Solid polymer films exhibiting handedness-switchable, full-color-tunable selective reflection of circularly polarized light

Article abstract of DOI:10.1021/ja504808r

Poly(quinoxaline-2,3-diyl)s bearing (S)-2-methylbutyl, n-butyl, and 8-chlorooctyl groups as side chains were synthesized to fabricate dry solid polymer thin films. These films exhibited selective reflection of right-handed circular polarized light (CPL) in the visible region after annealing in CHCl₃ vapor at room temperature. The handedness of reflected CPL was inverted to the left after annealing in 1,2-dichloroethane vapor. It was also found that the color of a particular single film along with the handedness of reflected CPL were fully tuned reversibly, upon exposure of the film to the vapor of various mixtures of chloroform and 1,2-dichloroethane in different ratios.

Full text of DOI:10.1021/ja504808r

Communication
pubs.acs.org/JACS
Solid Polymer Films Exhibiting Handedness-Switchable, Full-Color-
Tunable Selective Reflection of Circularly Polarized Light
Yuuya Nagata,† Keisuke Takagi,† and Michinori Suginome*,†,‡
†Department of Synthetic Chemistry and Biological Chemistry, Graduate School of Engineering, Kyoto University, Kyoto 606-8501,
Japan
‡CREST, Japan Science and Technology Agency (JST), Katsura, Nishikyo-ku, Kyoto615-8510, Japan
S
* Supporting Information
achieved with dry solid polymer film on the basis of selective
reflection.
ABSTRACT: Poly(quinoxaline-2,3-diyl)s bearing (S)-2-
methylbutyl, n-butyl, and 8-chlorooctyl groups as side
chains were synthesized to fabricate dry solid polymer thin
films. These films exhibited selective reflection of right-
handed circular polarized light (CPL) in the visible region
after annealing in CHCl3 vapor at room temperature. The
handedness of reflected CPL was inverted to the left after
annealing in 1,2-dichloroethane vapor. It was also found
that the color of a particular single film along with the
handedness of reflected CPL were fully tuned reversibly,
upon exposure of the film to the vapor of various mixtures
of chloroform and 1,2-dichloroethane in different ratios.
We herein report the preparation of helical-polymer-based
solid films that show selective reflection of CPL according to the
helical cholesteric superstructure, which, in turn, originates from
the helical structure of the macromolecules.16 The color and the
handedness of reflected CPL are both tunable by altering the
composition of the monomers and also by altering the nature of
the solvent vapor that is applied during the solvent vapor
annealing process. Of note is that a single film having a certain
monomer composition exhibits full-color tunability with control
of handedness of CPL by applying varying solvent vapors during
the annealing process.
Poly(quinoxaline-2,3-diyl) (PQX) molecules are a unique
class of helical polymers; they possess a nanoscale, rigid rod-like
helical backbone.17 The unique molecular structure allows them
hysical interaction of light with a cholesteric nanostructure
Pwhose pitch falls within a range of wavelengths of visible to serve as mesogenic segments for liquid crystals.18 PQXs
bearing chiral side chains show a unique solvent-dependent
inversion of their helical sense.19 We have reported that the
helical scaffold of PQX serves as a highly efficient chiral reaction
environment. The chirality is switchable by a solvent effect,20
leading to the production of a pair of enantiomeric products from
a single chiral catalyst in various reactions.21
light shows coloration by selective reflection of circularly
polarized light (CPL). The wavelength (λ) of reflected CPL is
determined by the product of the average refractive index (n) and
the pitch (P) of the cholesteric structure.1 Such structural
coloration is widely found in nature, as exemplified by the
colored skin tissue of jeweled beetles2 and a certain type of plant.3
Synthetic materials exhibiting selective-reflection-based physical
color are attracting increasing attention for applications in full-
color electronic papers,4 sensing devices,5 low-threshold lasing,6
and “copy-safe” printing for security purposes.7 Various liquid
crystalline materials have been developed, taking advantage of
the ready formation of cholesteric superstructures with small
chiral molecules.8 Liquid crystalline materials that exhibit
reversible change of their color and handedness of CPL by
external stimuli have been reported.9 They are expected to be
useful as new materials for color information technology.10
Furthermore, efforts have been devoted to preparing solid, dry
cholesteric materials for long-term as well as thermal stability of
coloration. For this purpose, several polymer-based systems have
been reported.11 However, due to the solid nature, it is difficult to
acquire both tunability of color and the handedness of CPL. For
instance, the materials require continuous heating,12 solvent
adsorption,13 or cross-linking14 to retain the acquired physical
colors, failing to provide dry materials with stable coloration with
full-color tunability. Although dicholesteryl ester is reported to
be a nonpolymer-based solid film that changes its reflection
wavelength reversibly upon changing the temperature applied to
melt it, it does not show the switch of CPL handedness.15 To
date, reversible switching of the handedness of CPL has not been
Newly synthesized random copolymers P(x*/0/z) bearing
(S)-2-methylbutyl and 8-chlorooctyl groups as side chains
(Figure 1a) formed an M-helical polymer backbone in
chloroform (CHCl3) solutions at room temperature. The
screw-sense excess of the M-helical structure varied with the
ratio (x/z) of the two monomers. Higher screw-sense excess was
observed for the PQX bearing more chiral units. Thin films of
those PQXs were then fabricated (Figure 1b). A concentrated
solution thereof in xylene (0.6 g/mL) was applied on a glass
substrate. After drying, the films were transparent, with no color,
as a result of lack of absorption by PQX in the visible light region.
