Overview: A new class of red-emitting fluorescent dyes
Biomedical imaging often relies on fluorescent dyes to illuminate structures inside tissues. Traditional blue and green dyes face two major challenges: light scattering in tissue and interference from endogenous signals, which reduces image clarity at depth. Red and near-infrared (NIR) emissions offer a solution because they penetrate tissue more effectively and provide higher-contrast images of deep structures such as tumors. Yet red dyes have historically suffered from poor brightness and instability, with quantum yields frequently hovering around the single-digit percentages. A team of MIT chemists has now introduced a novel strategy that tackles these limitations by leveraging a positively charged boron center, a borenium ion, stabilized by a tailored ligand. The result is a family of red-to-NIR emitters that are both brighter and more robust than many previous boron-based systems.
From instability to stability: stabilizing borenium ions
In the new approach, researchers attach the reactive borenium cation to a surrounding ligand. This stabilization enables the molecules to exist in solid films, powders, and crystals while maintaining strong red-to-NIR emission. The stabilization is crucial because boron-based emitters have long suffered from intrinsic instability and non-radiative losses governed by the energy gap law. By carefully tuning the ligand environment and exploiting exciton coupling with counter anions, the team managed to shift absorption and emission toward the infrared while preserving high brightness.
Exciton coupling drives the red shift
The researchers explored how the negatively charged counterions interact with the borenium cation. These interactions give rise to exciton coupling, a phenomenon that can adjust the energy landscape of the molecule. The effect is a concerted shift of both absorption and emission toward longer wavelengths. Importantly, this red-shift comes with a notable boost in quantum yield, making the dyes—now in the red to near-infrared range—more efficient emitters than many conventional red dyes.
Performance highlights: brightness, efficiency, and stability
Reported quantum yields for these red/NIR emitters are in the thirties percent range, a remarkable improvement for boron-centered dyes in this spectral window. This level of efficiency is especially meaningful for biomedical imaging, where signal strength translates directly into clearer images from deeper tissues. In addition to solution-based forms, the researchers demonstrated that the dyes can be integrated into solids, films, and colloidal systems, broadening potential applications beyond simple injections.
Potential biomedical applications
The immediate vision is to encapsulate these borenium-containing materials within biodegradable polymers, enabling injection as fluorescence imaging dyes. Initial collaborations with MIT chemistry colleagues and the Broad Institute aim to test these materials in cellular contexts, gauging their ability to reveal intricate structures inside living cells with minimal background noise. Beyond cellular imaging, temperature-responsive behavior suggests several other compelling uses. The materials could function as molecular thermometers, signaling whether cargo has been exposed to unsuitable temperatures during storage or transport—an essential feature for vaccines and biologics.
Beyond imaging: smart materials and devices
Researchers propose additional avenues, including polymer-based thin films that could be used in organic light-emitting diodes (OLEDs) or flexible electronic displays. The combination of high near-IR quantum yield and environmental stability makes this dye class attractive for smart materials, sensors, anticounterfeiting technologies, and other optoelectronic applications.
What comes next
Looking forward, the MIT team plans to push the color emission further toward longer near-IR wavelengths by incorporating extra boron atoms. While this strategy holds promise, it introduces stability challenges that the researchers are addressing with novel carbodicarbene frameworks to preserve robustness. The overarching goal remains clear: create bright, stable red-to-NIR fluorophores that can be safely used in vivo for high-contrast imaging of tumors and other deep-tissue features, enabling better diagnostics and more precise treatment planning.
Experts outside the study recognize the significance of achieving both brightness and stability in the red-NIR region. As one chemist notes, the combination of high quantum yields, tunable emission, and environmental resilience could make these dyes a linchpin for future biomedical imaging and related technologies.
