A study published in Angewandte Chemie International Edition on January 22nd, 2025, by Suguru Ito, an Associate Professor of Engineering at Yokohama National University, along with collaborators from various institutions, explores the potential of circularly polarized luminescence (CPL) in advanced technologies such as 3D displays and security printing.
The monomer-emissive crystal of 1 and the excimer-emissive crystal of 2 exhibits (+)-CPL and transform by grinding into excimer-emissive amorphous states that retain (+)-CPL. Image Credit: Yokohama National University
The study investigates how mechanical stimuli, specifically grinding, affect the CPL properties of chiral small organic molecules, leading to mechanochromic luminescence (MCL).
Certain advanced technologies, including 3D displays, biosensing, and security printing, can leverage circularly polarized luminescence (CPL), which arises when specific molecules are exposed to UV light. The CPL's electric field exhibits a spiral rotation.
Mechanical alterations to these molecules, such as grinding, can trigger a transition that results in a reversible change in emission color, known as mechanochromic luminescence (MCL). To enhance CPL efficiency post-grinding, researchers examined two readily available compounds to analyze the changes in CPL properties following mechanical treatment.
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.
Suguru Ito, Associate Professor, Yokohama National University
Chiral small organic molecules are advantageous for generating CPL. Chirality refers to the property of a molecule that prevents it from being superimposed on its mirror image. This characteristic is crucial for CPL, as it involves light that rotates in a circular manner. The chirality of a molecule can influence the direction of light rotation.
Gaining insights into other molecular properties that affect CPL behavior could assist researchers in developing improved design strategies for chiral organic molecules. A key focus is the advancement of mechanochromic CPL. Previous studies on mechanochromic CPL indicated that the grinding necessary to induce color changes also led to structural alterations in the crystals, diminishing CPL.
The researchers utilized two molecular compounds known as chiral pyrenylprolinamides 1 and 2, which display distinct luminescence colors in their crystalline forms. Both compounds are designed to incorporate an amino acid that imparts chirality, a pyrene group for monomer and excimer emissions that regulates energy output, an amide group for hydrogen bonding, and a substituent R that influences the crystal arrangement. The first pyrenylprolinamide is a tert-butoxycarbonyl (Boc) derivative, while the second is a 2,2,2-trichloroethoxycarbonyl (Troc) derivative.
The findings provided insights into the optimal molecular design for achieving mechanochromic CPL based on desired characteristics.
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 promote 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.
Suguru Ito, Associate Professor, Yokohama National University
Looking forward, the researchers aim to establish design 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,” Ito further noted.
Shin Wakiyama and Hao Chen from Yokohama National University, Masato Abekura and Hidehiro Uekusa from Tokyo Institute of Technology, and Ryoya Ikemura and Yoshitane Imai from Kindai University are additional contributors to this research.
JST PRESTO, JSPS KAKENHI, and JST CREST supported this research.
Journal Reference:
Ito, S. et al. (2025) Contrasting Mechanochromic Luminescence of Enantiopure and Racemic Pyrenylprolinamides: Elucidating Solid-State Excimer Orientation by Circularly Polarized Luminescence. Angewandte Chemie International Edition. doi.org/10.1002/anie.202422913