Theoretical Study On The Luminescence Properties Of Thermally Activated Delayed Fluorescence Molecules

Organic light-emitting diodes?OLEDs?have attracted attention from all walks of life in recent years due to their advantages such as light weight,good flexibility,low cost,and wide display range.They have been playing an important role in today's organic display and lighting fields.At the same time,luminescent materials of OLEDs have also experienced different stages of development,and their performance has been continuously improved.In order to solve the above problems of traditional fluorescent materials and phosphorescent materials,recently developed third-generation light-emitting materials,thermally activated delayed fluorescence?TADF?materials,have attracted much attention.TADF material can avoid the pollution problem caused by heavy metals.Its main luminescence mechanism is as follows:The energy difference between the singlet and triplet excited states of TADF material is usually small,and the relatively small energy difference is conducive to realizing an effective intersystem crossing process,through which the triplet excitons can be transferred to the singlet excited state,followed by fluorescence emission based on the radiative transition.The exciton utilization rate of TADF materials can also reach 100%.At present,a large number of thermally activated delayed fluorescent materials have been reported experimentally,but the theoretical study of their luminescence mechanism is still lacking.In this thesis,we mainly conduct a systematic detailed theoretical research on the luminescence mechanism of thermally activated delayed fluorescent materials using the first-principles calculations.In this work,the molecular environments in solution and in thin film are simulated using the continuous polarization model?PCM?and combined quantum mechanics/molecular mechanics?QM/MM?methods,respectively.Based on the calculation and analysis of molecular energy levels,structural changes during excitation,Huang Kun factors,recombination energies,intermolecular interactions as well as the excited state dynamics information,the luminescence mechanism of TADF molecules is revealed.The details are as follows:?1?Study on aggregation-inducing effects of chiral molecules.We have performed detailed calculations on the geometric structures and luminescence mechanisms of two circularly polarized enantiomeric luminescent molecules?R/S-BN-AF,R/S-BN-CF?in toluene and in solid phase.The results show that apart from the electron circular dichroism?ECD?,there is no significant difference in the luminescent properties of the two enantiomers of the chiral molecule.Comparing the luminescent properties of the molecules in different environments,it was found that the radiation rate of the two molecules was significantly increased in the solid phase,while the non-radiation decay rate was significantly suppressed,revealing the aggregation-induced emission?AIE?characteristics of the molecules.At the same time,the intersystem crossing rate and reverse intersystem crossing rate are comparable to the radiation rate of the S₁ state,indicating that the molecules exhibit obvious TADF luminescence properties.The luminous efficiency of the molecule R-BN-CF is much higher than that of S-BN-AF,revealing that different electron donor groups could directly affect the molecular excited state dynamics and the luminescence efficiency.?2?Study on the aggregation-induced luminescence mechanism of efficient undoped delayed fluorescent emitter:PXZ2PTO.Recently,a new type of non-doped thermally activated delayed fluorescent material PXZ2PTO with D-A structure was synthesized experimentally.In this material,phenoxazine works as the electron donor and the 10-phenyl-10H-phenothiazine-5,5-dioxide?2PTO?group was first reported as the electron acceptor.The 2PTO group are based on the oxidation of PTZ group,while PTZ group usually works as traditional electron donor.We have used the PCM model and the QM/MM method to study the unique luminescence mechanism of PXZ2PTO molecules in detail.Studies have shown that due to the introduction of 2PTO group as electron donor,PXZ2PTO exhibits a highly stereoscopic three-dimensional structure,in which the electron donor part is almost perpendicular with the electron acceptor unit.In the solid phase,due to the influence of the rigid environment,the unique stereoscopic structure of PXZ2PTO causes a high steric hindrance between adjacent molecules and hinders the molecular rotations.Thus,the non-radiation rate of PXZ2PTO in solid phase is two orders of magnitude lower than that in toluene.The luminescence characteristics of PXZ2PTO molecule is enhanced through the introduction of 2PTO group.Our theoretical research work reasonably explained the experimental results and provided some theoretical support for the design of efficient TADF emitters in the future.?3?Study on the influence mechanism of donor unit on the singlet-triplet energy gap.For TADF materials,the small energy difference between singlet and triplet excited states as well as the strong spin-orbit coupling are crucial for realizing the intersystem crossing processes.A comparative study was conducted for two chiral molecules Cz Ph Trz and Czp Ph Trz with different electron donor units.We calculated the geometric and electronic structures of the two molecules in detail to study the effect of electron donors on the energy difference between singlet and triplet excited states.Studies have shown that compared with Cz Ph Trz,due to the introduction of carbazolophane?Czp?as the electron donor unit,the molecule Czp Ph Trz are more likely to undergo a large twist between the electron donor and the electron acceptor during the excitation.As a result,the overlapping degree of the frontier orbitals of the molecule is reduced,and the energy difference between the singlet and triplet states becomes smaller.The reduction of the energy difference is beneficial for the molecule to achieve delayed fluorescence emission through the intersystem crossing process.