Excimer sites

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. 

The use of copolymers incorporating aromatic species offers a unique opportunity to determine the rate parameters governing intramolecular excimer formation in macromolecules using the extrapolation techniques oudined in method (1) above. Furthermore it is possible, in principle, to remove the ccmcentration dependence from the rate coefficient assigned to excimer formatnn. The problem encountered in the use of ct lymers is the selection of an propriate function to describe the concentration of chromophores which occupy potential excimer sites in the molecule. The formulation of appropriate concentration terms have been discussed with respect to description of relative excimer to monomer emission efficiencies in steady state conditions 56) -pijg concentration term must account not only for the geometric considerations implied in the excimer formatnn process but also for the ex-tmt to which m ation is capable of populating such sites. 

It will be noted that the kinetic scheme adapted above reverse dissociation of excimer sites, the experimental evidence for this being the triple exponential decay of the monomer fluorescence and the gated time-resolved fluorescence spectra, reported here and seen earlier " in the homopolymer. 

In recent years the fluorescence from poly(N-vinylcarbazde) has received much attention, partly due to its photoconducting properties and partly due to the unique photophysical processes which it displays. In constrast to other aromatic containing polymers, no emission from monomer — like moieties has been observed Even in solution the broad unstructured fluorescence profile is resolved into two spectrally distinct excimer sites One of these, with emission maximum at 420 nm,... 

F%. 37. Kinetic scheme for formation and inteiconversion of excimer sites in polyfN- vinyl carbazole)... 

Local motions of a, a)-bis( 1-pyrene) alkanes and pyrene -labelled poly(methyl methacrylate) in solution have been measured by pico-second excimer fluorescence spectroscopy In this study the formation of the polymers during polymerisation could be accurately maintained by measuring excimer fluorescence lifetime. The photophysical properties of poly (N-vinylcarbazole) have been interpreted on the basis of a study in diastereoisomers of 2,4-di(N-carbazolyl)t In this work it is concluded that 95% of the excitation energy occurs with chromophores associated with specific excimer sites. Excimer formation in various polyvinyl carbazole... 

The model which has been developed (2>1) to fit these results involves formation of excimers via thermally activated phenyl group motion. Such motion is restricted in a rigid matrix, and "pre-form-ed" excimer sites, where little phenyl motion would be required for collapse to the excimer, are evidently rare. 

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. 

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. 

At least in regard to excited singlets, the 0° in most of these polymers may generally reflect (a) the probability that the photon is absorbed by an S-unit adjacent to a weak link, usually an MMA-unit, and (b) the probability that the absorbed photon will migrate to an excimer site before it reaches a weak link. Superficially, for an individual polymer chain in solution, a competition for the absorbed energy exists between the excimer site, which is an energy sink, and the MMA-unit, which is a weak link. 

The energy sinks thus far considered have been excimer sites, photooxidation products, or adventitious impurities. 

Excimer quenching accompanies the formation of Product I in irradiated polystyrene films, but the rates of change of excimer fluorescence intensity in vacuum and in air are quite different. They are contrasted in Figure 3. Photoconversion of excimer sites, which act as... 

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... 

P.L. Egerton, J. Trigg, E.M. Hyde, and A. Reiser, Photocycloaddition at excimer sites in a solid polyester of p phenylenediacrylic acid, Macromol. 14, 100 (1981). 

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." ... 

M. Graley, A. Reiser, A.J. Roberts, and D. Phillips, Excimer fluorescence as a probe into the solution behavior of a polyester of p phenylenediacrylic acid, Macromolecules 14, 1752 (1981). P.L. Egerton, J. Trigg, E.M. Hyde, and A. Reiser, Photocycloaddition at excimer sites in a solid polyester of p phenylenediacrylic acid, Macromolecules 14, 100 (1981). 

Obviously, excimer formation represents a serious obstacle to energy migration, since the excimer site itself functions as a trap, and, after excitation, is mostly deactivated by emission of a photon rather than by energy transfer to a neighboring donor moiety (Texc< z h)- Moreover, any effect on coil density exerted by the choice of temperature or solvent can dramatically effect the efficiency of energy trapping. 

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