Exciton Transfer Strong Coupling

In the strong coupling case, the transfer of excitation energy is faster than the nuclear vibrations and the vibrational relaxation ( 10 12 s). The excitation energy is not localized on one of the molecules but is truly delocalized over the two components (or more in multi-chromophoric systems). The transfer of excitation is a coherent process9 the excitation oscillates back and forth between D and A and is never more than instantaneously localized on either molecule. Such a delocalization is described in the frame of the exciton theory10 . 

Given the density of Chi in PSI, it is immediately obvious that any detailed description of energy transfer in this system will need to handle both weakly coupled (Forster-type) and strongly coupled (excitonic) systems self consistently. It is also evident that discerning rate limiting steps and optimizing principles may not be straightforward. 

Strong coupling would not be expected for many fluid solvents but weak coupling could lead to energy transfer from solvent to solvent molecule at rates in excess of diffusion rates. This could occur as a result of singlet or even triplet exciton migration. 

Strong coupling case Coulombic interactions between electrons induce the conversion of an excited molecule to its ground state with simultaneous excitation of another molecule from its ground state to the excited state. This phenomenon, the molecular exciton, takes place with a relaxation time of less than 10 14 sec by transfer or migration of energy from a molecule to an adjacent molecule. 

Singlet excitonic energy transfer between chls is most commonly discussed in terms of the two limiting cases of very strong and very weak electronic coupling (J) between donor (D) and acceptor (A) transition dipoles [159-161]. J(cm" ) may be calculated by the expression given by Pearlstein [162]... 

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