A Multiaddressable Dyad with Switchable Cyan/Magenta/Yellow Colors for Full-Color Rewritable Paper

Article abstract of DOI:10.1002/chem.201801692

Reversible multicolor displays on solid media created from single-molecule pigments are a long-awaited goal. Herein, a new and simple molecular dyad, which can undergo switchable cyan (C), magenta (M), and yellow (Y) color changes in both solution and the solid state upon exposure to light, water/acid, and nucleophiles, has been designed and synthesized. The stimuli used herein can be applied independent of each other, which is beneficial for color changes without mutual interference. For comparison, mixtures of the two molecular switching motifs that form the basis of the dyad were also studied. The dyad greatly outperforms the corresponding mixed system with respect to reversible color switching on the paper substrate. Its potential for full-color rewritable paper with excellent reversibility has been demonstrated. Legible multicolor prints, that is, high color contrast and resolution, good dispersion, and excellent reversibility, were achieved by using common water-jet and light-based printers. This work provides a very promising approach for the further development of full-color switchable molecules, materials, and displays.

Full text of DOI:10.1002/chem.201801692

A Journal of
Accepted Article
Title: A Multi-Addressable Dyad with Switchable CMY Colors for Full-
Color Rewritable Papers
Authors: Tianyou Qin, Jiaqi Han, Yue Geng, Le Ju, Lan Sheng, and
Sean Xiao-An Zhang
This manuscript has been accepted after peer review and appears as an
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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
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content of this Accepted Article.
To be cited as: Chem. Eur. J. 10.1002/chem.201801692
Link to VoR: http://dx.doi.org/10.1002/chem.201801692
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A Multi-Addressable Dyad with Switchable CMY Colors for Full-
Color Rewritable Papers
Tianyou Qin,[a] Jiaqi Han,[b] Yue Geng,[b] Le Ju,[b] Lan Sheng,*[b] and Sean Xiao-An Zhang*[a]
Abstract: Reversible multicolor displays on solid media using single
molecule pigments have been a long-awaited goal. Herein, a new
and simple molecular dyad, which can undergo switchable CMY
color changes both in solution and solid substrate upon exposure to
light, water/acid, and nucleophiles, is designed and synthesized. The
stimuli used in this work can be applied independent of each other,
which is beneficial for color changes without mutual interference. As
a comparison, the mixtures of the two molecular switching motifs
forming the basis of the dyad were also studied. The dyad greatly
outperforms the corresponding mixed system with respect to
reversible color-switching on the paper substrate. Its potential for
full-color rewritable paper with excellent reversibility has been
demonstrated. Legible multicolor prints, that is, high color contrast
and resolution, good dispersion, excellent reversibility, were
achieved using common water-jet and light-based printers. This work
provides a very promising approach for further development of full-
color switchable molecules, materials and displays.
light,[6-12] water,[16-18] and metal ions.[22] Nevertheless, in many
cases, the communication of complex messages and images
needs to be presented in full color in order to satisfy the
demands of the readers. In this context, the development of full-
color rewritable papers is especially relevant.
Multi-addressable molecular switches are attractive as
candidates that could endow materials with multichromic display
capabilities. These molecules are capable of reversibly changing
their colors by stimuli-promoted isomerization reactions. One
strategy to achieve a full-color display with these types of
molecules is to mix together three independent molecular
switches, each of which is capable of displaying either cyan (C),
magenta (M) or yellow (Y), respectively.[6,9,23,24] This strategy is a
simple way to achieve multicolor capabilities in both solution and
solid media. Another complementary strategy is to incorporate
three or more molecular color switching motifs into a single
molecule. Such a single molecule would possess several
potential advantages over the above mentioned mixed system,
namely, avoidance of phase separation which is necessary for
producing high resolution images, exhibiting consistent color
balances across large areas, and so on. Even despite these
merits, there are only a few single molecule systems[25-29]
capable of multichromic display that have been successfully
implemented. One among these is a full-color display that relied
on a fused dithienylethene trimer demonstrated by Professor Irie
and co-authors[26]. Nevertheless, these reported systems only
exhibited multicolor switching in solution, and rarely
demonstrated its color switching in a solid substrate. Probable
technological obstacles include: 1) the challenge to design and
synthesize switchable molecules capable of displaying multiple
colors, especially the three primary colors, under different
stimuli; 2) complications caused by the surrounding
microenvironment present in the solid state, wherein the
molecules may operate differently than they do in solution; 3)
most reported multicolor display systems are based on full-photo
mode molecules[25,26,28], which might cross-interfere with each
other when imaging on a solid substrate.
