Reversible dynamic full-color phototuning with a new chiral molecular switch containing linking group between chiral center and photosensitive group

Article abstract of DOI:10.1080/15421406.2019.1595670

In this study, a new photoresponsive chiral molecular switch, where azobenzene was not directly connected to the axially chiral binaphthyl group, was successfully synthesized and used to induce helical superstructures in the achiral nematic liquid crystal

Full text of DOI:10.1080/15421406.2019.1595670

Molecular Crystals and Liquid Crystals
ISSN: 1542-1406 (Print) 1563-5287 (Online) Journal homepage: https://www.tandfonline.com/loi/gmcl20
Reversible dynamic full-color phototuning with
a new chiral molecular switch containing linking
group between chiral center and photosensitive
group
Lang Qin, Jiaqi Liu, Xinlei Pang, Jia Wei & Yanlei Yu
To cite this article: Lang Qin, Jiaqi Liu, Xinlei Pang, Jia Wei & Yanlei Yu (2018) Reversible
dynamic full-color phototuning with a new chiral molecular switch containing linking group between
chiral center and photosensitive group, Molecular Crystals and Liquid Crystals, 676:1, 50-58, DOI:
10.1080/15421406.2019.1595670
To link to this article: https://doi.org/10.1080/15421406.2019.1595670
Published online: 11 Jul 2019.
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MOLECULAR CRYSTALS AND LIQUID CRYSTALS
018, VOL. 676, NO. 1, 50–58
2
https://doi.org/10.1080/15421406.2019.1595670
Reversible dynamic full-color phototuning with a new
chiral molecular switch containing linking group between
chiral center and photosensitive group
Lang Qin, Jiaqi Liu, Xinlei Pang, Jia Wei, and Yanlei Yu
Department of Materials Science and State Key Laboratory of Molecular Engineering of Polymers,
Fudan University, Shanghai 200433 China
ABSTRACT
KEYWORDS
Azobenzene; Cholesteric
liquid crystals; Chiral
molecular switch;
Photoisomerization
In this study, a new photoresponsive chiral molecular switch, where
azobenzene was not directly connected to the axially chiral
binaphthyl group, was successfully synthesized and used to induce
helical superstructures in the achiral nematic liquid crystal (LC) host
E7. Photoisomerization of the chiral molecular switch was carried out
in both organic solvent and LC host upon light irradiation, contribu-
ting to the HTP difference at different photostationary states. The
phototunable property of chiral molecular switch in E7 allowed
dynamic, rapid and reversible tuning of the reflection color in choles-
teric LC phase over the entire visible spectrum. The strategy breaks
the restriction in common structures of photoresponsive chiral
molecular switches, providing exciting insight into design for novel
binaphthyl azobenzene derivatives with axial chirality.
1. Introduction
Apart from pigments and dyes, a profusion of colors in nature originate from the inter-
action of periodic biological structures with light, such as beetle carapaces, peacock feath-
ers and butterfly wings [1–4]. Structural colors do not fade over time due to their unique
structure-dependent property, which accordingly arouse extensive attention of scientists to
exploit artificially synthesized structural colors. The self-assembled helical superstructure
in cholesteric liquid crystals (CLCs) makes these materials promising for generating struc-
tural colors. The most outstanding character of CLCs is its selective reflection of light,
which can be tuned through multiple external stimuli (heat [5], light [6–12], electric field
[13–17], etc.). Thanks to the fast responsiveness, non-destructive property and remote
precise controllability of light, phototunable CLCs are widely studied.
The most commonly used method to obtain phototunable CLCs is to dope a small
amount of photoresponsive chiral molecular switches into a nematic liquid crystal (LC)
host, where molecular chirality is transferred to phase chirality and the resulting helical
superstructures selectively reflect light according to Bragg’s law [18]. The selective
reflection wavelength k is defined by k ¼ np, where n is the average refraction index of
LC materials and p is the pitch length of the helical superstructure. The ability of a
CONTACT Jia Wei
weijia@fudan.edu.cn
Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/gmcl.
ß 2019 Taylor & Francis Group, LLC

