Highly Tunable Multicolor Water-Jet Rewritable Paper Based on

Oct 18, 2018 - Rewritable paper based on switchable molecules has attracted great attention in both academic research and marketplace. However, most ...
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Applications of Polymer, Composite, and Coating Materials

Highly Tunable Multi-color Water-jet Rewritable Paper Based on Simple New-type Dual-addressable Oxazolidines Tianyou Qin, Lan Sheng, and Sean Xiao-An Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b13660 • Publication Date (Web): 18 Oct 2018 Downloaded from http://pubs.acs.org on October 22, 2018

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Highly Tunable Multi-color Water-jet Rewritable Paper Based on Simple New-type Dual-addressable Oxazolidines Tianyou Qin,† Lan Sheng,*‡ and Sean Xiao-An Zhang*†

† State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun, 130012, P. R. China.

‡ College of Chemistry, Jilin University, Changchun, 130012, P. R. China.

KEYWORDS. rewritable paper, hydrochromism, halochromism, multicolor display, inkfree printing.

ABSTRACT. Rewritable paper based on switchable molecules has attracted great attention in both academic research and marketplace. However, most available

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switchable dyes have single switchable color state only, which can’t meet the longawaited multicolor reversible displays. Herein, through simple introduction of phenolic hydroxyl group, we develop a series of new oxazolidines with one switch unit which could reversibly display two different as well as their mix-gradient colors by treating with water and mild acid, respectively, both in solution and in a solid substrate. The structures and mechanism for formation of the two colors had been studied in detail via UV-Vis/NMR spectroscopy, skillfully designing contrast molecules, and kinetics experiments. This multiple switchable colors of the dyes have been further applied to construct rewritable paper for ink-free printing with multi-/gradient-color display.

Introduction

Oxazolidines (OXs) are a class of unique and important molecular switches with various applications, such as, enhanced nonlinear optical materials,1,2 ion/small molecule detection and label,3-5 electrochromic materials,6 CO2 capture,7 due to their interesting properties of photo-/electro-/acido-addressability. Recently, their hydrochromic property

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was also discovered and successfully applied to water-jet rewritable paper.8 Although OXs show great potential in these important fields, they only show single switchable color between their ring-open and ring-closed state under external stimuli (i.e. light, electricity, acid, water). Such mono-color switching ability greatly limits their development in multi-addressable applications, for instance, logic gates and multi-color display.

In general, combination of different color-switching units into a single molecule is an effective way to endow molecules with multi-stimuli responsive properties.9-12 And this method really expands the application of OXs.13-16 However, design and synthesis of simple molecules with only one switching unit capable of multichromic display is still a challenge and there are limited successful examples.17,18 In addition, most of the reported works only exhibited multicolor switching in solution, and rarely demonstrated their color switching in a solid substrates because complications caused by the surrounding microenvironment present in the solid state, wherein the molecules may operate differently than they do in solution.

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Above mentioned considerations and our interest in developing multi-addressable switch dyes drive us to elaborate simple multi-stimuli-responsive OX systems to fulfill the requirements for multi-color display both in solution and solid state, which would be beneficial to achieve the long-awaited goal for multi-color reversible display materials with simple molecule.8,19-29 Inspired by the observation from our previous work8 that substituents on conjugated skeleton would have great influence on ring-opening ability and color display of OXs, a facile strategy that introduction of easily dissociated phenol hydroxyl group accompanying with changes of electro negativity onto skeleton of OXs for endowing them with multi-color switching ability has sprouted.

In this work, we designed and synthesized a series of simple OX molecules containing a phenolic hydroxyl group to demonstrate the strategy. The introduction of phenolic hydroxyl group not only retains halochromism of OXs, but also is beneficial to improve their hydrochromism via dissociation of phenolic hydroxyl, which would result in a change of π-conjugation in ring-open forms of OXs and provide a different color from

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their halochromism (Scheme 1). Furthermore, these OXs were managed to apply in multi-color display rewritable paper.

Scheme 1. Schematic representation of the design for OXs.

