Conjugation enhancement of intramolecular exciton migration in poly(p-phenylene ethynylene)s

Article abstract of DOI:10.1021/ja051936g

Efficient energy migration in conjugated polymers is critical to their performance in photovoltaic, display, and sensor devices. The ability to precisely control the polymer conformation is a key issue for the experimental investigations and deeper unders

Full text of DOI:10.1021/ja051936g

Published on Web 06/23/2005
Conjugation Enhancement of Intramolecular Exciton Migration
in Poly(p-phenylene ethynylene)s
Evgueni E. Nesterov,† Zhengguo Zhu, and Timothy M. Swager*
Contribution from the Department of Chemistry and Institute for Soldier Nanotechnologies,
Massachusetts Institute of Technology, Cambridge, Massachusetts 02139
Received March 25, 2005; E-mail: tswager@mit.edu
Abstract: Efficient energy migration in conjugated polymers is critical to their performance in photovoltaic,
display, and sensor devices. The ability to precisely control the polymer conformation is a key issue for the
experimental investigations and deeper understanding of the nature of this process. We make use of specially
designed iptycene-containing poly(p-phenylene ethynylene)s that display chain-extended conformations
when dissolved in nematic liquid crystalline solvents. In these solutions, the polymers show a substantial
enhancement in the intrachain exciton migration rate, which is attributed to their increased conjugation
length and better alignment. The organizational enhancement of the energy transfer efficiency, as determined
by site-selective emission from lower energy traps at the polymer termini, is accompanied by a significant
increase of the fluorescence quantum yield. The liquid crystalline phase is a necessary requirement for
these phenomena to occur, and when the temperature was increased above the nematic-isotropic transition,
we observed a dramatic reduction of the energy transfer efficiency and fluorescence quantum yield. The
ability to improve the exciton migration efficiency through precise control of the polymer structure with
liquid crystalline solutions demonstrates the importance of a polymer’s conformation for energy transfer,
and provides a way to improve the energy transporting performance of conjugated polymers.
Introduction
intramolecular exciton migration in isolated polymer chains may
indeed be very efficient.6
The ability of conjugated polymers to function as gain media
in electro-optical and sensory devices is dependent on the
efficient transport of excited states (excitons) along the polymer
chain. We have been specifically interested in facile exciton
migration in conjugated polymers in general, and poly(p-
phenylene ethynylene)s (PPEs) in particular, that allows energy
absorbed over large areas to be funneled into traps created by
the binding of analytes, resulting in signal amplification in
sensory devices.1-3 The energy migration in conjugated poly-
mers can occur both intramolecularly and intermolecularly. In
the case of dilute solutions, the intramolecular process dominates
in the form of a one-dimensional exciton random walk along
isolated chains.4 Much higher efficiency can be reached in
polymer aggregates and in solid films, where the energy
migration occurs as a three-dimensional process by both
intramolecular and intermolecular pathways. The interplay
between these two pathways has received much attention in the
literature, with the intrachain migration being sometimes
considered slow and inefficient as compared to its interchain
counterpart.5 However, recent experiments have shown that the
Emissive polymer films with modest to high quantum yields
generally have limited electronic interactions between polymer
chains, and in this case interchain energy migration is generally
accepted to occur through the dipole-induced dipole mecha-
nisms. The three-dimensional nature of energy migration in films
usually leads to longer exciton diffusion lengths, but often is
accompanied by formation of low-emissive intermolecular
species, resulting in diminished emission quantum yields.7 A
detailed understanding of intramolecular energy transfer in
conjugated polymers is presently elusive and is complicated by
the conformational complexities that are typically associated
with conjugated polymers in solution and in thin films. An
exception is a previous study4a wherein the rate of energy
transfer was investigated on PPEs assembled into discrete
multilayers with precise control of polymer conformation and
alignment. This study indicated that energy transfer was much
(5) (a) Beljonne, D.; Pourtois, G.; Silva, C.; Hennebicq, E.; Hertz, L. M.; Friend,
R. H.; Scholes, G. D.; Setayesh, S.; Mu¨llen, K.; Bre´das, J. L. Proc. Natl.
Acad. Sci. U.S.A. 2002, 99, 10982-10987. (b) Nguyen, T.-Q.; Wu, J.; Doan,
V.; Schwartz, B. J.; Tolbert, S. H. Science 2000, 288, 652-656. (c) Wang,
C. F.; White, J. D.; Lim, T. L.; Hsu, J. H.; Yang, S. C.; Fann, W. S.; Peng,
K. Y.; Chen, S. A. Phys. ReV. B 2003, 67, 035202.
† Current address: Department of Chemistry, Louisiana State University,
Baton Rouge, LA 70803.
