Clay Minerals (1999) 34, 213-220

Photochromism of azobenzene in the hydrophobic interlayer spaces of

dialkyldimethylammonium- fluor-tetrasilicic mica films

M. O G A W A , M. H A M A * AND K. K U R O D A * t

PRESTO, Japan Science and Technology Corporation and Institute of Earth Science, Waseda University, Nishiwaseda 1-6-1, Shinjuku-ku, Tokyo 169-8050, *Department of Applied Chemistry, Waseda University, Ohkubo 3-4-1, Shinjuku-

ku, Tokyo 169-8555, t Kagami Memorial Laboratory for Materials Science and Technology, Waseda University, Nishiwaseda 2-8-26, Shinjuku-ku, Tokyo 169-0051, Japan

(Received 6 November 1997; revised 1 March 1998)

ABSTRACT: Photochemical isomerization of azobenzene intercalated in the hydrophobic, interlayer spaces of swelling fluor-tetrasilicic micas exchanged with dialkyldimethylammonium ions, with the alkyl chain length from 10 to 18, was investigated. Thin films of the organoammonium-mica-azobenzene intercalation compounds were obtained by depositing a suspension of the organoammonium-micas (prepared using a toluene/methanol solution of azobenzene) on quartz substrates. The intercalated azobenzene showed reversible photochromic reactions induced by UV and visible light irradiation. The fraction of the photochemically formed c/s-isomer in the photostationary states decreased with a decrease in temperature. The observed change in the photochemical reactions is thought to reflect changes in the states of the dialkyldimethylammonium ions in the interlayer space of the swelling fluor-tetrasilicic mica.

Intercalation of guest species into layered inorganic solids is one method of producing ordered inorganic-organic assemblies with unique micro- structures controlled by host-guest and guest-guest interactions (Whittingham & Jacobson, 1982; Mfiller-Warmuth & Sch611horn, 1994). Recently, photophysical and photochemical properties of intercalation compounds have been investigated extensively (Ogawa & Kuroda, 1995). The smectite group, one of a number of possible layered hosts, provides attractive features including large surface area, swelling behaviour, and ion-exchange proper- ties which can organize organic guest species (Theng, 1974; Van Olphen, 1977). Since the smectites are transparent in the visible wavelength region and form stable colloidal suspensions and oriented films, their uses for photochemical studies have attracted increasing interest (Ogawa & Kuroda, 1995; Thomas, 1988), and the photo-

chromism of viologen (Miyata et al., 1987) and fulgide (Adams & Gabbutt, 1990) in the interlayer space of layered silicates has been reported. Photochromic behaviour is environmentally sensi- tive (Diirr & Bouas-Laurent, 1990), thus photo- chromism of organic substances in solid matrices has been investigated to to both elucidate and modify the photochromic behaviour. Indeed, the photochromic behaviour of the intercalated species was different from that observed in solution or other solid matrices, suggesting the applicability of layered clay minerals as a novel support to control photochromic properties.

We have been interested in the organization of azo dyes in the hydrophobic interlayer spaces of organoammonium-silicates (Ogawa, 1996; Ogawa et al., 1991, 1996). The photochromism of azobenzene and its derivatives due to cis-trans isomerization has been widely investigated (Diirr &

�9 1999 The Mineralogical Society

214 M. Ogawa et al.

Bouas-Laurent, 1990). Photocontrol of chemical and physical functions of various supramolecular systems has been studied vigorously using photo- chemical configurational change of azobenzene derivatives (Kumar & Neckers, 1989). The intro- duction of poorly water-soluble species into the hydrophilic interlayer space of layered silicates has been achieved by exchanging the interlayer cations with cationic surfactants. These organoammonium- clays have been studied as a precursor for pillared clays (Galarneau et al., 1995), selective adsorbates (Boyd et al., 1988a,b; Lee et al., 1990), membranes (Okahata & Simizu, 1989), catalyst supports (Hu & Rusling, 1991), and photoactive species (Seki & Ichimura, 1990; Tomioka & Itoh, 1991; Takagi et al., 1991; Ahmadi & Rusling, 1995; Ogawa et al., 1992b; 1993; 1995), among others (Lagaly, 1981; Ogawa & Kuroda, 1997).

