Hydration-Facilitated Fine-Tuning of the AIE Amphiphile Color and Application as Erasable Materials with Hot/Cold Dual Writing Modes

Article abstract of DOI:10.1002/anie.201911845

Hydration water greatly impacts the color of inorganic crystals, but it is still unknown whether hydration water can be utilized to systematically manipulate the emission color of organic luminescent groups. Now, metal ions with different hydration abilit

Full text of DOI:10.1002/anie.201911845

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Accepted Article
Title: Hydration-Facilitated Fine-Tuning the Color of AIE Amphiphile
and It’s Application as Erasable Materials with Hot/Cold Dual
Writing-Modes
Authors: Hongjun Jin, Hongpeng Li, Zhiyang Zhu, Jianbin Huang,
Yunlong Xiao, and Yun Yan
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201911845
Angew. Chem. 10.1002/ange.201911845
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http://dx.doi.org/10.1002/ange.201911845

Angewandte Chemie International Edition
10.1002/anie.201911845
RESEARCH ARTICLE
Hydration-Facilitated Fine-Tuning the Color of AIE Amphiphile
and It’s Application as Erasable Materials with Hot/Cold Dual
Writing-Modes
Hongjun Jin, Hongpeng Li, Zhiyang Zhu, Jianbin Huang, Yunlong Xiao, Yun Yan*
This paper it to dedicated to the 20 Anniversary of Aggregation Induced Emission (AIE)
Abstract: It has long been recognized that hydration water greatly
impacts the color of inorganic crystals, but it is still unknown whether
hydration water can be utilized to systematically manipulate the
emission color of organic luminescent groups. Herein, we report that
metal ions with different hydration ability allow fine-tuning the
emission color of a fluorescent group displaying aggregation induced
emission (AIE). Because the hydration water can be removed
facilely by gentle heating or mechanical grinding and re-gained by
solvent fuming, rewritable materials can be fabricated both in the
hot-writing and cold-writing modes. This hydration-facilitated strategy
will open up a new vista in fine-tuning the emission color of AIE
molecules based on one synthesis and in the design of smart
luminescent devices.
physical treatment. Therefore, it would be very promising if the
fluorescence can be manipulated via the hydration/ dehydration
process. In addition, variation of the coordinating metal ions may
add another tunable parameter toward the luminescence.
However, so far, this facile strategy hasn’t been adopted in the
design of luminescent materials.
In this work we show that by attaching a coordinating head to a
luminescent group displaying aggregation induced emission
(AIE),[
21-24]
the emission color of the fluorophore can be fine-
tuned between blue and yellowish green, simply by coordinating
to metal ions with different hydration ability. Further study
reveals that all the coordinating complexes, regardless of the
used metal ions, end in yellow as the hydration water is removed
by mild heating or mechanical grinding (Scheme 1). Thus we are
able to create the maximum color change in the process of
hydration/ dehydration simply by using appropriate coordinating
metal ions. Finally, we show that color change from blue to
yellow can be employed in erasable materials both in the mode
of hot-writing and cold-writing, corresponding to heating and
grinding triggered water loss, respectively. We expect the
present strategy opens a new paradigm in fine-tuning the
emission colors of AIE molecules based on one synthesis and in
the design of smart luminescent devices.
Introduction
Many inorganic crystals in nature contain a certain amount of
coordinating water, which induces energy level splitting of the
central metal ions. This results in absorption of visible light of
certain wavelength, so that the crystal displays complementary
color.[1] Well-known examples are CuSO
•5H O and CoCl •6H O.
4 2 2 2
They are blue and pink, but become white and blue, respectively,
as the coordinating water is lost upon heating. Recently, with the
extensive research on metal-organic framework (MOF),[2-8] it is
found that hydration water shows a miraculous impact on the
fluorescent color of the organic ligands in MOF materials.[9,10]
However, so far hydration water hasn’t been utilized to
manipulate the emission color of various luminescent groups.
