J O U R N A L O F C H E M I S T R Y Materials Controlled microstructures of amphiphilic cationic azobenzenemontmorillonite intercalation compounds Makoto Ogawa*a,b and Ayako Ishikawab aPRESTO, Japan Science and T echnology Corporation (JST) and bInstitute of Earth Science, Waseda University, Nishiwaseda 1–6-1, Shinjuku-ku, T okyo 169–50, Japan The intercalation of two amphiphilic cationic azobenzene derivatives into the interlayer space of montmorillonite has been conducted by the ion exchange reactions between sodium montmorillonite and p-(v-trimethylammoniopentyloxy)-p¾- (dodecyloxy)azobenzene bromide or p-(v-trimethylammoniodecyloxy)-p¾-(octyloxy)azobenzene bromide.X-Ray diVraction and elemental analysis results indicated that the dye cations were intercalated into the interlayer space of montmorillonite.The spectral properties as well as the X-ray diVraction results have revealed that the adsorbed azo dye cations form so-called J-like aggregates with mono- and bi-layers in the interlayer space of montmorillonite. The orientation of the chromophore was controlled by host– guest and guest–guest interactions. The intercalated azo dyes exhibit reversible trans–cis photoisomerization by UV and visible light irradiation.Intercalation of guest species into layered inorganic solids is a method of producing ordered inorganic–organic assemblies with unique microstructures controlled by host–guest and guest–guest interactions.1,2 Among possible layered solids, the smectite group of layered clay minerals provides attractive features such as large surface area, swelling behavior, and ion exchange properties for organizing organic guest species.3,4 The organization of photoactive species on the surface of smectites has been investigated to probe the surface properties of smectites as well as to construct novel photofunctional supramolecular systems,5,6 since the photoprocesses of the photoactive species are environmentally sensitive.7,8 Along this line, photochromic reactions of organic dyes in the interlayer space of smectites have been reported.9–17 We have been interested in the photochemistry of azobenz- C8H17 O C10H20 N N O N+ CH3 CH3 CH3 C12H25 O C5H10 N N O N+ CH3 CH3 CH3 C8AzoC10N+ C12AzoC5N+ Scheme 1 Molecular structures of the amphiphilic azo dyes used enes in the interlayer spaces of layered silicates.14–17 The construction of photoresponsive supramolecular systems based on the photochemical isomerization of azobenzene has been silicates through electrostatic attractions between the negaintensively studied.18 The photoisomerization of azobenzene tively charged surface of the silicate layer and the cationic dyes in the interlayer space may lead to novel photoresponsive as well as dye–dye interactions.inorganic–organic nanocomposites. The hydrophobic modification of the surface properties of Experimental smectites by the intercalation of surfactants11–16,19,20 has been conducted for the introduction of azobenzenes into the inter- Materials layer spaces of smectites.11–16,19,20 Although the intercalated Sodium montmorillonite (Kunipia F, Kunimine Industries Co.; azobenzenes isomerize eVectively in the hydrophobic interlayer reference clay sample of The Clay Science Society of Japan) space of organoammonium silicates,14–16 it has been diYcult was used as the host material.The cation exchange capacity to evaluate and control the location and the orientation of the (c.e.c.) of the Na-montmorillonite is 119 mequiv./100 g of clay.intercalated azo dyes in the hydrophobic interlayer space. The two amphiphilic azo dyes C12AzoC5N+Br and In order to overcome this limitation, a cationic azobenzene C8AzoC10N+Br were purchased from Sogo Pharmaceutical derivative has been used as the guest species.17 In this paper, Co. and used without further purification. we report the intercalation of two amphiphilic azobenzene derivatives p-(v-trimethylammoniopentyloxy)-p¾-(dodecyloxy)- Sample preparation azobenzene bromide (C12AzoC5N+) and p-(v-trimethylammoniodecyloxy)- p¾-(octyloxy)azobenzene bromide, Intercalation of C12AzoC5N+ and C8AzoC10N+ into mont- (C8AzoC10N+) (molecular structures are shown in Scheme 