In our previous study, solid-solid reactions have been used to achieve intercalation of p-aminoazo- benzene and p-dimethylaminoazobenzene into orga- noammonium-montmorillonite and fluor-tetrasilicic micas. The intercalated azo dyes exhibited rever- sible photoisomerization in the hydrophobic inter- layer spaces at room temperature. In this study, dialkyldimethylammonium-fluor-tetrasilicic micas have been used as the host materials in order to incorporate azobenzene (the molecular structure is shown in Fig. 1) as well as to react with them in the interlayer spaces. It has been suggested that the intercalated dialkyldimethylammonium ions exhibit gel-to-liquid crystal phase transitions (Seki & Ichimura, 1990; Okahata & Shimizu, 1989; Hu & Rusling, 1991; Ahmadi & Rusling, 1995; Vaia et al., 1994). Since phase transitions may affect the photochemistry of the adsorbed azo dyes, the photochemical reactions of azobenzene intercalated in the hydrophobic interlayer spaces of the organoammonium-silicates have been studied at different temperatures.

E X P E R I M E N T A L

Materials

A swelling mica, fluor-tetrasilicic mica, TSM, (supplied by Topy Industries Co.), was used as the host material after removing non-expandable impu- rities by dispersion-sedimentation. The cation exchange capacity was 100 mEq/100 g clay (Ogawa et al., 1992a). Dialkyldimethylammonium (abbreviated to 2C,2CIN +-, where n denotes the carbon number in the alkyl chain; n-10, 12, 14, and 18) chlorides were obtained from Tokyo Kasei Industries Co. and used without further purification. Azobenzene (Tokyo Kasei Industries Co.) was used after recrystallization from ethanol.

Sample preparat ion

The 2C,2C1N+-TSMs were prepared by the conventional ion-exchange method in which Na- TSM was dispersed in aqueous solutions of appropriate organoammonium salts and subsequently washed with ethanol/deionized water mixtures repeatedly until a negative AgNO3 test was obtained.

The organoammonium-TSMs were dispersed in a toluene/methanol solution of azobenzene at room temperature and the suspension was cast on a quartz substrate and dried in air, so that thin films formed on the substrate. In a typical experiment, 0.1 g of 2C182ClN+-TSM was dispersed in 10ml of azobenzene solution (4.9x 10 -4 M). The thickness of the films, determined by a surface profilometer, was a few gm.

Characterization

X-ray powder diffraction patterns of the products were recorded on a Rigaku R1NT 1100 diffract- ometer using Mn filtered Fe-K~ radiation. Absorption spectra of the films were recorded on a Shimadzu UV-3100PC spectrophotometer. The thin films of organoammonium-TSMs were used as reference, in order to reduce the effect of scattering on the absorption spectra and thus the spectra shown are difference spectra. The compositions of the products were determined by elemental (C,H,N) analysis (Yanaco MT-3).

Photochemical reactions

FIG. 1. The molecular structure of azobenzene. The photochemical reaction of the intercalated

azobenzene was achieved using UV and visible

Photochromism of azobenzene in organoclays

light irradiation from a 500 W super high pressure Hg lamp. (USHIO USH-500D) A band pass filter (Toshiba UV-D35), with the transmittance centred at 350 nm, was used for isolating the UV light. For the cis-to-trans reverse reactions, a sharp cut filter, I-IOYA L42 (cut off wavelength 420 nm) was used to obtain visible light. The reactions were monitored by the change in absorbance of the trans-isomer of azobenzene. A sample film was set in a cryostat with optical windows (Oxford DN- 1704), and photochemical reactions were performed at constant temperatures from 100 to 360 K.

