Alkali Metal Ion-enhanced Threading of a Perylenediimide-bound Polymer Chain through a Double-stranded Spiroborate Helicate with a Bisporphyrin Unit

Abstract

Threading of a poly(ethylene glycol) (PEG) chain linked to a perylenediimide unit at one end with a bulky stopper at the other end through an inner cavity of a bisporphyrin-based doublestranded spiroborate helicate was remarkably accelerated in a specific solvent, in which the PEG chain enfolded sodium cations coordinating to the negatively charged spiroborate helicate, thus forming a 1:1 inclusion complex with the perylenediimide sandwiched between the bisporphyrin unit. The threading of a short or long axle molecule through an inner space of macrocycles provides a class of unique mechanically-interlocked molecules, namely, rotaxanes that have become one of the key structural motifs to develop artificial molecular machines 1 and functional nano-and biorelated intelligent materials.2 Such rotaxane-like structures are widely utilized in sophisticated biological events, such as DNA replication, 3 in which a single-stranded DNA chain is threaded through a toroidal-shaped polymerase enzyme while moving along the strand. 4 Nolte, Rowan, and co-workers took advantage of this unique biological rotaxane formation and succeeded in the construction of an artificial rotaxane system composed of a catalyticallyactive porphyrin-based macrocycle that threaded onto a reactive polymer chain during oxidation along the polymer, 5 and further proposed a unique threading mechanism of porphyrin-based macrocycles onto a viologen-appended polymer chain bearing a bulky stopper at one end as a model system, 6 through which the macrocycles thread from the polymer chain in one direction to form a 1:1 inclusion complex with the terminal viologen residue of the polymer. Although a large number of supramolecular rotaxane and pseudorotaxane formations have been reported, 1,2,7 biomimetic pseudorotaxane complex formation between macrocycles and polymers focusing on such a polymer threading process before complexation remains limited. 6We previously reported a double-stranded spiroborate helicate with a bisporphyrin unit in the middle (1 Na2 , Figure 1a) that formed a 1:1 inclusion complex with an electron-deficient aromatic guest, such as 1,4,5,8-naphthalenetetracarboxylic dianhydride (NDA). This complexation is accompanied by unique unidirectional dual rotary and twisting motions of the helicate triggered by the guest-encapsulation. 8,9 We envisioned that the macrocyclic helicate 1 Na2 would also form an inclusion complex with an analogous polymer-bound electron-deficient aromatic (b) Schematic representation of the inclusion complex formation between a double-stranded bisporphyrin helicate 1 Na2 and a perylenediimide (PDI) derivative carrying a polymer chain and a bulky substituent (stopper) at each end. Inclusion complexation is entirely possible by one-way threading of the polymer chain through the bisporphyrin cavity of 1 Na2 due to the bulky stopper at the other end of PDI.