Pyrene aggregates

In a film, however, molecular mobility is severely limited, so that excimer fluorescence must arise mainly from pairs or groups of pyrene molecules that were approximately in the excimer configuration when the film was cast. Thus, the intensity of the excimer emission is also an indication of the local concentration of pyrene in the cast film. If the pyrene aggregates, we expect that the excimer fluorescence would increase with aggregation. This system can be used to look at the aggregation of very low concentrations of a small molecule dye in a polymer film, and potentially detect molecular aggregation before it would be observable by other tech-... 

Single polymer micelles were observed at low polymer concentrations in aqueous media whereas at higher concentrations both inter and intrapolymeric aggregation took place. Above the critical solution temperature excimer formation decreased due to disruption of the pyrene aggregates. 

Keywords Pyrene aggregates and excimer Associative thickeners Fluorescence Blob Model Model free analysis Fluorescence Surfactants... 

In the vast majority of instances, the pyrene monomer fluorescence decays acquired with aqueous solutions of Py-WSPs, particularly when the pyrene labels are randomly incorporated into a WSP, are always multiexponential in nature. This experimental observation is a consequence of the distribution of distances between an excited pyrene and a ground-state pyrene or pyrene aggregate formed in water that leads to a distribution of rate constants for excimer formation by diffusion [30-32, 34]. Unfortunately, the analysis of multiexponential decays is notoriously difficult to handle and unless the decay times resulting from the photophysical processes are well resolved (i.e., separated by a factor of at least 2) as is the case for pyrene end-labeled short alkyl oligomers, the parameters retrieved from a triexponential fit should be considered with utmost caution [32]. Indeed early reports [38, 39] on the analysis of the fluorescence decays of Py-WSP based on the DMD model [40] or one of its variants introduced originally by Zachariasse to deal with the multiexponential decays of pyrene end-labeled oligomers yielded sets of parameters whose validity has been questioned [41]. 

Besides the polarity of the environment experienced by the pyrene monomer, the level of pyrene aggregation could be inferred from the IA + ratios obtained from... 

To this end, relationships were established between the four fractions /dift(A), /free(A),/ 2W, and/Eo(A) (/el(A) is redundant since it equals 1 -/diff(A) -/foeW -fkiW -/eoW) obtained at each excitation wavelength and the parameters / , f Q, cmW, and ceoW- Thus, these relationships yielded four independent equations that depended on six variables. The molar absorbance coefficient of the pyrene monomer in water cmW was determined by preparing a Py-PDMA sample with a very low pyrene content of 6 pmol g (Py(6)-PDMA). Py(6)-PDMA generated a minimum level of pyrene aggregation which allowed the determination of cmC ) from the absorption spectrum. Equating the absorbance of an aqueous solution of a Py-PDMA sample determined experimentally to the concentrations of pyrene species present in solution provided a fifth equation that enabled the determination of the molar fractions / e /k fm most importantly, eEo( )> the molar absorbance coefficient of a pyrene aggregate. 

Together 150 monomer fluorescence decays and 150 excimer fluorescence decays were acquired and fitted globally according to the FBM to yield six sets of fractions /diff(A), /freeW, fkiW, /eoW, and /elW for each polymer solution. The procedure described above was applied to these six sets of /eoW. and /elW values which varied with the pyrene content and polymer concentration of the Py-PDMA samples. However, the procedure accounted successfully for these variations yielding a tight set of CeoW values for a range of 2ex between 325 and 350 nm with 1 nm increments (Fig. 15). This study provided the first spectrum of the molar absorbance coefficient CeoW of pyrene aggregates in water. The Pa value obtained from the e X) spectrum equaled 1.5 0.1, much lower than the Pa value of 2.5 found for the isolated pyrene monomers of Py(6)-PDMA. 

This success led us to determine the ceoC-I) spectrum of pyrene aggregates formed by another Py-WSP, namely Pyi-PEO(2.5K) in water [72]. Since Pyi-PEO(2.5K) is known to form micelles in water, excimer formation took place through diffusive encounters between solvated Pyi-PEO(2.5K) chains and PyiPEO(2.5K) micelles, rapid rearrangement of the pyrene moieties inside a micelle, or direct excitation of pyrene aggregates. In other words, the pyrene labels... 

Fig. 15 Molar absorption coefficient of the pyrene monomer (cm(. ). solid line) and pyrene aggregates (ceo(2)) for Py(263)-PDMA with a concentration of 4.8 g L open triangle), Py (479)-PDMA with a concentration of 0.01 g L open square), 0.08 g L open diamond), and 1.8 g L filled triangle), and Py(645)-PDMA with a concentration of 0.09 g L filled square) and 2.2 gL filled diamond) [69]. Reprinted with permission from J.Pltys. Chem.B2iH)S, 112, 15301-15312. Copyright 2014 American Chemical Society ...

The eo(A) spectra obtained for the pyrene aggregates of Py-PDMA [69] and Pyi-PEO(2.5K) [43] are compared in Fig. 16. As for the Py-PDMA aqueous solutions, the ceoCA) spectrum of pyrene aggregates generated by concentrated solutions of Pyi-PEO(2.5K) was substantially broader than that of Pyj-PEO... 

Siu H, Duhamel J (2005) The importance of considering non-fluorescent pyrene aggregates for the smdy of pyrene-labeled associative thickeners by fluorescence. Macromolecules... 

Siu H, Duhamel J (2008) Molar extinction coefficient of pyrene aggregates in water. J Phys... 

Siu H, Duhamel J (2012) Molar absorption coefficient of pyrene aggregates in water generated by a poly(ethylene oxide) capped at a single end with pyrene. J Phys Chem B 116 1226-1233... 

Pyrene aggregation and steric requirements result in increased Tg while only slightly altering Tm... 

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