Full Color Light Responsive Diarylethene Inks for Reusable Paper

Article abstract of DOI:10.1002/adfm.201600032

“Digitalization” represents one approach to shift society's dependence on paper-based communication. However, thus far, this tactic has not had a significant impact on global paper consumption, which has risen over the past few decades. The escalating demand of paper making and consumption has resulted in an intensified negative effect on the environment. Because of this, the development of rewritable paper or erasable ink appears to be an ideal approach to alleviate the increasing demand for paper. In the investigation described herein, novel light-stimulated (UV–vis), reversible color switching, photochromic diarylethene (DE) derivatives are designed, which serve as cyan, magenta, and yellow colored ink materials for full color ink-jet printing. The structures of the DE derivatives are unique in that they contain hydrophilic ethylene glycol chains that enable them to be compatible with aqueous based, ink-jet printing systems. The results of these studies demonstrate that the new DE derivatives can be used in a printing system based on the “write–erase–write” concept that utilizes the same paper multiple times. The approach appears to be ideal for reducing the negative environmental consequences of paper production and consumption.

Full text of DOI:10.1002/adfm.201600032

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Full Color Light Responsive Diarylethene
Inks for Reusable Paper
Woomin Jeong, Mohammed Iqbal Khazi, Dong-Hoon Park, Young-Sik Jung,
and Jong-Man Kim*
creation of electronic based communica-
tion methods that substitute for paper has
not been as rapid as was expected, which
is evidenced by threefold increase in the
global consumption of paper over the past
few decades.[1,2]
“Digitalization” represents one approach to shift society’s dependence on
paper-based communication. However, thus far, this tactic has not had a
significant impact on global paper consumption, which has risen over the
past few decades. The escalating demand of paper making and consumption
has resulted in an intensified negative effect on the environment. Because
of this, the development of rewritable paper or erasable ink appears to be an
ideal approach to alleviate the increasing demand for paper. In the inves-
tigation described herein, novel light-stimulated (UV–vis), reversible color
switching, photochromic diarylethene (DE) derivatives are designed, which
serve as cyan, magenta, and yellow colored ink materials for full color ink-jet
printing. The structures of the DE derivatives are unique in that they contain
hydrophilic ethylene glycol chains that enable them to be compatible with
aqueous based, ink-jet printing systems. The results of these studies demon-
strate that the new DE derivatives can be used in a printing system based on
the “write–erase–write” concept that utilizes the same paper multiple times.
The approach appears to be ideal for reducing the negative environmental
consequences of paper production and consumption.
According to findings made in recent
surveys, most documents containing
information recorded on paper are dis-
posed after a one-time use. This trend is
creating serious and growing environ-
mental problems associated with deforest-
ation, solid waste, and chemical pollution
in air, water, and on land.[3,4] As a result,
much attention has been given to the
development of rewritable paper that can
be used multiple times. If successful, the
new technology would help bring about
a balance between the socioeconomic
advancement of society and environmental
protection. In this context, several types
of rewritable papers and erasable inks
have been described recently. One elegant
approach to this problem utilizes a rewritable composite paper,
containing a photocatalyzed, color switchable redox dye. In this
system, ink-free replication of a text/pattern created by using
a preprinted photomask is possible.[5] In another remarkable
effort, a hydrochromic dye embedded paper was designed for
water-jet printing where water serves as the trigger for color
switching.[6] By taking advantage of their tunable refractive
index property, photonic crystals have been utilized to fabricate
rewritable paper. In this system, a well-ordered photonic coating
is embedded on substrate for pattern-on-demand printing using
water,[7] hygroscopic salts,[8] and silicon fluid[9] as inks. Further-
more, a wide range of photoresponsive color switchable func-
tionalized azobenzenes,[10,11] flugides,[12,13] bisthienylethenes,[14]
spironapthooxazine,[15] and spiropyrans[16] have been explored in
potential rewritable paper applications. In similar approaches,
phenomenon involving reversible halochromism of oxazine
derivatives[17] and thermochromism of leuco dyes[18,19] has been
employed to develop rewritable paper systems.
