Self-assembly and luminescence of oligo(p-phenylene vinylene) amphiphiles

Article abstract of DOI:10.1021/ja047210m

We have synthesized a series of amphiphilic molecules consisting of oligo(phenylene vinylene) (OPV) asymmetrically end-substituted with a hydrophilic poly(ethylene glycol) (PEG) segment and a hydrophobic alkyl chain. This amphiphilic structure induces sel

Full text of DOI:10.1021/ja047210m

Published on Web 12/03/2004
Self-Assembly and Luminescence of Oligo(p-phenylene
vinylene) Amphiphiles
James F. Hulvat,† Marina Sofos,† Keisuke Tajima,† and Samuel I. Stupp*,†,‡,§
Contribution from the Department of Materials Science and Engineering, Department of
Chemistry, and Feinberg School of Medicine, Northwestern UniVersity, EVanston, Illinois 60208
Received May 12, 2004; E-mail: s-stupp@northwestern.edu
Abstract: We have synthesized a series of amphiphilic molecules consisting of oligo(phenylene vinylene)
(OPV) asymmetrically end-substituted with a hydrophilic poly(ethylene glycol) (PEG) segment and a
hydrophobic alkyl chain. This amphiphilic structure induces self-assembly into both thermotropic and lyotropic
lamellar liquid crystalline (LC) phases. The molecules form strongly fluorescent, self-supporting gels in
both water and polar organic solvents, even at high concentrations on the order of 30 wt %. These self-
assembled structures have been characterized by small-angle X-ray scattering (SAXS), differential scanning
calorimetry (DSC), and polarized optical microscopy (POM). Photoluminescence (PL) is influenced by the
structure of the material, with enhanced emission in the LC state due to assembly of the chromophore in
confined two-dimensional layers. Self-assembly controlling molecular aggregation at the nanoscale could
significantly improve the performance of OPV-based materials in optoelectronic devices.
Introduction
their well-defined chemical structure facilitates tuning of
electronic properties.7-9 Use of well-defined oligomers reduces
Previous research efforts have led to the synthesis of many
conjugated polymers, particularly derivatives of poly(phenylene
vinylene) (PPV), soluble in organic solvents and easily processed
into films with great promise as organic electronic materials.1-4
Controlling the nanoscale structure of rodlike conjugated
polymers has proven difficult. However, supramolecular order
plays a critical role in device performance, as both charge
mobility and luminescent efficiency are influenced by molecular
aggregation and structural defects.5,6 π-Conjugated oligomers,
first investigated as model compounds for conjugated polymers,
are now widely studied for use in optoelectronic devices because
defect density while enabling more control over the effective
conjugation length. In particular, oligo(p-phenylene vinylene)s
(OPVs) are being investigated for use in solar cells and light-
emitting diodes (LEDs) due to their stability, high luminescent
efficiency, and ease of synthesis.10,11 OPVs with solubilizing
substituents combine the low-cost, solution-based processing
of conjugated polymers with the improved structural control
inherent to oligomers.6,9,11,12
One approach to control nanoscale structure in conjugated
polymers has been to design molecules that could exhibit
thermotropic or lyotropic liquid crystalline (LC) behavior.4,13
Among these are the “hairy-rod”-type liquid crystalline polymers
† Department of Materials Science and Engineering.
‡ Department of Chemistry.
§ Feinberg School of Medicine.
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J. AM. CHEM. SOC. 2005, 127, 366-372
10.1021/ja047210m CCC: $30.25 © 2005 American Chemical Society

Self-Assembly of OPV Amphiphiles
A R T I C L E S
Scheme 1. Synthesis of OPV Amphiphilea
a Reaction conditions: (i) DPTS/EDIC, CH2Cl2. (ii) P(OEt)3, 130 °C. (iii) K2CO3, 18-crown-6, acetone, 56 °C. (iv) LDA, cyclohexane/THF, -78 °C. (v)
DIBAL-H, Et2O, 0 °C.
(LCPs), in which flexible alkyl side chains added for solubility
induce nanophase separation of the rigid, conjugated polymer
backbone.3,14 A more direct approach is to incorporate calamitic
mesogens as side chains on a conjugated polymer backbone.15,16
Both methods can result in layered, smectic ordering of the
polymer, but only micron-sized domains are generally obtained
due to the viscosity and rigidity of the polymer’s extended
backbone conjugation, which inhibits formation of the large
monodomains needed in device applications.2,16
asymmetrically end-substituted OPVs have been reported, and
their mesophase behavior has not been extensively character-
ized.8,19,21
We report here the synthesis and characterization of a novel
series of amphiphiles consisting of a well-defined OPV trimer
asymmetrically end-substituted with a hydrophobic alkyl chain
and a hydrophilic poly(ethylene glycol) (PEG). These molecules
form lyotropic and thermotropic LC phases in polar solvents,
and we can tune the solubility and mesophase structure by
controlling the length of the PEG block. With a sufficiently
long PEG, OPV amphiphiles become water soluble, a significant
advantage for large-scale processing of materials. Though a
number of water-soluble PPV polymers have been reported,22
only a few examples of water-soluble OPVs have been reported,
and these were solubilized through the use of ionic substitu-
ents.23 With the nonionic amphiphile reported here, liquid
crystallinity is used to control OPV aggregation, influencing
exciton mobility, fluorescence, and potentially leading to
improved charge carrier mobility in heterojunction solar cells
or enabling more efficient, polarized emission from organic
light-emitting diodes (OLEDs).
