Substituent position effect on photochromic properties of aldehyde-functionalized dithienylethenes

Article abstract of DOI:10.1080/15421406.2020.1856502

Three symmetrical dithienylethenes bearing two electron-withdrawing aldehyde groups have been synthesized. And their structures were confirmed by ¹H NMR, ¹³C NMR and HRMS (ESI). Investigation on photochromic properties indicated that

Full text of DOI:10.1080/15421406.2020.1856502

Molecular Crystals and Liquid Crystals
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Substituent position effect on photochromic
properties of aldehyde-functionalized
dithienylethenes
Jingjing Yuan & Yu Bao
To cite this article: Jingjing Yuan & Yu Bao (2020) Substituent position effect on photochromic
properties of aldehyde-functionalized dithienylethenes, Molecular Crystals and Liquid Crystals,
712:1, 24-30, DOI: 10.1080/15421406.2020.1856502
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MOLECULAR CRYSTALS AND LIQUID CRYSTALS
2020, VOL. 712, NO. 1, 24–30
https://doi.org/10.1080/15421406.2020.1856502
Substituent position effect on photochromic properties of
aldehyde-functionalized dithienylethenes
Jingjing Yuan and Yu Bao
Department of Traditional Chinese Medicine, Hubei College of Chinese Medicine, Jingzhou, PR China
KEYWORDS
ABSTRACT
Aldehyde; dithienylethene;
photochromism; substituent
position effect
Three symmetrical dithienylethenes bearing two electron-withdraw-
ing aldehyde groups have been synthesized. And their structures
were confirmed by 1H NMR, 13C NMR and HRMS (ESI). Investigation
on photochromic properties indicated that they had good photo-
chromic behavior with excellent fatigue resistance upon irradiation
with UV or visible light. And it was found that the aldehyde substitu-
ent position had a significant effect on their photochromic proper-
ties. The DFT calculations further validated those experimental
results for their photochromic behavior. Furthermore, they may be
used as versatile building blocks to construct novel photochro-
mic materials.
Introduction
In recent decades, photochromic compounds, such as spiropyran, azobenzene, diaryle-
thene and so on, have drawn increasing attention due to potential applications in
molecular devices and optical memory systems [1,2]. Among them, diarylethenes with
heterocyclic aryl groups, especially those bearing two thiophene groups have been exten-
sively investigated because of their excellent irreversible thermal stability, high sensitivity
and remarkable fatigue resistance [3–11]. The photochromic properties of the dithieny-
lethene depended on several factors, such as the conformation of the ring-open isomer,
electron donor/acceptor substituents, and the p-conjugation length of the heteroaryl
groups, etc [12–14]. Among these factors, the effect of electron substituents was the
most direct and practicable one for the design of a photoactive dithienylethene with
CONTACT Jingjing Yuan
18062366418@163.com
Department of Traditional Chinese Medicine, Hubei College of
Chinese Medicine, Jingzhou 434020, PR China
ß 2020 Taylor & Francis Group, LLC

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
25
tunable properties [15–18]. Dithienylethenes bearing phenyl groups were the most com-
mon class of examples which can be substituted by different electron-withdrawing/
donating substituents at different positions of the benzene ring in order to modulate
their photochromic properties [19]. Apart from the fact that various substituent groups
had great influence on the photochromic behaviors of dithienylethenes, the position of
the substituent groups could affect its photochromism significantly, which can be uti-
lized for the fine tuning of the optical properties of dithienylethenes [20–24].
Nevertheless, searching new dithienylehene scaffolds with excellent regulation of photo-
chromism is still highly desirable due to their potential application. To the best of our
knowledge, the aldehyde group is a strong electron-withdrawing moiety which have
been used in many functional materials. In additon, the formyl group can function as
not only electron acceptor (A) but a classical reaction moiety to carry on a series of
reaction, such as Reduction, Oxidation, Aldol condensation, Knoevenagel condensation,
etc. Herein, three symmetrical dithienylethenes bearing two aldehyde groups were pre-
sented in order to: (1) evaluate the effect of the position of the CHO substitution on
dithienylethene; (2) utilization as potential building blocks to construct novel functional
photochromic materials. As expected, investigation on photochromic properties indi-
cated that dithienylethenes 1–3 had good photochromic behavior with excellent fatigue
resistance upon irradiation with UV or visible light. Moreover, results showed that the
aldehyde substituent position had a significant effect on their photochromic properties.
Experimental
General procedures and materials
All manipulations were carried out under a nitrogen atmosphere by using standard
Schlenk techniques unless otherwise stated. THF was distilled under nitrogen from
sodium/benzophenone. 1, 2-bis (5-chloro-2-methylthiophen-3 -yl)cyclopent-1-ene 2 [25]
and dithienylethenes 1 [26] and 3 [27] were prepared by modified literature methods.
All other starting materials were obtained commercially as analytical-grade and used
without further purification. The relative quantum yields were determined by compar-
ing the reaction yield with the known yield of the compound 1, 2-bis (2-methyl-5-phe-
1
nyl-3- thienyl)perfluorocyclopentene [28]. H and 13C NMR spectra were collected on
1
German BRUKER AVANCE III 400 MHz. H and 13C NMR chemical shifts are relative
to TMS. High resolution mass spectra were obtained on Thermo Scientific TM Q
ExactiveTM (ESI mode). UV-vis spectra were obtained on a Persee TU-1810 UV-Vis
spectrophotometer. In the photoisomerization reaction, UV light irradiation (254 nm)
was carried out using a ZF5UV lamp, and visible light was irradiated using a LZG
220 V 500 W tungsten lamp (k > 402 nm) with cutoff filters.
Synthesis of dithienylethene (2)
To a solution of 4 (537 mg, 1.0 mmol) in anhydrous THF (10 mL) was added n-BuLi
(0.88 mL of 2.5 M solution in hexane, 2.2 mmol) under nitrogen at 0 ꢀC and stirred for
2 h at 0 ꢀC. Then B(OBu)3 (0.82 mL, 3.0 mmol) was added slowly and stirred for 6 h at
room temperature, which was used in the following Suzuki cross coupling reaction

