Tetraphenylethene-substituted benzothiadiazoles: AIE and TICT properties, tunable intramolecular conjugation and application in detecting trace water in organic solvents

A series of tetraphenylethylene-based benzothiadiazole derivatives (TPEB-o-NO2, TPEB-m-NO2, and TPEB-p-NO2) were designed and synthesized by adjusting the linkage model of nitro group. Different substituent positions of nitro group lead to different intramolecular conjugation of compounds TPEB-o-NO2, TPEB-m-NO2, and TPEB-p-NO2, thus resulting in different photophysical properties. Their maximum emission gradually red-shifted when changing the connected position of nitro group from ortho-position to meta-position and to para-position. TPEB-o-NO2, TPEB-m-NO2, and TPEB-p-NO2 exhibit interesting solvatochromic effects and “on–off–on” optical switch behavior owing to the combined action of AIE and TICT. Moreover, TPEB-m-NO2 and TPEB-p-NO2 were successfully applied as highly sensitive sensors in quantitatively detecting trace water in tetrahydrofuran (THF), dioxane (Diox) and ethyl acetate (EA). The DL of TPEB-m-NO2 and TPEB-p-NO2 are, 0.039% and 0.014% for THF, 0.021% and 0.009% for EA, 0.033% and 0.018%(v/v) for Diox, respectively. It is worth mentioning that TPEB-m-NO2, and TPEB-p-NO2 can act as sensitive indicator for monitoring trace water fraction in organic solvent rectification, which is much more convenient and intuitive to estimate the real-time water content approximately than traditional method.