Langmuir 1999, 15, 413-418
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Molecular Alignment and Photodimerization of 4′-Chloro-4-stilbenecarboxylic Acid in Hydrotalcite Clays: Bilayer Formation in the Interlayers Ryo Sasai,† Nobuaki Shin’ya,† Tetsuya Shichi,† Katsuhiko Takagi,*,† and Kunihiko Gekko‡ Department of Crystalline Materials Science, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan, and Department of Materials Science, Faculty of Science, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8526, Japan Received June 15, 1998. In Final Form: November 2, 1998 It was observed that the photolysis of 4′-chlorostilbenecarboxylic acid (CSC) in N,N-dimethylformamide (DMF) yielded mainly a syn head-to-head dimer. The molecular alignment in hydrotalcite interlayers has been investigated by electric linear dichroism (ELD) at room temperature in order to clarify the correlation between the photocyclodimerization and the molecular alignment of CSC intercalated in hydrotalcite interlayers in DMF. ELD analysis in combination with X-ray diffraction analysis of this hydrotalcite complex involving CSC revealed the CSC molecules to be intercalated as a double layer, the tilt angle of the molecular plane being ca. 39.4°. The suggested molecular parallel packing model was found to be in good agreement with the stereochemistry of the photodimerization.
Introduction Investigation of the photochemistry of microporous crystals has been an interesting subject for researchers in various fields utilizing microheterogeneous surfaces and interfaces that accommodate photoactive species in their sterically restricted spaces.1,2 Quite a few reports have recently appeared elucidating the characteristic photofunctions of organized organic supramolecules on heterogeneous surfaces. Inorganic substances such as layered metal oxides,3-6 zeolites,7-11 and clay minerals12-18 are well-known as microporous crystals. In particular, clay minerals, which are ubiquitous lamellar aluminosilicates, can intercalate various molecular species, charged or neutral, due to their ion-exchange properties. This intercalation of the guest species is based on the electrostatic interaction between the ion-exchangeable sites of the clay interlayer surface and the ionic guest molecules.19 * To whom correspondence should be addressed. E-mail: ktakagi@ apchem.nagoya-u.ac.jp. (1) Kalyanasundaram, K. Photochemistry Microheterogeneous Systems; Academic Press Inc.: Orlando, FL, 1987. (2) Ramamurthy, V. Tetrahedron 1986, 42, 5753. (3) Synder, L. R.; Ward J. W. J. J. Phys. Chem. 1966, 70, 3941. (4) Oclkrug, D.; Flemming, W.; Fullemann, R.; Gunther, R.; Honnen, W.; Krabichler, G.; Shafer, M.; Uhl, S. Pure Appl. Chem. 1986, 58, 1207. (5) Turro, N. J. Tetrahedron 1986, 43, 1589. (6) Breuer, H. D.; Jacobs, H. Chem. Phys. Lett. 1980, 73, 172. (7) Breck, D. W. Zeolite Molecular Sieves; Wiley: New York, 1974. (8) Robo, J. A. Zeolite Chemistry and Catalysis; ACS Monograph Series 171; American Chemical Society: Washington, DC, 1976. (9) Van Hoff, J. H. C. In Chemistry and Chemical Engineering of Catalytic Process; Prins, R., Schuit, G. C. A., Eds.; NATO Advanced Studies Institute E39; Sijthoff & Noordhoff Publishers: The Netherlands, 1980; p 161. (10) Casal, H. L.; Scaiano, J. C. Can. J. Chem. 1985, 63, 1308. (11) Casal, H. L.; Scaiano, J. C. Can. J. Chem. 1984, 62, 628. (12) Grim, R. E. Clay Mineralogy; McGraw-Hill: New York, 1953. (13) Barrer, R. M. A Clays Clay Miner. 1989, 37, 385. (14) Thomas, J. K. Acc. Chem. Res. 1988, 21, 275. (15) Pinnavaia, T. J. Science 1983, 220, 365. (16) Nakamura, T.; Thomas, J. K. Langmuir 1987, 3, 234. (17) Schoonheydt, R. A.; Pauw, P. D.; Dominique, V.; Vliers, D.; DeSchrijver, F. C. J. Phys. Chem. 1984, 88, 5113. (18) Viaene, K.; Schoonheydt, R. A.; Cra¨tzel, M.; Kunyima, B.; DeSchryver, F. C. Langmuir 1988, 4, 749.
