Two-photon probe of forbidden exciton states in symmetric aggregates of asymmetric molecules Original Research Article

Two-photon absorption (TPA) spectra of molecular aggregates are calculated for asymmetric molecules with a difference static dipole between the ground and excited states. If the molecules are aligned symmetrically such that their difference static dipoles cancel out as a whole, some of dipole-forbidden exciton states are two-photon allowed. This effect is important not only for probing forbidden excitons as well as for resonance enhancement of optical nonlinearity. Examples of dimers, and linear and circular aggregates are discussed. As an interesting application, the TPA spectrum is calculated for circular light-harvesting antenna complexes in photosynthetic bacteria.