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Using this absorption band, we evaluated the degree of discoloring by changing the concentration ratios of the developer and FD to find out the optima...
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In Situ Observation of Thermochromic Behavior of Binary Mixtures of Phenolic Long-Chain Molecules and Fluoran Dye for Rewritable Paper Application Satoshi Yamamoto,*,‡ Hiromi Furuya,‡ Kyoji Tsutsui,‡ Satoru Ueno,† and Kiyotaka Sato†

CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 7 2256–2263

AdVanced Technology R&D Center, Ricoh Co., Ltd., Tsuzuki-ku, Yokohama 224-0035, Japan, and Graduate School of Biosphere Science, Hiroshima UniVersity, Higashi-hiroshima, 739-8528, Japan ReceiVed September 19, 2007; ReVised Manuscript ReceiVed April 25, 2008

ABSTRACT: This paper reports experimental results of in situ observations of thermochromic processes of two binary mixture systems of 1-(4-hydroxyphenyl)-3-octadecylurea (PU18) + fluoran dye (FD), and 4-hydroxy-N-octadecylbenzamide (PC18) + FD, which were obtained by using FT-IR spectroscopy, reflective optical density, and thermal and X-ray diffraction (XRD) techniques. As the FD, 2′-anilino-6′-di-n- butylaminospiro [isobenzofuran-1(3H),9′-[9H] xanthene]-3-one was employed. We called the longchain molecules of PU18 and PC18 developers, since the FD molecules became either colored or discolored depending on whether the FD and developer mixture formed a supramolecular complex or the FD and developer were separately crystallized. The supramolecular complex, in which FD was colored by opening its lactone ring, was formed when the molten mixture of FD-developer was quenched from elevated temperature to chilled temperature. The separate crystallization of the FD and developer occurred by heating the supramolecular complex. The present study focused to clarify the detailed structure of the supramolecular complex and its decomposition mechanism that determines the discoloring rate of FD. Small-angle XRD patterns showed that the supramolecular complexes formed lamellar structures with long spacing values of 42.0 Å (PU18/FD) and 40.7 Å (PC18/FD). However, wide-angle XRD patterns showed broad peaks, indicating disordered lateral packing of the long-chain molecules in the supramolecular complex. The colored supramolecular complexes were thermodynamically metastable, and decomposition occurred by heating to elevated temperatures, making FD discolored. We found that, in the two mixtures of PU18/FD and PC18/FD, the decomposition of the supramolecular complexes included two stages of structural transformations of the developers: (a) the formation of a liquid-crystallike structure occurred soon after the developer-FD complex was decomposed, and (b) crystallization occurred at elevated temperatures. In accordance with these transformation processes, the degree of discoloring of FD was developed, and a perfect discoloring state was achieved after the crystallization of the developer was completed. The present findings provided new ideas to design the thermochromic mixture system for a high-speed rewritable recording paper system using the FD-developer mixtures. Introduction In recent years, widespread digitization of information has given rise to a new role for printed papers as the medium of display of the information kept in data storage devices such as IC tags, magnetic tapes, etc. Usually, most of such informationdisplaying papers are used one time and discarded then. It is required, however, to develop a printing system that permits images to be printed, then erased and printed again; for example, rewritable function as an environmentally friendly system that is suitable for the digital information age.1,2 Several candidates for rewritable and electronic display systems have been proposed such as microcapsule-type electrophoretic display,3 twist ball display,4 and two-color fluoran dyes/liquid crystal type display,5 toner display,6 electronic ink display,7 and so on. In 1994, we proposed a rewritable recording system using thermochromic behavior of binary mixtures of octadecylphosphonic acid (developer) and a fluoran dye (FD).2a FDs, called leuco dyes,8,9 are usually employed in thermal print systems using acidic phenolic molecules, although this system is irreversible. By contrast, our rewritable recording function was performed by repeating thermochromic properties of the developer-FD mixture as schematically shown in Figure 1.2a In the first, the developer and FD were mixed and melted at a high temperature. Then the mixture was quenched to form the molecular complex of the developer and FD. At this stage, a * Corresponding author. E-mail: [email protected]. ‡ Advanced Technology R&D Center, Ricoh Co., Ltd. † Hiroshima University.

