Visible-Light-Driven Halogen Bond Donor Based Molecular Switches: From Reversible Unwinding to Handedness Inversion in Self-Organized Soft Helical Superstructures

Article abstract of DOI:10.1002/anie.201913977

Visible-light-driven molecular switches endowing reversible modulation of the functionalities of self-organized soft materials are currently highly sought after for fundamental scientific studies and technological applications. Reported herein are the des

Full text of DOI:10.1002/anie.201913977

A Journal of the Gesellschaft Deutscher Chemiker
Angewandte
Chemie
International Edition
www.angewandte.org
Accepted Article
Title: Visible Light-Driven Halogen Bond Donor Based Molecular
Switches: From Reversible Unwinding to Handedness Inversion
in Self-Organized Soft Helical Superstructures
Authors: Hao Wang, Hari Krishna Bisoyi, Bing-Xiang Li, Michael
McConney, Timothy Bunning, and Quan Li
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201913977
Angew. Chem. 10.1002/ange.201913977
Link to VoR: http://dx.doi.org/10.1002/anie.201913977
http://dx.doi.org/10.1002/ange.201913977

10.1002/anie.201913977
Angewandte Chemie International Edition
COMMUNICATION
Visible Light-Driven Halogen Bond Donor Based Molecular
Switches: From Reversible Unwinding to Handedness Inversion
in Self-Organized Soft Helical Superstructures
Hao Wang, Hari Krishna Bisoyi, Bing-Xiang Li, Michael E. McConney, Timothy J. Bunning, and Quan
Li*
Abstract: Visible light-driven molecular switches endowing reversible
modulation of the functionalities of self-organized soft materials are
currently highly sought after for fundamental scientific studies and
technological applications. Herein, we report the design and synthesis
of two novel halogen bond donor based chiral molecular switches that
are able to exhibit reversible photoisomerization upon exposure to
visible light of different wavelengths. These halogen bond donor
based chiral molecular switches induce photoresponsive helical
superstructures, i.e., cholesteric liquid crystals, when doped into the
commercially available room temperature achiral liquid crystal host
5CB which also acts as a halogen bond acceptor. The induced helical
superstructure containing the molecular switch with terminal iodo
atoms exhibits visible light-driven reversible unwinding, i.e., a
cholesteric-nematic phase transition. Interestingly, the molecular
switch with terminal bromo atoms confers reversible handedness
inversion to the helical superstructure upon irradiation with visible light
of different wavelengths. This visible light-driven reversible
handedness inversion, enabled by halogen bond donor based
molecular switch, is unprecedented. These halogen bond donor chiral
molecular switches would inspire the development of visible light-
driven advanced photonic materials based on specific molecular
interactions and beyond.
wavelength of interest. Such CLCs are responsive to various
stimuli, including temperature, electric field, magnetic field,
mechanical field, and/or light.[3] Among all stimuli, light is
particularly fascinating as it holds spatial, temporal, precise and
non-invasive advantages. A magnificent number of light-driven
molecular switches have been synthesized and applied in various
areas of materials science. To broaden the scope of light-driven
chiral switches, we recently demonstrated full-color reflection
tuning by light or temperature using a halogen bonded chiral
switch.[4] The halogen bond, owing to its special characteristics
such as linearity, tunability and hydrophobicity, has been
employed as
a powerful tool in designing self-organized
supramolecular functional materials.[5] To further explore the
potential of the halogen bond in the development of functional
CLCs, it is attractive to design halogen bond donor chiral
switches.
In contrast with the large number of chiral switches enabling
dynamic tuning of the selective reflection of CLCs, endowing light-
directing reversible phase transition and handedness inversion
using a pure enantiomer molecular switch is a fascinating
research topic as it demands optimal conformation change of the
molecular switch.[6] Light-induced phase transition requires a
decrease in the helical twisting power (HTP) of the chiral switch
to close to zero. For light-driven handedness inversion, it is
necessary that the two isomeric forms of the chiral switch induce
CLCs with opposite handedness.[7] Both reversible phase
transition and handedness inversion have great potential in novel
optical device applications. The utility of visible light
photoisomerization can avoid the potential degradation caused by
harmful UV light to both materials and operating personnel.[8-13]
Nature is abundant with light-driven phenomena which have
served as great inspiration for the development of light-driven
functional materials for basic scientific studies and advanced
technological applications. Recently, visible light-driven dynamic
functional soft materials have been drawing tremendous
attention. In this context, photo-tunable self-organized helical
superstructures of cholesteric liquid crystal (CLC) containing light-
driven chiral molecular switches or motors have yielded
remarkable achievements.[1] CLCs with tunable attributes have
been extensively utilized in the development of polarizers,
reflectors, optical filters, tunable lasers, beam steering devices,
and photodisplays.[1,2] The most convenient and economical
approach to fabricate CLCs is to add chiral dopant into
commercially available achiral nematic liquid crystal (LC) host at
the appropriate concentration to induce selective reflection in the
Currently, the most common commercially available achiral
nematic LC hosts are cyano-based materials, including 5CB, E7
and ZLI-1132. The electronegative region at the end of cyano
(N≡C-) group in the host mesogens has been long overlooked. It
is known that the cyano group readily interacts with the
electrophilic region of a halogen atom of another molecular entity
to form a halogen bonded complex with a linear bond angle.[14,15]
The halogen bonding complexes have been observed before.[14-
17]
However, CLCs based on halogen bonding between the
photoresponsive chiral dopant as the halogen bond donor and the
nematic LC host as the acceptor has not been explored. Herein,
we report the synthesis and characterization of two novel visible
light-driven halogen bond donor based chiral molecular switches
and their use in the development of functional CLCs capable of
exhibiting either reversible unwinding or handedness inversion in
a halogen bond acceptor achiral nematic host (5CB) (Scheme 1).
[*]
H. Wang, Dr. H. K. Bisoyi, Dr. B. Li, Prof. Q. Li
Advanced Materials and Liquid Crystal Institute and Chemical
Physics Interdisciplinary Program, Kent State University, Kent, Ohio
44242, USA
Email:qli1@kent.edu
Dr. M. E. McConney, Dr. T. J. Bunning
Materials and Manufacturing Directorate, Air Force Research
Laboratory, Wright-Patterson AFB, Ohio 45433, USA
Supporting information for this article is given via a link at the end of
the document.
This article is protected by copyright. All rights reserved.

