Reversed photochromism reactivity of malononitrile attached bisthienylthene

Article abstract of DOI:10.1080/15421406.2018.1466534

An electron acceptor, malononitrile, was attached to bisthienylethene derivative. The significant influence of electron nature result a reversed phtochromism. Investigation of MT in THF demonstrates the reversible ring-open and ring-close. Upon irradiation of 405?nm laser, the solution color changed from yellowish to colorless, which is different from most of the photochromic colored phenomenon. Upon 254?nm irradiation, the original yellowish state can be recovered. Due to the attachment of malononitrile, the photo-/thermal-stability were enhanced. The ring-open/ring-closed forms were optimized by Dmol³. The distance between photocyclizing atoms in aptiparallel conformation meets the requirement for photochromic reaction. And the calculated absorption values were also in agreement with the experimental ones.

Full text of DOI:10.1080/15421406.2018.1466534

Molecular Crystals and Liquid Crystals
ISSN: 1542-1406 (Print) 1563-5287 (Online) Journal homepage: http://www.tandfonline.com/loi/gmcl20
Reversed photochromism reactivity of
malononitrile attached bisthienylthene
Xiaochuan Li, Yujie Han, Myeong Jin Kim & Young-A Son
To cite this article: Xiaochuan Li, Yujie Han, Myeong Jin Kim & Young-A Son (2018) Reversed
photochromism reactivity of malononitrile attached bisthienylthene, Molecular Crystals and Liquid
Crystals, 662:1, 147-156, DOI: 10.1080/15421406.2018.1466534
To link to this article: https://doi.org/10.1080/15421406.2018.1466534
Published online: 04 Jun 2018.
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MOLECULAR CRYSTALS AND LIQUID CRYSTALS
, VOL. , –
https://doi.org/./..
Reversed photochromism reactivity of malononitrile attached
bisthienylthene
Xiaochuan Li a,b, Yujie Hana, Myeong Jin Kimc, and Young-A Sonc
aCollaborative 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; bHenan Key Laboratory of Green Chemical Media and Reactions, School of Chemistry and Chemical
Engineering, Henan Normal University, Xinxiang, Henan, P. R. China; cBK, Department of Advanced Organic
Materials Engineering, Chungnam National University, South Korea
ABSTRACT
KEYWORDS
Bisthienylethene;
malononitrile; photo- and
thermal-stability;
photocyclizing atoms;
reversed photochromism
An electron acceptor, malononitrile, was attached to bisthienylethene
derivative. The significant influence of electron nature result a reversed
phtochromism. Investigation of MT in THF demonstrates the reversible
ring-open and ring-close. Upon irradiation of 405 nm laser, the solution
color changed from yellowish to colorless, which is different from most
of the photochromic colored phenomenon. Upon 254 nm irradiation,
the original yellowish state can be recovered. Due to the attachment
of malononitrile, the photo-/thermal-stability were enhanced. The
ring-open/ring-closed forms were optimized by Dmol3. The distance
between photocyclizing atoms in aptiparallel conformation meets the
requirement for photochromic reaction. And the calculated absorption
values were also in agreement with the experimental ones.
Introduction
Molecules that respond to the external stimuli and could precede a reversible transforma-
tion between two distinct configurations/confirmations have the potential to significantly
influence on the physical properties of molecules, which could be developed in numerous
materials science and structural biological technologies [1–5]. This potential is based on the
reversibly change in their electronic and topological characteristics. Thus, a switching ele-
ments or dynamic components in various optoelectronic devices or smart materials could be
established [6, 7]. Photons are particularly appealing tools because modern laser pulse can
be used to achieve fast response times, to focus a fine-tuned stimulus on a small localized
domains without significant diffusion and to trigger photochemical events. Photochromism is
referred as that molecule that can interconvert between different isomers have unique absorp-
tion spectra when stimulated with light. Accompanied with the conformation change, it is also
resulted variations of other practical physical properties such as luminescence (switch “on” or
“off”), reactivity, chirality, signal encoding, and electronic communication [8–18]. Modula-
tion of these properties is of significant in functional device fabrication. Up to now, three or
CONTACT Prof. Xiaochuan Li
University, East Jianshe Rd. , Xinxiang, Henan , China; Young-A Son
lixiaochuan@htu.edu.cn
School of Chemistry and Chemical Engineering, Henan Normal
yason@cnu.ac.kr Department of Advanced
Organic Materials Engineering, Chungnam National University,  Daehakro, Yuseong, Daejeon , South Korea.
Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/gmcl.
©  Taylor & Francis Group, LLC

