Research Article Cite This: ACS Appl. Mater. Interfaces 2017, 9, 38032-38041
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Endowing Hydrochromism to Fluorans via Bioinspired Alteration of Molecular Structures and Microenvironments and Expanding Their Potential for Rewritable Paper Guan Xi,† Lan Sheng,*,‡ Ivan Zhang,† Jiahui Du,† Ting Zhang,† Qiaonan Chen,† Guiying Li,§ Ying Zhang,§ Yue Song,‡ Jianhua Li,∥ Yu-Mo Zhang,‡ 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 § Key Laboratory for Molecular Enzymology and Engineering of the Ministry of Education, School of Life Sciences, Jilin University, Changchun, 130012, P. R. China ∥ Department of Chemistry and Pharmacy, Zhuhai College of Jilin University, Zhuhai, 519041, P. R. China S Supporting Information *
ABSTRACT: Interest and effort toward new materials for rewritable paper have increased dramatically because of the exciting advantages for sustainable development and better nature life cycle. Inspired by how nature works within living systems, herein, we have used fluorans, as a concept verification, to endow original acidochromic, basochromic or photochromic molecules with broader properties, such as switchable with solvent, water, heat, electricity, stress, other force, etc., via simplified methods (i.e., via variation of submolecular structure or microenvironments). The hydrochromic visual change and reversible behavior of selected molecules have been explored, and the primary mechanism at the atomic or subatomic level has been hypothesized. In addition, several newly demonstrated hydrochromic fluorans have been utilized for water-jet rewritable paper (WJRP), which exhibit great photostability, high hydrochromic contrast, and fast responsive rate and which can be reused at least 30 times without significant variation. The water-jet prints have good resolution and various colors and can keep legibility after a few months or years. This improved performance is a major step toward practical applications of WJRP. KEYWORDS: rewritable paper, water-jet prints, hydrochromism, fluoran dye, dynamic equilibrium, microenvironment
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INTRODUCTION Paper plays a very important role in helping people read, observe, collect information, record, and communicate. However, in the past several decades, excessive consumption of paper and prints for human’s increasing needs has contributed to serious environment problems (e.g., deforestation, soil erosion, and global warming).1 Rewritable paper that can be used multiple times is an effective solution that has attracted increasing attention. Eminent researchers have demonstrated that materials with color change via heat,2,3 light,4−10 electricity,11,12 stress,13−15 pH,16,17 and water18−21 or other solvents22−24 can be key components for rewritable paper. When water-jet rewritable paper (WJRP) and its associated water-jet printing were first proposed and demonstrated,18 they excited global interest and media attention. WJRP incorporated hydrochromic dyes for printing rewritable paper, thereby protecting forest for better global environment. Water-jet printing used water as the only ink, which could be © 2017 American Chemical Society
refilled in the cartridge of the printer and which could avoid the frequent replacement of ink cartridge, integrate well with current inkjet printing techniques, and prevent blocking of the needle tip of the printer. However, there are some problems that hinder WJRP’s practical applications. First, hydrochromic materials are still very limited, containing photonic crystals19,25−27 and few dyes,16,18 which greatly restricts the selection of colors for desired water-jet prints. In addition, the existing “hydrochromic ink”28 is actually white pigment and can undergo water induced physical change from opaque to transparent to reveal the hidden color. Though it is currently used in color changeable umbrellas or raincoats, etc., it is not suitable for water-jet printing and WJRP because the quantity of water needed to reveal the color is extremely large and the Received: August 17, 2017 Accepted: October 12, 2017 Published: October 12, 2017 38032
DOI: 10.1021/acsami.7b12363 ACS Appl. Mater. Interfaces 2017, 9, 38032−38041
Research Article
