Energy transfer triplet singlet

The rather stable diacyl peroxides such as dibenzoyl or phthaloyl peroxide have attracted special interest as some of their reactions, mostly not chain reactions, are chemiluminescent. Triplet-singlet energy transfer is very often involved, and emission generally occurs only when a fluorescer is present since the primary excited products cannot emit in the visible range of the spectrum. 

D 1A —- 1D + 1 A (triplet-singlet energy transfer). This type of transfer leads to phosphorescence quenching of the donor (e.g. phenanthrene (T) + rhod-amine B (S)). 

The fluorescer of choice for counting chemienergized triplet states via triplet-singlet energy-transfer chemiluminescence has been 9,10-dibromoanthracene (DBA). Like DP A, it is readily available and easily purified unlike DPA it has a relatively low fluorescence quantum yield, that is, 0dba about 0.10 and is temperature- and solvent-dependent. For reliable triplet yields, the fluorescence quantum yields of DBA should be measured under the conditions at which the chemienergized carbonyl product K is generated. 

Triplet quenchers having triplet energies well below that of naphthyl halides and halogeno-biphenyls do not quench the triplet-state reactions of these molecules in the normal way, and it has been speculated that some reactionenhancing process counterbalances energy transfer.252 A study of triplet -> singlet energy transfer between benzophenone and perylene in vitreous solution... 

The activation energy in this case varied from 87 to 93 kJ/mol in different solvents. From the temperature dependence, several competitive reaction paths for this dimethyl-dioxetanone decomposition were deduced, all having a biradical as first intermediate. Heavy-atom effects often play a role in dioxetan chemiluminescence. If DBA is used as fluorescer, the quantum yield is markedly greater than that observed when DPA is used - although the latter has a fluorescence efficiency of 0.89, compared with 0.1 for DBA. In both cases triplet-singlet energy transfer is the origin of the chemiluminescence. 

From these and additional results from other compounds of this type it was concluded that the chemiluminescence of (19) and (22) is caused by intramolecular triplet-singlet energy transfer of a mixed dipole-dipole and exchange character. This is demonstrated by the fact that the fluorescence from the singlet-state of the emitter-moiety is not governing the chemiluminescence of these compounds and that, moreover, the most important excited state of the phthalate moieties is the first excited triplet state [38]. Whereas in the compounds (19) and (22) the hydrazide group and the fluorescer group can alter their relative spatial position by (possibly restricted) rotation around the CH2-bonds, this is not possible in paracyclophane-type compounds like (23). 

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