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
51
Figure 1. Schematic illustration of selective reflection of light in cholesteric liquid crystal and photo-
tuning of pitch in helical superstructures.
chiral molecular switch to twist an achiral nematic LC is described as helical twisting
ꢀ
1
power (HTP,b) following the equation b ¼ (pc) , where c is the molar or weight con-
centration of chiral molecular switches. The isomerization of chiral molecular switches
under light irradiation causes change in HTP values at different photostationary states
and thus controls reflection wavelength of CLCs (Figure 1), opening doors to applica-
tions in fields of tunable color reflectors and filters [12,19–21], optically addressed flex-
ible displays [6,9,22,23], etc. The key character of those applications lies in the dynamic
phototuning of reflection color over the entire visible light region, which requires pho-
toresponsive chiral molecular switches to possess proper initial HTP values as well as
HTP differences among different photostationary states.
Currently binaphthyl azobenzene derivatives with axial chirality are a group of inten-
sively studied photoresponsive chiral molecular switches, because the axial chirality
caused by the rotational barrier around naphthyl-naphthyl bond provides significant
HTP values and azobenzene leads to suitable HTP differences among various states.
Nevertheless, the existing chiral molecular switches that meet the above requirements
mostly share the general structure, i.e. direct connection of azobenzene to the 2, 2’ posi-
tions of axially chiral binaphthyl group [6,9–12], in which it is not easy to involve diver-
sified azobenzene structure. The monotonic structures of these chiral molecular switches
make it difficult to optimize structures towards more excellent properties. Besides, rela-
tively high cost of crude materials (i.e. 1,1’-binaphthyl-2,2’-diamine) impede the applica-
tions of CLC for it is unrealistic to synthesize these chiral molecular switches on a large
scale. Therefore, it is highly necessary to explore new molecules to rich the structure of
chiral molecular switches, meeting the different requirements for application.
Here we report a novel photoresponsive chiral molecular switch 4 (Figure 2) differing
from the common structures mostly reported. By adding one methylene spacer and
rigid benzene between axially chiral 1, 1’-binaphthyl group and azobenzene, chiral
molecular switch 4 allows structural diversity of photoresponsive chiral molecular
switches. The design of this chiral molecule is based on the following points: 1) the
steric hindrance resulting from the rigid benzene between 1, 1’-binaphthyl group and
azobenzene as well as the rotational barrier from the naphthyl-naphthyl bond (i.e. the
dihedral angle h) play an important role in transferring the axially molecular chirality to
LC phase [24], 2) conformational isomers of azobenzene offers HTP differences at

5
2
L. QIN ET AL.
Figure 2. Synthetic route of chiral molecular switch 4.
different photostationary states, 3) the rigid benzene between 1, 1’-binaphthyl group
and azobenzene enhances geometrical variation of the chiral molecular switch under
photoisomerization and hence induces larger HTP change, 4) two long methylene
chains greatly improve its solubility in nematic LC host.
2. Experimental
Materials and Methods
In the syntheses of chiral molecular switch, common reagents and solvents were com-
1
13
mercially available and used without further purification. H NMR and C NMR spec-
tra were recorded on 500 MHz Liquid NMR (AVANCE III HD) spectrometer in CDCl3
or DMSO. Mass spectra were taken by Bruker McriOTOF11 Mass Spectrometer and
Voyager-DE STR Mass Spectrometer. DSC (TA Q2000 Differential Scanning
Calorimetry) was used to measure melting points, where heating and cooling rates were
ꢁ
ꢀ1
both 5 C min . UV-Vis absorption spectrum was measured by Perkin Elmer Lambda
50 Spectrometer. Circular dichroism (CD) spectra were measured by MM-450 CD
6
spectrophotometer. Disclination lines, fingerprint texture and reflection color were
recorded by Leika DM2500p Polarized Optical Microscopy. Reflection spectra and ther-
mal relaxation were carried out by PG2000 PRO/NIR 2500 reflection spectroscopy.
Synthesis of chiral molecular switch 4
4-Bromoaniline (5g, 29 mmol) was dissolved in a solution of 6 mL concentrated HCl
and 40 mL H O. A solution of sodium nitrite (2.07g, 29 mmol) in 40 mL H O was
2
2

