Chemical and photochemical reactions under restricted geometry conditions: Similarities and differences

• We consider the applicability of Pseudophase Model to photochemical reactions in the presence of receptors.
• Using a previously developed treatment, we conclude that this application is only formal in the sense that apparent parameters are obtained.
• The relation between apparent and true parameters is established.

Chemical reactivity under restricted geometry conditions can be described through the equations of the Pseudophase Model (a two state model). This model is based on the assumption of an equilibrium between these two states (free and bound) which is not perturbed by the reactive event. This implies that the reaction in which these states participate must be slow, in relation to the exchange processes between the two states. This condition holds in the case of chemical reactions, but does not hold for very rapid photochemical reactions. However, the Pseudophase Model seems to be applicable, apparently, in the case of excited state reactions. According to this a similar formal behavior is observed in chemical and photochemical reactions. However, a closer inspection of the data reveals important differences between the two types of reactions, in particular in the meaning of the parameters appearing in the equations of the Pseudophase Model. These similarities, and differences, are considered in this work using, as representative examples, the reactions between [Fe(CN)5(4-CNpy)]3− and [Co(NH3)4(pzCO2)]2+ (chemical) and the quenching of pyrene by the iodide ion (photochemical), under restricted geometry conditions. These conditions are due to the presence of receptors (CTACl micelles and DNA, respectively) in the reaction media. An explanation for these facts is given.

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