Poly , phosphorescence excitation spectra

The PL spectrum of a thin film of poly(3,6-dibenzosilole) 31 at 77 K exhibited a 0-0 transition at 3.5 eV and a second maximum at 3.3 eV (excitation at 4.4 eV) [41]. The phosphorescence emission spectrum at 77K consists of a broad band exhibiting vibronic structure (excitation at 3.9 eV). The polymer triplet energy level was taken to be the onset of triplet emission at 2.55 eV. This is considerably higher than the triplet energy of commonly used polyfluorenes (2.1 eV) [10,46] making it a host for phosphorescent emitters without the risk of energy back-transfer onto the polymer. 

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). 

With a glassy solution of poly-1-vinylnaphthalene, the delayed emission spectrum has been shown to consist of an emission having a mean lifetime of approximately 80 ms at the normal fluorescence wavelength, in addition to the phosphorescence having a mean lifetime of about 2 s [159]. The delayed fluorescence did not appear in the spectrum of 1-ethylnaphthalene. With the polymer it was found to be inhibited by piperylene, a well-known triplet quencher. These results have been explained by mutual annihilation of two excited triplet states produced by the absorption of two photons by the same polymer molecule. They are considered as strong evidence for migration of the excited triplet state in poly-1-vinylnaphthalene. In polyacenaphthalene, however, which is chemically very similar to poly-1-vinylnaphthalene (see p. 409), no delayed fluorescence could be detected in the same experimental conditions [155]. 

Poly(butadiene) is transparent to sunlight unless thermally oxidized, when it becomes photo-unstable. It has been established that, upon thermal oxidation, the polymer becomes phosphorescent at low temperatures, with emission and excitation spectra (Figure 33) typical of an a3-unsaturated carbonyl (128). If the sample is photolrradlated the phosphorescence spectrum changes into that typical of a saturated carbonyl. Clearly a complete photooxlda-tlon mechanism must Incorporate reactions to explain such observations, but given the very great sensitivity of the Ixmlnescence... 

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