Use of conjugated, liquid crystalline oligomers can improve
ordering, and several LC oligomers based on substituted OPV
rods have been reported.17 Many are analogues of conjugated
LCPs, with flexible alkyl chains grafted laterally onto the
molecule.6,18 However, this introduces bulky, sterically hindering
groups that disrupt coplanarity within the π-conjugated system.19
End-substitution is preferred for this reason; however, there are
few reports of end-substituted OPVs that form LC phases.20
Typically, these are symmetrically substituted to simplify their
synthesis. While asymmetric substitution can improve ordering
of OPV mesogens, only a few examples of truly amphiphilic,
Results and Discussion
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Macromolecules 1999, 32, 3034-3044.
Synthesis. The syntheses of OPV amphiphiles are shown in
Scheme 1 and described in detail in the Experimental Methods.
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K. M.; Tang, C. W. Chem. Mater. 2003, 15, 4352-4360. (b) Culligan, S.
W.; Geng, Y. H.; Chen, S. H.; Klubek, K.; Vaeth, K. M.; Tang, C. W.
AdV. Mater. 2003, 15, 1176-1180. (c) Precup-Blaga, F. S.; Schenning, A.
P. H. J.; Meijer, E. W. Macromolecules 2003, 36, 565-572. (d) Tinh, N.
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Maddux, T.; Li, W. J.; Yu, L. P. J. Am. Chem. Soc. 1997, 119, 844-845.
(18) Zhu, W.; Li, W. J.; Yu, L. P. Macromolecules 1997, 30, 6274-6279.
(19) Tao, Y.; Donat-Bouillud, A.; D’Iorio, M.; Lam, J.; Gorjanc, T. C.; Py, C.;
Wong, M. S. Synth. Met. 2000, 111, 417-420.
(21) (a) Watakabe, A.; Kunitake, T. Thin Solid Films 1990, 186, L21-L24. (b)
Wang, H. B.; Wang, H. H.; Urban, V. S.; Littrell, K. C.; Thiyagarajan, P.;
Yu, L. P. J. Am. Chem. Soc. 2000, 122, 6855-6861. (c) Wang, H.; You,
W.; Jiang, P.; Yu, L.; Wang, H. H. Chem.sEur. J. 2004, 10, 986-993.
(22) (a) Peng, Q. G.; Yang, J. L.; He, Q. G.; Bai, F. L. Synth. Met. 2003, 135,
163-164. (b) Li, H. M.; Xiang, C. H.; Li, Y. L.; Xiao, S. Q.; Fang, H. J.;
Zhu, D. B. Synth. Met. 2003, 135, 483-484. (c) Fujii, A.; Sonoda, T.;
Fujisawa, T.; Ootake, R.; Yoshino, K. Synth. Met. 2001, 119, 189-190.
(d) Wang, H. L.; McBranch, D. W.; Donohoe, R. J.; Xu, S.; Kraabel, B.;
Chen, L. H.; Whitten, D.; Helgeson, R.; Wudl, F. Synth. Met. 2001, 121,
1367-1368. (e) Fujii, A.; Sonoda, T.; Yoshino, K. Jpn. J. Appl. Phys.,
Part 2 2000, 39, L249-L252. (f) Wang, D. L.; Lal, J.; Moses, D.; Bazan,
G. C.; Heeger, A. J. Chem. Phys. Lett. 2001, 348, 411-415.
(23) (a) Liu, B.; Gaylord, B. S.; Wang, S.; Bazan, G. C. J. Am. Chem. Soc.
2003, 125, 6705-6714. (b) Gaylord, B. S.; Wang, S. J.; Heeger, A. J.;
Bazan, G. C. J. Am. Chem. Soc. 2001, 123, 6417-6418. (c) Stork, M.;
Gaylord, B. S.; Heeger, A. J.; Bazan, G. C. AdV. Mater. 2002, 14, 361-
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Sci. Eng., C 1999, 10, 83-86.
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(b) Wong, M. S.; Samoc, M.; Samoc, A.; Luther-Davies, B.; Humphrey,
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6414.
9
J. AM. CHEM. SOC. VOL. 127, NO. 1, 2005 367

A R T I C L E S
Hulvat et al.
Table 1. LC Mesophase Transitions Observed for OPV
Amphiphiles
c
OPV-na
M
w (g/mol)b
Mw/Mn
Tm
(
°
C)
TLC
(°
C)d
Tc (°
C)e
OPV-8
350
550
750
1100
2000
1.01
1.02
1.02
1.03
1.02
<rt
<rt
30
42
49
122
124
126, 137
109, 120
100, 109
160
151
146
131
123
OPV-12
OPV-16
OPV-24
OPV-45
a The n is the average number of repeat units in PEG blocks. b Average
molecular weight of PEG block. c Polydispersity of OPV amphiphiles
(determined by GPC). d Liquid crystalline mesophase transition(s). e Clear-
ing temperature of LC phase.
The conjugated vinylene bond was formed using the Horner-
Emmons reaction between phosphonate 2 and aldehyde 4 at a
low temperature to obtain trans-phenylene vinylene 5.
