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Fragility of and Robustness by Quantum Coherence in Exciton Dynamics


Excitons formed in photosynthetic light harvesting complexes and many organic molecular environments are of Frenkel-type, which delocalize mainly through coherent superposition of localized molecular excitations.  Quantum coherence accompanying the dynamics of such excitons is fragile in the sense that specific and long lasting phase relations are difficult to find due to the complexity of interactions and the presence of various dephasing mechanisms.  However, in many cases, the signature of quantum coherence does not entirely disappear, and can even manifest itself in the realization of exciton dynamics that are robust against disorder and fluctuations.  This talk presents two complementary theoretical approaches capable of capturing such positive effects of quantum coherence, generalized master equation for modular exciton density (GME-MED) and polaron-transformed quantum master equation (PQME).  Application of GME-MED to light harvesting 2 (LH2) complexes of purple bacteria elucidates an intricate interplay between quantum delocalization of excitons and hidden effects of molecular constraints involving hydrogen bonding, which taken together can explain the optimality of natural sizes of LH2.  Application of PQME to generic donor-bridge-acceptor model systems illustrates how the effects of quantum coherence can result in robust exciton dynamics in the intermediate regime of system-bath coupling.