Multicolored fluorescence variation of a new carbazole-based AIEE molecule by external stimuli

Article abstract of DOI:10.1039/d0cp02783a

In this article, we design and synthesize a new carbazole-based molecule, Cz2CN, with a twisted D-A structure, using the carbazole group as the donor and a dicyanoethylene fragment as the acceptor. Such a twisted D-A structure endows Cz2CN with two charac

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Page 1 of 7Journal Name
DOI: 10.1039/D0CP02783A
Cite this: DOI: 10.1039/c0xx00000x
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ARTICLE TYPE
Multicolored Fluorescence Variation of a New Carbazole-Based AIEE
Molecule by External Stimuli
Yan Liu,ae Aisen Li,be Zhimin Ma,c Weiqing Xu,b Zhiyong Ma,*a and Xinru Jiad
5
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
In this article, we design and synthesize a new carbazle-based molecule Cz2CN with twisted D-A structure, using carbazole group as
the donor and dicyanoethylene fragment as the acceptor. Such twisted D-A structure endows Cz2CN with two characteristic emission
bands, LE emission and ICT emission. Cz2CN is enabled with AIEE, solvatochromism, different response to anisotropic shearing force
10 and isotropic hydrostatic pressure, due to the sensitivity of ICT emission to aggregation, solvent polarity and mechanical force.
Aggregation benefits the ICT emission, leading to the AIEE of Cz2CN in THF/water system. Polar solvent can help stabilize the ICT
excited states and make the ICT emission red shift. The original crystalline powder is strongly fluorescent with high quantum yield of
40.4%. The single crystal of Cz2CN is obtained and dimers without π-π interactions among carbazole groups contribute to the strong
emission. Anisotropic shearing alters the emission of Cz2CN powder from sky blue (474 nm) to green (520 nm). The single crystal of
15 Cz2CN undergoes a distinct multicolored variation from sky-blue (476 nm) to green (510 nm) and further to orange (590 nm) by
isotropic hydrostatic pressure. The emission of the original powder/crystal is dominant by LE emission in HLCT. Upon force/pressure,
the ICT emission becomes dominated and red shifts progressively. To the best of our knowledge, Cz2CN is an interesting carbazole-
based molecule that shows simultaneous AIEE, solvatochromism and force-induced multicolored variation.
20
50 hydrostatic pressure, although these factors are of great
significance in the applications of these materials.
Introduction
Smart organic luminescence materials mean that such materials
25 can respond to external stimuli and give out some signal.1-3 Solid-
state smart organic materials with adjustable and stimuli-
responsive luminescence have been drawing considerable
attention due to their broad applications in organic light-emitting
diodes, sensors, display systems, anti-counterfeiting and data
30 storage.4-8 However, many organic π-conjugated molecules with
strong fluorescence in solution become non-fluorescent in the
CN
O
O
NC
H
NC
CN
N
F
55
N
NH4OAC, AcOH, 100℃,24h
t-BuOK, DMF
110℃,13h
N
toluene, reflux
CzC=O
Cz2CN(74%)
solid state because of the notorious aggregation caused quenching
10
(ACQ).9,
In 2001, Prof. Tang discovered the aggregation-
Scheme 1. The synthetic route of Cz2CN.
induced emission enhancement (AIEE) and provided a new
35 strategy to develop strongly emissive solids by tuning nonplanar
molecular structure and regulating supramolecular interactions.11-
16 Carbazole, as an important luminescence group, has to be faced
with ACQ owing to its coplanar structure, which hampers its
wide application. How to acquire carbazole-based luminescent
40 molecules with AIEE is challenging and needs to be solved.
60
Carbazole group is widely utilized as an electron donor to
fabricate luminogens.40-43 As reported, luminogens with twisted
donor-acceptor (D-A) structure often show unique energy band
owing to the existence of intercrossed hybridized local and
65 charge transfer (HLCT) excited state.44-51 Modifying the electron
acceptor and the conjugated spacer between donor and acceptor
can easily change the luminescent features of D-A molecules.46,
52It is well known that restriction of intramolecular motions (RIM)
is proposed as the mechanism for the AIE/AIEE effect, so AIEE
70 luminogens glow weakly in dilute solution due to intramolecular
motion but become highly emissive in solid state because the
intermolecular π-π stacking is avoided, and the twisted geometry
help in reducing the π-π staking interaction. Hence, our design
concept is to introduce highly twisted acceptor to prevent the
75 whole molecules from being too close and finally to achieve
Mechanical force, as an easily obtainable and eco-friendly
stimulus, can perturb the molecular orientation, molecular
conformation and intermolecular interactions and finally lead to
an emission color change.17-19 Mechanical force includes different
45 forms, for example, grinding, shearing, stretching, tension and
hydrostatic pressure.20-30 Some force-responsive carbazole
derivatives have been reported.31-39 However, there is nearly no
literature that has reported that carbazole derivatives show
responsiveness to both anisotropic shearing force and isotropic
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AIEE. Meanwhile, the molecular conformation and the 65 Figure 1. (a) Fluorescent images of Cz2CN in mixed solvents of
intermolecular interaction can be disturbed by mechanical force,
leading to the ICT emission shift and finally achieving
multicolored switch. 45, 53-56
THF/water with different water fraction (fw) under 365 nm UV
light; (b) UV-Vis absorption and (c) fluorescence (λex=365 nm)
spectra of Cz2CN (20 μM) in THF/water mixtures with different
fw.
