A new synthesis protocol for photochromic triarylethenes and their multifunctional derivatives

Article abstract of DOI:10.1080/00397911.2020.1793206

A synthesis protocol was presented and applied in the preparation of a series of new photochromic triarylethene (TAE) derivatives cherishing bifunctional groups. The titled molecules all perform photochromism in the rare yellow absorption band in both solution and solid. Some of them show classical aggregation-induced emission (AIE) fluorescence properties, and the fluorescence exhibits distinctive photo-controllable performance. The synthesis and design of photochromic triarylethenes could provide a new alternative for achieving solid photochromic and photo-controllable fluorescent materials based on simple structures and convenient synthesis process.

Full text of DOI:10.1080/00397911.2020.1793206

Synthetic Communications
An International Journal for Rapid Communication of Synthetic Organic
Chemistry
ISSN: (Print) (Online) Journal homepage: https://www.tandfonline.com/loi/lsyc20
A new synthesis protocol for photochromic
triarylethenes and their multifunctional
derivatives
Liying Yan , Wei Ding , Lijun Wang , Qingyu Dou & Qianfu Luo
To cite this article: Liying Yan , Wei Ding , Lijun Wang , Qingyu Dou & Qianfu Luo (2020): A new
synthesis protocol for photochromic triarylethenes and their multifunctional derivatives, Synthetic
Communications, DOI: 10.1080/00397911.2020.1793206
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https://doi.org/10.1080/00397911.2020.1793206
A new synthesis protocol for photochromic triarylethenes
and their multifunctional derivatives
Liying Yan, Wei Ding, Lijun Wang, Qingyu Dou, and Qianfu Luo
Key Laboratory for Advanced Materials, Institute of Fine Chemicals, School of Chemistry and Molecular
Engineering, East China University of Science and Technology, Shanghai, China
ABSTRACT
ARTICLE HISTORY
Received 2 January 2020
A synthesis protocol was presented and applied in the preparation
of a series of new photochromic triarylethene (TAE) derivatives cher-
ishing bifunctional groups. The titled molecules all perform photo-
chromism in the rare yellow absorption band in both solution and
solid. Some of them show classical aggregation-induced emission
(AIE) fluorescence properties, and the fluorescence exhibits distinct-
ive photo-controllable performance. The synthesis and design of
photochromic triarylethenes could provide a new alternative for
achieving solid photochromic and photo-controllable fluorescent
materials based on simple structures and convenient synthesis pro-
cess.
KEYWORDS
Aggregation-induced emis-
sion; fluorescence;
photochromism; synthesis;
triarylethene
GRAPHICAL ABSTRACT
Introduction
Photochromic diarylethenes play a crucial role in functional materials, which have
aroused enormous attention because of their distinctive isomers and unique physical
and chemical properties.[1,2] Plenty of diarylethene derivatives with high fatigue resist-
ance, good thermal stability, fast response and high quantum efficiency have been
extensively studied for versatile applications in information storage, chemical sensors,
molecular machines, biological macromolecules, etc.[3–15] Masahiro Irie reported the
preparation and photochromism of thermally stable diarylethenes in 1988.[16] Since
then, researchers have developed a variety of preparation methods for different
CONTACT Qianfu Luo
luoqf@ecust.edu.cn
Key Laboratory for Advanced Materials, Institute of Fine Chemicals,
School of Chemistry and Molecular Engineering, East China University of Science and Technology, Shanghai
200237, China.
Supplemental data for this article can be accessed on the publisher’s website.
ß 2020 Taylor & Francis Group, LLC

