Highly efficient room-temperature phosphorescence achieved by gadolinium complexes

Boxun Sun a, Chen Wei *a, Huibo Wei *b, Zelun Cai a, Huanyu Liu a, Zhiyu Zang a, Wenchao Yan a, Zhiwei Liu a, Zuqiang Bian *a and Chunhui Huang a
aCollege of Chemistry and Molecular Engineering, Peking University, 202 Chengfu Road, Beijing 100871, P. R. China. E-mail: bianzq@pku.edu.cn; c.wei@pku.edu.cn
bJiangsu JITRI Molecular Engineering Institute Co., Ltd., 88 Xianshi Road, Changshu 215500, P. R. China. E-mail: weihuibo@163.com

Received 25th July 2019 , Accepted 9th September 2019

First published on 9th September 2019


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.


Phosphorescence with a lifetime of the excited state ranging from microseconds to hours for both organic and inorganic materials has been investigated for applications in anti-counterfeiting,1 bio-imaging and sensing,2,3 and solid-state lighting.4 Among them, the organic phosphorescent materials are of special interest due to their mild synthesizing conditions, tunable emissions and easy compatibility with organic semiconductor devices.

To enhance the spin-forbidden transitions of phosphorescence, different strategies were used,5 one of which is introducing heavy atoms into organic systems, such as iodine into pure organic molecules6 or iridium into complexes,7 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 the metal to ligand charge transfer (ML3CT) transitions or intraligand triplet states (3IL). Distorted structures in triplet states may also promote the spin–orbit coupling rate, including examples of arylboronic esters12 and N-benzoyl-carbazole.13 Lanthanide ions can improve spin–orbit coupling as well because of their heavy atom effect. 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 usually higher than (or close to) the complicated excited energy levels of lanthanide ions.14 Nevertheless, gadolinium(III) ions possess the highest energy level for the first excited state (32[thin space (1/6-em)]000 cm−1) among the lanthanide(III) ions,15 which means no energy transfer occurs for most complexes. Besides, gadolinium(III) ions have seven unpaired f electrons, and show the highest spin multiplicity of 8 and the strongest spin–orbit coupling in the lanthanide series, implying that they are an ideal candidate for obtaining organic phosphorescence. But to date, the triplet emissions of gadolinium complexes have been usually observed at very low temperature16 and gadolinium phosphorescence with a high quantum yield has barely been reported at room temperature in the existing work.17

In this work, bright phosphorescence emissions were successfully obtained at room temperature based on a series of gadolinium(III) complexes with varied ligand structures (shown in Fig. 2). Our observation of phosphorescence behavior of gadolinium(III) complexes started with a tridentate ligand, L3, that was reported by our group for constructing a europium(III) complex with a high photoluminescence quantum efficiency.18 This tridentate ligand coordinates to the gadolinium(III) ion in the ratio of three to one, with no solvent coordination in the first coordination sphere, which can be proved by the crystal structure of Gd(L3)3 (Gd3) (Fig. S1). The elimination of solvent coordination helps to avoid quenching by O–H vibration from a solvent. Under a quite low temperature (77 K), the frozen-state ethanol solution of Gd3 (1 × 10−5 M) can produce a bright green emission. However, as the temperature is gradually increased, the emission disappears 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 (shown in Fig. S2). The reason is usually considered to be the unfreezing of molecular vibration (which would quench the emissions) with increased temperature.19

The solid state can afford more restricted surroundings for organic molecules. However, the emission of the gadolinium(III) complex cannot be observed even in the solid state (Fig. S3). Concentration quenching is regarded as the reason of such a phenomenon because the long-lifetime excited states may cause a serious triplet–triplet annihilation (TTA) in the solid powder state.20 As a substitute, polymers have a relatively rigid structure compared with the solution state, which can restrict the collision of molecules and reduce the rate of nonradiative decay. So we dispersed the gadolinium(III) complex in a polymethyl methacrylate (PMMA) matrix (0.1% to 2%, w/w) by drop casting its PMMA solution on a piece of quartz glass. However, this is still not enough since the triplet oxygen molecules in air may also quench the long-lived phosphorescence,21 leading to a quite faint emission. When such a film was placed inside a glove box under a complete nitrogen atmosphere, a bright green light was observed under a 365 nm UV excitation (Fig. S3). Therefore, we further encapsulated this PMMA film by covering its surface with another quartz slice and sealing the surrounding gaps with wax (to cut off oxygen) in the glove box (Fig. 1). As expected, a bright luminescence was still observed even when such an encapsulated film was taken outside.


image file: c9dt03050f-f1.tif
Fig. 1 Scheme of fabrication and encapsulation process of a PMMA film doped with title gadolinium(III) complexes.

image file: c9dt03050f-f2.tif
Fig. 2 The molecular structures of ligands.

