Ultrafast X-ray Spectroscopy of Vibronically Mediated Electron Transfer

Ultrafast transient X-ray spectroscopy is becoming increasingly useful for the study of ultrafast dynamics, partly due to the elemental and orbital selectivity of the methods. Understanding the photophysics of transition metal complexes on the ps and fs timescale, especially the interplay between the electronic and vibrational components, is key to the future development of long-lived and effective photosensitisers. Chapter 1 is an overview of transition metal complexes, ultrafast spectroscopy studies, and common ultrafast phenomena. Ultrafast optical, vibrational and X-ray spectroscopy methods, as well as computational methods for simulating core-level spectra are outlined.

Chapter 2 covers the study of Pt(II) diimine thiolates via fs-TA and tr-XANES, supported by TDDFT and DFT/ROCIS simulations. Evidence is presented for the formation of the thiyl radical and three-electron, sulfur-sulfur bond at early times, followed by delocalisation of CT character across both thiolate ligands. The partial metal-to-ligand charge transfer character of the excited state is confirmed, by this character is lost after ∼10 ps.

Chapters 3 and 4 cover fs-TA, FLUPS, tr-XES and tr-XAS studies of heterolpetic Cu(I) diimine diphosphine complexes. Evidence is presented to suggest ultrafast ISC competing with IC and the photoinduced structural distortion on a fs/ps timescale, whilst tr-XES suggests there are limited changes in the excited state character after 100 fs.

Chapters 5, 6 & 7 cover fs-TA, tr-XAS and singlet oxygen yield measurements for a selection of Cr complexes. Long-lived Cr tris-diimines are used to showcase µs tr-XAS capabilities at I-18, DLS. Additionally, novel Cr carbene complexes are noted for their high singlet oxygen yield, potential light-activated cytotoxicity and possible use in photodynamic therapy.