Gold-Colored Organic Crystals of an Azobenzene Derivative

Article pubs.acs.org/Langmuir

Gold-Colored Organic Crystals of an Azobenzene Derivative Yukishige Kondo,* Akiko Matsumoto, Kengo Fukuyasu, Kazuya Nakajima, and Yutaka Takahashi Department of Industrial Chemistry, Faculty of Engineering, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku, Tokyo 162-8601, Japan ABSTRACT: A new azobenzene derivative, bis[4-(3methylbutoxy)phenyl]diazene (DC-azo), was synthesized as yellow powder through a three-step reaction. DC-azo was crystallized from a mixture of acetone/water in the form of highly insulating gold-colored crystals with a specular reflectance of 21% over the wavelength range of 520−800 nm. The CIELAB color space of the crystals indicated that the crystals were slightly more reddish and less yellowish than metallic Au. The crystals retained their gold coloration under dry conditions and were stable up to 383 K. The X-ray diffraction (XRD) measurements estimated the long-range periodical thickness in the crystal to be 1.12 nm, thereby indicating that the DC-azo molecules were oriented such that the azobenzene moieties showed a tilt angle of 32.2° against the crystal surface. The gold-colored crystals of DC-azo could be a potential substitute for the conventional metallic paints.

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INTRODUCTION The pursuit of technologies to create noble metals such as gold and silver from base metals (i.e., alchemy) has contributed considerably to the development of modern chemistry. In particular, gold has fascinated people since ancient times because of its worth and beauty. In fact, gold-colored origami is the most popular form of origami among children, who practice creating origami figures using folded colored paper found in commercial origami sets. Recently, we succeeded in synthesizing gold-colored organic crystals from an azobenzene derivative, bis[4-(2dimethylaminoethoxy)phenyl]diazene (DN-azo).1 Although some polymers are known to be golden in color,2 there have been very few reports describing gold-colored low-molecularweight (MW) materials.3 The only reports of gold-colored materials from low-MW compounds have been from Ogura et al.4−6 They found that the conductive crystals of π-conjugated compounds with a thiophene−pyrrole−thiophene chemical skeleton produced a gold coloration. DN-azo has an azobenzene skeleton; therefore, we can propose that DN-azo is a new class of low-MW compounds that can produce goldcolored crystals. However, when the gold-colored crystals of DN-azo were placed under dry conditions, they because yellow (Figure 1c).1 Metallic paints that yield painted films having metallic luster contain metal fine particles. Thus, the painted films obtained from conventional metallic paints are usually heavy. When metallic paints are applied to large vehicles and aircrafts, such heavily painted films bring about enormous mileage and extra carbon dioxide emission. In this regard, the gold-colored crystals from DN-azo are advantageous because the density of the organic compounds is much smaller than that of metals. Furthermore, painted films from metallic paints shield radio waves because the films are conductive owing to the presence © 2014 American Chemical Society

Figure 1. Visual appearance of azobenzene compounds. (a) DC-azo powder, (b) DC-azo crystal, (c) DN-azo crystal after being placed in a dry desiccator, and (d) DC-azo crystal after being placed in an oven, followed by placement in a desiccator.

of metal fine particles. This hinders the application of metallic paints in communication devices including cell phones. Therefore, the isolated gold-colored crystals from DN-azo are Received: January 4, 2014 Revised: March 15, 2014 Published: April 1, 2014 4422

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Scheme 1. Synthesis of DC-azo

