Synthesis and application of reversible fluorescent photochromic molecules based on tetraphenylethylene and photochromic groups

Article abstract of DOI:10.1039/C8NJ05388J

Tetraphenylethylene-naphthopyran (TPE-NP) and tetraphenylethylene-spirooxazine (TPE-SO) with conjugated photochromic groups and fluorophores were designed and synthesized. Both of them exhibit the comprehensive effects of aggregation-induced emission (AIE

Full text of DOI:10.1039/C8NJ05388J

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Synthesis and application of reversible fluorescent
photochromic molecules based on
Cite this: New J. Chem., 2019,
tetraphenylethylene and photochromic groups†
43, 617
a
a
a
a
b
Shanchao Peng, Jiahui Wen, Mingtan Hai, Zhou Yang, * Xiaotao Yuan,*
Dong Wang,
a
a
a
Hui Cao and Wanli He
Tetraphenylethylene-naphthopyran (TPE-NP) and tetraphenylethylene-spirooxazine (TPE-SO) with conju-
gated photochromic groups and fluorophores were designed and synthesized. Both of them exhibit the
comprehensive effects of aggregation-induced emission (AIE) and photochromism while different
photochromic groups lead to a unique photochromic rate and fluorescence intensity. Among them, TPE-NP
has significant photochromic and fluorescence effects with good stability. Therefore, it could be used as an
anti-counterfeiting ink and is expected to be used in fluorescence switches in the future.
Received 24th October 2018,
Accepted 24th November 2018
DOI: 10.1039/c8nj05388j
rsc.li/njc
1
4–17
emission (AIE).
TPE-based materials are easy to synthesize
Introduction
and modify, and exhibit high AIE effect values (the extent of the
emission enhancement) and excellent solid-state fluorescence.
Therefore, the combination of photochromic molecules with
1
2
Since the first discovery of photochromism in 1867 by Fritsche,
great progress has been made in the research of organic and
inorganic photochromic materials. Research on various organic TPE is envisioned to achieve a new type of multi-responsive
3
4
photochromic systems such as diarylethene, spiropyran,
luminescent material. Qi et al. reported multicolored tunable
4
5
6
spirooxazine, fulgide, and azobenzene gradually emerged. emission anti-counterfeiting inks and super-resolution imaging
Naphthopyrans (NPs) are well known as an important class of agents by blending spiropyran-functionalized with a distyryl-
organic photochromic compound because of their ease of anthracene derivative. Li et al. conjugated dithienylethene and
1
2
preparation, high coloration, rapid color reversibility and good tetraphenylethene as a novel switchable fluorescent material,
7
–9
fatigue resistance.
Spirooxazine compounds have special which has potential for utilization in localization-based super-
photochromic properties; in particular, their two isomers have resolution imaging as an alternative optical nanoimaging
1
8
outstanding fatigue resistance.
agent. Therefore, it can be imagined that the conjugation
In most research regarding luminescence as a detection or of TPE with naphthopyran or spirooxazine by a covalent bond
10
readout signal, the photochromism is investigated in solution.
will enhance their performance and extend their application
However, optoelectronic applications of photo-switches often fields.
11
require the photoswitch to be in a solid or aggregated state.
Inspired by the above design strategy, we synthesized two
The primary challenge in solid-state photoswitches concerns tight new molecules (TPE-NP and TPE-SO) through covalent connec-
molecular packing in aggregated molecules, which will impede tion (Scheme 1). TPE-NP and TPE-SO show distinct photo-
12
the cis–trans transformation. Thus, providing a large free volume chromic and AIE properties. These molecules not only can
in the solid state is vital to promote effective photoswitching. emit bright fluorescence, but also demonstrate reversible
13
Recently, tetraphenyl-ethylene (TPE) and its derivatives with a solid-state luminescence switching under alternating UV light
twisted p-conjugated structure have attracted substantial research and visible light treatment.
interest. It’s highly 3D structure avoids strong intermolecular p–p
stacking; this particular phenomenon is aggregation-induced
Results and discussion
Design and synthesis
a
b
School of Materials Science and Engineering, University of Science & Technology
Beijing, Beijing, China. E-mail: yangz@ustb.edu.cn
TPE-NP and TPE-SO were synthesized by a series of chemical
reactions according to Scheme 1. The detailed synthetic procedure
and characterization are described in the Experimental section
and ESI.†
School of Chemistry and Biological Engineering, University of Science &
Technology Beijing, Beijing, China
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/
c8nj05388j
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Scheme 1 Synthetic routes to TPE-NP and TPE-SO.
