A Cationic Azobenzene Dye-Montmorillonite Intercalation Compound

Preparation and Photochemical Behavior of a Cationic Azobenzene Dye-Montmorillonite Intercalation Compound WAN Tao, XU Huihua, YUAN Yi, HE Wenqiong(College of Materials and Chemistry & Chemical Engineering, Chengdu University of Technology, Chengdu 610059, China)Abstract: Montmorillonite/cationic azobenzene dye(p -(δ-triethylammoniobutoxy)-p ′-methyl- azobenzene bromide) intercalation compounds were prepared by the conventional ion exchange method. As compared with that of pure cationic azo-dye, the thermal stability of the intercalated dye was greatly enhanced, and the absorption band corresponding to azobenzene group in intercalated dye shifted towards longer wave length by 38 nm. This could be ascribed to the strong conjugation of cationic azo-dye supramolecular order structure(J cluster) confined in a nanoscale space of montmorillonite interlayer gallery. UV/vis spectra data show that the intercalated azo dye in the montmorillonite interlayer space exhibited reversible trans-to-cis photoisomerization and daylight cis-to-trans back reaction. FTIR indicates the successful intercalation ofcationic azo-dye into the montmorillonite interlayer.Key words:Na-montmorillonite; azobenzene; intercalation; photoisomerizationDOI 10.1007/s11595-006-3466-71 IntroductionO r g a n i c /i n o r g a n i c n a n o c o m p o s i t e s b y intercalation technique using layered silicate have attracted much attention due to their specific properties such as photocatalysis, photoluminescence, photochromism and optical nonlinearity [1-7].Azobenzenes are well-known photochromic dyes capable of trans–cis isomerization. The photochromic reactions of these compounds in the interlayer space of montmorillonite have been extensively investigated [1-3], and the photochromic behavior has been found to be different from that observed in solutions or other solid matrixes. In order to facilitate the introduction of azobenzene into the interlayer space, the silicates were made hydrophobic by the intercalating alkylammonium surfactants [4]. It was found that the orientation of alkylammonium ions depends largely on their packing density and the length of the alkyl chains. Lagaly et al [8] postulated that the alkyl chains lie flat on the silicate surface in “monolayer, bilayer, pseudotrimolecular arrangement” (through kink formation) or radiate away from the surface “paraffin-type”, depending on the charge density and the chain length. A l t h o u g h t h e i n t e r c a l a t e d a z o b e n z e n e s photoisomerized effectively in the hydrophobic interlayer space of the organoammonium-silicates [5],it was difficult to evaluate and control the location and the orientation of the intercalated azo dyes. In order to overcome this limitation, amphiphilic cationic azobenzene derivatives have been used as the guest species [6].In this paper, the intercalation of a cationic azo dye, p -(δ-triethylammoniobutoxy)-p ′-methyl-azobenzene bromide (shown in Scheme 1) into montmorillonite was reported. The spectral properties as well as the X-ray diffraction results of the supramolecular systems revealed the information on the interlamellar organization and orientation of the cationic azo dye intercalated into the layered silicate. In the present system, the intercalated J-like aggregate azo dye in the interlayer silicate exhibited trans-to-cis photoisomerization and daylight cis-to-trans back reaction, as evidenced by the basal spacing as well as the visible absorption spectra of the intercalation compounds.2 Experimental 2.1 Materials The Na-montmorillonite was a reference clay sample provided by the Qinghe Chemical Plant of China. The cation-exchange capacity of Na montmorillonite was 70 mequiv/100 g of clay. The cationic azo dye, p -(δ-triethylammoniobutoxy)-p′-Scheme 1 Molecular structure of p -(δ-triethylammoniobutoxy)-p ′-methyl-azobenzene bromidemethyl- azobenzene bromide(AZ+) was used after the recrystallization from ethanol.2.2 Preparation of the GTL-montmorillonite intercalation compoundIntercalation of p-(δ-triethylammoniobutoxy)-p′-methyl-azobenzene bromide(AZ+) into montmorillonite was carried out by a conventional ion exchange reaction as followed: 2 g sample of Na-montmorillonite was dispersed in a mixture of 100 mL water and 60 mL ethanol by stirring at 70 ℃ for 1 h and sonication for 10 min. It was added dropwise by stirring the required amount of p-(δ-triethylammoniobutoxy)-p′-methyl- azobenzene bromide dissolved in 40 mL of ethanol, and the reaction mixture was stirred at 70 ℃ for 24 h. At the end of the reaction, the mixture was centrifuged. The yellowish solid precipitate was washed with water and dried at room temperature. The intercalation compounds obtained were dispersed in PVP solutions, and casted on glass substrates to obtain supported films. The films were used for the photochemical studies. 