The film formed on a glass substrate was exposed to CHCl3 vapor
for 4 min at 20 °C and then dried under vacuum. Although
copolymers P(50*/0/50) and P(25*/0/75) as well as
homopolymers P(100*/0/0) and P(0*/0/100) still had no
color, one copolymer, P(75*/0/25), did show blue color (Table
1, entries 1−5). This was eventually known to arise from selective
reflection of visible light. It should be noted here that the film
reflected right-handed CPL selectively over left-handed CPL,
suggesting the formation of a right-handed helical supra-
Received: May 14, 2014
Published: June 18, 2014
© 2014 American Chemical Society
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Journal of the American Chemical Society
Communication
Initially, PQX ternary co-100mers P(x*/y/25) containing
various ratios of chiral (x) and achiral units (y), along with a
fixed number of chlorooctyl units (z = 25), were prepared to
investigate the role of chiral units (Table 1, entries 6−16). As
observed for the binary polymers, the polymers in CHCl3
solution showed M-helical structures whose screw-sense purity
increases as the ratio of chiral units increases (Figure 2, solid
Figure 1. Synthesis (a) and fabrication of solid polymer films (b) of
terpolymers P(x*/y/z).
Table 1. The gabs Values (CHCl3 and 1,2-DCE Solutions)
Determined by CD Measurements and d-Spacing Values
Obtained From XRD and Reflection Maximum Wavelength
Figure 2. CD spectra of polymers P(x*/y/25) (x + y = 75, x = 75, 50, 40,
30, 28, 25, 22, 20, 18, 16, and 15) in CHCl3 (solid line) or 1,2-DCE (line
with circles).
Rmax (Solid Polymer Films Annealed in CHCl3 or 1,2-DCE) of
Terpolymers P(x*/y/z)
P(x*/y/z)
CHCl3
1,2-DCE
lines) in accordance with the Green’s theory.23 The color of the
thin films after CHCl3 vapor annealing varied from blue to red
through green according to the ratio of chiral monomer units
(Figure 3a). The change in color was clearly correlated to the
screw-sense excess of the helical macromolecules and, in turn, to
the degree of the screw-sense of the cholesteric-like super-
structures. The mean reflection wavelength was red-shifted as the
number of chiral units decreased, resulting in coverage of the
XRD d
R
XRD d
R
maxb
a
maxb
a
entry
x
y
z
(nm)
(nm)
(nm)
(nm)
e
f
e
e
1
100
75
50
25
0
0
0
25
50
75
100
25
25
25
25
25
25
25
25
25
25
25
10
15
20
30
35
−
ND
−
−
c
c
2
0
1.59
<400
1.81
e
<400
e
f
e
3
0
−
ND
−
−
−
−
e
f
e
e
4
0
−
ND
−
e
c
e
e
5
0
−
−
−
d
d
6
50
40
30
28
25
22
20
18
16
15
0
25
35
45
47
50
53
55
57
59
60
75
65
60
55
45
40
1.60
1.59
1.58
1.58
1.58
1.58
1.58
1.58
1.58
1.57
<400
411
440
490
521
540
573
612
633
643
1.65
1.61
1.57
1.57
1.57
1.55
1.57
1.54
1.51
1.58
<400
406
451
466
549
567
630
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
f
ND
e
−
−
e
e
f
e
f
−
ND
−
ND
f
25
25
25
25
25
1.53
1.53
1.57
1.63
1.67
405
424
1.52
1.54
1.56
1.61
1.63
ND
512
530
585
596
443
601
>700
a
Average intermolecular distance calculated from the XRD peak
b
c
positions 2θ. Wavelength of maximum selective reflection. Annealing
time was set to 20 min. Annealing time was set to 10 min. Not
measured. Not detected.