Aggregation-induced emission properties
UV light. Therefore, the isomerizations of TPE-NP and TPE-SO
in dichloromethane were measured. It was easy to find from the
UV/vis absorption of TPE-NP and TPE-SO in Fig. 2, that the
two compounds can undergo reversible photoisomerization
between their unconjugated closed form (CF) isomer and
conjugated open form (OF) isomer under the irradiation of
a 365 nm ultraviolet lamp. The absorption peak at 362 nm for
TPE-NP-CF (tetraphenylethylene-naphthopyran-closed form)
gradually decreased and a new absorption peak at 490 nm
assigned to the OF appeared and increased with exposure
duration (Fig. 2a). It is clearly seen that the absorption peak
at 292 nm and 354 nm for TPE-SO-CF gradually decreased and a
new absorption peak at 466 nm assigned to the OF appeared
and increased under continuous UV irradiation (Fig. 2b). The
OF absorption peak of TPE-NP (484 nm) has an obvious redshift
compared with the OF absorption peak of TPE-SO (466 nm),
which implies that the TPE-NP-OF has a better p-conjugated
effect than that of TPE-SO-OF (tetraphenylethylene-spirooxazine-
open form). Interestingly, TPE-NP can much more easily undergo
photoisomerism compared to TPE-SO, because TPE-NP can
achieve the maximum ring-open ratio after 10 s UV irradiation,
whereas TPE-SO needs nearly 24 min. Thus, the different linking
groups between NP and SO will lead to the different photochromic
behaviors and capacity. The reason may be that the worse
p-conjugation between TPE and spirooxazine-OF in TPE-SO-OF
TPE-NP and TPE-SO are two potential fluorophores because of
the large Stokes shift (TPE-NP: labs = 360 nm, lem = 480 nm.
TPE-SO: labs = 350 nm, lem = 475 nm). The AIE properties of
TPE-NP and TPE-SO were measured in their THF/water solution
and shown in Fig. 1. When the water fraction (f
w
) is lower than
70%, the PL curve is almost parallel to the horizontal coordi-
nate, which indicated that there is no emission in pure THF
solution. However, when a large amount (470%) of water
(a poor solvent) was added to its THF solution, the peak of
TPE-NP at 480 nm and the peak of TPE-SO at 475 nm can be
clearly found. The mechanism for this AIE phenomenon
has been thoroughly studied and the increased fluorescence
intensity attributed to the formation of molecular aggregates,
in which the intramolecular free rotation has been blocked.
In the meantime, the fluorescence intensity of TPE-SO was
significantly higher than that of TPE-NP by comparing
the fluorescence spectra and fluorescence images of the two
molecules.
Photochromic properties in the solution state
Similar to the reported spirooxazine and naphthopyran
molecular switches, their isomerization can be triggered by
(tetraphenylethylene-naphthopyran-open form) reduces the
Fig. 1 (a and b) Fluorescent images of TPE-NP and TPE-SO, respectively,
in different water fraction (f
and d) Fluorescence spectra of TPE-NP and TPE-SO (10
respectively, in THF/H O mixtures with different f values.
w
) mixtures under 365 nm UV illumination.
Fig. 2 Time-dependent UV/vis absorption spectra of (a) TPE-NP and
(b) TPE-SO (10 M) in dichloromethane recorded under 365 nm hand-
held ultraviolet lamp irradiation.
ꢀ5
ꢀ5
(
c
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Fig. 3 Photos of TPE-NP-CF and TPE-NP-OF powders in visible and
under UV light (365 nm) against a white background. (a) Photograph under
visible light for TPE-NP-CF, (b) photograph under visible light for TPE-NP-OF,
and (c) fluorescence image of TPE-NP-CF and TPE-NP-OF.
Scheme 2 Schematic of TPE-NP-CF and TPE-NP-OF showing photo-
chromism and fluorescence/quenching in solution and the ‘‘aggregate’’
state respectively.
stability of the SO moiety by aggregating the positive charge
of the nitrogen cation of indoline and makes photochromism
1
9
more difficult. In contrast, TPE-NP-OF has good stability and
is prone to photochromism.
