Syntheses, structures and photophysical properties of heterotrinuclear Zn2Ln clusters (Ln = Nd, Eu, Tb, Er, Yb)

Article abstract of DOI:10.1039/c000783h

Heterotrinuclear Zn₂Ln (Ln = Nd 2, Eu 3, Tb 4, Er 5, Yb 6) clusters [(Znq₂)₂](μ-CH₃COO){Ln(hfac) ₂} (q = 8-hydroxylquinolinate, hfac = hexafluoroacetylacetonate) have been synthesized. The Zn₂Ln framework is ligated by two q ligands featuring μ-phenoxo and two q ligands featuring μ₃-phenoxo coordination modes, and one μ-CH₃COO^- anions. Since the short intramolecular separations of Zn...Ln (ca. 3.354-3.373 A) allow energy transfer from Znq₂-based sensitizers to the Ln ^III centres through two energy transfer pathways, the lanthanide luminescence is indeed lighted up by excitation of the Znq ₂-based chromopores. Photophysical measurements revealed that these Zn₂Ln complexes exhibit the so-called dual emission originating from both Znq₂-based luminophores and lanthanide emitters. By virtue of the dual luminescence with complementary colours, the Znq₂-based cyan emission and Eu^III-centred red luminescence are combined to generate a white-light emission in the Zn ₂Eu (3) complex.

Full text of DOI:10.1039/c000783h

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Syntheses, structures and photophysical properties of heterotrinuclear Zn2Ln
clusters (Ln = Nd, Eu, Tb, Er, Yb)† ‡
Hai-Bing Xu,*a,b Ye-Teng Zhong,a Wei-Xiong Zhang,a Zhong-Ning Chen*b and Xiao-Ming Chen*a
Received 12th January 2010, Accepted 23rd April 2010
First published as an Advance Article on the web 20th May 2010
DOI: 10.1039/c000783h
Heterotrinuclear Zn2Ln (Ln = Nd 2, Eu 3, Tb 4, Er 5, Yb 6) clusters [(Znq2)2](m-CH3COO){Ln(hfac)2}
(q = 8-hydroxylquinolinate, hfac = hexafluoroacetylacetonate) have been synthesized. The Zn2Ln
framework is ligated by two q ligands featuring m-phenoxo and two q ligands featuring m3-phenoxo
coordination modes, and one m-CH3COO- anions. Since the short intramolecular separations of
III
˚
Zn ◊ ◊ ◊ Ln (ca. 3.354–3.373 A) allow energy transfer from Znq2-based sensitizers to the Ln centres
through two energy transfer pathways, the lanthanide luminescence is indeed “lighted up” by excitation
of the Znq2-based chromopores. Photophysical measurements revealed that these Zn2Ln complexes
exhibit the so-called “dual emission” originating from both Znq2-based luminophores and lanthanide
emitters. By virtue of the dual luminescence with complementary colours, the Znq2-based cyan
emission and EuIII-centred red luminescence are combined to generate a white-light emission in the
Zn2Eu (3) complex.
diketonates are ideal candidates as light-harvesting chromophores
Introduction
for sensitization of visible and NIR luminescence from LnIII
ions.1c,8 Generally, complexation of b-diketonates to LnIII ions
Due to lanthanides’ unique electronic structures, optical proper-
ties, and magnetic properties, well-designed lanthanide complexes
have been widely used in magnetic resonance image, biological
assays, light-emitting diodes, telecommunications, optical fibers,
light amplifiers, lasers, etc.1–3 However, as the lanthanide f–
f transitions are Laporte forbidden, inducing low absorption
coefficients, the photophysical properties of lanthanide ions are
markedly dependent on their environment.4 To achieve efficient
emissions, adjacent strongly absorbing chromophores as light
harvesters are often employed to populate the lanthanide excited
states.5,6 Such chromophores could also protect the LnIII centre
from solvent molecules which can quench lanthanide emissions
and lifetimes.7
gives the corresponding neutral, unsaturated tris complexes [Ln(b-
diketonate)3], for LnIII ions commonly prefer high coordination
numbers (8 or 9). Thus only diketonate ligands affording a neutral
complex can not protect LnIII ions from solvent molecules. More-
over, Ln(hfac)3(solv)n units are almost non-emissive if irradiated
with an excitation wavelength above 350 nm.9–11 Therefore, other
well designed chromophores should take part in the coordination
to form more stable complexes.
8-Hydroxyquinolines (Hq) and their derivatives are important
ligands for luminescent metal complexes. These complexes often
exhibit efficient luminescence that can be adjusted in colour
from blue to the infrared region by modification of the ligands
or incorporation of other adequate metal ions.12 It has been
demonstrated that 8-hydroxyquinolines and their derivatives are
the most promising reagents for NIR-emitting lanthanides,13–16
because of their potential use as active components in optical
amplifiers or organic light-emitting diodes (OLEDs) operating at
telecom wavelengths.17 Meanwhile, tetrameric [(Znq2)4] (1) and its
derivatives are one class of the most promising candidates as elec-
tric luminescence (EL) materials.18 It has been shown that a lower
operating voltage was observed on the (Znq2)4-based OLEDs com-
pared with that of tri(8-hydroxyquinolinato)aluminium(III) (Alq3),
due to the efficient electron transport through the molecules.
Actually, the device with [(Znq2)4] as the EL material has a
luminance up to 16200 cd m-2.18b In contrast, these derivatives have
sparsely been used as antenna chromophores or energy donors for
sensitization of lanthanide luminescence.
