Enhancing the stability of photocatalytic systems for hydrogen evolution in water by using a tris-phenyl-phenanthroline sulfonate ruthenium photosensitizer

Abstract

The family of ruthenium tris-bipyridine complexes remains among the most widely used molecular photosensitizers (PSs) to drive catalytic reactions using the energy of light. However, the main drawback of such PSs is the poor stability of their oxidized and reduced forms subject to ligand dissociation, especially in water that causes relatively fast deactivation of the photocatalytic systems. We were able to improve the stability and efficiency of a Ru based photocatalytic system for hydrogen production in water by using the water-soluble Na₄[Ru((SO₃Ph)₂phen)₃] derivative (RuSPhphen, (SO₃Ph)₂phen = disodium (1,10-phenanthroline-4,7-diyl)bis(benzenesulfonate)) in place of regular [Ru^II(bpy)]₃Cl₂ (Rubpy, bpy = 2,2′-bipyridine) PS. RuSPhphen was tested with the [Co^III(CR14)Cl₂]Cl (Co) catalyst and ascorbate (HA⁻) as a sacrificial electron donor under visible-light irradiation. The RuSPhphen absorption coefficient being twice as high compared to Rubpy and the excited-state lifetime being much longer, while keeping almost similar potentials, more efficient intermolecular electron transfers have been observed allowing the concentration of Ru PSs to decrease by 5-fold, i.e. to 100 μM, compared to previous studies with the Rubpy/Co/HA⁻/H₂A photocatalytic system. A substantially enhanced H₂-evolving photocatalytic activity was obtained with RuSPhphen directly correlated to its better stability over Rubpy. With 1.1 M H₂A/HA⁻, at catalyst concentrations of 10 an 5 μM, the H₂ production is two times higher compared to that obtained with Rubpy. When the H₂A/HA⁻ concentration is decreased to 0.1 M, the stability of both Ru PSs is further improved although RuSPhphen still systematically outperforms Rubpy whatever the catalyst concentration, with TONs reaching up to 4770 at 5 μM catalyst. A faster electron transfer of the RuSPhphen excited state to HA⁻ has been observed by time-resolved luminescence compared to that of Rubpy, that could be ascribed to its much longer lifetime. In addition, a much higher rate constant for back electron transfer from the RuSPhphen˙⁻ reduced state to HA˙ was determined by nanosecond transient absorption spectroscopy that could contribute to the higher stability of RuSPhphen during photocatalysis. The greater stability of RuSPhphen over Rubpy could also be correlated to the geometry of the SPhphen ligand that makes it less prone to dissociation in water in its reduced state.