Aggregation-induced emission (AIE)-active highly emissive novel carbazole-based dyes with various solid-state fluorescence and reversible mechanofluorochromism characteristics

Article abstract of DOI:10.1016/j.dyepig.2019.01.057

Three new carbazole-based fluorescent molecules 1–3 functionalized with tetraphenylethene have been successfully synthesized, and these compounds have high thermal stability, and they exhibited different fluorescence in solid states with the superior lumi

Full text of DOI:10.1016/j.dyepig.2019.01.057

Accepted Manuscript
Aggregation-induced emission (AIE)-active highly emissive novel carbazole-based
dyes with various solid-state fluorescence and reversible mechanofluorochromism
characteristics
Fei Zhao, Zhao Chen, Congbin Fan, Gang Liu, Shouzhi Pu
PII:
S0143-7208(18)32576-2
https://doi.org/10.1016/j.dyepig.2019.01.057
DYPI 7330
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 22 November 2018
Revised Date: 17 January 2019
Accepted Date: 18 January 2019
Please cite this article as: Zhao F, Chen Z, Fan C, Liu G, Pu S, Aggregation-induced emission (AIE)-
active highly emissive novel carbazole-based dyes with various solid-state fluorescence and reversible
mechanofluorochromism characteristics, Dyes and Pigments (2019), doi: https://doi.org/10.1016/
j.dyepig.2019.01.057.
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ACCEPTED MANUSCRIPT
Aggregation-induced emission (AIE)-active highly emissive novel
carbazole-based dyes with various solid-state fluorescence and
reversible mechanofluorochromism characteristics
Fei Zhao, Zhao Chen**, Congbin Fan, Gang Liu, Shouzhi Pu*
Synopsis
Three carbazole-based fluorescent molecules were successfully synthesized.
These luminogens showed color-tunable solid-state fluorescence, high thermal
stability,
typical
aggregation-induced
emission,
and
reversible
mechanofluorochromism characteristics.

ACCEPTED MANUSCRIPT
Aggregation-induced emission (AIE)-active highly emissive novel
carbazole-based dyes with various solid-state fluorescence and
reversible mechanofluorochromism characteristics
Fei Zhao, Zhao Chen**, Congbin Fan, Gang Liu, Shouzhi Pu*
Jiangxi Key Laboratory of Organic Chemistry, Jiangxi Science and Technology
Normal University, Nanchang 330013, PR China.
Tel: +86-791-83831996 Fax: +86-791-83831996
** Corresponding author E-mail: chenzhao666@126.com
* Corresponding author E-mail: pushouzhi@tsinghua.org.cn
ABSTRACT
Three new carbazole-based fluorescent molecules 1-3 functionalized with
tetraphenylethene have been successfully synthesized, and these compounds have
high thermal stability, and they exhibited different fluorescence in solid states with
the superior luminescence quantum yields of 99.04% (1), 98.90% (2) and 39.83% (3).
Their aggregation-induced behaviors were explored by the study of
photoluminescence spectroscopy. The results showed that luminogens 1-3 exhibited
remarkable aggregation-induced emission effect. Furthermore, their distinct
mechanical stimulus-responsive fluorescence characteristics were also surveyed by
solid-state photoluminescence spectroscopy. Interestingly, the various emitting
colors of these luminogens could be changed into the same green, and the
repeatabilities of their mechanochromic luminescence behaviors were outstanding,
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and the powder X-ray diffraction results indicated that the reversible conversion
from a crystalline to an amorphous state was responsible for the obvious
mechanofluorochromism phenomena of compounds 1-3. This work will be valuable
for the exploitation of mechanical-force sensors with typical aggregation-induced
emission feature.
