Watching Photoactivation in a Ru(II) Chromophore–Catalyst Assembly on TiO2 by Ultrafast Spectroscopy

Abstract

This paper examines the ultrafast dynamics of the initial photoactivation step in a molecular assembly consisting of a chromophore (denoted [RuaII]2+) and a water-splitting catalyst (denoted [RubII]2+) anchored to TiO2. Photoexcitation of the chromophore is followed by rapid electron injection from the Ru(II) metal-to-ligand charge-transfer (MLCT) excited state. The injection process was followed via the decay of the bpy radical anion absorption at 375 nm. Injection is ∼95% efficient and exhibits multiple kinetic components with decay times ranging from <250 fs to 250 ps. Electron injection is followed by the transfer of the oxidative equivalent from the chromophore to the catalyst (ΔG = −0.28 eV) with a transfer time of 145 ps. In the absence of subsequent photoexcitation events, the charge-separated state undergoes electron-transfer recombination on the microsecond time scale.