Impact of position of electron withdrawing cyano groups on nonlinear optical properties of centrosymmetric donor-π-acceptor system

Two symmetrically substituted phenylenevinylene D-A-D′-A-D type siblings, (2Z,2′Z)-2,2′-(2,5-dimethoxy-1,4-phenylene)bis(3-(4-(dimethylamino)phenyl)acrylonitrile) (↑-dscn) and (2Z,2′Z)-3,3′-(2,5-dimethoxy-1,4-phenylene)bis(2-(4-(dimethylamino)phenyl)acrylonitrile) (↓-dscn), are prepared. We investigate the effect of different but symmetrical location of these cyano groups in vinylene bridges on the 1-photon and 2-photon absorption behaviors. We report that the closeness of CN group on the vinyl group to the central phenyl ring in ↑-dscn induces an intramolecular geometric distortion between the central phenyl ring and vinylene group, preventing the effective π-conjugation length in ground and excited states. Thus, the transition energy that is observed in 1-photon absorption and fluorescence is larger in ↑-dscn than in ↓-dscn. This leads to a different intramolecular charge distribution, as a result of which the linear and nonlinear optical properties strongly depend on the position of acceptors. These results are theoretically unraveled in terms of charge transfer pathways, charge distribution, and charge distribution differences on transition.