Volkhard May

Ultrafast excitation energy transfer dynamics in photosynthetic pigment–protein complexes

Abstract Photosynthetically active membranes of certain bacteria and higher plants contain antenna systems which surround the reaction center to increase its absorption cross section for the incoming sun light. The excitation energy created in the antenna pigments is transferred via an exciton mechanism to the reaction center where charge separation takes place. Sub-picosecond laser spectroscopy makes it possible to follow the initial dynamic events of excitation energy (exciton) motion and exciton relaxation in real time. On the other hand, the success of structure resolution opened the door to the microscopic understanding of spectroscopic data and to an appreciation of the structure–function relationship realized in different systems. Here, it will be demonstrated how the combination of microscopically based theoretical models and numerical simulations pave the road from spectroscopic data to a deeper understanding of the functionality of photosynthetic antenna systems. The density matrix technique is introduced as the theoretical tool providing a unified description of the processes which follow ultrafast laser excitation. This includes in particular coherent exciton motion, vibrational coherences, exciton relaxation, and exciton localization. It can be considered as a major result of recent investigations that a theoretical model of intermediate complexity was shown to be suitable to explain a variety of experiments on different photosynthetic antenna systems. We start with introducing the structural components of antenna systems and discuss their general function. In the second part the formulation of the appropriate theoretical model as well as the simulation of optical spectra is reviewed in detail. Emphasis is put on the mapping of the complex protein structure and its hierarchy of dynamic phenomena onto models of static and dynamic disorder. In particular, it is shown that the protein spectral density plays a key role in characterizing excitation energy dissipation. The theoretical concepts are illustrated in the third part by results of numerical simulations of linear and nonlinear optical experiments for three types of antennae: the peripheral light-harvesting complex 2 of purple bacteria, the Fenna–Mathew–Olson complex of green bacteria, and the light-harvesting complex of photosystem II of green plants.

Dissipative vibrational dynamics in a curve–crossing system

The density matrix theory is utilized for the description of ultra fast optical properties and related vibrational wave packet dynamics of molecular systems in condensed media. As an example, optically induced vibrational wave packets in the so‐called curve–crossing system are considered. Such a system goes beyond the standard treatment of optical phenomena since the vibrational wave packet moves in a double well potential and is subject to environmental influences like wave function dephasing and relaxation. The complete theoretical description has been carried out in a representation of the vibrational wave functions of the diabatic states which refer to the two coupled vibrational surfaces. Solving the corresponding density matrix equations by numerical methods allows us to incorporate the static coupling between the crossed surfaces in a nonperturbative manner. Standard projection operator technique is used to treat environmental contributions up to the second order. For the case of a bilinear couplin...

Nonadiabatic donor–acceptor electron transfer mediated by a molecular bridge: A unified theoretical description of the superexchange and hopping mechanism

Nonadiabatic bridge-assisted electron transfer (ET) is described by a set of kinetic equations which simultaneously account for the sequential (hopping) as well as the superexchange mechanism. The analysis is based on the introduction of a certain reduced density operator describing a particular set of electron-vibrational levels of the molecular units (sites) involved in the transfer act. For the limiting case of intrasite relaxations proceeding fast compared to intersite transitions a set of rate equations is obtained. This set describes the time evolution of the electronic site populations and is valid for bridges with an arbitrary number of units. If the rate constants for the transition from the bridge to the donor as well as to the acceptor exceed those for the reverse transitions the ET reduces to a single-exponential process with an effective forward and backward transfer rate. These effective rates contain a contribution from the sequential and a contribution from the superexchange mechanisms. A ...

Theory of ultrafast photoinduced heterogeneous electron transfer: Decay of vibrational coherence into a finite electronic–vibrational quasicontinuum

Photo-induced electron transfer from a surface attached dye molecule to the band levels of a semiconductor is modeled via an electronic–vibronic quasicontinuum. The description enables one to obtain a fairly accurate expression for the decay of the excited molecular state, including initial vibronic coherences. The model accounts for (a) the effect of a finite band width, (b) variations in reorganization energy and electronic coupling, (c) various energetic positions for the injecting level, (d) different initial vibrational wave packets in the excited state, and (e) two vibrational modes participating in the electron transfer process. Most cases are studied numerically and can be reasonably well understood from the obtained decay expression.

Vibrational effects in laser-driven molecular wires.