In previous papers, we have reported on the stereoselective photocyclodimerizations of 4-stilbenecarboxylates and stilbazolium ions intercalated in clay minerals.20-24 The stereoselective photocyclodimerization of these intercalated molecules was explained in terms of their orientation in the clay interlayers. Specifically, the packing behavior of the molecular aggregates of the oriented guest molecules in the clay interlayers was investigated by means of various physicochemical methods.25-28 However, the orientation configuration of the guest molecules on the clay interlayer surfaces has yet to be explained by other independent methods such as electric linear dichroism (ELD). ELD is a suitable and powerful method for studying the colloidal suspension of the clay with intercalated guest molecules in its interlayers.29,30 Moreover, the average molecular configuration on the clay surface can be obtained by the analysis of the ELD data. This ELD method has already been applied to some of the clay-guest molecular complex suspensions by other researchers, and important, interesting results have been reported.29-32 However, the application of the ELD technique to the clay-guest (19) Theng, K. G. The Chemistry of Clay-Organic Reactions; Adam Hilger: London, 1974. (20) Takagi, K.; Shichi, T.; Usami, H.; Sawaki, Y. J. Am. Chem. Soc. 1993, 115, 4339. (21) Usami, H.; Takagi, K.; Sawaki, Y. J. Chem. Soc., Perkin Trans. 2 1990, 1723. (22) Takagi, K.; Usami, H.; Fukaya, H.; Sawaki, Y. J. Chem. Soc., Chem. Commun. 1989, 1174. (23) Usami, H.; Takagi, K.; Sawaki, Y. Bull. Chem. Soc. Jpn. 1991, 64, 3395 (24) Usami, H.; Takagi, K.; Sawaki, Y. J. Chem. Soc., Faraday Trans. 1992, 77. (25) Yamanaka, S.; Kanamaru. F.; Koizumi, M. J. Phys. Chem. 1975, 79, 9, 1285. (26) Raupach, M.; Emerson, W. W.; Slade, P. G. J. Colloid Interface Sci. 1979, 69, 398. (27) McBride, M. B. Clays Clay Miner. 1985, 33, 510 (28) Breen, C.; Adams, J. M.; Rieckel, C. Clays Clay Miner. 1985, 33, 275. (29) Fredericq, E.; Houssier, C. Electric Dichroism and Electric Birefringence; Clarendon Press: Oxford, 1973. (30) Stoylov, S. P. Colloid Electrooptics: Theory, Techniques, Applications; Academic Press: New York, 1991.
10.1021/la980699a CCC: $18.00 © 1999 American Chemical Society Published on Web 12/30/1998
414 Langmuir, Vol. 15, No. 2, 1999
molecular complex suspension is not yet sufficient in estimating the molecular configuration in the clay interlayers. Herein, we report the molecular configuration of 4′-chloro-4-stilbenecarboxylic acid intercalated in hydrotalcite clay (abbreviated as LDH) against the LDH plane obtained from ELD analysis as well as the correlation between the photocyclodimerization and its molecular alignment by ELD and X-ray diffraction analyses. Experimental Section The UV and visible spectra were recorded on a Jasco V-550 spectrophotometer at room temperature. High-performance liquid chromatography (HPLC) analyses were carried out with a Shimadzu HPLC instrument with a photomultiplier detector fixed at 250 nm using a 30-cm SIL column (Jasco) with CH2Cl2 as the eluent. The NMR spectra were taken with a Varian Gemini200 NMR instrument. X-ray powder diffraction (abbreviated as XRD) analysis was carried out with an X-ray diffractometer (Rigaku Denki) with Ni-filtered Cu KR radiation. Materials. 4′-Chloro-4-stilbenecarboxylic acid (abbreviated as CSC) was prepared by the reaction of (4-carbomethoxybenzyl)triphenylphosphonium bromide with 4-chlorobenzaldehyde. A hydrotalcite clay with CSC molecules as the exchangeable anion was synthesized according to a modified procedure detailed in previous literature.20 Two kinds of aqueous solutions of (A) a mixture of AlCl3 and MgCl2 with total amounts of 1.0 mol/dm3 ([Mg2+]/[Al3+] ) 3) and (B) a mixture of sodium 4′-chloro-4stilbenecarboxylate ([CSC-]/[Al3+] ) 1) and NaOH (1.0 mol/dm3) were continuously mixed at a total flow rate of 10 mL/min, carefully adjusting the pH of the reaction mixture from 10.5 to 11.0 by adding the 1.0 mol/dm3 NaOH aqueous solution. The mixture was thoroughly stirred during the reaction at room temperature and then heated to 60 °C for 2 h. The precipitate was passed through a membrane filter (pore size 0.45 µm) and washed with deionized water to remove the unreacting species and byproducts involved in the mixture. The precipitate was filtered and dried overnight in vacuo. The layered structure of the synthetic hydrotalcite with CSC molecules was confirmed by powder XRD analysis. The amounts of the CSC molecules intercalated into hydrotalcite interlayers were ca. 100%. Irradiation of Hydrotalcite with CSC molecules in N,NDimethylformamide. The synthesized hydrotalcite including the CSC molecules (30 mg) was dispersed in 10 mL of N,Ndimethylformamide (abbreviated as DMF) by