BIMETALLIC SESQUIFULVALENE COMPLEXES - COMPOUNDS WITH UNUSUALLY LARGEHYPERPOLARIZABILITY

In order to construct bimetallic organometallic donor-acceptor complex es with nonlinear optical (NLO) properties, the bimetallic sesquifulva lene 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 sho w 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 g roup P2(1)/c. The organometallic fragments in 2a adopt a transoid conf ormation, those in 2b and 2c are cisoid. The cyclo-C-5 and cyclo-C-7 r ings 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-ele ctron oxidation (>0 mV vs. FeH/FcH(+), FcH = ferrocene) and an irrever sible one-electron reduction (less than or equal to - 900 mV), which a re assigned to the ferrocenyl and [(eta(7)-(CH6)-H-7)Cr(CO)(3)](+) uni ts, respectively. UV/vis spectra reveal low-energy absorptions for 2a- c at about lambda = 600 nm, showing strong negative solvatochromism. T hese absorption maxima are related to a charge-transfer excitation bet ween the ferrocenyl and the [(eta(7)-C7H6)Cr(CO)(3)](+) groups, indica ting considerable nonlinear optical properties. Determination of the f irst hyperpolariz-ability beta by means of hyper Rayleigh scattering y ields exceptionally large beta values: beta(2b) = 570 x 10(-30) esu an d beta(2c) = 320 x 10(-30) esu. These unexpectedly large beta values a re explained by resonance enhancement.