Usually, b-diketonate ligands such as hexafluoroacetylacetone
(Hhfac) are used to act as an antenna to effectively transfer light
energy to the metal for sensitized lanthanide emission, because
they exhibit intense absorptions in the UV-vis region. Therefore, b-
aMOE Laboratory of Bioinorganic and Synthetic Chemistry/State Key Lab-
oratory of Optoelectronic Materials and Technologies, School of Chemistry
and Chemical Engineering, Sun Yat-Sen University, Guangzhou, 510275,
China. E-mail: haibingxu@fjirsm.ac.cn, cxm@mail.sysu.edu.cn
bState Key Laboratory of Structural Chemistry, Fujian Institute of Research
on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian,
350002, China
† Electronic supplementary information (ESI) available: The chromaticity
coordinates of 3 once irradiated with 360 nm in dichloromethane at
298 K. CCDC reference numbers 752291, 752292, 752293 for compounds
2–4, respectively. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c000783h
Among the heterometallic d–f compounds reported to date,
Ir(dfppy)2 antenna chromophores were adopted to populate the
excited state of the EuIII ion,19 which emits at a sufficiently high
energy to act as a donor for sensitizing EuIII in the complex.
Interestingly, the final Eu-centered red emission combines with
the residual broad blue emission of the Ir(dfppy)2 chromophore
‡ Crystal
Data
for
2·0.5CH2Cl2·0.25C6H14
(CCDC752291):
C50H33.50ClF12N4NdO10Zn2, Mr = 1388.74, monoclinic, space group
˚
˚
˚
C2/c, a = 34.7227(18) A, b = 18.4700(10) A, c = 24.1252(13) A, b =
◦
3
-1
˚
125.0233(8) , V = 12670.4(12) A , Z = 8, rc = 1.456 g cm , m(Mo-Ka) =
1.687 mm-1, T = 298(2) K, 32525 reflections collected, 8827 unique (Rint
=
0.0276), R1 = 0.0551, wR2 = 0.1519 for 12444 reflections with I > 2s(I),
GOF = 1.027
5676 | Dalton Trans., 2010, 39, 5676–5682
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to give an almost white-light emission. This fact inspires us to
take the advantage of the “dual emission” for developing new
functional materials. If there is unfavourable spectral overlapping
between the emission band of the donor chromophore and
lanthanide(III) absorption bands of f–f transitions, only a partial
energy transfer procedure can be processed from the excited
antenna chromophore to the LnIII-based emitter. Therefore, two
simultaneously emissive excited states would be operative in this
system at room temperature,20 being particularly suitable for
generating white-light emission by combining both the donated-
based cyan-light emission and the sensitized EuIII-centered red-
light emission.21
We recently reported bis(2-methyl-8-hydroxylquinoline)-
aluminium(III) (Al(Mq)2) chromophores as light-harvesting
antenna sensitizer for achieving sensitized lanthanide
luminescence.21 Herein we report the crystal structures and
properties of a new family of lanthanide complexes formed with
Hhfac and Znq2-based metalloligand.18 Upon coordination, these
complexes demonstrate several superior luminescence properties,
including highly efficient transfer of electronic excitation from
the ligand to metal centre and sufficient protection of the
lanthanide ion against nonradiative deactivation from the
environment. Therefore, heteronuclear complexes [(Znq2)2](m-
CH3COO){Ln(hfac)2} (Ln = Nd 2, Eu 3, Tb 4, Er 5, Yb 6)
were synthesized and characterized, which exhibit sensitized
lanthanide emissions, and white-light emission properties for 3.
Fig. 1 Perspective drawing of 2 with atom-labeling scheme (poppy, Nd;
green, Zn; red, O; blue, N; gray, C). The solvent molecules, H atoms and
F atoms on the trifluoromethyl are omitted for clarity.
and the bond angle of Zn–O–C associated with the m3-phenoxo
coordination mode are elongated and larger compared with those
associated with the m-phenoxo coordination mode, respectively.
˚
The separations of Zn1 ◊ ◊ ◊ Nd1 and Zn2 ◊ ◊ ◊ Nd1 are 3.3540(7) A
˚
and 3.3735(6) A, respectively. Because of steric requirements, the
Syntheses and characterisation
two Znq2 units in 2 are oriented at opposite directions. The
˚
Zn1 ◊ ◊ ◊ Zn2 distance is ca. 3.241 A. The triangular Zn2Nd core
The precursor 1 was synthesized by reaction of Hq and
ZnCl2·6H2O in ethanol, which was isolated by precipitation
after neutralization with triethylamine and washed with water,
ethanol, and diethyl ether, then purified by recrystallization.
The heterometallic Zn2Ln clusters were prepared by reaction of
Ln(hfac)2(CH3COO)(H2O)2 with 1 in dichloromethane solution.
Crystallization by layering n-hexane onto the corresponding
dichloromethane solutions afforded Zn2Ln complexes as yellow
or yellowish-green crystals or microcrystals.
Selected bond lengths and angles of 2–4 (CCDC 752291,
752292, and 752293) are presented in Table S1 (see ESI†). The sto-
ichiometries of 2–4 have been confirmed by X-ray crystallography
to be [(Znq2)2](m-CH3COO){Ln(hfac)2} (Fig. 1). Compounds 2–4
are isomorphous, and only the crystal structure of 2 is described
here. In 2, the NdIII ion is eight-coordinated to oxygen atoms
with two from the hydroxy groups of q featuring the m-phenoxo
coordination mode, two from the hydroxy groups of q featuring
the m3-phenoxo coordination mode, two from hfac to furnish a
distorted square-antiprism. Each of the two Zn(II) ions adopts
an octahedral coordination environment with two chelating q,
one m3-phenoxo of q and one m-CH3COO- anions. In 2, there
are two kinds of q. One features a m-phenoxo coordination mode
in which the bond lengths of O1–C8, N1–Zn1 and O1–Nd1 are
is bicapped at opposite sides by two m3-phenoxo oxygen atoms
into a trigonal bipyramid.