Scheme 2 illustrates the photochromism of TPE-NP and
the mechanisms of solution quenching and enhanced solid Fig. 4 Illustration of TPE-NP as an anti-counterfeiting ink on a TLC plate
state fluorescence. Non-radiative decay occurs due to the free printed with ‘‘USTB’’. (a) The unprinted image taken under visible light, and
(
b) its fluorescence image. (c) The printed image taken under visible light
internal motions of molecules in solution, resulting in quenching
and non-fluorescence. Conversely, the solid state results in
restriction of intramolecular rotations (RIR) of the molecules,
after 40 s UV irradiation, and (d) its fluorescence image.
20
and subsequent enhanced emission. After transformation into
the opened form, the new peak at around 480 nm in the
absorption spectra overlaps with the TPE emission and energy
transfer occurs, which leads to the quenching of the emission.
This process can explain the ‘‘on-state’’ and ‘‘off-state’’ phenom-
enon in the solid state of TPE-NP-OF and TPE-NP-CF. Photos of
the two isomeric forms of the solid state (Fig. 3) show changes in
color and fluorescence in the open and closed forms. TPE-NP-CF
appears yellow-green (Fig. 3a) under visible light. After 40 s UV
(
Fig. 4c) under visible light, and this effect was mainly caused
by photochromism. It could also be found that the hollowed
out part was black (Fig. 4d) but the other part hadn’t changed
under UV light, as in the case described in Fig. 4c, and all these
phenomena could be explained by Scheme 2. In addition, these
changes can easily be observed with the naked eye, and the
printed pattern could be restored to its original state under
ambient conditions for 20 s. This color reversibility could be
repeated many times. What’s more, the corresponding fluores-
cence image showed clearer signals than the visual photo of
TPE-NP in this process. The good solid-state photochromic
properties of TPE-NP make it a promising anti-counterfeiting
ink with either visual color or emission changes.
(
365 nm) irradiation, TPE-NP-CF isomerized to TPE-NP-OF, which
is yellow-brown (Fig. 3b) under visible light and bright blue-white
Fig. 3c) under a 365 nm hand-held ultraviolet lamp.
(
Photoswitchable patterns
We applied TPE-NP as an anti-counterfeiting ink due to its good
photochromic properties. Firstly, the TLC plate was dipped into
Conclusions
TPE-NP solution, and after drying in the surrounding environ- In summary, we report
a photochromic naphthopyran-
ment, nothing could be found on the TLC paper under visible functionalized tetraphenylethylene derivative, TPE-NP, and
light (Fig. 4a), whereas it showed blue-green fluorescence under spirooxazine-functionalized tetraphenylethylene derivative,
365 nm hand-held UV lamp irradiation (Fig. 4b). Next, the TLC TPE-SO. Both of them could photoswitch in absorption and
paper was covered by a plate with a hollow out ‘‘USTB’’, and fluorescence under alternating UV/visible treatment. A comparison
then the hollowed out part was exposed to UV light (365 nm) for of the two molecules indicates that the different linking groups
40 s. A pale brown pattern could be observed on the TLC paper between TPE and the photochromic groups will result in different
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p-conjugate systems, which eventually produce different photo- ethyl acetate. The organic layer was dried over anhydrous sodium
chromic and fluorescence properties. This reversible fluorescence sulfate. After moving the solvent under reduced pressure,
switch for TPE-NP and TPE-SO may have great potential for the residue was chromatographed on a silica gel column with
applications such as anti-counterfeiting inks and fluorescence CH Cl /n-hexane (v/v = 1 : 2) as an eluent to give a faint
2
2
1
sensors.
white powder with 90% yield. H NMR (500 MHz, CDCl , TMS):
3
d = 7.17–7.18(d, 2H), 7.11–7.14(m, 9H), 7.04–7.08(m, 6H),
6
.99–7.01(d, 2H), 6.61–6.67(dd, 1H), 5.67–5.71(d, 1H), 5.19–
5.22(d, 1H), 5.19–5.22(d, 1H). HRMS (MALDI-TOF MS): m/z
22) calculated: 358.2, found: 358.3.
