A series of novel 1H-indole-7-carbohydrazide derivatives with photoswitching and AIE properties: "On-off" fluorescence sensors for Cu2+

In order to exploit novel fluorescent sensors with photoswitching properties, six fluorescent molecules were synthesized via coupling 1H-indole-7-carbohydrazide and corresponding substituted formaldehyde through an acylhydrazone bond. All title compounds were characterized via¹H, ¹³C NMR, and elemental analysis, the single crystal structure of compound IC-PD was obtained via X-ray diffraction. The compounds IC-EL and IC-4PZ exhibited aggregation-induced emission (AIE) characteristics and also with regular responses to temperature and alkalinity in the solvent system dominated via deionized water. Probes IC-PL, IC-4PZ, and IC-PD could be used as fluorescence probes to detect copper (Ⅱ) with the LODs of 0.37, 0.27, and 0.26 μM in concentration of 0-50 μM, especially extraordinary bits of complex IC-PD-Cu²⁺ could also exhibited bright yellow in tetrahydrofuran (THF), which ensured the convenience of detection for Cu²⁺ ion. The exploration on the combination mechanism of representative compound IC-PD and Cu²⁺ ion via FT-IR and ¹H NMR, which confirmed that the proton of hydrazide was protonated via Cu²⁺ ion, resulting in the excited state intramolecular proton transfer (ESIPT) process was interfered. When IC-PD was irradiation to the 302 nm photostationary state (PSS), the fluorescence emission intensity of IC-PD in THF was slightly enhanced, accompanied with the increased sensitivity of detection for Cu²⁺. Although the improvement was not significant, it was also provided a design foundation for the development of stimuli responsive fluorescent materials with photoswitching properties.