New insight into π-π interactions: realization of full color emission from blue to red under hydrostatic pressure without exogenous intramolecular charge transfer

Non-covalent interactions between aromatic moieties, especially pi-pi interactions, play a key role in organic luminescent materials. Pyrene often serves as a common luminogen, forming discrete dimers for fundamental research. In this work, pyrene-based materials with different pi-pi stackings in the crystal state were chosen and combined with high-pressure technique to explore the relationship between pi-pi stacking and corresponding photophysical properties. Xanthene was introduced as the template frame to quantitatively control the distance between two pyrene units of 2,7-di-tert-butyl-9,9-dimethyl-4,5-di(pyren-1-yl)-9H-xanthene (X2P). Next, alkynylene substituents were introduced for improving molecular flexibility (2,7-di-tert-butyl-9,9-dimethyl-4,5-bis(pyren-1-ylethynyl)-9H-xanthene, X2EP). High pressure technique was utilized for regulating pi-pi interactions gradually and continuously. As a result, these four crystals, including pyrene, X2P, X2EP-B and X2EP-N, showed red shifts of emission with different degrees in response to the decreased pi-pi distance and enhanced pi-pi interactions under high pressure, in which X2EP exhibited remarkable tri-color changes from blue to red with higher contrast and enhanced sensitivity than X2P. Therefore, this work not only reveals that the photophysical properties of pyrene-based materials are highly related to the variation of the pi-pi distance, but also provides a strategy for the fabrication of piezochromic materials (PCMs) with excellent performance by combining molecular rigidity and flexibility, which can be extended to other fluorophores besides pyrene.