The excitation and emission spectra of the encapsulated PMMA film were measured at room temperature. We can see broad peaks with a large Stokes shift of above 130 nm, originating from the excitation of the singlet state of L3 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 a much weaker spin–orbit coupling. Y3 showed a similar excitation spectrum to Gd3 but a quite different emission spectrum at a shorter wavelength, which was assigned as fluorescence emission (Fig. S4). The emission lifetime of Gd3 is 9.52 ms while the lifetime of Y3 is 14.1 ns, which also proves the phosphorescence nature of Gd3 emission. The lifetime of Gd3 is quite long because 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.

Furthermore, we expanded such Gd-based room temperature phosphorescence to other gadolinium(III) complexes. To achieve emission covering the entire visible light range, we introduced 4 other ligands with different triplet states. The structures of ligands are shown in Fig. 2. L1 and L4 were used as ligands to sensitize other lanthanide(III) ions in our previous work.18,22 The phosphorescence color of Gd1 is orange and that of Gd4 is sky-blue, which can be further tuned by suitable ligand structure modification. To increase the triplet energy of L1, we introduced the cyan group at 3-position of the 4-hydroxyl-1,5-naphthyridine segment, a strategy that was used by our group in previous work23 to reduce the highest occupied molecular orbital (HOMO) energy level and to increase the energy gap (derivative named as L2, synthetic route seen in Scheme S1). An increased triplet energy of ∼1000 cm−1 was obtained and the emission color was changed from orange to yellow (Gd1 to Gd2, Table 1, Fig. 3). The Triazole group was also integrated to modify the structure of L4, which can also increase the energy gap of the ligand by changing the pyridine cycle to a smaller one (from L4 to L5).


image file: c9dt03050f-f3.tif
Fig. 3 Phosphorescence emission spectra (a) and decay curve (b) of gadolinium(III) complexes in doped PMMA film (0.5%, w/w) encapsulated by wax at room temperature. λex = 410 nm, 360 nm, 370 nm, 290 nm and 300 nm for Gd1 to Gd5, respectively.
Table 1 Luminescence characterization of the title gadolinium(III) and yttrium(III) complexes in PMMA filmsa
  λ em (nm) CIE coordinatesb τ (ms) PLQY (%)
a The encapsulated PMMA films were prepared at a concentration of 0.5% (w/w) except for Gd5 which was 0.2%. Excitation wavelengths were 410 nm, 370 nm, 290 nm, 360 nm and 300 nm for Gd1/Y1 to Gd5/Y5, respectively. b A way of expressing colors. CIE stands for Commission Internationale de L'Eclairage.
Gd1 602 (0.57, 0.43) 1.75 18
Gd2 568 (0.46, 0.52) 2.11 20
Gd3 521 (0.29, 0.55) 9.52 66
Gd4 465 (0.21, 0.32) 4.41 22
Gd5 464 (0.18, 0.27) 8.58 54
Y1 513 (0.34, 0.49) 1.07 × 10−5 15
Y2 468 (0.29, 0.36) 7.26 × 10−6 31
Y3 453 (0.19, 0.21) 1.41 × 10−5 47
Y4 335 (0.20, 0.24) 1.89 × 10−6 32
Y5 398 (0.15, 0.09) 5.23 × 10−6 32


From the emission spectra of the five gadolinium(III) complexes (Fig. 3), it can be seen that the phosphorescence 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 have longer triplet emission lifetimes than those of the 3MLCT transitions24 but shorter than those of the heavy-atom free organic molecules.25

Absolute quantum yields of the encapsulated PMMA films with a doping concentration of 0.1% to 2% were obtained using an integrating sphere under ambient conditions. The results are shown in Table S3, which display excellent quantum yields (beyond 18% for all and best 68%) at their optimized concentration. As far as we know, the values are among the highest results of the ligand-based room-temperature phosphorescent materials.26 The appropriate concentration for the complexes is about 0.5%. Higher concentration may result in quenching by adjacent molecules through the TTA process while too low concentration may lead to partial dissociation of the complexes in PMMA. One should notice that the PLQYs of Gd1 and Gd2 are relatively low compared to those of other gadolinium(III) complexes. This is because the lower energy transitions (orange and yellow) are easier to be quenched compared to the higher energy transitions (blue and green).

In summary, we have developed a new strategy to achieve organic room-temperature phosphorescence by employing gadolinium(III) complexes. Special design of molecular structures ensures saturation coordination of the gadolinium(III) complexes, excluding the solvent quenching. To reduce the concentration quenching, the complexes are doped into a polymer in a rigid environment. The 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 phosphorescence emissions with emission 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 investigated in our lab.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This research is supported through grants from the National Basic Research Program of China (Grant 2017YFA0205100), National Natural Science Foundation of China (Grant 21621061), Key Project of Science and Technology Plan of Beijing Education Commission (Grant KZ201910028038), and Natural Science Foundation of Beijing Municipality (Grant 2172017).

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Footnote

Electronic supplementary information (ESI) available. CCDC 1937605. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/C9DT03050F

This journal is © The Royal Society of Chemistry 2019