Structural control in thin layers of poly(p-phenyleneethynylene)s: Photophysical studies of Langmuir and Langmuir-Blodgett films

Article abstract of DOI:10.1021/ja0200600

We present the relationship between the spatial arrangement and the photophysical properties of fluorescent polymers in thin films with controlled structures. Eight surfactant poly(p-phenyleneethynylene)s were designed and studied. These detailed studies

Full text of DOI:10.1021/ja0200600

Published on Web 06/08/2002
Structural Control in Thin Layers of
Poly(p-phenyleneethynylene)s: Photophysical Studies of
Langmuir and Langmuir-Blodgett Films
Jinsang Kim,†,‡ Igor A. Levitsky,† D. Tyler McQuade,† and Timothy M. Swager*,†
Contribution from the Department of Chemistry and Department of Materials Science and
Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139
Received January 10, 2002
Abstract: We present the relationship between the spatial arrangement and the photophysical properties
of fluorescent polymers in thin films with controlled structures. Eight surfactant poly(p-phenyleneethynylene)s
were designed and studied. These detailed studies of the behavior of the polymers at the air-water interface,
and of the photophysical properties of their transferred LB films, revealed key structure-property
relationships. Some of the polymers displayed π-aggregates that are characteristic of an edge-on structure
at the air-water interface. Monolayer LB films of these polymers showed greatly reduced quantum yields
relative to solution values. Other polymers exhibited a highly emissive face-on structure at the air-water
interface, and did not form π-aggregates. The combination of pressure-area isotherms and the surface
pressure dependent in situ UV-vis spectra of the polymers at the air-water interface revealed different
behavioral details. In addition, the UV-vis spectra, fluorescence spectra, and quantum yields of the LB
films provide design principles for making highly emissive films.
Introduction
considered. Among these are the polymer’s absorption and
emission spectra, degree of organization, bandwidth, band gap,
A comprehensive understanding of the electronic states of
conjugated polymers is pivotal to the continued development
of these materials. In general, the electronic structure of isolated
chains is well understood and is readily approximated by
oligomeric compounds due to the limited effective chromophore
dimensions in these materials. However, there is invariably
electronic coupling between polymer chains, and these second-
ary interactions often dominate a material’s redox potential, band
gap, fluorescence efficiency, and electrical energy transport.1
The transport properties of conjugated polymers, their most
interesting feature, are extremely sensitive to interpolymer
interactions. This fact follows from the necessity of transfer of
charge or excitations between polymer chains.
Much of the technological promise of conjugated polymers
rests on their emissive properties. Applications include elec-
troluminescent displays,2 organic lasers,3 and sensors.4 Our
group’s interest in conjugated polymers for the amplification
of sensory signals4a requires optimization of their transport
properties. Within this framework, many factors need be
emission quantum yield and lifetimes, and energy transport
dynamics. The evolution of electronic structure from the
constituent chromophores of a polymer is reasonably well
understood; however, interchain interactions have less predict-
able consequences. Nevertheless, strong electronic coupling
between polymer chains has the prospect to increase intermo-
lecular energy transfer. Most often strong interpolymer inter-
actions give rise to distinct red shifts in the absorption spectra
and generally produce less emissive (quenched) materials.5,6 It
is generally assumed that polymers prefer to organize with
cofacial π interactions, and structural studies support this fact.7-9
However, the detailed role of interpolymer interactions on
luminescent polymers’ absorption and emission properties has
not been systematically studied due to the difficulty in control-
ling interpolymer arrangement. Greater understanding of chain-
chain interactions in conjugated polymers is clearly necessary
(4) (a) Swager, T. M. Acc. Chem. Res. 1998, 31, 201. (b) Yang, J. S.; Swager,
T. M. J. Am. Chem. Soc. 1998, 120, 5321. (c) Yang, J. S.; Swager, T. M.
J. Am. Chem. Soc. 1998, 120, 11864. (d) Kim, J.; McQuade, D. T.; McHugh,
S. K.; Swager, T. M. Angew. Chem., Int. Ed. 2000, 39, 3868. (e) McQuade,
D. T.; Pullen, A. E.; Swager, T. M. Chem. ReV. 2000, 100, 2537.
(5) (a) Jenekhe, S. A.; Osaheni, J. A. Chem. Mater. 1994, 6, 1906. (b) Jenekhe,
S. A.; Osaheni, J. A. Science 1994, 265, 765. (c) Jenekhe, S. A. AdV. Mater.
1995, 7, 309. (d) Osaheni, J. A.; Jenekhe, S. A. J. Am. Chem. Soc. 1995,
117, 7389.
(6) McQuade, D. T.; Kim, J.; Swager, T. M. J. Am. Chem. Soc. 2000, 122,
5885.
(7) Weder, C.; Wrighton, M. S. Macromolecules 1996, 29, 5157.
(8) Li, H.; Powell, D. R.; Hayashi, R. K.; West, R. Macromolecules 1998, 31,
52.
* To whom correspondence should be addressed. E-mail: tswager@
mit.edu.
† Department of Chemistry.
‡ Department of Materials Science and Engineering.
(1) (a) Cornil, J.; Heeger, A. J.; Bredas, J. L. Chem. Phys. Lett. 1997, 272,
463. (b) Cornil, J.; dos Santos, D. A.; Crispin, X.; Silbey, R.; Bredas, J. L.
J. Am. Chem. Soc. 1998, 120, 1289. (c) Bredas, J. L.; Cornil, J.; Beljonne,
D.; dos Santos, D. A.; Shuai, Z. Acc. Chem. Res. 1999, 32, 267.
(2) 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.
(9) (a) Halkyard, C. E.; Rampey, M. E.; Kloppenburg, L.; Studer-Martinez, S.
L.; Bunz, U. H. F. Macromolecules 1998, 31, 8655. (b) Levitus, M.;
Schmieder, K.; Ricks, H.; Shimizu, K. D.; Bunz, U. H. F.; Garcia-Garibay,
M. A. J. Am. Chem. Soc. 2001, 123, 4259-4265.
(3) (a) Hide, F.; Schwartz, B. J.; Diazgarcia, M. A.; Heeger, A. J. Chem. Phys.
Lett. 1996, 256, 424. (b) Hide, F.; Diazgarcia, M. A.; Schwartz, B. J.;
Anderson, M. R.; Pei, Q. B.; Heeger, A. J. Science 1996, 273, 1833.
