Polaronic effects at finite temperatures in the B850 ring of the LH2 complex

Abstract

Energy transfer and relaxation dynamics in the B850 ring of LH2 molecular aggregates are described, taking into account the polaronic effects, by a stochastic time-dependent variational approach. We explicitly include the finite temperature effects in the model by sampling the initial conditions of the vibrational states randomly. This is in contrast to previous applications of the variational approach, which consider only the zero-temperature case. The method allows us to obtain both the microscopic dynamics at the single-wavefunction level and the thermally averaged picture of excitation relaxation over a wide range of temperatures. Spectroscopic observables such as temperature dependent absorption and time-resolved fluorescence spectra are calculated. Microscopic wavefunction evolution is quantified by introducing the exciton participation (localization) length and the exciton coherence length. Their asymptotic temperature dependence demonstrates that the environmental polaronic effects range from exciton self-trapping and excitonic polaron formation at low temperatures to thermally induced state delocalization and decoherence at high temperatures. While the transition towards the polaronic state can be observed on the wavefunction level, it does not produce a discernible effect on the calculated spectroscopic observables.

Benchmarking the stochastic time-dependent variational approach for excitation dynamics in molecular aggregates

“…This makes it completely feasible to apply for large molecular complexes, as shown in [41]. Since the averaging is performed over completely independent realizations, the approach takes full advantage of parallel computing techniques and is very suitable for running on supercomputers.…”

“…The sTDVA, introduced in Ref. [41] and analyzed in detail here, does not require any secondary bath and allows to treat both high and low frequency phonons on the same level, explicitly tracking the degrees of freedom. We must highlight another key difference between earlier contributions and the present work.…”

“…Stochasticity comes into play only by properly sampled random initial conditions. This approach was described in detail in [41] and here we outline the main points.…”

“…The parameters {α n (t), λ q (t)} constitute a set of complex functions, which are optimized through the variational procedure to minimize the deviation from the exact solution of the Schrödinger equation. We obtain a set of differential equations for the parameters, allowing us to propagate the initial state of the system [41]:…”

Dynamics of coherence, localization and excitation transfer in disordered nanorings

“…Recently, non-linear spectra of the LH2 has been obtained through atomistic modeling and a combination of quantum-classical methods [29]. Polaronic effects in the LH2 at finite temperatures have been recently analyzed [30], but the authors did not model excitation transfer including dissipative terms and their analysis of static disorder is scarce. The single Davydov D 2 trial wave function employed in that work is the simplest form of soliton-like trial states and only yields reliable results in the strong exciton-phonon coupling regime [31].…”