Self-assembly of amphiphilic poly(phenylene ethynylene)s in water-potassium dodecanoate-decanol lyotropic liquid crystals

Article abstract of DOI:10.1039/b814598a

Poly(phenylene ethynylene)s bearing a high density of branched amphiphilic side-chains self-assemble at the air-water interface and in water-potassium dodecanoate-decanol lyotropic liquid crystals. The Royal Society of Chemistry.

Full text of DOI:10.1039/b814598a

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Self-assembly of amphiphilic poly(phenylene ethynylene)s in
water–potassium dodecanoate–decanol lyotropic liquid crystalsw
Jean Bouffard and Timothy M. Swager*
Received (in Austin, TX, USA) 21st August 2008, Accepted 17th September 2008
First published as an Advance Article on the web 1st October 2008
DOI: 10.1039/b814598a
Poly(phenylene ethynylene)s bearing a high density of branched
amphiphilic side-chains self-assemble at the air–water interface
and in water–potassium dodecanoate–decanol lyotropic liquid
crystals.
The achievement of complete control over the conformation of
conjugated polymer single chains and their assembly into
functional structures remains an elusive goal, despite its
relevance to the understanding of energy transfer and con-
ductivity and the importance of these properties in applica-
tions ranging from PLEDs to CP-based solar cells to
biosensors.1 In a plurality of cases, structural control in the
condensed phase is achieved empirically wherein the annealing
of polymer films may or may not lead to the desired improve-
ment in properties.2
Elegant strategies previously developed for the control of
conjugated polymer chain conformation, alignment and as-
sembly are accomplished in templated matrices such as
stretch-aligned polymers,3 nematic liquid crystals,4 roll-cast
block copolymers,5 mesoporous silica,6 designed supramole-
cular aggregates,7 and controlled deposition from an air–water
interface.8 Herein, we describe the extension of the last stra-
tegy to include polar interfaces of lyotropic liquid crystals
(LLCs).
Surfactants typically self-assemble upon dissolution in
water to form the well-known soap micelles. When many of
such amphiphiles are dissolved in higher concentrations, the
micelles can coalesce and/or self-assemble into a large number
of liquid crystalline phases (e.g. nematic, cubic, hexagonal,
bicontinuous and lamellar), which have been extensively stu-
died for a wide array of binary (e.g. water–amphiphile),
ternary (e.g. water–amphiphile–hydrophobe) and higher order
mixtures.9
Scheme 1 Conjugated polymer amphiphiles P1–P3.
and/or propensity of the polymers to self-aggregate in LLC
media. Polymers P2 and P3 were designed and synthesized (see
ESIw for details) to circumvent these limitations by incorpo-
rating a larger density of branched10 amphiphilic side-chains
along the poly(phenylene ethynylene) (PPE) backbone.
Non-ionic conjugated polymer amphiphiles such as P1a and
P1b (Scheme 1),8c bearing a comparatively low density of
linear side-chains, have previously been shown to self-assem-
ble in 2D nematic phases at the air–water interface.8d,e How-
ever attempts to introduce P1a–b within lyotropic liquid
crystalline lattices have been unsuccessful, presumably due
to the low density of amphiphilic side-chains, limited solubility
The amphiphilic properties of polymers P2 and P3 were first
investigated by the Langmuir–Blodgett technique.8c,d Solu-
tions of P2 or P3 in CHCl3 spread evenly at the air–water
interface to form very stable self-assembled monolayers. Ex-
trapolated surface area per repeating unit of 200 A2 and
230 A2 were found for P2 and P3, respectively.11 These values,
together with the characteristic shape of the P–A isotherm and
in situ spectroscopy (Fig. 1; see ESIw for details) are consistent
with a polymer chain conformation wherein the phenylene
units lie cofacial with the air–water interface (i.e. face-on
Department of Chemistry, Massachusetts Institute of Technology,
77 Massachusetts Avenue, Cambridge, MA, USA.
E-mail: tswager@mit.edu; Fax: +1 617 253 7929;
Tel: +1 617 253 4423
w Electronic supplementary information (ESI) available: Experimental
details including synthesis and characterization, absorption, fluores-
cence and NMR spectra. See DOI: 10.1039/b814598a
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Fig. 2 Micrographs (100Â magnification) of lyotropic liquid crystal
textures under cross-polarizers: (a) LLC-1; (b) P2 in LLC-1; (c) LLC-
2; (d) P3 in LLC-2.
Fig. 1 Pressure–area isotherms of P2 (dashed) and P3 (plain) at the
air–water interface.
structure). As the monolayers are compressed, folding into
multilayers is observed at 44 mN mÀ1 (122 A2) for P2 and
