A highly enantioselective receptor for N-protected glutamate and anomalous solvent-dependent binding properties

Article abstract of DOI:10.1002/1521-3773(20021115)41:22<4225::AID-ANIE4225>3.0.CO;2-3

Solvation of the macrocycle by polar solvents leads to strong binding of an N-protected excitatory amino acid neurotransmitter. A 1:1 complex forms highly enantioselectively between the macrocycle and N-Boc-L-glutamate (as the dicarboxylate anion, see pic

Full text of DOI:10.1002/1521-3773(20021115)41:22<4225::AID-ANIE4225>3.0.CO;2-3

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S
olvent-induced aggregation of optically active poly(phenylene ethynylene)s (PPEs) can follow different
pathways, depending on the ability of the PPEs to stabilize the chiral macrostructure. A new approach has
been developed that utilizes chirality in combination with ™pillarlike∫ structural elements for the first time
to create strong electronic coupling systematically while maintaining high quantum yields. For more
information see the following publication by Swager et al.
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that interchain interactions in poly(phenylene ethynylene)
species (PPEs) give new low energy bands in the UV/Vis
absorption spectra.[8] Recently, a theoretical account by
Brÿdas, et al.[9] and an unexpected result reported by us[10]
have suggested that an oblique orientation between neighbor-
ing transition dipole moments of conjugated polymers might
prevent fluorescence quenching upon aggregation. Indeed,
based upon the exciton-coupling model,[11] a parallel orienta-
tion of polymer chains is expected to result in cancellation of
transition dipoles to give a forbidden S0 S1 transition, whereas
coupled chromophores with oblique organizations should
exhibit an allowed S0 S1 transition. Considering that polymer-
backbones twist, which is promoted by introducing chirality,
may introduce such an oblique arrangement we embarked on
a program to produce chiral PPEs that would self-assemble
into stable aggregates with high quantum yields.
These studies build on a detailed understanding of the
electronic spectra of PPEs deduced through studying ordering
and phase transitions in Langmuir films, which allowed for a
deconvolution of the effects of backbone planarization
(conjugation length) and interpolymer interactions (aggrega-
tion).[12] To promote high degrees of aggregation, we initially
investigated simple structures, such as 1, which contains chiral
Three-Dimensional Electronic Delocalization
in Chiral Conjugated Polymers**
Steffen Zahn and Timothy M. Swager*
Transport properties are the central performance determi-
nant of electronic materials in most applications. Strong,
extended electronic interactions have long been understood
to be critical to obtaining molecules and polymers with high
charge-carrier mobility. It has been further revealed recently
that strong electronic coupling is the dominant contributor to
intramolecular exciton transport in solutions of some con-
jugated polymers.[1] As a result, the production of extended
electronic interactions both within and between electronic
polymers is critical to the optimization of their transport of
charge and excitons, which underpins future applications in
transistors,[2] electroluminescent devices,[3] sensors,[4] and
photovoltaic devices.[5] However, in devices requiring lumi-
nescence and/or exciton transport, researchers have generally
avoided the strong interpolymer electronic coupling that will
enhance transport because of the strong self-quenching that
nearly exclusively accompanies these interactions. Self-
quenching lowers the light output of luminescent devices
and also lowers the diffusion
length of excitons, which reduces
sensory responses. As a result,
most researchers seeking to cre-
ate highly emissive films of elec-
tronic polymers use large struc-
tures such as dendrimers[6] or
smaller rigid scaffolds[7] to pre-
vent interchain interactions. It
can therefore be stated that pres-
ent design principles for main-
taining high emission quantum
yields in conjugated polymers
have been diametrically opposed
to those for the optimization of
charge and exciton transport. To
address this limitation we present herein a general design for
producing strongly interacting polymer chains with 3D
electronic interactions while maintaining high luminescence
efficiency. This ability to optimize charge and exciton trans-
port simultaneously in conjugated polymers has broad
implications for optoelectronic devices. Herein we describe
experimental results that demonstrate that 3D electronic
interactions enhance exciton transport.
