Page - 58 - in Advanced Chemical Kinetics

1. Introduction In nature, there are a number of processes indispensable for lifemaintenance that begins with light absorption. Fromthis startingpoint, several chemical changes, rankedbyproba- bilities of occurrences, are triggered to give a product. In this process, molecular photophysical andphotochemical processes occur simultaneously, competing to eachother for theexcessenergy.Ontheotherhand, thesecompetingprocessesarealsocollaboratingto each other, since they occur through electronic excited state reactants that originate elec- tronic excited state intermediates. Based on the structures and characteristics of these excited electronic states intermediates, newmechanisms can be proposed, yet involving dissociations, isomerization, bond cleavages, nevertheless, taking into account that these excitedspeciespresentpeculiarelectronicdistributionand, therefore, involvephotophysical activation and deactivation mechanisms, that arise from their interaction with light, all governed by newand challenging kinetic laws. In this sense, the peculiar characteristic of the kinetic laws involved in molecular photophysical processes is that electronic excited species that can be reached by light absorption are considered unstable, and to achieve a more stable electronic configuration, excess energy is liberated by radiative and/or non- radiativeunimoleculardecays. The photophysical processes that occur immediately following the light absorption aim to ensure themechanisms to achieve the best energetic configuration to: (1) lead to the reactive excited intermediate, fromwhichthephotochemistrycanoccuror (2)achievethe fasterwayto release the excess energy and to retrieve the initial reactant. They can all be defined in a Jablonski diagram [1] (Figure 1) and their corresponding rate expressions can be obtained fromthere. Figure1. Jablonskidiagrampresenting themajor radiativeandnon-radiativeprocessesand their rates. Advanced Chemical Kinetics58