with two ring electrodes (ring diameters of 6 mm and 11 mm) at 25 °C. Synthesis. Azobenzene derivative 1, bis[4-(3-methylbutoxy)phenyl]diazene, referred to as DC-azo, was synthesized as shown in Scheme 1. All of the resultant compounds were characterized using 1H NMR and MS. Intermediate 3 was synthesized following a previous method.1 DC-azo was synthesized as follows: the reaction mixture of 3 (3.56 g, 16.6 mmol), K2CO3 (16.08 g, 116.3 mmol), 1-bromo-3methylbutane (25 g, 166 mmol), and acetone (35 mL) was refluxed for 48 h. After K2CO3 was filtered out of the reaction mixture, the solvent (acetone) was evaporated. DC-azo was obtained as a gold solid by the recrystallization of the crude product from a mixture of acetone and water, followed by drying under reduced pressure. Yield 4.24 g (72%). 1H NMR (solvent (CD3)2SO) δ 0.94 (12H, doublet (d), J = 12 Hz), 1.78−1.83 (2H, multiplet (m)), 1.67 (4H, m, J = 7 Hz), 4.11 (4H, triplet (t), J = 8 Hz), 7.11 (4H, d, J = 8 Hz), 7.83 (4H, d, J = 8 Hz). HRMS(FAB+) m/z: obsd, 355.2390 [M + H]+; calcd, 355.2386 [M + H]+. Preparation of Gold-Colored Crystals. To a solvent mixture of water (8 mL) and acetone (20 mL) was added DC-azo (0.05 g). After the mixture was heated to 80 °C to dissolve DC-azo, it was allowed to stand for 24 h to crystallize DC-azo at room temperature (ca. 25 °C). Then, the mixture was placed in a refrigerator (3 °C) for 24 h to complete the crystallization of DC-azo. The resultant gold-colored crystals were recovered by filtration over a piece of round filter paper (diameter, Ø = 21 mm).

superior to metallic paints as well as the aforementioned conductive organic crystals discovered by Ogura et al. In light of the information above, the crystals of DN-azo have potential application potency whereas the crystals’ color changes from gold to yellow under dry conditions. It is known that DN-azo molecules produce a golden color as they form J aggregates stabilized by the adsorption of water molecules to the amino groups in the molecule in the crystals.1 When the gold-colored DN-azo crystals are dried, the adsorbed water molecules are evaporated to collapse the J-aggregates, leading to the crystals’ color change from gold to yellow.1 Therefore, we recently synthesized a new compound that is structurally analogous to DN-azo and has no amino groups and finally explored the synthesis of long-lasting gold-colored crystals from low-MW compounds. We describe herein a novel low-MW compound that yielded gold-colored crystals even under dry conditions. This novel compound, which does not suffer from the drawbacks of the DN-azo crystals, can be a potential material for fabricating metal-colored films on the surfaces of various solid materials.

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EXPERIMENTAL SECTION

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Materials. 1-Bromo-3-methylbutane was obtained from TCI (Tokyo, Japan), and K2CO3 and acetone were purchased from Kanto Chemical (Tokyo, Japan). These reagents were all used as received. The water used in this study was high-purity H2O (Milli-Q pure water; R = 18 MΩ·cm). An Au-coated microscope slide (Thermo Spectra-Tech Inc., Shelton, CT) was used as a thin film for Au evaporated on a glass slide. Measurements. A Bruker DPX-400 spectrometer was used to record the 400 MHz 1H NMR spectra of the synthesized compounds at 30 °C; fast atom bombardment mass spectrometry (FAB-MS) measurements were performed using a JEOL JMS-SX102A mass spectrometer. The specular reflectance, total reflectance, and CIE L*a*b* (CIELAB) color space7 of the gold-colored materials were measured at 25 °C using a JASCO V-570 spectrophotometer equipped with a JASCO ILN-472 integration sphere (incident light angle 5°). Xray diffraction (XRD) measurements were performed on a Rigaku Ultima IV X-ray diffractometer at a scan rate of 0.5° s−1 over 2θ ranges from 3 to 80° at 25 °C using monochromatic Cu Kα radiation (λ = 0.154178 nm, 40 kV, 40 mA) in which DC-azo crystals were placed on a nonreflecting silicon substrate. The melting point and decomposition temperature of the sample were measured using a Rigaku Thermoplus TG8120 at a rate of 1.0 K·min−1. The surface conductivity was measured using a Hiresta IP HT260 (Mitsubishi Chemical Co. Ltd.)

RESULTS AND DISCUSSION Preparation of Gold-Colored DC-azo Crystals. DC-azo was obtained as a yellow solid (Figure 1a) that easily dissolved in acetone and methanol but remained insoluble in water. The recrystallization of DC-azo from a mixture of acetone/water as described in the Experimental Section yielded gold-colored crystals. Figure 1b demonstrates the gold-colored crystals of DC-azo recovered by filtration, after recrystallization, on a piece of filter paper. As shown in Figure 1b, the gold-colored DC-azo crystals seem to have the same color and luster as metallic gold (Au). The luster of the DC-azo crystals was compared to that of the metallic Au thin film. As seen in Figure 2, the specular reflectance of the gold-colored DC-azo crystals started to increase at 480 nm and became constant above 550 nm, producing a value of 21% in the range of 550−800 nm. This indicated that the DC-azo crystals absorbed the wavelengths for colors in the range from green to red more favorably than that for yellow. Although the maximum specular reflectance of DC-