colored state was formed by opening a lactone ring of FD. Then the mixture was heated to cause fractionated crystallization of the developer and FD, revealing a discolored state. Further heating brought back the initial stage of the molten mixture. With reference to thermochromic mixtures using the developer and leuco FDs, several types of thermochromic mixtures including two or three components have been investigated.10 In addition, structural information of FD with an acidic phenolic developer has recently been clarified.11 In the studies so far reported, little information has been obtained for discoloring mechanisms corresponding to structural changes, and more importantly, the rate of discoloring processes are an order of time (minute) which is too slow for an industrially applicable printing system that needs coloring/discoloring rates of the order of milliseconds. For this purpose, further research of new types of developers that enable higher discoloring rates is needed. The discoloring rate using octadecyl phosphonic acid (P18)/ FD mixture, that was proposed in the first,2a was of an order of minutes. We assumed that binding energy between P18 and FD may be too strong, and the rate of the fractional crystallization of P18, that decomposes the molecular complex of the developer and FD, might be low. Therefore we have searched for different types of long-chain alkyl developers. In this paper, we present a developer having polar phenol structures and long chain molecules where urea (-NHCONH-) and carbamoyl groups (-CONH-) were placed, as shown in Figure 2.12 By using the two developers, we obtained the rate of discoloring of the order of hundreds of milliseconds.

10.1021/cg7009034 CCC: $40.75  2008 American Chemical Society Published on Web 06/13/2008

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Figure 1. Conceptual illustration of thermochromic coloring and discoloring processes of developer-fluoran dye (FD) mixture.

Figure 2. (a) Conversion of colored and discolored states of fluoran dye, in which closed and open lactone rings are shown. (b) Three developer molecules; octadecyl phosphonic acid (P18), 4-hydroxy-N-octadecylbenzamide (PC18), and 1-(4-hydroxyphenyl)-3-octadecylurea (PU18).

In the present study, we observed the mechanisms of coloring/ discoloring processes of the developer/FD mixtures using 1-(4hydroxyphenyl)-3-octadecylurea (PU18) and 4-hydroxy-Noctadecylbenzamide (PC18) as the developers. This was enabled by an increased rate of crystallization of the developers, compared with that of octadecylphosphonic acid (P18). FT-IR spectroscopy, optical density changes, heat analysis, and synchrotron radiation X-ray diffraction (SR-XRD) were applied to observe the formation and decomposition of the developer/ FD molecular complex. A particular interest was focused on the discoloring processes caused by crystallization of the developer during the heating process by using the SR-XRD technique that has been applied to observe very complicated processes of binary mixing behaviors of long-chain molecules.13 Experimental Section General. Unless otherwise noted, materials were obtained from commercial suppliers and employed without further purification. 2′Anilino-6′-di-n-butylaminospiro [isobenzofuran-1(3H), 9′-[9H]xanthene]3-one (FD) was used from commercially available reagent (Yamamoto Chemicals, Tokyo). Melting points and phase transformation temperatures were recorded on Seiko EXSTAR 6000 and DSC 6200 (Seiko Instruments, Tokyo) at 5 °C/min, calibrated using Indium under an atmosphere of dry nitrogen. The reflective optical density (OD) was measured with an optical reflection densitometer (Macbeth Co. Ltd., RD-914, USA).

