Tetrakis(trialkylsilylethynylphenyl)ethenes: mechanofluorochromism arising from steric considerations with an unusual crystal structure

Abstract

Mechanofluorochromism (MFC) arising from extended partial planarization of a conjugated molecular framework through the transfer of mechanical stress by terminal bulky groups is demonstrated. Thus the tetrakis(triisopropylsilyl-4-ethynylphenyl)ethene [TPE-(TIPS)₄] luminophore, synthesized for the first time, is shown to exhibit MFC when the threshold pressure exceeds about 1.3 MPa. An analogous luminophore, tetra(trimethylsilyl-4-ethynylphenyl)ethene [TPE-(TMS)₄], with smaller bulky groups at the periphery of TPE, synthesized for comparative studies, was also observed to exhibit MFC. MFC is observed under visible and UV light excitation. The color change is reversed upon annealing or under solvent fumigation conditions. The luminophores show high quantum yields in the solid state ranging from 60 to 74%. Very interestingly, TPE-(TMS)₄ exhibited quite an unusual crystal structure, where no intermolecular interactions attributed to mechanofluorochromism are observed. The solid state optical absorption and CP-MAS ¹³C NMR measurements strongly suggested that the mechanism of MFC could have originated from partial planarization of the molecule upon grinding/shearing, facilitated by the bulky peripheral/terminal groups. These high solid state emitting luminophores with MFC properties could find applications in the areas of solid state lighting and sensors. The synthetic design, in principle, should enable MFC as a function of pressure (through variation in steric handle) and temperature (through electronic and conformational handles). The novel and rational synthetic design, in principle, should enable MFC in a family of organic molecules as a function of pressure and temperature.