Fluorescence studies of furanoxy radicals: intramolecular and intermolecular processes

Fluorescence properties of furanoxy radicals were used to investigate intramolecular relaxation and intermolecular quenching processes involving these organic doublet species. The emission spectra of several substituted furanoxy radicals are given; maxima lie in the 615-660-nm region. Excited-state lifetimes were measured and lie in the 6-13-ns range. There were solvent effects on the lifetimes as well as on the absorption and emission spectra. The temperature dependence of the fluorescence lifetimes was studied and found to be small, E{sub a} = 0.35 kcal/mol, compared to that in benzyl radical. Rate constants of internal conversion (and fluorescence quantum yields) were estimated and found to follow a previously proposed correlation with Taft polar substituent constants. In addition to the intramolecular photophysics, quenching studies were carried out. In particular, the furanoxy radicals were used to quench the triplet of two polycyclic aromatics where transfer of energy was allowed and where sensitized fluorescence was observed (k{sub q} {approximately} 6 {times} 10{sup 9} M{sup {minus}1} s{sup {minus}1}). Details of an improved method for making measurements of k{sub q} between two transients are given. Recombination kinetics of the radicals were also measured using 5,5{prime}-bis(furanones) as precursors of the radicals.