The effect of meta versus para substitution on the efficiency of chemiexcitation in the chemically triggered electron-transfer-initiated decomposition of spiroadamantyl dioxetanes

A similar viscosity dependence of the CIEEL efficiencies for the para- and meta-substituted spiroadamantyl dioxetanes 1 has been observed, which implies that an electron-transfer mechanism operates through solvent-caged species for both regioisomers. The pronounced difference in the chemiexcitation yields for the meta- and para-substituted dioxetanes is rationalized in terms of the much larger (ca. 200-fold) rate constant for the electron back-transfer (BET) step to afford the excited meta CIEEL emitter. An in-depth kinetic analysis of the viscosity effect on the excited-state generation for the para versus meta regioisomers supports this conclusion.