Thermally activated delayed fluorescence (TADF) has recently emerged as one of the most attractive methods for harvesting triplet states in metal-free organic materials for application in organic light emitting diodes (OLEDs). A large number of TADF molecules have been reported in the literature with the purpose of enhancing the efficiency of OLEDs by converting non-emissive triplet states into emissive singlet states. TADF emitters are able to harvest both singlets and triplet states through fluorescence (prompt and delayed), the latter due to the thermally activated reverse intersystem crossing mechanism that allows up-conversion of low energy triplet states to the emissive singlet level. This allows otherwise pure fluorescent OLEDs to overcome their intrinsic limit of 25% internal quantum efficiency (IQE), which is imposed by the 1:3 singlet-triplet ratio arising from the recombination of charges (electrons and holes). TADF based OLEDS with IQEs close to 100% are now routinely fabricated in the green spectral region. There is also significant progress for blue emitters. However, red emitters still show relatively low efficiencies. Despite the significant progress that has been made in recent years, still significant challenges persist to achieve full understanding of the TADF mechanism and improve the stability of these materials. These questions need to be solved in order to fully implement TADF in OLEDs and expand their application to other areas. To date, TADF has been exploited mainly in the field of OLEDs, but applications in other areas, such as sensing and fluorescence microscopies, are envisaged. In this review, the photophysics of TADF molecules is discussed, summarising current methods to characterise these materials and the current understanding of the TADF mechanism in various molecular systems.

Here, a comprehensive photophysical investigation of a the emitter molecule DPTZ‐DBTO2, showing thermally activated delayed fluorescence (TADF), with near‐orthogonal electron donor (D) and acceptor (A) units is reported. It is shown that DPTZ‐DBTO2 has minimal singlet–triplet energy splitting due to its near‐rigid molecular geometry. However, the electronic coupling between the local triplet (³LE) and the charge transfer states, singlet and triplet, (¹CT, ³CT), and the effect of dynamic rocking of the D–A units about the orthogonal geometry are crucial for efficient TADF to be achieved. In solvents with low polarity, the guest emissive singlet ¹CT state couples directly to the near‐degenerate ³LE, efficiently harvesting the triplet states by a spin orbit coupling charge transfer mechanism (SOCT). However, in solvents with higher polarity the emissive CT state in DPTZ‐DBTO2 shifts below (the static) ³LE, leading to decreased TADF efficiencies. The relatively large energy difference between the ¹CT and ³LE states and the extremely low efficiency of the ¹CT to ³CT hyperfine coupling is responsible for the reduction in TADF efficiency. Both the electronic coupling between ¹CT and ³LE, and the (dynamic) orientation of the D–A units are thus critical elements that dictate reverse intersystem crossing processes and thus high efficiency in TADF.

The recent progress in the theory of organic molecules exhibiting thermally activated delayed fluorescence (TADF) is discussed with a particular focus upon their application in highly efficient purely organic light emitting diodes (OLEDs).