Photoluminescent europium-containing inner sphere conducting metallopolymer

Article abstract of DOI:10.1021/ja077626a

The oxidative electrochemical polymerization of a new Eu(III) complex that contains 3,4-ethylenedioxythenyl (EDOT) groups has been carried out, resulting in the formation of an inner sphere conducting metallopolymer. Preliminary photophysical measurements

Full text of DOI:10.1021/ja077626a

Published on Web 01/10/2008
Photoluminescent Europium-Containing Inner Sphere Conducting
Metallopolymer
Xiao-Yan Chen, Xiaoping Yang, and Bradley J. Holliday*
Department of Chemistry and Biochemistry and Center for Nano and Molecular Science and Technology,
The UniVersity of Texas at Austin, 1 UniVersity Station, A5300, Austin, Texas 78712-0165
Received October 3, 2007; E-mail: bholliday@cm.utexas.edu
Scheme 1. Electrochemical Polymerization of
Europium-Containing Monomer to Conducting Metallopolymer
The development of light-emitting materials for solid-state
lighting applications has been a topic of intense research activity
in recent years.1 Polymer light-emitting diodes (PLEDs), in
particular, have been studied extensively since the first fabrication
of a functioning diode utilizing a conjugated polymer as the elec-
troluminescent material in 1990.2 Compared to inorganic electro-
luminescent materials, polymer materials have several attractive
features including mechanical strength, flexibility, ease of process-
ing, and the potential to be of low cost.3
While many advances have been made, long-standing issues
including limited lifetime, low quantum efficiencies, and poor color
purity continue to plague many conjugated polymer materials/
devices. For example, high photoluminescence efficiencies (>50%)
often do not translate into high PLED electroluminescence efficien-
cies (<5%) due to formation of non-emissive triplet excitons.4
Additionally, obtaining high color purity from organic polymer
materials is difficult due to broad emission peaks.5 These drawbacks
of conjugated polymers have triggered efforts to use lanthanide ions
as the emissive materials in PLEDs.6 If successful, the energy of
the singlet and triplet excited states can be harvested to excited
states of the f orbitals of the lanthanide ions and subsequently
generate light. In theory, internal efficiencies of 100% should thus
be possible. In addition, lanthanide metal ions exhibit extremely
sharp emission bands due to the shielding of the 4f orbitals by the
5s and 5p shells.
Seminal work in this area has primarily focused on polymer
systems doped or blended with lanthanide complexes.6 However,
blending may not always give rise to uniformly dispersed and
thermodynamically stable compositions. A more effective, but much
less explored, approach involves the covalent coupling of a
lanthanide complex to a polymer backbone.7 This route rules out
phase separation or aggregation during device processing/operation.
Furthermore, energy transfer in a covalently bound system is
anticipated to be more efficient due to the close proximity of the
two components. Despite such encouraging premises, only non-
conducting polymers with the lanthanide complex linked as a
pendant unit have been explored.7 Conducting polymers have been
used in many device applications including electrical conductors,
nonlinear optical devices, sensors, batteries, and memory devices
and should therefore offer advantages in electroluminescent appli-
cations. Herein, we report the first example of a new class of
photoluminescent conducting metallopolymers with direct electronic
communication between the metal center and the polymer backbone.
Our approach to the development of solid-state light-emitting
materials involves the design and synthesis of well-defined
conducting metallopolymers that incorporate lanthanide complexes
in an inner sphere, or Wolf Type II, fashion (Scheme 1).8 As such,
we aim to take full advantage of the properties of both components
with high efficiency due to the direct electronic interface this
configuration creates. To accomplish this goal, the new ligand, 3,8-
bis(3,4-(ethylenedioxy)thien-2-yl)-1,10-phenanthroline (L) in which
the 1,10-phenanthroline (Phen) moiety serves as a metal binding
group and 3,4-(ethylenedioxy)thiophene (EDOT) as a polymerizable
group, was prepared. Additionally, we have chosen europium(III)
for our initial studies considering the ease of characterization the
pure red europium emission provides (∼615 nm) and the rich
literature base on this class of emissive metal complexes.9
The europium â-diketonate complex Eu(DBM)3(H2O)2, where
DBM ) dibenzoylmethanido, was covalently linked to the nitrogen
donors from the Phen moiety of L to form monomer 1 (Scheme
1). This complex has been fully characterized by 1H and 13C NMR
spectroscopy, mass spectrometry, combustion analysis, and single-
crystal X-ray diffraction (Figure 1). The molecular structure of 1
(Figure 1) is the first reported structure of a complex containing
both a lanthanide ion and EDOT electropolymerizable units. The
eight-coordinate Eu(III) ion lies at the center of a slightly distorted
square antiprism that is defined by the six oxygen atoms from the
DBM ligands and the two nitrogen atoms from L. The Eu-Nav
bond distance (2.577(4) Å) is longer than the Eu-Oav bond distance
(2.330(3) Å). This trend and these bond distances are consistent
with the data of the complex Eu(DBM)3Phen (Eu-Nav 2.656 Å;
Eu-Oav 2.359 Å).10
Monomer 1 has been electropolymerized to form poly-1 (Scheme
1) as an electrode-confined film onto a variety of working electrode
surfaces (Supporting Information). Cyclic voltammetry of 1 over
a window of +1.25 to -1.25 V (vs Fc/Fc+) resulted in the growth
of a polymer film that has a reversible wave with E1/2 ) -0.55 V.
The first scan exhibits two oxidative peaks at ipa of ∼0.4 and 1.0
V whose positions steadily become more positive with increasing
Figure 1. ORTEP diagram of 1 showing the labeling scheme of selected
atoms at 30% probability level. Hydrogen atoms, solvent molecules, and
phenyl rings of the dibenzoylmethanido ligands are omitted for clarity.
9
1546
J. AM. CHEM. SOC. 2008, 130, 1546-1547
10.1021/ja077626a CCC: $40.75 © 2008 American Chemical Society

