Towards tunable molecular switches of organic compounds

Newly synthesized organic molecules can be tuned to emit different colors depending on their molecular structure in the crystalline form.

Molecular switches are chemicals with a molecular structure that can be shifted between two or more stable configurations in response to changes in their environment. They are particularly interested in the development of molecular computers, molecular machines, and drug delivery systems. Compounds with conformational isomers—identical molecular formulas but different molecular structures—can make very effective molecular substitutions.

Researchers at Hokkaido University and Kyushu University have developed a technique to synthesize a potential molecular switch from anthraquinodimethanes (AQDs), a group of highly dense organic molecules. The study, led by Associate Professor Yusuke Ishigaki at Hokkaido University and Associate Professor Toshikazu Ono at Kyushu University, is published in the journal Chemical Frontier.

“AQD is a tightly packed type of ethylene, a molecule with carbon-carbon double bonds surrounded by large chemical groups,” explains Ono. “They have two common isomers, folded and twisted forms. They are of great interest as molecular switches, because sterically hindered double bonds can provide isomers that absorb and emit different wavelengths of light.

AQD generally adopts the most stable folded or bent form, making it difficult to isolate pure samples of other isomers to study their properties. The researchers overcome this obstacle by designing flexible AQD derivatives that can more easily and stably form different isomers.

The synthesized derivatives are capable not only of stably forming bent and folded isomers, but also of other isomeric forms, when recrystallized in different solvents. The researchers carried out a detailed analysis of the strains to fully understand their properties.

In the crystalline state, each of these isomers absorbs and emits a different frequency of light, which is due to the difference in the distribution of electrons in the isomer molecule. Interestingly, the absorption and emission of light change when the crystals are ground into an amorphous solid, and following treatment with an appropriate solvent can produce native crystals or other crystals with various colors.

“This work is the first report on the isolation of several isomeric forms of AQD,” concludes Ishigaki. “Different absorption and emission frequencies of light, and more importantly, the ability to modulate absorption and emission by external stimuli, make this compound an excellent candidate for molecular switch development.”