With few exceptions, strong electronic interactions between
chains of conjugated polymers (often accompanying aggre-
gation) dramatically lower their quantum efficiency. Previous
studies on carefully designed Langmuir films have revealed
alkyloxy groups. Poly(2,5-bis[2-(S)-methylbutoxy]-1,4-phen-
ylene ethynylene) was previously investigated.[13] However,
our results contrasted significantly from the earlier studies,[14]
which prompted a detailed investigation. The addition of poor
solvents to a solution of the conjugated polymer generates
stable solutions of very small nonscattering particles[15] that
allow quantitative studies of the optical properties by avoid-
ing the scattering, reflection, and waveguiding of the emitted
light that complicate thin-film studies. To develop a compre-
hensive understanding of polymers containing chiral alkyloxy
side chains, we undertook the systematic investigation of the
58 different compositions shown in Scheme 1. While a de-
tailed description of all of these materials is beyond the scope
of this present contribution, we have established that molec-
ular weight is a critical variable and that lower molecular-
weight materials (Mn < 12000) gave different results from
higher molecular-weight compounds.
[*] Prof. T. M. Swager, Dr. S. Zahn
Department of Chemistry
Massachusetts Institute of Technology
Cambridge, Massachusetts 02139 (USA)
Fax : (þ 1)617-253-7929
We chose to study the well-defined polymer 1, because it
yielded consistent results that are representative of the entire
family of polymers. The addition of methanol (ꢀ 40%) to
E-mail: tswager@mit.edu
[**] We are grateful for research funding from the Department of Energy,
Army Research Office, and NASA.
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Scheme 1. Structures of the polymers under investigation.
chloroform solutions of 1 introduces an band at 492 nm in the
UV/Vis absorption spectrum (Figure 1) caused by an aggre-
gate. Consistent with our previous studies, the fluorescence is
strongly quenched by the aggregate. Commensurate with the
appearance of the band arising from the aggregate at low
methanol concentrations, we see circular dichroism (CD)
bands indicative of a chiral structure.[16] The large peak in the
CD spectrum at low methanol concentrations is coincident
with the band caused by aggregation and confirms that the
new aggregate chromophore is chiral.[17] The other features in
the CD spectrum indicate the presence of a second chromo-
phore at higher energy, which has the same sign in the CD.
The inflection point at 454 nm is the same as the absorption
maxima of unaggregated and less-conjugated segments of the
polymer, which indicates the presence of exciton-coupled CD
(ECCD).[18] The small negative CD peak at 467 nm in the
composite spectrum at low methanol concentrations is the
result of cancellation of the strong positive second Cotton
Effect (CE) from the aggregate chromophore and the
negative first CE from the exciton-coupled polymers.
The most striking feature of the behavior of 1, and indeed
related structures in Scheme 1, is that above methanol
concentrations of 50% the CD drops off dramatically, while
the aggregate peak continues to grow (UV/Vis). This effect
can be understood as shown schematically in Figure 2a c.
Initially a chiral aggregate forms with a small angle between
polymer chains to give an organization reminiscent of a
cholesteric liquid crystal. However, the polymer ultimately
favors a stronger aggregate structure with coincident align-
ment of polymer chains and the helical structure is lost. We
note that in previous studies this behavior was not ob-
served.[13] The resultant organization gives a low or nonexis-
tent dihedral angle between polymer chains and, as expected,
the fluorescence quantum yield drops to only < 5% (F ¼
0.04) of its original value.
Figure 1. Circular dichroism (a), absorption (b), and fluorescence (c) spec-
tra of (S)-1 recorded in good solvent/poor solvent mixtures at 258C (100:0
denotes 100 v/v% of chloroform and 0 v/v% of methanol, 20:80 indicates a
solvent mixture consisting of 20 v/v% chloroform and 80 v/v% methanol).
To stabilize a strongly aggregated chiral and emissive
organization of polymers, we have investigated specific
architectures that prevent aggregated chains from achieving
a collinear structure. We had previously developed the
polymers 2 containing the pentiptycene group and found that
this group was excellent in preventing chain aggregation in
spin-cast films.[19] We have found, however, that addition of
methanol (30%) to solutions of 2 gives aggregates with
significantly quenched emission (F ¼ 0.21). This aggregate is
much slower to assemble than 1 and we hypothesized that the
polymer chains assemble into an interlocking structure, in
which the polymer chains are constrained in the clefts
between pentiptycene groups.
