Ruthenium Alkynyl Complexes in Non-Linear Optics

Abstract

Non-linear optical (NLO) materials are able to modify the propagation characteristics of light. Such materials have a range of potential applications in advanced technologies and are therefore of considerable interest. This account summarizes the development of one class of organometallics as potential NLO materials, namely ruthenium alkynyl complexes. These are available in high yields by straightforward synthetic procedures and have good thermal and environmental stability. In studies ranging from small molecules (molecular weights ~1000) to second-generation dendrimers (with molecular weights of more than 20000), the author’s group and collaborators have assayed the NLO effects in complexes with a variety of ‘multipolar’ charge distributions (dipolar, quadrupolar, octupolar), revealing that ruthenium alkynyl complexes can be engineered to display record and near-record values of the parameters responsible for various interesting NLO effects. In particular, recent studies driven by the current focus on optimizing molecular multiphoton absorption cross-sections have afforded several examples with world-record values of these key coefficients. The author’s group has also shown that the fully reversible redox processes undergone by many ruthenium alkynyl complexes are a distinctive feature that can be exploited to afford molecular NLO switches, because the different and reversibly accessible redox forms of the complexes exhibit measurably different NLO responses. This unique type of switching has been extended in two ways to afford molecular switches with multiple accessible NLO states. First, ruthenium alkynyl complexes have been subjected to various ‘orthogonal’ (independent) switching stimuli (specifically oxidation–reduction, protonation–deprotonation, and photoisomerization), affording complexes that function as NLO switches with up to six distinct NLO states. Second, heterobimetallic complexes coupling ruthenium alkynyl and iron alkynyl centres have been prepared that exhibit multiple redox-accessible NLO states.