Excimer reverse dissociation

Figure 4 Fit to the decay of Poly(VBuPBD) at 350 nm and 25 C assuming transient quenching and excimer reverse dissociation.

It is of interest to note that within the limit of acoiracy of these experiments, monomer decay curves (Fig. 22) were single exponential, whereas Sdieme 1 predicts dual exponentiality (Eq. 65). The results thus imply that in pdyslyrene reverse dissociation ( feedback ) of the excimer is not of importance. This point is amplified by time-resolved fluorescence spectra which show that late- ted >ecti a (see experimental section) are composed exclusively of excimer emission (Fig. 23). The same is true in poly(a-methylstyrene) In view of more recent work mi other vinyl aromatic ptdymers, it would be of interest to study pdy(styrene) further with more sophisticated techniques. 

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. 

Nevertheless, why the transient quenching model is less appropriate for Poly(VPPO) than it is for Poly(VBuPBD) is still unclear but it is perhaps significant that its excimer decay (both direct emission and reverse dissociation) is considerably more intense than Poly(VBuPBD) which would be expected to more clearly expose the inappropriateness of assuming that only one excimer conformation and decay time is present. 

One clear deficiency in Equation 15 is that reverse dissociation of the excimer is omitted, although there is spectroscopic evidence that it does occur slightly in polystyrene. Inclusion of an additional exponential term to account for this phenomenon may well improve the fitting of Equation 15, but this method will only with difficulty be distinguished from other simpler methods. 

The results presented above may be compared with previous studies on these polymers. It has been suggested that more than one spectrally distinct excimer species may exist in some naphthalene-containing polymers (6,22). Analyses of the spectral surfaces for the polymers studied in this work clearly indicate that only one spectrally distinct monomer and one excimer species contribute to emission. Phillips and co-workers (3,23) also found that the monomer fluorescence decay from PACE requires a minimum of three exponential terms to provide adequate fitting. They explained their results on the basis of a kinetic model involving two temporally distinct excited monomers (M3, M2 ) which can form the excimer (D ) as outlined in Scheme 1. Reverse dissociation of the excimer was also considered important. 

In a prdiminary study on the time-resolution of fluorescence in pdy(l-vinyl naphthalaie) the kinetics were constrained to fit Scheme 1, yielding values of monomer decay times in methylene chloride sdution of 7.4 and 43.1 ns. Late-gated spectra indicated that reverse dissociation of the excimer occurred. With improvements in techniques, these studies have been greatly amplified recently. In particular, studies on copolymers have permitted more detailed analysis of the concentration dependence of excimer formation, and improved statistical analyses have permitted refined modelling of the kinetics. We will discuss at some length one of these papers, and summarize results on other systems. 

Once %ain, time-resolved studfes on copdymers have shown the existence of two kinetkally distinct monomer ecies, an isdated chromophore capable only of excimer formation through long-range interactions and a monomer enable of excimer fc mation which is also populated by reverse dissociation . An alternative explanation of observed behaviour is that two excimer species may exist which... 

Step 5 is the excimer formation step. Step 6, which is the reverse process involving the dissociation of the excimer into an excited and a ground state monomer, may need an energy of activation. 

Figure 9 shows the temperature dependence of the recovered kinetic rate coefficients for the formation (k bimolecular) and dissociation (k unimolecular) of pyrene excimers in supercritical CO2 at a reduced density of 1.17. Also, shown is the bimolecular rate coefficient expected based on a simple diffusion-controlled argument (11). The value for the theoretical rate constant was obtained through use of the Smoluchowski equation (26). As previously mentioned, the viscosities utilized in the equation were calculated using the Lucas and Reichenberg formulations (16). From these experiments we obtain two key results. First, the reverse rate, k, is very temperature sensitive and increases with temperature. Second, the forward rate, kDM, 1S diffusion controlled. Further discussion will be deferred until further experiments are performed nearer the critical point where we will investigate the rate parameters as a function of density. 

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