Correlated-electron description of the photophysics of thin films ofπ-conjugated polymers

We extend the Mulliken theory of ground-state charge transfer in a donor-acceptor complex to excited-state charge transfer between pairs of identical $\pi$-conjugated oligomers, one of which is in the optically excited state and the other is in the ground state, leading to the formation of a charge-transfer exciton. Within our theory, optical absorptions from the charge-transfer exciton should include a low-energy intermolecular charge-transfer excitation, as well as distinct intramolecular excitations from both the neutral delocalized exciton component and the Coulombically bound polaron-pair component of the charge-transfer exciton. We report high-order configuration-interaction calculations for pairs of oligomers of polyparaphenylenevinylene (PPV) that go beyond our previous single configuration-interaction calculation and find all five excited-state absorptions predicted using heuristic arguments based on the Mulliken concept. Our calculated excited-state absorption spectrum exhibits strong qualitative agreement with the complete wavelength-dependent ultrafast photoinduced absorption in films of PPV derivatives, suggesting that a significant fraction of the photoinduced absorption here is from the charge-transfer exciton. We make detailed comparisons to experiments and a testable experimental prediction.