Theoretical study of two-photon absorption properties of a series of double-layer paracyclophane derivatives

The equilibrium geometries, electronic structures, and one- and two-photon absorption properties of a series of paracyclophane derivatives have been determined by using the AM1 and ZINDO methods. The results show that the paracyclophane core as a multidimensional tunneling barrier remarkably increases the two-photon absorption cross section of molecules. As far as this series of paracyclophane derivatives is concerned, there exists a nonconventional "through-space" charge transfer. Our theoretical findings are consistent with recent experimental observations. It is found that the molecular length plays the most crucial role in the one-photon absorption intensity and the two-photon absorption cross section. For molecules with a given framework, a symmetrical structure with strong donor groups can result in a maximum two-photon absorption cross section. The three-state approximation is applicable to this series of paracyclophane derivatives. It is notable that paracyclophane-based molecules may afford advantages in the tradeoff of nonlinearity and transparency by generating a strong NLO response while providing a favorable displacement of the region of transparency.