Nanotechnologies / Materials
Smart devices, Security inks, Sensors

Hysteretic control of near-infrared transparency using stableradical cation
Department of Chemistry, Graduate School of Engineering Science

Shuichi Suzuki
Professor Takeshi Naota
Associate Professor

Abstract
Molecules that absorb near-infrared (near-IR) light (over 700 nm)
have attracted as fundamental components of various practicalapplications, such as information-storage materials, optical filters,
and security inks. In this study, we have succeeded in developing
aggregates of radical cations that exhibit drastic changes in nearIR transparency by temperature control.

Nanotechnologies / Materials▍ Background & Results

https://researchmap.jp/read0210228
https://researchmap.jp/read0013723▍ Significance of the research and Future perspective

By providing near-IR optical properties with “forward” and “backward” directional responses to stimuli, it is possible to achieve
highly-complicated information processes for invisible opticalsensors and security. The radical cation can be “painted” on various media and used in on-demand sensors, because they can bemelted. Furthermore, radical molecules frequently show interesting
magnetic properties and electrical conductivity by external stimuli,enabling them to be applied in many fields in materials chemistry.

Radicals are chemical species that are used as reagents fororganic synthetic reactions such as polymerization and intramolecular cyclization reactions. Their high reactivity generally promotes
the disappearance of their unique functionality in the aggregatedstate. Fortunately, recent developments in related chemistry enabled us to design radical pairs exhibiting interesting electronicand magnetic properties derived from their unusual electronicstructures. It is possible to modulate the radical function when
the radicals consisting of the radical pair can be displaced evenin condensed states. However, it is generally difficult to change
the structure in the condensed state where molecular motions are
remarkably restricted. We have succeeded in overcoming this difficulty by utilizing liquescent radical cations, and have found newproperties of the radical molecules.
Radical molecules frequently exhibit long-wavelength absorption
properties because of existence of the singly-occupied molecularorbitals. Furthermore, the radical pair species are known to show
longer wavelength absorption until near-IR region due to the existence of relatively strong inter-radical interactions. We consideredthat molecule-based materials with controllable properties of nearIR absorption/transmission could be created when the changes
in the structures of the radical pair could be induced by external
stimuli/environment. Consequently, a bis(trifluoromethanesulfonyl)
imide salt of dihydrophenazine radical cation was found to show
hysteretic phase transitions over a wide-temperature range withdrastic changes in the near-IR transparency under neat and ambient conditions without changes in visible color. During the heatingand slow cooling process, its electronic and magnetic properties
were altered clearly and repeatedly changed between solid andliquid states. The liquid state was transparent to near-IR light, but
the solid state was opaque, despite both samples exhibiting a similar green color under room light. This feature allowed us to control
visibility and legibility using visible and near-IR light under specifictemperature conditions. UV/vis/near-IR and electron spin resonance spectroscopies revealed that these drastic changes wereattributable to the dynamic dissociation and association of theπ-dimer structure of the radical cation accompanying with the solid-liquid phase transitions even under the condensed conditions.

P a t e n tSuzuki, Shuichi; Naota, Takeshi et al. Hysteretic Control of Near-infrared Transparency Using a Liquescent Radical Cation. Angew. Chem. Int. Ed. 2021, 60(15), p. 8284-8288, doi: 10.1002/anie.202016930
http://www.chem.es.osaka-u.ac.jp/soc/URL
https://youtube.com/playlist?list=PLAb-4nGLCCqs1uJkBWLSOcyudMOia7IcV
K e y w o r d open-shell molecule, stimuli responsiveness, liquescent molecule, near-IR property
Tr e a t i s e