Fine-tuning the molecular conformation and packing structures of coumarin-based luminogens to achieve distinct piezochromic properties upon mechanical grinding and under hydrostatic pressures

Abstract

Organic luminogens (OLs) exhibiting piezochromic (PC) properties have drawn much attention owing to their great application potentials. Both mechanical grinding (MG) and hydrostatic pressures (HP) can induce PC behaviors of OLs, and it is highly desirable to combine the two strategies to study the PC properties of OLs for comprehensively exploring their application scopes and deeply understanding the intrinsic PC mechanisms. In this work, four coumarin derivatives with different substituents at 3- or 4-positions are designed and synthesized to investigate their PC properties by MG and under HP. By MG, two materials show PL shifts, and the PL of the other two molecules barely change. In contrast, under HP, these molecules all exhibit PL shifts, but with different pressure coefficients. In addition, they show different reversibility of PL change after releasing HP. The different molecular conformation and packing structure changing manners of the materials, indicated by single-crystal and powder X-ray diffraction patterns, and in situ PL lifetime analysis, are anticipated to induce distinct PC behaviors upon disparate force stimulus. Our study indicates that fine-tuning the functionalization position of coumarin derivatives is a powerful strategy to engineer their molecular conformation and packing structures, thus developing versatile pressure-responsive OLs.