Reversible light-directed red, green, and blue reflection with thermal stability enabled by a self-organized helical superstructure

Article abstract of DOI:10.1021/ja302772z

Adding external, remote, and dynamic control to self-organized superstructures with desired properties is an important leap necessary in leveraging the fascinating molecular subsystems for employment in applications. Here two novel light-driven dithienylethene chiral molecular switches possessing remarkable changes in helical twisting power during photoisomerization as well as very high helical twisting powers were found to experience photochemically reversible isomerization with thermal stability in both isotropic organic solvents and anisotropic liquid crystal media. When doped into a commercially available achiral liquid crystal host, the chiral switch was able to either immediately induce an optically tunable helical superstructure or retain an achiral photoresponsive liquid crystal phase whose helical superstructure was induced and tuned reversibly upon light irradiation. Moreover, reversible light-directed red, green, and blue reflection colors with thermal stability in a single thin film were demonstrated.

Full text of DOI:10.1021/ja302772z

Communication
pubs.acs.org/JACS
Reversible Light-Directed Red, Green, and Blue Reflection with
Thermal Stability Enabled by a Self-Organized Helical Superstructure
†
‡
,†
Yannian Li, Augustine Urbas, and Quan Li*
†
Liquid Crystal Institute, Kent State University, Kent, Ohio 44242, United States
‡
Materials and Manufacturing Directorate, Air Force Research Laboratory WPAFB, Ohio 45433, United States
*
S Supporting Information
opportunities as well as challenges in fundamental science that
open the door for many applications such as tunable color
ABSTRACT: Adding external, remote, and dynamic
control to self-organized superstructures with desired
properties is an important leap necessary in leveraging
the fascinating molecular subsystems for employment in
applications. Here two novel light-driven dithienylethene
chiral molecular switches possessing remarkable changes in
helical twisting power during photoisomerization as well as
very high helical twisting powers were found to experience
photochemically reversible isomerization with thermal
stability in both isotropic organic solvents and anisotropic
liquid crystal media. When doped into a commercially
available achiral liquid crystal host, the chiral switch was
able to either immediately induce an optically tunable
helical superstructure or retain an achiral photoresponsive
liquid crystal phase whose helical superstructure was
induced and tuned reversibly upon light irradiation.
Moreover, reversible light-directed red, green, and blue
reflection colors with thermal stability in a single thin film
were demonstrated.
6
7
filters, tunable LC lasers, and optically addressed displays that
8
require no driving electronics and can be flexible.
It is known that color change is a key feature of modern
information display technology, which is currently achieved by
the employment of subpixelated color (red-green-blue) filters
overlaid onto pixelated LC light valves that effectively control
the transmission of light through the color filter. Much effort is
devoted toward developing red, green, and blue (RGB)
reflection colors in cholesteric LC thin films, since light-
directed RGB color change would be a viable alternative,
whereas the dimensions of a subpixelated color (R, G, or B) in
a single thin film can be limited only by the dimensions of light.
However, to date reversible light-directed RGB color control
driven by a chiral dopant has been limited to recent reports
3
,4,8d
such as those employing chiral azobenzene dopants
and
5
overcrowded alkene as dopants, where all the photoresponsive
dopants experienced photoisomerization but were accompanied
by competing thermal back relaxation. For example, azoben-
zenes can transform from trans form to cis form upon UV light
irradiation whereas the reverse process can occur thermally or
photochemically. The slow but unavoidable thermal relaxation
gives rise to the problem of lack of stability and controllability.
