Fluorescence properties of 3- and 4-trifluoroacetylamino-1,8- naphthalimides: Solvent-controlled switching of fluorescence color and response to metal-cations

The effects of solvents and metal cations on the fluorescence spectra of 3- (1) and 4- (2) trifluoroacetylamino-1,8-naphthalimides were investigated. In MeCN, these naphthalimides displayed a violet ∼ blue fluorescence based on their neutral amide form (1, 2). Whereas in DMSO, they gave off a yellow ∼ orange emission due to their amide anions (1^-, 2^-). In MeOH, naphthalimide 2 emitted a dual fluorescence (λ_FL 436 and 556 nm) to display a white luminescence. Naphthalimides 1, 2 are essentially insensitive to metal cations in their neutral amide form. In contrast, the fluorescence of the amide anions 1^-, 2^- was quenched by metal cations. The efficiency of the quenching correlated with the Lewis acidity of the metal cations; a stronger Lewis acid tended to display a more effective fluorescence quenching. Therefore, the fluorescence color and the sensitivity to metal cations of the naphthalimides were effectively switched by the solvent.