(6) (a) Mu¨ller, J. G.; Lemmer, U.; Raschke, G.; Anni, M.; Scherf, U.; Lupton,
J. M.; Feldmann, J. Phys. ReV. Lett. 2003, 91, 267403. (b) Mu¨ller, J. G.;
Lupton, J. M.; Feldman, J.; Lemmer, U.; Scherf, U. Appl. Phys. Lett. 2004,
84, 1183-1185. (c) Lupton, J. M.; Craig, M. R.; Meijer, E. W. Appl. Phys.
Lett. 2002, 80, 4489-4491. (d) Padmanaban, G.; Ramakrishnan, S. J. Am.
Chem. Soc. 2000, 122, 2244-2251. (e) Guillet, T.; Berre´har, J.; Grousson,
R.; Kovensky, J.; Lapersonne-Meyer, C.; Schott, M.; Voliotis, V. Phys.
ReV. Lett. 2001, 87, 087401.
(1) McQuade, D. T.; Pullen, A. E.; Swager, T. M. Chem. ReV. 2000, 100,
2537-2574.
(2) (a) Zhou, Q.; Swager, T. M. J. Am. Chem. Soc. 1995, 117, 7017-7018.
(b) Swager, T. M. Acc. Chem. Res. 1998, 31, 201-207.
(3) Bunz, U. H. F. Chem. ReV. 2000, 100, 1605-1644.
(4) (a) Levitsky, I. A.; Kim, J.; Swager, T. M. J. Am. Chem. Soc. 1999, 121,
1466-1472. (b) Levitsky, I. A.; Kim, J.; Swager, T. M. Macromolecules
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J. AM. CHEM. SOC. 2005, 127, 10083-10088
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Nesterov et al.
Scheme 1. Synthesis of the End-Capped Polymer 1
a Reagents and conditions: (a) p-benzoquinone, xylenes, reflux, 3 h; (b) concentrated HBr, AcOH, reflux, 10 min; (c) aqueous KBrO3, reflux, 5 min, 65%
over three steps; (d) AcOH, reflux, 66 h, 17%; (e) Me3SiCtCLi, THF, 0f25 °C, 16 h; (f) SnCl2‚2H2O, acetone-AcOH, 25 °C, 24 h; (g) NaOH, MeOH-
THF, 1 h, 49% over three steps; (h) n-BuLi (1 equiv), THF, -70 °C, 3 h, then add I2, -70 to 25 °C, 12 h, 82%; (i) phenylacetylene (1 equiv), Pd(PPh3)4,
CuI, i-Pr2NH-toluene, 55 °C, 24 h, 73%; (j) Me3SiCtCH, Pd(PPh3)4, CuI, i-Pr2NH-toluene, 80 °C, 36 h; (k) KOH, MeOH-THF, 25 °C, 40 min, 71%
over two steps; (l) Pd(PPh3)4, CuI, i-Pr2NH-toluene, 90 °C, 24 h, then add 4 (0.2 equiv), 90 °C, 12 h, 58%.
faster in the plane defined by each layer of the polymer chains
as compared to the direction normal to the chains, thereby
suggesting that intramolecular energy transfer is faster than the
intermolecular process. Nevertheless, intermolecular energy
transfer is often considered to be faster than intramolecular
energy transfer because dipolar energy transfer mechanisms,
known in small-molecule systems as Fo¨rster energy transfer,
are assumed to be dominant. This assumption is largely based
on the fact that through-space dipolar interactions scale as 1/R6,
where R is the interchromophore distance, and hence in
agglomerated weakly coupled systems the distances are shorter
between effective chromophores on neighboring polymer chains,
which favors interchain energy transport. However, when there
is a strong electronic coupling between chromophores, as in
extended systems like PPEs, there is also the possibility of facile
energy transfer due to direct orbital overlap, which is generally
referred to as the through-bond Dexter energy transfer mech-
anism.8
intramolecular energy migration, we have devised methods for
the study of isolated polymers in dilute solution with precise
conformations. An important method in these investigations is
to measure energy transfer in π-conjugated polymers that have
low band gap termini. This approach, first demonstrated by one
of us,9 has been used previously to investigate intramolecular
energy transfer in isotropic solutions and amorphous films.5a,10
We report herein energy transfer studies on polymers dissolved
in thermotropic nematic liquid crystals, and demonstrate that
energy transfer is enhanced in ordered phases with chain-
extended structures and increased π-conjugation. This level of
control over polymer conformations has not been achieved
previously, and has the potential to shed light on the relative
importance of dipolar and Dexter mechanisms for intramolecular
energy migration in conjugated polymers.
