Molecular Probe for a Fluorous Medium: Long-Lived Phosphorescence of α-Diketones in Perfluoromethylcyclohexane at Room Temperature

Abstract

We found that α-diketones (2,3-butanedione (BD) and 1-phenyl-1,2-propanedione (PPD)) were very suitable luminescence probes for studying the properties of a perfluorinated solvent (i.e., fluorous solvent; perfluoromethylcyclohexane (PFMC)), since these compounds were soluble in PFMC and showed long-lived phosphorescence even at room temperature. The phosphorescence lifetime (τ_p) of BD in PFMC (650 µs) was much longer than that in cyclohexane (CH, 270 µs). The longer τ_p value of BD in PFMC was ascribed to the variation of the intersystem crossing rate constant (k_isc) from the excited triplet state (T₁) to the ground state (S₀) with the solvent. Some possible reasons for the change in k_isc were discussed in terms of solute-solvent interactions. Furthermore, by utilizing phosphorescence quenching of BD by pyrene, we determined a rate constant of the diffusion-controlled reaction in PFMC. Characteristic behaviors of mixing/separation processes between PFMC and a common organic solvent observed by Schlieren photographs were also reported.