Aldehyde 6 was obtained by reduction of the cyano group in
5 with diisobutylaluminum hydride. To avoid solubility prob-
lems with unsubstituted OPV, we chose the coupling reaction
of PEG-phosphonate 2 and aldehyde 6 as the last step. While
the lengths of the OPV segment and alkyl tail were kept
constant, the PEG block was varied from an average of 8 to 45
repeat units in order to study its effect on mesophase behavior
(PEG Mw ) 350-2000 g/mol, Mw/Mn ) 1.02).
Thermotropic Mesophases. The OPV amphiphiles synthe-
sized were found to be thermotropic LCs, with the mesophase
structure highly dependent on the length of the PEG block.
Below OPV-16, the molecules form liquid crystals at room
temperature. As the length of the hydrophilic block increases,
the PEG crystallizes and the thermal range of the mesophase
narrows. Table 1 summarizes the molecular weight and thermal
transitions observed for each amphiphile. Transition tempera-
tures were determined from differential scanning calorimetry
(DSC) (scans shown in Supporting Information).
Figure 1. (a) Small-angle X-ray scattering of OPV amphiphiles. Inset shows
interlayer spacing as a function of PEG length determined from the first-
order diffraction peak. Dashed line is the fully extended length of one
molecule. (b) Wide-angle X-ray diffraction of OPV amphiphiles.
Small-angle X-ray scattering (SAXS) indicates a smectic
structure for all the amphiphiles studied. Figure 1 shows small-
and wide-angle X-ray diffraction patterns for the OPV am-
phiphiles, with the interlayer spacing plotted in the inset of
Figure 1a as a function of PEG length. The interlayer spacing
is approximately equal to the fully extended lengths of the
molecules, indicating significant interdigitation and/or tilt within
a bilayer smectic structure. The higher-order peaks observed
between 2 and 4° 2θ disappear with longer PEG chains, as does
the sharp wide-angle peak at 4.3 Å, which is likely to correspond
to the breakup of OPV ordering within the smectic layers. Two
other peaks appear at 4.6 and 3.8 Å, which may arise from the
crystal structure of the PEG block as these peaks were also
observed in control PEG-alkyl diblock molecules without OPV.
The X-ray data indicate that at room temperature OPV am-
phiphiles with short PEG chains form an interdigitated bilayer
smectic phase with a highly ordered OPV layer. As the length
of the hydrophilic segment increases, PEG crystallization
disrupts the OPV layer, resulting in a less-ordered structure.
The OPV amphiphiles show distinctive, strongly birefringent
mesophase textures when analyzed by polarized optical mi-
croscopy (POM). These textures were compared with known
ones in the literature;24,25 however, the similarities among higher-
ordered smectic phases and the complexity of the bilayer
structure needed further characterization for structural determi-
Figure 2. Phase diagram of transitions observed in DSC as a function of
PEG length, with preliminary phase assignments.
nation. In Figure 2, preliminary phase assignments are proposed
for the OPV layer of the LC, based on X-ray and DSC results
as well as POM textures.
The competing influence of the OPV and PEG segments on
amphiphile aggregation is clear from the phase diagram. A
distinct transition occurs above OPV-12 as the size of the
hydrophilic PEG exceeds that of the hydrophobe (Mw ) 494
g/mol for the alkyl-OPV). Figure 3 shows a mosaic birefringence
texture, similar to that of the smectic B (SB) mesophase24
observed for shorter PEG lengths.
Here, SB ordering is driven by OPV aggregation, while less-
ordered smectic C (SC) and smectic A (SA) phases result when
longer PEG blocks frustrate order within the OPV sublayers.
At lower temperatures, a texture similar to that of the crystalline
smectic G (CrG) mesophase is observed, in which OPV
molecules are hexagonally ordered in crystal-like layers with
positional and orientational order, but retaining the rotational
and diffusional motion of a liquid.24 In longer OPV amphiphiles
at room temperature, crystallization of the hydrophilic PEG
(24) Kumar, S. Liquid Crystals; Cambridge University Press: Cambridge, 2001.
(25) Gray, G. W.; Goodby, J. W. G. Smectic Liquid Crystals: Textures and
Structures; Leonard Hill: London, 1984.
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Self-Assembly of OPV Amphiphiles
A R T I C L E S
Figure 3. Mosaic birefringence texture of the OPV-12 amphiphile at 130
°C, observed between crossed polarizers.
Table 2. UV-vis Absorption and Photoluminescence (PL) of OPV
Amphiphiles
λ
abs (nm)a
λ
PL (nm)b
λ
abs (nm)c
λ
PL (nm)d
λ
PL (nm)e
THF soln
THF soln
THF film
THF film
CH3CN film
OPV-8
375
377
375
376
375
467
467
466
468
468
308
309
320
322
335
510
508
508
506
507
508
507
504
468
467
OPV-12
OPV-16
OPV-24
OPV-45
a Absorption in THF solution (375 nm excitation). b PL emission
maximum in THF solution (375 nm excitation). c Absorption in films spin-
coated from THF (310 nm excitation). d PL emission maximum in films
spin-coated from THF (310 nm excitation). e PL emission maximum in films
spin-coated from CH3CN (310 nm excitation).
block inhibits molecular rotation, as occurs in the transition from
the CrG to the CrH mesophase. While this transition increases
ordering of the hydrophilic sublayer, crystallization of the PEG
block may actually disrupt packing in the OPV sublayers, due
to differences in the preferred packing arrangement of the two
segments. This disruption of the OPV sublayer should be evident
in the spectroscopic behavior of the OPV amphiphiles.