2.3 Photochemical reactionsThe photochemical reactions of the intercalated p-(δ-triethylammoniobutoxy)-p′-methyl-azobenzene bromide were carried out by UV irradiation with a 500 W super-high pressure Hg lamp. The reactions were monitored by the change in the absorbance of the trans-isomer of the azobenzene.2.4 CharacterizationThermogravimetric analysis(TGA) of the intercalation compound was conducted on a Dupont 2100 instrument. The temperature program was 10.0 K/min from 20 ℃ to 800 ℃ in N2 atmosphere. The mass loss with increasing temperature as well as its first derivative (DTG) that represented the change in decomposition rate were plotted. X-ray powder diffraction patterns were obtained on a Mac Science, MXP3 diffractometer using monochromatic Cu Kα radiation(λ=0.15418 nm). To determine the peak positions, the diffractograms were fitted with a split Pearson VII function (TOPAS P profile fitting program, Bruker AXS) and the basal-plane spacing (d001) was calculated by applying Bragg’s law. UV-visible absorption spectra were recorded on a Thermo Spectronic Genesys 10 series spectrophotometer with a quarts cuvette possessing an optical path length of1 cm. Infrared spectra were taken with a 510 Nicolet FTIR spectrometer.3 Results and Discussion3.1 Microstructural analysesFig.1 shows the X-ray powder diffraction patterns for pristine Na-montmorillonite and AZ+-montmorillonite intercalation compounds. As compared with that of pristine Na-montmorillonite, the basal spacing of the intercalation compounds increased from 1.23 nm to 1.87 nm, indicating successful intercalation of AZ+ into the interlayer of Na-montmorillonite by a cation exchange reaction. The interlayer distance(Δd) could be obtained by subtracting the thickness of an individual layer of montmorillonite (0.96 nm) from the observed basal spacings(d001), and the tilt angle(θ) of AZ+ in the interlayer of Na-montmorillonite could be calculated as follows:Δd=d001–0.96=(I C+I G)sinθ (1)where, I C is the hydrophobic chain length of AZ+ ;I G is the length of hydrophilic polar head of AZ+.The structure of AZ+, as simulated by Gaussian 98 software, is shown in Fig.2, and I C and I G of AZ+obtained from Fig.2, were 2.01 nm and 0.25 nm, respectively. As calculated from Eq(1), Δd of the intercalation compounds was 0.91 nm and the corresponding tilt angle of AZ+ was 26.4°, suggesting an interdigitated monolayer of AZ+ with the molecular long axis inclined to the silicate layer, as schematically shown in Fig.3(c).As Maged reported[9]that short alkyl chains, such as cationic dyes, could lie flat on the substrate surface as long as there was enough space. However, with increasing amounts of the organic molecules intercalated into the silicate interlayers, the organic chains forced the layers to enlarge their basal-plane spacing. At a certain length and number of chains, the molecules adopted an ordered state due to increasing chain interactions and packing density. To minimize their conformational entropy and maximize their packing density, the chains attached to platelets facing each other interdigitated. The average organic molecular axis was inclined to the montmorillonite surface normal by an angle, and thus the interdigitatedmonolayer was formed, as shown in Fig.3c.By embedding the AZ+-montmorillonite into a polyvinylpyrrolidone (PVP) solution, a thin film was obtained. The basal spacing was the same as that of the powdered sample, suggesting that the arrangement of the intercalated AZ+ did not change during the film preparation.The UV-visible absorption spectra of a 1.0×10–4 M chloroform solution of AZ+ and AZ+-montmorillonite are shown in Fig.4. The absorption band of the AZ+-montmorillonite intercalation compound due to the trans-azobenzene chromophore was observed at around 504 nm, which was significantly shifted from that of AZ+chloroform solution (466 nm). In the molecular assembly, the chromophore interacted to give aggregated states and the dye-dye interactions caused both bathochromic and hypsochromic spectral shifts depending on the microstructures[10].According to Kasha’s molecular exciton theory[11], the observed absorption band bathochromic shift of the AZ+-montmorillonite was ascribable to J-aggregates (head-to-tail aggregates) of the intercalated AZ+. This was consistent with the proposed orientation (Fig.3(c)) speculated from the X-ray diffraction result. In summary, from these observations on the variation of the basal spacings and the UV-visible absorption spectra, it is shown that the intercalated AZ+ cations from an interdigitated monolayer gave a J-aggregate in the interlayer space of montmorillonite.3.2 BehaviorsAfter UV irradiation for 30 min, the absorbance band of the AZ+-montmorillonite intercalation compound due to the trans isomer (at around 504 nm) decreased (Fig.5), indicating an isomerization reaction induced by UV irradiation. However, when the