1. Introduction
Paralleling advances made in technology, electronic-based com-
munication and digitalization have grown significantly to a
point where they are an integral part of the lives of most human
beings. In spite of these advances, society still relies on paper
as the key material used to communicate and record informa-
tion. Paper has played an extremely important role in recording
history and enabling the cultural development of society. The
W. Jeong, D.-H. Park, Prof. J.-M. Kim
Department of Chemical Engineering
Hanyang University
222 Wangsimni-ro, Seongdong-gu
Seoul 133-791, South Korea
E-mail: jmk@hanyang.ac.kr
Dr. M. I. Khazi, Prof. J.-M. Kim
Institute of Nano Science and Technology (INST)
Hanyang University
In the investigation described below, we devised and tested
a new strategy for the design of light-responsive ink for rewrit-
able paper, which is based on the photochromic switching
properties of diarylethenes (DEs). DEs are known to undergo
light-stimulated (UV–vis) reversible photochemical cyclization
and cycloreversion reactions, which are accompanied by color
switching from colored closed to colorless open ring forms.
222 Wangsimni-ro, Seongdong-gu
Seoul 133-791, South Korea
Dr. Y.-S. Jung
Bio-Organic Science Division
Korea Research Institute of Chemical Technology
Daejeon 305-606, South Korea
DOI: 10.1002/adfm.201600032
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Figure 1. Structures of the hydrophilic DEs. Light-stimulated (UV–vis) reversible isomerization, colorless-ring open form 1a–3a & colored-ring closed
form 1b–3b.
Among various types of photochromic molecules, DEs are
the most promising class in terms of their thermal stability
and fatigue resistance, as well as their ability to participate in
repeatable color switching cycles.[20–32] Specially, photochromic
performance of nonionic water soluble DEs inspired us to
design erasable ink system.[33,34] The results of the study show
that well designed hydrophilic DE derivatives (Figure 1) can be
readily synthesized and that they can be employed in erasable
inks for rewritable paper that can be used in multiple “print–
erase–print” cycles.
2.2. Synthesis of DE Derivatives
The initial goal of this effort was to develop the light-
responsive, cyan colored dye 1a as an ink to replace traditional,
high color intensity black ink. The strategy utilized for the syn-
thesis of 1a, depicted in Figure 2, begins with 4-bromo-5-meth-
ylthiophene-2-carbaldehyde (4). This substance was prepared
in high yield by formylation of 2,5-dibromo-3-methylthiophene
using n-butyllithium and dimethylformamide (DMF). The alde-
hyde group in 4 was protected as its diethyl acetal derivative by
reaction with triethyl orthoformate to generate 5,[36–38] which
was then lithiated by using n-butyllithium. The organolithium
intermediate was then reacted with octafluorocyclopentene fol-
lowed by acid hydrolysis using trifluoroacetic acid to form photo-
chromic diformyl substituted DE 6.[39,40] Jones oxidation of 6
produced the corresponding dicarboxylic acid derivative 7,[41]
which upon treatment with oxalyl chloride and a catalytic amount
of DMF gave the acid chloride intermediate, which was directly
used for coupling with octaethylene glycol monomethyl ether to
generate the target DE 1a (Supporting Information).
Next, we extended our synthetic strategy to prepare the
respective magenta and yellow colored DEs 2a and 3a as light-
responsive inks for full color ink-jet printing. For magenta, thi-
anaphthene derivative 2a was synthesized starting from 3-meth-
oxythiophenol by employing multistep synthetic route[42–44]
(Supporting Information). In case of yellow (3a), synthetic
strategy similar to 1a was employed (for scheme and experi-
mental details, see the Supporting Information). The struc-
tures of synthetic 1a, 2a, and 3a were confirmed by using IR,
1H NMR, 13C NMR, and high-resolution mass spectra (HRMS)
spectral methods (Figures S2–S10, Supporting Information).