2
L. YAN ET AL.
EtO
S
S
O
HO
S
O
S
S
S
R
X
R
X
N
S
H
4
P
(
O
A
O
O
O
A
r
C
E
c
,
O
t
)
A
C
3
c
R
(HO)2B
l
,
O
P
H
y
O
S
S
A
l
C
l
Br
3
COOEt
S
O
S
R
S
S
Addition
Z
n
/
H
R
S
S
R
C
R
R
l
HO
OH
H
O
ClOC
S
A
c
X
R
H
O
n
o
i
t
a
u
t
Ti0
S
S
i
t
N
s
,
b
u
N
C
S
K
R
O
O
O
S
S
CN
CN
HN
OMe
S
S
S
Scheme 1. General approaches to the synthesis of photochromic diarylethenes.
molecular structures and applications: Fan et al.[17] synthesized some analogous cis-trans
isomerized diarylethenes from corresponding ketones. Uchida’s group used P(OEt)3 as a
catalyst and got the photochromic diarylethenes from thiophene derivatives.[18] Apart
from these, heterocyclic diarylethenes were obtained by bonding oxamyl with acid
chloride,[19] and it could be synthesized from acyloin as well.[20] It was also an effective
way to yield diarylethenes by combining the alkenyl halides and aromatic com-
pounds.[21–25] In addition, Lucas and Feringa et al.[26–30] developed a series of diaryle-
thenes by McMurry coupling reaction from diketone compounds. Another route to
synthesize photochromic diarylethenes was the addition reaction of olefins and
alkynes.[31–34] Furthermore, Sud[35] obtained the diarylethenes through the substitution
of the aromatic alkyne compounds, while they were also prepared from ethyl acetoace-
tate derivatives.[36–39] Dehydration of alcohol compounds to obtain olefinic compounds,
the diarylethenes were got from pinacol by Dinesh and coworkers in the same way.[40]
These are, to the best of our knowledge, the most common methods to construct the
diarylethene molecules. Thienyl diarylethenes were taken as examples, and the reported
synthetic routes of diarylethenes are shown in Scheme 1.
The researches mentioned above have made important contributions in the develop-
ment and application of photochromic diarylethenes, because they not only constructed
a variety of promising functional materials but also were applied in many new fields.
However, some molecular designs are defective, for example, there are cis-trans configu-
rations in structural design, which might bring difficulty to the purification of the prod-
ucts. Otherwise, extra rings have to be introduced to fix the ethylene bridges in order to

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3
R
R
R
(
)
R
O
OBu
...
B
OBu
Photochromic Triarylethenes
starting material
Photochromic Diarlethenes
= aryl group
Scheme 2. The schematic diagram for the synthesis of triarylethenes.
avoid the unwanted isomers, which will increase extra work in preparation or confining
the spatial configuration of the photochromic molecules. These inconveniences in prep-
aration and purification may limit their applications. In view of the above consideration,
we manage to design a kind of simple photochromic diarylethene system that funda-
mentally avoids the production of cis-trans isomers and does not require the extra add-
ition of rings to immobilize ethylene bridge (Scheme 2). Furthermore, considering the
practical limitations of fluorescence quenching of common diarylethene in solution and
aggregation, diverse functional groups of aggregation-induced emission[41–51] have fur-
ther been introduced to the molecules through the combination of tristyrene (AIE unit)
and the diarylethene (photochromic unit) which sharing the same vinyl bridge. Results
show that this design is beneficial to the synthesis of asymmetric photochromic mole-
cules. Interestingly, the target compounds perform obvious photochromism and thermal
reversibility in both solution and aggregation. Some of them exhibit both optically-con-
trolled fluorescent performance and AIE properties. These compounds are desirable for
the applications in solid luminescent materials. This design could provide a new idea
for constructing photochromic and fluorescent materials based on simple structures and
cyclization mode.
Results and discussion
Photochromic behaviors in solution and solid
As expected, all of the three target compounds showed noteworthy photochromic fea-
tures in both solution and aggregation. Firstly, the photochromic performances of the
three heterotriarylethenes (TAE-1o, TAE-2o and TAE-3o) were investigated in solution
at room temperature, the photoinduced absorption spectra and corresponding color
changes of TAE-1o in THF solution were shown in Figure S13(a). The solution of the
open-ring isomers is colorless and do not show any optical absorption in the visible
light region. While TAE-1o turned the rare yellow for photochromic diarylethenes after
it was treated with UV light, which was attributed to the formation of the closed-ring
isomers by a mechanism of the cyclization[52] between the thiophene and benzene at
the same side (Scheme 3). The backward reaction took place upon irradiation with
more than 400 nm light. TAE-1o was chose as an example to show their photochromic