Photophysical properties
The UV-vis absorption data of 1, 2, and Ln(hfac)2(CH3COO)-
(H2O)2 are presented in Fig. 2. In dichloromethane solution, 1
displays two intense absorption bands at ca. 250 and 367 nm,
which are most likely to be the intraligand transition of q ligands.18a
For Nd(hfac)2(CH3COO)(H2O)2, there is only a single intense
absorption band at ca. 300 nm assigned as intra-ligand p–p* (hfac)
transitions.9–11 In the electronic spectra of 2, there are three sets
˚
˚
˚
ca. 1.324(6) A, 2.071(5) A and 2.363(3) A, respectively, and the
bond angles of Zn1–O1–C8 is 111.6(4)◦. The other one features
a m3-phenoxo coordination mode in which the bond lengths
˚
˚
of O2–C17, N2–Zn1, O2–Nd1 are 1.337(5) A, 2.104(4) A and
˚
2.503(3) A, respectively, and the bond angles of Zn1–O2–C17
and Zn1–O2–Zn2 are 113.2(3)◦ and 93.59(11)◦, respectively. It is
interesting to note that the bond lengths of C–O, N–Zn, O–Nd
Fig. 2 The UV-vis absorption spectra of 1 (short dash), 2 (solid) and
Nd(hfac)2(CH3COO)(H2O)2 (short dot) in dichloromethane.
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Table 1 Absorption and emission data of compounds 1–6 at 298 K
l
abs /nm
Compounds Medium (e/dm3 mol-1 cm-1)
l
em/nm (tem/ms) Uem (%)
1
2
Solid
480 (18.5 ns)
482 (9.3 ns)
CH2Cl2 250 (155200), 367
(16800)
Solid
14.9b
0.4b
486 (9.4 ns)a
1061 (15.1 ms)
490 (8.9 ns)a
CH2Cl2 253 (137400), 314
(28700), 367 (12500)
1061 (9.8 ms)
490 (2.2 ns)a
612 (436.5 ms)
486 (8.5 ns)a
3
4
5
6
Solid
CH2Cl2 253 (127000), 313
(25600), 367 (11400)
1.3c
612 (602.3 ms)
496 (3.2 ns)a
543 (601.5 ms)
496 (7.6 ns)a
Fig. 3 Normalized emission spectra of 1 (black), 2 (green), 3 (red), and 4
(magenta), 5 (blue) and 6 (cyan) in dichloromethane solutions at 298 K.
Solid
CH2Cl2 253 (139700), 313
(29100), 367 (12700)
543 (135.6 ms)
490 (8.0 ns)a
1535 (weak)
490 (3.7 ns)a
in microsecond ranges in both solid state and dichloromethane
solution at room temperature, which is much longer than those of
Ln(hfac)3(H2O)2 and Lnq3.14,15b,22,23 This fact can be attributed to
the fully exclusion of solvent molecules coordination to LnIII ions,
which are the most important lanthanide luminescent quenchers,24
by the eight-coordination of diketonate and 8-hydroxyquinoline
ligands.
Solid
CH2Cl2 253 (141000), 313
(30900), 365 (13100)
0.3b
1535 (weak)
490 (8.0 ns)a
980 (23.8 ms)
490 (4 ns)a
Solid
CH2Cl2 253 (139500), 313
(24400), 365 (11700)
2.7b
1.1d
The lifetime of residual ZnII chromophore-based emission is
8.9 0.1 ns for Zn2Nd (2), 3.7 0.1 ns for Zn2Er (5) and 4.0
0.1 ns for Zn2Yb (6) in dichloromethane at ambient temperature
(Table 1). The Zn-bound aryl chromophores to Ln(III) energy-
transfer rates kET can be estimated by the equation kET = 1/t -
1/t0, where t is the ‘partially quenched’ lifetime in the presence of
an energy-acceptor, and t0 (10.5 0.1 ns) is the ‘unquenched’
lifetime of the reference Zn2Gd complex which lacks energy
transfer from the ZnII-based antenna triplet states.25 The energy
980 (22.6 ms)
All data collected in solution are measured at the concentration of
about 5 ¥ 10-5 M.a The residual emission and lifetime due to Znq2-based
antenna chromophore. b The quantum yield of 1 and the residual Znq2-
based emission of 2–6 were determined relative to that of Fluorescein
(Uem = 0.79) in H2O with excitation at 400 nm. c The quantum yields
of 3 in degassed dichloromethane were determined relative to that of
Ru(bpy)3(PF6)2 (Uem = 0.062) in degassed acetonitrile. d The quantum
yields of Yb complexes in dichloromethane solutions are estimated by the
equation ULn = tobs/t0, in which tobs is the observed emission lifetime and
t0 is the radiative or ‘natural’ lifetime with t0 = 2 ms for YbIII. These values
refer to the lanthanide-based emission process only and take no account
of the efficiency of intersystem crossing and energy transfer processes.
transfer rates (kET) can thus be calculated as kET = 1/tZnNd
-
1/tZnGd = 1.71 ¥ 107 s-1 for Zn2Nd (2), 1.75 ¥ 108 s-1 for Zn2Er (5),
and 1.55¥ 108 s-1 for Zn2Yb (6) species.
It is interesting to note that the energy transfer from Zn-bound
aryl chromophores to Yb(III) is faster than to Nd(III). So, we need
to discuss the mechanism of energy transfer from the chromophore
to the metal centre. Yb(III) could be sensitised by a redox-based
mechanism that does not require donor/acceptor overlap, but this
only works if the excited state of the Zn-bound donor is a good
electron acceptor.26,27 A combination of a cyclic voltammogram
(Fig. S2, ESI†), and the information of the excited state energy
(Fig. S1, ESI†) could clarify this issue.
of absorption bands, one is from the intra-ligand transition of q
ligands at ca. 253 nm; the second is from hfac at ca. 314 nm; the
third is from the intra-ligand transition of q centered at ca. 367 nm
trailing to 420 nm.