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7710
J. AM. CHEM. SOC. 2002, 124, 7710-7718
10.1021/ja0200600 CCC: $22.00 © 2002 American Chemical Society

Thin Layers of Poly(p-phenyleneethynylene)s
A R T I C L E S
Chart 1
to reveal under which circumstances these interactions will lead
to emissive10 or quenched π-aggregated materials. Added fidelity
is required to understand the circumstances, wherein proximal
chain-chain interactions can lead to excited-state aggregation
and produce strong exciplex emissions.
Investigations are best performed when organized assemblies
of polymers can be prepared with predictable or manipulatable
conformations and intermolecular interactions. In this regard,
combination of surfactant polymer design and the Langmuir-
Blodgett (LB) technique11 is very useful. We have recently
synthesized surfactant poly(p-phenyleneethynylene)s (PPEs) and
used a LB trough to create spatially well-defined Langmuir
films.12 In these previous studies, we identified three specific
geometries. The first involves cofacial organization of the
π-plane with the air-water interface, referred to as the face-on
structure. The second geometry wherein the plane of the
conjugated π-system lies normal to the air-water interface is
referred to as the edge-on structure. The third one is the zipper
structure, alternating face-on and edge-on structures. In our
earlier systems,13 we determined that the face-on structure
provided a dynamic phase with liquid crystalline characteristics
at the air-water interface and was readily transferred to a
support to give highly aligned materials. Chain alignment plays
an important role in Fo¨rster energy transfer since those processes
are highly dependent upon the coincidence of transition dipoles
between donors and acceptor chromophores.14,15 The edge-on
structure gives crystalline aggregates on the LB trough that are
not amendable to organization by flow fields or anisotropic
compression. Nevertheless, these edge-on structures represent
an ideal situation for the organization of materials into well-
defined cofacial π-aggregated structures. In both cases, the
structures can be manipulated by applying pressure with the
LB trough. This exquisite structural control, as well as the ability
to transfer materials monolayer by monolayer, provides the most
controlled preparation of conjugated polymer assemblies to date.
The LB method also creates an extraordinary venue for
investigations directed at interfacing conjugated polymers with
water-soluble elements, and thereby establishes a firm founda-
tion from which to construct novel biosensory materials.
Herein we describe studies of PPEs designed to have specific
surfactant characteristics. We have broadened the scope of
structures that display edge-on or face-on structures and have
discovered conditions wherein the applied pressure can ma-
nipulate the polymer chromophores at the air-water interface.
We have also systematically studied how interpolymer interac-
tions affect the fluorescent polymers’ ground and excited states
in structurally defined monolayer films and solid solutions of
the polymers in PMMA. The approaches described herein have
broad applicability to a range of conjugated polymer structures
and illustrate the utility of LB techniques for the elucidation of
the structure-property relationships in conjugated polymers.
were investigated (Chart 1). Palladium-catalyzed cross-coupling
methods figure prominently into the synthesis of these poly-
mers.16,17 The preparation of polymers 1, 3, 4, and 6 was
reported elsewhere.4d,12,13,23 Polymers 2 and 8 were synthesized
as described in eq 1. The preparation of 9 was reported
elsewhere.12 Monomer 9 was further reacted with 1-((triethylene
glycol monomethyl ether)oxy)-4-decyloxy-2,5-diiodobenzene or
1,4-bis(N,N-dioctylcarbamoyl)-2,5-diiodobenzene to produce
polymers 2 and 8, respectively.
Polymer 5 was synthesized from the palladium coupling
reaction of 10 with 1,4-bis(N,N-dioctylcarbamoyl)-2,5-diiodo-
benzene (eq 2). Pentaethylene glycol ditosylate was reacted with
1,4-diiodo-2,5-dihydroxybenzene under standard Williamson
ether conditions, followed by palladium-catalyzed coupling
(trimethylsilyl)acetylene and deprotection to give 2,5-diethynyl-
p-phenylene-20-crown-6 (10).
Results and Discussion
Building upon our designs of PPEs that adopt face-on or edge-
on geometries at the air-water interface,12,13 eight polymers
(10) Deans, R.; Kim, J.; Machacek, M.; Swager, T. M. J. Am. Chem. Soc. 2000,
122, 8565.
(11) Wegner, G. Thin Solid Films 1992, 216, 105.
(12) Kim, J.; Swager, T. M. Nature 2001, 411, 1030.
(13) Kim, J.; McHugh, S. K.; Swager, T. M. Macromolecules 1999, 32, 1500.
(14) Fo¨rster, T. Ann. Phys. 1948, 2, 55.
(16) Sonogashira, K.; Tohda, Y.; Hagihara, N. Tetrahedron Lett. 1975, 4467.
(17) Swager, T. M.; Gil, C. J.; Wrighton, M. S. J. Phys. Chem. 1995, 99, 4886.
(15) Levitsky, I. A.; Kim, J.; Swager, T. M. J. Am. Chem. Soc. 1999, 121, 1466.
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J. AM. CHEM. SOC. VOL. 124, NO. 26, 2002 7711

A R T I C L E S
Kim et al.
As shown in eq 3, polymer 7 was synthesized from the
coupling reaction of 11 with 1,4-bis(N,N-dioctylcarbamoyl)-2,5-
diiodobenzene. 1,4-Bis[2-(2-hydroxyethoxy)ethoxy]-2,5-diiodo-
benzene was subjected to palladium-catalyzed coupling (tri-
methylsilyl)acetylene followed by deprotection to give 1,4-bis-
[2-(2-hydroxyethoxy)ethoxy]-2,5-diethynylbenzene (11).
Our designs allowed us to control the interpolymer arrange-
ment and intrapolymer conformations at the air-water interface.
Polymers 1, 2, and 3 have the edge-on structure at the air-
water interface due to the presence of hydrophilic groups on
only one side of each phenyl ring. Polymers 4-8 comprise a
second group that have a face-on structure with a symmetric
para disposition of side groups. The edge-on polymers have
π-aggregation between polymer chains, but the face-on polymers
do not.
