Intramolecular Charge Transfer-Boosted π–π Stacking for Rapid Screening of the d Electron Number in Transition Metals

The d electrons largely affect the physicochemical properties of transition metal, and its rapid screening is significant to the understanding and regulation of metal-based materials. In this study, we proposed an imidazole-anchored naphthalenediamine-based fluorescent probe for rapid screening of d electrons of transition metal ions. The d electron-regulated fluorescence quenching is attributed to the fact that d electron number-enhanced probe-metal binding, which promotes the intramolecular charge transfer and benefits the face-to-face π–π stacking of the naphthalene motif. The universality of d electron-regulated fluorescence is validated by imidazole-anchored phenylenediamine with a smaller π-electron delocalization system. In addition, the d electron number-related fluorescence character is applied for the differentiation of transition metal ions based on a sensor array.