Substituent effects on the decomposition of chemiluminescent tricyclic aromatic dioxetanes

Three tricyclic 1,2-dioxetane derivatives, 1a, 2a and 3a were synthesized from their corresponding 1,4-dioxin acenaphthylene compounds, 1, 2 and 3, by reaction with singlet-oxygen ((1) O2 ) in dichloromethane. Evidence for the formation of the dioxetanes 1a, 2a and 3a is provided by the chemiluminescence (CL) that corresponds to the emission from the electronically excited diesters 1b*, 2b* and 3b*, which are decomposed thermally from the dioxetanes 1a, 2a and 3a, respectively. The highly strained 1,2-dioxetane ring decomposes from a twisted geometry by simultaneous cleavages of the O-O and C-C bonds, producing the electronically excited diester that emits CL. It was observed that the CL from compound 2a is red-shifted relative to that of compounds 1a and 3a suggesting a higher degree of stabilization for the excited state by the electron-donating methoxy group. Also, a study of the solvent effect on fluorescence shows a significant red-shift in compound 2b, indicating a more polar excited state. The kinetics of the thermal decomposition of the 1,2-dioxetanes clearly demonstrate that the CL characteristics of compound 2a are quite different from those of compounds, 1a and 3a. These results are consistent with the proposed intramolecular chemically initiated electron exchange luminescence (CIEEL) mechanism which is triggered by the electron-donating group of compound 2a.