R E S U L T S A N D D I S C U S S I O N

Preparation of 2Cn2C1N+-TSMs

The basal spacings of the 2Cn2C1N+-TSMs and the amounts of the adsorbed 2Cn2C1N + are listed in Table 1. The amounts of 2C,,2CtN + adsorbed confirmed that the exchangeable cations were replaced almost quantitatively by 2C~2C1 N+ in all the samples investigated. There was a linear relation- ship between the d value and carbon number in the alkyl chains, which, given the observed basal spacings and the size of the alkyl-chain, suggests that the intercalated 2Cn2CIN + formed paraffin-type aggregates in the interlayer space. Two types of arrangement of the intercalated 2C,2C1N + were expected; one was monomolecular coverage and the other was bimolecular coverage with the alkylchains inclined to the silicate sheet. It was not possible to determine which type of arrangement occurred here.

Intercalation of azobenzene into 2C ~82C lN+-TSM

The XRD patterns of the products obtained by the reactions between 2C182CIN+-TSM and azoben-

215

zene (at a molar ratio of azobenzene to 2C182C1N + of 2) showed an expansion in the basal spacing from 3.4 to 3.9 nm, indicating that azobenzene had been intercalated into the interlayer space of 2Cls2C~N+-TSM. No further increase in the basal spacing was observed when an excess amount of azobenzene was added. The solid-state intercalation ofp-aminoazobenzene, naphthalene, anthracene and pyrene into organoammonium-montmorillonites and TSMs also exhibited a limited interlayer expansion (Ogawa et al., 1991; 1992b, 1993, 1995, 1996).

It is very difficult to accommodate the large amounts of guest species achieved in the present studies into various surfactant aggregates in solution and at solid surfaces. It is considered that the layered silicates support the surfactants and maintain a layered organization even with such a large number of guest species. There is a possibility that azobenzene adsorbs at the external surface, but the relative contribution of the external surface to the interlayer space should be very small and therefore negligible.

In order to understand the reaction, the suspen- sion of the 2C~s2C1N+-TSM in a methanol/toluene mixture containing azobenzene was centrifuged. The supernatant liquid contained azobenzene and the precipitate was not coloured. This suggests that azobenzene molecules were intercalated during solvent evaporation. The intercalation of p-aminoa- zobenzene and p-dimethylaminoazobenzene into various organoammonium-montmorillonites and TSMs by solid-solid reactions has already been reported (Ogawa et al., 1991, 1996). Since the products were obtained as powders, the intercala- tion compounds were embedded in poly(vinyl alcohol) films for the photochemical studies. In this study, the products were obtained as films so that the photochemical studies could be carried out without any pretreatment. Although the films were

TABLE 1. The amounts of the adsorbed 2Cn2C1N+.

Sample Basal spacing, Amounts of adsorbed nm 2C,,2C1N +, mEq/100 g clay

Na-TSM 1.26 2C182ClN+-TSM 3.40 97 2C142CIN+-TSM 2.80 99 2C 122C ~ N+-TSM 2.59 97 2CI02C1N+-TSM 2.39 97

~D

o r ~

34. Ogawa et al.

slightly turbid, they were still useful for the photochemical studies.

Photochemical isomerization of azobenzene in the interlayer space of 2Cle2C1N+-TSM

The loading of azobenzene in the photochemical study, was fixed at l tool azobenzene to 20 mol 2C182C1N + in order to avoid guest-guest interac- tions. The resulting intercalation compound showed no significant change in the basal spacing resulting from the small amount of the azobenzene present. The absorption spectrum of the 2Cjs2C1N+-TSM - azobenzene intercalation compound is shown in Fig. 2a. The absorption band attributable to trans- azobenzene appeared near 325 rim, confirming the successful incorporation of azobenzene in the 2C1s2CxN+-TSM film.

The change in the absorption spectrum of the 2C182C1N+-TSM-azobenzene in t e rca la t ion compound film upon irradiation with UV and visible light is shown in Fig. 2. The absorbance at 325 nm due to the trans-isomer decreased and the absorption band of cis-azobenzene appeared near 427 nm. The absorption spectrum of the photosta- tionary state appeared after 5 min (Fig. 2b) under the experimental conditions employed in this study. Subsequent irradiation with visible light resulted in the recovery of the absorption band due to the trans-isomer. Figure 2c shows the absorption spectrum of the 2Cls2C1N+-TSM-azobenzene inter- calation compound after visible light irradiation for 1 rain. Further irradiation led to the complete recovery of the absorption spectrum recorded before the UV irradiation (Fig. 2a) and this reversible spectral change was observed repeatedly.