We considered that an interlocking structure, such as that of
2, would stabilize chiral aggregates by preventing a coinci-
dence of strongly interacting polymer chains. Studies of the
chiral analogue 3 confirm the restricted nature of the aggre-
Angew. Chem. Int. Ed. 2002, 41, No. 22
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Figure 2. Illustration of three stages of aggregation of 1 (a) (c) and 3 (d)
(f). Top: 1 (a) and 3 (d) dissolved in neat chloroform. Both polymers are
highly solvated and there are no interactions between polymer chains.
Middle: Aggregation of 1 occurred and the rigid-rod PPEs form a lamella
structure within each plane (b). The chiral side chains guide the polymers
into a chiral macrostructure as depicted. The formation of the optically
active macrostructure is guided by the influence of the chiral side chains.
Polymer 3 aggregates analogously to 1, but because of the presence of the
pentiptycene groups, a slightly irregular interlocked structure results (e).
Macrostructure 3 is shown in a two-layer graphic for simplification and
clarity. Bottom: The initial chiral macrostructure of 1 has been ™un-
twisted∫ (c), which is favorable as it maximizes p p stacking in the edge-on
conformation. The dihedral angle assumes a very small value, affording a
very weak dichroic signal and low fluorescence quantum yield. Polymer 3
self-assembles into a tighter structure (f) by incorporating the polymer into
the clefts of the pentiptycene groups. Because the ™untwisting∫ motion
observed in 1 is hindered, polymer 3 is able to maintain its optically active
structure and its high fluorescence quantum yield. The inset illustrates the
anticipated chiral gridlike structure.
Figure 3. Circular dichroism (a), absorption (b), and fluorescence (c) spec-
tra of (R)-3 recorded in good solvent/poor solvent mixtures at 258C (100:0
denotes 100 v/v% of chloroform and 0 v/v% of methanol, 30:70 indicates a
solvent mixture consisting of 30 v/v% chloroform and 70 v/v% meth-
anol).[24]
gated state; the structure of the CD and absorption spectra
evolve slowly over approximately five minutes. Although the
absorption spectra are similar to those of 1, with more fine
structure, the CD spectra are very different (Figure 3). Like
that of 1 in the aggregated state (ꢀ 30% methanol), the
spectrum of 3 displays two dominant contributions that are
associated with the ™aggregate∫ band at lmax ¼ 460 nm and the
band arising from unaggregated species at 430 nm. However,
there are key differences between 1 and 3. Most striking is the
change in shape and even sign of the CD bands upon forming
the tightest aggregate structure in high methanol concentra-
tions (ꢀ 50%). We believe this complication is the result of
the polymer chains being constrained in interlocked irregular
structures as shown in Figure 2e. The position of the CD band
and the near coincidence of the inflection point with the lmax
of the aggregate indicates an ECCD effect that only originates
from the interpolymer interaction of the planar chromo-
phores. Hence it appears that only well-structured (planar)
polymer chains are incorporated into the grid. Portions of the
material that probably have a less ordered and therefore a less
conjugated electronic structure are not incorporated into the
gridlike structure because of their higher steric demands that
make them unsuitable to fit in the cleft of the polymer. These
™solutionlike∫ portions of the material communicate poorly
with each other electronically and therefore give rise to a
broad CD band at 430 nm, which is difficult to analyze.
The CD signal and the aggregate band in the absorption
spectrum are both consistent with interpolymer electronic
interactions. However, as shown in Figure 3, in contrast to
other polymers, 3 maintains the majority of its fluorescence
intensity with aggregation (F ¼ 0.61).[25] The CD does not
diminish at high methanol concentration and, interestingly,
upon initial aggregation the intensity is slightly lower than the
more highly aggregated form. Control experiments utilizing
the opposing enantiomer to 3 yielded UV/Vis and fluores-
cence spectra of identical shape and intensity, and a CD
spectrum that differs only in the signs of the Cotton effects.
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Furthermore, a solution containing 50% (S)-3 and 50% (R)-3
gave absorption spectra similar to pure solutions of (S)-3 or
(R)-3, but no CD signals, as expected. Consistent with
chirality being a critical element to maintain a high quantum
yield, this aggregate maintained a F of only 0.3. Achiral
aggregates present in the 50:50 mixture of enantiomeric
polymers are probably responsible because the long-range
dipole dipole interactions responsible for the excitonic cou-
pling are extended beyond nearest neighbors.
