Aromatic polymers excimers

Complex formation is important in photophysics. Two terms need to be described here first, an exciplex, which is an excited state complex formed between two different kinds of molecules, one that is excited and the other that is in its grown state second, an excimer, which is similar to exciplex except that the complex is formed between like molecules. Here, we will focus on excimer complexes that form between two like polymer chains or within the same polymer chain. Such complexes are often formed between two aromatic structures. Resonance interactions between aromatic structures, such as two phenyl rings in PS, give a weak intermolecular force formed from attractions between the pi-electrons of the two aromatic entities. Excimers involving such aromatic structures give strong fluorescence. 

Onium salts have been widely used as an acid generator for photo-, EB, and x-ray resist. In addition, aromatic polymers such as novolak and polyhydroxystyrene have been often used as a base polymer for EB and x-ray resist. The reaction mechanisms in a typical resist system have been investigated by pulse radiolysis [43,52,77-88], SR exposure [79,80,83-85], and product analysis [88]. Figure 6 shows the acid-generation mechanisms induced by ionizing radiation in triphenylsulfonium triflate solution in acetonitrile. The yields of products from electron beam and KrF excimer laser irradiation of 10 mM triphenylsulfonium triflate solution in acetonitrile are shown in Fig. 7 to clarify the... 

Excimer fluorescence from polychromophoric compounds in rigid systems, while easy to detect, is difficult to interpret. The transient response of the excimer can be empirically characterized by the limiting lifetime T p. In the absence of processes which convert D to M, this limiting lifetime is the reciprocal of k . We will examine xa]D for PS, P2VN, and other aromatic polymers to see if there is any difference between fluid and rigid solution at room temperature. 

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)... 

Energy transfer in polymers has been studied in the pure solid state, in heterogeneous systems (e.g. polymer blends), in liquid solutions and in solid solutions. The last case, which will be considered here, provides relatively simple and clear experimental conditions since interactions between the macromolecules can be excluded by dilution and molecular movement is severely restricted by low temperature and rigid environment. Thus, excitonic energy transfer can be studied without competing molecular movement. The luminescence of dilute, solid solutions of aromatic polymers is not dominated by excimers - in sharp contrast to the other modes of observation - so that side group fluorescence and phosphorescence can be observed. This does not mean, however, that exciton trapping processes are absent in these systems. 

The absence of low energy excimer in solid solutions of PVCA and other aromatic polymers can easily be explained by the need of thermal activation to form a sandwich-pair (excimer-forming site) of two neighboring bulky aromatic groups. This has been shown by Frank for P2VN and Poly(4-vinylbiphenyl) (21,39). 

In many synthetic polymers, particularly the vinyl aromatic type, excimer formation is significant. In the simplest case of excimer formation in free molecules in fluid... 

They propose that in excimer-forming vinyl aromatic polymers in fluid solution, the... 

As a consequence of such investigations, it has been proposed [124—126] that two kinetically distinct monomer species exist in addition to excimer in aromatic polymers. Two independent kinetic schemes have been proposed [124—127], which are descriptive of the heterogeneity of monomer sites in such polymers (see Scheme 2.3). 

Excitation fluorescence is the principle of the fluorescence techniques used for studying polymer blends. The method comprises of three steps incorporation of an excimer, its excitation, and recording the excitation delay. The excimer can be an aromatic polymer component of the blend (viz., PS, poly(viny 1-dibenzyl), polyvinylnaphthalene, an aromatic group grafted onto the macromolecular chain, etc.), or it can be added as probe molecule (e.g., anthracene). There are three possibilities for the aromatic rings to form excimers intramolecular adjacent, intramolecular nonadjacent, and intermolecular types. Each of these types is sensitive to different aspects of the chain conformation and environment, thus, sensitive to blend miscibility effects. The most important of these for studies of polymer blends is the intermolecular, usually identified from concentration measurements (Wiruiik et al. 1988). 

The excimer laser irradiation of synthetic fibers made of highly absorbing polymers, e.g. aromatic polymers such as PET and aramids as well as aliphatic polymers such as polyamide-6 and -6.6 (PA), generates a... 

Most simple aromatic polymers like polystyrene, polyvinylnaphthalene and poly(N-vinylcarbazole) exhibit both exciton migration and excimer trapping. 

Excimer fluorescence involving a complex (excited state-ground state) between adjacent or non-adjacent fluorescent units with the same polymer chain or intermolecular association between units on different chains can also be studied to assess phase behavior and the level of mixing in polymer blends. These studies generally involve the addition of low concentrations of aromatic polymers (capable of fluorescence) to non-fluorescent polymers. Excimer fluorescence is favored by phase separation, because the intermolecular associations of the fluorescent polymer will be shielded by the miscible non-fluorescent polymer, in which case monomer emission will be more dominant. This technique was developed and demonstrated initially by Frank and coworkers [333-335]. 

In the absence of impurities or other energy traps introduced into a macromolecule during synthesis, excimer forming sites constitute the natural energy trap in aromatic polymers. 

Studies of intramolecular excimer formation in bichromophoric systems, as described by Professor De Schryver in this publication, have been invaluable in provision of information relevant to understanding of the photophysics of macromolecules and in the early years led to the generalisation of Hirayama C3] known as the "n = 3 rule". Whilst the n=3 rule is not strictly obeyed it has focussed the thoughts of workers seeking elucidation of macromolecular photophysics especially for the case of vinyl aromatic polymers. 

For quasi-one-dimensional, two-dimensional, and three-dimensional diffusional processes, other forms are appropriate [10,11-14], Thus very extensive theoretical and picosecond experimental work on electronic excited state transport in finite volumes of randomly distributed molecules has been reported, which shows that there are significant deviations in the behaviour of finite volume systems compared with the infinite volume systems considered above. The treatment is mathematically complex and the results will not be given here explicitly [12-14], Frederickson and Franck [15] have used this treatment to suggest possible forms for the decay of monomer and growth and decay of excimer fluorescence in vinyl aromatic polymers where electronic energy migration might be a dominant process. This treatment is presented elsewhere in the volume, and will be discussed briefly below. 

---