Certain advanced technologies, such as 3D displays, biosensing, and security printing, can utilize circularly polarized luminescence (CPL), which is produced when specific types of molecules are irradiated with UV light. The electric field of the CPL rotates in a spiral shape. Mechanical changes to these molecules, such as grinding, can induce a transition that creates a reversible emission color change. This is called mechanochromic luminescence (MCL). To improve CPL efficiency after grinding, researchers tested two different readily available compounds to understand how the CPL properties changed upon grinding.
The results were published in Angewandte Chemie International Edition on January 22.
“Circularly polarized luminescence (CPL) exhibited by chiral molecules holds great promise for applications in areas such as three-dimensional displays and security printing. However, research on mechanical-stimuli-induced CPL switching in solid-state materials has been limited. In particular, a significant challenge lies in the substantial reduction of CPL efficiency when crystals are amorphized by mechanical stimuli,” said Suguru Ito, an associate professor of engineering at Yokohama National University in Yokohama, Japan.
Chiral small organic molecules have been found to be very beneficial for producing CPL. Chiral means that the molecule cannot be superimposed on its mirror image. Chirality is important for CPL because it is a light that rotates in a circular fashion. The chirality of the molecule can help direct the direction of the light’s rotation. Understanding the other molecular properties that can determine CPL behavior could help researchers create better design strategies for chiral organic molecules. An important step is developing mechanochromic CPL. When mechanochromic CPL has been studied in the past, the grinding required to induce the color change also induced structural changes to the crystals that weakened the CPL.
The researchers used two molecular compounds called chiral pyrenylprolinamides 1 and 2, which exhibit different luminescence colors in the crystal state. They are both designed to include an amino acid that gives the molecule its chiral shape, a pyrene group for both monomer and excimer emission which controls the amount of energy produced, an amide group for hydrogen bonding, and a substituent R which controls the arrangement in the crystal state. The first pyrenylprolinamide is a tert-butoxycarbonyl (Boc) derivative. The second pyrenylprolinamide is a 2,2,2-trichloroethoxycarbonyl (Troc) derivative.
The results helped identify the best way to design the molecules to produce mechanochromic CPL depending on the desired properties. “We have demonstrated for the first time that the excimer chirality rule can be applied to acquire structural information about excimers formed in the amorphous state. Most remarkably, in contrast to previous reports where CPL nearly vanished after grinding, this study shows that the stacked pyrenes by intermolecular hydrogen bonds promotes excimer emission even in the amorphous states. These insights provide new design guidelines for mechanochromic CPL molecules, advancing the development of practical solid-state CPL materials,” said Ito.
Looking ahead, researchers hope to set guidelines. “The next step is to establish general design guidelines for molecules that enable solid-state CPL switching through mechanical stimuli. Our ultimate goal is widespread implementation of materials with switchable solid-state CPL for applications such as three-dimensional displays and security printing,” said Ito.
