Photoinduced electron transfer in N,N-bis(pyridylmethyl)naphthalenediimides: study of their potential as pH chemosensors

The change in fluorescent properties of a series of N,N-bis(pyridylmethyl)naphthalenediimides (BIPy-NDIs) as function of pH were investigated. The naphthalenediimide dyes displayed OFF–ON pH sensing properties owing to photoinduced electron transfer in the pH range from 1.7 to 4.1. The fluorescence enhancement of the chemosensors studied is based on the hindering of photoinduced energy-electron transfer (PET) from pyridine ring to the naphthalene fluorophore by protonation. Moreover, using density functional theory theoretical calculations of molecular orbitals, it was verified that protonation nitrogen atom in pyridine ring inhibits the PET process. The best selective response for monitoring pH in the presence of different metal ions, was exhibited by BIPy-NDI 1B. In addition, 1B was applied for determination of pH in real samples of commercial vinegars. The results were consistent with those obtained by glass electrode method, indicating that the new probe could be a practical pH indicator in strongly acidic conditions.