Aryl vinyl polymer

Photophysics of Excimer Formation in Aryl Vinyl Polymers... 

The objective of this review is to characterize the excimer formation and energy migration processes in aryl vinyl polymers sufficiently well that the excimer probe may be used quantitatively to study polymer structure. One such area of application in which some measure of success has already been achieved is in the analysis of the thermodynamics of multicomponent systems and the kinetics of phase separation. In the future, it is likely that the technique will also prove fruitful in the study of structural order in liquid crystalline polymers. 

Photophysical Species and Monochromophoric Processes in Aryl Vinyl Polymers 34... 

Our motivation for offering a further consideration of excimer fluorescence is that it is a significant feature of the luminescence behavior of virtually all aryl vinyl polymers. Although early research was almost entirely devoted to understanding the intrinsic properties of the excimer complex, more recent efforts have been directed at application of the phenomenon to solution of problems in polymer physics and chemistry. Thus, it seems an appropriate time to evaluate existing information about the photophysical processes and structural considerations which may influence excimer formation and stability. This should help clarify both the power and limitations of the excimer as a molecular probe of polymer structure and dynamics. 

One point which can be settled qualitatively, however, is whether singlet exciton migration does in fact occur in the aryl vinyl polymers. It will be shown that available evidence supports energy migration as an important feature of the photophysics of polystyrene (PS), poly(l-vinyl naphthalene) (P1VN), and poly(2-vinyl naphthalene) (P2VN), the homopolymers which are the subject of the majority of the review. 

The most widely used nomenclature in the field of luminescence of aromatic molecules is that proposed by Birks. 3) The photophysical species and processes in this scheme which are encountered in rigid systems of aryl vinyl polymers are described in Tables 1-4. Triplet excimers have been omitted since it will be shown later that all triplet states play minor roles in the room-temperature, air saturated P2VN blends studied recently 2). 

Table 1. Intrinsic Photophysical Species in Aryl Vinyl Polymers in a Rigid Environment...

What are the absorbing species in the aryl vinyl polymers ... 

How does the UV absorbance of an aryl vinyl polymer differ from that of the matching monochromophoric compound, given that the polymer is free from defects and impurities ... 

Bichromophoric model compounds can provide such standards, since these compounds can be highly purified and characterized by techniques inapplicable to polymers. For example, the racemic compound 2,4-diphenylpentane models the syndiotac-tic dyad of PS, while the meso 2,4-diphenylpentane models the isotactic dyad of PS. The 2,4-diarylpentanes are the best models of aryl vinyl polymers, since the populations of the rotational conformers of the model compounds are similar to those of the dyads of the polymer. 

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

One method of directly determining QM and Q for excimer-forming compounds involves the relationship direct method requires a number of samples of an aryl vinyl polymer with differing molecular weights. If QM and Q are assumed to be independent of molecular weight, then they can again be obtained from the (pD vs. polymer samples. The rate can be determined if FM is assumed to be identical to that of the monochromophoric compound. It follows that kM = kFM/QM. While these methods are rarely used in the literature, it is worthwhile to review the assumptions that Qm and Q are independent of solvent, or of the molecular weight and structure of the compound. 

How much does triplet-triplet annihilation, kMXX, enhance the fluorescence yield of aryl vinyl polymers, relative to the fluorescence yield of the analogous monochromophoric compounds ... 

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)). 

While the study of diarylpentanes is helpful in understanding the conformational behavior of aryl vinyl polymers, a simple weighting of the properties of the model compounds by the tacticity of the polymer does not yield the properties of the polymer. For example, the presence of dl dyads surrounding a meso dyad will suppress the tt conformer in the meso dyad 14fl). Thus, in order to obtain the fraction of tt meso conformers within an atactic P2VN sample, it is necessary to resort to a Monte Carlo calculation utilizing an extended product of statistical weight matrices, 26). 

The third group ofpolychromophoric compounds to be discussed are homopolymers in which the pendant rings are separated from the backbone by one or more atoms. The polymers of allyl arenes, which lack only the n = 3 ring spacing of aryl vinyl polymers, have been studied very little. The fluorescence spectrum of poly(l-allyl-naphthalene) in dilute dichloromethane solution has been reported 28). Like 1-ethyl-naphthalene, the maximum intensity was seen at 337 nm, but a weak, broad shoulder was also recorded for the polymer at 410 nm. The fluorescence ratio Iu/IM for poly(l-allylnaphthalene) was only 1/100 th the value for P1VN 28). The excimeric nature of the 410 nm emission in the allyl-based polymer has not been confirmed, since neither the lifetime nor the excitation spectrum of this fluorescence band are known. 

The review of the structure of excimers in aryl vinyl polymers can be concluded as... 

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