10.1002/anie.201913977
Angewandte Chemie International Edition
COMMUNICATION
photoisomerization when exposed to visible light of different
wavelengths.
The
chiral
switch
2
exhibits
similar
photoisomerization behavior (see Figure S2).
Figure 1. (A) UV-vis absorption spectra of switch 1 in various states in solution
(10-5 M in hexane). (B) The switching cycles of absorption intensity at 339 nm
upon alternating irradiation using 530 nm and 440 nm light sources.
The chirality induction ability of the molecular switches was
investigated using commercially available room temperature
nematic LC host 5CB. Here it should be noted that while the chiral
molecular switches act as halogen bond donors, the host 5CB
acts as halogen bond acceptor. We doped 1.0 mol% of the chiral
molecular switches to obtain two different cholesteric mixtures
and determined the HTPs of 1 and 2 (Table 1) using Cano-Wedge
method (Figure S4 and Figure S5). The Cano wedge cell studies
suggested that the CLC containing dopant 1 undergoes a
reversible cholesteric-nematic transition whereas the CLC
containing dopant 2 exhibits reversible handedness inversion.
From Table 1, it is clear that switch 1 shows higher HTP value
compared to 2 in initial state. This trend could be attributed to the
stronger halogen bonding ability of iodo compound compared to
the bromo counterpart. Density functional theory calculations
support the above observation (Figure 2).
Scheme 1. Top: Chemical structures of visible light-driven halogen bond donor
based chiral molecular switches 1 and 2 and achiral nematic LC 5CB which
functions as both the halogen bond acceptor and host medium. Bottom:
Schematic representation of reversible unwinding and handedness inversion in
the self-organized helical superstructure upon visible light irradiation.
Table 1. HTP values (µm-1, molar ratio) of the chiral molecular switches (S)-1
and (S)-2 in nematic host 5CB at various states. The negative and positive signs
represent left- and right-handed induced CLCs, respectively.
The synthesis and characterization of the two photoresponsive
visible light-driven chiral azoarenes are presented in the
Supporting Information (SI). Both the new compounds have been
characterized using nuclear magnetic resonance (NMR)
spectroscopy, and high-resolution mass spectrometry.
Switches
Initial state
-26.9
PSS 440 nm
-12.8
PSS 530 nm
~0
1
2
-23.2
-7.3
+13.4
The photoisomerization behavior of the two chiral molecular
switches was studied in solution (10-5 M in hexane). The UV-vis
absorption spectra of switch 1 shows a main absorption band with
a maximum at 339 nm, which corresponds to the π-π* transition
band (Figure 1). The band also covers the visible range because
of the red-shift of the π-π* transition due to ortho fluoro-
substitutions on the outer phenyl groups. We also observed the
band separation of the n-π* transition of the trans- and cis-
isomers (Figure S1), which facilitates the visible light-driven
reversible photoisomerization. By using green light of 530 nm, we
observed that the π-π* absorption intensity decreases, indicating
trans-to-cis photoisomerization. The reverse process can be
achieved by shining blue light at 440 nm. Overall, the chiral
molecular switch is capable of exhibiting reversible
Figure 2. Electron density maps of the binaphthyl tetrafluoroazobenzene
photoresponsive chiral dopants (1 and 2) as the halogen bonding donor and
5CB host as the acceptor.
This article is protected by copyright. All rights reserved.

