Two NDI-Based Multi-Stimuli-Responsive Coordination Polymers: Photo-Electro-Thermochromism, Detection of Amines/Antibiotics, and Applications in Message Encryption and Anticounterfeiting

Two naphthalene diimide (NDI)-based coordination polymers (CPs), namely {[Cd(BENDI)]·DMF·H2O}n (MDJNU-25) and {[Cd(BENDI)]·DMA·H2O}n (MDJNU-26), were successfully synthesized via a solvothermal method using Cd(NO3)2·4H2O and N,N′-bis(4-ethylphenyl)-1,4,5,8-naphthalene diimide (H2BENDI) in DMF and DMA, respectively. Both CPs exhibit reversible photochromic, electrochromic, and thermochromic properties. Structural analysis reveals that the variation in solvent modulates the crystal packing, resulting in different donor–acceptor (D–A) distances between MDJNU-25 and MDJNU-26. Notably, MDJNU-26 possesses a shorter D–A distance and a faster photoresponse rate than MDJNU-25. Furthermore, both CPs exhibit obvious color changes and high sensitivity to ammonia vapor and organic amines. Fluorescence sensing reveals that elevated antibiotic concentrations enhance quenching, with tetracycline hydrochloride giving the strongest response. The quenching mechanism is attributed to the synergistic effects of the internal filter effect (IFE), fluorescence resonance energy transfer (FRET), and photoinduced electron transfer (PET). TDDFT calculations were used to explore the photochromic and antibiotic-sensing mechanisms. These CPs perform excellently in message encryption and QR code anticounterfeiting, making them promising candidates for multistimuli-responsive systems.