Chemiluminescence of cerium(IV)–rhodamine 6G–phenolic compound system

The oxidation reaction between cerium(IV) and rhodamine 6G in sulfuric acid medium underwent weak chemiluminescence (CL). The effects of 53 organic compounds of interest on cerium(IV)–rhodamine 6G chemiluminescence were investigated by a flow injection procedure, and 32 phenolic compounds were found to enhance CL. Phenolic compounds mainly include phenols, polyphenols, phenolic acids, hydroxycinnamic acids and flavonoids. The correlation between CL and molecular structure was systematically studied. It was noteworthy that phenolic hydroxyls were the main active groups for the generation of CL. The magnitude of CL was related to the type and position of substituents in the benzene ring. The maximal emission wavelength of CL spectra for all tested phenolic compounds was at about 555 nm, and luminophors were assigned to rhodamine 6G. Based on the studies of kinetic process and the spectra of CL, fluorescence and UV–vis absorption, a CL mechanism has been proposed to be due to that rhodamine 6G and phenolic compound are oxidized by cerium(IV) to form the excited-state cerium(III), which transfers energy to rhodamine 6G, resulting in light emission. However, on the other hand, if the oxidation products of some phenolic compounds such as benzoquinone or ketone could significantly quench the emissive rhodamine 6G via energy transfer, no light emission was observed for such compounds as hydroquinone and catechol.