An easy-to-make strong white AIE supramolecular polymer as a colour tunable photoluminescence material

Article abstract of DOI:10.1039/c8tc04540b

An easy-to-make supramolecular polymer was successfully constructed by self-assembly of tripodal tri-(pyridine-4-yl)-functionalized trimesic amide (DTB) in DMSO-H₂O binary solutions. The supramolecular polymer DTB showed strong white aggregatio

Full text of DOI:10.1039/c8tc04540b

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Materials Chemistry C
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An easy-to-make strong white AIE supramolecular
polymer as a colour tunable photoluminescence
material†
Cite this: J. Mater. Chem. C, 2018,
6, 13331
Yan-Qing Fan,a Juan Liu,*b Yan-Yan Chen,a Xiao-Wen Guan,a Jiao Wang,a
a
a
a
Hong Yao,
You-Ming Zhang,
Tai-Bao Wei *a and Qi Lin
*
An easy-to-make supramolecular polymer was successfully constructed by self-assembly of tripodal
tri-(pyridine-4-yl)-functionalized trimesic amide (DTB) in DMSO–H2O binary solutions. The supramole-
cular polymer DTB showed strong white aggregation-induced emission (AIE) in the solid state as well as
in water suspensions. DTB could also form a stable supramolecular polymer hydrogel (DTB-G) in pure
water. Interestingly, DTB showed excellent coordination ability for rare earth metal ions (Tb3+, Eu3+
,
La3+, Th4+ and Ce3+) and formed the corresponding complexes (DTB-Ms). Interestingly, by introducing
the above-mentioned rare earth metal ions, the fluorescence colour of the obtained DTB-Ms showed
corresponding variations. By inseting UV-LED lamps (l = 365 nm) into different rare earth metal
ion-coordinated xerogel DTB-Ms in small test tubes, a series of LED devices with multicolour lights were
obtained. Therefore, DTB and DTB-Ms can act as fluorescence colour-tunable photoluminescence materials.
Received 7th September 2018,
Accepted 15th November 2018
DOI: 10.1039/c8tc04540b
rsc.li/materials-c
emission systems such as polymers,10 metal organic frameworks,11
and small molecules.12 However, it is still a challenge to
Introduction
In recent years, multicolour photoluminescence materials1 have develop small-molecule luminophores that emit white light in
attracted significant research interest due to their extensive the solid state.
applications in lighting devices and display media,2 optical
The rare-earth metal ions Tb3+, Eu3+, La3+, Ce3+ and Th4+
sensors3 and switches.4 In the quest for white-light emitting play pivotal roles in many fields such as medical diagnostics,
materials,5 supramolecular systems6 displaying different stimuli- optical imaging,13 and luminescent sensing.14 Due to their
responsive emission colours are attracting considerable attention. excellent coordination ability and fluorescence properties, they
Meanwhile, white light-emissive materials have also attracted are used by many scientists. A series of rare earth metal ions
increasing attention owing to their potential application in lighting have been applied as photoluminescence materials.15 However,
devices and display media.7 As white-light emission covers the as a heavy metal ion waste, these ions may lead to long-term
whole visible region from 400 to 700 nm, the generation of white- potential threat to soils, groundwater and human beings;16
light emission commonly requires simultaneous emission of thus, the detection and separation of these rare-earth metal
three primary RGB colours (red, green and blue) or at least two ions are very important.
complementary colours.8 However, serious problems are encoun-
In view of these observations and as a part of our research
tered such as spectral instability and bad colour reproducibility.9 interest in supramolecular functional materials,17,18 herein,
To date, the most common strategy to solve these problems we reported a simple supramolecular polymer that was self-
is to develop small-molecule luminophores to emit white assembled from an easy-to-make tripodal gelator tri-(pyridine-
light efficiently in the solid state. In recent years, a variety of 4-yl)-functionalized trimesic amide (DTB). DTB could form a
approaches have been reported to develop efficient white-light supramolecular polymer hydrogel (DTB-G) in water and showed
strong white aggregation-induced emission (AIE). Interestingly,
a Key Laboratory of Polymer Materials of Gansu Province, College of Chemistry and
Chemical Engineering, Northwest Normal University, Lanzhou 730070, China.
