Inkjet-Printable Hydrochromic Paper for Encrypting Information and

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Inkjet Printable Hydrochromic Paper for Encrypting Information and Anti-counterfeiting Varun Kumar Singh, Ramesh Kumar Chitumalla, Sai Kishore Ravi, Yaoxin Zhang, Yongjie Xi, Vijayvenkataramana Sanjairaj, Chun Zhang, Joonkyung Jang, and Swee Ching Tan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b08054 • Publication Date (Web): 06 Sep 2017 Downloaded from http://pubs.acs.org on September 11, 2017

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ACS Applied Materials & Interfaces

Inkjet Printable Hydrochromic Paper for Encrypting

Information

and

Anti-

counterfeiting

Varun Kumar Singh,1 Ramesh Kumar Chitumalla,2 Sai K. Ravi,1 Yaoxin Zhang,1 Yongjie Xi,3 Vijayvenkataramana Sanjairaj,4 Chun Zhang,3 Joonkyung Jang2 and Swee Ching Tan*,1

1

Department of Materials science and Engineering, National University of Singapore, 9

Engineering Drive 1, Singapore 117575

2

Department of Nanoenergy Engineering, Pusan National University, Busan, Republic of

Korea, 609735

3

Department of Physics, National University of Singapore, 2 Science Drive 3, Singapore

117551

4

Department of Mechanical Engineering, National University of Singapore, 9 Engineering

Drive 1, Singapore 117575

Keywords: Hydrochromic, rewritable, aggregation, anti-counterfeiting, security printing

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ABSTRACT Developing rewritable papers has gathered immense interest in recent times in view of developing sustainability in print media without exhausting the environmental resources. We herein present a rapid and facile procedure for the fabrication of a communication media by treating the surface of a paper with synthetic organic molecules, after which plain water could be used as an ink to print and reprint numerous times on the treated-paper before disposing it. Interestingly, as the paper comes in contact with water, the molecules are driven to reorganize in a slip-stacked arrangement. This alters their ground and excited state properties by hydrogen-bond assisted non-radiative decay, the changes associated with which are visible to naked eye. The changes evolved are sensitive to the solubility parameter of the solvent, and thermally reversible; thus embarking the hydrochromic property to the paper. Against a background of concerns over rising counterfeiting and leak of confidential information, prospects for encrypted communications and anti-counterfeiting is herein demonstrated.

INTRODUCTION Preserving confidentiality of senstive data has always been a major concern in a wide range of organizations. Confidential information reaching wrong hands could have catastrofic effects. Currently, many countries have laws to protect confidential information in the workplace. Government departments, businesses and corporations have a lot of confidential information which needs to be encrypted while it is being circulated confidentially. Disseminating information of high sensitivity is not only limited to big organizations, but also has become part of our daily life such as, banks mailing the security passwords and other confidential information to their customers by post. Even when the confidential information has landed safely in the recipient’s hands, the process of shedding the paper to keep the privacy is still found to be tedious and a lot of paper with encrypted information has been wasted as a result.


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Even common people prefer to maintain some level of privacy when assessing and circulating personal information among themselves. Another important application of circulating confidential information is the use of hidden textual information or symbolic security markers for currency notes or top secret documents to prevent clandestine activities.1-3 Companies as well as enforcement agencies need to make sure that their trademarks are protected and proper anti-counterfeiting policies are implemented. Technologies such as 3D holograms, biometric markers, inks and smart cards can be employed to prevent proliferation of counterfeiting4. However, in order to implement this, such technologies must be cost-effective, resistant, durable and environmentally safe. Furthermore, if such materials are rewritable, they can help in reduction of waste paper and environmental pollution associated with deforestation for paper production. Recent progress in materials science and engineering has yielded rewritable imaging technologies that are functionalized in some cases for high density optical memory systems, self-erasing images, water jet rewritable paper, photochromic paper, and volume holography. Kim and coworkers introduced a hydrochromic paper for human sweat pore mapping utilising a conjugated polymer polydiacetylene.5,6 Later on a rewritable paper based on hydrochromic dyes was demonstrated where in the information written using water is visible under ambient light and served as ink free writing media.7 In 2004, Park and his group demonstrated a rewritable photoimage on the polymer film, based on a photochromic molecule for applications in rewritable optical memory media.8 Later, Grzybowski and co-workers demonstrated a self-erasing material based on aggregation of photoresponsive metal nanoparticles where the nanoparticles were coated with trans-azobenzene groups which isomerized to cis-azobenzene on exposure to UV light thereby producing a colour change in the images.9 However, the prevailing colour change stimuli in these studies are mainly UV/Vis which lead to potential safety issues. Besides the organogels used as matrix for the nanoparticles are hazardous to environment limiting the practical applications of these ACS Paragon Plus Environment


