23 research outputs found
Partially polaron-transformed quantum master equation for exciton and charge transport dynamics
Polaron-transformed quantum master equation (PQME) offers a unified framework
to describe the dynamics of quantum systems in both limits of weak and strong
couplings to environmental degrees of freedom. Thus, PQME serves as an
efficient method to describe charge and exciton transfer/transport dynamics for
a broad range of parameters in condensed or complex environments. However, in
some cases, the polaron transformation (PT) being employed in the formulation
invokes an over-relaxation of slow modes and results in premature suppression
of important coherence terms. A formal framework to address this issue is
developed in the present work by employing a partial PT that has smaller
weights for low frequency bath modes. It is shown here that a closed form
expression of a 2nd order time-local PQME including all the inhomogeneous terms
can be derived for a general form of partial PT, although more complicated than
that for the full PT. All the expressions needed for numerical calculation are
derived in detail. Applications to a model of two-level system coupled to a
bath of harmonic oscillators, with test calculations focused on those due to
homogeneous relaxation terms, demonstrate the feasibility and the utility of
the present approach.Comment: 17 pages, 5 figure
Delocalized excitons in natural light harvesting complexes
Natural organisms such as photosynthetic bacteria, algae, and plants employ
complex molecular machinery to convert solar energy into biochemical fuel. An
important common feature shared by most of these photosynthetic organisms is
that they capture photons in the form of excitons typically delocalized over a
few to tens of pigment molecules embedded in protein environments of light
harvesting complexes (LHCs). Delocalized excitons created in such LHCs remain
well protected despite being swayed by environmental fluctuations, and are
delivered successfully to their destinations over hundred nanometer length
scale distances in about hundred picosecond time scales. Decades of
experimental and theoretical investigation have produced a large body of
information offering insights into major structural, energetic, and dynamical
features contributing to LHCs' extraordinary capability to harness photons
using delocalized excitons. The objective of this review is (i) to provide a
comprehensive account of major theoretical, computational, and spectroscopic
advances that have contributed to this body of knowledge, and (ii) to clarify
the issues concerning the role of delocalized excitons in achieving efficient
energy transport mechanisms. The focus of this review is on three
representative systems, Fenna-Matthews-Olson complex of green sulfur bacteria,
light harvesting 2 complex of purple bacteria, and phycobiliproteins of
cryptophyte algae. Although we offer more in-depth and detailed description of
theoretical and computational aspects, major experimental results and their
implications are also assessed in the context of achieving excellent light
harvesting functionality. Future theoretical and experimental challenges to be
addressed in gaining better understanding and utilization of delocalized
excitons are also discussed.Comment: 53 pages, 15 figure
Modified Fermi's golden rule rate expressions
Fermi's golden rule (FGR) serves as the basis for many expressions of
spectroscopic observables and quantum transition rates. The utility of FGR has
been demonstrated through decades of experimental confirmation. However, there
still remain important cases where the evaluation of a FGR rate is ambiguous or
ill-defined. Examples are cases where the rate has divergent terms due to the
sparsity in the density of final states or time dependent fluctuations of
system Hamiltonians. Strictly speaking, assumptions of FGR are no longer valid
for such cases. However, it is still possible to define modified FGR rate
expressions that are useful as effective rates. The resulting modified FGR rate
expressions resolve a long standing ambiguity often encountered in using FGR
and offer more reliable ways to model general rate processes. Simple model
calculations illustrate the utility and implications of new rate expressions.Comment: 11 pages, 4 figure
Fundamental trade-off between the speed of light and the Fano factor of photon current in three-level lambda systems
Electromagnetically induced slow-light medium is a promising system for
quantum memory devices, but controlling its noise level remains a major
challenge to overcome. This work considers the simplest model for such medium,
comprised of three-level -systems interacting with bosonic bath, and
provides a new fundamental trade-off relation in light-matter interaction
between the group velocity of light and the Fano factor of photon current due
to radiative transitions. Considering the steady state limits of a newly
derived Lindblad-type equation, we find that the Fano factor of the photon
current maximizes to 3 at the minimal group velocity of light, which holds true
universally regardless of detailed values of parameters characterizing the
medium.Comment: 14 pages, 5 figure
General Chemical Reaction Network Theory for Olfactory Sensing Based on G-Protein-Coupled Receptors : Elucidation of Odorant Mixture Effects and Agonist-Synergist Threshold