Upon irradiation at lex ª 400 nm, 1 emits cyan luminescence at
ca. 480 nm in both the solid state and dichloromethane solution
(Table 1). The room-temperature luminescence of 1 is ascribed
to the intra-ligand [p–p* (q)] ILCT character.18a Upon excitation
at 300 nm < lex < 420 nm, centered at 400 nm, complexes 2, 5
and 6 exhibit luminescence characteristics of the corresponding
NdIII, ErIII, YbIII ions, respectively. For 3 and 4 (Ln = Eu and
Tb), severely spectral overlapping (Fig. 3) between unquenched
Znq2-based emission and lanthanide-centered emission excluded
the possibility to estimate accurately the radiative lifetime tr, and
the luminescence quantum yield Uem. As the excited-state energy
of 1 is about 21000 cm-1 (Fig. S1, ESI†), which is around that of
the emissive excited state of Tb(III) at the 5D4 level. It means that
energy transfer from the excited-state energy of the Znq2 units to
Tb(III) is inefficient in the Zn2Tb complex (Fig. 3), for there exists
fast thermally activated back energy-transfer to the Znq2 centres.
The emissive lifetimes (Table 1) of lanthanide luminescence are
The driving force -DG for the forward energy transfer may be
2
estimated with the equation DGET = (Eox - Ered) - Es - e0 /ea,28
where Eox is the oxidation potential of the Znq2 donor (1.13 V),
Ered is the reduction potential of the acceptor (Yb3+; -0.88 V for
Zn2Yb in CH2Cl2), Es is the singlet state energy of the donor group
(2.6 eV from the shortest wavelength emission maximum at 77 K),
2
and e0 /ea represents the attraction between the resulting radical
2
ion pair. With reference to the exciplex e0 /ea ª 0.15 eV,27 it serves
as an upper limiting condition in our case. Since the driving force
DGET = (1.13 + 0.88) -2.6 + 0.15 is about -0.44 eV, it means that
the electron redistribution mechanism for energy transfer is likely
to be feasible, the results above demonstrate that Yb(III) sensitised
by a redox-based mechanism is operative in 6.
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It is well known that the Zn2+ ion with d10 configuration would
generally not be involved in the electron transfer process and
contribute to sensitization of the lanthanide centres.20b,29 Indeed,
the emission of Znq2-based choromophore originates from the
electronic transitions in the quinolinolate ligands, and the ZnII
ions have a role of improving the efficiency of the compound.
There are two q ligands chelated with each ZnII ion, therefore,
the intramolecular separations of ZnII ◊ ◊ ◊ LnIII could be viewed as
the energy transfer distance from the Znq2-based sensitizers to
the lanthanide centres. Furthermore, the emission spectra in the
UV-vis region of 1 resembled that of 3, but were different to that
of the deprotonated 8-hydroxylquinolinate (q-) (Fig. 4). There is
no doubt that it is the Znq2-based chromophores, rather than
the deprotonated 8-hydroxylquinolinate ligands that transfer the
energy to the lanthanide centres in these Zn2Ln clusters.
from the Znq2-based chromophores to the lanthanide centres in
these clusters.
The white-light emission test for 3 was also measured at
ambient temperature with irradiation at 365 nm in a dark
environment (Fig. 5). In line with the two precursors (1 and
Eu(hfac)2(CH3COO)(H2O)2), three samples are dissolved in aer-
ated dichloromethane solution at low concentration. In our
experiment, a Znq2-based chromophore with emission to 480 nm
was selected as the metalloligand sensitizes for lanthanide lumi-
nescence, ensuring this emission to be one of the complementary
emission bands to that of the EuIII emitter at ca. 610 nm.
As a partial transfer of energy is operative from the excited
Znq2-centered luminophores to the EuIII-based ones, due to less
spectral overlapping between the emission spectra of Znq2-based
luminophores and the EuIII absorption bands of f–f transitions, the
residual Znq2-based cyan light emission at ca. 500 nm and Eu(III)-
chromophores red light emission at ca. 610 nm are combined
to generate a white-light upon irradiation at lex ª 365 nm. The
quantum yield of the white-light emission is about 1.3% and the
chromaticity coordinates is CIE = (0.19, 0.14), falling within the
white region of the International Commission on Illumination
(CIE) 1931 colour space chromaticity diagram (Fig. S4, see
ESI†).30 Obviously, it is the partial transfer of energy from the
excited Znq2-centered luminophores to the Eu-based ones that
make it practicable to combine the complementary light emissions
for white light emission.
Fig. 4 Emission spectra of q- (dash), 1 (short dot) and 3 (solid line) in
dichloromethane solutions at 298 K by excitation at 300 nm.
As the precursor Ln(hfac)2(CH3COO)(H2O)2 is non-emissive at
excitation lex > 350 nm,9–11 the lanthanide emissions in Zn2Ln
complexes should be sensitized by energy transfer from Znq2-
based light-harvesting chromophores with near-UV irradiation at
350 nm < lex < 440 nm. It is noteworthy that the intermolecular p–
p stacking of q ligands leads to the red shift of the corresponding
absorption and excitation spectra, thus the lanthanide emission
in Zn2Ln complexes can be achieved with excitation wavelengths
longer than 350 nm (Fig. S3, ESI†).
Fig.
Eu(hfac)2(CH3COO)(H2O)2 (short dot) and luminescence images
of (cyan), (white), and Eu(hfac)2(CH3COO)(H2O)2 (red) in
dichloromethane solutions with irradiation at 365 nm at ambient
temperature.
5
Emission spectra of
1
(short dash),
3
(solid), and
The intramolecular ZnII ◊ ◊ ◊ NdIII distances (< 3.4 A) are short
˚
1
3
enough, the donor and acceptor components are connected by the
conjugated q ligands directly, lanthanide-based luminescence are
sensitized (Fig. 3) by the Znq2-based choromophore excited state,
and the calculations of donor/acceptor spectroscopic overlap for
Fo¨rster energy transfer (3.6 ¥ 10-14 cm3 M-1) is likely to be negligible
compared with the electronic coupling for Dexter energy transfer
(3.9 ¥ 10-4 cm).25 Therefore, the energy transfer from Zn-bound
aryl chromophores is likely to be operative through Dexter energy
transfer except for 6.
