Hyperconjugative and Inductive Perturbations in Poly(p-phenylene vinylenes)

Article abstract of DOI:10.1021/ja038472b

New polymers having high solid-state fluorescence quantum yields and the ability to tune their electron affinity without effecting their band gap using hyperconjugative interactions is reported. The novel three-dimensional poly(phenylene vinylenes) having [2.2.2] bicyclic ring systems shown were synthesized, and the different hyperconjugative perturbations provide differential fluorescence sensory quenching responses to electron-rich and electron-deficient analytes in solution and solid thin films. Copyright

Full text of DOI:10.1021/ja038472b

Published on Web 12/24/2003
Hyperconjugative and Inductive Perturbations in Poly(p-phenylene vinylenes)
Youngmi Kim, Zhengguo Zhu, and Timothy M. Swager*
Department of Chemistry, Massachusetts Institute of Technology, 77 Massachusetts AVenue,
Cambridge, Massachusetts 02139
Received September 11, 2003; E-mail: tswager@mit.edu
Scheme 1 a
Poly(p-phenylene vinylene) (PPV) and its derivatives are among
the most extensively studied organic semiconductive polymers.
Despite their processablity, high luminescence, and structural
diversity, several challenges remain for further applications. As in
many conjugated polymers (CPs), the fluorescence quantum yields
of PPVs are substantially lower in the solid state due to interchain
interations.1 Successful approaches to enhance the solid-state
emission efficiency of CPs include the incorporation of bulky side
2
chains or rigid three-dimensional moieties. Another key issue to
be addressed is the tuning of the electron affinity of CPs to control
3
their work functions and charge transporting properties. Tradition-
ally, atoms with lone pair electrons capable of electron-donating
or electronegative groups connected directly to the π-system have
been utilized to modify their electron affinity. Although direct
attachment of such groups can produce large effects, steric repulsion
of bulky substituents at the phenylene or vinylene subunits often
induce deviations from planarity and decrease conjugation.4
We are interested in developing new CP designs that produce
high fluorescence quantum yields and also tune electron affinity.
Simultaneously, we seek architectures for the covalent attachment
of the CPs to peptides, nucleic acids, or antibodies for biosensor
a
(
a) NaBH4, 2-propanol, reflux. (b) dimethylacetylenedicaboxylate or
hexafluoroacetylene, xylene, 140 °C. (c) NBS, AIBN, CCl4, reflux. (d)
t
KO Bu, THF, rt.
Table 1. Summary of Molecular Weight and Photophysical Data
Abs λmax
GPC
(M
(nm)
(log ꢀ)
em λmax
(nm)
τ
(ns)
polymer
n
)
PDI
Φ
F
5
4
5
5
4a (THF)
1.2 × 10
6.8 × 10
7.9 × 10
2.5
401 (3.83)
401
403 (3.48)
405
413 (4.32)
414
473, 498
507
471, 497
506
469, 499
477, 511
0.58
0.42
0.86
0.43
0.76
0.61
1.16
0.75
0.62
applications that avoid potentially deleterious barriers or traps in
4
4
4
a (film)
b (THF)
b (film)
4d (THF)
d (film)
the electronic structure caused by conformational disorder. Herein,
we report novel CPs having three-dimensional structures that display
highly efficient solid-state fluorescence and demonstrate how groups
with hyperconjugation and inductive interactions can be used to
tune their electron affinity.
2.6
2.1
4
To perturb the electronic structure of CPs without interrupting
conjugation by adding steric bulk in the plane of polymer backbone,
we designed a new [2.2.2] bicyclic ring system that contains an
electron-deficient double bond that can interact with the polymer
(
N,N-dimethyl p-toluidine (DMT)) and electron-deficient (2,4-
dinitrotoluene (DNT)) aromatic compounds. All of thin films
displayed the largest quenching response (Figure 1) to DNT despite
-4
the fact that it has lower vapor pressure (1.47 × 10 mmHg) than
6
backbone in a hyperconjugative fashion (Scheme 1). Compounds
-1
DMT (1.78 × 10 mmHg). This result is likely due to the former’s
3a and 3b, which have ester or trifluoromethyl groups appended
strong π-acid character that favors association with electron-
to the alkene of the bicyclic ring system, were synthesized and
2
donating π-electron systems. As shown in Figure 1 the relative
quenching response of 4a, 4b, and 4d reflects the expected
t
then polymerized by reaction with excess KO Bu to give polymers
4
a and 4b (Scheme 1). Ester groups in polymer 4a included both
hyperconjugative and inductive effects with 4b being the most
oxidizing and 4d being the most reducing. Hence, 4b gives the
methyl and (30%) tert-butyl groups, with the latter being produced
by transesterification under the polymerization conditions. The
triptycene polymer 4d represents an electron-rich model polymer
for the comparison with relative electron-poor polymers 4a and
7
strongest relative response to DMT and the weakest relative
response to DNT. Correspondingly, 4d displays the opposite
behavior having a larger response relative to that of the other
polymers to DNT and a weaker relative response to DMT. Polymer
4a exhibits responses intermediate to those of 4b and 4d.
4b. The absorption and emission maxima of polymers 4a and 4b
are extremely similar (Table 1). The insensitivity of 4a’s spectra
to partial transesterfication with tert-butyloxy groups confirms that
the [2.2.2] system tolerates bulky groups without reducting
conjugation length. High fluorescence quantum yields were ob-
served for all of the polymers in THF solution and in thin films.
The latter feature is attributed to the greatly reduced interchain
interactions enforced by the three-dimensional frameworks.1
The effect of hyperconjugative perturbations on the sensory
properties was determined by investigating fluorescence quenching
responses of thin films with exposure to vapors of electron-rich
To further investigate the quenching behavior, we conducted
solution Stern-Volmer quenching studies and determined the rates
of static and dynamic quenching by performing steady-state and
8
time-resolved experiments (Figure 2). Static quenching, involving
a preformed complex, does not reduce the excited-state lifetime,
whereas dynamic quenching, resulting from diffusion, lowers the
lifetime.
The trends in the solution Stern-Volmer rate constants, sum-
marized in Table 2, contrast markedly to those from our thin film
452
9
J. AM. CHEM. SOC. 2004, 126, 452-453
10.1021/ja038472b CCC: $27.50 © 2004 American Chemical Society

