AbstractThe use of a triazatriangulenium organic dye known as TATA+ as a photosensitizer in a completely precious metal-free photocatalytic system for the reduction of protons to H2 was investigated. When used in association with the cobalt catalyst [Co(CR)(H2O)2]3+ (10 μM) and ascorbate (0.1 M) as a sacrificial electron donor, the system can achieve turn-over-numbers greater than 4000 upon visible light irradiation (400-700 nm) at pH 4.5 with a photosensitizer concentration of 250 μM. In terms of stability and activity, TATA+ exceedingly outperforms any organic photosensitizer and the complex [Ru(bpy)3]2+ which is often used as a benchmark. Where other recently reported organic photosensitizers rapidly decompose from their reduced state in acidic conditions, TATA+ is stable in its reduced radical state and shows little decomposition after 46 hours of irradiation. The excellent stability of the reduced state can be attributed to the delocalisation of the radical across the planar structure and stabilization from three electron-donating nitrogen atoms.
The synthesis of cobalt and iron complexes containing polypyridine-(bis)imine ligands as potential proton reduction catalysts was also attempted. The complexes were formed in their +2 oxidation state and showed reduction processes, as required for catalysis, in the cyclic voltammograms. [Co(LPI)(MeCN)2](BF4)2 (LPI = phenanthroline-(bis)imine ligand) displayed a reversible reduction potential of -1.45 V vs SCE for the E(CoII/CoI) redox couple suggesting that it may be active towards proton reduction if used in conjunction with [Ru(bpy)3]2+. [Fe(LPI)Cl2] displayed a reversible reduction potential of -0.94 V vs SCE for the E(FeII/FeI) redox couple which suggests that it could be used in conjugation with TATA+. Two other iron complexes were reported, [Fe(LPI)(EtOH)2](BF4)2 and [Fe(LBI)(EtOH)2](BF4)2 (LBI = bipyridine-(bis)imine ligand), however the process for the E(FeII/FeI) redox couples were irreversible.
|Date of Award||2019|