Fine-tuning of luminescent colors of an organic material is
important to achieve multi-color displays and to meet the need of
next generation light-emitting materials.[11-15] To obtain light-
emission of an appropriate wavelength remains, however, a
challenge.[16,17] Usually, the luminescence of an organic
compound is manipulated by changing the chemical structure of
the luminescent or substituting groups,[18-20] which can be
achieved through complicated organic synthesis. Compared to
the organic synthesis strategy, the approach of hydration and
dehydration of a coordinating compound only involves one step
Scheme 1. Illustration of hydration-facilitated reversible fine-tuning the
emission color by coordinating of metal ions with different hydration ability.
H. Jin, H. Li, Prof. J. Huang, Prof. Y. Xiao, Prof. Y. Yan
Beijing National Laboratory for Molecular Sciences (BNLMS)
College of Chemistry and Molecular Engineering
Peking University
Beijing 100871, China
Email: yunyan@pku.edu.cn
Supporting information for this article is given via a link at the end of
the document.
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Angewandte Chemie International Edition
10.1002/anie.201911845
RESEARCH ARTICLE
Scheme 2. Synthesis of PBFL.
solving the notorious fluorescent quenching in organic solid-
state materials. Up to now, literatures have shown that PBF has
three fluorescent colors (blue, green and yellow) which are
related to the aggregation states and can be switched by
Results and Discussion
The hydrophobic diphenyldibenzofulvene (PBF) was chosen as
the fluorophore in this study, which was tethered to a hydrophilic
coordinating head (L) with three coordinating sites (Scheme 2,
and SI for synthesis details). PBF has been found to show
unusual aggregation-induced emission (AIE),[25,26] perfectly
-
grinding, heating and solvent fuming.[
27 30]
The coordinating head
L is reported to interact with most metal ions except the alkaline
family,[31] thus rendering judicious choice on the metal ions with
different hydration ability.
8000
6000
4000
2000
0
a
2+
2+
+
2+
2+
2+
2+
Mg , Ca , K , Sr , Ba , Mn , Pb
f
PBFL-Mg
2+
1
0
0
0
0
0
.0
2
+
PBFL-Ca
2
+
PBFL-Sr
.8
.6
.4
.2
2+
PBFL-Ba
.0
4
00
450
500
550
600
2
4
6
8
10
Wavelength(nm)
2 deg)
b
c
d
e
Figure 1. (a) Normalized fluorescent emission spectra of 0.5mM PBFL-M2+ dispersed in acetonitrile-water mixed solvent (1:1, v/v) and PBFL-2K+ in acetonitrile
excited at 350nm. The inset picture is the fluorescent photo of PBFL-M2+ under 365nm UV lamp. SEM images of the precipitate of (b) PBFL-Mg2+, (c) PBFL-Ca2+
,
and (d) PBFL-Ba2+. (e) HRTEM image of PBFL-Mg2+. (f) XRD patterns for PBFL-Mg2+, PBFL-Ca2+, PBFL-Sr2+ and PBFL-Ba2+
.
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Angewandte Chemie International Edition
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RESEARCH ARTICLE
The obtained PBFL is a coordinating amphiphilic AIE molecule.
The fresh solution of PBFL in acetonitrile-water mixed solvent
peaks recovered to their original position. This clearly verified
that the hydration water may trigger conformation change of the
PBF group, and the extent of the conformational change
depends on the hydration ability of the metal ions binding to the
head of the PBFL molecule. This conclusion is further
consolidated by the fact that the maximum fluorescent emission
of the PBF group in the hydrated precipitates systematically
changes with increasing the hydration ability of the metal ions.
