An “electron lock” toward the photochromic activity of phenylacetylene appended bisthienylethene

Article abstract of DOI:10.1080/15421406.2020.1743450

Photochromic core structure of BTEs was configured in the design of BT (2,3-Bis(2,5-dimethylthiophen)?5-phenylethynylthiophene) and NT (2,3-Bis(2,5-dimethylthiophen)?5-(4-methoxyphenyl)?1,3-butadiene-1,1,4,4-tetracarbonitrile). BT exhibits regular photoch

Full text of DOI:10.1080/15421406.2020.1743450

Molecular Crystals and Liquid Crystals
ISSN: 1542-1406 (Print) 1563-5287 (Online) Journal homepage: https://www.tandfonline.com/loi/gmcl20
An “electron lock” toward the photochromic
activity of phenylacetylene appended
bisthienylethene
Xiaochuan Li, Qiongyan Cai, Junna Zhang, Hyorim Kim & Young-A Son
To cite this article: Xiaochuan Li, Qiongyan Cai, Junna Zhang, Hyorim Kim & Young-A Son (2020)
An “electron lock” toward the photochromic activity of phenylacetylene appended bisthienylethene,
Molecular Crystals and Liquid Crystals, 706:1, 141-149, DOI: 10.1080/15421406.2020.1743450
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MOLECULAR CRYSTALS AND LIQUID CRYSTALS
020, VOL. 706, NO. 1, 141–149
2
https://doi.org/10.1080/15421406.2020.1743450
An “electron lock” toward the photochromic activity of
phenylacetylene appended bisthienylethene
a
a
a
b
b
Xiaochuan Li
, Qiongyan Cai , Junna Zhang , Hyorim Kim , and Young-A Son
a
Henan Key Laboratory of Organic Functional Molecule and Drug Innovation, Collaborative Innovation
Center of Henan Province for Green Manufacturing of Fine Chemicals, Key Laboratory of Green
Chemical Media and Reactions, Ministry of Education, School of Chemistry and Chemical Engineering,
b
Henan Normal University, Xinxiang, Henan, P. R. China; Department of Advanced Organic Materials
Engineering, Chungnam National University, Daejeon, S. Korea
ABSTRACT
KEYWORDS
Electron distribution; LUMO;
photochromism; photochro-
mic reactivity;
Photochromic core structure of BTEs was configured in the design of
BT (2,3-Bis(2,5-dimethylthiophen)ꢀ5-phenylethynylthiophene) and
NT (2,3-Bis(2,5-dimethylthiophen)ꢀ5-(4-methoxyphenyl)ꢀ1,3-butadi-
ene-1,1,4,4-tetracarbonitrile). BT exhibits regular photochromic
behavior in hexane when excited with UV (254 nm) and visible light
tetracarbonitrile
(
>400 nm). The solution can be cycled between pink colors and col-
orless. Correspondingly, NT, with a more electron-withdrawing
group attached, the photochromic reactivity was totally quenched.
It is the lowered electron density on reactive carbon atoms in
LUMO that influence the photochromism, which is one of the key
elements to photochromism. Apart from the distance between
reactive atoms, electron distribution on reactive atoms is another
important key to the configuration of photochromic dyes.
1. Introduction
Switching function in a molecule scale is of great importance in the next generation of
information storage and corresponding smart logic application based on processing big
data. Therefore, establishing a reversible switch in chemistry became the new task of
chemists and physicists. Switching molecules can be defined as a transformer that can
reversibly change between two distinct configurations when exerting external stimuli,
such as photo, electricity, magnetism, heat, etc. This transformation associated with a
series of physical properties changes, which can be developed in material science and
structural biological technologies [1–8]. Basically, this switching character is based on
the reversible change in their electronic and topological characteristics. The computer
disk currently used is established inorganic/organic switching elements with dynamic
components in optoelectronic devices [3,9]. It is obvious that photos are the appealing
one among various stimuli ways due to fast response time and nondestructive
CONTACT Xiaochuan Li
lixiaochuan@henannu.edu.cn
Henan Key Laboratory of Organic Functional Molecule and
Drug Innovation, Collaborative Innovation Center of Henan Province for Green Manufacturing of Fine Chemicals, Key
Laboratory of Green Chemical Media and Reactions, Ministry of Education, School of Chemistry and Chemical
Engineering, Henan Normal University, Xinxiang, Henan, P. R. China; Young-A Son
yason@cnu.ac.kr
Department
of Advanced Organic Materials Engineering, Chungnam National University, Daejeon, 305-764, S. Korea.
ß 2020 Taylor & Francis Group, LLC

