## 3 Results and Discussion  
The GPC traces of each of the Py-PDMS samples were recorded at an excitation wavelength of 344 nm (Figure 2). The broad peak centered at 25 mL in all GPC traces is for the Py-PDMS sample, while the spike at approximately 30 mL is thought to be the Py-ODMS species, followed by a spike at approximately 34 mL corresponding to residual PAA. The assignment of the spike eluting last was confirmed by injecting a known sample of PAA into the GPC (Figure 2A). The molar fractions of pyrene labels present as Py-PDMS, Py-ODMS, or PAA in the samples were determined by integration of the peak areas in the GPC traces (Table 1). They indicate that 11 \(\pm\) 1 mol % of the pyrene labels attached to the PDMS belong to the Py-ODMS species. As shown in the upper portion of Figure 2C, it was possible to collect a fraction that was free of Py-ODMS and PAA (Py(3.5)-PDMS(f)) by collecting the front part of the peak of the Py(3.5)-PDMS sample. The Py(3.5)-PDMS(f) sample was employed later to validate the corrections to account for the presence of Py-ODMS and PAA that were applied to the fluorescence data with the unfractionated Py-PDMS samples. Since ODMS is too short to accommodate more than one pyrene label per chain, these oligomers do not form excimers and act as a fluorescent impurity in the characterization of the LRPCD of the Py-PDMS samples. Their negative impact on the fluorescence measurements is clearly illustrated in Figure 3, where the fluorescence spectra of all Py-PDMS samples are presented, including that of Py(3.5)-PDMS(f), the fractionated Py(3.5)-PDMS sample. The fluorescence spectra were normalized at 375 nm, the wavelength for the 0-0 emission band of the pyrene monomer. The normalized fluorescence spectra of the non-fractionated Py-PDMS samples showed a continuous increase in PEF with increasing pyrene content. However, since the SSF spectra cannot distinguish among the different pyrene species in solution, the extent of PEF probed in a SSF spectrum is affected by the emission from the Py-ODMS and PAA species. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Py-PDMS Sample** & \(f_{\rm PDMS}\) & \(f_{\rm ODMS}\) & \(f_{\rm PAA}\) \\ \hline Py(0.9)-PDMS & 78 & 8 & 14 \\ Py(3.4)-PDMS & 88 & 9 & 2 \\ Py(3.5)-PDMS & 87 & 10 & 3 \\ Py(5.2)-PDMS & 90 & 9 & 1 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Molar fractions of pyrene species present as Py-PDMS (\(f_{\rm PDMS}\)), Py-ODMS (\(f_{\rm ODMS}\)), and PAA (\(f_{\rm PAA}\)). How the presence of Py-ODMS affected the fluorescence spectra is clearly illustrated in the fluorescence spectrum obtained for Py(3.5)-PDMS(f). As shown in Figure 2C, the Py(3.5)-PDMS(f) sample was obtained by collecting the solution eluting between 21 and 25 mL following an injection of Py(3.5)-PDMS into the preparative GPC. In so doing, the contributions from the Py-ODMS and PAA species that were present in the Py(3.5)-PDMS sample and eluted in the GPC trace at 30 and 34 mL, respectively, were eliminated in Py(3.5)-PDMS(f). Since these two pyrene species contributed mostly to the fluorescence of the pyrene monomer, the excimer signal in the normalized SSF spectrum of Py(3.5)-PDMS(f) was 3.3, 2.7, and 2.9 times larger than that for the unfractionated Py(3.5)-PDMS sample in THF, DMF, and dioxane, respectively. Such a major difference in the measured fluorescence intensity of the excimer leads to the conclusion that the fluorescence spectra obtained from the unfractionated Py-PDMS samples do not reflect accurately the PEF from pyrene labels covalently attached to PDMS. Consequently, our study focused mainly on the analysis of the TRF decays. It is also noteworthy that the Py-PDMS samples yield the least PEF in DMF compared to THF and dioxane (Figure 3). An increase in PEF can be attributed to an increase in pyrene aggregation or a decrease in solvent viscosity.