“…Firstly, similarly to their structural analogues, hexarhenium clusters [{Re 6 Q 8 }L 6 ] m (Q = S or Se), these complexes demonstrate high chemical and photostability of the cluster cores {Mo 6 X 8 } 4+ that are primarily responsible for their triplet excited state photoluminescence (i.e. 4,5,11,15 Additionally, the metal cluster complexes can also act as powerful photosensitisers in the singlet oxygen ( 1 O 2 ) generation, [5][6][7][8][16][17][18] which makes them especially interesting for applications associated with the generation of singlet oxygen in vivo, such as, for example, photodynamic therapy (PDT). Secondly, their broad emission spectra extend in the red/near infra-red region (from ∼550 to more than 950 nm) 4-13 overlapping the so called "optical tissue window" (650 − 900 nm), where the boundaries of the region are defined by the minimal light absorption of hemoglobin (< 650 nm) and water (> 900 nm).…”