d
e
f
molecular structure which corresponds to the cholesteric phase
of liquid crystals.1 The relationship between the screw-sense of
the copolymer (left-handed) and the twist-sense of the
supramolecular structure (right-handed) was in agreement
with Straley’s theory.22
To obtain insight into the observation of the selective
reflection, a “spectator monomer” that has an achiral n-butyl
side chain with no ω-chlorine atom was introduced as the third
monomer into the polymer backbone, in addition to the
monomers used for the synthesis of the binary copolymers.
Figure 3. Difference reflection spectra of P(x*/y/25) annealed in (a)
CHCl3 or (b) 1,2-DCE vapor. Photographs are taken through left- or
right-handed CPL filters. Scale bar = 1.0 mm.
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Journal of the American Chemical Society
Communication
entire visible region, 400−650 nm, by changing the number of
chiral units from 50 to 15.
formation of the organized structure, with mean distances
ranging from 1.5 to 1.7 nm. It is presumed that the XRD signals
correspond to the distances of the two neighboring polymer
main chains, because the diameter of PQXs is estimated to be
around 2 nm (by molecular model calculation, see SI). The
results of XRD measurements therefore suggest that the mean
distance between the polymer main chains increases with an
increase in the proportion of chlorooctyl monomer units, due to
the steric and/or electronic effect of the chlorooctyl side chains.
Consequently, the pitch length of the cholesteric structure is
elongated, leading to the observed bathochromic shift of the
reflected wavelength. We observed parallel results with the 1,2-
DCE-vapor-treated thin films, although they all reflected left-
handed CPL (Figure 4. lines with circles). By contrast, XRD
measurements of a series of co-100mers P(x*/y/25), in which
the number of 8-chlorooctyl side chains are fixed, showed almost
the same diffraction peaks (Table 1, entries 6−15). This clearly
suggests that the ratio of the chlorooctyl monomer is the
determinant of the mean distance between the polymer main
chains.
We have thus far shown that the reflection wavelength (color)
is controllable by polymer structures with a varying ratio of chiral
and/or chlorooctyl monomer units and that the handedness of
the reflected CPL is switchable by changing the solvent used for
vapor annealing. In particular, the reflection wavelength crucially
depends on the screw-sense purity of the helical polymer
backbones. Because the findings of our previous study suggested
that the macromolecular screw-sense purity is tunable by
changing the ratio of the two solvents,19 we became interested
in the preparation of a single film where the color and
handedness of reflected CPL are fully tunable by the solvent
effect alone.
In 1,2-dichloroethane (1,2-DCE) solutions, we observed an
inverted, right-handed helical structure for the same series of
PQX ternary co-100mers P(x*/y/25) in circular dichroism
(CD) spectra (Figure 2, dotted lines). As-casted films obtained
from their xylene solutions were treated with 1,2-DCE vapor at
room temperature for 4−20 min. The films again exhibited
varying colors, from blue to red, as shown by the difference
reflection spectra (Figure 3b). It should be noted that left-handed
CPL was reflected by these polymer films treated with 1,2-DCE
vapor, suggesting that their cholesteric structure was inverted to
the right. It seems likely that the handedness of the cholesteric
superstructures depends on the sense of the helix of the helical
polymer backbone. We found that the formation of P- and M-
helical superstructures was essentially reversible. A CHCl3 vapor-
treated thin film of P(30*/45/25), which reflected right-handed
blue CPL, was exposed to 1,2-DCE vapor at room temperature
for 4 min. Although the dried thin film still reflected blue light
(∼450 nm), the handedness of reflected CPL was inverted to the
left (See SI). This reversible switch of handedness of the CPL can
be repeated by exposing the film to either CHCl3 or 1,2-DCE
vapor, alternately, without any change in the mean reflection
wavelength for each CPL.