26
J. YUAN AND Y. BAO
Scheme 1. Synthetic route of dithienylethenes 1-3.
Scheme 2. Photochromism of dithienylethenes 1-3.
without any workup. Then, the resulting reddish solution was added dropwise to a flask
containing 5 (368 mg, 2.0 mmol), Pd(PPh3)4 (100 mg, 0.082 mmol) in THF (15 mL) and
Na2CO3 solution (15 mL, 2 M) at 60 ꢀC. Subsequently, the mixture was refluxed for 24 h
under an N2 atmosphere. The solution was cooled to room temperature and extracted
with EA (3 ꢁ 40 mL), and the combined organic layer was washed with the saturated
NaCl solution (2 ꢁ 50 mL). The organic layer was dried over Na2SO4, filtered and con-
centrated in vacuo. And the residue was purified by column chromatography (PE: EA
1
¼ 9:1) to afford compound 2 as a gray solid in a yield of 73%. H NMR (400 MHz,
CDCl3) d 10.04 (s, 2H), 8.02 (s, 2H), 7.77 ꢂ 7.74 (m, 4H), 7.52 (t, J ¼ 8.0 Hz, 2H), 7.16
(s, 2H), 2.89 (t, J ¼ 8.0 Hz, 4H), 2.17 ꢂ 2.10 (m, 2H), 2.04 (s, 6H). 13C NMR (100 MHz,
CDCl3) d 192.08, 138.07, 136.91, 136.87, 135.59, 135.44, 134.76, 130.96, 129.49, 128.18,
125.98, 124.99, 38.45, 23.01, 14.45. HRMS (ESI-TOF) m/z: [M þ H]þ Calcd for
C29H24O2S2 468.1218; Found 468.1227.
Results and discussion
Photochromic properties of dithienylethenes 1–3
The photoisomerization behavior of the dithienylethenes 1–3 in CH2Cl2 was investi-
gated at room temperature. These dithienylethene derivatives underwent photoisomeri-
zation between ring-open isomers and ring-closed isomers upon alternating irradiation
with UV light (254 nm) and visible light (> 402 nm), as illustrated in Scheme 2. As
shown in Figure 1A, the absorption maximum of dithienylethene one was observed at
246 nm (e ¼ 8.38 ꢁ 104 L molꢂ1 cmꢂ1), which was ascribed to intramolecular p - p
ꢃ
transition [29]. Upon irradiation with 254 nm UV light, a new absorption band at

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
27
Figure 1. Absorption spectral changes of dithienylethenes 1-3 with 254nm UV and >402 nm Vis light
irradiation in DCM (2.0 ꢁ 10ꢂ5 mol/L), (A) spectral changes for 1; (B) spectral changes for 2; (C) spec-
tral changes for 3; (D) optical response rate monitored at the maximum absorption wavelength in the
visible region for ring-closed isomers 1c-3c.
Figure 2. Fatigue resistance of dithienylethenes 1-3 with 254 nm UV and >402 nm Vis light irradi-
ation in DCM (2.0 ꢁ 10ꢂ5 mol/L), (A) fatigue resistance for 1 (518 nm); (B) fatigue resistance for 2
(544 nm); (C) fatigue resistance for 3 (596 nm).
518 nm (e ¼ 1.59 ꢁ 104 L molꢂ1 cmꢂ1) appeared along with an obvious color change
from colorless to pink, as a result of formation of the corresponding ring-closed isomer
1c. Furthermore, a well-defined isosbestic point was observed at 293 nm, which revealed
that 1o was cleanly converted to the photocyclized product. Upon irradiation with
k > 402 nm visible light, the pink ring-closed isomer 1c underwent a cycloreversion
reaction and returned to the initial ring-open isomer 1o. In particular, dithienylethene
one showed very good reversibility and no apparent deterioration (about 3%) was