Spectroscopic Characterization. The structural characteriza-
tion of these systems demonstrates how variation in PEG length
can change the aggregation state of OPV segments. This should
influence the optical and electronic properties of these materials
to a greater extent than is possible with standard solution-based
processing of OPVs. Thus, UV-vis absorption and photo-
luminescence (PL) spectroscopy were used to investigate the
effect of PEG length on OPV aggregation in solutions and in
thin films (Table 2).
Dilute, well-solvated solutions of OPV amphiphiles in tet-
rahydrofuran (THF) showed absorption and PL emission at 375
and 467 nm, respectively, with no effect of PEG length (Figure
4). However, in spin-coated films, the length of PEG segments
had a significant effect on the λabs of UV absorption. All films
showed enhanced vibronic structure and an absorption blue-
shift, indicating H-type aggregation with parallel alignment of
the OPV transition dipole moments,26 consistent with a low tilt
angle and no interdigitation of the OPV segments within the
bilayer smectic structure. This results in two distinct OPV layers
(Figure 4c) in which exciton coupling can occur between
molecules within a layer, but not between adjacent layers. The
longer the PEG block, the smaller the λabs blue-shift, as
Figure 4. (a) Absorption and (b) photoluminescence of OPV amphiphile
solutions and films of varying PEG length. All spectra are normalized to
the same intensity to facilitate comparison of peak shape. (c) Bilayer packing
model showing alkyl (red), OPV (green), and PEG (blue) layers. Inset arrows
show approximate orientation of OPV transition dipole.
crystallization of longer PEGs should disrupt the smectic layers,
thereby reducing OPV aggregation. Film PL depends on both
solvent and PEG length, as observed by the PL emission
maximum (λPL) of films spin-coated from CH3CN, which shifts
from 504 nm for OPV-16 to 468 nm for longer amphiphiles
(Figure 4b). In shorter OPV amphiphiles, aggregation and
exciton coupling within the highly ordered OPV layer could
explain the red-shifted λPL. At the same time, we propose that
structural disorder of the OPV induced by crystallization of
longer PEGs reduces aggregation and limits energy transfer
between OPV chromophores, enhancing emission and leading
to the observed PL spectra of OPV-45 films, which is almost
identical to that of the molecule in dilute solution. It is at first
counterintuitive that crystallization of one segment of the
molecule (PEG) could increase disorder in another segment
(OPV). However, this is consistent with the spectroscopic data
and is reasonable considering the very different crystal structures
of the two separate portions of the molecule, which likely
prevent the amphiphile from adopting a packing geometry that
is simultaneously favorable for both PEG and OPV. Thus, as
the length of the PEG segment increases, its equilibrium
structure dominates the overall behavior of the amphiphile, at
the expense of the highly ordered OPV-driven structure present
in shorter amphiphiles.
(26) Whitten, D. G. Acc. Chem. Res. 1993, 26, 502-509.
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J. AM. CHEM. SOC. VOL. 127, NO. 1, 2005 369

A R T I C L E S
Hulvat et al.
Table 3. Lyotropic LC Clearing Temperature and Lamellar Layer
Spacing
solventa
Tc (°
C)
d
100 (nm)b
H2O
>100
68
11.0
11.1
11.3
13.1
DMSO
CH3CN
DMF
55
52
a Mesophases formed with 40 wt % OPV-45 in the listed solvent. b Layer
spacing determined from first-order diffraction peak in SAXS.
Figure 6. PL spectra for aqueous solutions of OPV amphiphiles at various
concentrations (Ex ) 375 nm). Spectra are normalized (at 475 nm) to
facilitate comparison. Inset shows maximum intensity versus concentration,
with CMC and LC transitions indicated.
PL intensity is directly proportional to concentration up to 0.1
wt %, beyond which a 30 nm red shift and exponential decrease
in PL emission is observed, consistent with OPV aggregation
above the critical micelle concentration (CMC) of the solution.
A sharp shoulder appears at 505 nm, reaching a maximum at
10 wt % amphiphile, which could be vibronic in nature or due
to J-aggregate formation. At 30 wt %, LC mesophase formation
induces a surprising change in the shape of the PL spectrum.
The peak at 505 nm diminishes, and the emission blue shifts
toward that of dilute solutions, possibly due to confinement of
the OPV within the layered LC structure, thus limiting
intermolecular energy transfer.
Figure 5. (a) Aqueous OPV amphiphile gel under visible (left) and 365
nm UV light (right). (b) Birefringent defect in homeotropically aligned
aqueous LC. (c and d) Shear induced homogeneous alignment of aqueous
LC. Switches uniformly from (c) bright to (d) dark on 45° rotation of crossed
polarizers.
Aqueous Self-Assembly and Lyotropic Phases. OPV am-
phiphiles are soluble in most polar organic solvents and, for
OPV-24 or longer, are soluble in water as well. At high
concentration (>30 wt %), they form lyotropic LC phases that
likely consist of a solvent-swelled PEG layer and an aggregated
OPV-alkyl layer. One expects, based on the Israelachvili packing
model, that the bulky PEG chains of the longer amphiphiles
would increase the curvature of the hydrophobic-hydrophilic
interface, forming hexagonal or cubic mesophases.27,28 Only
lamellar structures are observed, however, possibly because OPV
aggregation frustrates hydrophobic collapse, increasing the
effective molecular cross section at the interface.
OPV-45 was used for the study of lyotropic LC gels since
longer PEGs imparted better solubility and more promising PL
behavior. Table 3 details results of DSC and SAXS on this
amphiphile in solvents where mesophase behavior was observed.
