Bimetallic sesquifulvalene complexes - Compounds with unusually large hyperpolarizability

In order to construct bimetallic organometallic donor-acceptor complexes with nonlinear optical (NLO) properties, the bimetallic sesquifulvalene complexes [{CpFe(eta(5)-C5H4)}Z{eta(7)-C7H6)Cr-(CO)(3)}]BF4 (2BF(4)) have been synthesized with different bridging functions Z (2a: Z = -; 2b: Z = C-2; 2c: Z = (E)-C2H2). X-ray structure determinations show that 2aBF(4) and 2cBF(4) crystallize in acentric space groups (P4(1), P2(1)2(1)2(1)), whereas crystalline 2bBF(4), has the centric space group P2(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 sesqui-fulvalene ligands are almost coplanar; this allows an optimum electronic interaction between the ferrocenyI donor and the [(eta 7-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 (less than or equal to - 900 mV), which are assigned to the ferrocenyl and [(eta(7)-(CH6)-H-7)Cr(CO)(3)](+) units, respectively. UV/vis spectra reveal low-energy absorptions for 2a-c at about lambda = 600 nm, showing strong negative solvatochromism. These absorption maxima are related to a charge-transfer excitation between the ferrocenyl and the [(eta(7)-C7H6)Cr(CO)(3)](+) groups, indicating considerable nonlinear optical properties. Determination of the first hyperpolariz-ability beta by means of hyper Rayleigh scattering yields exceptionally large beta values: beta(2b) = 570 x 10(-30) esu and beta(2c) = 320 x 10(-30) esu. These unexpectedly large beta values are explained by resonance enhancement.