Solvatochromic effects on the electron exchange chemiluminescence (CIEEL) of spiroadamantyl-substituted dioxetanes and the fluorescence of relevant oxyanions

A comparative spectral study on the intramolecular chemically initialed electron exchange luminescence (CIEEL) of five spiroadamantyl-substituted dioxetanes by various enzymatic and chemical triggers (alkaline phosphatase, HO-, and F-) in protic (H2O, D2O, and MeOH) and aprotic (MeCN and DMSO) solvents and the fluorescence of the authentic methyl m-oxybenzoate anion in the same media is reported. The present study reveals that the CIEEL spectral maxima are independent of the reaction medium and the means of triggering, whereas the fluorescence spectrum of the authentic CIEEL emitter depends on the solvent. In the aprotic solvents the CIEEL and the fluorescence spectra coincide, while in the protic media the fluorescence maxima of the authentic methyl m-oxybenzoate ion are blue-shifted by ca. 50 nm relative to those of the CIEEL emission. Neither exciplex formation nor specific environmental influence of the enzyme on the chemiexcited oxybenzoate accounts for the spectral shift between the CIEEL and the fluorescence emissions. The substitution pattern of the fluorophore is decisive for the observed solvent effects. For the odd-substituted methyl oxybenzoate, in which the oxy and the ester functionalities are cross-conjugated, a blue shift applies in the protic media, but not for the even-patterned methyl oxynaphthoate, with the oxy and ester groups extendedly conjugated. These divergent spectral observations are rationalized in terms of hydrogen-bonding effects, which are qualitatively substantiated by the semiempirical AM1 calculations.