C O M M U N I C A T I O N S
Figure 2D shows the photophysical profiles of a film of poly-1
deposited on ITO-coated glass. The absorption spectrum displays
a very broad redox-dependent band characteristic of the extended
aromatic system of a conducting metallopolymer structure which
is red shifted from that of L and 1. The emission spectrum measured
with 375 nm as the excitation wavelength is characteristic of
5
7
Eu(III) emission. The five expected components of the D0 f FJ
transitions are well-resolved and the hypersensitive 5D0 7F2
f
transition dominates the spectrum. This is indicative of a low-
symmetry coordination environment around the Eu(III). Most
5
7
notably, poly-1, shows exclusiVely the Eu(III)-centered D0 f FJ
emission bands, and the ligand emission from the organic backbone
completely disappears, presumably indicating more efficient energy
transfer. The excitation spectrum of poly-1 is less broad and blue
shifted relative to the absorption spectrum, suggesting that the
energy transfer takes place from a localized excited state. This is
consistent with energy transfer from the Phen portion of the polymer
backbone to the Eu(III).
In summary, we have demonstrated the synthesis of the first
photoluminescent lanthanide conducting metallopolymer. This
structure, which displays pure metal-based photoluminescence via
stimulated excitation, is assembled from a well-characterized
europium-containing monomer via controlled electropolymerization.
These results bode well for the development of high color purity
PLEDs from this new class of materials. Furthermore, this approach
represents a novel perspective on the use of luminescent conducting
metallopolymers for a wide range of light-emitting applications.
Figure 2. (A) Electrochemical scan rate dependence of poly-1 (Fc/Fc+ is
the redox couple of ferrocene). Inset: plot of linear current increase versus
scan rate. (B) Photophysical properties of L recorded in CH2Cl2 at room
temperature. (C) Photophysical properties of 1 recorded in CH2Cl2 at room
temperature. (D) Photophysical properties of poly-1 recorded as a thin film
on ITO coated glass at room temperature. UV ) absorbance profile, ex )
excitation profile, and em ) emission profile.
scans (Figure S1). The resulting electroactive polymer films were
characterized using electrochemical methods (Figure 2A), UV-
vis spectroscopy (Figure 2D), and X-ray photoelectron spectroscopy
(XPS). As shown in Figure 2A, the peak current of an electrode-
confined film of poly-1 in pure electrolyte solution varies linearly
with the rate of the electrochemical scan up to 500 mV/s. This
behavior is indicative of a strongly adsorbed electroactive thin film
which is not limited by the ionic flux of counteranions. The XPS
data were used to determine the film composition and metal
coordination environment (Supporting Information). The Eu 3d3/2
and Eu 3d5/2 peaks are observed at 1165.2 and 1135.2 eV,
respectively, corresponding well to the expected values for Eu(III)
bound to oxygen.11 The S 2p peak is also found at 164.3 eV.
Quantitative XPS analysis reveals that the film has an atomic ratio
of Eu:S ) 1:1.85, which is in agreement with the stoichiometric
molar ratio of the monomer (1:2.03 by XPS) and proposed film
structure.
Acknowledgment. We gratefully acknowledge the Welch
Foundation (F-1631), the PRF/ACS (47022-G3), the THECB (ARP
003658-0010-2006), the UT-CNM and UT-Austin for financial
support.
Supporting Information Available: Experimental details for the
synthesis and characterization of L, 1, and poly-1; electrochemical and