1) morillonite was carried out by a conventional ion exchange into montmorillonite and the photochemical reactions of the method in which an aqueous suspension of montmorillonite azo dyes in the interlayer spaces are reported.A series of was mixed with an ethanol solution of C12AzoC5N+Br- or amphiphilic azo dyes with variable alkyl chain length have C12AzoC5N+Br- (0.014 M) and the mixture was allowed to been synthesized and the formation of self-assembled structures react for one day at 70 °C.The amount of the added dye was has been observed in aqueous solutions and in films.21–25 In 1.2 times excess of the cation exchange capacity of the clay, the present system, it seems possible to control the orientation since excess amounts of amphiphilic species may be adsorbed as a salt (intersalation). After centrifugation, the resulting of the chromophore in the interlayer space of swelling layered J.Mater. Chem., 1998, 8(2), 463–467 463yellowish solid was washed with ethanol and dried under is ascribable to the diVerence in the size of the dye cations as well as the amounts of the adsorbed dyes. reduced pressure at room temperature. The intercalation compounds were dispersed in toluene with sonication and casted The compositions of the products were determined by elemental analyses as C, 27; N, 3% for the C12AzoC5N+ on quartz substrates, so that thin films were obtained.The thin films are used for the photochemical studies. montmorillonite intercalation compound and C, 26; N, 3% for the C8AzoC10N+ montmorillonite intercalation compound. From the elemental analyses, the amounts of the adsorbed azo Characterization dyes were determined as ca. 110 and 100 mequiv./100 g clay X-Ray powder diVraction patterns of the products were for the C12AzoC5N+ and the C8AzoC10N+ montmorillonite recorded on a Rigaku RAD-IA diVractometer using monochrointercalation compounds, respectively. These values indicate matic Cu-Ka radiation. Absorption spectra of the films were that the cation exchange between sodium ions and recorded on a Shimadzu UV-3100PC spectrophotometer. The C12AzoC5N+ or C8AzoC10N+ ions occurred almost quantitatcomposition of the products were determined by the CHN ively.As observed for the intercalation of long chain alkylamanalysis (Yanaco MT-3). monium ions into smectites, the two amphiphilic azo dyes preferred to occupy the interlayer space of montmorillonite to Photochemical reactions replace the interlayer sodium ions eVectively.From the observed basal spacings and the sizes of The photochemical reaction of the intercalated azobenzene C12AzoC5N+ and C8AzoC10N+ ions, the orientation of the was conducted by UV and visible light irradiation with a intercalated dye cations can be discussed. Supposing that the 500W super high pressure Hg lamp (USHIO USH-500D).A alkyl chains of the two amphiphilic azo dyes were fully band-pass filter, Toshiba UV-D35, with transmittance centered extended, two types of orientation can be expected from the at 350 nm, was used to isolate the UV light. For the cis–trans observed basal spacings. One is an interdigitated monomolecu- reverse reactions, a sharp cut filter, HOYA L42 (cut-oV wavelar layer of the dyes with the alkyl chains inclined to the length 420 nm) was used to obtain visible light.The reactions silicate sheet. The other is a bilayer coverage of the dyes with were monitored by the change in the absorbance of the transtheir alkyl chains inclined to the silicate sheet. Note that the isomer of the azobenzene.A sample film was set in a cryostat tilt angles are diVerent in the two models. with optical windows (Oxford DN-1704), and the photochem- The intercalation of the amphiphilic azo dyes alters the ical reactions were performed at constant temperatures between surface properties of montmorillonite to strongly organophilic 100 and 400 K for a single sample. as has been observed for the long chain alkylammonium smectites.26–28 Consequently, the C12AzoC5N+ and Results and Discussion C8AzoC10N+ montmorillonite intercalation compounds swell in organic solvents such as toluene and chloroform. Thin films In the reaction between C12AzoC5N+Br- and Nawere obtained by casting the suspension in toluene onto a montmorillonite, a yellowish solid was obtained.The XRD quartz substrate.The X-ray diVraction patterns of the films pattern of the product is shown in Fig. 1( b), together with that are shown in Fig. 1(c) and (e) for the C12AzoC5N+ and of Na montmorillonite [Fig. 1(a)]. The basal spacing of the C8AzoC10N+ montmorillonite intercalation compounds, product was 2.4 nm, indicating an interlayer expansion of respectively. The basal spacings of the films (2.4 and 2.5 nm 1.4 nm.