7 research outputs found
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/PMe<sub>3</sub> 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:
[Fe<sub>2</sub>(μ-S<sub>2</sub>C<sub>3</sub>H<sub>6</sub>)Â(CO)<sub>4</sub>(CN)<sub>2</sub>]<sup>2–</sup> (<b>1</b>) and
[Fe<sub>2</sub>(μ-S<sub>2</sub>C<sub>2</sub>H<sub>4</sub>)Â(CO)<sub>4</sub>(CN)<sub>2</sub>]<sup>2–</sup> (<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/PMe<sub>3</sub>-containing models. We find no evidence for
fast (ca. 150 ps) relaxation seen in the all-CO and CO/PMe<sub>3</sub> 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
Linkage Isomerization via Geminate Cage or Bimolecular Mechanisms: Time-Resolved Investigations of an Organometallic Photochrome
The extent of the photoinitiated
linkage isomerization of dicarbonylÂ(3-cyanomethylpyridine-κ<i>N</i>)Â(η<sup>5</sup>-methylcyclopentadienyl)Âmanganese
(<b>4</b>) to dicarbonylÂ(3-cyano-κ<i>N</i>-methylpyridine)Â(η<sup>5</sup>-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 dicarbonylÂ(η<sup>5</sup>-methylcyclopentadienyl)Âmanganese
(<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 dicarbonylÂ(η<sup>5</sup>-methylcyclopentadienyl)Âmanganese
fragment
Charge Carrier Dynamics and Mobility Determined by Time-Resolved Terahertz Spectroscopy on Films of Nano-to-Micrometer-Sized Colloidal Tin(II) Monosulfide
TinÂ(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 cm<sup>2</sup>/V·s for
the smallest SnS crystals to 20.3 cm<sup>2</sup>/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 Fe<sub>2</sub>(μ-S<sub>2</sub>C<sub>3</sub>H<sub>6</sub>)Â(CO)<sub>4</sub>(PMe)<sub>2</sub> (<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 (RuO<sub>2</sub>) 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<sup>–1</sup>). 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
RuO<sub>2</sub> 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 Fe<sub>2</sub>(μ-S<sub>2</sub>C<sub>3</sub>H<sub>6</sub>)Â(CO)<sub>6</sub> and Fe<sub>2</sub>(μ-S<sub>2</sub>C<sub>2</sub>H<sub>4</sub>)Â(CO)<sub>6</sub> 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 Fe<sub>2</sub>(μ-S<sub>2</sub>C<sub>3</sub>H<sub>6</sub>)Â(CO)<sub>6</sub> were studied with density functional theory (DFT) calculations.
For Fe<sub>2</sub>(μ-S<sub>2</sub>C<sub>3</sub>H<sub>6</sub>)Â(CO)<sub>6</sub> excited with 266 nm, long-lived signals (Ï„
= 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 Fe<sub>2</sub>(μ-S<sub>2</sub>C<sub>3</sub>H<sub>6</sub>)Â(CO)<sub>6</sub> with a semibridging CO is reminiscent
of the geometry of the Fe<sub>2</sub>S<sub>2</sub> 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