4
L. YAN ET AL.
S
S
UV
VIS/
S
S
TAE-1c
TAE-1o
Scheme 3. The photochromic reaction of TAE-1.
behaviors. It was clear that upon irradiation with UV light (254 nm), the THF solution
of TAE-1o turned yellow quickly. Meanwhile, a new absorption peak at 460 nm
appeared along with the decrease of the absorption band at around 265 nm, and the
photostationary state was achieved after irradiation for 2 min. What’s more, a clear isos-
bestic point was observed at 285 nm, which indicated the reversible two-component
photochromic reaction.[53,54] Moreover, the yellow solution of the photostationary was
bleached entirely when exposed to visible light. The other two compounds exhibited the
similar photochromic behaviors (Figure S13(b,c)).
Compounds TAE-1, TAE-2 and TAE-3 exhibited obvious photochromic properties
in the solid powders as well. The color changes of them in solid powders before and
after photochromism were illustrated in Figures 1 and 2. Taken TAE-2 for instance to
describe its photochromic behavior (Figure 1). With the irradiation of 254 nm UV light
for several minutes, the color of TAE-2 changed from white (TAE-2o) to orange yellow
(TAE-2c), while upon irradiation with visible light, the orange yellow powders reverted
to white gradually. It indicated that the color change of TAE-2 in solid powders was
reversible. TAE-1 and TAE-3 performed the similar photochromic behavior in solid
powders. It is worth noting that after UV-light irradiation (254 nm), the solid fluores-
cence emission of TAE-3o changed from blue to green, while the green fluorescence
returned to blue again when it was exposed with visible light (Figure 2). As shown in
the fluorescence emission spectra of TAE-3 (Figure 3), compound TAE-3 emitted
strong fluorescence at about 405 nm before ultraviolet light irradiation. With the exten-
sion of ultraviolet light exposure time, the fluorescence intensity at 405 nm gradually
decreased, and new fluorescence emission peak appeared in the range of 460–650 nm.
This change could be restored to the original state after irradiation of visible light,
which indicated that the compound has undergone reversible photochromism during
the illumination process. The change in fluorescence may be caused by the expansion of
conjugate range after photochromism. This distinctive photo-controllable fluorescence is
desirable for solid fluorescent materials.
Thermal reversibility
All of the closed-ring isomers, TAE-1c, TAE-2c and TAE-3c, were also thermally
reversible, which could transfer to their open forms in dark. Taken TAE-2c as an
example (Figure S14(b)), the THF solution of TAE-2c (5.0 ꢀ 10ꢁ5 molꢂLꢁ1) with photo-
stationary state was kept in the dark for about 6 hours, that maximum absorption at

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5
Figure 1. Photochromic behaviors of TAE-1 and TAE-2 in solid powders.
Figure 2. Photochromism and photo-controllable fluorescence images of TAE-3 under 365 nm in
solid powders.
468 nm decayed to the original spectra, accompanied by the orange solution fading to
colorless. It meant that the closed-ring form, TAE-2c, could recover to the open-ring
form (TAE-2o). Similarly, the maximum absorption at 460 nm and 470 nm of TAE-1c
and TAE-3c in THF (5.0 ꢀ 10ꢁ5 molꢂLꢁ1) decayed to the original spectra when kept in
dark for 400 min and 350 min respectively with corresponding color changes.
Light reversibility and fatigue resistance
The recycling of the photochromic process of the three target compounds were illus-
trated as a function of alternate exposure to UV (254 nm) and visible light (500 nm).
Taken TAE-2 as an example (Figure S16), the maximum absorption of photostation-
ary state decayed significantly after it was tested 5 cycles, which indicates that TAE-
2 has a certain photoreversible property but shows obvious fatigue after 5 photocyc-
lization and cycloreversion cycles. TAE-1 and TAE-3 showed similar reversibility
and fatigue resistance (Figures S15 and S17). The main fatigue process of photochro-
mic compounds are caused by the photostable byproduct formations. While the

