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 [(Me8Fcv)2bpy] or 4-(diphenylamino)­phenyl [(Dapv)2bpy]} are used to
prepare eight new complexes with ZnIICl2, ZnII(OAc)2, or fac-ReICl­(CO)3 centers. The recently reported complex fac-ReICl­(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/Me8Fc complexes, accompanied by quasireversible
or irreversible ligand-based reductions. The Re complexes also show
irreversible ReII/I waves. Single-crystal X-ray structures
are reported for (Me8Fcv)2bpy, ZnIICl2[(Me8Fcv)2bpy], ZnII(OAc)2[(Fcv)2bpy]·CHCl3, and fac-ReICl­(CO)3[(Me8Fcv)2bpy]·0.5CHCl3. 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
β0. 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 βtot increases in the ZnIICl2 series in
the order R = Fc < Me8Fc < 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 βxxy tensor components.