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(22) Kostov, K. L.;Marinova, Ts. S. Surf.Sci. 1987, 184, 359. (23) Demuth, J. E. Surf.Sci. 1977,69, 365. (24) Barteau, M.A.; Madix, R. J. Surf.Sci. 1982, 115, 355. (25) Stuve, E. M.; Madix, R. J.; Sexton, B. A. Surf.Sci. 1982, 123,491. (26) Parmeter, J. E.; Hills, M. M.;Weinberg, W. H. J. Am. Chem. Soc. 1988, 110,1952. (27) Guo, X.; Hoffman, A.; Yates, J. T., Jr. J. Chem. Phys. 1989, 90, 5187. (28) Matsushima, T.Surf.Sci. 1985, 157, 297. (29) Imbihl, R.; Demuth, J. E. Surf. Sci. 1986. 173, 395. (30) Weissman, D.L.;Shek, M. L.;Spiar, W. E.S w f . Sci. 1980,92, L59.
(31) Kesmodel, L.L.; Waddill, G. D.; Gates, J. A. Surf.Sci. 1984,138, 464. (32) Szabo, A.; Kiskinova, M.;Yates, J. T., Jr. J. Chem. Phys. 1989,90, 4604. (33) Davis, J. L.; Barteau, M. A. Sur/. Sci. 1988,197, 123. (34) Davis, J. L.; Barteau, M. A. Surf.Sci. 1992,268, 11. (35) Davis, J. L.; Barteau, M. A. Surf. Sci. 1989, 208, 383. (36) Davis, J. L.; Barteau, M. A. Surf Sci. 1991, 256, 50. (37) Stuve, E. M.; Madix, R. J. Surf.Sci. 1985, 160, 293. (38) Conrad, H.; Ertl, G.; Kiippen, J.; Latta, E.E. Surf.Sci. 1977, 65, 245.
Photochemical Hole Burning of 1,4-Dlhydroxyanthraquinone Intercalated In a Pillared Layered Clay Mineral Makoto Ogawht Tokuhiko Handa? Kazuyuki K u r ~ d a , * .Chuzo ~ Kate; and Toshiro Tanit Department of Applied Chemistry, Waseda University, Ohkubo-3, Shinjuku- ku. Tokyo 169, Japan, and Electrotechnical Laboratory, Umezono 1 - 1-4, Tsukuba-shi. Ibaraki 305, Japan (Received: March 19, 1992; In Final Form: May 27, 1992)
A t e ~ ~ y l a ~ n i ~ p o n i t ~ 1 , ~ y ~ intercalation o x y ~ compound ~ q ~ owas n prepared e and a persistent spectral zero-phonon hole was obtained at liquid helium temperatures by Kr' laser light irradiation (520.8 nm). This is the first example of persistent hole formation of 1,44ihydroxyanthraquinone in crystalline inorganic matrices. In spite of the high concentration of 1,4-dihydroxyanthraquinone(ca. 1.5 mol kg-I), a narrow hole with the initial width of 0.25 cm-' (4.6K) was obtained with no distinct decrease in burning efficiency (>1.0X 10-4) if compared with those doped in ordinary polar polymers or organic glasses. The microscopic geometry of the compound is discussed on the basis of hole formation properties as well as X-ray powder diffraction and I3C NMR observation.
Introduction Photochemical hole burning (PHB) is a site-selective and persistent photobleaching in an inhomogeneously broadened a b sorption band induced by resonant laser light irradiation at cryogenic temperatures. It has attracted increasing attention recently partly due to its possible applicability to high-density frequency domain optical storage, in which a more than lo3 times higher storage density than that of the on-going optical disk system would in principle be available. There remain, however, many obstacles to be overcome in materials optimization as well as in the issue of the storage system and architecture before its realization.' Among many efforts, a search for new materials is important because the hole formation processes depend significantly on the nature of host-guest systems. For the formation of persistent holes, the existence of both a solid matrix and a photoreactive molecule is essential. In this study, we prepared a tetramethylammonium (TMA)-saponite1,4dihydroxyanthraquinone (DAQ) intercalation compound and investigated its PHB reaction to show the merits of an ordered matrix for a PHB material. DAQ is one of the molecules most extensively used as a PHB probe and its hole formation has been observed in lots of amorphous matrices.*-* It has been suggested that the PHB reaction of DAQ is due to the breakage of internal hydrogen bond(s) and the subsequent formation of external hydrogen bond(s) to proton acceptor(s) in a matrix (Figure Saponite, the idealized structure is shown in Figure Id, is a layered clay mineral which can accommodate a wide variety of organic substances in its interlayer space to form intercalation compounds? There are some attractive features as a host material for PHB reactions: It is colorless and transparent in the visible region and its interlayer surfaces are capable of accepting hydrogen bonds. Moreover, the surface properties of + Waseda
University.