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
53
ꢁ
added dropwise and stirred at 0 C for 2 h. Then a solution of phenol (2.82 g, 29 mmol)
and NaOH (1.2 g, 30 mmol) in 40 mL H O was added dropwise. The solution was
2
ꢁ
stirred at 0 C for 2 h. The suspension was then acidified with HCl solution and filtered.
The precipitate was washed and dried over magnesium sulfate to get crude intermediate
1. The mixture of crude intermediate 1, 1-bromohexane (4.79 g, 29 mmol) and potas-
sium carbonate (4.81g, 34.8 mmol) were dissolved in 100 mL DMF and heated to reflux
ꢁ
ꢁ
at 110 C for 4 h. The resulting mixture was cooled to room temperature (25 C) and
added great amount of H O to obtain precipitate. The precipitate was purified by chro-
2
matography (petroleum ether: dichloromethane ¼ 4:1) to yield intermediate 2 as an
ꢁ
1
orange solid (6.6 g, 18.33 mmol, yield 63.2%). m.p. 92.75 C; H NMR (500 MHz,
CDCl ): d 7.90 (d, J ¼ 9.0 Hz, 2H), 7.75 (d, J ¼ 9.0 Hz, 2H), 7.62 (d, J ¼ 9.0 Hz, 2H), 7.00
3
(
0
1
d, J ¼ 9.0 Hz, 2H), 4.04 (t, J ¼ 13.0 Hz, 2H), 1.82 (m, 2H), 1.48 (m, 4H), 1.36 (m, 2H),
1
3
.92 (t, J ¼ 14.0 Hz, 3H); C NMR (500 MHz, CDCl ): d 162.04, 151.55, 146.72, 132.23,
3
24.92, 124.47, 124.07, 114.80, 68.44, 31.57, 29.16, 25.70, 22.60, 14.02; MALDI-TOF MS
(
M þ H) calcd for C H BrN O: 361.1, found: 361.1
1
8
21
2
The mixture of intermediate 2 (2.12 g, 5.89 mmol) and 4-(hydroxymethyl)phenylbor-
onic acid (1.34 g, 8.83 mmol) in 20 mL toluene was added a solution of potassium car-
bonate (4.88 g, 35.34 mmol) in 10 mL H O. The heterogeneous mixture was stirred at
2
ꢁ
room temperature (25 C) under an atmosphere of nitrogen. Then 10 droplets of meth-
yltrioctylammonium and tetrakis(triphenylphosphine)palladium(0) (0.68g, 0.59 mmol)
ꢁ
were added. The reaction was heated to reflux at 90 C for 12 h. The solvent was
removed under reduced pressure and the residue was purified by chromatography
(
7
dichloromethane) to yield intermediate 3 as a yellow solid (1.61 g, 4.14 mmol, yield
0.3%). m.p. 91.37 C; H NMR (500 MHz, DMSO): d 7.90 (d, J ¼ 9.0 Hz, 2H), 7.88 (d,
ꢁ
1
J ¼ 9.0 Hz, 2H), 7.85 (d, J ¼ 8.5 Hz, 2H), 7.71 (d, J ¼ 8.5 Hz, 2H), 7.43 (d, J ¼ 8.5 Hz,
2
H), 7.11 (d, J ¼ 9.0 Hz, 2H), 4.55 (s, 2H), 4.08 (t, J ¼ 13.0 Hz, 2H), 1.74 (m, 2H), 1.44
13
(
1
6
m, 2H), 1.32 (m, 4H), 0.88 (t, J ¼ 14.5 Hz, 3H); C NMR (500 MHz, DMSO): d 162.04,
51.69, 146.81, 143.05, 142.73, 137.97, 127.85, 127.59, 126.97, 125.02, 123.33, 115.62,
8.62, 63.14, 31.42, 29.05, 25.57, 22.47, 14.28; MALDI-TOF MS (M þ H) calcd for
C H N O : 389.2, found: 389.2
2
5
28 2 2
The mixture of intermediate 3 (0.5 g, 1.28 mmol) and diisopropyl azodicarboxylate
0.62 g, 3.09 mmol) was added dropwise to a mixture of (S)-(-)-1,1’-Bi-2,2’-naphthol
(
(
ꢁ
0.15 g, 0.52 mmol) and triphenylphosphine (0.81 g, 3.09 mmol) in THF at 50 C under
ꢁ
an atmosphere of nitrogen. Then the reaction was heated to reflux at 90 C for 12 h.
The solvent was removed under reduced pressure and the residue was purified by chro-
matography (petroleum ether: dichloromethane ¼ 1:4) to yield chiral molecular switch
ꢁ
1
4
as an orange solid (0.35 g, 0.34 mmol, 66.0%). m.p. 119.49 C; H NMR (500 MHz,
CDCl ): d 8.03 (m, 8H), 7.94 (d, J ¼ 9.0 Hz, 2H), 7.89 (d, J ¼ 8.5 Hz, 4H), 7.65 (d,
3
J ¼ 8.5 Hz, 4H), 7.50 (d, J ¼ 9.0 Hz, 2H), 7.46 (d, J ¼ 8.0 Hz, 4H), 7.39 (d, J ¼ 8.5 Hz,
4
4
H), 7.12 (d, J ¼ 8.0 Hz, 4H), 7.03 (d, J ¼ 8.5 Hz, 4H), 5.15 (m, 4H), 4.06 (t, J ¼ 13.5 Hz,
13
H), 1.83 (m, 4H), 1.49 (m, 4H), 1.36 (m, 8H), 0.92 (t, J ¼ 14.0 Hz, 6H); C NMR
(500 MHz, CDCl ): d 161.77, 154.94, 151.97, 147.02, 142.50, 139.70, 136.51, 134.10,
3
1
1
33.87, 130.93, 129.89, 127.60, 127.47, 127.07, 126.44, 125.10, 124.77, 123.28, 123.01,
17.54, 116.03, 114.76, 71.00, 68.42, 31.59, 29.19, 25.71, 22.61, 14.03; MALDI-TOF MS
(
M þ H) calcd for C H N O : 1027.5, found: 1027.5
7
0
66 4 4