(The thickness of the silicate layer of montmorillonite for the C12AzoC5N+ and C8AzoC10N+ montmorillonite inter- is 9.6 A ° .) A yellowish solid was also obtained by the reaction calation compounds, respectively) were same as those observed between C8AzoC10N+Br- and Na-montmorillonite. The XRD for the powdered samples, indicating that the arrangements of pattern of the product [Fig. 1(d)] shows a basal spacing of the intercalated azo dyes did not change during the film 2.5 nm, which indicates an interlayer separation of 1.5 nm. The preparation and the solvents employed for the films prep- diVerence in the basal spacings between the two compounds aration are completely removed. Although the films are slightly turbid, they are still useful for photochemical studies.The visible absorption spectrum of the C12AzoC5N+ montmorillonite intercalation compound film is shown in Fig. 2(a). In the absorption spectrum, a band due to the trans-azobenzene chromophore was observed at ca. 385 nm, which is shifted towards longer wavelength relative to that (355 nm) of monomeric C12AzoC5N+ in a dilute ethanol solution of Fig. 1 The X-ray powder diVraction patterns of (a) sodium montmoril- Fig. 2 The absorption spectra of (a) the C12AzoC5N+ montmorillonite lonite, (b) and (c) C12AzoC5N+ montmorillonite intercalation compound: powder (b) and cast film (c); (d) and (e) C8AzoC10N+ intercalation compound and (b) the C8AzoC10N+ montmorillonite intercalation compound montmorillonite intercalation compound: powder (d) and cast film (e) 464 J.Mater. Chem., 1998, 8(2), 463–467C12AzoC5N+Br. The absorption spectrum of the C8AzoC10N+ montmorillonite intercalation compound film is shown in Fig. 2( b). A broad absorption band centered at 373 nm was observed in the absorption spectrum. Compared to that of the dye solution (the absorption maximum of a dilute C8AzoC10N+Br solution appeared at 355 nm), the absorption band due to the p–p* transition of trans-azobenzene shifted to longer wavelength.It should be noted that the absorption band of the C8AzoC10N+ montmorillonite intercalation compound film was observed at a shorter wavelength than that of the C12AzoC5N+ montmorillonite intercalation compound film. A C12AzoC5N+ montmorillonite intercalation compound with a C12AzoC5N+ loading of 0.1 of the c.e.c.showed a basal spacing of ca. 1.3 nm, indicating the adsorbed dyes arranged parallel to the silicate layers. This intercalation compound was orange, diVerent from that (yellow) of the intercalation compound in which the interlayer cations were replaced almost quantitatively. The adsorbed dyes interact with the surface of silicate layer when the adsorbed amount is low, while the dye–dye interactions are dominant at high loading.In the molecular assembly, the chromophore interacts to give aggregated states and the dye–dye interactions cause both bathochromic and hypsochromic spectral shifts depending on the microstructures. According to Kasha’s molecular exciton theory,29 the observed bathochromic shifts of the absorption bands of the intercalation compounds were ascribable to J-like aggregates of the intercalated C12AzoC5N+ and C8AzoC10N+ ions in the interlayer space of montmorillonite.The spectral shifts reflect the orientation of the dipoles in the aggregates; smaller spectral red shifts are expected for the aggregates with larger tilt angles of the dipoles.29 The diVerence Fig. 3 Proposed microstructures of (a) the C12AzoC5N+ montmoril- in the wavelength of the absorption maxima observed for the lonite intercalation compound and (b) the C8AzoC10N+ montmoril- C12AzoC5N+ and C8AzoC10N+ montmorillonite intercalation lonite intercalation compound.