* Electrotechnical Laboratory.
saponite such as adsorption property can be altered by co-intercalation of appropriate guest species. To be a molecularly dispersed system is another basic prerequisite for composing effcient PHB materials to avoid line broadening due to energy transfer. For this purpose, saponite was modified by pillaring with TMA ions to obtain indepmdent micro pore^^^.^' in which DAQ molecules were incorporated at a monomolecular level without aggregation. ExperimeaW Section
Synthetic saponiteizin powder form (100 mesh, cation exchange capacity; 71 mequiv/100 g of clay, Kunimine Industries Co., Japan) was used as the host material. Typical size of the particles seems to be roughly 100 pm in diameter with 10 layer thickness. DAQ (WAKO Pure Chemical Industries Co.) was used after recrystallization from chloroform. TMA chloride (Tokyo Kasei Industries Co.) was used without further purification. TMA-saponite was prepared by an ion exchange reaction wing an aqueous solution of TMA chloride and used after washing with deionized water repeatedly until a negative result of AgN03 test was obtained. Then, the TMA-saponite-DAQ intercalation compound was prepared by mixing the TMA-saponite into a saturated acetone solution of DAQ and stirring at room temperature for 1 day followed by washing with n-hexane. For the PHB experiments, the TMA-saponiteDAQ intercalation compound was dispersed in deionized water and cast on a glass s u b strate to form a thin film. Samples were oooled down to liquid helium temperature region in a temperature-variable cryostat (CF 1204,Oxford) and the holes are burnt with a krypton ion laser (Spectra Physics 165) with a solid Etalon operated at 520.8 nm. Monochromatic light (Jobin Yvon, THR 1500)from a tungsten halogen lamp (150 W) was transmitted through the sample and detected by a cooled photomultiplier (Hamamatsu Photonics, R943-01) with phasesensitive method. The overall resolution was 0.03 cm-'. Temperatures of the samples were monitored by caribrated Au (0.08
0022-3654/92/2096-8116$03.00/00 1992 American Chemical Society
The Journal of Physical Chemistry, Vol. 96, No. 20, 1992 8117
Photochemical Hole Burning
c
3
Silicate sheet
Silicate sheet
TMA
Silicate sheet
It
0.95 nm
-/i
Q.H/O
b)
Ground State
Photoproducts
e) Silicate sheet
DAQ Silicate sheet
I/
Figure 1. Schematic structures of the TMA-saponite-DAQ intercalation compound: (a) original saponite, (b) the TMA-saponite; (c) the TMAsaponite-DAQ intercalation compound. Insert: Layered structure of saponite (d) and the proposed photochemical reaction of DAQ (e).
wt % Fe) vs Chrome1 thermocouple. The accuracy of the temperature measurements is K0.05 K while its resolution is much higher. Results and Discussion The basal spacing of the TMA-saponite determined by X-ray diffraction was 1.43 nm, indicating that the TMA-pillared interlayer spacing is 0.47 nm. (The thickness of each of the silicate layers of saponite is 0.96 nm.9) The amount of adsorbed TMA,
which was determined by elemental analysis, was 68 mequiv/ 100 g of clay, indicating that almost 96%Na+ ions are exchanged by TMA ions. The inductively coupled plasma emission (ICP) spectroscopy showed that only a trace amount of Na+ remained in the TMA-saponite intercalation compound. Thus, it was concluded that TMA ions substituted most of the interlayer Na+ ions and therefore the intercalated TMA can provide sites or micropores for adsorption of DAQ in the interlayer as is shown in Figure lb.
8118 The Journal of Physical Chemistry, Vol. 96, No. 20, 1992
By the reaction with DAQ, the basal spacing increased further from 1.43 to 1.46 nm, which was also observed by X-ray diffraction, indicating the expansion of the interlayer spacing from 0.47 to 0.50 nm. In the infrared spectrum of the TMA-saponiteDAQ intercalation compound, though the overlapping of other lines such as from H 2 0 make it fairly hard to distinguish, the absorption bands due to aromatic ring frequencies of DAQ were detected at 1587 and 1457 cm-'. This may confirm that DAQ molecules are incorporated within the micropores without decomposition. The chemical shifts observed in the I3CCP MAS NMR spectrum showed the existence both of DAQ (165.4.133.3, and 125.5 ppm relative to TMS) and TMA (57.2 and 55.6 ppm)I3 in the product. The observed chemical shifts attributed to DAQ were almost similar to those of DAQ in DMSO-d6,14indicating no existence of distinctive chemical bonds between TMA and DAQ. As for the chemical shifts attributed to TMA, the observed values of 57.2 and 55.6 ppm in a TMA-saponittDAQ system are clearly different from those in TMA-saponite (58.4 and 56.7 ppm). This suggests the occurrence of substantial chemical change in the surrounding of TMA. In the TMA-saponite-DAQ intercalation compound, TMA ions are surrounded by both DAQ while they are surrounded only by H 2 0 in the latter and HzO, system. On the basis of these facts and takmg into account the molecular size and shape of DAQ and the geometry of the micropore of TMA-saponite, DAQ was intercalated into the interlayer site of the TMA-saponite with the molecular plane nearly perpendicular to the silicate sheet (Figure IC). The amount of adsorbed DAQ was ca. 3.7 wt %, which was determined by subtracting the amount of TMA from the total organic content for the TMA-saponiteDAQ intercalation compound obtained by the elemental analysis. Considering the geometry of the micropore of TMA-saponite and the available surface area (ca. 470 m2g-l, which was determined by subtracting the TMA occupied area (ca. 310 m2 g-') from the total surface area of saponite (780 m2g-l of clay)), and once again taking into account the molecular size of DAQ or its projected area (ca. 0.22 nm2), up to two adsorbed DAQ molecules could occupy each micropore in the TMA-saponite. However, the volume of two DAQ molecules seems rather tight for the space and the occupation may require some distortion in the compound, which should eventually introduce electronic coupling between DAQ and/or TMA. No such evidence is obtained in the IR and I3CNMR spectra of DAQ in the compound as is mentioned above. Therefore, we postulate that most of the micropores are occupied with a single DAQ molecule in the present system. The intercalation compound contains a certain amount of adsorbed water (
