Excimer forming site

It seems to be natural to suspect if these excimer forming sites were the effective trapping center also for hole carriers. 

Excimer-forming site (EFS) in ground singlet state... 

It is clear that the sandwich-dimer studies discussed above apply to P1VN, not P2VN, since no photodimerization has been observed in bis(2-naphthyl)alkanes 10 and ethers 39). Nevertheless, the UV absorbance of naphthyl sandwich dimers, like that expected for [3.4] or [3.5] naphthalenophanes, differs from that of isolated molecules only for X > 325 nm. The same slight difference in UV absorbance probably occurs for excimer-forming sites. 

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]. 

Geometrical disorder, molecular packing, excimer-forming sites... 

Since It will be necessary to distinguish between triplet excitons which are mobile from those which are trapped at an excimer forming site, the symbol T will be used to represent molar concentrations of the former and for the latter. If more than one type of trapped triplet is present an additional numerical subscript will also be used. i.e. or, in general, T j. 

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). 

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. 

We note at the outset that there are two types of excimer formation in the 1x10 M-PEG aqueous solution. The first type of excimer forming site (EFS) results intermolecularly by association between chromophores from two different polymer chains. The number of these sites is directly dependent on the local concentration of labeled chains. The second type of excimer site arises from association between aromatic rings on the same polymer chain. In the present case, this means that cyclization of the labeled PEG chain must occur. 

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. 

Excimer forming sites in poly(methylphenylsiloxane) have been found... 

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