Chemists develop stable acenes for organic light-emitting diodes in various colors

MIT chemists have devised an innovative method to enhance the stability of acenes, chains of fused carbon-containing rings renowned for their unique optoelectronic properties. These chains serve as valuable semiconductors and can be fine-tuned to emit diverse colors of light, making them promising candidates for organic light-emitting diodes (OLEDs).

The color emitted by an acene is intrinsically linked to its length. However, the challenge lies in the fact that as these molecules extend, their stability diminishes, limiting their widespread use in light-emitting applications. The MIT team, led by Robert Gilliard, the Novartis Associate Professor of Chemistry, has now overcome this hurdle, paving the way for the synthesis of stable acenes with varying lengths.

This groundbreaking approach enables the creation of molecules emitting light across the spectrum, from red and orange to yellow, green, and blue. The versatility achieved through this method enhances the potential deployment of acenes in a multitude of applications.

Addressing the reactivity challenges inherent in this class of molecules, Gilliard highlights the study’s dual focus on stability and the development of compounds with a customizable range of light emission. Lead author Chun-Lin Deng, a research scientist at MIT, spearheaded this research, and their findings are detailed in the journal Nature Chemistry.

Colorful molecules

Acenes, composed of benzene molecules intricately fused in a linear arrangement, serve as valuable semiconductors and field-effect transistors due to their electron-rich nature and efficient charge transport. Recent advancements have explored the enhanced electronic properties of acenes by replacing some carbon atoms with boron and nitrogen—a process known as “doping.” However, both traditional and doped acenes share a common challenge: instability when exposed to air or light.

Traditionally, synthesizing acenes requires a glovebox, a sealed container, to shield them from air exposure, preventing breakdown initiated by oxygen, water, or light. The susceptibility to unwanted reactions increases with the length of the acene molecules. To address this stability issue, Professor Robert Gilliard and his team at MIT employed a ligand called carbodicarbenes, previously utilized to stabilize borafluorenium ions in a study published last year.

In this study, the researchers developed a novel synthesis method, incorporating carbodicarbenes into boron and nitrogen-doped acenes. The addition of this ligand induced a positive charge in the acenes, significantly improving their stability while imparting distinctive electronic properties. This innovative approach allowed the creation of acenes capable of emitting various colors, determined by their length and the attached chemical groups to the carbodicarbene.

Crucially, this method expanded the color range beyond the typical blue emitted by most synthesized boron and nitrogen-doped acenes. Gilliard emphasizes the importance of red emission, particularly in biological applications like imaging, where blue light interference from human tissue poses challenges. The breakthrough in producing acenes emitting red light opens up possibilities for a broader range of applications.

Better stability

An additional crucial attribute of these acenes is their remarkable stability in both air and water. This is particularly noteworthy as boron-containing charged molecules with a low coordination number often exhibit high instability in water. The newfound stability of these acenes in water enhances their potential utility in medical applications, such as imaging.

Professor Robert Gilliard expresses enthusiasm about the versatility of the reported compounds, highlighting their ability to be suspended in water, opening up a plethora of possibilities. The research team aims to explore further improvements by incorporating different types of carbodicarbenes, anticipating the creation of acenes with enhanced stability and quantum efficiency—measuring the emitted light’s quantity.

The future trajectory involves collaboration with MIT professor Marc Baldo to integrate these novel acenes into single-fission-based solar cells, a promising avenue for improving efficiency by producing two electrons from one photon. Furthermore, the potential applications extend to light-emitting diodes (LEDs) for television and computer screens. Organic LEDs, being lighter and more flexible than traditional counterparts, offer advantages like brighter images and lower power consumption.

Despite being in the early stages of application development, Gilliard emphasizes the smooth device fabrication potential due to the stability of these compounds. Tiow-Gan Ong, deputy director of the Institute of Chemistry at the Academia Sinica in China, commends the research, recognizing its innovative approach in paving the way for highly stable light-emitting materials and miniature energy-harvesting devices, describing it as a promising advancement.

Source: Massachusetts Institute of Technology

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