Intramolecular triplet energy transfer, effect

Figure 2.11. Effect of separation distance on the rate constant for intramolecular triplet energy transfer as calculated for (a) Dexter electron exchange with /S = 0.1, (b) Dexter electron...

The direct irradiation at 254 nm of the allenes (67) in hexane gives the molecule in its triplet state. Bridging within this state affords the intermediate biradical (68). Bond formation then yields the major products (69) and (70). Minor products (71) and (72) are also formed. Irradiation at X. > 280 nm fails to yield products. However, acetophenone sensitization is effective. The authors suggest that in this instance the vinyl moiety is the chromophore which is excited and that intramolecular energy transfer is involved. The yields obtained from the reactions are shown in Table 1. 

---