Developing iron and ruthenium complexes for potential nonlinear optical applications

Nonlinear optical (NLO) chromophores containing transition metals have been investigated widely because they offer fascinating combinations of physical properties and great scope for molecular engineering. This review summarises our recent studies with NLO metallochromophores, focused largely on complex salts of iron or ruthenium. Highlights of this work include: (i) the extension of our solution-state studies with RuII ammine chromophores to the first demonstration of redox-induced switching of bulk NLO behaviour; (ii) heterobimetallic ligand-bridged RuII/ReI complexes for which Stark spectroscopy provides useful data; (iii) FeII cyanide complexes that show very strongly medium-dependent and pH-switchable NLO responses; (iv) FeII or RuII tris(2,2′-bipyridyl) derivatives that combine very large quadratic and cubic NLO properties; (v) unusual ferrocenyl “diquat” species that show multiple redox states, 2D NLO responses and a polar crystal form in one case. These studies reveal extensive and logical correlations between charge-transfer absorption, electrochemical and NLO behaviour, with the experimental observations being supported by theoretical calculations in several instances.

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► Various NLO chromophores containing iron or ruthenium have been studied.
► Redox-switching of both molecular-level and bulk NLO behaviour is demonstrated.
► Using HRS and Stark spectroscopy affords detailed structure-activity relationships.
► Tris-chelate derivatives combine very large quadratic and cubic NLO properties.
► Observed properties are rationalised via DFT and other calculations.