ABSTRACT
This chapter discusses different models for dynamically controlled relaxations, and also discusses applications to porous systems. It focuses on the geometric nature of porous systems as reflected through the relaxation dynamics of excited probe molecules placed in the porous systems. The chapter presents two different scenarios for relaxation in donor-acceptor systems embedded in porous solids. It has been repeatedly recognized that for a wide class of disordered materials the relaxation of initially excited probe molecules cannot be expressed in terms of a single decay rate. The chapter formulates the problem of the relaxation of an excited molecule which transfers its excitation to randomly distributed static defects. It provides a microscopic example for a parallel relaxation mechanism which exhibits a stretched exponential decay. The chapter provides target relaxation model, first for quenchers randomly walking on regular lattices. It extends the results to fractal structures and to cases which include waiting time distributions.
