Investigation of a Photochromic Diarylethene with Electron-acceptor Attached

Article abstract of DOI:10.1080/15421406.2015.1096440

In this contribution, an electron acceptor attached diarylethene derivative was synthesized and fully characterized. The photochromic behavior was investigated in THF. Under exposure of UV and visible light cycles, the solution color can be switched betwe

Full text of DOI:10.1080/15421406.2015.1096440

Molecular Crystals and Liquid Crystals
ISSN: 1542-1406 (Print) 1563-5287 (Online) Journal homepage: http://www.tandfonline.com/loi/gmcl20
Investigation of a Photochromic Diarylethene with
Electron-acceptor Attached
Xiaochuan Li, Jingli Xu, Yaya Zhang & Young-A Son
To cite this article: Xiaochuan Li, Jingli Xu, Yaya Zhang & Young-A Son (2015) Investigation of
a Photochromic Diarylethene with Electron-acceptor Attached, Molecular Crystals and Liquid
Crystals, 621:1, 102-111, DOI: 10.1080/15421406.2015.1096440
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Date: 24 December 2015, At: 06:41

Molecular Crystals and Liquid Crystals, Vol. 621: pp. 102–111, 2015
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2015.1096440
Investigation of a Photochromic Diarylethene with
Electron-acceptor Attached
XIAOCHUAN LI1,2,1,∗ JINGLI XU,1 YAYA ZHANG,1
AND YOUNG-A SON3,∗
1Collaborative Innovation Center of Henan Province for Green Manufacturing of
Fine Chemicals, Key Laboratory of Green Chemical Media and Reactions,
Ministry of Education, Henan Normal University, Xinxiang, Henan, P. R. China
2Henan Key Laboratory of Green Chemical Media and Reactions, School of
Chemistry and Chemical Engineering, Henan Normal University, Xinxiang,
Henan, P. R. China
3BK21, Department of Advanced Organic Materials Engineering, Chungnam
National University, Daejeon 305-764, South Korea
In this contribution, an electron acceptor attached diarylethene derivative was syn-
thesized and fully characterized. The photochromic behavior was investigated in THF.
Under exposure of UV and visible light cycles, the solution color can be switched
between colorless and yellow smoothly. Fatigue resistance measurements could be re-
peated 50 times with an acceptable degradation. Due to the electron acceptor attached
to the framework of diarylethene, the photo- and thermal-stability were enhanced both.
A full-photo mode switch can be established based on the well-defined states by ex-
ternal excitation. The molecular structures of ring-open and ring-closed form were
optimized by Dmol3. The distance between photocyclizing atoms in aptiparallel confor-
mation meets the requirement for photochromic reaction. And the calculated absorption
wavelengths were also in agreement with the experimental values.
Keywords: diarylethene; photochromism; fatigue resistance; photo- and thermal-
stability; photocyclizing atoms
Introduction
Small functional molecules play vital roles in material science and bio-technology develop-
ment. It has been well documented and developed toward the application in molecular logic
[1], emission materials [2–6], fluorescent sensors and bioprobes [7–12], photoelectronic
conversion devices [13–16], and optical switches [17–20]. Optical switches are powerful
tools for signal conversion in molecular level, which is potential for super massive infor-
mation storage media, biomedical signal/image processing, and microprocessors. Among
various types of optical switches, diarylethenes (DAEs) is the promising candidate due
∗Address correspondence to Prof. Young-A Son, Department of Advanced Organic Materi-
als Engineering, Chungnam National University, 220 Gung-dong, Daejeon, 305-764, South Korea,
E-mail: yason@cnu.ac.kr and Prof. Xiaochuan Li, School of Chemistry and Chemical Engineer-
ing, Henan Normal University, East Jianshe Rd. 46, Xinxiang, Henan, 453007, China, E-mail:
lixiaochuan@henannu.edu.cn.
[272]/102