E-mail: linqi2004@126.com, weitaibao@126.com
DTB showed excellent coordination ability for rare earth metal
ions (Tb3+, Eu3+, La3+, Th4+ and Ce3+) and formed corresponding
complexes (DTB-Ms). Also, by introducing the above-mentioned
rare earth metal ions, the fluorescence colour of the obtained
DTB-Ms showed corresponding variation. Therefore, DTB could
act as a fluorescence colour-tunable photoluminescence material.
Moreover, by coordination with Tb3+, Eu3+, La3 and Ce3+, DTB could
b College of Chemical Engineering, Northwest University for Nationalities,
Lanzhou 730070, China. E-mail: liujuan656@126.com
† Electronic supplementary information (ESI) available: Experiment details,
synthesis of DTB, 1H NMR, 13C NMR and MS spectra, other materials. See DOI:
10.1039/c8tc04540b
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form stable supramolecular polymer metallo-hydrogels in
pure water. Meanwhile, DTB also showed excellent separation
properties for Tb3+, Eu3+, La3+ and Ce3+
.
Results and discussion
As shown in Scheme S1 (ESI†), tripodal DTB was first synthe-
sized by a simple one-step reaction between trimesoyl chloride
and 4-aminopyridine. DTB was dissolved in DMSO (1 Â
10À3 M); however, with an increasing amount of pure water
in the DTB–DMSO solution, the solution showed strong white
emission (Fig. 2). As shown in Fig. 1c, a dilute DMSO solution
of DTB exhibited no Tyndall effect. However, with the addition
of water into the dilute solution of DTB, a clear Tyndall effect
could be observed. Moreover, in the SEM image (Fig. 1 and
Fig. S12, ESI†), the DTB xerogel obtained from DMSO–H2O
binary solution showed a fibriform structure. These results indicated
that with the addition of H2O in the DTB–DMSO solution, DTB
underwent a self-assembly process, forming a stable fibriform
supramolecular polymer. The self-assembly process induced
strong white emission. Therefore, the white light emission is an
aggregation-induced emission (AIE). Interestingly, DTB could
form a stable supramolecular hydrogel (c = 1 M) (Fig. 1a). The
DTB hydrogel (Fig. 1a) and DTB xerogel (Fig. 3a) also showed
strong white emission. Moreover, the water solubility of DTB is
very poor. DTB was almost insoluble in water. Therefore, in this
study, DTB was suspended in water, and we carried out the
corresponding research by using DTB suspensions.
Fig. 2 (a) Photographs of DTB solutions containing different water contents.
(b) Fluorescence spectra of DTB solutions containing different water contents.
concentration-dependent 1H NMR. As shown in Fig. 2, with the
addition of water, aggregation-induced emission including
white-light emission was observed, and great fluorescence increase
was observed at 475 nm after addition of water into the DTB–
DMSO solution. This result indicated that DTB could self-assemble
in water. The self-assembly mechanism was also investigated by
The possible assembly mechanism (Fig. 1) of DTB was
investigated by fluorescence spectroscopy, IR, XRD, SEM and
1
concentration-dependent H NMR (Fig. 3). In the concentration-
dependent 1H NMR spectra of DTB, with increasing concentration
of DTB, the proton signals of H1 (11.27 ppm) and H2 (8.86 ppm) on
DTB showed downfield shifts. This indicated that hydrogen bonds
existed in the self-assembly process of DTB. In addition, in the
powder X-ray diffraction (PXRD) pattern of DTB xerogel (Fig. S9,
ESI†), there are many sharp peaks at 2y from 15 to 30 degrees,
indicating that DTB has a long-range ordered structure in the
xerogel. Meanwhile, as shown in Fig. 1 and Fig. S12 (ESI†), the
Fig. 1 (a) The photograph of DTB hydrogel in water (1 M) and proposed
self-assembly mechanism of DTB. (b) Proposed coordination mechanism
of DTB with rare earth metals. (c) Photos showing the Tyndall effect of DTB
(a: in DMSO; b: in a binary solution of DMSO and H2O).