C O MMU N I C A T I O N S
stability of polymers 4a, 4b, and 4d to conditions associated with
solid-phase peptide synthesis.10 Polymer 4c is best prepared from
4
a by treatment with aqueous acid and THF. Conjugated polymers
often exhibit reactivity with strong electrophiles such as trifluoro-
acetic acid (TFA); however, exposure of 4b and 4d in CH Cl
2
2
solutions of TFA or immersion of solids in neat TFA results in no
apparent reduction/modification of their emissions. Methylene
chloride solutions 4a are quenched with the addition of TFA;
however, its fluorescence was immediately and completely recov-
ered without any spectral shift after neutralization with pyridine.
In summary, we have synthesized three-dimensional polymers
that have novel structures and hyperconjugative/inductive electronic
perturbations. The differences in the solution and thin film sensory
responses of these polymers reveal the complexities associated with
any comparison of quenching sensitivities. This latter point can be
dramatically illustrated wherein comparisons of sensitivities of poly-
Figure 1. Stern-Volmer plots of polymers 4a (9), 4b (b), and 4d (×) in
spin-cast films with DMT (A) and DNT (B) vapor.
(
phenylene ethynylene)s and polysiloles to TNT have been found
11
2
to be the same order of magnitude in solution; however, thin films
of poly(phenylene ethynylene)s exhibit orders-of-magnitude more
sensitive responses.12
Acknowledgment. This research was supported by NASA and
by the U.S. Army through the Institute for Soldier Nanotechnolo-
gies, under Contract DAAD-19-02-D-0002 with the U.S. Army
Research Office.
Figure 2. Stern-Volmer plots of polymers 4a (9), 4b (b), and 4d (×)
with N,N-dimethyl p-toluidine (DMT) in THF. The lifetime measurements
are shown in the inset.
Table 2. Quenching Constants of Polymers 4a, 4b, and 4da
-
1
-1
(M-1 s-1
)
Supporting Information Available: Experimental procedures and
photophysical study data (PDF). This material is available free of charge
via the Internet at http://pubs.acs.org.
polymer
quencher
K
D
(M )
K
S
(M )
k
q
8
4
4
4
4
4
4
a
b
d
a
b
d
DMT
DMT
DMT
DNT
DNT
DNT
0.80
5.19
2.99
11.00
7.60
8.00
0.92 ( 0.58
2.49 ( 0.60
0.94 ( 0.67
86 ( 65
108 ( 93
25 ( 15
6.9 × 10
7.0 × 10
4.8 × 10
9.4 × 10
1.0 × 10
1.3 × 10
9
9
9
1
1
References
0
0
(
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See Supporting Information for details of experimental conditions. (KD,
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(
(
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(
(
6) There are many examples of hyperconjugative interactions in [2.2.2]
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4
s
(7) The quenching sensitivities to nitroaromatics of the polymers described
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(8) Lakowicz, J. R. Principles of Fluorescence Spectroscopy; Plenum Press:
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(
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(
10) Polymer 4d is completely insensitive to acid deprotection conditions typical
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electron-rich PPEs tend to undergo irreversible decomposition.
(
11) Sohn, H.; Sailor, M. J.; Magde, D. and Trogler, W. C. J. Am. Chem. Soc.
2
003, 125, 3821-3830
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Emerging sensor applications of CPs require conjugation to
biorecognition elements, and to this end we have tested the acid
9
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