Comprehensive Fluorescence-based Comparative Studies of the Inclusion Properties of Molecular Hosts

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Supramolecular chemistry involves structures consisting of two or more molecules held together by non-covalent intermolecular interactions. The simplest example of a supramolecular system is a host-guest inclusion complex, in which a guest molecule becomes included within the cavity of a larger, cage-like host molecule. Our group has established the comparative use of two isomeric fluorescent guests, 1-anilino-8-naphthalene sulfonate (1,8-ANS) and 2-anilino-6-naphthalene sulfonate (2,6-ANS), which differ only in shape. Previous work involved studies of the properties of the two most widely used families of molecular hosts, cyclodextrins and cucurbiturils, which illustrated the value of using these two guests in comparative studies. In this proposal, the binding of these two guests as well as 7-methoxycoumarin (7MC) will be explored with two other families of macrocyclic hosts, namely calixarenes and pillarenes. These four families of hosts have varying cavity sizes, shapes, and properties. In the first part of this proposal, the inclusion of 1,8- and 2,6-ANS in calixarenes and pillarenes will be studied in aqueous solution, to complement previous work and give an extensive study of these four families of hosts in water. The second part of the proposal involves a similar extensive set of comparative binding studies of these four families of hosts in nonaqueous solution, to firmly establish the role of solvent in host-guest inclusion. We recently published a study of the inclusion of both ANS by a pillarene host in nonaqueous solvents, which showed strong dependence of the binding on the solvent used, and which established our ability to do these nonaqueous studies. The third part of this proposal involves the use 7MC as guest, which is more compact in shape and size compared to ANS. The binding of this guest in the four families of hosts will provide further understanding of their comparative binding properties. The major objective of this research project is to establish an extensive understanding of the cavity properties, guest preferences, and driving forces of inclusion amongst these four important families of hosts in both aqueous and nonaqueous solution, and to establish general and specific structure-binding relationships. Host-guest inclusion in solution has found extensive research and industrial applications, such as in drug delivery and consumer products. The suite of these four macrocyclic families provides an arsenal of hosts from which to choose, and it is essential to understand their comparative binding properties. There has been no previous such large scale comprehensive study with these hosts; the innovative and ambitious research proposed herein making use of three different guests, four different families of hosts, and both aqueous and nonaqueous solvents, will provide a deep understanding of this phenomenon, which will be essential for efficient and transformative academic and industrial applications of host-guest inclusion.