Pure white-light and yellow-to-blue emission tuning in single crystals of Dy(iii) metal-organic frameworks

Article abstract of DOI:10.1039/c4cc01763c

Direct white-light emission was first achieved in a single phase material of a Dy(iii) metal-organic framework, which also shows tunable yellow-to-blue photoluminescence upon variation of excitation wavelengths.

Full text of DOI:10.1039/c4cc01763c

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Pure white-light and yellow-to-blue emission
tuning in single crystals of Dy(III) metal–organic
frameworks†
Cite this: Chem. Commun., 2014,
50, 7702
Received 9th March 2014,
Accepted 23rd May 2014
Qing-Yuan Yang,a Kai Wu,a Ji-Jun Jiang,a Chien-Wei Hsu,b Mei Pan,*a
Jean-Marie Lehnab and Cheng-Yong Su*a
DOI: 10.1039/c4cc01763c
www.rsc.org/chemcomm
Direct white-light emission was first achieved in a single phase emissions from other Ln atoms, such as Sm(III) and Dy(III), are usually
material of a Dy(III) metal–organic framework, which also shows rather weak. Dy(III) possesses appropriate f–f transitions which are
tunable yellow-to-blue photoluminescence upon variation of exci- able to emit colors in blue (480 nm), yellow (573 nm) and red
tation wavelengths.
(661 nm) regions upon adequate excitation. Usually yellow emission
dominates over the other two emissive colors. Therefore, if a Dy-MOF
White-light-emitting diodes (WLEDs) have wide applications in can be designed to emit yellow (Dy-based) and blue (ligand-based)
lasers, indicators, backlights, displays, etc.1,2 Apart from the tradi- colors simultaneously, white-light emission can be achieved through a
tional method of blending multi-LEDs (for example, mixing LEDs BY dichromatic strategy on the balance of the blue (B) and yellow (Y)
with three primary colors red, green and blue, i.e. RGB), single colors. This will provide an alternative way to utilize Ln-MOFs as
component (SC) white-light-emitting materials provide an alterna- potential candidates of single-phase white phosphors. Nevertheless,
tive approach to fabricate WLEDs characteristic of uniform and due to the less efficient luminescence in most Dy-MOFs compared
well-balanced color, as well as easy-manipulation and low-cost.3–6 with the Eu/Tb counterparts,12 little attention has been paid to the
In recent years, lanthanide containing metal–organic frameworks photoluminescence (PL) of Dy-MOFs, especially SC white-light
(Ln-MOFs) have aroused special interest in the design of SC white- emitting and color tuning properties.
light and color-tunable materials, taking advantages of the intrinsic
We herein report a multiple-color photoluminescent Dy-MOF,
multiple-colored f–f emissions of Ln atoms with efficient antenna [Dy(TETP)(NO3)3]ꢀ4H2O (1ꢀ4H2O, TETP = 1,10,100-((2,4,6-triethyl-
effect, sharp band and long lifetime.
benzene-1,3,5-triyl)tris(methylene))tris(pyridin-4(1H)-one)), in which
To reach this goal, Ln atoms that could emit pure visible lights, e.g. the ligand TETP can play dual functions. On one hand, it can
red Eu(III) and green Tb(III) emissions, have been most frequently effectively sensitize Dy centers to produce yellow light; on the other
applied. In principle, if these two primary colors are further combined hand, it is strongly blue luminescent with its blue emission in
with blue luminescence, which is often contributed by the ligand- Dy-MOF being sufficient to balance yellow emission of Dy to generate
based emission, white-light emitting phosphor can be achieved in a white-light, offering a Commission Internationale de l’Eclairage (CIE)
single phase Ln-MOF containing mixed Eu and Tb centers.7–10 So far, chromaticity coordinate of (0.33, 0.35). Moreover, by varying the
most of the white-light emitting Ln-MOFs are constructed on the excitation wavelengths, yellow-to-blue PL color-tuning can be readily
basis of such trichromatic RGB strategy; however, studies on the accomplished depending on the variation of the intensity ratios
white-light emission of Ln-MOFs with other Ln(III) centers remain between the characteristic yellow emission of Dy atoms and the blue
quite rare.11 This is probably due to the fact that visible color luminescence of the TETP ligand.
