Nonlinear Optical and Related Properties of Iron(II) Pentacyanide Complexes with Quaternary Nitrogen Electron Acceptor Units

Three new complex salts Na_2[Fe^(II)(CN)_5(L)] [L ) N-methyl-4-{E,E-4-(4-pyridyl)buta-1,3-dienyl}pyridinium, N-methyl- 4-{E,E,E-6-(4-pyridyl)hexa-1,3,5-trienyl}pyridinium, or N-methyl-1,4-bis-{E-2-(4-pyridyl)ethenyl}] benzene have been prepared. These compounds have been characterized by using various techniques including electronic absorption spectroscopy and cyclic voltammetry, allowing their properties to be compared with those of the known complexes where L = N-methylpyrazinium (Mepyz^+), N-methyl-4,4^′-bipyridinium, or N-methyl-4-[E-2-(4-pyridyl)ethenyl]pyridinium. Molecular quadratic nonlinear optical (NLO) responses have been determined by using hyper-Rayleigh scattering (HRS) at 1064 nm, and also via Stark (electroabsorption) spectroscopic studies on the intense, visible d f π^* metal-to-ligand charge-transfer (MLCT) bands. The MLCT transitions show large red-shifts on moving from aqueous to methanol solutions, and these excitations are associated with relatively large static first hyperpolarizabilities β_0. Single crystal X-ray structures of two hydrated materials containing the complex anion [Fe^(II)(CN)_5(Mepyz^+)]^(2-) have been determined, and both of these show some degree of alignment of the constituent complex dipoles.