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X. LI ET AL.
Scheme. Atypicalbisthienyletheneinitsopenand closed forms, thecyclopenteneringbeingthebridging
group. The reactive carbon atoms are in red.
more photochromic elements, together with fluorescent units, were integrated together for
multi-color modulation have been documented, which could be applied to photo-switchable
full-color displays [19–22]. And the multi-response toward external stimuli advances the
device configuration of smart-logic displays.
Among various types of photo switches, bisthienylethenes (BTE), is one of most promis-
ing candidate due to its high quantum yield, favorable photo-thermal stability, and highly
fatigue resistance [23]. The skeleton of BTE possesses the hexatriene framework, with
which a reversible conrotatory 6π-cyclization reaction could be occur upon photo stim-
uli (Scheme 1) [24, 25]. Up to now, structural variation of symmetric and asymmet-
ric BTEs have been intensively developed and investigated. Generally, the modification is
focused in two aspects, the swing units and the bridge. For the bridge variation, it is lim-
ited in contrast to the swing units. Typical bridge units are perfluorocyclopentene, maleic
anhydride, maleic imide, or aromatic heterocyclic. However, higher degree of aromatic-
ity of the central unit could lead to rapidly thermal back reaction due to the loss of aro-
matic stabilization energy upon photo-cyclization from the ring open to close isomers [26].
In this contribution, a strong electron withdrawing group, malononitrile, is attached to
the bridge thiophene ring by condense reaction. And the photo-physical properties were
investigated.
Experimental
General procedures and materials
The solvents used in the reaction were carefully dried according to the standard procedure and
˚
stored over 4A molecular sieve. All the reagent-grade chemicals were purchased from Sigma-
Aldrich CO. LLC./Alfa Aesar-A Johnson Matthey Company (South Korea) and used without
further purification. All synthesized compounds were routinely characterized by TLC and
1H NMR. TLC was performed on aluminum-backed silica gel plates (Merck DC. Alufolien
Kieselgel 60 F254).
1H/13C NMR and high resolution mass spectroscopy
1H and 13C nuclear magnetic resonance (NMR) spectra were recorded on a Bruker
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 (doublet), t (triplet), and m (multiplet) as well as b (broad). Coupling constants (J) are given
in Hertz. High resolution mass spectra (HRMS) were measured on a Bruker autoflex X III
instrument.

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
149
Scheme . The synthesis of MT.
UV-Vis spectra
The absorption spectra were measured with a PERSEE TU-1900 and an Agilent 8453 spec-
trophotometer. 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,
and a fixed amount of these concentrated solutions (2 × 10−5 M) were added to each exper-
imental 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. 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 2,3-Bis(2,5-dimethylthiophen-3-yl)-5-methylenemalononitrile-thiophene
(MT) is depicted in Scheme 2. Commercially available 2,5-dimethylthiophene was used
as the starting materials and it was conveniently bromized. Then, it was transformed to
2-(2,5-dimethylthiophen-3-yl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (4) when treated
with n-BuLi and 2-isopropyl-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (3). Photochromic
unit, 2,3-bis(2,5-dimethylthiophen-3-yl)thiophene (5), was obtained by Suzuki coupling
method between 4 and 2,3-dibromothiophene [27]. Aldehyde was introduced by treating
5 with n-BuLi/DMF. Finally, the target compound MT was obtained by condensing 6 and
malononitrile in ethanol.
3-bromo-2,5-dimethylthiophen (2)
1H NMR (400 MHz, CDCl3): δ (ppm): 6.54 (1H, s), 2.38(3H, s), 2.31(3H, s). 13C NMR
(100 MHz, CDCl3): δ (ppm): 137.0, 132.1, 128.8, 108.4, 15.9, 14.7.

