(Invited) Luminescent Mechanochromic Gold Complex Exhibiting Phase Transition Between Crystalline Phases
Authors: Seki Tomohiro 
Published: February 2020

Mechanochromic luminescence is referred to as solid-state emission color changes upon mechanical stimulation, i.e., grinding, pressing and ball-milling. For mechanochromic compounds, changes in molecular arrangement and noncovalent bonding interactions patterns take place upon mechanical stimulation, and these changes resulted in the switching of photoluminescence property of mechanochromic compounds. These materials are applicable to diplays, force-sensors, memory devices, and etc. One of the pioneering mechanochromic compounds was reported in 2005, and more than 500 reports have been known for this research field in the past ten years. However, the research on luminescent mechanochromic compounds still depend on serendipitous discoveries and derivatization of known mechanochromic compounds. This is caused by the absence of rational design strategies of the mechanochromic compounds. One reason for the absence of the rational design is resulted from the fact that most mechanochromic compounds so far have shown crystal-to-amorphous phase transition. These mechanochromic materials are unfavorable for the determination of the mechanism of emission color changes upon grinding. Few mechanochromic compounds have been reported to exhibit crystal-to-crystal phase transitions. In this case, a deeper insight into structure–property relationship can be obtained, which can lead to establish the rational design/functionalization strategies of mechanochromic compounds.
So far, our group have investigated luminescent mechanochromism of a series of aryl gold isocyanide complexes and reported various unique mechanochromic properties. Although we reported various gold complexes exhibiting prominent mechanochromism, it is still challenging for us to construct the mechanochromic compounds with desired functionality.
Here, we will report a novel “screening approach” to establish mechanochromic compounds exhibiting a crystal-to-crystal phase transition. In this approach we first prepared 48 para-substituted (R1) phenyl[para-substituted (R2) phenyl isocyanide]gold(I) complexes R1

-
R2
(six kinds of R1 and eight R2 substituents). For these 48 complexes, we then performed three-step screening experiments. The first screening experiment was focus on selection of emissive R1

-
R2
complexes under UV light. This experiment gave 37 emissive R1

-
R2
complexes. For these 37 R1

-
R2
complexes, the second screening experiment was performed where the luminescent mechanochromic behaviors were evaluated through emission spectroscopy. Then, we identified 28 mechanochromic complexes. As the third screening experiment, we evaluated mechano-induced changes in the powder diffraction patterns of 28 complexes. Then, we newly found CF3

-
CN exhibited a crystal-to-crystal phase transition upon mechanical stimulation. We also successfully prepared two distinct single crystals of CF3

-
CN corresponding to mechanically unprocessed and processed phases. Theoretical calculations indicated that the red-shifted emission of CF3

-
CN in response to mechanical stimulation is caused by the formation of aurophilic interactions.

Figure 1