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 (R(1)) phenyl[para-substituted (R(2)) phenyl isocyanide]gold(I) complexes designated R(1)-R(2) (six R(1) and eight R(2) substituents) and then performed three-step screening experiments. The first screening step was selection of emissive complexes under UV light, which gave 37 emissive R(1)-R(2) 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 R(1)-R(2) 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.