Europium complexes of a novel ethylenedioxythiophene-derivatized bis(pyrazolyl)pyridine ligand exhibiting efficient lanthanide sensitization

Article abstract of DOI:10.1021/ic902481y

A new class of highly luminescent nine-coordinated europium(III) tris(β-diketonate) bis[(ethylenedioxythiophene)pyrazolyl]pyridine (L) complexes has been synthesized and the photophysical properties studied: 1 =Eu(hfac)₃(L); 2=Eu(tta)₃(L); 3=Eu(btfac)₃(L). The solid-state structure of complex 1 has been determined by single-crystal X-ray crystallography and shows the geometry of the local coordination environment around the Eu^III ion to be a slightly distorted tricapped trigonal prism. Luminescence lifetimes were found to be 581, 473, and 576 μs for complexes 1-3, respectively. Absolute quantum yields for complexes 1-3 were measured as 16.4 ± 1.4%, 27.5 ± 1.2%, and 22.2% ± 0.3%, respectively.

Full text of DOI:10.1021/ic902481y

Inorg. Chem. 2010, 49, 2035–2037 2035
DOI: 10.1021/ic902481y
Europium Complexes of a Novel Ethylenedioxythiophene-Derivatized
Bis(pyrazolyl)pyridine Ligand Exhibiting Efficient Lanthanide Sensitization
Julie M. Stanley, Xunjin Zhu, Xiaoping Yang, and Bradley J. Holliday*
Department of Chemistry and Biochemistry and Center for Electrochemistry,
The University of Texas at Austin, 1 University Station, A5300, Austin, Texas 78712-0165
Received December 14, 2009
A new class of highly luminescent nine-coordinated europium(III)
tris(β-diketonate) bis[(ethylenedioxythiophene)pyrazolyl]pyridine
(L) complexes has been synthesized and the photophysical
properties studied: 1=Eu(hfac)3(L); 2=Eu(tta)3(L); 3=Eu(btfac)3-
(L). The solid-state structure of complex 1 has been determined by
single-crystal X-ray crystallography and shows the geometry of the
local coordination environment around the EuIII ion to be a slightly
distorted tricapped trigonal prism. Luminescence lifetimes were
found to be 581, 473, and 576 μs for complexes 1-3, respectively.
Absolute quantum yields for complexes 1-3 were measured as
16.4 ( 1.4%, 27.5 ( 1.2%, and 22.2% ( 0.3%, respectively.
that maximize the efficiency of the energy-transfer process,
which is crucial in the development of systems that result in
high quantum yields from lanthanide-centered emission. For
this to occur, the energy of the excited state of the ligand must
be greater than, but of similar energy to (within 2000 cm-1),
that of the lanthanide excited state.7 Other important factors
in accomplishing efficient energy transfer include limitation
of the nonradiative energy loss and maximization of the
population of the triplet excited state of the sensitizing
ligand.5
Seminal work by de Bettencourt-Dias on the functionali-
zation of the sensitizing ligands with thiophene and thio-
phene derivatives7-10 has demonstrated a decrease in the
energy level of the triplet state of the ligand and an increase in
the luminescence quantum yield when these groups are
present. These effects are attributed to the increased conjuga-
tion of the system and the enhanced spin-orbit coupling
due to the presence of sulfur atoms, known as the heavy-atom
effect.5,11 The net result is an improved match between the
ligand triplet excited state and the excited state of the
lanthanide ion and an increase in the population of the ligand
triplet excited state, which results in a more efficient energy-
transfer process and higher quantum yields of lanthanide
luminescence.
Luminescentlanthanide complexes have interesting photo-
physical properties that have been utilized in areas such as
biological probes and sensors, fluoroimmunoassays, and
emitting layers in polymer light-emitting diodes.1-4 Shielding
of the f orbitals by the 5s2 and 5p6 shells results in narrow
linelike emissions of optically pure colors and long radiative
lifetimes. However, the f-f transitions that result in light
emission from the lanthanide are spin- and parity-forbidden,
thus requiring the use of antenna molecules to indirectly
excite the metal center.5
This indirect excitation, also known as the antenna effect,
takes advantage of coordinated ligands that transfer energy
from ligand-centered excited states to the metal center,
resulting in lanthanide ion luminescence. In 1942, Weiss-
mann observed that the use of organic ligands in europium
complexes increased the luminescence intensity from the
lanthanide ion when the complexes were irradiated with
ultraviolet (UV) light.6 Ideal ligands, also referred to as
sensitizers, include multidentate, aromatic chromophores
For our systems, we developed and synthesized a new class
of luminescent compounds consisting of europium(III) tris-
(β-diketonate) 2,6-bis[4-(3,4-ethylenedioxythien-2-yl)pyrazol-
1-yl]pyridine (L) complexes in which the bis[(3,4-ethylene-
dioxythiophene)pyrazolyl]pyridine moiety serves as the metal
binding site. The organic backbone is an aromatic, tris-
chelating ligand bound to the lanthanide ion through the
nitrogen atoms of the pyridine and pyrazole rings. The
inclusion of three anionic β-diketonate ligands brings the
coordination number of the trivalent europium complex to
*To whom correspondence should be addressed. E-mail: bholliday@
cm.utexas.edu.
