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Development of Molybdenum and Tungsten Cluster Complexes for Singlet Oxygen Generation and Biological Implications

Subject Area Solid State and Surface Chemistry, Material Synthesis
Term from 2019 to 2023
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 410292974
 
Final Report Year 2023

Final Report Abstract

Octahedral metal halide clusters [{M6I8}L6]2-(M = Mo, W; L = inorganic or organic ligand) are known for their unique photophysical properties, including a bright red photoluminescence and a luminescence-quenching behavior in the presence of molecular oxygen, appearing in solid state and in solution. Within this process, excited of cluster-triplet-states can relax as phosphorescence, or produce singlet-oxygen in the presence of molecular oxygen, by energy transfer into triplet-states of O2. Hence, cluster compounds of this type are useful photosensitizers for the generation of singlet-oxygen that is useful in several applications, such as photodynamic therapy, photodynamic inactivation (bacteria, viruses, fungi), oxygensensing, and so on. It has been experimentally evidenced that long emission lifetimes and strong oxygen quenching are related to cluster compounds bearing strong electron-withdrawing anionic ligands in apical positions. Our studies on clusters bearing neutral (acetonitrile) ligands showed short lifetimes and the unneglectable quenching behavior for [W 6I8(CH3CN)6]4+. Comparative electron densities calculations (using ELF) on different clusters revealed that short lifetimes and weak oxygen-quenching is related to higher d-electron density at the metal centers of [W 6I8(CH3CN)6]4+. High d-electron densities are attributed to a low energetic splitting of the emitting triplet sublevels and thus a higher thermal population of the highest sublevel featuring an allowed transition into the ground state (“short lifetime”). Octahedral metal halide clusters with anionic organic ligands usually suffer from poor hydrolytic stability, limiting their applications. However, the newly developed neutral cluster [W 6I12(C6H5CN)2], bearing two benzonitrile ligands has revealed high hydrolytic as well as thermal stability. The high stability stems from a dense packing of the clusters in the crystal structure with H-I-bridges between adjacent units. The photocatalytic efficiency of [W 6I12(C6H5CN)2] was demonstrated for rhodamine B as a model compound in aqueous solution. The high stability and the proven photocatalytic activity make this cluster compound an interesting material for application in photodynamic inactivation. To study photodynamic inactivation of bacteria and fungi we incorporated (TBA)2[W 6I8(CO2CF3)6] and (TBA)2[W 6I8(C7H7SO3)6] into silicone matrices. Upon irradiation with visible light these samples showed significant light-induced antimicrobial activity, as exemplified and evaluated for several relevant samples. During the past, it has been shown by us that an energy transfer is not only possible from a cluster to molecular oxygen, but also onto a rare-earth ion. For this purpose, synthesized GdW 6Cl15, TbW 6Cl15 and EuW 6Cl14 via solid-state reactions. The photoluminescence spectra recorded from crystalline samples solely showed tungsten chloride cluster emission.

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