Ab initio study of one-photon and two-photon absorption properties of 2,5-bis[4-(2-arylvinyl)phenyl]-1,3,4-oxadiazoles

The one-photon and two-photon absorption properties of two symmetrical 2,5-bis[4-(2-arylvinyl)phenyl]-1,3,4-oxadiazoles synthesized recently are investigated by use of the analytic response theory at DFT level. In the visible region, the maximal one-photon absorption of the molecules occurs in the first excited state. It is found that both one- and two-photon absorptions are extremely sensitive to the geometry of molecules, and the two-photon absorption cross section can be dramatically reduced by conformation distortion. The comparison with experimental measurement demonstrates that the HF geometry is more realistic. By analyzing the charge-transfer process when the molecules are excited from the ground to excited states, one finds the π center in the two compounds plays a different role for their optical properties. The calculated values however are smaller than measurements because the two-step two-photon absorption appears when long laser is applied.