This work presents a general chemical reaction network theory for olfactory
sensing processes that employ G-protein-coupled receptors as olfactory
receptors (ORs). The theory is applicable to general mixtures of odorants and
an arbitrary number of ORs. Reactions of ORs with G-proteins, both in the
presence and the absence of odorants, are explicitly considered. A unique
feature of the theory is the definition of an odor activity vector consisting
of strengths of odorant-induced signals from ORs relative to those due to
background G-protein activity in the absence of odorants. It is demonstrated
that each component of the odor activity defined this way reduces to a
Michaelis-Menten form capable of accounting for cooperation or competition
effects between different odorants. The main features of the theory are
illustrated for a two-odorant mixture. Known and potential mixture effects,
such as suppression, shadowing, inhibition, and synergy are quantitatively
described. Effects of relative values of rate constants, basal activity, and
G-protein concentration are also demonstrated
Nonadiabatic derivative couplings through multiple Franck-Condon modes dictate the energy gap law for near and short-wave infrared dye molecules
Near infrared (NIR, 700 - 1,000 nm) and short-wave infrared (SWIR, 1,000 -
2,000 nm) dye molecules exhibit significant nonradiative decay rates from the
first singlet excited state to the ground state. While these trends can be
empirically explained by a simple energy gap law, detailed mechanisms of the
nearly universal behavior have remained unsettled for many cases. Theoretical
and experimental results for two representative NIR/SWIR dye molecules reported
here clarify an important mechanism of such nature. It is shown that the first
derivative nonadiabatic coupling terms serve as major coupling pathways for
nonadiabatic decay processes exhibiting the energy gap law behavior and that
vibrational modes other than the highest frequency ones also make significant
contributions to the rate. This assessment is corroborated by further
theoretical comparison with possible alternative mechanisms of intersystem
crossing to triplet states and also by comparison with experimental data for
deuterated molecules
Nonequilibrium generalization of F\"{o}rster-Dexter theory for excitation energy transfer
F\"{o}rster-Dexter theory for excitation energy transfer is generalized for
the account of short time nonequilibrium kinetics due to the nonstationary bath
relaxation. The final rate expression is presented as a spectral overlap
between the time dependent stimulated emission and the stationary absorption
profiles, which allows experimental determination of the time dependent rate.
For a harmonic oscillator bath model, an explicit rate expression is derived
and model calculations are performed in order to examine the dependence of the
nonequilibrium kinetics on the excitation-bath coupling strength and the
temperature. Relevance of the present theory with recent experimental findings
and possible future theoretical directions are discussed.Comment: published in {\it Chemical Physics} (special issue on Photoprocesses
in Multichromophoric Molecular Assemblies
On the Temperature Dependence of Molecular Line Shapes Due to Linearly Coupled Phonon Bands â€
Comparative Computational Study of Electronic Excitations of Neutral and Charged Small Oligothiophenes and Their Extrapolations Based on Simple Models
This work reports electronic excitation
energies of neutral and
charged oligothiophenes (OTn) with repeat
unit n = 2–6 computed by routinely used semiempirical
and time-dependent density functional theory (TD-DFT) methods. More
specifically, for OTn, OTn+, and OTn–, we calculated
vertical transition energies for electronic absorption spectroscopy
employing the Zerner’s version of intermediate neglect differential
overlap method for structures optimized by the PM6 semiempirical method
and the TD-DFT method with three different functionals, B3LYP, BVP86,
and M06-2X, for structures optimized by the ground-state DFT method
employing the same functionals. We also calculated vertical transition
energies for the emission spectroscopy from the lowest singlet excited
states by employing the TD-DFT method for the structures optimized
for the lowest singlet excited states. In addition to computational
results in vacuum, solution phase data calculated at the level of
polarizable continuum model are reported and compared with available
experimental data. Most of the data are fitted reasonably well by
two simple model functions, one based on a Frenkel exciton theory
and the other based on the model of independent electrons in a box
with sinusoidal modulation of potential. Despite similar levels of
fitting performance, the two models produce distinctively different
asymptotic values of excitation energies. Comparison of these with
available experimental and computational data suggests that the values
based on the exciton model, while seemingly overestimating, are closer
to true values than those based on the other model. This assessment
is confirmed by additional calculations for a larger oligomer. The
fitting parameters offer new means to understand the relationship
between electronic excitations of OTs and their sizes and suggest
the feasibility of constructing simple coarse-grained exciton-bath
models applicable for aggregates of OTs