Monodisperse oligo(phenylene vinylene) ligands on CdSe quantum dots: Synthesis and polarization anisotropy measurements

Article abstract of DOI:10.1021/ja077409+

Well-defined oligo(phenylene vinylene) (OPV) molecules with phosphine oxide chain-ends were prepared and used for surface functionalization of CdSe quantum dots. Interestingly, OPV-covered CdSe nanostructures exhibit photoluminescence emission properties

Full text of DOI:10.1021/ja077409+

Published on Web 02/02/2008
Monodisperse Oligo(phenylene vinylene) Ligands on CdSe Quantum Dots:
Synthesis and Polarization Anisotropy Measurements
P. K. Sudeep, Kevin T. Early, Kevin D. McCarthy, Michael Y. Odoi, Michael D. Barnes,* and
Todd Emrick*
Polymer Science & Engineering Department and Department of Chemistry, UniVersity of Massachusetts,
Amherst, Massachusetts 01003
Received September 27, 2007; Revised Manuscript Received January 13, 2008; E-mail: mdbarnes@chemistry.umass.edu;
tsemrick@mail.pse.umass.edu
Scheme 1
Nanocomposites composed of π-conjugated polymers and semi-
conductor nanoparticles have attracted enormous recent interest in
the context of optoelectronic and photovoltaic devices because of
their combined optoelectronic properties and facile solution
processibility.1-4 Electroluminescent materials fabricated by casting
films from solutions of blended poly(phenylene vinylene) (PPV)
and CdSe quantum dots are promising examples of such nanocom-
posites, exhibiting low operating voltage and voltage-dependent
emission.1 However, nanoparticle-polymer blends typically suffer
from phase separation and nanoparticle aggregation, as well as the
presence of insulating surfactant ligands on the particle surface.
Both issues can be addressed effectively and simultaneously by
surface functionalizing the nanoparticles with the polymer of
interest, as seen in films of quantum dots functionalized with
polythiophene or PPV ligands.5-8 The improved and interesting
properties of CdSe-polymer nanocomposite materials prepared
through this surface functionalization design, relative to simply
blended materials, warrant further exploration of this concept.
In prior work, we prepared oligo(phenylene vinylene) (OPV)-
CdSe hybrid nanocomposites by polymerization of suitable OPV
precursors from arylbromide-functionalized quantum dots. This
“grafting-from” polymerization leads to a polydispersity of chain
lengths emanating from the nanoparticles. Thus, we undertook a
synthesis of well-defined OPV ligands for quantum dot surface
functionalization by “grafting-to” (i.e., ligand exchange). In prin-
ciple, this approach enables better tuning of OPV absorption and
emission properties to those of the quantum dots. Moreover, this
method refines the size distribution of a given nanostructure sample,
precluding size variation due to ligand length heterogeneity, to more
closely mirror the size distribution of the core nanoparticles. Here
we describe the synthesis and single nanoparticle spectroscopy of
such nanostructures, revealing a surprising correlated directionality
in absorption and emission moments from the quantum dot core,
suggesting new ways for these species to be used in optoelectronic
devices.
The OPV ligands for this study were prepared by the stepwise
process shown in Scheme 1, utilizing conditions reported by
Jorgensen and co-workers,9 but installing a di-n-octylphosphine
oxide group onto the OPV chain-end for coordination to the
nanoparticle surface. Diethylphosphonate ester-functionalized ben-
zaldehyde 3 was prepared by palladium-catalyzed Heck coupling
of phenylbromide 1 and vinylbenzene 2. Acetal protection of 3 gave
4, which was converted to 6 (denoted OPV-1) by Horner-
Wadsworth-Emmons (HWE) coupling with phosphine oxide-
functionalized benzaldehyde 5. The longer oligomer 7 (OPV-2)
was generated by coupling the deprotected version of 6 with
phosphonate 4. Both OPV-1 and OPV-2 were purified by column
chromatography, eluting with dichloromethane/methanol mixtures.
The HWE couplings to give OPV-1 and OPV-2 were achieved in
70 and 50% yields, respectively. Detailed syntheses and charac-
terization are given in the Supporting Information. Characterization
of OPV-1 and OPV-2 by 1H NMR spectroscopy revealed the
characteristic trans-vinylic signal at 6.97 ppm (J ) 16.2 Hz) and a
doublet from the methylene group attached to the phosphorus at
3.15 ppm. 31P NMR spectroscopy showed a single peak at 47 ppm.
Tri-n-octylphosphine oxide (TOPO)-covered CdSe quantum dots
were prepared according to literature methods,10 then subjected to
ligand exchange with OPV-1, going through pyridine-covered
particles (Scheme 1, bottom). Pyridine-covered quantum dots were
dispersed in a THF solution of OPV-1 and heated to 55 °C for 24
h under argon atmosphere. The OPV-1-covered quantum dots
obtained were precipitated repeatedly in chloroform/methanol
mixtures to remove unbound ligands. The solid-state emission
spectrum of thin films of OPV-1-covered CdSe (350 nm excitation)
showed emission primarily from the quantum dots, reflecting an
energy transfer from the OPV to the quantum dots, in accord with
our prior studies on CdSe nanoparticles covered with polydisperse
phenylene vinylenes.7 The excitation spectrum of OPV-1-CdSe,
recorded by collecting the emission of quantum dots (at 560 nm),
resembles the OPV absorption spectrum, further supporting the
proposed energy transfer process. Longer oligomers such as OPV-2
are desired for ligand coverage but, so far, give particles with low
solubility, possibly due to unfavorable sterics associated with using
these longer oligomers in the grafting-to approach.
9
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10.1021/ja077409+ CCC: $40.75 © 2008 American Chemical Society

