Photoionization of [Ru(bpy)₃]²⁺:  A Catalytic Cycle with Water as Sacrificial Donor

Abstract

The photoionization of the tris-2,2′-bipyridyl ruthenium(II) ion [Ru(bpy) 3 ] 2+ in water was investigated by laser-flash photolysis at 308, 355, and 532 nm up to very high excitation intensities I exc (500. . .900 mJ/cm 2 ). By single-pulse absorption and two-pulse luminescence measurements, it was established that the mechanism is cyclic and comprises three steps: excitation of [Ru(bpy) 3 ] 2+ to the metal-to-ligand charge-transfer (MLCT) excited state *[Ru(bpy) 3 ] 2+ , ionization of *[Ru(bpy) 3 ] 2+ by a second photon to give a hydrated electron e aq •and the oxidized complex [Ru(bpy) 3 ] 3+ , and photoreduction of [Ru(bpy) 3 ] 3+ by water. No other species is involved in the reaction. Experiments on chemically generated [Ru(bpy) 3 ] 3+ yield direct evidence for the photoreduction step and show that it is completed in less than 30 ns. The concentrations of [Ru(bpy) 3 ] 2+ , *[Ru(bpy) 3 ] 2+ , [Ru(bpy) 3 ] 3+ , and e aq•at the end of the laser pulse were measured as functions of I exc . Closedform expressions for these dependences were derived by a kinetic treatment. Very good simultaneous fits to the concentrations of all species were obtained with three global kinetic parameters only. A decision as to whether the photoreduction leads back to [Ru(bpy) 3 ] 2+ or to *[Ru(bpy) 3 ] 2+ is kinetically impossible in that system, but evidence pointing to a delayed generation of *[Ru(bpy) 3 ] 2+ by this reaction step is presented, which would also explain short-time anomalies of the luminescence reported in the literature. The quantum yield φ 2 of the photoionization step is the same at 308 and 355 nm, is independent of pH, and is an order of magnitude higher (0.016 ( 0.001) than previously thought. The quantum yield φ 3 of the photoreduction is comparable to φ 2 at pH 7 but lower by a factor of 14 at pH 0.

Multi‐Photon Excitation in Photoredox Catalysis: Concepts, Applications, Methods

“…Two‐pulse experiments demonstrate that [Ru(bpy) 3 ] 3+ converts back to [Ru(bpy) 3 ] 2+ (either in its 3 MLCT or its ground state) under UV excitation. This photoreduction of [Ru(bpy) 3 ] 3+ occurs with a quantum yield of 0.023 at 355 nm and liberates hydroxyl radicals, as confirmed by scavenging experiments …”

An “All‐Green” Catalytic Cycle of Aqueous Photoionization

“…[13] As the main plot of Figure 1 a shows, the intensity dependence of MLCT formation is given by a saturation curve, and more than 90 % of GS can be converted to MLCT within the flash duration even with the weaker of our two lasers. [18] The inset of Figure 1 a demonstrates this chemical stability of both MLCT and *MLCT: Producing variable amounts of MLCT and *MLCT with a first flash, then allowing them to deactivate completely, and then probing the concentration of GS with a second flash of constant intensity gives the same result regardless of the intensity of the first flash. [13] MLCT absorbs weakly at 532 nm (see Section S2), but the resulting higher excited state *MLCT is shortlived on the timescale of our laser flashes, and this further excitation does not lead to the decomposition of the complex and specifically not to ionization.…”

“…[13] MLCT absorbs weakly at 532 nm (see Section S2), but the resulting higher excited state *MLCT is shortlived on the timescale of our laser flashes, and this further excitation does not lead to the decomposition of the complex and specifically not to ionization. [18] The inset of Figure 1 a demonstrates this chemical stability of both MLCT and *MLCT: Producing variable amounts of MLCT and *MLCT with a first flash, then allowing them to deactivate completely, and then probing the concentration of GS with a second flash of constant intensity gives the same result regardless of the intensity of the first flash. 1 b) Reductive quenching of MLCT by the sacrificial donor D À sac yields OER, which is then bleached by a second green photon.…”

Table of Contents

“…Unquenched MLCT reverts to the GS without side reactions, accompanied by an emission that allows its quantitative monitoring 13. MLCT absorbs weakly at 532 nm (see Section S2), but the resulting higher excited state *MLCT is short‐lived on the timescale of our laser flashes, and this further excitation does not lead to the decomposition of the complex and specifically not to ionization 18. The inset of Figure 1 a demonstrates this chemical stability of both MLCT and *MLCT: Producing variable amounts of MLCT and *MLCT with a first flash, then allowing them to deactivate completely, and then probing the concentration of GS with a second flash of constant intensity gives the same result regardless of the intensity of the first flash.…”