The pressure-area (PA) isotherms of polymers 1-8 at the
air-water interface confirm our assertions of the polymers’
orientational preferences (Figure 1). The extrapolated areas per
phenyleneethynylene group for polymers 1, 2, and 3 range from
32 to 37 Å2.18 Combining this area with one phenylene-
ethynylene unit length of 7 Å provides the average distance
between polymer main chains of 4.6 to 4.9 Å for an edge-on
structure. In contrast, polymers 4-8 exhibit areas of 175-230
Å2 per each repeating unit that has two phenyleneethynylene
units. Considering a face-on structure, we calculated a distance
between adjacent main chain acetylene carbons of 12.1-16.4
Å. The sustainable maximum pressure above which monolayers
fold into multilayers was approximately 40 mN/m for the edge-
on polymers, a value 10 mN/m higher than those for the face-
on polymers. It is intuitive that the edge-on structure, which
provides a larger contact area between polymers, sustains a
larger surface pressure than does the face-on organization. One
would expect a similar behavior for macroscopic objects such
as boards organized in two dimensions with a vertical (edge-
on) or horizontal (face-on) structure. The steep slopes of the
P-A isotherms of the edge-on polymers also indicate the
expected low compressibility associated with this structure.
The difference between extrapolated area per repeating unit
of 4 and 5, 210 and 175 Å2, respectively, provides insight into
the arrangement of adjacent polymers at the air-water interface.
Polymer 5 has the same 1,4-bis(N,N-dioctylcarbamoyl)phenyl
group as 4 but a smaller macrocycle in its repeating unit. To
achieve the minimum area arrangement in two dimensions, the
bulkier 1,4-bis(N,N-dioctylcarbamoyl)phenyl repeating units
Figure 1. Pressure-area isotherms of polymers.
likely are positioned next to a macrocycle containing units of
adjacent chains to give an interdigitated structure.
The area per repeating units at which the polymers fold into
multilayers is similar for polymers 4-7 (125-140 Å2) but is
much smaller for polymer 8 (90 Å2), indicating more compress-
ible nature of polymer 8. An explanation is that the main chain
phenyl rings of polymers 4-7 maintain a face-on structure until
the monolayers fold into multilayers at 30 mN/m. However,
polymer 8 undergoes a two-stage transformation. It begins in a
face-on structure, but as it is compressed, the orientation of the
1-((triethylene glycol monomethyl ether)oxy)-4-decyloxyphen-
ylene groups, which have surfactant characteristics, rotates to
an edge-on structure.12 The reluctance of the hydrophobic
repeating group to adapt an edge-on structure produces an
intermediate structure with alternating edge-on and face-on
residues, which we refer to as a zipper structure as shown in
Scheme 1.12 Therefore, the first slope of 8’s PA isotherm is
smaller than those of the other face-on polymers. After an
intermediate (ca. 16 mN/m) transition point, the polymers
become less compressible with a steep slope until the mono-
layers fold into multilayers at 30 mN/m. We synthesized two
different molecular weights of polymer 8. One is the polymer
with a lower molecular weight (Mn ) 16 700) that is used for
this contribution, and the other one we used in previous studies12
has a higher molecular weight (Mn ) 293 000). The lower
molecular polymer shows less featured PA isotherm and lower
(18) The extrapolated area per repeating unit for polymer 3 was divided by 2
because there are two phenyleneethynylene groups on the repeating unit.
9
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Thin Layers of Poly(p-phenyleneethynylene)s
A R T I C L E S
Scheme 1
maximum surface pressure (30 vs 37 mN/m) as compared to
the higher molecular weight one because chain ends would
disrupt the interlocking zipper structure. Likewise, we previously
reported12 that diacetylene defects in this structure destroy long-
range registry between neighboring polymer chains and the
presence of these groups prevents well-defined phase transitions.
The in situ UV-vis spectra versus surface pressure of the
Langmuir polymer films strongly support the above PA
isotherm-based explanation of the arrangements and conforma-
tions of the polymers. At the air-water interface, the face-on
polymers have a flat geometry with a maximum conjugation
length. The in situ UV-vis spectra of the face-on polymers at
the air-water interface are red-shifted about 30 nm from these
in solution, showing surface-induced conjugation length in-
creases (compare Figures 2 and 3).12,19-21 The absorption λmax
of polymers 4-7 is blue-shifted, and the shape of the spectra
becomes less structured during the compression (Figure 2). This
disturbance of the π-π conjugation system is likely due to
interpolymer steric interactions that create a more heterogeneous
distribution of conjugation lengths. The case of polymer 8 is
again different and is characteristic of a zipper organization
where the absorption λmax blue shifts by 37 nm relative to its
initial uncompressed face-on structure. The in situ absorption
λmax of the edge-on polymers at the air-water interface is
essentially constant during compression because there is no
perturbation of the π-π conjugation system due to a highly
packed crystalline monolayer structure (Figure 2).
As described above, we have demonstrated control of
intramolecular conformation and interpolymer arrangement of
the fluorescent polymers by rational surfactant designs and the
types of interactions that occur with increased surface pressure.
In the following section, the nature of each peak of the in situ
UV-vis spectra, the red-shifted absorption peaks of transferred
LB films, and corresponding fluorescent spectra are elaborated.
Interpolymer Effects on the Ground and Excited States.
Both the edge-on and the face-on Langmuir films were
transferred onto hydrophobic and hydrophilic substrates. The
transfer ratios at a surface pressure of 20 mN/m for polymers
1-3, displaying an edge-on structure, were quantitative (>95%)
for both upstrokes and downstrokes. However, the transfer ratios
of the downstrokes for polymers 4-8 depended on the area per
repeating unit rather than on the surface pressure. For example,
the second downstroke transfer ratios of polymer 4 at 18 mN/m
were negligible (<10%). In contrast, the second downstroke
for polymer 5 at the same surface pressure gave an 80% transfer
ratio. The area/repeating unit of polymer 4 at 18 mN/m is 175
Å2, while that of polymer 5 is 145 Å2. Therefore, at 18 mN/m
the hydrophobic octyl side chains of polymer 5 are more
Figure 2. Surface pressure dependent in situ UV-vis spectra of polymers
1-8 at the air-water interface. The surface pressure was gradually increased
from bottom solid line to top solid line (0, 18, 38 mN/m for polymer 1; 0,
27, 40 mN/m for polymer 2; 0, 20, 40 mN/m for polymer 3; 0, 18, 27, 29
mN/m for polymer 4; 0, 18, 30 mN/m for polymer 5; 0, 6, 18, 27 mN/m
for polymer 6; 0, 18, 32 mN/m for polymer 7; 0, 9, 15, 17, 23, 26, 30
mN/m for polymer 8) until the monolayer folds into a multilayer (dashed
line). The UV-vis spectra of polymers 2 and 4-8 are completely reversible
upon cycles of compression and expansion.
compressed than those of polymer 4, which leads to greater
extension of the chains from the surface and a more hydrophobic
surface with better transfer ratios. This effect is confirmed by
transferring polymer 4 at an area per repeating unit of 145 Å2,
and the second downstroke transfer ratio was greater than 80%.