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materials. In addition, the ambient light can accidentally convert back the photochemically generated states into original forms, hence, posing additional issue to their perpetuation. Kim and coworkers demonstrated a direct writting of fluorescent patterns by applying shear force to a supercooled film of a diketopyrrolopyrrole dye.10 Recently, concept of mechanochromic luminogens has been introduced for ink free writing where a crystalline twisted molecular structure changes colour on applying pressure and erases on solvent exposure.11 However, the technology is not user friendly enough as the solvents used for erasing are either flammable or can be found only with designated chemical suppliers. Therefore, new colour switching mechanisms to develop rewritable papers are highly needed. Organic dyes are capable of self-organization into extended supramolecular arrays as a result of oriented dye aggregation which results from the intermolecular interactions such as π-π stacking, electrostatic, ion-dipole and hydrogen bonding between individual molecules and changes their optical properties.12-13 Organic dyes that exhibit strong photoluminescence in solution may lose it in the solid state or upon self-assembly in a solvent.14 Such stimuli responsive photoluminescent materials possess extra security features, making them potential materials to prevent tampering and counterfeiting. Herein, we report the synthesis of a diketopyrrolopyrrole dye and its fabrication into a rewritable paper which stands out as medium for secure communications wherein water is used as ink and the supramolecular organization (slipped-stacking induced aggregation) of the dye on contact with water serve as high contrast secret writing. The unique features of this rewritable paper include: (i) use of water as ink. As water is a renewable resource and possesses no risk to the environment. Its introduction and removal can be safely achieved. In addition, use of water cuts down the cost of expensive ink cartridges substantially. Moreover, water can be incorporated as printing ink in fully developed inkjet printing techniques making this process even more practical and useful. (ii) The written information can be destroyed on


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demand and paper can be used many times. This cuts down the wastage of paper enormously and therefore, simultaneously meets the global need of environment protection.

RESULTS AND DISCUSSION The chosen organic dye capable of self-organization was Diketopyrrolopyrrole (DPP) dye. DPP and its derivatives represent an important class of pigments as they are used in paints, colour inks and plastics owing to their brilliant colour and exceptional high stability upon exposure to light, weather and heat.15-17 Recently, modified DPPs have been developed as functional materials for optical and electronic devices18-19, field-effect transistors20, polymer solar cells21, organic light-emitting diodes22 and applications requiring two-photon absorption23,

owing

to

their

large

extinction

coefficients

and high fluorescence quantum yields which make them ideal for applications in sensors. DPP dye was prepared by condensation reaction of dimethyl succinate with thiophene-2carbonitrile in tert-amyl alcohol (Scheme S1, Supporting Information). Subsequently, Nalkylation of DPP dye proceeded in DMF which resulted in its increased solubility in common organic solvents which was then unambiguously characterized by 1H NMR and 13C NMR spectroscopy. Desired HD-1 dye was then obtained by bromination of N-alkylated thiophene DPP using N-Bromosuccinimide at room temperature. The UV-Vis absorption and fluorescence emission of HD-1 in THF (Figure 1a, d) show all the intrinsic features of DPP dye. The absorption maximum of monomeric HD-1 appears at 566 nm (absorption coefficient: ε = 4.17 x 104 cm-1M-1), while the fluorescence emission spectrum exhibits a maximum at 579 nm. Thus, HD-1 shows a stokes shift of 13 nm. The emission band is relatively narrow with full width at half maximum (FWHM) of 25 nm, as opposed to 70 nm for the absorption band. In our design, slipped-stacking induced aggregation in the presence of water and its reversal upon water evaporation was the basic reaction that occurred during writing and erasing steps, therefore, first the optical changes of ACS Paragon Plus Environment


