Rationally designed molecular D-A-D triad for piezochromic and acidochromic fluorescence on-off switching

We report herein a new class of piezochromic luminescence (PCL) material with an acid stimulus response based on a donor (D)-acceptor (A)-donor (D) molecular triad, m-BHHDCS (1), synthesized via a rational design strategy. Acid-responsive harmane derivatives were rationally introduced into the unique D-A-D platform to obtain dual-stimuli responsive properties, i.e. acidochromic and piezochromic behaviors. Upon exposure to mechanical force and/or acid treatment, the pristine powders of 1 showed rapid and instant fluorescence turn-on with discernible intensity (Phi(1B) = 9% and Phi(1C) = 5%, respectively). In order to test more practical applications, we utilized light as a stimulus source using a photo-acid generator (PAG) instead of acid, and thus successfully demonstrated an intriguing optical recording medium orthogonally activated by mechanical force and light. Moreover, based on the in-depth combined experimental and computational studies, we could conclude that the dual-stimuli responsive properties of 1 originated from a structurally controlled photo-induced electron transfer (PET) process in the molecular assemblies, that is inhibited by mechanical force and acid. Mechanical force destroys the characteristic stacked structure, so that the ultrafast PET is inhibited and fluorescence is recovered. In contrast, acid covalently binds to the harmane appendages in the specific stacked arrangement of 1, which efficiently blocks PET. We believe that this system will inspire the development of a new class of smart PCL materials, and furthermore provide important insights into solid-state emission and stimuli-response behaviors.