Synthetization Process Yields Green Light-Emitting Idolizines

Chemists from the Peoples’ Friendship University of Russia (RUDN University) have introduced a new approach for synthesizing indolizines, a group of organic substances with biological and optical properties. The method, which uses pyridinium salts (electrically neutral molecules with both positively and negatively charged “poles”) and enaminones (a chemical compound), demonstrated the ability to emit green light, giving it potential applications in biomedicine.

Indolizines contain two carbon cycles and a single nitrogen atom. They are used to produce substances and materials including dyes and solar panels in addition to drugs for treating tumors and diabetes.

Though indolizines do not occur naturally and must be constructed in laboratory settings, their construction does rely on chemical reactions (specifically, cycloadditions, in which multiple molecules or different parts of the same molecule cause a cyclization reaction). The reactions involve pyridinium salts — the poles of which balance out one another.

In the new work, The RUDN University team determined a distinct reaction in which a pyridinium salt undergoes two other, consecutive processes (cyclodimerization and cyclocondensation) instead of cycloaddition. The reaction, once complete, produced indolizines with fluorescent properties.

Indolizines are a group of substances with biological and optical properties. A team of chemists from RUDN University developed a new approach to the synthesis of indolizines using pyridinium salts and enaminones. The new substances turned out to be able to emit light in the green range, which can be useful for medical applications. Courtesy of RUDN University.

The scientists found that pyridinium salts containing a methyl group and a certain, distinct binding tended to enter into an unexpected domino reaction with present enaminones, said Alexey Festa, a senior lecturer in the Department of Organic Chemistry at RUDN University.

In altering the ratio of components to an existing process for producing indolizine and creating optimal conditions for the process, a team of chemists from RUDN University, Moscow State University, and KU Leuven (Belgium) initially achieved a 50% reaction yield. When the scientists added pyridinium salts, the reaction yield increased to 82%. The team observed its two-stage reaction with x-ray structural analysis.

Of all those produced in the work, eight of the new indolizines demonstrated the capability of intensive fluorescence, which occurred when photons drove electrons to higher energy levels (excited the electrons). A release of energy accompanied fluorescence in these instances, in the form of reduced-energy (and increased-wavelength) photons.

The wavelength of light the indolizines emitted amounted to 505 to 528 nm, corresponding to the green range of the spectrum, and making indolizines a promising substance for the manufacture of fluorescent tags used to study biological objects, the scientists said. The indolizines also emitted light with a high quantum yield, Festa said, indicative of the process’s efficiency.

The team additionally reported that the indolizines it synthesized were effective in absorbing emissions in the range of 403 to 420 nm — in the blue/violent range bordering UV.

The research was published in Molecules (www.doi.org/10.3390/molecules25184059).