Molecular Design And Performance Simulation Of OLED Emitters For Thermally Activated Delayed Fluorescence Mechanism

The new generation of organic light-emitting diodes(OLED)based on the thermally activated delayed fluorescence(TADF)mechanism has received widespread attention in recent years,owing to not only 100%internal quantum efficiency(IQE)but also low cost,high stability,high color purity and other advantages.The new mechanism makes OLED technology take an important step towards high performance and low cost direction.The most important condition for realizing the TADF mechanism is that the singlet-triplet energy splitting(△EST)can not be too high(<0.1 eV),which enables the triplet excitons convert into singlet excitons by reverse intersystem crossing(RISC)process.Till now electron donor(D)-electron acceptor(A)molecular structure is a popular strategy to construct TADF emitters,for it can effectively separate the highest occupied molecular orbital(HOMO)and the lowest unoccupied molecular orbital(LUMO),thus decrease the△EST.With the rapid development of computer science and technology,theoretical molecular design and simulation play an important role in scientific research.Using density functional theory to simulate at molecular level and combine with the key experimental results to predict and verify the molecular properties attract more and more researchers’ attention.In this paper,we designed TADF molecules with small △EST based on the first principle method.All the calculations were carried out on Gaussian09 package,including the molecular stable conformations,the electronic structures and properties.The significant conclucions were acquired as follows:(1)D-A molecules contain "pseudo-plannar" fragments have double stable conformations,among which the near-orthogonal conformation shows smaller overlap and △EST.Besides,increasing the rigidity of the D group or the steric hindrance between D and A fragments can reduce the stabilization energy of the near-orthogonal conformation.(2)The adiabatic excitation energy method which considers the relaxation effect gives a perfect result consistent with the experimental one for carbazole-based D-A molecules.In addition,B3LYP functional is a general and accurate way to predicting△EST.(3)When the polarity of the solvents increase,the geometries of ground state and excited state do not change significantly,while the △EST,the oscillator strengh and the transition dipole moment decrease regularly.