Excimer formation in polymers

Several interesting theoretical papers have appeared dealing with molecular dynamics and excimer formation in polymer systems. Frank and coworkers have developed a model to describe the transport of electronic excitation energy in polymer chains. The theory applies to an isolated chain with a small concentration of randomly placed chromophores, and a three-dimensional transport model was used to solve the problem which is based on a diagrammatic expansion of the transport Green function. (The Green function is related to time-dependent and photostationary depolarization and to transient and steady-state trap fluorescence.) The analysis is shown to be... 

To observe complex formation in monofunctional systems, rather high concentrations may be needed, while in the blchromophorlc systems under investigation, two chromophores are near each other even in very dilute solutions. This makes it possible to investigate the excited state behavior of such complexes at high dilution, although the chain, linking the chromophores, may Impose new restrictions on Intramolecular processes. An extensive review on intramolecular exclmers, including excimer formation in polymers, has been made recently by Klopffer (9). Recent developments in this area have broadened substantially the scope of intramolecular complex formation. 

In the publications on excimer formation in polymers to date, the vast majority have concentrated on homopolymers or copolymers having pendant aromatic chromophores such as phenyl or naphthyl groups. Polymers and copolymers based on 1-vinylnaphthalene, styrene, 2-vinylnaphthalene and N-vinylcarbazole have probably received the most attention while polymers based on vinyltoluene, acenaphthalene, vinylpyrene, 2-naphthylmethacryl ate, and a number of other monomers have also been studied, but to a lesser extent. Excimer formation in such polymer systems is especially favorable when the interacting species are "nearest neighbors" pendant to the polymer backbone and separated by three carbon atoms. However, excimers have also been reported for copolymer systems where the interactive chromophores are separated by a larger number of atoms. 

The inability of the classical Birks scheme [ 123] to describe excimer formation in polymer systems in organic media (see Scheme 2.2) has long been recognized. 

SINGLET ENERGY MIGRATION, TRAPPING AND EXCIMER FORMATION IN POLYMERS... 

The influence of the intramolecular distribution of chromophores upon the spectral characteristics of polymer luminescence is evident in the works of Nishijima [42] and David et al [43]. The latter workers were the first to attempt to characterize the extent of excimer formation in polymers in terms of the microcomposition of the macromolecule. It was argued that in styrene-methyl methacrylate copolymers excimer formation required energy migration from the site of absorption to that suitable for excimer formation. It was proposed that excimer formation was dominated by interactions between adjacent chromophores upon the polymer chain and that the excimer site concentration was consequently proportional to the fraction of pairs of styrene residues in the co-pol3nner faa. ... 

The Birks scheme is generally inapplicable to description of intramolecular excimer formation in polymers. 

Three types of experiments have been applied to the study of diffusion controlled cyclization of polymers. These are excimer formation in polymers containing pyrene groups at both chain ends [Py-polymer-Py ], exciplex formation in polymers containing a pyrene at one end and a dimethylaminophenyl group at the other [DMA polymer-Py i ], and triplet-triplet [TT] annihilation between anthracene groups at both ends of a polymer [A -polymer-A 3]. A fourth obvious approach, as yet to be reported, is intramolecular phosphorescence quenching in a polymer... 

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