Fluorescence Emission and Absorption Spectra of Naphthalene in Cyclohexane Solution: Evidence of a Naphthalene–Oxygen Charge–Transfer Complex

Molecular fluorescence is typically reduced or "quenched" by dissolved oxygen present in solutions. Quenching mechanisms are broadly categorized as dynamic quenching, and static quenching. Dynamic quenching involves collisions between oxygen and the fluorophore. Static quenching occurs when the fluorophore forms a nonfluorescent complex with oxygen. In a previous study, oxygen quenching of naphthalene showed evidence of static quenching, although the mechanism was not known. The earlier work was limited to low oxygen concentrations (∼2 mM) in cyclohexane, leading to relatively large uncertainties in the value of the static quenching constant. In the present study, oxygen concentrations up to ∼50 mM were used, resulting in a more accurate determination of the static quenching constant. Absorption spectra obtained at higher oxygen concentrations revealed the spectrum of a new species, which we interpret as that of a naphthalene-oxygen charge-transfer complex. The binding energy of the naphthalene-oxygen complex was estimated and compared with that of the benzene-oxygen complex. In the method described herein, the equilibrium (or association) constant for the naphthalene-oxygen complex was assumed to be the static quenching constant, as determined from graphs of relative fluorescence intensities vs oxygen concentration using the Stern-Volmer formalism. The values of the complex molecular absorption coefficients were determined from graphs of absorbance vs oxygen concentration using the value of the static quenching constant from fluorescence data. This method differs from a commonly used method in which graphs of absorbance vs oxygen concentration are used to determine the product of the equilibrium constant and the complex absorption coefficient. The latter method can lead to substantial errors in both these quantities if the spectrum of the free molecule and complex overlap significantly.