The photochemistry and dynamics of a model compound of the active site of the [FeFe]hydrogenase enzyme system have been studied on a wide range of time scales using a unique combination of femtosecond time-resolved infrared spectroscopy, nanosecond time-resolved infrared spectroscopy, and steady-state UV-FTIR methods. Using three different solvents, heptane, acetonitrile, and cyanoheptane, we have observed the rapid formation of solvent adduct species from the first solvation shell of the solute following photolysis of a carbonyl ligand and global fitting techniques have been employed to provide new insights into the ultrafast dynamics of this process. In addition, the use of solvent mixtures has enabled the observation of competitive ligand substitution processes at the newly created coordination site on time scales of a few nanoseconds, shedding new light on the chemical behavior of these enzyme models.

Ultrafast transient 2D-IR (T-2D-IR) spectroscopy has been used to study the photolysis products of the [FeFe]hydrogenase enzyme model compound (μ-propanedithiolate)Fe(2)(CO)(6) in heptane solution following irradiation at ultraviolet wavelengths. Observation of coupling patterns between the vibrational modes of the photoproduct species formed alongside examination of the appearance time scales of these signals has uniquely enabled assignment of the photoproduct spectrum to a single pentacarbonyl species. Comparison of the vibrational relaxation rate of the photoproduct with that of the parent is consistent with the formation of a solvent adduct at the vacant coordination site, while anisotropy data in conjunction with density functional theory simulations indicates substitution in an axial rather than equatorial position. No firm evidence of additional short-lived intermediates is seen, indicating that the subsequent chemistry of these species is likely to be strongly defined by the nature of the first solvation shell.

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The vibrational dynamics of (μ-propanedithiolate)Fe(2)(CO)(4)(CN)(2)(2-), a model compound of the active site of the [FeFe]-hydrogenase enzyme, have been examined via ultrafast 2D-IR spectroscopy. The results indicate that the vibrational coupling between the stretching modes of the CO and CN ligands is small and restricted to certain modes but the slow growth of off-diagonal peaks is assigned to population transfer processes occurring between these modes on timescales of 30-40 ps. Analysis of the dynamics in concert with anharmonic density functional theory simulations shows that the presence of CN ligands alters the vibrational relaxation dynamics of the CO modes in comparison to all-carbonyl model systems and suggests that the presence of these ligands in the enzyme may be an important feature in terms of directing the vibrational relaxation mechanism.