Investigation of a Photochromic Diarylethene
[273]/103
to its high quantum yield in conversion between bistable state, favorable photo-thermal
stability, and highly fatigue resistance [21].
The framework of DAEs possesses a hexatriene framework, with which a reversible
conrotatory 6π-cyclization photocyclization reaction can proceed efficient upon suitable
light stimuli (Scheme 1). Up to now, the rational design of symmetric or asymmetric DAEs
has been fully development and investigated by modifying the aromatic rings, mainly
the thiophene rings [22]. In addition, it is should not to be neglected that the central
ethane bridges is also a key element for the versatility the DAEs architectures. To date,
the central bridge units of most DAEs varied but limited to five-membered cyclopentene
unit or its strong electron-withdrawing analogues, such as perfluorocyclopentene, maleic
anhydride, maleic imide, and other five membered heteroaromatics. Six membered aromatic
ring also can be an alternative selection as the central bridge unit, such as cyclohexene,
1,10-phenanthroline, benzothiadiazole, and naphthalimide units. However, highly degree
of aromaticity of the six-membered central bridge ring facilitates the undesirable thermal
back reaction in the dark as a result of the significant loss of aromatic stabilization energy
upon photocyclization form the open to the closed isomers. Therefore, low aromaticity of
the central ethene bridge unit (five-membered central bridging) is expected to lead to a
higher thermal stability of the closed isomers. In addition, five-membered central bridging
ethene units can enhance the quantum yield in its closed-ring isomer and longer absorption.
Here, we report a DAE derivative with a strong electron-withdrawing group attached and
investigate its photo-physical properties.
Scheme 1. Photochromic reaction of diarylethenes.
Experimental
General Procedures and Materials
The solvents used in the reaction were carefully dried according to the standard procedure
and stored over 4Å molecular sieve. All the reagent-grade chemicals were purchased
from Sigma-Aldrich CO. LLC. (South Korea) and used without further purification. All
the synthesized compounds were routinely characterized by TLC and NMR. TLC was
performed on aluminum-backed silica gel plates (Merck DC. Alufolien Kieselgel 60 F254).
1H and 13C NMR Spectroscopy
1H and 13C nuclear magnetic resonance (NMR) spectra were recorded on a Brucker AM-
400 spectrometer operating at frequencies of 400 MHz for proton 100 MHz for carbon in
CDCl3. Proton chemical shifts (δ) are relative to tetramethylsilane (TMS, δ = 0) as internal
standard and expressed in parts per million. Spin multiplicities are given as s (singlet), d

104/[274]
X. Li et al.
(doublet), t (triplet), and m (multiplet) as well as b (broad). Coupling constants (J) are given
in Hertz.
Mass and High Resolution Mass Spectra (HRMS)
The mass spectra measured on a LC-MS (Waters UPLC-TQD) mass spectrometer. High
resolution mass spectra (HRMS) were measured on a Brucker microOTOF II Focus instru-
ment.
UV-Vis and Emission Spectra
The absorption spectra were measured with a PERSEE TU-1900 and an Agilent 8453 spec-
trophotometer. Emission spectra were measured with Shimadzu RF-5301PC fluorescence
spectrophotometer. The solvents used in photochemical measurement were spectroscopic
grade and were purified by distillation. The stock solution of compounds (2 × 10−3 M) was
prepared in THF, and a fixed amount of these concentrated solutions were added to each
experimental solution. All the experiments were done repeatedly, and reproducible results
were obtained. Prior to the spectroscopic measurements, solutions were deoxygenated by
bubbling nitrogen through them.
Theoretical Calculations
For the theoretical study of excited state photo-physics of the compound, the DMol3 program
packaged in Material Studio (Accelrys Software Inc., United States) was used [23, 24]. The
ground state geometries and the frontier molecular orbital of the compound were calculated
using the density function theory (DFT) with the B3LYP hybrid functional and the double
numerical plus d-functions (DNP) atomic orbital basis set.
Synthesis
The synthesis of target diarylethene, 2-(4-(3,4-bis(2,5-dimethylthiophen-3-yl)cyclopent-
3-en-1-yl)benzylidene) malononitrile (abbreviated as DM), derivative was outlined in
Scheme 2. The total synthesis was started from 4-bromobenzaldehyde (1), a very cheap
commercial chemical. Firstly, 4-bromobenzaldehyde was condensed with ethyl acetoacetate
at room temperature with piperidine as catalyst and gave 2. β-arylglutaric (3) was obtained
by hydrolysis under 50% KOH aqueous solution. Refluxing 3 and sulfoxide chloride in
benzene gave diacid chloride (4). Acylation of 2,5-dimethylthiophene with 4 yielded 1,5-
diketone (5). Diarylethene backbone was formed by a ring-closure reaction of 1,5-diketone
via McMurry coupling reaction, yielding 6. The active Br atom of 6 can be transformed to
aldehyde (7) with n-BuLi/DMF. The totally synthesis form 1 to 7 is based on our previous
experience [25].
7 (80 mg, 0.2 mmol) and malononitrile (20 mg, 0.3 mmol) dissolved in toluene (20 mL).
One drop of piperidine and a catalytic amount of acetic acid were added to the above mixture.
The mixture was refluxed until 7 disappeared on TLC plate. The mixture was poured into
dichloromethane 50 mL. It was washed with water (20 mL × 3). The organic layers were
combined, dried over anhydrous MgSO4, and the solvent was evaporated in vacuum. The
crude product was purified by column chromatography (Silica gel, EtOAc/hexane) afforded
DM (67 mg, 76%).

