Facile control of chiral packing in poly(p-phenylenevinylene) spin-cast films

Article abstract of DOI:10.1021/ma047418s

The facile control of chiral packing in poly(p-phenylenevinylene) (PPV) spin-cast films was investigated. The circular dichroism (CD) spectroscopy was used to demonstrate that spin-cast films from chiral π-conjugated polymer can be easily and selectively fabricated from different solvents to have correspondingly different architectures with opposite handedness. By varying the spin-casting solvent or annealing conditions, three distinct film architectures can be prepared from the PPV. The architectures included two chiral organizations with opposite handedness and a disordered assembly that does not exhibit and chiral packing.

Full text of DOI:10.1021/ma047418s

4054
Macromolecules 2005, 38, 4054-4057
mation of the polymer at various degrees of aggregation
(Figure 1a).16-20 The strong bisignate Cotton effects
centered at the π-π* transition (around 430 nm in the
UV-vis absorption spectra) are indicative of exciton
coupling21 between obliquely oriented neighboring poly-
mer backbone chains, suggesting the existence of a
chiral supramolecular organization within the aggre-
gates.22 In 100% chloroform, aggregation of the polymer
did not occur, so as expected, no bisignate CD couplets
were observed. In relatively nonpolar cosolvent mixtures
(70:30 and 60:40 CHCl3:MeCN) a positive CD couplet
is observed, whereas in polar cosolvent mixtures (e50:
50 CHCl3:MeCN) a negative CD couplet is observed.23,24
This solvent-induced inversion of Cotton effects has been
previously observed for chiral aggregate solutions of
polythiophene13,19,20,25 and polysilane.17 Yashima et al.
attributed this phenomenon in polythiophene to the
formation of two types of π-stacked, chiral supramo-
lecular assemblies: a cholesteric liquid crystalline-type
(i.e., helical) assembly of coplanar chains and a stack
of twisted backbone chains.19 Thus, for example, a
cholesteric assembly may be dominant in nonpolar
solvents (e.g., 60:40 CHCl3:MeCN which exhibited right-
handed chirality), while the twisted stack assembly may
be dominant in more polar solvents (e.g., 50:50 CHCl3:
MeCN which exhibited left-handed chirality). However,
it is not known exactly what type of architecture is
responsible for each observed handedness.
When the aggregate-free chloroform solution was
spin-cast into a thin film, no chiral organization was
found to exist in the resulting film (Figure 1b). The
disordered assembly in the solid state is a direct
consequence of the absence of ordered chiral aggregates
in good solvents. Several groups observed that it was
important to thermally anneal films in order to develop
chiroptical properties in the solid state.26 Additionally,
Liu et al. recently described a method of increasing the
degree of interchain interactions in MEH-PPV films by
exposing the films to saturated organic solvent vapor,9
which induces a plasticization effect and reduces the
glass-transition temperature of polymers.27 Consistent
with these studies, thermally annealing a film of PPV1
(at 45 °C for 30 min) in the presence of chloroform
vapor28 resulted in a bisignate CD couplet, suggesting
that the polymer chains self-assembled from a disor-
dered state to a more thermodynamically favored chiral
organization. This CD spectrum has a similar shape to
the negative CD couplet observed for the tightly ag-
gregated polymer in poor solvents (e50:50 CHCl3:
MeCN). Thus, the annealed film and the tightly aggre-
gated polymer solutions both consist of a predominantly
left-handed chiral organization.
Facile Control of Chiral Packing in
Poly(p-phenylenevinylene) Spin-Cast Films
Andrew Satrijo and Timothy M. Swager*
Department of Chemistry, Massachusetts Institute of
Technology, Cambridge, Massachusetts 02139
Received December 15, 2004
Revised Manuscript Received March 24, 2005
The control of film morphology is of critical impor-
tance in the fabrication of conjugated polymer-based
devices such as light-emitting diodes, field effect tran-
sistors, photodiodes, and photovoltaic cells.1 To optimize
the performance and efficiency of these devices, it is
crucial to obtain a better understanding of the polymer’s
interchain interactions and aggregation behavior in the
solid state. Many groups have recently studied the
morphological properties of poly(p-phenylenevinylene)
(PPV) derivatives, which are among the most exten-
sively investigated conjugated polymers for electronic
applications due to their stability, easy processability,
and good electrical and optical properties.1 The mor-
phology and electrooptical properties of conjugated
polymer films are highly dependent on the deposition
technique,2 choice of solvent,3-7 polymer concentration,5
and annealing process.4,6,8,9 Notably, Schwartz et al.
observed that the degree of aggregation in a poly[2-
methoxy-5-(2′-ethylhexyloxy)-1,4-phenylenevinylene]
(MEH-PPV) solution could be directly transferred into
the film state by spin-casting deposition.6 Generally,
PPV films with higher degrees of interchain contact
have lower luminescence quantum yields, higher prob-
abilities of exciton-exciton annihilation, lower rates of
photobleaching, and greater charge carrier mobilities.
Currently, the nature of polymer aggregation in spin-
cast films is still unclear. One method to monitor
aggregation behavior both in solution and in the solid
state is by employing circular dichroism (CD) spectro-
scopy, a widely used technique for the conformational
analysis of chiral molecules and materials.10 In this
study, we used CD spectroscopy to demonstrate that
films spin-cast from a chiral π-conjugated polymer can
be easily and selectively fabricated from different
solvents to have correspondingly different chiral archi-
tectures with opposite handedness.
Fujiki et al. recently reported that σ-conjugated
polysilane films can be selectively prepared to contain
either P- or M-helices by drop-casting from isooctane
or hexane solutions above or below the helix-helix (P-
M) inversion temperature.7 In another account, Meijer
et al. reported that the cooling rate of a heated chiral
polythiophene film determined its chiral organization.11
By immersing the hot film into a cold water bath, they
were able to freeze-in a metastable assembly that
possessed a chirality that was opposite to that of a film
that was slowly cooled.
The chiral PPVs synthesized for this study are shown
in Scheme 1. To induce optical activity in the conjugated
backbone, enantiomerically pure chiral side chains were
incorporated into the polymers.12
By adding a poor solvent (e.g., polar acetonitrile) to a
solution of PPV1 dissolved in a good solvent (e.g.,
nonpolar chloroform), it is possible to study the confor-
Using the same polymer, we wanted to fabricate a
film having a chiral architecture with the opposite
handedness. Unfortunately, spin-cast films from good
solvent-poor solvent mixtures, such as CHCl3/MeCN or
CHCl3/MeOH, exhibited a significant amount of light
scattering and inhomogeneity due to precipitation dur-
ing the spin-coating process. A survey of various sol-
vents revealed that in 1,2-dichloroethane (DCE) the
polymer exhibited a strong positive CD couplet (Figure
2a) similar to that observed in 60:40 CHCl3:MeCN. The
lower solubility of PPV1 in 1,2-dichloroethane relative
to chloroform29 allowed the formation of stable ag-
* Corresponding author. E-mail: tswager@mit.edu.
10.1021/ma047418s CCC: $30.25 © 2005 American Chemical Society
Published on Web 04/15/2005

