Waldemar Adam and Dirk Reinhardt
Photooxygenation of the methoxy-, siloxy- and acetoxy-substituted adamantylideneacridanes 3 afforded the corresponding dioxetanes 4. Thanks to the spiroadamantyl-substitution, these dioxetanes were sufficiently persistent to allow their isolation and full characterization. The activation parameters (Ea, log A, ωH‡, ωS‡) of the direct chemiluminescence for the methoxy-substituted derivatives 4a–c were determined by standard isothermal kinetic methods. The fluoride-ion- and base-induced decomposition of the siloxy- and acetoxy-substituted dioxetanes 4g,h,j,k was shown to involve intramolecular CIEEL emission. The CIEEL quantum yields (ΦCIEEL) were independent of the nature of the protective group, but marked differences were observed between the 2- and the 3-substituted derivatives; the latter are about two orders of magnitude more efficient. The difference in the CIEEL quantum yields was attributed to the distinct fluorescence properties of the corresponding emitters 7 since fluorescence from the 2-substituted derivative 7(2) is too small to be measurable, while for the 3-substituted derivative 7(3) the fluorescence quantum yields (ΦFl) are as much as a few percent. AM1 calculations were conducted on the oxy-substituted acridone emitters 7 to explore the reasons. Presumably, the dominant charge-transfer excitation of the acridone chromophore is not appreciably perturbed by the oxy substituent for both regioisomeric emitters 7(2,3). Thus, the similar singlet excitation yields (ΦS) for the regioisomeric oxy-substituted spiroacridane dioxetanes 6 generated on triggering reveal that they do not follow the odd/even rationale established for the related oxy-substituted benzoates and naphthoates.