spectroscopic details; X-ray diffraction tables; and crystallographic data
for 1 (.cif). This material is available free of charge via the Internet at
http://pubs.acs.org.
References
(1) EmissiVe Materials: Nanomaterials; Advances in Polymer Science 199;
Springer: Berlin, 2006 and references therein.
(2) Burroughes, J. H.; Bradley, D. D. C.; Brown, A. R.; Marks, R. N.;
MacKay, K.; Friend, R. H.; Burn, P. L.; Holmes, A. B. Nature 1990,
347, 539.
(3) (a) Forrest, S. R. Nature 2004, 428, 911. (b) Gustafsson, G.; Cao, Y.;
Treasy, G. M.; Klavetter, F.; Colaneri, N.; Heeger, A. J. Nature 1992,
357, 477.
(4) Segal, M.; Baldo, M. A.; Holmes, R. J.; Forrest, S. R.; Soos, Z. G. Phys.
ReV. B 2003, 68, 075211/1.
(5) Kido, J.; Okamoto, Y. Chem. ReV. 2002, 102, 2357.
(6) (a) de Bettencourt-Dias, A. Dalton Trans. 2007, 2229. (b) Zhang, T.; Xu,
Z.; Qian, L.; Tao, D. L.; Teng, F.; Gao, X.; Xu, X. R. Chem. Phys. Lett.
2005, 415, 30. (c) Kang, T. S.; Harrison, B. S.; Foley, T. J.; Knefely, A.
S.; Boncella, J. M.; Reynolds, J. R.; Schanze, K. S. AdV. Mater. 2003,
15, 1093. (d) Kang, T. S.; Harrison, B. S.; Bouguettaya, M.; Foley, T. J.;
Boncella, J. M.; Schanze, K. S.; Reynolds, J. R. AdV. Funct. Mater. 2003,
13, 205. (e) McGehee, M. D.; Bergstedt, T.; Zhang, C.; Saab, A. P.;
O’Regan, M. B.; Bazan, G. C.; Srdanov, V. I.; Heeger, A. J. AdV. Mater.
1999, 11, 1349.
(7) (a) Ling, Q.; Yang, M.; Wu, Z.; Zhang, X.; Wang, L.; Zhang, W. Polymer
2001, 42, 4605. (b) Lenarets, P.; Storms, A.; Mullens, J.; D’Haen, J.;
Go¨rller-Walrand, C.; Binnemans, K.; Driesen, K. Chem. Mater. 2005, 17,
5194.
The photophysical properties of L, 1, and poly-1 have been
studied under a variety of conditions (Figure 2B-D). The absorption
spectrum of L (Figure 2B) displays a broad band with some vibronic
structure from 325 to 400 nm (λmax ) 365), which is red shifted
compared to Phen (λmax ) 290 nm) due to the extended conjugation.
Furthermore, the emission and excitation spectra are consistent with
fluorescence emission (Φfl ) 0.40) from the direct population of
the singlet excited state. The absorption maximum of 1 at 353 nm
and the shoulder around 405 nm are attributed to the combination
of the singlet-singlet π-π* transitions of the DBM and L ligands
(Figure 2C). Upon selective excitation at 420 nm, the emission
profile of 1 displays two distinct features: a broad band centered
at 460 nm, residual fluorescence from the coordinated ligands, and
the characteristic sharp peaks associated with the 5D0
f
7F0-4
transitions of the Eu(III) ion between 575 and 700 nm (Figure 2C).
The excitation spectrum of 1 was recorded while monitoring the
emission intensity at 610 nm. The observed transitions correspond
to the absorption profile of the combined π-π* transitions of the
ligands at 405 nm thus confirming that energy transfer takes place
from the ligands to the Eu(III) ion. The overall luminescent quantum
yields for the emission from the Eu(III) ion and the coordinated
ligands in 1 in CH2Cl2 are 0.11 and 0.21, respectively.
(8) (a) Wolf, M. O. J. Inorg. Organomet. Polym. Mater. 2006, 16, 189. (b)
Holliday, B. J.; Swager, T. M. Chem. Commun. 2005, 23.
(9) Modern Aspects of Rate Earths and Their Complexes; Sastri, V. S., Bu¨nzli,
J.-C., Rao, V. R., Rayudu, G. V. S., Perumareddi, J. R., Eds.; Elsevier:
Amsterdam, 2003.
(10) Ahmed, M. O.; Liao, J. L.; Chen, X.; Chen, S. A.; Kaldis, J. H. Acta
Crystallogr. 2003, E59, m29.
(11) Mercier, F.; Alliot, C.; Bion, L.; Thromat, N.; Toulhoat, P. J. Electron
Spectrosc. Relat. Phenom. 2006, 150, 21.
JA077626A
9
J. AM. CHEM. SOC. VOL. 130, NO. 5, 2008 1547