Excimers constraines

In summary, all available evidence suggests that the intermolecular excimers of naphthalene compounds have a sandwich structure in which the ring planes are parallel and the molecular axes are aligned. While the intermolecular excimer appears to adopt the eclipsed sandwich structure in solution, there may be differences in the structure of excimers constrained by hydrocarbon links or by rigid matrices. These constrained excimers will be considered next. 

The serious drawback of the methods of evaluation of fluidity based on intermolecular quenching or excimer formation is that the translational diffusion can be perturbed in constrained media. It should be emphasized that, in the case of biological membranes, problems in the estimation of fluidity arise from the presence of proteins and possible additives (e.g. cholesterol). Nevertheless, excimer formation with pyrene or pyrene-labeled phospholipids can provide interesting in-... 

There is a substantial entropy decrease AS associated with intermolecular excimer formation, as given in the tabulation by Birks 71). For all solvents (except 95% ethanol 75), the value AS ss —20 cal/mole-K was observed for naphthalene and its derivatives. For comparison, the entropy of fusion of unsubstituted aromatic hydrocarbons such as naphthalene falls in the range of —8 to —15 e.u. The large loss of entropy in the intermolecular excimer formation process indicates a very constrained symmetric structure. 

Phane compounds, i.e. two chromophores held face-to-face by at least two hydrocarbon links (4) Sandwich dimers, which are chromophore pairs produced by photolysis of photodimers in rigid matrices and (5) Bichromophoric compounds having a single saturated hydrocarbon linkage, which form intramolecular excimers as allowed by the rotational isomers of the linkage. In each case, we will utilize the intermolecular excimer formed in solution as the standard against which the properties of constrained excimers will be measured. 

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. 

It is obvious, that the conformation of a section of polystyrene which is required to make two neighboring phenyl residues lie parallel to one another (at a distance of about 2.5A°) would be characterized by a prohibitive potential energy and such a conformation would not be expected to be significant in the ground state. This conclusion is also supported by the fact that polystyrene exhibits the normal UV spectrum of alkylbenzene, while benzene rings constrained to lie parallel to one another at such a short separation have characteristically distorted absorption spectra (14). We must, therefore, conclude that excimers are formed by a conformational transition during the lifetime of the excited state. This transition is assisted by the large exother-micity of excimer formation which is characterized by AH values of -5.1 and -3.9 kcal/mole for benzene and toluene, and by -5.8 and -6.9 kcal/mole for naphthalene and the methylnaphthalenes (11). 

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