2. Results and Discussion
2.1. Design of Hydrophilic DEs
Based on knowledge about the photochromism of DEs com-
bined with our objective to develop erasable full color inks for
ink-jet printing, we designed hydrophilic photochromic sub-
stances for use as cyan, magenta, and yellow primary colors
in inks. The core structures of the DEs were selected based
on existing knowledge about the substances that undergo
light-stimulated color switching (Figure S1, Supporting
Information). The results of a literature survey indicated
that the colors of the ring-closed isomers of these substances
could be readily manipulated by simply changing the posi-
tion of sulfur atom in the thiophene ring.[35] Accordingly, the
photochromic DEs 1a, 2a, and 3a (Figure 1) were designed
to produce cyan, magenta, and yellow colors, respectively,
upon UV irradiation. The required hydrophilicity of the mol-
ecules used in the inks was installed by incorporating cova-
lently linked, polar ethylene glycol chains of proper length
to the DE cores. The ethylene glycol moieties not only make
the molecules suitable for use as ink material but it also
offer advantages over ionic salts and sulfonic acid group in
terms of long term stability, nontoxicity, and environmental
friendliness.
2.3. Photochromic Properties
Photochromic properties of DEs 1a–3a, all of which are highly
soluble in water, were examined. These substances in aqueous
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Figure 2. Synthetic route for the preparation of DE 1a.
solutions were observed to undergo light stimulated, revers-
ible isomerization accompanied by significant color-switching.
Inspection of the absorption spectra and images displayed
We also investigated the photofatigue properties of 1a–3a
in water at room temperature.[47,48] Figure S12 (Supporting
Information) illustrates the irradiation time dependent absorb-
ance changes of 1a–3a at the absorption maximum of their
closed-ring isomers by applying UV light (254 nm) for 6 h in dark.
The absorption maxima for 1b and 2b significantly declined over
the time and decayed about 50% after 90 and 230 min of irradia-
tion time, respectively, indicating by-product formation from the
closed-ring isomer. In contrast, compound 3b does not show any
significant decay for 6 h of irradiation. Upon alternating irradia-
tion with UV (254 nm) and visible light (>450 nm), photocycliza-
tion and cycloreversion cycle of 3a could be repeated for ten times
without noticeable change in absorption maxima. Significant loss
of photochromic material was observed for 1a and 2a after a few
switching cycles (Figure S13, Supporting Information).
in Figure 3a–c shows that the open-ring isomers 1a, 2a, and
−1
3a have absorption maxima at 256 (ε: 34 × 103
m
cm−1),
cm−1)
−1
−1
m
269 (ε: 18.6 × 103
m
cm−1), and 333 (ε: 12.6 × 103
nm, respectively. Upon UV light irradiation, colorless solu-
tions of 1a, 2a, and 3a transform to respective cyan, magenta,
and yellow colored solutions, as a consequence of formation
of the respective closed-ring isomers 1b, 2b, and 3b. Absorp-
tion maxima of the closed-ring isomers 1b, 2b, and 3b are 592
−1
−1
(ε: 8.2 × 103
m
cm−1), 533 (ε: 10.4 × 103
m
cm−1), and 441
−1
(ε: 4.6 × 103
m
cm−1) nm, respectively. Visible light irradia-
tion of the colored solutions containing the ring-closed isomers
regenerates the original colorless solutions that have identical
absorption spectra to that of 1a, 2a, and 3a.
The quantum yields for cyclization and cycloreversion reac-
tion, and conversion ratio were measured in water at room tem-
perature by reference method.[45,46] Conversion ratios (C. R.)
from the open- to the closed-ring form, upon irradiation with
UV light, are 90%, 14%, and 18% for 1a, 2a, and 3a, respectively.
The quantum yield of the cyclization reaction of 1a (Φ: 0.65) was
determined to be much higher than its cycloreversion process
(Φ: 0.025). While the quantum yields for cyclization reaction of
2a (Φ: 0.088) and 3a (Φ: 0.45) were lower than cycloreversion
process (Φ: 0.11, 0.64). Quantum yield, absorption maxima,
and their coefficients for the open- and closed-ring isomers
were also calculated in ethyl acetate (Table 1). The solvent effect
of 1a–3a for cyclization process has been studied in series of
aprotic solvents and protic solvents in Figure S11 (Supporting
Information).
2.4. Ink Formulation
To achieve optimal printing performance, the nonionic surfactant
Brij 78 was utilized in the ink formulation. The inks, individu-
ally containing DEs 1a–3a, were prepared using the following
generic formulation: 1 g (10 wt%) of DE, 0.5 g (5 wt%) of Brij
78, and 8.5 mL (85 wt%) of deionized water. The solutions were
sonicated for 30 min to ensure that they are homogeneous, and
then irradiated with a hand-held laboratory UV lamp (254 nm,
1 mW cm−2) for 30 min. The colorless solution containing 1a,
2a, and 3a were observed to change to cyan (I1), magenta (I2),
and yellow (I3), respectively (Figure 3g). Absorption maxima for
ink solutions I1–I3 were compared with the commercial ink (HP
703 tricolor) solutions (Figure S14, Supporting Information).