The effects of the spatial arrangement on the electronic ground
states of the polymers are revealed in the absorption spectra of
the polymers’ LB films (Figure 3) as well as in the in situ
absorption spectra of polymers’ Langmuir films at the air-water
interface (Figure 2). As mentioned earlier, the edge-on structure
gives crystalline aggregants at the onset. Therefore, Langmuir
and LB films of the edge-on polymers show a new red-shifted
absorption λmax that is 30-40 nm to the red of the solution
values. The spectra are unchanged when the monolayers fold
into multilayers indicating that the aggregates are formed in a
monolayer state. To further investigate the origin of the red-
shifted absorption spectra of the edge-on polymers, three spin-
cast films of polymer 1 in poly(methyl methacrylate) (PMMA)
with different weight ratios from 10-3/1 to 10-1/1 (polymer
1/PMMA) were prepared. As the concentration of polymer 1
(19) Lucht, B. L.; Mao, S. S. H.; Tilley, T. D. J. Am. Chem. Soc. 1998, 120,
4354.
(20) Zhang, Q. T.; Tour, J. M. J. Am. Chem. Soc. 1997, 119, 9624.
(21) Miteva, T.; Palmer, L.; Kloppenburg, L.; Neher, D.; Bunz, U. H. F.
Macromolecules 2000, 33, 652.
9
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A R T I C L E S
Kim et al.
Figure 5. Fluorescence spectra of polymer 1 (a) normalized spectra in
various environments, (b) in B at different excitation wavelength. The mass
ratio of polymer 1/PMMA in the spin-cast films is A ) 10-1/1, B ) 10-2
1, C ) 10-3/1, respectively.
/
of these two peaks in absorption spectra indicates the coexistence
of monomer-like and aggregated regions in the film.22
Fluorescence spectra provide additional information about the
relationship between the different polymer’s excitation charac-
teristics and film structures. The fluorescence spectra of
monolayer LB films of the edge-on polymers 1-3 clearly
demonstrate that all emission emanates from π-aggregated
excited states. Figure 5a shows the fluorescence spectra of
polymer 1 in solution, in spin-cast films with PMMA, and in
monolayer LB films. As the concentration of polymer 1 in
PMMA matrix increases and the aggregation band in absorption
spectra increases (Figure 4), the short wavelength (solution-
like) peak disappears, and a long wavelength peak grows in,
thereby indicating the formation of π-aggregates. Excitation at
different wavelengths resulted in the intensity redistribution
between these two peaks (Figure 5b), proving that these two
peaks belong to different excited and ground states. Therefore,
we can exclude excimer species. In the case of polymer 1, the
fluorescence peak of π-aggregated PMMA films perfectly
matched those of the LB film, and similar trends for polymers
2 and 3 were observed. Therefore, we conclude that the
polymers 1-3 have both solution-like and π-aggregated regions
in PMMA solid solutions, LB films, and Langmuir films, but
all the fluorescence is emitted only from the aggregated states.
This latter result is due to a fast energy migration from solution-
like regions to the aggregated regions that have lower energy
Figure 3. UV-vis spectra of polymers 1-8 in solution (solid line) and in
LB films (dashed line). All LB films are monolayers on hydrophobic
substrates unless otherwise noted.
Figure 4. UV-vis spectra of polymer 1/PMMA (weight ratio) spin-cast
films.
increases in these films, the intensity of the higher wavelength
absorption increases and is further red-shifted (Figure 4). These
results are consistent with strong intermolecular π-stacking
interactions promoted by the edge-on geometry. The broad peak
at ca. 440 nm is similar to the solution spectra and represents
a distribution of conjugation lengths. Therefore, the presence
(22) (a) Blatchford, J. W.; Gustafson, T. L.; Epstein, A. J.; VandenBout, D. A.;
Kerimo, J.; Higgins, D. A.; Barbara, P. F.; Fu, D. K.; Swager, T. M.;
MacDiarmid, A. G. Phys. ReV. B 1996, 54, R3683. (b) Blatchford, J. W.;
Jessen, S. W.; Lin, L. B.; Gustafson, T. L.; Fu, D. K.; Wang, H. L.; Swager,
T. M.; MacDiarmid, A. G.; Epstein, A. J. Phys. ReV. B 1996, 54, 9180.
9
7714 J. AM. CHEM. SOC. VOL. 124, NO. 26, 2002

Thin Layers of Poly(p-phenyleneethynylene)s
A R T I C L E S
Scheme 2
and act as excitation traps.22 Therefore, the quantum yields of
π-aggregates are low due to efficient self-quenching which is
typical of most conjugated polymers (vide infra).
The face-on polymers have different absorption spectra
depending on whether their LB films are transferred onto a
hydrophobic or a hydrophilic substrate (Figure 3). These results
are in accord with our previous investigations and are related
to structural differences of the transferred films. As we
previously reported for polymer 4, monolayers of other face-
on polymers 5-8 on a hydrophilic substrate also have a smooth
flat surface.13 This structure is stable due to favorable strong
anchoring interactions between the polar surface and the oxygen
moieties in the polymer. In contrast, monolayers of the face-on
polymers on a weak anchoring hydrophobic substrate reconstruct
into “nanofibrils” to relieve the high surface energy of the polar
macrocycles as described in Scheme 2 and shown in Figure
6.13 Because these monolayers cannot have π-aggregation when
they are tightly bound (anchored) to hydrophilic substrates, no
aggregation peaks are observed in their UV-vis spectra.
However, for nanofibriled monolayer films on hydrophobic
substrates, the formation of an aggregation peak depends on
the structure of the hydrophilic repeating group. Macrocycles
containing polymers 4 and 5 display solution-like absorption
spectra without any aggregation peak after assembly into
nanofibrils. The spectra of 4 and 5 are slightly red-shifted from
those in solution due to an increase in conjugation length
imposed by the surface. The lack of π-aggregation is due to
the macrocycles in 4 and 5 that by virtue of their structure
prevent direct interpolymer interactions between the conjugated
aromatic rings. It follows that the folded Langmuir or multilayer
LB films of 4 and 5 did not show the π-aggregation peak.