Complex 1ꢀ4H2O was prepared through a simple wet chemical
method by reaction of TETP ligand and Dy(NO3)3ꢀ6H2O salt in a
a MOE Laboratory of Bioinorganic and Synthetic Chemistry, State Key Laboratory of
water–acetone solvent system, affording colorless crystals in reason-
Optoelectronic Materials and Technologies, Lehn Institute of Functional Materials,
able yield (58%). The phase purity of the bulky sample of 1ꢀ4H2O has
School of Chemistry and Chemical Engineering, Sun Yat-Sen University,
been confirmed by powder X-ray diffraction in comparison with its
single-crystal data simulation (Fig. S1, ESI†). Complex 1 crystallizes
in the space group Pna21 with the asymmetric unit consisting of one
Guangzhou 510275, China. E-mail: panm@mail.sysu.edu.cn,
cesscy@mail.sysu.edu.cn
b
´
´
Laboratoire de Chimie Supramoleculaire, Institut de Science et d’Ingenierie
´
´
´
Supramoleculaires (ISIS), Universite de Strasbourg, 8 allee Gaspard Monge,
ꢁ
Dy(III), one TETP and three coordinating NO3 groups. The Dy(III)
Strasbourg 67000, France
centers lie in a tricapped trigonal prismatic geometry,ꢁcoordinated
by three TETP through terminal O and three NO3 groups in
chelating mode (Fig. 1a). Meanwhile, each tripodal TETP connects
† Electronic supplementary information (ESI) available: Synthetic, photophysical
and crystallographic details. CCDC 990178. For ESI and crystallographic data in
CIF or other electronic format see DOI: 10.1039/c4cc01763c
7702 | Chem. Commun., 2014, 50, 7702--7704
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Fig. 1 Crystal structure of complex 1: (a) coordination environment of the
central Dy(III) atom and surrounding ligands, and (b) 3D framework along the a
axis, the solvent molecules, counter anions and H atoms are omitted for clarity
(yellow for Dy, blue for N, red for O, and gray for C atoms, respectively).
Fig. 2 Solid-state PL spectrum of 1 (lex = 365 nm). The emission curve is
superimposed on the white-light spectrum. Inset: photograph of white
emissive Dy-MOF in a sample vial and CIE chromaticity coordinates.
three different Dy(III) with metal and ligand centers as nodes and
pyridone arms as linkers, thus generating an intricate three-
dimensional (3D) framework in (10, 3)-d (or utp) topology13 as seen
in Fig. 1b.