5
4
L. QIN ET AL.
3. Results and Discussion
Chiral molecular switch 4 was prepared in a facile synthesis route (Figure 2) starting
from 4-bromoaniline which first reacted with phenol to give the azobenzene intermedi-
ate 1, followed by coupling with 1-bromohexane to afford intermediate 2. Intermediate
2
was then coupled with 4-(hydroxymethyl)phenylboronic acid through Suzuki Cross-
coupling Reaction [25] to obtain intermediate 3. Intermediate 3 was treated with the
cheap axially chiral binaphthyl source (S)-(-)-1,1’-bi-2,2’-naphthol to yield the target
1
molecule 4 by Mitsunobu Reaction [26]. These molecules were well-identified by H
1
3
NMR, C NMR and high resolution MS.
Containing azobenzene groups in the structure, chiral molecular switch 4 exhibited
fast and reversible photoresponsive behavior in both organic solvent and nematic LC
host. A solution of chiral molecular switch 4 in CH Cl underwent fast trans-cis photoi-
3
ꢀ
2
somerization upon 365 nm light irradiation (3 mW cm ), transferring from the initial
state ((trans, trans)-4 in dominant ratio) to the 365 nm photostationary state ((cis, cis)-4
in dominant ratio) in 24 seconds (Figure 3(a)). This was evidenced by the decrease in
the maximum absorption around 365 nm indicating p-p* transition together with the
*
increase in the absorption around 450 nm representing n-p transition. Similarly, the
reverse cis-trans isomerization process from the 365 nm photostationary state to the
530 nm photostationary state was achieved through 530 nm visible light irradiation
ꢀ
5
Figure 3. (a) Changes in UV-Vis absorption spectra of chiral molecular switch 4 in CH Cl (2 ꢂ 10
3
ꢀ
1
ꢀ2
mol L ) under 365 nm light irradiation (3 mW cm ); (b) Changes in CD spectra of chiral molecular
ꢀ4
ꢀ1
switch 4 in CH Cl (1 ꢂ 10 mol L ) under 365 nm light and 530 nm light irradiation.
3