The emission for all the dehydrated sample centered at 540 nm
(Figure 2c), whereas this value blue-shifted to 494 nm, 488 nm,
473 nm, and 468 nm, respectively, as the hydration ability is
(
1:1, v/v) is clear with almost no fluorescent emission. However,
upon addition of metal ions with different hydration ability, such
2
+
2+
2+
2+
as Mg , Ca , Ba , at the molar ratio of PBFL/M =1:1, the
solution immediately turns turbid and produces precipitates with
different fluorescence (Figure 1a). It is interesting to find the
emissions can be fine-tuned continuously from blue to yellowish
green simply by varying metal ions. Elemental analysis
confirmed that the molar ratio between PBFL and M2+ is 1:1 in all
the precipitates (Table S1). SEM images in Figure 1b-d show
the formation of different self-assembled structures (more
images in Figure S1). All these structures display typical lamellar
Bragg diffractions (Figure 1f and Figure S2). High-resolution
2
+
2+
increased by varying metal ions in the sequence of Ba , Sr ,
Ca2+ and Mg (Figure 1a).
2+
2
+
+
2+
TEM (HRTEM) images for PBFL-Mg , K , and Ca (Figure 1e
and S3) shows clear strips, confirming the formation of lamellar
structures. The lamellar thickness, d, obtained from the Bragg
diffractions, is in perfect agreement with the interlamellar
distance obtained from HRTEM. These d values (Table 1) are all
close to 2 times of the extending length of PBFL (2.18 nm),
Table 1. d values obtained from Bragg diffractions for the complexes formed
with PBFL and alkaline-earth metal ions before and after dehydration.
Sample [a]
d/nm, before heating
d/nm, after heating
PBFL-Mg2+
4.3
4.1
4.0
3.8
3.9
4.0
4.0
4.0
indicating that the lamellae are stacked from the bilayers of
PBFL-Ca2+
-
2
+
[32 37]
PBFL-M , just as those in other amphiphilic systems.
PBFL-Sr2+
It is noticed that the bilayer thickness in Table 1 decreases
regularly from 4.3 nm to 3.8 nm as the radii of the alkaline-earth
metal ions increase. This order is in line with the decreased
hydration ability of the alkaline-earth metals. For the alkaline-
earth metal series, the hydration enthalpies are -1920, -1620, -
PBFL-Ba2+
a
b
2
+
1
00
95
90
PBFL-Mg
1
.0
2
+
PBFL-Ca
2
+
PBFL-Sr
480 and -1360 kJ•mol , for Mg , Ca , Sr , Ba2+
-
1
2+
2+
2+
0.8
0.6
0.4
0.2
2+
2
1
PBFL-Ba
+
PBFL-Mn
respectively.[38] In line with this decreasing tendency, the number
of hydration water, which is obtained from the thermogravimetric
analysis (TGA) (Figure 2a), for each PBFL-M2+ pair also
decreases, are 5.0, 4.0, 2.4, and 1.6, respectively. We therefore
speculate that the bilayer thickness is mainly affected by the
different number of hydration water binding to the metal ions. To
test this hypothesis, the PBFL-M2+ complexes were heated to
remove the hydration water. Strikingly, all the dehydrated PBFL-
M2+ complexes displayed the same yellow fluorescence (Figure
2+
PBFL-Pb
2
+
PBFL-Mg
PBFL-Ca2+
8
8
7
5
0
5
2
+
PBFL-Sr
2+
2+
PBFL-Ba
PBFL-Mn
2+
PBFL-Pb
0
.0
400
450
500
550
600
40
80
120
T( C)
160
200
o
Wavelength(nm)
c
d
00
1
2000
1
2+
heated PBFL-Mg
2+
PBFL-Mg
N blowing
2
2+
1
0000
heated PBFL-Ca
2+
heated PBFL-Sr
95
2+
heated PBFL-Ba
8
6
4
2
000
000
000
000
0
90
2b), indicating that the emission change is solely triggered by
85
80
the extent of hydration, rather than the difference in the
electronic structure of metal ions. The same yellow emission
also suggests that the PBF groups have adopted the same
molecular conformation in all the dehydrated systems.[26-30] In
0
20
40
60
80
100
120
2
4
6
8
10
Time (min)
2 deg)
Figure 2. (a) TGA curves of PBFL-M2+ powder. (b) Normalized fluorescent
line with this, the bilayer thickness of the all the dehydrated
PBFL-M2+ became almost the same( Table 1 and Figure 2c).
emission spectra of the heated PBFL-M2+ dispersing in acetonitrile excited at
3
50nm. The inset is the photograph of PBFL-M2+ powder after heating under a
3
65nm UV lamp. (c) XRD patterns of the heated PBFL-M2+ powders. (d) TGA
It is notable that hydration triggered emission change is
curve of the PBFL-Mg2+ powder at the room temperature of 22±2 oC under
00 mL•min-1 flow. The inset is photograph of the PBFL-Mg2+ powder after
blowing under a 365nm UV lamp.
