Theory of excited-state absorption in phenylene-based π-conjugated polymers

Within a rigid-band correlated electron model for oligomers of poly-(paraphenylene) (PPP) and poly-(paraphenylenevinylene) (PPV), we show that there exist two fundamentally different classes of two-photon A_g states in these systems to which photoinduced absorption (PA) can occur. At relatively lower energies there occur A_g states which are superpositions of one electron–one hole (1e–1h) and two electron–two hole (2e–2h) excitations, that are both comprised of the highest delocalized valence-band and the lowest delocalized conduction-band states only. The dominant PA is to one specific member of this class of states (the mA_g). In addition to the above class of A_g states, PA can also occur to a higher energy kA_g state whose 2e–2h component is different and has significant contributions from excitations involving both delocalized and localized bands. Our calculated scaled energies of the mA_g and the kA_g agree reasonably well to the experimentally observed low- and high-energy PA’s in PPV. The calculated relative intensities of the two PA’s are also in qualitative agreement with experiment. In the case of ladder-type PPP and its oligomers, we predict from our theoretical work an intense PA at an energy considerably lower than the region where PA’s have been observed currently. Based on earlier work that showed that efficient charge-carrier generation occurs upon excitation to odd-parity states that involve both delocalized and localized bands, we speculate that it is the characteristic electronic nature of the kA_g that leads to charge generation subsequent to excitation to this state, as found experimentally.