Keywords: Carbazole; Tetraphenylethene; High thermal stability; Different
fluorescence; Aggregation-induced emission; Mechanofluorochromism
1. Introduction
Organic luminescence materials exhibiting bright fluorescence in the solid state
have received considerable attention in recent decades in view of their promising
applications in optoelectronic devices, sensors and display devices [1-6]. In
particular, mechanofluorochromic functionalized materials, displaying different
luminescent colors induced by mechanical force, have been attracting continuing
interests from researchers because of their broad applications in the fields of pressure
sensors, data storage and rewritable media [7-17]. Generally, the molecular packing
of mechanochromic luminescence molecules can be adjusted by various
intermolecular interactions such as π-π interactions, hydrogen bonding and halogen
bonding, and no chemical structure damage appear in mechanofluorochromic
luminogens during their reversible luminescence change processes [18-24]. To date,
numerous mechanical stimulus-responsive materials have been reported. However,
the types of the corresponding mechanochromic materials are limited, and examples
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of high-efficiency mechanochromic smart fluorescent materials with obvious color
change before and after stimulating are still inadequate. Indeed, a lot of effort has
been made to exploit high-contrast mechanofluorochromic materials. There is no
doubt that strong solid-state emission is a crucial factor for the development of
mechanochromism materials with clear color contrast. Unfortunately, the majority of
traditional emissive materials exhibit very bright luminescence in their dilute
solutions, but the bright luminescence is often weakened or quenched at high
concentrations, a well-known phenomenon referred to as aggregation-caused
quenching (ACQ) [25], and the ACQ effect largely hinders the exploitation of
high-performance mechanical force-responsive materials. Excitingly, Tang et al.
discovered the aggregation-induced emission (AIE) phenomenon [26], which is
commonly caused by the restriction of intramolecular motions[27,28]. Luminophors
with AIE effect can overcome ACQ, and achieve strong solid-state emission. Indeed,
the exploitation of AIE-active fluorogenic molecules has attracted intense research
interest due to their potential applications in the fields of biomedical imaging,
sensors and organic light emitting diode [29-37]. For example, AIE-active fluorogens
can be designed to exhibit very weak fluorescence in aqueous media, and their
fluorescence will be turned on upon interacting with target analytes, which is
extremely significative for biosensing and imaging applications of these fluorogenic
molecules with AIE feature. Until now, luminogens simultaneously possessing AIE
and reversible mechanochromism characteristics are still scarce, not to mention
highly thermal stable luminogens with these interesting properties. Therefore, the
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discovery of AIE-active mechanochromic highly thermal stable compounds is a
significative and challenging research topic, and it deserves great attention.
Carbazole-based derivatives are very valuable candidates in the fields of
photoelectronic devices and sensors [38-45]. Nevertheless, the notorious ACQ effect
limits the rapid development of these materials with carbazole skeletons. Thus, it is
also a very important and urgent challenge to synthesize AIE-active luminogens
based on a carbazole scaffold structure. In this work, we designed and prepared three
novel carbazole-based tetraphenylethene derivatives with different substituents
(Chart 1). Aggregation-induced behavior and mechanofluorochromic feature of these
highly emissive dyes were systematically investigated by photoluminescence
spectroscopy, dynamic light scattering and powder X-ray diffraction. Indeed, all
these different solid-state light-emitting compounds showed superior thermal
stability, outstanding AIE and reversible mechanochromic fluorescence
characteristics.
Chart 1. The molecular structures of carbazole-based compounds 1-3.
2. Materials and methods
2.1. Experimental
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General: All manipulations were carried out under an argon atmosphere by using
standard Schlenk techniques, unless otherwise stated. The starting materials
3,6-dibromo-9H-carbazole, (4-(trifluoromethyl)phenyl)boronic acid, phenylboronic
acid, (4-methoxyphenyl)boronic acid, n-(4-iodophenyl)acetamide, nitrobenzene and
tetrabutylammonium bromide (TBAB) purchased from J&K Chemical were used as
received. All other starting materials and reagents were obtained as analytical-grade
from commercial suppliers and used without further purification. Compounds 1-1
[46], 1-2 [46], 1-3 [46] and 1-4 [47] were prepared by procedures described in the
1
corresponding literatures. H NMR (400 MHz) and 13C NMR (100 MHz) spectra
1
were collected on American Varian Mercury Plus 400 spectrometer (400 MHz). H
NMR spectra were reported as followed: chemical shift in ppm (δ) relative to the
chemical shift of TMS at 0.00 ppm, integration, multiplicities (s = singlet, d =
13
doublet, t = triplet, m = multiplet), and coupling constant (Hz). C NMR chemical
shifts reported in ppm (δ) relative to the central line of triplet for CDCl3 at 77 ppm,
Mass spectra were obtained on a Bruker AmaZon SL Ion Trap Mass spectrometer.
