A Cyclometalated Cobalt(III) Triazolylidene Photosensitiser Complex Exhibiting Enhanced Light-Harvesting and Low Temperature 3MC State Luminescence

The exploration of the novel photochemistry offered by Earth-abundant first row transition metal complexes as replacements for photosensitiser complexes based on rare and expensive 4d and 5d metals is a highly topical area of contemporary inorganic chemistry. Here we report the synthesis and optoelectronic characterisation of the cyclometalated ‘abnormal’ carbene complex [Co(btzph)2]+ (btzphH = 1,3-bis(1-butyl-3-methyl-1,2,3-triazol-4-yl)benzene) which exhibits an intense 1MLCT absorption band (l = 421 nm, e = 7550 dm3 mol-1 cm-1) and a Co(III)/Co(IV) oxidation process at +0.46 V vs Fc/Fc+ in acetonitrile. Ultrafast transient absorption spectroscopic measurements reveal that the 3MLCT excited state is depopulated on timescale of 650 fs to populate a 3MC state, which then decays to the ground state with a lifetime of 1.5 ns. The complex is emissive from the 3MC state at 77 K (max = 700 nm) in EtOH/MeOH (4:1) glass. Using this wavelength to approximate a value for E0-0 of 1.77 eV, an excited state oxidation potential Eox* of ca. -1.3 V can be estimated, indicating that the complex may be able to act as a competent photoreductant in stark contrast to recently highlighted potently photooxidising Co(III) diimine complexes.