The Chemiluminescence of Organic Compounds

The chemiluminescence of organic compounds in solution is briefly surveyed with some emphasis on the mechanisms which result in relatively efficient population of an excited state of a product molecule. This is done in terms of the relation = x 'E 1F with particular reference to peroxide compositions. iNTRODUCTION The study of chemiluminescence mechanism in a persistent and systematic fashion is an activity largely confined to the last decade. This is not surprising when one considers that the problems associated with ground—excited state interactions must be multiplied by the complexities of a vigorous oxidative reaction for a full description of a chemiluminescent reaction. A general survey of the phenomenon discloses that, with almost no exceptions, oxidation is involved, the mechanism ranging from the relative simplicity of an electron transfer reaction to the confusion of a many step, bond cleavage process. The excitation step in certain bioluminescent reactions is of particular interest as an example of the latter kind. Although luminescence in the gas phase has been comparatively well understood for a long time, there are various reasons why the problem of chemiluminescence in solution must be approached differently. All gas phase reactions emitting light involve dior tn-atomic molecules and often atom recombination. The restriction of the degrees of freedom in small molecules and the near absence of collision in the gas phase leads to a non-equilibrium distribution of the energy derived from the exothermic reaction. If one assumes that the reactants in an exothermic reaction in solution are, within the lifetime of an excited species, in equilibrium with the surrounding solvent, then it is obvious that in this case vibrational energy, no matter how great, is an unlikely source of electronic excitation. Thus of the various mechanisms for organic chemiluminescence, electron transfer is particularly attractive. Conversely, there is no well proven instance of the emission of light from a radical recombination reaction in solution, although the bonds formed in such a process usually release more than sufficient energy to populate an electronically excited state. A further consequence of the conditions under which most organic chemiluminescent reactions occur is the observation that emission is from the first excited singlet state, the triplet state being easily quenched. However, there