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HD-1 THF solution in the presence of water was tested. As can be seen with the increase of water fraction the monomeric band at 566 nm decreased in intensity and a new bathochromically shifted band appeared at 618 nm, a red shift of 52 nm, with an isosbestic point at 596 nm suggesting a slipped-stacking induced aggregation (Figure 1b). The decrease in intensity of monomeric band at 566 nm band is accelerated with an increase in water content, suggesting that water indeed promotes the self-organization of HD-1 dye. The intensity ratio of new band at 618 nm and monomeric band at 566 nm shows a gradual increase after the water fraction, Fw passed 30% (Figure 1c). Note that the actual optical density over 80% water fraction, Fw is underestimated here, because only a small fraction of light at 618 nm is absorbed by the aggregates. Figure 1d depicts the fluorescence response of HD-1 in varied water fractions. The number of photons emitted (cf. integral of fluorescent intensities at 579 nm) decreased as more aggregates formed with increasing water fractions, presumably originating from ultrafast relaxation of exciton in DPP aggregates. Visual change in the physical appearance was noticed from pink to purple colour upon addition of water 5050% (v/v) and the photoluminescence also quenched (Figure 1e). Main driving force for the self-organization behavior of HD-1 upon hydration involves combined effect of π-π stacking between the aromatic core of HD-1 and hydrogen bonding interaction between dye and water molecules. Similar aggregation behavior in aqueous media and hydrogen bond interactions between the solute and solvent molecules is experienced by several research groups over the years where self-assembly often resulted in micelles and nanofibers formation, resulting from hydrogen bonding complexation and interactions.24-27 Therefore, we explored the optical changes of HD-1 in solvents like methanol, ethanol, acetonitrile which differ in their hydrogen bonding donor (HBD) ability and different polarity. The hydrogen bonding donor ability of water, methanol, ethanol and acetonitrile are 1.17, 0.93, 0.83 and 0.19, respectively28 and corroborated well with the tendency for self-organization. The spectra dynamics observed in all solvents can be explained in terms of hydrogen bond interactions ACS Paragon Plus Environment

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between the two C=O ends of HD-1and the solvent. The optical changes of HD-1 in water were comparable to other solvents where the drop in intensity of monomeric band was relatively low and no new band formation was observed (Figure 2a-d). No significant visual change in the colour of the solution was noticed for all the solvents except water. We also probed the absorption analysis in deuterium oxide to assess the hydrogen bonding mechanism which showed similar optical changes as water. The absorbance at 618 nm is larger in this case than THF-H2O. It was reported that a hydrogen bonding interaction between D2O and solute molecules is stronger than that of water28, therefore, the optical changes in the water and deuterium oxide system also indicate that self-organization is indeed triggered by the hydrogen bonding interaction between dye and selectively diprotic solvent molecules or simply aqueous solutions. The frequency of changes in the monomeric band also correlates with α parameter of the solvent pointing to hydrogen bonding interactions in the ground state. This conclusion was made from an analysis of the Hansen parameters of the solvent (Figure S2 and Table S1). The hydrogen bonding in liquids is a dynamic interaction and the bond breaking and formation takes place on a ultrafast timescale of few picoseconds.29-30 Therefore, rapid fluctuations in the stoichiometry and the structure of hydrogen bond complexes in protic solvents is expected. Consequently, the optical changes of HD-1 in solvents with α < 1.17 can be attributed to the formation of a loose hydrogen bond complex between HD-1 and the solvent. On similar ground, more pronounced optical changes observed in water and D2O can be assigned to the formation of a tight hydrogen bond complex. When the non-fluorescent aggregated solution (blue colour) was heated at 600C, within few seconds the colour of the solution changed to original pink colour where only monomeric form existed and also the fluorescence was completely recovered when watched under a UV lamp (365 nm). This also points to the acceleration of the hydrogen bond assisted non-radiative decay of excited molecules and is discussed in detail elsewhere.31-33 The observed self-assembly in water in ACS Paragon Plus Environment


