Realization of distinct mechano- and piezochromism by regio-isomerization of cyanostilbene isomers

Three cyanostilbene isomers DCSB-In, DCSB-IO and DCSB-Ou with different cyano positions have been designed and synthesized. The –CN regio-isomerization in this system results in distinct aggregation-induced optical properties, mechano- and piezochromic luminescent properties. By shifting –CN unit from β to α position, molecules convert from a typical ACQphore (DCSB-Ou) to AIEEgen (DCSB-In). It is noteworthy that DCSB-Ou with β-CN groups exhibits an obvious mechanochromic (MC) property, whereas no photoluminescent (PL) shift is observed from DCSB-In upon grinding though accompanied with the crystalline-amorphous phase transition like most reported MC materials. Moreover, all isomers show piezochromic (PC) properties, and the PL of both DCSB-In and DCSB-IO red-shifts consistently and almost linearly dependent on the increased hydrostatic pressures, whereas DCSB-Ou experiences two stages of fluorescent change with more than 200 nm of wavelength shift. With only regio-isomerization but varied MC and PC performances, these cyanostilbene analogues as the model molecules provide a platform to explore the distinct MC and PC mechanisms. Photophysical properties, structural analysis, theoretical calculations and in-situ tests were comparatively studies to establish the origin of MC and PC phenomenon. The subtle modulation of –CN position could significantly influence the intermolecular interaction, thus control the π-stacking mode in solid-state, and endow molecules with different stimuli dependent fluorescent properties.