Near-IR Core-Substituted Naphthalenediimide Fluorescent Chemosensors for Zinc Ions: Ligand Effects on PET and ICT Channels

Near-IR (NIR) emission can offer distinct advantages for both in vitro and in vivo biological applications. Two NIR fluorescent turn-on sensors N,N'-di-n-butyl-2-(N-{2-[bis-(pyridin-2-ylmethyl)amino]ethyl})-6-(N-piperidinyl)naphthalene-1,4,5,8-tetra-carboxylic acid bisimide and N,N'-di-n-butyl-2-[N,N,N'-tri(pyridin-2-ylmethyl)amino]ethyl-6-(N-piperidinyl)naphthalene-1,4,5,8-tetra-carboxylic acid bisimide (PND and PNT) for Zn2+ based on naphthalenediimide fluorophore are reported. Our strategy was to choose core-substituted naphthalenediimide (NDI) as a novel NIR fluorophore and N,N-di(pyridin-2-ylmethyl)ethane-1,2-diamine (DPEA) or N,N,N'-tri(pyridin-2-ylmethyl)ethane-1,2-diamine (TPEA) as the receptor, respectively, so as to improve the selectivity to Zn2+. In the case of PND, the negligible shift in absorption and emission spectra is strongly suggestive that the secondary nitrogen atom (directly connected to the NDI moiety, N-1) is little disturbed with Zn2+. The fluorescence enhancement of PND with Zn2+ titration is dominated with a typical photoinduced electron-transfer (PET) process. In contrast, the N' atom for PNT can participate in the coordination of Zn2+ ion, diminishing, the electron delocalization of the NDI moiety and resulting in intramolecular charge-transfer (ICT) disturbance. For PNT, the distinct blue-shift in both absorbance and fluorescence is indicative of a combination of PET and ICT processes, which unexpectedly decreases the sensitivity to Zn2+. Due to the differential binding mode caused by the ligand effect. PND shows excellent selectivity to Zn2+ over other metal ions, with a larger fluorescent enhancement centered at 650 nm. Also both PND and PNT were successfully used to image intracellular Zn2+ ions in the living KB cells.