Large steric hindrance amide modified benzothiadiazole: High contrast mechanochromic luminescence and volatile acid vapor response

Three mechanochromic luminescent (MCL) materials were obtained by modifying the 5, 6 and 4, 7 positions of benzothiadiazole (BTD) with two sterically hindered amides and two aromatic groups, respectively. The Stokes shifts of the compounds were larger in different solvents (>6000 cm−1), especially 3c in dimethyl sulfoxide up to 8900 cm−1. Upon response to mechanical force, the fluorescence of 3c had high contrast (75 nm) before and after milling, and the emission changed from bright yellow light (533 nm) to orange-red light (608 nm). Meanwhile, strong fluorescence emission intensities were observed before and after grinding (quantum yield: 56%, 46% respectively). The MCL reasons of the compounds were explained in detail by theoretical calculation, single crystal X-ray diffraction (XRD), powder XRD and differential scanning calorimetry (DSC) analysis. In addition, the MCL materials also showed fluorescence responsiveness to the vapor of volatile acids. [Display omitted] •Three solid fluorescent materials were designed by modifying the 5, 6 and 4, 7 positions of benzothiadiazole.•The materials show high-contrast mechanochromic luminescence properties and high quantum yield.•The MCL mechanism were carried out by theoretical calculation, single crystal XRD and powder XRD.•The materials are sensitive to acid in solution and in the gaseous state.