Luminescence and charge transfer. Part 3. The use of chromophores with ICT (internal charge transfer) excited states in the construction of fluorescent PET (photoinduced electron transfer) pH sensors and related absorption pH sensors with aminoalkyl side chains
7-Methoxycoumarin, 3-aminobenzimidazo[2,1-a]benz[de]isoquinolin-7-one and 4-nitroaniline derivatives, useful as chromatographic derivatizing agents, dyes and solvent polarity indicators respectively, are easily transformed into optical pH sensors 3a–c, 4a–c and 5a–c respectively by attachment of a proximal, non-adjacent amine moiety. The availability of ICT (internal charge transfer) excited singlet states in each of these sensor families leads to protonation-induced wavelength shifts of their electronic absorption spectra. A similar phenomenon is observed in those fluorescence emission spectra where the proton-free form of the sensor is weakly emissive due to the sluggishness of the designed PET (photoinduced electron transfer) process. Except for these differences, sensors 3a–c and 4a–c show the expected pH dependence of fluorescence quantum yields, thus extending the scope of the fluorescent PET pH sensor design logic to electronic push-pull fluorophores besides aromatic hydrocarbons.