Results and Discussion
Synthesis of End-Capped Polymer 1. We synthesized the
pentiptycene-incorporating PPE 1 (Scheme 1), end-capped with
10-(phenylethynyl)anthracenyl groups that create low-energy
emissive traps.9 The efficiency of the exciton migration to the
terminal groups can be evaluated by measuring their fluores-
From the preceding discussion it is clear that energy transfer
in conjugated polymers is complex and depends on the
organization of the materials as well as their electronic structure.
To gain a better understanding of the factors that control
(8) (a) Murphy, C. B.; Zhang, Y.; Troxler, T.; Ferry, V.; Martin, J.; Jones, W.
E., Jr. J. Phys. Chem. B 2004, 108, 1537-1543. (b) List, E. J. W.; Leising,
G. Synth. Met. 2004, 141, 211-218. (c) Hennebicq, E.; Pourtois, G.;
Scholes, G. D.; Herz, L. M.; Russell, D. M.; Silva, C.; Setayesh, S.;
Grimsdale, A. C.; Mu¨llen, K.; Bre´das, J.-L.; Beljonne, D. J. Am. Chem.
Soc. 2005, 127, 4744-4762.
(9) Swager, T. M.; Gil, C. J.; Wrighton, M. S. J. Phys. Chem. 1995, 99, 4886-
4893.
(10) Ego, C.; Marsitzky, D.; Becker, S.; Zhang, J.; Grimsdale, A. C.; Mu¨llen,
K.; MacKenzie, J. D.; Silva, C.; Friend, R. H. J. Am. Chem. Soc. 2003,
125, 437-443.
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Conjugation Enhancement of Intramolecular Energy Transfer
A R T I C L E S
Figure 1. Normalized absorption (A) and emission (B) spectra of polymer 1. Emission spectra were acquired upon excitation at 370 nm. Fluorescence
quantum yields are the following: CH2Cl2 solution, 0.60; spin-cast film, 0.08; LC solution, 0.90. Extinction coefficients (at the PPE backbone maxima, per
molar repeating unit): 13 000 (CH2Cl2 solution), 15 000 (LC solution). The lines marked with arrows correspond to emission spectra obtained at 450 nm
excitation (direct excitation of the anthracenyl group).
cence intensity, with higher relative intensity corresponding to
more efficient migration. The tetra-tert-butylpentiptycene units
in the backbone of 1 limit interchain interactions in thin films,11
and also promote the polymer’s solubility in thermotropic liquid
crystalline solvents.12 In addition, the pentiptycene moiety has
a substantial “internal free volume”,13 which by design facilitates
the polymers’ alignment in nematic liquid crystalline media.14
Polymer 1 was prepared via a Sonogashira coupling protocol
in two subsequent steps. The polymerization was carried out
with 10% excess of diiodo monomer 3 to ensure that at the end
of polymerization the polymer chain was terminated by iodine
groups. The initially formed polymer was end-capped in situ
by reacting with an excess of the acetylene reagent 4 (synthetic
details are given in the Supporting Information). The polymer
prepared by this procedure was analyzed by 1H NMR and GPC
and found to be completely end-capped (number-averaged
molecular weight Mn ) 24.5 kDa and a polydispersity index
1.69, according to GPC).15
Energy Transfer Studies. The UV/vis absorption spectrum
of a dilute (0.005%) CH2Cl2 solution of 1 displays an intense
absorption centered at 377 nm as well as a much weaker
absorption at 466 nm, corresponding to the anthracenyl-based
terminal groups (see Figure 1A). The polymer was found to be
highly fluorescent in solution (quantum yield 0.60). The
fluorescence spectrum parallels the absorption spectrum in that
it consists of two bands (Figure 1B). The major band, centered
at 420 nm, is characteristic for the emission from dominant PPE
backbone, while the less intense band at 500 nm represents the
end-group emission. When the solution is excited at 370 nm as
shown in Figure 1B, essentially all of the light is absorbed by
the PPE backbone, and hence the end-group emission is the
result of intramolecular exciton migration. Under these condi-
tions, the efficiency of energy transfer is relatively low and was
estimated by spectral deconvolution to be approximately 7%.16
The fact that the PPE is behaving as an antenna for the end
groups is confirmed by the much weaker end-group emission
observed when the anthracenyl group is excited directly at 450
nm with the same photon flux (Figure 1b).
