Excimer-forming polymers

The analysis of fluorescence quenching in excimer-forming polymers embodied by Eqs. (7)—(9) contains a number of unresolved flaws. First, the decay schemes of P2VN U,19S) and other polymers 44 45 151,158,195) in solution require at least three decay terms to adequately interpret the fluorescence behavior over a range of temperatures. While additional work on the decay response of polymers as a function of [Q] is needed, the current quenching scheme is probably too simplistic. Second, an... 

Quenching rates derived for excimer-forming polymers are questionable, given that the decay kinetics of unquenched samples are uncertain. More reliable values of kQM can be obtained for non-excimeric polymers. 

In synthetic polymers, the interpretation is necessarily more difficult The form of Equation 4 and Equation 5 requires that the kinetics of formation and decay of complexes are modelled adequately by rate-constants and that they take place in a homogeneous medium. If, as in synthetic polymers, the population of excimer trap sites, may occur through energy migration or rotational diffusion, a rate-constant may not be an adequate representation of the process, some time-dependent parameter being required (see below.) Heterogeneity may also play an important role. Thus in earlier work the fluorescence decay of excimer-forming polymers was modelled adequately by a scheme based upon simple excimer kinetics to which had been added terms to account for the occurrence in co-polymers of monomer sites which, by their isolation, could not form excimers (4-10). For polymers which contain isotactic and syndiotactic sequences, or rather, are made up of meso and racemic triads (14), the kinetics may be similarly a superimposition of simple schemes appropriate for the different sequences. 

Our interest in polymer photodegradation has led us to an investigation of the competition for migrating energy by chain impurities and "defects" such as excimer-forming sites. To isolate the intrachain phenomena, photooxidation of polymer solutions in dimethoxymethane have been carried out. As an example of a very photostable excimer-forming polymer, PS has been selected. Since poly(methyl methacrylate) (PMMA) is known to be far less stable than PS on a quanta-absorbed basis, MMA units have been incorporated into the chain to act as weak links. For energy sinks, 1-VN and 2-VN units have been made part of the chain. 

An analysis of the time-resolved fluorescence in excimer-forming polymer systems can also prove informative in elucidating the conformations of macromo-lecular chains in solution these data are particularly relevant in terms of the structures adopted by polyelectrolytes in aqueous media. In this latter context, it is useful at this stage to briefly overview the development of the kinetic schemes to describe excited state interactions in fluorescent polymer solutions. 

The interpolymer association is a distinctively different property of exciplex forming polymers from that of excimer forming polymers. The reason must be attributed to the presence of the ground state interaction in the former polymers. Interpolymer excimer formation is, however, facilitated by the aid of hydrophobic interaction in water. Polyionenes bearing anthryl groups (J2) form both inter- and intrapolymer excimer in water (2s). The excimer intensity decreases with increasing hydrophobic interaction. All experimental results indicate that weak intermolecular interactions almost undetectable in small molecule systems are amplified enormously in polymer systems in dilute solutions. 

Polyesters bearing 9-anthryl or 1-pyrenyl groups originally prepared as photocurable and excimer forming polymers (7) exhibit the tendency of singlet energy migration. The... 

In our early work, it became clear that simple Birks kinetic schemes representing excimer formation and decay were inadequate for modelling fluorescence in excimer forming polymers [17-22]. The empirical observation that multiple (dual and triple) exponential terms could model successfully the decay of monomer and excimer fluorescence in poly(vinyl naphthalenes) led us to propose simple models which... 

Indeed in toluene at 22 , the cyclization rate constant for the exciplex forming polymer is substantially larger than that for the excimer forming polymer, figure 9. I find it remarkable that such a subtle feature of the experiment is effectively predicted by the WF model. While the difference in the slopes of the two lines in the log-... 

The method is based on the fact that the rate of conformational change required for excimer formation depends on the free volume induced by the segmental motions of the polymer occurring above the glass transition. DIPHANT (compound 3 in Figure 8.3) was used as an excimer-forming probe of three polymer samples consisting of polybutadiene, polyisoprene and poly(dimethylsiloxane).a)... 

This investigation shows that it is indeed possible to study the flexibility of polymer chains in polymer matrices by means of excimer-forming probes and that the rotational mobility of these probes reflect the glass transition relaxation phenomena of the polymer host matrix, in agreement with the appropriate WLF equation. 