6
L. YAN ET AL.
600
500
400
300
200
100
0
UV
Vis
UV Vis
400
500
600
Wavelength/nm
Figure 3. Changes in fluorescence spectra of TAE-3 in THF (5.0 ꢀ 10ꢁ5 M) upon irradiation at 254 nm.
introduction of methyl groups at the 4-positions of the thiophene rings may improve
the fatigue resistance of these compounds due to the methyl substituents are consid-
ered to prevent rearrangement of the thiophene rings to the six-membered con-
densed ring.[1]
Aggregation-induced emission properties
In addition to the obvious photochromic behaviors and thermal reversibility in both
solution and solid powders, TAE-3 had excellent AIE properties simultaneously. To
investigate the AIE properties of the target compounds, their fluorescence spectra in
H2O/THF mixture solution with different water fractions were recorded. Given these
spectra data observations, TAE-3 exhibited typical AIE properties, as shown in Figure 4.
The fluorescent emission of TAE-3 could not be observed in pure THF solvent (kex ¼
310 nm), but it was increased gradually with the increasing of the water fraction because
of the aggregation in the aqueous medium which restricting the intramolecular rota-
tion.[46] As the fluorescence spectra and images (Figure 4) show that the fluorescence
emission intensity of TAE-3 (5 ꢀ 10ꢁ5 M) in pure THF was close to 0 (kex ¼ 310 nm),
but it increased slowly when the water fraction increased from 20 vol% to 60 vol%. The
fluorescence emission intensity increased observably when the water fraction reached to
70 vol%, and finally reach the peak when the water fraction is 80 vol%. TAE-3’s emis-
sion was turned on gradually when the water fraction increased and accompanied by
blue fluorescence, as a result of the aggregation in the aqueous medium.[55] The test
results of laser light scattering showed that the radius of aggregate particle is around
100 nm, and the particle size distribution is narrow enough, which indicates that the
solution is stable and no insoluble particles are precipitated out directly (Figure 5).

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7
800
600
400
200
0
Water fraction/vol%
80 %
70 %
60 %
40 %
20 %
0 %
350
400
450
500
550
600
Wavelength/nm
Figure 4. Fluorescence spectra and images of TAE-3 in H2O/THF mixture solution with different water
fractions, the inset shows the fluorescence photographs of TAE-3 with different water fractions.
1.0
0.8
0.6
0.4
0.2
0.0
0.1
1
10
100
1000
10000
Radius [ nm ] (unweighted, log.)
Figure 5. Laser light scattering of TAE-3 in H2O/THF mixtures (80% water content).
Experimental
All reagents were purchased commercially and used without further purification unless
1
otherwise mentioned. H NMR spectra (400 MHz) and 13C NMR spectra (101 MHz)
were recorded on a Bruker AV400 spectrometer using CDCl3 as solvent and tetrame-
thylsilane as internal standard. All HRMS spectrometric analyses were tested on a LCT
Premier XE mass spectrometer. The UV light for irradiation was generated by LED
sources and the lamp power is 5 W. Absorption and fluorescence spectra were recorded

8
L. YAN ET AL.
Br
Br
O
CBr4
NH2NH2.H2O
6
OBu
B
OBu
S
3
S
S
S
S
TAE-1
Scheme 4. The synthesis protocol for photochromic triarylethenes.
on Agilent Technologies Cary 60 UV-Vis and Varian Cray Eclipse fluorescence spectro-
photometer, respectively.
Generally, the photochromic diarylethenes obtained from two ketone molecules need
to take the following process: the acylation of aromatic compounds as the first step, fol-
lowed by a Friedel–Crafts reaction to obtain the precursors, finally, the target com-
pounds were obtained by McMurry coupling reaction. The shortcomings of this method
are that the cis-trans isomerization of the products are difficult to separate, and the low
yield of single product due to cis-trans isomers. Moreover, this method is not suitable
to the synthesis of asymmetric diarylethenes.[56] Another class of diarylethenes needs an
extra ring to fix the ethylene bridge, which leads to an increase in synthetic steps and
workload.[26] The synthetic protocol in this paper circumvented these shortcomings. It
mainly included two steps. The commercial aryl ketone was reacted first with hydrazine
and carbon tetrabromide respectively to get the key intermediate dibromocarbene. Then
the titled triarylethenes could be conveniently obtained by the classic Suzuki coupling
reaction (Scheme 4).
The photochromic triarylethene TAE-1 was obtained firstly as colorless transparent
liquid in 66% yield through this method. Furthermore, another two new triarylethene
derivatives TAE-2 and TAE-3 were achieved successfully by replaced the aryl group of
the aryl ketone with more huge functional blocks, triptycene and tetra-arylethene. The
synthetic routes are shown in Scheme 5. The intermediate carbine was prepared by
revised procedures reported previously.[57,58]
Reagents and conditions: (a) NH2NH2ꢂH2O, HOAc, C2H5OH; (b) CBr4, CuI; DMSO-
NH3; (c) Pd(PPh3)4, K2CO3, THF, H2O.
Synthesis of 3,30-(2-phenylprop-1-ene-1,1-diyl)bis(2,5-dimethylthiophene) (TAE-1)
Compound 6 (450 mg, 1.64 mmol), anhydrous K2CO3 (1.38 g, 10.00 mmol), and
Pd(PPh3)4 (180 mg, 0.16 mmol) were added into a dry 50 mL two-necked flask orderly.