The UV-adsorption of ligand TETP in solution shows a strong
pp* band at around 265 nm, and relatively weak intra-ligand
charge transfer (ILCT)10 above 300 nm originating from pyridone
donors. These ligand transitions are merged and even expanded
beyond 400 nm in the solid-state (Fig. S2, ESI†). The solid-state
emission spectrum of TETP displays a broad asymmetric emission
band at around 430 nm with decay lifetimes t = 1.2 and 2.2 ns
(Fig. S3, ESI†), revealing the blue nature of the ligand-centered (LC)
luminescence which can only be excited above 300 nm, thereof,
dominated by ILCT. Upon excitation at 365 nm at room tempera-
ture, the solid sample of 1 displays multiple emission peaks at 430,
480, 573 and 661 nm. The first broad band centered at 430 nm is
attributable to the reserved ILCT emission of TETP, while the three
narrow peaks originate from f–f transitions between the Dy(III)
Scheme 1 Schematic representation of energy absorption, migration,
emission (plain arrows) and dissipation (dotted arrows) processes in
complex 1. LC, ligand-centered; MC, metal-centered; ILCT, intra-ligand
charge transfer; S, singlet state; T, triplet state; A, absorption; F, fluores-
cence; P, phosphorescence; nr, nonradiative; ISC, intersystem crossing;
ET, energy transfer.10b
4
6
6
lowest emitting state F9/2 and ground levels of H15/2, H13/2 and
6H11/2. The nature of these emissions has been confirmed by the however, the blue LC emission can only be excited in the ILCT
measurement of their decay dynamics. The 430 nm peak has a spectral region. These ETs are subject to influence of excitation
decay feature with t = 0.8 and 3.2 ns, typical of LC emission. And energy. As shown in excitation spectra of 1 (Fig. S6, ESI†), MC
the main peak at 573 nm has a decay lifetime of 31 ms, character- emission (573 nm) can be effectively sensitized via ligand singlet
istic of Dy-centered emission (Fig. S4, ESI†). Remarkably, combi- adsorption (o300 nm), or moderately by ILCT and hypersensitized
nation of these four emissions in complex 1 is properly balanced to f–f transitions (300–400 nm), while this ILCT excitation in the
result in white-light output, offering a CIE coordinate of (0.33, 300–400 nm region also generates LC emission. Monitoring at
0.35) which is close to that of pure white light (0.33, 0.33). 430 nm reveals a wide excitation band resembling that of pure TETP,
The collected emissions of 1 (lex = 365 nm, detection span: and shows a structured profile with a few troughs corresponding
400–800 nm) give an absolute quantum yield of B7%, which is to f–f adsorptions of Dy(III). This implies that ILCT excitation has no
among the highest value for the Dy(III) complex.14 The white-light efficient antenna effect as singlet transition of TETP, leading
can also be directly detected by the naked eye (Fig. 2, inset).
to LC emission accompanied with f–f emissions. Therefore, fine
The PL mechanism and energy transfer (ET) processes involved color-tuning is achievable through a dichromatic approach upon
in this Dy-MOF may be complicated as illustrated in Scheme 1. variation of excitation wavelengths. At proper wavelength, the relative
The PL spectrum of isostructural Gd-MOF (Fig. S1 and S5, ESI†) intensity between MC and LC emissions can be balanced to give
indicates a triplet energy level of TETP at B23 000 cmꢁ1 (T = 77 K, white-light output directly from the single phase of Dy-MOF.
t = 0.7 and 6.9 ms), which is adequate for ET to the 4F9/2 level of Dy
Based on the above discussion, this Dy-MOF can be considered
(B21 500 cmꢁ1).14 Therefore, after TETP absorbs the UV light at to contain two kinds of emissive centers, i.e. Dy atoms emitting
either pp* (singlet transition, o300 nm) or ILCT (300–400 nm) mainly yellow (573 nm) color and TETP components emitting blue
bands, it can transfer the energy directly15 or via its triplet state color (430 nm), of which the luminescent intensities depend on the
to the Dy center to generate the metal-centered (MC) emission; ET efficiency at varied excitation wavelengths. As a consequence,
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Fig. 3 Emission spectra of 1 excited at different lex. Insets: chromaticity
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excitation wavelengths. As seen in Fig. 3 and Fig. S6 (ESI†), when
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430 nm indicate that the nature of LC emission at this temperature
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In summary, we have successfully synthesized a 3D Dy-MOF
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wavelengths. It is noteworthy that pure white-light emission
has been realized by this Dy-MOF in a single phase for the first
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We thank the 973 Program of China (2012CB821701), the NSFC
projects (91222201, 21373276, 21121061, 21173272), the NSF of
Guangdong (S2013030013474), the FRF for the Central Universities,
and the RFDP of Higher Education of China for funding.
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7704 | Chem. Commun., 2014, 50, 7702--7704
This journal is ©The Royal Society of Chemistry 2014