Highly efficient roomerature phosphorescence achieved by gadolinium complexes

Article abstract of DOI:10.1039/c9dt03050f

A new family of room temperature phosphorescent materials with emission lifetimes in microseconds has been reported in this work. Phosphorescence of gadolinium complexes with emission color from blue to orange has been obtained at room temperature with a maximum photoluminescence quantum yield of 66%, benefiting from appropriate molecular structures and favorable encapsulation methods.

Full text of DOI:10.1039/c9dt03050f

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Douglas W. Stephen et al.
N-Heterocyclic carbene stabilized parent sulfenyl, selenenyl,
and tellurenyl cations (XH , X = S, Se, Te)
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DOI: 10.1039/C9DT03050F
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Highly Efficient Room-Temperature Phosphorescence Achieved by
Gadolinium Complexes
Received 00th January 20xx,
Accepted 00th January 20xx
a
a
b
a
a
a
a
Boxun Sun, Chen Wei,* Huibo Wei,* Zelun Cai, Huanyu Liu, Zhiyu Zang, Wenchao Yan, Zhiwei
DOI: 10.1039/x0xx00000x
a
a
a
Liu, Zuqiang Bian* and Chunhui Huang
A new family of room temperature phosphorescence materials usually higher than (or close to) the complicated excited energy levels
with emission lifetimes in microseconds was reported in this work. of lanthanide ions.14 Nevertheless, gadolinium(III) ion possesses the
-
1
highest energy level for the first excited state (32,000 cm ) among the
lanthanide(III) ions,15 which means no energy transfer occur for most
complexes. Besides, gadolinium(III) ion has seven unpaired f
electrons, which shows the highest spin multiplicity of 8 and the
strongest spin−orbit coupling in the lanthanide series, implying that it
is an ideal candidate for obtaining organic phosphorescence. But up
to now, the triplet emissions of gadolinium complexes were usually
just observed at very low temperature16 and gadolinium
phosphorescence with high quantum yield has barely been reported at
room temperature in the existing works.17
Phosphorescence of gadolinium complexes with color from blue to
orange was obtained at room temperature with maximum
photoluminescence quantum yield of 66%, benefiting from the
proper molecular structure and favorable encapsulation methods.
Phosphorescence with lifetime of excited state in microseconds to
hours both for organic materials and inorganic materials has been
investigated in the use of anti-counterfeiting,1 bio-imaging and
sensing,2-3 and solid-state lighting4. Among them, organic
phosphorescent materials are of special interest for their mild
synthesizing condition, tunable emissions and easy compatibility with
organic semiconductor devices.
In this work, bright phosphorescent emissions were successfully
obtained at room temperature based on a series of gadolinium(III)
complexes with varied ligand structure (shown in Fig. 2). Our
observation about phosphorescence behavior of gadolinium(III)
complexes started with a tridentate ligand, L3, that was reported by
our group for constructing europium(III) complex with high
photoluminescence quantum efficiency.18 This tridentate ligand
coordinates with the gadolinium(III) ion in the ratio of three to one,
with no solvent coordination in the first coordination sphere, which
To enhance the spin-forbidden transitions of phosphorescence,
5
different strategies were used, one of which is introducing heavy
6
atom to organic systems, such as iodine to pure organic molecules or
7
iridium to complexes , for improving the spin-orbit coupling and
accelerating intersystem crossing. Organometallic complexes
employing copper(I),8 silver(I),9 ruthenium(II)10 and osmium(II)11
were also used to harvest triplet emissions originating from metal to
3
can be proved by the crystal structure of Gd(L3) (Gd3) (Fig. S1). The
ligand charge transfer (ML CT) transitions or intraligand triplet states
3
(3
eliminating of solvent coordination helps to avoid quenching by O-H
IL). Distorted structures in triplet state may also promote spin−orbit
coupling rate, including examples of arylboronic esters12 and N-
benzoyl-carbazole.13 Lanthanide ions can improve spin-orbit coupling
vibration from solvent. Under quite low temperature (77K), the
-5
frozen-state ethanol solution of Gd3 (1×10 M) can give bright green
emission. However, as the temperature gradually increased, the
emission disappeared quickly. This phenomenon can be clearly seen
from the varied-temperature phosphorescence spectra of the
gadolinium(III) complex in solution from 77 K to 200 K (displayed in
Fig. S2). The reason is usually considered as unfreeze of molecular
vibration (which would quench the emissions) with increased
temperature.19
as well because of their heavy atom effects. However, when organic
molecules coordinate to lanthanide ions, energy transfer might happen
from ligands to central lanthanide ions and only emissions from
lanthanide ions are obtained, because the triplet energy of ligands is
aCollege of Chemistry and Molecular Engineering, Peking University, 202 Chengfu
Road, Beijing 100871, P. R. China. Email: bianzq@pku.edu.cn, c.wei@pku.edu.cn.