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
55
ꢀ
2
*
*
(
10 mW cm ) within 210 seconds. Apart from p-p and n-p transition bands of azo-
benzene chromophore, the bands between 250-350 nm represented short-axis polariza-
1
1
tions ( L and L ) of the 1, 1’-binaphthyl group. Circular dichroism (CD) spectroscopy
b
a
was also used to identify the conformation of chiral molecular switch 4 and the effect
of photoisomerization on conformational change. As shown in Figure 3(b), the positive
cotton effect in CD spectrum proved the absolute S configuration of chiral molecular
switch 4. The CD spectrum showed two main regions of absorption: a) 250-350 nm
with strong Cotton effect and b) over 350 nm with relatively weak Cotton effect. The
1
1
former represented short-axis polarizations ( L and L ) of the 1, 1’-binaphthyl group
b
a
*
*
and the latter was caused by p-p and n-p transition of azobenzene chromophore,
which was also indicated in UV-Vis spectrum as well. Upon 365 nm light irradiation,
the absolute S configuration did not change, but intensity changed in both two main
regions. CD band changes between 250-350 nm clearly indicated dihedral angle change
in 1, 1’-binaphthyl group, while excitations of azobenzenzene over 350 nm in the CD
spectrum illustrated trans-cis photoisomerization. In other words, light irradiation can-
not change the overall configuration of chiral molecular switch 4, but dihedral angle
change upon photosiomerization is essential for creating HTP differences at different
photostationary states [21,24,27].
When doped in a commercially available achiral nematic LC host E7 at a low concen-
tration, chiral molecular switch 4 can induce self-organization of helical superstructures
and formation of cholesteric LC phase. The HTP values were measured by Grandjean-
Cano method [28]. Disclination lines were observed under polarized optical microscope
and distance between them was measured for further calculation of HTP values. The
ꢀ
1
ꢀ1
initial HTP of chiral molecular switch 4 was 67 lm and decreased by 29 lm upon
65 nm light irradiation, indicating the enlargement of helical pitch in CLC phase. After
exposure to 530 nm light, the helical pitch decreased and the HTP value of chiral
3
ꢀ
1
molecular switch 4 returned to 66 lm , which was almost equal to its initial value
Figure 4). Though the initial HTP of chiral molecular switch 4 is not very high, it
(
didn’t hinder the excellent phototuning performance of the chiral switch in LC phase.
And this problem could be solved conveniently by introducing rod-like cyclohexyl-
phenyl moieties that resemble the LC host structure, since they have better interaction
with LC host and may bring about higher HTP values [29]. The reversible phototuning
of pitches in helical superstructures as well as HTP values of chiral molecular switch 4
lay foundations for phototunable reflection color change.
Figure 4. Polarized optical microscopy images of 0.47 mol% chiral molecular switch 4 in E7 in wedge
cells, showing disclination lines and pitch changes upon light irradiation. (a) Initial state; (b) 365 nm
photostationary state; (c) 530 nm photostationary state.

