Polymer intramolecular excimer fluorescence

Box 8.2 Intramolecular excimer fluorescence for probing the mobility of bulk polymers... 

Intramolecular Excimer Fluorescence Studies in Polymers Carrying Aromatic Side Chains. Some years ago, it was shown that certain excited aromatic molecules may form a complex with a similar molecule in the ground state, which is characterized by a structureless emission band red-shifted relative to the emission spectrum of the monomer. The formation of such complexes, called "exclmers", requires the two chromophores to lie almost parallel to one another at a distance not exceeding about 3.5A° (11). Later, it was found that Intramolecular excimer formation is also possible. In a series of compounds of the type C5H (CH2)jiC H5, excimer fluorescence, with a maximum at 340nm, was observed only for n 3 -all the other compounds had emission spectra similar to toluene, with a maximum at about 280nm (12). Similar behavior was observed in polystyrene solutions, where the phenyl groups are also separated from one another by three carbon atoms (13). 

Ve have described three fluorescence techniques for monitoring the cure of epoxy resins. The first one is based on intramolecular excimer fluorescence, the second one is based on the enhancement of fluorescence intensity with the medium viscosity, and the third one is based on the measurement of the translational diffusion coefficient of a fluorescent probe. Finally, we have demonstrated the fluorescence monitoring of the formation of a polyimide polymer. 

With the increased level of understanding of the cyclization dynamics as monitored by intramolecular excimer fluorescence, it is now possible to extend this probe to the study of systems more complex than dilute solutions. One such situation involves the structure and dynamics of macromolecular complexes formed between polymeric proton donors and acceptors in aqueous solution. For example, there has been widespread interest in the complexes formed between poly(ethylene glycol) and poly(acrylic acid) or poly(methacrylic acid) in aqueous solution (34, 35). A second, complicated morphological problem is to describe the configurational behavior of polymer chains adsorbed on colloidal particles. This research has relevance to the understanding of steric stabilization. One system of particular interest is the interaction of poly(ethylene glycol) and colloidal silica (36-40). 

How is the monomer fluorescence of aryl vinyl polymers or intramolecular excimer-forming compounds distinguished from that of monochromophoric compounds ... 

A similar but smaller intramolecular quenching effect was seen by Phillips and co-workers 44,4S) for 1-vinylnaphthalene copolymers incapable of excimer fluorescence. The monomer fluorescence lifetime of the 1-naphthyl group in the methyl methacrylate copolymer 44) was 20% less than the lifetime of 1-methylnaphthalene in the same solvent, tetrahydrofuran. However, no difference in lifetimes was observed between the 1-vinylnaphthalene/methyl acrylate copolymer 45) and 1-methylnaphthalene. To summarize, the nonradiative decay rate of excited singlet monomer in polymers, koM + k1M, may not be identical to that of a monochromophoric model compound, especially when the polymer contains quenching moieties and the solvent is fluid enough to allow rapid intramolecular quenching to occur. 

The conformational statistics of asymmetric vinyl chains such as P2VN are well-known 126). The rotational conformers of isotactic (meso) dyads are entirely different from those of syndiotactic (dl) dyads. Frank and Harrah132) have described each of the six distinct conformers for meso and dl dyads, using the t, g+ and g nomenclature of Flory 126). Excimer-forming sites (EFS) are found in the tt and g g+ meso states, and in the degenerate tg , g t dl state. Because the rotational conformers of compounds such as l,3-bis(2-naphthyl)propane do not match those of either the iso-or syndiotactic dyads of P2VN, the propane compounds make poor models of aryl vinyl polymers. However, the rate constants of fluorescence and decay of the intramolecular excimer in polymers can usually be determined from the propane compounds (but see the exceptional case of PVK and its models133)). 

