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– (<b>1</b>) and [Fe₂(μ-S₂C₂H₄)­(CO)₄(CN)₂]^2– (<b>2</b>). 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

Time-Resolved Vibrational Spectroscopy

Time-Resolved Vibrational Spectroscop

Linkage Isomerization via Geminate Cage or Bimolecular Mechanisms: Time-Resolved Investigations of an Organometallic Photochrome

The extent of the photoinitiated linkage isomerization of dicarbonyl­(3-cyanomethylpyridine-κ<i>N</i>)­(η⁵-methylcyclopentadienyl)­manganese (<b>4</b>) to dicarbonyl­(3-cyano-κ<i>N</i>-methylpyridine)­(η⁵-methylcyclopentadienyl)manganese (<b>5</b>) was examined by time-resolved infrared spectroscopy on picosecond to microsecond time scales in room temperature isooctane to determine the extent the isomerization occurs as a geminate cage rearrangement. We previously reported that a substantial part of the conversion between <b>4</b> and <b>5</b> must be a bimolecular reaction between a solvent coordinated dicarbonyl­(η⁵-methylcyclopentadienyl)­manganese (<b>3</b>) and uncoordinated 3-cyanomethylpyridine. For the purpose of designing a molecular device, it would be desirable for the photoisomerization to occur in a geminate cage reaction, because the faster the isomerization, the less opportunity for side reactions to occur. In this study, assignments of transients are identified by comparison with transients observed for model reactions. Within 100 μs after photolysis of <b>4</b> in isooctane, no <b>5</b> is observed. Instead, the solvent coordinated <b>3</b> is observed within 25 ps after irradiation. The formation of <b>5</b> is observed only in the presence of 9 mM 3-cyanomethylpyridine but not until 10–50 μs after irradiation of <b>4</b>. Within the limits of detection, these results indicate the conversion of <b>4</b> to <b>5</b> occurs exclusively via a bimolecular reaction of 3-cyanomethylpyridine with solvent coordinated <b>3</b> and not a geminate cage reaction between 3-cyanomethylpyridine and the dicarbonyl­(η⁵-methylcyclopentadienyl)­manganese fragment

Charge Carrier Dynamics and Mobility Determined by Time-Resolved Terahertz Spectroscopy on Films of Nano-to-Micrometer-Sized Colloidal Tin(II) Monosulfide

Tin­(II) monosulfide (SnS) is a semiconductor material with an intermediate band gap, high absorption coefficient in the visible range, and earth abundant, nontoxic constituent elements. For these reasons, SnS has generated much interest for incorporation into optoelectronic devices, but little is known concerning the charge carrier dynamics, especially as measured by optical techniques. Here, as opposed to prior studies of vapor deposited films, phase-pure colloidal SnS was synthesized by solution chemistry in three size regimes, ranging from nanometer- to micrometer-scale (SnS small nanoparticles, SnS medium 2D nanosheets, and SnS large 2D μm-sheets), and evaluated by time-resolved terahertz spectroscopy (TRTS); an optical, noncontact probe of the photoconductivity. Dropcast films of the SnS colloids were studied by TRTS and compared to both thermally annealed films and dispersed suspensions of the same colloids. TRTS results revealed that the micrometer-scale SnS crystals and all of the annealed films undergo decay mechanisms during the first 200 ps following photoexcitation at 800 nm assigned to hot carrier cooling and carrier trapping. The charge carrier mobility of both the dropcast and annealed samples depends strongly on the size of the constituent colloids. The mobility of the SnS colloidal films, following the completion of the initial decays, ranged from 0.14 cm²/V·s for the smallest SnS crystals to 20.3 cm²/V·s for the largest. Annealing the colloidal films resulted in a ∼20% improvement in mobility for the large SnS 2D μm-sheets and a ∼5-fold increase for the small nanoparticles and medium nanosheets

Time-Resolved Infrared Studies of a Trimethylphosphine Model Derivative of [FeFe]-Hydrogenase