Elemental analyses (C, H, N) were carried out with a PE CHN 2400 analyzer. The
absorption spectra were measured on an Agilent 8453 UV/Vis spectrophotometer.
Fluorescence spectra were recorded on
a
Hitachi-F-4600 fluorescence
spectrophotometer. XRD studies were recorded on a Shimadzu XRD-6000
diffractometer using Ni-filtered and graphite-monochromated Cu Kα radiation (λ =
1.54 Å, 40 kV, 30 mA). The N, N-dimethyl formamide (DMF)/water mixtures with
various water fractions were prepared by slowly adding ultra-pure water into the
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DMF solution of samples. Absolute fluorescence quantum yields and fluorescence
lifetimes were measured by Edinburgh FLS1000 spectrometer. Dynamic light
scattering (DLS) measurements were performed by NanoBrook 90Plus (Brookhaven
Instruments). Column chromatographic separations were carried out on silica gel
(200-300 mesh). TLC was performed by using commercially prepared 100-400 mesh
silica gel plates (GF254) and visualization was effected at 254 nm.
2.2. Synthesis
2.2.1. Synthesis of compounds 1, 2 and 3. Into a 50 mL, two-necked, round-bottom
flask were placed 1-4 (1234 mg, 3.0 mmol), Pd2(dba)3 (57.4 mg, 0.1 mmol), and
t-BuONa (384.4 mg, 4.0 mmol). The flask was evacuated under a vacuum and then
flushed with dry nitrogen three times. Toluene (30 ml), 1-1 (546.5 mg, 1.0 mmol) or
1-2 (410.5 mg, 1.0 mmol) or 1-3 (470.6 mg, 1.0 mmol), and toluene solution of
(t-Bu)3P (2 ml) were injected into the flask, and the mixture was refluxed for 48 h
and then cooled to room temperature. The solution was poured into water (60 ml)
and extracted with dichloromethane three times. The combined organic layers were
washed with brine, dried (Na2SO4), and concentrated in vacuo. In the end, the
residues were purified by silica-gel column chromatography, affording the expected
solid product 1 (white), 2 (yellowish) or 3 (yellow) in a yield of 80%, 75% or 76%,
1
respectively. 1: H NMR (400 MHz, CDCl3): δ (ppm) = 8.42 (s, 2H), 7.83 (d, J = 8
Hz, 4H), 7.74-7.68 (m, 6H), 7.50 (d, J = 8 Hz, 2H), 7.38 (d, J = 8 Hz, 2H), 7.20-7.03
(m, 32H), 6.96-6.88 (m, 8H). 13C NMR (100.6 MHz, CDCl3): δ (ppm) = 147.2,
145.4, 145.3, 144.0, 143.6, 143.5, 141.6, 140.9, 140.5, 139.0, 132.4, 132.1, 131.4,
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131.3, 130.6, 129.2, 128.9, 128.6, 128.2, 127.7, 127.7, 127.6, 127.5, 126.5, 126.4,
125.8, 124.0, 123.8, 123.7, 123.1, 119.1, 110.6. ESI-MS (m/z): 1207.3 [M]+. Anal.
Calcd. for C84H56F6N2: C, 83.56; H, 4.68; N, 2.32. Found: C, 83.49; H, 4.76; N, 2.26.
