Chromophores based on silica-supported oxo-bridged heterobimetallic groups have recently shown themselves to be promising alternatives to metal oxide semiconductors in photocatalytic solar fuel production by Frei and co-workers. In these materials, visible light is absorbed by metal-to-metal charge transfer (MMCT) units that can be coupled with multi-electron transfer catalysts. Such MMCT chromophores can side-step the problems of oxidative degradation of other metallo-organic photosensitizers such as [Ru(bpy)3]2+. Inspired by these heterobimetallic MMCT assemblies, we are investigating transition metal-substituted polyoxometalates (POMs) as molecular inorganic MMCT chromophores, which as molecular species offer greater possibilities for structural/spectroscopic characterization and tuning. POMs are excellent candidates for electron-accepting chromophores because of their oxidative and thermal stability and ability to accommodate multiple heterometals, but their photophysical properties have not yet been systematically explored. To this end, we have studied the electronic structure and excited state dynamics of cobalt-containing Keggin POMs: Co-centered [CoIIW12O40]6- and [CoIIIW12O40]5- , and Si-centered, Co-substituted [SiCoII(H2O)W11O39]6- and [SiCoIII(H2O)W11O39]5-. These compounds provide a convenient platform for investigation of the effects of the transition metal coordination site and oxidation state on POM excited state properties. Femtosecond transient absorption spectroscopy, supported by UV-visible spectroscopy, cyclic voltammetry, X-ray crystallography and DFT calculations, reveals that the excited states of the CoII-containing POMs arise from MMCT transitions from CoII to W. Notably, the excited state tetrahedral CoII-centered POM is far longer-lived (t = 420 ps) than where the CoII atom is located in a pseudo-octahedral addendum site (t = 1.3 ps). Short-lived states (t < 6 ps) arising from non-MMCT transitions are observed in the two CoIII-containing POMs. The dramatically extended lifetime for the CoII-centered POM is ascribed to the increased ground-to-excited state structural change permitted by the coordinatively flexible central site and reduced orbital overlap of Co with the polytungstate acceptor framework.

Electronic structure and excited state dynamics of cobalt-containing Keggin polyoxometalates

246th National Meeting (2013)

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