Bimetallic Sesquifulvalene Complexes-Compounds with Unusually Large Hyperpolarizability β

In order to construct bimetallic organometallic donor–acceptor complexes with nonlinear optical (NLO) properties, the bimetallic sesquifulvalene complexes [{CpFe(η5‐C5H4)}Z{η7‐C7H6)Cr‐(CO)3}]BF4 (2BF4) have been synthesized with different bridging functions Z (2a: Z = ‐ 2b: Z = C2; 2c: Z  (E)‐C2H2). X‐ray structure determinations show that 2a BF4 and 2c BF4 crystallize in acentric space groups (P41, P212121), whereas crystalline 2b BF4 has the centric space group P21/c. The organometallic fragments in 2a adopt a transoid conformation, those in 2b and 2c are cisoid. The cyclo‐C5 and cyclo‐C7 rings of the sesquifulvalene ligands are almost coplanar; this allows an optimum electronic interaction between the ferrocenyl donor and the [η7‐C7H6)Cr(CO)3]+ acceptor moiety in the crystalline state. Cyclovoltammetric studies show an electrochemically reversible one‐electron oxidation ( > 0 mV vs. FeH/FcH+, FcH = ferrocene) and an irreversible one‐electron reduction (≤–900 mV), which are assigned to the ferrocenyl and [(η7‐C7H6)Cr(CO)3]+ units, respectively. UV/vis spectra reveal low‐energy absorptions for 2a–c at about λ=600 nm, showing strong negative solvatochromism. These absorption maxima are related to a charge‐transfer excitation between the ferrocenyl and the [(η7‐C7H6)Cr(CO)3]+ groups, indicating considerable nonlinear optical properties. Determination of the first hyperpolarizability β by means of hyper Rayleigh scattering yields exceptionally large β values: β(2b) = 570 × 10−30 esu and β(2c) = 320 × 10−30 esu. These unexpectedly large β values are explained by resonance enhancement. An order of magnitude or more separates of the first hyperpolarizabilities β of bimetallic sesquifulvalene complexes A and the values recorded for other ferrocene derivatives. This effect is explained by a charge‐transfer excitation from the cyclo‐C5 to the cyclo‐C7 moiety to give B, which produces a resonance enhancement of the hyperpolarizability β (for Z = (E)‐CHCH‐, β = 320 × 10−30 esu; for Z = CC,β = 570 × 10−30 esu).