8 research outputs found
Photodriven Charge Separation Dynamics in CdSe/ZnS Core/Shell Quantum Dot/Cobaloxime Hybrid for Efficient Hydrogen Production
Photodriven charge-transfer dynamics and catalytic properties
have
been investigated for a hybrid system containing CdSe/ZnS core/shell
quantum dots (QDs) and surface-bound molecular cobaloxime catalysts.
The electron transfer from light-excited QDs to cobaloxime, revealed
by optical transient absorption spectroscopy, takes place with an
average time constant of 105 ps, followed a much slower charge recombination
process with a time constant of ≫3 ns. More interestingly,
we also observed photocatalytic hydrogen generation by this QD/cobaloxime
hybrid system, with >10 000 turnovers of H<sub>2</sub> per
QD in 10 h, using triethanolamine as a sacrificial electron donor.
These results suggest that QD/cobaloxime hybrids succeed in coupling
single-photon events with multielectron redox catalytic reactions,
and such systems could have potential applications in long-lived artificial
photosynthetic devices for fuel generation from sunlight
Ligand Mediation of Vectorial Charge Transfer in Cu(I)diimine Chromophore–Acceptor Dyads
In this work, we present the photoinduced
charge separation dynamics
of four molecular dyads composed of heteroleptic Cu(I)bis(phenanthroline)
chromophores linked directly to the common electron acceptor naphthalene
diimide. The dyads were designed to allow us to (1) detect any kinetic
preference for directionality during photoinduced electron transfer
across the heteroleptic complex and (2) probe the influence of excited-state
flattening on intramolecular charge separation. Singular value decomposition
of ultrafast optical transient absorption spectra demonstrates that
charge transfer occurs with strong directional preference, and charge
separation occurs up to 35 times faster when the acceptor is linked
to the sterically blocking ligand. Further, the charge-separated state
in these dyads is stabilized by polar solvents, resulting in dramatically
longer lifetimes for dyads with minimal substitution about the Cu(I)
center. This unexpected but exciting observation suggests a new approach
to the design of Cu(I)bis(phenanthroline) chromophores that can support
long-lived vectorial charge separation
The Hydrogen Catalyst Cobaloxime: A Multifrequency EPR and DFT Study of Cobaloxime’s Electronic Structure
Solar fuels research aims to mimic photosynthesis and
devise integrated
systems that can capture, convert, and store solar energy in the form
of high-energy molecular bonds. Molecular hydrogen is generally considered
an ideal solar fuel because its combustion is essentially pollution-free.
Cobaloximes rank among the most promising earth-abundant catalysts
for the reduction of protons to molecular hydrogen. We have used multifrequency
EPR spectroscopy at X-band, Q-band, and D-band combined with DFT calculations
to reveal electronic structure and establish correlations among the
structure, surroundings, and catalytic activity of these complexes.
To assess the strength and nature of ligand cobalt interactions, the
BF<sub>2</sub>-capped cobaloxime, Co(dmgBF<sub>2</sub>)<sub>2</sub>, was studied in a variety of different solvents with a range of
polarities and stoichiometric amounts of potential ligands to the
cobalt ion. This allows the differentiation of labile and strongly
coordinating axial ligands for the Co(II) complex. Labile, or weakly
coordinating, ligands such as methanol result in larger <i>g</i>-tensor anisotropy than strongly coordinating ligands such as pyridine.
In addition, a coordination number effect is seen for the strongly
coordinating ligands with both singly ligated LCo(dmgBF<sub>2</sub>)<sub>2</sub> and doubly ligated L<sub>2</sub>Co(dmgBF<sub>2</sub>)<sub>2</sub> . The presence of two strongly coordinating axial ligands
leads to the smallest <i>g</i>-tensor anisotropy. The relevance
of the strength of the axial ligand(s) to the catalytic efficiency
of Co(dmgBF<sub>2</sub>)<sub>2</sub> is discussed. Finally, the influence
of molecular oxygen and formation of Co(III) superoxide radicals LCo(dmgBF<sub>2</sub>)<sub>2</sub>O<sub>2</sub><sup>•</sup> is studied.