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Probable Structure of DC-azo Crystals. To evaluate the structure of gold-colored DC-azo crystals, a 1-D XRD spectrum was recorded. The result is shown in Figure 4. Several peaks

Figure 2. Specular reflectance of DC-azo crystals and a metallic gold (Au) thin film.

azo was smaller than that of the Au thin film, the curve obtained by plotting the specular reflectance against the wavelength of the gold-colored DC-azo showed the same profile as that of the corresponding curve of Au. This meant that the gold-colored DC-azo crystals possessed a golden luster. In terms of luster, DC-azo crystals were superior to DN-azo crystals because the maximum specular reflectance of DC-azo and DN-azo crystals was 21 and 15%, respectively. Figure 3 shows the CIELAB color space of the DC-azo crystals and Au thin film. The L* value indicates the degree of

Figure 4. One-dimensional XRD pattern of DC-azo crystals.

were observed at regular intervals (2θ = 7.86, 15.64, 23.48, 31.46, 39.56, 47.90, 56.50, and 65.48°). The ratio of 2θ values of the peaks was 7.86:15.64:23.48:31.46:39.56:47.90° = 1:2:3:4:5:6, indicating that lamellar structures existed in the crystals. The layer thickness of a lamellar structure can be calculated using Bragg’s law as follows

d=

nλ 2 sin θ

(2)

where n is an integer and d is the layer thickness of a lamellar structure. d was calculated to be 1.12 nm for the crystal. The crystals would be primarily formed by the stacking of 1.12-nmthick molecular layers consisting of DC-azo. The optimization of the molecular structure of DC-azo through MM2 calculations (ChemBio3D Ultra 13) provided a molecular length of 2.1 nm (minimized steric energy: 1.15863 × 105 J· mol−1). As shown in Figure 5, DC-azo molecules would be

Figure 3. CIELAB color space of DC-azo crystals and the Au thin film.

lightness of a sample, whereas a* and b* values show the degrees of redness and yellowness of the sample. The L*, a*, and b* values of DC-azo crystals were 79.4, 9.4, and 51.4, respectively. Compared to the color of the Au thin film, the color of the DC-azo crystals was slightly reddish and greenish. The color difference (ΔEab) between DC-azo crystals and the Au film was calculated using eq 1: ΔEab = [(ΔL*)2 + (Δa*)2 + (Δb*)2 ]1/2

Figure 5. Probable structure of DC-azo crystals.

(1)

The ΔEab value between the DC-azo crystals and Au film was 15 because the crystals were slightly more reddish and less yellowish than metallic Au. It is known that human beings cannot visually recognize the color difference between two samples if the ΔEab value between the samples is less than 1.8 Thus, one will be able to detect the color difference between DC-azo gold crystals and the metallic Au film. We plan to bring the color of the DC-azo gold crystals closer to that of Au in the future.

oriented with the azobenzene moieties at a tilt angle of 32.2° against the crystal surface. The resultant molecular orientation provided evidence of the existence of monolayers with a thickness of 1.12 nm. A number of monolayers would be stacked in the gold-colored crystals either in the herringbone structure or in another structure. Long-Lasting Gold Color. The melting point of DC-azo was 399 K (126 °C), and the temperature of decomposition 4424