We have recently demonstrated that dissolving iptycene-
incorporating conjugated polymers in nematic liquid crystals
(LCs) results in an increased conjugation length with the
completely extended (uncoiled) polymer chains aligned along
the LC director.14 The absorption spectrum of a dilute (0.05%)
solution of the polymer 1 in 1-(trans-4-hexylcyclohexyl)-4-
isothiocyanatobenzene (6CHBT, Tm ) 12.4 °C, TNI ) 42.4 °C)
at room temperature showed a dramatically red-shifted PPE
backbone absorption band with a sharp maximum at 421 nm
(bathochromic shift of ∼44 nm as compared to CH2Cl2 solution,
Figure 1A). The anthracenyl end-group absorption was also red-
shifted, although to a lesser extent (∼10 nm). Excitation spectra
acquired under the detection of the anthracenyl end-group
emission showed similar behavior, with the PPE-centered
excitation band being significantly bathochromically shifted in
the LC solution as compared to those in isotropic solution and
thin film (Figure S2, Supporting Information). The fluorescence
spectrum displayed a modest 10 nm red shift of both the PPE
and the end-group emission bands, as compared to those of the
isotropic CH2Cl2 solution (Figure 1B). Concomitant to these
bathochromic shifts is a dramatic 5-fold increase of the
efficiency of energy transfer (up to 36%) in the LC solution.
This substantial enhancement of the energy transfer is ac-
companied by a large increase of the total fluorescence quantum
yield, which is almost quantitative at 0.90 (cf. with the value
of 0.60 for the isotropic CH2Cl2 solution).
These dramatic changes are the result of the highly uniform
alignment of the polymer chains and the increased electronic
(11) Yang, J.-S.; Swager, T. M. J. Am. Chem. Soc. 1998, 120, 11864-11873.
(12) Williams, V. E.; Swager, T. M. Macromolecules 2000, 33, 4069-4073.
(13) (a) Long, T. M.; Swager, T. M. AdV. Mater. 2001, 13, 601-604. (b) Long,
T. M.; Swager, T. M. J. Am. Chem. Soc. 2002, 124, 3826-3827.
(14) Zhu, Z.; Swager, T. M. J. Am. Chem. Soc. 2002, 124, 9670-9671.
(15) The conclusion that 1 was completely end-capped was supported by
comparison of an average degree of polymerization (DP) estimated using
the 1H NMR end-group analysis (∼23) and the value obtained from the
GPC analysis (∼25), which are in excellent agreement with each other.
(16) We define the efficiency of energy transfer as a measure of the probability
that an exciton formed on the PPE backbone is transmitted to the
anthracenyl end group. It was estimated by comparing the emission
spectrum of 1 to that of phenyl end-capped model polymer 5 at constant
absorbance at the 370 nm excitation wavelength (see Supporting Information
for more details).
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Nesterov et al.
Figure 3. Comparison of absorption spectra of anthracenyl end-capped
polymer 1 (solid line) and phenyl end-capped model polymer 5 (dashed
line) in the LC solution.
Figure 2. Polarized emission spectra of a LC solution of polymer 1 with
the excitation light polarized parallel to the nematic LC director. The solid
line shows the emission spectra acquired with the emission channel polarizer
parallel to the polarization plane of excitation light (I|), while dashed line
correspond to the spectrum obtained with the emission channel polarizer
perpendicular to it (I ). The latter spectrum is “G-factor” corrected. The
fluorescence anisotropy ([I|/I - 1]/[I|/I + 2]) at 420 nm was calculated
to be 0.79.