C O M M U N I C A T I O N S
1C shows the transient for a single OPV-1-CdSe nanostructure
undergoing 4π rotations of E (lines and markers), along with a
sin2 θ fit (dashed line) taken over 50 s (angular integration ∼14°).
This transient shows a remarkable reproducibility in phase of the
polarization response, and >90% extinction of emission for µabs
orthogonal to E, making these nanoparticles attractive as potential
robust polarization-sensitive optical switches. We note that all the
CdSe-OPV-1 particles observed gave very deep modulation depth,
a result inconsistent with a random distribution of 2-D degenerate
dipole emitters, in which an appreciable fraction of dots would give
near zero modulation depth.
In summary, OPV-covered quantum dots were prepared and
characterized as single nanostructures. Defocused optical imaging
and excitation polarization anisotropy measurements separately
reveal a l-D character to both the absorption and emission dipole
moments, a novel effect in functionalized quantum dot systems.
While the origin of this effect is not clearly understood, we speculate
that the appearance of linear polarization in the QD emission is
associated with photoinduced charge transfer from the ligand to
the QD surface.18,19 The pinned excess surface charges act to break
the spherical symmetry and thus the 2-D degeneracy. While further
experiments are necessary to confirm this, we note that this effect
might be exploited to produce novel polarization modulation
devices.
Figure 1. (A) Defocused fluorescence image of OPV-CdSe nanostructure,
showing a spatial intensity pattern characteristic of a linear dipole oriented
in the x-y plane. (B) Schematic showing a polarized excitation geometry
of absorption moment (µabs), laser electric field (E), and separation angle
(θ). (C) PL intensity of a single CdSe-OPV nanostructure during 4π full
rotations of E (lines and markers), showing nearly 100% extinction for
perpendicular excitation, along with a sin2 θ fit (dashed line).
Acknowledgment. The authors acknowledge financial support
of the National Science Foundation (CHE-0239486 Materials
Research Science and Engineering Center, and Center for
Hierarchical Manufacturing) and the Department of Energy.
Interestingly, OPV-1-covered CdSe nanostructures exhibit ab-
sorption and emission properties characteristic of linear dipoles,
similar to those found in organic dyes11 and collapsed polymer
chains.12 Figure 1A shows the spatial photon distribution from a
single OPV-1-CdSe nanostructure under slight defocusing in a
high numerical aperture (NA) optical system (Inverted Nikon TE300
microscope, 1.4NA) excited in epi-illumination geometry using a
405 nm GaN diode laser. These images, along with aberration-
corrected numerical simulations (Supporting Information), provide
evidence of a highly linear electric dipole radiator.13 The dipole
images are filtered spectrally to exclude any photoluminescence
emission from the OPV ligands, such that the dipole emission
distribution originates only from the pseudo-spherical CdSe core.
A typical dipole image is shown in Figure 1A. Thus, it appears
that the presence of OPV ligands on the quantum dot surface results
