Luminescent Metal–Organic Framework-Based Fluorescence Turn-On and Red-Shift Sensor toward Al3+ and Ga3+: Experimental Study and DFT Calculation

A dual-functional sensor based on a two-dimensional metal–organic framework (MOF), namely, {[Cd(BBZB)(2,6-NDC)]·CH3OH}n (JXUST-6, BBZB = 4,7-bis(1H-benzimidazole-1-yl)-2,1,3-benzothiadiazole and 2,6-NDC = 2,6-naphthalenedicarboxylate), has been designed and synthesized under solvothermal conditions. JXUST-6 exhibits good stability in aqueous solution, boiling water, and N,N-dimethylformamide (DMF) solution. Luminesecence studies of JXUST-6 reveal the existence of fluorescence red shift toward Al3+ and Ga3+ accompanied by turn-on fluorescence response. After gradually adding Al3+ solution to JXUST-6 suspended solution, the emission peak moved from 505 to 529 nm and triggered the color conversion from yellow to yellow-green under 365 UV light irradiation. Similarly, the addition of Ga3+ to JXUST-6 suspended solution caused the enhancement of fluorescence intensity as well as the red-shift from 505 to 530 nm. The detection limits of JXUST-6 toward Al3+ and Ga3+ ions are 0.081 and 0.047 ppm, respectively. Actually, the test paper can be used to sense Al3+ and Ga3+ ions, thereby expanding the potential application scope of JXUST-6. Importantly, JXUST-6 represents the rare turn-on MOF toward Ga3+. The red-shift effect of JXUST-6 toward Al3+ and Ga3+ could account for the formation of a strong exciplex between JXUST-6 and Al3+ and Ga3+. The theoretical caculation suggests that the occurrence of strong intramolecular charge transfer leads to turn-on effect toward Al3+ and Ga3+ ions.