sonication in a quartz tube with a septum cap. After argon gas was bubbled for 20 min at room temperature, the suspension was irradiated with a 300 W high-pressure Hg lamp for 6 h under magnetic stirring. By addition of concentrated HCl, the irradiated suspension was acidified and the clay layer structure was decomposed, inducing a precipitation of free carboxylic acid as a white powder. The acidified solution was extracted with several portions of CH2Cl2/ethyl ether (2:1), followed by methylation with CH2N2 in ethyl ether and the evaporation of CH2N2 in vacuo. The obtained product was dissolved in CHCl3 and was analyzed with HPLC and 1H NMR. Electric Linear Dichroism Analysis. Ten milligrams of synthetic hydrotalcite intercalating CSC molecules was dispersed in 10 mL of DMF by sonication. The large particles in the dispersion were removed by centrifugation at 2000 rpm for 10 min. Electric linear dichroism (ELD) analysis of the supernatant of the sample was performed at room temperature in the wavelength range of 270-400 nm and with a field strength of E e 24 kV/cm on a self-made apparatus which is able to detect both parallel (∆A|) and perpendicular (∆A⊥) dichroism signals separately. Details of this ELD apparatus have been described in previous literature.33-35 (31) Yamagishi, A.; Taniguchi, M.; Takahashi, M.; Asada, C.; Matsushita, N.; Sato, H. J. Chem. Phys. 1994, 98, 7555. (32) Windsor, S. A.; Tinker, M. H. Abstracts of the 8th International Symposium on Colloid and Molecular Electrooptics; Russia, 1997. (33) Yamaoka, K.; Matsuda, K. Macromolecules 1981, 14, 595. (34) Yamaoka, K.; Yamamoto, Y.; Fujita, Y.; Ojima, N. J. Phys. Chem. B 1997, 101, 837. (35) Yamaoka, K.; Ojima, N.; Fujita, Y. J. Phys. Chem., in press.
Sasai et al. The reduced electric linear dichroism (∆A/A) of the suspension is defined as33-35
∆A/A ) (A| - A⊥)/Α
(1)
where ∆A| ) A⊥ - A is expressed in terms of the absorbances in the presence and absence of the electric field for the monochromatic light beam linearly-polarized parallel to the field direction. As with the case of perpendicularly polarized light, the difference can be defined as ∆A⊥ ) A⊥ - A. Since a guest molecule consists of several overlapping absorption bands, the observed absorption spectrum is represented by a composite of a number of these partial bands. Thus, the reduced dichroism of a suspension at a given wavelength and in the applied electric field E can be expressed as34-36
∆A A
)
( ) ∆A A
∑A [(∆A/A) ] j
s j
j
Φ(E) )
∑A
s
Φ(E)
(2)
j
j
where (∆A/A)s is the saturated or intrinsic reduced dichroism at infinitely higher field strengths, Aj is the absorbance of the jth partial band, and Φ(E) is the orientation function of the clay particles at a given field strength, E, and is independent of the wavelengths. Since the shape of a clay particle is assumed to be a platelet, the value of Φ(E) becomes -0.5 at infinitely higher field strengths. The saturated reduced dichroism in eq 2 is expressed as follows34-36
( ) () ∆A A
)
s
3
∑A (ν) (3 cos
2
j
θj - 1)
j
∑
2
(3) Aj(ν)
j
where θj is the angle between the direction of the jth optical transition dipole moment and the orientation axis, which should coincide with the normal (z) line of the clay plate, while relating to the roll angle ΘR and the tilt angle ΘT of the planar guest molecule, which possesses only in-plane π-π* transition moments. The θ angle is given as34-36
cosθ ) -cos ΘT sin ΘR sin ξ + sin ΘT cos ξ
(4)
where ξ is the angle between the transition moment, projected onto the molecular plane, |mb|, and the roll (y) axis, the sign being taken to be positive for the clockwise rotation from the positive y axis around the z axis, as indicated in Figure 1. The signs of ΘR and ΘT are taken to be positive for a clockwise rotation, as viewed from the negative to positive directions of the x and y axes in Figure 1. The wavelength dependence of the measured (∆A/A)s is simulated with the aid of eq 3, by calculating the best-fitting angles of θj by means of computer-aid-iterations. In this procedure, however, angles must be obtained for each transition moment involved in the isotropic absorption spectrum, which decomposes into the component bands of the guest molecule. In the present work, the values of ξ were calculated from theoretical data.37 The profile of the absorption band Aj in eq 3 was approximated by the Gaussian function based on the wavenumbers (but not wavelengths) as:
{
[
Aj(ν) ) Amax,j exp -(4 ln 2)
]}
ν - νmax,j δj
2
(5)
where Aj(ν) is the jth partial band at the wavenumber of ν, Amax,j is the maximum absorbance, νmax,j is the position of the band (36) Ojima, N.; Fukudome, K.; Yamaoka, K. J. Sci. Hiroshima Univ., Ser. A 1995, 59, 119. (37) Molina, V.; Merch n, M.; Roos, B. O. J. Phys. Chem. A 1997, 101, 3478.