in a one-dimensional emission dipole moment, in contrast with the
ordinarily observed 2-D degenerate emission moments.14,15 To rule
out a potential contributing dipole emission from high aspect ratio
particles (nanorods), which are known to exhibit such linear
emission geometries,16 transmission electron microscopy (TEM) was
performed, confirming the spherical nature of the particles following
ligand exchange with OPV-1.
To further probe the one-dimensional nature of this transition,
polarization anisotropy measurements were performed on single
OPV-covered nanostructures, similar to studies by Chung et al.14c
on quantum dots and Hu et al. on flexible MEH-PPV chains.17
Figure 1B depicts the experimental excitation geometry; the linearly
polarized electric field of the excitation laser E is rotated at a
constant rate in the x-y plane, thereby sweeping angles θ between
the absorption transition moment and E. For a linear absorber, this
results in a sin2 θ intensity variation during constant rotation. Figure
Supporting Information Available: Experimental procedures and
characterization of compounds in Scheme 1. This material is available
free of charge via the Internet at http://pubs.acs.org.
References
(1) Huynh, W. U.; Dittmer, J. J.; Alivisatos, A. P. Science 2002, 295, 2425.
(2) Colvin, V. L.; Schlamp, M. C.; Alivisatos, A. P. Nature 1994, 370, 354.
(3) Coe, S.; Woo, W. K.; Bawendi, M.; Bulovic, V. Nature 2002, 420, 800.
(4) Solomeshch, O.; Kigel, A.; Saschiuk, A.; Medvedev, V.; Aharoni, A.;
Razin, A.; Eichen, Y.; Banin, U.; Lifshitz, E.; Tesslerc, N. J. Appl. Phys.
2005, 98, 074310.
(5) Liu, J.; Tanaka, T.; Sivula, K.; Alivisatos, A. P.; Fre´chet, J. M. J. J. Am.
Chem. Soc. 2004, 126, 6550.
(6) Milliron, D. J.; Alivisatos, A. P.; Pitois, C.; Edder, C.; Frechet, J. M. J.
AdV. Mater. 2003, 15, 58.
(7) Odoi, M. Y.; Hammer, N. I.; Sill, K.; Emrick, T.; Barnes, M. D. J. Am.
Chem. Soc. 2006, 128, 3506.
(8) Hammer, N. I.; Early, K. T.; Sill, K.; Odoi, M. Y.; Emrick, T.; Barnes,
M. J. Phys. Chem. B 2006, 110, 14167.
(9) Jorgensen, M.; Krebs, F. C. J. Org. Chem. 2004, 69, 6688.
(10) Peng, Z. A.; Peng, X. G. J. Am. Chem. Soc. 2001, 123, 183.
(11) Bartko, A. P.; Dickson, R. M. J. Phys. Chem. B 1999, 103, 11237.
(12) Mehta, A.; Kumar, P.; Dadmun, M. D.; Zheng, J.; Dickson, R. M.;
Thundat, T.; Sumpter, B. G.; Barnes, M. D. Nano Lett. 2003, 3, 603.
(13) Hellen, E. H.; Axelrod, D. J. Opt. Soc. Am. 1987, 4, 337.
(14) (a) Brokmann, X.; Coolen, L.; Dahan, M.; Hermier, J. P. Appl. Phys. Lett.
2004, 93, 107403. (b) Patra, D.; Gregor, I.; Enderlein, J.; Sauer, M. Appl.
Phys. Lett. 2005, 87, 101103. (c) Chung, I. H.; Shimizu, K. T.; Bawendi,
M. G. Proc. Natl. Acad. Sci. U.S.A. 2003, 100, 405.
(15) Efros, A. L.; Rosen, M.; Kuno, M.; Nirmal, M.; Norris, D. J.; Bawendi,
K. Phys. ReV. B 1996, 54, 4843.
(16) Hu, J. T.; Li, L. S.; Yang, W. D.; Manna, L.; Wang, L. W.; Alivisatos,
A. P. Science 2001, 292, 2060.
(17) Hu, D. H.; Yu, J.; Wong, K.; Bagchi, B.; Rossky, P. J.; Barbara, P. F.
Nature 2000, 405, 1030.
(18) , Greenham, N. C.; Peng, X.; Alivisatos, A. P. Phys. ReV. B 1996, 54,
17628.
(19) Ginger, D. S.; Greenham, N. C. Phys. ReV. B 1999, 59, 10622.
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