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Ultrafast Dynamics of Carbon Dioxide Reduction Photocatalysts Studied with Equilibrium and Transient Two-Dimensional Infrared Spectroscopy

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Ultrafast Dynamics of Carbon Dioxide Reduction Photocatalysts Studied with Equilibrium and Transient Two-Dimensional Infrared Spectroscopy


L. M. Kiefer1, K. J. Kubarych1*

1Department of Chemistry, University of Michigan, 930 N. University Ave., Ann Arbor, MI, USA

The photocatalytic reactions that leverage light energy to power chemical transformations operate on a vast range of time scales, from femtosecond light absorption to much slower substrate binding, release and catalyst regeneration. Considerable progress has been made in the last decade to use ultrafast multidimensional spectroscopy to study ground electronic state equilibrium dynamics in systems ranging from small molecules and peptides to liquids and proteins. Advances have come slower in using transient versions of these methods to study excited electronic states on the same footing as their well-understood ground states. It is particularly important to be able to study the vibrationally equilibrated excited states of photocatalysts, since those are states relevant to catalysis reaction dynamics. We have used both equilibrium and non-equilibrium, transient two-dimensional infrared (2D-IR) spectroscopy to investigate the ground S0 and long-lived (~60 ns) excited triplet metal-to-ligand-charge-transfer state (3MLCT) of several ReCl(CO)3bpy CO2 reduction catalysts in a series of solvents.[1-3] Transient 2D-IR spectroscopy enables direct comparison of solvation dynamics on distinct electronic states. This talk will summarize our previous findings regarding the differences in spectral dynamics for the S0 and 3MCLT, where we find a pronounced slowdown in solvation dynamics as reported by spectral diffusion that we have attributed to the ~8 Debye change in the molecular dipole moment.[1-2] Additionally, we will discuss both chemical substitutions on the bipyridyl ligand,[3] as well as the role of preferential solvation in controlling the electron transfer from the sacrificial donor triethanolamine (TEOA). Our data suggest preferential solvation by the polar TEOA co-solvent, which has important implications for the question of whether or not the primary charge transfer process is diffusion controlled.

References:

[1] L. M. Kiefer, J. T. King, K. J. Kubarych, J. Phys. Chem. A 118, 9853 (2014).

[2] L. M. Kiefer, J. T. King, K. J. Kubarych, Acc. Chem. Res. 48, 1123 (2015).

[3] L. M. Kiefer, K. J. Kubarych, J. Phys. Chem. A 119, 959 (2015).