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(η 5 ‐C 5 H 4 )}Z{η 7 ‐C 7 H 6 )Cr‐(CO) 3 }]BF 4 (2BF 4 ) have been synthesized with different bridging functions Z ( 2a : Z = ‐ 2b : Z = C 2 ; 2c : Z ( E )‐C 2 H 2 ). X‐ray structure determinations show that 2a BF 4 and 2c BF 4 crystallize in acentric space groups ( P 4 1 , P 2 1 2 1 2 1 ), whereas crystalline 2b BF 4 has the centric space group P 2 1 / c . The organometallic fragments in 2a adopt a transoid conformation, those in 2b and 2c are cisoid . The cyclo ‐C 5 and cyclo ‐C 7 rings of the sesquifulvalene ligands are almost coplanar; this allows an optimum electronic interaction between the ferrocenyl donor and the [η 7 ‐C 7 H 6 )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 [(η 7 ‐C 7 H 6 )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 [(η 7 ‐C 7 H 6 )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.