Leveraging Excited-state Coherence to Probe Molecular Mechanisms of Ultrafast Excited-state Evolution in Transition Metal-based Chromophores

Transition metal-based chromophores represent an important class of compounds for applications in a variety of light-to-chemical energy conversion strategies. Compounds based on ruthenium and iridium represent the vast majority of systems currently being employed in this context, but their elemental scarcity has compelled researchers to explore alternatives based on the more earth-abundant elements of the first transition series. A fundamental difference in the photophysics of these less expensive, more scalable options - namely ultrafast relaxation of the initially formed excited states to lower energy, metal-centered excited states - represents a significant scientific challenge that must be overcome if these systems are to enjoy wider applicability. Our research program focuses on a synergystic approach combining synthesis and ultrafst time-resolved absorption spectroscopy to elucidate structure-dynamic correlations to guide chromophore design. This presentation will focus specifically on our use of sub-50 fs methods to generate excited-state vibronic coherences that can report on the structural dynamics occuring as the system evolves from the initially formed excited state to the lower-energy states that give rise to their bimolecular reaction chemistry. This approach has allowed us to identify subsets of the compound's 3N-6 vibrational degrees of freedom that we believe serve to define the reaction coordinate for ultrafast excited-state evolution, thereby providing a roadmap for synthetically tailoring how this class of compounds absorbs and non-radiaitively dissipates energy.