Darius Abramavicius

Direct evidence of quantum transport in photosynthetic light-harvesting complexes

The photosynthetic light-harvesting apparatus moves energy from absorbed photons to the reaction center with remarkable quantum efficiency. Recently, long-lived quantum coherence has been proposed to influence efficiency and robustness of photosynthetic energy transfer in light-harvesting antennae. The quantum aspect of these dynamics has generated great interest both because of the possibility for efficient long-range energy transfer and because biology is typically considered to operate entirely in the classical regime. Yet, experiments to date show only that coherence persists long enough that it can influence dynamics, but they have not directly shown that coherence does influence energy transfer. Here, we provide experimental evidence that interaction between the bacteriochlorophyll chromophores and the protein environment surrounding them not only prolongs quantum coherence, but also spawns reversible, oscillatory energy transfer among excited states. Using two-dimensional electronic spectroscopy, we observe oscillatory excited-state populations demonstrating that quantum transport of energy occurs in biological systems. The observed population oscillation suggests that these light-harvesting antennae trade energy reversibly between the protein and the chromophores. Resolving design principles evident in this biological antenna could provide inspiration for new solar energy applications.

Vibrational vs. electronic coherences in 2D spectrum of molecular systems

Two-dimensional spectroscopy has recently revealed the oscillatory behavior of the excitation dynamics of molecular systems. However, in the majority of cases there is considerable debate over what is actually being observed: excitonic or vibrational wavepacket motion or evidence of quantum transport. In this letter we present a method for distinguishing between vibrational and excitonic wavepacket motion, based on the phase and amplitude relationships of oscillations of distinct peaks as revealed through a fundamental analysis of the two-dimensional spectra of two representative systems

Distinctive character of electronic and vibrational coherences in disordered molecular aggregates

Abstract Coherent dynamics of coupled molecules are effectively characterized by the two-dimensional electronic spectroscopy. Depending on the coupling between electronic and vibrational states, oscillating signals of purely electronic, purely vibrational or mixed character are observed with the help of oscillation maps, constructed from time-resolved spectra. Amplitude of beatings caused by electronic coherences is heavily affected by energetic disorder and consequently electronic coherences are quickly dephased. Beatings with vibrational character weakly depend on the disorder, ensuring their long-time survival. We show that detailed modeling of two-dimensional spectroscopy signals of molecular aggregates provides direct information on the origin of the coherent beatings.

Tight-binding model of the photosystem II reaction center: application to two-dimensional electronic spectroscopy

We propose an optimized tight-binding electron-hole model of the photosystem II (PSII) reaction center (RC). Our model incorporates two charge separation pathways and spatial correlations of both static disorder and fast fluctuations of energy levels. It captures the main experimental features observed in time-resolved two-dimensional (2D) optical spectra at 77K: peak pattern, lineshapes and time traces. Analysis of 2D spectra kinetics reveals that specific regions of the 2D spectra of the PSII RC are sensitive to the charge transfer states. We find that the energy disorder of two peripheral chlorophylls is four times larger than the other RC pigments.

Simulations of the Two-Dimensional Electronic Spectroscopy of the Photosystem II Reaction Center

We report simulations of the two-dimensional electronic spectroscopy of the Q(y) band of the D1-D2-Cyt b559 photosystem II reaction center at 77 K. We base the simulations on an existing Hamiltonian that was derived by simultaneous fitting to a wide range of linear spectroscopic measurements and described within modified Redfield theory. The model obtains reasonable agreement with most aspects of the two-dimensional spectra, including the overall peak shapes and excited state absorption features. It does not reproduce the rapid equilibration from high energy to low energy excitonic states evident by a strong cross-peak below the diagonal. We explore modifications to the model to incorporate new structural data and improve agreement with the two-dimensional spectra. We find that strengthening the system-bath coupling and lowering the degree of disorder significantly improves agreement with the cross-peak feature, while lessening agreement with the relative diagonal/antidiagonal width of the 2D spectra. We conclude that two-dimensional electronic spectroscopy provides a sensitive test of excitonic models of the photosystem II reaction center and discuss avenues for further refinement of such models.

Excitation dynamics and relaxation in a molecular heterodimer

Abstract The exciton dynamics in a molecular heterodimer is studied as a function of differences in excitation and reorganization energies, asymmetry in transition dipole moments and excited state lifetimes. The heterodimer is composed of two molecules modeled as two-level systems coupled by the resonance interaction. The system-bath coupling is taken into account as a modulating factor of the molecular excitation energy gap, while the relaxation to the ground state is treated phenomenologically. Comparison of the description of the excitation dynamics modeled using either the Redfield equations (secular and full forms) or the Hierarchical quantum master equation (HQME) is demonstrated and discussed. Possible role of the dimer as an excitation quenching center in photosynthesis self-regulation is discussed. It is concluded that the system-bath interaction rather than the excitonic effect determines the excitation quenching ability of such a dimer.

Advancing Hierarchical Equations of Motion for Efficient Evaluation of Coherent Two-dimensional Spectroscopy

To advance hierarchical equations of motion as a standard theory for quantum dissipative dynamics, we put forward a mixed Heisenberg-Schrodinger scheme with block-matrix implementation on efficient evaluation of nonlinear optical response function. The new approach is also integrated with optimized hierarchical theory and numerical filtering algorithm. Different configurations of coherent two-dimensional spectroscopy of model excitonic dimer systems are investigated, with focusing on the effects of intermolecular transfer coupling and bi-exciton interaction.

Excitation Energy Transfer and Quenching in a Heterodimer: Applications to the Carotenoid–Phthalocyanine Dyads

The dynamics of a molecular heterodimer composed of a long-lived excitation donor and a short-lived acceptor (quencher) is examined. In order to consider various dynamical regimes without any restrictions on the system parameters, the energy transfer is modeled employing the hierarchical equations of motion, while the relaxation to the ground state is treated by assuming a phenomenological spontaneous nonradiative decay rate. Time scales of the resulting two-exponential evolution are investigated as functions of the energy gap and the resonance coupling between the monomeric constituents of the dimer. Relevance of the present analysis to the recent experimental findings on artificial carotenoid-phthalocyanine dyads is discussed. By examining the first two time scales of the reported time-resolved spectra, it is shown that upon the increase of carotenoid conjugation length its first excited state approaches the first excited state of phthalocyanine from above, thereby inducing a remarkable quenching. The proposed model also provides a unified treatment of quenching in the regimes previously distinguished as energy transfer and excitonic state formation.

Acceleration of charge separation by oscillations of the environment polarization

A new model explaining fast charge separation from optically achievable charge transfer (CT) states in molecular complexes is suggested. The idea is based on the assumption that the polarization reaction to the charges generated in the initial CT state can be divided into two evolution paths along fast and slow reaction coordinates. A possibility of the charge transfer rate enhancement by the oscillations of the system along the slow polarization coordinate is demonstrated.

Electronic Coherence and Structure of Biological Aggregates Probed By Coherent Multidimensional Spectroscopy

Using multidimensional spectroscopy, we explored the energy-transfer and charge-separation pathways in photo-systems, quantum entanglement effects in nature, and aggregation mechanism of amyloid fibrils related to neurodegenerative diseases.