2
completely reversible at room temperature. If we use N flow
1
N
2
instead of heating, removal of the hydration water occurred as
well(Figure 2d), which also leads to the dehydrated yellow
emissive PBFL-M2+ complexes(inset in Figure 2d). All the yellow
N
2
It is amazing that the hydration on the remote coordinating head
has triggered conformational change of the PBF group at the
chain end at ambient temperature, which is in high analogy to
the allosteric effect occurs in nature.[36,39,40] Usually, variation of
emissive powders, no matter obtained by heating or by N
2
blowing, recovered their original emission after rehydrating the
PBFL-M2+ complexes by fuming with ethanol/ water mixed
solvent for 30 minutes(Figure S4), and the X-ray diffraction
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Angewandte Chemie International Edition
10.1002/anie.201911845
RESEARCH ARTICLE
the conformation of PBF group may occur upon heating, which
triggers irreversible phase transition from the green to yellow
emissive crystal. Crystal structure analysis revealed that the
torsion angles between the two phenyl rings in the PBF group
for the yellow emission and green emission state are different.[27]
Obviously, the current study suggests that hydration at a remote
site away from the PBF group has triggered similar
conformational change, as illustrated in Scheme 3.
(Figure 3a). The film was prepared by mixing PBFL-Mg2+ powder
with PDMS before solidification. After solidification, a transparent
film displaying blue-green emission was obtained. Images could
be thermally written using a soldering pen, and then be erased
by simply wetting or fuming with water/ethanol mixed solvent for
about 30 min. The writing and erasing could be conducted
repeatedly (Figure 3b-c).
It is interesting to find that the coordinating water can also be
removed under mechanical grinding. Figure 4a shows that the
blue PBFL-Mg2+ powder becomes yellow after grinding, and it
recovers blue immediately when fumed with ethanol/ water
mixed solvent. XRD measurements (Figure 4b) reveal that the
grinding has destroyed all the ordered lamellar structure, which
is in clear contrast with the retained lamellar structures when
treated with heating. However, TGA measurements (Figure S6)
reveal that the yellow powder has the same curve with the one
obtained by heating, and elemental analysis suggests that the
two samples have exactly the same composition (Table S1).
This result indicates that the yellow emission is only related to
the hydration state of PBFL-Mg2+ complex, rather than the
lamellar packing. It is clear that the grinding process has
removed the solvent coordinated to the metal ions, which results
in conformation change of the PFB group.
Scheme 3. Illustration of the hydration-facilitated reversible conformation
change of PBF group.
a
.0
c
orignal
1
0
0
0
0
0
heated
rehydrated
.8
.6
.4
.2
.0
4
00
450
500
550
600
Wavelength(nm)
.
8000
before grinding
after grinding
rehydrated
a
b
b
6
000
000
4
Figure 3. (a) Normalized fluorescent emission spectra of PBFL-Mg2+ before
heating, after heating, and rehydrated by fuming with ethanol/ water,
respectively. Insets are the corresponding photos under 365 nm UV lamp. (b)
Illustration of the hot writing process: PBFL-Mg2+ powder was dispersed into
liquid PDMS monomer and casted on a glass slide, then heated at 60 °C for 4
hours to obtain a transparent blue emissive film. Yellow images could be
obtained on the blue fluorescent film when written with a soldering pen around
2000
0
2
4
6
8
10
2
 deg)
Figure 4. (a) Photograph of PBFL-Mg2+ powder after grinding and its
rehydrated one under 365nm UV lamp. (b) XRD patterns of the original PBFL-
Mg2+ powder before grinding (black line), the ground powder (red line) and the
2
50 °C. The image can be erased by wetting or fuming with water/ ethanol
rehydrated powder (blue line), respectively. The inset is the procedures of
repeating writing and erasing process of the PBFL-Mg2+ casted on a filter
mixed solvent about 30 min. (c) Experimental results of the repeated writing
and erasing process on the PDMS film doped with PBFL-Mg2+. Photos were
taken under 365 nm UV lamp.
