Unexpected Emission Properties of a 1,8-Naphthalimide Unit Covalently Appended to a Zn-Salophen

We report the synthesis, characterization, and binding properties of a Zn-salophen complex, 1a, functionalized with a 1,8-naphthalimide unit. Unexpectedly, the emission spectrum of 1a shows a remarkable quenching of the band assigned to the naphthalimide unit. To better understand this phenomenon, a supramolecular model system constituted by a symmetric Zn-salophen and a pyridyl derivative of 1,8-naphthalimide, 1b2a, was investigated. We propose the existence of a photoinduced energy transfer process between the naphthalimide (donor) and the salophen (acceptor) units in 1b2a. A similar process must be operative in the covalent receptor 1a. Nevertheless, the results deriving from steady-state fluorescence experiments do not rule out the occurrence of a photoinduced electron transfer process as alternative pathway for the quenching. We also describe the chemosensing properties of receptor 1a and the supramolecular system 1b2a towards acetate. The nonsymmetrically substituted salophen receptor 1a only transduces the binding of the anion to the Zn metal center in significant spectroscopic changes in its absorption spectrum. On the other hand, we exploit the strong emission quenching experienced by the naphthalimide component in the supramolecular complex 1b2a to detect anions (e.g. acetate) by means of a typical turn-on fluorescent indicator displacement assay.