The ultrafast dynamics of transition metal complexes represents a rapidly expanding area of research at the nexus of physical and inorganic chemistries. While the time scale of electron and energy transfer processes in intramolecular donor-acceptor assemblies can constitute a practical reason for the need to use femtosecond time-resolved spectroscopic methods, the availability of ultrashort (i.e., &lt; 50 fs) excitation sources opens up interesting new possibilities for probing the mechanisms of ultrafast processes with regard to electronic-vibrational coupling. This presentation will focus on our efforts to leverage such ultrashort pulses as a means of identifying specific nuclear degrees of freedom that contribute to and/or drive excited-state evolution in coordination complexes. Results will be presented for complexes of the form Cr(acac')3 (where acac' is a derivative of acethylacetonate). Sub-50 fs excitation of these compounds within the 4A2 ---&gt; 4T2 ligand-field absorption band reveals oscillatory features in the transient absorption data corresponding to quantum oscillations arising from the creation of a wavepacket on the 4T2 excited-state potential energy surface. Analysis of these data using linear predictive single valued decomposition (LPSVD) indicate dephasing times for these oscillations that are too fast to be purely electronic and too slow to be purely vibrational. The specific nature of the atomic motions associated with these so-called vibronic coherences - which by definition couple electronic and nuclear coordinates - was then used as the basis for synthetic modifications of the compound with the goal of altering the time scale of excited-state evolution. Data on several systems will be presented that demonstrate this sort of approach can indeed provide the means for using ultrafast time-resolved spectroscopy to control of the time scale and mechanism of ultrafast excited-state dynamics through targeted synthetic design.

Vibronic coherence as a mechanistic probe for ultrafast dynamics in transition metal-based chromophores.

255th National Meeting (2018)

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