2: 1H NMR (400 MHz, CDCl3): δ (ppm) = 8.39 (s, 2H), 7.74-7.67 (m, 6H), 7.48 (t, J
13
= 8 Hz, 6H), 7.41-7.33 (m, 4H), 7.19-7.03 (m, 32H), 6.95-6.87 (m, 8H). C NMR
(100.6 MHz, CDCl3): δ (ppm) = 146.8, 145.4, 144.0, 143.7, 143.5, 142.0, 141.0,
140.8, 140.6, 138.8, 133.5, 132.3, 131.4, 128.8, 127.7, 127.6, 127.5, 127.3, 126.6,
126.5, 126.4, 126.4, 125.6, 124.1, 123.9, 123.5, 118.8, 110.2. ESI-MS (m/z): 1071.4
[M]+. Anal. Calcd. for C82H58N2: C, 91.93; H, 5.46; N, 2.61. Found: C, 91.98; H,
1
5.50; N, 2.54. 3: H NMR (400 MHz, CDCl3): δ (ppm) = 8.33 (d, J = 4 Hz, 2H),
7.67-7.61 (m, 6H), 7.46-7.38 (m, 4H), 7.18-7.02 (m, 36H), 6.95-6.87 (m, 8H), 3.88
(s, 6H). 13C NMR (100.6 MHz, CDCl3): δ (ppm) = 158.7, 146.7, 145.4, 144.0, 143.7,
143.5, 140.8, 140.7, 140.6, 138.8, 134.7, 133.1, 132.3, 131.4, 128.3, 127.7, 127.6,
127.5, 126.5, 126.4, 126.4, 125.3, 124.2, 123.9, 123.5, 118.4, 114.3, 110.2, 55.4.
ESI-MS (m/z): 1131.4 [M]+. Anal. Calcd. for C84H62N2O2: C, 89.17; H, 5.52; N, 2.48.
Found: C, 89.10; H, 5.61; N, 2.43.
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F
F
F
F
F
F
F
F
F
F
F
F
F
F
F
B(OH)2
I
NHAc
H2SO4, H2O
K2CO3, CuI,
Nitrobenzene
TBAB, toluene,
Pd(PPh3)4, K2CO3(aq)
THF, MeOH
N
N
NHAc
NH2
1-1
Br
Br
B(OH)2
I
NHAc
H2SO4, H2O
THF, MeOH
N
N
K2CO3, CuI,
Nitrobenzene
TBAB, toluene,
Pd(PPh3)4, K2CO3(aq)
N
H
NHAc
NH2
H3CO
OCH3
1-2
H3CO
OCH3
H3CO
B(OH)2
I
NHAc
H2SO4, H2O
THF, MeOH
N
K2CO3, CuI,
Nitrobenzene
TBAB, toluene,
Pd(PPh3)4, K2CO3(aq)
N
NHAc
NH2
1-3
F
F
F
F
F
F
Br
N
N
1-4
1-1
Pd2(dba)3, t-BuONa, P(t-Bu)3
1
H3CO
OCH3
Br
Br
N
N
N
N
1-4
1-4
1-2
1-3
Pd2(dba)3, t-BuONa, P(t-Bu)3
Pd2(dba)3, t-BuONa, P(t-Bu)3
2
3
Scheme 1. Synthesis routes of compounds 1-3.
3. Results and discussion
3.1. Synthesis
As shown in Scheme 1, the carbazole-based tetraphenylethene derivatives with
different substituents were prepared in 80%, 75% or 76% yield respectively
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according to the substitution reaction of intermediate product 1-1, 1-2 or 1-3 and
intermediate product 1-4.
3.2. Aggregation-induced emission (AIE) characteristics of compounds 1-3
To investigate the AIE behaviors of compounds 1-3, the UV-Vis absorption
spectra of luminogens 1-3 (20 µM) in DMF-H2O mixtures with different water
fractions (fw) were studied initially (Supporting information: Figs. S1-S3). Indeed,
level-off tails in the visible region were observed as the fw increased. This interesting
phenomenon is commonly associated with the well-known Mie scattering effect, and
it is the signal of nano-aggregate formation [48]. Next, the photoluminescence (PL)
spectra of 1-3 (20 µM) in DMF-H2O mixtures with various water contents were
systematically explored. As can be seen in Fig. 1, luminogens 1-3 are almost
non-emissive in pure DMF, and the corresponding fluorescence quantum yield is
1.15% for 1, 0.48% for 2, or 2.03% for 3, respectively. However, when the water
content in the DMF solution was increased to 30%, a new green emission band was
observed, with a maximum (λmax) at 505 nm for 1, 507 nm for 2, or 508 nm for 3,
and a strong green luminescence with the related fluorescence quantum yield up to
68.01% (1), 67.92% (2) or 59.90% (3) could be observed as the fw value was
increased to 90%.
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Fig. 1. PL spectra of the dilute solutions (2.0 × 10-5 mol L-1) of luminogen 1 (a),
2 (b) or 3 (c) in DMF-H2O mixtures with various water contents (0%-90%).