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Figure 3. a–c) UV–vis spectra of aqueous solutions of compounds 1–3 (1 × 10−4 mol L−1) in open form (solid black line) and at photostationary states
(PSS), (solid colored line) formed by irradiation with UV light (254 nm for compounds 1, 2 and 365 nm for compound 3). The inset in each figure shows
color change of the photochromic solution. d–f) Particle size distributions of DE inks I1–I3 obtained by using DLS. g) Photographs of UV-irradiated
DE inks I1–I3 and their extracted color samples below. h) CIE coordinates of DE inks I1–I3.
2.5. Particle Size Distribution
also show existence of self-assembled nanospheres with
average spherical diameters <5 nm (Figure S15 and Table S1,
Supporting Information).
Amphiphilic substances display self-assembly properties in
aqueous environments. Consequently, dynamic light scattering
(DLS) analysis was carried out to determine particle size distri-
butions of ink solutions I1–I3. The results show the existence
of self-assembled nanospheres with respective average spher-
ical diameters of 3.9 nm 1.0, 3.8 nm 1.0, and 4.2 nm 0.8
(Figure 3d–f). DLS measurements on 1a–3a (open-ring form)
in pure water and 1a–3a (open-ring form) in DE ink solutions
2.6. Viscosity Measurements
The viscosities of the ink solutions I1–I3 were measured at
room temperature and were found to be in the range accept-
able for ink-jet printing with 2.4, 3.0, and 2.7 cP, respectively.
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Table 1. Photochromic properties of DE derivatives 1–3.
max [nm]/ε [103
m
−1 cm−1
]
Φc)
Compound
Solvent
C. R.d) [%]
λ
Open-ring isomera)
Closed-ring isomerb)
Cyclization Φo-c
0.65
Cycloreversion Φc-o
1
2
3
Water
Ethyl acetate
Water
256/34
592/8.2
0.025
0.24
0.11
0.25
0.64
0.40
90
84
14
99
18
37
257/34.4
269/18.6
268/23
593/10.4
0.41
533/10.4
0.088
Ethyl acetate
Water
534/12.7
0.59
333/12.6
331/15
441/4.6
0.45
Ethyl acetate
443/6.8
0.53
a)
b)
λ
(absorption maximum) and ε (absorption coefficient) of open-ring isomers;
λ
(absorption maximum) and ε (absorption coefficient) of closed-ring isomers;
max
max
c)Quantum yields of cyclization reaction (Φo-c) and cycloreversion reaction (Φc-o); d)Photoconversion ratio from the open- to the closed-ring isomers at photostationary
state.
These values are in good agreement compared to respective
commercial inks. Measurements of 1a–3a (open-ring form) in
DE ink solutions did not show any significant difference in the
viscosity (Table S2, Supporting Information).
observed to completely disappear upon irradiation with the
Xe lamp for 30 s. Each “print–erase” cycle was found to take
place without reduction in the quality of the paper. The nature
of the “print–erase” cycle was also probed by using absorp-
tion spectroscopy (Figure 5b). Visible light irradiation of the
printed-paper results in the nearly complete disappearance of
absorption in the 500–700 nm region. In Figure 5c are shown
absorbance changes at 592 nm during ten consecutive “print–
erase” cycles. The legibilities of printed text and images were
found to last for more than several months under dark con-
ditions (Figure S16, Supporting Information). Importantly,
printed images on paper, stored under dark conditions for
several months, are also readily erased by using visible light
irradiation. Finally, similar results were observed for text and
images printed with magenta (I2) ink (Figure S17, Supporting
Information).
2.7. Commission Internationale de l′Éclairage (CIE)
Colors of formulated inks are depicted in CIE color space.