Undoubtedly, the discovery of new light-driven chiral molecular
switches capable of having good thermal stability as well as
reversible phototuning of RGB reflection color and beyond in
an induced helical superstructure is of great importance to
practical applications. Furthermore, it is highly desirable to
direct RGB color changes with only small amounts of a light-
driven chiral switch since a high concentration of chiral dopant
can often lead to phase separation and coloration and alter the
desired physical properties of the LC host. This requires the
dopant to have high HTP as well as a significant difference in
HTP among the various states of the switch.
ontrolling the dynamic nature of molecular self-
organization with an external stimulus is a challenge in
C
science and technology that, when overcome, could lead to a
breakthrough in the creation of new intelligent molecular
devices. Among all the stimuli such as electric field, magnetic
field, mechanical stress, temperature, or chemical reaction, light
is particularly fascinating due to its advantages of remote,
spatial, and temporal controllability. Light-driven chiral
molecular switches or motors in liquid crystal (LC) media
capable of self-organizing into optically tunable helical
superstructures undoubtedly represent such a striking exam-
1
−5
ple.
Such a system can be achieved by doping photo-
responsive chiral molecules into an achiral nematic LC host to
self-organize into an optically tunable helical superstructure, i.e.
a photoresponsive cholesteric LC phase. The resulting
macroscopic helical superstructure can selectively reflect light
according to Bragg’s law and be tuned by light. The central
wavelength λ of the selective reflection is defined by λ = np,
where p is the pitch length of the helical structure and n is the
average refraction index of the LC material. The ability of a
chiral dopant to twist an achiral nematic LC phase, i.e. helical
Dithienylethenes are a well-known family of photochromic
compounds. Upon UV irradiation, they can transform from a
colorless ring-open form to its colored ring-closed form. Their
reverse isomerization process is thermally stable and occurs
only photochemically by visible light irradiation. The excellent
fatigue resistance and superior thermal stability of both open
and closed forms make dithienylethenes especially promising
candidates for technological applications. However, their
applications as chiral dopants are limited by either low HTPs
−1
twisting power (HTP, β), is expressed as β = (pc) where c is
the chiral dopant concentration. The isomerization of dopant
molecules upon light irradiation can control the HTP and
reflection wavelength λ of the cholesteric phase, providing
Received: March 21, 2012
Published: May 25, 2012
©
2012 American Chemical Society
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Journal of the American Chemical Society
Communication
9
or/and minor HTP changes in photoisomerization. Here we
report two new enantiomeric dithienylethene switches with
axial chirality, (S,S)-1 and (R,R)-1 (Figure 1), which not only
possess very high HTPs at its initial state but also show a
remarkable increase in HTP from open form to its closed form
upon UV irradiation (Figure 2).
closed form. Accordingly, there was a gradual decrease seen in
the maximum absorption at 282 nm with a concomitant
appearance of a new absorption band around 550 nm. The
photostationary state (PSS310 nm) was reached in 60 s. The
irradiated state was thermally stable and was able to
photochemically switch back to its initial state by visible light
irradiation at 550 nm for 15 min (see Supporting Information
Figure S9). The excellent fatigue resistance was confirmed by
repeated irradiation of the solution with UV and visible light.
No obvious degradation was observed after many cycles (see
Supporting Information Figure S10).
As expected, the CD spectra of (S,S)-1 and (R,R)-1 exhibited
a mirror image relationship (Figure 3). The strongest
Figure 3. CD spectra changes of (S,S)-1 and (R,R)-1 (3 μM in
hexane) upon UV irradiation at 310 nm.
bisignated exciton couplet between 200 and 240 nm is due to
Figure 1. Photoisomerization of dithienylcyclopentene chiral switches
1
the coupling of the two B transitions located on distinct
b
(S,S)-1 or (R,R)-1. Note: The photoisomerization generates two
naphthalene rings, while the couplet at around 280 and 310 nm
diastereomers with (S,S) and (R,R) configuration of the two new chiral
centers (see Supporting Information Figure S7). Inset: color change
from colorless to purple upon UV irradiation.
1
1
10
is related to the L and L transitions of naphthalene. Upon
a
b
UV irradiation, the exciton couplet at 230 nm weakened
significantly while a shoulder exciton developed at 260 nm. The
distinct changes gave clear evidence that the chiroptical
properties of this compound can be modulated by light.