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this case is indeed enthalpically driven process which is indicated by the dissociation of dye assemblies upon heating. Computational studies To optimize ground state geometries and illustrate the interaction of water with HD-1 molecules, we performed density functional theory (DFT) calculations using Gaussian 09 Suit.34 The optimized geometries of the HD-1 and its dimer are depicted as Figure S3 and S4. We first calculated the electron density distribution in the HOMO and LUMO states and found that both frontier orbitals are symmetric and delocalized over the whole chromophore (Figure 3a). This shows that the molecule HD-1 has a non-polar character and the shifts observed in the absorption regions can therefore, not be due to solvatochromism, rather aggregation seems to be a dominant process. Calculations show that a water molecule binds on HD-1 (Figure S5) through a hydrogen bond with a bond length of 1.893 Å and binding energy of ca. 0.4 eV. The low binding energy suggests that the water molecule can be easily desorbed, consistent with the reversible change in the absorption of dye solution upon heating. First principles investigation also well supported the optical changes of HD-1 in THF with the addition of water. The simulated absorption maxima of HD-1 (557 nm) is in excellent agreement with that of the experiment (566 nm). To investigate the induced aggregation of HD-1 and influence of water on the photophysical behavior of DPP dye, we modelled dimer2H2O (Figure 3b) and dimer-4H2O (Figure 3c). The UV-Vis spectra of these two dimer systems were obtained in THF solution employing implicit polarizable continuum model as implemented in Gaussian 09.35,36 The addition of explicit water molecules to HD-1 have not yielded any absorption beyond 600 nm as observed in the experiment. Therefore, we considered a dimer of HD-1 in the presence of water to model the induced aggregation of HD1. We modeled two dimer systems, one with two explicit water molecules, and another with four explicit water molecules to study the influence of increased water fraction on the optical behavior of DPP dye. To facilitate the H-bonding interactions, we have added the water ACS Paragon Plus Environment

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molecules in the proximity of the oxygen atoms of the DPP dye. By employing the time dependent DFT formalism, we have simulated the UV-Vis absorption spectra of the dimers in THF using implicit polarizable continuum model. It is interesting to note that these dimers have resulted in the absorption beyond 600 nm. Further, the intensity of the absorption peak ca. 640 nm is decreased as the water fraction increased, which is evidenced from the observed oscillator strengths (Figure 3d). The absorption beyond 600 nm is predominantly due to the HOMO LUMO excitation. Noteworthy, the observed redshifted absorption beyond 600 nm upon aggregation can be attributed to the reduced bandgap caused by the lowered LUMO level. The simulated eigenvalues of the frontier molecular orbitals of HD-1 and its dimers are given in Table S2. Inspired by the hydrochromic property of HD-1, we pursue imaging material for secret or hidden communications using water as ink source. To begin with, a filter paper used as substrate was coated with HD-1 solution and dried. HD-1dye coated paper appeared slight bluish in colour when seen under a UV lamp (365 nm) even without contact with water. Close to our experience, same phenomenon was observed by Sun et al. while working on polyoxometalate (POM) based rewritable paper.37 They notice that while writing with water the images smeared and were rather unclear. We observed that polyhydoxyl groups of paper interaction with the coated dye could be the reason for bluish colour under UV lamp. To presume we dried the dye coated paper on hot plate at 800C for few seconds and the paper then seen under UV lamp (365 nm) showed bright fluorescence. It was made clear that passivation of the hydrogen bonding of polyhydroxyls of paper was must to improve the contrast and quality of hydrochromic paper. For the passivation of paper substrate, a solution of PEG 20000 was coated first, followed by drying step and then HD-1 layer was coated by drop casting and dried (See Figure S6 for procedure). The resulting paper showed bright fluorescence and, a high contrast on exposure to water when seen under UV lamp (Figure 4a). Similar changes were observed in the solution experiment (Figure 4b).UV-Vis spectroscopy ACS Paragon Plus Environment