10.1002/anie.201913977
Angewandte Chemie International Edition
COMMUNICATION
Inspired by the large HTP change and transition to the
nematic phase observed in the Cano wedge cell of the iodo-
based sample, we filled the CLC mixture (1.0 mol% of 1 in 5CB)
into a 20 µm homeotropic cell and observed the typical
fingerprint texture (Figure 3). Upon 530 nm (3.5 mW/cm2)
irradiation for 60s, the helix unwinds resulting in a nematic phase
confirmed by conoscopy method. The nematic to cholesteric
state transition could be achieved by blue light (440 nm, 2.5
mW/cm2) irradiation (Figure S6). The reversible visible light-
driven cholesteric to nematic transition has also been
ascertained using planar cells (Figures S7-S9).
Figure 4. POM of handedness inversion of the CLC containing 1.0 mol% of 2
in a homeotropic cell upon 530 nm light irradiation.
The helical sense and inversion process were further
characterized by miscibility testing using a sample prepared by
the contact method. We filled the CLC mixture containing 2 (1.0
mol% in 5CB) and the known left-handed CLC formed by
cholesteryl oleyl carbonate in a 15 µm homeotropic cell (Figure
5). Initially, the test CLC exhibits continuous contact with the
known left-handed CLC, indicating the initial helical sense is left-
handed. After 20 s irradiation (530 nm), the test CLC on the left
undergoes unwinding until the helix disappears completely
(Figure 5D). After 120 s of irradiation, the helix reappears and
forms a clear discontinuous boundary with the known left-
handed CLC at 140 s, indicating the newly appeared CLC is
right-handed (Figure 5F). The whole process illustrates that the
CLC undergoes handedness inversion upon irradiation with
green light.
Figure 3. POM images of CLC containing 1.0 mol% of 1 in a 20 µm
homeotropic cell and its transition to nematic phase upon 530 nm light
irradiation.
Interestingly, we have found that the CLC mixture containing
switch 2 in 5CB exhibits handedness inversion upon visible light
irradiation, which is caused by the change in molecular
conformation upon photoisomerization. The occurrence of
handedness inversion through a compensated nematic state in
the Cano wedge cell is shown in Figure S6.
We used a homeotropic cell to characterize the light-induced
handedness inversion of the CLC mixture containing 1.0 mol%
of switch 2 (Figure 4). The CLC mixture was filled into a 15 µm
homeotropic cell while in the isotropic state and then cooled to
room temperature where it exhibits a typical finger-print texture.
Upon green (530 nm) light exposure, the fingerprint texture
disappeared quickly (20 s) and a transient nematic phase was
confirmed with conoscopic observation (Figure 4D). Continued
irradiation at 530 nm resulted in the reappearance of the
fingerprint texture, indicating the formation of another
cholesteric phase with opposite handedness. The reverse
process can be achieved by blue light irradiation at 440 nm
(Figure S10).
Figure 5. POM images of contact tests between a CLC mixture containing 1.0
mol% of switch 2 and the left-handed cholesteryl oleyl carbonate.
We have fabricated gratings from the CLC containing dopant
2 by applying an electric field (sine wave of alternative current,
voltage of 2.2 V) to a planar cell with 15 µm cell gap (Figure 6).
The grating period could be reversibly tuned and switched off
This article is protected by copyright. All rights reserved.

