Site-isolated luminescent europium complexes with polyester macroligands: Metal-centered heteroarm stars and nanoscale assemblies with labile block junctions

Article abstract of DOI:10.1021/ja0261269

The synthesis of a series of polymeric Eu(III) complexes with polyester ligands, along with supporting emission spectra, luminescence lifetimes, and, for a Eu block copolymer film, atomic force microscopy (AFM) data, is presented. Dibenzoylmethane was der

Full text of DOI:10.1021/ja0261269

Published on Web 06/25/2002
Site-Isolated Luminescent Europium Complexes with Polyester
Macroligands: Metal-Centered Heteroarm Stars and Nanoscale Assemblies
with Labile Block Junctions
Jessica L. Bender,† Perry S. Corbin,† Cassandra L. Fraser,*,† David H. Metcalf,†
Frederick S. Richardson,† Edwin L. Thomas,‡ and Augustine M. Urbas‡
Department of Chemistry, UniVersity of Virginia, McCormick Road, P.O. Box 400319,
CharlottesVille, Virginia 22904-4319, and Department of Materials Science and Engineering,
Massachusetts Institute of Technology, Cambridge, Massachusetts 02139
Received March 7, 2002
Luminescent block copolymers with selectively positioned
chromophores are prepared in a single step by chelation of
dibenzoylmethane and bipyridine macroligands to europium(III)
ions. Placement of metals at the block junction in the molecular
architecture implies localization of metals at microdomain bound-
aries upon self-assembly into hierarchically structured composites.
Many lanthanide complexes are noted for their luminescence arising
from f-f transitions generated by direct metal excitation, or more
commonly via the “antenna effect”, namely, energy transfer from
ligand excited states to the metal.1 Europium-based systems are of
special interest for optical excitation and emission studies because
they exhibit high luminescent quantum efficiencies, and the details
of Eu(III) (4f-4f) excitation and emission spectra and excited state
reaction kinetics are particularly sensitive to structural details of
the coordination environment. These features have been exploited
in biological contextsse.g., in time-resolved fluoroimmunoassays2
and structural probes1asand also make lanthanide chromophores
(Eu, red; Tb, green) the focus of organic light emitting diode
(OLED) materials for new display technologies.3 To facilitate
processing and to enhance properties, it is advantageous to
“package” luminophores in polymer shells.4 Metal site isolation in
polymer matrixes may prevent self-quenching and inhibit ready
access by small molecules such as water, which dissipates energy
nonradiatively through OH stretching modes. Self-assembly of
inorganic/organic composites, selective positioning of functionality
on small length scales, and the ability to respond to environmental
stimuli and transition between distinct states are attractive features
for advanced materials.5 A molecular design that shows much
promise for addressing these challenges is described.
One class of ligands that has figured prominently in both funda-
mental studies and applications development involving lanthanides
is the â-diketonates.6 Numerous derivatives are known and steric
and electronic properties are easily modified by modular syntheses.
Tris(dibenzoylmethane)Eu(III) and other â-diketonate complexes
are common7 and easily produced by reaction of the ligand with
EuCl3 in the presence of a base (e.g., Et3N). For extension to a
polymeric analogue, dibenzoylmethane was functionalized with a
hydroxyl site (dbmOH, 1) for generating a biocompatible poly-
(lactic acid) macroligand (dbmPLA, 2; Mn(NMR) ) 8.4 kDa;
Mw(MALLS) ) 7.0 kDa, PDI ) 1.12) by tin-catalyzed ring opening
polymerization (Figure 1).8 A Eu(III) complex, Eu(dbmPLA)3, 3,
was prepared by using a mixed solvent system to accommodate
both the metal salt and the polymer.
Figure 1. Dibenzoylmethane initiator 1 and PLA macroligand 2.
As a preliminary screen of sample homogeneity and lumines-
cence properties, unpolarized emission spectra (Figure 2a) and
lifetime data (Table 1) were recorded for Eu(dbmPLA)3 (3) and
Eu(dbm)3, for comparison. Analysis of the emission decay for these
species reveals both short and longer lifetime components. Given
the Lewis acidity of six-coordinate Eu(III) dbm complexes and the
variety of Lewis basic groups in the reaction medium, there are
many plausible explanations for the multiple species that are
observed. Residual solvent (H2O, CH3OH), hydroxyl groups on the
polymer chain ends, or other oxygen donors on the main chain
could serve as donor ligands (Figure 2a, X), increasing the
coordination number of certain Eu centers and perhaps shortening
their luminescence lifetimes. Interestingly, the relative weighting
of the longer lifetime component (Table 1, RW2) is greater for
polymeric complex 3 relative to Eu(dbm)3, and for films versus
solutions of 3. The previously noted polymer shell effect may
explain these observations.4
Because lanthanide tris(dbm) complexes readily form higher
coordinate adducts with additional donor ligands,7,9 they are an ideal
platform for metal template assisted block copolymer synthesis.
Provided that traditional coordination chemistry extends to poly-
meric ligands, when two different kinds of macroligands (e.g., dbm
and bpy10) are combined with lanthanide salts, heteroarm stars with
metals at the block junction can form in a single step. In films,
these systems are expected to form nanoscale assemblies with
metals at the microdomain interfaces, presenting new possibilities
for photonic materials in optical information processing.11 Moreover,
because Eu and other lanthanides are labile metals, facile ligand
dissociation can cause the block copolymer to fragment at the block
junction.12 For films, this implies a transition from microstructured
materials to macrophase-separated homopolymer blends (i.e., of
Eu(dbmA)3 and bpyB2) in the affected regions.
To test these ideas, namely, if metal-centered heteroarm star
block copolymers and their nanostructured films can be produced,
and whether changes at the molecular level can be transmitted to
assembly structures, the five-arm heteroarm star Eu(dbmPLA)3-
* Author to whom correspondence should be addressed. E-mail: fraser@
virginia.edu.
† University of Virginia.
‡ Massachusetts Institute of Technology.
9
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J. AM. CHEM. SOC. 2002, 124, 8526-8527
10.1021/ja0261269 CCC: $22.00 © 2002 American Chemical Society

