Multicolour- and high-colour-contrast switching in response to force and acid vapour by introducing an asymmetric D–π–A–π–D structure

Abstract

An asymmetric D–π–A–π–D phenothiazine derivative (DPIAN) with cyano and imine as electron-withdrawing groups exhibited three fluorescence colours, green, orange and red, under mechanical force and thermal or solvent annealing stimuli. The shifts of absorption and emission spectra reached 88 and 102 nm, respectively. The analogue with cyano and amine moieties changed its fluorescence colour from yellow to orange. Only 20 and 34 nm shifts in absorption and fluorescence spectra were observed after grinding. The results suggested that introducing another phenothiazine by a weak electron-withdrawing imine not only induces hypsochromic absorption and emission bands in crystals but also results in a large bathochromic shift under mechanical force stimuli. More interestingly, the red ground solid of DPIAN under formic acid vapour transformed into a blue solid, and this transformation relates to an absorption spectral shift of more than 200 nm compared with that of the pristine solid. The blue solid converted to red and then yellow again after fuming with NH₃ vapour and then organic solvent. So, multi-colour and high-colour-contrast switching in response to double stimulus sources could be achieved.