Excimer fluorescence, polystyrene

Following the initial observation113 of excimer fluorescence from dissolved polystyrene, Hirayama114 has reported a systematic survey of the fluorescence spectra of the di- and triphenylalkanes shown in Figure 14. In addition to the normal molecular spectrum of the phenyl group exhibited by all the molecules listed, an excimer band is exhibited by those systems in which the planar phenyl groups are separated by exactly three carbon atoms or a distance of 2.54 A in the trans propane chain. Since there is no evidence of a... 

Solid Films. The excimer fluorescence of solid films of polystyrene was observed using pulse radiolysis. The decay curves of the excimer fluorescence observed at 340 nm for solid films of polystyrene and CMS are shown in Figures 4(a) and (b), respectively. The lifetime of the excimer fluorescence of polystyrene agree with the reference data (12). In CMS, the initial yield decreases and the decay rate of excimer fluorescence increases with increasing chloromethylation ratio of CMS. These experimental results indicate that the chloromethyl part of CMS quenches the excimer of CMS and scavenges the precursors of the excimer as described below. 

CMS and Polystyrene Solutions in Cyclohexane. Both monomer and excimer fluorescences were observed in the pulse radiolysis of polystyrene solution in cyclohexane. The decay curves of monomer and excimer fluorescences at 287 and 360 nm are shown in Figures 7(a) and (b), respectively. Energy migration on the polymer chain has been discussed elsewhere (15). The dependences of the decay of monomer fluorescence and the rise of excimer fluorescence on the... 

Figure 7. The decay curves obtained from pulse radiolysis of polystyrene solution in cyclohexane (a) monomer and (b) excimer fluorescence monitored at 287 nm and 360 nm, respectively.

The absorption band around 520 nm is very similar to that of polystyrene excimer (2,3,5). The decay follows first order kinetics with a lifetime of 20 ns. The decay rate agrees with that of the excimer fluorescence and excimer absorption. The longer life absorptions, attributed to the triplet states and free radicals (2,5), were observed at wave lengths <400 nm, although the anionic species of polystyrene with the absorption maximum at 410 nm as seen in solid films (cf. Figure 5) was not observed. Figure 9 shows the absorption spectrum observed in the pulse radiolysis of CMS solution in cyclohexane. 

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. 

This value agrees with the lifetime of the excimer of polystyrene. The intensity of the excimer fluorescence decreased with increasing chloromethylation ratio. Electrons produced in cyclohexane, one of the precursors of the excimer, were scavenged by chloromethylated part of polystyrene. Absorption spectra of the excited states and the polymer radicals were measured by laser photolysis of the cyclohexane solutions. The results are summarized in Fig. 10 [67]. 

Very recently Kouchi et al. constructed an ion beam pulse radiolysis system and use it for the study of the LET effect in irradiated polystyrene thin films [106]. The nanosecond pulsed MeV ion beam with the variable repetition rate was obtained by chopping ion beams from a Van de Graaff. Time profiles of the excimer fluorescence from polystyrene thin films, excited by He+ impact, were... 

In ion-beam irradiated polystyrene, some kinds of reactive intermediates are produced. The excimer, which is one of the reactive intermediates, emits intense fluorescence. Thus, we measured the time profiles of the excimer fluorescence (328.5 nm) from ion irradiated polystyrene thin films. One of the results is shown in Fig. 6a and b for irradiation with 0.6 MeV He+ (several hundred pA beam current). In Fig. 6a, the irradiation time dependence of the excimer fluorescence intensity is shown. In Fig. 6b, the time profile (I) was recorded with an irradiation time of 0 s-139 s (low dose-time profile), and (II) in the irradiation time of 1839 s- 3839 s (high dose-time profile). The following experimental results were obtained. 

Fig. 7. Time profiles of the excimer from polystyrene resist films (0.5 pm thick) irradiated with ions. These time profiles were not influenced by the quenching seen in Fig. 6. The straight lines correspond to the fluorescence lifetime obtained by the electron pulse radiolysis study of polystyrene [38], From Ref. 35...

Deep UV Resist Reactions of CMS. The laser photolysis studies on CMS and polystyrene solutions in cyclohexane were carried out at 248 nm using a KrF excimer laser. The intensity of monomer and excimer fluorescence of CMS becomes weaker with increasing chloromethylation ratio, but the lifetime of the excimer is essentially independent of the chloromethylation ratio being almost the same as the excimer lifetime of polystyrene (20 ns). The lifetime of the monomer fluorescence has not been investigated by nanosecond laser photolysis because of the short lifetime (about 1 ns) (20,21) of the singlet. 

