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The importance of the modular structure 'fluor-spacer-amine' is pointed out for the design of fluorescent molecular sensors for p H according to the principle of photoinduced electron transfer (PET). Anthracen-9-yl methylamines (24) and some azacrown ether analogues (1 5 and 23) are examined in this context. They show p H -dependent fluorescence quantum yields describable by eqn. ( 5) while all other electronic spectral parameters remain essentially pH-invariant. The range of pK, values of these sensors are understandable in terms of macrocyclic effects and the transmission of electric fields across the anthracene short axis. Phase-shift fluorometric determination of the fluorescence lifetimes of these sensors allows the calculation of the rate constant of PET in their proton-free form t o be 1 Ole-1 0" s-', with the diamines 23 and 24b exhibiting the faster rates.