Molecular Recognition in Lamellar Solids and Thin Films

“Molecular recognition” describes the selective, and usually noncovalent, binding of a molecule or ion to a complementary host. Such specific associations between molecules are ubiquitous in biology and include the vital functions of catalysis, immune response, transcription, replication, and biochemical signaling. They also lie at the heart of several practical chemical technologies, including chemical sensing, separations, and catalysis. In light of the important place that molecular recognition holds in chemistry, the design and discovery of better hosts remain a worthy challenge. Historically, this challenge has been taken up by organic chemists, who have elaborated the principles of host-guest chemistry and have synthesized a wide variety of hosts with high affinity and selectivity.1 It has been correctly noted, however, that molecular recognition by itself is not enough.2 To be useful, host-guest interactions must operate in concert with other processes. In chemical sensing, for example, the binding event must be coupled to a signal transduction mechanism. Sometimes this can be achieved by using a soluble, small molecule host, as in the case of fluorescent chemosensors.3 More often, however, some kind of macromolecular scaffoldingsa membrane, an electrode, a conducting polymer, to name a few possibilitiessis needed to allow the binding event to be detected in the outside world. Similar considerations apply to the design of supramolecular catalysts, in which molecular association must occur near a catalytic center or, in the case of a bimolecular reaction, in such a way as to bring the reactant molecules together.4 Solids in general present more opportunities for signal transduction and catalysis than do individual organic molecules in solution. The hard part is making solids with molecular specificity. This Account describes work that our group and others have recently undertaken in the design of lamellar inorganic hosts. The idea behind this research is to combine the best of both worlds: to take inspiration and design from the world of organic host-guest chemistry, and to combine them with materials that have the virtues of crystallinity, ultrahigh surface area, and modular synthesis from easily manipulated metals, ligands, and templates. So far our work has concentrated on problems of chiral separations and small molecule sensing; however, it should be noted that the catalytic and electronic properties associated with inorganic materials might lend themselves readily to other applications, once the design principles of extended inorganic hosts are better elaborated.