Behavior of Excited Molecules of Aromatic Hydrocarbons in the Liquid State. II. Benzene and p-Xylene Quenching by O2 and CCl4

Results are presented on the efficiency of quenching of excited molecules of benzene and p‐xylene by O₂ and CCl₄ as a function of the concentration of these aromatics in cyclohexane. At each aromatic concentration, the aromatic decay‐time ratio $\text{τ(0)\hspace{0.17em}/\hspace{0.17em}τ(}{\mathrm{CCl}}_4)$ and steady‐state fluorescence intensity ratio $\text{I(0)\hspace{0.17em}/\hspace{0.17em}I(}{\mathrm{CCl}}_4)$ [where $\text{τ(0)}$ and $\text{I(0)}$ refer to the aromatic decay time and fluorescence intensity at zero quencher concentration] are linear functions of quencher concentration, i.e., a Stern–Volmer relation is observed. Further, the quenching constant determined from decay‐time and steady‐state data are equal to each other at each aromatic concentration. Finally, the efficiency of quenching increases with aromatic concentration, the quenching constant for the undiluted aromatic being about two times greater than the quenching constant for very dilute solutions. The results can be explained in terms of the monomer–excimer mechanism discussed in Paper I. This explanation, however, is not unique and does not eliminate the possibility of excitation migration, but indicates that excimers may play an important role in excitation transfer.