Ruthenium(II)-nitrosyl polypyridinyl Complexes: from NO/ON Isomerization to NO delivery
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Ruthenium(II)-nitrosyl polypyridinyl Complexes: from NO/ON Isomerization to NO delivery Pascal G. Lacroix and Isabelle Malfant Laboratoire de Chimie de Coordination du CNRS, 205 route de Narbonne, 31077 Toulouse, France
ABSTRACT Ruthenium-nitrosyl (RuII(NO)) complexes are stable in the dark, but exhibit a unique photoreactivity which can lead either to a solid state isomerization from RuII(NO) to RuII(ON), or to a nitric oxide (NO·) release in solution. From our recent discovery of a high yield of isomerization (> 92%) in [RuII(py)4Cl(NO)](PF6)2, we have developed a computational strategy aimed at designing switchable nonlinear optical (NLO) material with high contrast (large difference in the on / off NLO response) in the solid state. Our synthetic targets are terpyridine based RuII chromophores in which various substituents can be introduced to adjust the NLO response which, at best, should be vanishing in the off state. Alternatively, these complexes can undergo a photo-induced NO· release in solution, a possibility which becomes increasingly appealing in relation to the discovery of the numerous biological roles of NO·, in the context of the emergence of the photodynamic therapy. A promising fluorene-terpyridine RuII(NO) complex was investigated, which could find an additional interest in relation to its capability for releasing NO· by a two-photon absorption process. INTRODUCTION The concept of molecular switch is currently attracting a growing interest either from an academic point of view or for its fascinating perspectives in the analysis of biological molecular mechanism and in the design of molecular devices with potential applications in various areas of material science [1]. Despite these challenging perspectives, the issue of switching the nonlinear optical (NLO) properties has been addressed only recently, after the convincing report by Coe et al. that a ruthenium(II) complex could have its molecular NLO response (E) almost completely vanishing after oxidation into a ruthenium(III) analogue, in which the Ruthenium → ligand charge transfer is suppressed [2]. Indeed, the NLO response of most NLO chromophores can be related to a set of few electronic transitions exhibiting non-centrosymmetric (“push-pull”) charge transfers between electron donor and an electron acceptor substituents linked by a S-conjugated bridge. Each transition contributes to the final E value through simplified two-level model as follows [3]: (1)
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In this equation, fi, 'Pi, and Ei are the oscillator strength, the difference between ground and excited state dipole moment and the energy of the ith transition, respectively (ħZ being the energy of the incident laser beam). In (1), E is assumed to be a vector oriented in the direction of the charge transfer 'Pi. Vanishing E values are usually obtained either by removing the donor, the acceptor or the bridge, by means of chemical reactions (e.g. redox process [4], protonation, or caption of ion [5]). At a more practical level, switching the properties of an operating device should ult
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