The experimental results are compared with a comprehensive set of
DFT calculations on Co(dmgBF<sub>2</sub>)<sub>2</sub> model systems
with various axial ligands. Comparison with experimental values for
the “key” magnetic parameters such as <i>g</i>-tensor and <sup>59</sup>Co hyperfine coupling tensor allows the
determination of the conformation of the axially ligated Co(dmgBF<sub>2</sub>)<sub>2</sub> complexes. The data presented here are vital
for understanding the influence of solvent and ligand coordination
on the catalytic efficiency of cobaloximes
The Hydrogen Catalyst Cobaloxime: A Multifrequency EPR and DFT Study of Cobaloxime’s Electronic Structure
Solar fuels research aims to mimic photosynthesis and
devise integrated
systems that can capture, convert, and store solar energy in the form
of high-energy molecular bonds. Molecular hydrogen is generally considered
an ideal solar fuel because its combustion is essentially pollution-free.
Cobaloximes rank among the most promising earth-abundant catalysts
for the reduction of protons to molecular hydrogen. We have used multifrequency
EPR spectroscopy at X-band, Q-band, and D-band combined with DFT calculations
to reveal electronic structure and establish correlations among the
structure, surroundings, and catalytic activity of these complexes.
To assess the strength and nature of ligand cobalt interactions, the
BF<sub>2</sub>-capped cobaloxime, Co(dmgBF<sub>2</sub>)<sub>2</sub>, was studied in a variety of different solvents with a range of
polarities and stoichiometric amounts of potential ligands to the
cobalt ion. This allows the differentiation of labile and strongly
coordinating axial ligands for the Co(II) complex. Labile, or weakly
coordinating, ligands such as methanol result in larger <i>g</i>-tensor anisotropy than strongly coordinating ligands such as pyridine.
In addition, a coordination number effect is seen for the strongly
coordinating ligands with both singly ligated LCo(dmgBF<sub>2</sub>)<sub>2</sub> and doubly ligated L<sub>2</sub>Co(dmgBF<sub>2</sub>)<sub>2</sub> . The presence of two strongly coordinating axial ligands
leads to the smallest <i>g</i>-tensor anisotropy. The relevance
of the strength of the axial ligand(s) to the catalytic efficiency
of Co(dmgBF<sub>2</sub>)<sub>2</sub> is discussed. Finally, the influence
of molecular oxygen and formation of Co(III) superoxide radicals LCo(dmgBF<sub>2</sub>)<sub>2</sub>O<sub>2</sub><sup>•</sup> is studied.
The experimental results are compared with a comprehensive set of
DFT calculations on Co(dmgBF<sub>2</sub>)<sub>2</sub> model systems
with various axial ligands. Comparison with experimental values for
the “key” magnetic parameters such as <i>g</i>-tensor and <sup>59</sup>Co hyperfine coupling tensor allows the
determination of the conformation of the axially ligated Co(dmgBF<sub>2</sub>)<sub>2</sub> complexes. The data presented here are vital
for understanding the influence of solvent and ligand coordination
on the catalytic efficiency of cobaloximes
Charge Separation Related to Photocatalytic H<sub>2</sub> Production from a Ru–Apoflavodoxin–Ni Biohybrid
The direct creation
of a fuel from sunlight and water via photochemical
energy conversion provides a sustainable method for producing a clean
source of energy. Here we report the preparation of a solar fuel biohybrid
that embeds a nickel diphosphine hydrogen evolution catalyst into
the cofactor binding pocket of the electron shuttle protein, flavodoxin
(Fld). The system is made photocatalytic by linking a cysteine residue
in Fld to a ruthenium photosensitizer. Importantly, the protein environment
enables the otherwise insoluble Ni catalyst to perform photocatalysis
in aqueous solution over a pH range of 3.5–12.0, with optimal
turnover frequency 410 ± 30 h<sup>–1</sup> and turnover
number 620 ± 80 mol H<sub>2</sub>/mol hybrid observed at pH 6.2.