To illustrate the advantages of a 3D structure for the design
of optoelectronic devices, the performance of 3 as a self-
amplifying sensory polymer was investigated under solution
and solid-state conditions by means of Stern Volmer quench-
ing experiments (Figure 4). In solution, fully aggregated 3
displayed a 15-fold steeper quenching slope toward the
conducting studies as a function of film thickness. Because of
limited exciton transfer between polymer chains, which limits
exciton diffusion, 2 displayed a dramatic 45% decrease in
sensitivity in thicker films (OD ¼ 0.3) relative to thinner films
(OD ¼ 0.04).[23] In contrast, 3 displays only a 10% decrease in
quenching sensitivity in thick films (OD ¼ 0.2) relative to thin
films (OD ¼ 0.02).
To summarize, the high quantum yield of 3 in its aggregated
state results from the formation of a helical grid structure and
an oblique orientation of the transition dipole moments of the
neighboring polymer chains. We have discovered that rigid
scaffolds prevent collinear aggregation of polymer chains.
Chirality has been used for the first time to create strong
electronic coupling systematically while maintaining high
fluorescence efficiency. These chiral, 3D interactions produce
sensitive sensory materials with excellent quantum yields.
Similar approaches will probably enhance the performance of
electronic polymers for many other optoelectronic-device
applications.
Received: July 8, 2002 [Z19683]
[1] A. Rose, C. G. Lugmair, T. M. Swager, J. Am. Chem. Soc. 2001, 123,
11298.
[2] H. Sirringhaus, P. J. Brown, R. H. Friend, M. M. Nielsen, K. Bech-
gaard, B. M. W. Langeveld-Voss, A. J. H. Spiering, R. A. J. Janssen,
E. W. Meijer, P. Herwig, D. M. De Leeuw, Nature 1999, 401, 685.
[3] R. H. Friend, R. W. Gymer, A. B. Holmes, J. H. Burroughes, R. N.
Marks, C. Taliani, D. D. C. Bradley, D. A. dos Santos, J.-L. Brÿdas, M.
Logdlund, W. R. Salaneck, Nature 1999, 397, 121; S. Sibley, M. E.
Thompson, P. E. Burrows, S. R. Forrest, Optoelectron. Prop. Inorg.
Compd. 1999, 29.
Figure 4. Stern Volmer plots of 3 in a nonaggregated form (*) and an
aggregated form (*) in solutions at OD ¼ 0.09 (a). Stern Volmer plot of 2
(*) and 3 (*) in films of OD ¼ 0.03 upon exposure to TNT vapors (b).
[4] D. T. McQuade, A. E. Pullen, T. M. Swager, Chem. Rev. 2000, 100,
2537.
[5] C. J. Brabec, N. S. Sariciftci, J. C. Hummelen, Adv. Funct. Mater. 2001,
11, 15.
[6] T. Sato, D.-L. Jiang, T. Aida, J. Am. Chem. Soc. 1999, 121, 10658.
[7] J.-S. Yang, T. M. Swager, J. Am. Chem. Soc. 1998, 120, 5321.
[8] D. T. McQuade, J. Kim, T. M. Swager, J. Am. Chem. Soc. 2000, 122,
5885.
[9] J.-L. Brÿdas, J. Cornil, D. Beljonne, D. A. dos Santos, Z. Shuai, Acc.
Chem. Res. 1999, 32, 267.
[10] R. Deans, J. Kim, M. Machacek, T. M. Swager, J. Am. Chem. Soc.
2000, 122, 8565.
