Triplet excited states energy transfer from

During irradiation polymers may undergo different photophysical processes, such as photon absorbtion and activated singlet entities formation, generation of free radicals from singlet and triplet excited states, energy transfer between donor and acceptor molecules present in the system or nonradiative transitions of intercrossing systems with excited triplet states formation. 

The transient absorption formed from laser flash photolysis of 15 was not quenched in oxygen-saturated solutions, presumably because the photoenolization to form 16 takes place from a singlet excited state of 15. Furthermore, Schworer and Wirz did not observe any transient absorption that can be attributed to the formation of the triplet excited state of 15 from direct irradiation. In contrast. Hurley and Testa used energy transfer to estimate that the triplet excited state of 15 is formed in 67% yield,whereas Takezaki et al. have measured the yields for forming the triplet excited state of 15 to be slightly higher or above 80%. They estimated the lifetime of the lowest triplet excited state of 15 to be 350 ps in ethanol. 

When the triplet is an excited state, energy transfer occurs to form singlet oxygen. Ground state triplets react with oxygen by a spin-allowed process which, for carbenes in particular, produce carbonyl oxides [64], It seems that triplet nitrenes react with oxygen slowly. This will be discussed more fully later. Here we examine the products formed from reaction of photolysis of phenyl azide in the presence of oxygen. 

Finally, in several cases the radiation has a frequency lower than required for the direct excitation of reactants. In this case excitation is still possible if a sensitiser is present this can absorb the radiation, be converted from a singlet to a triplet excited state and transfer energy to a reactant, say A (the respective triplet excitation energies being, of course, in the correct relation). A simple scheme in this case (S is the sensitiser) is... 

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