Phosphorescent dienones

Since the phosphorescence emission from (6) (68.8 kcal/mole) is very similar in energy and vibrational structure to benzophenone, and has a short lifetime (0.5 msec), it was proposed that the photorearrangement takes place via the triplet state. A Zimmerman-like mechanism is as follows for the formation of the cyclopropyl ketone (7) from dienone (6) ... 

The fluorescence and phosphorescence excitation and emission spectra of commercial polypropylene and poly(4-methylpent-l-ene) are examined using a fully compensated spectrofluorimeter. The excitation spectra of the polymers are compared with the absorption spectra of model chromo-phores of those believed to be present in the polymers. The fluorescence emission is associated primarily with the presence of enone and the phosphorescence is associated with dienone impurity chromophoric units. Bromination of cold hexane extracts of the polymers reduces significantly the intensity of the fluorescence confirming the presence of ethylenic unsaturation. The behavior of the luminescent enone and dienone groups during irradiation under sunlight-simulated conditions is examined also. Possible mechanisms for the participation of these chromophoric units in the photooxidation of the polymers are discussed. 

In Figure 3 we compare the polymer phosphorescence excitation spectra with other possible phosphorescent carbonyl chromophores known to be present in the polymer (1,8,9,10). The absorption of a typical dienone (14) or -al), trans,trans-hexa-2,4-dienal, more closely matches that of the polymer excitation than does a typical long chain aliphatic aldehyde or ketone (14). 

Figures 1.6 and 1.7 show fluorescence and phosphorescence of polyolefins, respectively [69]. Figure 1.6 shows that fluorescence of polyolefins cannot be attributed to the presence of polynuclear aromatic hydrocarbons (e.g. naphthalene), because the positions of emitted bands differ significantly. On the other hand phosphorescence emission (Fig. 1.7) shows the presence of a,)S-unsaturated carbonyl groups. These results indicate the presence in polyolefins of enone and/or dienone chromophoric groups which are responsible for the observed luminescence.

These authors observed phosphorescence from 4,4-diphenylcyclohexadie-none at 77°K (0-0 band at 68.8 kcal per mole) and pointed out that the similarity between the phosphorescence spectrum of the dienone and that of benzophenone indicates that the lowest triplet state of the former compound is n — IT. This evidence suggests that a triplet state of the dienone may be involved in the unsensitized rearrangement but is not sufficient to justify Zimmerman and Swenton s conclusion that such a state is demanded. Interestingly, naphthalene (triplet 0-0 band at 61 kcal per mole) sensitizes rather than quenches Reaction (110). Such sensitization has also been observed by Caspar and Hammond (unpublished results) in a similar system and can be best interpreted as involving singlet excitation transfer from naphthalene to the dienone. 

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