(7) de Bettencourt-Dias, A. Inorg. Chem. 2005, 44, 2734–2741.
(8) de Bettencourt-Dias, A.; Viswanathan, S.; Ruddy, K. Cryst. Growth
Des. 2005, 5, 1477–1483.
(9) Viswanathan, S.; de Bettencourt-Dias, A. Inorg. Chem. 2006, 45,
10138–10146.
(10) de Bettencourt-Dias, A.; Viswanathan, S.; Rollett, A. J. Am. Chem.
Soc. 2007, 129, 15436–15437.
(11) Sager, W. F.; Filipescu, N.; Serafin, F. A. J. Phys. Chem. 1965, 69,
1092–1100.
(1) Lanthanide Probes in Life, Chemical and Earth Sciences Theory and
;
Practice; B€unzli, J.-C. G., Choppin, G. R., Eds.; Elsevier: Amsterdam, The
Netherlands, 1989.
(2) Faulkner, S.; Natrajan, L. S.; Perry, W. S.; Sykes, D. Dalton Trans.
2009, 3890–3899.
(3) de Bettencourt-Dias, A. Dalton Trans. 2007, 2229–2241.
€
(4) Bunzli, J.-C. G. Chem. Lett. 2009, 38, 104–109.
€
(5) Bunzli, J.-C. G.; Piguet, C. Chem. Soc. Rev. 2005, 34, 1048–1077.
(6) Weissmann, S. I. J. Chem. Phys. 1942, 10, 214–217.
r
2010 American Chemical Society
Published on Web 01/29/2010
pubs.acs.org/IC

2036 Inorganic Chemistry, Vol. 49, No. 5, 2010
Scheme 1. Synthesis of 1-3
Stanley et al.
nine. This architecture provides a rigid environment around
the europium ion, helping to limit nonradiative deactivation
pathways and exclude solvent interactions that can lead to
the quenching of lanthanide luminescence.5 The exclusion of
water is extremely important for lanthanide complexes be-
cause luminescence from the excited state is easily quenched
by the harmonics of the O-H oscillator.5 Our approach
involves the coupling of 3,4-ethylenedioxythiophene (EDOT)
to bis(pyrazolyl)pyridine in order to optimize the match be-
tween the ligand triplet excited state and the excited state of
the EuIII ion by taking advantage of the extended conjugation
resulting from the addition of the thiophene derivative and
the increased electron richness of EDOT over that of thio-
phene alone. In addition, the sulfur atoms present in EDOT
induce greater spin-orbit coupling because of the heavy-
atom effect, resulting in a higher population of the ligand
triplet state and a more efficient energy transfer to the metal
center. Herein we report the synthesis, characterization,
and photophysical properties of three nine-coordinated
trivalent europium complexes: Eu(hfac)3(L) (1; hfac =
1,1,1,5,5,5-hexafluoro-2,4-pentanedionate); Eu(tta)3(L) (2;
tta = 2-thenoyltrifluoroacetonate); Eu(btfac)3(L) (3; btfac =
4,4,4-trifluoro-1-phenyl-1,3-butanedionate).
Figure 1. (A) ORTEP diagram of 1 showing the labeling scheme of
selected atoms at a 30% probability level. Hydrogen atoms are omitted
˚
for clarity. Selected bond lengths (A) and angles (deg) are as follows:
Eu1-O6 2.362(3), Eu1-O5b 2.395(3), Eu1-O5a 2.411(3), Eu1-O6b
2.412(3), Eu1-O6a 2.423(3), Eu1-O5 2.481(3), Eu1-N1 2.559(4), Eu1-
N5 2.563(4), Eu1-N3 2.630(4); N1-Eu1-N5 118.74(12), N1-Eu1-N3
61.21(11), N5-Eu1-N3 60.71(12), O6-Eu1-N1 145.89(11), O5b-
Eu1-N1 68.98(11), O5a-Eu1-N1 70.70(11), O6b-Eu1-N1 138.34(11),
O6a-Eu1-N1 107.05(12), O5-Eu1-N1 81.91(11), O6-Eu1-N5
75.40(12), O5b-Eu1-N5 107.05(12), O5a-Eu1-N5 139.69(12),
O6b-Eu1-N5 66.62(12), O6a-Eu1-N5 131.17(12), O5-Eu1-N5
73.47(12), O6-Eu1-N3 134.29(11), O5b-Eu1-N3 70.13(12), O5a-
Eu1-N3 127.07(11), O6b-Eu1-N3 97.08(11), O6a-Eu1-N3 144.87(11),
O5-Eu1-N3 82.92(11). (B) Coordination environment around EuIII in 1.
