Keggin and Dawson-type polyoxometalates (POMs) decorated by organometallic [cyclometalated ruthenium(II) polypyridine complex] or organic (pyrene) chromophores were prepared by postfunctionalization of hybrid disilylated POM platforms. The connection is made in a very efficient and modular way via Sonogashira coupling reactions, which provide a rigid linkage between the POM and the photoactive centers. Electronic properties have been inferred from electrochemical and photophysical studies and reflect poor electronic interactions between both partners. The presence of the POM leads to luminescence quenching of the chromophores, which was attributed to an intramolecular electron transfer from the chromophore to the POM. The rate of this process is much faster in the POM-pyrene than in the POM-Ru system. It depends on the driving force dictated by the redox potentials of both partners but also in the case of the POM-Ru system on the presence of the metallacycle, which acts as a molecular insulator and delays the intramolecular electron transfer. In the POM-Ru system, a comparative study of the luminescence quenching showed that the electron transfer is still more important in the covalently bonded hybrids than in systems where the POM and the ruthenium complexes are assembled via electrostatic interactions.
A rotaxane containing a ruthenium bisphenanthroline complex, acting as an axis, and a macrocycle incorporating a 2,2'-bipyridine (bpy) unit, threaded by the axis, has been synthesized. The bisphenanthroline ligand is such that its ruthenium(II) complexes possess a clearly identified axis, making such compounds ideal components of rotaxanes constructed around an octahedral ruthenium(II) center, which serves as a template. The ring is threaded by the axial ruthenium(II) precursor complex, to afford the corresponding pseudorotaxane in moderate yield. The X-ray structure analysis of this compound reveals the threaded nature of the complex. The length of the threaded ring (35 atoms in the periphery) is too short to allow easy threading of the axis through the macrocycle. As a consequence, an isomer is also obtained for which the axial ruthenium complex is attached in an exo fashion. (1)H NMR studies have been carried out, which reveal various conformational equilibria for the pseudorotaxane. Light-induced decoordination of the bpy-containing cyclic fragment was shown to be quantitative and to lead to the free ring and the axial ruthenium(II) complex, regardless of the starting compound (pseudorotaxane or exo isomer). Finally, the real rotaxane could be prepared, although it could not be separated from its exo isomer.
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