Theoretical aspects of the effects of coupled coherent-incoherent dynamics on the primary processes of photosynthesis

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

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.

Coupling of exciton motion in the core antenna and primary charge separation in the reaction center

The relation between exciton motion in the LH1 antenna and primary charge separation in the reaction center of purple bacteria is briefly reviewed. It is argued that in models based on hopping excitons described strictly by Förster theory, transfer-to-trap-limited kinetics is quite unlikely according to the relation between the exciton trapping kinetics and 'N', the size of the photosynthetic unit in such models. Because the results of several recent experiments have been interpreted in terms of transfer-to-trap limited kinetics, this presents a conflict between these experimental interpretations and strictly Förster-based theoretical models. Two possible resolutions are proposed. One arises from the random phase-redistribution trapping kinetics of partially coherent excitons, a kinetics uniquely independent of both N and the rate constant for primary charge separation in the reaction center. The other comes from multiple-pathways models of the multipicosecond nonexponentiality of the decay of P(*), the electronically excited primary electron donor in the reaction center. In these models, because it depends only on a certain averaged electron-transfer time constant, the exciton lifetime may be relatively insensivive to variations of individual electrontransfer rate constants-thereby undercutting the argument appearing in recent literature that by default the exciton kinetics must be transfer-to-trap limited.