Excimer fluorescence, compounds origin

Since Forster s original work, a large number of aromatic compounds, including benzene, naphthalene, and anthracene, have been found to have concentration-dependent fluorescence spectra under some conditions. Most of these excimers are not as stable a.s the pyrene prototype, and require lower temperatures or higher concentrations to be observed.29 Some crystals also exhibit excimer emission. Crystalline pyrene, for example, has only a single structureless fluorescence band of the same energy as its excimer emission in solution.30... 

Excimers (excited dimers) are formed by pairs of molecules or atoms that do not signihcantly interact in the ground state, but are weakly bonded in the excited state. The bonding in the excimer takes place between an excited molecule and a ground state molecule of the same species. Its origin is in the change of orbital symmetry that accompanies excitation and leads to cooperative (positive) orbital overlap and hence to bonding between the two systems. Examples in resist systems can be found in aromatic and heteroaromatic molecules used in photoactive compounds. Excimers were hrst observed by Eorster and Kasper in 1954 when they observed two kinds of fluorescence in fairly concentrated solutions (10 M) of pyrene. ... 

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