The scanned images (PNG format) of ink were normalized
in Adobe Photoshop (despeckle, dust and scratches, median
filters) to minimize artifacts introduced by scanner hard-
ware noise to determined CIE coordinates.[49,50] CIE coor-
dinates of cyan (I1), magenta (I2), and yellow (I3) at (x,y) =
(0.230,0.103), (0.445,0.203), and (0.442,0.546), respectively
(Figure 3h).
Next, we explored techniques to obtain full color images.
Owing to the low color intensity of the yellow ink (I3), direct
ink-jet printing of a full color image with cyan (I1), magenta
(I2), and yellow (I3) inks using a commercial tricolor cartridge
does not lead to high quality full color images. Consequently,
the optimal composition of the ink was created by adding
appropriate amounts of yellow (I3) ink to both cyan (I1) and
magenta (I2) inks. The optimum molar ratios were found to
be I1:I3 = 3:7 for the new cyan MI1 and I2:I3 = 7:3 for the
new magenta MI2 inks. Figure 6 displays a full color image
printed using these ink combinations. As expected, the full
color image can be erased by using visible light irradiation and
a new erasable full color image can be printed on the same
paper.
2.8. Ink-Jet Printing
The excellent dispersion properties of the DE derivatives in
water are compatible with those required for ink-jet printing. In
order to demonstrate the quality of print obtained by using the
inks, text was printed on conventional A4 size paper using the
light-responsive cyan (I1) ink. For this purpose, a commercial
black ink cartridge was filled with I1 ink and ink-jet printing
was carried out using a common office printer (HP Deskjet
Ink Advantage K209) (Figure 4a). In order to obtain informa-
tion about the printing resolution, the quality of I1 ink-printed
images on a paper substrate were compared to those gener-
ated using conventional black ink (Figure 4b,c). Inspection of
the images demonstrates that the resolution and color intensity
of the light-responsive ink-printed image are ideal for general
reading purposes, and the legibility of the prints can last at least
3 h under ambient condition.
Importantly, the printed text, formed by using ink I1, is
completely erased by using visible light irradiation (35 W
Xe lamp) (Figure 4a). To demonstrate that the erasable light-
responsive ink formulations are applicable to rewriteable
paper, repeated “print–erase” cycles were carried out with cyan
(I1) ink (Figure 5a). Visible light from a 35 W Xe lamp at a
1 cm distance above the printed image was used for erasing
the text and patterns created using ink I1. The image was
3. Conclusion
The photochromic performances of 1a–3a in aqueous solutions
were found to be excellent, with generating the corresponding
three primary colors, cyan, magenta, and yellow, upon UV
irradiation. Brij 78 was found to be an effective stabilizer of
ink formulation. The average particle size of 3.7–4.2 nm was
observed for DE inks (I1–I3), caused by the presence of eth-
ylene glycol pendant, made the inks compatible with typical
ink cartridge nozzles (no clogging of the nozzle vicinity of the
ink-jet head), reduced image unevenness and blurring, and
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Figure 4. a) Scanned images of cyan-color prints obtained using an ink-jet printer with a black cartridge filled with I1 (200 μL). Image and text were
half-erased by using visible light. Scale bars, 3 cm. b,c) The resolution of printed text using the light-responsive cyan ink (b) is compared to the text
printed with a commercial inkjet printer (c). Scale bars, 1 cm.
contributed to higher optical densities of the printed images.
Viscosity values were also in acceptable range of aqueous-based
inks, for high quality ink-jet printing. Printed text, arising from
using cyan (I1) and magenta (I2) inks, was found to have a
good resolution and patterns created from the inks have sharp
edges. Printing of satisfactory images was possible even after
prolonged storage (>one month) of the light-responsive inks
loaded in a cartridge.
We observed for the first time that these primary colored inks
can be used to produce full-color images on conventional paper.
The printed full-color text/pattern can be erased by irradiation
with visible light without affecting the quality of the paper.
Also, “print–erase” cycles can be carried multiple times with
no deterioration of the quality of the images. We believe that
the substances explored in this effort will be useful in devel-
oping aqueous-based ink for ink-jet printing. This approach
is ideally suited to rewriting paper multiple times without any
resource-intensive processing. An additional meritorious fea-
ture of the new ink is that, even when the printed image is
stored in the dark for several months, it can be readily removed
by irradiation with visible light.
