Chiral ethylhexyl substituents for optically active aggregates of π-conjugated polymers

Article abstract of DOI:10.1021/ja068461t

We report an efficient synthesis of chiral (2S)-ethylhexanol for functionalizing and solubilizing conjugated polymers. The α-substituted chiral ethylhexyl side chains were obtained through a powerful and flexible asymmetric synthesis using pseudoephedrine as a chiral auxiliary. The dependence of the properties of conjugated polymers on molecular structure is investigated by circular dichroism, fluorescence, and absorption spectroscopy on two new chiral conjugated polymers, poly(3,3-bis((S)-2-ethylhexyl)-3,4-dihydro-2H- thieno[3,4-b][1,4]dioxepine) (PProDOT((2S)-ethylhexyl)₂) and poly(3,3-bis((S)-2-methylbutyl)-3,4-dihydro-2H-thieno[3,4-b][1,4]dioxepine) (PProDOT((2S)-methylbutyl)₂). The properties of PProDOT((2S)- ethylhexyl)₂) differ significantly from those of its methylbutyl analog as investigated by chiral aggregation providing insight into the role of interchain interactions in these subsecond switching electrochromic polymers.

Full text of DOI:10.1021/ja068461t

Published on Web 08/09/2007
Chiral Ethylhexyl Substituents for Optically Active Aggregates
of π-Conjugated Polymers
Christophe R. G. Grenier,† Subi J. George,‡ Thomas J. Joncheray,†
E. W. Meijer,‡ and John R. Reynolds*,†
Contribution from The George and Josephine Butler Polymer Laboratory, Department of
Chemistry, Center for Macromolecular Science and Engineering,
GainesVille, Florida 32611-7200, and Laboratory of Macromolecular and Organic Chemistry,
Technische UniVersiteit EindhoVen, P.O. Box 513, 5600 MB EindhoVen, The Netherlands
Received November 25, 2006; Revised Manuscript Received June 24, 2007; E-mail: reynolds@chem.ufl.edu
Abstract: We report an efficient synthesis of chiral (2S)-ethylhexanol for functionalizing and solubilizing
conjugated polymers. The R-substituted chiral ethylhexyl side chains were obtained through a powerful
and flexible asymmetric synthesis using pseudoephedrine as a chiral auxiliary. The dependence of the
properties of conjugated polymers on molecular structure is investigated by circular dichroism, fluorescence,
and absorption spectroscopy on two new chiral conjugated polymers, poly(3,3-bis((S)-2-ethylhexyl)-3,4-
dihydro-2H-thieno[3,4-b][1,4]dioxepine) (PProDOT((2S)-ethylhexyl)2) and poly(3,3-bis((S)-2-methylbutyl)-
3,4-dihydro-2H-thieno[3,4-b][1,4]dioxepine) (PProDOT((2S)-methylbutyl)2). The properties of PProDOT((2S)-
ethylhexyl)2) differ significantly from those of its methylbutyl analog as investigated by chiral aggregation
providing insight into the role of interchain interactions in these subsecond switching electrochromic polymers.
Introduction
The use of circular dichroism (CD) spectroscopy provides a
powerful tool for examining aggregation and ordering behavior
in CPs.5 Here, we report on the first chiral ethylhexyl-func-
tionalized PProDOTs with EC switching comparable to its race-
mic analogues and a sharp transition as a function of potential.3
The impact of the chemistry we report in synthesizing the side-
chain precursor is of broad interest as, to date, only one CP, a
polyfluorene with chiral 2-ethylhexyl side chains, has been
reported.6 In that case, the side chain was obtained by enzymatic
catalysis giving low yields (10%) and providing little flexibility
(only one enantiomer is accessible) in preparing analogues.
