Copolymers excimer formation

The influence of the intramolecular distribution of chromophores upon the spectral characteristics of polymer luminescence is evident in the works of Nishijima [42] and David et al [43]. The latter workers were the first to attempt to characterize the extent of excimer formation in polymers in terms of the microcomposition of the macromolecule. It was argued that in styrene-methyl methacrylate copolymers excimer formation required energy migration from the site of absorption to that suitable for excimer formation. It was proposed that excimer formation was dominated by interactions between adjacent chromophores upon the polymer chain and that the excimer site concentration was consequently proportional to the fraction of pairs of styrene residues in the co-pol3nner faa. ... 

Almgren, M., J. Alsins, and P. Bahadur. 1991. Fluorescence quenching and excimer formation to probe the micellization of a poly(ethylene oxide)-poly(propylene oxde)-poly(ethylene oxide) block copolymer, as modulated by potassium uoride in aqueous solillHngmuir7 446—450. 

It has been reported that in ethyl acetate and dichloroethane solution, the position of the excimer band is concentration dependent The interpretation of solvent effects is complex. Since the compactness of the pdymer coil will affect the efficiency of energy migration and the ccmcentration of aromatic species in conformations suitable for excimer formation, sdvent effects are to be expected in polymers in which excimer formation is the result of nearest-neighbour interactions, as is the case in styrene as shown in studies on styrene-methyl methacrylate copolymers ... 

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 sdutiMi ci 7.4 and 43.1 ns. Late-gated spectra indicated that reverse dissociaticm of the excimer occurred. With improvements in techniques, these studies have been greatly an lified recently. In particular, studies on copolymers have permitted more detailed analysis of the concentration dependence of excimer formation, and improved statistical analyses have permitted reEned modelling of the kinetics. We will discuss at some length one of these papers, and summarize rearlts on other sterns. 

The use of copolymers incorporating aromatic species offers a unique opportunity to determine the rate parameters governing intramolecular excimer formation in macromolecules using the extrapolation techniques oudined in method (1) above. Furthermore it is possible, in principle, to remove the ccmcentration dependence from the rate coefficient assigned to excimer formatnn. The problem encountered in the use of ct lymers is the selection of an propriate function to describe the concentration of chromophores which occupy potential excimer sites in the molecule. The formulation of appropriate concentration terms have been discussed with respect to description of relative excimer to monomer emission efficiencies in steady state conditions 56) -pijg concentration term must account not only for the geometric considerations implied in the excimer formatnn process but also for the ex-tmt to which m ation is capable of populating such sites. 

The principal difference between considerations of fluotescenre behaviour of acenaphthylene polymers VI and those of the photophysical characteristics of vinylaro-matic polymers is that whilst the latter sterns may form excimers throu interactions between nearest-neighbour chromophores on the polymer chain, steric restrmnts preclude such a mechanism in polymers of acenaphthylene It has been suggested that the dominant mechanism for excimer formation in acenaphthylene derived systems involves interactions between next to nearest nei bours. Such a proposal has been validated for copolymers of acenaphthylene with methyl methacrylate and methyl acrylate and is reinforced by the observation of excimer emission in alternating copolymers incorporating acenaphthylene chromophores in steady state excitation. 

Once again, time-resolved studies on copolymers have shown the existence of two kinetically distinct monomer species, an isolated chromophore capable only of excimer formation throu long-range interactions and a monomer capable of excimer formation which is also populated by reverse disrociation . An alternative explanation of observed behaviour is that two excimer species may exist which... 

Phillips, D., Roberts, A. J., Soutar, I. Transient decay studies of photophysical processes in aromatic polymers, III. Concentration dependence of excimer formation in copolymers of acenaphthylene and methyl methacrylate. Eur. Polym. J. 17,101 (1981)... 

Recent studies in our laboratory were aimed at defining more closely the conditions governing Intramolecular excimer formation in dilute polymer solutions (15). An alternating copolymer of styrene with maleic anhydride or methylmethacrylate showed no excimer emission, confirming that interactions of other than neighboring phenyl residues made no significant contribution to... 

Excimer formation and energy migration and its relationship toward an understanding of molecular mobility in polymers remains to be the nost prolific area of study in luminescence analysis. Measurements on excimer fluorescence from copolymers of polystyrene with various acrylates has shown that excimer formation is directly related to the... 

In the publications on excimer formation in polymers to date, the vast majority have concentrated on homopolymers or copolymers having pendant aromatic chromophores such as phenyl or naphthyl groups. Polymers and copolymers based on 1-vinylnaphthalene, styrene, 2-vinylnaphthalene and N-vinylcarbazole have probably received the most attention while polymers based on vinyltoluene, acenaphthalene, vinylpyrene, 2-naphthylmethacryl ate, and a number of other monomers have also been studied, but to a lesser extent. Excimer formation in such polymer systems is especially favorable when the interacting species are "nearest neighbors" pendant to the polymer backbone and separated by three carbon atoms. However, excimers have also been reported for copolymer systems where the interactive chromophores are separated by a larger number of atoms. 

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). 

PolyCACN) has a rigid chain structure yet can form excimers with alternate units along the chain (8), or by stacking in a helical conformation. Excimer formation has been reported for alternate copolymers of ACN with styrene (9) and for ACN with maleic anhydride CIO). The situation is different for 2-vinylnaphthalene since alternating copolymers of 2VN with methyl methacrylate or methacrylic acid did not form excimers, yet random copolymers of the same systems showed excimer fluorescence Cll). Only random copolymers of ACN were prepared in this work. 

ACN had the longest migration length of 70A compared to 63 A for PACN using equation (1). The above percentages correspond to an overall monomer composition of 1 1 and 4 1. Random poly(ACN-co-AN) showed monomeric fluorescence at 355 nm and excimer fluorescence at 405 nm. The Iq/Im ratios were 0.88 and 0.61 for the 47 and 80 ACN copolymers, respectively, and the same value for PACN was 0.52. The incorporation of photo-inactive units such as AN in the copolymer increase chain flexibility and allows more excimer formation. 

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