Page - 64 - in Advanced Chemical Kinetics

When the probability of occurrence of non-radiative energy transfer is 50%, a critical dis- tance, calledForster radius, is reachedand it isdefinedas thedistance inwhich the transfer rate, kDA, is equivalent to the donor fluorescence lifetime, when in the absence of the acceptor,τD �1: ð9Þ Thecriticaldistance ismuch longer than thebonddistances and theenergy transfer is said to bea long-distanceenergy transfer. 3.4.Dexterenergy transfer Themechanism of electronic energy transfer that involves the electron transfer between the electronic excited state of the donor to the unoccupied excited state of the acceptor, simulta- neouslytothetransferofanelectronof theelectronicgroundstateof theacceptor tothepoorly occupiedelectronicgroundstateof thedonor, characterizinganelectronexchangemechanism is theDexter energy transfer. Since it is an exchange interaction, it needs anoverlapbetween thewavefunctionsof thedonorandtheacceptor tooccur. The rate of the electron exchange is proportional to the ratio between the donor-acceptor distanceandthesumof theirVanderWaals radii. ð10Þ The donor-acceptor distance, in this case, is short, corresponding to distances of a complex formation.Thesemechanismsare illustrated inFigure4. Figure4. Diagrams illustrating the (A)Forster resonantenergy transferand(B)Dexterenergy transfermechanisms. Advanced Chemical Kinetics64