(a)

I

216

200 300 400 500 600

Wavelength / nm

Fro. 2. The absorption spectra of the 2Cls2C1N+-TSM-azobenzene intercalation compound. The molar ratio of azobenzene to 2Cls2CIN + is 1/20: (a) before UV irradiation; and (b) after the UV irradiation at the

photostationary state. Spectrum (c) was recorded after visible light irradiation for 1 rain.

Photochromism o f azobenzene in organoclays 217

Therefore, the change in the absorption spectra upon UV irradiation followed by visible light irradiation was attributed to the photoisomerization of the intercalated azobenzene.

Since the thermally promoted reverse reactions took more than a day for complete recovery of the absorption spectrum at room temperature, an estimate of the reaction yields can be made. The change in absorbance of the trans-isomer suggests that the fraction of the photochemically formed cis-

isomer was ca. 80% at the photostationary state at room temperature. This result indicates that the interlayer space of the 2CI82C1N+-TSM offered an environment where the intercalated azobenzene isomerized effectively.

The photochemical reaction was conducted at different temperatures (in the range 100-360 K) and Fig. 3 shows the absorption spectra at the

photostationary states. High temperature may cause desorption of the azobenzene from the intercalation compound, so that the maximum temperature was restricted to 360 K in the present study.

The variation in the fraction of the photochemi- cally formed cis-isomer (Fig. 4) suggests a change in the fluidity of the matrices surrounding the azobenzene molecules. The fractions of the photo- chemically formed cis-isomer were ca. 80% and did not change significantly above the discontinuity at 325 K, while the fraction of the c/s-isomer decreased as the temperature decreased below 325 K. It has been observed that the quantum yields of trans-to-cis

photoisomerization of azobenzenes in organic solvents decreases as the temperature decreases (Rau, 1990; Malkin & Fischer, 1962). Therefore, the gradual decrease in the fraction of the cis -isomer below 325 K is thought to reflect the change in the

o

I P I I

200 250 300 350 400 450

Wavelength / nm

FIG. 3. The absorption spectra of the 2Cas2C1N+-TSM-azobenzene intercalation compound at the photostationary state. The molar ratio of azobenzene to 2CIs2CIN + is 1/20: (a) before UV irradiation at 100 K and after UV

irradiation at 100 K (b), 200 K (c), 250 K (d) and 300 K (e).

218 M. Ogawa et al.

1 0 0

o

o

o

8O

6O

4O

2O

0 J i

8 0 1 4 0 2 0 0

J

i i

2 6 0 3 2 0 3 8 0

Temperature / K

J F~6. 4. The temperature dependence of the fraction of the photochemically formed cis-isomer at the photo- stationary state for the 2C182C1N+-TSM-azobenzene intercalation compound. The molar ratio of azobenzene to 2Cls2CIN + is 1/20. Arrow indicates the phase

transition temperature.

microviscosity of the environment surrounding the azobenzene molecules.

The change in the state of surfactants in the interlayer spaces of smectites has been reported previously (Seki & Ichimura, 1990; Okahata & Shimizu, 1989; Hu & Rusling, 1991; Ahmadi & Rusling, 1995). Changes in the permeability of the 2Cls2ClN+-silicate composites (Okahata & Shimizu, 1989), the thermal discolouration kinetics of the intercalated spiropyrane (Seki & Ichimura, 1990), the luminescence of the intercalated pyrenes (Ahmadi & Rusling, 1995) and the catalytic activity (Hu & Rusling, 1991) have been observed at a certain temperature and attributed to the change in state of the intercalated surfactant aggregate between gel and liquid crystalline states. Phase transition temperatures have been reported around 326-327 K for 2Cls2CaN+-montmorillonites (Seki & Ichimura, 1990; Okahata & Shimizu, 1989; Hu & Rusling, 1991; Ahmadi & Rusling, 1995). In the present system, the temperature dependence indi- cated a change at 325 K and this value is in agreement with those reported previously within the experimental accuracy (Table 2).