The concentration used corresponds to a water-PEG ratio
of 1.9:1 (w/w) or 4.6 water molecules per PEG repeat unit. This
should result in full hydration of PEG, with hydrogen bonding
saturated and nearly all water in the bound state.29 As the solvent
is largely immobilized, the mixture forms a highly viscous,
transparent gel. Figure 5a is a photograph of the aqueous, self-
supporting gel in an upturned vial, illuminated by either visible
or UV light. The gels are strongly fluorescent despite the high
concentration of OPV.
Lamellar LC order was confirmed in the aqueous OPV
amphiphile gel by POM and SAXS. When heated to 40 °C, the
LC phase aligns homeotropically on glass (Figure 5b), likely
due to strong interaction between PEG and the hydrophilic glass
surface. The LC phase can also be oriented homogeneously
through shear induced alignment (Figure 5c,d). Uniformly
aligned regions several centimeters in diameter have been
prepared, which is a significant improvement over thermotropic
films where domain sizes are generally <100 µm (Figure 3).
This suggests a facile route to align OPV amphiphile films by
casting from a dilute aqueous solution. As water evaporates,
the molecules undergo a transition through an LC phase, yielding
ordered, aligned films on drying. SAXS confirmed that lamellar
structure of the aqueous gel is retained on drying, though the
layer spacing increases from 11 to 15 nm (Figure 7). This likely
is due to rearrangement of the PEG chain, which typically adopts
a 7/2 helical conformation (7 monomer repeat units per 2 turns)
on drying with a pitch of 1.93 nm,30
giving an expected length
of 12.5 nm for OPV-45. Thus, the observed length is consistent
with that of a lamellar packing model with a fully interdigitated
PEG layer.
Figure 6 shows PL of aqueous OPV amphiphile solutions.
At low concentrations, λPL is 473 nm, showing a small
solvatochromic shift from the 468 nm peak in THF (Table 2).
To confirm LC order is responsible for the novel photo-
luminescence of OPV amphiphile gels, we examined its
temperature dependence. Aqueous gels remain in the LC phase
up to the solvent’s boiling point, thus, the temperature depen-
dence of PL was determined for a 40 wt % gel of OPV-45 in
dimethyl sulfoxide (DMSO) instead of water. At this composi-
(27) Israelachvili, J. N. Intermolecular and Surface Forces, 2nd ed.; Academic
Press: San Diego, CA, 1992.
(28) Kunieda, H.; Umizu, G.; Yamaguchi, Y. J. Colloid Interface Sci. 1999,
218, 88-96.
(29) (a) Borodin, O.; Bedrov, D.; Smith, G. D. J. Phys. Chem. B 2002, 106,
5194-5199. (b) Smith, G. D.; Bedrov, D.; Borodin, O. Phys. ReV. Lett.
2000, 85, 5583-5586.
(30) Yang, R.; Yang, X. R.; Evans, D. F.; Hendrickson, W. A.; Baker, J. J.
Phys. Chem. 1990, 94, 6123-6125.
9
370 J. AM. CHEM. SOC. VOL. 127, NO. 1, 2005

Self-Assembly of OPV Amphiphiles
A R T I C L E S
Experimental Methods
General. Melting and mesophase transition temperatures were
determined using a TA Instruments 2920 DSC/TGA. Ten milligram
samples were placed in hermetically sealed Al pans and cycled three
times at 5 °C min-1. Transition temperatures were determined from
the second heating cycle (all were fully reversible). Thermogravimetric
analysis (TGA) showed an onset of decomposition at 190 °C on heating
at 10 °C min-1 in air, which was well above the clearing temperature
for all OPV amphiphiles investigated. Additionally, the stability of the
molecule against thermal degradation was verified by NMR, which
showed no change after heating the molecule to 180 °C for 4 h in air.
Purity and polydispersity of the final product were determined using a
Waters 2690 GPC in THF calibrated with polystyrene standards. A
single peak was obtained for each amphiphile. POM was performed
using a Leitz Laborlux 12POL polarizing microscope with a thermo-
statically controlled heating stage and a 35 mm film camera. Samples
were sealed with a 75 µm spacer between pre-cleaned glass slides,
heated to isotropization, and then cooled at 0.3 °C min-1 to obtain
identifiable LC textures. SAXS spectra were collected on samples sealed
in quartz capillary tubes using a Rigaku Cu KR source at 33 kV and a
2D Bruker CCD detector calibrated with silver behenate. Wide-angle
powder X-ray diffraction patterns were collected on a Scintag XDS2000
automated diffractometer with a Cu KR source operating at 40 kV.
UV-vis absorption was studied using a Cary 500 UV-vis-NIR
spectrometer operating in double-beam mode. PL spectra were collected
on a PC1 spectrofluorimeter in right angle geometry. Both instruments
were fitted with heated, thermostatically controlled sample holders. For
film measurements, 1 wt % solutions were spin-coated at 2000 rpm on
quartz plates, yielding films 300 nm thick. For PL, film samples were
placed at 45° to the detector. The excitation wavelength was chosen at
the absorption maximum from UV-vis, but PL spectral shape was
found to be relatively insensitive to excitation wavelength. Dilute
solutions for fluorescence in THF were prepared by adjusting the
concentration to an absorption of <0.1. For high-concentration solutions
and LC gels, ultrashort (10 µm) path length quartz holders were used
to allow adequate transparency. To ensure complete dissolution of the
OPV amphiphile, lyotropic LC gels were prepared by dissolving the
material at 10 wt % and then evaporating solvent to achieve the desired
concentration. Samples were then heated in sealed vials to homogenize.
