Excitons and interband excitations in polyphenylenes

A microscopic model which elucidates the essential physics of the photoexcitations of the polyphenylenes is introduced. Recognizing that these excitations are derived from the local excitations of the phenylene monomer, it allows for the effects of the ‘correlation-energy gaps’ Un between charged and charge-neutral excitations, and the dipole-dipole interaction Vdd between neighboring monomers. It leads to the generic appearance of three principal, dispersing, absorption bands whose relative spacings in energy are largely set by Un and the electronic bandwidth W, thereby establishing direct experimental access to these microscopic parameters. There is a fourth, dipole-forbidden, singlet excitation band which can be rendered allowed by distortion of local D6h symmetry. A family of four distinct, tightly bound, triplet state excitons is also found.