Fig. 3 Partial concentration-dependent 1H NMR spectra of the mixture of
DTB in DMSO-d6: (a) 1.37 Â 10À2 M; (b) 1.82 Â 10À2 M; (c) 2.28 Â 10À2 M;
(d) 2.73 Â 10À2 M; (e) 3.18 Â 10À2 M.
13332 | J. Mater. Chem. C, 2018, 6, 13331--13335
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morphology of DTB, investigated by a scanning electron micro-
scope (SEM), is an orderly fibriform structure. These results
indicated that DTB could self-assemble to form a supramolecular
polymer with an orderly fibriform structure by hydrogen bonds
(Fig. 1a).
Then, we introduced rare earth metal ions into water suspensions
of DTB to investigate the photoluminescence tunable properties of
DTB. Interestingly, upon the addition of 1.0 equiv. of Tb3+, Eu3+,
La3+, Ce3+ and Th4+ into the water suspension of DTB (2.2 Â 10À6 M),
through coordination with rare earth metal ions, DTB could
assemble into supramolecular metallo-hydrogels (DTB-Ms) with
Tb3+, Eu3+, La3+ and Ce3+ in pure water (Fig. S5, ESI†). More
interestingly, the critical gelation concentration (CGC) was
lower than 2.2 Â 10À6 M [about 0.5% (10 mg mLÀ1 = 1%)],
whereas the gel–sol transition temperature (Tgel) was in the
range from 51.3 to 76.1 1C (Table S1, ESI†). However, with the
addition of Th4+ into the water suspension of DTB, the suspension
changed into a clear colourless water solution (Fig. S5, ESI†) and
did not form a metallogel.
Interestingly, by introducing rare earth metal ions into
DTB, the xerogel of DTB-Ms showed multicolour tunable photo-
luminescence properties. As shown in Fig. 4a and 5, compared
with the strong white fluorescence emission of DTB xerogel,
due to the coordination with rare earth metal ions, the fluores-
cence emission of solid powder or xerogel of DTB-Ms changed
from white to multicolour photoluminescence. In addition, as
shown in Table 1, the xerogel of DTB and DTB-Ms (DTB-Tb,
DTB-Eu, DTB-La and DTB-Th) and solid powder of DTB-Th with
different colour fluorescence emissions have higher fluorescence
quantum yields.
Fig. 5 Fluorescence spectra of xerogel and solid powder of DTB and
DTB-M (for xerogel, Ms are Tb3+, Eu3+, La3+ and Ce3+; DTB-Th is solid
powder of DTB-Th complex).
Table 1 The fluorescence quantum yields (Ys) of xerogel DTB and
DTB-Ms (for xerogel, Ms are Tb3+, Eu3+, La3+ and Ce3+; DTB-Th is the
solid powder of the DTB-Th complex)
State
Samples
YS
1
2
3
4
5
6
DTB
0.452
0.501
0.483
0.595
0.492
0.100
DTB-Tb
DTB-Eu
DTB-Th
DTB-La
DTB-Ce
xerogels or solid powder have potential applications as multi-
colour photoluminescence materials.
