Synthesis and properties of new thiourea-functionalized poly(propylene imine) dendrimers and their role as hosts for urea functionalized guests

Five generations of poly(propylene inline) dendrimers have been modified by palmityl and adamantyl endgroups via a thiourea linkage. The synthesis of the thiourea dendrimers DAB-dendr-(NHCSNHAd)(n) and DAB-dendr-(NHCSNHC16H33 )(n) (n = 4, 8, 16,32, 64) proceeds smoothly via the amino-terminated DAB d endrimer :ind the adamantyl and palmityl isothiocyanates, respectively. The properties of the thiourea dendrimers have been studied by IR and H-1 NMR, including relaxation (T1, T2) measurements. The thiourea dendrimers are us ed as multivalent hosts for a number of guest molecules containing a termin al urea-glycine unit in organic solvents. The host-guest interactions have been investigated using 1D- and NOESY-NMR. These investigations show that t he guest molecules bind to the dendritic host via thiourea (host)-urea (gue st) hydrogen bonding, and ionic bonding between the terminal guest carboxyl ate moiety and the outer shell tertiary amines of the dendrimer. The abilit y to bind guest molecules of the adamantyl- and palmitylthiourea dendrimers has been compared with their respective urea containing dendrimer analogue s, by NMR-titration, and competition experiments. Upon complexation, the th iourea dendrimer hosts show a larger downfield NH. shift than the correspon ding urea dendrimer hosts, indicative of stronger hydrogen bonding in the c omplexed state. Furthermore, microcalorimetry has been used to determine bi nding constants for formation of the host-guest complexes; the binding cons tants are typically in the order of 10(4) M-1. Both NMR and microcalorimetr ic studies show that the thiourea dendrimers bind the urea containing guest s with somewhat higher affinity than the corresponding urea dendrimers.