Based on these considerations, integrating switchable
molecular units capable of responding orthogonally to different
stimuli into a single molecule might be a more effective method
to realize multicolor displays on solid media. Relative to other
stimuli, light and water in this role can be delivered to precise
locations, are obviously nontoxic, environmentally friendly, and
integrate well with current laser-jet and ink-jet printing
technologies. Light and water stimuli can be applied totally
independent of each other without mutual interference. Taking
into account these characteristics, water- and light-responsive
switching units seemed like a good choice from a design point of
view. Moreover, from the perspective of atom economy and
reducing the potential for deleterious influences among a
Introduction
Main Text Paragraph. Although electronic media is widely used
in daily life, paper, with its long history, still plays an extremely
important role in information transmission and knowledge
inheritance. Society still enjoys to read, write and paint with
paper. Most paper is usually disposed of just after one-time
reading, however, paper production and recovery processes are
associated with significant environmental problems including
deforestation, solid waste disposal issues and chemical
pollution.[1] For these reasons, rewritable ink[2] and rewritable
paper[3], which can allow a single piece of paper to be reused
multiple times, are now attracting much attention from the
perspective of sustainable development. To date, numerous
examples[2-22] have been reported in this field which make use of
both organic and inorganic materials. Among these works,
interconversion between colorless and one or two different
colors on a sheet of paper have been demonstrated by
employing various external stimuli, such as electricity,[2] heat,[3,4]
[a]
Dr. T. Qin, Prof. S. X. –A. Zhang
State Key Laboratory of Supramolecular Structure and Materials
College of Chemistry,
Jilin University
Changchun, 130012, P. R. China
E-mail: seanzhang@jlu.edu.cn; Fax: +86 431 85153812
J. Han, Y. Geng, Le Ju, Dr. L. Sheng
College of Chemistry, Jilin University
Changchun, 130012, P. R. China.
[b]
E mail: shenglan17@jlu.edu.cn; Fax: +86 431 85153812.
Supporting information for this article is given via a link at the end of
the document.
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10.1002/chem.201801692
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hydroxy group of OX with the carboxyl group of SP making use
of
(1-ethyl-3-(3-dimethylaminopropyl)-carbodiimide
as
a
condensation reagent (details of the synthesis are in the
supporting information). The six-atom non-conjugated bridge
between the two units is necessary to ensure each switching
unit’s ability to act independently.
As a control, mixtures of OX and SP (OX+SP), compounds
which constitute the core switching units of dyad 1 were also
studied. The photochromic and hydrochromic properties for both
dyad 1 and the OX+SP mixture in ethanol solutions were
investigated first (Figure 2). Without external stimuli, the
solutions of 1 and OX+SP are both initially colorless (Figure 2a
and 2d, black lines). Upon UV irradiation, the colorless
solutions of 1 and OX+SP transform to magenta. These
colorations are accompanied by the appearance of absorption
bands centered at 552 and 537 nm for dyad 1 and OX+SP,
respectively (Figure 2a and 2d, magenta lines). Both colored
solutions could be returned back to their colorless states upon
visible light irradiation (Figure 2a and 2d, gray lines). These
color and spectral changes arise from the SP unit, a hypothesis
which was verified by the respective UV tests of OX and SP
(Figure S1). In comparison to the mixed system, there is a 15
nm red shift for the absorption band of dyad 1. This subtle shift
likely arises from the non-conjugated N-substitution of the SP
Figure 1. Illustration of the structural design for dyad 1 and the color-changing
isomerizations for each subunit.
plurality of structural units, incorporating fewer and simpler
switching motifs into one system is more beneficial. Therefore, in
this work, we designed and synthesized a simple new dyad 1
with dual-switching capabilities that allowed it to undergo
changes in color using water and/or light stimuli (Figure 1).
