Low-lying two-photon excitations in poly(p-phenylene vinylene) and its derivatives

We study the low-lying two-photon excited states in nonlinear optical processes in poly(p-phenylene vinylene) (PPV) and its derivatives. We find that, within the Pariser–Parr–Pople (PPP) model, the mAg state in (unsubstituted) PPV is exactly the lowest two-photon excited state (i.e., the 21Ag+ state). Due to the breaking of charge-conjugation symmetry, the 2Ag in substituted PPVs, originating from the 11Ag−, is weakly two-photon allowed, while the mAg has its origin from the 21Ag+, hence giving a strong peak in the two-photon absorption spectra.