“Phospholipids can be modified by certain chemical reactions, which have a significant impact on the color and fluorescence efficiency of the molecule. Another special feature of these substances is their fan-like structure,” explains Koenig. When these molecules are dissolved in a solvent and exposed to ultraviolet light, they do not fluoresce. The absorbed energy is released in the form of rotational motion, causing the molecules to spin like a propeller in the solvent. However, in the crystalline state, the ability to rotate is very limited, which causes the material to fluoresce strongly under ultraviolet light. This behavior is known as aggregation-induced emission (AIE).
In the recently published paper, Niels Koenig and colleagues show a novel AIE-based phospholipid reaction, which opens access to a new class of materials. Phospholipids can be modified under mild conditions by isocyanates, a reactive class of substances composed of the elements nitrogen, oxygen and carbon, which are inexpensive and widely available due to their industrial applications in polymers and biochemistry. This reaction, which seems to contradict classical organic chemistry, is characterized by high yield and excellent atomic economy.
The optical properties of the new materials were investigated in collaboration with the Institute for Surface Engineering (IOM) in Leipzig, as well as the Center for Nanotechnology (CeNTech) and the University of Münster (WWU). It turned out that the simple modification significantly increased the fluorescence efficiency compared to the original materials. This is due to the formation of a unique interaction between parts of the molecular framework, which greatly strengthens the molecule in the solid state and leads to stronger fluorescence. Thus, the new modification method makes a significant contribution to understanding the concept of AIE and can serve as a tool for synthesizing effective new pigments for screens or as markers for biomolecules.