(a¾) Less plausible model for compounds suggests two diVerent orientations of the interca- the C12AzoC5N+ montmorillonite intercalation compound, with the lated azo dyes.For the C12AzoC5N+ montmorillonite system, C12AzoC5N+ arranged as an interdigitated monolayer. Note that the distance between the adjacent chromophores is larger in this model. a greater spectral shift is observed, indicating the smaller tilt angle of the azobenzene dipoles in the intercalated dye aggregates. The absorption band observed for the C8AzoC10N+ montmorillonite system showed the smaller spectral shift com- molecular structures.21–23 C8AzoC10N+ has been reported to form a H-aggregate in cast films as revealed by the X-ray pared to that observed for the C12AzoC5N+ montmorillonite system, suggesting a greater tilt angle of the azobenzene dipoles diVraction and the hypsochromic shift of the absorption band in the visible spectrum.On the contrary, C8AzoC10N+ ions in the interlayer space of montmorillonite. As discussed previously, two possible orientations of the form J aggregates which show a bathochromic shift of the absorption band upon intercalation into the interlayer space intercalated species are proposed from the gallery height and the size of the dye; one is a monomolecular layer and the other of montmorillonite.This observation implies that the states of the dyes in the interlayer spaces are controlled by the electro- is a bimolecular coverage in the interlayer spaces. Since the basal spacings of the two intercalation compounds are similar, static attractions between the negatively charged silicate surfaces and the cationic dyes as well as the dye–dye interactions.the tilt angles in the bilayer assembly must be larger than that in the monolayer aggregate. Consequently, the bilayer structure The photochemical reaction of the intercalated azobenzene has been investigated by UV and visible light irradiation. of the intercalated C12AzoC5N+ [as shown in Fig. 3(a)] seems to be a plausible model to explain the observed spectral shift.Fig. 4 shows the change in the absorption spectra of the C12AzoC5N+ montmorillonite intercalation compound upon On the contrary, the intercalated C8AzoC10N+ ions are thought to form an interdigitating monomolecular layer in the UV and visible light irradiation. After UV irradiation, the band due to the trans-isomer (at ca. 385 nm) decreased [spectra interlayer space of montmorillonite as shown in Fig. 3( b). The diVerences in the microstructures of the C12AzoC5N+ (b) and (c) in Fig. 4 were recorded after UV irradiation for 20 and 50 min, respectively], indicating trans–cis isomerization. and C8AzoC10N+ montmorillonite systems are ascribed to the location of the azobenzene chromophore in the amphiphilic UV irradiation for a longer period did not cause any further spectral change.The absorption band ascribable to the cis- ions. The C8AzoC10N+ ions are able to adopt an interdigitating monolayer without producing void spaces. On the contrary, isomer appeared at 330 nm. Upon visible light irradiation, the absorption spectrum recovered [Fig. 4(d) shows the absorption for the C12AzoC5N+ ions to form an interdigitating monolayer, the distance of adjacent azobenzene chromophores must be spectrum recorded after visible light irradiation for 13 min].This spectral recovery was also observed thermally. Reversible larger than that expected for the interdigitating C8AzoC10N+ monolayer to weaken the dye–dye interactions [as sche- spectral changes were repeatedly observed. The ratio of the cis-isomer formed by UV irradiation at the photostationary matically shown in Fig. 3(a¾)]. As a consequence, the C12AzoC5N+ ions form a bilayer in the interlayer space of state at room temperature was ca. 60% from the change in the absorption band due to the trans-isomer. montmorillonite as shown in Fig. 3(a). Shimomura and co-workers have extensively investigated A similar change in the absorption spectra was observed for the C8AzoC10N+ montmorillonite intercalation compound.the preparation and the organization of a series of amphiphilic azo dyes with variable alkyl chain length