Monomer trap emission

On the other hand, the excimer emission because it is 80% non-correlated with monomer trap emission and because it is effectively quenched in the copolymers even at low temperatures, must largely arise from a mobile precursor. The activation energy for hopping of this precursor is implied to be <10 cm l. This is not unreasonably low(12,17), and indeed, the zero-point energy of the phenyl chromo-phore could in principle allow completely activationless hopping (tunneling) at reasonable rates. Determination of the true situation will require measurements at still lower temperatures, which are now in progress. We note that the polystyrene emission spectrum at 4.2K reported in (Id) indicates a monomer/excitner intensity ratio nearly the same as our 20K spectra. 

Before we leave this section we should note that our explanation of the monomer emission as due to a special pre-formed shallow trap suggests that the low-temperature monomer/excimer emission intensity should be morphology dependent, hence dependent upon sample history, molecular weight, etc. This is indeed the case and we have found that different batches of polymer, aged samples, or samples prepared in different ways than described, may give slightly different intensity ratios at 20K. However, our results for samples prepared as described are completely reproducible and the temperature dependences remain qualitatively the same for deviant samples. 

It is clear from our results that the initial excitation in solid polystyrene must be extremely mobile in contrast to the case of the polymer in solution. However, it is now clear that available experimental results do not directly examine the mobile excitation. In particular, observed "monomer-like" emission is due to a shallow trap, which vitiates a previous analysis (Id,18) that assumed it to be due to the mobile excitation. Direct determination of mobile excitation dynamics will require extension of photophysical measurements to the picosecond time regime and efforts by us to accomplish this goal are now in progress. 

We think that this shallow trap monomer is predominantly "preformed" and achieved by the initial absorption of a photon. Thus, comparison of Figures 6 and 12 shows that monomer emission is decreasing considerably less rapidly than excimer emission at low temperature. The activation energy is consistent with expectation (2, 1, 16) for a phenyl rotation and the activated process is likely then suc a rotation into a conformation where energy-transfer to a neighboring phenyl group can occur. We have not attempted to correlate the population of the shallow trap with chain dyads because we suspect that interactions with phenyl groups of adjacent chains are much more important in the solid polymer. 

Figure 31. Calculated time scales of fluorescence decay in a PS-I monomer as a function of emission wavelength [97]. Excitation is at 640-660nm, and the panels show the amplimdes of eigenvalues of the rate matrix for four different detection wavelengths. The amplitudes clearly cluster into four groups < 100 fs, 300 fs, 2-3 ps, and 38 ps, with the latter representing the overall trapping time.

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