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9
OBu
S
B
OBu
O
Br
Br
a)
b)
3
S
c)
S
10:
15:
=
=
=
TAE-2
TAE-3
=
Scheme 5. The synthetic routes of the target compounds TAE-2, TAE-3.
5 mL of water and 15 mL of distilled THF were added into the flask subsequently, the
argon protection device was installed, and replaced the gas of the flask 3 times with
argon. The reaction system gradually warmed up in the oil bath, when the temperature
rose to 60 ꢃC, borate 3 was added to the flask with a syringe in a short period of time,
and the temperature was increased to 80 ꢃC continuously. The solution was refluxed
overnight and cooled to room temperature, after which H2O (20 mL) was added and
extracted with DCM (20 mL ꢀ 3), The organic layer was separated and subsequently
dried over anhydrous Na2SO4. The sodium sulfate was filtered off, and the solvent was
removed by evaporation under vacuum. The residue was purified by column chroma-
1
tography (SiO2, PE) to give TAE-1 (366 mg, 66%) as colorless transparent liquid. H
NMR (400 MHz, CDCl3, ppm): d ¼ 7.19–7.10 (m, 5H), 6.35 (s, 1H), 6.19 (s, 1H), 2.38
(s, 3H), 2.28 (s, 3H), 2.25 (s, 3H), 2.05 (s, 3H), 1.76 (s, 3H). 13C NMR (101 MHz,
CDCl3, ppm): d ¼ 143.84, 139.54, 138.65, 137.74, 135.47, 134.63, 132.10, 129.06, 128.81,
127.98, 127.91, 127.30, 126.46, 21.97, 15.57, 15.49, 14.04. HRMS (ESIþ): m/z calcd for
C21H22S2 [M þ H]þ: 339.1241; found: 339.1227.
Synthesis of 3,3’-(2-((9r,10r)-9,10-dihydro-9,10-[1,2]benzenoanthracen-2-yl)prop-1-
ene-1,1-diyl)bis(2,5-dimethylthiophene) (TAE-2)
Compound 10 (274 mg, 0.61 mmol), anhydrous K2CO3 (1.38 g, 10.00 mmol), and
Pd(PPh3)4 (37 mg, 0.031 mmol) were added into a dry 50 mL two-necked flask orderly.
5 mL of water and 15 mL of distilled THF were added into the flask subsequently, the
argon protection device was installed, and replaced the gas of the flask 3 times with
argon. The reaction system gradually warmed up in the oil bath, when the temperature
rose to 60 ꢃC, borate 3 was added to the flask with a syringe in a short period of time,
and the temperature was increased to 80 ꢃC continuously. The solution was refluxed
overnight and cooled to room temperature, after which H2O (20 mL) was added and

