42 research outputs found
Distinguishing the roles of energy funnelling and delocalization in photosynthetic light harvesting
Photosynthetic complexes improve the transfer of excitation energy from
peripheral antennas to reaction centers in several ways. In particular, a
downward energy funnel can direct excitons in the right direction, while
coherent excitonic delocalization can enhance transfer rates through the
cooperative phenomenon of supertransfer. However, isolating the role of purely
coherent effects is difficult because any change to the delocalization also
changes the energy landscape. Here, we show that the relative importance of the
two processes can be determined by comparing the natural light-harvesting
apparatus with counterfactual models in which the delocalization and the energy
landscape are altered. Applied to the example of purple bacteria, our approach
shows that although supertransfer does enhance the rates somewhat, the
energetic funnelling plays the decisive role. Because delocalization has a
minor role (and is sometimes detrimental), it is most likely not adaptive,
being a side-effect of the dense chlorophyll packing that evolved to increase
light absorption per reaction center
Generalised Marcus Theory for Multi-Molecular Delocalised Charge Transfer
Although Marcus theory is widely used to describe charge transfer in
molecular systems, in its usual form it is restricted to transfer from one
molecule to another. If a charge is delocalised across multiple donor
molecules, this approach requires us to treat the entire donor aggregate as a
unified supermolecule, leading to potentially expensive quantum-chemical
calculations and making it more difficult to understand how the aggregate
components contribute to the overall transfer. Here, we show that it is
possible to describe charge transfer between groups of molecules in terms of
the properties of the constituent molecules and couplings between them,
obviating the need for expensive supermolecular calculations. We use the
resulting theory to show that charge delocalisation between molecules in either
the donor or acceptor aggregates can enhance the rate of charge transfer
through a process we call supertransfer (or suppress it through subtransfer).
The rate can also be enhanced above what is possible with a single molecule by
judiciously tuning energy levels and reorganisation energies. We also describe
bridge-mediated charge transfer between delocalised molecular aggregates. The
equations of generalised Marcus theory are in closed form, providing
qualitative insight into the impact of delocalisation on charge dynamics in
molecular systems
Environment-assisted quantum transport in ordered systems
Noise-assisted transport in quantum systems occurs when quantum
time-evolution and decoherence conspire to produce a transport efficiency that
is higher than what would be seen in either the purely quantum or purely
classical cases. In disordered systems, it has been understood as the
suppression of coherent quantum localisation through noise, which brings
detuned quantum levels into resonance and thus facilitates transport. We report
several new mechanisms of environment-assisted transport in ordered systems, in
which there is no localisation to overcome and where one would naively expect
that coherent transport is the fastest possible. Although we are particularly
motivated by the need to understand excitonic energy transfer in photosynthetic
light-harvesting complexes, our model is general---transport in a tight-binding
system with dephasing, a source, and a trap---and can be expected to have wider
application
Delocalisation enables efficient charge generation in organic photovoltaics, even with little to no energetic offset
Organic photovoltaics (OPVs) are promising candidates for solar-energy
conversion, with device efficiencies continuing to increase. However, the
precise mechanism of how charges separate in OPVs is not well understood
because low dielectric constants produce a strong attraction between the
charges, which they must overcome to separate. Separation has been thought to
require energetic offsets at donor-acceptor interfaces, but recent materials
have enabled efficient charge generation with small offsets, or with none at
all in neat materials. Here, we extend delocalised kinetic Monte Carlo (dKMC)
to develop a three-dimensional model of charge generation that includes
disorder, delocalisation, and polaron formation in every step from
photoexcitation to charge separation. Our simulations show that delocalisation
dramatically increases charge-generation efficiency, partly by enabling
excitons to dissociate in the bulk. Therefore, charge generation can be
efficient even in devices with little to no energetic offset, including neat
materials. Our findings demonstrate that the underlying quantum-mechanical
effect that improves the charge-separation kinetics is faster and
longer-distance hops between delocalised states, mediated by hybridised states
of exciton and charge-transfer character
Recommended from our members
Environment-Assisted Quantum Transport in Ordered Systems
Noise-assisted transport in quantum systems occurs when quantum time evolution and decoherence conspire to produce a transport efficiency that is higher than what would be seen in either the purely quantum or purely classical cases. In disordered systems, it has been understood as the suppression of coherent quantum localization through noise, which brings detuned quantum levels into resonance and thus facilitates transport. We report several new mechanisms of environment-assisted transport in ordered systems, in which there is no localization to overcome and where one would naively expect that coherent transport is the fastest possible. Although we are particularly motivated by the need to understand excitonic energy transfer in photosynthetic light-harvesting complexes, our model is general—transport in a tight-binding system with dephasing, a source and a trap—and can be expected to have wider application.Chemistry and Chemical Biolog
Benchmarking calculations of excitonic couplings between bacteriochlorophylls
Excitonic couplings between (bacterio)chlorophyll molecules are necessary for
simulating energy transport in photosynthetic complexes. Many techniques for
calculating the couplings are in use, from the simple (but inaccurate)
point-dipole approximation to fully quantum-chemical methods. We compared
several approximations to determine their range of applicability, noting that
the propagation of experimental uncertainties poses a fundamental limit on the
achievable accuracy. In particular, the uncertainty in crystallographic
coordinates yields an uncertainty of about 20% in the calculated couplings.
Because quantum-chemical corrections are smaller than 20% in most biologically
relevant cases, their considerable computational cost is rarely justified. We
therefore recommend the electrostatic TrEsp method across the entire range of
molecular separations and orientations because its cost is minimal and it
generally agrees with quantum-chemical calculations to better than the
geometric uncertainty. We also caution against computationally optimizing a
crystal structure before calculating couplings, as it can lead to large,
uncontrollable errors. Understanding the unavoidable uncertainties can guard
against striving for unrealistic precision; at the same time, detailed
benchmarks can allow important qualitative questions--which do not depend on
the precise values of the simulation parameters--to be addressed with greater
confidence about the conclusions
Measuring Energetic Disorder in Organic Semiconductors Using the Photogenerated Charge-Separation Efficiency
Understanding and quantifying energetic disorder in organic semiconductors continues to attract attention because of its significant impact on the transport physics of these technologically important materials. Here, we show that the energetic disorder of organic semiconductors can be determined from the relationship between the internal quantum efficiency of charge generation and the frequency of the incident light. Our results for a number of materials suggest that energetic disorder in organic semiconductors could be larger than previously reported, and we advance ideas as to why this may be the case
Quantum Algorithm for Molecular Properties and Geometry Optimization
It is known that quantum computers, if available, would allow an exponential
decrease in the computational cost of quantum simulations. We extend this
result to show that the computation of molecular properties (energy
derivatives) could also be sped up using quantum computers. We provide a
quantum algorithm for the numerical evaluation of molecular properties, whose
time cost is a constant multiple of the time needed to compute the molecular
energy, regardless of the size of the system. Molecular properties computed
with the proposed approach could also be used for the optimization of molecular
geometries or other properties. For that purpose, we discuss the benefits of
quantum techniques for Newton's method and Householder methods. Finally, global
minima for the proposed optimizations can be found using the quantum basin
hopper algorithm, which offers an additional quadratic reduction in cost over
classical multi-start techniques.Comment: 6 page