Molecular probe for a fluorous medium:: Long-lived phosphorescence of α-diketones in perfluoromethylcyclohexane at room temperature

We found that alpha-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 (tau(rho)) of BD in PFMC (650 mu s) was much longer than that in cyclohexane (CH, 270 mu s). The longer tau(rho), value of BD in PFMC was ascribed to the variation of the intersystem crossing rate constant (k(isc)) from the excited triplet state (T-1) to the ground state (So) with the solvent. Some possible reasons for the change in ki,, 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 rnixing/separation processes between PFMC and a common organic solvent observed by Schlieren photographs were also reported.