Synthesis of 1-(4-Hydroxyphenyl)-3-octadecylurea (PU18). 4-Aminophenol (98% purity, 4.0 g, 37 mol) in anhydrous toluene (100 mL) was added dropwise to the solution of octadecyl isocyanate (98% purity, 10.2 g, 34 mmol) in anhydrous toluene (300 mL) while stirring in an Ar atmosphere at 60 °C, and stirring was continued at this temperature for 3 h. Then, the mixture was cooled to room temperature, and the reactant mixture was crystallized. The solid reactant was then collected by filtration under reduced pressure. After removal of the solvent in vacuo, purification of the crude product was done twice by recrystallization in ethyl acetate (200 mL). By this process, pure white crystals of the long-chain alkyl compound PU18 (12.8 g, 92%) were obtained. The melting point was 145.3-147.8 °C; EI-MS: m/z: [M]+ 404; elemental analysis calcd (%) for C25H44N2O2: C 74.21, H 10.96 N, 6.92; found: C 74.18, H 10.95, N, 6.88. Synthesis of 4-Hydroxy-N-octadecylbenzamide (PC18). N,N′Diisopropylcarbodiimide (4.8 g, 38 mol) was added dropwise to 4-hydroxybenzoic acid (5.3 g, 38 mol) and 1-hydroxybenzotriazole (4.9 g, 36 mol) in anhydrous THF (300 mL) while stirring in an Ar atmosphere at 60 °C, and stirring was continued at this temperature for 0.5 h. Then, octadecyl amine (97% purity, 9.8 g, 36 mol) was slowly added to the reaction mixture. The mixture was stirred for 3 h and cooled to room temperature, and the reactant mixture was crystallized. The solid reactant was then collected by filtration under reduced pressure. After removal of the solvent in vacuo, purification of the crude product was done twice by recrystallization in ethyl acetate (200 mL) to obtain pure white crystals of long-chain alkyl compound PC18 (12.0 g, 85%). The melting point was 104.2-105.8 °C; EI-MS: m/z: [M]+ 389; elemental analysis calcd (%) for C25H43NO2: C 77.07, H 11.12, N 3.60; found: C 76.96, H 11.10, N, 3.64.

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Figure 3. Apparatus of rapid scanning method for the monitoring discoloring process with FT-IR. In addition to the two developers of PU18 and PC18, octadecylphosphonic acid (abbreviated P18) was employed as a reference.2a Preparation of Colored Mixed State. 2′-Anilino-6′-di-n-butylaminospiro[isobenzofuran-1(3H), 9′-[9H]xanthene]-3-one was used as a fluoran FD (FD), indicating colored-discolored reversible reactions shown in Figure 2. The lactone ring is open in the presence of a weak acid or electron acceptor to form a zwitterionic structure, increasing the conjugation and lowering the energy of the π-π* transition. Therefore, the fluoran FD readily turns colored as it comes in contact with a phenol and a phosphonic compound. Colored composites of the developer and FD mixtures were prepared by rapid cooling of the molten developer/FD mixture through the following processes: the powdered mixtures (several milligrams) of developer and FD were sandwiched between two pieces of slide glass and heated and melted at 190 °C, then quickly soaked in cold water below 5 °C. One side of the slide glass was removed carefully. After air-drying, the thickness of each solid film was approximately 10-25 µm. Thus the collected solids were employed as starting colored samples for the SR-XRD press-forming disks (φ ) 2 mm, thickness ) 1 mm), DSC and powder XRD samples using the above-referenced three types of developers, PU18/FD, P18/FD, and PC18/FD. During the heating process, the discoloration reaction revealed in the decrease in the optical density was initiated, as the temperature approached the melting point of the developer, and further heating decreased the optical density. Subsequent heating increased the optical density, because the complete melting of the developer caused close contact between the developer and FD. Therefore, we defined the discoloring temperature as the temperature at which the optical density showed the minimum value. The discoloring temperatures were 75 °C (P18/FD), 85 °C (PC18/FD), and 120 °C (PU18/FD) (see below). Infrared Spectroscopy. FT-IR spectra were obtained with FT/IR670 PLUS (JASCO, Tokyo) using a regular reflection method. Thin aluminum sheet (12 µm) was employed as the reflection mirror for the heat conductivity. Colored composites placed on the aluminum sheet were prepared as follows: colored mixtures were sandwiched between an aluminum sheet and slide glass, and spread over the hot plate (190 °C) and quenched (5 °C). Thus formed colored samples were removed from the slide glass and then air-dried. The thickness of colored sample on the sheet was 1-3 µm. Rapid scan of IR spectroscopy (20 scans per second) was performed by using a sample holder system shown in Figure 3. Rapid heating to cause the discoloring was done by putting hot oil droplets (ethylene glycol, EG) on the top of aluminum sheet. The temperature of the oil droplets (75-120 °C) was measured by a thermocouple inserted to the droplet. The colored developer-FD mixture was adhered beneath an aluminum sheet, which was placed on the side of a heat-insulated stage. Then, we recorded the FT-IR spectra during the discoloring process at the moment when the oil droplet at various temperatures was put on the aluminum sheet. The colored and discolored states were determined by observing the FT-IR band at 1755 cm-1, which is due to CdO stretching vibrational mode of a lactone ring. No absorption corresponds to the colored (open ring) state and the strongest absorption corresponds to the complete discolored state. Using this absorption band, we evaluated the degree of discoloring by changing the concentration ratios of the developer and FD to find out the optimal concentration ratio of the two components. For this purpose, we calculated the band strength by measuring the whole area of absorption during the spectral area from 1700 to 1800 cm-1. Powder X-ray Diffraction Measurements. Laboratory-scale X-ray diffraction measurements were performed with a RINT1100 (Cu KR) (RIGAKU, Tokyo) at room temperature, calibrated by palmitic acid (CH3(CH2)15-COOH) crystal at the small-angle scattering. The SR-XRD measurements were done using a beam line 9C and 15A at the Photon