Excimer fluorescence in solid pure polystyrene is observed exclusively ... 

Intramolecular Excimer Fluorescence Studies in Polymers Carrying Aromatic Side Chains. Some years ago, it was shown that certain excited aromatic molecules may form a complex with a similar molecule in the ground state, which is characterized by a structureless emission band red-shifted relative to the emission spectrum of the monomer. The formation of such complexes, called "exclmers", requires the two chromophores to lie almost parallel to one another at a distance not exceeding about 3.5A° (11). Later, it was found that Intramolecular excimer formation is also possible. In a series of compounds of the type C5H (CH2)jiC H5, excimer fluorescence, with a maximum at 340nm, was observed only for n 3 -all the other compounds had emission spectra similar to toluene, with a maximum at about 280nm (12). Similar behavior was observed in polystyrene solutions, where the phenyl groups are also separated from one another by three carbon atoms (13). 

There continues to be extensive interest in latexes and micellar systems. The structure of acrylic latex particles has been investigated by non-radiative energy transfer by labelling the co-monomers with fluorescent acceptor-donor systems. Phase separations could also be measured in this way. Excimer fluorescence has been used to measure the critical micelle temperature in diblock copolymers of polystyrene with ethylene-propylene and the results agree well with dynamic light scattering measurements. Fluorescence anisotropy has been used to measure adsorption isotherms of labelled polymers to silica as well as segmental relaxation processes in solutions of acrylic polymers. In the latter case unusual interactions were indicated between the polymers and chlorinated hydrocarbon solvents. Fluorescence analysis of hydrophobically modifled cellulose have shown the operation of slow dynamic processes while fluorescence... 

Excimer formation and energy migration and its relationship toward an understanding of molecular mobility in polymers remains to be the nost prolific area of study in luminescence analysis. Measurements on excimer fluorescence from copolymers of polystyrene with various acrylates has shown that excimer formation is directly related to the... 

Here excimer fluorescence from phenyl-phenyl interactions in PS is the main experimental observable. This blend was selected because it has been demonstrated by numerous other techniques that miscible one-phase blends may be prepared by solution casting from toluene solvent. [2,3] Moreover, the blend may be forced to phase separate by thermal means, leading to a two phase system. In addition, we will consider results for blends of poly(2-vinyl naphthalene) (P2VN) with low molecular weight poly(cyclohexyl methacrylate) (PCMA) and polystyrene (PS). 

Excimer quenching accompanies the formation of Product I in irradiated polystyrene films, but the rates of change of excimer fluorescence intensity in vacuum and in air are quite different. They are contrasted in Figure 3. Photoconversion of excimer sites, which act as... 

Excimer formation has been studied in polystyrene and poly(a-alkylstyrenes)189 (PS), poly(vinylcarbazole),139>140 poly-(2-vinylnaphthalene), and poly-(4-vinyl-biphenyl).141 For polystyrene films, David et a/.189 showed that the fluorescence yield increased with increasing crystallinity, at both ambient temperature and 77 K. The contribution of excimer fluorescence yield increased in the sequence atactic (0.7) < atactic oriented (0.60) < isotactic amorphous (0.28) < isotactic crystallized (0.01), with normal yields relative to excimer given in parentheses. Similar results were obtained for poly(vinylcarbazole), PVCZ, although the contribution of excimer fluorescence at 77 was independent of crystallinity. The results can be interpreted in terms of electronic energy migration to low-energy defect sites from which excimer emission can occur. In PVCZ copolymers with fumaronitrile (10), diethyl fumarate (11), and diethyl maleate (12) (Scheme 6),... 

Another method involves excimer fluorescence as a molecular probe see Section 2.9. The question may be raised as to whether polymer blends will become more miscible if the differences in their solubility parameters are reduced. Excimer fluorescence provides some evidence see Rgure 4.14 (52). Here, 0.2 wt.% of poly(2-vinyl naphthalene), P2VN, is dispersed in a series of poly (alkyl methacrylates). These include the following, which are identified in Figure 4.14 by acronym methyl, PMMA ethyl, PEMA n-propyl, PnPMA isopropyl, PiPMA n-butyl, PnBMA isobutyl, PiBMA . yec-butyl, PsBMA ferf-butyl, PtBMA phenyl, PPhMA, isobomyl, PiBoMA benzyl, PBzMA and cyclohexyl, PCMA. Two other host polymers were polystyrene, PS, and poly(vinyl acetate), PVAc. 

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