As well-known, carbazole tends to form π-π interaction due to
its coplanar structure, which is detrimental to strong fluorescence
in the solid state. Unexpectedly, Cz2CN displays remarkable
aggregation-induced emission enhancement (AIEE) property
(Figure 1a&Figure S4). We carried out fluorescence and
5
In this article, we design and synthesize a new carbazle-based
molecule Cz2CN with twisted D-A structure, using carbazole
group as the donor and dicyanoethylene fragment as the acceptor.
Such twisted D-A structure endows Cz2CN with two
characteristic emission bands, LE emission and ICT emission.
70
10 Cz2CN is enabled with AIEE, solvatochromism, different
response to anisotropic shearing force and isotropic hydrostatic
pressure, due to the sensitivity of ICT emission to aggregation,
solvent polarity and mechanical force. Aggregation benefits the
ICT emission, leading to the AIEE of Cz2CN in THF/water
15 system. Polar solvent can help stabilize the ICT excited states and
make the ICT emission red shift. The original crystalline powder
is strongly fluorescent with high quantum yield of 40.4%. The
single crystal of Cz2CN is obtained and unique dimers without π-
π interactions among carbazole groups contribute to the strong
20 emission. Anisotropic shearing alters the emission of Cz2CN
powder from sky blue (474 nm) to green (520 nm). The single
crystal of Cz2CN undergoes a distinct multicolored variation
from sky-blue (476 nm) to green (510 nm) and further to orange
(590 nm) by isotropic hydrostatic pressure. The emission of the
25 original powder/crystal is dominant by LE emission in HLCT.
Upon force/pressure, the ICT emission becomes dominated and
red-shifts progressively. To the best of our knowledge, Cz2CN is
an interesting carbazole-based molecule that shows simultaneous
AIEE, solvatochromism and force-induced multicolored variation.
30 This study is expected to pave a new way for further
understanding the relationship between the D-A molecular
structure and stimuli-responsive properties and lay the foundation
for their application.
75 absorption spectra to verify the AIEE activity. In the diluted THF
solution, the maximum absorption band and the emission peak of
Cz2CN were located at 275 nm and ~415 nm (average lifetime,
τa= 1.40 ns), respectively. When lifting the water fraction (fw), the
Mie scattering effect was observed in the absorption spectra,
80 revealing that aggregated particles took shape (Figure 1b). As fw
increased, the emission at 415 nm weakened while a new
emission band at 550 nm (τa=4.71 ns, Figure S5) appeared and
enhanced gradually (Figure 1c). The dynamic light scattering
(DLS) study was performed on the solution of Cz2CN in 99%
85 water/THF(v/v) to investigate the particle size distribution. As
shown in Figure S6, and one can note that their sizes are
concentrated at 312.3 nm in average, indicating that the emission
enhancement of Cz2CN originates from the formation of
nanoaggregates. According to our previous work,55-57 the 415 nm
90 emission and the 550 nm emission are attributed to the LE
emission of carbazole and ICT emission, respectively. Obviously,
the D-A system of Cz2CN are both ICT-active and AIEE-active.
Upon gradual addition of water (as a poor solvent), significant
aggregation occurs and the AIEE effect dominates the ICT effect
95 with the consequence that the ICT emission at 550 nm rises
(Figure 1c). Therefore, aggregation prohibited the non-radiation
channels and benefited the ICT emission, finally resulting in the
strong fluorescence with high quantum yield (Table S2).
35 Results and discussion
100
The target molecule Cz2CN is composed of a carbazole group
and a dicyanoethylene fragment bridged by a benzene ring.
