Discovery of novel materials for hydrogen gas production has prompted new avenues of research towards developing hydrogen as an alternative to fossil fuel. Optimizing candidate materials’ properties for high hydrogen generation per unit weight, rapid kinetics, and high turnover for catalytic systems are still difficult to attain. To aid in this research, we employ infrared (IR) spectroscopy as a measurement tool to better understand the intrinsic vibrational modes and complex molecular dynamics of di-iron hydrogenase mimics (see Figure) of a light-driven hydrogen-producing catalyst. These measurement modalities permit one to evaluate systems capable of meeting efficient hydrogen production levels. In this work, ultrafast time-resolved infrared and FTIR spectroscopy are applied to measure internal vibrational energy reorganization dynamics and identify specific vibrational modes of model species for the reactive center of the hydrogenase protein in active bacteria. DFT methods have also been employed to help with mode assignments. Through collaboration with Prof. Christopher Stromberg and his students from the Department of Chemistry, Hood College, various CO- and CN-derivatized versions of the Fe-Fe hydrogenase active site have been investigated using these measurement tools. Our measurements have identified specific vibrational modes in the mid-to-far (THz) infrared spectral regions for the different isomeric forms of these mimics. Time-resolved IR measurements, employing UV and visible photolysis, reveals different dynamics including CO-ejection and Fe-Fe bond extension occurs. Current studies include measurement of specifically substituted CN-containing homologs to better understand hydrogen generation reaction yields and transient species.
Time-resolved infrared (TRIR) measurements on the picosecond to microsecond timescales and FTIR transmission spectroscopy in the far-infrared (THz) spectral region are being applied as a means to monitor light-initiated vibrational mode dynamics in model hydrogenase species. Synthesis of the model species is conducted at Hood College using literature methods while THz FTIR spectroscopy and TRIR measurements are conducted at NIST. Theoretical modeling of Raman, IR and transient species’ vibrational modes is conducted at Hood and NIST using Gaussian 03 Density Functional Theory. To date, detailed vibrational mode assignments for the Fe-Fe(CO)6 ethyl-bridged and propyl-bridged (see Figure b) and c) below) species have been performed and published. Studies of these same samples under UV and visible excitation conditions have helped identify transiently vibrationally hot species, CO-loss products and electronically excited states that extend the Fe-Fe bond distance to create strongly absorbing transient state in room temperature n-hexane solution.
Molecular structures and mimics for the active site of Di-Iron Hydrogenase. a) active site of bacterial protein connected through an Fe-S linkage, b) hexacarbonyl mimic with a propyl disulphide bridge and c) same as b) with an ethyl bridge. Similar CN-containing compounds are also under investigation.
Examples of TRIR spectra in the CO-stretching region (near 5 microns) are shown in the Figure below. Evolution of transient IR spectra taken several picoseconds after excitation through 750 ps time delays are shown.
TRIR spectra for the propyl-bridged Fe-Fe(CO)6-Hydrogenase model compound using a) 532 nm pump in n-hexane at 293 K, b) 355 nm pump, IR probe spectra, and c) steady-state FT-IR spectrum of the compound for reference.
Lead Organizational Unit:pml
Prof. Christopher Stromber, Hood College, Frederick MD
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