Jozef Hodyl PhD Thesis - Theses - Flinders University

In addition, successful immobilisation of these receptors on a silica surfacecomplicates the characterisation of the guest uptake process. 136 In particular, thesilica immobilised receptor materials cannot be quantitated to obtain bindingconstants as this requires dissolution of both the host and the guest to set upconditions for a rapidly established equilibrium between the bound and free guest. 165Taking this point into account, it was decided firstly to synthesise a structurally andchemically similar model receptor, 49, that could be used to test the suitability of athree-walled binding cavity. In this molecular receptor model the place of the linkergroup would be taken by an (S)-hydroxypropyl pendant arm, as shown in Figure 2.1,and because of its solubility anion binding constants could be measured andcompared with those found for a comparable first generation receptor such as 26.49Figure 2.1 Schematic depiction of the structural features of the molecular receptor model, 49,highlighting its correspondence with silica attached receptor 94: blue, three phenoxyderived pendant arms, red, octa-coordination with cadmium(II) and green, (S)-hydroxypropyl pendant arm, simulating the linker group.Conceptually, the three-walled cavity appears to have an "open gate" throughwhich included species are likely to escape through increased contact with thesolvent. Furthermore, the reduction in the number of aromatic species forming thewalls of the cavity may result in a proportional loss of possible π-π interactions,presupposing, of course, that hydrogen bonding interactions between the host O-H36