Having found the dependence of the reflection wavelength on
the ratio of the chiral monomer units, we then examined the role
of the chlorooctyl monomer units. Maintaining the number of
chiral units x at 25, the ratio (y/z) of the spectator units and
chlorooctyl units in ternary co-100mer P(25*/y/z) was changed
(Table 1, entries 17−21). All the co-100mers P(25*/y/z)
showed almost constant CD spectra, either in CHCl3 (M-helix,
gabs ≃ −1.6 × 10−3) or 1,2-DCE (P-helix, gabs ≃ −0.5 × 10−3)
(See SI). These results clearly demonstrated that the screw-
senses of the PQXs depend solely on the ratio of the chiral
monomer units. It was, however, interesting to observe a
bathochromic shift of the reflected wavelength by the thin film
upon increasing the content of the chlorooctyl monomer units by
both CHCl3 vapor treatment (Figure 4, solid lines). For instance,
CHCl3-vapor-annealed films of P(25*/60/15) and P(25*/45/
30) exhibited blue (424 nm) and red (601 nm) color with
reflection of right-handed CPL.
We chose to consider P(30*/45/25), which showed reflection
of right-handed blue CPL on exposure to 100% CHCl3 vapor
(Figure 5a). Increasing the ratio of 1,2-DCE to 33% and 50%
Figure 5. Difference reflectance spectra of P(30*/45/25) after solvent
vapor annealing by using mixed solvent of 1,2-DCE and CHCl3.
Photographs of the annealed polymer films were shown in the inset.
Scale bar = 1.0 mm.
Figure 4. Difference reflection spectra of P(25*/y/z) annealed in
CHCl3 (solid line) or 1,2-DCE vapor (line with circles).
resulted in a bathochromic shift of the reflected wavelength,
showing green (b for 33% 1,2-DCE) and red (d for 50% 1,2-
DCE) color. Further increases in the ratio of 1,2-DCE caused the
visible light reflection to disappear. It is presumed that the
reflected wavelength became longer than that in the visible light
region (>700 nm) in association with a decrease in the excess of
left-handed helical structures. When the ratio of 1,2-DCE was
To determine the origin of the difference in reflected
wavelength, X-ray diffraction (XRD) measurements were
conducted for a series of co-100mers P(25*/y/z) (Table 1,
entries 17−21, see also SI). Each co-100mer showed a diffraction
peak, whose angle 2θ decreased with an increase in the ratio of
the chlorooctyl monomer. The values of 2θ suggested the
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further increased to 90% (e), a right-handed helical structure
with left-handed helical superstructure becomes predominant,
resulting in reflection of left-handed red CPL. Blue shift of
reflection of left-handed CPL was observed with an increase in
the ratio of 1,2-DCE (f−h), resulting in the observation of green
(g) and blue (h) color. Both the wavelength and the chirality of
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the mixed solvents for the vapor annealing process.
In summary, we have synthesized PQXs bearing (S)-2-
methylbutyl, n-butyl, and 8-chlorooctyl groups as side chains.
Polymer thin films, fabricated from these terpolymers by solvent
casting, exhibited selective reflection of right-handed CPL in the
visible region after annealing in CHCl3 vapor at room
temperature. With a decrease in the ratio of (S)-2-methylbutyl
units, the mean wavelength of the selective reflection was red-
shifted. After annealing in 1,2-DCE vapor, the handedness of
reflected CPL was inverted to the left. We propose that the
increase in the number of chlorooctyl units increases the distance
between two polymer main chains, resulting in a red-shift of the
structural color by elongation of the cholesteric pitch. It was
ultimately demonstrated that a particular single film exhibited full
color tunability with switch of handedness of CPL by changing
the ratio of 1,2-DCE and CHCl3 used in the solvent vapor
annealing. We believe that this study will open up new
possibilities for the development of new chirality-switchable
materials,24 not only through their unique macromolecular
structures but also the highly ordered supramacromolecular
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ASSOCIATED CONTENT
* Supporting Information
Experimental procedures and spectral data. This material is
available free of charge via the Internet at http://pubs.acs.org.
■
S
AUTHOR INFORMATION
Corresponding Author
suginome@sbchem.kyoto-u.ac.jp
■
(19) (a) Yamada, T.; Nagata, Y.; Suginome, M. Chem. Commun. 2010,
46, 4914. (b) Nagata, Y.; Yamada, T.; Adachi, T.; Akai, Y.; Yamamoto,
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Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
We thank Shun Tanaka and Makoto Uno for help with
experiments and analyses. Financial support for this research
was provided by the Japan Science and Technology Corporation
(CREST, “Development of High-Performance Nanostructures
for Process Integration” Area).
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