Effect of π-conjugate units on the ferrocene-based complexes: Switchable second order nonlinear optics controlled by redox stimuli

• The reduced centers should take place preferentially at the π-conjugated units.
• The order of is in roughly consistent with anisotropy polarizability.
• This open/close molecular motion allows efficient molecular switch.
• The β enhancement is related to the intramolecular interlayer charge transfer.

The control of rotational motion, using an external impulse (such as redox), has been intensively pursued as nonlinear optics switch. Four redox responsive ferrocene-based actuators with varied π-conjugated units have been systematically investigated by the density functional theory. The open structures of these complexes with antiparallel orientation of both π-conjugated units are due to the large charge repulsion effect, while the closed forms are stabilized by the intramolecular π–π stacking interactions.The rotational motion can be triggered by reduction process preferentially centered on the π-conjugated units. The polarizabilities and first hyperpolarizabilities of complexes [3]closed2+ and [4]closed2+ are significant enhanced, which are respectively related to the electronic spatial extent and intramolecular interlayer charge transfer occurred between two respective π-conjugated units. Overall, the present work shows that one can design the molecular nonlinear optical switches characterized by a large contrast varying from zero to a large value along the reversible transformations.

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