DECOMPOSiTION OF DIOXETANES: A UNIQUE PROBE INTO MECHANISM AND ENERGY TRANSFER PROCESSESt

3,4-Dimethyl-3,4-di-n-butyl-1,2-dioxetane breaks down thermally to give singlet excited 2-hexanone (5 per cent) and triplet 2-hexanone (3.5 per cent). This is in contrast to the very high yield of triplet acetone reported from tetramethyl dioxetane. The yields of excitation products are determined by measurement of the amounts of Norrish Type II photochemical products from 2-hexanone. The results are confirmed by quenching of the thermal process with 1,3-hexadiene. Photolysis of the dioxetane (>369 nm) gives 4 per cent singlet 2-hexanone and 2.2 per cent triplet. The results are consistent with either a diradical intermediate or a concerted process of a very unusual type. Photosensitized decomposition of this dioxetane produces an intriguing variation in the yields of photochemical product. The product ratio is charac-teristic of triplet and ground state product formation only; no excited singlet is produced. The highest yield of product is formed with biacetyl. Higher and lower energy sensitizers give less of the triplet product and more ground state product. The reasons for this seem to differ; the lower energy sensitizers produce very little excited state. The higher energy sensitizers quantitatively give triplet 2-hexanone as a product, but energy transfer from this excited product back to ground state sensitizer is very rapid. The rates of this process have been measured and exceed the diffusion-controlled rate, which suggests that the back transfer occurs within a solvent cage. The effect of solvent viscosity is consistent with this assumption. By this technique, rates of energy transfer exceeding 1012 s ' have been determined. The dependence of this rate on energy separation shows that the intrinsic rate of energy transfer falls off as the gap between the donor and acceptor increases. The formation of sensitizer and acceptor within the solvent cage permits these exceedingly high rates to be measured directly. This system thus provides a unique probe for energy transfer mechanisms