Photexcitation of Aqueous Ruthenium(II)-tris-(2,2‘-bipyridine) with High-Intensity Femtosecond Laser Pulses

Abstract

We report a femtosecond pump-probe study on the photochemistry of concentrated aqueous solutions of [RuII(bpy)3]2+, as a function of pump power (up to 2 TW/cm2) at 400 nm excitation. The transient absorption spectra in the 345-660 nm range up to 1 ns time delay enable the observation of the following photoproducts: the triplet 3MLCT (metal-to-ligand-charge-transfer) excited state, the reduced form [RuII(bpy)3]+, the oxidized species [RuIII(bpy)3]3+, and the solvated electron e(aq). The 3MLCT state is formed within the excitation pulse and undergoes vibrational relaxation in 3-5 ps, as evidenced by the shift of the ligand-centered (LC) absorption band below 400 nm. Even at the highest pump powers, the majority of e(aq) originates from multiphoton ionization of [RuII(bpy)3]2+ and not from the solvent, generating [RuIII(bpy)3]3+ as a byproduct. At 10 ps time delay, the total concentration of the three product species is balanced by the depleted concentration of [RuII(bpy)3]2+, even at the highest fluences used, indicating that no further reaction products significantly contribute to the overall photochemistry. On the 100 ps time scale, most probably diffusion-controlled reduction of ground-state [RuII(bpy)3]2+ by solvated electrons occurs, next to recombination between e(aq) and [RuIII(bpy)3]3+.

Electron Localization Dynamics in the Triplet Excited State of [Ru(bpy)3]2+ in Aqueous Solution

“…We cannot excluded that the fate of the system is decided at very early times, as the 1 MLCT state has a lifetime of 30 fs, and appears vibrationally cold (at least if one considers the Franck-Condon modes). [11] Our results seem in accord with the reported vibrational cooling time of 3-5 ps in the 3 MLCT state [32] and call therefore for a reexamination of the depolarization dynamics.…”

Electronic transient spectroscopy from the deep UV to the NIR: unambiguous disentanglement of complex processes

“…As similar problems have been tackled in the past with transient IR spectroscopy, we would like to ask ourselves how the two methods with state-of-the-art setups 4,[10][11][12][60][61][62] compare. For the comparison a gure of merit is helpful.…”

Photoinduced Structural Dynamics of Molecular Systems Mapped by Time-Resolved X-ray Methods

“…All six 4d electrons of Ru 2+ are in the lower t2g orbital while the eg orbital is empty. Upon excitation of the 1 MLCT state, an electron is transferred from the metal t2g orbital to the bpy ligand, populating the long-lived (~600 ns) 3 MLCT state 160 and opening up a new channel for a (2p-4d(t2g)) core transition. In refs 91,159 , all the features of the electronic structure changes were observed: i) the blue shift of the 2p-4d(eg) transition due to the oxidation state change from Ru 2+ to Ru 3+ ; ii) the appearance of a new resonance at the 2p-4d(t2g) transition, due to the hole created in the t2g orbital.…”