On the other hand, the Znq2-based luminescence is still observed
at ca. 500 nm although it is significantly weakened compared with
that in the precursor 1. As shown in Table 1, the quantum yields
of the Znq2-based emission (2.7–0.3%) in Zn2Ln complexes are
much lower compared with that in 1 (14.9%). This fact further
suggests that substantial energy transfer is most likely operating
Conclusions
A new family of heterometallic Zn2Ln clusters (Ln = Nd 2,
Eu 3, Tb 4, Er 5, Yb 6) have easily been synthesized by
reaction of the [(Znq2)4] and LnIII precursors in CH2Cl2–n-hexane.
These trinuclear complexes [(Znq2)2](m-CH3COO){Ln(hfac)2} are
featuring with both m-phenoxo and m3-phenoxo coordination
modes of the bridging q ligands. From the short intramolecular
˚
separations of Zn(II) ◊ ◊ ◊ Nd(III) (ca. 3.35–3.37 A), two Zn-bound
aryl chromophores to Ln(III) energy-transfer pathways are opera-
tive in Zn2Ln simultaneously through the Dexter energy transfer
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mechanism, except that the electron redistribution mechanism is
operative for 6. The suitable separations afford ways to optimize
the energy transfer process. It is the partial transfer of energy
from the excited Znq2-centered luminophores to the lanthanide
emitter that makes it a reality to generate white-light emission
for the Zn2Eu complex by combination of the residual Znq2-based
emission at ca. 500 nm and the EuIII-chromophores emission at ca.
610 nm. Meanwhile, on irradiating these complexes, the excited
Znq2-based moieties in the Zn2Ln complexes transfer energy
to the lanthanide chromophores and induce the corresponding
lanthanide emission, respectively.
3
(Ln
=
Eu). Anal. Calcd for C48H29F12N4Zn2EuO10·
0.5CH2Cl2·0.25C6H14: C, 43.00; H, 2.42; N, 4.01. Found: C, 43.24;
-1
=
H, 2.35; N, 3.92. IR (KBr, cm ):1661s (C O). Yield: 75%.
4
(Ln
=
Tb). Anal. Calcd for C28H29F12N4Zn2TbO10·
0.5CH2Cl2·0.25C6H14: C, 42.79; H, 2.41; N, 3.99. Found: C, 43.09;
-1
=
H, 2.56; N, 3.79. IR (KBr, cm ): 1664 s (C O). Yield: 72%.
5
(Ln
=
Er). Anal. Calcd for C48H29F12N4Zn2ErO10·
0.5CH2Cl2·0.25C6H14: C, 42.54; H, 2.39; N, 3.97. Found: C, 41.99;
-1
=
H, 2.07; N, 3.82. IR (KBr, cm ): 1661 s (C O). Yield: 70%.
6
(Ln
=
Yb). Anal. Calcd for C48H29F12N4Zn2YbO10·
0.5CH2Cl2·0.25C6H14: C, 42.36; H, 2.38; N, 3.95. Found: C, 42.54;
-1
=
H, 2.55, N, 3.87. IR (KBr, cm ): 1662 s (C O). Yield: 62%.
Experimental section
Physical measurements
General procedures
Elemental analyses (C, H, N) were carried out on a Perkin-Elmer
model 240C automatic instrument. Electrospray mass spectra
(ES-MS) were recorded on a Finnigan LCQ mass spectrometer
using dichloromethane–methanol as the mobile phase. UV-vis
absorption spectra were measured on a Perkin-Elmer Lambda
25 UV-Vis spectrometer. Infrared spectra were recorded on a
Magna 750 FT–IR spectrophotometer with KBr pellets. 1HNMR
spectra were measured on a Bruker Avance III (400 MHz)
spectrometer using SiMe4 as the internal reference. Emission
and excitation spectra in the UV-vis region were recorded on
a Perkin-Elmer LS 55 luminescence spectrometer with a red-
sensitive photomultiplier type R928. The emission quantum
yields (U) of 3 in degassed dichloromethane solutions at room
temperature were calculated by Us = Ur(Br/Bs)(ns/nr)2(Ds/Dr)
All manipulations were performed under dry argon atmosphere
using Schlenk techniques and vacuum-line systems. The solvents
were dried, distilled and degassed prior to use except those for
spectroscopic measurements which were of spectroscopic grade.
Zinc chloride hexahydrate, 8-hydroxyquinoline (Hq), hexafluo-
roacetylacetone (Hhfac) were commercially available.
Ln(hfac)2(CH3COO)(H2O)2 (Ln = Nd, Eu, Tb, Er, Yb) were
prepared by modification of the literature procedures31,32 as follows
(see ESI†): Ln2O3 were dissolved in CH3COOH, after filtration,
the filtrates were evacuated to afford Ln(CH3COO)3. To an
aqueous solution of lanthanide(III) acetates (pH = 5–7) were added
dropwise 2.2 equiv of Hhfac with stirring at room temperature for
3 h. The precipitates were filtered, washed with an excess of water,
and dried under vacuum to afford the products without further
purification.