b
Jiangsu JITRI Molecular Engineering Institute Co., Ltd., 88 Xianshi Road,
Changshu 215500, P. R. China. Email: weihuibo@163.com.
Electronic supplementary information (ESI) available. CCDC 1937605. For ESI and
crystallographic data in CIF or other electronic format see See
DOI: 10.1039/x0xx00000x
Solid state can afford more restricted surroundings for organic
molecules. However, emission of the gadolinium(III) complex still
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cannot be observed even in solid state (Fig. S3). Concentration Furthermore, we expanded such Gd-based roomViewteAmrtipcleerOatnulinree
quenching is regarded as the reason of such phenomenon because the phosphorescence to other gadolinium(III) DcoOmI: p10le.1x0e3s9./CT9oDTa0c3h0ie5v0Fe
long-lifetime excited states may cause serious triplet-triplet annealing emission covering all visible light range, we introduced 4 other
(
TTA) process in solid powder state.20 As a substitute, polymers have ligands with different triplet states. The structures of ligands are
relatively rigid structure comparing with solution state, which can shown in Fig. 2. L1 and L4 were used as ligands to sensitize other
restrict the collision of molecules and reduce the rate of nonradiative lanthanide(III) ions in our previous work.18,22 The phosphorescence
decay. So we dispersed the gadolinium(III) complex in a polymethyl color of Gd1 is orange and that of Gd4 is sky-blue, which can be
methacrylate (PMMA) matrix (0.1% to 2%, w/w) by drop casting its further tuned by proper ligand structure modification. To increase the
PMMA solution on a piece of quartz glass. However, this is still not triplet energy of L1, we introduced cyan group at 3-position of the 4-
enough since the triplet oxygen molecules in the air may also quench hydroxyl-1,5-naphthyridine segment, a strategy that was used by our
the long-lived phosphorescence,21 leading to a quite faint emission. group in previous work23 to reduce the highest occupied molecular
When such film was taken into a glove box with totally nitrogen orbital (HOMO) energy level and to increase the energy gap
atmosphere, bright green light can be seen under UV 365 nm (derivative named as L2, synthetic route seen in Scheme S1).
excitation (Fig. S3). Therefore, we further encapsulated this PMMA Increased triplet energy of ~1000 cm-1 is obtained and the emission
film by covering another quartz slice on its surface and sealing the color is changed from orange to yellow (Gd1 to Gd2, Table 1, Fig.
surrounding gaps with wax (to cut off oxygen) in glove box (Fig. 1). 3). Triazole group was also integrated to modify the structure of L4,
As expected, the bright luminescence can still be observed even when which can also increase the energy gap of the ligand by changing the
such encapsulated film was taken outside.
pyridine cycle to a smaller one (from L4 to L5).
Fig. 2. The molecular structures of ligands.
From the emission spectra of the five gadolinium(III) complexes (Fig.
