Excimer-exciton migration

III. EXCIMER-EXCITON MIGRATION IN CRYSTALS A. Experimental Information... 

Fluorescence techniques have been demonstrated in recent reviews 1( to be powerful methods for obtaining detailed information on the molecular structure of biopolymers and synthetic polymers. The objective of the present review is to concentrate on two aspects of the photophysics of synthetic polymers — excimer formation and singlet exciton migration. Both topics have been considered recently 11 but in less detail. 

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. 

Another characteristic of excimers in polymers is that they have a lower excitation energy than the isolated monomer, which implies that excimer sites in a polymer matrix can act as exciton traps. This is the reason polystyrene films, for example, exclusively emit excimer fluorescene although the concentration of the excimer sites is low, on the order of 1 mol%." In these systems, the excimer sites are supplied with excitation energy by exciton migration. Of particular interest is the fact that the concentration of excimer sites tends to limit the exciton migration range in these systems." ... 

Most simple aromatic polymers like polystyrene, polyvinylnaphthalene and poly(N-vinylcarbazole) exhibit both exciton migration and excimer trapping. 

As well known, so-called excimer sites exist in poly-N-vinylcarbazole. It is well established that these excimer sites are the efficient traps for the singlet and triplet excitons, which migrate along the polymer chain. The structure of these sites are thought to be a special conformation having a pair of carbazolyl groups arranged parallel each other. 

It has been suggested that in the solid state electronic energy migrates rapidly from the initially excited chromophore to excimer tr sites by either exciton diffusion or single-step mechanisms. 

The 0% derived from these data are summarized in Table I. With the single exception of PS-B, the PS homopolymers are more photostable than the copolymers. Among the random copolymers, the quantum yield for scission increases as the proportion of S-units decreases. The least stable copolymers are those in which the S-units are separated by one or more MMA-units excimer fluorescence is not observed in these copolymers (1). As the ratio of excimer fluorescence to fluorescence increases, the quantum yield for scission decreases. Wherever a migrating singlet exciton is trapped at an excimer site, it is less likely to contribute to the scission process. 

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