Using time-resolved Ru L-edge X-ray absorption spectroscopy to capture photoinduced transient electronic structure in a solar cell dye molecule

Understanding the electronic structure of transition metal dye molecules used in dye-sensitized solar cells (DSSC) is critical for determining the functioning of these devices. Ru$^{II}$ dyes such as Ru(dcbpy)$_{2}$(NCS)$_{2}$ (termed RuN3) have been components in some of the most effective DSSCs. We use synchrotron-based picosecond Ru L-edge X-ray absorption spectroscopy (XAS) to monitor changes the changes in the electronic structure of RuN3$^{4-}$ that accompany the $^{1}$A$_{1}$ to $^{3}$MLCT conversion initiated with a 400 nm light pulse. The results are interpreted by simulating the Ru L$_{3}$ X-ray absorption spectra of the $^{1}$A$_{1}$ and $^{3}$MLCT states with time-dependent density functional theory (TD-DFT). We observe the formation of the Ru$^{III}$ oxidation state of RuN3$^{4-}$ within the 70 ps time resolution of our experiment. The TD-DFT simulation allows us to assign a spectral feature in the Ru L-edge spectrum as a probe of the electronic structure of the NCS ligands due to overlap between Ru 4d and NCS $\pi $* orbitals. A 1.2 eV blue shift in this feature between ground and $^{3}$MLCT state corresponds to depletion in charge density on the NCS ligands in the excited state. We will discuss in detail the local electronic structure around the Ru atom in the transient $^{3}$MLCT state measured by time-resolved XAS.