A color-tunable europium complex emitting three primary colors and white light

Full text of DOI:10.1002/anie.200901266

Communications
DOI: 10.1002/anie.200901266
Fluorophores
A Color-Tunable Europium Complex Emitting Three Primary Colors
and White Light**
Guangjie He, Dong Guo, Cheng He, Xiaolin Zhang, Xiuwen Zhao, and Chunying Duan*
The manufacture of new full-color displays is one of the main
tasks in flat-panel display systems and lighting technology.[1]
Different applications place different demands on emitted
light: in some cases a white-light source is needed,[2] and in
others pure colors are necessary.[3] Thus, white emission
should ideally be composed of three (blue, green, and red) or
two (blue and yellow) primary colors and cover the whole
visible range from 400 to 700 nm, and the emitter should have
the ability to emit the primary colors simultaneously with
equal intensities to produce white light and the pure colors
separately in a tunable way.[4] Considerable interest exists for
such color-tunable materials, which can be used to define or
modify environments, moods, and brands.[5] Traditional meth-
ods of such white light generation typically rely on mixing
various primary colors from different emitting materials.[6] An
alternative approach for the generation of efficient (white)
light sources is to use a single-component emitter, which can
have advantages such as greater stability, better reproduci-
bility, no phase separation, and simpler fabrication process-
es.[7,8] Although a few materials show white-light emission as a
single-emitting component, none has been reported to
produce well-separated blue, green, and red emissions
beside white light.[8a] Since energy transfer typically quenches
one or more of the emission pathways and thereby restricts
the transitions that define the output spectrum,[9] the design of
color tunable single-component emitters requires readily
tailorable different fluorophores and fine-tuning of the
energy-transfer processes between the different fluorophores.
On the other hand, lanthanide-containing materials,
which exhibit excellent sharp-emission luminescence proper-
ties with suitable sensitization, have attracted considerable
interest and been effectively used in designing white-emitting
nanoparticles.[10] With judiciously chosen red- (EuIII, PrIII,
SmIII), green- (TbIII, ErIII), and blue-emissive (TmIII , CeIII,
DyIII) ions doped in an suitable host, it is possible to obtain
phosphors which emit across the entire visible spectrum with
high color purity.[11] Specifically, an EuIII-containing single-
component complex has been reported to offer white-light
emission in a carefully designed system which only allows
partial energy transfer between the sensitizing fluorophore
and the EuIII center.[12] Herein we report the design and
synthesis of a new fluorophore that exhibits tunable emission
of three primary colors (blue, green, and red) and white light,
by combining an EuIII moiety as the origin of red light with an
organic ligand that comprises a blue-emitting coumarin
fluorophore and a green-emitting Rhodamine 6G fluoro-
phore.
Coumarin-Rhodamine CR1 was synthesized by reaction
of 7-diethylamino-2-oxo-2H-chromen-3-carboxylic chloride
and N-(Rhodamine-6G)lactamethylenediamine and recrys-
tallized from ethanol as yellow crystals. Single-crystal X-ray
structural analysis confirms the coexistence of two fluoro-
phores in CR1 (Supporting Information Figure S1), whereby
the Rhodamine 6G moiety is in a luminescence-inactive ring-
closed tautomeric form.[13] Europium compound CR1-Eu (1)
was prepared by refluxing ligand CR1 and [Eu(tta)3] in THF
(tta = 1,1,1-trifluoro-3-(2-thenoyl)acetone) and purified by
recrystallization as an amorphous yellow powder. Elemental
1
analysis and H NMR spectroscopic characterization suggest
the chemical formula [Eu(tta)2(CR1)2](tta) for 1 (Figure 1).
Figure 1. Proposed structure of the white-emitting dye 1 showing the
fragments of the coumarin (blue), Rhodamine 6G (green), and EuIII-
based (red) primary-color-emitting fluorophores.
Fourier transform IR spectroscopic studies on 1 and CR1
(Supporting Information Figure S3) showed a significant
change in the stretching bands of the carbonyl group at
1699 and 1685 cmÀ1, which suggests participation of the
carbonyl groups in coordination to EuIII. Additionally, the
ESI-MS spectrum of 1 (Supporting Information Figure S4)
exhibited only one intense peak at m/z 1993.61, which was
assigned to the [Eu(tta)2(CR1)2]+ cation according to the
exact comparison of the intense peak with the simulation
based on natural isotopic abundances, which indicates for-
mation of complex 1 and its stability in solution.
[*] G. He, Dr. D. Guo, Dr. C. He, Dr. X. Zhang, X. Zhao, Prof. C. Duan
State Key Laboratory of Fine Chemicals, Dalian University of
Technology, Dalian, 116012 (P. R. China)
Fax: (+86)411-8370-2355
E-mail: cyduan@dlut.edu.cn
[**] This work is supported by the National Natural Science foundation
of China (20571041 and 20501016) and the Start-up Fund of The
Dalian University of Technology.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200901266.
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The absorption spectra of 1, CR1, [Eu(tta)3], and acidified
N-(Rhodamine-6G)lactamethylenediamine are shown in
Figure 2. Free CR1 displays an intensive absorption band in
the visible region centered at 415 nm that arises from the
Figure 3. Emission spectra of 1 in acetonitrile solution (50 mm) show-
ing the three individual blue (blue line), green (green line), red (red
line), and nearly white (pink line) emissions when excited at 415, 525,
360, and 388 nm, respectively. The right picture shows the three
primary colors and nearly white color corresponding to each case. The
white line shows the pure white emission of 1 (0.2 mm) when excited
at 388 nm in acetonitrile solution.
