Migration singlet excitons

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. 

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. 

One point which can be settled qualitatively, however, is whether singlet exciton migration does in fact occur in the aryl vinyl polymers. It will be shown that available evidence supports energy migration as an important feature of the photophysics of polystyrene (PS), poly(l-vinyl naphthalene) (P1VN), and poly(2-vinyl naphthalene) (P2VN), the homopolymers which are the subject of the majority of the review. 

A useful measure for exciton migration is the diffusion length L = (Dto)1/2. Experimental data show that for Frenkel excitons in molecular crystals at room temperature the diffusion coefficient D 10 3 cm2/s and the lifetime of singlet excitons to 10 8s. This gives a typical diffusion length L 10 6cm (for anthracene crystals L 5 10 6cm). 

Energy transfer is a ubiquitous phenomenon in molecular crystals in which singlet and triplet excitons with lifetimes of nanoseconds and up to 10-100 milliseconds, respectively, migrate incoherently via either dipole-dipole or exchange interaction. Typical intermolecular jump times are 1 ps for singlet excitons and 10 ps for triplets [2], This implies that a singlet (triplet) exciton can visit some... 

The results from the experiment under various temperatures indicated that the first step is a temperature-dependent migration process of singlet excitons, which can be a... 

In polysilanes, conduction takes place by hole rather than electron transport. Holes can be injected from an extended source (such as irradiated selenium) or generated within the polysilane film. The latter process is thought to involve photogeneration of a singlet exciton which migrates to the surface of the film and there generates an electron-hole pair. 

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