CzC=O was gained via the typical aromatic nucleophilic
substitution
reaction
between
carbazole
and
1-(4-
40 fluorophenyl)ethan-1-one. Cz2CN was acquired as a light yellow
powder with a yield of 74% by a typical condensation reaction,
keeping CzC=O and malononitrile in the solvent of acetic acid at
100 oC for 24 h. The molecular structure and geometry of Cz2CN
105
110
115
1
is confirmed by H NMR, 13C NMR, HR-MS and single crystal
45 data. The synthetic procedures and the molecular characterization
are available in the Supporting Information (Scheme 1, Figure
S1-S3, Table S1).
50
55
60
Figure 2. (a) Fluorescent images of Cz2CN in different organic
solvent under 365 nm UV light; (b) UV-Vis absorption and (c)
fluorescence (λex=365 nm) spectra of Cz2CN (20 μM) in
120 different organic solvent.
Cz2CN is featured with a Donor-Acceptor structure. Firstly,
quantum mechanical computations via DFT method were
performed to gain the highest occupied molecular orbital (HOMO)
125 and the lowest unoccupied molecular orbital (LUMO) of Cz2CN
(Figure S7). The electron cloud of the HOMO is distributed at the
carbazole group while that of the LUMO is mainly confined over
the dicyanoethylene fragment, indicating that Cz2CN has a
separated D-A structure, which enables the molecule to undergo a
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typical ICT process. Secondly, we tested the photophysical
property in different organic solvent (Figure 2a), and the
corresponding data were illustrated in Table S3. In
dichloromethane, THF, acetone, ethanol and acetonitrile, the ICT
style forms in the single crystal. Viewing along a axis and c axis,
the dimers can be observed clearly (Figure S10&S12). There are
strong interactions between the carbazole group (head) and the
dicyanoethylene fragment (tail) (Figure 3b), such as C-H…π
5
emission of Cz2CN was detected at ~550 nm (Figure S8) and the 70 (2.805 Å and 2.840 Å). The spacer benzene ring interacts with
intensities differed widely (Figure 2c). As documented,45 ICT
emission is highly sensitive to external environment, which is in
consistence with our data. In this case, the ICT process is also
allergic to solvent polarity but shows something different. To
each other via enhanced π-π interactions (3.379 Å, 3.379 Å) and
also displays interaction with carbazole through C-H…π (2.858
Å). However, no interaction among carbazole groups is observed.
The unique dimer structure can confine the molecular motion due
10 better evaluate the effects of solvents on the emission features, 75 to the strong intermolecular interations and also prevent excitons
the relationship between the solvent polarity parameter and the
Stokes shift according to the Lippert-Mataga plot was
investigated (Figure S9), which displays an almost linear
correlation of the Stokes shift with solvent polarity, reflecting the
from quenching by keeping the whole molecules from being too
close, leading to strong light emission. In addition, the methyl
groups effectively decrease the fractional of free volumes within
the crystal lattice and subsequently suppress the non-radiative
15 larger charge separation and higher dipole moment in the excited 80 decay pathways of the excited structural relaxation.
state than in the ground state. Even though Cz2CN showed
solvatochromism, the ICT emission was quite weak in polar
solvents due to free intramolecular motion. Given the AIEE
property of Cz2CN, intensity of ICT emission much more
Cz2CN shows solid-state stimuli-responsive property. The
mechanofluochromic data are summarized in Table S4.As
depicted above, emission of the original powder was located at
474 nm with a sky-blue color (quantum yield: Φ=0.404). The
20 depends on the aggregation state than the solvent polarity. It is 85 emission band had a short lifetime of 7.01 ns (Figure S14a),
worth noting that Cz2CN is an interesting D-A molecule that
possesses both AIEE property and solvatochromism.
assigned to prompt fluorescence. The powder of Cz2CN was
sensitive to external force. When shearing by a blunt pestle, the
emission red-shifted to 520 nm (Φ=0.214) with a brilliant green
and became broad. The new emission band was also ascribable to
90 prompt fluorescence due to its short lifetime of 3.53 ns (Figure
S14b). Why the quantum yield decreased after shearing was
probably due to that π-π interactions formed among carbazole
groups. Moreover, there are many intermolecular interactions
among crystalline powder so that the nonradiative relaxation
95 channel is prohibited, finally resulting in a high quantum yield.
Meanwhile, the nonradiative decay rate (knr) of Cz2CN powder
increased a lot (from 82.37×106 S-1 to 219.38×106 S-1) by simply
grinding, so the QY of the ground powder decreased.The green
color would be totally restored to the original sky-blue and the
100 474 nm emission band appeared again after being treated by
dichloromethane vapor (Figure 4a-d). Thus, the abovementioned
data unearth that Cz2CN displays reversible mechanofluochromic
behavior. We deduced that the fluorescence at 474 nm and the
fluorescence at 520 nm were both attributed to the hybridized
105 local and charge transfer excited emission band (HLCT). This
assumption would be discussed in detail hereinafter.
