Quenching Rates and Mechanisms

In the absence of added quencher (Q), the rate of change of the concentration of the excited state M is given by 

In the presence of added quencher, the rate now becomes that given by 

Under steady-state concentration conditions, the relative concentrations of the excited states are given by 

Since the intensity of the emission is proportional to the steady state concentration, 

The quenching reaction can occur by a number of different pathways. Three commonly observed bimolecular quenching pathways are energy transfer, oxidative quenching, and reductive quenching. Energy transfer can occur when the triplet 

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Also in the case of Cu.lO", where a [2]catenand-type moiety is present, a two-step protonation reaction occurs, but in this case the luminescence of such moiety is quenched by the Cu-based one [66]. Unfortunately, it is not possible to determine the quenching rate and mechanism for a variety of reasons, including (i) the instability of the protonated forms under laser irradiation in CH2CI2 [56] (ii) the lack of electrochemical potentials of the protonated forms and (iii) the weakness of the MLCT emission band of the Cu-based moiety, which is partially masked by the tail of the emission of the protonated subunit, albeit quenched. However, the energy transfer mechanism is thermodynamically allowed and probably active. 

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