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  Ribeiro da Silva Neto, Wagner: Triplet Metal-to-ligand Charge Transfer (³MLCT) Dynamics of the N3-Dye Photosensitizer in water probed by Time-dependent X-Ray Absorption Spectroscopy at picosecond timescales. , University of Leipzig, Dissertation, 2024

Open Access Version

Abstract:
The N3-Dye — [Ru(dcbpy)₂(NCS)₂]^−4 — is a well-known photosensitizer largely used in solar-cells for improving photo-current output. This capability stems from a ³MLCT long-lasting state, which permits electron injections into the solar cell’s substrate in the subpicosecond range. The 3MLCT state has largely been studied, and it is known to stem from a transition from the t2g metal-centered orbitals to π∗ orbitals derived from the dicarboxylic-bipyridine ligands (dcbpy) (i.e., a t2g^5 π∗^1 state), assuming a pseudo octahedral symmetry. This dynamics has been investigated by pump-probed techniques—mainly UV-Vis and X-ray absorption spectroscopy (at the metal L edges), focusing on the injection times and (or) the electronic interactions with the substrate. In this work, the ³MLCT state is investigated in water via time-resolved X-ray absorption spectroscopy at the N K-edge for the first time and supported by TD-DFT calculations. The obtained pump-probe features allow the assignment of the t2g-hole by both NCS- and dcbpy-N 1s core states and also evidence the localization of the 3MLCT state in one of the dcbpy ligands with an unambiguous transition, which is shifted to lower energies due to the core-hole screening, and valence and geometry synergetic effects. As also seen in the ground state spectrum, the transient X-ray spectrum is also sensitive to the dx2+y2 and dz2 orbitals, and these transitions also appear shifted towards lower energies in the transient measurements, directly correlating to core-hole screening effects seen by the DFT calculations. From the time-dependent XAS spectrum, delay traces were acquired at 398.73, 399.33, and 400.86 eV and fitted according to a kinetic model assuming solely ground state photoconversion to the 3MCLT as the 1MLCT cannot be tracked in the picosecond timescale due to the fast ISC to the ground state (≈ 100 fs). The obtained lifetime from the delay trace measurements appears to be solvent-dependent when compared to different solvation environments and reported lifetimes. Possible pathways to understanding these emerging properties are outlined here.