their
λem and the fluorescence lifetime (τ ∼ 0.6 ns) are the same,
only the quantum yield is significantly different.
If the fluorescence spectra of Pb and Pc are examined, the
first conclusion that emerges, is that excitation of the OPV
side chain results in an (almost) complete energy transfer
from the OPV side chain to the PDTP backbone.
The observed emission therefore arises from the polymer
backbone, regardless whether the side chain or backbone
is excited, and gives information on the macro- and/or
supramolecular structure in which the polymer backbone is
present.
The shape of the emission of Pb is again rather insensitive
toward the solvent and excitation wavelength (375 or
550 nm) used and resembles the spectra of Pgal. The emissive
decay (analyzed globally) is bi- or triexponential, from which
one contribution (0.04 ns) can be attributed to artifacts or
quenching. At both excitation wavelengths the main con-
tribution shows a fluorescence lifetime of ∼0.73 and ∼0.84 ns
in good (THF) and poor (THF/Et2O) solvent, respectively.
When excited at 375 nm, a third component of 1.09 ns (THF)
and 1.31 ns (THF/Et2O) is observed. Upon increasing the
emission wavelength from 560 to 640 nm (THF/Et2O) or
from 600 to 665 nm (THF), the contribution of this compo-
nent slightly increases. This component is probably due to
aggregates of stacked chromophores. One should however
note that over the wavelength range where fluorescence
decays are analyzed, the fluorescence is mainly due to the
component with a decay time of ∼0.8 ns. This is again due to
the fact that at the excitation wavelength used molecules in a
coiled conformation are excited to a major extent (375 nm)
or even nearly exclusively (550 nm). Moreover, the long-
living contribution only becomes visible if the side chain
is excited, which efficiently transfers its energy to the
solvent
λ
ex (nm)
λem (nm)
τ (ns)
Φf
9b
9c
Pgal
THF
THF
THF
toluene
THF
375
375
550
550
375
550
375
550
375
550
375
480
490
610
610
600
600
600
600
625
610
600
700
590
680
1.28
1.32
0.15; 0.59
0.67
0.72
0.26
0.15
0.16
0.21
0.10
0.17
0.31
0.33
0.16
0.10; 0.62
Pb
Pc
0.04;a 0.73; 1.09
0.03;a 0.47; 0.84
0.23; 0.84; 1.31
0.03; 0.89
THF/Et2O
THF
0.67
0.06;a 0.66
0.76, 1.02
0.76, 1.02
THF/Et2O
550
0.09; 0.79
0.11; 1.01
0.16
a Can be attributed to artifact or quenching processes.
∼0.15 and ∼0.6 ns. The ratio of the pre-exponential factors
of the first and second decay time is much smaller than unity
and does not depend on the wavelength (for a complete
survey of all fluorescence data, see Supporting Information,
Table S1). This suggests that in both solvents the emission
originates from a single species. The nonexponential nature
of the fluorescence decay is probably due to energy transfer
to a nonfluorescent traps. If those findings are combined
with the decrease in fluorescence quantum yield from THF
to toluene, it can be concluded that light absorption in THF
almost completely originates from isolated chromophores
while in toluene about one-third of the light is absorbed
by nonfluorescent, stacked (aggregated) chromophores (or
aggregates with a very small fluorescence quantum yield),
which is consistent with a helical conformation in which the
chromophores lay on top of each other. The fraction of
chromophores in the helical conformation will however be
significantly larger than 1/3 as the helix absorbs at much
longer wavelengths than 550 nm. Unfortunately, although

Article
Macromolecules, Vol. 43, No. 5, 2010 2167
Figure 7. Visualization of the structure of (left) Pb in a cis-configuration, (middle) Pc in a cis-configuration and (right) Pc in a planar, trans-
configuration. The cis-configuration promotes a helical conformation, while the trans-configuration can result in lamellar stacks. The simulations were
done with Hyperchem.
Figure 8. Schematic overview of the conformational changes in OPV-substituted PDTPs.
backbone. This indicates that energy transfer populates the
stacked chromophores more efficiently than direct excitation
of the backbone at 550 nm. The latter is confirmed by the
overal drop in fluorescence quantum yield (in spite of
the nearly complete excitation transfer) upon excitation of
the side chains. It should also be noted that these emission
data only indicates that the chromophores are stacked; they
do not provide any information whether this stacking occurs
intra- or intermolecularly. Therefore, it does not conflict
with the results of the UV-vis and CD experiments.
The increase of the ∼0.6 ns fluorescence decay time of Pgal
to ∼0.8 ns can be attributed to the decrease of motional
freedom the DTP segments in Pb due to the increased size of
the side chain. The increase of τ upon decreasing the solvent
quality can be attributed to the decrease of the solvent
polarity (THF versus THF/Et2O), rather than it reflects a
difference in macro- or supramolecular structure. For the
same reason;the decrease of the solvent polarity, the emis-
sion spectra in THF/Et2O are blue-shifted compared with
neat THF. The decrease of the quantum yield in a poor
solvent, finally, can be attributed to the fact that the possible
aggregated chromophores in the helix have a very low
fluorescence quantum yield. This suggests that, in a poor
solvent a larger part of the light is absorbed by or transferred
to aggregates.
The emission spectrum of Pc in THF again resembles the
spectra of Pgal both upon excitation at 375 and 575 nm. The
fluorescence decays of Pc obtained upon excitation at
375 nm in THF can be analyzed globally from 625 until
675 nm as a monoexponential decay (τ ∼ 0.66 ns), demon-
strating that only one emitting type of chromophores is
present; i.e., the backbone in its random coil conformation.