Results and Discussion

Halochromic and hydrochromic properties of 1a in acetonitrile (CH3CN) solution were investigated first. As shown in Figure 1a, the colorless solution displayed different colors after adding acid (i.e. trifluoroacetic acid (TFA)) and water, respectively. That is, yellow

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for halochromism and magenta for hydrochromism. The former is accompanied by appearance of an absorption band centered at 453 nm, which could be modulated by adding different equivalent TFA (Figure 1a, yellow lines); the latter generated a new absorption band centered at 556 nm, whose intensity enhanced gradually with increasing water content (Figure 1a, magenta lines). These results suggest that 1a transformed to different structures after treating with acid or water as expected. To confirm the role of hydroxyl (-OH) in these color- and structure-switching processes, 2a and 2b which have similar structure to 1a with just different donor groups (i.e. -OMe, -OCH (CH3)2) instead of -OH on ethylene phenyl were synthesized and studied as control ones.30 Similar phenomena were observed for the case of 2a and 2b after addition of TFA, whose absorption bands around 450 nm (Figure 1b and 1c, yellow lines). Increasing the water contents of CH3CN solutions, there were little color and weak absorption band changes observed for 2a and 2b. Besides, the generated absorptions for 2a and 2b were around 400-500 nm, similar to their halochromic spectra. These weak absorptions indicate that the hydrochromism of 2a and 2b were seriously inhibited because of the poor donor groups (i.e. -OMe, -OCH(CH3)2) on their

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ethylene phenyls, according to our previous work.8 Conversely, these results suggest that a stronger donor group than phenolic hydroxyl, that is phenolate anion, on the ethylene phenyl endows hydrochromism to 1a. On the basis of color and spectral changes, structures for halochromism and hydrochromism were given as 1a-H and 1aW, respectively (Figure 1a). As a result, our strategy that introduction of easily dissociated phenolic hydroxyl group with changes of electro negativity onto OXs brings their hydrochromism different from their halochromism works. It’s also noticed that the OH should be at the para-position of the phenyl ring which could better participate in conjugation of the donor-acceptor structure (Figure S1).

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Figure 1. UV-Vis spectra of a) 1a, b) 2a, c) 2b in acetonitrile (C = 1 × 10-5 M) upon addition of varying equivalent of TFA and in variable mixtures of acetonitrile and water with increasing percentage of water (C = 1 × 10-5 M).

Our speculation on different structural changes of 1a with addition of acid and water was further supported by 1H NMR spectroscopy (Figure 2). Due to the serious overlap of the signals for 1a in the mixture of deuterated acetonitrile (CD3CN) and water (D2O) (Figure S2), we carried out the NMR experiments in deuterated methanol (CD3OD), in

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which 1a could not only undergo halochromism and hydrochromism as same as in acetonitrile, but also more sensitive to water (Figure S3). 1a was observed to be completely in its ring-closed form in deuterated methanol (CD3OD) (Figure 2a). When adding deuterated TFA (TFA-d) to the CD3OD solution, 1a transformed to its ring-open form 1a-H completely (Figure 2b). Upon introduction of deuterium oxide (D2O), a new set of signals which was similar to the spectrum of 1a-H was observed (Figure 2c, magenta). Howbeit these signals were shifted to an upfield region relative to the spectrum of 1a-H. Especially, the aromatic proton f and g (magenta) were shifted to an upfield region relatively to corresponding signals of 1a (black). This NMR analysis further confirms the formation of a ring-open form 1a-W with phenolate anion as stronger donor group.

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Figure 2. Partial 1H NMR spectra of 1a in a) CD3OD, b) CD3OD with CF3CO2D (1.5 eq.), c) CD3OD / D2O (1 / 4, v / v).

To further gain insight into the hydrochromic mechanism of 1a, kinetics for effects of concentration and water content on the hydrochromism were studied. The kinetics fit well with mono-exponential curves (Figure S4 and S5) and specific rate constants were calculated and shown in the right of Figure 3. Increasing the concentration of 1a could