HPLC grade solvents for LC experiments (THF, CH3CH, and DMSO)
were degassed prior to use, and DI water was purified using a Millipore
filtration system.
Figure 7. SAXS spectra of aqueous (blue) and dried (red) films of OPV-
45, showing lamellar periodicity. Dashed line is magnified 10× to show
higher-order peaks.
Figure 8. (a) PL spectra of 40 wt % DMSO gel at various temperatures
(Ex ) 375 nm). (b) Wavelength (red) and intensity of PL maximum versus
temperature.
The OPV-8 amphiphile was synthesized by the following procedure.
Other OPV-n amphiphiles were synthesized by a similar procedure
beginning with longer poly(ethylene glycol) methyl ethers. Unless
otherwise noted, all starting materials were obtained from commercial
suppliers and used without further purification. 4-(Dimethylamino)-
pyridinium-4-toluenesulfonate (DPTS) was prepared according to
tion, the amphiphile crystallizes at room temperature, forms a
lamellar LC between 38 and 68 °C, and an isotropic solution
above. Figure 8 shows PL spectra for the DMSO gel at various
temperatures. Strikingly, a 4-fold increase in PL intensity and
a 25 nm blue shift occur on formation of the LC mesophase.
On isotropization, PL intensity returns to its previous level. The
transition is fully reversible, and similar results were obtained
in other solvents, such as acetonitrile. While fluorescence varied
significantly as a function of temperature, absorption did not,
indicating that the observed behavior is not due to changes in
scattering or absorption by the gel. Instead, it appears that
photoluminescence is enhanced due to the ordered structure of
the lyotropic liquid crystal, which could alter the OPV’s
aggregation state or limit exciton coupling and migration within
the OPV sublayers of the lamellar structure. The ability to
control the nanoscale structure and aggregation of OPV through
amphiphilic self-assembly, as demonstrated in these systems,
may prove useful in enhancing the performance of phenylene
vinylene-based molecules for a variety of organic electronics
applications.
1
literature.31 The H NMR and 13C NMR spectra were recorded on a
Varian Unity 400 (400 MHz) or Unity 500 (500 MHz) spectrometer
using the solvent proton signal as standard. Mass spectra were obtained
on a Micromass Quattro II atmospheric pressure ionization (API) triple
quadrupole mass spectrometer. Matrix-assisted laser desorption ioniza-
tion-time-of-flight mass spectrometry (MALDI-TOF MS) was per-
formed on an Applied Biosystems Voyager-DE Pro. Silica for flash
chromatography was ICN Silitech 32-63 D 60 A.
Poly(ethylene glycol) Methyl Ether-4-bromomethyl Benzoate (1).
R-Bromo-p-toluic acid (1.07 g, 5.00 mmol, 1.00 equiv), poly(ethylene
glycol) methyl ether (Mw ) 350 g/mol, 1.75 g, 5.00 mmol, 1.00 equiv),
DPTS (1.56 g, 5.30 mmol, 1.06 equiv), and CH2Cl2 (150 mL) were all
combined in a flask with a stirring bar. 1-(3-(Dimethylamino) propyl)-
3-ethylcarbodiimide hydrochloride (1.44 g, 7.53 mmol, 1.50 equiv) was
added, and the reaction mixture was stirred for 24 h at room
temperature. The solution was washed with a 5% aqueous solution of
citric acid and a saturated aqueous solution of NaCl. The organic layer
was collected, dried with MgSO4, filtered, and concentrated in vacuo.
(31) Granier, T.; Thomas, E. L.; Gagnon, D. R.; Karasz, F. E.; Lenz, R. W. J.
Polym. Sci., Part B: Polym. Phys. 1986, 24, 2793-2804.
9
J. AM. CHEM. SOC. VOL. 127, NO. 1, 2005 371

A R T I C L E S
Hulvat et al.
The product was subjected to a column chromatography using 5%
MeOH/CH2Cl2 as the eluant to afford the product (2.2 g, 4.0 mmol,
80% yield). 1H NMR (400 MHz, CDCl3) δ: 8.06 (d, 2H, J ) 7.9 Hz),
7.47 (d, 2H, J ) 8.6 Hz), 4.63 (s, 2H), 4.50 (t, 2H, J ) 4.9 Hz), 3.85
(t, 2H, J ) 4.6 Hz), 3.67 (m, 24H), 3.39 (s, 3H). 13C NMR (500 MHz,
CDCl3) δ: 166.0, 142.8, 130.4, 130.2, 129.0, 72.5, 71.9, 70.7, 70.6,
70.5, 70.2, 64.3, 61.6, 59.0, 32.3.
was evaporated in vacuo, and a yellow solid precipitated out in the
water layer and was collected by filtration. The crude product was
subjected to a column chromatography using CH2Cl2 to afford the
product as a yellow solid (8.4 g, 21 mmol, 58% yield). 1H NMR (400
MHz, CDCl3) δ: 9.98 (s, 1H), 7.85 (d, 2H, J ) 7.9 Hz), 7.62 (d, 2H,
J ) 7.9 Hz), 7.48 (d, 2H, J ) 8.6 Hz), 7.22 (d, 1H, J ) 16.5 Hz), 7.00
(d, 1H, J ) 16.5 Hz), 6.91 (d, 2H, J ) 8.6 Hz), 3.98 (t, 2H, J ) 6.7
Hz), 1.79 (m, 2H), 1.45 (m, 2H), 1.26 (m, 16H), 0.88 (t, 3H, J ) 6.7
Hz). 13C NMR (500 MHz, CDCl3) δ: 191.9, 159.9, 144.2, 135.2, 132.1,
130.5, 129.3, 128.5, 126.8, 125.2, 115.1, 68.4, 32.2, 29.9, 29.7, 29.6,
29.5, 26.3, 22.9, 14.4. APCI-MS m/z 393.4 (M+).
