A comparative study of the redox and excited state properties of (nBu(4)N)(2)[Mo6X14] and (nBu(4)N)(2)[Mo6X8(CF3COO)(6)] (X = Cl, Br, or I)

The excited-state dynamics, luminescence, and redox properties of a series of hexanuclear molybdenum cluster complexes, (nBu(4)N)(2)[Mo6X14] and (nBu(4)N)(2)[Mo6X8(CF3COO)(6)] (X = Cl, Br, or I), were investigated. Substitution of the apical halogen ligands for the trifluoroacetate ligands increased the oxidation potentials and induced a blue shift in the absorption and luminescence bands as well as a considerable increase in the luminescence quantum yields for heavy inner ligands. Time-resolved transient absorption measurements showed that the intersystem crossing from the excited singlet states is ultrafast with time constants ranging between <120 fs and 1.68 ps and leads to hot triplet states. The following cooling occurred at a ps time scale and was assigned to electronic redistribution within the emissive triplet state sublevels. The formation of singlet oxygen, O-2((1)Delta(g)), suggested earlier on the basis of photooxidation experiments for some complexes, was revised by direct measurements of O-2((1)Delta(g)) phosphorescence. We showed the effects of the attached ligands on key physico-chemical and photophysical parameters of the title complexes. The synthesis and structural characterisation of a new cluster complex, (nBu(4)N)(2)[Mo6Br8(CF3COO)(6)], completed the series. Our results demonstrated that the complexes with heavy inner ligands (Br, I) and apical trifluoroacetate ligands were photochemically and electrochemically stable, highly luminescent, and good sensitisers of O-2((1)Delta(g)).