10
L. YAN ET AL.
extracted with DCM (20 mL ꢀ 3), The organic layer was separated and subsequently
dried over anhydrous Na2SO4. The sodium sulfate was filtered off, and the solvent was
removed by evaporation under vacuum. The residue was purified by column chroma-
tography (SiO2, PE: DCM ¼ 60: 1, V/V) to give TAE-2 (234 mg, 75%) as colorless
1
transparent liquid. Mp 177.3–181.4 ꢃC H NMR (400 MHz, CDCl3, ppm): d ¼ 7.36–7.31
(m, 4H), 7.19–7.18 (d, J ¼ 7.6 Hz, 1H), 7.09 (s, 1H), 6.98–6.96 (m, 4H), 6.81–6.79 (dd, J1
¼ 1.6 Hz, J2 ¼ 7.6 Hz, 1H), 6.32 (s,1H), 6.00 (s, 1H), 5.34 (s, 1H), 5.21 (s, 1H), 2.36 (s,
3H), 2.23 (s, 3H), 2.07 (s, 3H), 2.00 (s, 3H), 1.60 (s, 3H); 13C NMR (101 MHz, CDCl3,
ppm): d ¼ 145.38, 145.23, 144.58, 143.36, 140.32, 139.29, 138.39, 136.96, 135.11, 134.01,
132.01, 131.83, 128.73, 127.81, 127.14, 125.05, 125.00, 124.91, 124.16, 123.43, 123.41,
122.65, 54.15, 53.73, 21.42, 15.29, 14.94, 13.83, 13.58; HRMS (ESIþ): m/z calcd for
C35H30S2 [M þ Na]þ: 537.1687; found: 537.1679.
Synthesis of 3,3’-(2-(4-(1,2,2-triphenylvinyl)phenyl)prop-1-ene-1,1-diyl)bis(2,5-
dimethylthioph-ene) (TAE-3)
Compound 15 (266 mg, 0.50 mmol), anhydrous K2CO3 (1.38 g, 10.00 mmol), and
Pd(PPh3)4 (60 mg, 0.05 mmol) were added into a dry 50 mL two-necked flask orderly.
5 mL of water and 15 mL of distilled THF were added into the flask subsequently, the
argon protection device was installed, and replaced the gas of the flask 3 times with
argon. The reaction system gradually warmed up in the oil bath, when the temperature
rose to 60 ꢃC, borate 3 was added to the flask with a syringe in a short period of time,
and the temperature was increased to 80 ꢃC continuously. The solution was refluxed
overnight and cooled to room temperature, after which H2O (20 mL) were added and
extracted with DCM (20 mL ꢀ 3). The organic layer was separated and subsequently
dried over anhydrous Na2SO4. The sodium sulfate was filtered off, and the solvent was
removed by evaporation under vacuum. The residue was purified by column chroma-
tography (SiO2, PE) to give TAE-3 (168 mg, 57%) as colorless transparent liquid. Mp
1
93.2–94.6 ꢃC. H NMR (400 MHz, CDCl3, ppm): d ¼ 7.14–7.07 (m, 9H), 7.05–6.98 (m,
6H), 6.89–6.82 (m, 4H), 6.31 (s, 1H), 6.15 (s, 1H), 2.36 (s, 3H), 2.28 (s, 3H), 2.24 (s,
3H), 1.99 (s, 3H), 1.71 (s, 3H). 13C NMR (101 MHz, CDCl3, ppm): d ¼ 144.11, 144.06,
141.95, 141.90, 141.11, 140.94, 139.63, 138.65, 137.50, 135.42, 134.58, 132.10, 132.05,
131.61, 131.59, 130.87, 129.07, 128.12, 128.01, 127.88, 127.84, 127.28, 126.72, 126.61,
21.59, 15.50, 14.31, 13.98; HRMS (ESIþ): m/z calcd for C41H36S2 [M þ Na]þ: 615.2156;
found: 615.2153.
Conclusions
In summary, a new synthesis strategy of photochromic triarylethenes with bifunctional
groups was developed and a series of new triarylethenes without cis-trans were con-
structed. The novel design ensures a simple structure with two independent functional-
ities. The target compounds were conveniently obtained by straightforward synthesis
and easy to purify. Results showed that they exhibited obvious photochromism in the
rare yellow absorption bands in both solution and aggregation. Furthermore, TAE-3

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11
performs distinctive photo-controllable fluorescence and AIE properties. These com-
pounds are potential applications in solid photochromic and fluorescent materials.
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