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was also used to probe the reversibility of colour change on paper (Figure 4c). In the absence of water, coated paper shows characteristic absorption maximum at 568 nm. The addition of water reduced the intensity of absorption maximum with an increase in the absorbance around 620 nm. Upon slow heating of paper at 600C to evaporate water, the spectrum returned to original within few seconds. These results were encouraging from the applications point of view. Upon introduction of water the fluorescence quenched which reappeared on subsequent evaporation of water. Considering the repeatability, fluorescence of the hydrochromic paper was recorded at 582 nm by repetitive writing with water and erasing by heating which showed that the rewritable hydrochromic paper could be used over 20 times with minimal loss in paper’s fluorescence intensity (Figure 4d).

Water responsive hidden Security mark to prevent counterfeiting A practical application of the water responsive fluorescent security marker is for checking the authenticity of currency and documents on the spot and therefore, helps in preventing currency counterfeiting and document duplication. Currently used currency notes use fluorescent labels which are not responsive to water and are duplicated and faked easily. The security marker label made using our dye compound responds to water exposure and as a result the fluorescence of the dye is quenched. Figure 5a shows how the marks made on the currency note stop fluorescing on contact with water. The first photo shows the currency note under normal light and the second picture shows the fluorescent security marks under UV light at 365 nm. In the third picture, fluorescent security mark changes colour on spraying with water. The water exposure currency note can be heated again to remove water and the currency becomes reusable. This invention provides a double level security protection for the currency. First, is the hidden fluorescent marks which are not visible in the normal light and the second, is colour change of the fluorescent label on exposure to water.


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As this hydrochromic property can readily be applied to currency notes, it would also be crucial to check if it is safe to use. To ensure that the dye used to make the Hydrochromic paper is hazardless, we have carried out the biological cell culture tests and studied the cell viability at different concentrations of the dye solution (Figure 5b). The cells used in this study are HeLa cells. HeLa cells are the oldest and most commonly used human cell line and is remarkably durable and prolific which warrants its extensive use in scientific research. These cells were cultivated at 37oC under humidified 5% CO2 in Dulbecco’s modified Eagle’s medium, supplemented with 10% fetal bovine serum and 1% penicillin–streptomycin. The cytotoxicity of the dyes was evaluated by determining cell viability after 24 h of incubation with various final concentrations (solvent only, 0.5 µg/ml, 1 µg/ml, 5 µg/ml and 10 µg/ml). The number of viable cells was determined by estimating their mitochondrial reductase activity using the tetrazolium-based colorimetric method (MTT conversion test) with a Microplate Reader at a wavelength of 570 nm. Results indicate that the dye is non-toxic to the cells, even at high concentrations (10 µg/ml) and, therefore, safe to use.

Inkjet printable Hydrochromic paper using water as Ink Against a background of increasing concerns over environmental pollution and deforestation, reuse of paper for printing and other documentation seems economical and environmentally pleasant solution. The Hydrochromic paper can be used to print secret messages or information using an ordinary inkjet printer. To do this, the cartridge of the printer was emptied and washed to remove any ink left in the cartridge and was then filled with water. The printer use only water as ink. This makes the invention more user friendly to government or private agencies which rely on the use of secret documentation or hidden communications. Figure 6 e-f shows the image of Merlion and a letter printed with inkjet printer using water. Sequential writing and erasing using water with PDS Stamps



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Since the fluorescent dye changes colour on exposure to water, an important application of this invention could be for secret communications. The Hydrochromic paper can be used to create images which may carry hidden information to the receiver and will be visible only when seen under UV light. The confidential messages created on the paper self destroy in hours time or can be destroyed on demand by heating and the paper becomes reusable again The paper can be used several times and just use water as ink to write. The hydrochromic paper can also be used to send hand written information. To do this, we filled an ordinary pen with water to make WATER-PEN and demonstrated writing different letters on the paper (Figure 6g-u and Figure S7 ). Images created on Hydrochromic paper self-erase within 7 h The time required to dry the message written on the hydrochromic paper was noticed by illumination under UV lamp. Figure 6j show that the message written on the hydrochromic paper can self destroy within 7 hours and the paper can be used again. The message can be destroyed on demand by heating the paper using a hot air gun or simply heating on a hot plate. In summary, we have presented a new approach towards rewritable imaging media based on reversible self-assembly of DPP dyes in water. The use of water as ink source is promising from environmental and safety point. The images created or handwritten showed high contrast with the substrate paper thus maintaining clarity, moreover, the images could be printed using an ordinary inkjet printer making this technique user friendly. The paper could be rewritten over twenty times and also, created images can survive several hours but the duration of the images can be easily enhanced by using a mixture of water and ethylene glycol. The designed hydrochromic paper is promising candidate for secret communications and anticounterfeiting.

