Abstract
The novel photosensitizer [Ru(S–Sbpy)(bpy)2]2+ harbors two distinct sets of excited states in the UV/Vis region of the absorption spectrum located on either bpy or S–Sbpy ligands. Here, we address the question of whether following excitation into these two types of states could lead to the formation of different long-lived excited states from where energy transfer to a reactive species could occur. Femtosecond transient absorption spectroscopy identifies the formation of the final state within 80 fs for both excitation wavelengths. The recorded spectra hint at very similar dynamics following excitation toward either the parent or sulfur-decorated bpy ligands, indicating ultrafast interconversion into a unique excited-state species regardless of the initial state. Non-adiabatic surface hopping dynamics simulations show that ultrafast spin–orbit-mediated mixing of the states within less than 50 fs strongly increases the localization of the excited electron at the S–Sbpy ligand. Extensive structural relaxation within this sulfurated ligand is possible, via S–S bond cleavage that results in triplet state energies that are lower than those in the analogue [Ru(bpy)3]2+. This structural relaxation upon localization of the charge on S–Sbpy is found to be the reason for the formation of a single long-lived species independent of the excitation wavelength.
Highlights
Natural photocatalytic systems include a light-harvesting complex, where absorption occurs, covalently linked to a site of catalytic activity
Energy transfer from the absorptive to the reactive site is achieved via a multitude of proton-coupled electron transfers that form a complicated network of singleand multi-step subreactions themselves.[1−4] The efficiency of these energy and electron transfers is increased in biological photosynthesis by a supramolecular arrangement, which connects the light-harvesting unit to the catalytic site via a cascade of electron transporters operating near the thermodynamical optimum.[5,6]
Two different sets of excited states are initially populated via absorption, each reminiscent of transferring an electron from the ruthenium center to one of the two types of ligands present: the parent bpy ligands or the modified S−Sbpy ligand, which are experimentally accessible at 430 and 520 nm, respectively
Summary
Natural photocatalytic systems include a light-harvesting complex, where absorption occurs, covalently linked to a site of catalytic activity. The bpy ligand itself has been found to exhibit both π-donating and π-accepting features in iron complexes.[16] The low-energy photon absorption leads to excitation of a singlet state of metal-to-ligand charge-transfer (1MLCT) character, oxidizing the ruthenium center and reducing the ligands whereto the electrons are excited. From this initial 1MLCT excited state, ultrafast intersystem crossing into the triplet manifold is observed in less than 50 fs[17−19] followed by a descent into the lowest 3MLCT state from which phosphorescence is observed or an electron could be transferred to another species. The phosphorescence decays in hundreds of nanoseconds,[21] providing ample time for interaction with other species whereto the energy or electron is transferred
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