1
42
X. LI ET AL.
Scheme 1. Open and closed forms of typical bisthienylethene structure. The reactive carbon atoms
are in red.
processing. Photochromism is of photo-controllable switching molecule science, refer-
ring to as interconverting molecule between two isomers. When stimulated, there exist
unique absorption spectra. Hidden behind this toggle is luminescence, reactivity, chiral-
ity, etc. [10–20]. Photochromic elements, together with other functions, have been inte-
grated to achieve photo-controlled multi-color modulation [21–23]. One stimulus
induced multi-response is indispensable for the development of smart-logic systems.
It has been established several switching building blocks and the most promising
one is bisthienylethenes (BTE) [24]. The most fascinating property is a highly bistable
state [3]. The skeleton of BTE possesses the hexatriene framework, with which a revers-
ible conrotatory 6p-cyclization reaction could have occurred upon photo stimuli
(Scheme 1) [25–27]. Based on the hexatriene framework, variation of multi-functional-
ized BTEs were configured and documented. The bridge ring of BTE has been engi-
neered as perfluorocyclopentene, maleic anhydride, maleic imide, etc. However, higher
degree of aromaticity of the central unit could lead to unstable of photo-cyclization
isomer [28]. In this contribution, introduction of electron withdrawing group to the
central unit of BTE significantly change the electron distribution and suppression of the
photochromism.
2. Experiment
2.1. General procedures and materials
The solvents used in the reaction were carefully dried according to the standard proced-
ure and stored over 4 Å molecular sieve. All the reagent-grade chemicals were purchased
from Sigma-Aldrich CO. LLC. (South Korea) and used without further purification. All
synthesized compounds were routinely characterized by TLC and NMR. TLC was per-
formed on aluminum-backed silica gel plates (Merck DC. Alufolien Kieselgel 60 F254).
2.2. NMR spectroscopy and high resolution mass spectra (HRMS)
Nuclear magnetic resonance (NMR) spectra were recorded on a Brucker AM-400 spec-
trometer operating at frequencies of 400 MHz for proton in CDCl . Proton chemical
3
shifts (d) are relative to tetramethylsilane (TMS, d ¼ 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. The mass spectra measured on a LC-MS (Waters UPLC-TQD) mass spectrom-
eter. High resolution mass spectra (HRMS) were measured on a Brucker micrOTOF II
Focus instrument.

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
143
Scheme 2. The synthesis of BT and NT.
2.3. UV-Vis
The absorption spectra were measured with a PERSEE TU-1900 spectrophotometer.
The solvents used in photochemical measurement were spectroscopic grade and were
ꢀ
3
purified by distillation. The stock solution of compounds (2 ꢁ 10 M) was prepared
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.
3
. Synthesis
Scheme 2 shows the synthesis of 2,3-Bis(2,5-dimethylthiophen)ꢀ5-(4-phenyl)ꢀ1,3-butadiene-
,1,4,4-tetra-carbonitrile (NT). 2,5-dimethylthiophene, a commercially available reagent,
1
was first brominated by NBS (N-bromosuccinimide). Then, it was treated with n-BuLi
and 2-isopropyl- 4,4,5,5-tetramethyl-1,3,2-dioxaborolane (3), yielding 2-(2,5-dimethylthio-
phen-3-yl)ꢀ4,4,5,5-tetramethyl-1,3,2-dioxaborolane (4). Photochromic skeleton, 2,3-
bis(2,5-dimethyl-thiophen-3- yl)thiophene (5), was obtained by Suzuki coupling method
between 4 and 2,3-dibromothiophene [29]. Subsequently, five was iodinated and coupled
with phenylacetylene under Sogonashira condition, yielding BT. Finally, BT mixes with
tetracyanoethylene at room temperature yielding NT. Overall, the configuration of the
core structure of BTE is constituted by three thiophene ring, which is different from the
conventional BTE unit with electron withdrawing bridging unit, such as perfluorocyclo-
pentene, maleic anhydride, maleic imide [24]. The coupling between dibromo heterocycle
and corresponding thiophene derivatives by Suzuki other coupling strategies can be used
to configure the core structure unit for desired photochromism [30–33].
3
.1. 2,3-Bis(2,5-dimethylthiophen)-5-phenylethynylthiophene (BT))
In a flamed dried Schlenk flask, 2,3-Bis(2,5-dimethylthiophen)ꢀ5-henylethynylthiophene
200 mg, 0.48 mmol), iodide copper (9 mg), Pd(PPh ) (28 mg) were mixed with
(
3
4

