Organic crystal materials that exhibit circularly polarized luminescence developed by YNU, Kindai University, and Science Tokyo ― A path for the realization of practical solid-state materials emitting circularly polarized light

A joint research group including Associate Professor Suguru Ito of Yokohama National University (YNU), Professor Yoshitane Imai of Kindai University, and Professor Hidehiro Uekusa of the Institute of Science Tokyo developed a chiral organic crystal material that exhibits circularly polarized luminescence (CPL) with a long wavelength when rubbed. In the past, the efficiency of CPL was greatly reduced after rubbing, which caused the crystals to collapse and turn into an amorphous state. Through original molecular design, the problem of efficiency reduction was solved and the structure of the excited dimer (excimer) that exhibits CPL in an amorphous state was successfully clarified. These results are expected to lead to the development of solid-state CPL materials for use in 3D displays, security printing, and other applications. The results were published in the online edition of Angewandte Chemie International Edition.

Among organic molecules, there exist chiral molecules that cannot be superimposed on each other. Chiral molecules that absorb light such as ultraviolet light and emit visible light exhibit CPL, emitting right- and left-handed circularly polarized light. CPL is expected to be utilized in light sources for displays that show 3D images and inks for security printing to prevent counterfeiting.

There has been active research on organic molecular crystals that exhibit mechanochromic luminescence (MCL), in which the color of the emitted light changes under mechanical stimuli such as rubbing. However, until recently, research into switching the CPL of solid-state materials by mechanical stimulation was slow, partly due to the difficulty of measuring CPL in the solid state. A particular problem is that the efficiency of CPL is greatly reduced when crystals collapse under mechanical stimulation, resulting in an amorphous state in which the periodic arrangement of molecules is lost.

The research group designed and synthesized chiral molecules 1 and 2, which consisted of proline--a type of amino acid--linked to a pyrene ring, a luminescent polycyclic aromatic hydrocarbon. They found that crystals consisting of molecules of only one form (enantiopure crystals) and crystals containing molecules of two forms (racemic crystals) exhibited contrasting MCL. When rubbed with a laboratory spatula, the emission wavelength of the enantiopure crystal of molecule 1 changed more significantly than that of the racemic crystal, whereas the opposite behavior was observed for molecule 2. Compared to the luminescence from monomers in which the pyrene ring is isolated in the crystal (monomer luminescence), the luminescence from stacked dimers has a longer wavelength (excimer luminescence). Further, in the amorphous state, excimer luminescence occurs at even longer wavelengths, which is found to be the reason for the different wavelength change in MCL. Furthermore, the enantiopure crystals of molecule 1 and molecule 2 showed left-handed (S-body) CPL with left-handed rotation (positive sign), and positive CPL was also observed from the amorphous state after rubbing.

By applying the excimer chirality rule, which has been proposed as a theory to explain the CPL sign in solutions, the researchers were able to explain this phenomenon as pyrene rings overlapping each other in a clockwise twisted position, forming excimers in the amorphous solid. This explanation was also supported by quantum chemical calculations. The pyrene rings of molecules 1 and 2 overlapped due to the formation of intermolecular hydrogen bonds at the amide sites. Hence, unlike that in conventional chiral organic molecules, CPL was observed well even in the amorphous state.

These results provide new design guidelines for the research and development of solid-state CPL materials for applications such as 3D displays and security printing. Solid-state materials whose CPL can be switched in response to mechanical stimuli are expected to find applications in areas such as switching 3D images and rewriting cryptographic information. Research on solid-state CPL is still in its infancy. It is expected that the knowledge gained in this study can be utilized to develop practical solid-state materials whose CPL can be switched in response to various external stimuli, not limited to mechanical stimuli.

Journal Information
Publication: Angewandte Chemie International Edition
Title: Contrasting Mechanochromic Luminescence of Enantiopure and Racemic Pyrenylprolinamides: Elucidating Solid-State Excimer Orientation by Circularly Polarized Luminescence
DOI: 10.1002/anie.202422913