Organic light-emitting diodes (OLEDs) have their performance limited by the number of emissive singlet states created upon charge recombination (25%). Recently, a novel strategy has been proposed, based on thermally activated up-conversion of triplet to singlet states, yielding delayed fluorescence (TADF), which greatly enhances electroluminescence. The energy barrier for this reverse intersystem crossing mechanism is proportional to the exchange energy (ΔEST ) between the singlet and triplet states; therefore, materials with intramolecular charge transfer (ICT) states, where it is known that the exchange energy is small, are perfect candidates. However, here it is shown that triplet states can be harvested with 100% efficiency via TADF, even in materials with ΔEST of more than 20 kT (where k is the Boltzmann constant and T is the temperature) at room temperature. The key role played by lone pair electrons in achieving this high efficiency in a series of ICT molecules is elucidated. The results show the complex photophysics of efficient TADF materials and give clear guidelines for designing new emitters.

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.

Regio- and conformational isomerization are fundamental in chemistry, with profound effects upon physical properties, however their role in excited state properties is less developed. Here two regioisomers of bis(10H-phenothiazin-10-yl)dibenzo[b,d]thiophene-S,S-dioxide, a donor–acceptor–donor (D–A–D) thermally-activated delayed fluorescence (TADF) emitter, are studied. 2,8-bis(10H-phenothiazin-10-yl)dibenzo[b,d]thiophene-S,S-dioxide exhibits only one quasi-equatorial conformer on both donor sites, with charge-transfer (CT) emission close to the local triplet state leading to efficient TADF via spin-vibronic coupling. However, 3,7-bis(10H-phenothiazin-10-yl)dibenzo[b,d]thiophene-S,S-dioxide displays both a quasi-equatorial CT state and a higher-energy quasi-axial CT state. No TADF is observed in the quasi-axial CT emission. These two CT states link directly to the two folded conformers of phenothiazine. The presence of the low-lying local triplet state of the axial conformer also means that this quasi-axial CT is an effective loss pathway both photophysically and in devices. Importantly, donors or acceptors with more than one conformer have negative repercussions for TADF in organic light-emitting diodes.

Article type: Full PaperHighly efficient TADF OLEDs; how the emitter-host interaction controls both the excited state species and electrical properties of the devices to achieve near 100% triplet harvesting and high efficiency.Vygintas Jankus, * (OLEDs). Molecules that have a charge transfer (CT) excited state can potentially achieve this through the mechanism of thermally activated delayed fluorescence 2 (TADF). Here, it is shown that a D-A charge transfer molecule in the solid state, can emit not only via an intramolecular charge transfer (ICT) excited state, but also from exciplex states, formed between the molecule and the host material. OLEDs based on one of our previously studied D-A-D molecules in a host TAPC achieves >14% external electroluminescence yield and shows nearly 100% efficient triplet harvesting. In these devices it is unambiguously established that the triplet states are harvested via TADF, but more interestingly these results are found to be independent of whether the emitter is the ICT state or the D-A-D/host exciplex. IntroductionArtificial lighting is an essential part of our lives, which consumes 19% of the planet's electricity usage, and cheaper ways of creating energy, and more efficient devices that use less electricity, are needed to reduce energy costs, and greatly cut CO 2 production. Ultraefficient lighting will play an important role in achieving this. In particular, white organic light emitting diodes (OLEDs) could become an integral part of the new lighting technologies; however, alternatives to Ir based phosphors, and especially much improved deep blue OLED emitters, are needed in order to give high quality, efficient white OLEDs that are not reliant on scarce rare-earth metals.Two types of excited states are created when charge recombines in an OLED -singlet and triplet excitons, but only the singlets directly give light, which fundamentally limits external OLED efficiency to 5%. [1] Thus the efficiency can be increased fourfold if the non-emissive triplets can be utilised. Currently, phosphorescent heavy metal complexes are used to 'harvest' the triplet states and generate light. [2][3][4] Unfortunately, pushing the metal-to-ligand charge transfer excited state of these complexes into the blue opens a non-radiative pathway via the 3 metal d-orbitals, which limits efficiency [5] , and makes the complexes thermally and photochemically unstable. [6] The production of singlets via triplet-triplet annihilation (TTA)i.e. triplet fusion (TF), has also been demonstrated in OLEDs, [7,8] but it has been shown that triplet annihilation can contribute to both an increase and a loss in yield in OLEDs, [9] and the maximum theoretical external quantum efficiency (EQE) 12.5%, when accounting for emission arising from TF, has not been reached. However, deep blue emission is essential to achieve the required colour rendering and efficiency in white OLEDs for lighting applications, therefore, other processes that can convert triplets to singlets must be found.An alternative way to convert t...