Polymers 6 and 8 have acyclic polar residues, and the in situ
absorption spectra showed π-aggregation peaks after the Lang-
muir monolayers are folded into multilayers (Figure 2). Because
of the reversible nature of the PA isotherm, the π-aggregation
does not persist upon removal of the surface pressure. The
absorption spectra of these polymer nanofibril monolayers on
hydrophobic substrates also show the additional π-aggregation
peaks at longer wavelengths, 464 and 467 nm, respectively,
values about 50 nm red to their solution λmax (Figure 3).
Interestingly, the extent of π-aggregation in their nanofibril LB
Figure 7. Polarized UV-vis spectra of (a) polymer 6 and (b) polymer 7.
films is responsive to its environment. Dipping a nanofibril
monolayer of 6 with a π-aggregation peak in water produces a
solution-like UV-vis spectra. The π-aggregation peak is then
reestablished with drying. Because the cores of the nanofibrils
are hydrophilic (Scheme 2), water dilates the nanofibrils, thereby
diminishing intermolecular interactions between polymers.
Consistently, hydrophobic solvents are not absorbed into the
fibrils, and the same film retains the aggregation peak upon
dipping in hexane.
The polarized UV-vis spectra of nanofibril monolayer films
of polymers 6 and 7 in Figure 7 provide additional information
about the aggregate regions. For polymer 6, the aggregation
peak at 464 nm in the parallel polarized UV-vis spectrum along
the polymer alignment direction is of greater intensity than the
solution-like peak at 438 nm. In contrast, the intensity of the
aggregation peak in perpendicularly polarized spectra is less
intense relative to the solution-like peak, indicating that the
aggregate regions are better aligned along the dipping direction
than are the nonaggregated regions. This result is intuitive
because coincident alignment of polymer chains promotes
aggregation. Even though it lacks macrocycles, nanofibrils of
polymer 7 do not form π-aggregation peaks in absorption
spectrum. It appears that the hydroxyl groups play an important
role in reducing intermolecular interaction in the nanofibrils by
strongly holding water molecules in the cores of the nanofibrils.
This explanation is also supported by the behavior of 6 and 8
that form π-aggregates in dry nanofibrils, but then deaggregate
when immersed in water. Folded Langmuir films of polymer 7
did not show additional π-aggregation peaks either (Figure 2).
However, multilayer LB films of polymer 7 display a π-ag-
gregation peak that increased with dehydration in a vacuum.
Figure 6. Atomic force microscope image (500 × 500 nm) of a nanofibril
monolayer LB film of polymer 4 and a sectional profile across the
nanofibrils. Nanofibrils are aligned along the dipping direction.
9
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A R T I C L E S
Kim et al.
Table 1. Photophysical Data of the Polymersa
fluorescence λmax (nm)
quantum yield (ΦF)
optimized
polymer
A /A
|
in solution in LB film in solution (A) in LB film (B) ΦF ratio (B/A)
1
2
3
4
5
6
7
8
3.0
3.6
3.0
6.3
5.0
5.5
4.3
1.4
476
471
474
457
456
455
456
457
493
505
500
465
459
472
466
474
0.43
0.34
0.36
0.54
0.41
0.46
0.53
0.44
0.002
0.007
0.01
0.19
0.17
0.14
0.17
0.10
0.005
0.02
0.03
0.35
0.41
0.30
0.32
0.23
a See Experimental Section for the detailed information. LB films in this
table were monolayer films on hydrophobic substrates.
(Figure 8a). However, polymers with a tendency to aggregate
showed significant spectral changes. In Figure 8b, increasing
concentrations of polymer 8 in spin-cast films with PMMA
induced the evolution of fluorescence spectra from solution-
like behavior to those characteristics of π-aggregation. The
fluorescence spectrum of a spin-cast film at a low concentration
(10-2/1, polymer 8/PMMA) clearly showed a solution-like
shoulder that decreases at higher concentrations of polymer 8.
Interpolymer Effects on Fluorescence Quantum Yield of
Polymer Films. Table 1 shows the fluorescence quantum yields
of the polymers in chloroform solution and in monolayer LB
films on a hydrophobic substrate, and the ratio thereof (i.e.,
quantum yield in LB films/quantum yield in solution). The
fluorescence quantum yields of the polymers in solution are
similar and range from 0.34 to 0.54. The organizations have a
larger effect, and monolayer LB films of face-on polymers have
at least an order of magnitude higher quantum yield as compared
to those of edge-on polymers.
Generally, monolayers on hydrophilic substrates have much
smaller fluorescence quantum yields than monolayers on
hydrophobic substrates. For example, the quantum yields of
monolayers of polymers 1, 4, and 6 on a hydrophilic substrate
are ∼0.0015, 0.05, and 0.05, respectively. The lower quantum
yields of the monolayers on hydrophilic substrates may arise
from stronger interactions between the polymers’ hydrophilic
groups and the substrates.
By comparing the ratio of quantum yield in LB films/quantum
yield in solution, we can see the influence of intermolecular
interactions on the quantum yield. First, the strong π-aggregating
edge-on polymers display self-quenching and extremely low
quantum yields. Second, the face-on polymers, which are devoid
of strong π-aggregates, experience varying degrees of inter-
polymer interactions in nanofibrils. The ratio of quantum yield
in LB films/quantum yield in solution has a tendency to increase
as intermolecular interactions in nanofibrils decrease. Macro-
cycles of polymers 4 and 5 reduce intermolecular interactions
in the nanofibrils significantly, resulting in the best ratio of
quantum yield in LB films/quantum yield in solution.
Figure 8. Fluorescence spectra of polymers (a) 4 and (b) 8. The mass
ratio of polymer/PMMA in the spin-cast films is A ) 10-1/1, B ) 10-2/1,
C ) 10-3/1, respectively.
As shown in Figure 7b, these multilayer films show π-aggregate
absorptions that are polarized along the direction of the polymer
alignment.
