Saving paper with switchable ink

Article abstract of DOI:10.1016/j.dyepig.2014.02.019

An oxazine auxochrome and a carbazole chromophore can be integrated within the same covalent skeleton to generate a halochromic molecular switch. Upon addition of acid, the oxazine ring opens to bring the carbazole fragment in conjugation with a 3H-indolium cation. This structural transformation shifts the main absorption of the carbazole chromophore from the ultraviolet to the visible region and, as a result, is accompanied by the appearance of an intense red color. This species can be formulated into an ink to print colored patterns on conventional paper. Upon treatment with base, however, the oxazine ring closes to restore the initial colorless state and erase the printed pattern. In fact, the very same sheet of paper can be recycled for multiple printing and erasing steps. Thus, this structural design for switchable inks can evolve into viable operating principles to enable innovative printing technologies and reduce drastically paper consumption.

Full text of DOI:10.1016/j.dyepig.2014.02.019

Dyes and Pigments 106 (2014) 71e73
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Short communication
Saving paper with switchable ink
*
Jaume Garcia-Amorós, Subramani Swaminathan, Françisco M. Raymo
Laboratory for Molecular Photonics, Department of Chemistry, University of Miami, 1301 Memorial Drive, Coral Gables, FL 33146-0431, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
An oxazine auxochrome and a carbazole chromophore can be integrated within the same covalent
skeleton to generate a halochromic molecular switch. Upon addition of acid, the oxazine ring opens to
bring the carbazole fragment in conjugation with a 3H-indolium cation. This structural transformation
shifts the main absorption of the carbazole chromophore from the ultraviolet to the visible region and, as
a result, is accompanied by the appearance of an intense red color. This species can be formulated into an
ink to print colored patterns on conventional paper. Upon treatment with base, however, the oxazine
ring closes to restore the initial colorless state and erase the printed pattern. In fact, the very same sheet
of paper can be recycled for multiple printing and erasing steps. Thus, this structural design for
switchable inks can evolve into viable operating principles to enable innovative printing technologies
and reduce drastically paper consumption.
Received 16 January 2014
Received in revised form
19 February 2014
Accepted 21 February 2014
Available online 11 March 2014
Keywords:
Carbazole
Halochromism
Molecular switch
Switchable ink
Oxazine
Ó 2014 Elsevier Ltd. All rights reserved.
Heterocycle
The average consumption of paper worldwide currently ap-
proaches 400 million tons per year [1]. This overwhelming figure
poses serious environmental concerns and is, for the most part, a
consequence of printing and writing. In spite of the on-going digital
revolution and the constant presence of electronic gadgets in our
daily routines, we seem to be still relying very much on books,
magazines, newspapers, photocopies and any possible iteration of
printed documents on a regular basis. A viable strategy to alleviate
the pressing demand for paper, and its raw sources, is the devel-
opment of rewritable inks. The identification of pigments that
could easily be removed, or at least decolored, from the surface of
conventional paper would offer the opportunity to recycle sheets
multiple times and reduce consumption drastically. In this context,
the development of switchable organic dyes [2] would be partic-
ularly valuable. Indeed, the level of sophistication reached by
modern organic synthesis, together with the need to avoid toxic
heavy metals, is definitely encouraging the formulation of all-
organic inks [3e5]. The further design of operating principles to
erase the color of their constituent components with appropriate
stimulations, after printing, would also ensure the rewritable
character essential for recycling. In fact, a few remarkable examples
of rewritable inks, based on switchable organic dyes, have been
reported in the literature already [6e12].
In the wake of our work on halochromic compounds [13], we
envisaged the possibility of developing switchable inks based on
the opening and closing of oxazine rings under the influence of acid
and base respectively. Specifically, we designed a molecular switch
(1 in Fig. 1) incorporating an oxazine auxochrome and a carbazole
chromophore within its covalent skeleton and prepared this com-
pound in a single synthetic step (Fig. S1). Upon addition of acid, the
oxazine ring of 1 opens to form 2 and bring the carbazole appendage
in conjugation with a 3H-indolium cation. This structural trans-
formation is accompanied by significant changes in the visible re-
gion of the absorption spectrum (Fig. S6) and the appearance of an
intense red color (Fig. 1). The process can be fully reversed with the
addition of base to restore the original and colorless state.
The pronounced changes in color associated with the reversible
interconversion of 1 and 2 suggest that these halochromic trans-
formations can indeed be exploited to develop switchable inks.
Specifically, a solution of the colored form can be used to print a
given pattern on conventional paper. Treatment with base can then
switch the colored form to the colorless one and erase the printed
information. As a result, the very same sheet of paper can subse-
quently be recycled for further printing. In order to test these
operating principles, 1 (300
acetonitrile (250 L) and ethanol (250
with the addition of TFA (10 L). The resulting red solution was
m
g) was dissolved in a mixture of
m
mL) and then switched to 2
* Corresponding author.
E-mail address: fraymo@miami.edu (F.M. Raymo).
m
http://dx.doi.org/10.1016/j.dyepig.2014.02.019
0143-7208/Ó 2014 Elsevier Ltd. All rights reserved.