Substitution with branched alkyl chains is a highly effective
method for inducing solubility in otherwise insoluble conjugated
polymers (CPs) as their bulkiness efficiently reduces interchain
interactions. CPs substituted with 2-ethylhexyl side chains have
been widely used, such as poly(2-methoxy,5-(2-ethylhexy-
loxy),1,4-phenylenevinylene) (MEH-PPV) which is one of the
most studied and efficient polymers in light-emitting and
photovoltaic applications.1 Recently, our research group reported
the synthesis of spray processable, regiosymmetric ethylhexyl-
substituted poly(3,4-propylenedioxythiophene)s, including PPro-
DOT-(2-ethylhexyl)2 and PProDOT-(CH2O-2-ethylhexyl)2.2,3
These polymers were used in the fabrication of high-contrast,
subsecond switching and high coloration efficiency electro-
chromic (EC) devices. Similar polymers substituted with linear
alkyl chains yielded poorer EC properties.4 It was observed that
the 2-ethylhexyl-substituted polymers undergo a very sharp
transition in luminance and absorption spectrum as a function
of potential.
In this work, two soluble enantiomerically pure disubstituted
poly(3,4-propylenedioxythiophenes) (PProDOTs), PProDOT-
((2S)methylbutyl))2 (P1) and PProDOT-((2S)-ethylhexyl)2 (P2)
were synthesized by Grignard metathesis polymerization.7 Chiral
(2S)-ethylhexanol (4) was obtained by enantioselective alkyla-
tion of an acylated pseudoephedrine, modifying a procedure
outlined by Myers and co-workers .8 This flexible method, using
inexpensive materials, gives high yields and allows the efficient
synthesis of a variety of chiral primary alcohols with variable
substituents on the R-position, along with other derivatives
including chiral acids, acyl chlorides, and aldehydes. Both
enantiomeric products can be obtained by using either enan-
tiomerically pure d- or l-pseudoephedrine.
† University of Florida.
‡ Technische Universiteit Eindhoven.
(1) (a) Hoppe, H.; Glatzel, T.; Niggemann, M.; Schwinger, W.; Schaeffler, F.;
Hinsch, A.; Lux-Steiner, M. C.; Sariciftci, N. S. Thin Solid Films 2006,
511-512, 587-592. (b) Friend, R. H.; Gymer, R. W.; Holmes, A. B.;
Burroughes, J. H.; Marks, R. N.; Taliani, C.; Bradley, D. D. C.; Dos Santos,
D. A.; Bredas, J. L.; Logdlund, M.; Salaneck, W. R. Nature 1999, 397,
121-128. (c) Sirringhaus, H.; Tessler, N.; Friend, R. H. Science 1998,
280, 1741-1744. (d) Kraft, A.; Grimsdale, A. C.; Holmes, A. B. Angew.
Chem., Int. Ed. 1998, 37, 402-428.
(5) Zahn, S.; Swager, T. M. Angew. Chem., Int. Ed. 2002, 41, 4225-4230.
(6) Oda, M.; Nothofer, H. G.; Scherf, U.; Sunjic, V.; Richter, D.; Regenstein,
W.; Neher, D. Macromolecules 2002, 35, 6792-6798.
(7) Loewe, R. S.; Khersonsky, S. M.; McCullough, R. D. AdV. Mater. 1999,
11, 250.
(8) (a) Myers, A. G.; Yang, B. H.; Chen, H.; Gleason, J. L. J. Am. Chem. Soc.
1994, 116, 9361-9362. (b) Myers, A. G.; Yang, B. H.; Chen, H.;
McKinstry, L.; Kopecky, D. J.; Gleason, J. L. J. Am. Chem. Soc. 1997,
119, 6496-6511.
(2) Welsh, D. M.; Kloeppner, L. J.; Madrigal, L.; Pinto, M. R.; Thompson, B.
C.; Schanze, K. S.; Abboud, K. A.; Powell, D.; Reynolds, J. R.
Macromolecules 2002, 35, 6517-6525.
(3) Reeves, B. D.; Grenier, C. R. G.; Argun, A. A.; Cirpan, A.; McCarley, T.
D.; Reynolds, J. R. Macromolecules 2004, 37, 7559-7569.
(4) Cirpan, A.; Argun, A. A.; Grenier, C. R. G.; Reeves, B. D.; Reynolds, J.
R. J. Mater. Chem. 2003, 13, 2422-2428.
9
10694
J. AM. CHEM. SOC. 2007, 129, 10694-10699
10.1021/ja068461t CCC: $37.00 © 2007 American Chemical Society

Chiral Substituents for
π
-Conjugated Polymers
A R T I C L E S
Scheme 1. Synthesis of (2S)-Ethylhexanol from l-Pseudoephedrine
Scheme 2. Synthesis of Chiral Polymers P1 and P2 from Enantiomerically Pure (2S)-Methylbutanol and (2S)-Ethylhexanol
Results and Discussion
heating to obtain full dissolution. In CH2Cl2, P2 is fully soluble,
whereas P1 is not.