150
X. LI ET AL.
2-(2,5-dimethylthiophen-3-yl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (4)
1H NMR (400 MHz, CDCl3): δ (ppm): 7.28 (1H, s), 7.03 (1H, s), 6.48 (1H, s), 6.43 (1H, s),
2.38 (6H, s), 2.03 (3H, s), 2.01 (3H, s). 13C NMR (100 MHz, CDCl3): δ (ppm): 136.7, 135.1,
125.4, 122.3, 88.9, 24.5, 16.1, 15.6.
2,3-Bis(2,5-dimethylthiophen-3-yl)thiophene (5)
1H NMR (400 MHz, CDCl3): δ (ppm): 7.29 (1H, s), 7.04 (1H, s), 6.47 (1H, s), 6.44 (1H, s), 2.36
(6H, s), 2.01 (3H, s), 1.99 (3H, s). 13C NMR (100 MHz, CDCl3) δ (ppm): 141.6, 135.4, 135.1,
134.1, 133.9, 133.1, 132.4, 130.9, 128.1, 127.8, 126.9, 15.2, 15.1, 13.7, 13.6.
2,3-Bis(2,5-dimethylthiophen-3-yl)thiophenecarbaldehyde (6)
1H NMR (400 MHz, CDCl3): δ (ppm): 9.88 (1H, s), 7.68 (1H, s), 6.47 (1H, s), 6.41 (1H, s), 2.38
(6H, d, J = 4.4 Hz), 2.06 (6H, d, J = 15.2 Hz). 13C NMR (100 MHz, CDCl3) δ (ppm): 182.8,
144.4, 140.9, 138.9, 136.2, 136.0, 135.9, 135.6, 133.6, 131.8, 129.6, 127.0, 126.6, 15.1, 15.0, 14.0,
13.7.
2,3-Bis(2,5-dimethylthiophen-3-yl)-5-methylenemalononitrile-thiophene (MT)
Malononitrile (72 mg, 1.09 mmol) and 6 (200 mg, 0.6 mmol) were dissolved in ethanol
(15 mL) and stirred for 30 min. Then, sodium acetate (73 mg, 0.89 mmol) was added to the
above solution. The mixture was stirred at room temperature. After the 6 was fully reacted, as
confirmed by TLC, the solvent was evaporated. The residue was loaded onto the silica gel col-
umn chromatography. Pure sample was obtained with petroleum ether as the eluent, affording
brown oil 190 mg (83%).
1H NMR (400 MHz, CDCl3): δ (ppm): 7.76 (1H, s), 7.67 (1H, s), 6.49 (1H, s), 6.39 (1H, s),
2.38 (6H, s), 2.07 (3H, s), 2.04 (3H, s); 13C NMR (100 MHz, CDCl3): δ (ppm): 150.5, 146.8,
140.8, 137.1, 136.8, 136.4, 136.3, 134.2, 132.9, 130.8, 128.8, 126.8, 126.4, 114.2, 113.3, 15.1,
15.0, 14.2, 13.7. MS (EI, 70 eV) m/z 380 (M+); HRMS: [M+] C20H16N2S3 requires 380.0476;
found [M+] 380.0473.
Results and discussion
The photochromic behavior of MT is investigated in THF solution with the concentration
2.0 × 10−5 M at room temperature. The absorption change was shown in Fig. 1(a) irradiated
with 405 nm laser light (hand-held LED lamp, 405 nm/50 mW). Before irradiation with
405 nm light, the longest absorption maximum is observed at 410 nm (ε = 2.5 × 104 M−1
cm−1), which is fitting well to the commercially available LED laser lamp (405 nm). This
absorption band steps into visible region and exhibits yellowish color. Together with the
shoulder band at the blue side (330 nm, ε = 1.8 × 104 M−1 cm−1), S0→S1 transition could
be assigned with the bands on the high- and low-energy side corresponding to 0–1 and 0–0
vibration transition, respectively. Upon irradiation with 405 nm light, the 410 nm absorption
decreases gradually to totally disappearance. The absorption around 280 nm also significantly
decreases and reaches to optically transparent almost. In contrast, a significant absorption
increase is observed around 240 nm (ε = 2.4 × 104 M−1 cm−1) and 295 nm (ε = 1.4 ×
104 M−1 cm−1). Finally, a photo-stationary state (PSS) is reached after 7 min irradiation of