10.1002/anie.201913977
Angewandte Chemie International Edition
COMMUNICATION
Stimuli Responsive Materials: From Well-Defined Nanostructures to
Applications (Ed: Q. Li), Wiley, Hoboken, 2013; c) Y. Wang, Q. Li. Adv.
Mater. 2012, 24, 1926; d) L. Wang, Q. Li. Chem. Soc. Rev. 2018, 47,
1044; e) H. K. Bisoyi, Q. Li. Chem. Rev. 2016, 116, 15089; f) H. K. Bisoyi,
Q. Li. Acc. Chem. Res. 2014, 47, 3184; g) H. K. Bisoyi, T. J. Bunning, Q.
Li. Adv. Mater. 2018, 30, 1796512; h) L. Wang, Q. Li. Photochromism
into Nanosystems: towards Lighting up the Future Nanoooworld. Chem.
Soc. Rev. 2018, 47, 1044; i) L. Wang, A. M. Urbas, Q. Li. Adv. Mater.
2018, 1801335; j) R. S. Zola, H. K. Bisoyi, H. Wang, A. M. Urbas, T. J.
Bunning, Q. Li. Adv. Mater. 2019, 31, 1806172.
using visible light irradiation. Diffraction patterns have been
obtained by passing red laser light (632.8 nm) through both right
and left-handed cholesteric gratings.
[2]
[3]
a) H. Wang, H. K. Bisoyi, A. M. Urbas, T. J. Bunning, Q. Li. J. Am. Chem.
Soc. 2019, 141, 8078; b) J. Li, H. K. Bisoyi, S. Lin, J. Guo, Q. Li. Angew.
Chem. Int. Ed. 2019, 58, 16052; Angew. Chem. 2019, 131, 16198; c) J.
Fan, Y. Li, H. K. Bisoyi, R. S. Zola, D. Yang, T. J. Bunning, D. A. Weitz,
Q. Li, Angew. Chem. Int. Ed. 2015, 54, 2160; Angew. Chem. 2015, 127,
2188.
Figure 6. POM images of a lying CLC helix containing 1.0 mol% switch 2 in a
15 μm planar cell under 2.2 V AC electric field. (A) PSS440nm, (B) transient
nematic phase, (C) PSS530nm. The insets are the corresponding diffraction
gratings.
a) Y. Li, C. Xue, M. Wang, A. M. Urbas, Q. Li. Angew. Chem. Int. Ed.
2013, 52, 13703; Angew. Chem. Int. Ed. 2013, 125, 13948; b) L. Wang,
H. Dong, Y. Li, R. Liu, Y. Wang, H. K. Bisoyi, L. Sun, C. Yan, Q. Li. Adv.
Mater. 2015, 27, 2065; c) Z. Zheng, Y. Li, H. K. Bisoyi, L. Wang, T. J.
Bunning, Q. Li. Nature. 2016, 531, 352; d) L. Wang, K. G. Cuevas, A.
Urbas, Q, Li. Adv. Optical Mater. 2016, 4, 247; e) C. Yuan, W. Huang, Z.
Zheng, B. Liu, H. K. Bisoyi, Y. Li, D. Shen, Y. Lu, Q. Li. Sci. Adv. 2019,
5, aax9501; f) K. G. Gutierrez-Cuevas, L. Wang, Z. Zheng, H. K. Bisoyi,
G. Li, L. Tan, R. A. Vaia, Q. Li, Angew. Chem. Int. Ed. 2016, 55, 13090;
Angew. Chem. 2016, 128, 13284; g) L. Zhang, L. Wang, U. S. Hiremath,
H. K. Bisoyi, G. C. Nair, C. V. Yelamaggad, A. M. Urbas, T. J. Bunning,
Q. Li, Adv. Mater. 2017, 29, 1700676.
In conclusion, we have designed and synthesized two axially
chiral fluorinated azoarene based molecular switches. These
halogen bond donor based molecular switches undergo
reversible photoisomerization upon exposure to visible light of
different wavelengths. These chiral molecular switches induce
visible light-driven helical superstructures when added into the
halogen bond acceptor room temperature achiral nematic host
5CB. As the size of the terminal halogen atom decreases, the
HTP of the switches decreases. The chiral dopant containing
terminal iodo atoms shows a higher HTP than the compound
containing terminal bromo atoms. The cholesteric mixture
containing compound 1 exhibits reversible helix unwinding, i.e.,
a cholesteric to nematic phase transition upon irradiation with
visible light. The cholesteric phase obtained using chiral switch
2 undergoes reversible handedness inversion upon exposure to
visible light of different wavelengths. We have also
demonstrated cholesteric gratings which exhibit light diffraction
using the CLC containing 2. The grating period could be tuned