It should be noted that the limitation associated with low
color intensities of images printed using the yellow colored
ink needs to be overcome in order to avoid the need for
multiple printing events. It also should be emphasized that
printing with cyan ink alone gives a high quality, readily
erasable image that is good for general reading purposes
(Figure S18 and Movie S1, Supporting Information). Further
advancements in structural modifications of diarylethene
derivatives and optimization of printing condition are under
progress. The proof of concept arising from the results of the
current study should open a new avenue to the development
of erasable full color inks that are compatible with common
office inkjet printers.
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Figure 5. a) Scanned cyan-color images after repeated print–erase cycles on the same paper using ink-jet printer with black cartridge filled with cyan
(I1) ink. Scale bars, 3 cm. b) Visible absorption spectra of a cyan-color print before (blue line) and after (black line) visible light irradiation. The size of
the print is 4 cm by 4 cm. c) A plot of absorbance of a printed image measured during ten consecutive print–erase cycles. Printed images are erased
by using visible light irradiation and printed again on ink-jet printer.
Figure 6. Scanned full-color images after repeated print–erase cycles on the same single paper using ink-jet printer with tricolor cartridge filled with
DE inks cyan (MI1), magenta (MI2), and yellow (I3). To obtain full-color image identical to that of one printed using commercial HP ink-jet ink, each
image was printed five times. Scale bars, 2 cm.
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4. Experimental Section
Materials and Instruments: All chemicals were purchased from Sigma-
Aldrich, Korea and Tokyo Chemical Industry, Korea, were of highest
commercial quality, and were used without further purification. All
reactions were carried out under a nitrogen atmosphere using dry
solvents under anhydrous conditions, unless otherwise noted. 1H NMR
spectra were recorded on a Varian Unitylnova (300 MHz) spectrometer.
Proton chemical shifts were reported in ppm (δ) relative to internal
tetramethylsilane (TMS, δ 0.0 ppm) or with the solvent reference relative
to TMS (CDCl3, δ 7.26 ppm). Splitting patterns were indicated as
s, singlet; d, doublet; t, triplet; m, multiplet. 13C NMR spectra were recorded
on a Varian VNMRS (600 MHz) spectrometer with proton decoupling.
Carbon chemical shifts were reported in ppm (δ) relative to TMS with
the CDCl3 (δ 77.0 ppm) as the internal standard. HRMS were recorded
on a Jeol JMS-700 LTQ-Orbitrap-Ms (ESI+). IR spectra were recorded on a
Thermal Nicolet Nexus 670 spectrometer. Thin-layer chromatography was
performed on Merck silica gel plates (60 F254; Merck) using UV light as
the visualizing agent and basic aqueous potassium permanganate as the
developing agent. Merck silica gel (60–120 mesh size) was used for column
chromatography. Nomenclature used for all compounds was assigned
with help of ChemBio Draw Ultra 12.0 software. UV–vis absorption spectra
were measured on a single beam Agilent 8453 spectrophotometer. All
optical measurements were performed using a quartz cell. DLS data were
collected using a MalvernZEN3600. Reflectance spectra were recorded on
an USB2000 miniature fiber-optic spectrometer (ocean optics). Viscosity
was measured with a RheoSense microVISC viscometer. 35 W Osram Xe
arc lamp was used as visible light source.
Ink Formulation and Ink-Jet Printing: Ink was formulated in the manner
described in the text above and a 254 nm UV-light source was used for
irradiation. The HP Deskjet Ink Advantage K209 a-z printer was used
for printing on regular A4 size paper with 80 g m−2. The commercial
cartridges (black and tricolor, HP 703) were used after washing with
methanol, sonication (2 h), and drying by N2 blowing. Ink solutions
were loaded into cartridges (200 μL) and printed using default standard
settings. 35 W Xe lamp was used as the visible light source to erase
printed text and images. For “print–erase” cycles, printed text or images
were erased for 1 min. All printed and corresponding erased paper were
scanned using a HP Deskjet Ink Advantage K209 a-z printer with 600 dpi
(300 dpi was selected for print–erase cycle).
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Acknowledgements
W.J. and M.I.K. contributed equally to this work. The authors thank
the National Research Foundation of Korea for financial support
(No. 2014R1A2A1A01005862).
Received: January 4, 2016
Revised: March 18, 2016
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