Structural differences in monolayer LB films of the face-on
polymers also affect emission spectra. The nanofibril monolayers
of polymers 6 and 8 on hydrophobic substrates, with an
aggregation peak in their absorption spectra, have structured
fluorescence spectra with three peaks. On the other hand, the
UV-vis spectra of the nanofibriled monolayers of polymers 4,
5, and 7 on hydrophobic substrates do not show aggregation,
and their fluorescence spectra are less structured. In the case of
polymer 7, we observed that the aggregation peak increased
when the number of layers in the LB films increased, indicating
a gradual increase of intermolecular interactions. We observed
the same effect of intermolecular interactions on the fluorescence
spectra of multilayer films of polymer 7. From a relatively
featureless solution-like shape for a monolayer film, the spectra
developed structure as the number of layers increased. Ag-
gregation and highly ordered segments are mutually reinforcing
and produce narrowed and structured fluorescence spectra.
Energy migration to the aggregates also facilitates the band
narrowing by reducing the contribution of solution-like fluo-
rescence.
Conclusions
Fluorescence studies of spin-cast films of these polymers in
PMMA solid solutions are consistent with the UV-vis analysis,
that polymers 4, 5, and 7 do not form aggregates, whereas
polymers 6 and 8 do. Figure 8 shows fluorescence spectra of
polymers 4 and 8 in solution, in spin-cast films with PMMA,
and in LB films on both hydrophobic and hydrophilic substrates.
As the concentration of polymer 4 in a spin-cast film with
PMMA increased, the emission λmax undergoes slight red shifts
We have demonstrated control of the spatial arrangement of
PPEs by designed chemical structures and the LB method. This
precise control of the spatial arrangement of conjugated
polymers allowed us to elucidate aggregation mechanisms and
the effect of π-aggregation on the spectroscopic properties of
conjugated polymers. Through extensive structure-property
correlations, we have firmly established spectroscopic features
associated with nonaggregated polymers with various conjuga-
9
7716 J. AM. CHEM. SOC. VOL. 124, NO. 26, 2002

Thin Layers of Poly(p-phenyleneethynylene)s
A R T I C L E S
ΦF ) [(AsFunu )/(AuFsns2)]Φs,26 where subscripts s and u refer to
standard and unknown samples, respectively. A is the optical density
at the excitation wavelength, F is the integrated area of the fluorescence
spectrum, and n is the refractive index. We assumed that the refractive
indices of LB films are the same and the refractive index of quinine
sulfate solution is the same as pure water.
2
tion lengths. In well-defined LB films and various solid
solutions, we demonstrated that strong π-aggregates present in
fluorescent polymer thin films result in quenching due to
efficient energy migration from nonaggregated regions to the
aggregated ones. Intermolecular interactions in nanofibrils also
affect the photophysical properties of the polymers. Macrocycles
attached to the polymer backbone significantly reduce inter-
molecular interactions in the solid state, thereby increasing
quantum yield relative to polymers without macrocycles. In well-
defined monolayer LB films, orders of magnitude different
quantum yields were obtained depending on spatial arrangement
of polymers even though the solution quantum yields are similar
to each other. These comprehensive results provide important
design principles for fabricating highly luminescent polymer
films. Because chemical modification can give surfactant
character to other conjugated polymers, this method is poten-
tially applicable to general conjugated polymers.
Polymer 2. A 10 mL Schlenk flask equipped with a stir bar was
charged with 1-((triethylene glycol monomethyl ether)oxy)-4-decyloxy-
2,5-diiodobenzene (73.1 mg, 0.11 mmol, 1 equiv), 1-((triethylene glycol
monomethyl ether)oxy)-4-decyloxy-2,5-diethynylbenzene (9) (51.6 g,
0.11 mmol, 1.03 equiv), and copper(I)iodide (3.9 mg, 20.5 µmol, 0.07
equiv). The flask was placed under argon, and tetrakis(triphenylphos-
phine)-palladium (0) (8 mg, 6.92 µmol, 0.18 equiv) was added under
a nitrogen atmosphere. Toluene (3.0 mL) and diisopropylamine (DIPA)
(1.25 mL, 8.91 mmol, 79 equiv) were successively added by syringe,
and the mixture was stirred at room temperature for 30 min. As the
mixture became viscous, toluene (2 mL) was added, after which the
mixture was heated to 60 °C for 24 h. The polymer solution was then
precipitated in methanol, filtered, and rinsed with hot methanol, giving
polymer 2 as an amorphous orange solid. 1H NMR (300 MHz,
CDCl3): δ 7.06 (s, 1H), 7.01 (s, 1H), 4.24 (br m, 2H), 4.04 (br m,
2H), 3.92 (br m, 2H), 3.78 (br m, 2H), 3.62 (br m, 4H), 3.51 (br m,
2H), 3.35 (br m, 3H), 1.86 (br m, 2H), 1.64-1.10 (br m, 14H), 0.87 (t,
J ) 6.9 Hz, 3H). GPC: Mn ) 55 900; PDI ) 2.9.
Experimental Section
General. Air- and moisture-sensitive reactions were carried out in
flame-dried glassware using standard Schlenk-line or drybox techniques
under an inert atmosphere of dry argon. All chemicals used were of
reagent grade and were purchased from Aldrich unless otherwise noted.
Anhydrous toluene was used from Aldrich Kilo-lab metal cylinders.
CH2Cl2 and THF were used directly from Aldrich Sure-seal bottles.
Diisopropylamine was distilled over solid KOH pellets and degassed
by three freeze-pump-thaw cycles. Tetrakis(triphenylphosphine)-
palladium (0) and trans-dichlorobis(triphenylphosphine)-palladium (II)
were purchased from Strem chemicals and used as received. (Tri-
methylsilyl)acetylene was purchased from Farchan Laboratories and
2,5-Diiodo-p-phenylene-20-crown-6. This compound was a side
product of a reaction. Flash chromatography (20% ethyl acetate/5%
methanol/75% hexane) afforded a white solid. H NMR (250 MHz,
CDCl3): δ 7.32 (s, 2H), 4.41-4.21 (m, 4H), 3.88-3.72 (m, 4H), 3.60-
3.53 (m, 8H), 3.33 (s, 4H). 13C NMR (125 MHz, CDCl3): δ 153.43,
124.57, 87.23, 71.68, 70.99, 70.89, 70.51. Anal. Calcd for C16H22I2O6:
C, 34.06; H, 3.93. Found: C, 34.05; H, 4.02.
