Resonance energy transfer reaction kinetics

The maximum fluorescence quantum yield is 1.0 (100 %) every photon absorbed results in a photon emitted. Compounds with quantum yields of 0.10 are still considered quite fluorescent. The fluorescence lifetime is an instance of exponential decay. Thus, it is similar to a first-order chemical reaction in which the first-order rate constant is the sum of all of the rates (a parallel kinetic model). Thus, the lifetime is related to the facility of the relaxation pathway. If the rate of spontaneous emission or any of the other rates are fast, the lifetime is short (for commonly used fluorescent compounds, typical excited state decay times for fluorescent compounds that emit photons with energies from the UV to near infrared are within the range of 0.5-20 ns). The fluorescence lifetime is an important parameter for practical applications of fluorescence such as fluorescence resonance energy transfer. There are several rules that deal with fluorescence. 

No observable product was detected from the N02 /N02 reaction at low kinetic energies, placing an upper limit of 10" cm sec on the rate constant for an ion-molecule reaction from this collision. However, the use of isotopically labeled reactants demonstrated that the near-resonant charge-transfer reaction... 

This formula was first derived in ref. 6 when calculating the kinetics of donor luminescence decay in the presence of the randomly, i.e. chaotically, located acceptors under the condition n N and on the assumption of the resonance exchange mechanism of energy transfer. Similar equations were later used for the analysis of experimental data on the kinetics of electron tunneling reactions obtained under conditions of the chaotic distribution of the reagents and at n < N. As a rule, only the first term of the exponent in eqn. (23) has been taken into account, which is equivalent to employing the previously mentioned (see Sect. 2.1) stepwise approximation of the function 0(R,t) = exp[- 1V(jR)(]. In this case, one obtains... 

The free radical thus formed is resonance-stabilized. The addition of a new monomer to this free radical will decrease the resonance energy. The free radical formed by reaction (20-46) does not, therefore, start a new polymer chain. The polymer free radical, so to speak, commits suicide. A reaction of this kind can be classified kinetically as termination but chemically as transfer (see also Section 20.3.1). 

Conflicting thermodynamic and kinetic answers are particularly striking in the case of aniline. This compound is thermodynamically stable with a resonance energy similar to benzene, but it is kinetically unstable with a high reactivity due to the possibility of an easy electron-transfer reaction to oxygen. Reactivity is related to the HOMO and LUMO energies. Bird showed that the hardness of a molecule that is half of the HOMO—LUMO gap is related to the REPE. 

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