With our ever-increasing reliance on mobile phones and portable technology, it’s never been more crucial to discover efficient and stable light-emitting compounds that can curtail energy consumption. Marc Etherington, Ph.D., tells us how his research is helping to identify novel emitters that could soon find their way into our digital screens.
In today’s world, we’re becoming ever more reliant upon screen technology – from televisions and computer displays at home to mobile phones and other portable technology. It’s hard to imagine life without digital displays. But have you ever stopped to think about the technology that goes into our screens? Did you know that the blue glow of tonic water might help to inspire the next generation of screen technology?
Dr. Etherington, assistant professor at Northumbria University, UK, has spent his career researching organic semiconductors, particularly those with light-emitting properties. Through fluorescence spectroscopy, Dr. Etherington has been discovering novel light-emitting compounds that could improve the efficiency and longevity of our phone or TV displays. We recently spoke to him about his groundbreaking research, and how the HORIBA Fluorolog-QM has been supporting his investigations.
“The main goal of my research is developing more efficient, stable light-emitting materials for use in displays, but also for bioimaging and sensing purposes,” he said. “Efficient light-emitting compounds with high stability are crucial for numerous technologies, but especially in displays – nobody wants to replace their phone or TV after a couple of months because the colors are fading. Similarly, for sensing applications, the light-emitting compounds are going to be exposed to a variety of conditions or environments, and users will expect predictable behavior throughout.”
In discovering light-emitting compounds with better efficiency and high stability, digital displays can be made more energy-efficient and longer-lasting. This can improve energy consumption, reduce waste, and reduce the impact of digital displays on the environment.
“I don't think we’re ever going to see a reduction in our use of portable technology, but by making display technology more efficient and longer-lasting, it’s possible to reduce their impact on the environment. More efficient displays not only decrease the drain on battery but the energy required to power the devices is also reduced. It's not just about making sure that the user doesn't have to charge their phone every day. It's also about how much energy we're consuming on a global scale.”
One of the biggest problems to solve when seeking to improve the efficiency of digital displays is a lack of stable blue light-emitting compounds. Blue fluorescent compounds used in modern screens are much less efficient at emitting light than those in the red or green part of the spectrum, and it’s also less stable. In practical terms, this means that the blue pixels in your display degrade much faster than the red or green, causing a relatively bigger drain on the battery and shortening the life of the display.
“Finding a stable blue emitter is the crucial challenge that’s been there for some time now. It’s harder to find a stable compound in the blue emission range versus red or green, purely because of the relative energies of different colored light. While light toward the red end of the spectrum has a longer wavelength and lower energy, when we get to the blue end, the wavelength and energy of the light increases to a level that’s sufficient to start breaking the bonds of the materials.”
Dr. Etherington’s research has focused on the N-alkylation of amine-based fluorophores [1] and thermally activated delayed fluorescence (TADF) [2], which are different mechanisms that can be used to improve the efficiency of OLEDs. Much of his recent research, however, has been based on an organic molecule called quinine.
You may have heard of quinine as a component of tonic water – it’s also a well-known antimalarial compound. You may not be aware, however, that quinine was one of the first compounds officially studied for fluorescence in the 1850s. Quinine emits blue light – which is why your gin and tonic might have a bluish hue on a sunny day.
“We’ve been investigating quinine and a host of other organic compounds in an attempt to find more stable blue light emitters. By looking at the structure and activity of these compounds, we can use this information to find similar materials that people perhaps wouldn't expect to end up in their TV displays. There are many everyday organic compounds with fluorescent properties – olive oil, for example, glows a bright red color under UV radiation. While these are perhaps not the most efficient emitters, by studying them we can begin to unpick characteristics that make good emitters, and design fluorophores based upon these characteristics.”
Dr. Etherington spoke about the need to identify some of the molecular structure-property relationships to better understand the link between structural molecular properties and light-emitting behavior.
“Unlike our current understanding of drug compounds, for example, the structure-activity relationship of fluorescent compounds is poorly defined. This is perhaps because when looking for light-emitting compounds, we screen a tiny fraction of the number of molecules that would be tested in a drug discovery campaign. It's a lot more difficult to undertake large-level screening in photophysics, but determining molecular structure-activity relationships is something we’re really interested in.”