Supporting Information


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Experimental section, synthesis of HD-1 and 1H NMR, additional supporting figures, optical characteristics, DFT calculation results and tables. “This material is available free of charge via the Internet at http://pubs.acs.org.”

Corresponding Author S. C. Tan ([email protected]) Notes The authors declare no competing financial interest. Acknowledgements S.C.T. acknowledges the financial support from MOE AcRF 1 (R-284-000-134-112 and R284-000-129-133).

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of

Fluorination

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Microstructure


and

Performance

of


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Diketopyrrolopyrrole-Based Polymer Solar Cells. J. Polym. Sci., Part B: Polym. Phys. 2017, 55, 49-59. 22. Qiao, Z.; Xu, Y.; Lin, S.; Peng, J.; Cao, D. Synthesis and Characterization of Red Emitting Diketopyrrolopyrrole-alt-Phenylenevinylene Polymers. Synth. Met., 2010, 160, 1544-1550.. 23. Jiang, Y.; Wang, Y.; Hua,J. L.; Qu,S. Y.; Qian, S.; Tian, H. Synthesis and Two Photon Absorption Properties of Hyperbranched Diketopyrrolopyrrole Polymer with Triphenylamine As the Core. J. Polym. Sci., Part A: Polym. Chem., 2009, 47, 44004408. 24. Dereka, B.; Rosspeintner,A.; Krzeszewski, M.; Gryko,D. T.; Vauthey, E. SymmetryBreaking Charge Transfer and Hydrogen Bonding: Toward Asymmetrical Photochemistry. Angew Chem. Int. Ed., 2016, 55 (50), 15624-15628. 25. Wurthner, F.; Thalacker, C.; Sautter, A. Hiearchial Organization of Functional Perylene Chromophores to Mesoscopic Superstructures by Hydrogen Bonding and pipi Interactions. Adv. Mater. 1999, 11, 754-758. 26. Jonkheijm, P.; Schoot, P. V.; Schenning,A. P. H. J;. Meijer, E. W. Probing the Solvent Assisted Nucleation Pathway in Chemical Self Assembly. Science, 2006, 313, 80. 27. Sautter, A.; Thalacker,C.; Heise, B.; Wurthner, F. Hydrogen Bond-Directed Aggregation of Diazadibenzoperylene Dyes in Low Polarity Solvents and the Solid State. PNAS, 2002, 99, 4993. 28. Kamlet, M. J.; Abboud, J.-L. M.; Abraham,M. H.; Taft, R. W. Linear Solvation Energy Relationships. 23. A Comprehensive Collection of the Solvatochromic Parameters, T*, a, and 0, and Some Methods for Simplifying the Generalized Solvatochromic Equation. J. Org. Chem., 1983, 48, 2877-2887.