Synthesis and Characterization. Chiral (2S)-ethylhexanol
was prepared as described in Scheme 1 starting by the acylation
of (1R,2R)-pseudoephedrine using butyric anhydride. Diaste-
reoselective alkylation of the enolate generates 2, which can
be hydrolyzed under acidic conditions to give (2S)-chiral
ethylhexanoic acid. Reduction with lithium aluminum hydride
yields (2S)-ethylhexanol. The enantiomeric excess was deter-
mined by chiral GC of the Mosher ester, being 96% (see the
Supporting Information). (2S)-Ethylhexanal and (2S)-ethylhex-
anoyl chloride, useful intermediates in Wittig or Friedel-Craft
reactions, were also synthesized, demonstrating the versatility
of the synthesis.
Solution Properties. The optical properties of P2 were
studied by UV-vis absorption, fluorescence, and CD spectros-
copy (Figure 1). In xylene solution,9 the polymer is highly
fluorescent with a quantum yield of 0.43.10 When DMF, a poor
solvent for the polymer, is added, only a slight red-shift of the
absorbance is observed at xylene/DMF ratios higher than 60/
40, along with a small decrease of the fluorescence quantum
yield down to 0.28. No CD signal is observed, indicating that
the polymer chains are molecularly dissolved. When additional
poor solvent is added, a sharp change of optical properties is
Both monomers for P1 and P2 were obtained by a modified
procedure used for the synthesis of the racemic ProDOTs and
described in Scheme 2.3 For P1, commercial (2S)-methylbutanol
was used. After tosylation of the chiral alcohols, malonic ester
synthesis was carried out to yield, after LiAlH4 reduction,
intermediates 5 and 7. Transetherification of 7 and 5 with
3,4-dimethoxythiophene afforded oxidatively polymerizable
ProDOT((2S)-methylbutyl)2 (8) and ProDOT((2S)-ethylhexyl)2
(6). Following bromination with NBS, the polymers P1 and
P2 were obtained by nickel-catalyzed Grignard metathesis
polymerization.7
After Soxhlet extraction with methanol and hexane to remove
polar impurities and low molecular weight polymers, P1 and
P2 were dissolved in CHCl3. The polymers have weight-average
molecular weights of 13 800 and 14 200 g mol-1, respectively,
with a polydispersity index of 1.6 for both. Although the
polymers have similar molecular weights, P2 displays a higher
solubility in CHCl3, toluene, and THF than P1, which requires
Figure 1. Solvatochromism of P2 (PProDOT((2S)-ethylhexyl)2) in xylene/
DMF solutions (C ) 8 × 10-6 M): (a) UV absorption; (b) CD spectrum;
(c) fluorescence; (d) superposition of the CD spectrum and absorbance
derivative of an aggregated 30/70 xylene/DMF solution.
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A R T I C L E S
Grenier et al.