Excitation wavelength = 365 nm. The inset shows the corresponding fluorescence
images of 1 (a), 2 (b) or 3 (c) in pure DMF as well as 90% water fraction under 365
nm UV light.
Fig. 2. Decay curves of luminogens 1-3 (505 nm for 1, 507 nm for 2 and 3) in
DMF-water mixtures with 90% volume fraction of water. Concentration：2.0 × 10-5
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mol L-1.
Fig. 3. Size distribution curves of 1 (a), 2 (b) and 3 (c) in DMF-water mixtures
with 90% volume fraction of water. Concentration：2.0 × 10-5 mol L-1.
In addition, as shown in Fig. 2, the emission lifetime of 1, 2 or 3 in 90% water
fraction is 4.32 ns, 4.41 ns or 4.22 ns, respectively. There is no doubt that water is a
nonsolvent for luminogens 1-3, and thus an increase in the water content of mixed
solvent caused the aggregate formation. In fact, the nano-aggregates obtained were
characterized by dynamic light scattering (DLS) experiments (Fig. 3). Therefore, the
bright green fluorescence of 1-3 was attributed to aggregate formation. Clearly, 1-3
belong to AIE-active luminous molecules.
3.3. Solid-state fluorescence characteristics and thermal stability of compounds 1-3
To investigate the solid-state fluorescence behaviors of luminogens 1-3, the
solid-state PL spectra of 1-3 were studied.
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Fig. 4. (a) Solid-state PL spectra of compounds 1-3. The fluorescence images of
luminogens 1-3 under 365 nm UV light: (b) the solid sample of 1; (c) the solid
sample of 2; (d) the solid sample of 3.
As evident from Fig. 4, the emission spectrum of 1 with two trifluoromethyl
groups showed an emission band with λmax at 465 nm, corresponding to a blue
emission under 365 nm UV light, and 2 showed a broad emission band with λmax at
490 nm, and it exhibited blue-green fluorescence upon UV illumination at 365 nm.
However, 3 with two methoxyl groups exhibited green luminescence with a λmax at
497 nm. On the other hand, to evaluate the thermostability of the compounds 1-3,
their thermogravimetric analyses (TGA) were carried out. As presented in Fig. 5,
luminogens 1-3 are thermally stable and possess high onset degradation temperatures
(Td) ranging from 480 to 540℃.
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Fig. 5. TGA thermograms of solid samples 1-3.
3.4. The mechanofluorochromic behaviors of compounds 1-3
Subsequently, the mechanochromic fluorescence characteristics of compounds
1-3 were further researched by PL spectroscopy. As shown in Fig. 6, the solid sample
of 1 showed blue luminescence, and its fluorescence quantum yield (QY) is 99.04%.
However, upon grinding of the corresponding solid sample using a pestle, a new
emission peak with λmax at 510 nm was observed, and the strong blue emission was
converted into the green luminescence (QY = 61.92%). Meanwhile, as shown in Fig.
S4, the emission lifetime of before or after grinding is 2.52 ns or 2.50 ns,
respectively. On the other hand, upon treatment of the ground sample with
dichloromethane solvent, the green-emitting sample could be reverted to its initial
blue color. Therefore, 1 exhibited obvious and reversible mechanofluorochromic
behavior. Furthermore, this reversible mechanochromic fluorescence could be
repeated numerous times without obvious changes by repeated grinding-exposure
(Fig. S5). The possible mechanism of mechanofluorochromic phenomenon of 1 was
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examined by powder X-ray diffraction (PXRD) experiment. As can be seen in Fig. 7,
the XRD pattern of the as-synthesized sample exhibited a lot of sharp diffraction
peaks, indicative of its crystalline nature. Interestingly, after grinding, the clear and
sharp reflection peaks vanished, which implied that grinding resulted in the
transformation from a crystalline to an amorphous state. In contrast, the original
intense diffraction peaks reappeared after treatment by exposure to
dichloromethane vapor, suggesting that the crystalline phase was restored. Thus, the
PXRD measurements indicated that a crystal-to-amorphous phase conversion was
responsible for the reversible mechanofluorochromism phenomenon of compound 1.
