Many transition metal complexes have metal-to-ligand charge transfer (MLCT) excited states with superior photochemical properties such as high absorption in the solar spectrum and long excited-state lifetimes, making them suitable for energy conversion. A better understanding of photoinduced electron transfer in these complexes would allow us to achieve higher energy conversion efficiency and enable photocatalytic reactions that are inaccessible by other trigger sources. Prototypical examples for studying these phenomena are bimetallic copper (I) and ruthenium (II) complexes, where the metals are electronically coupled through a large π-conjugated bridging ligand.  Photoexcitation of the MLCT band in these complexes leads to electron transfer to the bridging ligand, where it may then migrate across the ligand or lead to further electron/hole transfer between the metal centers.  Previous studies have used optical transient absorption (OTA) to observe this electron transfer, though considerable overlap between the two metallic absorption bands can lead to difficulties in interpretation.  In contrast, x-ray near-edge absorption (XANES) spectroscopy is element-specific and sensitive to local environments, making x-ray transient absorption (XTA) a promising technique for a more direct observation of electron transfer on the ultrafast (femtosecond) time scale.  In particular, as the tetrapyridophenazine bridging ligand contains nitrogen atoms in three different atomic environments, XANES measurements reveal three distinct and well-separated absorption features which can be used to monitor precisely the movement of the electron across the complex at very short timescales.<br/> <br/>In this talk, we present experimental x-ray absorption spectra for a number of these complexes.  Density functional theory (DFT) calculations are used to reproduce these spectra, allowing us to explain the origin and shape of absorption features, which are largely attributed to the delocalization of electron-accepting orbitals and stabilization of nitrogen 1s orbitals.  Furthermore, excited-state geometry optimizations of various charge-transfer states allow for the prediction of nuclear relaxation pathways in the complex.  Finally, real-time time-dependent DFT simulations allow for the explicit simulation of intramolecular electron transfer and the prediction of the transfer rate.

Ab initio modeling of ultrafast intramolecular electron transfer

ACS Spring 2022

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