Luminescence and charge transfer. Part 2. Aminomethyl anthracene derivatives as fluorescent PET (Photoinduced Electron Transfer) sensors for protons

The importance of the modular structure 'fluor-spacer-amine' is pointed out for the design of fluorescent molecular sensors for pH according to the principle of photoinduced electron transfer (PET). Anthracen-9-yl methylamines (24) and some azacrown ether analogues (15 and 23) are examined in this context. They show pH-dependent fluorescence quantum yields describable by eqn. (5) while all other electronic spectral parameters remain essentially pH-invariant. The range of pK_a 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 to be 10¹⁰-10¹¹ s^-1, with the diamines 23 and 24b exhibiting the faster rates.