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decrease the hydrochromic rate of 1a (Figure 3a). This may be the result that increase of intermolecular interaction among 1a molecules at high concentration leads to a decline in the dissociation degree of phenolic hydroxyl to phenolate anion for hydrochromism of 1a.31 The kinetic data also shows that the hydrochromic rate of 1a would be accelerated by increasing the water content, which facilities dissociation of phenolic hydroxyl (Figure 3b). Interesting, it was observed that increasing 10 percent water content would almost double the kinetic rate constant of hydrochromism, a phenomenon which is similar to the effect of temperature on reaction rate constant.32 Noticeably, the hydrochromic rate of 2a (instead of -OH in 1a with -OMe) was very slow (Figure S6). These results demonstrate that the dissociation of phenolic hydroxyl group would play an important role in the hydrochromic ring-opening reaction of 1a. It is worth mentioning that the degree of dissociation for phenolic hydroxyl of 1a is very low according to its dissociation constant (pKa ≈ 10)33, howbeit the mount and rate for the formed 1a-W isomer upon addition of water were very large and fast. Its hydrochromic rate is in the same order of magnitude with the result of 3a which contains a group (i.e. -N(CH2CH3)2) having similar electronic donating ability to oxygen anion on

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the benzene ring without dissociation process (Figure S7). Additionally, the formation of similar hydrochromic ring-open form of 1a could also be achieved in MeCN by adding propitiate mount of triethylamine (Figure S8). Hence, there should be some base that could promote the dissociation of phenolic hydroxyl group of 1a. According to previous work8, the quick dynamic equilibrium between ring-closed form (RCF) and ring-open form (ROF) of these oxazolidines could happen in solution at room temperature no matter with or without water. Due to the ROF containing an ethanolate group (CH2CH2O-), which should be alkaline, we called them as transient bases and demonstrated its possibility of nucleophilic addition with CO27. Thus, to our understanding, the initial base is the transiently formed ROF of the oxazolidine dyes (i.e. 1a – 1d) without dissociation of phenol hydroxyl. This speculation was further confirmed by the fact that the absorption bands around 400-500 nm indicating ROFs of 2a and 2b have a slight increase as water content increases (Figure 1b and 1c). Taking account of that ethanolate group is well-known sensitive to water (e.g. hydrolysis of sodium ethoxide can generate sodium hydroxide) and there are plenty of water around the molecules, thus it’s feasible to occur the reaction between the transient base of ROF

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and water resulting in formation of ROF of 1a with phenolic hydroxyl group and hydroxyl in the solution. On the one hand, as the small size and strong alkalinity of the hydroxyl, it rapidly and continuously promotes the dissociation of other ring-closed 1 and subsequent hydrochromic ring-opening processes. On the other hand, dissociation of ROF of 1 with phenolic hydroxyl group for formation of 1a-W could occur as well. We speculate that dissociation and the ring-opening processes are mutual promotion and interdependence, just like a pseudo synergistic reaction (Figure S9). All of these processes facilitate the mount and rate for the formed 1a-W isomer upon addition of water were so fast that seems once generating the ring-open form by introduction of water, the hydrochromic ring-opening process would take place continuously and quickly.

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Figure 3. Kinetic data of a) different concentration of 1a in 99.9% H2O and b) 1a (C = 1 × 10-5 M) in CH3CN / H2O with different volume ratio.

To further verify our strategy, derivatives 1b – 1d with -OH on vinyl benzene ring were synthesized and investigated. Upon introduction of acid and water, they also could undergo halochromism and hydrochromism, respectively (Figure 4). Addition of TFA to

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the solution of 1b – 1d, the colorless solutions changed to yellow color and the new corresponding absorption bands around 430 nm could be adjusted by modulating the volume of TFA (Figure S10), which were the same with 1a. Interestingly, for 1b, a concomitant absorption band around 500 to 600 nm for its hydrochromic form also appeared after addition of 1 equivalent of TFA to its MeCN solution. It might be because the moderately increased hydrogen-donating capacity of the solvent environment upon addition of 1 equivalent of TFA is beneficial to hydrolysis of phenol hydroxyl within 1b which further facilities hydrochromism of 1b. This speculation was further confirmed by the fact that 1b also has absorption band around 500 to 600 nm for its hydrochromic form in protonic solvents (e.g. ethanol, methanol and glycol) with a certain hydrogen-donating capacity, while that band didn't appear in aprotic solvents (e.g. CH3CN, ethyl acetate and tetrahydrofuran) with poor or nearly no hydrogen-donating capacity

(Figure S11). Increasing water contents in CH3CN, the

colorless solutions of 1b, 1c and 1d gradually changed to blue, magenta and orangered, respectively. In particular, the orange-red color solution of 1d induced by water arose from the mixed absorption bands appeared at 430 and 541 nm (Figure 4c), which correspond to the ring-open forms with phenolic hydroxyl and phenolate anion as donors, respectively.

