CHAPTER 6: Photophysics of Thermally Activated Delayed Fluorescence in Organic Molecules

Thermally Activated Delayed Fluorescence (TADF), also known as E-type delayed fluorescence, is currently a topic of great interest in molecular photophysics. Numerous TADF emitters have been designed in recent years. Mainly due to their ability to harvest triplet states using metal-free organic molecules, which offer the opportunity to enhance the efficiency of organic light emitting diodes (OLEDs). This is achieved by the up-conversion of low energy non-emissive triplet states into higher energy, emissive singlet states, using thermal energy in a purely intramolecular process. The TADF mechanism, therefore, enables fluorescent OLEDs to overcome their intrinsic limitation of 25% internal quantum efficiency (IQE), imposed by the 1:3 singlet triplet ratio appearing from charge recombination. OLEDS with IQEs close to 100% are now routinely fabricated in the blue and green emissive regions, but red emitters still show relatively lower efficiencies. Major challenges also persist concerning the full understanding of the TADF mechanism, which need to be resolved to improve the stability of these materials, and fully implement TADF in OLEDs. This will also facilitate expanding their applicability to other areas. TADF has so far been exploited mainly in OLEDs, however, applications in biological imaging and sensing are envisaged. In this paper, the photophysics of TADF molecules is reviewed, covering both the theory and experimental characterization in different molecular systems, focusing the discussion on the mechanism of reverse intersystem crossing (rISC).