Photophysical properties of Co(iii) photosensitizers with phenothiazine-based ligands

Abstract

The ultrafast decay inherent to metal complexes with a 3d⁶ configuration limits their application as photosensitizers. Despite recent advances in improving the photophysical properties of these complexes, existing ligand designs restrict further modification and are often synthetically challenging. Here, we show how sulfur-bridged ligands can be used to tune the structural and photophysical properties in Co(III) photosensitizers. Two complexes, CoS ([Co(PTZIm₂)₂]PF₆) and CoSO₂ ([Co(PTZO₂Im₂)₂]PF₆), adopt facial geometries due to a less rigid ligand backbone compared to other pincer-type ligands. The lowest-lying absorption bands of both CoS and CoSO₂ display metal/ligand-to-ligand charge-transfer (M + L)LCT character with different contributions from the sulfur-bridged ligand. TD-DFT analysis indicates that CoSO₂ has a lower contribution from the phenothiazine moiety to the band at 400 nm. The sulfur oxidation state also affects the electronic density at the metal center, with CoS showing a lower M^IV/III oxidation potential. Transient absorption experiments reveal that fast non-radiative decay channels are facilitated in CoS. However, a photoactive long-lived component (8.0 ns) is also observed. Oxidation of phenothiazine extends the lifetimes of short-lived components in CoSO₂, where both electronic and structural effects may be playing a role. These findings demonstrate that the photophysical properties of Co(III) complexes can be modulated by variation of the sulfur oxidation state to achieve different photophysical properties of the complexes.