8 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 β-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 ≤1.3, and all are much faster
than the decay of <sup>1</sup>P–C<sub>60</sub>–CyP by
unichromophoric processes. The fullerene excited state accepts an
electron from P and donates singlet excitation energy to CyP. The
P<sup>•+</sup>–C<sub>60</sub><sup>•–</sup>–CyP charge-separated state transfers an electron to CyP to
produce a final P<sup>•+</sup>–C<sub>60</sub>–CyP<sup>•–</sup> state. The same state is formed from P–C<sub>60</sub>–<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>•+</sup>–C<sub>60</sub><sup>•–</sup>–CyP
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
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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
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
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 “inverted
parabola” 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 fullerene–porphyrin 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
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 1–10 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
Förster dipole–dipole mechanism. A pyridyl-bearing fullerene
moiety self-assembles to the form of the hexad containing zinc porphyrins
to yield an antenna–reaction 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>•+</sup>–C<sub>60</sub><sup>•–</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
Controlling Surface Defects and Photophysics in TiO<sub>2</sub> Nanoparticles
Titanium dioxide (TiO<sub>2</sub>) is widely used for photocatalysis
and solar cell applications, and the electronic structure of bulk
TiO<sub>2</sub> is well understood. However, the surface structure
of nanoparticulate TiO<sub>2</sub>, which has a key role in properties
such as solubility and catalytic activity, still remains controversial.
Detailed understanding of surface defect structures may help explain
reactivity and overall materials performance in a wide range of applications.
In this work we address the solubility problem and surface defects
control on TiO<sub>2</sub> nanoparticles. We report the synthesis
and characterization of ∼4 nm TiO<sub>2</sub> anatase spherical
nanoparticles that are soluble and stable in a wide range of organic
solvents and water. By controlling the temperature during the synthesis,
we are able to tailor the density of defect states on the surface
of the TiO<sub>2</sub> nanoparticles without affecting parameters
such as size, shape, core crystallinity, and solubility. The morphology
of both kinds of nanoparticles was determined by TEM. EPR experiments
were used to characterize the surface defects, and transient absorption
measurements demonstrate the influence of the TiO<sub>2</sub> defect
states on photoinduced electron transfer dynamics
Spectroscopic Analysis of a Biomimetic Model of Tyr<sub>Z</sub> Function in PSII
Using
natural photosynthesis as a model, bio-inspired constructs
for fuel generation from sunlight are being developed. Here we report
the synthesis and time-resolved spectroscopic analysis of a molecular
triad in which a porphyrin electron donor is covalently linked to
both a cyanoporphyrin electron acceptor and a benzimidazole–phenol
model for the Tyr<sub>Z</sub>-D<sub>1</sub>His190 pair of PSII. A
dual-laser setup enabled us to record the ultrafast kinetics and long-living
species in a single experiment. From this data, the photophysical
relaxation pathways were elucidated for the triad and reference compounds.
For the triad, quenching of the cyanoporphyrin singlet excited state
lifetime was interpreted as photoinduced electron transfer from the
porphyrin to the excited cyanoporphyrin. In contrast to a previous
study of a related molecule, we were unable to observe subsequent
formation of a long-lived charge separated state involving the benzimidazole–phenol
moiety. The lack of detection of a long-lived charge separated state
is attributed to a change in energetic landscape for charge separation/recombination
due to small differences in structure and solvation of the new triad
Modulating Short Wavelength Fluorescence with Long Wavelength Light
Two
molecules in which the intensity of shorter-wavelength fluorescence
from a strong fluorophore is modulated by longer-wavelength irradiation
of an attached merocyanine–spirooxazine reverse photochromic
moiety have been synthesized and studied. This unusual fluorescence
behavior is the result of quenching of fluorophore fluorescence by
the thermally stable, open, zwitterionic form of the spirooxazine,
whereas the photogenerated closed, spirocyclic form has no effect
on the fluorophore excited state. The population ratio of the closed
and open forms of the spirooxazine is controlled by the intensity
of the longer-wavelength modulated light. Both square wave and sine
wave modulation were investigated. Because the merocyanine–spirooxazine
is an unusual reverse photochrome with a thermally stable long-wavelength
absorbing form and a short-wavelength absorbing photogenerated isomer
with a very short lifetime, this phenomenon does not require irradiation
of the molecules with potentially damaging ultraviolet light, and
rapid modulation of fluorescence is possible. Molecules demonstrating
these properties may be useful in fluorescent probes, as their use
can discriminate between probe fluorescence and various types of adventitious
“autofluorescence” from other molecules in the system
being studied