Reversible full-color displays, i.e., magenta, blue, and yellow
colors, making use of the dyad by employing different stimuli
were investigated in solution as well as in solid substrates. As a
comparison, simple mixtures of the two molecular switching
motifs forming the basis of the dyad were also studied. The
application of the dyad in multicolor rewritable paper was
developed.
subunit necessary for synthesizing dyad
1 (Figure S2).
Surprisingly, addition of excess NaOH to the solution of dyad 1
after UV irradiation caused the magenta solution to change to a
yellow color, an observation which is correlated to a decrease in
the absorption band centered at 552 nm and appearance of a
new band at 416 nm (Figure 2a, yellow line). Additionally, the
Results and Discussion
1. Investigation of the multi-stimuli chromic properties of
dyad 1 and the mixed system in solution
Spiropyrans (SPs) are unique photochromic switches, which are
known to undergo light-stimulated reversible isomerization
between their colorless (ring-closed) and colored (ring-open)
forms, and are easy to synthesize. Hence, SPs have been
widely used in light responsive materials.[30] Oxazolidines (OXs)
have been reported as a new class of photochromic and
halochromic molecular switches, which have been mostly
investigated in the context of enhanced nonlinear optical
materials.[31] Recently, our group has demonstrated that OXs
could also be reversibly switched by introduction and removal of
water both in solution and solid substrates, resulting in obvious
color changes. This feature has been employed for the
construction of “water-jet” rewritable paper, making use of
standard printer-ink cartridges filled with water.[16] Even though
OXs and SPs are both photochromic molecules, according to
the literature, the photochromic ability of OXs always depends
on their specific molecular structures and the particular ambient
conditions, while the photochromism of the SP unit is more
independent of structure and the surrounding microenvironment.
In an attempt to obtain
a single molecule capable of
Figure 2. Absorption spectra for 1 and the mixed OX+SP system in ethanol,
respectively, with a, d) after UV- (254nm) / visible-light irradiation, adding
excess NaOH after UV exposure, and adding TFA after NaOH, b, e) different
water contents (%), and c, f) acid/base. The concentration was 2 × 10-5 M for
each molecule. The structural changes shown below for SP and OX are
responsible for the different color-switching behavior under external stimuli.
hydrochromism- and photochromism-multicolor displays, herein
we have designed and synthesized dyad 1, which consists of an
OX unit for blue color and an SP unit for magenta color display
(Figure 1). Dyad 1 was prepared by condensation of the
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yellow color can return almost completely back to the original
magenta color by adding an appropriate amount of trifluoroacetic
and halochromic switching behavior in solution using mutually
compatible stimuli bringing about magenta and blue colors,
respectively. An additional reversible yellow color change was
acid (TFA) (Figure 2a, light magenta line).
A similar
phenomenon was also observed in the mixed system (Figure 2d, also obtained by means of a nucleophilic attack employing
yellow and light magenta lines). The generation of yellow
color may be the result of formation of MCOH via nucleophilic
addition[32] of a hydroxide to the electrophilic imine of the open-
ring form of the SP switch (Figure S3). The corresponding
structural changes in SP that result in the magenta and yellow
colors are shown below Figure 2d.
Next, the hydrochromic properties for dyad 1 and OX+SP
were each tested. Increasing the content of water in the ethanol,
solution resulted in little color changes for the case of dyad 1,
and only a weak absorption peak was observed in the visible
region (Figure 2b). In contrast, the mixed system of OX+SP
showed a bright blue color when increasing the water content
(Figure 2e). Noticeably, during the experiment, the solution
NaOH and recovered with TFA. Thus, three primary colors —
magenta, yellow and blue — for the dyad 1 can be obtained.
Structural transformations for the dyad 1 during these processes,
which were further confirmed by NMR and mass spectroscopy
(Figure S6, S7), and the corresponding color changes observed
in its solutions are shown in Figure 3. The mixed SP+OX
system, on the other hand, has the advantage over dyad 1 in
that it is still capable of demonstrating hydrochromic behavior in
solution.