25
30
35
40
45
110
115
120
Figure 3. (a) The molecular geometry of Cz2CN; (b) the unique
dimer with strong interactions and (c) the unit cell in the single
crystal. White：Hydrogen, Grey：Carbon, Blue: Nitrogen.
50
Interestingly, Cz2CN demonstrated strong solid-state
luminescence. The original crystalline powder was obtained
from vacuum evaporation and gave out shiny sky-blue
fluorescence. The emission band was centred at 474 nm; the
55 quantum yield was quite high and calculated to be 40.4%.
Generally, the solid-stated luminescent property is closely
relevant to the stacking mode. Luckily, the single crystal of
Cz2CN was successfully cultivated by slow evaporation of
petroleum ether into the dichloromethane solution. Cz2CN
60 crystallizes according to the space group of P 1 21/c 1 (No. 14)
and the single crystal is in the monoclinic crystal system with
four molecules in each unit cell (Figure 3c, Figure S10,
S11&S12). The dihedral angle between carbazole group and
benzene ring is 40.0o and Cz2CN adopts a twisted geometry
65 (Figure 3a). Obviously, a kind of dimer in a head-tail packing
125
Figure 4. Optical images of (a) original sky-blue powder, (b)
green powder after shearing, (c) restored powder by DCM vapor;
the upper is under room light; the bottom is under UV light. (d)
Fluorescent spectra of original powder (black line), powder after
130 grinding (red line) and restored sample (blue line); (e) DSC
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Page 4 of 7
curves of the original powder, the ground powder.
To gain sight into the mechanochromic mechanism, X-ray
diffraction (XRD) and differential scanning calorimetry (DSC)
disappeared and phase transition occurred from crystalline form
to amorphous state, resulting in the planarization of Cz2CN.
Planarization is conducive to the ICT emission, leading to the red
were carried out to explore the microstructure variation. For the 15 shift. Upon treatment of DCM vapor, the XRD pattern would
5
original powder, a number of sharp scattering peaks were
detected and these peaks were consistent with the simulated
pattern based on the single crystal data, suggesting that the
original powder has identical packing mode with the single
return to the original state (Figure S15). DSC thermograms
further evidenced the phase transition. The original powder
exhibited two endothermic peaks at 168 oC and 175 oC, implying
an unexpected crystal-crystal transition. After grinding, a new
crystal. The molecular structure of Cz2CN was stuck in the 20 cold-recrystallization peak appeared at 63 oC and two
10 twisted geometry. In this regard, the LE emission was dominated
in the HLCT emission. After shearing, these sharp peaks
endothermic peaks at 160 oC and 171 oC (Figure 4e, Figure
S16&S17).
25
30
35
40
45
50 Figure 5. (a) Fluorescent images of the Cz2CN single crystal at different pressure. The corresponding normalized fluorescent spectra of
the Cz2CN single crystal during (b) compression, (c) decompression.
To clearly see the mechanofluochromism of Cz2CN and
quantify the relationship between the emission shift and the
55 external force, the single crystal sample was subjected to high
pressure by using the diamond anvil cell (DAC) device with
due to the D-A structure of Cz2CN and eventually reached 590
nm. It is rational that the increasing pressure compels the adjacent
molecules to approach much closer and makes the molecular
geometry of Cz2CN more planarized, leading to the red shift of
silicone oil as the pressure-transmitting medium (PTM). In-situ 80 ICT emission. The emission split originates from the separation
fluorescence spectra and in-situ micrographs under different
pressures were recorded to dynamically observe the color
60 alteration. Remarkably, Cz2CN demonstrated fascinating
pressure responsiveness. The single crystal underwent a distinct
of hybridized local and charge transfer excited states. When the
high pressure was released, the emission split disappeared and the
overall fluorescence could not return to 476 nm (Figure 5b&5c
and Figure S18), indicating that the high pressure would damage
multicolored variation from sky-blue to green and further to 85 the single crystal structure
orange when the hydrostatic pressure increased (Figure 5a).
Initially, the sample afforded a dominant sky-blue fluorescence at
65 476 nm, in accordance with the original powder sample. The
emission wavelength change could be divided into two stages. At
the first stage, when the pressure was below 2.59 GPa, the
emission wavelength gradually shifted from 476 nm to 510 nm.
