C O M M U N I C A T I O N S
exciton migration via the Dexter mechanism is more prevalent than
Fo¨rster migration, Figure 3. A Dexter mechanism implies that the
main chain of the graft polymer does not prevent donor dyes from
having overlapping wave functions.
From the slope of the plot, the average exciton was calculated
to hop approximately 3800 times before being quenched.20 Utilizing
reported crystal structures for 1, the average molecular distance
(centroid-to-centroid of the oxadiazole ring) was found to be 9.7
Å.21 Assuming that the rate of energy transfer between donor and
acceptor is infinite,5 the lower limit for the diffusion length was
found to be 600 Å (diffusion length ) (distance between hops) ×
(number of hops)1/2). This diffusion length is twice that found
previously in graft polymer systems and a third of that reported
for crystalline films.16
In conclusion, luminescent chemosensors can be constructed of
small-molecule chromophores and still achieve amplification similar
to that found in conjugated polymer films. Signal amplification can
be achieved in both crystalline and donor-graft polymer films.
Crystalline films provide much better amplification, but create films
with very poor mechanical properties. Conversely, the donor-graft
polymer films have excellent mechanical properties, but provide
attenuated sensitivity. However, the sensitivity observed in the
donor-graft polymer films is still better than similar approaches
using conjugated polymers.18 Using the improved optical and
mechanical properties of the donor-graft polymer films, the energy
migration mechanism was investigated and, unexpectedly, appears
to be exciton hopping between chromophores. Energy hopping can
occur in the polymer films suggesting that grafted dyes could be
used as alternatives to conjugated polymers in some applications.
Acknowledgment. We thank Cornell University, the Dreyfus
Foundation, the Sokol Family, 3M, and NYSTAR, for support.
Figure 3. A plot of Q versus concentration ratio for a spin cast film in
which 6 is suspended in 5.
control due to inhomogeneous crystallization of the compounds onto
the quartz plate. The error associated with each measurement
prohibited a detailed investigation of the energy migration mech-
anism and prevents the films from being used as quantitative
indicators. To solve these problems, donor dyes were grafted to a
methacrylate polymer, 5, synthesized via a radical polymerization
of 4. Films of the grafted donor provided excellent optical properties
and were mechanically robust.
Mixtures of 5 and 2 were constructed and responded to TFA in
the same manner as the crystalline films, except that the amplifica-
tion observed was approximately 20-fold instead of 80-fold. Since
the absolute concentration of 3 cannot easily be measured, a
quantitative analysis of the energy-migration mechanism within
these films was impossible. To reduce the system’s complexity and
obviate the risk of 3 being deprotonated or not fully protonated, 2
was methylated with methyl iodide and ion-exchanged to create 6
for use as a model of 3. The spectral properties and films of 5 and
6 are identical to those of films of 5 and 3, but by using 6 a more
precise mixture can be created.
Twelve mixtures of 5:6 ranging from 10:1 to 10000:1 were spin
cast onto quartz plates, creating a solid solution of 6 dispersed in
5. The luminescence spectra indicated that energy migrated from
5 to 6. Each spectrum was multimodal, having bands resulting from
the emission of 5 and 6. Again the amplification was determined
by measuring the emission resulting from 6 when excited at 320
nm (excitation of the grafted dye) and 380 nm (direct excitation of
6). In the worst case, 10 000:1, an enhancement of only 4 was
observed, and in the best case, 300:1, the signal enhancement was
17. Nevertheless, as can be seen in Figure 2, amplification of 17 is
still significant and is better than any previously reported system
of this type.18
Supporting Information Available: Film preparation, synthesis,
X-ray data, tabulated data, and quantum yields (PDF). This material is
References
velop.htm
(2) Kenkre, V. M.; Parris, P. E.; Schmid, D. Phys. ReV. B 1985, 32, 4946.
(3) Guillet, J. Polymer Photophysics and Photochemistry; Cambridge Press:
New York, 1985.
(4) Turro, N. J. Modern Molecular Photochemistry; University Science
Books: Sausilito, CA, 1991.
(5) Dexter, D. L. J. Chem. Phys. 1953, 21, 836.
(6) Fo¨rster, Th. Faraday Soc. Discuss. 1959, 27, 7.
(7) Swager, T. M. Acc. Chem. Res. 1998, 31, 201.
(8) Swager, T. M.; Gil, C. J.; Wrighton, M. S. J. Phys. Chem. 1995, 99, 4887.
(9) McQuade, D. T.; Pullen, A. E.; Swager, T. M. Chem. ReV. 2000, 7, 2537.
(10) (a) Nguyen, T.-Q.; Wu, J.; Doan, V.; Schwartz, B. J.; Tolbert, S. H. J.
Am. Chem. Soc. 2000, 288, 652. (b) DiCerare, N.; Pinto, M. R.; Schanze,
K. S.; Lakowicz, J. R. Langmuir 2002, 18, 7785. (c) Gaylord, B. S.;
Heeger, A. J.; Bazan, C. C. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 10954.
(11) Cotts, P. M.; Swager, T. M.; Zhou, Q. Z. Macromolecules 1996, 29, 7323.
(12) Chen, L.; McBranch, D. W.; Hsing-Lin, W.; Helgeson, R.; Wudl, F.;
Whitten, D. G. Proc. Natl. Acad. Sci. U.S.A. 1999, 96, 12287.
(13) Wolf, H. C. In Organic Molecular Crystals; Reineker, P., Haken, H., Wolf,
H. C., Eds.; Springer-Verlag: Berlin, 1983; p 2.
To better understand the mechanism of energy transfer within
these films, each of the 12 spectra resulting from excitation of 5:6
mixtures were curve-fitted, and each band was integrated. These
integrations were used to calculate the quenching factor (Q; I-
(accep)/I(donor)‚(Φ(donor)/Φ(accep).)15 A plot of Q versus the con-
centration ratio of the acceptor provides evidence of the energy
migration mechanism. Energy migration for Dexter and Fo¨rster pro-
cesses has different dependences on donor/acceptor distances. For
a Dexter process, quenching decays linearly as the acceptor con-
centration decreases. Alternatively, quenching decays nonlinearly
for a Fo¨rster mechanism.15 For the system reported here, a plot of
Q versus the acceptor concentration ratio is linear, indicating that
(14) Klo¨pffer, W. J. Chem. Phys. 1969, 50, 1689.
(15) Pivovarov, A. P.; Kaplunov, M. G.; Yakushchenko, I. K.; Belov, M. Y.;
Nikolaeva, G. V.; Efimov, O. N. Russ. Chem. Bull. Int. Ed. 2002, 51, 67.
(16) Klo¨pffer, W. J. Chem. Phys. 1969, 50, 2337.
(17) Lamp intensity at 320 and 380 nm was controlled with a neutral density
filter.
(18) McQuade, D. T.; Hegedus, A. H.; Swager, T. M. J Am. Chem. Soc. 2000,
122, 12389.
(19) Q ) 0.66mC, where m ) number of hops and C ) concentration of
acceptor. The value m is equal to the slope from the plot in Figure 3
divided by 0.66. See refs 12 and 15 for detailed discussions.
(20) Kuznetsov, V. P.; Patsenker, L. D.; Lokshin, A. I.; Tolmachev, A. V.
Funct. Mater. 1996, 3, 460.
JA035218A
9
J. AM. CHEM. SOC. VOL. 125, NO. 37, 2003 11155