Theory of excited-state absorption in phenylene-basedπ-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 ${\mathrm{mA}}_g).\mathrm{}$ In addition to the above class of $A_g$ states, PA can also occur to a higher energy ${\mathrm{kA}}_g$ 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 ${\mathrm{mA}}_g$ and the ${\mathrm{kA}}_g$ 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 ${\mathrm{kA}}_g$ that leads to charge generation subsequent to excitation to this state, as found experimentally.