Phosphorescent carbonyl chromophores

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). 

In photo-oxidized polymers, HAS may meet excited chromophores (Ch ) such as residues of polymerization catalysts or carbonyls arising in oxidized PO. Attempts were made to obtain information on the ability of HAS or derived NO to quench Ch [213, 214], The phosphorescence emissions from sensitizers anthraquinone, benzophenone or benzhydrol were not affected by HAS. These data indicate that quenching ability is lacking in the HAS mechanism [214],... 

T uminescence studies of commercial polymers have provided valuable -L information on the nature of some of the light absorbing chromophoric impurities believed to be responsible for sunlight-induced oxidation (1-13). The luminescence (fluorescence and phosphorescence) from commercial polyolefins has been attributed to the presence of impurity carbonyl groups (1,2,5,6,8), and recent work on polypropylene has indicated that these groups are conjugated with ethylenic unsaturations... 

Polymer Luminescence Spectra. Figure 1 shows typical fluorescence and phosphorescence excitation and emission spectra obtained from commercial polypropylene film (or powder). Poly(4-methylpent-l-ene) exhibits similar spectra to those of polypropylene. The excitation spectrum for the fluorescence has two distinct maxima at 230 and 285 nm while that of the phosphorescence has only one distinct maximum at 270 nm with rather weak and diffuse structure above 300 nm. It is clear from these results that the fluorescent and phosphorescent chromophoric species cannot be the same. This, of course, does not rule out the fact that both may arise from carbonyl emitting species, as will be shown later, since these chromophoric groups when linked to ethylenic unsaturation can have quite distinct absorption (14) and emission spectra (15,16,17). 

This, therefore, diminishes the possibility of a simple a,/3-unsaturated carbonyl chromophore being responsible for the majority of the phosphorescence emission. However, these groups may indeed phosphoresce (16,17) but they are overlaid evidently by a much stronger emitting impurity species. 

Moreover, If as has been suggested, similar emitting chromo-phores might appear in such diverse polymers as, for example, poly(propylene), poly(butadiene), and poly(amldes), where aS-un-saturated carbonyls have been identified by luminescence, the method loses some credibility. The presence of more than one type of emitting chromophore is a further complication, which may however provide further evidence of the type of polymer. Inspection of Table 11 reveals that, for example, In the polyamide Nylon 66 in chip form fluorescence is observed in the region of 417 nm, whereas phosphorescence Is seen at 400 nm. Clearly phosphorescence cannot be to the red of the fluorescence thus the phosphorescent species must be different from that which fluoresces. 

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.

Ultraviolet and visible light absorption spectroscopy can be used to identify chromophores (e.g. benzene rings and carbonyl groups) and to determine the lengths of sequences of conjugated multiple bonds in polymers. It also can be used to analyse polymers for the presence of additives such as antioxidants or for detection of residual monomer(s). Additionally, fluorescence and phosphorescence techniques are important in studies of polymer photophysics. 

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