A mechanochromic luminescent material with aggregation-induced emission: Application for pressure sensing and mapping

In this study, we report a new compound, (E)-4-(((2-hydroxynaphthalen-1-yl)methylene)amino)-3-methylbenzoic acid (HNMB), which shows aggregation-induced emission property as well as intramolecular charge transfer (ICT) nature. In addition, it exhibits unique mechanochromic luminescence (MCL). The HNMB solid powder emits strong emission but shows quenching effect together with bathochromic-shift after grinding, suggesting a high contrast ratio up to 1420%. Through crystallographic analysis, the relationship between MCL nature and molecular packing mode is verified: Molecules in crystalline phase adopt the J-type coupling based on less overlapped π⋯π stacking, in which multiple intermolecular interactions mainly including C-H⋯π, C-H⋯O and hydrogen bonding, help to stabilize such packing mode. When these interactions are destructed by mechanical force, the packing would be disassembled, activating the MCL behavior. Such working mechanism only needs weak external force capable of destructing intermolecular interactions, rendering the MCL material highly sensitive to pressure. As a practical application, a film sensor for pressure detection is designed based on HNMB, which gives a linear relation between the emission intensity and the external pressure in a lower range. The detection limit of the film sensor is 27.24 Mpa, suggesting high sensitivity. In addition, pressure mapping with high contrast ratio is obtained by surface plot, making this pressure sensor a reliable candidate to be instrumented for various applications.