Table 1. Peaks (λmax, nm) and Onsets (in eV) of the Chiral Alkyl
PProDOTs Absorption Spectra upon Aggregation Induced by
Solvatochromism in Xylene/DMF Solutions
P1
P2
λ
max 100% xylene
onset 100% xylene
max 30% xylene
onset 30% xylene
556, 603 nm
1.98 eV
576, 628 nm
1.88 eV
553, 595 nm
2 eV
559, 608 nm
1.95 eV
λ
observed, with a further red-shift of the absorption spectrum
by 10 nm. The vibronic features become more resolved,
suggesting more ordered, planar chains. In addition, the
fluorescence quantum yield drops sharply to less than 0.04 at a
40/60 xylene/DMF ratio. In addition, a strong CD signal
corresponding to the π-π* absorption appears (Figure 1b). The
anisotropy factor in absorption (defined as ∆ꢀ/ꢀ) reaches a
maximum of gabs ) 1.8 × 10-2 at 625 nm and -1.4 × 10-2 at
498 nm. The signal shape and anisotropy factor in absorption
are similar to the CD signals observed in aggregated chiral poly-
(alkylthiophenes), indicating that PProDOT((2S)-ethylhexyl)2
forms similar interchain chiral helices.11 This is further supported
by the planar geometry of the polymer chains, shown by the
presence of well-resolved vibronic features in the absorption
spectra both in solution and film state. Such planarity of
dioxythiophene-based polymers and oligomers is arising from
intramolecular sulfur-oxygen interactions along with mesomeric
effects.12 Although the Davydov splitting is not resolved, the
close match between the bisignate CD spectrum with the first
derivative of the absorption spectrum (Figure 1d) is in full
agreement with a splitting of the excited state into two exciton
levels via small Davydov interactions, consistent with studies
on other polythiophenes.11
Figure 2. (Top) Temperature-dependent CD intensity of (a) P1 (λ ) 637
nm) and (b) P2 (λ ) 617 nm) in 50/50 xylene/DMF mixture (C )
8 × 10-6 M). (Bottom) CD spectrum for (c) P1 and (d) P2 in 50/50
xylene/DMF mixture (C ) 8 × 10-6 M) as prepared and slowly cooled
from 100 °C.
Solvatochromism on P1 solutions indicates that P1 also forms
similar chiral interchain aggregates (gabs ) 1.6 × 10-2 at 642
nm) with a strong match between the CD spectrum and the first
derivative of the absorbance (Supporting Information). However,
as shown in Table 1, P1 displays a stronger bathochromic shift
(100 meV) than P2 (50 meV) upon aggregation.
Figure 3. Comparison between CD spectrum and first derivative of the
absorption spectrum for P2 (a) and P1 (b).
Complementary thermochromism experiments on 50/50 xy-
lene/DMF solutions of P1 and P2 were performed and are
presented in Figure 2. At this solvent mixture ratio, both P1
and P2 are aggregated at room temperature. Upon heating
aggregated solutions of P2 above 70 °C, the formation of the
chiral aggregates is fully reversed to the molecularly dissolved
state (Figure 2b). In higher DMF content solutions, the transition
to the molecularly dissolved is moved to higher temperatures,
becoming irreversible above 60% DMF content. In the case of
P1 in 50/50 xylene/DMF, the aggregates could not be com-
pletely reversed to the molecularly dissolved state even at
100 °C (Figure 2a). Aggregated solutions with higher xylene
content were also prepared but did not yield full reversibility.
Following the heating cycle to 100 °C, both 50/50 xylene/DMF
solutions of P1 and P2 were slowly cooled down. In the case
of P2, molecularly dissolved 100 °C, the aggregates reformed
with a full recovery of the CD signal at room temperature
(Figure 2d). Solutions of P1, remaining aggregated at 100 °C,
lead to a significantly decreased CD signal when slowly cooled
back to room temperature, indicating an irreversibility and a
more complex aggregation process (Figure 2c).13
Solid-State Properties. To examine the chiral properties of
the polymers in the solid state, films of P1 and P2 were spray-
coated from toluene. Films of P2 yield strong CD signals, as
presented in Figure 3a. The shape of the CD signals (with
maximum anisotropy factors of gabs ) +2.9 × 10-2 at 627 nm
and -2.2 × 10-2 at 501 nm) is very similar with that in solution,
though the absorptions are broadened and red-shifted. Here, the
maxima of the CD coincide with those in the UV-vis (Figure
4), which is in contrast to other known polythiophenes.11,14 As
(9) Xylene used in this study is a mixture of ortho, meta, and para isomers.
(10) Sulforhodamine in absolute ethanol (φ ) 0.9) was used as standard.
(11) (a) Langeveld-Voss, B. M. W.; Janssen, R. A. J.; Meijer, E. W. J. Mol.
Struct. 2000, 521, 285-301. (b) Langeveld-Voss, B. M. W.; Janssen, R.