72
J. Garcia-Amorós et al. / Dyes and Pigments 106 (2014) 71e73
Fig. 1. Reversible interconversion of 1 and 2 under the influence of acid and base and photos of the corresponding MeCN solutions.
applied with a capillary tube within a defined region of a pattern
pre-printed on a conventional sheet of paper. The corresponding
photo (a in Fig. 2) clearly reveals the red color of the applied ink.
Furthermore, the colored area remains virtually unchanged for
several months after storage of the printed sheet under ambient
illumination at room temperature [14]. However, the color can be
erased by immersing the sheet of paper in a diluted aqueous so-
lution of potassium carbonate (15 mM) for 15 min. Consistently, a
photo (b in Fig. 2), taken after oven drying at 80 ꢀC for 5 min, shows
the complete decoloration of the printed area. At this point, the
colored solution of 2 can be applied again in the very same region to
restore color (c in Fig. 2) and then the sheet of paper can be treated
with base to erase it once again (d in Fig. 2). In fact, coloration and
decoloration steps can be reiterated several times without
damaging the printed paper. Thus, such a switchable ink offers the
opportunity to recycle the very same sheet for multiple printing
and erasing steps and, in principle, would permit to save a signif-
icant amount of paper. For example, the use of a single sheet for just
four times translates already into a 75% reduction in paper con-
sumption. Furthermore, an additional advantage of this switchable
ink is the possibility to recover printed information. After treatment
with base to erase color (b and d in Fig. 2), the deleted pattern can
be fully restored by immersing the sheet of paper in an aqueous TFA
solution (Fig. S7).
Our results demonstrate that halochromic transformations
can be the basis for the development of switchable inks. Indeed,
they offer the opportunity to reiterate printing and erasing steps
on conventional paper and, hence, to recycle the very same
sheet multiple times. Thus, these findings can lead to the realiza-
tion on an entire family of rewritable inks and, possibly, enable
valuable printing technologies for the drastic reduction of paper
consumption.
Fig. 2. Photos of a conventional sheet of paper after the application of a MeCN/EtOH (1:1, v/v) solution of 2 (1.2 mM) with a capillary tube exclusively in the indicated region (a and
c) and immersion of the entire sheet in aqueous K2CO3 (15 mM) for 15 min and drying at 80 ꢀC for 5 min (b and d).

J. Garcia-Amorós et al. / Dyes and Pigments 106 (2014) 71e73
73
[7] Kishimura A, Yamashita T, Yamaguchi K, Aida T. Rewritable phosphorescent
paper by the control of competing kinetic and thermodynamic self-
assembling events. Nat Mater 2005;4:546e9.
[8] Horiguchi T, Koshiba Y, Ueda Y, Origuchi C, Tsutsui K. Reversible coloring/
decoloring reaction of leuco dye controlled by long-chain molecule. Thin Solid
Films 2008;516:2591e4.
[9] Yamamoto S, Furuya H, Tsutsui K, Ueno S, Sato K. In situ observation of
thermochromic behavior of binary mixtures of phenolic long-chain molecules
and fluoran dye for rewritable paper application. Cryst Growth Des 2008;8:
2256e63.
Acknowledgments
The National Science Foundation (CAREER Award CHE-0237578,
CHE-0749840 and CHE-1049860) is acknowledged for supporting
our research program. JGA is grateful for a Beatriu de Pinós post-
doctoral grant from the Generalitat de Catalunya (Spain, grant
2011 BP-A-00270).
[10] Kohno Y, Tamura Y, Matsushima R. Simple full-color rewritable film
with photochromic fulgide derivatives. J Photochem Photobiol A 2009;201:
98e101.
Appendix A. Supplementary data
[11] Yoon B, Ham DY, Yarimaga O, An H, Lee CW, Kim JM. Inkjet printing of con-
jugated polymer precursors on paper substrates for colorimetric sensing and
flexible electrothermochromic display. Adv Mater 2011;23:5492e7.
[12] Sheng L, Li M, Zhu S, Li H, Xi G, Li YG, et al. Hydrochromic molecular switches
for water-jet rewritable paper. Nat Commun 2014;5. 3044e1e8.
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.dyepig.2014.02.019.
[13] (a) Tomasulo M, Sortino S, Raymo FM.
A new family of photochromic
References
compounds based on the photoinduced opening and thermal closing of [1,3]
oxazine rings. J Photochem Photobiol A 2008;200:44e9;
(b) Deniz E, Tomasulo M, Cusido J, Sortino S, Raymo FM. Fast and stable
photochromic oxazines for fluorescence switching. Langmuir 2011;27:
11773e83.
[1] RISI. Annual historical data e world pulp; 2010.
[2] Feringa BL, Brown WL, editors. Molecular switches. Weinheim: Wiley-VCH;
2011.
[3] Drake JAG. Chemical technology in printing and imaging systems. Cambridge:
RSC; 1993.
[4] Leach RH, Pierce RJ. The printing ink manual. London: Blueprint; 1993.
[5] (a) Kunjappu JT. Essays in ink chemistry. New York: Nova Science Publishers;
2001;
(b) Kunjappu JT. Ink chemistry. Chem Br 2003;39:22e5.
[6] Fudouzi H, Xia N. Photonic papers and inks: color writing with colorless
materials. Adv Mater 2003;15:892e6.
[14] Although no change in color could be detected visually over this period of
time, the actual image permanence rating was not determined and, at this
stage, it is not clear how the lightfastness of this switchable formulation
compares to that of commercial inks. For examples of the quantitative
assessment of ink lightfastness, see: Chovancova A, Fleming III PD, Howell P,
Rasmusson A. Color and lightfastness performance of different epson ink sets
J Imaging Sci Technol 2005;49:652e9.