paper: writing with a glass rod gently, and erasing by fuming with water/
ethanol mixed solvent. Photos were taken under 365 nm UV lamp.
Both the hydrated and the dehydrated PBFL-M2+ powder can be
stable for months in ambient environment. Only strong desiccant,
Finally, we show that the grinding triggered color change
such as P
2
O
5
, can remove the hydration water within 12 hours in
2+
enables direct cold-writing on the native film of PBFL-Mg . Such
vacuum (Figure S5). Reversible transition between hydration
and dehydration occurs only when heating/air blowing and
mixed solvent fuming is applied alternatively. The sufficient
stability and excellent reversibility of the two luminescent states
allow us to create two-color photoluminescence switching
materials. For instance, an erasable film capable of hot-writing
can be made with the PBFL-Mg2+ system, since it offers the
largest color change in the hydration/ dehydration process
a PBFL-Mg2+ film was prepared by loading a thin layer of PBFL-
Mg2+ powder onto a filter paper. The surface of filter paper is
coarse enough so that the PBFL-Mg2+ powder can be entrapped
into the gaps between the paper fibers. Writing with a glass stick
at room temperature on the PBFL-Mg2+ paper would lead to a
yellow image on the bright blue emissive background. This
image could be easily erased upon fuming with mixed water/
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Angewandte Chemie International Edition
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RESEARCH ARTICLE
Thermogravimetric analysis (TGA) was carried out on TA Instrument
Q600 SDT at a heating rate of 10ꢀ°C•min-1 under nitrogen flow of 100
mL•min-1 from room temperature to 800°C.
ethanol vapor, and repeated writing and erasing is possible,
(
Inset in Figure 4b), too
Conclusion
Differential scanning calormetry (DSC) wad carried on TA Instrument
Q100 DSC at a heating rate of 5ꢀ°C•min-1 under nitrogen flow of 50
mL•min-1 from room temperature to 300°C.
In summary, we demonstrated a new strategy of hydration
facilitated fine-tune of fluorescent color using the amphiphilic
coordinating PBFL. Upon coordinating to metal ions with
Elemental Analysis (EA) of the PBFL-M2+ powders were carried out on
Vario EL elemental analyzer with 0.1% accuracy for C, H and N.
2
+
different hydration ability, PBFL-M exhibited almost continuous
fluorescent color change from blue to yellowish green. When the
coordinating water was removed by heating, desiccation, or
grinding, all the PBFL-M2+ complexes give out the same yellow
emission, which can be reverted to their original state when
rehydrated. The molecular conformation may simply be adjusted
by coordinating water. The interconversion between the
hydrated and dehydrated states results in a reversible two-color
photoluminescence switching. Within the help of this hydration-
facilitated fluorescent color-tuning strategy, it’s very easy to get
the largest emission change for a potential application as high
performance optical recording material.
Acknowledgements
The authors are grateful to National Natural Science Foundation
of China (Grant No. 91856120, 21573011,21633002) and the
Beijing National Laboratory for Molecular Sciences (BNLMS) for
financial support.
Keywords: Aggregation-Induced-Emission • hydration water •
coordination • fine-tune • emission color •
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-
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Entry for the Table of Contents
RESEARCH ARTICLE
Hydration water on metal ions can be
utilized to reversibly fine-tune the
emission colour of organic
Hongjun Jin, Hongpeng Li, Zhiyang Zhu,
Jianbin Huang, Yunlong Xiao, Yun Yan*
luminescent groups.
Page No. – Page No.
Title
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