(Poly(ethylene glycol) Methyl Ether)-4-(2-(4-(2-(4-dodecyloxy-
phenyl)-(E)-1-ethenyl)phenyl)-(E)-1-ethenyl)-1-benzoate (OPV-8).
OPV-8 amphiphile was prepared with aldehyde 6 (0.36 g, 0.92 mmol,
1.0 equiv), phosphonate 2 (0.5 g, 0.9 mmol, 1 equiv), and LDA (1.5
M solution in cyclohexane, 0.74 mL, 1.1 mmol, 1.2 equiv) via the
Horner-Emmons condition similar to that of the reaction for compound
5. The crude product was extracted with CH2Cl2 and subjected to a
column chromatography using 5% MeOH/CH2Cl2 to afford the product
as a yellow solid (0.56 g, 0.66 mmol, 72% yield). Mw/Mn ) 1.01.
MALDI-TOF MS m/z 900.2 (M + Na+). 1H NMR (400 MHz, CDCl3)
δ: 8.05 (d, 2H, J ) 8.6 Hz), 7.58 (d, 2H, J ) 8.6 Hz), 7.52 (d, 4H, J
) 3.1 Hz), 7.46 (d, 2H, J ) 8.5 Hz), 7.23 (d, 1H, J ) 16.5 Hz), 7.14
(d, 1H, J ) 16.5 Hz), 7.11 (d, 1H, J ) 16.5 Hz), 6.98 (d, 1H, J ) 16.5
Hz), 6.91 (d, 2H, J ) 8.6 Hz), 4.45 (t, 2H, J ) 4.5 Hz), 3.99 (t, 2H,
J ) 6.7 Hz), 3.85 (t, 2H, J ) 4.6 Hz), 3.65 (m, 24H), 1.79 (m, 2H),
1.45 (m, 2H), 1.27 (m, 16H), 0.89 (t, 3H, J ) 6.7 Hz). 13C NMR (500
MHz, CDCl3) δ: 166.5, 159.3, 142.2, 138.0, 135.8, 131.1, 130.4, 130.0,
129.0, 128.9, 128.0, 127.4, 127.3, 126.8, 126.5, 126.0, 114.9, 72.1,
70.9, 70.8, 69.5, 68.3, 64.3, 59.2, 32.1, 29.9, 29.8, 29.6, 29.6, 29.5,
26.3, 22.9, 14.4.
Poly(ethylene glycol) Methyl Ether-4-((diethylphosphono)methyl)
Benzoate (2). Benzyl bromide 1 (1.0 g, 2.1 mmol, 1.0 equiv) and
triethyl phosphite (0.72 mL, 4.2 mmol, 2.0 equiv) were placed in a
flask with a magnetic stirring bar. A distillation apparatus was attached
to collect ethyl bromide formed along with the reaction. The mixture
was immersed in an oil bath and heated to 130 °C for 24 h. The reaction
mixture was cooled, diluted with Et2O, and washed with H2O. The
organic layer was dried with MgSO4, filtered, and concentrated in vacuo.
The resulting oil (1.29 g, 2.10 mmol, 100% yield) was used for the
1
next Horner-Emmons reaction without further purification. H NMR
(400 MHz, CDCl3) δ: 8.00 (d, 2H, J ) 7.9 Hz), 7.37 (d, 2H, J ) 7.3
Hz), 4.45 (t, 2H, J ) 4.9 Hz), 4.01 (t, 2H, J ) 6.7 Hz), 3.63 (m, 24H),
3.19 (d, 2H, J ) 22.0 Hz).
Diethyl-4-cyanobenzyl Phosphonate (3). Compound 3 was prepared
with R-bromo-p-tolunitrile (10 g, 51 mmol, 1.0 equiv) and triethyl
phosphite (9.6 mL, 56 mmol, 1.1 equiv) via an Arbuzov condition
similar to that of the reaction for 1 as a colorless oil (12.9 g, 51.0
1
mmol, 100% yield). H NMR (400 MHz, CDCl3) δ: 7.60 (d, 2H, J )
7.9 Hz), 7.41 (d, 2H, J ) 7.9 Hz), 4.03 (m, 4H), 3.19 (d, 2H, J ) 22.6
Hz), 1.25 (t, 6H, J ) 7.0 Hz).
4-Dodecyloxybenzaldehyde (4). 4-Hydroxybenzaldehyde (5.0 g, 41
mmol, 1.0 equiv), potassium carbonate (8.5 g, 61 mmol, 1.5 equiv),
dodecyl bromide (12.3 g, 49.2 mmol, 1.20 equiv), and 18-crown-6 (1.0
g, 4.1 mmol, 0.10 equiv) were placed in a flask with a magnetic stirring
bar and a cooling column and were dissolved in 50 mL of acetone.