shifts are too large to be simply a solvatochromic effect, and
indicate a highly planarized polymer backbone conformation
in the LC solution. This conclusion is in a good agreement both
with our own previous results19a and with the literature data
obtained with PPEs,19b as well as with small-molecule model
systems.19c On the basis of the experimental observation that at
the low (0.005-0.05%) polymer concentrations in the LC
solution the efficiency of energy transfer is concentration-
independent, and the fact that we are dealing with a true, not
aggregated, solution, one can safely rule out an intermolecular
contribution to the increased energy transfer efficiency. Thus,
the increased energy transfer to the terminal groups in the LC
solution is attributed to the enhanced intrachain exciton migra-
tion. Of the two components of energy transfer, dipolar (Fo¨rster-
like) mechanisms do not depend significantly on the electronic
conjugation, while the Dexter through-bond exciton migration
is expected to be greatly affected by the electronic conjuga-
tion.9,20 A strict deconvolution of the two mechanisms in this
case is not possible; however, there are considerations that lead
us to hypothesize that the Dexter mechanism may be more
important than previously appreciated. The role of Dexter energy
transport is further supported by our earlier studies20 that
revealed an enhancement in intrachain energy transfer with
longer excited state lifetimes, a result that is in direct conflict
with present theories which focus essentially exclusively on
dipolar mechanisms.8c,21 Furthermore, earlier studies on singlet
energy transfer over longer distances in conjugated donor-
acceptor dyads revealed that this process cannot be satisfactorily
described by using the Fo¨rster mechanism alone, without
including the electron-exchange Dexter mechanism.22 In random
coil conformations, intramolecular Fo¨rster-like energy transfer
is thought to be facilitated by a reduction in the distance between
the effective donor and acceptor chromophores. In the LC
solution, 1 has a completely extended, effectively rigid-rod chain
conjugation within the polymer backbone in the ordered LC
phase. The fluorescence of the polymer in LC solution was
found to be highly anisotropic, with the emission intensity being
polarized parallel to the direction of the nematic LC director
(Figure 2). In total, our observations are indicative of 1
exhibiting a chain-extended, highly conjugated planarized
conformation in the nematic phase. The role of the pentiptycene
structure is clearly evident when these results are compared to
a recent report17 wherein LC solutions (possibly dispersions)
of simple conjugated polymers were found to display broadened
emission bands relative to those for CH2Cl2 solutions and
exhibited modest polarization (fluorescence anisotropy ≈ 0.3).
Such spectral broadening and low order parameters are indica-
tive of a highly disordered polymer conformation (and poten-
tially aggregation) in the LC solvent, in stark contrast to our
results.18 The greatly enhanced conjugation length of 1 in the
LC solvent is evident from the significant bathochromic shifts
of the absorption and (to the lesser extent) emission spectra.
The lower sensitivity of the emission wavelength to changes in
the solvent is to be expected because energy transfer processes
generally lead to domination of a conjugated polymer’s emission
by minority low energy segments in the polymer chain.7 The
high extent of the electronic conjugation in the polymer
dissolved in the LC solvent is also clearly seen from comparing
absorption spectra of 1 and the model polymer 5, possessing
the same backbone structure as 1 but with phenyl end-capping
groups (Figure 3). Both spectra are very similar; however, the
anthracenyl end-capped polymer 1 has an additional well-defined
absorption maximum at the band edge (421 nm) which the
polymer 5 does not show. The additional lower energy absorp-
tion peak is likely to originate from strong electronic coupling
between the conjugated PPE backbone and the low band-gap
terminal anthracenyl group. Regardless, the observed spectral
(19) (a) Kim, J.; Swager, T. M. Nature 2001, 411, 1030-1034. (b) Wang, Y.;
Erdogan, B.; Wilson, J. N.; Bunz, U. H. F. Chem. Commun. 2003, 1624-
1625. (c) Schmieder, K.; Levitus, M.; Dang, H.; Garcia-Garibay, M. A. J.
Phys. Chem. A 2002, 106, 1551-1556.
(20) Rose, A.; Lugmair, C. G.; Swager, T. M. J. Am. Chem. Soc. 2001, 123,
11298-11299.
(17) Fritz, K. P.; Scholes, G. D. J. Phys. Chem. B 2003, 107, 10141-10147.
(18) It was recently demonstrated that the iptycene-incorporating conjugated
polymers, if used as dopants, are capable of increasing the order parameter
of nematic LCs due to the same “internal free volume” minimization effect
(Araoka, F.; Shin, K.-C.; Takanishi, Y.; Ishikawa, K.; Takezoe, H.; Zhu,
Z.; Swager, T. M. J. Appl. Phys. 2003, 94, 279-283).