28
J. YUAN AND Y. BAO
Table 1. Photochromic parameters of dithienylethene 1-3 in DCM at 298 K (2.0 ꢁ 10ꢂ5 mol/L).
kmax/nma
(e ꢁ 104)
Open
kmax/nmb
（e ꢁ 104)
PSSd
Uc
Compounds
uo-c
uc-o
1
2
3
246 (8.38)
247 (6.58)
357 (6.40)
518 (1.59)
544 (1.47)
596 (2.85)
0.356
0.232
0.526
0.0079
0.0065
0.0092
aAbsorption maxima of open-ring isomers;.
bAbsorption maxima of closed-ring isomers;.
cQuantum yields of open-ring (uc-o) and closed-ring isomers (uo-c);.
dPhotostationary state, respectively.
observed after repeating the above process eight times, indicating excellent fatigue resist-
ance (Figure 2A). The cyclization and cycloreversion quantum yields of 1 were 0.356
(uo-c) and 0.0079 (uc-o), respectively (Table 1). Similar photochromic properties were
obtained when solution of dithienylethenes 2 and 3 in DCM was irradiated with the
same light, as shown in Figures 1B,C and 2B,C and Table 1. The absorption spectral
parameters of dithienylethenes 1–3 in DCM were summarized in Table 1. These data
showed that the aldehyde substituent position had a significant effect on their photo-
chromic properties, mainly including maximum absorption wavelength, molar absorp-
tion coefficients, optical response rate and quantum yields. Among these three isomers,
the absorption maximum of the para-substituted derivative 3c (596 nm) is the longest
in DCM, while that of ortho-substituted derivative 1c (518 nm) is the shortest, which
exhibited 78 nm red-shifts. However, these results were inconsistent with those for
methoxy substituents where the absorption maximum of meta-substituted diarylethene
was the shortest, and that of ortho-substituted one was the longest [20]. As presented in
Table 1, the molar absorption coefficients of ring-closed isomers 1c–3c increased in the
order of meta- < ortho- < para- substitution by the aldehyde group. Furthermore, their
optical response rate was compared, and it was found that the response rate of para-
substituted derivative was the fastest and that of meta- substituted one was the slowest
(Figure 1D). As shown in Table 1, the cyclization quantum yields of dithienylethenes
1–3 were much higher than their corresponding cycloreversion quantum yields. The
cyclization quantum yield of the para-substituted derivative one was the biggest (uo-c
0.526), that of the meta-substituted derivative two is the smallest (uo-c ¼ 0.232).
¼
DFT theoretical calculation
In order to gain an insight into the electronic features and photoreactivity of 1–3, their
optimized molecular geometries and electron densities have been calculated by depend-
ꢃ
ent density functional theory (DFT) in Gaussian 09 B3LYP/6-31G level [30–32]. As
shown in Figure 3, the open-isomers of 1o–3o presented classical antiparallel conform-
ation. Moreover, the HOMO orbital energy of 1o–3o was mainly localized on the cen-
tral dithienylethene unit, while the orbital energy of the LUMO was largely distributed
over the thiophene and formyl substituted benzene moieties due to great effect of elec-
tron deficient formyl group, which exhibited Acceptor-DTE-Acceptor type molecular
skeleton. And higher HOMO-LUMO band gap (3.35 eV for 1o, 3.54 eV for 2o, and
3.53 eV for 3o) for their ring-open isomers was obtained. For the ring-closed isomers
1c–3c, they presented an almost planar conjugated skeleton, and their HOMO and

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
29
Figure 3. The energy-minimized structures and Frontier molecular orbital profiles of ring-open and
ꢃ
ring-closed isomers of 1-3 based on DFT calculations at the B3LYP/6-31G level by using the
Gaussian 09 program.
LUMO coefficient was nearly delocalized on the whole molecular backbone. As
expected, 1c–3c presented a narrower energy band gap (1.28 eV for 1c, 1.62 eV for 2c,
and 1.39 eV for 3c) compared with that of the corresponding ring-open isomers due to
the extended p conjugation system. Accordingly, the theoretical calculations further vali-
dated the above experimental results of the photochromic behaviors for 1–3.
Conclusion
In summary, three dithienylethene derivatives appending two electron-withdrawing
aldehyde groups were prepared. They displayed good photochromic behaviors and
excellent fatigue resistance upon irradiation with UV or visible light. And it was found
that the aldehyde substituent position had a remarkable effect on their photochromic
properties, mainly including maximum absorption wavelength, molar absorption coeffi-
cients, optical response rate and quantum yields. The DFT theoretical calculations

30
J. YUAN AND Y. BAO
further validated these experimental results for photochromic behaviors. Moreover, they
can be utilized as versatile building blocks to construct novel photochromic materials.
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