1
2,5-Diethynyl-p-phenylene-20-crown-6 (10). A 50 mL Schlenk
flask equipped with a stir bar was charged with 2,5-diiodo-p-phenylene-
20-crown-6 (0.564 g, 1 mmol, 1 equiv), trans-dichlorobis(triphen-
ylphosphine)-palladium (II) (14 mg, 19.9 µmol, 0.02 equiv), and
copper(I) iodide (5.7 mg, 29.9 µmol, 0.03 equiv). The flask was placed
under argon, and then toluene (10 mL) and diisopropylamine (20 mL,
0.14 mol, 130 equiv) were successively added. The orange solution
was treated with (trimethylsilyl)acetylene (TMSA) (0.31 mL, 2.2 mmol,
2.2 equiv) and stirred at 70 °C for 48 h. The solvent was removed in
vacuo, and the residue was dissolved in CH2Cl2. The black mixture
was filtered through a 1 in. plug of silica gel and eluted using ethyl
acetate. The filtrate was removed in vacuo to yield a black oil that was
chromatographed (10% CH2Cl2/25% ethyl acetate/70% hexane, Rf )
0.24) to afford 2,5-((trimethylsilyl)ethynyl)-p-phenylene-20-crown-6 as
a yellow solid (0.34 g, 67%).
A 100 mL two-necked round-bottomed flask equipped with a stir
bar was charged with 2,5-((trimethylsilyl)ethynyl)-p-phenylene-20-
crown-6 (0.24 g, 0.48 mmol, 1 equiv) and methanol (10 mL). The flask
was capped and argon bubbled through the solution for 45 min.
Tetrabutylammonium fluoride hydrate (TBAF) (0.30 g, 1.14 mmol, 2.4
equiv) was then added to the flask under argon, and the mixture was
stirred at room temperature for 12 h. The red solution was then
concentrated in vacuo, and the residue was partitioned between CH2-
Cl2 (100 mL) and water (50 mL). The organic layer was washed with
saturated aqueous NaCl (50 mL), and then dried (MgSO4), and
concentrated in vacuo. Flash chromatography (50% CH2Cl2/45%
hexane/5% methanol, Rf ) 0.25) afforded 10 (0.13 g, 77%) as a light
yellow solid. 1H NMR (250 MHz, CDCl3): δ 7.11 (s, 2H), 4.46-4.26
(m, 4H), 3.81-3.78 (m, 4H), 3.67-3.52 (m, 8H), 3.38 (s, 4H), 3.34
(s, 2H). 13C NMR (75 MHz, CDCl3): δ 154.33, 119.80, 114.04, 82.54,
80.30, 71.72, 71.00, 70.95, 70.88, 69.75 ppm. HR-MS: calcd for
C20H24O6 (M+), 360.1573; found, 360.1567.
1
used as received. H and 13C NMR spectra were taken in CDCl3 with
1H chemical shifts reported relative to internal tetramethylsilane (0.00
ppm) and 13C chemical shifts reported relative to CDCl3 (77.00 ppm).
2,5-Diiodo-4-decyloxyanisole,13 2,5-diethynyl-4-decyloxyanisole,13 1,4-
diiodo-2,5-dihydroxybenzene,23 1,4-bis (N,N-dioctylcarbamoyl)-2,5-
diiodobenzene,23 1,4-bis[2-(2-hydroxyethoxy)ethoxy]-2,5-diethynyl-
benzene,6 and polymers 1,13 3,4d 4,23 and 612 were synthesized according
to the literature procedures.
Polymer molecular weights were determined with a Hewlett-Packard
1100 series HPLC equipped with a PLgel mixed-C column (5 µ) using
THF as the mobile phase at a rate of 1 mL/min. Gel permeation
chromatography (GPC) measurements were made relative to mono-
disperse polystyrene standards purchased from Polymer Laboratories.
This technique may give relative molecular weights higher than the
actual values for rigid-rod polymers. The overestimate of the molecular
weight of PPEs by GPC is largest for low molecular weight samples,
and high molecular weight samples have a very modest correction.24
LB thin films were prepared on a 601 M LB trough equipped with
vertical dipping mechanism from NIMA Technology, Ltd., using
purified water (18 MΩ) from a Barnstead Nanopure system. Substrates
were 18 × 18 mm glass microscope cover slides treated according to
the literature.13 UV-vis spectra were obtained on a Hewlett-Packard
8453 diode array spectrophotometer. To collect UV-vis spectra versus
surface pressure of polymers’ films at the air-water interface, a LB
trough with a window was placed in a vertically rotated UV spectro-
photometer. Fluorescence spectra were measured at room temperature
using a SPEX Fluorolog-τ2 spectrofluorometer. Fluorescence quantum
yields in chloroform solutions and monolayer LB films were determined
relative to equiabsorbing quinine sulfate solution (∼10-6 M in 1 N
H2SO4, ΦF ) 0.55)25 and spin-cast films of ∼10-3 M 9,10-dipheny-
lanthracene in PMMA (ΦF ) 0.83),5d respectively, by using the equation
Polymer 5. A 10 mL flame-dried Schlenk flask equipped with a
stir bar was charged with 1,4-bis (N,N-dioctylcarbamoyl)-2,5-diiodo-
(23) Zhou, Q.; Swager, T. M. J. Am. Chem. Soc. 1995, 117, 7017.
(24) Cotts, P. M.; Swager, T. M.; Zhou, Q. Macromolecules 1996, 29, 7323.
(25) Demas, J. N.; Crosby, G. A. J. Phys. Chem. 1971, 75, 991.
(26) Eaton, D. F. Pure Appl. Chem. 1988, 60, 11
9
J. AM. CHEM. SOC. VOL. 124, NO. 26, 2002 7717

A R T I C L E S
Kim et al.
benzene (86.5 mg, 100 µmol, 1 equiv), 2,5-diethynyl-p-phenylene-20-
crown-6 (10) (36 mg, 99.9 µmol, 1 equiv), and copper(I)iodide (2.3
mg, 12 µmol, 0.12 equiv). The flask was placed under argon, and
tetrakis-(triphenylphosphine)palladium (0) (8.1 mg, 7 µmol, 0.07 equiv)
was added under a nitrogen atmosphere. Toluene (1 mL) and diiso-
propylamine (1.12 mL, 8 mmol, 80 equiv) were successively added
by syringe, and the mixture was stirred at 70 °C for 60 h. The cooled
polymer solution was precipitated in methanol, filtered, and rinsed with
warm methanol. After drying under high vacuum, polymer 5 was
Polymer 7. A 10 mL Schlenk flask equipped with a stir bar was
charged with 1,4-bis[2-(2-hydroxyethoxy)ethoxy]-2,5-diethynylbenzene
(11) (30 mg, 91.3 µmol, 1.05 equiv) and 1,4-bis (N,N-dioctylcarbam-
oyl)-2,5-diiodobenzene (75 mg, 87 µmol, 1 equiv). Tetrakis-(triphen-
ylphosphine)palladium (0) (3.1 mg, 4.35 µmol, 0.05 equiv) and
copper(I) iodine were added to the flask under a nitrogen atmosphere.
