Excitons Forster rate

Figure 14. Downhill exciton relaxation rates as a function of energy gap between two monomers, predicted by the Forster model (open triangles), the traditional Redheld model (open circles), and the modihed Redheld model (hlled circles). In (a) the electronic couphng, Ju = lOOcm and in b) Ju =20cm . The reorganization energy, if = lOOcm and the spectral density is represented by a Gaussian correlation function with Zg = 100 fs [163],...

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. 

The first improved theory addressing the weakly coupled, or non-adiabatic electronic excitation transfer was the semiclassical vector model proposed by Forster [15]. It was further developed and refined by Levinson [16], Kasha [17], and others [18], who sometimes referred to it as the molecular exciton theory . Notably, this was the first successful attempt to link the rate of electronic excitation transfer with readily available experimental parameters, such as the absorption spectrum of the... 

When the exciton states are localized on individual Chls, the expression reduces to the well-known Forster formula. If Jcutoff oo this holds for all transfer rates. On the other hand, when Jcutoff 0, the expression reduces to that derived from modified Redfield theory. It can be shown analytically that the forward and backward rates satisfy the detailed balance condition... 

T p y+l)/k0j, where k j is the RC trapping rate and V is the ratio of the probabilities of finding the exciton in the antenna system and RC. It can be shown that from nine experimental observables (fluorescence and phosphorescence intensities and quantum yields, 7) qj) only one is independent. In all the transfer regimes, the observables depend only on V which is in general a function of time, intramolecular rate constants, size of the photosynthetic unit and initial conditions. Therefore, V (t) is the maximum information obtainable from the observables. These and further results representing general theoretical answers to problems l)-5) were illustrated on the case of the bacterial photosynthesis (Rhodopseudomonas viridis) where they are valid for the whole range of the physically acceptable values of the Forster radius. 

Forster transfer The rate for direct exciton transfer between a donor molecule, D, and an acceptor molecule, A, is determined by the Fermi Golden Rule expression,... 

---