Large second-order optical polarizabilities in mixed-valency metal complexes

THE potential development of optoelectronic devices based on the nonlinear polarization of molecular materials has aroused much recent interest 1,2 . The search for large second-order electric susceptibilities (that is, proportional to the square of an applied electric field) has concentrated on acentric organic or organometallic chromophores with an organic π-electron system coupling electron donor and acceptor groups 3–6 . It is conceivable that mixed-valence compounds characterized by an intervalence charge-transfer (IVCT) transition 7 , in which the donor and acceptor centres are both metal atoms, might also have the potential to provide a large second-order response 8 , but this possibility has not been widely explored. Here we report the first hyperpolarizability, β, of a bimetallic complex ion, [(CN) 5 Ru-μCN-Ru(NH 3 ) 5 ] - (I in Fig. 1), and a novel organometallic analogue, [(η 5 -C 5 H 5 )Ru(PPh 3 ) 2 -μ-CN-Ru(NH 3 ) 5 ] 3+ (II). Measurements of β (which is related to the bulk second-order response) in solution at a wavelength of 1,064 nm using the newly developed hyper-Rayleigh scattering technique 9,10 give values greater than 10 –27 e.s.u., which are among the largest reported for solution species. The ease with which the energy of the IVCT transition can be modified suggests that there may be considerable potential for this class of chromophore in nonlinear optical devices.