A. J.; Christiaans, M. P. T.; Meskers, S. C. J.; Dekkers, H. P. J. M.; Meijer,
E. W. J. Am. Chem. Soc. 1996, 118, 4908-4909.
(12) (a) Grenier, C. R. G.; Pisula, W.; Joncheray, T. J.; Mullen, K.; Reynolds,
J. R. Angew. Chem., Int. Ed. 2007, 46, 714-717. (b) Turbiez, M.; Frere,
P.; Allain, M.; Videlot, C.; Ackermann, J.; Roncali, J. Chem.sEur. J. 2005,
11, 3742-3752. (c) Apperloo, J. J.; Groenendaal, L.; Verheyen, H.;
Jayakannan, M.; Janssen, R. A. J.; Dkhissi, A.; Beljonne, D.; Lazzaroni,
R.; Bredas, J. L. Chem.sEur. J. 2002, 8, 2384-2396. (d) Spencer, H. J.;
Skabara, P. J.; Giles, M.; McCulloch, L.; Coles, S. J.; Hursthouse, M. B.
J. Mater. Chem. 2005, 15, 4783-4792.
(13) (a) Satrijo, A.; Meskers, S. C. J.; Swager, T. M. J. Am. Chem. Soc. 2006,
128, 9030-9031. (b) Leclere, P.; Surin, M.; Viville, P.; Lazzaroni, R.;
Kilbinger, A. F. M.; Henze, O.; Feast, W. J.; Cavallini, M.; Biscarini, F.;
Schenning, A. P. H. J.; Meijer, E. W. Chem. Mater. 2004, 16, 4452-4466.
(14) Langeveld-Voss, B. M. W.; Christiaans, M. P. T.; Janssen, R. A. J.; Meijer,
E. W. Macromolecules 1998, 31, 6702-6704.
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Chiral Substituents for
π
-Conjugated Polymers
A R T I C L E S
Figure 5. Spectroelectrochemistry of P1 and P2 films spray-coated from
toluene in 0.1 M TBAP/PC. Potentials are reported against Fc/Fc+
pseudoreference.
Figure 4. (a) CD and (b) UV-vis absorption spectra evolution with doping
(I2) and undoping (NH2NH2 vapor) cycles of P2 (PProDOT-((2S)-
ethylhexyl)2) spray-coated films on glass substrates; inset to (a) shows
magnification of the 600-900 nm region, and inset to (b) shows the
difference in UV absorption between film and aggregated solution.
examined, indicating the polymer was dedoped to the polaron
state, as noted by the presence of an absorption peak centered
at 950 nm (full spectrum in the Supporting Information). No
CD signal is observed for this band showing the polaron
absorption is not CD active. Spray-coated films of P1 also show
CD reversibility following iodine doping and hydrazine neu-
tralization cycles (Supporting Information) despite the much
lower CD values and limited thermoreversibility of the chiral
aggregates in solution.
The above experiment demonstrates that during the casting
process, both P1 and P2 chains spontaneously self-assemble.
The assemblies are reversible upon redox cycling, as no changes
in CD signal or shift in absorption spectrum are observed. This
result is unique to the branched alkyl PProDOTs as linear alkyl
and ether linked alkyl PProDOTs synthesized in our group were
shown to retain a “solution conformation” with little or no shift
in absorption spectrum during casting.2,3 For these polymers, a
bathochromic shift is only observed upon redox switching and
the color of the films changes from a red color to a blue-purple
(PProDOT-Hx2, PProDOT-Bu2)2 or a red-purple color
(PProDOT(CH2O-2-ethylhexyl)2).3
Spectroelectrochemistry experiments were carried out on P2
and P1 (Figure 5). A distinct optical change is observed with a
200 mV potential increase for P2 as noted by the marked
spectra. In the case of P1, a wider 350 mV potential window is
required to obtain similar optical changes. In PProDOT-Hx2,
the potential window required is even larger, the polymer
requiring more than 600 mV to achieve similar optical changes.3
The band gaps for spray-coated films of P1 and P2 are 1.88
and 1.92 eV, respectively, close to the onsets found in
aggregated solutions. Cyclic voltammetry at a slow scan rate
(5 mV/s) of the two films also indicates an oxidation process
over a much larger range of potential in the case of P1
(Supporting Information). Four point probe conductivity mea-
surements carried out on spray-coated thick films that were gas-
phase doped with iodine yielded a conductivity of 4 S cm-1
for P1 and 0.3 cm-1 for P2.