Figure 4. FT-IR spectra of colored mixture (developer-to-FD ) 8:1) of (a) PU18 and FD, (b) discolored PU18/FD mixture, (c) colored PU18/FD mixture, (d) PC18 and FD, (e) discolored PC18/FD, (f) colored PU18/FD. Factory (PF), a synchrotron radiation facility in the National Laboratory for High-Energy Physics (KEK), Tsukuba, Japan. The PH operates at 2.5 GeV; the X-ray wavelength (λ) is 0.15 nm. Small-angle X-ray scattering (SAXS) and wide-angle X-ray scattering (WAXS) patterns were recorded simultaneously every 10 s with two gas-flown onedimensional, position-sensitive detectors.

Results and Discussion Reflection FT-IR Spectra. Figure 4 shows the FT-IR spectra of the powder samples of PC18, PU18, FD, the discolored and colored mixtures of PC18/FD, and the discolored and colored mixtures of PU18/FD. In the two mixtures, the molar ratio of the developers and FD was 8:1. The FT-IR spectra of the PC18, PU18, and FD samples exhibited quite complicated patterns. It was hardly possible to correspond the all spectral band to vibrational modes of the two developers. However, it was obvious that the bands around 1650 cm-1 of PC18 and PU18 corresponded to CdO stretching modes of the carbamoyl group and urea group, respectively,14 and the band at 1755 cm-1 of FD was due to CdO stretching vibrational mode of the lactone ring in the discolored state.15 Taking this band as the reference, we evaluated the degree of coloration/discoloration by changing the concentration ratios of the developer/FD at room temperature and the rate of discoloring during the heating process of the mixture at the fixed concentration ratio of the developer and FD, as explained below. Figure 4b,e showed that the FT-IR patterns of the discolored states of the two mixtures were simple super positions of those of the developers and FD. This means that there are no specific developer-FD interactions in the discolored state. However, the FT-IR patterns in the colored states of the two mixtures

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Figure 5. (a) Illustration of the area of coloring and discoloring around CdO vibrational lactone ring of dye, (b) percentages of colored state with various molar developer-to-dye ratios. Stoichiometry of the colored PU18/FD and PC18/FD complex were estimated and selected as 8:1. The error bars represent standard deviation over three experiments.

were largely different from those in the discolored state, as the absorption band at 1755 cm-1 disappeared, and the bands around 1600 cm-1 and other areas were deformed. These changes indicated the formation of supramolecular complex of the developer/FD mixture which causes the opening of the lactone ring of FD (Figure 2a). A previous FT-IR study of the molecular complex formation of phenol derivatives and leuco dye11 showed that opening of the lactone ring formed COO- instead of COOH when weakly acidic phenol derivatives were attached to the leuco dye. In such a case, symmetric and antisymmetric stretching vibration modes must appear at 1360 and 1640 cm-1, respectively. However, the band at 1640 cm-1 was hindered by the presence of the absorption bands of the developers, but the band at 1360 cm-1 was detectable, as shown in Figure 4c,f indicated by arrows. The band at 1360 cm-1 coexists with small absorption bands of the developers, and using this absorption band for the evaluation of the discoloring and coloring conversion was inadequate. Therefore, the band at 1755 cm-1 was employed as the reference spectrum. In our previous study of the octadecyl phosphonic acid (P18)/ FD mixture,2a we found the most optimal concentration ratio of P18/FD to form the supramolecular complex showing the highest degree of discoloring. This ratio of P18/FD was 7/1, indicating the most stable stoichiometric ratio of the developer/ FD supramolecular complex. Such a stoichiometric relationship was also found in the mixtures of PC18/FD and PU18/FD. We calculated relative ratios of the absorption intensity, measured as the area of the spectral band at 1755 cm-1 in the colored (Acolor) and discolored (Adiscolor) states, as shown in Figure 5a. The percentage of colored state was defined as (1 - Acolor/ Adiscolor) × 100, and measured for the developer/FD mixtures with different molar ratios from 1/1 to 10/1. Acolor was measured soon after quenching the molten mixture to ice temperature, and Adiscolor was measured after heating the quenched mixture to 85 and 120 °C for PC18/FD and PU/FD, respectively. As shown in Figure 5b, it was evident that the percentage of colored state increased by increasing the concentration of the developer, and reached the maximum value around the developer/ FD ratio of 8/1. The maximum value was maintained by increasing the developer/FD ratios from 8/1 to 10/1. From this result, we can conclude that the supramolecular complex revealing the highest colored state is formed by the combination of eight developer molecules with respect to one FD molecule. This makes a contrast to the combination of bisphenol A developer and a FD mixture having the optimal molar ratio of 4/1.13 The difference may be rationalized by considering that

