Ever since my very first day of graduate school almost fifteen years ago, I’ve been fascinated with fluorescent and phosphorescent organic materials. The difference between the two isn’t readily appreciated by most people, but an explanation goes to the heart of why these materials are interesting to scientists. Fluorescence is a behavior that entails absorption of light (the excitation step) followed by immediate release of light (the emission step). There’s only a very short time delay between the two steps. Phosphorescence also has an excitation step and an emission step, but halfway through there’s a pause. The excited molecule crosses over into an intermediate step and stays there for a split second before tumbling back down to it’s normal electronic phase.
This short delay is critical for the use of organic materials in devices that emit light, or LEDs. LEDs are being used in all manner of thin displays and computer screens, ranging from smart phones to TVs. In the solid state (a powder, or film, which is how LEDs are manufactured) normal organic materials lose a lot of their excitation energy to heat, instead of reemitting that energy as light. It was a problem I ran into over and over during my work in graduate school. The percentage of light energy lost to nonproductive ends can be given as the “quantum yield”. A quantum yield of 100% means that every photon absorbed is reemitted as light; a quantum yield of 0% means that no light is reemitted, and it’s all lost as heat. Most organic films only have a very low QY of fluorescence, meaning that an LED produced from organic materials is not going to be efficient.
The route I used in my graduate studies, and the method chosen by most other researchers in this area, was to combine organic materials with metal phosphors. These materials accept the extra energy from the excited organic material and release light via the phosphorescence route. Because the yield of this emission is higher than for organic fluorescence, the efficiency of the LED can be increased. However, metal phosphors are very limited in the shades of light color they can emit. Organic materials are much more variable, but they have a small quantum yield. It’s been a problem I’ve faced for years, which is why I was so excited to read a new article published in the prestigious journal Nature: Chemistry which outlines a way of producing 100% organic phosphors.
The researchers from Michigan took advantage of a property called electronegativity, which is the theory that lies behind the old rule “opposites attract”. Parts of the organic molecule used in the publication consisted of carbon-oxygen double bonds. Because the oxygen has a greater demand for electrons (when compared to carbon), there is a deficiency of electron charge on the carbon atom. This manifests itself as a small amount of positive charge on the carbon atom. Elsewhere in the molecule were halogen atoms such as chlorine and bromine that possessed an excess of electric charge, which showed up as a small negative charge. These two opposite charges attracted each other. The resulting stabilization (called a halogen bond) stabilized the molecular “jitter” that normally overwhelms the natural phosphorescence that organics possess. As a result, films of the material possessed a large quantum yield without the need for extra added metal phosphors.
This discovery is important because organic chemists like myself are very skilled at manipulating molecules to produce a certain color light; practically any wavelength of visible light can be produced. The problem has always been the low quantum yield in the solid material, which meant that metals had to be added, which were very limited in the colors of light they could emit. Now that a method is available to produce metal-free organic LEDs with a high quantum yield, the result should be a leap forward in the LED field. This will result in cheaper and better displays for all of our electronic devices.
The source of this article can be found at:
Bolton, O., et al. “Activating efficient phosphorescence from purely organic materials, by crystal design”. Nature: Chemistry, 2011, 3, 205-210.