1
44
X. LI ET AL.
triethylamine (5 mL) under nitrogen atmosphere. After stirring for 30 min, phenylacety-
lene (48 mg, 0.48 mmol) was added and stirred for another 12 h. Then the mixture was
poured into water (30 mL) and extracted with dichloromethane (3 ꢁ 20 mL). The
organic phase was combined and dried over anhydrous MgSO . After the solvent was
4
evaporated in vacuum, the residue was loaded to the column. Column separation (silica
gel 200–300 mesh, hexane) produced 150 mg deep red solid (77%).
1
H NMR (400 MHz, CDCl ) d 7.55 (m, 2H), 7.37 (m, 3H), 7.22 (s, 1H), 6.49 (s, 1H),
3
1
3
6
1
9
4
.42 (s, 1H), 2.38 (s, 6H), 2.07 (s, 6H). C NMR (100 MHz, CDCl ) d 135.5, 135.7,
3
35.4, 134.9, 134.5, 134.4, 133.2, 132.5, 131.4, 129.9, 128.4, 127.4, 127.0, 123.0, 121.1,
þ
þ
3.6, 82.7, 15.2, 14.0. EI /MS m/z 427 [M] . HRMS calcd for C H S 404.0727, found
24
20 3
þ
27.0692 [M þ Na] .
3
.2. 2,3-Bis(2,5-dimethylthiophen)-5-(4-phenyl)-1,3-butadiene- 1,1,4,4-tetra-
carbonitrile (NT)
In a Schlenk tube with inner gas, tetracarbonitrile (32 mg, 0.25 mmol) and BT (100 mg,
.25 mmol) dissolve in dichloromethane 20 mL. The mixture is stirred at room tempera-
0
ture for 48 h. And then it is poured into water (30 mL). After the organic phase is sepa-
rated and dried over MgSO , it is condensed and loaded on the column. Purification is
4
on silica gel (200–300 mesh) with dichloromethane:hexane (2:1) as the eluent, yielding
5
3 mg deep red solid (40%).
H NMR (400MHz, CDCl ) d 7.8 (s, 3H), 7.69 (s, 1H), 7.58 (s, 2H), 6.49 (s, 1H), 6.35
1
3
1
3
(s, 1H), 2.37 (s, 6H), 2.07 (s, 6H). C NMR (100MHz, CDCl ) d 207.1, 166.6, 156.7,
3
1
1
5
49.1, 139.8, 138.2, 137.4, 137.3, 136.8, 134.8, 131.5, 131.1, 130.3, 130.1, 129.4, 128.2,
26.7, 126.2, 112.9, 112.0, 111.6, 111.0, 87.7, 77.8, 31.0, 29.7, 15.1, 14.4, 13.8. EIþ/MS m/z
þ
þ
55 [M] . HRMS calcd for C H N S 532.0850, found 555.0804 [Mþ Na] .
3
0 20 4 3
4. Result and discussion
The molecular structure of BT possesses the hexatriene skeleton, which is the typical
photochromic structure character of BTEs. With phenylacetylene appended at the 5-pos-
ition of BT, the p-conjugate system is expanded. Both the bridging thiophene group
and the triple bond are electron-rich units. This is a benefit to gather the electron dens-
ity to the photochromic hexatriene part, which is significant to the photochromic
reactivity. Partial NMR spectra of BT and NT are shown in Fig. 1. With or without the
addition of tetracarbonitrile to the triple bond, the core structural unit was identical to
each other. However, the typical proton signal in NMR is the proton at the b-position
of two sides thiophen rings, which does not overlap with the proton of the aromatic
benzene ring in the downfield. For BT, two single peaks can be found at 6.49 and
6.42 ppm, which can be assigned to the b-photos of side thiophene ring. The two peaks
were separated by 0.07 ppm When tetracarbonitrile added, the electron-withdrawing
character of carbonitrile significantly shifted the electron distribution of molecular skel-
eton, then the magnetic field. Therefore, in NT, the b-proton signals of two side thio-
phene rings (6.49 and 6.35 ppm) were still single peaks, and the separation between the
two photons was enlarged to 0.14 ppm. Simultaneously, the proton signals of benzene
rings turned to be more complex due to the polarized magnetic field.