using [Ru(bpy)3](PF6)2 in acetonitrile as the standard (Uem
=
0.062); while the quantum yield of 1 and the residual Znq2-based
emission of 2–6 were determined relative to that of Fluorescein
(Uem = 0.79) in H2O. In the above-mentioned formulae, the
subscripts r and s denote reference standard and sample solutions,
respectively; and n, D and U are the refractive index of the
solvents, the integrated intensity and the luminescence quantum
[(Znq2)4] (1). A mixture of the ligand (220 mg, 1.5 mmol) and
ZnCl2·6H2O (122 mg, 0.5 mmol) in degassed ethanol (20 mL) was
refluxed for 3 h. The mixture was cooled to room temperature
and neutralized using triethylamine. Water (70 mL) was added
and the precipitated complex was filtered off. The filter cake was
washed thoroughly with water, ethanol, and diethyl ether, and
dried under vacuum to provide the desired product, which was
then purified by recrystallization with CH2Cl2 and n-hexane. Yield:
76%. Anal. Calcd. for C72H48N8O8Zn4·2CH2Cl2·H2O: C, 55.46; H,
3.40;N, 6.99. Found:C, 55.55;H, 4.09;N, 7.25. ESI–MS(CH3OH–
DMSO): m/z (%) 1437 ([M+Na]+). 1H NMR (400 MHz, DMSO):
8.60 (s, 8H, q), 8.35 (d, 8H, J = 8.0 Hz; q), 7.50 (s, 8H, q), 7.34
(s, 8H, q), 6.90 (d, 16H, J = 7.2 Hz; q). 13C NMR (DMSO, ppm):
161.7(s, 8C, q), 145.7 (s, 8C, q), 140.1(s, 8C,q), 139.2 (s, 8C, q),
130.2(s, 8C, q), 129.8 (s, 8C, q), 121.9 (s, 8C, q), 112.9 (s, 8C, q),
110.5 (s, 8C, q).
yield, respectively.33,34 The quantity B is calculated by B = 1–10-AL
,
where A is the absorbance at the excitation wavelength and L is
the optical path length. All the solutions used for determination
of emission lifetimes and quantum yields were prepared under
vacuum in a 10 cm3 round-bottom flask equipped with a side arm
1 cm fluorescence cuvette and sealed from the atmosphere by a
quick-release Teflon stopper. The solutions used for luminescence
determination were prepared after rigorous removal of oxygen
by three successive freeze–pump–thaw cycles. Calculations of the
energy transfer rate constants for Fo¨rster and Dexter mechanisms
and the relevant spectroscopic overlap integrals were performed
on PhotoChemCAD version 2.1 with the orientation factor k2
taken as 2/3 for statistical reasons.25 The cyclic voltammogram
(CV) was made with a potentiostat/galvanostat model 263A in
dichloromethane solutions containing 0.1 M (Bu4N)(PF6) as the
supporting electrolyte. The CV was performed at a scan rate of 100
mV s-1. Platinum and glassy graphite were used as the counter and
working electrodes, respectively, and the potential was measured
against a Ag/AgCl reference electrode.
[(Znq2)2](l2-CH3COO){Ln(hfac)2} (Ln = Nd, Eu, Tb, Er, Yb).
These Zn2Ln complexes were prepared by addition of 2 equiv
of Ln(hfac)2(CH3COO)(H2O)2 (Ln = Nd, Eu, Tb, Er, Yb) to 1
equiv of 1 in dichloromethane solutions with stirring for one hour.
After filtration, the concentrated dichloromethane solutions were
layered with n-hexane to afford the products as yellow crystals,
the yields were estimated based on the precursor 1.
Crystal structural determination
2
(Ln
= Nd). Anal. Calcd for C48H29F12N4Zn2NdO10·
0.5CH2Cl2·0.25C6H14: C,43.24; H, 2.43; N, 4.03. Found: C, 43.04;
X-ray diffractions of 2, 3 were performed on a Bruker Smart Apex
CCD diffractometer while 4 was performed on a Rigaku Mercury
-1
=
H, 2.51; N, 4.24. IR (KBr, cm ): 1660 s (C O). Yield: 78%.
5680 | Dalton Trans., 2010, 39, 5676–5682
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CCD diffractometer by the w scan technique at room temperature
11 T. K. Ronson, T. Lazarides, H. Adams, S. J. A. Pope, D. Sykes, S.
Faulkner, S. J. Coles, M. B. Hursthouse, W. Clegg, R. W. Harrington
and M. D. Ward, Chem.–Eur. J., 2006, 12, 9299.
12 M. Albrecht, M. Fiege and O. Osetska, Coord. Chem. Rev., 2008, 252,
812.
˚
using graphite-monochromated Mo-Ka (l = 0.71073 A) radia-
tion. Lp corrections were carried out in the reflection reduction
process. The structures were solved by direct methods and
successive difference Fourier syntheses. The hydrogen atoms were
generated geometrically with isotropic thermal parameters. The
structures were refined on F2 by full-matrix least-squares methods
using the SHELXTL program package.35 The refinements were
13 (a) D. Imbert, S. Comby, A.-S. Chauvin and J.-C. G. Bu¨nzli, Chem.
Commun., 2005, 1432; (b) S. Comby, D. Imbert, A.-S. Chauvin and
J.-C. G. Bu¨nzli, Inorg. Chem., 2006, 45, 732; (c) M. Albrecht, R.
Fro¨hlich, J.-C. G. Bu¨nzli, A. Aebischer, F. Gumy and J. Hamacek,
J. Am. Chem. Soc., 2007, 129, 14178; (d) N. M. Shavaleev, R. Scopelliti,
F. Gumy and J.-C. G. Bu¨nzli, Inorg. Chem., 2008, 47, 9055; (e) N. M.
Shavaleev, R. Scopelliti, F. Gumy and J.-C. G. Bu¨nzli, Inorg. Chem.,
2009, 48, 2908.
14 (a) R. Van Deun, P. Fias, P. Nockemann, A. Schepers, T. N. Parac-
Vogt, K. Van Hecke, L. van Meervelt and K. Binnemans, Inorg. Chem.,
2004, 43, 8461; (b) F. Artizzu, P. Deplano, L. Marchio`, M. L. Mercuri,
L. Pilia, A. Serpe, F. Quochi, R. Orru`, F. Cordella, F. Meinardi,
R. Tubino, A. Mura and G. Bongiovanni, Inorg. Chem., 2005, 44,
840.
