NEW ASPECTS OF THE PHOTO CHEMISTRY OF N,PI-ASTERISK-EXCITED STATES

New aspects of the photochemistry of n,pi-excited states have emerged from the detailed examination of the hydrogen abstraction reaction of singlet-excited states and from the computational examination of the interaction between triplet-excited ketones and molecular oxygen. Firs t, the strong dependence of the fluorescence lifetimes of the azoalkan e 2,3-diazabicyclo [2.2.2] oct-2-ene (DBO) on the chemical environment , which is caused by quenching along the pathway for hydrogen abstract ion, is employed to monitor inclusion into supramolecular assemblies. P-Cyclodextrin was selected as a prototypal host. The novel fluorescen t probe DBO, the first one with an n,pi electronic configuration, dis plays properties, which are distinct form those of the more commonly e ncountered pi,pi fluorescent probes. Besides other advantages, the mo st important one appears to be the chemical, selectivity of DBO, since fluorescence quenching is mediated through a (partial) abstraction of hydrogen atoms from the surrounding C-H and O-H bonds, not by the app arent polarity or acidity of the environment. Second, a computational study has addressed the role of the putative trioxetane in the oxygen quenching of triplet-excited ketones. The data demonstrate that the tr ioxetane is no viable reaction intermediate, since the singlet biradic al derived from addition is predicted to be more stable. In view of th e computed data, previous studies, which presented arguments against t he chemical addition of oxygen to excited ketones, require more detail ed attention.