Although upon excitation at 550 nm global analysis of the
decays obtained between 610 and 665 nm requires a second
decay time, the small value of this decay time (0.06 ns)
suggests that this component has to be attributed to an
artifact or trapping by a nonfluorescent trap. The contri-
bution of this component does moreover not depend upon
the analysis wavelength and is very small. Furthermore,
excitation at both 375 and 550 nm yields a similar fluores-
cence quantum yield (31-34%). All these data suggest that
upon excitation at 375 or 550 nm only emission from isolated
chromophores is obtained in THF. The fact that the fluore-
scence lifetime is somewhat higher than for Pgal can again be
attributed to slower segmental motions due to the larger
side chain.
In a poor solvent (THF/Et2O), two emission bands are
observed with maximum at 596 and 688 nm, the latter being
the most intense. While, considering the results obtained for
Pb, the first band can be attributed to isolated chromo-
phores, the second band;being absent in Pb;must be due
to stacked chromophores. Upon excitation at 375 nm global
analysis of the fluorescence decays from 600 until 700 nm
yields a clear biexponential decay with decay times of 0.76
and 1.02 ns. Over this wavelength range the amplitude of the
slowly decaying component increases from 12% to 30%.
Analysis of the fluorescence obtained upon excitation at
550 nm reveals, except for a minor short-living contribution
originating from artifacts or quenching, similar results. This
confirms the hypothesis that the emission near 590 nm can be
attributed to isolated chromophores in some residual coil-
like polymer chains, while the emission near 700 nm arises
from stacked chromophores of aggregated polymer chains.
One should note that, considering the absorption spectra of
coiled and stacked chromophores, excitation at 500 nm will
bias the excitation of coiled chromophores. The increase of
the fluorescence lifetime of the polymer and the correspond-
ing blue shift of the emission in its coil-like conformation
from THF to THF/Et2O (0.66 and 0.78 ns) can again be
attributed to the decrease in polarity of the solvent. The
quantum yield of fluorescence amounts 34% in THF, but is
lower in the poor solvent mixture (16%), reflecting again the

2168 Macromolecules, Vol. 43, No. 5, 2010
Vanormelingen et al.
low fluorescence quantum yield of the aggregated chromo-
phores, already suggest for Pb. Since the fluorescence life-
time in a poor solvent is higher, the decrease in quantum yield
cannot be attributed to quenching processes, but must
instead originate from a lower oscillator strength of the
emitting chromophore in a stacked configuration.
helical senses. In contrast, OPV-PDTPs of which the R-position
of the OPV is substituted, cannot adopt a helical conformation
due to steric restrictions; instead, the polymer strands planarize
and stack in poor solvents. In both cases, the OPV side chains can
be chirally aligned in poor solvents by the polymer backbone and
they contribute significantly to the optical properties of the
material.
It can therefore be concluded that emission spectroscopy
indicates that the same species, i.e. isolated chromophores,
emit in Pgal and Pc in a good solvent. Slightly different
emission properties are found for Pgal and Pb in poor
solvents: in those cases, also (intramolecularly) aggregated
chromophores are found. In Pc in poor solvent, however,
significantly different emission properties, such as a distinct
emission band with a corresponding longer fluorescence
decay time, are measured, which can be attributed to
(intermolecularly) aggregated chromophores. Furthermore,
the amplitudes of the slower decaying component becomes
larger at longer wavelengths (see Supporting Information,
Table S1) Therefore, the fluorescence experiments confirm
the proposed macromolecular and supramolecular behavior
of the polymers.
Acknowledgment. We are grateful to the Katholieke
Universiteit Leuven (GOA) and the Fund for Scientific Research
(FWO-Vlaanderen) for financial support. L.P. is a doctoral
fellow of IWT-Vlaanderen and G.K. is a postdoctoral fellow of
the Fund for Scientific Research (FWO-Vlaanderen).
Supporting Information Available: Figures showing con-
centration dependent UV-vis and/or CD spectra of Pa-c, text
giving an analysis of pre-exponential factors of the fluorescence
decays of 9a,b and Pgal, Pb, and Pc with a table of data, and
figures showing NMR spectra of the new compounds. This
material is available free of charge via the Internet at http://
pubs.acs.org.
Helical Conformation versus Aggregates. It is clear that Pa,
Pb, and Pc show fundamentally different chiroptical proper-
ties, which results from the fact that completely different
structures (random coils, helices, aggregates) are formed. In
order to explain the origin of this totally different behavior in
poor solvents, the conformation of Pb in a helical and Pc in a
helical and planar conformation is visualized (Figure 7). This
clearly reveals that a helical conformation is sterically
allowed for Pb, but that the additional alkoxy substituents
in Pc prevent this conformation, since steric repulsion be-
tween the alkoxy groups of adjacent monomeric units would
force the substituent to adopt a twisted orientation. In this
way, a helical backbone conformation, in which the pendant
OPV side chains are stacked, is prevented. In contrast, a
coplanar conformation poses no restrictions and is sterically
allowed. Therefore, these calculations suggest that Pc does
not adopt a helical conformation since this is sterically not
allowed.
It can therefore be concluded that PDTPs substituted with
OPVs equipped with a gallic acid moiety, adopt an un-
ordered, coiled structure in a good solvent and show a
tendency to fold into a helical conformation upon decreasing
the solvent quality, unless this latter option is impeded by
steric hindrance originating from R-substitution (Figure 8).
It must be noticed that in both ways, the OPV side chains can
be (chirally) organized by the polymer backbone. This has
been visualized in Figure8.
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In conclusion, the conformation and chiroptical properties of
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solvent, organizing the OPV side chains at the periphery, pro-
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OPV moiety is equipped with chiral (alkyl) groups which are
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€
€
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