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
145
Figure 1. Partial NMR spectra of BT (left) and NT (right) in the downfield.
Figure 2. Absorption changes of BT in hexane (1 ꢁ 10ꢀ5 M).
The photochromic behavior of BT is investigated in a different organic medium,
including hexane, dichloromethane, tetrahydrofuran (THF), and acetonitrile, under
irradiation of 254 nm light. Different polarity environment must lead to different
absorption profiles. However, the photochromic wavelength at a longer wavelength side
is same as each other with the peak value around 570 nm. With thiophene and phenyl
rings constituting the whole molecular skeleton, there must be evenly electron density
distribution. Therefore, hexane, the non-polar solvent, would be the best choice for
photochromic behavior investigation. Fig. 2 shows the spectral change in hexane when
excited with UV light. In hexane, there exists strong absorption before 325 nm.
Therefore, hand held 254 nm UV lamp is well-matched to the excitation requirement of
BT. Upon 254 nm irradiation UV light irradiation, a gradual absorption band increase
*
can be found with the band center around 570 nm in hexane, corresponding to the p-p

1
46
X. LI ET AL.
Figure 3. Absorption of NT in chloroform. (1ꢁ 10ꢀ5 M).
transition. Meanwhile, a decrease at 265, 330 nm is concomitant with the increase at
5
70 nm. When excited with 254 nm, the absorption spectra were recorded every 10 s.
Generally, the absorption band contributed by ring-open form is at UV region and
extended to visible region once the ring-closed form generated due to the delocalization
of p-electrons to cyclizing thiophene rings. 570 nm’ absorption turns BT’s hexane solu-
tion to a pink color. Well-defined isosbestic points can be determined to be 285
ꢀ
1
ꢀ1
ꢀ1
ꢀ1
(
M
e ¼ 4.8 ꢁ 104 M cm ), 318 (e ¼ 5.9 ꢁ 104 M cm ), and 338 nm (e ¼ 6.1 ꢁ 104
ꢀ
1
ꢀ1
cm ). It indicates that the photochromic transformation only occurs between two
components in the photochromic reaction. Contrary to the above photochromic pro-
cess, the pink solution can be decolorized by irradiation with visible light (ꢂ400 nm,
LED lamp) and the band around 570 nm decreases gradually until its disappearance
totally. Simultaneously, the pink solution was decolored to totally transparent. And this
coloration/decoloration can be cycled by UV/Vis excitation cycle. The conversion effi-
ciency (a ) between ring-open and ring-closed forms at the photo-stationary state,
ps
induced by UV light irradiation, is estimated to be 0.37, which can be calculated follow-
ing the equation based on the absorbance of the two forms [29]. According to the
already established molecular configuration of BTEs, the attaching of phenylacetylene
does not quench the photochromic reactivity or shift the photochromic wavelength. At
least, the electron distribution around the reactive carbon of the two side thiophene
ring was not lowered, which is a key requirement for photochromic activity [34].
It is well-known that a carbon–carbon triple bond can be added with tetracarbonitrile
efficiently, which can introduce an electron-withdrawing group and significant change
of the electron distribution. BT mixed with tetracarbonitrile at room temperature and
yielded NT efficiently. Simply structural analysis shows that the addition of tetracarbo-
nitrile does not alter the photochromic core. However, the photochromic reactivity of
NT was totally quenched. Fig. 3 shows the absorption of NT in chloroform. A new
ꢀ
1
ꢀ1
strong absorption peak was observed at 603 nm (e ¼ 3.8 ꢁ 104 M cm ) and a shorter
wavelength band 230–400 nm. No matter how to irradiate the NT solution or alter the
solvent polarity, there is no absorption change that happened as that of BT in hexane.
Based on the analysis of the absorption of BT and NT, we found that there is a signifi-
cant overlap of the photochromic wavelength of BT (475–650 nm) and the strong