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Figure 4. UV-Vis spectra of a) 1b, b) 1c and c) 1d in acetonitrile (C = 1 × 10-5 M) upon addition of excess TFA and in variable mixtures of acetonitrile and water with increasing percentage of water (C = 1 × 10-5 M).

Encouraged by the color switching behavior of 1a – 1d in solution, the color-switching properties of 1a – 1d were investigated next in a solid matrix. Drawing on the experience of our previous work, a three-layer structure of paper was designed. Filter paper was chosen as the substrate upon which 1 was loaded (Figure 5a). The filter paper acting as matrix layer 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 of the dye. PEG was also introduced into the next imaging layer with dye in order to prevent its aggregation. We next examined the halochromic and hydrochromic behaviors of the 1a-based paper, which were monitored by UV-vis reflectance spectroscopy with integrating sphere (Figure 5b). In the absence of any external stimuli, the paper appears white. Treating the paper with volatile acetic acid (AcOH), acting as a weak and safe acid, instead of TFA, resulted in the paper taking on

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a yellow color, and a reflectance band was observed in the visible region around 464 nm. It indicates that 1a loading on the paper can still retain the halochromic property. Upon introduction of water, the paper displayed a magenta color accompanied with appearance of a reflectance band centered at 564 nm. Based on the understanding of the mechanism in solution, these changes of color and spectrum would arise from the formation of 1a-H for the former and 1a-W for the latter, respectively. Removing out acid and water by heating, the colored paper could be returned to its initial state. After 10 times reversible write-erase cycles, the paper still works well (Figure 5c, d). Microscopy images reveal that aggregation was not observed on the paper substrates before and after treatment with water and acid for developing the yellow and magenta colors, respectively (Figure 5e). Meanwhile, it’s convenient to obtain blend colors by tuning the concentration of acid treating on our paper, which accompanied with intensity changes of absorption peak at 464 and 564 nm (Figure S12). These experiments demonstrate that the 1a-based paper could act as a potential multi-color rewritable material. Moreover, the color of the paper is also pH sensitive (Figure S13). It could continuous display yellow, orange, pink, magenta and white colors during the pH from 1 to 11, the

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nature which endows its potential application in the field of pH detection like pH paper. Papers loaded 1b – 1d using similar procedures to 1a were also prepared and they still took on the hydrochromic and halochromic properties. Without external stimuli, the paper was almost colorless. Upon introduction of water and acid, the paper can also exhibit two different colors, respectively (Figure 5f – 5h and S14).

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Figure 5. a) Schematic illustrations of the three-layer structural design for the rewritable paper. b) Reflectance UV−vis spectra of the paper integrated with 1a before (gray trace) and after addition of water (magenta trace) and 5% acetic acid (yellow trace). A plot of

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the reflectance versus the number of cycles as the paper is cycled through c) water (564 nm) and d) acetic acid (464 nm) spraying (write) and heating at 70 oC (erase), respectively. e) Images of the initial state, after addition of water (magenta state) and after treating with 1% AcOH (yellow state) of the 1a based paper as imaged under a microscope. Scale bar: 200 μm. Reflectance spectra and photos for the halo- and hydrochromism of f) 1b, g) 1c and h) 1d on paper.

To further confirm the practical application of our rewritable paper in multi-color display, we next managed to print colorful patterns in rewritable paper with current inkjet technology by replacing the ink with water and 1% AcOH aqueous solution (Figure 6a). Taking rewritable paper loaded 1a as an illustration, we could print magenta (Figure 6b) and yellow (Figure 6c) color patterns with water- and acid-jet printing, respectively. Furthermore, elaborate patterns displaying multiple and gradient colors could also be achieved by tuning the water- and acid-jet dosage to regulate the concentration of acid printing on paper (Figure 6d, S12). Noticeably, these colorful pictures can be easily erased by heating. The higher the heating temperature, the faster the erasure. At 70 oC