5
6
L. QIN ET AL.
A mixture of 6.0 mol% chiral molecular switch 4 in nematic LC host E7 was capil-
lary-filled in a 5 lm thick cell with planar alignment layer to induce the formation of
homogenous aligned CLC phase. A black board was introduced on one side of the LC
cell in order to avoid background light interference. Measured by reflection spectra, the
wavelength of refection light ranged from blue light region across the entire visible
ꢀ
2
spectrum to near infrared region after 365 nm light irradiation at 3 mW cm light
intensity (Figure 5). The reverse process from near infrared region to blue light region
was achieved by 530 nm light irradiation at the same light intensity within several sec-
onds, or it was realized by thermal relaxation in the dark at room temperature in
approximately 28 h. As is shown in Figure 5, the reflection spectra have no palpable
drawbacks such as fluctuation in peak intensity or distortion of peak shape. The cycle
was repeated many times without obvious fatigue.
In addition, color change of the resulting CLC film induced by light irradiation was
observed and recorded. When a mixture of 5.9 mol% chiral molecular switch 4 in nem-
atic LC host E7 was capillary-filled in a 5 lm thick planar anchoring cell, reflection
color of the resulting CLC film shifted rapidly between blue and red upon UV or visible
light irradiation, covering the whole visible spectrum (Figure 6). The entire process was
easily captured by a polarized optical microscope in reflective mode and no phase separ-
ation or coloration was observed. By carefully controlling the irradiation time, ratio of
trans/cis isomers in chiral molecular switch 4 could be tuned continuously.
Accordingly, color almost changed sequentially from blue to red in 18 seconds upon
ꢀ
2
3
65 nm light irradiation at intensity of 3 mW cm , while the reverse process was
achieved in 35 seconds upon 530 nm light irradiation at the same light intensity.
Phototunable reflection color change over the visible spectrum is an ideal property for
Figure 5. Reflection spectra of 6.0 mol% chiral molecular switch 4 in nematic LC host E7 in a 5 lm
ꢀ
2
thick planar anchoring cell. (a) Upon 365 nm light irradiation at intensity of 3 mW cm ; (b) Upon
30 nm light irradiation at the same light intensity.
5

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
57
Figure 6. Reflection color change of 5.9 mol% chiral molecular switch 4 in nematic LC host E7 in a
lm thick planar anchoring cell under polarized optical microscope. (Top) Color change from blue to
5
ꢀ2
red in 18 seconds upon 365 nm light irradiation at intensity of 3 mW cm ; (Bottom) The reverse pro-
cess achieved in 35 seconds upon 530 nm light irradiation at the same light intensity. (Bottom) Upon
530 nm light irradiation at the same light intensity.
optical devices. This rapid and reversible phototunable behavior could be mainly attrib-
uted to the excellent solubility of chiral molecular switch 4 in E7 and proper HTP dif-
ference between two photostationary states. The excellent solubility of chiral molecular
switches is also a property urgently demanded for display applications.
4. Conclusions
In conclusion, we have successfully synthesized a novel photoresponsive chiral molecu-
lar switch 4 that can phototune reflection color reversibly and rapidly over visible spec-
trum. The unique structure of chiral molecular switch 4 broke through restriction in
the classic structure of photoresponsive chiral molecular switches where azobenzene was
directly connected to axially chiral binaphthyl group. Chiral molecular switch 4 was
able to induce helical superstructures in the achiral nematic LC host E7 and thus form
the cholesteric LC phase. Photoisomerization of chiral molecular switch 4 was carried
out in both organic solvent and LC host upon light irradiation, contributing to the HTP
difference at different photostationary states. The phototunable property of chiral
molecular switch 4 in E7 allowed dynamic, rapid and reversible phototuning of the
reflection color in CLC phase over the entire visible light region. These results provide
exciting insight into design for novel binaphthyl azobenzene derivatives with axial chir-
ality and widen applications of CLCs in optical displays.
Funding
This work is financially supported by the National Natural Science Foundation of China
(51573029, 21734003), National Key R&D Program of China (2017YFA0701302) and Natural
Science Foundation of Shanghai (17ZR1440100), Innovation Program of Shanghai Municipal
Education Commission (Grant No. 2017-01-07-00-07-E00027).

5
8
L. QIN ET AL.
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