(21) Bre´das, J. L.; Beljonne, D.; Coropceanu, V.; Cornil, J. Chem. ReV. 2004,
104, 4971-5003.
(22) (a) Gust, D.; Moore, T. A.; Moore, A. L.; Devadoss, C.; Liddell, P. A.;
Hermant, R.; Nieman, R. A.; Demanche, L. J.; DeGraziano, J. M.; Gouni,
I. J. Am. Chem. Soc. 1992, 114, 3590-3603. (b) Devadoss, C.; Bharathi,
P.; Moore, J. S. J. Am. Chem. Soc. 1996, 118, 9635-9644.
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Conjugation Enhancement of Intramolecular Energy Transfer
A R T I C L E S
Figure 4. Simplified representation of polymer 1 in isotropic and LC
solutions. In isotropic solution, the conformational disorder in the polymer
backbone prevents efficient intrachain exciton migration, thus resulting in
predominant emission from the PPE backbone. In nematic LC solution,
the electronic conjugation on the straightened and planarized polymer chains
is higher, which leads to the enhanced intrachain exciton migration toward
the terminal groups with a concomitant increase in the termini’s emission.
Figure 6. Fluorescence spectra of 1 recorded in chloroform-methanol
mixtures with increasing fraction of methanol. Right vertical axis: Fluo-
rescence quantum yield at each point. Inset: Magnified fluorescence
spectrum in 99% methanol.
films, 1’s fluorescence is almost completely dominated by the
end-group emission, although the absorption spectrum showed
no significant change from the solution spectrum (Figure 1).
The high efficiency of exciton migration in this case is enhanced
by intermolecular three-dimensional Fo¨rster energy transfer
processes, resulting in the expected longer exciton diffusion
length, as discussed earlier. However, this effect is accompanied
by a significant drop of the fluorescence quantum yield (down
to 0.08 as compared to 0.60 in dilute isotropic solution, and
0.90 in the nematic LC solution).
We also followed changes in the optical properties of the
polymer 1 at different extents of aggregation. These studies are
best performed by creating stable solutions of nanoscopic
nonscattering particles, thereby allowing quantitative studies of
the polymer optical properties in an aggregated state.25 This is
accomplished by the addition of methanol to a chloroform
solution of the polymer 1. These aggregates display no
significant change in 1’s absorption spectra, indicating that
aggregation does not cause strong intermolecular electronic
interactions between the PPE chains. The fluorescence spectra
reveal a substantial decrease of intensity of both emission bands
(Figure 6). We observe a rapid intensity decrease of the PPE
backbone 420 nm band as the quality of solvent diminishes,
accompanied by a much slower decrease of the 500 nm
anthracenyl band. At very high methanol concentrations (above
80%), the fluorescence pattern (as well as the emission quantum
yield) becomes similar to those of spin-cast films. In this
strongly aggregated state, the 420 nm PPE-centered emission
band drops to 0.7% of its initial intensity, while the intensity
of the 500 nm anthracenyl-centered band decreases to only about
50% of its initial value. This differential quenching can be
caused by more efficient energy transfer in the three-dimensional
framework of aggregated state; however, it is also important to
consider that the bulk of PPE and anthracenyl end groups exhibit
dramatically different rates of nonradiative decay.26 To decon-
volute the effects of differential quenching, we obtained the
emission spectra of 1 using direct excitation of the end groups
Figure 5. Fluorescence spectra of polymer 1 in the nematic LC phase at
25 °C (solid line) and after heating the same solution to its isotropic phase
at 60 °C (dashed line).
conformation, in which the effective interchromophore distance
is increased. In LC solution, dipolar energy transfer can also
be enhanced by two additional factors: smaller Stokes shifts
which will increase the spectral overlap integral,23 and improved
(more parallel) alignment of transition dipole moments.24 This
may increase the dipolar coupling between the effective chro-
mophores required for the Fo¨rster-type mechanism. However,
the 1/R6 distance dependence is a more dominating parameter.
It appears likely that the facilitated energy transfer may result
from an enhancement in the intrachain Dexter energy transfer,
which results from the stronger electronic coupling along the
polymer backbone in the ordered LC phase, and is weakened
in the disordered isotropic solution (Figure 4). This critical
dependence on the LC-induced order can further be simply
demonstrated by heating the LC solution of 1 above its clearing
point, when upon entering the resultant isotropic phase, 1’s
conjugation length was decreased (blue shift in the emission),
and the energy transfer efficiency to the terminal groups was
reduced to about 10% (Figure 5).