Molecular Alignment and Photodimerization in Hydrotalcite
Langmuir, Vol. 15, No. 2, 1999 415 Table 1. Photocyclodimerization of 4′-Chlorostilbene-4-carboxylate in the Presence of Hydrotalcite Clay in DMF irradiation conversion
product yields (%)
solvent
ca (mg/mL)
time (h)
(%)
cis
synHH
synHT
DMF waterb
3 2.2
6 4
100 97
7.5 1
92.2 87
0.3 6
antiHH
a Symbol c is mass concentration of hydrotalcite clay. b These data are cited from literature: Takagi K.; et al. J. Am. Chem. Soc. 1993, 115, 4339.
Figure 1. Coordinate system of a guest for rotational operation. The guest plane lies on the Cartesian coordinates (x, y, z) with the x axis as the tilt axis, the y axis as the roll axis, and the z axis as the normal axis to the guest plane.
Figure 2. X-ray diffraction patterns of the LDH clay powder (a) in the absence and (b) the presence of CSC molecules into the interlayers. Arrows display the position of d001 diffractions. maximum, and δj is the half-intensity bandwidth. The isotropic absorption spectrum was decomposed into partial bands, each specified with a set of parameters (Amax, νmax, and δ). To remove any ambiguities and to facilitate a unique deconvolution, the second-derivative spectrum, d2Aj(ν)/dν2, was utilized.34,35
Results and Discussion X-ray Diffraction Analysis of Synthetic Hydrotalcite with CSC Molecules. Figure 2 shows the XRD patterns of hydrotalcite (a) in the absence and (b) in the presence of the CSC molecules. The gallery heights were calculated by subtracting the Mg(OH)2 sheet thickness of 0.48 nm from the (001) basal spacings, which are displayed by arrows in Figure 2. The gallery height in the absence of the CSC molecules was 0.31 nm. However, the gallery
Figure 3. (a) Transient electric linear dichroism signals, ∆A|/A and ∆A⊥/A at 310 nm. Applied field strength E ) 14.3 kV/cm and ∆A|/A ) -0.05 and ∆A⊥/A ) 0.02. (b) Field strength dependence of the steady-state reduced electric dichroism ∆A/A at 310 nm. Circles are experimentally obtained. Solid line is the best-fitting SUSID orientation function.
height of the synthetic hydrotalcite with the CSC molecules expanded up to 1.95 nm. These results indicate that the CSC molecules have been efficiently intercalated into the synthetic hydrotalcite clay interlayers. Furthermore, the gallery height of 1.95 nm was larger than the molecular length of a CSC molecule (1.48 nm). The possibility that the CSC molecules are intercalated as interdigitated or inclined double layer structures on the synthetic hydrotalcite interlayers must also be considered. Photocyclodimerization of Hydrotalcite with CSC Molecules in DMF. Table 1 shows the product yields of the photocyclodimerization. In our previous studies, the irradiation of CSC molecules in the presence of hydrotalcite clays dispersed in water stereoselectively produced both syn head-to-head (syn-HH) and syn head-to-tail (syn-HT)
416 Langmuir, Vol. 15, No. 2, 1999
Figure 4. Deconvolution and synthesis of isotropic absorption (lower) and ELD (upper) spectra. Experimental data are shown by the signs of circles, partial absorption bands are broken lines, and simulation data are by solid lines. At the bottom, the decomposed component bands with their optical parameters in Table 2 were assigned with the values of the angle θ in degrees, with which the ELD spectrum was synthesized (solid line at top).
