Chain-length dependent nematic ordering of conjugated polymers in a liquid crystal solvent

Article abstract of DOI:10.1021/ja805017m

In this communication we demonstrate the dependence of the solute order parameter on the solute molecular weight for polymer solutes dissolved in liquid crystalline solvents. Using ensemble absorption polarization spectroscopy together with single molecul

Full text of DOI:10.1021/ja805017m

Published on Web 08/19/2008
Chain-Length Dependent Nematic Ordering of Conjugated Polymers in a
Liquid Crystal Solvent
Alexei Tcherniak,† David Solis, Jr.,† Saumyakanti Khatua,† Andrew A. Tangonan,‡ T. Randall Lee,‡
and Stephan Link*,†
Departments of Chemistry, Rice UniVersity, Houston, Texas 77005, and UniVersity of Houston,
Houston, Texas 77204-5003
Received July 9, 2008; E-mail: slink@rice.edu
The nanoscale ordering of conjugated polymers plays an important
role in enhancing the performance of organic electronic devices.1-5
Key to this effort is the use of long-range supramolecular interactions
to create organized self-assembled structures.6-8 In particular, liquid
crystalline (LC) solvents are ideal templates for inducing the large-
scale nematic ordering of conjugated polymers.9,10 According to
Onsager,11 the degree of alignment in a nematic mixture is enhanced
for rodlike solutes that are longer than the LC solvent molecules. This
prediction has recently been verified for a conjugated polymer solute
in an LC solvent using single molecule spectroscopy.12,13 Correlated
long-range anisotropic solvation forces are thought to be responsible
for a nearly perfect polymer alignment-an effect termed “cooperative
anisotropic solvation” (CAS).12 However, it is not well established
how much longer the polymer solute has to be to experience an
enhanced ordering.
In this communication, we demonstrate the dependence of the
Figure 1. UV-vis absorption spectra of MEH-PPV (Mn ) 12 100) in the
nematic and isotropic phases of 5CB. Inset: Dichroic ratio D as a function
of Mn. Lines are given as guides to the eyes.
solute order parameter on the solute molecular weight (Mn). Using
ensemble absorption polarization spectroscopy together with single
molecule fluorescence polarization measurements, we have deter-
mined the order parameter of the conjugated polymer MEH-PPV
(poly[2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene]) in
5CB (4-cyano-4-n-pentylbiphenyl) as a function of chain length.
For comparison to the solvent order14 and to approximate an
oligomer, we also show results for the dye R6G.
The degree of solute alignment for five MEH-PPV samples and
R6G was characterized using absorption spectroscopy. Figure 1
shows the polarized absorption spectra for MEH-PPV with Mn )
12 100 in the nematic and isotropic phases. The absorption intensity
in the isotropic phase Aiso agrees well with the isotropic average,
A ) A| + 2A|, verifying the homogeneous solvation of the MEH-
PPV. The dichroic ratio, D ) A|/A|, is larger than 1, indicating a
parallel alignment of the conjugated polymer backbone with the
LC director.15,16 The dichroic ratios determined at the absorption
maximum increase with increasing Mn (inset) due to a better
alignment of the longer polymer chains.
The weight-dependent ordering of MEH-PPV was further
confirmed using single molecule fluorescence polarization spec-
troscopy. Figure 2 shows single molecule fluorescence polarization
distributions for two MEH-PPV samples with Mn ) 53 000 and
Mn ) 3900. The mean polarization increases with increasing Mn
as further illustrated in the inset.
Figure 2. Single molecule fluorescence polarization distributions for MEH-
PPV samples with Mn ) 53 000 and Mn ) 3900. Inset: Mean polarization
as a function of Mn. Results of a previously measured sample with Mn )
110 000 are included for comparison (open symbol).12 Lines are given as
guides to the eyes.
alignment of the main internal polymer axis relative to the LC
director expressed by the angle R. SC is a measure of the orientation
of the chain segments with respect to the main polymer axis defined
The alignment and conformation of an uncollapsed polymer chain
containing a small number of defects can be quantitatively expressed
by two coupled order parameters, the orientation and conformation
order parameters, SO and SC.12,13 By analogy to the solvent order
parameter describing the orientation of the LC solvent molecules
as SC
)
1/2 3 cos2 ꢀ - 1 , where ꢀ is the angle between the
individual segments and the main polymer axis (Figure 3).
Experimentally, the conformation order parameter is related to the
mean polarization of the molecular transition dipoles of a single
polymer chain that is composed of multiple chromophores orientated
along the chain segments.
1
with respect to the LC director, SO ) /2 3 cos2 R - 1 gives the
† Rice University.
‡ University of Houston.
9
12262 J. AM. CHEM. SOC. 2008, 130, 12262–12263
10.1021/ja805017m CCC: $40.75
2008 American Chemical Society