Doping (S,S)-1 into an achiral nematic LC host E7 even at a
very low concentration (0.7 wt %) can immediately induce a
chiral nematic mesophase with a characteristic fingerprint
texture (Figure 4A). Interestingly, when doped at a lower
Figure 2. A schematic mechanism of the reflection wavelength of light-
driven chiral molecular switch (S,S)-1 or (R,R)-1 in an achiral nematic
LC media reversibly tuned by light. (a) Light-driven chiral switch; (b)
achiral nematic host; (c and d) photoresponsive chiral nematic phase
with pitch length decrease from c to d. (A) Immediately induce helical
superstructure c and (B) retain a photoresponsive achiral phase whose
helical superstructure is induced and tuned reversibly with light
irradiation.
Figure 4. Crossed polarized optical textures of 0.7 wt % (A) and 0.4%
(B, C, and D) (S,S)-1 in E7 in a 5 μm thick homeotropic aligned cell.
Upon UV irradiation at 310 nm with different time: 0 s (A, B), 5 s (C),
and 30 s (D). Inset: Conoscopic texture (B).
The chiral switch (S,S)-1 and its enantiomer (R,R)-1 were
prepared in a facile synthesis. Their chemical structures were
concentration (0.4 wt %), the resulting mixture retained an
achiral nematic phase, as evidenced by the characteristic black
texture in a homeotropic aligned cell with the inset conoscopic
observation (Figure 4B). Upon irradiation at 310 nm, the
fingerprint texture gradually appeared (Figure 4C and 4D),
indicating the formation of a cholesteric phase and the increase
in HTP. The reverse process happened upon visible light
irradiation at 550 nm.
1
13
identified by H and C NMR, high resolution MS, and
elemental analysis (see Supporting Information). They
exhibited photochemically reversible but thermally stable
behavior in both organic solvents and liquid crystal media.
For example, the solution of (S,S)-1 in hexane was colorless,
which is consistent with its UV−vis spectrum, i.e. no absorption
in the visible region at the initial state (see Supporting
Information Figure S8). Upon UV irradiation at 310 nm, the
solution color changed immediately from colorless to fresh
purple due to its photoisomerization from the open form to
The HTPs of (S,S)-1 and (R,R)-1 in two commercially
available nematic LC hosts and the changes in HTP upon light
11
irradiation were determined by the Grandjean−Cano method
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Communication
and summarized in Table 1. (S,S)-1 induced a right-handed
helix whereas (R,R)-1 induced a left-handed helix. Upon UV
Table 1. HTPs of Chiral Dopants (S,S)-1 and (R,R)-1 at
Different States in Nematic LC Host
HTP βM (μm−1)
dopant
LC host
initial
PSSUV
PSSvis
Δβ/β
(
S,S)-1
S,S)-1
R,R)-1
R,R)-1
E7
+104
+109
−104
−109
+153
+156
−153
−156
+105
+113
−105
−113
47%
43%
47%
43%
(
(
(
5CB
E7
5CB
irradiation, the HTP of (S,S)-1 in E7 increased from 104 to 153
−1
μm which could be almost switched back to its initial state
−1
with 105 μm HTP upon visible light irradiation at 550 nm. Of
significance are the unusually high HTP values and the
remarkable increase in HTP upon UV irradiation that (S,S)-1
and its enantiomer (R,R)-1 exhibited in the two LC hosts.
It is established that the dihedral angle (θ) of two
naphthalene rings in binaphthyl derivatives plays a key role in
their cholesteric induction abilities. The unusual high HTPs of
these two compounds might originate from the bridged
binaphthyl unit, which is significantly more efficient for helicity
induction than unbridged ones due to its rigid structure and
Figure 5. (Top) Reflection color images of 7.7 wt % (S,S)-1 in E7 in a
5 μm thick planar cell taken from a polarized reflective mode
2
microscope. (A) Upon UV irradiation at 310 nm (30 mW/cm ) and
2
followed by (B) upon visible irradiation at 550 nm (30 mW/cm ).