The variation in the d values for the 2Cas2C1N +- silicates reported previously should be noted. The

basal spacings of the 2Cls2C1N+-montmorillonites are summarized in Table 2. The d values (4.24 rim) for the 2Cag2CiN+-clays reported by Seki & Ichimura (1990), 4.83 nm (Okahata & Shimizu, 1989), and 4.3 nm (Ahmadi & Rusling, 1995) are larger than that for the 2Cls2CIN+-TSM), 3.40 nm, observed for the present system. Since the thickness of the alumino-silicate sheet of montmorillonite is similar to that of TSM, the difference in the basal spacings must correspond to the difference in the orientation of the 2C182C1 N+ ions, which may cause the difference in the phase transition temperature. In the present system, the amount of the intercalated 2C,2C1 N+ was just equivalent to the cation exchange capacity of the TSM as shown in Table 1. It is known that excess of guest species can be accommodated in the interlayer spaces of smectites as a salt and this phenomenon is referred as 'intersalation'. The large d values reported in the literature (Seki &Ichimura, 1990; Okahata & Shimizu, 1989; Ahmadi & Rusling, 1995) may be due to the intersalation and/or the difference in the effective surface layer charge density. Although the difference did not cause a change in the phase transition temperatures, the effect of 'intersalation' on the properties of the complexes should be investigated further.

The cis-to-trans reverse reaction has been investigated at 100 K, but the intercalated azo dye did not isomerize (trans-to-cis) at that temperature. Therefore, the film was UV irradiated at room temperature (the c/s-fraction was ca. 80%) and cooled to 100 K. The visible light irradiation at 100 K resulted in the effective recovery of the absorption band due to trans-isomer. This observa- tion indicates that the cis-to-trans reactions do not require fluid surrounding, while the trans-to-cis reactions were significantly restricted in rigid environments as observed for the photochemistry of azobenzene in organic solvents (Rau, 1990; Malkin & Fischer, 1962).

The effects o f the alkyl chain length of the interlayer dialkyldimethylammonium ions on the photochemistry of the intercalated azobenzene

In order to examine the effects of the alkyl chain length on the photochemistry of the intercalated azobenzene, the 2Cn2CtN+-TSM-azobenzene inter- calation compounds, where n -10, 12 and 14, were prepared by the Same procedure used for the

Photochromism of azobenzene in organoclays 219

TABLE 2. Basal spacings and gel to liquid-crystalline phase transition temperatures for the 2C182C1N+-silicates.

Host Phase Basal Technique Reference transition spacings temp/K (nan)

Montmorillonite a) 327 4.24 Thermal discolouration Seki & Ichimura of photomerocyanie (1990) to spiropyran

Montmorillonite a) 327 4.83 Permeation of fluorescent Okahata & Shimizu probe b) and DSC (1989)

Bentonite 326 4.3 Pyrene luminescence Ahmadi & Rusling (1995)

Bentonite 327 not given Electrochemical reduction Flu & Rusling (1991) of Trichloroacetic acid

TSM 328 3.4 Photochromism of azobenzene This study

a) Kunipia G, Kunimine Industries Co. b) 1-(1,3,4,5-tetrahydroxycyclohexanecarboxyamido)naphthalene

preparation of the 2Cla2C1N+-TSM-azobenzene system and the photochemical reactions of the intercalated azobenzene were studied. The molar ratios of the loaded azobenzene to 2Cn2C1 N+ ions were adjusted to 1/20. The absorption band due to trans-azobenzene, around 326 nm (similar to that observed for the 2C182CaN+-TSM-azobenzene intercalation compound) was also present in the 2C10_ 142C 1N+-TSM-azobenzene systems.