Despite the technical problems in the latter film study, we conclude that there is no intramolecular excimer formation in the compounds of Richards et al.143, and probably little intermolecular excimer formation in the pure films. The absence of an effect of solvent power 25) on the possible excimer fluorescence of the R = CH3 polymer may not be significant, since little change in the coil dimensions would be expected for the short ( 300 backbone atoms) polymers 143> which were studied. Additional work is needed on the fluorescence of such polymers having higher molecular weights, different aryl substituents (R = 2-naphthyl, for example), and fewer adventitious impurities. 

Intramolecular excimer emission, the polarization of fluorescence, nonradiative energy transfer and the use of medium-sensitive fluoropliores has been used to study the conformational mobility of polymers in dilute solution, the interpenetration of drain molecules, the association of polymers with each other or with small species and the cooperative transition of certain poly-carboxylic acids from a compact to an expanded state. 

Figure 1 illustrates the total fluorescence matrix in contour format for BuPBD euid poly (VBuPBD) at 298K. These spectra graphically illustrate the above conclusions and the symmetry of the broadened profile of the polymer emission with respect to the emission/excitation axes is convincing evidence for the excimeric nature of the species responsible for the low energy intensity component. In addition the emission of both polymers reduces to that of the unassociated monomeric chromophore upon dispersal in a glassy (MeTHF) matrix at 77K. The effect is illustrated for poly (VBuPBD) in Figure 2. This observation is consistent with the formation of intramolecular excimers by a conformational sampling mecheutism.

Having established the existence of the excimer emission of NDI based polyurethanes in solution, and realizing that the intramolecular excimer forming naphthyl carbamate groups are located on the backbone of the polymer, it becomes apparent that an excellent opportunity exists for chain conformational studies as a function of solvent. Figure 10 shows the steady-state fluorescence spjectra of NDI-650 in four solvents with distinctively different solvating power. In each case (curves a-d) both monomer and excimer emission are observed however, tlie ratios of excimer to monomer emission reflect conformational differences of the NDI-650 polymer in the solvent employed. The excimer to monomer intensity ratio... 

Clearly, this is a simplistic view, since there is a considerable variation in 0g among the PS homopolymers. Yet the excimer fluorescence to fluorescence ratio in DMM solutions is very similar from polymer to polymer. A possible alternative, or additional, route to chain scission may involve intramolecular triplet energy migration and transfer processes. It has been shown (J ) that intramolecular triplet migration is facile in both PS and P(S-alt-MMA), but that sequences of two or more MMA-units interrupt the migration process. If an excited triplet trapping site can release its excess energy without bond scission,... 

The specific fluorescent probe technique used in the present research is excimer formation between pyrene groups terminally attached to both ends of poly(ethylene glycol) (PEG) chains. Excimer formation in such a labeled polymer may result from both intramolecular and intermolecular interactions. A large body of literature over the past decade deals with intramolecular cyclization as detected by fluorescence techniques. This work has come largely from three groups. Cuniberti and Perico (24, 25) were the first to explore the possibility that excimer fluorescence could be used as a probe of end-to-end cyclization dynamics. Later, Horie and co-workers (26, 27) used triplet annihilation as a probe of cyclization in long chains. By... 

Excimer to Monomer Intensity Ratio. Our approach toward the determination of this information is to focus on the configurational behavior of the pyrene-terminated PEG. Two sets of experiments were performed to separate the intramolecular behavior of the isolated Py-PEG-Py chains from the intermolecular interactions that occur between labeled PEG chains. First, we performed photostationary fluorescence measurements of IJI for fully labeled PEG as a function of the molar ratio of the poly(carboxylic acid) to that of the PEG. These results will contain both intermolecular and intramolecular contributions to the excimer fluorescence. Next, we performed analogous measurements with the same total amount of PEG but with only 1% having pyrene labels. The selection of the relative amount of the tagged polymer is somewhat arbitrary, but it appears to be sufficiently low to provide a reasonable approximation to the behavior of the isolated, labeled PEG chain. These fluorescence results are then subtracted from those for the fully tagged system, leaving just the intermolecular contribution. 

STEADY-STATE FLUORESCENCE STUDIES OF INTRAMOLECULAR EXCIMER FORMATION IN POLYMERS... 

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