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29. Fournier, J. A.; Carpenter, W.; De Marco, L.; Tokmakoff, A. Interplay of Ion-Water and Water-Water within the Hydration Shells of Nitrate and Carbonate Directly Probed with 2D IR spectroscopy. J. Am. Chem. Soc., 2016, 138, 9634-9645. 30. Shinokita, K.; Cunha, A. V.; Jansen, T. L. C.; Pshenichnikov, M. S. Hydrogen Bond Dynamics in Bulk Alcohols. J. Chem. Phys., 2015, 142, 212450. 31. Flom, S. R.; Barbara, P. F. Proton Trasfer and Hydrogen Bonding in the Internal Conversion of S1 Anthraquinones. J. Phys. Chem., 1985, 89, 4489-4494. 32. Fita, P.; Fedoseeva, M.; Vauthey, E. Ultrafast Excited State Dynamics of Eosin B: A Potential Probe of the Hydrogen-Bonding Properties of the Environment. J. Phys. Chem. A, 2011, 115, 2465-2470. 33. Richert, S.; Mosquera Vazquez, S.; Grzybowski, M.; Gryko, D. T.; Kyrychenko, A.; Vauthey., E. Excited State Dynamics of an Environment Sensitive Push-Pull Diketopyrrolopyrrole: Major Differences Between the Bulk Solution Phase and the Dodecance/Water Interface. J. Phys. Chem. B, 2014, 118, 9952-9963. 34. Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria,G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone,V.; Mennucci,B.; Petersson, G. A.; H. Nakatsuji, M. Caricato, X. Li, H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng, J. L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, J. A. Montgomery, Jr., J. E. Peralta, F. Ogliaro, M. Bearpark, J. J. Heyd, E. Brothers, K. N. Kudin, V. N. Staroverov, R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J. C. Burant, S. S. Iyengar, J. Tomasi, M. Cossi, N. Rega, J. M. Millam, M. Klene, J. E. Knox, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski, R. L. Martin, K. Morokuma, V. G. Zakrzewski, G. A. Voth, P. Salvador, J. J. Dannenberg, S. Dapprich, A. D.



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Figure 1. a) Molecular structure of HD-1. b) UV-Vis absorption changes of HD-1 in varied THF-water fractions (0-90) %, (c = 2 x 10-5 M). c) Plots of relative absorbance intensity at different THF-water fractions. d) Fluorescence changes of HD-1 in varied THF-water


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fractions, c = 1 x 10-8 M. e) Photographs of HD-1 solution in different THF-water fraction, under ambient light (top) and UV lamp (below).



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Figure 2. UV-Visible absorption changes of HD-1 in (a) THF/D2O (b) THF/Acetonitrile (c) THF/Methanol (d) THF/Ethanol, varied volume ratio (0 to 90%)


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Figure 3. a) Frontier molecular orbitals (HOMO and LUMO) of HD-1 computed at the B3LYP/6-31G(d,p) level of theory. b) Molecular geometry of the dimer-2H2O. Lateral view (left) and front view (right). c) Molecular geometry of the dimer-4H2O. Lateral view (left) and side view (right). d) Simulated optical changes of HD-1 in THF with the addition of water, (i) HD-1; (ii) HD-1 with two water molecules; (iii) HD-1 with four water molecules.



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Figure 4. a) Hydrochromic behaviour observed in solution form. b) Illustration of hydrochromic nature of dye coated paper. c) UV-Vis absorption change of hydrochromic paper upon heating at 600C. d) Plot of fluorescence intensity change in paper on repetitive erasing by heating at 600C and writing by spraying water, cycles.


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Figure 5. a) Demonstration of detection method for anti-counterfeitng. Images become nonfluorescent on contact with water. Images were seen under 365 nm UV lamp. b) MTT assay of cytotoxicity of HD-1 by the cell viability of HeLa cells incubated with various concentrations, 0.5-10 µg/mL.



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Figure 6. a-f) Inkjet printed images using water (a-d: schematic, e-f: digital photographs). g-j) Design drawn with a water filled pen (g-i: schematic, j: digital photographs). j, The image in the hydochromic paper self-erase in 7 hours. k-u) Images created on hydrochromic paper with water-ink using PDS stamps. ∆ means heat. (k-m: schematic, n-q: digital photographs, r-u: digital photographs under ambient light).


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Inkjet Printable Hydrochromic Paper for Encrypting Information and Anticounterfeiting

A stand-alone communication media to restraint secret information efflux and rising counterfeiting is developed. Essentially, high sensitivity to solubility parameter of the solvent is found to induce a visual colour change in the HD-1 dye. The effect originates from the hydrogen bond assisted non-radiative decay which is thermally reversible, thus embarks a hydrochromic property to the paper.


Inkjet-Printable Hydrochromic Paper for Encrypting Information and

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