Efficient Singlet Oxygen Generation from Polymers Derivatized with Hexanuclear Molybdenum Clusters

The molybdenum(II) chloride cluster Mo6Cl12 has been immobilized on 2% cross-linked poly(4-vinylpyridine) (PVP-Mo6Cl12) by the axial ligation of two pendant pyridines of the polymer to the hexanuclear metal core. Polymers with different cluster loadings exhibit the characteristic broad band emission of Mo6Cl12 at λmax = 765 ± 3 nm, and luminescence decays are composed of short- and long-lifetime components, which are consistent with the presence of cis- and trans-disubstituted cluster species, respectively. The lifetime of the cis form is too short to permit its bimolecular reaction with oxygen. However, the long-lifetime component is quenched efficiently by O2, and Stern−Volmer analyses of emission lifetimes yield quenching rate constants of (2−10) × 105 M-1 s-1 for cluster loadings in the range (3.5−0.3) × 10-3 g/g of PVP, respectively. The quenching rate constant increases to ∼50 × 105 M-1 s-1 for all polymers in MeOH regardless of cluster loading. In aqueous solutions (pH 3−6) intermediate quenching rate constants are observed. These results correlate with the swelling properties of the PVP polymer in various solvents where greater swelling is manifested in greater accessibility of oxygen to the cluster active site and hence increased quenching rate constants. Oxygenated solutions of PVP-Mo6Cl12 and alkenes photoreact to yield oxidized olefins. The products and product distributions indicate an energy transfer quenching reaction between oxygen and electronically excited polymer-bound clusters to produce singlet oxygen.