Photodynamics of Asymmetric Di-Iron-Cyano Hydrogenases Examined by Time-Resolved Mid-Infrared Spectroscopy

Two anionic asymmetric Fe-Fe hydrogenase model compounds containing a single cyano (CN) and five carboxyl (CO) ligands, [Et₄N][Fe₂(μ-S₂C₃H₆)(CO)₅(CN)₁] and [Et₄N][Fe₂(μ-S₂C₂H₄)(CO)₅(CN)₁], dissolved in room-temperature acetonitrile, are examined. The molecular asymmetry affects the redox potentials of the central iron atoms, thus changing the photophysics and possible catalytic properties of the compounds. Femtosecond ultraviolet excitation with mid-infrared probe spectroscopy of the model compounds was employed to better understand the ultrafast dynamics of the enzyme-active site. Continuous ultraviolet lamp excitation with Fourier transform infrared (FTIR) spectroscopy was also used to explore stable product formation on the second timescale. For both model compounds, two timescales are observed; a 20-30 ps decay and the formation of a long-lived photoproduct. The picosecond decay is assigned to vibrational cooling and rotational dynamics, while the residual spectra remain for up to 300 ps, suggesting the formation of new photoproducts. Static FTIR spectroscopy yielded a different stable photoproduct than that observed on the ultrafast timescale. Density functional theory calculations simulated photoproducts for CO-loss and CN-loss isomers, and the resulting photoproduct spectra suggest that the picosecond transients arise from a complex mixture of isomerization after CO-loss, while dimerization and formation of a CN-containing Fe-CO-Fe bridged species are also considered.

Photodynamics of [FeFe]-Hydrogenase Model Compounds with Bidentate Heterocyclic Ligands

Two asymmetrically structured model compounds for the hydrogen-generating [Fe-Fe]-hydrogenase active site were investigated to determine the ultrafast photodynamics, structural intermediates, and photoproducts compared to more common symmetric di-iron species. The bidentate-ligand-containing compounds studied were Fe2(μ-S2C3H6)(CO)4(bipy), 1, and Fe2(μ-S2C3H6)(CO)4(phen), 2, in dilute room temperature acetonitrile solution and low-temperature 2Me-THF matrix isolation using static FTIR difference and time-resolved infrared spectroscopic methods (TRIR). Ultraviolet-visible spectra were also compared to time-dependent density functional theory (TD-DFT) to ascertain the orbital origins of long wavelength electronic absorption features. The spectroscopic evidence supports the conclusions that only a propyl-bridge flip occurs in low-temperature matrix, while early time CO ejection leads to the formation of solvated isomeric species on the 25 ps time scale in room temperature solution.

Ultrafast Photodynamics of Cyano-Functionalized [FeFe] Hydrogenase Model Compounds

[FeFe] hydrogenases are efficient enzymes that produce hydrogen gas under mild conditions. Synthetic model compounds containing all CO or mixed CO/PMe3 ligands were previously studied by us and others with ultrafast ultraviolet or visible pump-infrared probe spectroscopy in an effort to better understand the function and interactions of the active site with light. Studies of anionic species containing cyano groups, which more closely match the biological active site, have been elusive. In this work, two model compounds dissolved in room-temperature acetonitrile solution were examined: [Fe2(μ-S2C3H6)(CO)4(CN)2]2- (1) and [Fe2(μ-S2C2H4)(CO)4(CN)2]2- (2). These species exhibit long-lived transient signals consistent with loss of one CO ligand with potential isomerization of newly formed ground electronic state photoproducts, as previously observed with all-CO and CO/PMe3-containing models. We find no evidence for fast (ca. 150 ps) relaxation seen in the all-CO and CO/PMe3 compounds because of the absence of the metal-to-metal charge transfer band in the cyano-functionalized models. These results indicate that incorporation of cyano ligands may significantly alter the electronic properties and photoproducts produced immediately after photoexcitation, which may influence the catalytic activity of model compounds when attached to photosensitizers.

Femtosecond Laser Eyewear Protection: Measurements and Precautions for Amplified High Power Applications

Ultrafast lasers have become increasingly important as research tools in laboratories and commercial enterprises suggesting laser safety, personal protection and awareness become ever more important. Laser safety eyewear are typically rated by their optical densities (OD) over various spectral ranges, but these measurements are usually made using low power, large beam size, and continuous beam conditions. These measurement scenarios are vastly different than the high power, small beam size, and pulsed laser beam conditions where ultrafast lasers have extremely high peak powers and broad spectra due to the short pulse durations. Many solid-state lasers are also tunable over a broad wavelength range, further complicating the selection of adequate laser safety eyewear. Eighteen laser eyewear filter samples were tested under real-world conditions using a Ti:Sapphire regenerative amplifier with output pulses centered at 800 nm running from 2 Hz to 1 KHz repetition rate. The typical maximum peak laser irrandiance employed was ca. 3 TW/cm² (800 nm wavelength, 450 uJ/pulse with 80 fs FWHM pulse duration) or less when damage occurred, depending on the sample. While many samples maintained their integrity under these test conditions, many plastic samples showed signs of failure which reduced their OD, in some cases transmitting 4 to 5 orders of magnitude higher than expected. In general, glass filters performed significantly better than plastic filters, exhibiting less physical damage to the substrate and less absorber degradation.

Photochemical Dynamics of a Trimethyl-Phosphine Derivatized [FeFe]-Hydrogenase Model Compound

Though there have been many studies on photosensitizers coupled to model complexes of the [FeFe]-hydrogenases, few have looked at how the models react upon exposure to light. To extract photoreaction information, ultrafast time-resolved UV/visible pump, IR probe spectroscopy was performed on Fe₂(μ-S₂C₂H₄)(CO)₄(PMe₃)₂ (2b) dissolved in heptane and acetonitrile and the photochemical dynamics were determined. Excitation with 532 and 355 nm light produces bleaches and new absorptions that decay to half their original intensity with time constants of 300 ± 120 ps and 380 ± 210 ps in heptane and acetonitrile, respectively. These features persist to the microsecond timescale. The dynamics of 2b are assigned to formation of an initial set of photoproducts, which were a mixture of excited-state tricarbonyl isomers. These isomers decay into another set of long-lived photoproducts in which approximately half the excited-state tricarbonyl isomers recombine with CO to form another complex mixture of tricarbonyl and tetracarbonyl isomers.

Femtosecond Laser Eyewear Protection: Measurements and Precautions

Ultrafast laser systems are becoming more widespread throughout the research and industrial communities yet eye protection for these high power, bright pulsed sources still require scrupulous characterization and testing before use. Femtosecond lasers, with pulses naturally possessing broad-bandwidth and high average power with variable repetition rate, can exhibit spectral side-bands and subtly changing center wavelengths, which may unknowingly affect eyewear safety protection. Pulse spectral characterization and power diagnostics are presented for a 80 MHz, Ti⁺³:Sapphire, ≈ 800 nm, ≈40 femtosecond oscillator system. Power and spectral transmission for 22 test samples are measured to determine whether they fall within manufacturer specifications.