Investigation of a Photochromic Diarylethene
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Scheme 2. Synthesis of 2-(4-(3,4-bis(2,5-dimethylthiophen-3-yl)cyclopent-3-en-1-yl)benzylidene)
malononitrile (DM).
1H NMR (400 MHz, CDCl3) δ 7.87 (d, J = 8.0 Hz, 2H), 7.74 (s, 1H), 7.49 (d, J =
8.0 Hz, 2H), 6.43 (s, 2H), 3.69 (m, 1H), 3.26 (dd, J = 8.0 Hz, 2H), 2.86 (dd, J = 8.0 Hz,
2H), 2.36 (s, 6H), 1.89 (s, 6H); 13CNMR (100 MHz, CDCl3): δ 159.5, 155.3, 135.4, 134.4,
132.8, 132.7, 131.2, 128.9, 128.1, 125.7, 113.9, 112.8, 81.4, 46.0, 42.6, 15.1, 14.1. EI+/MS
m/z 440[M]+. HRMS calcd for C27H24N2S2 440.1381, found 440.1384.
Result and Discussion
The photochromic behavior of DM was investigated in THF solution with the concentration
4.5 × 10−5 M at room temperature. The absorption change was shown in Figure 1a irradiated
with 254 nm (hand-held UV lamp, 8W). Before irradiation with 254 nm light, the longest
absorption maximum was observed at 254 nm (ε = 2.4 × 104 M−1cm−1), which was
exactly match to the hand-held UV lamp (254 nm), generally used in the lab for TLC
detection. Together with the shoulder peak (287 nm, ε = 6.6 × 103 M−1cm−1), all the
absorption bands were shorter than 325 nm, locating in the invisible region. Therefore, the
solution color was colorless. Upon irradiation with 254 nm UV light, a new absorption
band appeared between 400 and 500 nm, centered at 447 nm. At the same time, the 254 nm
absorption decreased accompanied with the increase of 447 nm absorption. At last, a
plateauing of the 447 nm band at 150s of 254 nm irradiation was observed, corresponding
to the photostationary state (PSS, A = 0.126) (Figure 1b). The solution color changed from
colorless to yellowish. Irradiating with visible light (broad-band light greater than 400 nm),
the absorption band in the visible (400-500 nm) decreased until the totally disappearance
of this absorption band. The resulting yellowish solution color decolorized to colorless.
Obvious isosbestic points (λ = 236, 272 nm) were found and suggest the existence of only
two species in solution, which in this case are the ring-open and ring-closed isomers of DM.
Based on the character mentioned above, a full-photo mode switch could be established
according to the well-distinguished states by external excitation.
The photochromic reaction upon alternating irradiation of UV/Vis was shown in
Scheme 3. Generally, the absorption of the ring-open form appeared at a shorter wave-
length, while that of the ring-closed form would shift to longer wavelength due to the

106/[276]
X. Li et al.
Figure 1. a) Absorption change of DM in THF (4.5 × 10−5 M) upon irradiation with 254 nm light.
Insert: Enlarged absorption change of DM in the 350–500 nm. B) Increase of absorbance at 447 nm
with respect to irradiating time.
delocalized π-electrons. In theory, the dithienylethene molecule has two conformations
with the two thiophene rings in mirror symmetry (parallel conformation) and in C2 sym-
metry (anticonformation). The photoisomerization can only proceed from the antiparallel
confirmation. In general, the population ration of the two conformations in solution is 1:1.
According to the well-defined isosbestic point, the relative conversion of photocyclization
was determined to be 0.33 at the photostationary state [26].
The fatigue resistance of DM was investigated at room temperature. In THF, DM
was irradiated alternatively with 254 nm and visible light (<400 nm), respectively. In
each cycle, PSS state was reached with 254 nm irradiation and then bleached to colorless
completely. As shown in Figure 2a, the coloration/decoloration could repeat 50 cycles with
17% degradation, indicating an excellent fatigue resistance. Photo-stability measurements
were performed under the UV light exposure. A gradual decrease of absorbance at 272 nm

