Synthesis and Photophysical Properties of Polymer-Bound Hexanuclear Molybdenum Clusters

The coordination of [Mo6Cl8 i(SO3CF3)6 a]2− (1) and [Mo6Cl8 iCl4 a(EtOH)2 a] (2) to poly(evinylpyridine) (PVP) produces crosslinked materials. Formation of these materials occurs by displacement of axial ligands ((a) in Fig. 1) SO3CF3 − or EtOH on the clusters by the vinylpyridine moieties. The availability of six coordination sites in 1 and two coordination sites in 2 offers control over the degree of crosslinking and solubility of the materials. Similarly, varying the ratio of cluster coordination sites (C) to polymer-pendant pyridine ligands (P) yields materials with a wide range of solubilities and glass transition temperatures (Tg). Materials with C:P ratios of 1:100 are highly soluble in CH2Cl2 and MeOH, with Tg values slightly higher than that of pure PVP. Unswellable materials with no discernible Tg are obtained with C:P ratios of 1:5 and lower, indicating a high degree of crosslinks. Transient emission spectroscopy reveals that polymer-bound cluster 1 lacks the luminescent properties characteristic of many [Mo6Cl8]4+ based clusters. Conversely, polymer-bound cluster 2 displays intense luminescence and retains the unusually long-lived excited-state observed for the free clusters in solution. The emission lifetime of PVP-bound 2 fits a biexponential decay, with τl1 = 90 μs (60 %) and τ2 = 8 μs (40 %), while [Mo6Cl8 iCl6 a]2− has a uniexponential decay of τ = 156 μs in CH2Cl2 solution.