To rule out any possibility that intermolecular aggregation
is responsible for the increased energy transfer to the end groups,
we have investigated aggregates and thin films of 1. In solid
(23) Turro N. J. Modern Molecular Photochemistry; University Science
Books: Mill Valley, CA, 1991.
(25) (a) Langeveld-Voss, B. M. W.; Janssen, R. A. J.; Christiaans, M. P. T.;
Meskers, S. C. J.; Dekkers, H. P. J. M.; Meijer, E. W. J. Am. Chem. Soc.
1996, 118, 4908-4909. (b) Zahn, S.; Swager, T. M. Angew. Chem., Int.
Ed. 2002, 41, 4226-4229.
(24) Levitus, M.; Garcia-Garibay, M. A. J. Phys. Chem. A 2000, 104, 8632-
8637.
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at 450 nm (shown as the dashed lines in Figure 1b). This
provided an “internal quantum yield standard”, enabling us to
estimate the efficiency of the energy transfer. In solution, the
direct excitation of the PPE backbone resulted only in 2-fold
increase of the end-group emission intensity relative to that
gained by direct excitation of the end groups. In thin films and
in strongly aggregated poor solvent solutions, upon excitation
at the wavelength of PPE-centered absorption we observe a
much larger 18-fold increase in end-group emission intensity.
This clearly indicates that the efficiency of energy transfer
increases by an order of magnitude with the transformation from
a one-dimensional (solution) to a three-dimensional thin-film/
aggregated structure.27
From the current studies, it can be concluded that inter-
molecular aggregation results in an overall decrease of the
fluorescence quantum yield. The fact that we observed a
substantial increase of the quantum yield in LC solution rules
out aggregation as an explanation for the enhanced energy
transfer. It is apparent that the more efficient energy transfer in
nematic LC solutions is entirely the result of the improved
intrachain energy transfer due to the conjugation enhancement.
The increase of the polymer emission quantum yield in the LC
solvent as compared to that in isotropic solutions is a remarkable
observation which has no precedence. We can attribute it to an
effective rigidification of the polymer chains due to conforma-
tional restrictions imposed by the surrounding more highly
organized LC medium. This should diminish vibrationally/
rotationally coupled electronic deactivation processes, therefore
reducing the rate of nonradiative decay. In addition, the
organized LC environment reduces the rate of collisional
deactivation (quenching) of the excited state by effectively
“insulating” the polymer molecules.
Conclusions
We demonstrated that intrachain energy transfer in conjugated
polymers can be enhanced in the nematic LC solutions as a
result of the increasing electronic conjugation length. We have
further shown that the reduction in conformation disorder in
the nematic LC phase can give rise to greatly enhanced quantum
yields. LC solutions provide a means to organize polymers into
forms that optimize their performance and also can be used to
study the effects of conformational order on their transport
properties. This novel approach can be used in the design of
new conjugated polymer-based sensing platforms that utilize
facile intrachain exciton migration to improve their chemosen-
sory performance without compromising the fluorescence ef-
ficiency.
Acknowledgment. This research was supported by the
National Science Foundation and the U.S. Army through the
Institute for Soldier Nanotechnologies, under contract DAAD-
19-02-D-0002 with the U.S. Army Research Office.
(26) Because of the much larger size of the polymer backbone compared to the
size of the terminating group, the rate of (collisional) quenching of an
exciton in the backbone is expected to be much higher. This may result in
nonproportional quenching of the emission, with the backbone emission
displaying greater quenching.
(27) A statistical analysis (Montroll, E. W. J. Phys. Soc. Jpn. 1969, 26 (Suppl.),
6-10) showed that in the case of an exciton random walk, the number of
steps required for the exciton to find a trap decreases from 1/c2 for a one-
dimensional array to 1/c for a three-dimensional network, where c is the
concentration of randomly distributed traps. We believe this statistical
reasoning to be critical if not dominant in the enhancement of exciton
migration efficiency in the three-dimensional network.
Supporting Information Available: Detailed synthetic pro-
cedures and additional experimental data. This material is
available free of charge via the Internet at http://pubs.acs.org.
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