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 π–π 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 π–π stackings in the crystal state were chosen and combined with high-pressure technique to explore the relationship between π–π 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 π–π 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 π–π distance and enhanced π–π 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 π–π 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.