Syntheses, characterization and optical limiting properties of heterometallic cluster-containing polymers

Transition metal carbonyl clusters incorporating group 6 (molybdenum, tungsten) and iridium atoms in a tetrahedral or butterfly-shaped four-atom cluster core, together with carbonyl, cyclopentadienyl and alkyne ligands, have been synthesized and incorporated into oligourethanes, and their optical limiting properties assessed by open-aperture Z-scan (ns pulses, 523 nm) and time-resolved pump-probe studies (ps pulses, 527 nm). The Z-scan studies reveal that the tetrahedral [M₂Ir₂] cluster cores (M = Mo, W) displayed a greater effective nonlinear absorption coefficent β ₂ than the [MoIr₃] cores; the tungsten example, W ₂Ir₂(CO)₁₀(η-C₅H ₅)₅, exhibited the highest response. Substitution at the cyclopentadienyl group (including incorporation into a polymer backbone) had little effect on the response measured. A time-resolved investigation of the alkyne-adduct Mo₂Ir₂(μ₄-η ²-MeC₂Ph)(CO)₈(η-C₅H ₄Me)₂ using picosecond pulses at 527 nm reveals optical-power-limiting behaviour that results from electronic processes [specifically, a fast nonlinear absorption process followed by reverse saturable absorption involving long-lived (>1000 ps) metastable excited states].