Polystyrene excimer formation

For the distyrylbenzene carbon-centered tetramer 46b, the fluorescence spectrum in the solid him differs from the spectra in solution or in a polymer matrix due to excimer formation [93]. A concentration of 5% in a polystyrene matrix is sufficient for a distinct broadening of the emission. For the higher homologue 46c, a fluorescence maximum of 472 nm was measured in freshly prepared films. If the film is thermally annealed, the spectrum shifts to 511 nm, probably due to intermolecular arrangement that favors excimer formation. 

The only head-to-head polymer which has been examined for excimer fluorescence is polystyrene 25). Unfortunately, the synthetic route to this polymer leaves a number of stilbene-based structures in the sample, which have a lower-energy singlet state than either PS monomer (285 nm) or excimer (330 nm). Thus, fluorescence from these intrinsic stilbene traps was seen in the spectra of head-to-head PS in pure films and, to a lesser extent, in fluid solution. In the latter, the fluorescence of PS monomer was predominant, and the small amount of stilbene fluorescence was increased when a nonsolvent (methanol or cyclohexane) was added to the 2-methyl-tetrahydrofuran solution. In films of the polymer, stilbene fluorescence was the major spectral band, although some PS excimer fluorescence was also present in the spectrum. No monomer fluorescence at 285 nm was detected from films. Given the impure nature of the head-to-head PS sample, no conclusions on excimer formation in these systems could be drawn. 

Intramolecular Excimer Fluorescence Studies in Polymers Carrying Aromatic Side Chains. Some years ago, it was shown that certain excited aromatic molecules may form a complex with a similar molecule in the ground state, which is characterized by a structureless emission band red-shifted relative to the emission spectrum of the monomer. The formation of such complexes, called "exclmers", requires the two chromophores to lie almost parallel to one another at a distance not exceeding about 3.5A° (11). Later, it was found that Intramolecular excimer formation is also possible. In a series of compounds of the type C5H (CH2)jiC H5, excimer fluorescence, with a maximum at 340nm, was observed only for n 3 -all the other compounds had emission spectra similar to toluene, with a maximum at about 280nm (12). Similar behavior was observed in polystyrene solutions, where the phenyl groups are also separated from one another by three carbon atoms (13). 

Polymer blend studies have attracted much less interest. Strong excimer formation in blends of polystyrene with l,4-bis(4-a-cyano-styryl)-2,5-dio-... 

In cyclohexane, for example, the rate constant for exciplex formation was found to be identical to that of excimer formation for pyrene end-terminated polystyrenes. In tolerence simplex formation was much faster and was preceded by electron-transfer. In the case of a-anthrylpolystyrene end to end dimerisation was found to be diffusion controlled... 

Excimer formation and energy migration and its relationship toward an understanding of molecular mobility in polymers remains to be the nost prolific area of study in luminescence analysis. Measurements on excimer fluorescence from copolymers of polystyrene with various acrylates has shown that excimer formation is directly related to the... 

From consideration of molecular models and the geometric requirements for excimer formation, we have concluded that the trans, trans meso rotational dyad is the predominant EPS in the aryl vinyl polymers. [5,22] The identification of the EPS trap with a particular rotational dyad state was a critical factor because it opened the way to utilize the powerful rotational isomeric state theory of Flory [23] to calculate the EPS population for the isolated PS chains. This trap population is relatively small in polystyrene. 

It is obvious, that the conformation of a section of polystyrene which is required to make two neighboring phenyl residues lie parallel to one another (at a distance of about 2.5A°) would be characterized by a prohibitive potential energy and such a conformation would not be expected to be significant in the ground state. This conclusion is also supported by the fact that polystyrene exhibits the normal UV spectrum of alkylbenzene, while benzene rings constrained to lie parallel to one another at such a short separation have characteristically distorted absorption spectra (14). We must, therefore, conclude that excimers are formed by a conformational transition during the lifetime of the excited state. This transition is assisted by the large exother-micity of excimer formation which is characterized by AH values of -5.1 and -3.9 kcal/mole for benzene and toluene, and by -5.8 and -6.9 kcal/mole for naphthalene and the methylnaphthalenes (11). 

Investigations into singlet—singlet energy transfer processes in polymers have been mainly concerned with polymers containing aromatic chromo-phores such as polystyrene, polyvinylnaphthalene, polyacenaphthalene and polyvinylcarbazole. With these, excimer formation is possible as a consequence of the interaction of an excited and a ground state chromo-phore. In the presence of a suitable acceptor, energy transfer in the polymer can occur either from an isolated excited state or from an excimer... 

Rate constant it dm of excimer formation in polystyrene oligomers as a function of the degree of polymerization N... 

The modulation technique mentioned above has been used to identify triplet excimers in 1,2-benzanthracene and 1,2 3,4-dibenzanthracene at high solute concentrations167 and the differences between luminescence from naphthalene in fluid solution in the temperature range 353—173 and naphthalene in a rigid solution at 77 have been ascribed to phosphorescence from a triplet excimer.168 Excimer formation in solid poly-(2-vinylnaphthalene) and polystyrene is found to be dependent on the temperature at which the film is cast, and a statistical model based on the rotational isomeric state approximation has been used to formulate an expression for the fraction of excimer sites in the solid systems.168 Kinetic equations for dimer formation and decay, based on the statistical mechanics of ideal gases, have been obtained. These equations, derived from the N-atom von Neumann equation, take into account both bimolecular and termolecular equations.157 158 160... 

Excimer formation has been studied in polystyrene and poly(a-alkylstyrenes)189 (PS), poly(vinylcarbazole),139>140 poly-(2-vinylnaphthalene), and poly-(4-vinyl-biphenyl).141 For polystyrene films, David et a/.189 showed that the fluorescence yield increased with increasing crystallinity, at both ambient temperature and 77 K. The contribution of excimer fluorescence yield increased in the sequence atactic (0.7) < atactic oriented (0.60) < isotactic amorphous (0.28) < isotactic crystallized (0.01), with normal yields relative to excimer given in parentheses. Similar results were obtained for poly(vinylcarbazole), PVCZ, although the contribution of excimer fluorescence at 77 was independent of crystallinity. The results can be interpreted in terms of electronic energy migration to low-energy defect sites from which excimer emission can occur. In PVCZ copolymers with fumaronitrile (10), diethyl fumarate (11), and diethyl maleate (12) (Scheme 6),... 

Frank and L.A. Harrah, Excimer formation in vinyl polymers. II. Rigid solutions of poly(2 vinylnaphthalene) and polystyrene, J. Chem. Phys. 61, 1526 (1974). 

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