“Pocket Dendrimers” as Nanoscale Receptors for Bimolecular Guest Accommodation

Abstract

A new series of dendrimer receptors was prepared by combining a (tetraphenylporphinato)zinc(II) core and benzyl ether type dendritic substituents. Since one direction of the (tetraphenylporphinato)zinc(II) was not substituted by a dendritic residue, the resulting unsymmetrical dendrimers have “pockets” available for access of external substrates. Molecular modeling, NMR measurements, and zinc‐coordination experiments revealed that the third‐generation dendrimer of this type exhibited characteristic inclusion of coordinative pyridine guests. When diamidopyridine moiety was introduced into the dendrimer pocket, a thymine derivative was bound through complementary hydrogen bonding. Two different kinds of substrates, pyridine and thymine derivatives, were simultaneously accommodated in the nanoscale pocket and bimolecular guest accommodation was realized with the designed dendrimer receptor.

Macromolecular semi-rigid nanocavities for cooperative recognition of specific large molecular shapes

“…We also examined a flexible dendrimer composed of a benzylether structure (ZnG4d) with the same topological architecture around the zinc porphyrin core, but with a very small free volume percentage (45%) 38 . As shown in previous studies, flexible dendrons exhibit exclusive host-guest binding 18,[20][21][22] . The host ZnG4d allowed binding only of small guest molecules (1 and 4) with one or no phenyl group (Fig.…”

“…In the two-dimensional spectrum, many cross-peaks were observed among the aromatic protons of the dendron subunits as intramolecular nuclear Ö verhauser effect (NOE) couplings. In addition, a clear cross-peak between the t Bu protons of the binding guest molecule (20) and a specific aromatic proton signal of the dendron subunit was also observed as intermolecular NOE coupling (Fig. 5d).…”

“…The idea was that they would act as large containers, allowing encapsulation of many guest molecules simultaneously [15][16][17] . However, the previous dendrimers were found to be suitable only for identification of simple and small guests [18][19][20][21][22] . Although dendrimers are potential containers for a larger guest or many guest molecules, they can become compact through back-folding of their terminal monomers towards the inside of the cavity, leading to loss of the cavity [23][24][25] .…”

“…The dendrimer shell exhibited a positive effect on the binding of Y-shaped guests, as can be observed from the value of the binding constant ratio K G4a /K G1a (up to 4.2 for 11). Previous reports on host-guest chemistry involving dendrimer architecture have shown that an increase in the generation number results in a severe decrease in binding affinity because of the shell effect [18][19][20][21][22] . In fact, the dendrimer host under discussion exhibits unusual enhancement on host-guest complexation.…”

“…As the phenylazomethine monomer had two non-equivalent aromatic rings at syn-and anti-positions, the NMR proton signals for every aromatic ring in one dendron subunit could not be reconciled. On the basis of this, the t Bu protons of the binding guest molecule (20) were thought to be close to a specific arene in the dendrimer cavity without any tilt or rotation. It is noteworthy that the generation-3 dendrimer RuG3a did not show any NOE cross-peak with 20, suggesting that the fourth layer of the dendrimer contributed to guest stabilization, as noted above.…”