Reversible coloring/decoloring reactions of thermochromic leuco dyes controlled by a macrocyclic compound developer

In this study, we examine macrocyclic compounds to determine whether they can provide a safer replacement and stable complex for BPA in thermochromic dyes. To achieve this objective, a series of macrocyclic compounds, Methyl-N-benzylhexahomotriazacalix[3]arene (MeAC3), p-Chloro-N-benzylhexahomo-triazacalix[3]arene (ClAC3), α-Cyclodexdrin (α-CD), β-Cyclodexdrin (β-CD), p-tert-Butylthiacalix[4]arene (TC4), Calix[4]arene (C4), and Resorcin[4]arene (RC4), were synthesized. Among these macrocyclic compounds, RC4 was determined to be the most appropriate candidate to replace BPA as the developer material used in thermochromic dyes. In tests of prepared thermochromic dyes, RC4 had results similar to those of BPA, achieving the best protonation/deprotonation equilibria and providing a dark contrast with the thermochromic dye. DFT calculations also showed stable complexes between RC4 and CVL via hydrogen-bond interactions.


Introduction
Thermochromic (temperature-sensitive) dyes (TC) are materials which change color as a function of temperature [1]. They can be used in several different applications such as sensors, thermal indicators, memory storage devices, security inks, and dyes for solar cells and other luminescent switches of solid-state materials [2][3][4][5]. Normally, leuco dye-based TC composites have three components, including a color former (leuco dye), a color developer, and a solvent. The color changes in this system result from two competing reactions, one between the dye and the developer and the other between the solvent and the developer. Common color formers, such as spirolactone, fluoranes, spiropyrans, or fulgides, are electron-donating compounds [6]. One of the various types of color formers, crystal violet lactone (CVL), is a classic halochromic dye which has been widely used as a chromogenic reagent in thermochromic systems [7]. Bis-phenol A (BPA) has been intensively used as a color developer because of its efficiency and low cost [8]. However, previous research has shown that BPA exposure can potentially cause various detrimental health effects such as heart disease, breast cancer, infertility, and neuro-developmental disorders [10]. Skin penetration/absorption of BPA for someone simply holding thermal printing papers can reach 71 mg/day [9]. In addition, the absence of stability of a complex system (three components) is also a problem in leuco dye applications. Therefore, many researchers are interested in finding new substrates to serve as suitable color developers. Recently, linear-type and hyperbranched-type polyphenol derivatives have been used as developer materials for temperature-sensitive dyes [11,12]. However, it was found that this type of polymer has a few disadvantages such as poor molecular weight control, high polydispersities, and low blocking efficiencies [13]. The results of these studies have been difficult to interpret, and analysts have reached different conclusions about its use as color developer. Macrocyclic compounds are known as traditional supramolecules based on the specific channel where molecular recognition is possible. Normally, their structure, composed of phenol units and alcohol groups, can be used as a novel developer material. To the best of our knowledge, there have not been any studies about using this type of developer material for temperature-sensitive dyes. In this work, we study various macrocyclic compounds. (MeAC3, ClAC3, TC4, C4, α-CD, β-CD, and RC4) as color developers, as they have good stability, exact molecular weights, and good complexation properties.

Instrumentation and apparatus
1 H-NMR spectra were measured using a Varian 400 MHz spectrometer in CDCl 3 with TMS as an internal reference. Fluorescence spectra in solution were measured with a Perkin Elmer LS 50B spectrometer. Absorbance spectra were recorded with a Perkin Elmer Lambda 25 UV-Vis spectrometer. FT-IR spectra were recorded with a Bruker Tensor 27 FT-IR spectrometer. SEM measurements were performed with a Zeiss (LEO) 1450VP microscope. A thermogravimetric analysis (TGA) was carried out using a SDT Q600 from TA Instruments.

Synthesis of p-tert-Butylthiacalix[4]arene (TC4)
TC4 was synthesized in a manner similar to that described in the literature with a slight adjustment [15]. A solution of p-tert-butylphenol (64.5 g, 0.43 mol), elemental sulfur S 8 (27.5 g, 0.86 mol), and NaOH (8.86 g, 0.215 mol) in tetraethylene glycol dimethyl ether (19 mL) was stirred under nitrogen. The stirred mixture was gradually heated to 230 °C over a period of 4 h and kept at this temperature for a further 3 h with concomitant removal of the evolving hydrogen sulfide by using a slow stream of nitrogen gas. The resulting dark red product was cooled to an ambient temperature and diluted with toluene and ether mixture. Then, 0.5 M aq. sulfuric acid was added to the solution using vigorous stirring to get a suspension. The precipitate was collected by filtration, recrystallized with chloroform, and dried in vacuo (100 °C, 4 h) to obtain an essentially pure sample of TC4 (28.53 g, 36.7%). This resulted in 1 H NMR (400 MHz, CDCl 3 ): δ 9.56 (s, 4H, OH), 7.52 (s, 8H, ArH), and 1.33 (s, 36H, t-C 4 H 9 ). 13