Macromolecules, Vol. 38, No. 10, 2005
Communications to the Editor 4055
Scheme 1. Synthesis of Chiral PPV Derivatives PPV1 and PPV2a
a (a) (S)-3,7-Dimethyloctyl bromide,13 K2CO3, KI, 2-butanone, reflux; (b) n-BuLi, TMEDA, hexane, rt; (c) B(OMe)3, 0 °C; (d)
aqueous HCl; (e) 2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane, 0 °C; (f) Pd(PPh3)4, K2CO3, DMF, PhMe, H2O, EtOH, 90
°C; (g) Pd(PPh3)4, Cs2CO3, p-dioxane, 90 °C; (h) SOCl2, CH2Cl2; (i) KOtBu, THF.
Figure 1. CD and UV-vis spectra of PPV1 as (a) solutions
in CHCl3:MeCN (v:v) and (b) a spin-cast film, before (solid line)
and after (dashed line) annealing. Insets: proposed packing
models of (left) left-handed twisted stack and (right) right-
handed cholesteric assembly.
Figure 2. CD and UV-vis spectra of PPV1 as (a) a solution
in 1,2-dichloroethane and (b) a corresponding spin-cast film
without any annealing.
Similar to the DCE solution, the corresponding spin-
cast films also displayed strong positive CD couplets
(Figure 2b), indicative of a predominantly right-handed
chiral organization. It is apparent that the spin-casting
process kinetically traps the polymer chains in their
solution conformation.4-6 Thermal annealing in the
gregates in solution. The DCE solution, as well as the
polar CHCl3:MeCN solutions (Figure 1a), exhibited red-
tailing in the UV-vis spectra, signifying the presence
of light-scattering aggregate particles.

4056 Communications to the Editor
Macromolecules, Vol. 38, No. 10, 2005
significantly alter the shape of the positive CD couplet,
but it did increase the signal’s intensity, suggestive of
a minor reorganization of the film’s chiral packing.
In conclusion, we have shown that incorporating
bulky, interlocking side groups into a chiral π-conju-
gated polymer can help to assemble ordered, chiral
architectures in films spin-cast from a good solvent (e.g.,
chloroform). Additionally, we have demonstrated that
from one chiral PPV, three distinct film architectures
can be prepared: a disordered assembly that does not
exhibit any chiral packing and two chiral organizations
with opposite handedness. This ability to easily control
the stacking organization in a π-conjugated polymer film
may facilitate the optimization of its electrooptical
properties and aid in future studies of luminescence
quenching in the aggregated state.
Acknowledgment. We thank the NSF for financial
support. A.S. also thanks NSERC Canada and the MIT-
DuPont Alliance for graduate fellowships.
Supporting Information Available: Details of experi-
mental procedures and the synthesis and characterization of
polymers. This material is available free of charge via the
Internet at http://pubs.acs.org.
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Figure 3. CD and UV-vis spectra of PPV2 as (a) solutions
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and after (dashed line) annealing.
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