Bond Rupture in LMCT Excited States

The Physical Chemistry

Ligand-to-metal charge transfer (LMCT) electronic transitions are exhibited in transition metal complexes with metals in high oxidations states (Cu(II), Fe(III), etc.) coordinated by “oxidizable” ligands ranging from Cl- to MeO-. This light-induced homolytic bond cleavage process is reminiscent of the fundamentals of the photolysis of molecular diatomics like halogen gases or disulfides, but with the flair of photoinduced redox transitions common to transition metal complexes. Despite the superficial similarities to the much better understood metal-to-ligand charge transfer transitions (MLCT), the photoinduced dynamics following LMCT transitions are difficult to rationalize in the context of MLCT excited states. What is clear is that there is an ultrafast rate competition on the sub-100 femtosecond timescale for either photoinduced ligand dissociation or ultrafast nonradiative decay, a competition for which the intrinsic branching ratio and underlying photochemical rules are unknown.

The Kudisch Lab aims to build foundational knowledge in the sphere of LMCT-initiated photoinduced dynamics by directly time-resolving the ratio of these ultrafast branching pathways and reconciling these results with photochemical quantum yield measurements. Applying extremely high time resolution spectroscopies to a variety of increasingly complex high oxidation state metal complexes we hope will eventually lead us to understand the factors limiting overall photolytic bond dissociation yields and subsequently bottom-up design more photochemically efficient inorganic photoreagents.

Relevance to Synthetic Photochemistry

There has been an explosion of reports utilizing LMCT-initiated photocatalysis and photochemistry to generate functionalizable chemical intermediates, for C-H activation, and even for polymer degradation. One of the best reasons to use an LMCT-based photochemical activation platform is the ability to use earth-abundant transition metals like Fe, Cu, or Mn, instead of typical Ir-, Ru-, or Os-based photosensitizers. New LMCT-initiated transition metal complexes are being discovered every year, despite little fundamental predictability as to which complexes will catalyze the reaction of interest. With our foundational work into these photoinduced dynamics, we hope to engineer highly efficient and highly photoreactive photocomplexes to replace precious metal photosensitizers in photoredox catalysis.