Development of new thiocyanate-free Ruthenium(II) dyes bearing isoquinoline chromophores for hydrogen production via water splitting

Four thiocyanate-free ruthenium(II) dyes, [Ru^II(4, 4-dicarboxylic acid-2,2-bipyridine)₂(L1 or L2)](PF₆)] and [Ru^II(tetraethyl[2,2′-bipyridine]-4,4′-diylbis(phosphonate))₂(L1 or L2)](PF₆)] with different anchoring groups, coded as Ru1–Ru4, were synthesized and characterized by NMR spectroscopy and mass spectrometry, and their electrochemical and photophysical properties were investigated. Density functional theory (DFT) calculation was used to gain insight into the factors responsible for photocatalytic properties of Ru1–Ru4 as effective photosensitizers. Relative to common ruthenium dyes, such as [Ru(bpy)₃]²⁺ and N719, the absorption spectra of our dyes with triphenylamine moiety are broader with enhanced molar absorptivities (ε), especially, in the region between 400 nm and 650 nm. This is attributed to the extended π-conjugation system by inserting electron-donating triphenylamine to isoquinoline through thiophene moiety in their cyclometalating (C^∧N) ligands, in which strong intramolecular charge transfer (ICT) ability was induced. All the new ruthenium(II) complexes were used as photosensitizers for visible-light driven hydrogen production reactions by attaching to platinized TiO₂ nanoparticles (Pt–TiO₂) in the presence of sacrificial electron donor (SED) ascorbic acid (AA) in purely aqueous solution. A H₂ turnover number (TON) up to 14232 was demonstrated after 236 h irradiation. Complexes with tetraethyl[2,2′-bipyridine]-4,4′-diylbis(phosphonate) anchoring groups were found to be outperform to those with classical 4, 4-dicarboxylic acid-2,2-bipyridine, which may be one of the crucial steps in designing superior ruthenium(II) photosensitizers for hydrogen generation from water.