(Bottom) Reflective spectra of 7.7 wt % (S,S)-1 in E7 in a 5 μm thick
12
planar cell at room temperature. (Left) Upon UV irradiation at 310
quite narrow dihedral angle (around 60°). The handedness of
the cholesteric phase observed for (S,S)-1 is also in accordance
with the fact that S configuration binaphthyl derivatives
normally induced a left-handed helix in CLCs with θ > 90°,
while right-handedness is only observed when θ < 90°. As
indicated by CD spectra changes (Figure 3), although the
binaphthyl units are fixed with a methylene tether and thus
have less flexibility, a slight variation of the dihedral angle is also
2
nm (30 mW/cm ) for different times. (Right) Upon visible light
2
irradiation at 550 nm (30 mW/cm ) for different times.
13
possible (Δθ ∼5°). The dramatic increase in HTP upon UV
irradiation can be ascribed to this dihedral angle change and the
possible overall conformation change of the molecule caused by
photoisomerization.
Encouraged by the high HTPs as well as the remarkable
variation in HTP value upon light irradiation, a mixture of 7.7
wt % of (S,S)-1 in LC E7 was capillary-filled into a 5 μm thick
planar aligned cell which was painted black on one side.
Surprisingly, reversible light-directed RGB reflection colors
with thermal stability were achieved in the cell (Figure 5 top).
The reflection central wavelength was around 630 nm at the
initial state. Upon UV irradiation, its reflection wavelength was
tuned to 530 nm in only 10 s and further reached a
photostationary state in 25 s with a reflection central
wavelength at 440 nm (Figure 5 bottom). This photostationary
state was thermally stable and was able to photochemically
switch back to a nearly initial state by visible light irradiation at
Figure 6. Real cell images of a 8 μm thick planar cell (2.1 cm ×2.5 cm)
filled with 7.7 wt % (S,S)-1 in E7.
respectively. Moreover, the optically addressed images can be
erased by light irradiation when desired, and the cell is
rewritable for many times due to the excellent fatigue
resistance.
In conclusion, two novel enantiomeric light-driven dithieny-
lethene chiral switches with axial chirality were designed and
synthesized. They possessed a remarkable change in HTP
during photoisomerization as well as an unusually high HTP,
which are significantly larger than the HTPs previously
reported for chiral dithienylethenes. The switches exhibited
photochemically reversible switching between open and closed
forms with good thermal stability in both isotropic organic
solvents and anisotropic LC media. When doped in an achiral
nematic LC with a larger concentration, the switch was able to
immediately induce a helical superstructure that was reversibly
tunable upon light irradiation. However, when doped in an
achiral nematic LC with a lower concentration, it retained an
achiral LC phase whose helical superstructure was induced
upon light irradiation and vice versa. Furthermore, reversible
5
50 nm within 2 min. The reversible tuning of reflection across
RGB reflection colors was repeated many times without
noticeable degradation. When this cell was stored in the dark
at any irradiated state, no observable change was found in both
reflection color and reflection wavelength, even after one week,
which results from the excellent thermal stability of (S,S)-1.
Furthermore, three primary RGB colors can be observed
simultaneously in a single thin film based on different UV
irradiation times (Figure 6 D and E) facilitated by masking at
different areas: red, no irradiation; green, irradiated for 10 s;
blue, irradiated for 25 s. After driving the background color to
blue by UV irradiation, the red and green reflection colors can
be recorded through visible light irradiation for different times
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Journal of the American Chemical Society
Communication
light-directed red, green, and blue reflection colors with thermal
stability in a single thin film were demonstrated. Of significance
are the superior thermal stability and excellent fatigue resistance
of the light-directing tuning of the self-organized helical
superstructure in a controllable way, which are important for
practical applications.
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ASSOCIATED CONTENT
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C NMR. This material is available free of charge via the
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AUTHOR INFORMATION
■
Corresponding Author
qli1@kent.edu
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(
ACKNOWLEDGMENTS
■
This work is supported by the Air Force Office of Scientific
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