Fluorescence of molecular excimers

Abstract

The phenomena of excimer fluorescence are most thoroughly studied in solutions of polycyclic aromatic hydrocarbons (PAHs). Therefore, excimer formation is usually treated with reference to molecules of this class of compounds. In the literature, only a qualitative picture of the energetics of excimer formation is given. It is assumed that dimerization of electron-excited and unexcited molecules is followed by the splitting of molecular electron-excited levels. PAH molecules are characterized by two lower excited levels: {sup 1}L{sub a} and {sup 1}L{sub b} (according to Platt`s classification). The S{sub 1}* state of some PAH compounds (e.g., naphthalene, phenanthrene, pyrene) is {sup 1}L{sub b}, while in other PAH (anthracene, naphthacene, perylene, etc.) it is the {sup 1}L{sub a} state. It is assumed that the {sup 1}L{sub a}-level is split more significantly than the {sup 1}L{sub b} level. Therefore, for all PAH investigated the excimer state is described as a lower-lying component of the split {sup 1}L{sub a} level. Quantum-chemical consideration of the splitting of electron levels of PAH molecules in excimers is undertaken. Unfortunately, in this case the description is also of a qualitative character. In the cited work, a correlation is noted between the energy of the {sup 1}L{sub a} state ofmore » the molecule and the wave number corresponding to the maximum of the emission band of the excimer. However, it does not give the wavelength of the maximum of the excimer band for some PAH, in other words, a definite dependence of the position of this band on molecular structure. The present work is devoted to a search for an answer to this question.« less