MOLECULAR PHOTOIONIC SWITCHES WITH AN INTERNAL REFERENCE CHANNEL FOR FLUORESCENT PH SENSING APPLICATIONS

The second-generation fluorescent PET (Photoinduced Electron Transfer) pH sensors 4 and 5 are built up from the first generation system (alk ylaminomethyl anthracene) by attaching 3-amino-1,8-naphthalimide as a second fluorophore via an alkyl chain of 2 or 6 methylene units, Irrad iation of 4 and 5 in acidic solution at 367 nm mainly excites the anth racene fluorophore and the resulting emission arises from both fluorop hores in comparable proportions, The anthracene emission decreases sha rply as the pH is increased whereas the 3-amino-1,8-naphthalimide fluo rescence is less affected, The ratio of these two emissions reflects t he environmental pH, Excitation spectra monitoring 3-amino-1,8-naphtha limide emission at 550 nm shows that electronic energy transfer (EET) from anthracene to 3-amino-1,8-naphthalimide is maintained at high eff iciency across the pH range, with a further small increase at basic pH , The behavior in basic solution is most naturally understood in terms of an electron exchange mechanism facilitated by the intervening amin o receptor which gives rise to PET in first generation systems.