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

Abstract

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 Ru(II) solar cell dye, Ru(dcbpy)2(NCS)2 (termed N3). We measure the Ru L-edge XA spectra of the singlet ground ((1)A1) and the transient triplet ((3)MLCT) 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 eg, and NCS π* orbitals are stabilized by 2.0, 1.0, and 0.6 eV, respectively, in the transient (3)MLCT state of the dye. These results highlight the role of the NCS ligands in governing the oxidation state of the Ru center.

Atomic-Scale Perspective of Ultrafast Charge Transfer at a Dye–Semiconductor Interface

“…The strong concentration of the HOMO on the central Ru atom and the thiocyanate ligands is consistent with previous experimental and theoretical studies. 18,19,23,24,32,36 In an intuitive picture, the photoinduced removal of an electron from the HOMO of the dye molecule is expected to lead to a significant increase in inner-shell binding energies in those parts of the molecule where most of the HOMO electron density is localized. This is consistent with our experimental finding that the excitation-induced chemical shift is much more prominent in the Ru 3d doublet than in the C 1s carboxyl and pyridine signals.…”

“…In the 3 MLCT state, the excited electron is mostly located on the bipyridine ligands, which is consistent with recent calculations for solvated N3 molecules. 18 In the fully ionized configuration, the hole density largely mimics the HOMO of the ground-state molecule ( Figure 3B), while the electron is assumed to be completely removed from the system. The electronic charge distribution of the interfacial CT state is constrained to simulate a configuration in which the excited electron is transferred to the ZnO substrate but not yet further separated from the dye cation.…”

Core and valence excitations in resonant X-ray spectroscopy using restricted excitation window time-dependent density functional theory

“…However, ligand field multiplet studies have found that spin-orbit interaction does not affect L-edges too much, 91 and other spin-free DFT studies have also given good L-edge results. 92,93 Very recently, it has been shown that the Ru L 3 edge including scalar relativistic corrections via the zeroth order regular approximation can be modeled reliably with REW-TDDFT. 66 So spin-orbit interaction is neglected in this study.…”

Tracking reaction dynamics in solution by pump–probe X-ray absorption spectroscopy and X-ray liquidography (solution scattering)

“…166,204,206 In this regard, metal L-edge spectroscopy based on the dipoleallowed 2p  3d transitions is more informative because they directly probe unoccupied valence orbitals with high resolution owing to smaller broadening of their spectral features by almost ten times compared with those of 1s-excitations. 147,[207][208][209] Specifically, TRXAS at transition-metal L-edges directly probes changes in ligand-field splitting of metal d orbitals and spin states via selective excitation of 2p 1/2 and 2p 3/2 electrons to vacancies in the 3d valence orbitals of metals with spin-orbit splitting. For the Fe(II) SCO processes, fast ligand-cage dilation and spin-state changes should be coupled to changes in Fe3d/N-2p hybridizations caused by weakening of the Fe-N bonds as well as associated changes in the ligand-field electron configurations.…”