Synthesis and photophysical properties of polymer-bound hexanuclear molybdenum clusters

The coordination of [Mo6Cl8i(SO3CF3)(6)(a)](2-) (1) and [(Mo6Cl8Cl4a)-Cl-i(EtOH)(a)(2)] (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 (T-g). Materials with C:P ratios of 1:100 are highly soluble in CH2Cl2 and MeOH, with T-g values slightly higher than that of pure PVP. Unswellable materials with no discernible T-g 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 tau(1) = 90 mu S (60 %) and tau(2) = 8 mu s (40 %), while [(Mo6Cl8Cl6a)-Cl-i](2-) has a uniexponential decay of tau = 156 mu s in CH2Cl1 solution.