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(on a heating platform), it takes ca. 15 to 20 seconds for erasing clearly the color patterns. Moreover, in our experiment, we found that the heat generated by a printer is enough to erase the patterns. The retaining time of prints is another important factor we really care about. We found that the printed patterns only can be stored for 30 minutes at ambient condition (Figure S15). Draw on the experience of our previous work27, introduction of PVA in our material can prolong the display time to several days which is long enough to be used in practice (Figure S16). Yet, the color contrast needs to be improved further. At last, the stabilities of these dyes in solution and paper substrate have also been studied (Figure S17-S20). Results show that these dyes are very stable in dark within two weeks, while they would undergo different degrees of photodegradation under the extreme conditions (i.e. under sunlight). Especially for 1a and 1b, they almost lose the halochromic properties in MeCN after exposure to sunlight for 6 days. It seems that substituent groups have great influence on the photooxidation properties of such oxazolidine dyes in solution. That is the structures of 1a and 1b within stronger donor-acceptor than that of 1c and 1d causes their poorer photostabilities. However, under the common office light environment (i.e. indoor light), these dye molecules are relatively stable both in solution and on paper substrate. Therefore, it's better to keep our materials in dark and avoid exposure to sunlight when not in use. According to our previous work34, ultraviolet light is the main cause of photodegradation for oxazolidine dyes. Improvement of photostability of our materials by introduction of ultraviolet absorber additives will occupy an important position in our future work.

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Figure 6. a) Schematic illustration of practical application of the 1a based paper in multicolor ink-free printing. Examples of prints on the rewritable paper by b) pure water-jet printing for magenta color, c) 1%AcOH aqueous-jet printing for the yellow color, and d) water / 1%AcOH -jet combination printing for multiple colors. The numbers 1 (4) and 3

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indicates to colors obtained by water-jet printing with pure water and 1% AcOH aqueous solution, respectively; 2 indicates to a gradient color obtained by water / 1%AcOH -jet combination printing.

Conclusion In summary, we have designed and synthesized a series of new and simple OX switches through introduction of a para phenolic hydroxyl group which could exhibit halochromic and hydrochromic properties both in solution and a paper-based substrate displaying dual-color upon addition of mild acid (i.e. 1% AcOH) and water. The generation of phenolate anion as a strong conjugated donor group via dissociation of the phenolic hydroxyl group in the OXs was proved to play an important role in achieving their hydrochromism and leading to the structures and colors different from their halochromism. These results had been demonstrated by UV-Vis and NMR spectroscopy as well as skillfully designing contrast molecules. The hydrochromic mechanism was understood in detail through the kinetic experiments which suggest that increase of dissociation degree of 1a via raising the water content and decreasing the

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concentration of 1a would be conductive to accelerate the hydrochromism for 1a, and that the two-step hydrochromic reaction of 1a is quickly enough to be a pseudo synergistic reaction. Moreover, the switchable multi-color display extends the potential of these OXs to "water-jet" rewritable paper and ink-free printing. The rewritable paper has good reversibility, high resolution, and exhibiting multicolor display, as well as well compatible with current ink-jet technology. The strategy utilized herein demonstrates a promising approach for designing new and simple multi-addressable molecular switches, which could be further applied in multi-color display materials, logic gates and multi-information storage.

ASSOCIATED CONTENT

Supporting Information. The following files are available free of charge. Materials and instruments, detailed experimental procedures, characterization data for

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the molecules in this work. The other data mentioned in this work, such as the absorption spectra, reflective spectra and kinetic tests. (file type, i.e., PDF)

AUTHOR INFORMATION Corresponding Author

* [email protected] (S.X.-A.Z.)

* [email protected] (L.S.)

ORCID

Sean Xiao-An Zhang: 0000-0002-8412-3774

Lan Sheng: 0000-0003-1960-3111

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT

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We thank the National Postdoctoral Program for Innovative Talents (BX201600063) and the National Natural Science Foundation of China (No. 51603085, 21572079) for financial support.

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Table of Content. A highly tunable multi-color water-jet rewritable paper based on simple dual-addressable oxazolidines has been developed. Through simply introduction of phenolic hydroxyl group, the oxazolidines with only one switching unit can display two different as well as their multi gradient colors in paper substrate via “water-jet” printing.

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