Biphenyl or naphthalene-modified fluorene fluorogenic molecules with different solid-state emission and mechanofluorochromic behaviors

Abstract

The preparation of three-color mechanofluorochromic molecules is of crucial importance in the development of mechanically responsive smart materials with multicolor switching feature. Unfortunately, the related mechano-responsive luminogenic compounds are rare. In this work, four fluorene derivatives, which contain various sites of biphenyl or naphthalene, were synthesized. Interestingly, compounds 1–4 exhibited diverse high-brightness solid-state fluorescence and observable fluorescent changes in response to mechanical force stimulus. For fluorophor 1 in the solid state, upon light grinding, an orange-yellow to yellow-green mechanofluorochromic transition with the notable blue shift of 85 nm was noticed, and the yellow-green to yellow bathochromic mechanofluorochromic response was observed after vigorous grinding. Therefore, fluorogen 1 showcased an unfrequent bidirectional tricolored mechanofluorochromism with a blue shift followed by a red shift. As for luminogens 2–4 exhibited diverse two-color reversible red-shifted mechanofluorochromic responses. Furthermore, their mechanochromic emissive mechanisms of 1–4 were investigated by X-ray powder diffraction tests.