For the inaugural instance, investigators have illustrated that mechanoluminescence can take place devoid of a crystalline phase or polymer matrix by employing amorphous solids made up of copper complexes. Mechanoluminescence, the light emission instigated by mechanical stress such as grinding or friction, has conventionally been linked to rigid crystals. Nevertheless, the continual fracturing of these crystals reduces their size and lessens the luminance effect, thereby limiting their potential for application.
Historically, mechanoluminescence had been witnessed in crystalline substances when mixed with a polymer matrix. In 2020, a research group spearheaded by Julia Khusnutdinova and Ayumu Karimata at the Okinawa Institute of Science and Technology (OIST) showcased that a cluster of copper complexes displayed mechanoluminescence in both their crystalline states and when integrated into amorphous polymer films.
Building on these prior revelations, Khusnutdinova, Karimata, and their teammates have now identified mechanoluminescence in pure amorphous photoluminescent solids. This accomplishment was realized through the formulation of a set of heteroleptic copper(I) complexes, wherein copper atoms connect to two different ligands. In these materials, mechanoluminescence was activated not only by friction but also by non-destructive techniques such as the contact-separation method (for instance, peeling a polymer film off a surface) and by bending or twisting.
Notably, the mechanoluminescent response was persistently recorded across the entire collection of complexes, indicating that factors beyond those influencing crystalline materials might be significant. The researchers suggest that electron transfer induced by friction and contact electrification could account for the excitation of the metal complexes in these amorphous forms.
This discovery underscores that mechanoluminescence can manifest without the requirement for crystal fractures or a polymer matrix. It presents a simple method for producing mechanoluminescence under mild conditions, opening avenues for the creation of new mechanoluminescent systems without the need for complex crystal designs.