Encapsulation of Aromatic Guests in the Bisporphyrin Cavity of a Double-Stranded Spiroborate Helicate: Thermodynamic and Kinetic Studies and the Encapsulation Mechanism

Abstract

A double-stranded spiroborate helicate bearing a bisporphyrin unit in the middle forms an inclusion complex with electron-deficient aromatic guests that are sandwiched between the porphyrins. In the present study, we systematically investigated the effects of size, electron density, and substituents of a series of aromatic guests on inclusion complex formations within the bisporphyrin. The thermodynamic and kinetic behaviors during the guest-encapsulation process were also investigated in detail. The guest-encapsulation abilities in the helicate increased with the increasing core sizes of the electron-deficient aromatic guests and decreased with the increasing bulkiness and number of substituents of the guests. Among the naphthalenediimide derivatives, those with bulky N-substituents at both ends hardly formed an inclusion complex. Instead, they formed a [2]­rotaxane-like inclusion complex through the water-mediated dynamic B–O bond cleavage/reformation of the spiroborate groups of the helicate, which enhanced the conformational flexibility of the helicate to enlarge the bisporphyrin cavity and form an inclusion complex. Based on the X-ray crystal structure of a unique pacman-like 1:1 inclusion complex between the helicate and an ammonium cation as well as the molecular dynamics simulation results, a plausible mechanism for the inclusion of a planar aromatic guest within the helicate is also proposed.

Synthesis and cooperative guest binding of tetrameric porphyrin macrocycle

“…[13] Numerous studies have examined macrocyclic porphyrin hosts that bind their guest through dative bonds, [14] but the donor-acceptor host-guest complexes of macrocyclic porphyrin hosts are less studied. [15] During our study, [16] we developed an S-shaped tetrakisporphyrin host molecule connecting two cavities with a rigid butadiyne linker. [17] The cleft cavities of S-shaped tetrakisporphyrin encapsulate electron-deficient aromatic molecules through donor-acceptor and π-π stacking interactions.…”