Theoretical approach to large two-photon absorption cross section in extended π-conjugated systems

Two-photon absorption (TPA) properties of several pi-conjugated molecules are theoretically investigated by ab initio molecular orbital method. The TPA properties of two typical classes of pi-conjugated molecules, which represent centrosymmetric and noncentrosymmetric systems, are investigated. In case of centrosymmetric (quadrupolar) molecules, the TPA intensity is found to be governed not only by the conventional parity selection rule but also by a symmetry property called altemancy symmetry, which is specific to the pi-conjugated systems. It is found that in order for such symmetric pi-conjugated molecules to have large TPA activity at the lower TPA allowed excited states, breaking of the altemancy symmetry is effective. In case of noncentrosymmetric (dipolar) molecules, the total TPA cross sections are separated into each contributing term such as the dipolar term or three-state term. It is found that the dipolar term is predominant only for the lowest pi-pi* state while for the higher excited states the three-state terns become predominant. The effects of more structural factors such as intramolecular rotations on the TPA properties are discussed.