The mixture was refluxed for 24 h. After the mixture was cooled, it
was filtered and concentrated in vacuo. The crude product was subjected
to a column chromatography using CH2Cl2 to afford the product as a
pale yellow solid (10.3 g, 35.0 mmol, 87% yield). 1H NMR (400 MHz,
CDCl3) δ: 9.85 (s, 1H), 7.83 (d, 2H, J ) 8.6 Hz), 7.00 (d, 2H, J ) 8.6
Hz), 4.05 (t, 2H, J ) 6.4 Hz), 1.80 (m, 2H), 1.27 (m, 18H), 0.89 (t,
3H, J ) 6.7 Hz). 13C NMR (500 MHz, CDCl3) δ: 191.0, 164.5, 132.2,
130.0, 115.0, 68.7, 32.2, 29.9, 29.8, 29.6, 29.3, 26.2, 22.9, 14.4. APCI-
MS m/z 291.3 (M+).
4-(2-(4-Dodecyloxyphenyl)-(E)-1-ethenyl)-1-benzonitrile (5). Lithium
diisopropylamide mono(tetrahydrofuran) (LDA) (1.5 M solution in
cyclohexane, 28.3 mL, 42.5 mmol, 1.20 equiv) and 50 mL of THF
were placed in a dry flask using a magnetic stirring bar under N2 and
cooled to -78 °C. Phosphonate 3 (9.0 g, 35 mmol, 1.0 equiv), dissolved
in 50 mL of THF, was added dropwise into the precooled solution
with a dropping funnel. The reaction mixture was placed in a 0 °C ice
bath, and aldehyde 4 (10.3 g, 35.4 mmol, 1.00 equiv), dissolved in 50
mL of THF solution, was added dropwise into the mixture. The reaction
mixture was stirred overnight at room temperature and quenched by
adding an aqueous solution of acetic acid. THF was removed by
evaporation, and a yellowish-white solid that precipitated out in the
water layer was collected by filtration. The crude product was subjected
to a column chromatography using CH2Cl2 to afford the product as a
pale yellow solid (10.9 g, 28.0 mmol, 79% yield). 1H NMR (400 MHz,
CDCl3) δ: 7.61 (d, 2H, J ) 8.5 Hz), 7.54 (d, 2H, J ) 7.9 Hz), 7.46 (d,
2H, J ) 8.5 Hz), 7.16 (d, 1H, J ) 16.5 Hz), 6.94 (d, 1H, J ) 16.5
Hz), 6.90 (d, 2H, J ) 8.6 Hz), 3.98 (t, 2H, J ) 6.4 Hz), 1.79 (m, 2H),
1.45 (m, 2H), 1.26 (m, 16H), 0.88 (t, 3H, J ) 6.7 Hz). 13C NMR (500
MHz, CDCl3) δ: 160.0, 142.5, 132.7, 132.3, 128.5, 127.3, 126.8, 124.6,
119.4, 115.1, 114.8, 68.4, 32.2, 29.9, 29.7, 29.6, 29.5, 26.3, 22.9, 14.4.
APCI-MS m/z 390.5 (M+).
Conclusions
We have reported on a novel series of amphiphilic molecules
consisting of an OPV trimer end-substituted asymmetrically with
a hydrophobic alkyl chain and a hydrophilic ethylene glycol
chain, which self-assemble forming both thermotropic and
lyotropic liquid crystalline mesophases. By varying the length
of the ethylene glycol block, one can change the structure and
solubility of the amphiphile, altering its optoelectronic proper-
ties. Photoluminescence from films of the amphiphile with the
longest hydrophilic chain resembles that of molecules in dilute
solution, indicating that self-assembly dramatically alters the
aggregation behavior of the OPV chromophore. The layered
LC phase appears to inhibit OPV aggregation and reduce exciton
migration, leading to enhanced and blue-shifted photolumines-
cence. Formation of ordered OPV amphiphile mesophases
provides a facile route to prepare nanostructured films in which
the structural and optical properties are controlled to a degree
not possible with soluble PPV polymers or other substituted
OPVs.
Acknowledgment. This work made use of the Keck Biophys-
ics Facility, the Jerome B. Cohen X-ray Diffraction Facility,
and the Analytical Services Laboratory at Northwestern Uni-
versity. J.F.H. is supported by an NDSEG fellowship and a
Northwestern Presidential Fellowship. The work was supported
by the U.S. Department of Energy under Award DE-FG02-
00ER54810.
Supporting Information Available: Differential scanning
calorimetry, MALDI-TOF MS, and additional photolumines-
cence spectra of OPV amphiphiles. This material is available
free of charge via the Internet at http://pubs.acs.org.
4-(2-(4-Dodecyloxyphenyl)-(E)-1-ethenyl)-1-benzaldehyde (6). Ni-
trile 5 (10.6 g, 36.8 mmol, 1.00 equiv) was dissolved in 500 mL of
Et2O and cooled to 0 °C. Diisobutylaluminum hydride (1.0 M solution
in hexane, 55.2 mL, 55.2 mmol, 1.50 equiv) was added dropwise into
the solution using a dropping funnel. The solution was stirred at 0 °C
for 20 min and poured into 10% AcOH/H2O (500 mL). The ether layer
JA047210M
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372 J. AM. CHEM. SOC. VOL. 127, NO. 1, 2005