MATERIALS CHEMISTRY COMMUNICATION Second harmonic generation of dye aggregates in bentonite clay Thibaud Coradin,a Keitaro Nakatani,b Isabelle Ledoux,c Joseph Zyssc and Rene� Cle�ment*a aL aboratoire de Chimie Inorganique, U.R.A. 420, Universite� Paris-Sud, 91405 Orsay, France bP.P.S.M., Ecole Normale Supe�rieure de Cachan, U.R.A. 1906, avenue du Pdt Wilson, 94235 Cachan, France cFrance T elecom CNET Centre Paris B, L aboratoire de Bagneux, 196, avenue Henri Ravera, 92220 Bagneux, France Intercalation of the dyes was ascertained by powder X-ray diraction, using a Siemens diractometer with a Cu-Ka Intercalation of stilbazolium chromophores in a bentonite clay leads to the formation of dye aggregates exhibiting second anode.The diractograms of the four compounds show a relatively narrow 001 reflection (width ca. 2h=0.5°) indicating harmonic generation properties. a basal spacing of ca. 16A° . Assuming a van derWaals thickness of a clay layer around 9 A° ,10 a value of ca. 7A° can be derived for the thickness of the dye layer, which is consistent with the value found in the MPS3 intercalates. This suggests that the chromophores lie edge-on within the galleries.The amount of The use of organic–inorganic hybrid materials has already inserted dye also compares with the MPS3 analogues [from been extremely fruitful for producing new compounds with elemental analysis, C 11.8%, N 1.5% (by mass) for the non-linear optical (NLO) properties.1–4 We have recently bentonite–DAMS intercalate at maximum loading]. reported that intercalation of stilbazolium chromophores in Fig. 1 shows the evolution of the UV–VIS absorption spectra the hexathiohypodiphosphate MPS3 layered phases can give of the DAMS+ and DAES+ intercalates with increasing rise to NLO-active materials.5,6 A model involving the forma- chromophore concentrations. At low concentrations, both tion of dye J-aggregates in the interlamellar space of the host materials present a broad and slightly asymmetrical band with was then suggested.a maximum absorption around 460 nm, this wavelength being In order to study the influence of the nature of the host very close to the charge transfer band of the organic molecules lattice on the chromophore packing and its possible contri- in aqueous solution. As the amount of inserted cations bution to the NLO properties, we have undertaken the insertion of the same dyes in bentonite, a cation-exchangeable clay.In contrast to MPS3 intercalation chemistry, insertion in clays usually takes place at room temperature and permits monitoring of the concentration of inserted species. The synthesis of the 4-[4-(dimethylamino)-a-styryl]-1- methylpyridinium (DAMS+) iodide, and of the other derivatives (DAZOP+, DAES+, DEMS+) used in this work, has already been reported.6,7 The last three of these present only slight modifications from the DAMS+ skeleton to ensure a similar molecular quadratic hyperpolarisability b.8,9 Bentonite SPV (Comptoir de Mine�raux et de Matie`res Premie`res) with a cation exchange capacity (CEC) of 90 mequiv.per 100 g of the clay was used after equilibration with a sodium chloride solution.Aqueous suspensions with bentonite concentrations from 0.4 to 4 g l-1 and dye concentrations from 10-5 to 10-3 mol l-1 were allowed to stand overnight at room temperature. The strongly Fig. 1 Selected UV–VIS spectra of (a) bentonite–DAMS+ and coloured solids were collected by centrifugation, thoroughly (b) bentonite–DAES+ intercalates in a concentration range of 10-4 (spectrum 1) to 10-2 mol (spectrum 2) of dye per 100 g of clay washed with water and dried.J. Mater. Chem., 1997, 7(6), 853–854 853increases, two dierent behaviours are observed. In the tration dependence and the striking sensitivity to the chromophore structure of the aggregation process suggest a very tight DAMS+ case [Fig. 