The effect of substituent number on mechanochromic luminescence of β-diketones and the corresponding boron complexes

In this work, we designed and synthesized a series of beta-diketones and the corresponding boron complexes (including mono-substituted, meta-position di-substituted and tri-substituted compounds). The effect of substituent number on their photophysical properties and mechanochromic luminescence (ML) behavior in different states was systematically studied. As verified by experimental and calculated results, the mechanical responsive properties of the six molecules were found structurally dependent, which were mainly modulated by conjugation effect and steric effect derived from different substitution. The di-substituted m-bdk and m-BF2bdk displayed the most prominent ML behavior, while the mono-substituted molecules showed almost no response to mechanical force. Moreover, the tri-substituted compounds (tri-bdk and tri-BF2bdk) showed fast self-erasing properties, highlighting the promising applications in self-erasing or self-healing devices and optical force sensors with high temporal resolution. Our study not only provides an effective strategy for modulating the optical properties, ML performance and recovery time by tuning the substituent number at molecular scale for the first time, but also facilitates the in-depth understanding of structure property relationships in beta-diketonate based dyes.