bisphenol A has two phenolic OH groups, whereas PC18 and PU18 have one phenolic OH group. On the basis of this result, we employed the PC18/FD and PU18/FD mixtures with the ratio of 8/1 for the in situ FT-IR, optical density, and XRD measurements of the discoloring processes. Figure 6 shows time-resolved FT-IR spectral variation in the wavenumber range of 1600 to 2000 cm-1 of the P18/FD, PC18/ FD, and PU18/FD mixtures. The reference spectra at 1755 cm-1 were recorded with time soon after the colored film was exposed to elevated temperature (temperature jumping at time 0), by using the system shown in Figure 3. The elevated temperatures were chosen as 75 °C (P18/FD), 85 °C (PC18/FD), and 120 °C (PU18/FD) so that the degree of discoloration became the highest at every temperature for each mixture (see below). The P18/FD mixture increased the absorption intensity soon after the temperature jumping was done as shown in Figure 6a. However, Figure 6b,c show that the rate of increase in the absorption band intensity of the P18/FD mixture was much slower than those of the PC18/FD and PU18/FD mixtures. The percentage of colored state, (1 - Acolor/Adiscolor) × 100, was again calculated using the data of Figure 6a-c, as shown in Figure 6d. Within 200 ms, the percentage of colored state became below 10% for the mixtures of PC18/FD and PU18/FD and this value reached 5% at 1 s. By contrast, the percentage of colored state of the P18/FD mixture was still about 80% at 2 s. This result clearly showed that the use of PC18 and PU18 as the developer caused quite rapid discoloring process compared with P18. DSC and OD Measurements. Figure 7 shows the results of DSC heating thermopeaks and optical density (OD) values measured by heating the colored mixture film at the rate of 3 °C/min for the mixtures of PU18/FD and PC18/FD. The two mixtures showed the following common features: (a) OD rapidly decreased above the temperatures where the DSC heating scans showed exothermic peaks around 42 °C (PC18/FD) and 49 °C (PU18/FD), (b) further decrease in OD occurred around 50-95 °C (PC18/FD) and 50-110 °C (PU18/FD), (c) small DSC exothermic peaks appeared at 77 °C (PC18/FD) and 100 °C (PU18/FD), and (d) OD increased during heating up to 100 °C (PC18/FD) and 140 °C (PU18/FD). The results of (a) through (c) indicate the structural changes of the developer-FD mixtures associated with the discoloring processes, which seem to include two-stage processes. The process of (d) is due to melting of the developers, which caused the increase in optical density because of tight interactions between the developers and FD. We assume that the discoloring processes were caused by the

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Figure 6. Time-resolved FT-IR profiles during discoloring processes of the mixtures of (a) P18/FD, (b) P18/FD, (c) P18/FD, taken during instantaneous heating to colored mixtures at 75, 85, and 120 °C, respectively. (d) Time variation of degree of colored state during the instantaneous heating.

