Journal of Photochemistry and Photobiology A: Chemistry 278 (2014) 25–30 Contents lists available at ScienceDirect Journal of Photochemistry and Photobiology A: Chemistry journal h om epa ge: www.elsevier.com/locate/jphotochem Chemical and photochemical reactions under restricted geometry conditions: Similarities and differences Eva Bernal a , Beatriz Sarrion a , Antonio Barrios a , Pilar Perez a , Aila Jimenez a , Francisco Sanchez a , Manuel Lopez-Lopez b,∗ a Department of Physical Chemistry, Faculty of Chemistry, University of Sevilla, c/ Profesor García González s/n, 41012 Sevilla, Spain b Department of Chemical Engineering, Physical Chemistry and Organic Chemistry, Faculty of Experimental Sciences, University of Huelva, Avda. Tres de marzo s/n, Campus El Carmen, 21071 Huelva, Spain a r t i c l e i n f o Article history: Received 18 September 2013 Received in revised form 12 December 2013 Accepted 19 December 2013 Available online 2 January 2014 Keywords: Restricted geometry conditions Chemical and photochemical reactions Pseudophase Model a b s t r a c t 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(NH 3 ) 4 (pzCO 2 )] 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. © 2013 Elsevier B.V. All rights reserved. 1. Introduction There is a growing interest in the study of reactions under the so-called restricted geometry conditions (r.g.c.), that is, under con- ditions in which the chemical species of interest are forced to remain (at least in part) bound to some receptors such as micelles, cyclodextrins, and polymers [1]. This interest comes from the fact that the union of the species and the receptors produces a change in the properties of both, the receptor and the ligand. These changes can affect spectroscopic properties, chemical [2] and photochem- ical [3] reactivity. In fact, these changes in properties are what make the studies under r.g.c. interesting. Results in this field can, as a matter of fact, find applications in the fabrication of chemical and biochemical sensors [4], catalysis [4,5], enzymatic reactions [6], molecular electronics [7], etc. We are currently interested in the effects of r.g.c. on chemical [8] and photochemical [9] reactivity. These effects arise as a con- sequence of the changes in the free energies of the reactants (and ∗ Corresponding author. Tel.: +34 959218206. E-mail address: manuel.lopez@diq.uhu.es (M. Lopez-Lopez). transition states) when they are bound to a receptor, and can be quantified through an activity coefficient. Thus, the change in free energy of a reactant, R, as a consequence of the union to a receptor M (1) is given by: G = RT ln  R (a)  R = 1 1 + K [M] (b) (2) K being the equilibrium constant for the process in Eq. (1). It can be shown [10] that Eq. (2) produces the well-known equation of the Pseudophase Model [11]: k obs = k f + k b K [M] 1 + K [M] (3) 1010-6030/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jphotochem.2013.12.016