Probing the Electronic Structure of a Photoexcited Solar Cell Dye with Transient X-ray Absorption Spectroscopy

This study uses transient X-ray absorption (XA) spectroscopy and time-dependent density functional theory (TD-DFT) to directly visualize the charge density around the metal atom and the surrounding ligands following an ultrafast metal-to-ligand charge-transfer (MLCT) process in the widely used RuII solar cell dye, Ru(dcbpy)2(NCS)2 (termed N3). We measure the Ru L-edge XA spectra of the singlet ground (1A1) and the transient triplet (3MLCT) excited state of N34– and perform TD-DFT calculations of 2p core-level excitations, which identify a unique spectral signature of the electron density on the NCS ligands. We find that the Ru 2p, Ru eg, and NCS π* orbitals are stabilized by 2.0, 1.0, and 0.6 eV, respectively, in the transient 3MLCT state of the dye. These results highlight the role of the NCS ligands in governing the oxidation state of the Ru center.