Figure 5. Direction of optical transition moments of the guest molecule calculated from theoretical data in the literature. See text for details. Solid arrows indicate the coordinate axis: the x axis is a tilt axis, the y axis is a roll axis (cf. Figure 1). Arrows shown in broken line are illustrated as the respective transition moments. Numerals denote the approximate peak wavelengths in nanometers for the transition moments. Numerals in parentheses denote the angle ξ between the direction of the transition moment and the roll axis in degrees.
dimers among the four possible stereoisomers. In the present study, irradiation of hydrotalcite with the CSC molecules in DMF led exclusively to the yield of a syn-HH dimer. The resulting syn-HH and syn-HT ratio was 307.3: 1, higher than the ratio of 14.5:1 for the case of the photodimerization of CSC molecules intercalated in LDH in water. The photochemistry thus implies the molecular alignment of the CSC molecules in the clay interlayers to be in parallel orientation. These results indicate the dramatic effect of the solvent in controlling the photocy-
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Figure 6. Configuration model of CSC molecular intercalated aggregates in LDH interlayers. Distance between anion exchangeable sites, 0.54 nm, is the ideal value. The gallery height, 1.95 nm, is estimated from XRD analysis.
clodimerization of olefinic carboxylate anions as the direct result of the state of the molecular packing in the layers. (See later section for more details.) Field Strength Dependence of Reduced Dichroism. Figure 3a shows the measured ELD signals, ∆A|/A and ∆A⊥/A. The state of the CSC molecules intercalated in the clay interlayers remains unaltered by applying a pulse field (no electrochromic effect), based on the fact that the value of ∆A|/A coincides with the ∆A⊥/A value multiplied by a factor of -2. Figure 3b shows the field-strength dependence of the reduced dichroism ∆A/A plotted against E2 for the synthetic hydrotalcite with the CSC molecules suspended in DMF. The dichroism values tended to be saturated but never reached complete saturation even at high field strengths. To analyze the wavelength dependence of the reduced dichroism ∆A/A, the average degree of the orientation of the particles in suspension was computed at a given field. Since measurements of ∆A/A values at very high fields are difficult due to experimental conditions, the fitting of the observed ∆A/A values to a set of appropriate orientation functions was carried out. Some clay particles possess no permanent electric dipole moment, as verified by a recent reversing-pulse electric birefringence study.38-40 From the reversing-pulse electric birefringence studies of LDH dispersed in water, the LDH particles were found to possess no electric permanent dipole moment. Therefore, the theoretical SUSID orienta(38) Yamaoka, K.; Tanigawa, M.; Sasai, R. J. Chem. Phys. 1994, 101, 1625. (39) Sasai, R.; Yamaoka, K. J. Phys. Chem. 1995, 99, 17754. (40) Sasai, R.; Ikuta, N.; Yamaoka, K. J. Phys. Chem. 1996, 100, 17266.
Molecular Alignment and Photodimerization in Hydrotalcite
Langmuir, Vol. 15, No. 2, 1999 417
Figure 7. Mechanism of photocyclodimerization of CSC molecules intercalated into LDH interlayers.
