Synthesis. The phenothiazine substituted dendrimer (1) was prepared according to the published procedure for the lower generation dendrimers. The synthesis of the guest molecules 2 -5 is shown in Scheme 1 & 2. The phenylphosphonate (19) used in the synthesis of the two phosphonic acid guests was synthesized in 2 steps from chloroacetylchloride and phenol followed by an Arbuzov-reaction with trimethylphosphite (see also the experimental section). Binding studies. Initial experiments with the fluorescent host ( ) and the guests ( ) and ( ) in CHCl3 and CH2Cl2 showed, that the systems were not photochemically stable. The mechanism for the degradation is not clear, but the absence of the characteristic absorption band for the radical cation of phenothiazine at 515 nm indicates, that the degradation is not due to photoinduced electron transfer from the phenothiazine to the solvent, which could be expected for phenothiazines. However, NMR of the various combinations of guest and host (8) showed, that this technique could be used for determining the association constants. A typical titration curve is shown in figure , and upon fitting the measured data to a 1:1 stochiometry the association constants shown in table The guests 3 and 6 were isolated as their pyridinium salts (called 3b and 6b), which were easier to handle, than the free acids. The differences in association constants between the pincer and carboxylic (2), phosphonic (4) or sulfonate (3b) are much smaller, than what we found previously for an urea-based system with an adamantyl urea pincer system and ureidoacetic acids as guests, where large differences were observed, which essentially allowed discrimination between the different types of guests. The monoprotic phosphonic acid 5 shows no binding, which is interesting in view of the relatively high acidity of phosphonic acid mono esters (pKa = 1.6 for methane phosphonic acid monomethyl ester ).