Journal of Sol-Gel Science and Technology 26, 571–575, 2003 c  2003 Kluwer Academic Publishers. Manufactured in The Netherlands. Strategies for Spatially Separating Molecules in Mesostructured Sol-Gel Silicate Films PAYAM MINOOFAR, RAQUEL HERNANDEZ, ANNE-CHRISTINE FRANVILLE, SHINYE CHIA, BRUCE DUNN AND JEFFREY I. ZINK ∗ Department of Chemistry and Biochemistry, University of California, Los Angeles, CA 90095, USA zink@chem.ucla.edu Abstract. Three strategies for placing molecules in designated regions of mesostructured thin ﬁlms made by the sol-gel dip-coating technique are demonstrated. These strategies all involve one-step syntheses where all of the components are present in the sol from which the substrate is pulled. Silicate ﬁlms templated by ionic surfactants contain three spatially-separated regions: a silicate framework, an organic region formed by the hydrocarbon tails of the surfactants, and an intervening ionic interface formed by the charged surfactant head groups. The ﬁrst method exploits lipophilic interactions between the molecule and the micelle to place it in the organic region. The second method uses chemical bonding of multiple trialkoxysilane groups in a three dimensional array on the molecule to place it in the silica framework. The third method uses multiple functionality at opposite sides of the molecule to enable it to span two regions. Luminescent molecules are used, and spectroscopy monitors the formation. Keywords: sol-gel, mesostructure, luminescence, thin ﬁlm Deliberate and controlled placement of molecules in the spatially separated regions of mesostructured sili- cate materials is under active investigation [1–12]. The most commonly employed method involves backﬁlling or postdoping of MCM-41 type materials in which the silica is ﬁrst templated with surfactants, then emptied of these structure-directing molecules by calcination or solvent extraction, and ﬁnally re-ﬁlled by diffusion of the desired guest molecules into the empty pores [13– 18]. The location of the guest molecules is limited to the pores. An alternative method that allows one or more molecules to be placed in speciﬁed regions of the structure in a one-step dip-coating synthesis has recently been reported [1]. In the dip coating process a substrate is withdrawn slowly at a constant speed from a sol [12, 19, 20]. Upon withdrawal, a liquid ﬁlm becomes entrained on the surface of the moving substrate. This ﬁlm thins from gravitational draining, capillary-driven ﬂows and evaporation. The ﬁlm formation process is dynamic. ∗ To whom all correspondence should be addressed. Hydrolysis and condensation occur in tens of seconds, solvent evaporation is also rapid, and collapse of the gel network to the ﬁnal ﬁlm thickness (10 3 ˚ A) occurs in less than a minute [12, 19, 20]. Figure 1 is a schematic diagram of the transfor- mations that occur as the ﬁlm is drawn from a sol that contains surfactant molecules. The regions identi- ﬁed in the diagram are not sharply deﬁned but evolve continuously. The important assembly and chemical processes must occur sequentially and in the proper order; i.e., the micelles must form and achieve their long-range three-dimensional order before silicate con- densation cements the structure in place. In dip coat- ing, these processes are separated not only temporally but also spatially. This separation enables molecular probes to be used to monitor the assembly and the chemical changes that occur [12, 19, 20]. The ﬁrst materials that contained luminescent molecules local- ized in speciﬁc regions (micelle interior, framework) were those formed during dynamic spectroscopic studies using molecular probes in evolving ﬁlms [12].