ACS Applied Materials & Interfaces color contrast is low. Second, the retention time of current water-jet prints (22 h) is not long enough for specific needs. Third, the stability of the WJRP, referred to its resistance to the impact of moisture in the atmosphere and oxidation catalyzed by light, is still not good enough. Especially, the current hydrochromic magenta dye is too sensitive to ambient moisture, and as a result, the paper made using it easily changes to magenta prior to use. Therefore, it is highly desirable to discover new reversible hydrochromic dyes whose colors are more tunable and with great photostability. Learning from the ingenious solutions of nature is always a good choice to solve problems. Even though all the cells in a human body have been constructed by limited amino acids and base pairs, they endow enormous capabilities to respond to external stimuli, such as light, pressure, heat, sound, electricity, media, and so on. The same protein could have various functions to meet different needs. For example, it was found that RuBisCO had different enzymatic capabilities, as carboxylase for photosynthesis29 and as oxygenase for photorespiration.30 In addition, it is known that one molecule can possess various characteristics, expanding its applications in different areas. For instance, some photochromic molecules also have other functions, such as thermochromism,31 mechanochromism,32,33 acidochromism,34 hydrochromism,18,35 etc. This inspires us to explore more applications for molecular switches with superior performance, which is less common and mostly ignored before, to resolve practical problems and meet human needs. The next question is why different stimuli induce different responses in the same molecule in nature. Recent discoveries from outstanding colleagues36−40 and from our work33 indicated that isomerization of molecules can exist anywhere even at low temperature or in constrained crystal state. Additionally, newly reported exciting results in the molecular dynamics and quantum field41−43 enable us to be aware that dynamic equilibriums of structural alterations and tautomerizations seem to be essential behavior of molecules, both in biological macromolecules and unnatural small molecules. Stimulus (e.g., light, heat, force, acid) is a way to provide energy, which promotes dynamic equilibriums toward more favorable isomers among their dynamic intermediates resulting in a physical or chemical change at macro level. Moreover, the position of established complex equilibrium depends primarily on molecular structures and their microenvironments.44 All of these provide a theoretical basis for us to develop different types of molecular switches triggered by desired stimulus. Water, as a gift of nature, is involved in almost all biological activities on earth. It exists as both proton donor and acceptor with relatively large and dynamically ionizable dipoles within its intermolecular three-dimensional mobile network while interacting with other polarizable or ionizable molecules or molecular subunits.45,46 These processes of polarization or ionization are commonly accompanied by release of energy, which offers the possibility of using the dynamic proton donor or acceptor characteristics of water to modulate the equilibrium position of various molecular switches to realize hydrochromism (Scheme 1). However, hydrochromic dyes have been demonstrated only on limited molecules so far, such as oxazolidines18 and special spirooxazines,47 which are used as photochromic or acidochromic dyes.48,49 Theoretically, based on dynamic equilibriums, molecular switches, photochromism, acidochromism, basochromism, electrochromism, or mechanochromism, are capable of having hydrochromism with or
Scheme 1. Water as Proton Donor or Acceptor to Modulate the Equilibrium of Acidochromic or Basochromic Dyes to Realize Hydrochromism
without small structural modifications. Therefore, various molecular switches that were previously ignored, especially acidochromic and basochromic molecules, might have potential as hydrochromic dyes (Scheme 1). With this conjecture, fluoran dyes, as well-known and typical acidochromic and basochromic molecules,50−52 which also have excellent photostability, widely adjustable color range, and high molar absorption coefficients, may also be suitable hydrochromic dyes. However, systematic study on the hydrochromic properties of the fluoran dyes and their applications in rewritable paper is undeveloped so far. Herein, we selected/designed three classes of simple fluoran derivatives and prepared/synthesized six simple known or new molecules as examples, to verify our hypothesis of endowing the acido-/basochromic dyes with hydrochromism to suit human needs. First, we investigated effects of the molecular substituents and microenvironments on dynamic equilibrium of their existing stable states and tried to understand the switching mechanism from the atomic or submolecular level. Then, we screened them for proper hydrochromic dyes in WJRP. Thereafter, water-response rate, stability, and reversibility of the WJRP were investigated systematically. The retention time of the water-jet prints, which is the most important property for practical use of WJRP, was prolonged. Finally, legibility and resolution of the water-jet prints were tested on fluoran-based WJRPs.
