The unusual property of four-membered ring peroxide heterocycles ( 1 , 2 - dioxetane8) to produce electronically excited carbonyl fragment8 on thermal docomporition is discussed. The 8ynthetic tools for the preparation of such labile rubrtances and the analytic method8 for a~serring excitation yields are covered. Using the completo 8eries of methylated derivative8 of tho parent 1,2- dioxetano, their thermal 8tability and excited state production is rationalized in terms of a modified diradical mechanism. With the help of birdioxetanes, it is rhavn that generation of upper excited stater is improbable, do8pite the fact that sufficient energy is available for this purpose. Attachment of dioxetaner to biomolecules via mild erterificationr provides intereating opportunities for photobiological exploration.
The generation of electronically excited states by optical excitation and tho physical and chemical consequences of such excited states fall into the realm of photochemirtry, an active field of science with an ertablished tradition. On the other hand, bioluminescence and chemiluminescence, spectacular phenomena in which excited state8 are formed via biological and chemical transformations') , have received only during the last few decade8 detailed consideration. A well known example (Eq. 1 ) is firely luciferin autoxidation. In this care an elusive poroxido heterocycle is po8tulated (Eq. 21, namely the a-peroxylactone. Thir species is of such high energy content that on decarboxylation an electronically excited product is formod. The doexcitation manife8tr itrolf in the emission of light which i s identical to the fluoremcence observed for tho optically excited product molecule. Thua, enzymatic and also chemical autoxidationr (e.g. luminol) can mimic all tho facets of optical oxcitation. Intomwdiary energy-rich peroxide heterocycler roplace therewith radiation sourcorn such a8 8unlight and lamp.. Viewed from this perspective, such heterocyclam allow entering "photochemirtry through the backdoor".