Nonlinear Optical Chromophores with Two Ferrocenyl, Octamethylferrocenyl, or 4‑(Diphenylamino)phenyl Groups Attached to Rhenium(I) or Zinc(II) Centers

The compounds 4,4′-bis[(E)-2-R-vinyl]-2,2′-bipyridyl {R = ferrocenyl [(Fcv)_2bpy], octamethylferrocenyl [(Me_8Fcv)_2bpy] or 4-(diphenylamino)phenyl [(Dapv)_2bpy]} are used to prepare eight new complexes with Zn^(II)Cl_2, Zn^(II)(OAc)_2, or fac-Re^ICl(CO)_3 centers. The recently reported complex fac-Re^ICl(CO)_3[(Dapv)_2bpy] (Horvath, R. et al. Inorg. Chem. 2013, 52, 1304) is also studied. Electronic absorption spectra show intense d → π^* metal-to-ligand charge-transfer (MLCT) and π → π^* intraligand charge-transfer (ILCT) absorption bands, the relative energies of which correlate logically with the molecular structure. Cyclic voltammetry reveals a reversible oxidation wave for the Fc/Me_8Fc complexes, accompanied by quasireversible or irreversible ligand-based reductions. The Re complexes also show irreversible Re^(II/I) waves. Single-crystal X-ray structures are reported for (Me_8Fcv)_2bpy, Zn^(II)Cl_2[(Me_8Fcv)_2bpy], Zn^(II)(OAc)_2[(Fcv)_2bpy]·CHCl_3, and fac-Re^ICl(CO)_3[(Me_8Fcv)_2bpy]·0.5CHCl_3. Molecular first hyperpolarizabilities β are measured in DCM solutions via the hyper-Rayleigh scattering (HRS) technique at 1300 nm. Stark (electroabsorption) spectroscopic studies on only the MLCT bands in frozen butyronitrile allow the indirect estimation of lower limits for the overall static first hyperpolarizabilities β_0. Time-dependent density functional theory (TD-DFT) calculations on selected complexes confirm the expected assignments of their low energy absorption bands, with the best results obtained by using the M06 functional and Def2-TZVP/SVP/TZVPP mixed basis set. DFT predicts that the total static first hyperpolarizability β_(tot) increases in the Zn^(II)Cl_2 series in the order R = Fc < Me_8Fc < Dap, consistent with the HRS and Stark data. The computed β values increase substantially on moving from the gas phase to a DCM or MeCN solvent medium, and the essentially 2D nature of the chromophores leads to dominant β_(xxy) tensor components.