Polystyrene film, fluorescence spectra from

For the distyrylbenzene carbon-centered tetramer 46b, the fluorescence spectrum in the solid him differs from the spectra in solution or in a polymer matrix due to excimer formation [93]. A concentration of 5% in a polystyrene matrix is sufficient for a distinct broadening of the emission. For the higher homologue 46c, a fluorescence maximum of 472 nm was measured in freshly prepared films. If the film is thermally annealed, the spectrum shifts to 511 nm, probably due to intermolecular arrangement that favors excimer formation. 

Once laid, the polystyrene films were further purified by exhaustive extraction with methanol or n-heptane the progress of extraction was followed by the ultraviolet spectra of the extracts. These preirradiation extractions showed considerable variation in purity among the three polystyrenes in spite of reprecipitation measures. The degree to which solvents can remain with a 20/ film is suggested by the need of seven days of continuous methanol or n-heptane extraction to remove all of the extractable benzene from a film laid from that solvent and dried in vacuum at 65°C. for 24 hours. For film laid from methylene chloride, an optically clean n-heptane extract was obtained from the AIBN-ini-tiated sample within a few hours, but up to 48 hours were required for the benzoyl peroxide-initiated samples. The extracted 20/ polystyrene films were essentially non-absorbing above 285 m/, no absorption attributable to material other than polystyrene could be observed, and only one peak (337 m/ ) was seen in the fluorescence spectrum in methylene chloride. Once the films were purified by extraction, the products and wettability changes resulting from irradiation were the same for all polystyrene samples and were independent of the solvent from which the films were laid. 

Fluorescence spectra from a polystyrene film photolyzed in vacuum are shown in Figure 2. Similar but less intense spectra were observed in films irradiated in air. The Product I responsible for this spectrum was partially extractable from the film with methanol the fluorescence spectrum of the extract is shown also in Figure 2. Comparison of these spectra with those of a wide variety of reasonable model compounds suggests that Product I is related to 1,3-diphenyl-l,3-butadiene since the spectral match with 1,4-diphenyl-l,3-butadiene, shown in Figure 2, is quite close. Product I spectra were obtained also from the residual films after extraction, indicating that the diene moiety may form part of a photolyzed chain as well as exist as a short-chain fragment. Fluorescence spectra that could be related to higher polyenes were not detected in the vacuum exposures. In air exposures, however, the prompt emission spectra from films did exhibit a weak shoulder superimposed on the... 

The only head-to-head polymer which has been examined for excimer fluorescence is polystyrene 25). Unfortunately, the synthetic route to this polymer leaves a number of stilbene-based structures in the sample, which have a lower-energy singlet state than either PS monomer (285 nm) or excimer (330 nm). Thus, fluorescence from these intrinsic stilbene traps was seen in the spectra of head-to-head PS in pure films and, to a lesser extent, in fluid solution. In the latter, the fluorescence of PS monomer was predominant, and the small amount of stilbene fluorescence was increased when a nonsolvent (methanol or cyclohexane) was added to the 2-methyl-tetrahydrofuran solution. In films of the polymer, stilbene fluorescence was the major spectral band, although some PS excimer fluorescence was also present in the spectrum. No monomer fluorescence at 285 nm was detected from films. Given the impure nature of the head-to-head PS sample, no conclusions on excimer formation in these systems could be drawn. 

It has been shown [155,171] that the dependence of excimer emission intensity on acceptor concentration obeys the Stern—Volmer equation whether M or D is the donor, whereas a second-order equation is obtained if both types of excited state simultaneously act as donor. It seems that in poly-1-vinylnaphthalene and polyacenaphthalene films at room temperature, energy transfer to benzophenone occurs from M, although normal fluorescence cannot be detected in the emission spectrum of the polymers in these conditions [155]. Decay time measurements have shown that the excimers in solid polyvinylcarbazole are traps rather than intermediates in the energy transfer process [148]. With polystyrene, however, it has been clearly demonstrated that energy transfer to tetraphenylbutadiene occurs from both excimer and isolated excited chromophore [171]. 

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