Ferrocenyl Diquat Derivatives: Nonlinear Optical Activity, Multiple Redox States, and Unusual Reactivity

Four new dipolar cations have been synthesized, containing ferrocenyl electron donor groups and diquaternized 2,2′-bipyridyl (diquat) acceptors. To our knowledge, these are the first organometallic diquat derivatives to be reported and have been characterized as their PF_6^– salts by using various techniques including ^1H NMR and electronic absorption spectroscopies and cyclic voltammetry. UV–vis spectra show multiple intramolecular charge-transfer bands, and three reversible redox processes are observed for each compound. Molecular quadratic nonlinear optical (NLO) responses have been determined by using hyper-Rayleigh scattering at 1064 nm and Stark (electroabsorption) spectroscopic studies on the intense π→ π^* intraligand and d → π^* metal-to-ligand charge-transfer bands. The most active compounds have estimated, Stark-derived β0 values approaching that of the chromophore in the technologically important material (E)-4′-(dimethylamino)-N-methyl-4-stilbazolium tosylate. Single-crystal X-ray structures have been obtained for three of the salts, with one adopting the orthorhombic space group Aba2 and having potential for bulk NLO behavior due to its polar structure. Attempted crystallizations of the remaining chromophore revealed that it undergoes an unusual intermolecular formal Michael cycloaddition between an activated methyl group and a double bond, forming a dimeric species. This diastereomeric cyclic complex has also been characterized via single-crystal X-ray diffraction.