15 research outputs found
Photoinduced Electron and Energy Transfer in a Molecular Triad Featuring a Fullerene Redox Mediator
In
order to investigate the possibility of a fullerene acting as
an electron and/or singlet energy relay between a donor chromophore
and an acceptor, a triad consisting of a fullerene (C<sub>60</sub>) covalently linked to both a porphyrin energy and electron donor
(P) and a Ī²-tetracyanoporphyrin energy and electron acceptor
(CyP) was synthesized. Steady state and time-resolved spectroscopic
investigations show that the porphyrin first excited singlet state
donates singlet excitation and an electron to the fullerene and also
donates singlet excitation to the CyP. All three processes differ
in rate constant by factors of ā¤1.3, and all are much faster
than the decay of <sup>1</sup>PāC<sub>60</sub>āCyP by
unichromophoric processes. The fullerene excited state accepts an
electron from P and donates singlet excitation energy to CyP. The
P<sup>ā¢+</sup>āC<sub>60</sub><sup>ā¢ā</sup>āCyP charge-separated state transfers an electron to CyP to
produce a final P<sup>ā¢+</sup>āC<sub>60</sub>āCyP<sup>ā¢ā</sup> state. The same state is formed from PāC<sub>60</sub>ā<sup>1</sup>CyP. Overall, the final charge-separated
state is formed with a quantum yield of 85% in benzonitrile, and has
a lifetime of 350 ps. Rate constants for formation and quantum yields
of all intermediate states were estimated from results for the triad
and several model compounds. Interestingly, the intermediate P<sup>ā¢+</sup>āC<sub>60</sub><sup>ā¢ā</sup>āCyP
charge-separated state has a lifetime of 660 ps. It is longer lived
than the final state in spite of stronger coupling of the radical
ions. This is ascribed to the fact that recombination lies far into
the inverted region of the Marcus rate constant vs thermodynamic driving
force relationship
Hole Mobility in Porphyrin- and Porphyrin-Fullerene Electropolymers
Charge transport within films of several new types of
electropolymerized porphyrin and porphyrin-fullerene dyad polymers
was studied in order to obtain information on the suitability of these
organic semiconductors for applications in solar energy conversion,
sensor devices, etc. The films, prepared by electropolymerization
on a conductive substrate, were immersed in acetonitrile and studied
using chronocoulometric and cyclic voltammetric electrochemical methods.
The charge diffusion coefficients were found to be dependent upon
the electrolytic medium. Electrolyte anion size plays a significant
role in determining the rate of migration of charge through the polymers,
demonstrating that migration of positive charge is accompanied by
migration of negative counterions. Bulkier anions markedly decrease
the charge diffusion coefficient. This strong dependence suggests
that anion mobility is the rate-limiting process for diffusional charge
transport within the porphyrin polymer films and that the largest
rates obtained are lower limits to the intrinsic cation mobility.
With electrolytes containing the relatively small perchlorate anion,
charge diffusion coefficients of the porphyrin polymers were similar
to those reported for polyaniline under acidic conditions. The charge
diffusion coefficient for a zinc porphyrin polymer was found to decrease
2 orders of magnitude in the presence of pyridine, suggesting that
metal-containing porphyrins polymer films may have sensor applications.
Cation (hole) mobilities previously reported in the literature for
porphyrin-containing polymers with chemical structures quite different
from those investigated here were much smaller than those found for
the polymers in this study, but further investigation suggests that
the differences are due to choice of electrode size and material
All-Photonic Multifunctional Molecular Logic Device
Photochromes are photoswitchable, bistable chromophores which, like transistors, can implement binary logic operations. When several photochromes are combined in one molecule, interactions between them such as energy and electron transfer allow design of simple Boolean logic gates and more complex logic devices with all-photonic inputs and outputs. Selective isomerization of individual photochromes can be achieved using light of different wavelengths, and logic outputs can employ absorption and emission properties at different wavelengths, thus allowing a single molecular species to perform several different functions, even simultaneously. Here, we report a molecule consisting of three linked photochromes that can be configured as AND, XOR, INH, half-adder, half-subtractor, multiplexer, demultiplexer, encoder, decoder, keypad lock, and logically reversible transfer gate logic devices, all with a common initial state. The system demonstrates the advantages of light-responsive molecules as multifunctional, reconfigurable nanoscale logic devices that represent an approach to true molecular information processing units
Recommended from our members
Artificial Photosynthetic Reaction Center Exhibiting Acid-Responsive Regulation of Photoinduced Charge Separation
Nonphotochemical
quenching (NPQ) is a photoprotective regulatory
mechanism employed by many photosynthetic organisms to dynamically
modulate energy flow within the photosynthetic apparatus in response
to fluctuating light conditions. Activated by decreases in lumen pH
produced during periods of high photon flux, NPQ induces rapid thermal
dissipation of excess excitation energy. As a result, the rate of
charge separation (CS) decreases, thereby limiting the accumulation
of potentially deleterious reactive intermediates and byproducts.
