Ladderphanes and Related Ladder Polymers

Ladderphanes are defined as multiple layers of cyclophane where the tethers are part of the polymeric backbones. They can be symmetrical or unsymmetrical. Symmetric ladderphanes are obtained by ring opening metathesis polymerization (ROMP) of strained cycloalkenes, cyclopolymerization of bis-alkynes, topochemistry, or disulfide formation. Unsymmetric ladderphanes are synthesized by replication protocols or sequential polymerization. Ladderphanes easily form aggregates and assemble nicely as a highly ordered pattern on a graphite surface as revealed by scanning tunneling microscopic (STM) or TEM images. The covalent linkers in double stranded ladderphanes can be oligoaryls, aromatic or antiaromatic metallocycles, or three-dimensional organic or organometallic moieties as long as the span of these linkers is smaller than that of the monomeric species. Interactions between the adjacent linkers lead to various interesting photophysical properties. In the presence of chiral auxiliaries, one-handed helical ladderphanes can be obtained. Self-assembly and pairing of opposite charged species to form double helical ladder-like structures are briefly presented.