136 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
Plataformas digitales de la Gig Economy y su impacto en las relaciones laborales en el Perú: definiendo la presencia de la subordinación y la ajenidad
La investigación plantea como objetivo definir si resulta aplicable el marco del Derecho Laboral a los servicios presenciales prestados a través de la utilización de las Plataformas Digitales de la Gig Economy. En calidad de hipótesis, consideramos que sí resulta admisible la configuración de relaciones laborales en las prestaciones de servicios bajo comentario, en función a una nueva interpretación de los elementos de subordinación y ajenidad. Con la finalidad de verificar la hipótesis planteada, se ha efectuado una investigación mixta, combinando elementos de una investigación dogmático-jurídica y de una investigación socio-jurídica. A través de la investigación dogmático-jurídica ha sido posible concluir que los conceptos de subordinación y ajenidad sí pueden encontrarse presentes en los servicios presenciales desarrollados por medio de Plataformas Digitales de la Gig Economy, adecuándolos a esta novedosa forma de prestar servicios. Por su parte, la investigación socio-jurídica conllevó la ejecución de una serie de entrevistas a los colaboradores de diversas Plataformas Digitales de la Gig Economy presentes en el Perú – Uber, InDriver, PedidosYa y Rappi –; permitiendo verificar que, efectivamente, los servicios bajo comentario sí pueden configurar relaciones de naturaleza laboral en la realidad. La presente investigación nos ha llevado a concluir que los servicios presenciales que se brindan por medio de las Plataformas Digitales de la Gig Economy, según cada caso en particular y a las características inherentes a cada Plataforma, sí pueden exhibir diversos elementos y rasgos del trabajo dependiente, los cuales, analizados de manera conjunta, configuran efectivamente relaciones de naturaleza laboral que requieren encontrarse sujetas al marco legal del Derecho Laboral; corroborándose y confirmándose la hipótesis planteada en nuestra investigación
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
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
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
Non-Markovian stochastic description of quantum transport in photosynthetic systems
We analyze several aspects of the transport dynamics in the LH1-RC core of
purple bacteria, which consists basically in a ring of antenna molecules that
transport the energy into a target molecule, the reaction center, placed in the
center of the ring. We show that the periodicity of the system plays an
important role to explain the relevance of the initial state in the transport
efficiency. This picture is modified, and the transport enhanced for any
initial state, when considering that molecules have different energies, and
when including their interaction with the environment. We study this last
situation by using stochastic Schr{\"o}dinger equations, both for Markovian and
non-Markovian type of interactions.Comment: 21 pages, 5 figure
Atomistic study of the long-lived quantum coherences in the Fenna-Matthews-Olson complex
A remarkable amount of theoretical research has been carried out to elucidate
the physical origins of the recently observed long-lived quantum coherence in
the electronic energy transfer process in biological photosynthetic systems.
Although successful in many respects, several widely used descriptions only
include an effective treatment of the protein-chromophore interactions. In this
work, by combining an all-atom molecular dynamics simulation, time-dependent
density functional theory, and open quantum system approaches, we successfully
simulate the dynamics of the electronic energy transfer of the
Fenna-Matthews-Olson pigment-protein complex. The resulting characteristic
beating of populations and quantum coherences is in good agreement with the
experimental results and the hierarchy equation of motion approach. The
experimental absorption, linear and circular dichroism spectra and dephasing
rates are recovered at two different temperatures. In addition, we provide an
extension of our method to include zero-point fluctuations of the vibrational
environment. This work thus presents one of the first steps to explain the role
of excitonic quantum coherence in photosynthetic light-harvesting complexes
based on their atomistic and molecular description.Comment: 24 pages, 6 figure
Multiscale photosynthetic exciton transfer
Photosynthetic light harvesting provides a natural blueprint for
bioengineered and biomimetic solar energy and light detection technologies.
Recent evidence suggests some individual light harvesting protein complexes
(LHCs) and LHC subunits efficiently transfer excitons towards chemical reaction
centers (RCs) via an interplay between excitonic quantum coherence, resonant
protein vibrations, and thermal decoherence. The role of coherence in vivo is
unclear however, where excitons are transferred through multi-LHC/RC aggregates
over distances typically large compared with intra-LHC scales. Here we assess
the possibility of long-range coherent transfer in a simple chromophore network
with disordered site and transfer coupling energies. Through renormalization we
find that, surprisingly, decoherence is diminished at larger scales, and
long-range coherence is facilitated by chromophoric clustering. Conversely,
static disorder in the site energies grows with length scale, forcing
localization. Our results suggest sustained coherent exciton transfer may be
possible over distances large compared with nearest-neighbour (n-n) chromophore
separations, at physiological temperatures, in a clustered network with small
static disorder. This may support findings suggesting long-range coherence in
algal chloroplasts, and provides a framework for engineering large chromophore
or quantum dot high-temperature exciton transfer networks.Comment: 9 pages, 6 figures. A significantly updated version is now published
online by Nature Physics (2012
Computational Methodologies and Physical Insights into Electronic Energy Transfer in Photosynthetic Light-Harvesting Complexes
We examine computational techniques and methodologies currently in use to
explore electronic excitation energy transfer in the context of
light-harvesting complexes in photosynthetic antenna systems, and comment on
some new insights into the underlying physics. Advantages and pitfalls of these
methodologies are discussed, as are some physical insights into the
photosynthetic dynamics. By combining results from molecular modelling of the
complexes (structural description) with an effective non-equilibrium
statistical description (time evolution), we identify some general features,
regardless of the particular distribution in the protein scaffold, that are
central to light-harvesting dynamics and, that could ultimately be related to
the high efficiency of the overall process. Based on these general common
features, some possible new directions in the field are discussed.Comment: Invited Perspective Article for Phys. Chem. Chem. Phy
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