Herein, a molecular triad that functionally mimics the effects of
NPQ associated with an artificial photosynthetic reaction center is
described. Steady-state absorption and emission, time-resolved fluorescence,
and transient absorption spectroscopies have been used to demonstrate
a 1 order of magnitude reduction in the CS quantum yield via reversible
protonation of an excited-state-quenching molecular switch moiety.
As in the natural system, the populations of unquenched and quenched
states and therefore the overall yields of CS were found to be dependent
on acid concentration
Marcus Bell-Shaped Electron Transfer Kinetics Observed in an Arrhenius Plot
The Marcus theory of electron transfer
predicts a bell-shaped dependence
of the reaction rate on the reaction free energy. The top of the āinverted
parabolaā corresponds to zero activation barrier when the electron-transfer
reorganization energy and the reaction free energy add up to zero.
Although this point has traditionally been reached by altering the
chemical structures of donors and acceptors, the theory suggests that
it can also be reached by varying other parameters of the system including
temperature. We find here dramatic evidence of this phenomenon from
experiments on a fullereneāporphyrin dyad. Following photoinduced
electron transfer, the rate of charge recombination shows a bell-shaped
dependence on the inverse temperature, first increasing with cooling
and then decreasing at still lower temperatures. This non-Arrhenius
rate law is a result of a strong, approximately hyperbolic temperature
variation of the reorganization energy and the reaction free energy.
Our results provide potentially the cleanest confirmation of the Marcus
energy gap law so far since no modification of the chemical structure
is involved
Photonic Modulation of Electron Transfer with Switchable Phase Inversion
Photochromes may be reversibly photoisomerized between
two metastable
states and their properties can influence, and be influenced by, other
chromophores in the same molecule through energy or electron transfer.
In the photochemically active molecular tetrad described here, a porphyrin
has been covalently linked to a fullerene electron acceptor, a quinoline-derived
dihydroindolizine photochrome, and a dithienylethene photochrome.
The porphyrin first excited singlet state undergoes photoinduced electron
transfer to the fullerene to generate a charge-separated state. The
quantum yield of charge separation is modulated by the two photochromes:
one isomer of each quenches the porphyrin excited state, reducing
the quantum yield of electron transfer to near zero. Interestingly,
when the molecule is illuminated with white light, the quantum yield <i>decreases</i> as the white light intensity is increased, generating
an out-of-phase response of the quantum yield to white light. However,
when the same experiment is performed in the presence of additional,
steady-state UV illumination, a phase inversion occurs. The quantum
yield of electron transfer now <i>increases</i> with increasing
white light intensity. Such effects illustrate emergent complexity
in a relatively simple system and could find applications in molecular
logic, photochemical labeling and drug delivery, and photoprotection
for artificial photosynthetic molecules. The photochemistry leading
to this behavior is discussed
A De Novo Designed 2[4Fe-4S] Ferredoxin Mimic Mediates Electron Transfer
[Fe-S]
clusters, natureās modular electron transfer units,
are often arranged in chains that support long-range electron transfer.
Despite considerable interest, the design of biomimetic artificial
systems emulating multicluster-binding proteins, with the final goal
of integrating them in man-made oxidoreductases, remains elusive.