absorption spectroscopy probes all interchain interactions and
CD spectroscopy only probes chiral interchain interactions, we
speculate that this behavior arises from additional nonchiral
interchain interactions between chiral stacks forming during
solvent drying. This shift could also originate from the presence
of light-scattering effects, frequently observed in chiral poly-
fluorenes films.6 The presence of a negative CD signal in films
of P2 at longer wavelength, shown in Figure 3a, would be
consistent with this assumption. Spin-coated films of P2 from
o-dichlorobenzene (ODCB) also display a CD signal with
maxima matching the maxima in the UV-vis spectrum,
suggesting that this effect is not strongly affected by the
deposition method or the solvent. Spray-coated films of P1 from
toluene also yield a CD signal, but as shown in Figure 3b, it is
less intense than P2, with maximum anisotropy factor gabs
)
+3.2 × 10-3 at 637 nm and -1.2 × 10-3 at 512 nm. The
decreased anisotropy of P1 is consistent with the thermo-
chromism result of aggregated solutions showing a more delicate
aggregation process for P1 leading to more heterogeneous
aggregates.
Upon exposure of a spray-coated film of P2 to iodine, the
UV-vis spectrum indicates the polymer film is fully oxidized
to the bipolaron state (Figure 4b, full spectrum to 1800 nm in
the Supporting Information). The corresponding CD signal
(Figure 4a), seen in the visible range, is much less intense and
blue-shifted compared to that of the neutral film. This suggests
that undoped chromophores with short conjugation lengths
remain in the film and that the polymer chains retain a chiral
orientation in the doped state.15 A negative CD signal is also
observed between 700 and 900 nm (inset of Figure 4a),
suggesting the bipolaron band is also CD active. Upon exposure
of the films to hydrazine vapor, the neutral polymer CD signal
is fully recovered, demonstrating reversibility of the chiral
aggregates after an oxidation/charge neutralization cycle. Two
days after iodine oxidation, the UV-vis and CD spectra were
The sharp optical changes occurring over a narrow range of
potential are reminiscent of what was observed in racemic
PProDOT(2-ethylhexyl)2. To examine in more detail if the
presence of the enantiomerically pure side chains affect the
macroscopic properties, the electronic and optical properties of
(15) Koeckelberghs, G.; De Cremer, L.; Vanormelingen, W.; Verbiest, T.;
Persoons, A.; Samyn, C. Macromolecules 2005, 38, 4545-4547.
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A R T I C L E S
Grenier et al.
Table 2. Electronic and Optical Properties of P2 and Racemic
Equivalent
P2
racemic
chiral
Ep,a (mV)a
p,c (mV)a
max cast film
45
11
89
43
E
λ
618, 564, 523
1.93
0.4
617, 564, 524
1.92
0.3
Eg (optical, eV)
conductivity (S cm-1
)
b
a CV-determined anodic and cathodic peak potentials of drop-cast films
on a Pt button electrode measured in 0.1 M tetrabutylammonium perchlorate/
acetonitrile electrolyte vs Fc/Fc+. b Gas-phase doped with iodine for 1 h.
Figure 7. AFM topographic image of a film of P2 spin-coated at 2000
rpm from a 0.2 mg/mL ODCB solution. Bright regions correspond to the
polymer fibers and dark regions to the mica surface: (a) 5 × 5 µm2; (b)
magnification, 300 × 300 nm2; (c) height profile of the cross section shown
in Figure 6a; (d) cross section of Figure 6b showing the width of selected
fibers.
This observation suggests that the nanoribbons are formed of
stacks of interpenetrated chains, perpendicular to the mica
surface. The interconnected network of ribbons extends over
several micrometers (Figure 6a and 6b).
In the case of P2, nanoribbons are also observed (Figure 7),
with an average height for individual ribbons of 1.8 nm, fully
compatible with the calculated height of 1.5-2.2 nm. The
ribbons have widths, after tip correction,16 of 50-60 nm. The
ribbons, however, extend to a much shorter range, typically
250-500 nm, and are poorly connected together as shown in
Figure 7, parts a and b. Such a difference in morphology could
explain the difference in conductivity between the two polymers.