explosives dinitrotoluene (DNT) and trinitrotoluene (TNT)
than nonaggregated 3. In solid-state experiments, 3 was
compared to 2, a material with a self-amplified sensory
response arising from efficient exciton transport that is the
basis of new landmine detection technology with femtogram
detection limits.[7] Polymer 2 exhibits its best performance
when spin-coated from nonaggregated solutions to give films
of non-interacting chains. These films of 2 were compared
with films that were spin-coated from aggregated solutions of
3 (55:45 chloroform:methanol). Both films were uniform and
of identical optical density. Upon exposure to a static 10 ppb
vapor of TNT,[20] 3 displayed a Stern Volmer plot that is four
times steeper than that of 2. It should be noted that thin films
of 3 (optical density (OD) ¼ 0.04) exhibit a quenching
response of ꢁ 75% in 10 seconds that is significantly more
sensitive than previously published materials.[7,21 23] We attrib-
ute this observation to the improved diffusion length of
excitons in 3D-coupled chiral grids of 2. Our recent work[1]
suggested that strong electronic coupling provides the best
exciton transport and hence the excitonic coupling enhances
the interchain transport. Further, the highly organized
aggregate probably extends the conjugation length of the
polymer, which may also enhance exciton transport. We have
ruled out the possibility that the sensitivity enhancement is
caused by a higher partitioning of TNT to 3 than to 2 by
[11] This model is most properly used to understand only very weakly
interacting chromophores. However, it guided our studies. M. Kasha
in Spectroscopy of the Excited State (Ed.: B. Di Bartollo), Plenum,
New York, 1976, pp. 337 363.
[12] J. Kim, T. M. Swager, Nature 2001, 411, 1030.
[13] R. Fiesel, U. Scherf, Macromol. Rapid Commun. 1998, 19, 427.
[14] There are a number of contrasting results. However, in this earlier
report an absorption maximum of 420 nm (e ¼ 12400 Lmolꢂ1 cmꢂ1) is
reported, which is considerably (34 nm) blue-shifted relative to the
carefully characterized polymers investigated here, and to other
achiral polymers having the same electronic structure. Hence we
attribute the differences to the analysis in the previous studies of very
low molecular-weight oligomeric samples.
[15] B. M. W. Langeveld-Voss, R. A. Janssen, M. P. T. Christiaans, S. C. J.
Meskers, H. P. J. M. Dekkers, E. W. Meijer, J. Am. Chem. Soc. 1996,
118, 4908.
[16] A. F. M. Kilbinger, A. P. H. J. Schenning, F. Goldoni, W. J. Feast, E. W.
Meijer, J. Am. Chem. Soc. 2000, 122, 1820.
[17] B. M. W. Langeveld-Voss, D. Beljonne, Z. Shuai, R. A. J. Janssen,
S. C. J. Meskers, E. W. Meijer, J.-L. Brÿdas, Adv. Mater. 1998, 10, 1343;
B. M. W. Langeveld-Voss, R. A. J. Janssen, E. W. Meijer, J. Mol.
Struct. 2000, 521, 285.
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[18] N. Berova, K. Nakanishi in Circular Dichroism: Principles and
Applications (Eds.: N. Berova, K. Nakanishi, R. W. Woody), Wiley-
VCH, New York, 2000, pp. 337 382.
[19] We found that thin films of these materials do not aggregate even with
extended thermal annealing at 1608C. Possibly the PPE main chains
are collinear and kept apart by the presence of the pentiptycene
groups.
very general and includes diverse testing methodologies such
as solar radiation, mechanical stress, aggressive fluids, heat,
and others. Tested arrays are further analyzed by using high-
throughput screening (HTS) tools.
We demonstrated our strategy for high-throughput per-
formance screening of polymeric materials. Weathering is a
critical consideration in the development of new engineering
thermoplastics for outdoor applications.[4] A determining
factor in the outdoor weathering lifetime of these materials
is the received UV radiation dose. It leads to material
photodegradation that initially causes the loss of appearance
such as surface discoloration and gloss loss.[5,6] Other environ-
mental (humidity, temperature, frequency of light/dark cy-
cles) and material (catalyst residues, crystallinity, molecular
weight, end groups) factors do not appreciably affect the
appearance of engineering thermoplastics upon weather-
ing.[4,7] These factors, however, do have a pronounced effect
on other types of polymers.[4,6] Additives and pigments
(typical loadings of 0.01 5 wt%) greatly affect the stability
of engineering and other types of polymers.[8] Thus, studies of
polymer additive and polymer pigment combinations are of
great importance.
Resistance toward weathering presents a particular chal-
lenge for HTS since exposure times for adequate outdoor
weatherability testing are thousands of hours.[4] In principle,
acceleration of this process can be achieved either by
increasing the level of environmental stress or by very early
detection of material degradation. In laboratory weathering
methods, a dose equivalent to one year of exposure to
conditions in Florida can be reduced to about 1100 h.[9]
However, a further acceleration of materials weathering to
the HTS level becomes problematic because of loss of
correlation with traditional test methods.[4,8,10]
We use fluorescence imaging and spectroscopy to quantify
photodegradation in arrays of materials at short exposure
times that open up HTS capabilities in weathering studies.