C O M M U N I C A T I O N S
when exciting at 532 nm for the fluorescence polarization experi-
ments.18 Better agreement is indeed achieved if one calculates the order
parameter from the absorption intensities at 532 nm instead of the
absorption maximum, which gives 0.75 compared to 0.80 obtained
from the single molecule experiments.
It is worth considering the experimental Mn dependence in light
of possible practical applications of polymer-doped LC devices.19
A saturation of the absorption and fluorescence anisotropy occurs
at a polymer chain length of only 30 to 45 repeat units (∼10 to 15
times the length of a solvent molecule), giving a practical limit for
the size of the MEH-PPV solute because larger MEH-PPV polymer
chains show a significant decrease in solubility but do not increase
the polarization anisotropy. In contrast, stiffer poly(phenylene
ethylene)s with iptycene scaffolds designed to increase ordering
and solubility give a continuous increase of the order parameter
with Mn of 10K to over 200K.20
In summary, we have provided experimental verification of the
concept of CAS by showing how the order parameter for conjugated
polymers dissolved in a nematic LC scales with Mn. Ensemble
absorption polarization measurements agree well with results
obtained by single molecule fluorescence polarization spectroscopy,
indicating a large-scale ordering of the MEH-PPV solute in 5CB.
These results demonstrate that the increasing number of defects
for larger polymer weights inherently limits the polarization
anisotropy of the polymer solute. For applications in colored LC
displays, a further increase in the dichroic ratio could, in principle,
be achieved with a stiffer macromolecular solute.
Figure 3. Orientation SO and conformation SC order parameters as a function
of repeat units. Results of a previously measured sample with 423 repeat units
are included for comparison (open symbols).12 Green diamonds are data points
from ensemble absorption measurements. Lines are given as guides to the eyes.
The cartoon (not to scale) shows qualitatively how the ordering of a polymer
in an LC is enhanced with increasing polymer chain length.
By fitting the experimental fluorescence polarization distributions
(solid lines in Figure 2), we obtain the chain orientation order
parameter SO for a rod-shaped polymer solute in an anisotropic LC
solvation potential. The solvation potential depends linearly on the
number of repeat units,13,17 which were calculated from the
measured Mn, leaving the distribution of segment orientations (i.e.,
SC) as the only adjustable variable in this model. The results of
this analysis for the two order parameters are presented in Figure
3 for all investigated samples.
Analysis of the order parameter shows that the alignment
increases with increasing length, confirming the concept of CAS.
The conformation order parameter decreases from 1 for the single
chromophore R6G to 0.59 for MEH-PPV, independent of the MEH-
PPV chain length. The latter is in good agreement with the fact
that the percentage of tetrahedral defects (∼1-11%) is independent
of Mn as deduced from absorption and NMR spectra. While the
total anisotropic solvation energy increases with increasing chain
length, the total number of defects also increases, requiring more
energy to stretch a longer polymer chain. The competition between
anisotropic solvation and internal chain bending energy leads
therefore to the observed invariance of the conformation order
parameter. Nevertheless, all polymer chains are stretched in the
LC solvent compared to isotropic solution,10,15,18 as shown
previously by Monte Carlo simulation12,13 and verified here by an
up to 20 nm red shift of the absorption maxima in 5CB.
The advantage of using an LC solvent for ordering conjugated
polymers is that the alignment can be achieved over large length
scales,9 which is confirmed by the orientation order parameters obtained
from the polarized absorption spectra (green diamonds in Figure 3).
The order parameter of an absorbing solute is given by the polarization
anisotropy, PA ) (A| - A|)/(A| + 2A|), corrected by the angle ꢁ
between the transition dipole moment and the aligned molecular
axis.15,16 For ꢁ ) 0, SO ) PA, while for ꢁ > 0, SO ) PA * {2/(3 cos
ꢁ -1)}. Because the conformation order parameter is a measure of
the chain segment orientation and thus chromophore alignment for
nearly perfectly aligned chains, we used the angle associated with the
conformation order parameter in our analysis (i.e., ꢁ ) ꢀ). This
approach gives excellent agreement between the single molecule and
ensemble experiments.13,15 The MEH-PPV sample with Mn ) 3900
shows a difference for the two methods, which can be rationalized by
the photoselection of longer, straighter, and thus better aligned chains
Acknowledgment. Support was provided by the Robert A.
Welch Foundation (C-1664 to S.L., E-1320 to T.R.L.) and the
National Aeronautics and Space Administration (NCC-1-02038 to
T.R.L.). S.L. thanks 3M for a Non-Tenured Faculty Grant. D.S.
thanks the NSF and DoD for an REU fellowship (PHY-0453365).
Supporting Information Available: Sample preparations, chemical
structures, experimental setups, data analysis, NMR data, and absorption
spectra of MEH-PPV. This material is available free of charge via the
Internet at http://pubs.acs.org.
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