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Register a new userDielectric control of reverse intersystem crossing in thermally-activated delayed fluorescence emitters
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Thermally-activated delayed fluorescence (TADF) enables organic semiconductors with charge transfer (CT)-type excitons to convert dark triplet states into bright singlets via a reverse intersystem crossing (rISC) process. Here, we consider the role of the dielectric environment in a range of TADF materials with varying changes in dipole moment upon optical excitation. In a dipolar reference emitter, TXO-TPA, environmental reorganisation after excitation in both solution and doped films triggers the formation of the full CT product state. This lowers the singlet excitation energy by 0.3 eV and minimises the singlet-triplet energy gap ({Delta}EST). Using impulsive Raman measurements, we observe the emergence of two (reactant-inactive) modes at 412 and 813 cm-1 as a vibrational fingerprint of the CT product. In contrast, the dielectric environment plays a smaller role in the electronic excitations of a less dipolar material, 4CzIPN. Quantum-chemical calculations corroborate the appearance of these new product modes in TXO-TPA and show that the dynamic environment fluctuations are large compared to {Delta}EST. The analysis of the energy-time trajectories and the corresponding free energy functions reveals that the dielectric environment significantly reduces the activation energy for rISC, thus increasing the rISC rate by up to three orders of magnitude when compared to a vacuum environment.
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We study the relaxation dynamics and intersystem-crossing to the metastable state in laser-pumped tetra and hexa-coordinated nickel porphyrins. We use a ligand-field model which takes into account the crystal field created by the porphyrin ring and axial ligands. By accounting for the energy redistribution of the lattice vibrations of the metal-ligand stretch mode we get an irreversible decay within the order of the hundreds of femtoseconds timescale. We show how non-equilibrium time-dependent x-ray absorption atthe Ni K-edge measurements can elucidate the nature of the intermediate states involved in the decay. Understanding radiationless transitions in this system is of interest for their relevance in photocatalytic systems and photothermal sensitizers for cancer treatment.
Engineering a low singlet-triplet energy gap ({Delta}EST) is necessary for efficient reverse intersystem crossing (rISC) in delayed fluorescence (DF) organic semiconductors, but results in a small radiative rate that limits performance in LEDs. Here, we study a model DF material, BF2, that exhibits a strong optical absorption (absorption coefficient =3.8x10^5 cm^-1) and a relatively large {Delta}EST of 0.2 eV. In isolated BF2 molecules, intramolecular rISC is slow (260 {mu}s), but in aggregated films, BF2 generates intermolecular CT (inter-CT) states on picosecond timescales. In contrast to the microsecond intramolecular rISC that is promoted by spin-orbit interactions in most isolated DF molecules, photoluminescence-detected magnetic resonance shows that these inter-CT states undergo rISC mediated by hyperfine interactions on a ~24 ns timescale and have an average electron-hole separation of >1.5 nm. Transfer back to the emissive singlet exciton then enables efficient DF and LED operation. Thus, access to these inter-CT states resolves the conflicting requirements of fast radiative emission and low {Delta}EST.
Spin-spin interactions in organic light-emitting diodes (OLEDs) based on thermally activated delayed fluorescence (TADF) are important because radiative recombination is largely determined by triplet-to-singlet conversion, also called reverse intersystem crossing (RISC). Less obvious is the fact that the non-emissive triplet states are spin-polarized, e.g., by charge injection, and spin-selection rules prevent part of the triplet population from RISC. To explore the relationship between these two processes, we apply a two-frequency spin-resonance technique, which is essentially spectral hole burning, that directly probes electroluminescence. This allows us not only to independently confirm high spin-polarization, but also to distinguish between individual triplet exciplex states distributed in the OLED emissive layer. These states can be decoupled from the heterogeneous nuclear environment as a source of spin dephasing and can even be coherently manipulated on a spin-spin relaxation time scale T2* of 30 ns. Furthermore, we obtain the characteristic spin-lattice relaxation time T1 of the triplet exciplex in the range of 50 us, which is longer than the RISC time. We conclude that long spin relaxation time rather than RISC is an efficiency-limiting step for intermolecular donor:acceptor systems. Finding TADF emitters with faster spin relaxation will benefit this type of TADF OLEDs.
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 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{deg}. Surprisingly, an experimentally obtained energy gap of 72+/-5 meV can only be explained by torsion angles in the range of 70{deg}-75{deg}. 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.
Generating entangled graph states of qubits requires high entanglement rates, with efficient detection of multiple indistinguishable photons from separate qubits. Integrating defect-based qubits into photonic devices results in an enhanced photon collection efficiency, however, typically at the cost of a reduced defect emission energy homogeneity. Here, we demonstrate that the reduction in defect homogeneity in an integrated device can be partially offset by electric field tuning. Using photonic device-coupled implanted nitrogen vacancy (NV) centers in a GaP-on-diamond platform, we demonstrate large field-dependent tuning ranges and partial stabilization of defect emission energies. These results address some of the challenges of chip-scale entanglement generation.
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