New organic liquid provides efficient phosphorescence

The nostalgic “glow-in-the-dark” stars that twinkle on the ceilings of childhood bedrooms operate on a phenomenon called phosphorescence. Here, a material absorbs energy and later releases it in the form of light. However, recent demand for softer, phosphorescent materials has presented researchers with a unique challenge, as producing organic liquids with efficient phosphorescence at room temperature is considered difficult.

Now, researchers at the University of Osaka have attempted to tackle this problem by producing an organic liquid that phosphoresces in the ambient environment. This discovery is published in Chemical Science.

Traditional materials that can phosphoresce at room temperature contain heavy metal atoms. These phosphors are used to create the colored electronic displays we utilize every day, such as those in our smartphones. Organic materials, which contain carbon and hydrogen atoms (similar to materials found in nature), are more environmentally friendly.

However, organic molecules typically release the energy absorbed 1,000 times slower than metal molecules and need a rigid environment—for example, being arranged like a crystalline solid—to phosphoresce at room temperature. Crystalline materials are fragile and difficult to process.

“Organic liquids are ‘soft’ and can be easily deformed and processed,” explains lead author, Yosuke Tani. “However, creating organic liquids that phosphoresce at room temperature is difficult because liquids are flexible.”

One additional problem is that molecules in a liquid are so close together that the chromophores, which absorb the energy, can form aggregates and transfer the energy to other molecules, instead of releasing the energy as light. Overall, these issues can result in poor phosphorescence efficiency.

To overcome these challenges, the team designed an organic molecular skeleton with a phosphorescent backbone, referred to as 3-bromo-2-thienyl diketone, to which a special group of molecules was attached—the dimethyloctylsilyl group—or DMOS. Attaching a single DMOS group proved beneficial, as this resulted in a liquid that was stable at room temperature. Even more interestingly, attaching two DMOS groups disrupted molecular aggregation and prevented weakening of the phosphorescence.

The designed molecule can produce phosphorescence rapidly, owing to its design, created by the team with efficiency in mind. The quantum yield, the measure of efficiency in photochemical reactions, is the highest known for an organic liquid, registering at about three times the efficiency of other organic liquids.

“The color of the light emitted by solids and liquids is typically quite muted, whereas our material is a vivid yellow,” reports Takuji Ogawa, senior author. “This characteristic in our designed molecule is a testament to its efficiency.”

It is hoped that these improvements in phosphorescence will benefit any application of an organic liquid. It is noted that having organic materials that are both phosphorescent and flexible will lead to new developments in electronic displays, particularly for those that can be bent or stretched to ensure functionality for wearable electronic devices.