Regiocontrol of Fluorescent PET (Photoinduced Electron Transfer PC) pH Sensors

摘要

The design of these fluorescent PET (Photoinduced Electron Transfer) sensors for protons was based on the 'Fluorophore-Spacer-Receptor' format which has previously been shown to be effective in the construction of fluorescent sensors for various chemical species. This presentation shows the importance of kinetic control factors in the design of fluorescent PET sensors, based on the 4-amino-1,8-naphthalimide fluorophore. Two sets of molecules with opposite connectivities of the 'spacer-receptor' assembly to the fluorophore have been studied. ΔGPET is nearly zero for both sets of molecules. However due to their 'push-pulp a-electron system (4-amino donor and 1,8-naphthalimide acceptor) an ICT (Internal Charge Transfer) is set up in the lowest excited singlet state. Due to the creation of this electric field upon photoexcitation acceleration or inhibition of a transiting electron can occur. Molecules with the 'spacer-receptor' connected at the 4-amino position display 'switching on' of fluorescence upon protonation (factor of 25) as the proton-induced suppression of PET leads to fluorescence as the predominant deactivation pathway. However, molecules with the 'spacer-receptor' connected at the 1,8-naphthalimide position show only a very slight reduction of fluorescence upon protonation which is attributed to quenching via an intramolecular hydrogen bonded structure. The presence of hydrogen bond donors near the negative pole of ICT excited states is known to result in fluorescence quenching. These results show the importance of kinetic control factors as well as thermodynamic factors in the design of fluorescent PET sensors.