## A recent study of Al Khateeb et al

Excitation energy migrates (EM — energy migration) visiting different donor molecules before it is transferred to any acceptor. This is the AM966 for the so called hopping model of forward nonradiative energy transfer (FET-HM) [109]. The hopping model elaborated by Bojarski included: 1) the theory of photoluminescence quenching by foreign absorbing substances (PLQ) in the presence of excitation energy migration, 2) the theory of photoluminescence concentration quenching (PLCQ) as a result of appearance of dimers and 3) the theory of concentration depolarization of photoluminescence (CDPL). All these theories take into account the existence of energy migration in the donor set. These theories have been in many ways confirmed by experimental results [110], [111], [112] and [113].

In the frames of PLCQ theory Bojarski and Domsta [109] have obtained the following expression for relative donor quantum yield:equation(10)ηDη0D=1−fγ1−αα0fγand in the framework of CDPL theory the following expression for relative donor emission anisotropy has been obtained:equation(11)rDr0D=1−αfγwhereequation(12)fγ=πγexpγ21−erfγequation(13)α=γD/γα=γD/γwhere γ is the reduced concentration. It is the sum of reduced concentrations of donors and acceptors, respectively:equation(14)γ=γD+γA=π2CDC0DD+CAC0DA.