In poly(3-hexylthiophene) (P3HT), increasing evidence suggests
that the increase of charge mobility with increasing molecular
weight arises from a high degree of interconnectivity between
small, highly ordered domains.18
Figure 6. AFM topographic image of a film of P1 spin-coated at 2000
rpm from a 0.2 mg/mL ODCB solution. Bright regions correspond to the
polymer and dark regions to the mica surface: (a) 5 × 5 µm2; (b)
magnification, 300 × 300 nm2; (c) height profile of the cross section shown
in (a); (d) cross section of (b) showing the width of selected fibers.
both polymers were compared. As presented in Table 2 for P2,
the properties of both racemic and chiral polymers are found to
be almost identical, showing that the enantiomeric purity has
little or no effect on the macroscopic properties.
Morphology Studies. In the previous sections, we have
examined the microscopic ordering of the two chiral polymers.
CD experiments show that the (2S)-ethylhexyl polymer P2,
although having more bulky side chains, displays strong
microscopic ordering both in aggregated solution and the solid
state. Surprisingly, the polymer was found to have an order of
magnitude lower conductivity than P1.
Conclusions
In conclusion, we introduce a powerful and versatile synthesis
to prepare the most often used enantiomerically pure side chains
which solubilize π-conjugated polymers. Using this method, we
synthesized a functionalized poly(3,4-propylenedioxythiophene)
P2 having two chiral ethylhexyl side chains and compared its
properties to the chiral methylbutyl derivative P1. The enan-
tiomerically pure side chains allow us to follow the aggregation
process by CD spectroscopy, as the two polymer forms chiral
helical aggregates similar to chiral alkyl polythiophenes. CD
analysis of the more soluble P2 indicates that the bulkier side
chains allow for a greater control of the microscopic ordering
in aggregated solution. CD experiments on films of P1 and P2
also suggest that P1 forms more heterogeneous aggregates than
P2. For both polymers, the chiral aggregates were found to be
fully reversible upon chemical oxidation and reneutralization
cycles.
To explain this difference, we examined the two polymers
morphologies by AFM. Films of P1 and P2 were both spin-
coated onto a mica surface from a 0.2 mg/mL solution in 1,2-
dichlorobenzene (2000 rpm). AFM study of P1 films reveals
the presence of interconnected nanoribbons (Figure 6). The
individual ribbons have an average height of 1.7 nm and average
width of 50 nm (dimensions after correction for tip-broadening
effect16). The height of the ribbons is compatible with the height
of the P1 chains, estimated between 1.4 and 2 nm for chains
with trans conformation of adjacent thiophenes and an extended
conformation for the alkyl chains.17 The width of the ribbons
largely exceeds the length of the chains expected from the GPC
data yielding an estimated length of 11-15 nm for P1 chains.
(18) (a) Kline, R. J.; McGehee, M. D.; Kadnikova, E. N.; Liu, J. S.; Frechet, J.
M. J.; Toney, M. F. Macromolecules 2005, 38, 3312-3319. (b) Brinkmann,
M.; Rannou, P. AdV. Funct. Mater. 2007, 17, 101-108. (c) Yang, H. C.;
Shin, T. J.; Yang, L.; Cho, K.; Ryu, C. Y.; Bao, Z. N. AdV. Funct. Mater.
2005, 15, 671-676.
(16) Samori, P.; Francke, V.; Mangel, T.; Mullen, K.; Rabe, J. P. Opt. Mater.
1998, 9, 390-393.
(17) Width of the chains, estimated using Hyperchem software is given as a
range as it is dependent on the conformation of the seven-membered ring.
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Chiral Substituents for
π
-Conjugated Polymers
A R T I C L E S
The differences in microscopic ordering could explain the
broader potential window required for obtaining complete optical
changes during oxidation. However, such differences are
unlikely to account for the lower conductivities of P2 films.
The lower conductivities are rather explained by different
morphologies of the polymer films, as P1 forms an intercon-
nected network of nanoribbons, whereas P2 forms more isolated
nanoribbons.
Acknowledgment. We gratefully acknowledge funding of this
project from the AFOSR (Grant FA9550-06-1-0192).
Supporting Information Available: Full synthetic procedures
and characterization, along with absorbance, emission, and
CD spectra for PProDOT((2S)-methylbutyl)2). This material
is available free of charge via the Internet at http://
pubs.acs.org.
JA068461T
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J. AM. CHEM. SOC. VOL. 129, NO. 35, 2007 10699