1(a)], a new band arises around 550 nm, close packing of the organic molecules, consistent with the which grows stronger and narrower than the previous one as nearly 100 nm shift of the charge-transfer band observed for the maximum dye concentration is reached.In contrast, the the intercalates. We are currently studying the photophysical bentonite–DAES+ spectra [Fig. 1(b)] show no equivalent feaproperties of these materials to obtain more information ture, the bandshape remaining essentially the same when concerning the size and structure of these aggregates.concentration is increased. Finally, the DAZOP+ cation presents the same behaviour as DAMS+ whereas DEMS+ seems We thank the Comptoir de Mine�raux et de Matie`res Premie`res to resemble DAES+. As already discussed,6 the strong red- (Paris) for the gift of bentonite. Support by European COST shift of the UV band of intercalated DAMS+ and DAZOR+ Action D4/0001/95 is kindly acknowledged.We are also grate- can be attributed to the formation of J-type aggregates. ful to Dr P. Lacroix (Toulouse) for stimulating discussions. Second harmonic generation (SHG) experiments were carried out on 100 mm sieved samples using the Kurtz–Perry powder technique11 operating at variable fundamental inten- References sity, the harmonic signal from the unknown powder being 1 S.Tomaru, S. Zembutsu, M. Kawachi and M. Kobayashi, J. Chem. plotted with respect to the harmonic emission from a reference Soc., Chem. Commun., 1984, 1207. urea powder.12 The laser source is a ns Nd–YAG pulsed laser 2 V.Ramamurthy and D. F. Eaton, Chem.Mater., 1994, 6, 1128. operating at 1.34 mm. Pure bentonite, bentonite–DAES+ and 3 B. Lebeau, C. Sanchez, S. Brasselet, J. Zyss, G. Froc and bentonite–DEMS+ did not generate any significant signals. In M. Dumont, New J. Chem., 1996, 20, 13. 4 R. Hoss, O. Ko�nig, V. Kramer-Hoss, U. Berger, P. Rogin and contrast, bentonite–DAMS+ and bentonite–DAZOP+ com- J.Hulliger, Angew. Chem., Int. Ed. Engl., 1996, 35, 1664. pounds were found to exhibit SHG signals whose intensity 5 P. G. Lacroix, R. Cle�ment, K. Nakatani, J. Zyss and I. Ledoux, increased with chromophore concentration to reach a maxi- Science, 1994, 263, 658. mum value of 0.25 times urea in the bentonite–DAMS+ case. 6 T. Coradin, R. Cle�ment, P. G. Lacroix and K. Nakatani, Chem.Upon comparison of these results with the previously Mater., 1996, 8, 2153. reported ones concerning MPS3 intercalates,7 the same dis- 7 S. R. Marder, J. W. Perry and C. P. Yakymyshyn, Chem. Mater., 1994, 6, 1137. crimination in UV–VIS spectra and NLO properties was 8 Non L inear Optical Properties of Organic Molecules and Crystals, observed. More precisely, only the intercalates containing ed.D. S. Chemla and J. Zyss, Academic Press, New York, 1987. aggregated dyes gave rise to SHG. The lower eciency, 9 Molecular Non L inear Optics: Materials, Physics and Devices, ed. as compared to MPS3–DAMS+, of the bentonite–DAMS+ J. Zyss, Academic Press, New York, 1994. compound may be attributed, at least in part, to the poor 10 Intercalation Chemistry, ed.M. S.Whittingham and A. J.Jacobson, Academic Press, New York, 1982. crystallinity of the clay samples. 11 S. K. Kurtz and T. T. Perry, J. Appl. Phys., 1968, 19, 3798. In conclusion, this work provides another example of a 12 P. D. Maker, Phys. Rev. A, 1970, 1, 923. host–guest system that exhibits SHG properties even though 13 D. Mo�bius, Adv. Mater., 1995, 7, 437. the host lattice is known to be centrosymmetrical. The nega- 14 R. Cohen and S. Yariv, J. Chem. Soc., Faraday T rans. 1, 1984, tively charged layers appear to favour the formation of highly 80, 1705. positive J-aggregates leading to a non-centrosymmetric arrangement of the dyes.13,14 Moreover, the strong concen- Communication 7/02078C; Received 25thMarch, 1997 854 J. Mater. Chem., 1997,