For the first time, a reversible light-induced charge-separated state
involving a Ni(I) intermediate was directly monitored by electron
paramagnetic resonance spectroscopy. Transient optical measurements
reflect two conformational states, with a Ni(I) state formed in ∼1.6
or ∼185 μs that persists for several milliseconds as
a long-lived charge-separated state facilitated by the protein matrix
Photocatalytic Hydrogen Production from Noncovalent Biohybrid Photosystem I/Pt Nanoparticle Complexes
A photocatalytic hydrogen-evolving system based on intermolecular electron transfer between native Photosystem I (PSI) and electrostatically associated Pt nanoparticles is reported. Using cytochrome c<sub>6</sub> as the soluble mediator and ascorbate as the sacrificial electron donor, visible-light-induced H<sub>2</sub> production occurs for PSI/Pt nanoparticle biohybrids at a rate of 244 μmol H<sub>2</sub> (mg chlorophyll)<sup>−1</sup> h<sup>−1</sup> or 21 034 mol H<sub>2</sub> (mole PSI)<sup>−1</sup> h<sup>−1</sup>. These results demonstrate that highly efficient photocatalysis of H<sub>2</sub> can be obtained for a self-assembled, noncovalent complex between PSI and Pt nanoparticles; a molecular wire between the terminal acceptor of PSI, the [4Fe−4S] cluster F<sub>B</sub>, and the nanoparticle is not required. EPR characterization of the electron-transfer reactions in PSI/Pt nanoparticle biohybrids shows blocked electron transfer to flavodoxin, the native acceptor protein of PSI, and presents evidence of low-temperature photogenerated electron transfer between PSI and the Pt nanoparticle. This work demonstrates a feasible strategy for linking molecular catalysts to PSI that takes advantage of electrostatic-directed assembly to mimic acceptor protein binding
Harnessing Intermolecular Interactions to Promote Long-Lived Photoinduced Charge Separation from Copper Phenanthroline Chromophores
Facilitating photoinduced electron transfer (PET) while
minimizing
rapid charge-recombination processes to produce a long-lived charge-separated
(CS) state represents a primary challenge associated with achieving
efficient solar fuel production. Natural photosynthetic systems employ
intermolecular interactions to arrange the electron-transfer relay
in reaction centers and promote a directional flow of electrons. This
work explores a similar tactic through the synthesis and ground- and
excited-state characterization of two Cu(I)bis(phenanthroline) chromophores
with homoleptic and heteroleptic coordination geometries and which
are functionalized with negatively charged sulfonate groups. The addition
of sulfonate groups enables solubility in pure water, and it also
induces assembly with the dicationic electron acceptor methyl viologen
(MV2+) via bimolecular, dynamic electrostatic interactions.
The effect of the sulfonate groups on the ground- and excited-state
properties was evaluated by comparison with the unsulfonated analogues
in 1:1 acetonitrile/water. The excited-state lifetimes for all sulfonated
complexes are similar to what we expect from previous literature,
with the exception of the sulfonated heteroleptic complex whose metal-to-ligand
charge-transfer (MLCT) lifetime in water has two components that are
fit to 10 and 77 ns. For the sulfonated complexes, we detected reduced
MV+• in both solvent environments following MLCT
excitation, but control measurements in 1:1 acetonitrile/water with
the unsulfonated analogues showed no PET to MV2+, indicating
that electrostatically driven supramolecular assemblies of the sulfonated
complexes with MV2+ facilitate the observed PET. Additionally,
the strength of the intermolecular interactions driving the formation
of these assemblies changes drastically with the solvent environment.
In 1:1 acetonitrile/water, PET occurred from both sulfonated complexes
with quantum yields (ΦET) of 2–3% but increased
to a remarkable 98% for the sulfonated heteroleptic complex with a
3 μs CS-state lifetime in water
Mechanistic Evaluation of a Nickel Proton Reduction Catalyst Using Time-Resolved X‑ray Absorption Spectroscopy
We report the light-induced electronic
and geometric changes taking
place in “real time” of a multimolecular [Ru(bpy)<sub>3</sub>]<sup>2+</sup>/[Ni(P<sup>Ph</sup><sub>2</sub>N<sup>Ph</sup><sub>2</sub>)<sub>2</sub>(CH<sub>3</sub>CN)]<sup>2+</sup>/ascorbic
acid photocatalytic system by time-resolved X-ray absorption spectroscopy
(tr-XAS) in the nano- to microsecond time regime. Using tr-XAS allows
us to observe the diffusion-governed electron transfer between the
excited photosensitizer and the nickel(II) proton reduction catalyst
on the nanosecond time scale followed by formation of a transient
distorted tetrahedral Ni(I) intermediate. A 50-fold increase in the
decay lifetime of the Ni(I) species, in the presence of the electron
donor, shows that the favored catalytic pathway occurs through reductive
quenching of the excited photosensitizer followed by electron transfer
to the catalyst. Lack of protonation of the Ni(I) amine groups within
our experimental tr-XAS time window suggests that proton binding is
the rate limiting step for H<sub>2</sub> photocatalysis by this system.
This study is supported by molecular orbital density functional theory
(DFT-MO) calculations providing relevant information for ongoing synthetic
efforts on “DuBois-type” nickel complexes as well as
mechanistic time-resolved understanding of the photoevolution processes
in the catalytic cycle for the rational design of molecular hydrogen-evolving
photocatalysts