Our libraries combine the variables of polymer composition,
pigment type, and exposure time, but are easily extendable to
incorporation of stabilizers and other additives at different
loadings that may affect the rate of degradation.
Unlike traditional measurements of weatherability-related
appearance based on color changes and gloss loss,[4,8b,11]
fluorescence analysis[12] advances the studies of polymeric
materials libraries produced by using high-throughput meth-
ods to a previously unavailable level of capabilities. Fluores-
cence detects trace amounts of weathering degradation
products and thus requires a considerably shorter testing
time. Measurements are performed on both lightly and highly
colored samples with a comparable level of sensitivity.
Because fluorescence light is isotropic, the whole combinato-
rial library is imaged without the need for an integrating
sphere or a goniophotometer. Different fluorescence excita-
tion emission conditions can be selected to analyze different
fluorescent species.
[20] The saturation vapor pressure of TNT at 228C is 8.02 î 10ꢂ6 mmHg or
about 10 ppb. Handbook of Physical Properties of Organic Chemicals
(Eds.: P. H. Howard, W. M. Meylan), CRC, Boca Raton, FL, 1997.
[21] For comparison, other electronic polymers reportedly decreased their
fluorescence intensity by 8.2% after exposure to similar vapor
concentrations with turbulent flow for 10 min (H. Sohn, R. M.
Calhoun, M. J. Sailor, W. C. Trogler, Angew. Chem. Int. Ed. 2001,
113, 2162; Angew. Chem. Int. Ed. 2001, 40, 2104). Hence 3, which was
studied under static conditions, is at least 560 times more sensitive.
[22] Y. Liu, R. C. Mills, J. M. Boncella, K. S. Schanze, Langmuir 2001, 17,
7452; G. A. Bakken, G. W. Kauffman, P. C. Jurs, K. J. Albert, S. S.
Stitzel, Sens. Actuators B 2001, 79, 1.
[23] Y.-S. Yang, T. M. Swager, J. Am. Chem. Soc. 1998, 120, 11864.
[24] The CD and absorption spectra end at 50:50 chloroform:methanol as
the rigid, gridlike aggregates precipitate readily at higher methanol
content. The fluorescence spectra could be recorded up to 30:70
chloroform:methanol as the concentration of the polymer was only 1/
10 of that of the absorption studies and therefore precipitation
occurred at higher methanol concentrations.
[25] Similarly, 3 had a photoluminescence efficiency which was four times
higher than that of 2 in films of identical optical density.
High-Throughput Multilevel Performance
Screening of Advanced Materials
Radislav A. Potyrailo* and James E. Pickett
High-throughput screening and combinatorial chemistry
have been successfully extended from pharmaceutical appli-
cations into other scientific disciplines.[1] Performance of
rapidly fabricated compounds is typically evaluated by relat-
ing results of high-throughput analysis to the intrinsic proper-
ties of starting materials or final products.[2] We report a
general scheme for screening of complex materials developed
by using high-throughput and combinatorial methods, where
simple screening for intrinsic properties of starting materials
or final compounds does not provide information about long-
term material performance.
Our general approach employs fabrication of materials
arrays followed by their testing against single or several
performance parameters at multiple levels. This testing
process imitates the end-use application and alters materials
properties in a detectable manner that is impossible to
quantitatively predict using existing knowledge.[3] Our scheme
of high-throughput evaluation of advanced materials fabri-
cated by using high-throughput and combinatorial methods is
[*] Dr. R. A. Potyrailo, Dr. J. E. Pickett
General Electric Company
Global Research Center
To demonstrate the applicability of our screening strategy,
three types of aromatic polymers (polycarbonate, PC; poly-
(butylene terephthalate), PBT; and their 45/55 wt% blend,
PC/PBT) were made in combination with two types of
pigments (rutile TiO2 and carbon black, CB) and tested under
Niskayuna, NY 12309 (USA)
Fax : (þ 1)518-387-5604
E-mail: Potyrailo@crd.ge.com
Supporting information for this article is available on the WWW under
http://www.angewandte.org or from the author.
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