Intramolecular charge transfer and sensing mechanism for a colorimetric fluoride sensor based on 1,8-naphthalimide derivatives

The sensing mechanism of a fluoride colorimetric chemosensor 4-(tert-butyldimethylsilyloxy)-N-butyl-naphthalimide has been studied with density functional theory and time-dependent density functional theory methods. The theoretical results suggest that the low barrier of the desilylation reaction is responsible for the rapid response speed to the fluoride anion of the chemosensor. The calculated vertical excitation energies in the ground state of the chemosensor and its desilylation product agree well with the experimental UV-Vis absorbance spectra. It is also found that the intramolecular charge transfer process of the first excited state of the desilylation product induces the redshift of the absorbance and fluorescence spectra of the desilylation product compared with that of the chemosensor. Further, the previously experimentally incorrect assignment of the H-1 NMR spectrum of the desilylation product has been rectified in the present theoretical study.