35 mN mÀ1 (150 A2) for P3. That folding into multilayers
occurs, especially for P2, at significantly higher pressures than
typically observed for face-on amphiphilic PPEs (e.g. P1b,
in the LLC mixtures and no precipitation is observed. Similar
optical textures (Fig. 2) are observed for samples containing
the polymers and those that do not, indicating that the
introduction of the polymers at these concentrations does
not significantly disrupt the nematic phases of LLC-1 and
LLC-2.
30 mN m )
À1 8b–e is interpreted as evidence that a higher density of
amphiphilic side-chains along the polymer backbone increases
the stability of the monolayer through an anchoring effect.
Water (D2O)–potassium dodecanoate–1-decanol (LLC-1;
67.5 : 26.2 : 6.3% w/w) and water (D2O)–potassium dode-
canoate–1-decanol–potassium chloride (LLC-2; 60.0 : 30.0 :
6.0 : 4.0% w/w) lyotropic liquid crystalline mixtures12 incor-
porating P2 and P3 were prepared from 1-decanol solutions
(B1 mg mLÀ1) of the polymers. The PPEs remain fully soluble
Polymers P2 and P3 remain highly fluorescent in the
lyotropic liquid crystalline mixtures. The absorption spectra
of P2 and P3 in LLC media are significantly red-shifted from
those in solution (Dlmax = 23 nm for P2 in LLC-1, 22 nm for
P3 in LLC-2) (Fig. 3, Table 1). Smaller bathochromic shifts
are similarly observed in the emission spectra (Dlmax = 7 nm
for P2 in LLC-1, 14 nm for P3 in LLC-2). These shifts,
together with a narrowing of the absorption band that is
Fig. 3 Normalized absorption (left) and emission (right) spectra of P2 (top) and P3 (bottom) in 1-decanol (plain), lyotropic liquid crystals
(dashed; LLC-1 for P2 and LLC-2 for P3), monolayers at the air–water interface (black squares) and thin solid films (open circles).
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Table 1 Summary of photophysical properties of P2 and P3
G. D. Joly and T. M. Swager, Chem. Rev., 2007, 107,
1339–1386.
P2
P3
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lmax, abs
nm
/
lmax, em
nm
/
lmax, abs
nm
/
lmax, em/
nm
CHCl3
387
382
406
405
387
407
420
417
424
424
422
422
417
427
460
451
449
451
450
453
467
467
467
466
1-DecOH
Air–water interface
LLC-1
LLC-2
LLC-3
Solid film
393, 427a 432
440, 472a 479
a
Aggregation peak.
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11 For the purposes of direct comparison with P1a–b and P3,
throughout this paper and in the ESI, the repeating unit for P2
has been doubled to include 2 phenylene ethynylene units.
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especially marked for P3, are indicative of a planarized poly-
mer chain conformation that extends p-conjugation relative to
the distribution of effective conjugation lengths found in
solution, as previously observed with PPEs and small-mole-
cule models.4,8c,d,13 The bathochromic shifts are not consistent
with polymer aggregates, for which a characteristic sharp
absorption peak is found another 20–23 nm to the red in solid
films. The absorption and emission spectra of the polymers in
LLC media are most similar to that observed in Langmuir
monolayers at the air–water interface, though the extent of the
bathochromic shift with respect to the solution value in LLC
media is inferior to that of the monolayers. We interpret these
as evidence that P2 and P3 reside at the polar interfaces
present within the LLC and adopt a p-extended face-on
conformation analogous to that found at the air–water inter-
face. Curvature of the polar interfaces within the LLCs and/or
a looser confinement of the polymer chain to the interface
would result in a less extended conjugation and a concomitant
bathochromic shift smaller than in Langmuir monolayers.
In summary, the branched amphiphilic PPEs P2 and P3
have been shown to form stable monolayers at the air–water
interface where the polymer chain adopts a planar face-on
structure. These polymers have been introduced in self-
assembled lyotropic liquid crystalline media and spectroscopic
data provide support for a similar planar face-on structure of
the polymer chains at polar interfaces within the LLC. Further
studies will include the crosslinking of conjugated polymers
self-assembled in LLCs to generate porous networks14 relevant
for vapour sensing and gas storage applications15 and the
confinement of conjugated polymers on other self-assembled
polar interfaces such as liposomes and cell membranes.16
This work was funded in part by NSERC (J.B.) and the
National Science Foundation. Dr Jessica Liao and Dr Juan Zheng
are acknowledged for the preparation of monomer precursors.
Notes and references
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