The flask was then evacuated and back filled with argon three times
before adding toluene (1.5 µL) and diisopropylamine (500 µL) via
cannula. The reaction was stirred while maintaining the temperature at
60 °C for 48 h. The reaction was cooled before diluting with methylene
chloride and subsequent washing with ammonium hydroxide. The
organic layer was concentrated to a yellow film that was redissolved
in methanol and precipitated with water. After drying, polymer 7 was
obtained as a fluorescent green-yellow solid. 1H NMR (300 MHz,
CDCl3): δ 7.51 (s, 2H), 6.95 (s, 2H), 4.14 (br s, 4H), 3.90 (br s, 6H),
3.82-3.52 (br m, 8H), 3.16 (br s, 6H), 2.19 (br), 1.82-1.02 (m, 48H),
0.86 (m, 12H). GPC: Mn ) 51 600; PDI ) 3.8.
Polymer 8. A 10 mL flame-dried Schlenk flask equipped with a
stir bar was charged with 1,4-bis (N,N-dioctylcarbamoyl)-2,5-diiodo-
benzene (86.5 mg, 100 µmol, 1 equiv), 1-((triethylene glycol mono-
methyl ether)oxy)-4-decyloxy-2,5-diethynylbenzene (9) (45.8 mg, 103
µmol, 1.03 equiv), and copper(I)iodide (2.3 mg, 12 µmol, 0.12 equiv).
The flask was placed under argon, and tetrakis-(triphenylphosphine)-
palladium (0) (8.1 mg, 7 µmol, 0.07 equiv) was added under a nitrogen
atmosphere. Toluene (2 mL) and diisopropylamine (1.12 mL, 8 mmol,
80 equiv) were successively added by syringe, and the mixture was
stirred at 70 °C for 43 h. The cooled polymer solution was precipitated
in methanol, filtered, and rinsed with warm methanol. After drying
under high vacuum, polymer 8 was obtained as a brown solid. 1H NMR
(300 MHz, CDCl3): δ 7.46 (s, 2H), 6.92 (s, 1H), 6.84 (s, 1H), 4.16 (br
s, 2H), 4.00-3.80 (m, 4H), 3.78 (m, 2H), 3.68 (m, 6H), 3.56 (m, 2H),
3.38 (s, 3H), 3.18 (br s, 6H), 1.80-1.03 (m, 64H), 0.89 (m, 15H) ppm.
GPC: Mn ) 16 700; PDI ) 2.4.
1
obtained as a brown solid. H NMR (300 MHz, CDCl3): δ 7.48 (s,
2H), 7.04 (s, 2H), 4.30 (br m, 4H), 3.84 (m, 4H), 3.70-3.40 (br m,
10H), 3.37 (m, 4H), 3.20 (br m, 6H), 1.70-1.03 (m, 48H), 0.86 (m,
12H). GPC: Mn ) 56 000; PDI ) 2.5.
1,4-Bis[2-(2-hydroxyethoxy)ethoxy]-2,5-diethynylbenzene (11).
1,4-Bis[2-(2-hydroxyethoxy)ethoxy]-2,5-diiodobenzene was combined
with trans-dichlorobis(triphenylphosphine)-palladium (II) (39 mg, 56
µmol, 0.03 equiv) and copper(I)iodine (21 mg, 110 µmol, 0.06 equiv)
in a flame-dried flask equipped with a stir bar. The flask was evacuated
and back filled with argon three times. The solids were dissolved/
suspended in toluene (25 mL) and diisopropyle amine (652 µL). The
(trimethylsilyl)actylene (0.580 mL, 4.1 mmol, 2.2 equiv) was then
added, and the reaction was stirred and heated to 60 °C for 20 h. Once
the reaction was cooled to room temperature, saturated ammonium
chloride solution was added, and the biphasic mixture was stirred for
30 min. The layers were then separated after dilution with ether. The
organic layer was washed four times with saturated ammonium chloride
solution, dried with magnesium sulfate, filtered, and concentrated to
give a dark brown oil that was chromatographed on silica gel (50%
hexane/50% ethyl acetate).
The bistrimethylsiylacetylene (850 mg, 1.8 mmol, 1 equiv) was
deprotected by reaction with aqueous potassium hydroxide (200 mg,
3.6 mmol, 2 equiv; dissolved in 1 mL of DDI water) in degassed THF
(17 mL) and methanol (13.3 mL). The solution was stirred for 48 h.
The reaction was then poured into 50 mL of ether. The organic layer
was separated and washed with water three times (40 mL). The organic
layer was dried with magnesium sulfate and concentrated to provide
an off-white solid that was recrystallized from ethanol to provide 11.
1H NMR (300 MHz, CDCl3): δ 7.00 (s, 2H), 4.16 (m, 4H), 3.88 (m,
4H), 3.74 (m, 4H), 3.69 (m, 4H), 3.36 (s, 2H), 2.21 (br s, 2H). 13C
NMR (75 MHz, CDCl3): δ 153.76, 118.03, 113.44, 82.99, 79.38, 72.53,
69.39, 69.22, 61.79 ppm. HR-MS: calcd for C18H22O6 (M+), 334.1416;
found, 334.1424.
Acknowledgment. The authors thank the Office of Naval
Research, the Center for Materials Science and Engineering at
MIT (NSF MRSEC), and Draper Lab for generous financial
support, and Mr. Davide L. Simone for providing 2,5-diiodo-
p-phenylene-20-crown-6. D.T.M. thanks the National Institute
of Health for a postdoctoral fellowship through NISMS.
JA0200600
9
7718 J. AM. CHEM. SOC. VOL. 124, NO. 26, 2002