Excited-state lifetime modulation in triphenylene-based conjugated polymers [5]

Full text of DOI:10.1021/ja016662l

11298
J. Am. Chem. Soc. 2001, 123, 11298-11299
Excited-State Lifetime Modulation in
Triphenylene-Based Conjugated Polymers
Scheme 1
Aimee Rose, Claus G. Lugmair, and Timothy M. Swager*
Massachusetts Institute of Technology
Department of Chemistry
Cambridge, Massachusetts 02139
ReceiVed July 19, 2001
Conjugated polymers are an extraordinary conduit for the
transport of electronic excitations1 to segments with the greatest
effective conjugation length. To provide the highest amplification
in sensory schemes5 we are interested in enhancing energy
migration. Such improvements will allow the excitation a greater
diffusion length and a higher probability of encountering a
receptor occupied by an analyte. The high efficiency of energy
-4
5-7
transfer in conjugated systems relative to systems with pendant
chromophores8 suggests that transport in these systems may be
enhanced by the strongly electronic coupled intrachain (Dexter-
type) processes in addition to the dipole-dipole (F o¨ rster-type)
processes that govern weakly interacting chromophores. To
determine the relative dominance of these types of processes we
have synthesized and studied polymers that by novel design have
longer excited-state lifetimes. Increases in lifetime imply a
reduction in transition dipole and should therefore decrease the
F o¨ rster rate, whereas transport by the Dexter mechanism should
be enhanced. We report herein that for isolated poly(arylene
ethynylenes) in solution that Dexter transport is dominant.
Poly(phenylene ethynylenes) (PPEs) have seen considerable
interest due to their rigid geometry and ability to effectively
transfer energy over long distances.10 To modify the lifetimes of
these systems we have chosen to incorporate into the polymer
backbone triphenylene chromophores with a well-known11 sym-
,9
desirable thin film photophysical properties, the present study is
restricted to solution measurements on isolated polymers.
The triphenylene monomer is synthesized as depicted in
Scheme 1 with an overall yield of 12%. Iodination of 1,2
dialkoxybenzene with periodic acid and iodine15 affords the 3,4-
dialkoxyiodobenzene (1), and a subsequent zinc/palladium (0)
coupling16 generates the 3,3′,4,4′-tetraalkoxybiphenyl (2) in high
yields. The latter reaction offers an alternative to the conventional
Ulman coupling which required exclusion of air and high
temperature and in our hands led to inferior yields. Iron (III)
trichloride and p-dimethoxybenzene afford cyclization to the
triphenylene. Oxidative demethylation with cerium ammonium
nitrate forms the quinone (3). A Diels-Alder reaction in neat
cyclohexadiene incorportates the bicyclooctane moiety. The
isolated double bond is hydrogenated, and oxidation with potas-
sium bromate generates the tetrasubstituted quinone (6). Lithiated
trimethlysilane acetylide adds to the quinone carbonyls, and
subsequent reductive dehydroxalation with tin dichloride recon-
stitutes the triphenylene ring system. Deprotection with base gives
the key diethynyl triphenylene monomer (5) which is copolymer-
ized with diiodo-funtionalized monomers using the Sonogashira-
metrically forbidden S
0 1
-S transition. Clearly, the triphenylene
wavefunction12 will be perturbed, and the stringent symmetry
responsible for its optical properties will be broken; however,
we considered that this strongly aromatic structure should retain
some of its individual identity and that poly(triphenylene ethy-
nylenes) (TPPEs) would have extended excited-state lifetimes.
In addition to their desired electronic properties triphenylene
materials also have a tendency to form π-stacked discotic liqu3,i1d4
crystalline phases that facilitate charge and energy transport.1
While our monomer design gives rise to polymers that exhibit
17
Hagihara reaction. The exceptions are polymers 7 and 7a which
1
8
are made through an oxidative coupling method. Resulting
polymers and some of their physical properties are tabulated
(
1) Samuel, I. D. W.; Crystall, B.; Rumbles, G. P. L.; Holmes, A. B.; Friend,
R. H. Chem. Phys. Lett. 1993, 213, 472.
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(Table 1).
(
Matched pairs of PPEs and TPPEs were synthesized for
Synth. Met. 1994, 64, 295.
comparative photophysical studies (Figure 1). The differences in
the molecular weights of the materials as synthesized were in
some cases very large; hence, gel permeation chromatography
was used to obtain materials with more closely matched molecular
weights. The results have high self-consistency, and TPPEs exhibit
extended excited-state lifetimes over their PPE analogues as
depicted in Figure 2. The TPPEs demonstrate about a 30% lifetime
increase without severely compromising quantum yield. The
combination of Φ and τ data demonstrate that the enhanced
(
3) B a¨ ssler, H.; Schweitzer, B.; Huen, S. Acc. Chem. Res. 1999, 32, 173.
(
4) (a) Rothberg, L. J.; Yan, M.; Papadimitrakopoulos, F.; Galvin, M. E.;
Kwock, E. W.; Synth. Met. 1996, 80, 41. (b) Yan, M.; Rothberg, L. J.;
Papadimitrakopoulos, F.; Galvin, M. E.; Miller, T. M. Phys. ReV. Lett. 1994,
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5) (a) Zhou, Q.; Swager, T. M. J. Am. Chem. Soc. 1995, 117, 7017. (b)
Zhou, Q.; Swager, T. M. J. Am. Chem. Soc. 1995, 117, 12593. (c) Yang,
J.-S.; Swager, T. M. J. Am. Chem. Soc. 1998, 120, 5321.
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(b) van de Craats, A. M.; de Haas, M. P.; Warman, J. M. Synth. Met. 1997,
86, 2125.
(14) Markovitski, D.; Lecuyer, I.; Lianos, P.; Malthete, J. J. Chem. Soc.,
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(16) Jacob, P.; Callery, P. S.; Shulgin, A. T.; Catagnoli, N. J. Org. Chem.
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to the Study of Photoprocesses in Macromolecules; Cambridge University
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(9) (a) Byers, J. D.; Friedrich, M. S.; Friesner, R. A.; Webber, S. E. In
Molecular Dynamics in Restricted Geometries; Klafter, J., Drake, J. M., Eds.;
Wiley: New York, 1988; pp 99-144. (b) Kiserow, D. J.; Itoh, Y.; Webber,
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1
0.1021/ja016662l CCC: $20.00 © 2001 American Chemical Society
Published on Web 10/17/2001

