Exciton dynamics in light harvesting systems

We study the exciton wave functions and the optical properties of cylindrical molecular aggregates. The cylindrical symmetry allows for a decomposition of the exciton Hamiltonian into a set of effective one-dimensional Hamiltonians, characterized by a transverse wave number k2. These effective Hamiltonians have interactions that are complex if the cylinder exhibits chirality. We propose analytical ansatze for the eigenfunctions of these one-dimensional problems that account for a finite cylinder length, and present a general study of their validity. A profound difference is found between the Hamiltonian for the transverse wave number k2=0 and those with k2 not equal 0. The complex nature of the latter leads to chiral wave functions, which we characterize in detail. We apply our general formalism to the chlorosomes of green bacteria and compare the wave functions as well as linear optical spectra (absorption and dichroism) obtained through our ansätze with those obtained by numerical diagonalization as well as those obtained by imposing periodic boundary conditions in the cylinder's axis direction. It is found that our ansätze, in particular, capture the finite-length effect in the circular dichroism spectrum much better than the solution with periodic boundary conditions. Our ansätze also show that in finite-length cylinders seven superradiant states dominate the linear optical response.

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“…We finally note that inclusion of dynamic disorder through, e.g., a stochastic model, is of interest to study transport properties. 30…”

Section: Discussionmentioning

confidence: 99%

Chiral exciton wave functions in cylindrical J aggregates

Didraga

Knoester

2004

The Journal of Chemical Physics

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“…Early work by Pearlstein and co-workers also dealt with energy transfer in the purely incoherent and coherent regimes, 18,19 as well as the partially coherent regime, 20 and the current paper can be seen as an extension of that work using numerical methods. More complicated models have been applied to the problem of energy transport in the bacterial light-harvesting complex [21][22][23][24][25][26][27] and dendrimers. 28,29 Our goal is to use the simplest model, with a small number of free parameters, to treat the general problem of optimizing the energy trapping time in simple lightharvesting architectures.…”

Section: Introductionmentioning

confidence: 99%

The effects of connectivity, coherence, and trapping on energy transfer in simple light-harvesting systems studied using the Haken-Strobl model with diagonal disorder

Gaab

Bardeen

2004

The Journal of Chemical Physics

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The problem of electronic energy transfer in a network of two-level systems coupled to a single trapping site is investigated using a simple Haken-Strobl model with diagonal disorder. The goal is to illustrate how the trapping time T(trap), coherence time T(d), and molecular topology all affect the overall efficiency of a light-harvesting network. Several issues are identified that need to be considered in the design of an optimal energy transfer network, including the dephasing-induced decoupling the trap from the rest of the network, the nonlinear dependence of trapping rate on the coherence time, and the role of network size and connectivity in determining the effect of the coherence time on efficiency. There are two main conclusions from this work. First, there exists an optimum combination of trapping time and coherence time, which will give the most rapid population transfer to the trap. These values are not in general the shortest trapping time and the longest coherence time, as would be expected based on rate equation models and/or simple considerations from previous analytical results derived for the Haken-Strobl model in an infinite system. Second, in the coherent regime, where T(d) is longer than the other relevant timescales, population trapping in a finite system can be suppressed by quantum interference effects, whose magnitude is sensitive to the molecular geometry. Suggestions for possible methods of observing such effects are discussed. These results provide a qualitative framework for quantum coherence and molecular topology into account for the design of covalent light-harvesting networks with high energy transfer efficiencies.

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“…The pump-probe spectra of tubular J aggregates are very similar in nature to those of linear aggregates, which may be explained from an effective Pauli exclusion for excitons to reside on the same ring [23]. Further studies into time-resolved pumpprobe spectra and the interpretation in terms of exciton transport [24] are topics of current interest.…”

Section: Discussionmentioning

confidence: 89%

Excitons in tubular molecular aggregates

Didraga

Knoester

2004

Journal of Luminescence

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We present a brief overview of recent work on the optical properties of molecular aggregates with a tubular (cylindrical) shape. The exciton states responsible for these properties can be distinguished with regard to a transverse wave number, which directly relates to optical selection rules and polarization direction of the associated absorption line. We discuss two types of analytical solutions for the exciton wave functions and the associated absorption and dichroism spectra. r

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Exciton dynamics in light harvesting systems

Cited by 8 publications

References 10 publications

Chiral exciton wave functions in cylindrical J aggregates

Chiral exciton wave functions in cylindrical J aggregates

The effects of connectivity, coherence, and trapping on energy transfer in simple light-harvesting systems studied using the Haken-Strobl model with diagonal disorder

Excitons in tubular molecular aggregates

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