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 Zn 2+ 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 Zn 2+ . 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 Zn 2+ . The fluorescence enhancement of PND with Zn 2+ titration is dominated with a typical photoinduced electron‐transfer (PET) process. In contrast, the N 1 atom for PNT can participate in the coordination of Zn 2+ 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 Zn 2+ . Due to the differential binding mode caused by the ligand effect, PND shows excellent selectivity to Zn 2+ over other metal ions, with a larger fluorescent enhancement centered at 650 nm. Also both PND and PNT were successfully used to image intracellular Zn 2+ ions in the living KB cells.