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RESULTS AND DISCUSSION The three classes of fluoran molecules, such as rhodamine B (RhB), one dye black (ODB), and fluorescein (FLu), that are known as acido- or basochromic dyes have been fully investigated for fluorescent probes or display.53−57 They might be good candidates for obtaining the desired hydrochromism. According to our results and literature reports, the colored isomer of RhB is its preferential configuration in most cases,50 while ODB prefers its colorless lactone form and transforms to its colored zwitterion form only in strong acid.52 The general schematic diagram for dynamic equilibriums of fluorans between their colorless forms and colored forms, as well as their corresponding isomers, is shown in Figure 1a. The position of the equilibrium can be modulated via the functional groups or microenvironments. Most reports about the modification on the fluoran dyes are at the 2′ position for the switchable carboxyl group (Figure 1b).56,57 Considering that inter- or intramolecular dipole−dipole interactions among adjacent multiple atoms or subatoms are usually affected by their substituents, which further influence the existing equilibrium between their lactones and zwitterion forms. A strategy of introducing electron-donating (e.g., −NH2) or electron-withdrawing groups (e.g., −NO2) to phthalide ring of the fluoran dyes on the uncommon 3′ position was concerned (Figure 1b). It might hopefully change 38033
DOI: 10.1021/acsami.7b12363 ACS Appl. Mater. Interfaces 2017, 9, 38032−38041
Research Article
ACS Applied Materials & Interfaces
Figure 1. (a) Schematic representation of the equilibria between the neutral lactone forms and zwitterion forms of fluoran dyes. (b) The position of modification on phthalide ring of fluoran dyes in this work and literatures, respectively. (c) Structures of the investigated three classes of fluoran dyes.
the equilibrium position of fluorans, endow hydrochromism to them, and meanwhile the color of their ring-open conjugates would not be affected obviously. In view of the above considerations, six known or new fluoran molecules with/ without modification at 3′ position, which is adjacent to lactone (−CO−O−) subunits of the fluorans, that are RhB, NH2-RhB, NO2-RhB, ODB, NO2-ODB, and FLu, were selected/designed and prepared/synthesized (Figure 1c and Supporting Information). Quick solution tests of hydrochromism for these six molecules were conducted in MeCN and EtOH, respectively. From Figure 2a, we can find that all the molecules show obvious hydrochromic properties with good color intensity except ODB. Substituents at the 3′ position of fluorans play important roles on equilibrium of their lactones and zwitterions in different media (or microenvironments, that is, selected aprotic or protic solvents), and further affect their hydrochromism (Figure 2a and Figure S2−S4). Electron-donating groups (i.e., −NH2) make the fluorans more favorable to exist as colorless lactone forms (i.e., NH2-RhB versus RhB, RhB exists partially as zwitterionic form in EtOH showing obvious pink color), while electron-withdrawing groups (i.e., −NO2) accelerate the equilibrium shift to their colored zwitterionic forms (i.e., NO2-RhB versus RhB or NO2-ODB versus ODB; NO2-RhB exists partially as zwitterionic form even in MeCN showing pink color). This is presumably due to strong electronwithdrawing property of the nitro group next to its ortho lactones (−CO−O−), which can increase homodromous interatomic dipole−dipole interaction within the molecular subunits (i.e., −NO2 and −CO−O−) under influences of associated dipole media via dynamically variational van der Waals (VDW) force. The homodromous vibration further increases polarization and ionization of its C−O bond within phthalide ring, which facilitates to decrease activation energy for ring opening of fluorans (Figure 2b left and Movie S1). Whereas for the strong electron-donating amine group next to
Figure 2. (a) Photographs of different fluoran dyes in MeCN, EtOH (2 mL, 1 × 10−5 M) before (left) and after (right) addition of 2 mL of H2O at ambient conditions. (b) Schematics of homodromous and heteodromous dipole−dipole interaction among −NO2/−NH2, solvents of MeCN and their ortho lactones (−CO−O−) for NO2RhB and NH2-RhB, respectively. (c) Plots of maximum absorbance intensity for RhB, NH2-RhB, NO2-RhB, ODB, NO2-ODB, and FLu in visible region in their zwitterionic forms (C = 1 × 10−5 mol L−1) in MeCN/H2O against increasing percentage of water by volume from 0% to 90%.