Model compounds that structurally mimic the hydrogen-producing active site of [FeFe]-hydrogenases have been studied to explore potential ground-state electronic structure effects on reaction mechanisms compared to hexacarbonyl derivatives. The time-dependent behavior of Fe₂(μ-S₂C₃H₆)­(CO)₄(PMe)₂ (<b>A</b>) in room temperature <i>n</i>-heptane and acetonitrile solutions was examined using various ultrafast UV and visible excitation pulses with broadband IR-probe spectroscopy of the carbonyl (CO) stretching region. Ground- and excited-state electronic and CO-stretching mode vibrational properties of the possible isomers of <b>A</b> were also examined using density functional theory (DFT) computations. In <i>n</i>-heptane, 355 and 532 nm excitation resulted in short-lived (135 ± 74 ps) bands assigned to excited-state, CO-loss photoproducts. These bands decay away, forming new long-lived absorptions that are likely a mixture of isomers of both three-CO and four-CO ground-state isomers. These new bands grow in with a time scale of 214 ± 119 ps and persist for more than 100 ns. In acetonitrile, similar results are seen with a 532 nm pump, but the 355 nm data lack evidence of the longer-lived bands. In either solvent, the 266 nm pump data seem to also lack longer-lived bands, but the intensities are significantly lower in this data, making firm conclusions more difficult. We suggest that these wavelength-dependent excitation dynamics significantly alter potential mechanisms and efficiencies for light-driven catalysis

Static and Time-Resolved Terahertz Measurements of Photoconductivity in Solution-Deposited Ruthenium Dioxide Nanofilms

Thin-film ruthenium dioxide (RuO₂) is a promising alternative material as a conductive electrode in electronic applications because its rutile crystalline form is metallic and highly conductive. Herein, a solution-deposition multilayer technique is employed to fabricate ca. 70 ± 20 nm thick films (nanoskins), and terahertz spectroscopy is used to determine their photoconductive properties. Upon calcining at temperatures ranging from 373 to 773 K, nanoskins undergo a transformation from insulating (localized charge transport) behavior to metallic behavior. Terahertz time-domain spectroscopy (THz-TDS) indicates that nanoskins attain maximum static conductivity when calcined at 673 K (σ = 1030 ± 330 S·cm^–1). Picosecond time-resolved terahertz spectroscopy using 400 and 800 nm excitation reveals a transition to metallic behavior when calcined at 523 K. For calcine temperatures less than 523 K, the conductivity increases following photoexcitation (Δ<i>E</i> < 0) while higher calcine temperatures yield films composed of crystalline, rutile RuO₂ and the conductivity decreases (Δ<i>E</i> > 0) following photoexcitation

Time-Resolved Vibrational Spectroscopy of [FeFe]-Hydrogenase Model Compounds

Model compounds have been found to structurally mimic the catalytic hydrogen-producing active site of Fe–Fe hydrogenases and are being explored as functional models. The time-dependent behavior of Fe₂(μ-S₂C₃H₆)­(CO)₆ and Fe₂(μ-S₂C₂H₄)­(CO)₆ is reviewed and new ultrafast UV- and visible-excitation/IR-probe measurements of the carbonyl stretching region are presented. Ground-state and excited-state electronic and vibrational properties of Fe₂(μ-S₂C₃H₆)­(CO)₆ were studied with density functional theory (DFT) calculations. For Fe₂(μ-S₂C₃H₆)­(CO)₆ excited with 266 nm, long-lived signals (τ = 3.7 ± 0.26 μs) are assigned to loss of a CO ligand. For 355 and 532 nm excitation, short-lived (τ = 150 ± 17 ps) bands are observed in addition to CO-loss product. Short-lived transient absorption intensities are smaller for 355 nm and much larger for 532 nm excitation and are assigned to a short-lived photoproduct resulting from excited electronic state structural reorganization of the Fe–Fe bond. Because these molecules are tethered by bridging disulfur ligands, this extended di-iron bond relaxes during the excited state decay. Interestingly, and perhaps fortuitously, the time-dependent DFT-optimized exited-state geometry of Fe₂(μ-S₂C₃H₆)­(CO)₆ with a semibridging CO is reminiscent of the geometry of the Fe₂S₂ subcluster of the active site observed in Fe–Fe hydrogenase X-ray crystal structures. We suggest these wavelength-dependent excitation dynamics could significantly alter potential mechanisms for light-driven catalysis