At the second stage, when further lifting the pressure, the
70 emission band could split into two parts. A new group of
emission peaks at ~450 nm developed and the position kept
unchanged while the long-wavelength emission still red-shifted.
The new short-wavelength emission should be the LE emission of
carbazole groups because of its dull responsive to pressure. The
75 long-wavelength emission could be ascribed to the ICT emission
90
95
Figure 6. Raman spectra of the Cz2CN single crystal during (a)
the compression process and (b) the decompression process.
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† Electronic Supplementary Information (ESI) available: [details of
60 experimental sections, including synthesis, characterization, spectra,
SAXSS patterns and optical images]. See DOI: 10.1039/b000000x/
To monitor the molecular-level change, we conducted high-
pressure Raman spectra to monitor the Cz2CN single crystal. The
Raman spectra could track the molecular structure changes during
compression within the wavenumber range from 200 cm-1 to
2000 cm-1. As shown in Figure 6, most Raman vibration modes
exhibited blue-shifts as expected because of the decreased
interatomic distances and the increased effective force constants
upon compression.58, 59 When the pressure was increased to 6.50
10 GPa, no new Raman peaks appeared and all the Raman peaks
were retained, implying that only conformation change not phase
transition occur during the compression process. The increasing
pressure is not able to change the molecular structure of Cz2CN.
Once the pressure returned to atmosphere, the measured Raman
15 spectrum was identical with the original one (Figure S19),
suggesting the conformational change is reversible.
Based on the above results, several key points can be
concluded. (1) Cz2CN possesses two characteristic emissions, the
LE emission and the ICT emission, due to the twisted D-A
20 structure. (2) The ICT emission is more sensitive to the external
environment, such as solvent polarity, aggregation; particularly,
aggregation can enhance the ICT emission of Cz2CN. (3) The
solid-state emission color change upon force/pressure is
dominated by ICT emission, because mechanical stimuli is more
25 likely to disturb the intermolecular interactions and molecular
conformation which are both closely related to the ICT emission.
5
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30 solvatochromism and force-induced multicolored variation. The
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of Cz2CN composed of LE component and ICT component. In
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Acknowledgments
105
110
115
This work is financially supported by the Beijing Natural Science
40 Foundation (2182054), the National Natural Science Foundation
of China (21704002), the Big Science Project from BUCT
(XK180301), and the Fundamental Research Funds for the
Central Universities to Z. Y. Ma.
22.
23.
Notes and references
45 a Beijing State Key Laboratory of Organic-Inorganic Composites, College
of Chemical Engineering, Beijing University of Chemical Technology,
Beijing 100029, China. E-mail: mazhy@mail.buct.edu.cn
b State Key Laboratory for Supramolecular Structure and Materials,
Institute of Theoretical Chemistry, College of Physics, Jilin University,
50 Changchun 130012, China.
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DOI: 10.1039/D0CP02783A
For Table of Contents use only:
Multicolored
Fluorescence
Variation
of
a
New
Carbazole-Based AIEE Molecule by External Stimuli
Yan Liu,ae Aisen Li,be Zhimin Ma,c Weiqing Xu,b Zhiyong Ma,*a and Xinru Jiad
In this article, we design and synthesize a new carbazle-based molecule Cz2CN with twisted D-A
structure, using carbazole group as the donor and dicyanoethylene fragment as the acceptor. The
twisted D-A structure endows Cz2CN with two characteristic emission bands, LE emission and
ICT emission. Cz2CN is enabled with AIEE, solvatochromism, different response to anisotropic
shearing force and isotropic hydrostatic pressure, due to the sensitivity of ICT emission to
aggregation, solvent polarity and mechanical force. Aggregation benefits the ICT emission,
leading to the AIEE of Cz2CN in THF/water system. Polar solvent can help stabilize the ICT
excited states and enhance the ICT emission.
The original crystalline powder is strongly
fluorescent with high quantum yield of 40.4%. The single crystal of Cz2CN is obtained and
dimers without π-π interactions among carbazole groups contribute to the strong emission.
Anisotropic shearing alters the emission of Cz2CN powder from sky blue (474 nm) to green (520
nm). The single crystal of Cz2CN undergoes a distinct multicolored variation from sky-blue (476
nm) to green (510 nm) and further to orange (590 nm) by isotropic hydrostatic pressure. The
emission of the original powder/crystal is dominant by LE emission in HLCT. Upon
force/pressure, the ICT emission becomes dominated and red-shifts progressively. To the best of
our knowledge, Cz2CN is a rare carbazole-based molecule that shows simultaneous AIEE,
solvatochromism and force-induced multicolored variation.