2. Investigation of the multi-stimuli chromic properties of
dyad 1 and the mixed system on the solid substrate
Encouraged by its switching behavior in solution, the multi-
stimuli addressable color switching properties of dyad 1 were
investigated next in a solid matrix. Drawing on the experience of
our previous work,[16] a stimuli-responsive paper composed of a
three-layer structure was designed (Figure 4a). Filter paper was
chosen as the substrate upon which dyad 1 was loaded. First
the filter paper was covered with a layer of polyethylene glycol
(PEG, Mn: 20000), a polymer which passivates the hydroxy
groups of the paper and stabilizes the initial colorless state. PEG
was also introduced into the next imaging layer containing dyad
1 in order to prevent its aggregation. We next examined whether
1 retains the switching properties observed in solution. The
experiments were monitored by UV-vis reflective spectroscopy
with integral ball. Prior to any external stimuli, the paper appears
white, and almost no absorption in the visible region of the
spectrum is observed (Figure 4b, black line). Upon UV
irradiation, the paper takes on a magenta color which is
accompanied by the appearance of a new reflective peak
centered at 556 nm (magenta line). The magenta colored paper
could be returned to its initial white state by using visible light
irradiation and heat (gray line). These color changes result from
structural changes of the SP subunit (Figure S8). The results of
these irradiation experiments indicate that the photochromism of
1 is retained in this paper-based matrix. The halochromism of 1
in the paper matrix was also tested. Volatile acetic acid
(CH3CO2H), acting as a weaker and safer acid than TFA, was
used in this experiment. Treating the paper with an aqueous
solution containing 0.01% CH3CO2H resulted in the blank paper
taking on a blue color, and a reflective band was observed in the
visible region around 594 nm (Figure 4c, blue line). Based on
the understanding of the mechanism in solution, these changes
in color and spectral features are a result of the ring-opening
transformation of the OX subunit. Unexpectedly, heat treatment
of the blue paper formed from addition of aqueous acetic acid
was unable to revert it completely to its initial colorless state.
However, the fact that such a small amount of aqueous acidic
solution could trigger the color change seemed to hint that the
microenvironment surrounding the dye is playing a crucial role in
the ring-opening reaction of the OX unit. In addition, the
hypothesized influence of poor solubility on the hydrochromic
containing dyad
1 becomes gradually more turbid with
increasing water content (Figure S4). This observation indicates
that the hydrochromism of dyad 1 might be inhibited by its poor
solubility resulting from its larger molecular structure. Taking into
account that OX is also known for its halochromism, and in order
to confirm the OX subunit still works as
a switch, the
halochromism of dyad 1 was investigated. Addition of TFA to a
solution of 1 did lead to a blue color change from an initially
colorless solution, as well as the appearance of a new
absorption band centered at 575 nm (Figure 2c, blue line). A
similar phenomenon was observed for the OX+SP system
(Figure 2f, blue line). This color change is attributed to the acid-
induced ring-opening of the OX unit, because SP does not have
absorption in that region under the same acidic conditions
(Figure S5). In addition, the blue solutions and their
corresponding spectra can completely be returned to their
colorless states by adding triethylamine (Et3N) (Figure 2c and 2f,
gray lines). These results suggest that the OX switching unit on
dyad 1 is still functional, even if its hydrochromism is lost in
solution likely as a result of its poor solubility. The structural
changes of the OX subunits that result in the blue-colored
solutions upon addition of water or acid are shown below in
Figure 2e and 2f, respectively.
The dyad 1 is capable of undergoing reversible photochromic
Figure 3. The structural transformations for dyad 1 and corresponding color
changes.
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Figure 4. a) Schematic illustrations of the three-layer structural design for the
rewritable paper based on 1. Reflective UV−Vis spectra of the 1-based paper,
b) before (black trace) and after UV irradiation (254 nm, magenta trace), c)
before (black trace) and after (blue trace) addition of 0.01% CH3CO2H, d)
before (black trace) and after (blue trace) addition of water, e) before (black
trace) and after (yellow trace) adding imidazole and heating, and f) before
(black trace) and after (purple trace) treating with both water and UV light. G)
Photograph of the rewritable paper stimulated with water (blue C), imidazole-
heat (yellow Y), UV light (magenta M), and the corresponding CIE Lab
coordinates.
