Luminophores with a New Recipe

The study involved specialists from Togliatti State University, the A.N. Nesmeyanov Institute of Organoelement Compounds of the Russian Academy of Sciences (Moscow), and the Ufa Institute of Chemistry of the Russian Academy of Sciences. The problem the team of scientists solved was the difficulty of controlled production of specific organic molecules – pyrazoles. These compounds are in high demand in the pharmaceutical, materials science, and chemical industries, as they are used to create drugs, luminophores, and other functional materials. However, the synthesis of pyrazoles often results in a mixture of "twins" – regioisomers, molecules similar in structure but different in properties. Separating such mixtures is extremely difficult.

Chemists have successfully found conditions under which the reaction proceeds strictly in a single, desired direction, allowing them to obtain the target "glowing" compounds – 5-styrylpyrazoles – with extremely high selectivity (up to 97%) and quantitative yields of up to 98%. This phenomenally high result means the reaction is nearly complete, with minimal losses due to side processes, evaporation, purification, etc.

The developed approach is based on the preliminary modification of the starting materials with amines – organic compounds often used in chemistry as catalysts or bases. The synthesis of 5-styrylpyrazoles requires two ingredients: conjugated enynones (highly reactive organic compounds) and hydrazine (a nitrogen-hydrogen compound).

When scientists combined them according to old recipes, the reaction proceeded blindly. The result was a useless mixture of two "twins" – the desired 5-styrylpyrazole and the unwanted 3-styrylpyrazole. The addition of piperidine as an auxiliary element completely changed both the process and the result.

"Enynones are complex and unpredictable. They have two chemically active centers, like 'velcro,' and it's unclear which one will bind first to the second ingredient. This was the main problem. The amine sort of 'closes' one of the 'velcro,' making the enynone more controllable and predictable. And when hydrazine attaches to it, the reaction proceeds along the desired pathway," notes one of the study's authors, Alexander Golovanov, Doctor of Chemical Sciences, Professor, and Head of the S.P. Korshunov Research Laboratory No. 13 "Organic Synthesis and Analysis" at the TSU Research Institute of Advanced Technologies. "After completing its task, the amine splits off, like a temporary scaffold, and almost 100% pure 5-styrylpyrazole is formed." The chemists successfully optimized the entire synthesis process to two steps, carried out in a single flask without isolating intermediate products. This method, known as "one-pot synthesis," not only simplifies the procedure but also makes it cost-effective."

Throughout the experiments, the researchers obtained a whole family of 26 different luminescent molecules in very high yields (up to 97-98%). All compounds exhibit intense fluorescence in the blue region of the spectrum. The maximum absolute quantum luminescence – a measure of luminescence efficiency – reached a record-breaking 0.7 (70%), comparable to the best commercial luminophores.

"Our method solves a long-standing problem for organic chemists: controlling regioselectivity in the synthesis of pyrazoles," commented Professor Alexander Golovanov. "We not only offer an effective tool for the laboratory production of such compounds but also open the possibility of their scaling and practical application. The highly luminescent properties of the resulting substances make them promising for the creation of new OLED materials, chemical sensors, and luminescent labels*."

The study was supported by the Ministry of Science and Higher Education of the Russian Federation. The scientists results were published in the prestigious international scientific journal The Journal of Organic Chemistry.

*Luminescent labels are luminous molecules or particles used to detect and visualize minute quantities of substances. They are indispensable in medicine for disease diagnosis, in biology for the study of living cells, and in forensics and industry for counterfeit protection and quality control.