When Naoya Oshima gently pressed the powder of a newly developed gold(I) compound between two microscope slides in order to measure its emission spectrum, he noticed a change in its luminescence properties. The subsequent investigation of this effect by the team from Hokkaido University1 showed subtle changes in molecular arrangement to be the origin of this mechanochromism.

Fig. 1: Photograph showing the material under UV light illumination. The part of the powder that was crushed shows yellow luminescence, contrasting with the blue luminescence from the original compound.

There are not many reports on luminescence mechanochromism—the luminescent color change of a compound as a result of mechanical grinding. Under visible light the gold(I) compound [(C6F5Au)2(μ-1,4-Diisocyanobenzene)] appears no different, and has a grey-yellowish appearance. However, under UV irradiation the compound shows blue luminescence that shifts to yellow when the material is gently crushed (Fig. 1). The change in color is fully reversible through the addition of a solvent. Once the solvent evaporates, the color change can be reintroduced through mechanical grinding.

Mechanochromism in another gold(I) compound has been previously observed, but changes in color occurred through a chemical reaction, while in the present experiments the origin is a more subtle interaction within the molecules of the solid compound. “The color change in our gold(I) compound is induced by changes in molecular arrangement, where weak bonds between gold atoms form,” says Masaya Sawamura, leader of the project.

Upon gentle grinding, the so-called aurophilic interaction between the gold atoms leads to a change in the intermolecular attraction that alters their bonding structure. This in turn causes shifts in the molecule’s electronic states and subsequent changes in their emission characteristics. The aurophilic interaction is broken again through the addition of a solvent. As the interaction is very weak, this process leads to no further modifications of the molecules and therefore is reversible.

These findings have fundamental implications. “The observation that mechanical stimuli can switch the intermolecular interactions is a powerful concept that could be used to design new materials with controllable emission properties,” says Sawamura. For example, mechanochromic materials with different colors could be designed, or even compounds where the color change is the result of a different external stimulus such as heating. Indeed, such materials appear promising for a variety of recording and sensing applications.