Figure 5. The plot of the reflectance of the rewritable paper loaded with 1
versus the number of write/erase cycles as the rewritable paper was
repetitively exposed to a) water spraying (write) and water removal (erase) by
means of wetting/heating at 70 oC, b) UV (write) and Vis-heat (erase)
irradiation, and c) spraying with an imidazole solution and heating at 70 oC
(write), and imidazole removal by heating at 100 oC under vacuum (erase). d)
Images of the initial state, after addition of water (blue state), after UV
irradiation (magenta state), and after treating with imidazole and heat (yellow
state) of the paper as imaged under a microscope. Scale bar: 200 μm.
switching behavior of 1 in solution may not be relevant in the
solid paper matrix. These considerations inspired us to
investigate the hydrochromism of
introduction of water, the paper displayed
1
once again. Upon
blue color
o
a
imidazole to the white paper accompa-nied by heating at 70 C,
accompanied by the observation of a reflective band centered at
593 nm (Figure 4d, blue line), observations which are the
consequences of the change in structure of the OX unit to its
ring-open form (Figure S9). Different from the acid-induced
coloration, we found that the colored paper stimulated by water
can be completely returned to its initial blank state by removal of
the water using heat (Figure 4d, gray line). These experiments
demonstrate that water is a much better stimulus than acetic
acid, endowing the paper-based matrix containing 1 with both
write and erase capabilities.
Even though the yellow color of 1 can be achieved by addition
of NaOH, complete recovery of the initial colorless state both in
solution and the paper matrix is very difficult (Figure S10).
These switching processes were stimulated by introduction of
strong alkali solutions (i.e. NaOH) and corrosive acid (i.e. TFA),
conditions of which are not ideal for everyday consumer
applications. Inspired by the fact that the imidazolyl group of
histidine residues functions as an important catalyst in biological
processes because of its nucleophilic charac-ter,[33,34] we tried
imidazole as the nucleophile instead of hydroxide. Upon the
introduction of imidazole, the yellow color is still observed.
Furthermore, with the help of heat, development of the yellow
color does not require pretreatment of the paper substrate with
UV irradiation. Upon introduction of an ethanolic solution of
the yellow color appeared, and a new reflective peak centered at
421 nm was observed (Figure 4e, yellow line). Different from
the yellow color observed when only the SP unit is combined
with NaOH, the yellow color of 1 in the paper-based matrix that
results from the introduction of imidazole arises from
nucleophilic additions at both the OX and SP subunits of 1
(Figure S11). In addition, the color could be erased by heating
o
the paper to 100 C under vacuum and leaving almost no trace
(Figure 4e, gray line). This reversible color change is due to
thermal decomposition of the imidazole adducts.[35] In addition,
when water and UV acted simultaneously on the paper matrix
incorporating 1, a purple color was obtained (Figure 4f). This
purple color is a mixture of magenta and blue that results from
the ring-opening isomerizations of both the OX and SP units,
respectively. It is worth mentioning that the similar paper
matrices prepared from the mixed OX+SP system with the same
concentraiton were unable to revert back to their initial states, no
matter what stimuli tested (Figure S12, last column). Even
though decreasing concentration of OX or SP or both of them
could improve its reversibility to a certain extent, the developed
colors (i.e. reflectance) upon stimulation were decreased as well
(Figure S12, S13). This result indicates that the switching
performances of the single molecule
1
outperform its
corresponding mixed system on solid media. This better
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that aggregation was not observed on the paper substrates
before and after treatment with water, UV light, and imidazole-
heat for developing the blue, magenta and yellow colors,
respectively (Figure 5d). These results support the notion that
formation of non-switchable aggregates of 1 are effectively
prevented with the aid of PEG in the imaging layer, and that the
single molecule system can avoid phase separations unlike the
corresponding mixed OX+SP system resulting in better
dispersion of the switchable dye molecules.
In brief, dyad 1 containing just two switch subunits could act
as a three-switch molecule and display three-primary colors
using water, light and nucleophile. Combining these stimuli with
standard printing technologies (i.e., water-jet printing and light
printing), 1-based paper is easy to achieve full-color display
printing and reuse (Figure 6a). In order to demonstrate the
practical application of this multicolor rewritable paper, we
printed colorful patterns containing blue, magenta, yellow and
purple. As shown in Figure 6b, we could print the blue and
magenta patterns with water and UV irradiation, respectively, in
a step-by-step fashion. For multicolor display, magenta was
printed with UV irradiation; blue and yellow were generated by
water-jet printing using water and imidazole inks, respectively.
