Synthesis and properties of [Pt(4-CO₂CH₃-py)₂(mnt)]: Comparison of pyridyl and bipyridyl-based dyes for solar cells

The dye complexes [Pt(4-CO₂R-py)₂(mnt)] (R = H (3a), CH₃ (3b)) and the precursor complexes [Pt(4-CO₂R-py)₂Cl₂] (2a, 2b) (py = pyridyl) were synthesised, characterised by electrochemical, spectroscopic, spectroelectrochemical (UV-vis-nIR and in situ EPR) and hybrid DFT computational methods and attached to a TiO₂ substrate to determine charge recombination kinetics. The results were compared to the bipyridyl analogues [Pt{X, X'-(CO₂R)-2,2'-bipyridyl}(mnt)], (X = 3 or 4). The electronic characteristics of the bis-pyridyl complex were found to be different to the bipyridyl complexes making the former harder to reduce, shifting the lowest-energy absorption band to higher energy and showing separate degenerate LUMO orbitals on the two pyridine rings. The latter point determines that the di-reduced pyridyl complex remains EPR active, unlike the bipyridyl analogue. Complex 3a attached to nanocrystalline TiO₂ shows a long charge recombination lifetime in comparison with the analogous complex with the ubiquitous 4,4'-(CO₂H)₂-bipyridyl ligand, suggesting that pyridyl complexes may possess some advantage over bipyridyl complexes in dye-sensitised solar cells.