Behavior of Excited Molecules of Aromatic Hydrocarbons in the Liquid State. I. Benzene and p-Xylene: Aggregation Effects

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Abstract
Experimental results of the effects of increasing aromatic concentration on the fluorescence spectrum and intensity of benzene and p‐xylene in cyclohexane are presented. Other authors have studied these systems but there is disagreement as to results and interpretations. In agreement with previous results, an increase in aromatic concentration results in the appearance of a new fluorescence band in benzene but has only a small effect on the spectrum of p‐xylene. The fluorescence spectrum of benzene at high concentrations, however, cannot be simply resolved into monomer and excimer spectra. Further, the increase in excimer fluorescence could not be correlated with the decrease in monomer fluorescence as predicted by a simple monomer–excimer mechanism. The luminescence decay time for both p‐xylene and benzene remains constant within 10% in going from very dilute solutions to undiluted aromatic. This constancy in decay time can be correlated with changes in the monomer fluorescence intensity by means of a monomer–excimer mechanism in which an approximate rapid equilibrium exists between the processes of excimer formation and excimer dissociation. Some discrepancies reported by other authors between experimental data and predictions of a monomer–excimer mechanism do not appear in our results, or we can remove some of these discrepancies, as shown in this paper, by proper choice of values for rate parameters. The results of this paper are applied to the analyses of external quenching data in Paper II.
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