One- and two-photon absorption properties of octupolar molecules with tetrahedral structure

We have theoretically examined two series of tetrahedral octupolar molecules that exhibit large two-photon absorption cross-sections in the visible region. The density functional theory B3LYP/6-31G* associated with ZINDO–SOS methods has been used to calculate the molecular geometries, electronic structure, one-photon and two-photon absorption properties. The computational results are in good agreement with the available experimental and theoretical values. It is found that the molecular conjugation length and the one-photon absorption intensity are quite strongly correlated factors, and a corresponding correlation for large two-photon absorption cross-section is equally strong. A most important role for altering the position of one-photon absorption is played by the substitution of the conjugated bridged NN for CHCH. The center with stronger electron-accepting ability is favorable for the efficient increase of two-photon absorption (TPA) cross-section in the higher energy region. While the molecules centered by C atom all exhibit large TPA cross section in a fairly wide range and this property may lead tetrahedral octupolar molecules centered by C atom to more nonlinear optical applications.