A new strategy of enhancing two-photon absorption in conjugated molecules: introduction of charged defects

The imaginary parts of γ(−ω;ω,ω,−ω) (Im γ), which describes two-photon absorption (TPA), are calculated for several conjugated molecules with charged defects by the sum-over-state (SOS) approach using the semi-empirical MO method. The Im γ values are found to be enhanced by introduction of charged defects more strongly than those of the corresponding neutral molecules. The main reasons can be 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 multi-functional TPA systems exhibiting both high electric conductivity and large TPA cross-sections in connection with a contribution of symmetric resonance structures with invertible polarization (SRIP).