The title ligand, L, was synthesized by Stille coupling of
2,6-bis(4-iodopyrazol-1-yl)pyridine and 2-(tributylstannyl)-
3,4-ethylenedioxythiophene in the presence of Pd(PPh3)2Cl2
1
and was characterized by melting point, H NMR, electro-
spray ionization mass spectrometry (ESI-MS), and elemental
analysis. Complexes 1-3 were prepared by reacting L and the
appropriate europium(III) tris(β-diketonate) dihydrate in
toluene (Scheme 1) and were characterized by melting point,
ESI-MS, and elemental analysis.
The solid-state structure of 1 was confirmed by single-
crystal X-ray crystallography (Figure 1A).12 The geometry
around the nine-coordinated EuIII ion is a slightly distorted
tricapped trigonal prism in which the metal ion lies at the
center (Figure 1B). It is defined by six oxygen atoms from the
β-diketonate ligands and three nitrogen atoms from L. The
average EuIII-N and EuIII-O bond distances are 2.600 and
Table 1. Photophysical Properties of L and Europium(III) Complexes 1-3 in
CH2Cl2 at Room Temperature
compound
λex (nm)
Φfl (%)
ΦEuIII (%)
τ (μs)
L
1
2
3
339
332
343
330
29.2 ( 1.3
6.1 ( 0.7
0.8 ( 0.3
1.4 ( 0.4
16.4 ( 1.4
27.5 ( 1.2
22.2 ( 0.3
581
473
576
˚
˚
2.412 A, respectively, and fall within the ranges [2.52-2.68 A
III
III
˚
(Eu -N) and 2.31-2.50 A (Eu -O)] reported for similar
(Figure 2A), the absorption spectrum closely follows the
excitation spectrum and displays a broad band (λmax=339 nm)
that is red-shifted compared to that of 2,6-bis(pyrazol-1-
yl)pyridine (λmax = 304 nm) because of the extended conju-
gation caused by the presence of the EDOT moieties. The
emission spectrum displays a broad band (λmax = 378 nm)
corresponding to ligand fluorescence, confirmed by a small
Stokes shift and a short excited-state lifetime. Upon cooling
to 77 K in a solvent glass, an increase in the emission intensity
and the presence of a previously absent vibrational structure
can be seen. These observations are attributed to the reduc-
tion of thermal nonradiative pathways available for energy
loss. Ligand phosphorescence is seen (Figure 2B) from 484 to
650 nm, and the energy of the ligand triplet state is measured
as 20 700 cm-1, which is 3700 cm-1 greater than that of the
structures.13-16
The photophysical properties of L have been studied in
solution and under a variety of conditions, the details of which
have been summarized in Table 1. At room temperature
(12) Crystal data for 1: C38H20EuF18N5O10S2, M = 1264.67, triclinic,
˚
˚
˚
space group P1, a = 13.175(3) A, b = 13.625(3) A, c = 14.527(3) A, R =
3
˚
68.77(3)°, β = 78.73(2)°, γ = 68.60(2)°, V = 2257.1(8) A , Z = 2, Dcalcd
=
1.861 g cm-3, μ=1.618 mm-1, T=153(2) K, R1=0.0418, R2=0.1014.
(13) Holz, R. C.; Thompson, L. C. Inorg. Chem. 1988, 27, 4640–4644.
(14) De Silva, C. R.; Wang, R.; Zheng, Z. Polyhedron 2006, 25, 3449–
3455.
(15) Li, X.-L.; Dai, F.-R.; Zhang, L.-Y.; Zhu, Y.-M.; Peng, Q.; Chen,
Z.-N. Organometallics 2007, 26, 4483–4490.
ꢀ
(16) Ziessel, R.; Diring, S.; Kadjane, P.; Charbonniere, L.; Retailleau, P.;
Philouze, C. Chem. Asian J. 2007, 2, 975–982.