and found that the Fig. 5 shows the change in the absorption spectra of the C8AzoC10N+ montmorillonite intercalation compound upon microstructures of the dye aggregates varied depending on the J. Mater. Chem., 1998, 8(2), 463–467 465Fig. 6 The temperature dependence of the fraction of the photochemically formed cis-isomer at the photostationary states for (O) the Fig. 4 The change in the absorption spectra of the C12AzoC5N+ C12AzoC5N+ montmorillonite and (D) the C8AzoC10N+ montmorilmontmorillonite intercalation compound: before (a) and after UV lonite intercalation compounds irradiation for (b) 20 and (c) 50 min; (d) after subsequent visible light irradiation for 13 min ture.Fig. 6 shows the variation of the fraction of the photochemically formed cis-isomer at the photostationary states at diVerent temperatures. These values decreased with decreasing temperature, suggesting that the motion of the azobenzene was restricted at lower temperatures. It has been reported that dialkyldimethylammonium ions in the interlayer space of silicates exhibit a gel-to-liquid crystal phase transition to aVect the photoprocess of the intercalated species and the permeability. 11,16,19,32,33 In the present system, the states of the amphiphilic azo dyes might aVect the observed temperature dependent photochemical reactions. Upon increasing the temperature above 340 K, the fraction of the cis-isomer at the photostationary state decreased as a result of competitive photochemical and thermal processes.Organoammonium-exchanged clays have been utilized as adsorbents for poorly water soluble species with specific selectivity. 34 The amphiphilic azo dye intercalated montmorillonites may find applications as novel adsorbents with photo-controllable selectivity.Fig. 5 The change in the absorption spectra of the C8AzoC10N+ montmorillonite intercalation compound: before (a) and after UV irradiation for (b) 16 min and (c) 32 min; (d) after subsequent visible Conclusions light irradiation for 8 min The intercalation of cationic amphiphilic azo dyes into the interlayer space of montmorillonite has been conducted by a UV and visible light irradiation.After UV irradiation, the conventional ion exchange method. The intercalated azo dyes band due to the trans-isomer (at around 373 nm) decreased formed J-like aggregates in the interlayer space of montmoril- [spectra (b) and (c) in Fig. 5 were recorded after UV irradiation lonite. Although the two dyes gave similar basal spacings, for 16 and 32 min, respectively], indicating trans-cis isomerizabsorption spectra showed diVerences in the microstructures ation.UV irradiation for a longer period did not cause any of the products. Two diVerent models, one a monomolecular further spectral change. Upon visible light irradiation, the the other a bimolecular layer, in the interlayer spaces, have absorption spectrum recovered [Fig. 5(d)].The ratio of the been proposed for the intercalation compounds and this diVercis- isomer formed by UV irradiation at the photostationary ence has been ascribed to the diVerence in the molecular state at room temperature was ca. 60% from the diVerence in structures of the dyes. The intercalated azo dyes exhibited the absorption spectra. photoisomerization at room temperature. It is worth noting that the azobenzene chromophore isomerized eVectively in the interlayer space of montmorillonite, The present work was partially supported by Waseda despite the fact that the azobenzene chromophore aggregates University as a Special Project Research.in the interlayer space. It has been pointed out that the isomerization of the azobenzene chromophore in a molecular assembly was restricted owing to the lack of free volume.In References order for the azobenzene chromophore to isomerize eVectively, 1 Intercalation Chemistry, ed. M. S. Whittingham and A. J. Jacobson, eVorts have been made by means of complexation with a Academic Press, New York, 1982. cyclodextrin cavity30 and with a polyion complex31 to create 2 Progress in Intercalation Research, ed. W.Mu�ller-Warmuth and suYcient room for photoisomerization. R. 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