irradiated sample was exposed to daylight for 24 h, a reversed isomerization reaction took place. Cis-siomers that formed with UV irradiation changed back to trans-isomers. As shown in Fig.5, the adsorption band due to the trans-isomer recovered its original intensity (Fig.5(b)). A reversible spectral change was observed repeatedly. This spectral change could be ascribed to the trans-to-cis photoisomerization and the daylight cis-to-trans back reaction of the intercalated AZ+.I t i s w o r t h n o t i n g t h a t t h e a z o b e n z e n e chromophore photoisomerized effectively in the interlayer space of montmorillonite, despite the fact that the azobenzene chromophore aggregated in the interlayer space, and the photoisomerization of azobenzene chromophore might be restricted.The thermal stability of AZ+-montmorillonite intercalation compound was evaluated using the TGA technique in programmed heating process(10 ℃/min). The results are shown in Fig.6. From the differential curves of TGA, the temperatures at which there was a maximum mass loss rate(T max), as well as the decompose onset temperature(T onset), was consideredto be a parameter for the estimation of the thermalstability.The TG and DTG thermogram for AZ+(Fig.6(a)) exhibits a gradual mass loss starting from ambient temperature and one decomposition peak at 258.9 ℃, with a corresponding mass loss of 24.6% and a DTGA of 15.33%•min–1. On the other hand, the TG and DTG thermogram for AZ+-montmorillonite intercalation compound (Fig.6(b)) exhibits two peaks; the first peak, attributed to the decomposition of AZ+, increased to 300.8 ℃, with a corresponding mass loss of 5.2% and a DTGA of 3.87%•min–1; the second dehydroxylation peak at 664.8 ℃ corresponded to the decomposition of structural hydroxyl groups in the aluminosilicate.It is noteworthy that an enhanced thermal stability of AZ+ could be achieved by way of intercalating it into the montmorillonite. As shown in Fig.6, T onset of AZ+ intercalated in the montmorillonite, increased from 248.5 ℃ to 290.6 ℃, accompanied by T max increasing from 258.9 ℃ to 300.8 ℃, as compared with that of pure AZ+. This remarkable enhancement of thermal stability of intercalated AZ+ molecules was attributed to their restricted mobility and their spatial confinement as well as the montmorillonite acting as a superior insulator and mass transport barrier to the volatile products generated during decomposition.3.3F T I R s t u d y o f t h e i n t e r c a l a t i o n compoundsFig.7 shows the FTIR spectra of the samples in the frequency range of 4 000-400 cm–1. In the FTIR spectrum of Na-montmorillonite, both stretching and bending vibrations of Al-OH-Al were observed at 3 629 and 915 cm–1. This could be considered to be characteristic of a dioctahedral smectite[12]. The bands at 3 445 and 1 640 cm−1 were the OH stretching and bending frequencies of the hydration water. The stretching vibration of Si-O groups was accompanied by two absorption bands at 1 154 and 1 090 cm–1. The tetrahedral bending modes were at 525 cm−1 for Si-O-Al, at 470 cm−1 for Si-O-Mg, and at 428 cm−1 for Si-O-Si[13]. In the FTIR spectra of AZ+-montmorillonite, all the bands belonging to the original bentonite were reserved. Besides, the bands at 2 950 and 2 865 cm−1 were assigned as the C-H stretching of methyl and methylene groups in AZ+ dye. On the other hand, the band at 1 499 cm−1 was due to deformation bands of the methyl and methylene groups attached to the cation N+ of AZ+. Therefore, the bands at 2 950, 2 865 and 1 499 cm−1 indicated the successful intercalation of AZ+ into the bentonite interlayer.4 ConclusionsThe intercalation of a cationic azo dye into the interlayer space of montmorillonite has been achieved by the conventional ion exchange between sodium-montmorillonite and p-(δ-triethylammoniobutoxy)-p′-methyl- azobenzene bromide(AZ+), as indicated by FTIR spectra. As compared with that of pure AZ+, the thermal stability of the intercalated dye was greatly enhanced, as indicated by TGA and DTGA curves, and the absorption band corresponding to azobenzene group in intercalated AZ+ shifted towards longer wave length by 38 nm, as shown by UV/vis spectra. This could be ascribed to the strong conjugation of AZ+ supramolecular order structure (J cluster) confined in a nanoscale space of montmorillonite interlayer gallery. The intercalated J-like aggregate AZ+ in the interlayer space exhibited reversible trans-to-cis photoisomerization and daylight cis-to-trans back reaction.References[1] M Ogawa, A Ishikawa. Controlled Microstructures ofAmphiphilic Cationic Azobenzene - Montmorillonite Interaction Compounds[J]. J. Mater. Chem. ,1998,8:463-467 [2] T Ikeda, T Sasaki, K Ichimura. Photochemical Switching ofPolarization in Ferroelectric Liquid-Crystal Films[J]. Nature, 1993, 361:428-430[3] Y Lansac,M A Glasser, N A Clark, et al. 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