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was 467 K (194 °C). The gold-colored crystals were held at 383 K for 5 h, followed by storage for 24 h in a desiccator (humidity 10%). Figure 1d depicts the state of the crystals after being placed in the desiccator. As seen, the crystal retained its gold coloration. Unlike in the case of the gold-colored crystals of DN-azo,1 DC-azo crystals were stable against heat and dryness. We reported that the gold-colored crystals of DN-azo were formed by the stacking of 1.51-nm-thick monolayers and the monolayers were stabilized by the water molecules (crystallized water molecules) adsorbed to the dimethylamino groups in the DN-azo molecule through H-bonding between (CH3)2Nδ−···δ+H−O−H.1 Dryness evaporated the adsorbed water molecules around the dimethylamino groups to disrupt the monolayer in DN-azo crystals as the color of the crystals turned to yellow from gold.1 Unlike in the case of DN-azo, DC-azo molecules have no amino groups that act as adsorption sites for water molecules; therefore, the 1.12-nm-thick monolayers of gold-colored DCazo crystals will be stabilized without the adsorption of water molecules, which brings about stability against heat and dryness. Because azobenzenes can undergo trans−cis photoisomerization under UV irradiation, the stability of DC-azo crystals under UV light was also evaluated. UV light (wavelength 365 ± 15 nm) was used to irradiate DC-azo crystals for 12 h at 25 °C using a Handy UV lamp SLUV-8 (ASONE, Japan). In terms of the CIELAB color space, the dependence of specular reflectance on the wavelength and the 1-D XRD pattern, there were no differences between DC-azo crystals before and after UV irradiation. Although UV light irradiation may cause trans−cis photoisomerization against molecules located at the outermost surface of DC-azo crystals, it will not affect the entire crystal structure and gold luster. Mechanism of Formation of Gold-Colored Crystals. It is well known that conductive polymers possess metallic luster.2,9 When the conductivity of DC-azo crystals was measured, it was less than the lower limit of measurement, indicating that the crystals have highly insulating properties. Thus, it is understood that the gold color of DC-azo is not attributed to electron mobility (free electrons). The metallic luster of insects such as morpho butterflies and jewel beetles is attributed to the structural color, which is dependent on the observation angle.10,11 Figure 6 shows the total reflectance of DC-azo gold crystals measured from three different detection angles (5, 30, and 45°). At any of the detection angles, the total reflectance began to increase at 480 nm and became constant beyond 580 nm. On comparing the total reflectance observed at the detection angle of 45° to those at 5 and 30°, the maximum value (constant total reflectance beyond 580 nm) at 45° was higher than those at 5 and 30°. However, the shape (profile) of the total reflectance versus wavelength curve was identical for all three different detection angles, thereby indicating that the color of the DC-azo crystals had almost no dependence on the observation angle. Thus, it was clear that the gold coloration of the DC-azo crystals was not based on the structural color.

Figure 6. Total reflectance of DC-azo crystals at three different detection angles of 5, 30, and 45° (incident light angle = 5°).

to at least 383 K. The investigation of the mechanism of formation of the gold coloration in DC-azo crystals is currently in progress.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +81-3-5228-8313. Fax: +81-3-5228-8313. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by KAKENHI (a JSPS grant-in-aid for scientific research (C)) (21550182). We are grateful to Mr. Masakatsu Kato and Mr. Yuki Nakagawa for their experimental support.

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REFERENCES

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CONCLUSIONS A new azobenzene derivative, bis[4-(3-methylbutoxy)phenyl]diazene (DC-azo), was successfully synthesized. DC-azo formed highly insulating gold-colored crystals that retained their gold coloration under dry conditions and were stable up 4425

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(-thienyl)-5-[5-(tricyanoethenyl)-2-theinyl]pyrroles. Bull. Chem. Soc. Jpn. 2002, 75, 2359−2370. (7) Fairchild, M. D. Color Appearance Models, 3rd ed.; John Wiley & Sons. Inc.: Chichester, West Sussex, U.K., 2013. (8) MacCormack, I. B.; Bowers, J. New White Gold Alloys. Gold Bull. 1981, 14, 19−24. (9) Kanatzidis, M. G.; Wu, C. G.; Marcy, H. O.; Kannewurf, C. R. Conductive-Polymer Bronzes. Intercalated Polyaniline in Vanadium Oxide Xerogels. J. Am. Chem. Soc. 1989, 111, 4139−4141. (10) Kinoshita, S.; Yoshioka, S.; Kawagoe, K. Mechanisms of Structural Colour in the Morpho Butterfly: Cooperation of Regularity and Irregularity in an Iridescent Scale. Proc. R. Soc. London, Ser. B 2002, 269, 1417−1421. (11) Vignolini, S.; Rudall, P. J.; Rowland, A. V.; Reed, A.; Moyroud, E.; Faden, R. B.; Baumberg, J. J.; Glover, B. J.; Steiner, U. Pointillist Structural Color in Pollia Fruit. Proc. Natl. Acad. Sci. U.S.A. 2012, 109, 15712−15715.

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