How is the monomer fluorescence of aryl vinyl polymers or intramolecular excimer-forming compounds distinguished from that of monochromophoric compounds ... 

One method of directly determining QM and Q for excimer-forming compounds involves the relationship direct method requires a number of samples of an aryl vinyl polymer with differing molecular weights. If QM and Q are assumed to be independent of molecular weight, then they can again be obtained from the (pD vs. polymer samples. The rate can be determined if FM is assumed to be identical to that of the monochromophoric compound. It follows that kM = kFM/QM. While these methods are rarely used in the literature, it is worthwhile to review the assumptions that Qm and Q are independent of solvent, or of the molecular weight and structure of the compound. 

The conformational statistics of asymmetric vinyl chains such as P2VN are well-known 126). The rotational conformers of isotactic (meso) dyads are entirely different from those of syndiotactic (dl) dyads. Frank and Harrah132) have described each of the six distinct conformers for meso and dl dyads, using the t, g+ and g nomenclature of Flory 126). Excimer-forming sites (EFS) are found in the tt and g g+ meso states, and in the degenerate tg , g t dl state. Because the rotational conformers of compounds such as l,3-bis(2-naphthyl)propane do not match those of either the iso-or syndiotactic dyads of P2VN, the propane compounds make poor models of aryl vinyl polymers. However, the rate constants of fluorescence and decay of the intramolecular excimer in polymers can usually be determined from the propane compounds (but see the exceptional case of PVK and its models133)). 

The dynamic RIS model, which was proposed before to investigate the dynamics of local conformational transitions in polymers, is elaborated to formulate the increase in the number of excimer-forming sites through rotational sampling. Application of the model to the meso and racemic diads in PS confirms the fact that conformational mobility of the chain plays a major role in intramolecular exclmer formation. Comparison with experiments demonstrates that the decay of the monomer fluorescence in styrene dimers is predominantly governed by the process of conformational transitions. 

For efficient hole transport the materials should be constructed such as to prevent dimer formation in both the ground and excited states. Excimer forming sites, in fact, may constitute traps for charge transport materials [45-47]. More recently, it has been shown that the host polymer can also affect the transport properties of the film [48]. 

The formation of intreunolecular excimers in polymer systems has aroused much interest in recent years (1). Perhaps most notable is the general observation that the reaction kinetics do not obey the accepted Birks scheme for low molecular weight systems (2)- In this scheme the fluorescence decay kinetics of the monomer (M) eind excimer (D) species Ccui be separated spectrally with fluorescence response functions of the form... 

The incompleteness of this description when applied to polymer systems has been previously explained by the evidence for isolated monomer sites which do not partake in the excimer forming process and which require the consideration of a third decay component, tj, i.e. 

Excimers, which are simply excited state conplexes formed from equivalent chemical species, one of which is excited prior to complexation, were first reported in small molecule systans. However, daring the past 20 years one of the more vigorous research areas in photophysics has been the investigation of excimers formed in polymers (1). In most of the cases reported to date, the excimer studies in polymer systems have been conducted on polymers bearing pendant aromatic chromophores. Only a few papers have been published on excimers formed from polymers with the two species participating in excimer formation spaced at relatively large... 

Having established the existence of the excimer emission of NDI based polyurethanes in solution, and realizing that the intramolecular excimer forming naphthyl carbamate groups are located on the backbone of the polymer, it becomes apparent that an excellent opportunity exists for chain conformational studies as a function of solvent. Figure 10 shows the steady-state fluorescence spjectra of NDI-650 in four solvents with distinctively different solvating power. In each case (curves a-d) both monomer and excimer emission are observed however, tlie ratios of excimer to monomer emission reflect conformational differences of the NDI-650 polymer in the solvent employed. The excimer to monomer intensity ratio... 

The absence of low energy excimer in solid solutions of PVCA and other aromatic polymers can easily be explained by the need of thermal activation to form a sandwich-pair (excimer-forming site) of two neighboring bulky aromatic groups. This has been shown by Frank for P2VN and Poly(4-vinylbiphenyl) (21,39). 

They propose that in excimer-forming vinyl aromatic polymers in fluid solution, the... 

In copolymers the exciton migration length is limited by the polymer structure available, and most importantly, by polymer conformation. When exciton migration involves excimers or exciplexes, long sequences of excimer-forming-sites (EFS) must be correctly orientated for maximum distance. 

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