Consistent with the mechanochromic behavior of 1, as shown in Fig. 8 and Fig. 9,
compounds 2 and 3 also showed similar mechanochromic fluorescence
characteristics, and their emission lifetimes of before and after grinding are shown in
Fig. S6 and Fig. S9. Moreover, the repeatabilities of their mechanofluorochromic
behaviors were also excellent (Fig. S7 and Fig. S10). In addition, as presented in Fig.
S8 and Fig. S11, the mechanism of their mechanofluorochromism phenomena was
also attributed to the switchable morphology changes between the crystalline and
amorphous states.
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Fig. 6. (a) Solid-state PL spectra of compound 1 before grinding, after
grinding, and after treatment with dichloromethane vapor. Excitation wavelength:
365 nm. Photographic images of 1 under 365 nm UV illumination: (b) the unground
solid sample. (c) the ground solid sample. (d) the solid sample after treatment with
dichloromethane vapor.
unground
ground
treated with CH Cl
2
2
10
20
30
40
50
2θ
(degree)
Fig. 7. XRD patterns of compound 1: unground, ground and after treatment
with dichloromethane vapor.
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Fig. 8. (a) Solid-state PL spectra of compound 2 before grinding, after grinding,
and after treatment with dichloromethane vapor. Excitation wavelength: 365 nm.
Photographic images of 2 under 365 nm UV illumination: (b) the unground solid
sample. (c) the ground solid sample. (d) the solid sample after treatment with
dichloromethane vapor.
Fig. 9. (a) Solid-state PL spectra of compound 3 before grinding, after grinding,
and after treatment with dichloromethane vapor. Excitation wavelength: 365 nm.
Photographic images of 3 under 365 nm UV illumination: (b) the unground solid
sample. (c) the ground solid sample. (d) the solid sample after treatment with
dichloromethane vapor.
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4. Conclusions
In summary, three novel carbazole-based tetraphenylethene-containing
luminogens with different substituents were designed and synthesized to investigate
their AIE, solid-state fluorescence, thermal stability, and mechanofluorochromic
characteristics. Noteworthily, compounds 1-3 showed high thermal stability and
bright solid-state fluorescence with outstanding luminescence quantum yield of
99.04%, 98.90% or 39.83%, and their solid-state emission could be adjusted by
various substituents. Furthermore, luminogens 1-3 showed typical AIE effect. In
addition, 1-3 also exhibited reversible mechanofluorochromism phenomena. Indeed,
the various solid-state emitting colors of these luminogens could be changed into the
same green, and the repeatabilities of their mechanochromic fluorescence behaviors
were excellent. The PXRD results indicated that the reversible conversion from a
crystalline to an amorphous state was responsible for the reversible
mechanofluorochromic characteristics of 1-3. This work will be beneficial for the
exploitation of highly thermally stable mechanical-force sensors with AIE feature.
Acknowledgments
The authors are grateful for the financial support from the National Natural
Science Foundation of China (21702079), the “5511” Science and Technology
Innovation Talent Project of Jiangxi Province (20165BCB18015), the key project of
Natural Science Foundation of Jiangxi Province (20171ACB20025), the youth
project of Natural Science Foundation of Jiangxi Province (20171BAB213004), the
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Young Talents Project of Jiangxi Science and Technology Normal University
(2017QNBJRC005), and the Project of the Science Fund of Jiangxi Education Office
(GJJ180593).
Supporting Information Available
UV/visible-absorbance spectra of compounds 1-3 (2.0 × 10-5 mol L-1) in DMF-water
mixtures with various water contents (0-90%). Solid-state PL spectra and
photographic images of compounds 2 and 3 in various solid states. Decay curves of
compounds 2 and 3 in various solid states. Repetitive experiments of
mechanochromic behaviors for compounds 2 and 3. XRD patterns of compounds 2
and 3 in various solid states, and Copies of NMR spectra and Mass spectra.
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Highlights
© Three novel carbazole-based tetraphenylethene-containing compounds were
synthesized.
© All these compounds showed high thermal stability and strong solid-state
fluorescence.
© These luminogens exhibited typical aggregation-induced emission characteristics.
© These luminogens with different substituents exhibited color-tunable solid-state
fluorescence.
© These luminogens exhibited highly reversible mechanofluorochromic behaviors.