Figure 7. The relationships between DSC heating thermopeaks (top) and optical density (OD) measurements (bottom) of two mixtures of PC18/FD and PU18/FD.

structural transformations including crystallization, polymorphic transformation, and melting of the developer molecules, which were clearly observed by the XRD experiments shown below. Although the two mixtures exhibited two-stage discoloring processes, the details are different between the PC18/FD and PU18/ FD mixtures, as noted in the following. In the case of the PC18/ FD mixture, the DSC peaks showed an endothermic peak that was

soon followed by an exothermic peak when the rapid discoloring process occurred at 42 °C. This is a typical behavior of meltmediated transformation, in which the complex of PC18 and FD mixture first melted, and after then PC18 was newly crystallized. The small DSC exothermic peak at 77 °C was not associated with detectable decrease in OD, although the XRD experiments indicated a polymorphic transformation (see below).

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Figure 8. XRD patterns of (a) PC18/FD and (b) PU18/FD mixtures in a colored state (red lines) taken after melting/quenching, and in a discolored state (blue lines) taken after heating to the colored state for 1 min.

Figure 9. In situ synchrotron XRD patterns of (a) colored PC18/FD and (b) PU18/FD while heating at 3 °C/min.

In the case of the PU18/FD mixture, a large exothermic peak at 49 °C caused a rapid decrease in OD, indicating the exothermic decomposition of the PU18/FD complex mixture. The DSC showed a very small endothermic peak before the occurrence of the exothermic peak, but clear melting-crystallization behavior, which was observed in the PC18/FD mixture, was not detectable in this case. This might be due to high rate of crystallization of PU18, whose exothermic peak did not allow revealing of the endothermic peak due to the decomposition of the PU18/FD mixture complex. At 100 °C where a relatively large exothermic peak was observed in the DSC heating process, OD remarkably decreased. This indicates that the polymorphic transformation of the PU18 crystals caused the change in OD through certain molecular interactions with FD. X-ray Diffraction Experiments. Figure 8 shows the XRD patterns of the colored and discolored states of the mixtures of PC18/FD (Figure 8a) and PU18/FD (Figure 8b) taken by the labo-scale equipment. The XRD patterns in the colored and discolored states were taken soon after the molten mixture was quenched at ice temperature, and by heating the colored mixtures to 85 °C (PC18/FD) and 120 °C (PU18/FD), respectively. In the two mixtures, the colored state showed a broad wide-angle X-ray scattering (WAXS) pattern with the short spacing value of 4.0 Å. In the small-angle X-ray scattering (SAXS) patterns, the PC18/FD mixture showed two peaks of 42.1 Å (001

reflection) and 19.5 Å (002 reflection), whereas the PU18/FD mixture showed peaks of 37.6 Å (001 reflection) and 18.9 Å (002 reflection). The presence of the broad WAXS pattern and sharp SAXS patterns with 001 and 002 reflections means the formation of a lamellar-type structure, in which hydrocarbon chains are packed in disordered hexagonal packing, as observed in P18/FD mixture of the colored state.2a In the discolored state, however, the SAXS patterns were composed of many reflections of low and high indices, all of which corresponded to the long spacing values of 53.8 Å (PC18/FD) and 52.5 Å (PU18/FD). In the case of the WAXS patterns, sharper peaks were detectable with the short spacing values of 4.3 and 3.8 Å in the PC18/FD mixture, and 4.6, 4.2, 4.0, and 3.9 Å in the PU18/FD mixture. This means that the crystal structure in the discolored state was largely different from those in the colored state in the two mixtures. The XRD patterns observed in the discolored state of the two mixtures are identical to those of the crystals of PC18 and PU18 (see Supporting Information). The temperature variation of the structural transformation of the mixtures was precisely observed in situ with the SR-XRD. Figure 9 shows the SR-XRD patterns taken during the heating from 20 to 90 °C (PC18/FD) and from 20 to 120 °C (PU18/ FD) at the rate of 3 °C/min. In the case of the PC18/FD mixture (Figure 9a), the XRD pattern of the colored state at 20 °C, that was identical to that shown in Figure 8, changed to the patterns

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Figure 10. Schematic model of discoloring processes of PU18/FD mixture.