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
151
Figure . (a) Absorption change of MT in THF (. × − M) upon irradiation with  nm laser light.
(b) Decrease of absorbance at  nm with respect to irradiation time.
405 nm light. The absorptivity at 240 and 295 nm is almost doubled with the molar extinction
coefficient 5.3 × 104 and 2.8 × 104 M−1 cm−1, respectively. Finally, the solution color changed
from yellowish to colorless completely. The newly appeared 240 nm band is fitting well to the
254 nm UV lamp (hand held UV lamp, 8 W) used in the lab for TLC detection. Irradiation
with 254 nm light, the absorption around 410 nm is appeared again gradually and the solution
color recovered to yellowish. Obvious isobestic points are found at 235, 265, 290, 318 nm.
It suggests the existence of only two species in the reversible process, which in this case are
the ring-open and ring-closed isomers of MT. It should be noted that the photochromic
behavior is different from most of the published diarylethene derivatives. The cyclization
and cycloreversion quantum yields of MT were determined to be 0.30 and 0.13. In contrast
with the photochromic behavior of 5 (reversible between colorless and colored state), the
photochromism of MT is reversible between colored and colorless state, which is defined as
“invisible photochromism” [28]. Figure 1(b) shows the absorption change at 410 nm with

152
X. LI ET AL.
Scheme . Photochromic reaction of MT.
respect to irradiation time and an excellent linear relationship could be established between
absorption and irradiation time, which indicates a smooth photochromic process.
The photochromic reaction upon alternation irradiation of 405/254 nm light is shown in
Scheme 3. In theory, the dithienylethene molecule has two conformations with the two thio-
phene rings in mirror symmetry (parallel conformation) and in C2 symmetry (unparallel con-
formation). The photo-isomerization can only proceed from the antiparallel confirmation,
which accounts for half of the totally two conformation. Generally, the absorption of the ring-
opened form appeared at a shorter wavelength side, while that of the ring-closed form would
shift to longer wavelength due to the delocalized π-electrons. However, it is reversed due to
the introduction of electron-acceptor unit. The absorption of ring-opened form appeared at
longer wavelength side in visible region. Once the ring-closed, the absorption band in visible
region disappeared, moving to invisible region (UV region). According to the well-defined
isobestic point, the relative conversion of photo-cyclization was determined to be 0.55 at the
photo-stationary state [29].
The photo- and thermal-stability were also investigated. The photo-stability measure-
ments were performed under the irradiation of UV light exposure (broad band, 240–400 nm,
16 W). The isobestic absorption of 318 nm was monitored for decomposition. The absorption
decrease indicates the degradation of MT induced by UV light Fig. 2(a). After 1 h’s exposure
of UV light, 5% of MT degraded, corresponding to an acceptable photo-stability. Thermal
stability of the ring-closed forms was also investigated at room temperature and 323 K in the
dark. Figure 2(b) shows thermal induced absorption increase at 410 nm. In the dark, the ring-
closed form is stable and the colorless solution could maintain for 24 h. With the temperature
rising, the thermal induced cycloreversion rate speeds up. The solution color recovered to
color state, reverting back to the ring-opened form. Therefore, the ring-closed form is both
light-sensitive and thermal-sensitive.
To better understand the geometric, electronic, and optical properties of MT, a compre-
hensive computational investigation was undertaken using Materials Studio software package.
To reduce the initial run times, the ground state energy-minimized structures were calculated
using DFT and the LDA/DN basis set. Further refinement and optimization of the structure
were achieved using time-dependent DFT (TDDFT) with B3LYP/DND basis set, as imple-
mented in the Dmol3 package [30, 31].
Figure 3 shows optimized antiparallel conformation, which is required for light-induced
conrotatory cyclization. The key factor for the reactivity of conrotatory cyclization is the dis-
˚
tance between the photo-cyclizing carbon atoms must shorter than 4.2 A [32–34]. Once the
˚
distance is larger than 4.2 A, light-induced conrotatory cyclization will be totally inhibited.
According to the optimized structure of MT, the distance between the two carbon atoms

