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We extend Mulliken's theory of ground state charge transfer in adonor-acceptor complex to excited state charge transfer between pairs ofidentical $\pi$-conjugated oligomers, one of which is in the optically excitedstate and the other in the ground state, leading to the formation of acharge-transfer exciton. Within our theory, optical absorptions from thecharge-transfer exciton should include a low energy intermolecularcharge-transfer excitation, as well as distinct intramolecular excitations fromboth the neutral delocalized exciton component and the Coulombically boundpolaron-pair component of the charge-transfer exciton. We report high orderconfiguration-interaction calculations for pairs of oligomers ofpoly-paraphenylenevinylene (PPV) that go beyond our previous singleconfiguration-interaction calculation and find all five excited stateabsorptions predicted using heuristic arguments based on the Mulliken concept.Our calculated excited state absorption spectrum exhibits strong qualitativeagreement with the complete wavelength-dependent ultrafast photoinducedabsorption in films of PPV derivatives, suggesting that a significant fractionof the photoinduced absorption here is from the charge-transfer exciton. Wemake detailed comparisons to experiments, and a testable experimentalprediction.

Correlated-electron description of the photophysics of thin films of $\pi$-conjugated polymers