with the SAXS pattern having the long spacing value of 61.0 Å and the WAXS patterns of the short spacing values of 4.3 and 3.7 Å at 40 °C. It is worth noting that this temperature is the same as that of the occurrence of the endothermic/exothermic DSC peaks shown in Figure 7a. Further heating caused the changes in the SAXS and WAXS patterns around 75 °C, where the long spacing value changed from 61.0 to 57.5 Å, and the short spacing values changed to 4.2 and 3.8 Å. This means that the two XRD patterns shown in Figure 8 correspond to those of the colored and discolored state, whereas there is another structure between the two states that is revealed during the transient state from colored to complete colored state during the heating process. Quite similar results were observed for the mixture of PU18/ FD, as shown in Figure 9b. The XRD pattern of the colored state at 20 °C changed to the SAXS pattern having the long spacing value of 63.7 Å and the WAXS patterns of the short spacing values of 4.3 and 3.9 Å at 45 °C. This temperature is the same as that of the occurrence of the exothermic DSC peaks and remarkable decrease in OD shown in Figure 7b. Further heating caused the changes in the SAXS and WAXS patterns around 100 °C, where the long spacing value changed from 55.4 Å, and the short spacing values changed to 4.5 and 4.1 Å. Similarly to the PC18/FD mixture, another structure was revealed between the colored and completely discolored states during the heating process. On the basis of the experimental data taken by FT-IR, DSC, OD, and XRD methods, we constructed a schematic model describing the discoloring behavior of the developer/FD mixtures during the decomposition processes of the supramolecular complexes, as shown in Figure 10. In the colored state, the developer/FD mixture forms a supramolecular complex, in which the FD molecules are placed in-between the lamellar layer of the developer molecules. Direct contact between the FD and developer molecules caused the opening of the lactone ring, leading to the colored state. The repeating distance of the supramolecular complex normal to the lamellar plane is a summation of the thicknesses of the FD molecule and single lamellae of the developer having an interdigitated structure. The hydrocarbon chains of the developer molecules in the lamellae are in a disordered conformation. Since this supramolecular complex is thermodynamically metastable, heating induced the decomposition of the supramolecular complex through two-stage structural transformation processes of the developer. In the first, bilayer-type crystals having the long-chain molecules arranged normal to the lamellar plane are formed by the decomposition of the supramolecular complex of the developer/FD mixture. This crystallization occurred around 40 and 48 °C in the PC18/ FD and PU18/FD mixtures, respectively. Further heating caused

second-stage transformation, which is a conversion into hightemperature polymorphic forms. In this transformation, longchain molecules are inclined against the lamellar plane around 75 and 100 °C in the PC18/FD and PU18/FD mixtures, respectively. The fractional crystallization of the developer caused lactone ring of FD closed to make the mixture discolored. The two-stage structural transformation shown in Figure 10 did not occur in the mixture of phosphponic acid (P18)/FD mixture, in which the fractional crystallization of P18 occurred once. It is highly reasonable to assume that the main reason for the high rate of discoloration of the PU18/FD and PC18/FD mixtures compared with the P18/FD mixture is ascribed to the two-stage structural transformation which may be caused by specific molecular interactions within the developer molecules. To compare the two mixtures of PC18/FD and PU18/FD, discoloring mechanisms illustrated in Figure 10 may be more applied to the latter mixture rather than to the former case, since the decrease in OD at the second transformation process was remarkable in the PU18/FD mixture, but less visible in the PC18/FD mixture. This might be due to structural relaxation or annealing effects of the PC18 crystals during the heating process. Conclusion We have synthesized phenolic-type developers and investigated thermochromic behavior of the mixtures of FD and developers from the conversion of colored to discolored state by using SR-XRD, thermal analysis, optical and FT-IR techniques. These results showed that the supramolecular complex structure exhibited the highest optical density at the developer-to-FD ratio of 8:1. Compared with the previously proposed phosphonic-type developer, the rate of discoloring became quite higher. These findings provide a new guideline to design the thermochromic mixture system for high-speed rewritable paper devices. Acknowledgment. We are deeply indebted to Professor Masaharu Nomura, High Energy Accelerator Organization, for helping with SR-XRD measurements, and Mr. Takeshi Sadoshima and Mr. Yosuke Bando, Hiroshima University, for their assistance in experimental observation. Supporting Information Available: Absorption spectral changes of P18/FD, PC18/FD, and PU18/FD solid films in Figures S1-S3. Stoichiometry analysis of PU18/FD in Figure S4. DSC data of single PC18 and PU18 in Figure S5 and S6. This information is available free of charge via the Internet at http://pubs.acs.org.

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