its ortho lactones (−CO−O−), heterodromous interatomic dipole−dipole offsetting interplay between adjacent subunits (i.e., −NH2 and −CO−O−) will surely decrease dynamic polarization and ionization of C−O bond within phthalide ring under influences of associated aprotic dipole media (Figure 2b right and Movie S2). This undoubtedly makes the zwitterionic forms of fluorans less stable except with help of partially protic microenvironment, which could alter original counteraction among adjacent subunits via media induced dynamically variational VDW force, or activation of strong ionizable polarity and resonated dynamic hydrogen bonding, like water or acid. These assumptions are strongly supported by theoretical calculations. Wherein, the calculated Gibbs energy barriers between lactone forms and transient states for both RhB and NO2-RhB are very low (5.87 and 5.69 kcal mol−1, respectively), indicating the ring-opening reactions for both of them are kinetically favorable. However, the free energy barrier between lactone forms and zwitterion forms for NO2-RhB (ΔG2 = 2.59 kcal mol−1) is lower than that of RhB (ΔG2 = 5.27 kcal mol−1), 38034
DOI: 10.1021/acsami.7b12363 ACS Appl. Mater. Interfaces 2017, 9, 38032−38041
Research Article
ACS Applied Materials & Interfaces which further indicates the zwitterion form of NO2-RhB is more stable than that of RhB. Meanwhile the zwitterionic form of NH2-RhB does not even exist in MeCN (Table S2 and Figures S5 and S6). To further investigate the relationship between structure and hydrochromic property of molecules, we studied the variation curve of their intensity of maximum absorption peaks for six fluoran dyes against water content in detail in a binary MeCN/ H2O solvent system. From Figure 2c, the hydrochromic properties of their water-sensitivity, color intensity (or molar extinction coefficient) and solubility are easy to be compared. The color intensity of RhBs (i.e., NO2-RhB, RhB, and NH2RhB) is better than that of FLu and ODBs (i.e., NO2-ODB and ODB) (Table S3). This indicates that better and stronger electron donating substituents (i.e., −N(CH2CH3)2) within large extended conjugated dipole of zwitterionic forms of fluorans can significantly enhance their color absorption efficiency. In the case of NO2-RhB versus RhB or NO2-ODB versus ODB, results indicate that electron-withdrawing substituents (i.e., −NO2) within the fluorans can surely improve water-sensitivity and solubility of the molecules. The extreme sensitivity of NO2-RhB and RhB to water and/or other protic media might be useful for some highly sensitive moisture sensor, since the water content needed for converting to their maximum colored forms is only ∼10 v/v %. The adverse effect of these highly sensitive hydrochromic dyes (i.e., RhB and NO2-RhB) is that they seem over sensitive to water, and their WJRPs will poorly resist to humidity, which is similar to previously reported hydrochromic oxazolidine dye 2.18 FLu turns from colorless to yellow gradually with water content increasing. These results indicate that NH2-RhB, NO2-ODB, and FLu may be suitable hydrochromic dyes for WJRPs because of their remarkable properties of moderate watersensitivity and high color intensity. In addition, the waterinduced color of these fluorans in solution nearly has no change over half of a month, which indicates the hydrochromic fluoran dyes have very great stability (Figures S7 and S8). From above discussions and our previous work,18 we can conclude that (1) acidochromic, basochromic, or photochromic molecules modified with suitable subunits (i.e., substituents) or in specific microenvironments (or media) can be potential hydrochromic dyes and that (2) introducing electron-donating groups to phthalide ring of hydro-sensitive fluorans (e.g., RhB to NH2-RhB) or electron-withdrawing groups to phthalide ring of hydro-passive fluorans (e.g., ODB to NO2-ODB) is an effective method for designing suitable fluoran hydrochromic dyes; 3) the variation for intensity of the maximum absorption peaks against water content of dyes is S curve, which seems to be more suitable hydrochromic dyes for WJRPs (such as NH2RhB, hydrochromic oxazolidine dyes 1 and 4).18 These results offer good reference for designing and screening suitable hydrochromic dyes for WJRPs. We take NH2-RhB as an example of acidochromic dye to show in detail the reversible hydrochromic behavior between its lactone and zwitterion isomers in mixed solution of MeCN/ H2O wherein water used as proton donor (Figure 3a). Without water, the solution was colorless and three hump absorption bands can be observed only in UV region (