Preparation of leuco dyes (LD1-LD7)
Leuco dyes containing CVL, macrocyclic compounds, and OD were prepared using the following procedure. First, CVL, macrocyclic compounds, and OD in a ratio of 1 (CVL), 6 (macrocyclic compounds), and 100 (OD) were added to a flask with a tight-fitting lid [18]. After the mixture had been heated at 160 °C for 30 s, the white 1-octadecanol solid in the prepared samples completely melted into a transparent liquid. The samples returned to their solid forms when cooled to room temperature. The leuco dye compounds obtained were pale-yellow, white, and blue solids. The macrocyclic compounds used for preparing leuco dyes were composed as follows:

Study of color differences in leuco dyes
Leuco dyes were heated from 30 to 80 °C, and color changes were recorded. A larger value of L* (lightness) indicates that the surface of the measured object was light and a smaller value indicates that it was dark. Red-green color changes are represented by a*, with positive values indicating red and negative values signifying green. Yellow-blue color changes are shown by b*, with positive values denoting that the surface of the object measured was yellow, and negative b* values indicating blue. The symbol C* indicates color chroma and h* indicates hue. Thus, color differences were calculated using the following formula: where ∆L*(brightness difference) = L1*-L2*, ∆a*(red and green difference) = a1*-a2*, and ∆b* (yellow blue difference) = b1*-b2*. L1*, a1*, and b1* are all initial values (30 °C), while L2*, a2*, and b2* are the values after the temperature change [19].

Macrocyclic compounds
All macrocyclic compounds were prepared using a condensation reaction slightly adapted from the literature [14][15][16][17]. The chemical structures of macrocyclic compounds were characterized by ATR-FTIR, 1 H-NMR, and mass measurements. As shown in Fig. 1, their structures are typically composed of phenol unit and alcohol groups. These can be used as color developers by acting as proton donors, changing the dye molecule between its leuco and protonated colored forms.

Leuco dye formation and properties
In order to investigate the color-forming abilities of synthesized macrocyclic compounds, mixtures of macrocyclic compounds CVL and OD in a ratio 1 (CVL): 6 (macrocyclic compounds): 100 (OD) were prepared. The mixture was then heated on a hot plate at 160 °C for 30 s until completely melted (molten mixture). All samples were brought back to solid form by cooling them to room temperature. Figure 2 shows photographs of the leuco dyes LD1-LD7 before and after heat treatment. It was found that only LD7 exhibited a blue to black color change when taken from room temperature to 160 °C. A color change in LD7 upon heating (160 °C) indicated that in the presence of RC4, CVL dyes underwent a structural change. They transformed from a closed-lactone form to an open-lactone form when heated due to the release of protons from developer materials, and thus turned black [11]. In addition, this colored/decolored reaction was reversible for more than several cycles. On the other hand, no CVL chemical, structural, or color changes occurred in other leuco dyes (LD1-LD6).
The opening of lactone rings in CVL dyes in LD7 was confirmed using ATR-FTIR measurements. The spectra of LD7 and pure compounds are shown in Fig. 3. Pure CVL ΔE * = (ΔL * ) 2 + (Δa * ) 2 + (Δb * ) 2 1∕2 exhibited C-C stretching in the aromatic ring at 1611 and 1440 cm −1 . A strong peak appearing at 1070 cm −1 was attributed to C-O stretching. The O-H stretching of RC4 appeared at 3316 cm −1 . Peaks at 2933 and 2859 cm −1 belonged to the -CH 2 asymmetric stretching vibration of an aliphatic chain. In the spectra of LD7, a sharp lactone stretching band (C = O) near 1740 cm −1 of LD7 clearly decreased and slightly shifted to 1755 cm −1 after heating because of a ring opening caused by protonation of the lactone structure of CVL dyes [20]. The color-forming reaction tests in LD7 confirm that RC4 is capable of complexation forming and opening the lactone ring of leuco dyes by releasing protons from phenolic groups.
Additionally, the thermal stability of macrocyclic compounds and leuco dyes (LD1-LD7) was studied using the TGA method. The TGA thermogram in Fig. 4a shows that the initial temperature causing a 10% weight loss in all macrocyclic compounds was above 250 °C. In the cases of ClAC3 and RC4, the TGA indicates that under normal atmospheric conditions, the macrocyclic compounds transformed into thermal-resistant residuals such as polymer [21]. Figure 4b shows that LD1-LD7 had a 10% loss weight at 240 °C, and just 20% of the original sample weight remained at 800 °C. A TGA curve of LD7 shows an increasing in percentage of weight loss (in which %residue decreased from 42 to 19%) which might be explained by complexation between RC4 and CVL leading to a decrease in thermal-resistant residual species.
The morphology and particle size distribution of the microcapsules were determined by means of SEM photographs. Figure 5 shows the microstructure of leuco dye pigments using BPA and RC4 as color developers. Figure 5a and 5b are SEM images of leuco pigments which have average diameters of 0.3-0.6 μm and were developed with BPA. Figure 5c and 5d are SEM images of leuco pigments which have average diameters of 2.8-3.5 μm and were developed with RC4. The SEM micrograph of RC4 shows an increase in crystallinity and particle size when compared to conventional BPA developers. These results might be attributed to π-π interactions and hydrogen bonding of RC4 and CVL in leuco dyes [22].

