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‐tetracarboxylic 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‐tetracarboxylic 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, N¹) 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 N¹ 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 blueshift 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.