Figure 2. UV/Vis absorption spectra of free [Eu(tta)3] (dotted), CR1
(dash-dotted), N-(rhodamine-6G)lactamethylenediamine with p-tolue-
nesulfonic acid (dashed), and 1 (solid line) in CH3CN solution.
coumarin chromophore (Supporting Information Figure S5.1)
and a weak band at about 302 nm, which is attributed to
absorption of the ring-closed tautomeric form of the Rhoda-
mine 6G moiety (Supporting Information Figure S5.2). The
UV/Vis absorption spectrum of 1 displays three main
absorption bands centered at 345, 415, and 525 nm. By
comparison of the absorption spectra of [Eu(tta)3], CR1, and
1, the first two were mainly assigned to the characteristic
absorption maxima of the Eu-TTA moiety and the coumarin
chromophore, respectively. The last band, which is absent in
CR1, should be assigned to the absorption of the ring-opened
tautomer of the Rhodamine 6G chromophore,[14] that is, the
carbonyl group of the Rhodamine 6G moiety is coordinated
to the Eu center and the attached spiro ring has opened to
make green emission of 1 possible.
Thus, 1 exhibits characteristic blue emission of the
coumarin fluorophore centered at 460 nm, green Rhodamine
emission centered at 550 nm, and typical red emission of the
EuIII ion with dominant peak at 612 nm when its solution in
acetonitrile (50 mm) is excited at 415, 525, and 360 nm,
respectively (Figure 3). The quantum yields of the blue,
green, and red emissions are about 0.056, 0.085 and 0.005,
respectively.[15] While a few organic dyes have been reported
that exhibit tunable emissive properties,[8a] the generation of
pure blue, green, and red emissions in a single-component dye
by simply changing the excitation wavelengths is extremely
appealing and significant. Such color-tunable materials may
be promising candidates for emitting “pure” white light by
controllably tailoring blue, green, and red primary colors that
fully span the entire visible spectrum.[5]
Figure 4. Luminescence spectra of 1 in acetonitrile solution plotted on
a CIE diagram with excitation wavelengths varying from 360 to 500 nm
(step size 5 nm) showing tunable chromaticity of visual emission
image. The red and blue lines represent concentrations of 0.20 mm
and 50 mm, respectively. Insert: CIE diagram of a solid thin film of 1
with excitation wavelengths varying from 360 to 500 nm (step size
5 nm).
region of the 1931 CIE diagram (for pure white x = 0.33, y =
0.33), the quantum yield of the whitest emission spanning the
entire visible spectrum is calculated as 0.025 when excited at
388 nm.[15] To the best of our knowledge, it is the first example
of a single-component dye that can emit blue, green, and red
primary colors as well as nearly white light.
Detailed emission and excitation spectra of 1 were
extensively examined to elucidate the energy-transfer mech-
anisms in this system (Supporting Information Figures S6–
S9). The typical EuIII emission at 612 nm with lifetime of
0.55 ms when excited at 360 nm can be assigned to the
excitation of the auxiliary organic ligand followed by energy
transfer to the EuIII ion. Excitation of TTA at 360 nm also
triggered the characteristic coumarin blue emission and
Rhodamine 6G green emission, beside the EuIII emission,
through multichannel energy transfer.[17] Moreover, the
As expected, the CIE chromaticity diagram[16] of 50 mm 1
(Figure 4, blue line) exhibited a color shift from red to blue
and then to green when the excitation wavelength varied from
360 to 500 nm (Supporting Information Figure S6). Clearly, 1
is able to create a nearly white emission to the eye with CIE
coordinates of (0.33, 0.24), which fall well within the white
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Communications
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should be attributed mainly to the equilibrium of these
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Interestingly, the quality of white emission could be
improved by increasing the concentration of 1 in acetonitrile
solution, which gave a better-balanced pure-white emission
with the chromaticity coordinate exactly positioned at (0.33,
0.33) when it was excited at 388 nm (Supporting Information
Figure S10). The spectrum of the pure white emission
(Figure 3, white line) comprises the three primary colors
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2 h (Supporting Information Figure S11). Furthermore, solid
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exhibited tunable luminescent properties, and the best
chromaticity with coordinates of (0.35, 0.36), when the thin
film was excited at 388 nm, lies within the white region of the
1931 CIE diagram (inset of Figure 4), demonstrating its
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primary colors (blue, green, and red) as well as white
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Received: March 6, 2009
Revised: May 19, 2009
Published online: July 7, 2009
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Keywords: chromophores · dyes/pigments · fluorescence ·
lanthanides
.
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[13] Crystal data for CR1: C42H45N5O5·C2H5OH; Mr = 745.90, mono-
clinic; P21/c; a = 19.1515(5), b = 12.9911(4), c = 15.9890(5) ꢇ,
b = 95.423(2)8, V= 3960.2(2) ꢇ3; Z = 4; 1calcd = 1.251 gcmÀ3; T=
293(2) K. The final refinement gave R1 = 0.0585, wR2 = 0.1541,
and GOF = 1.004 for 3626 observed reflections with I > 2s(I).
CCDC 720202 contains the supplementary crystallographic data
for this paper. These data can be obtained free of charge from
The Cambridge Crystallographic Data Centre via www.ccdc.
cam.ac.uk/data_request/cif.
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