Double‐stranded ladderphanes with C₂‐symmetric planar chiral ferrocene linkers

Abstract

Eight ladderphanes with C 2 -symmetric planar chiral ferrocene linkers are synthesized by ring opening metathesis polymerization of bisnorbornene monomers using Grubbs-I catalyst. The aminobenzoate in both monomers and polymers shows absorption maximum around 320 nm. Both monomers and polymers are Circular dichroism (CD) active. Little enhancements of CD profiles around 320 nm are observed for ladderphanes having chiral chloro-or phenyl-substituted ferrocene linkers. However, ladderphanes with a phenyl substituent on the cyclopendienyl ring exhibits enhancement of CD curves aound 240-300 nm. The congested phenyl moieties in adjacent linkers in this polymer might be well oriented such that interactions between these aromatic substituents on different monomeric units might provoke the enhancement of the CD curves in this region. When the methyl-substituted cyclopendienyl ligand is used for chiral ferrocene linkers, the ladderphanes exhibit two-fold enhancement of CD spectrum around 320 nm. This enhancement is further increased when the cyclopentadienyl ligand contains an additional phenyl substituent, owing to exciton coupling between aminobenzoate moieties in adjacent monomeric units. Moreover, the intensity of the CD curves in the region of 240-300 nm is significantly increased. These results suggest that the later polymer may adopt a posible helical structure.

A tribute to Professor Tien-Yau Luh

“…The linkers have to be rigid and connected to the polymeric backbones via ester linkages. They can be oligoaryls, 68,74 metallocycles, 75 three-dimensional organic 76 or organometallic 67,77,78 species as long as the span is smaller than that of the monomeric unit. When a flexible linker like a crown ether is used, cyclopolymerization may take place to form a single stranded polymer having hammock-like pendant (Eq 28).…”