-Bromo-1-nitro naphthalen-2-ol (3). 6-Bromo-2-naphthol
0.50 g, 2.24 mmol), NaOH (90 mg, 2.24 mmol) in water
10 mL), NaNO2 (0.16 g, 2.24 mmol), and H SO (0.16 mL,
Experimental section
Materials
28
(C H
6
Dimethylacetamide (DMAc) were distilled under vacuum from
phosphorus pentoxide. Pyridinium 4-toluenesulfonate (PPTS),
(
(
2
4
6
-bromo-2-naphthol, bromotriphenylethylene, 1,1-diphenyl-2-
propyne-1-ol trimethyl orthoformate, 4-vinylphenylboronic acid
-bromo-2-naphthol, indoline, tetrabutylammonium bromide
TBAB), palladium acetate (Pd(OAc) ) and tetrakis(triphenyl-
were purchased from Energy
2
.91 mmol) were reacted at 0 1C for 1 h. A solid precipitate was
; EtOAc : hexane,
: 20) gave 5 g as a yellow solid (0.41 g, 73%). The relevant
characterization is consistent with the literature.
obtained and column chromatography (SiO
1
2
6
(
2
3 4
phosphine) palladium Pd(PPh )
0
0
8
-Bromo-1,3,3-trimethylspiro[indoline-2,3 -naphtho[2,1-b]-
1,4]oxazine] (4). Indoline (0.940 mL, 5.31 mmol) and 6-bromo-
-nitro naphthalen-2-ol (1.323 g, 5.29 mmol) in ACN (50 mL).
Chemical, and all the other chemicals were purchased from
BeiJing Chemical Works and used as received without further
purification.
[
1
The solution was refluxed for 24 h before being concentrated
in vacuo. Purification by column chromatography (95 : 5,
Instruments
hexanes : EtOAc) afforded the desired spiropyran as a yellow
1
H NMR spectra were recorded on a 500 MHz BRAKER AVANCE
1
amorphous solid (0.818 g, 47%). H NMR (500 MHz, CDCl
3
,
3
III HD NMR spectrometer, by using CDCl as the solvent and
TMS): d = 8.45–8.47(d, 1H), 7.92(d, 1H), 7.77(s, 1H), 7.66–
tetramethylsilane (TMS) as an internal standard (d = 0.00 ppm).
MALDI-TOF MS was performed by an Agilent 1290-microTOF Q II.
UV/vis spectra were recorded in a quartz cuvette on a JASCO V-570
spectrophotometer. Fluorescence spectra were recorded using a
Shimadzu RF-5301 PC spectrometer.
7
1
3
.67(dd, 1H), 7.58–7.60(d, 1H), 7.25–7.28(m, 1H), 7.10–7.12(d,
H), 7.05–7.06(d,1H), 6.92–6.94(t, 1H), 6.60–6.61(d, 1H), 2.78(s,
H), 1.38(s, 3H), 1.37(s, 3H). HRMS (MALDI-TOF MS):
m/z (C H BrN O) calculated: 406.1, found: 406.1.
2
2
19
2
Synthesis of TPE-SO
-(4-Vinylphenyl)-l,2,2-triphenylethylene (1 mmol, 0.36 g), 8 -
Synthesis and characterization
0
1
0
Tetraphenylethene-naphthopyran (TPE-NP) and tetraphenylethene- bromo-1,3,3-trimethylspiro[indoline-2,3 -naphtho[2,1-b][1,4]oxazine]
dpirooxazine (TPE-SO) were prepared according to the synthetic (1 mmol, 0.35 g), K PO (3 mmol, 0.65 g), and Pd(OAc) as the
routes shown in Scheme 1. Compounds 1–4 were synthesized catalyst (200 mg) were dissolved in dry DMAc (5 mL). The
3
4
2
21–24
according to the literature method.
