19 research outputs found
Correlated Fluctuations and Intraband Dynamics of J-Aggregates Revealed by Combination of 2DES Schemes
The intraband exciton dynamics of molecular aggregates is a crucial initial step to determine the possibly coherent nature of energy transfer and its implications for the ensuing interband relaxation pathways in strongly coupled excitonic systems. In this work, we fully characterize the intraband dynamics in linear J-aggregates of porphyrins, good model systems for multichromophoric assemblies in biological antenna complexes. Using different 2D electronic spectroscopy schemes together with Raman spectroscopy and theoretical modeling, we provide a full characterization of the inner structure of the main one-exciton band of the porphyrin aggregates. We find that the redistribution of population within the band occurs with a characteristic time of 280 fs and dominates the modulation of an electronic coherence. While we do not find that the coupling to vibrations significantly affects the dynamics of excitonic coherence, our results suggest that exciton fluctuations are nevertheless highly correlated
Manipulation of Charge Delocalization in a Bulk Heterojunction Material Using a Mid-Infrared Push Pulse
In organic bulk heterojunction materials, charge delocalization has been
proposed to play a vital role in the generation of free carriers by reducing
the Coulomb attraction via an interfacial charge transfer exciton (CTX).
Pump-push-probe (PPP) experiments produced evidence that the excess energy
given by a push pulse enhances delocalization, thereby increasing photocurrent.
However, previous studies have employed near-IR push pulses in the range
0.4-0.6 eV which is larger than the binding energy of a typical CTX. This
raises the doubt that the push pulse may directly promote dissociation without
involving delocalized states. Here, we perform PPP experiments with mid-IR push
pulses at energies that are well below the binding energy of a CTX state
(0.12-0.25 eV). We identify three types of CTX: delocalized, localized, and
trapped. The excitation resides over multiple polymer chains in delocalized
CTXs, while is restricted to a single chain (albeit maintaining a degree of
intrachain delocalization) in localized CTXs. Trapped CTXs are instead
completely localized. The pump pulse generates a hot delocalized CTX, which
relaxes to a localized CTX, and eventually to trapped states. We find that
photo-exciting localized CTXs with push pulses resonant to the mid-IR charge
transfer absorption can promote delocalization and contribute to the formation
of long-lived charge separated states. On the other hand, we found that trapped
CTX are non-responsive to the push pulses. We hypothesize that delocalized
states identified in prior studies are only accessible in systems where there
is significant interchain electronic coupling or regioregularity that supports
either interchain or intrachain polaron delocalization. This emphasizes the
importance of engineering the micromorphology and energetics of the
donor-acceptor interface to exploit a full potential of a material for
photovoltaic applications
Efficiency of energy transfer in a light-harvesting system under quantum coherence
We investigate the role of quantum coherence in the efficiency of excitation
transfer in a ring-hub arrangement of interacting two-level systems, mimicking
a light-harvesting antenna connected to a reaction center as it is found in
natural photosynthetic systems. By using a quantum jump approach, we
demonstrate that in the presence of quantum coherent energy transfer and
energetic disorder, the efficiency of excitation transfer from the antenna to
the reaction center depends intimately on the quantum superposition properties
of the initial state. In particular, we find that efficiency is sensitive to
symmetric and asymmetric superposition of states in the basis of localized
excitations, indicating that initial state properties can be used as a
efficiency control parameter at low temperatures.Comment: Extended version of original paper. 7 pages, 2 figure
Distribution of entanglement in light-harvesting complexes and their quantum efficiency
Recent evidence of electronic coherence during energy transfer in
photosynthetic antenna complexes has reinvigorated the discussion of whether
coherence and/or entanglement has any practical functionality for these
molecular systems. Here we investigate quantitative relationships between the
quantum yield of a light-harvesting complex and the distribution of
entanglement among its components. Our study focusses on the entanglement yield
or average entanglement surviving a time scale comparable to the average
excitation trapping time. As a prototype system we consider the
Fenna-Matthews-Olson (FMO) protein of green sulphur bacteria and show that
there is an inverse relationship between the quantum efficiency and the average
entanglement between distant donor sites. Our results suggest that longlasting
electronic coherence among distant donors might help modulation of the
lightharvesting function.Comment: Version accepted for publication in NJ
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Dynamics of two-qubit entanglement in a self-interacting spin-bath
Controlling radiative heat flow through cavity electrodynamics
Cavity electrodynamics is emerging as a promising tool to control chemical processes and quantum material properties. In this work we develop a formalism to describe the cavity mediated energy exchange between a material and its electromagnetic environment. We show that coplanar cavities can significantly affect the heat load on the sample if the cavity resonance lies within the frequency region where free-space radiative heat dominates, typically the mid-IR at ambient temperature, while spectral filtering is necessary for having an effect with lower frequency cavities
Coherent Energy Transfer under Incoherent Light Conditions
Recent two-dimensional electronic spectroscopy (2DES)
experiments
have reported evidence of coherent dynamics of electronic excitations
in several light-harvesting antennae. However, 2DES uses ultrafast
coherent laser pulses as an excitation source; therefore, there is
a current debate on whether coherent excitation dynamics is present
under natural sunlight – incoherent – illumination conditions.
In this letter, we show that even if incoherent light excites an electronic
state with no initial quantum superpositions among excitonic states,
energy transfer can proceed quantum coherently if nonequilibrium dynamics
of the phonon environment takes place. Such nonequilibrium behavior
manifests itself in non-Markovian evolution of electronic excitations
and is typical of many photosynthetic systems. We therefore argue
that light-harvesting antennae have mechanisms that could support
coherent evolution under incoherent illumination
Energy Transfer in Light-Adapted Photosynthetic Membranes: From Active to Saturated Photosynthesis
In bacterial photosynthesis light-harvesting complexes, LH2 and LH1 absorb sunlight energy and deliver it to reaction centers (RCs) with extraordinarily high efficiency. Submolecular resolution images have revealed that both the LH2:LH1 ratio, and the architecture of the photosynthetic membrane itself, adapt to light intensity. We investigate the functional implications of structural adaptations in the energy transfer performance in natural in vivo low- and high-light-adapted membrane architectures of
Rhodospirillum photometricum
. A model is presented to describe excitation migration across the full range of light intensities that cover states from active photosynthesis, where all RCs are available for charge separation, to saturated photosynthesis where all RCs are unavailable. Our study outlines three key findings. First, there is a critical light-energy density, below which the low-light adapted membrane is more efficient at absorbing photons and generating a charge separation at RCs, than the high-light-adapted membrane. Second, connectivity of core complexes is similar in both membranes, suggesting that, despite different growth conditions, a preferred transfer pathway is through core-core contacts. Third, there may be minimal subareas on the membrane which, containing the same LH2:LH1 ratio, behave as minimal functional units as far as excitation transfer efficiency is concerned