The heat brought on the paper during the deposition of the
imidazole ink is sufficient for producing the yellow color. The
purple color can be displayed when UV and water act on the
same place. All of the patterns written could be erased.
Figure 6. a) Schematic illustration of full-color printing. b) Examples of
muliticolor images displayed on the rewritable paper by water-jet printing with
pure water for the blue color, a combination of an ethanol solution of imidazole
with UV irradiation for the yellow color, and UV irradiation only for the magenta
color.
Conclusions
In summary, we have designed and synthesized a new and
simple dual-switch molecule 1 consisting of hydrochromic
oxazolidine and photochromic spiropyran units, and studied its
color switching properties both in solution and a paper-based
substrate. Three primary colors of CMY, as well as a composite
purple color, for 1 can be written with excellent reversibility by
selective exposure from a suite of different stimuli including
water/acid, light, nucleophiles and heat. A mixed system
composed of the separate spiropyran and oxazolidine subunits
performance might be hinged upon the fact that the structure of
1 helps to prevent undesirable interactions between its two
subunits via their covalent linkage, behavior of which can still
take place in the case of the OX+SP physical mixture.
The dyad 1 recovers its hydrochromic property and is capable
of displaying a reversible change to yellow in the paper-based
substrate. Therefore, this paper is capable of displaying three
primary colors each in a reversible fashion using three different
stimuli. As shown in Figure 4g, a blue letter “C” written by water-
jet printing, a magenta letter “M” written by UV irradiation, and a
yellow letter “Y” hand-written with an imidazole solution and heat
were all obtained on one paper. The detailed information of the
colors is described by the CIE Lab coordinates.
(i.e., SP+OX) was also investigated for
a side-by-side
comparison. Even though the mixed system seems to have
better performance in terms of hydrochromic switching than the
new single molecule 1 at least in solution, the latter greatly
outperforms the former with respect to reversible color-switching
on the paper substrate. The hydrochromism of 1 that failed to be
observed in solution was successfully recovered when loaded,
with good dispersion, onto the paper substrate using the
assistance of a solid solvent (i.e., PEG). An unexpected
reversible display of a yellow color for 1-based paper was also
achieved by making use of imidazole as a nucleophile with
simultaneous heating, instead of the more commonly employed
strong alkali (i.e., NaOH) and corrosive acid (i.e., TFA)
conditions. The potential application of 1 for developing full-color
rewritable paper with good reversibility and uniformity has been
demonstrated. The stimuli and methods employed in the both
the construction as well as writing and erasing of the paper are
clean, economic and compatible with current ink-jet and laser-jet
3. Application in full-color display rewritable paper and its
performance
To further confirm the reversibility of the paper with respect to
reversion of the three primary colors of blue, magenta and
yellow, their corresponding reflectances at 593 nm, 556 nm, and
421 nm were measured after water-spraying/heating, UV-light
irradiation/visible-light irradiation and heating, and imidazole
introduction and heating/heating at 100 oC under vacuum,
respectively (Figure 5a-5c). After 10 write-erase cycles, the
paper still works well for each color. Microscopy images reveal
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promising approach for designing new multi-addressable
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color rewritable papers that use environmentally friendly ink-free
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This article is protected by copyright. All rights reserved.

10.1002/chem.201801692
Chemistry - A European Journal
FULL PAPER
Entry for the Table of Contents (Please choose one layout)
Layout 1:
FULL PAPER
Full-color rewritable paper based on a
newly designed dyad containing
photo- and hydro-chromic units has
been developed. The dyad can
display three primary colors of CMY
as well as a composite purple color
with excellent reversibility in both
solution and on paper substrates upon
exposure to different stimuli including
water/acid, light, nucleophiles and
heat.
Tianyou Qin, Jiaqi Han, Yue Geng, Le
Ju, Lan Sheng,* and Sean Xiao-An
Zhang*
Page No. – Page No.
A Multi-addressable Dyad with
Switchable CMY Colors for Full-color
Rewritable Papers
This article is protected by copyright. All rights reserved.