Schiff base-bridged TPE-rhodamine dyad: facile synthesis, distinct response to shearing and hydrostatic pressure, and sequential multicolored acidichromism

Abstract

In this article, we report a newly designed TPE-rhodamine dyad bridged by a Schiff base structure, which can be synthesized facilely and achieves AIEE, a distinct response to shearing and hydrostatic pressure and intriguing sequential multicolored acidichromism, owing to the imine group and spiro-lactam structure. RhBTPE displayed the AIEE property in a THF/H₂O system, due to water-induced aggregation and aggregation-suppressed nonradiative relaxation of the excited states. The single crystal of RhBTPE was obtained and the complex intermolecular interactions were analyzed. RhBTPE showed a distinct response to anisotropic shearing force and isotropic hydrostatic pressure. Under shearing force, the crystal powder underwent an emission color switch from blue (440 nm) to green (475 nm) and further to red (580 nm), which were ascribed to the LE emission, HLCT emission and ring-open species of rhodamine lactam, respectively. When subjected to hydrostatic pressure, the single crystal displayed a color change from blue to green and to dark color, because of the pressure-induced separation of HLCT and further pressure-induced emission quenching. More interestingly, RhBTPE realized a high-contrast sequential color switch from blue to yellow and further to red by fuming with TFA. The protonation of imine and rhodamine lactam played the key role in the multicolored change. To the best of our knowledge, RhBTPE is a rare dyad that achieves a sequential multicolored switch by using an external force and fuming with an acid, respectively. This study may contribute to developing a multi-stimuli responsive fluorescent switch with multicolored changes.