Quinoxaline based D-A-D molecules: high contrast reversible solid-state mechano- and thermo-responsive fluorescent materials

A new series of AEE-active D-A-D molecules by integrating a diphenylquinoxaline core with two aromatic chromophores such as anthracene or pyrene has been designed and synthesized. These highly pre-twisted molecules exhibited intriguing photophysical properties. The experimental and theoretical investigations revealed that their twisted conjugated skeleton is the key factor for showcasing the AEE property because of the restricted face-to-face co-planar arrangement and strong intermolecular interactions. Although, formulated compounds differ trivially in their chemical composition, they exhibit distinct stimuli-responsive behaviors in the solid state. The pyrene derivative exhibited high-contrast piezofluorochromic behavior with the emission colors reversibly changing from bluish green to yellow upon grinding and annealing. On the other hand, the anthranyl analogue showed a unique on-off switching of solid-state luminescence by heating and solvent fuming. This system provides a rare example of distinct thermal reversible fluorescence switching controlled by changing the molecular packing modes and the origin of this unique property was attributed to the formation of donor-acceptor intermolecular stacking which enables an intermolecular electron transfer quenching process. Based on XRD, DSC, and NMR studies, the observed piezofluorochromism of pyrene or thermochromism of anthracene derivatives is attributed to the result of solid-solid phase transition. The present findings would open the way for the development of novel organic luminescent solids that can be switched on and off by external thermal stimuli.