Communication
Inorganic Chemistry, Vol. 49, No. 5, 2010 2037
there is only one local symmetry environment around the
EuIII ion present.19 Absolute quantum yields of L and
complexes 1-3 were measured using an integrating sphere
(Table 1). We speculate that the observed trends in Φ and
τ over the series of 1-3 are due to a combination of
interrelated effects (e.g., cooperative heavy atom effect,
relative energy levels, ligand ε) but dominated by the relative
β-diketonate and EuIII excited-state energies. The trend of
longer excited-state lifetime and lower absolute quantum
yield with the larger energy gap (ΔE) between the β-diketo-
nate and the EuIII ion is borne out in 1-3 [ΔE (Â103 cm-1):
1(hfac)=5.0, 2(tta)=3.5, 3(btfac)=4.5].20,21 The observed
quantum yields fall within the ranges of thiophene-contain-
ing EuIII complexes previously reported: 0.1-3.6%,7 0.9-
3.1%,9 and 76.2%.10
In summary, we have demonstrated the design, synthesis,
and characterization of a new class of highly luminescent
lanthanide complexes that utilize a 2,6-bis(pyrazol-1-yl)-
pyridine ligand functionalized with 3,4-ethylenedioxythio-
phene in conjunction with β-diketones to indirectly excite the
europium metal center. These complexes show high quantum
yields and long radiative lifetimes, supporting the general
hypothesis that incorporation of thiophene, and thiophene
derivatives, into sensitizing ligands promotes the enhance-
ment of energy-transfer pathways that lead to lanthanide ion
radiative emission. This is important to the expansion of the
utilization of these and other similar complexes in biomedical
and materials science applications. Previous work has demon-
strated that similar compounds may be used to make thin
films via electropolymerization.22 Future efforts will be
focused in similar directions.
Figure 2. (A) Excitation, emission, and absorption spectra of L at room
temperature in CH2Cl2. (B) Excitation, emission, and absorption spectra
of L at 77 K in 2-methyltetrahydrofuran. (C) Excitation and absorption
spectra of 1-3 at room temperature in CH2Cl2. (D) Emission spectra of
1-3 at room temperature in CH2Cl2.
EuIII 5D0 excited state (17 000 cm-1).17 Although the energy
difference is slightly higher than the optimal value previously
reported,7,18 the probability of an efficient energy transfer
from the ligand to the metal center is still high.
The photoluminescent properties of complexes 1-3 have
been studied and are summarized in Table 1. Each of the
complexes is highly luminescent as a solid and in solution,
emitting a bright-red color when irradiated with a hand-held
UV lamp (λex = 365 nm). The absorption spectra of com-
plexes 1-3 are similar to that of the free ligand (L), with the
minor differences being attributed to contributions from the
bound β-diketonate ligands (Figure 2C). Through monitor-
ing of the emission intensity at 612 nm with varied excitation
wavelengths, the excitation spectrum was recorded. The
excitation profile of each complex closely mimics that of its
corresponding absorption spectrum, demonstrating that en-
ergy transfer occurs from L and the β-diketonate ligands to
the EuIII ion. The emission profiles of complexes 1-3 display
a broad band from 350 to 500 nm due to residual ligand
fluorescence and characteristic sharp peaks of 5D0 f 7F0-4
transitions of the EuIII ion from 575 to 725 nm (Figure 2D).
The symmetry of the coordination environment around the
EuIII ion is low in solution, which is consistent with the X-ray
crystal structure and confirmed by the domination of the
Acknowledgment. We gratefully acknowledge the Robert
A. Welch Foundation (Grant F-1631), the Petroleum
Research Fund administered by the American Chemical
Society (47022-G3), the National Science Foundation
(Grants CHE-0639239, CHE-0741973, and CHE-0847763),
the American Heart Association (Grant 0765078Y), the
Center for Nano & Molecular Science & Technology at
The University of Texas at Austin for financial support of
this research.
Supporting Information Available: Experimental details for
the syntheses of L and 1-3, details of photophysical measure-
ments, X-ray diffraction tables, and crystallographic data for 1
in CIF format. This material is available free of charge via the
Internet at http://pubs.acs.org.
spectra by the hypersensitive 5D0
f
7F2 transition. The
absence of splitting in the 5D0 f 7F0 transition indicates that
(20) Klink, S. I.; Hebbink, G. A.; Grave, L.; Oude Alink, P. G. B.;
van Veggel, F. C. J. M.; Werts, M. H. V. J. Phys. Chem. A 2002, 106, 3681–
3689.
€
(17) Lanaerts, P.; Gorller-Walrand, C.; Binnemans, K. J. Lumin. 2006,
(21) Lenaerts, P.; Driesen, K.; Van Deun, R.; Binnemans, K. Chem.
Mater. 2005, 17, 2148–2154.
(22) Chen, X.-Y.; Yang, X.; Holliday, B. J. J. Am. Chem. Soc. 2008, 130,
1546–1547.
117, 163–169.
(18) Meshkova, S. B.; Topilova, Z. M.; Bol’shoi, D. V.; Nazarenko, N. A.
J. Appl. Spectrosc. 2000, 67, 893–897.
(19) Carlos, L. D.; Videira, A. L. L. Phys. Rev. B 1994, 49, 11721–11728.