The influence of an electron-vibrational coupling on the laser control of electron transport through a molecular wire that is attached to several electronic leads is investigated. These molecular vibrational modes induce an effective electron-electron interaction. In the regime where the wire electrons couple weakly to both the external leads and the vibrational modes, we derive within a Hartree-Fock approximation a nonlinear set of quantum kinetic equations. The quantum kinetic theory is then used to evaluate the laser driven, time-averaged electron current through the wire-leads contacts. This formalism is applied to two archetypical situations in the presence of electron-vibrational effects, namely, (i) the generation of a ratchet or pump current in a symmetrical molecule by a harmonic mixing field and (ii) the laser switching of the current through the molecule.

Femtosecond pulse dependence of dissipation in molecular systems

Abstract Externally applied optical fields may influence molecular dynamics in a direct way alternating the reversible part of the dynamics but also in an indirect manner changing the dissipative processes. This indirect field influence is studied in the framework of the quantum master equation for the density matrix including a field-dependent Redfield tensor. Within a simple model appropriate to describe a dye molecule in solution, the field influence on molecular dynamics is studied for different laser-pulse intensities and lengths as well as bath correlation times. The possible external field control of dissipation is underlined.

THEORY OF EXCITON-VIBRATIONAL DYNAMICS IN MOLECULAR DIMERS

Abstract The exciton transfer in a molecular dimer embedded in a condensed medium is investigated theoretically. In order to include coherent vibrational dynamics a single effective mode per monomer is split off from the whole set of vibrational degrees of freedom of the dimer and the environment. The remaining modes are treated as a heat bath. To study the dissipative exciton transfer dynamics the density matrix formalism is applied. Choosing an appropriate exciton-vibrational basis set with respect to the two effective modes the theory is formulated in the related state representation. The general approach is applied to the exciton motion in a chlorophyll a / b dimer of the light-harvesting complex of the photosystem II of higher plants. The complex dynamic behavior following an ultrafast optical excitation in the absorption region of the chlorophyll b monomer is studied in detail.

Exciton exciton annihilation dynamics in chromophore complexes. II. Intensity dependent transient absorption of the LH2 antenna system

Using the multiexciton density matrix theory of excitation energy transfer in chromophore complexes developed in a foregoing paper [J. Chem. Phys. 118, 746 (2003)], the computation of ultrafast transient absorption spectra is presented. Beside static disorder and standard mechanisms of excitation energy dissipation the theory incorporates exciton exciton annihilation (EEA) processes. To elucidate signatures of EEA in intensity dependent transient absorption data the approach is applied to the B850 ring of the LH2 found in rhodobacter sphaeroides. As main indications for two-exciton population and resulting EEA we found (i) a weakening of the dominant single-exciton bleaching structure in the transient absorption, and (ii) an intermediate suppression of long-wavelength and short-wavelength shoulders around the bleaching structure. The suppression is caused by stimulated emission from the two-exciton to the one-exciton state and the return of the shoulders follows from a depletion of two-exciton population according to EEA. The EEA-signature survives as a short-wavelength shoulder in the transient absorption if orientational and energetic disorder are taken into account. Therefore, the observation of the EEA-signatures should be possible when doing frequency resolved transient absorption experiments with a sufficiently strongly varying pump-pulse intensity.

Femtosecond laser pulse control of multidimensional vibrational dynamics: Computational studies on the pyrazine molecule

The multiconfiguration time-dependent Hartree (MCTDH) method is combined with the optimal control theory (OCT) to study femtosecond laser pulse control of multidimensional vibrational dynamics. Simulations are presented for the widely discussed three-electronic-level vibronic coupling model of pyrazine either in a three or four vibrational coordinate version. Thus, for the first time OCT is applied to a four-coordinate system. Different control tasks are investigated and also some general aspects of the OCT-MCTDH method combination are analyzed.

Photoinduced switching of the current through a single molecule: effects of surface plasmon excitations of the leads.

The photoinduced switch of the current through a single molecule is studied theoretically by including plasmon excitations of the leads. A molecule weakly linked to two spherical nanoelectrodes is considered resulting in sequential charge transmission scheme. Taking the molecular charging energy (relative to the equilibrium lead chemical potential) to be comparable to the molecular excitation energy, an efficient current switch in a low voltage range becomes possible. A remarkable enhancement of the current is achieved due to simultaneous plasmon excitations in the electrodes. The behavior is explained by an increased molecular absorbance due to oscillator strength transfer from the electrode plasmon excitations and by a net excitation energy motion from the electrodes to the molecule.