C O M M U N I C A T I O N S
Figure 3. Representative phase contrast AFM image of a Eu(dbmPLA)3-
(bpyPCL2) block copolymer film (swelled with water vapor prior to analysis
to enhance contrast) with a schematic representation of the lamellar
morphology (A ) PCL, B ) PLA, b ) Eu center).
undergo microphase separation. Thin films of 4 were cast from
5% CH2Cl2 solution by slow evaporation on a mercury surface.
Phase contrast images show a lamellar microstructure with a period
of 17.5 nm (Figure 3). When ordered films of 4 were heated,
morphological changes consistent with thermally induced ligand
dissociation and fragmentation of the block copolymer at the block
junction were noted. More detailed investigation of this phenom-
enon, additional spectroscopic analyses, and further compositional
and architectural modification are underway.
Acknowledgment. We thank the National Science Foundation
(CHE-9733466, MIT Center for Materials Science and Engineering)
and the Petroleum Research Fund, administered by the ACS, for
partial support for this work. John E. McAlvin, Siddhartha R.
Shenoy, and Xufeng Wu are acknowledged for setting the stage
for this work.
Figure 2. Emission spectra for (a) Eu(dbmPLA)3 3 and (b) Eu(dbmPLA)3-
(bpyPCL2) 4 in CH2Cl2 solution (excitation λ: 465.8 nm).
Table 1. Luminescence Lifetimesa (τ1 and τ2) for Europium
Dibenzoylmethane Complexes and Their Bipyridine Adducts
Supporting Information Available: Experimental details for the
synthesis of Eu complexes, the measurement of emission spectra and
lifetimes for Eu complexes, and the AFM study of thin films of 4 (PDF).
This material is available free of charge via the Internet at http://
pubs.acs.org.
complex
τ1 (ms)
RW b (%)
τ2 (ms)
RW b (%)
1
2
Eu(dbm)3
0.021
0.108
0.119
0.102
0.111
86
52
13
100
100
0.302
0.349
0.371
14
48
87
Eu(dbmPLA)3 (3)
Eu(dbmPLA)3 c (3) film
Eu(dbm)3(bpy)
Eu(dbmPLA)3(bpyPCL2) (4)
References
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5
7
the D0 f F0 transition observed at 579.7 nm (Figure 2b). The
sharpness of this nondegenerate transition indicates that all the Eu
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for the dbmPLA/bpyPCL2 combination because they are comparable
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As further confirmation of adduct formation, AFM studies were
conducted to determine whether Eu(dbmPLA)3(bpyPCL2) films
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J. AM. CHEM. SOC. VOL. 124, NO. 29, 2002 8527