The photochemical reactions were conducted at selected temperatures and the fraction of cis-isomer formed at the photostationary state was determined from the change in the absorption spectra, which were similar to that observed for the 2C182C1N+- TSM-azobenzene in t e r ca l a t i on compound (Figs. 3,4) in that the fraction of cis- isomer decreased with decreasing temperature. There was a discontinuity around 290 K in the temperature dependence of the fractions of the photochemically formed cis-isomer for the 2C~22CaN+-TSM-azoben - zene intercalation compound. The fractions of cis- isomer were ca. 80% and did not change signif icant ly in the temperature range of 290-360 K. Similar changes in the fraction of the photochemically formed cis-isomer have been observed for other systems and the discontinuities were observed at 300 and 280 K for 2C142C1N +- and 2CI02C1N+-TSMs, respectively. Although the phase transition temperatures estimated from the temperature dependence of photoisomerization for the 2C142C1N+- and 2C122C1N§ were slightly different from those reported previously

(296 and 288 K for the 2C142C1N+- and 2C~22C1N+-TSMs, respectively), (Okahata & Shimizu, 1989), there is a tendency toward lower phase transition temperatures with a decrease in the alkyl chain length. All these observations indicate that the photochemistry of azobenzene intercalated in the 2Cn2CIN+-TSMs was influenced by the state of the surrounding surfactant (2Cn2C1N +) ions.

C O N C L U S I O N S

Azobenzene has been intercalated into the hydro- phobic interlayer spaces of the dialkyldimethyl- ammonium-swelling micas. The intercalated azobenzene exhibits reversible trans-cis isomeriza- tion upon UV irradiation and subsequent visible light irradiation. The fraction of the photochemi- cally formed cis-isomer at the photostationary states depends on the reaction temperature, suggesting a change in the state of the interlayer surfactants. The photochemistry of azobenzene can be controlled by its introduction into the organoammonium-silicates where the appropriate states of the intercalated alkylammonium ions can be controlled.

ACKNOWLEDGMENTS

The present work was partially supported by a Grant- in-Aid for Scientific Research from the Ministry of Education, Science and Culture. Waseda University also supported the study financially as a Special Research Project.

220 M. Ogawa et al.

REFERENCES

Adams J.M. & Gabbutt A.J. (1990) Interaction of smectites with organic photochromic compounds. J. Incl. Phenom. 9, 63-83.

Ahmadi M. & Rusling J. (1995) Fluorescence studies of solute microenvironment in composite clay-surfac- tant films. Langmuir, 11, 94-100.

Boyd S.A., Lee J.-F. & Mortland M.M. (1988a) Attenuating organic contaminant mobility by soil modification. Nature, 333, 345-347.

Boyd S.A., Mortland M.M. & Chiou C.T. (1988b) Sorption characteristics of organic compounds on hexadecyltrimethylammonium-smectites. Soil Sci. Soc. Am. J. 52, 652-657.

Dtirr H. & Bouas-Laurent H. (editors) (1990) Photochromism - - Molecules and Systems, Elsevier, Amsterdam.

Galameau A., Barodawalla A. & Pinnavaia T.J. (1995) Porous clay heterostructures formed by gallery- templated synthesis. Nature, 374, 529.

Hu N. & Rusling J.F. (1991) Surfactant-intercalated clay films for electrochemical catalysis. Reduction of trichloroacetic. Anal. Chem. 63, 2163.

lrie M. (1986) Pp. 269 310 in: Photophyical and Photochemical Tools in Polymer Science (M.A. Winnik, editor) Reidel Pub., Dordrecht.

Kumar G.S. & Neckers D.C. (1989) Photochemistry of azobenzene-containing polymers. Chem. Rev. 89, 1915-1925.

Lagaly G. (1981) Characterization of clays by organic compounds. Clay Miner. 16, 1-21.

Lee J.-F., Mortland M.M., Chiou C.T., Kile D.E. & Boyd S.A. (1990) Adsorption of benzene, toluene, and xylene by two tetramethylammonium-smectites having different charge densities. Clays' Clay Miner. 38, 113-120.

Malkin S. & Fischer E. (1962) Temperature dependence of photoisomerisation, part II. J. Phys'. Chem. 66, 2482-2485.

