Effects of Two Electron-Donating and/or -Withdrawing Substituents on Two-Photon Absorption for Diphenylacetylene Derivatives.

Two-photon absorption for diphenylacetylene (DPA) derivatives with two substituents (-OMe and/or -NO 2 ) at the 4,4'-position was investigated experimentally and theoretically. The two-photon absorption spectra and the two-photon absorption cross-sections σ (2) for DPA derivatives were obtained by optical-probing photoacoustic spectroscopy (OPPAS). The simulated two-photon absorption spectra of the DPA derivatives, obtained with the time-dependent density functional theory within the Tamm-Dancoff approximation, agreed well with the experimental ones. The mechanisms for enhancement of the σ (2) for centrosymmetric and non-centrosymmetric DPA derivatives were found to be different. The large σ (2) for centrosymmetric molecules (DPA-OMeOMe and DPA-NO 2 NO 2 ) results from the magnitude of the transition dipole moment, while for non-centrosymmetric molecules (DPA-OMeNO 2 ), it is enhanced by the smaller detuning energy. Information on two-photon absorption properties of DPA derivatives obtained in this study will be important for the molecular design of two-photon absorption materials.