Here, a novel deep blue emitter SBABz4 for use in organic light-emitting diodes (OLED) is investigated. The molecular design of the emitter enables thermally activated delayed fluorescence (TADF), which we examine by temperature-dependent time-resolved electroluminescence (trEL) and photoluminescence (trPL). We show that the dihedral angle between donor and acceptor strongly affects 2 the oscillator strength of the charge transfer state alongside the singlet-triplet gap. The angular dependence of the singlet-triplet gap is calculated by time-dependent density functional theory (TD-DFT). A gap of 15 meV is calculated for the relaxed ground state configuration of SBABz4 with a dihedral angle between the donor and acceptor moieties of 86°. Surprisingly, an experimentally obtained energy gap of 72±5 meV can only be explained by torsion angles in the range of 70°-75°. Molecular dynamics (MD) simulations showed that SBABz4 evaporated at high temperature acquires a distribution of torsion angles, which immediately leads to the experimentally obtained energy gap. Moreover, the emitter orientation anisotropy in a host matrix shows an 80% ratio of horizontally oriented dipoles, which is highly desirable for efficient light outcoupling. Understanding intramolecular donor-acceptor geometry in evaporated films is crucial for OLED applications, because it affects oscillator strength and TADF efficiency. IntroductionFluorescence-based organic light-emitting diodes (OLEDs) are known to reach a maximum internal quantum efficiency (IQE) int of 25% due to spin-statistics, while the incorporation of heavy metal atoms in emitting molecules enables the phosphorescence IQE of 100% due to efficient spin-orbit coupling. [1][2][3][4] The molecules based on thermally activated delayed fluorescence (TADF) can harvest non-radiative triplets via reverse intersystem crossing (RISC) and can also yield int of 100%.[5-9] Intuitively, one has to minimize the singlet-triplet energy gap in order to enhance the triplet-to-singlet up-conversion. [10,11] This condition is commonly fulfilled by separating electron and hole wave functions onto different moieties, so that their overlap is reduced. Large separation of electron and hole is realized in the molecules comprised of donor and acceptor units that are almost perpendicular to each other. The dihedral angle Θ between donor and acceptor determines the exchange energy and hence EST. Actually, one has to find a compromise for , because  = 90° means perfectly separated e-h wave functions and, consequently, 3 zero singlet-triplet gap, but, at the same time, the oscillator strength f for radiative transitions from the excited CT state to the ground state vanishes. [12]  can be calculated theoretically, but the optimum value for  is hard to predict a priori. The calculated dihedral angle  for an isolated molecule in its relaxed configuration can differ from the real value in a thin-film device prepared by thermal evaporation.Therefore, the experimentally measured activation energies can deviate s...

Solution-grown single crystals of furan/phenylene co-oligomer combine efficient charge transport properties and high fluorescence efficiency.

Thermally activated delayed fluorescence (TADF) emitters consisting of donor and acceptor molecules are potentially highly interesting for electroluminescence (EL) applications. Their strong fluorescence emission is considered to be due to reverse intersystem crossing (RISC), in which energetically close triplet and singlet charge transfer (CT) states, also called exciplex states, are involved. In order to distinguish between different mechanisms and excited states involved, temperature-dependent spin-sensitive measurements on organic light-emitting diodes (OLEDs) and thin films are essential. In our work we apply continuous wave (cw) and time-resolved (tr) photoluminescence (PL) spectroscopy as well as spin-sensitive electroluminescence and PL detected magnetic resonance to films and OLED devices made of three different donor:acceptor combinations. Our results clearly show that triplet exciplex states are formed and contribute to delayed fluorescence (DF) via RISC in both electrically driven OLEDs and optically excited films. In the same sample set we also found molecular triplet excitons, which occurred only in PL experiments under optical excitation and for some material systems only at low temperatures. We conclude that in all investigated molecular systems exciplex states formed at the donor:acceptor interface are responsible for thermally activated DF in OLEDs with distinct activation energies. Molecular (local) triplet exciton states are also detectable, but only under optical excitation, while they are not found in OLEDs when excited states are generated electrically. We believe that the weakly bound emissive exciplex states and the strongly bound non-emissive molecular triplet excited states coexist in the TADF emitters, and it is imperative to distinguish between optical and electrical generation paths as they may involve different intermediate excited states.

We designed and synthesized a new organic light-emitting diode (OLED) emitter, SBABz4, containing spiro-biacridine donor (D) in the core surrounded by two benzonitrile acceptors (A). The dual A-DxD-A structure is shown to provide pure-blue emission in relation to its single A-D counterpart. Time-resolved photoluminescence (TRPL) recorded in the broad dynamic range from solutions and solid films revealed three emission components: prompt fluorescence, phosphorescence, and efficient thermally-activated delayed fluorescence (TADF).The latter is independently proven by temperature-dependent TRPL and oxygen-quenching PL experiment. From the PL lifetimes and quantum yield, we estimated maximum external quantum efficiency of 7.1% in SBABz4-based OLEDs, and demonstrated 6.8% in a working device.