reaction mixture was heated to 110 1C in an oil bath and stirred
8
-Bromo-3,3-diphenyl-3H-benzo[f]chromene (1). 6-Bromo-2- for 24 h under a N atmosphere. After being cooled to room
2
naphthol (1 mmol) and 1,1-diphenyl-2-propyne-1-ol (1.1 mmol) temperature, the reaction mixture was poured into water and
in the presence of PPTS (0.05 mmol) and trimethyl ortho- filtered to get the precipitated solid. The product was chromato-
formate (2 mmol) in 1,2-dichloroethane (5 mL) were refluxed graphed on a silica gel column with petroleum ether/ethyl
for 3.5 h. After removing the solvent under reduced pressure, acetate (50 : 1 v/v) to give TPE-SO as a yellow solid (0.13 g,
1
the residue was chromatographed on a silica gel column with 20%). H NMR (500 MHz, CDCl
3
, TMS): d = 8.53–8.56(d, 1H),
2 2
CH Cl /n-hexane (v/v = 1 : 2) as an eluent to give a faint white 7.95(d, 1H), 7.88–7.91(dd, 1H), 7.77–7.80(d, 1H), 7.69–7.71(d, 1H),
1
powder with 90% yield. H NMR (500 MHz, CDCl , TMS): 7.31–7.33(d, 2H), 7.21(s, 1H), 7.20(s,1H), 7.02–7.16(m, 20H), 6.84–
3
d = 7.87–7.88(d, 1H), 7.82–7.84(d, 1H), 7.57–7.58(d, 1H), 7.53– 6.88(t, 3H), 6.70–6.72(d, 1H), 4.56(s, 1H), 2.90(s, 3H), 1.64(s, 3H),
7
7
1
4
.54(dd, 1H), 7.50–7.52(m, 4H), 7.34–7.37(m, 4H), 7.29– 1.03(s, 3H). HRMS (MALDI-TOF MS): m/z (C H N O) calculated:
50 40 2
.31(m, 1H), 7.28–7.29(m, 1H), 7.23–7.26(t, 2H), 6.31–6.32(d, 684.3, found: 683.3 [M ꢀ 1].
H). HRMS (MALDI-TOF MS): m/z (C25H17BrO) calculated:
12.1, found: 412.2.
Synthesis of TPE-NP
4
-(1,2,2-Triphenylvinyl)benzaldehyde (2). Bromotriphenyl- 1-(4-Vinylphenyl)-l,2,2-triphenylethylene (1 mmol, 0.36 g),
ethylene (2.01 g, 6 mmol) and 4-vinylphenylboronic acid 8-bromo-3,3-diphenyl-3H-benzo[f]chromene (1 mmol, 0.35 g),
1.33 g, 9 mmol) were dissolved in a mixture of toluene (40 mL), K PO (3 mmol, 0.65 g), and Pd(OAc) as the catalyst (200 mg)
(
3
4
2
TBAB (0.19 g, 0.6 mmol) and 1.2 M potassium carbonate aqueous were dissolved in dry DMAc (5 mL). The reaction mixture was
solution (10 mL). The mixture was stirred at room temperature heated to 110 1C in an oil bath and stirred for 24 h under a
for 0.5 h under N
5
2
gas followed by adding Pd(PPh
.3 ꢁ 10 mmol) and then heated to 90 1C for 24 h. After that the reaction mixture was poured into water and filtered to get the
mixture was poured into water and extracted three times with precipitated solid. The product was chromatographed on a
3
)
4
(60 mg,
2
N atmosphere. After being cooled to room temperature, the
ꢀ
3
6
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silica gel column with petroleum ether/ethyl acetate (50 : 1 v/v)
to give TPE-NP as a yellow solid (0.25 g, 39%). H NMR
8 T. V. Pinto, P. Costa and C. M. Sousa, ACS Appl. Mater.
Interfaces, 2016, 8, 7221–7231.
1
(
500 MHz, CDCl , TMS): d = 7.98–8.00(d, 1H), 7.92(d, 1H),
.75–7.77(d, 1H), 7.66–7.68(d, 1H), 7.59–7.60(d, 1H), 7.36–
9 S. Brazevic, M. Sliwa and Y. Kobayashi, J. Phys. Chem. Lett.,
2017, 8, 909.
.39(t, 3H), 7.29–7.33(m, 8H), 7.21–7.24(d, 1H), 7.05–7.16(m, 10 K. Uno, H. Niikura and M. Morimoto, J. Am. Chem. Soc.,
3
7
7
1
7H), 6.76(s, 1H), 5.71(s, 1H). HRMS (MALDI-TOF MS): m/z
38O) calculated: 690.3, found: 689.8.
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Conflicts of interest
1
There are no conflicts to declare.
1
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The financial support from the National Natural Science
Foundation of China (Grant no. 51673023, 51773017, and
8
51373024) and the Fundamental Research Funds for the Central
6
Universities (Grant no. FRF-GF-17-B12) is gratefully appreciated.
2
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