Communications to the Editor
J. Am. Chem. Soc., Vol. 123, No. 45, 2001 11299
Table 1. Physical Properties of Synthesized TPPEs and PPEs
The relationship between excited-state lifetime and energy
migration can be investigated through fluorescence depolarization
measurements. All polymers studied are high-molecular weight
materials and can be considered rotationally static over the
emission lifetime of the polymer. Therefore, energy migration is
the major contributor to the fluorescence depolarization in
conjugated polymers, and the exciton loses more of its initial
polarization as it diffuses along a disordered polymer chain. The
polarization value, P, has been determined from standard equation
P ) (Ivv - GIhv)/(Ivv + GIhv), where Ivv and Ihv are the intensities
of emissions detected parallel and perpendicular to the polarization
vector of the incident light and G is an instrumental correction
factor.
λ
maxabs
λmaxem
(nm)
a
pdia
b
τ (ns)c
(nm)
M
n
M
n
Φ
6
6
7
7
8
8
9
9
480
441
480
430
490
420
465
449
505
475
530
455
509
459
518
487
45000 2.5 102200
20900 2.0 41000
0.71
0.52
1.27
0.43
0.95
0.67
0.78
0.44
0.27
0.45
0.14
0.45
0.30
0.39
0.20
0.13
a
a
a
a
10500 1.3 199000
24000 1.8 108000
103000 1.9 325000
31400 2.0 253000
114000 1.3 256740
22000 3.0 177700
a
As synthesized. b After GPC fractionization for spectroscopy.
c
Monoexponential decay.
The full range of allowable excitation energies for each polymer
was investigated to separate depolarization due to energy migra-
tion from that due to absorption/emission dipole alignment. As
excitation energy is increased, both the TPPEs and PPEs display
lower P values, consistent with population of higher-energy
excitons that readily lower their energy by migration to lower-
energy states. Overall, wavelengths the TPPEs consistently reveal
lower P values (Figure 2a). The decrease in emission polarization
at higher energies may also be the result of exciting triphenylene-
localized transitions that have an angular displacement from the
emission dipole. In this case, polarization would be expected to
decrease more dramatically at certain intervals of wavelengths,
not continuously over the range investigated. In TPPEs, a near
linear decrease of polarization as a function of excitation
wavelength (Figure 2b) is observed supporting energy migration
as the major contributor to depolarization. For TPPEs, the lowest
attainable P values are a fraction of the P obtained when exciting
at λmax. In PPEs however, this value is only about half, indicating
the radiative rate of emission limits the extent of energy migration.
This limitation is overcome in the longer-lived TPPE systems as
near 0 P values are obtainable in most of the materials. Studies
of polarization as a function of chain length support these
conclusions.19 If the F o¨ rster mechanism dominated these systems,
then enhanced radiative rates in PPEs would encourage more
extensive migration. This is not observed, pointing to the Dexter
mechanism as the dominant intramolecular energy-transport
process in these materials.
Figure 1. TPPEs and their phenyl analogues. R is a 2-ethylhexyl group,
and all other side chains are n ) alkyl groups. Polymer 6 and 7a are
regio-random structures, and to maintain solubility at high-molecular
weights 7a was synthesized as a random copolymer.
In summary, we have reported a new method for the design of
conjugated polymers with longer excited-state lifetimes. The
triphenylene-based poly(arylene ethynylenes) display longer
excited-state lifetimes when compared to their phenyl analogues.
Enhanced fluorescence depolarization is also observed in TPPE
systems, indicating that more extensive energy migration has been
effected. Longer lifetimes foster both through-bond energy
migration and the population inversion necessary to observe
stimulated emission. These advances are currently being applied
to further amplify sensory responses.
Acknowledgment. We gratefully acknowledge financial support from
Bechtel Nevada, The Office of Naval Research, and DARPA.
Figure 2. Emission polarization for each polymer. TPPE (stripped bars)
and analogous PPE (solid bars): (a) upon excitation at λmax. (b) as a
function of excitation wavelength for 7 and 7a.
Supporting Information Available: Experimental details and char-
acterization data (PDF). This material is available free of charge via the
Internet at http://pubs.acs.org.
lifetimes are principally due to differences in radiative rates and
not differences in the nonradiative rates. Supporting our assertion
that the triphenylene group is responsible for the lifetime
enhancements, we find that 7 displays the largest enhancement
and is about 3 times that of 7a. This material also has the largest
fraction of triphenylene chromophore per repeat unit.
JA016662L
(19) Beyond a critical length the polymers are longer than the diffusion
length of the exciton, and P is a minimum value. For the TPPEs the molecular
weight needs to be much higher than that of the PPEs to observe the minimum
P value.