BANDGAP OSCILLATION IN POLYPHENANTHRENES

The electronic structures of carbon-based ladder polymers and polynucl ear aromatic hydrocarbons (PAHs) with acene-and phenanthrene-edge stru ctures are studied with an approximate molecular-orbital method. The d ifference between polyacene and polyphenanthrene can be derived from d etailed orbital interaction analyses of anthracene and phenanthrene. T he fragment molecular-orbital (FMO) method successfully characterizes the distinct electronic structures of the two small PAHs with differen t types of edges. We shed light on the electronic structures of ladder polymers with the phenanthrene-edge structure, PPh(n), in which n is the number of cis-polyene chains included. With an increase in width o f the ladder polymers, the bandgap of PPh(n) approaches zero with a be havior that has a periodicity of 3. The PPh(n) series are classified i nto three subgroups: small-gap (metallic) PPh(3m + 1), large-gap PPh(3 m + 2), and medium-gap PPh(3m), where m = 0, 1, 2,.... The oscillating behavior in the bandgap of the three subgroups is analyzed from the v iewpoint of interchain interactions in the frontier crystal orbitals.