tion functions for disklike particles39 can be employed to fit the observed data. In Figure 3b, the best-fitting theoretical curves were shown with a solid line. The reduced dichroism at infinitely high fields (∆A/A)inf was estimated as -0.195 from the curve fitting. This (∆A/A)inf value is related to the value of (∆A/A)s as follows: (∆A/A)s ) (-2)(∆A/A)inf. Therefore, the (∆A/A)s value was 0.39 at 310 nm. If a CSC molecule is perpendicular to the plane of the stacked clay layers, the (∆A/A)s value should be close to 3.0. Thus, the 310-nm transition moments of the CSC molecule can be assumed to be inclined at an angle of 49.6° relative to the clay plane. Wavelength Dependence of Saturated Reduced Dichroism. At a fixed electric field strength, the values of the reduced dichroism, ∆A/A, were measured at each wavelength in the absorption region. The wavelength dependence of the saturated reduced dichroism (∆A/A)s, which is termed the electric linear dichroism (ELD) spectrum, was calculated from the ∆A/A values according to eq 2. An upper curve in Figure 4 shows the ELD spectrum of the CSC molecules intercalated in the LDH interlyers. The ELD spectrum is not constant but is dependent on the wavelength within the 270-370 nm range with a maximum in the 300-340 nm region. This wavelength dependency leads to the important conclusion that the observed ELD spectrum is composed of a number of absorption bands of the CSC molecule. Therefore, the deconvolution of the isotropic absorption spectrum into partial bands and the determination of the directions for the optical transition dipole moments of these bands are necessary for a quantitative discussion in the configuration of CSC molecules intercalated in the LDH interlayers. Deconvolution of Isotropic Absorption. Figure 4 shows the deconvolution into component bands (broken bottom lines) in the observation of the absorption spectrum of CSC molecules in the LDH interlayers in the absence of an applied electric field. The observed ELD spectrum was simulated (top) by giving an appropriate value of the angle to each component band of the CSC molecule according to eq 3, as indicated with a numeral. An excellent agreement between the observed (circles) and calculated (solid line) values can be found. The principle of the deconvolution is to make the number of partial bands minimal in order to reproduce the observed spectrum; therefore, some partial bands may be composed of more than one component band, but no further attempt in deconvolution has been carried out in this work. Estimation of Tilt Angle ΘT and Roll Angle ΘR. To calculate the roll and tilt angles (ΘR and ΘT) of the plane of the CSC molecule intercalated into the LDH interlayer from eq 4, the angle in each CSC molecule must be known. Since no data for the transition moments are available for the CSC molecule adsorbed onto the LDH interlayers, the
Table 2. Optical Characteristic of 4′-Chlorostilbene-4-carboxylate and ELD Results statea
λmax (nm)
ξa (deg)
θ (deg)
ΘT (deg)
ΘR (deg)
11Bu 21Bu 61Bu 31Bu, 41Bu, 51Bu
341 328 310 285
1.0 -6.0 3.0 -52.0
51.0 49.5 50.5 48.0
39.4
27.2
a These data are cited from literature: Molina, V.; et al. J. Phys. Chem. A 1997, 101, 3478.
theoretical values of trans-stilbene were used in place of CSC for the angle ξ37 with consideration to the substitution of both the chloro and hydroxycarbonyl groups in the stilbene framework.41 Figure 5 shows the direction of the transition moments, the angle, and the wavelength at the position of the band maximum. The tilt and roll angles calculated from eq 4 are shown in Table 2. The values of ΘR and ΘT in Table 2 indicate that the plane of the CSC molecule is clearly tilted 39.4° and rolled 27.2° with respect to the LDH layer. Correlation between Molecular Alignment and Photodimerization. Figure 6 shows the configuration model of the CSC molecular aggregates packed in the LDH interlayers, constructed from the obtained ΘR and ΘT degrees and X-ray analyses. The CSC molecules are thought to form a double-layer structure more favorably than a single-layer structure in the LDH interlayers, as shown in Figure 6, judging from the fact that the molecular length of the CSC molecule is 1.48 nm. The above packing of the olefin molecules indicates that the syn-HH dimers are mainly produced as a natural consequence, since the photocyclodimerization occurs between adjacent, paralleloriented CSC molecules by UV irradiation. The production of syn-HT dimers is quite unlikely in such orientations, as shown in Figure 6. In the spatially controlled photocyclodimerization of CSC in hydrotalcite clay, an excited CSC molecule interacts with an adjacent CSC to form an excimer which is followed by the formation of a syn-HH dimer in the case of intercalation in the LDH interlayers, as shown in Figure 7. The CSC molecules intercalated in the LDH interlayers form a double layer structure, as shown in Figure 6. An excimer formation of CSC in the LDH interlayers was ascertained by the observation of an excimer emission at around 410 nm. Conclusions The present study was undertaken by employing an unprecedented method in which CSC molecules were (41) Silverstein, R. M.; Bassler, G. C.; Morrill, T. C. Spectrometric Identification of Organic Compounds; John Wiley & Sons: New York, 1991.
418 Langmuir, Vol. 15, No. 2, 1999
included in the interlayers during preparation. UV irradiation of this suspension primarily yielded syn-HH dimers as the main product. The formation of the syn-HH dimer correlates well with the double layer and parallel structure of CSC in the LDH interlayers (cf. Figure 6) which is evidenced by XRD and ELD analyses. Since the CSC molecules form a double layer in the LDH interlayers,
Sasai et al.
it was found that the syn-HH dimers can be stereoselectively produced by UV irradiation. Acknowledgment. This work was supported in part by the Grant-in-Aid from the Ministry of Education, Science and Culture of Japan. LA980699A
