Excitation migration

When the process can repeat itself so that the excitation migrates over several molecules, it is called excitation transport or energy migration. 

Electronic energy migration (or Hopping) The movement of electronic excitation energy from one molecular entity to another of the same species, or from one part of a molecular entity to another of the same kind (e.g. excitation migration between the chromophores of an aromatic polymer). The migration can happen via radiative or radiationless processes. 

When the excitation migrates to a trap such as P680 or P70o> this special Chi a dimer goes to an excited singlet state, as would any other Chi a. Because the trap chi cannot readily excite other chlorophylls by resonance transfer, it might become deexcited by the emission of fluorescence. However, very little fluorescence from the trap chi s is observed in vivo. This is explained by the occurrence of a relatively rapid photochemical event (see Eq. 5.5 trap chi + A — trap chl+ + A ) the donation within 10-10 s of an electron to an acceptor prevents the deexcitation of the trap chi s by fluorescence, which has a longer lifetime. 

Some simple fluorescence decay laws are given which result from models concerned with relaxation phenomena, heterogeneity, molecular motion and excitation migration in synthetic polymers exhibiting fluorescence. Application of such laws to decays from a poly (diacetylene) in rigid crystal form to polystyrene in fluid solution, and to vinyl naphthalene co-polymers in fluid solution are considered briefly. 

We conclude therefore that in the case of this highly-ordered, rigid polymer, time-resolved fluorescence measurements of excitation migration yield results which are compatible with an exact solution for one-dimensional diffusion. 

In situations where high concentrations of sensitizers exist the energy may be transferred to an activator ion. The excitation migrating on a lattice of sensitizers can be considered to be a localized exciton , sometimes referred to as a Frenkel or Davydov exciton i). Each energy transfer step between sensitizers can be treated by one of the ion-ion interaction mechanisms discussed previously. This may be either the same as or different from the mechanism of sensitizer-activator interaction. In characterizing this type of energy transfer it is important to describe both the dynamics of the... 

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