Redox switching dyes offer tunable fluorescence for advanced bioimaging and optical applications

The fluorescent molecules that absorb and emit light in the nearby infrared regions (NIR) and short wave (Swir) have a significant potential for various applications. These molecules can serve as markers to obtain images of biological tissues in the depths of the body, improve the efficiency of solar cells capturing more sunlight or incorporated into laser protection glasses to block the harmful radiation to reach the eyes. However, the design of organic compounds with a strong Nir broadcast is a challenge.

The energy absorbed by these molecules is often lost as heat due to vibrations in ch. To mitigate such losses and improve fluorescence, the heaviest atoms such as deuterium or fluoride should be introduced into the molecular structure, which adds complexity to the synthesis process.

The researchers at the Shibaura Institute of Technology have developed fluorescent coloring based on nitrogen rich that exhibit strong and tunable fluorescence between visible regions and Nir/Swir. The study, published online in the magazine ANGEWANDTE CHEMIE INTERNATIONAL EDITION On March 3, 2025, it was made by a multidisciplinary team led by Gary J. Richards of the Shibaura Institute of Technology, as well as Kazushi Nakada, Toshiki Tajima and Akiko Hori of the same Institute, and Jonathan P. Hill of the National Institute of Materials Sciences.

“Infrared fluorescent molecules have great potential in areas such as bioimaging and detection, but stronger emission at longer wavelengths is difficult to achieve with organic compounds,” explains Dr. Richards. “Our design allows compact molecular structures to exhibit dramatic fluorescence displaced in red and switching through a simple redox process, which can be achieved chemistry or electrochemically.”

Pyrazinacenos with four or five rings can change reversibly between two redox states: a completely oxidized form made of pyrazine rings, and a reduced shape that includes a dihydropirazine ring. However, both states emit only within the visible range, which limits its usefulness for infrared applications.

The largest pyrazinacens, with six or seven fused rings, can emit light closer to the NIR range. However, these molecules tend to remain in a stable reduced state and do not oxidize easily, which hinders reversible redox change.

To address these limitations, the researchers used a design strategy known as intramolecular load transfer. This approach links an electron donor group with an electron acceptance group through a conjugated molecular bridge π, changing the load distribution inside the molecule. This internal load transfer reduces the energy required for the absorption and emission of light, changing fluorescence towards longer wavelengths.

Using this strategy, the team added trifenylamine groups electron donors to smaller pyrazinaceans, creating two new active redox compounds: octaazatetracene (compound 1) and decaysaazapantcene (compound 2). In their reduced forms, compound 1 and compound 2 exhibited bright yellow and red fluorescence, respectively, with strong emissions visible at 560 Nm and 599 Nm, and quantum yields of 58% and 43%.

When oxidized, their emission spectra change to the Nir and Swir regions, with emission wavelengths at 847 Nm and 1,012 Nm, and quantum yields of 16.4% and 1.4%, respectively. In particular, these values ​​are well compared with those of existing Nir and Swir fluorescent dyes, highlighting their potential for image applications.

These fluorescent molecules that respond redox could be particularly useful to detect changes in redox conditions within cells, which is crucial to diagnose diseases such as cancer, where tumors often exhibit different redox environments compared to healthy tissue. When combining pyrazineos rich in nitrogen with an innovative molecular design, researchers have established a versatile platform to create fluorescent molecules that respond to redox changes.

“These compounds are the first examples of this strategy,” says Dr. Richards. “With more modifications of donor groups, we hope to develop more efficient infrared dyes for a wide variety of applications.”