Cooperative self-assembly of dendrimers via pseudorotaxane formation from a homotritopic guest molecule and complementary monotopic host dendrons

Interaction of the homotritopic guest 1,3,5-tris[p-(benzylammoniomethyl)phenyl]benzene tris(hexafluorophosphate) (1a) with dibenzo-24-crown-8 (DB24C8) leads to the sequential self-assembly of [2]-, [3]-, and [4]-pseudorotaxanes 7a, 8a, and 9a, respectively. The self-assembly processes were studied using NMR spectroscopy. In CD3CN and CD3COCID3 the individual association constants K-1, K-2, and K-3 for 1:1, 1:2, and 1:3 complexes were determined by several methods. Via Scatchard plots, the three NH2+ sites of la were shown to behave independently in binding DB24C8. K values (4.4 x 10(2), 1.4 x 10(2), and 41 M-1, respectively, in CD3CN) directly determined from signals for the individual complexes (7a, 8a, and 9a) were somewhat higher than those estimated from the Scatchard plot because of concentration dependence, but the ratios of association constants followed the expected statistical order (K-1:K-2:K-3 = 3:1:1/3). These are believed to be the first evaluations of association constants leading to a [4]-pseudorotaxane, In the less polar CDCl3, association constants could not be determined because similar to90% of the dissolved tritopic guest, which by itself is insoluble, was present as the fully loaded [4]pseudorotaxane 9a! Self-assembly of homotritopic guest 1 a with benzyl ether dendrons of the first, second, and third generations functionalized at the "focal point" with DB24C8 moieties (3-5) produces pseudorotaxane dendrimers. The self-assembly processes were studied using H-1 NMR spectroscopy. In CD3COCID3 for all three generations the individual association constants K-1, K-2, and K-3 for [2]-, [3]-, and [4]-pseudorotaxane complexes 7c-e, 8c-e, and 9c-e indicated that the self-assembly was cooperative; that is, the ratios of the individual association constants exceeded the expected statistical ratios. Scatchard plots confirmed this behavior. Self-assembly processes in the less polar CDCl3 were kinetically slow, requiring ca. 1, 2, and 3 days, respectively, for the first, second, and third generation systems to reach equilibrium with 1 a; the slow rate is attributed to the insolubility of the homotritopic guest 1 a in this medium and the steric demands of the resulting dendrimers. However, only dendrimers of 1:3 stoichiometry, that is, the nanoscopic [4]psoudorotaxanes 9, were formed! Moreover, it is noteworthy that the extent of dissolution of la (reflective of the overall association constant which is too high to measure) increases with generation number, presumably because of the more effective screening of the ionic guest by the larger dendrons and perhaps favorable pi-pi and CH-pi interactions. Such cooperative effects suggest a number of applications that can take advantage of the pH-switchable nature of these self-assembly processes.