Computational study
To further study the properties of CVL, RC4, and LD7, optimized geometries and HOMO and LUMO energies were computed using density functional theory (DFT) calculations at a B3LYP/LanL2DZ theoretical level through the Gaussian 09 program [23]. The lowest energy structures of CVL, RC4, and LD7 are shown in Fig. 6. It is clearly seen that the molecular geometries of CVL and RC4 found in LD7 distorted from their free forms and intermolecular hydrogen bonds (OH···O) appeared in the complex. The
In addition, the calculated energies of LD7 (−3005.7 a.u.) were lower than those of both CVL (−1322.7 a.u.) and RC4 (−1683.0 a.u.). As demonstrated in the optimized structures, it was found that RC4 could form stable complexes with CVL using complexation, as shown in Fig. 6. The electron distributions and orbital energies of the HOMO and LUMO of CVL (lactone form), and RC4 and LD7 (OD, RC4, CVL + (cationic form)) are presented in Fig. 7. In the cases of CVL and CVL + , the HOMO were all localized on a phenyl unit. However, the LUMO were localized on lactone units in CVL and uniformly distributed considerably smaller than that of CVL (0.141 eV) and RC4 (0.180 eV), and the cationic form in LD7 is more delocalized than the lactone form in CVL owing to the hydrogenbond interactions of the color former-developer [24].

Color differences in leuco dyes
To study the performance of LD7, firstly we checked the blue to brown black color changes upon heating process , leuco pigments with average diameters of 0.3-0.6 μm that were developed with BPA. 500 × (c), 10,000 × (d) and leuco pigments with average diameters of 2.8-3.5 μm that were developed with RC4 Fig. 6 The optimized structures of CVL, RC4, and LD7, obtained at a B3LYP/LanL2DZ level of theory with the lowest energy structures in a.u using UV-Vis spectroscopy by coating the leuco dyes on the glass plate and measuring before and after heat treatment (Fig. 8). It was showed that LD7 exhibit absorption band at around 525 to 680 nm, which is disappeared after applying thermal. This strong change is mainly complementary to the structural transformation of the LD7 molecule from the open-lactone form to the closed-lactone form via proton transfer from a phenolic and alcohol unit in the RC4. In this realize, RC4 is potentially a better and more effective developer material than other macrocyclic compounds in other leuco dyes (LD1-LD6) because RC4 in LD7 has more alcohol groups than another structure (LD1-LD6) causing dissociation constant (K) values that are greater than other structures, and a high percentage of weight loss and thus can be forceless at heating temperatures, but other macrocyclic compounds in leuco dyes (LD1-LD6) are infusible at heating temperatures owing to their cross-linked structures [11].
In addition, digital photography was used to evaluate the effect of temperature changes on color. The methodology used a bulk molten mixture of LD7 in vials, tempered in a water bath. After reaching desired temperature, the samples were removed from the water bath and an image was recorded using a digital camera. Then, images were transformed into trichromatic values XYZ, with consequent calculations of visual color differences. The trend of LD7 color difference variations based on temperature changes from 30 to 80 °C is shown in Fig. 9. The color of LD7 changed from blue to black as the temperature increased. When the molten mixture of LD7 was heated to 60 °C, the color changed significantly. The color of LD7 began to change around 60 °C.

Conclusion
We report on a series of macrocyclic compounds (MeAC3, ClAC3, TC4, C4, α-CD, β-CD, and RC4) which were prepared to investigate their feasibility as developer materials for temperature-sensitive dyes. After analyzing various macrocyclics, RC4 is found to be the most appropriate candidate for a novel color developer in temperature-sensitive dye thermal papers based on its coloring/ decoloring reactions. It is found that RC4 has a high static sensitivity and a dark contrast of leuco dye, similar to that found in BPA. This first successful demonstration of the use of macrocyclic compounds as developer materials for temperature-sensitive dyes opens up a new pathway for the development of macrocyclic compounds as replacements for problematic BPA developer materials.