Femtosecond Transient Absorption Anisotropy Study on [Ru(bpy)3]2 + and [Ru(bpy)(py)4]2 +. Ultrafast Interligand Randomization of the MLCT State

Ligand Rigidity Steers the Selectivity and Efficiency of the Photosubstitution Reaction of Strained Ruthenium Polypyridyl Complexes

While photosubstitution reactions in metal complexes are usually thought of as dissociative processes poorly dependent on the environment, they are, in fact, very sensitive to solvent effects. Therefore, it is crucial to explicitly consider solvent molecules in theoretical models of these reactions. Here, we experimentally and computationally investigated the selectivity of the photosubstitution of diimine chelates in a series of sterically strained ruthenium(II) polypyridyl complexes in water and acetonitrile. The complexes differ essentially by the rigidity of the chelates, which strongly influenced the observed selectivity of the photosubstitution. As the ratio between the different photoproducts was also influenced by the solvent, we developed a full density functional theory modeling of the reaction mechanism that included explicit solvent molecules. Three reaction pathways leading to photodissociation were identified on the triplet hypersurface, each characterized by either one or two energy barriers. Photodissociation in water was promoted by a proton transfer in the triplet state, which was facilitated by the dissociated pyridine ring acting as a pendent base. We show that the temperature variation of the photosubstitution quantum yield is an excellent tool to compare theory with experiments. An unusual phenomenon was observed for one of the compounds in acetonitrile, for which an increase in temperature led to a surprising decrease in the photosubstitution reaction rate. We interpret this experimental observation based on complete mapping of the triplet hypersurface of this complex, revealing thermal deactivation to the singlet ground state through intersystem crossing.

Photoinduced ligand dissociation follows reverse energy gap law: nitrile photodissociation from low energy 3MLCT excited states

A series of Ru(ii)-terpyridine complexes containing electron-donating bidentate ligands are able to effectively photodissociate nitrile ligands using red light. A spectroscopic investigation of these complexes reveal that they follow anti-energy gap law behavior, providing further evidence that population of ³LF excited states is not necessary for photoinduced nitrile dissociation.

An Insulating Al2O3 Overlayer Prevents Lateral Hole Hopping Across Dye-Sensitized TiO2 Surfaces

Three chromophores of the general form [Ru(bpy')₂(4,4'-(PO₃H₂)₂-2,2'-bipyridine)]²⁺, where bpy' is 4,4'-(C(CH₃)₃)₂-2,2'-bipyridine (Ru(dtb)₂P); 4,4'-(CH₃O)₂-2,2'-bipyridine (Ru(OMe)₂P), and 2,2'-bipyridine (RuP) were anchored to mesoporous thin films of TiO₂ nanocrystallites at saturation surface coverages to investigate lateral self-exchange Ru^III/II intermolecular hole hopping in 0.1 M LiClO₄/CH₃CN electrolytes. Hole hopping was initiated by a potential step 500 mV positive of the E_1/2 (Ru^III/II) potential or by pulsed laser (532 nm, 8 ns fwhm) excitation and monitored by visible absorption chronoabsorptometry and time-resolved absorption anisotropy measurements, respectively. The hole hopping rate constant k_R extracted from the potential step data revealed self-exchange rate constants that followed the trend: TiO₂|Ru(OMe)₂P (k_et = 1.4 × 10⁶ s^-1) > TiO₂|RuP (7.1 × 10⁵ s^-1) > TiO₂|Ru(dtb)₂P (6.5 × 10⁴ s^-1). Analysis of the anisotropy data with Monte Carlo simulations provided hole hopping rate constants for TiO₂|RuP and TiO₂|Ru(dtb)₂P that were within experimental error the same as that measured with the potential step. The hole hopping rate constants were found to trend with the TiO₂(e^-)|Ru^III → TiO₂|Ru^II charge recombination rate constants. The atomic layer deposition of an ∼10 Å layer of Al₂O₃ on top of the dye-sensitized films was found to prevent hole hopping by both initiation methods even though the chromophore surface coverage exceeded the percolation threshold and excited-state injection was efficient. The dramatic hole hopping turnoff was attributed to a larger outer-sphere reorganization energy for self-exchange due to the restricted access of electrolyte to the redox active chromophores. The implications of these findings for solar energy conversion applications are discussed.

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

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+.