A Screening Approach for the Discovery of Mechanochromic Gold(I) Isocyanide Complexes with Crystal-to-Crystal Phase Transitions
Mechanoinduced phase transitions of emissive organic crystalline materials have received much attention. Although a variety of such luminescent mechanochromic compounds have been reported, it is challenging to develop mechanochromic compounds with crystal-to-crystal phase transitions in which precise structural information about molecular arrangements can be obtained. Here, we report a screening approach to explore mechanochromic compounds exhibiting a crystal-to-crystal phase transition. We prepared 48 para-substituted (R1) phenyl[para-substituted (R2) phenyl isocyanide]gold(I) complexes designated R1–R2 (six R1 and eight R2 substituents) and then performed three-step screening experiments. The first screening step was selection of emissive complexes under UV light, which gave 37 emissive R1–R2 complexes. The second screening step involved evaluation of the mechanochromic properties by emission spectroscopy. Twenty-eight complexes were found to be mechanochromic. The third screening step involved preparation of single crystals, reprecipitated powders, and ground powders of the 28 mechanochromic R1–R2 complexes. The changes in the powder diffraction patterns of these complexes induced by mechanical stimulation were investigated. Two compounds exhibited a crystal-to-crystal phase transition upon mechanical stimulation, including the previously reported H–H complex. Single crystals of the as-prepared and ground forms of the newly discovered CF3–CN complex were obtained. Density functional theory calculations indicated that the mechanoinduced red-shifted emission of CF3–CN is caused by formation of aurophilic interactions. Comparison of the crystal structures of CF3–CN with those of the other complexes suggests that the weaker intermolecular interactions in the as-prepared form are an important structural factor for the observed mechanoinduced crystal-to-crystal phase transition.