Miyata H., Sugahara Y., Kuroda K. & Kato C. (1987) Synthesis of montmorillonite-viologen intercalation compounds and their photochromic behaviour. J. Chem. Soc., Faraday Trans. 1, 83, 1851-1858.

Mfiller-Warmuth W. & Sch611hom R. (editors) (1994) Progress in Intercalation Research. Kluwer Academic Publishers, Dordrecht.

Ogawa M. (1996) Preparation of a cationic azobenzene- montmorillonite intercalation compound and the photochemical behavior. Chem. Mater. 8, 1347-1349.

Ogawa M. & Kuroda K. (1995) Photofunctions of intercalation compounds. Chem. Rev. 95, 399-438.

Ogawa M. & Kuroda K. (1997) Preparation of Inorganic-Organic Nanocomposites through Intercalation of Organoammonium Ions into Layered Silicates. Bull. Chem. Soc. Jpn. 70,

2593-2618. Ogawa M., Wada T. & Kuroda K. (1995) Intercalation

of pyrene into alkylammonium-exchanged swelling layered silicates: The effects of the arrangements of the interlayer alkylammonium ions on the states of adsorbates. Langmuir, 11, 4598-4600.

Ogawa M., Aono T., Kuroda K. & Kato C. (1993) Photophysical probe study of alkylammonium-mont- morillonites. Langmuir, 9, 1529-1533.

Ogawa M., Fujii K., Kuroda K. & Kato C. (1991) Preparation of montmorillonite-p-aminoazobenzene intercalation compounds and their photochemical behavior. Mater. Res. Soc. Syrup. Proc. 233, 89 94.

Ogawa M., Kimura H., Kuroda K. & Kato C. (1996) Intercalation and the photochromism of azo dye in the hydrophobic interlayer spaces of organoammo- nium-fluor-tetrasilicic micas. Clay Sci. 10, 57-65.

Ogawa M., Nagafusa Y., Kuroda K. & Kato C. (1992a) Solid-state intercalation of acrylamide into smectites and Na-taeniolite. AppL Clay Sci. 7, 291-302.

Ogawa M., Shirai H., Kuroda K. & Kato C. (1992b) Solid-state intercalation of naphthalene and anthra- cene into alkylammonium-montmorillonites. Clays Clay Miner. 40, 485-490.

Okahata Y. & Shimizu A. (1989) Preparation of bilayer- intercalated clay films and permeation control responding to temperature, electric field, and ambi- ent pH changes. Langmuir, 5, 954-959.

Rau H. (1990) Photochromism - - Molecules and Systems (H. Dtirr & H. Bouas-Laurent, editors). Elsevier, Amsterdam.

Seki T. & lchimura K. (1990) Thermal isomerization behaviors of a spiropyran in bilayers immobilized with a linear polymer and a smectic clay Macromolecules, 23, 31-35.

Takagi K. Kurematsu T. & Sawaki Y. (1991) Intercalation and photochromism of spiropyrans on clay interlayers. J. Chem. Soc. Perkin Trans. 2, 1517 1522.

Theng B.K.G. (1974) The Chemistry of Clay-Organic Reactions. Adam Hilger, London.

Thomas J.K. (1988) Photochemical and photophysical processes on clay surfaces. Acc. Chem. Res. 21, 275 -280.

Tomioka H. & Itoh T. (1991) Photochromism of spiropyrans in organized molecular assemblies. Formation of J- and H-aggregates of photomerocya- nines in bilayer-clay matrices. J. Chem. Soc., Chem. Commun. 532-533.

Vaia R.A., Teukolsky R.K. & Gianello E.P. (1994) Interlayer structure and molecular environment of alkylammonium layered silicates. Chem. Mater. 6, 1017-1022.

Van Olphen H. (1977) An Introduction to Clay Colloid Chemistry, 2nd ed. Wiley-Interscience, New York.

Whittingham M.S. & Jacobson A.J. (editors) (1982) Intercalation Chemistry. Academic Press, New York.