ABSTRACT
In this chapter, the authors describe the theories of radiative and radiationless transitions. They discuss interpretations of the data derived from measurements of the relaxation rates of highly excited valence states of polyatomic molecules. The most convenient direct spectroscopic means of obtaining the rates of photophysical relaxation processes involves measuring the temporal decays and quantum yields of fluorescence and phosphorescence of radiative excited states. The authors review progress in cataloguing and understanding the photochemical and photophysical behaviour of the more highly excited valence electronic states of azulene and other nonalternant aromatic molecules, of the thiocarbonyls, and of selected porphyrins and metalloporphyrins. Almost a full century of research in the post-quantum-physics era has resulted in a vast literature on the photochemistry and photophysics of polyatomic molecules. The porphyrins and metalloporphyrins, which are ubiquitous in nature, have been the subject of intense spectroscopic, photophysical, and photochemical interest.
