Perylenes and polyenes: a second π-electron approximation

Expanded atomic sites with d-symmetry are introduced for the lowest π−π∗ excitations of perylene and longer rylenes, which reduce to linear chains in Hückel theory. A modified Pariser-Parr-Pople model for expanded sites accounts for similar splittings and different vibronic intensities in linear spectra of rylenes and polyenes, as well as reversed one- and two-photon thresholds. A second π-electron approximation maps N peri-linked naphthalenes into 4N-site polyenes and leads to excitons rather than bands in the polymer.