by visible light irradiation. The reversible handedness inversion
of the cholesteric mixture enables on demand switching of the
cholesteric grating. The molecular design of these visible light-
driven chiral switches should inspire the fabrication and
application of stimuli-responsive systems with novel properties.
[4]
[5]
[6]
H. Wang, H. K. Bisoyi, L. Wang, A. M. Urbas, T. J. Bunning, Q. Li. Angew.
Chem. Int. Ed. 2018, 57, 1627; Angew. Chem. 2018, 130, 1643.
H. Wang, H. K. Bisoyi, A. M. Urbas, T. J. Bunning, Q. Li. Chem. Eur. J.
2019, 25, 1369.
a) Y. Li, M. Wang, T. J. White, T. J. Bunning, Q. Li. Angew. Chem. Int.
Ed. 2013, 52, 8925; Angew. Chem. 2013, 125, 9093; b) M. Mathews, R.
S. Zola, S. Hurley, D. Yang, T. J. White, T. J. Bunning, Q. Li. J. Am. Chem.
Soc. 2010, 132, 18361.
[7]
a) H. Hayasaka, T. Miyashita, M. Nakayama, K. Kuwada, K. Akagi. J.
Am. Chem. Soc. 2012, 134, 3758; b) H. K. Bisoyi, Q. Li. Angew. Chem.
Int. Ed. 2016, 55, 2994; c) P. Chen, L. Ma, W. Hu, Z. Shen, H. K. Bisoyi,
S. Wu, S. Ge, Q. Li, Y. Lu. Nat. Commun. 2019, 10, 2518.
D. Bléger, J. Schwarz, A. M. Brouwer, S. Hecht. J. Am. Chem. Soc. 2012,
134, 20597.
[8]
[9]
O. S. Bushuyev, A. Tomberg, T. Friščić, C. J. Barrett. J. Am. Chem. Soc.
2013, 135, 12556.
[10] Y. Wang, A. M. Urbas, Q. Li. J. Am. Chem. Soc. 2012, 134, 3342.
[11] a) L. Qin, W. Gu, Y. Chen, J. Wei, Y. Yu. RSC Adv. 2018, 8, 38935; b) L.
Qin, W. Gu, J. Wei, Y. Yu. Adv. Mater. 2018, 30, 1704941; c) L. Qin, J.
Wei, Y. Yu. Adv. Optical Mater. 2019, 7, 1900430.
Acknowledgements
We thank the Air Force Office of Scientific Research and the Air
Force Research Laboratory for support.
[12] H. Huang, T. Orlova, B. Matt, N. Katsonis. Macromol. Rapid Commun.
2018, 39, 1700387.
[13] D. Zhao, X. Zhao, J. Wang, H. Peng, Y. Liao, X. Xie, I. I. Smalyukh, Y.
Yu. Macromol. Rapid Commun. 2019, 40, 1900037.
Conflict of interest
[14] J. Wu, Z. Li, M. Zhuo, Y. Wu, X. Wang, L. Liao, L. Jiang. Adv. Optical
Mater. 2018, 6, 1701300.
The authors declare no conflict of interest.
[15] J. Lieffrig, A. G. Niassy, O. Jeannin, M. Fourmigué. CrystEngComm.
2015, 17, 50.
Keywords: visible light • halogen bond • chiral molecular switch
• handedness inversion • helical superstructure
[16] J. Vapaavuori, A. Siikonen, V. Dichiarante, A. Forni, M. Saccone, T. Pilati,
C. Pellerin, A. Shishido, P. Metrangolo, A. Priimagi. RSC Adv. 2017, 7,
40237.
[1]
a) Photoactive Functional Soft Materials: Preparation, Properties, and
Applications (Ed: Q. Li), Wiley-VCH, Weinheim, 2019; b) Intelligent
[17] M. G. Sarwar, B. Dragisic, L. J. Salsberg, C. Gouliaras, M. S. Taylor. J.
Am. Chem. Soc. 2010, 132, 1646.
This article is protected by copyright. All rights reserved.

10.1002/anie.201913977
Angewandte Chemie International Edition
COMMUNICATION
COMMUNICATION
Hao Wang, Hari Krishna Bisoyi, Bing-
Xiang Li, Michael E. McConney, Timothy
J. Bunning, Quan Li*
Page No. – Page No.
Visible Light-Driven Halogen Bond
Donor Based Molecular Switches:
From Reversible Unwinding to
Handedness Inversion in Self-
Organized Soft Helical
Superstructures
Unprecedented visible light-driven reversible handedness inversion has been
accomplished in an induced photoresponsive self-organized helical superstructure
developed using a halogen bond donor based chiral molecular switch. Interestingly,
reversible visible light-driven unwinding of a helical superstructure has also been
achieved.
This article is protected by copyright. All rights reserved.