3
), it can be seen that the phosphorescent emissions cover from 450
nm to 650 nm, indicating that this is a common method to achieve
room temperature phosphorescence. The emission decay curves of all
the gadolinium(III) complexes were recorded (shown in Fig. 3), and
the lifetimes of these gadolinium(III) complexes and yttrium(III)
complexes are listed in Table 1. The lifetimes of yttrium(III)
complexes are in the range of nanoseconds while those of the
gadolinium(III) complexes are in milliseconds. The gadolinium(III)
complexes display longer triplet emission lifetimes than those of the
3MLCT transitions24 but shorter than those of the heavy-atom free
organic molecules.25
Fig. 1. Scheme of fabrication and encapsulation process of PMMA
film doped with title gadolinium(III) complexes.
The excitation and emission spectra of the encapsulated PMMA film
were measured at room temperature. We can see broad peaks with big
Stokes shift of more than 130 nm, originating from excitation of L3’s
singlet state and emission of its triplet (Fig. S4). For comparison, we
synthesized the yttrium(III) complex of L3(Y3), and prepared its
PMMA film in the same manner, considering that the yttrium(III) ion
has no electron in its 4f orbitals and shows much weaker spin-orbit
coupling. Y3 showed similar excitation spectrum compare to that of
Gd3 but quite different emission spectrum in shorter wavelength,
which was assigned as the fluorescent emission (Fig. S4). The
emission lifetime of Gd3 is 9.52 ms while the lifetime of Y3 is 14.1
ns, which can also prove the phosphorescent nature of Gd3 emission.
The lifetime of Gd3 is quite long because of the heavy atom effect of
gadolinium(III) ions is remarkable while the heavy atom effect of
yttrium(III) ions is not as significant as gadolinium(III) ions. In
consideration of the different Stokes shifts of Gd3 and Y3, we can
confirm that the emission of Gd3 is phosphorescence while the
emission of Y3 is fluorescence.
Table 1. Luminescence characterization of the title gadolinium(III)
and yttrium(III) complexes in PMMA filmsa
λem (nm)
602
568
521
465
464
513
468
453
CIE
τ (ms)
1.75
2.11
9.52
4.41
8.58
1.07ⅹ10
7.26ⅹ10
1.41ⅹ10
1.89ⅹ10
5.23ⅹ10
PLQY (%)
Gd1
Gd2
Gd3
Gd4
Gd5
Y1
Y2
Y3
Y4
Y5
(0.57, 0.43)
(0.46, 0.52)
(0.29, 0.55)
(0.21, 0.32)
(0.18, 0.27)
(0.34, 0.49)
(0.29, 0.36)
(0.19, 0.21)
(0.20, 0.24)
(0.15, 0.09)
18
20
66
22
54
15
31
47
32
32
-5
-6
-5
-6
-6
335
398
2
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a: The encapsulated PMMA films were prepared in the concentration coordination of the gadolinium(III) complexes, excludiVniegwtAhreticsleoOlvnelinnet
of 0.5% (w/w) except 0.2% for Gd5. Excitation wavelengths were 410 quenching. To reduce the concentration quencDhOinI:g1,0t.1h0e3c9o/Cm9pDlTe0x3e0s5a0rFe
nm, 370 nm, 290 nm, 360 nm and 300 nm.for Gd1/Y1 to Gd5/Y5,
respectively. b: CIE: A way of expressing colors.
doped into polymer with rigid environment. Encapsulation of the film
with quartz slices and wax keeps the film away from oxygen. By using
this method, the five gadolinium(III) complexes show bright
phosphorescent emissions with color from blue to orange. The PLQYs
of these complexes are among the high values of room-temperature
phosphorescent materials, showing potential in the areas of oxygen
sensing and solid lighting, which are being under investigated in our
lab.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This research is supported through grants from the National Key R&D
Program of China (2017YFA0205100 and 2016YFB0401001),
National Natural Science Foundation of China (Grant 21621061), and
Beijing Natural Science Foundation (Grant 2172017).
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Fig. 3. Phosphorescent emission spectra (a) and decay curve (b) of
gadolinium complexes in doped PMMA film (0.5%, w/w)
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DOI: 10.1039/C9DT03050F
Highly Efficient Room-Temperature Phosphorescence Achieved by Employing Gadolinium
Complexes, with Emission Color from Blue to Orange