Computational Insight Into Thermally Activated Delayed Fluorescence Emitters Based On D-A-A Structures

In the classical three-layer structure of organic light-emitting diode(OLED),the emitting layer is an important part which the luminescence properties of emitting materials determine the performance of OLED devices.As the first generation of luminescence materials applied in the OLED devices,traditional fluorescent materials can only make use of singlet excitons which the proportion of the excitons is 25%according to quantum statistics in the process of electroluminescence.The other excitons are triplet excitons which is waste because of spin forbidden.Hence,the internal quantum efficiency(IQE)of traditional fluorescent materials can only reach up to 25%.Low luminescent efficiency limits the development of the OLED devices.In comparison,phosphorescence materials as the second generation of emitting materials applied in the OLED devices,the introduction of transition metal atoms increases the spin orbit coupling(SOC)interaction between singlet and triplet states through the effect of heavy atom.The improvement of SOC can lead to the full use of all the excitons and internal quantum efficiency of the phosphorescent materials can reach up to 100%.However,heavy metals are expensive,rare and polluting the environment which results in the limitation of development of phosphorescent materials.As the third generation of luminescence materials applied in the OLED devices,thermally activated delayed fluorescence(TADF)materials have been proposed by researchers in recent years.The key of luminescent mechanism of this kind of materials is the reverse intersystem crossing(RISC)process.If the energy gap between the lowest singlet state and the lowest triplet state(?E_ST)is small enough,the triplet excitons can cross the energy barrier by absorbing the thermal energy from the surroundings,reach the singlet state through RISC process and emit to the ground state.Therefore,the corresponding IQE can also reach up to 100%.However,due to the severe efficiency rolling down,there are a few TADF-OLED devices whose luminous efficiency can be comparable with that of the phosphorescent OLED devices.Then,the‘hot exciton'type TADF materials reported by Ma et al can greatly improve the utilization rate of excitons.The exploration on the photophysical properties of‘hot exciton'type TADF materials provides a novel and rigorous theoretical support for the investigation on the luminescent process.New D-A-A type molecules reported by Xu group show more possibility for the design of high-efficiency TADF molecules.Hence,theoretical investigation on the structural design and photophysical properties of D-A-A type molecules will play a significant role in the development of high-efficiency TADF materials.By the formation of almost vertical dihedral angle between D and A,the highest occupied molecular orbital(HOMO)and the lowest unoccupied molecular orbital(LUMO)of traditional D-A TADF molecule are sufficiently divided.However,the emissive oscillator strength of the first singlet excited state and photoluminescent efficiency are reduced.Then,D-A-D type TADF molecules break the limitation of linear molecular design,obtain smaller?E_ST and more overlap of HOMO and LUMO which promote the increase of luminescent efficiency.Compared with D-A and D-A-D type molecules,the efficiency of RISC process of D-A-A type molecules is doubled.The corresponding radiative decay rate(k_r)in the lowest singlet state is improved by an order of magnitude.Furthermore,the nonradiative decay rate constants of singlet and triplet states are reduced by five times,and the external quantum efficiency(EQE)of the corresponding OLED devices are lifted up to 30%.Based on the above mentions,our work will be carried out as follow:(1)A series of novel molecules are theoretically designed based on pt BCz PO2TPTZ,including diphenylphosphine oxide(PO)group as the secondary receptor which the number of PO is 1 or 2.For the donor part,four stronger donors are used to constitute eight new molecules with high-lying RISC process,including dinitrogen miscellaneous naphthalene(DHPZ),phenothiazine(PTZ),phenoxazine(PXZ)and 9,9-dimethyl-9,10-dihydroacridine(DMAC).(2)With the comparison of the experimental and calculated values of the dihedral angle of D-A,the peaks of absorption spectrum,k_r of pt BCz PO₂TPTZ,the functional B3LYP and the basis set 6-31G(d,p)are used to explore the photophysical properties of novel designed molecules.By the optimization of geometrical structures and calculations of absorption and emission spectra of new molecules,it is indicated that there are luminescent processes from high-lying excited states of these new molecules.Among them,the oscillator strengths of the strongest absorption peaks of D-TRZ-2PO are significantly stronger than that of D-TRZ-PO,which suggests that there may be greater transition probability and vibration dipole during the excitation.In addition,the excited state energy level of DHPZ-TRZ-2PO is lower than that of other molecules,so it is more likely to perform the luminescent process from the high-lying excited state.(3)The luminous performance of D-TRZ-n PO are quantitatively estimated by calculating k_r of S_m?S₀,internal conversion rate(k_ic)of T_n?T₁,intersystem crossing rate(k_isc)and reverse intersystem crossing rate(k_risc)between S_m-T_n.DMAC-TRZ-n PO and PXZ-TRZ-n PO are almost non-emitting from the high-lying excited states because of the faster internal conversion(IC)process from T_n to T₁.PTZ-TRZ-n PO and DHPZ-TRZ-PO may be potential high-lying excited state fluorescence emitters due to their larger k_r than k_isc.Among these eight molecules,DHPZ-TRZ-2PO with blue emission can be considered as a potential high-lying excited state delayed fluorescence material due to its ultra-fast ISC and RISC processes and effective radiative decay.What's more,exploring the change of performance resulted from the geometrical variations of D-A-A type TADF molecules makes us get further understanding of structure-function relationship,which is beneficial for giving theoretical basis of material design.