Investigation of a Photochromic Diarylethene
[277]/107
Scheme 3. Photochromic reaction of DM.
was observed indicating the degradation of DM upon exposure to UV light (Figure 2b).
Totally, 19% of DM degraded after 10h hour’s exposure of UV light, corresponding to
moderate photo-stability. Thermal stability of the ring-closed forms was also investigated
with the temperature ranged from room temperature to 50◦C in the dark. Figure 2c shows the
thermal induced absorption decrease at 447 nm. In the dark, the ring-closed form is stable
and the solution color can maintain more than one day. With the temperature rising, the
thermal induced cycloreversion rate speeds up. The solution color decolorized to colorless
rapidly, reverting back to the ring-open form. Therefore, the ring-closed form is not only
visible light-sensitive but also thermo-sensitive.
To better comprehend the geometrical, electronic, and optical properties of DM, we
undertook a comprehensive computational investigation on the platform of Material Studio.
To reduce the run times in the first instance, the ground-state energy-minimized structures
were calculated using DFT and LDA/DN basis set. Further refinement and optimization on
structures were then undertaken using DFT/B3LYP/DNP basis set.
Figure 3 shows the optimized structure of DM. Due to the conformation requirement,
only the geometric structure of antiparallel conformation was optimized. A key factor for
the reactivity of conrotatory cyclization is the distance between the photocyclizing carbon
atoms. According to the optimized structure, the distance between the two carbon atoms
involved in the cyclization is 3.649Å, which is meet the requirement for photochromism
(distance less than 4.2 Å) [27, 28]. With the distance of photocyclizing carbon atoms larger
than 4.2 Å, the photochromic reaction will be suppressed completely. The absorption spectra
of ring-open and ring-closed form for DM were also calculated based on the optimized
ground state (Figure 4). For ring-open form of DM, the transition HOMO→LUMO is
possible with the oscillator strength (f) 0.547, corresponding to 272 nm absorption. It is
in agreement with experimental value (254 nm). The molecular ortibals (MO) topologies
of the ring-open form (Figure 4a) indicates that the HOMO and LUMO have π shapes.
In HOMO of ring-open form, the electron density distributed over the photochromic core
totally. Contrary to the HOMO distribution, the LUMO distribution of ring-open form
transferred to benzylidenemalononitrile unit due to the electron-withdrawing character. For
the ring-closed form, the electron transition is dominated by HOMO-1→LUMO+1 with
the oscillator strength (f) 0.823. Once the ring-closed, the energy gap for electron transition
was lowered to 2.77eV (ꢀEHOMO-1→LUMO+1) and led to the absorption band shift to longer
wavelength. The calculated absorption wavelength of ring-closed form is identical to that

108/[278]
X. Li et al.
Figure 2. a) Fatigue resistance of DM in THF at room temperature. b) Photo-stability of DM upon
exposure to UV light. c) Absorption change versus time with the temperature ranged from 25–50◦C.

Investigation of a Photochromic Diarylethene
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Figure 3. Optimized structure of DM and the distance between reactive carbon atoms.
Figure 4. Frontier orbitals for DM. a) ring-open form; b) ring-closed form.

110/[280]
X. Li et al.
of the experimental value (447) mentioned above. In addition, the electron density gathered
around in the photochromic core in the HOMO and LUMO distribution. By comparison of
the MO topologies between the ring-open and ring-closed form, the introduction of strong
electron acceptor unit enhanced the sensitivity toward 254 nm light. It also enhanced the
fatigue resistance of DM in solution.
Conclusions
In summary, a photochromic diarylethene derivative was designed and synthesized, and
was fully characterized with the NMR and mass spectra. Photochromic behavior was
investigated in THF solution in details. The longest absorption maximum of ring-open form
of DM was estimated to be 254 nm, which corresponds to our generally used hand-held
UV lamp. Upon irradiation with 254 nm light, the solution color changed from colorless to
yellowish. Visible light irradiation could decolorize the yellowish solution to the original
state. Under cycles of UV and Visible light exposure, the coloration/decoloration cycles
could be repeated 50 times with an acceptable degradation, attesting the excellent fatigue
resistance. Photo- and thermal-stability measurements show that DM in THF is stable at
room temperature. The ring-closed form of DM is thermal sensitive. Temperature rising will
lead to the cycloreversion accelerated. Based on the optimized antiparallel conformation,
the distance between the photocyclizing carbon atoms was estimated and shorter than the
threshold (4.2 Å), which is critical to the photochromic reactivity of diarylethenes. The
calculated MOs demonstrated the inherent relationship between fatigue resistance/photo-
/thermal-stability and the attaching of electron acceptor. Based on the principles, newly
designed functional diarylethenes were synthesized in progress.
Acknowledgments
This work was supported by the National Natural Science Foundation of China (grant no.
21272060), University Student Renovation Project (grant no. 20140166), and Program for
Innovative Research Team in Science and Technology in University of Henan Province
(15IRTSTHN 003). This study was supported by the Basic Science Research Program
through the National Research Foundation of Korea (NRF) funded by the Ministry of
Science, ICT and Future Planning (grant no. 2014006660).
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