Along with a collaborator, Dr. Etherington is currently studying a range of synthetic quinine analogs, to see if they can look at the structure and use this to predict the various analogs’ photophysical properties.
“We’re starting to see some trends. Building upon this, we’re also interested in what we can do with these materials outside of OLED displays – so we’re looking to develop novel light-emitting compounds for use in fluorescence detection systems in a biological setting as biosensors.”
Dr. Etherington confirmed the aims of this bioimaging and biosensor research. “For sensing applications, the key challenge is to find something that is sensitive for the parameter that you’re interested in but stable and insensitive to other properties that you may not want to detect.”
Fluorescence spectroscopy is central to Dr. Etherington’s research when characterizing a new material. Through the multifunctionality and exceptional sensitivity, enabled by the unique optical design, of his in-house Fluorolog-QM™ system, his research team has been able to push the boundaries of what we know about light-emitting compounds.
“When we find a new light-emitting compound to study, we need to establish its key photophysical properties: for example, we might want to determine the part(s) of the molecule responsible for emission, or the color of the light emitted. At this level, the Fluorolog-QM helps us to characterize and measure fluorescence with a great level of detail – we can see exactly which wavelengths are being emitted, and quantify the energy involved.”
“It’s also a very sensitive piece of kit for measuring low-level fluorescence: Sometimes, we might design an emitter that isn't so emissive, and for that, we need a high level of sensitivity to confirm a non-result or control measure. Because of this, the Fluorolog-QM is helping us to understand why some compounds perform better than others.”
"Not only the Fluorolog-QM have enabled Dr. Etherington to characterize the emission properties of light-emitting compounds, it has also enabled him to perform more advanced analyses.
“Thanks to the Fluorolog-QM, we’ve been able to look more closely at the excitation spectrum by comparing light going into, and how this affects the light emitted from the compounds under different conditions.”
Another key feature for Dr. Etherington is the system’s integrating sphere add-on, enabling him and his team to measure the photoluminescence quantum yield (PLQY) of a molecule. The PLQY value is the ratio of photons emitted versus the number of photons absorbed – and it's one of the main characteristics for energy efficiency.
“We can also resolve charge transfer states, which show the electron distribution in the molecule – this has enabled us to learn how structural changes like N-alkylation can be used to control certain fluorescence parameters like charge transfer, emission energy, and quantum yield.” [3]
Speaking about his future research plans, Dr. Etherington set out his goals to continue researching and developing novel light-emitting compounds that gave us insight into some of the potential applications in which they could be used.
“Once we can determine some of the structure-property relationships of different light-emitting compounds, we can start to apply what we know to design fluorophores for precise applications. So, we might be able to design a compound with maximum stability for use in an OLED display.”
“There’s also a lot of scope for use in biosensing applications: For example, we may be able to design fluorophores with sensitivity to pH – we could use them in a cellular environment to detect pH changes from cell to cell. If we can bring the sensitivity up to speed enough, we might be able to distinguish, for example, between cancerous versus non-cancerous cells, or at least a change in the environment that could be linked to it.”
The Fluorolog-QM will also be fundamental to this research moving forward, Dr. Etherington explained.
“It all stems back to the idea of structure-property relationships ― can we make these links? The only way to do it is by studying a wide range of molecules with the highest possible sensitivity, analyzing the very weakest fluorescence, and characterizing the most subtle of spectral changes. That’s what we’re hoping to achieve, starting with the quinine analogs.”
“Key to unpicking these structure-property relationships is having a reliable kit like the Fluorolog-QM. We need to have that level of sensitivity; both spectrally, and in terms of how much light it picks up. This is imperative because we must be 100 percent sure which structures are responsible for certain photophysical characteristics. We also rely upon that high level of competence if we want to establish general trends for this whole family of molecules.”
We have seen how the Fluorolog-QM has empowered Dr. Etherington’s research, and thanks to the modular nature of the instrument, it stands to be a key part of his research for years to come.
“I love the fact that (the Fluorolog-QM) is completely modular ― alongside HORIBA’s analyzers’ reliability and intuitive use, it’s actually one of the main reasons why I decided to purchase it initially. You can add on extra diffraction gratings and detectors to suit your needs ― we’re hoping to add the Time Correlated Single Photon Counting System in the near future. The modular nature of the Fluorolog-QM means that as my research grows, and my research team grows, the system can grow with it.”
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