The Mechanisms of Photochemical Reactions Quenching, Sensitization and Wavelength Effects

1 The Mechanisms of Photochemical Reactions Quenching, Sensitization and Wavelength Effects 

The nature of the reactive state (or states) is the first important point in the overall mechanism of a photochemical reaction. It is not merely of academic 

Quenching is observed as the lowering of a quantum yield of reaction with increasing concentrations of an additive Q. In the first place, this is simply an experimental observation which implies no particular quenching mechanism. Before the actual quenching of the reactive state by energy transfer can be established, it is necessary to eliminate certain apparent quenching effects  

Remembering that the absorbance is proportional to the concentration of each species, it is readily apparent that the neglect of the inner filter effect may be dangerous when the quencher is present in concentrations much higher than the reactant. It should not be forgotton that the additive may also contain absorbing impurities, even if it does not absorb itself. 

If the photochemical reaction is followed only by measuring the concentration of the light-absorbing ketone, it will indeed appear that the additive (acridine) acts as a quencher in reality it acts as a radical scavenger through a thermochemical reaction which is quite unrelated to the nature of the reactive excited state. 

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