Here, we report a novel bis-[4Fe-4S] cluster binding protein, DSD-Fdm,
in which the two clusters are positioned within a distance of 12 Ć
,
compatible with the electronic coupling necessary for efficient electron
transfer. The design exploits the structural repeat of coiled coils
as well as the symmetry of the starting scaffold, a homodimeric helical
protein (DSD). In total, eight hydrophobic residues in the core of
DSD were replaced by eight cysteine residues that serve as ligands
to the [4Fe-4S] clusters. Incorporation of two [4Fe-4S] clusters proceeds
with high yield. The two [4Fe-4S] clusters are located in the hydrophobic
core of the helical bundle as characterized by various biophysical
techniques. The secondary structure of the apo and holo proteins is
conserved; further, the incorporation of clusters results in stabilization
of the protein with respect to chemical denaturation. Most importantly,
this de novo designed protein can mimic the function of natural ferredoxins:
we show here that reduced DSD-Fdm transfers electrons to cytochrome <i>c</i>, thus generating the reduced cyt <i>c</i> stoichiometrically
Artificial Photosynthetic Reaction Center with a Coumarin-Based Antenna System
In photosynthesis, sunlight is absorbed
mainly by antenna chromophores
that transfer singlet excitation energy to reaction centers for conversion
to useful electrochemical energy. Antennas may likewise be useful
in artificial photosynthetic systems that use sunlight to make fuels
or electricity. Here, we report the synthesis and spectroscopic properties
of a molecular hexad comprising two porphyrin moieties and four coumarin
antenna chromophores, all organized by a central hexaphenylbenzene
core. Light absorbed by any of the coumarins is transferred to a porphyrin
on the 1ā10 ps time scale, depending on the site of initial
excitation. The quantum yield of singlet energy transfer is 1.0. The
energy transfer rate constants are consistent with transfer by the
FoĢrster dipoleādipole mechanism. A pyridyl-bearing fullerene
moiety self-assembles to the form of the hexad containing zinc porphyrins
to yield an antennaāreaction center complex. In the resulting
heptad, energy transfer to the porphyrins is followed by photoinduced
electron transfer to the fullerene with a time constant of 3 ps. The
resulting P<sup>ā¢+</sup>āC<sub>60</sub><sup>ā¢ā</sup> charge-separated state is formed with an overall quantum yield of
1.0 and decays with a time constant of 230 ps in 1,2-difluorobenzene
as the solvent
Photochemical Synthesis of a Water Oxidation Catalyst Based on Cobalt Nanostructures
New cobalt-based nanocomposites have been prepared by photoreduction of Co<sup>2+</sup> salts to generate cobalt nanoparticles deposited on carbon-based materials such as nanocyrstalline diamond and carbon felt. Spontaneous air oxidation converts the metal to Co<sub>2</sub>O<sub>3</sub> which has been tested as a water oxidation catalyst. This work demonstrates that the cobalt oxide nanostructures can be deposited on various carbon surfaces and can catalyze the four-electron oxidation of water to oxygen under anodic bias
Catalytic Turnover of [FeFe]-Hydrogenase Based on Single-Molecule Imaging
Hydrogenases catalyze the interconversion of protons and hydrogen according to the reversible reaction: 2H<sup>+</sup> + 2e<sup>ā</sup> ā H<sub>2</sub> while using only the earth-abundant metals nickel and/or iron for catalysis. Due to their high activity for proton reduction and the technological significance of the H<sup>+</sup>/H<sub>2</sub> half reaction, it is important to characterize the catalytic activity of [FeFe]-hydrogenases using both biochemical and electrochemical techniques. Following a detailed electrochemical and photoelectrochemical study of an [FeFe]-hydrogenase from Clostridium acetobutylicum (<i>Ca</i>HydA), we now report electrochemical and single-molecule imaging studies carried out on a catalytically active hydrogenase preparation. The enzyme <i>Ca</i>HydA, a homologue (70% identity) of the [FeFe]-hydrogenase from Clostridium pasteurianum, CpI, was adsorbed to a negatively charged, self-assembled monolayer (SAM) for investigation by electrochemical scanning tunneling microscopy (EC-STM) techniques and macroscopic electrochemical measurements. The EC-STM imaging revealed uniform surface coverage with sufficient stability to undergo repeated scanning with a STM tip as well as other electrochemical investigations. Cyclic voltammetry yielded a characteristic cathodic hydrogen production signal when the potential was scanned sufficiently negative. The direct observation of the single enzyme distribution on the Au-SAM surface coupled with macroscopic electrochemical measurements obtained from the same electrode allowed the evaluation of a turnover frequency (TOF) as a function of potential for single [FeFe]-hydrogenase molecules