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
147
Scheme 3. Photochromic reaction of BT and NT.
absorption band (500–700 nm) of NT. When NT was excited with UV light, there
would be an efficient energy transfer to the lower energy band (603 nm), which
quenched the photochromic activity of NT. We initially believed that the switch core
would be independent of the substitution to it. However, whether the molecules exhibit
a positive photochromic response to UV irradiation is closely related to the electronic
structure of the whole molecules. In NT, the tetracarbonitrile is a newly introduced flu-
orophore. The two photo-responsive regions will effectively compete for the lowest
energy transitions. The absorption around 603 nm outweighs the photochromic wave-
length of the switching unit around 570 nm. In fact, the photochromic activity of NT is
suppressed completely. By comparison of the structural variation of BT and NT, the
addition of tetracarbonitrile induce a significant change to the photochromic activity.
The photochromic activity of the switching unit was suppressed by an “electron lock.”
The suppressed photochromic activity is considered to be the electron distribution
induced by the tetracarbonitrile. It is predictable that the electron-withdrawing power of
tetracarbonitrile moves the electron density away from the side thiophene rings. To get
further insight into the electronic properties of BT and NT, we undertake a comprehensive
computational investigation using Materials Studio. The ground-state energy-minimized
structures are calculated using DFT and DND/B3LYP basis set [35,36]. The photochro-
mic reaction upon alternated irradiation of 254/>400 nm light is shown in Scheme 3.
Theoretically, there exist two conformations for the hexatriene structure with the two
thiophene rings in mirror symmetry (parallel conformation) and in C2 symmetry
(unparallel conformation). Only when the two side thiophene rings in antiparallel con-
formations, the photochromic reaction could occur. Once the ring-closed, a new absorp-
tion band in visible region appeared. One of the key requirements to the photochromic
activity is the distance between reactive carbon atoms, which has been determined to be
4.2 Å [37–39]. However, this distance is not the unique determinate factor for the
photochromic reactivity. The electron distribution, topology of the molecular orbitals,
must be considered. Fig. 4 shows the HOMO and LUMO distribution of BT and NT.
For BT, the LUMO distribution gathers around one of the reactive carbon atoms. It

1
48
X. LI ET AL.
Figure 4. HOMO and LUMO distribution of BT and NT in the ring-open forms.
indicates that the electron transition from HOMO to LUMO involves the reactive car-
bon atoms. The photochromic photocyclization reaction must be closely related with the
electron redistribution when excited with suitable light. Indeed, the photocyclization of
BT occurs smoothly upon excitation with 254 nm light, as demonstrated above. When
tetracarbonitrile was added to the triple bond, the electron density of NT in LUMO will
move away from the reactive carbon atoms. The significantly lower level of electron
density around the reactive carbon atoms leads to completely quenched photochromic
reactivity. Without the strong electron-withdrawing power toward the electron around
reaction atoms, BT maintains its photoactivity in photochromism.
5. Conclusions
In summary, photochromic core skeleton inserted molecules BT and NT are synthesized
and characterized by NMR and MS techniques. The photochromic behavior for both of
them is investigated. In hexane, BT exhibit regular photochromic properties with the
photochromic wavelength extending to 570 nm and corresponding pink color upon
irradiation with 254 nm light. Irradiated with visible light (>400 nm), the solution can
be decolorized and reverted to the originate state. On the contrary, NT, with tetracarbo-
nitrile introduction, the photochromism is quenched. HOMO/LUMO distribution ana-
lysis indicates that the lower electron density on the reactive carbon atom can
completely suppress the photoactive of photochromism. It is another key factor need to
be considered in the configuration of BTEs.
Funding
This work was supported by the National Natural Science Foundation of China [21772034]; the
Program for Innovative Research Team in Science and Technology of Colleges and Universities
in Henan Province [19IRTSTHN023, 20IRTSTHN005]. X. Li gratefully acknowledges the finan-
cial support of China Scholarship Council (CSC) for being a visiting scholar in Aoyama Gakuin
University [201908410053].
ORCID
Xiaochuan Li
http://orcid.org/0000-0002-6903-9574

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
149
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