Remarkable enhancement of two-photon absorption in cation molecules

We have calculated the imaginary parts of γ(-ω,ω,ω,-ω) (1mγ), which describe two-photon absorption (TPA), for several conjugated molecules with charged defects by the sum-over-state (SOS) approach using the semi-empirical MO method. The 1mγvalues are found to be remarkably enhanced by introduction of charged defects compared to those of similar-size neutral molecules. The main reasons can he ascribed to the decrease in the transition energy and the increase in the magnitude of transition moment between the first allowed excited state and the ground state. We propose novel systems with large TPA cross sections in connection with a contribution of symmetric resonance structures with invertible polarization (SKIP).