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
153
Figure . (a) Photo-stability of MT upon exposure to UV light; (b) absorption change versus time with the
temperature at  and  K.
˚
involved in the cyclization is 3.591A, which is meet the requirement for photochromism. The
distance between the reactive carbon atoms should be steric control of photochromic reac-
tivity. Apart from that, the electronic features control the photochromic reactivity in another
aspect. The nature of the states, topology of the molecule orbitals corresponding to the UV
peaks provides more detailed insights towards the photochromic reactivity Fig. 4. For ring-
open form, the transition of HOMO-3→LUMO and HOMO→LUMO+1 are contribute to
the strong absorption of 420 and 342 nm with the oscillator strength (f) 0.281 and 0.240,
respectively. It is agreement with the experimental value 410/330 nm. The molecular orbital
(MO) topology of the ring-open form Fig. 4(a) indicates that the orbitals have π shapes. The
electron density distributes over the photochromic group in LUMO+1, which has the bond-
ing character of to be formed C-C bond. Due to the electron-withdrawing nature, the LUMO
distributes toward the malononitrile group. For the ring-closed form, the electron transition
is dominated by HOMO-1→LUMO+2 with f being 0.240 Fig. 4(b). Once the ring-closed, the

154
X. LI ET AL.
Figure . Optimized structure of MT and the calculated between reactive carbon atoms. Hydrogen atoms
were omitted for clarity.
Figure . Frontier orbitals for MT. (a) Ring-open form; (b) ring-closed form.
energy gap for electron transition is enlarged to 4.12 eV and led to the absorption band shift
to higher energy band. The calculated absorption wavelength of ring-closed form is identical
to that of the experimental value (295 nm). In addition, the electron density gathered around
in the photochromic core in HOMO-1 and LUMO+2. By comparison of the MO topologies
between the ring-open and ring-closed form, the significant densities on the reactive carbons
moves to higher energy orbitals. This leads to the absorption of ring-closed form shifts to
shorter wavelength.
Conclusions
In summary, a robust photochromic bisthienylethene derivative was synthesized, and was full
characterized. Its photochromic behavior was investigated in THF solution. It is interesting
that the photochromic compound exhibit reversed photochromism. The ring-open form is
colored and changed to colorless when irradiated with 405 nm laser. Due to the introduction

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
155
of malononitrile, the strong absorption at longer wavelength was observed at 410 nm, which
is fitting well to the commercially available laser LED lamp. After the PSS state reached, the
410 nm absorption is nearly optical transparent and absorption band around 295 and 240 nm
enhanced. Upon irradiation with 254 nm light, the solution color changed from colorless to
yellowish, recovering the original color of MT. The photo- and thermal-stability measure-
ments show that MT is robust towards light and temperature. The calculation provides further
insight about the reversed photochromism. Strong electron-acceptor results the electron den-
sities in reactive carbons to higher energy orbitals and thus shifted absorption of ring-closed
form to shorter wavelength.
Funding
This work was supported by the National Natural Science Foundation of China (21272060, 21772034),
and the Program for Innovative Research Team in Science and Technology in University of Henan
Province (15IRTSTHN003). 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. NRF-2017R1E1A1A01074266).
ORCID
Xiaochuan Li http://orcid.org/0000-0002-6903-9574
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