Due to the excellent optical properties of rare earth metal
ion-coordinated DTB-Ms, the applications were primarily investi-
gated. As shown in Fig. 4b, by inseting UV-LED lamps (l = 365 nm)
into the different rare earth metal ion-coordinated xerogels or solid
powders of DTB-Ms in small test tubes and upon illumination,
these xerogels or solid powders emitted bright multicolour lights
such as white, red and green. Thus, a series of LED devices with
multicolour lights were obtained. Therefore, DTB and DTB-Ms
In addition, in view of excellent coordination ability of rare
earth metal ions with DTB and to explore the adsorption ability
of DTB for Tb3+, Eu3+, La3+ and Ce3+, the adsorption capacity of
DTB for Tb3+, Eu3+, La3+ and Ce3+ in water was assessed by
inductively coupled plasma (ICP) analysis. The DTB xerogel
(1 mg, 1 Â 10À7 mol) was suspended in a dilute water solution
of Tb3+, Eu3+, La3+ and Ce3+ (all concentrations are about 1 Â
10À5 M in 5.0 mL) and stirred for 10 min. Then, the suspension
was centrifuged at 10 000 rpm for 5 min, and the precipitate
was removed by filtration. The adsorption percentages of the
xerogel DTB for the above cations in water were assessed by ICP
analysis. As shown in Table 2, after the adsorption of these
cations by the xerogel of the host powder DTB in water, the
residual concentrations of Tb3+, Eu3+, La3+ and Ce3+ were in the
Table 2 The adsorption percentages of xerogel DTB for rare earth metal
ions
Ions
CI (mM)
CR (mM)
AP%
Tb3+
Eu3+
La3+
Ce3+
10
10
10
10
0.0796
0.0658
0.0978
0.0543
99.20
99.34
99.02
99.46
Fig. 4 (a) Fluorescence photographs of xerogel based on rare earth metal
ions. (b) Photographs of the 365 nm ultraviolet LED coated with the xerogel
of DTB and series of DTB-Ms (for xerogel, Ms is Tb3+, Eu3+, La3+ and Ce3+
DTB-Th is solid powder of DTB-Th complex) when the LED is turned on.
;
CI: initial concentration; CR: residual concentration; AP: absorbing
percentage.
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range of 5.43 Â 10À8 to 9.78 Â 10À8 M, and the adsorption Conflicts of interest
percentages of the host powder DTB for the above-mentioned
cations were up to 99.02–99.46%. These results indicated that
There is no conflict to declare.
DTB has excellent adsorption and separation capacities for
Tb3+, Eu3+, La3+ and Ce3+ in water.
Acknowledgements
The possible coordination mechanisms of DTB with Tb3+,
Eu3+, La3+ and Ce3+ were also investigated by IR, XRD and SEM.
The IR information of DTB and DTB-Ms was examined in the
solid state at room temperature to prove the coordination
mechanism. As shown in Fig. S6 (ESI†), the CQN vibration
This work was supported by the National Natural Science
Foundation of China (NSFC) (No. 21574104; 21662031; 21661028)
and the Program for Changjiang Scholars and Innovative Research
Team in University of Ministry of Education of China (No. IRT1177).
absorption peaks of pyridine for DTB appeared at 1508 cmÀ1
.
However, with the addition of Tb3+, Eu3+, La3+ and Ce3+ into
DTB, the CQN vibration absorption peaks shifted to 1515,
1522, 1515 and 1522 cmÀ1, respectively (Fig. S6–S8, ESI†); this
indicated that in the above-mentioned coordination inter-
action, N on pyridine acts as an electron-donating group and
forms stable metal coordination interaction with electron
acceptors (Tb3+, Eu3+, La3+ and Ce3+) (Fig. 1).
Moreover, the powder X-ray diffraction (PXRD) results
(Fig. S9–S11, ESI†) support the proposed coordination mechanism.
Taking DTB-Tb as an example, as shown in Fig. S9 (ESI†), XRD of
DTB-Tb xerogel shows peaks around 2y = 19.63, 22.57, 25.11 and
26.251 (d = 4.52, 3.94, 3.55 and 3.40 Å), which indicates that the
xerogel of DTB-Tb also exists in a long range ordered structure.
Finally, the morphologies of DTB-Ms were investigated by a
scanning electron microscope (SEM). SEM also supported the
proposed coordination mechanism of DTB-Ms (Fig. S13, ESI†).
The xerogel of DTB clearly appears as an orderly fibriform
structure (Fig. S12, ESI†). However, after coordination with rare
earth metal ions (Tb3+, Eu3+, La3+ and Ce3+) and forming DTB-Ms,
DTB-Ms presents as an agminated structure. The results also
indicated that there are strong coordination interactions between
DTB and rare earth metal ions (Tb3+, Eu3+, La3+ and Ce3+).
Notes and references
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supramolecular polymer DTB. DTB shows strong white aggregation-
induced emission (AIE). DTB also shows excellent coordination
ability for rare earth metal ions (Tb3+, Eu3+, La3+, Th4+ and Ce3+).
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