TADF-based Molecular Thermometers

In todays society, sensors build up an invisible network around us, making our lives easier and more comfortable. Street lights are turned on and off by sensors, air conditioners and refrigerators are controlled by sensors, weather forecasts are only possible due to the data generated by a worldwide network of sensors and also modern cars would not move without sensors. One vast class of sensors are optical ones, which are based on measuring light. Many optical sensors require dyes that change their properties depending on the environment they are exposed to. In the simplest form, this can be litmus paper, changing its colour depending on the pH of the solution it is dipped into, but also much more sophisticated systems are possible. Although many interesting and suitable dye classes are known, applications often require very specific properties to achieve a satisfying performance. The dyes may lack sensitivity towards changes in their environment, are not bright enough or are not stable enough. Research on OLEDs, known from display technology, has also lead to enormous advances in dye chemistry. The formerly used, long glowing, phosphorescent dyes nearly always contain expensive noble metals. These dyes are now being replaced by metal-free emitters that display delayed fluorescence. The new dyes are of special interest due to their modular design, short preparation procedures and the lack of expensive and ecologically unfavourable heavy and noble metals. The properties of these dyes, however, have not yet been adapted/are not yet suitable for optical sensing since the OLED technology has a focus on different properties. In order to be suitable for optical sensing, methods need to be developed to systematically adjust the properties of dyes to suit the requirements for a multitude of applications. In the project Thermally Activated Delayed Fluorescence-based Molecular Thermometers, the team from the Institute for Analytical Chemistry and Food Chemistry in Graz tries to answer the question of how the properties of dyes featuring delayed fluorescence can be systematically tuned and how this can be used to prepare dyes tailor-made for optical sensing. The systematic preparation of a variety of dyes and their thorough characterization are thought to give a deep insight into their structure-property relationship. The new dyes are expected to enable realization of new high performance optical temperature sensors, the parameter of primary importance in all areas of science and technology. Apart from this main aim, is also expected that the new insights into the structure-property relationship will provide benefits not only for analytical chemistry and optical sensing, but also for such important field as light production and conversion, OLED technology and photovoltaics.

Temperature affects all the processes on Earth and is thus one of the most frequently monitored parameters. Conventional tools, such as resistance thermometers or thermocouples, are widely used, however, they cannot cover the whole range of necessary applications. Luminescence thermometry (i.e., temperature measurement via emission of light by a specially designed material) offers several unique features that enable applications not feasible with other methods. Particularly, it can be used for mapping ("seeing") temperature distribution on surfaces or in 3D and for accessing temperature in very small objects such as capillaries or even living cells. Although many luminescent thermometers have been reported in literature and some have even been commercialized, the state-of-the-art materials generally suffer from low brightness and low sensitivity, which does not allow resolving comparably small temperature variations. This project was aimed at tackling the above limitations by providing a new generation of optical thermometers that rely on so-called thermally activated delayed fluorescence (TADF). This particular type of light emission, generated by dyes, has recently attracted considerable attention due to its high relevance for application in Organic Light Emitting Diodes (OLEDs) but has so far been rarely applied outside of this area. In the project, we successfully synthesized and thoroughly investigated several groups of luminescent dyes and designed optical thermometers based on the most promising candidates. This was achieved by incorporating the dyes into appropriate polymers. To address the growing demand in biological and medical research, we prepared a wide range of water-dispersible nanoparticles that proved highly promising for temperature visualization in small objects, including living cells. Importantly, the new materials are self-referenced, i.e., they provide a temperature reading that is independent of fluctuations in the light used for dye excitation and in the performance of the detector. Some of the new materials enable temperature visualization with simple consumer equipment, such as cameras and smartphones. Finally, we also designed a unique group of materials relying on a single dye molecule that enables simultaneous measurement of temperature and oxygen, one of the most important molecules to be monitored in science and industry.