Dexter energy transfer

Murphy CB, Zhang Y, Troxler T, Ferry V, Martin JJ, Jones WE Jr (2004) Probing Forster and Dexter energy-transfer mechanisms in fluorescent conjugated polymer chemosensors. J Phys Chem B 108 1537-1543... 

Experimentally, one of the main methods of distinction between the Forster and Dexter mechanisms in an energy transfer is a study of the distance dependence of the observed process. From Equation (2.32) it is evident that the rate of dipole-induced energy transfer, kfen/ decreases as d 6. This is typical of dipolar interactions and is reminiscent of the distance dependence of other such mechanisms, e.g. London dispersion forces. Therefore, the Forster mechanism can operate over large distances, whereas, in contrast, the rate of Dexter energy transfer, kden, falls off exponentially with distance. 

In the Forster mechanism, energy transfer occurs through dipolar interactions. This process is not coupled through bond interactions, and therefore orbital overlap and inter-component electronic coupling are unimportant. Dipole-dipole interactions may occur efficiently in systems where the donor and acceptor species are over 100 A apart, whereas Dexter energy transfer is typically efficient only up to distances of approximately 10 A. 

The second possibility of energy transfer is known as exchange type or Dexter energy transfer. Dexter ET is based on quantum mechanical exchange interactions, therefore it needs strong spatial overlap of the involved wavefunctions of D and A. Since the overlap of electronic wavefunctions decays exponentially with distance, it is expected that the rate constant koA decreases even more rapidly with distance R than observed in the case of singlet transfer. A schematic presentation of Dexter ET is shown in Fig. 21. Dexter ET occurs typically over distances which are similar to the van-der-Waals distance, i.e. R = 0.5 - Inm. The rate constant drops exponentially with the distance Rda between D and A ... 

The rates of the Forster energy transfer and Dexter energy transfer depend on the separation distance between the donor and the acceptor, shown qualitatively in Fig. 1.1. The former decreases as the inverse sixth power of the distance, whereas the latter falls off exponentially as the distance increases. Therefore, the Forster energy transfer is able to occur over very large distances, while Dexter energy transfer will give much greater rates at short distances and close contacts. 

From Eq. (3.6) we observe that the rate of energy transfer by electron exchange mechanism decreases exponentially with 2R/L. Thus, it will be negligibly small as R increases more than on the order of one or two molecular diameters. Hence the effective distances for Dexter energy transfer range between 10 and 15 A. 

Both the Forster and the Dexter energy transfer mechanisms require spectral overlap of the donor emission spectrum and the acceptor absorption spectrum. However, energy transfer is known to occur even in the absence of spectral overlap, resulting in effective quenching of excited states. As an example, we can cite the quenching of the fluorescence of aromatic hydrocarbons by dienes, a process which involves thermal deactivation of an excited state encounter complex, or exciplex, between D and A (Eq. (3.7)) ... 

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