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.

We report 2D electronic spectroscopy on the photosystem II core complex (PSII CC) at 77 K under different polarization conditions. A global analysis of the high time-resolution 2D data shows rapid, sub-100 fs energy transfer within the PSII CC. It also reveals the 2D spectral signatures of slower energy equilibration processes occurring on several to hundreds of picosecond time scales that are consistent with previous work. Using a recent structure-based model of the PSII CC [Y. Shibata, S. Nishi, K. Kawakami, J. R. Shen and T. Renger, J. Am. Chem. Soc., 2013, 135, 6903], we simulate the energy transfer in the PSII CC by calculating auxiliary time-resolved fluorescence spectra. We obtain the observed sub-100 fs evolution, even though the calculated electronic energy shows almost no dynamics at early times. On the other hand, the electronic-vibrational interaction energy increases considerably over the same time period. We conclude that interactions with vibrational degrees of freedom not only induce population transfer between the excitonic states in the PSII CC, but also reshape the energy landscape of the system. We suggest that the experimentally observed ultrafast energy transfer is a signature of excitonic-polaron formation.

A straightforward extension to the stochastic time-dependent variational approach allows the introduction of higher-order interaction effects to the Hamiltonian of an electronic-vibrational system. This is done using an Ansatz for the global wavefunction, describing vibrational wavepackets as squeezed coherent states (a generalized version of Davydov Ansatz). The approach allows quantum dynamics simulation and simulation of spectroscopic signals on anharmonic molecular potential surfaces. We calculate electronic and vibrational dynamics for a number of model systems, showing some results attributed to nonlinearities in spectroscopy experiments (such as breaking of mirror symmetry between absorption and fluorescence signals) and analyzing the influence of nonlinear effects on electronic energy transfer in multi-site aggregates.

Dynamics of excitonic polaron formation in molecular systems coupled to an overdamped bath are investigated using the Dirac-Frenkel variational principle and Davydov D1 Ansatz. Using a two-site model system we show that a few qualitatively distinct relaxation regimes of an optically created exciton are possible, depending on the timescale of bath fluctuations. A slow bath always leads to adiabatic polaron formation. Non-adiabatic exciton self-trapping occurs when the system is strongly coupled to a fast bath. Weak coupling to such bath does not perturb the excitonic picture. The complex system-bath dynamics can then be mapped to an effective model where the resonant coupling between sites is quenched during relaxation. The timescale of the polaron formation can be defined by the timescale of resonant coupling quenching, and is found to directly correlate with the bath relaxation time.

Representation of molecular vibrational degrees of freedom by independent harmonic oscillators, when describing electronic spectra or electronic excitation energy transport, raises unfavourable effects as vibrational energy relaxation becomes inaccessible. A standard theoretical description is extended in this paper by including both electronic-phonon and vibrational-phonon couplings. Using this approach we have simulated a model pigment-protein system and have shown that intermode coupling leads to the quenching of pigment vibrational modes, and to the redistribution of fluctuation spectral density with respect to the electronic excitations. Moreover, new energy relaxation pathways, opened by the vibrational-phonon interaction, allow to reach the electronic excited state equilibrium quicker in the naturally occurring water soluble chlorophyll binding protein (WSCP) aggregate, demonstrating the significance that the damping of molecular vibrations has for the intrarmolecular energy relaxation process rate. arXiv:1807.07314v1 [physics.chem-ph]

No abstract

Perturbative treatment of excitation dynamics in molecular systems with respect to external interactions with a dissipative environment is extensively used for the description of excitation energy transfer and relaxation. However the simulated dynamics becomes sensitive to a specific representation basis set, which makes the conclusions obscure and questionable. We revisit questions of excitation creation patterns, coherent dynamics, relaxation and detection from a theoretical viewpoint, and demonstrate that a mixture of specific requirements should be met to observe coherent phenomena and incoherent decay processes. We discuss how intermixing of coherent components in relaxation phenomena is related to a non-perturbative regime of dynamics leading to nonlinear feed-back effects where bath relaxation also affects excitation wavepackets. We also discuss how bath equilibration causes local heating effects which is often neglected in numerical simulations. The parameters reflecting the complexity of the processes are related to excitation delocalization patterns in various basis representations. While these seem to be auxiliary nonobservable features, their evaluation allows better investigation of the physical behavior of quantum relaxation processes in molecular aggregate systems.