Styrene fluorescence decay times

Rotational relaxation times of polymers in solution are generally such that probes with nanosecond fluorescence decay times suffice for measurement of p. For faster relaxation, picosecond time resolution is required, and this may have applications in the polymer field. One should therefore pay tribute to the extraordinarily elegant instrumentation that is now available for picosecond rotational relaxation measurements on smaller molecules (85-87). Much more viscous solutions can be studied if long-lived phosphorescence is used as the luminescence monitor, and such studies on a millisecond time scale have recently been carried out on poly(vinyl alcohol), poly(ethylmethacrylate), poly-(styrene), poly(butylmethacrylate), and poly(methylmethacrylate) using benzophenone and anthrone as probes (88). 

Figure 13 Calculated normalized fluorescence decay time distributions for a dye molecule embedded in poly(styrene) matrix for different percentages of holes. Reprinted with permission from Vallee, R. A. L. Marsal, P. Braeken, E. et al. J. Am. Chem. Soc. 2005,127(34), 12011 Copyright 2005 American...

Figure 15 (a) Fluorescence decay time trajectory of a single terylene diimide dye molecule embedded in a thin poly(styrene) film and (b) corresponding autocorrelation curve. 

Figure 11. Plot of the decay times and the relative intensities of the fluorescence of the styrene-TMA adduct as a function of excess energy (expressed as frequency shift from the system s origin). Note the different pattern observed for the intensity and lifetime in the case of exciplex-type emission. Adapted from Ref. [27].

Such behaviour has been reported in a wide range of polymers including both homo and copolymers labelled with napthalene (3 - 5) and styrene-methyl methacrylate copolymers ( 6). However, in all these cases no clear evidence for other than a single excimer species has emerged. In addition, a rise time in the fluorescence decay, which can be associated with excimer formation, has also been observed (7). 

Winnik et al. [53] used time-resolved fluorescence spectroscopy (direct non-radi-ative energy transfer experiments) to determine the interface thickness in films of symmetric poly(styrene-fc-methyl methacrylate) (PS-PMMA) block copolymers labeled at their junctions with either a 9-phenanthryl or a 2-anthryl group. The corrected donor fluorescence decay profiles were fitted to simulated fluorescence decay curves in which the interface thickness 8 was the only adjustable parameter. The optimum value of the interface thickness obtained was 6 = 4.8 run. In similar studies [54—57], the same authors determined the interface thickness value 6 = 1.6 nm in mixtures of two symmetrical poly(isoprene-b-methyl methacrylate) (PI-PMMA) block copolymers of similar molar mass and composition [54] the interface thickness value 8 = 1.1 nm for the lamellar structures formed in films of symmetric PI-PMMA diblock copolymers bearing dyes at the junctions [55] a cylindrical interface thickness value of d slightly smaller than 1.0 nm in films consisting of mixtures of donor- and acceptor-labeled PI-PMMA (29vol% PI) that form a hexagonal phase in the bulk state [56] and the interface thickness 8 = 5 run on the diblock copolymer poly(styrene-l>-butyl methacrylate)(PS-h-PBMA) [57]. 

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. 

Figure 9. Fluorescence spectra and decay characteristics of POS containing polystyrene. M, styrene monomer region, dual decay kinetics. D, styrene excimer region, triple decay characteristics (double fit shovm does not correlate with other wave lengths, thus meaningless). P is POS fluorescence, triple decay characteristics when styrene excited (see box), but single, t = 1.68 ns when excited directly. EGS is early-gated time-resolved spectrum which matches closely spectrum of P excited directly, and difference between late-gated spectrum LGS and known spectrum of D. ...

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