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 Run solar cell dye, Ru(dcbpy)(2)(NCS)(2) (termed N3). We measure the Ru L-edge XA spectra of the singlet ground ((1)A(1)) and the transient triplet ((MLCT)-M-3) excited state of N3(4-) 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 e(g), and NCS pi* orbitals are stabilized by 2.0, 1.0, and 0.6 eV, respectively, in the transient (MLCT)-M-3 state of the dye. These results highlight the role of the NCS ligands in governing the oxidation state of the Ru center.