˚
carried out by fixing the C–F distances (1.33
0.02 A) with
the occupancy factors of F1/F12 and F1b/F12b being 0.50,
respectively. Crystallographic data and the selected bond lengths
and bond angles of compounds 2–4 are listed in Table S1 and
Table S2 (see ESI†).
15 (a) G. B. Deacon, C. M. Forsyth, P. C. Junk, U. Kynast, G. Meyer, J.
Moore, J. Sierau and A. Urbatsch, J. Alloys Compd., 2008, 451, 436;
(b) M. Iwamuro, T. Adachi, Y. Wada, T. Kitamura, N. Nakashima and
S. Yanagida, Bull. Chem. Soc. Jpn., 2000, 73, 1359; (c) A. Meyers, A.
Kimyonok and M. Weck, Macromolecules, 2005, 38, 8671.
16 (a) L.-N. Sun, H.-J. Zhang, J.-B. Yu, S.-Y. Yu, C.-Y. Peng, S. Dang,
X.-M. Guo and J. Feng, Langmuir, 2008, 24, 5500; (b) L. Ning, M. I.
Trioni and G. P. Brivio, J. Mater. Chem., 2007, 17, 4464; (c) R. Kikkeri,
L. H. Hossain and P. H. Seeberger, Chem. Commun., 2008, 2127; (d) A.
Nonat, D. Imbert, J. Pe´caut, M. Giraud and M. Mazzanti, Inorg. Chem.,
2009, 48, 4207.
Acknowledgements
This work was financially supported by the NSFC (No. 20901077,
20821001 & 20821061), the 973 project (2007CB815304 &
2007CB815302) from MSTC.
References
17 (a) M. Ottonelli, G. Michele, M. Izzo, F. Rizzo, G. Musso, G.
Dellepiane and R. Tubino, J. Phys. Chem. B, 2005, 109, 19249; (b) J.
Ouyang, L. Li, Z. Tai, Z. Lu and G. Wang, Chem. Commun., 1997, 815;
(c) S. W. Magennis, A. J. Ferguson, T. Bryden, T. S. Jones, A. Beebyc
and I. D. W. Samuel, Synth. Met., 2003, 138, 463.
18 (a) L. S. Sapochak, F. E. Benincasa, R. S. Schofield, J. L. Baker, K. K. C.
Riccio, D. Fogarty, H. Kohlmann, K. F. Ferris and P. E. Burrows, J. Am.
Chem. Soc., 2002, 124, 6119; (b) Y. Hamada, T. Sano, M. Fujita, T. Fujii,
Y. Nishino and K. Shibata, Jpn. J. Appl. Phys., 1993, 32, L514; (c) Y.
Kai, M. Morita, N. Yasuoka and N. Kasai, Bull. Chem. Soc. Jpn., 1985,
58, 1631.
19 P. Coppo, M. Duati, V. N. Kozhevnikov, J. W. Hofstraat and L. D. Cola,
Angew. Chem., Int. Ed., 2005, 44, 1806.
20 (a) Z.-N. Chen, Y. Fan and J. Ni, Dalton Trans., 2008, 573; (b) H.-B.
Xu, H.-M. Wen, Z.-H. Chen, J. Li, L.-X. Shi and Z.-N. Chen, Dalton
Trans., 2010, 39, 1948.
1 (a) J.-C. G. Bu¨nzli and C. Piguet, Chem. Soc. Rev., 2005, 34, 1048; (b) S.
Comby and J.-C. G. Bu¨nzli, Handbook on the Physics and Chemistry
of Rare Earths, Vol. 37 (ed., K. A. Gschneidner, J.-C. G. Bu¨nzli and
V. K. Pecharsky), Elsevier B. V., 2007, p.217; (c) A. Thibon and V. C.
Pierre, Anal. Bioanal. Chem., 2009, 394, 107; (d) E. J. Werner, A. Datta,
C. J. Jocher and K. N. Raymond, Angew. Chem., Int. Ed., 2008, 47,
8568.
2 (a) Y. Hasegawa, Y. Wada and S. Yanagida, J. Photochem. Photobiol.,
C, 2004, 5, 183; (b) K. Kuriki and Y. K. Okamoto, Chem. Rev., 2002,
102, 2347; (c) S. Kaizaki, Coord. Chem. Rev., 2006, 250, 1804; (d) M. D.
Ward, Coord. Chem. Rev., 2007, 251, 1663.
3 (a) A. de Bettencourt-Dias, Dalton Trans., 2007, 2229; (b) A. Døssing,
Eur. J. Inorg. Chem., 2005, 1425; (c) T. Gunnlaugsson and J. P. Leonard,
Chem. Commun., 2005, 3114; (d) S. Faulkner, L. S. Natrajan, W. S. Perry
and D. Sykes, Dalton Trans., 2009, 3890.
4 J.-C. G. Bu¨nzli and G. R. Choppin, ed., Lanthanide Probes in Life,
Chemical and Earth Sciences: Theory and PracticeElsevier, Amsterdam,
1989.
21 H.-B. Xu, X.-M. Chen, Q.-S. Zhang, L.-Y. Zhang and Z.-N. Chen,
Chem. Commun., 2009, 7318.
22 F. Artizzu, L. Marchio`, M. L. Mercuri, L. Pilia, A. Serpe, F. Quochi, R.
Orru`, F. Cordella, M. Saba, A. Mura, G. Bongiovanni and P. Deplano,
Adv. Funct. Mater., 2007, 17, 2365.
23 (a) N. M. Shavaleev, S. J. A. Pope, Z. R. Bell, S. Faulkner and M. D.
Ward, Dalton Trans., 2003, 808; (b) M. Irfanullah and K. Iftikhar,
Inorg. Chem. Commun., 2009, 12, 296; (c) H.-B. Xu and H.-Y. Chao,
Inorg. Chem. Commun., 2007, 10, 1129.
5 S. I. Klink, H. Keizer and F. C. J. M. van Veggel, Angew. Chem., Int.
Ed., 2000, 39, 4319.
6 N. Sabbatini, M. Guardigli and J.-M. Lehn, Coord. Chem. Rev., 1993,
123, 201.
7 (a) G. E. Buono-Core, H. Li and B. Marciniak, Coord. Chem. Rev.,
1990, 99, 55; (b) T. Q. Ly and J. D. Woollins, Coord. Chem. Rev., 1998,
176, 451.
24 (a) M. P. O. Wolbers, F. C. J. M. van Veggel, B. H. M. Snellink-Rue¨l,
J. W. Hofstraat, F. A. J. Geurts and D. N. Reinhoudt, J. Chem. Soc.,
Perkin Trans. 2, 1998, 2141; (b) A. Beeby and S. Faulkner, Chem. Phys.
Lett., 1997, 266, 116; (c) D. Parker and J. A. G. Williams, J. Chem. Soc.,
Dalton Trans., 1996, 3613.
25 (a) T. Lazarides, D. Sykes, S. Faulkner, A. Barbieri and M. D. Ward,
Chem.–Eur. J., 2008, 14, 9389; (b) T. Lazarides, G. M. Davies, H. Adams,
C. Sabatini, F. Barigelletti, A. Barbieri, S. J. A. Pope, S. Faulkner and
M. D. Ward, Photochem. Photobiol. Sci., 2007, 6, 1152.
8 (a) A. Fratini, G. Richards, E. Larder and S. Swavey, Inorg. Chem., 2008,
47, 1030; (b) Y. Hasegawa, Y. Kimura, K. Murakoshi, Y. Wada, J.-H.
Kim, N. Nakashima, T. Yamanaka and S. Yanagida, J. Phys. Chem.,
1996, 100, 10201; (c) T.-S. Kang, B. S. Harrison, M. Bouguettaya, T. J.
Foley, J. M. Boncella, K. S. Schanze and J. R. Reynold, Adv. Funct.
Mater., 2003, 13, 205.
9 (a) H.-B. Xu, L.-X. Shi, E. Ma, L.-Y. Zhang, Q.-H. Wei and Z.-N.
Chen, Chem. Commun., 2006, 1601; (b) H.-B. Xu, L.-Y. Zhang, Z.-L.
Xie, E. Ma and Z.-N. Chen, Chem. Commun., 2007, 2744; (c) H.-B. Xu,
L.-Y. Zhang, Z.-H. Chen, L.-X. Shi and Z.-N. Chen, Dalton Trans.,
2008, 4664; (d) H.-B. Xu, X.-M. Chen, X.-L. Li, L.-Y. Zhang and Z.-N.
Chen, Cryst. Growth Des., 2009, 9, 569; (e) H.-B. Xu, J. Ni, K.-J. Chen,
L.-Y. Zhang and Z.-N. Chen, Organometallics, 2008, 27, 5665; (f) H.-B.
Xu, L.-Y. Zhang, L.-X. Shi, E. Ma, H.-Y. Chao and Z.-N. Chen, Inorg.
Chem., 2008, 47, 10744.
26 A. Abusaleh and C. F. Meares, Photochem. Photobiol., 1984, 39,
763.
27 (a) W. D. Horrocks Jr, J. P. Bolender, W.D. Smith and R. M. Supkowski,
J. Am. Chem. Soc., 1997, 119, 5972; (b) S. Faulkner, B. P. Burton-Pye,
T. Khan, L. R. Martin, S. D. Wray and P. J. Skabara, Chem. Commun.,
2002, 1668; (c) A. Beeby, S. Faulkner and J. A. G. Williams, J. Chem.
Soc., Dalton Trans., 2002, 1918.
10 (a) X.-L. Li, F.-R. Dai, L.-Y. Zhang, Y.-M. Zhu, Q. Peng and Z.-N.
Chen, Organometallics, 2007, 26, 4483; (b) X.-L. Li, L.-X. Shi, L.-Y.
Zhang, H.-M. Wen and Z.-N. Chen, Inorg. Chem., 2007, 46, 10892;
(c) J. Ni, L.-Y. Zhang and Z.-N. Chen, J. Organomet. Chem., 2009, 694,
339.
28 D. Rehm and A. Weller, Isr. J. Chem., 1970, 8, 259.
29 (a) C. Piguet, J.-C. G. Bu¨nzli, G. Bernardinelli, G. Hopfgartner, S.
Petoud and O. Schaad, J. Am. Chem. Soc., 1996, 118, 6681; (b) W.-K.
Lo, W.-K. Wong, W.-Y. Wong, J. Guo, K.-T. Yeung, Y.-K. Cheng, X.
This journal is
The Royal Society of Chemistry 2010
Dalton Trans., 2010, 39, 5676–5682 | 5681
©

View Article Online
Yang and R. A. Jones, Inorg. Chem., 2006, 45, 9315; (c) K. Hanaoka,
K. Kikuchi, H. Kojima, Y. Urano and T. Nagano, Angew. Chem., Int.
Ed., 2003, 42, 2996.
32 (a) T. Kido, Y. Ikuta, Y. Sunatsuki, Y. Ogawa and N. Matsumoto, Inorg.
Chem., 2003, 42, 398; (b) M. F. Richardson, W. F. Wagner and D. E.
Sands, J. Inorg. Nucl. Chem., 1968, 30, 1275.
33 J. V. Caspar and T. J. Meyer, J. Am. Chem. Soc., 1983, 105, 5583.
34 J. Q. Umberger and V. K. LaMer, J. Am. Chem. Soc., 1945, 67, 1099.
35 SHELXTL 6.10, Bruker Analytical Instrumentation, Madison,
Wisconsin, USA, 2000.
30 CIE (1932); Commission Internationale de I’Ecairage Proceedings,
1931, Cambridge University Press, Cambridge.
31 E. C. Constable and M. D. Ward, J. Chem. Soc., Dalton Trans., 1990,
1405.
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