Theoretical Investigations On New Conceptual Organic Electroluminescence

Organic opto-electronic materials have attracted wide attention due to their advantages of low cost,flexibility,etc.More and more novel luminescent materials are emerging.Such as high mobility emissive materials,organic room temperature phosphorescence materials,etc.The mechanism cannot be completely obtained by the experimental means alone.In this paper,the luminescence mechanism of novel luminescent materials is explored through first-principle calculations,and molecular design strategies are proposed.First,for the exciton utilization problem of organic electroluminescence,we propose a new mechanism to achieve 200%exciton utilization through three steps:1.The highly excited triplet excitons of 75%(T2)reach the singlet through the reverse intersystem crossing process;2.The singlet excitons(S1)generate the 200%triplet excitons(T1)through singlet fission process;3.The triplet excitons directly emit light or sensitize phosphorescent molecules through the Dexter energy transfer process.This process requires that the energy of T2 is greater than that of S1,and that it is more than twice the energy of T1.To this end,we propose a strategy to fulfill this energetic requirement.The ring with low T1 energy of the Baird aromaticity is used as the bridge.Connecting different donors and acceptors can regulate the energy of S1 and T2.Ultrafast spectra and sensitization experiments of the compound TPA-DBPrz fully confirm the results of the theoretical predictions.Second,for high mobility emissive molecules,we propose three-and four-state models for herringbone stacking and π-π stacking materials,respectively.It is found that when the electron and hole transfer integral(te and th)are of the same sign and sufficiently larger than the exciton coupling(J),the coupling between the bright Frenkel exciton and the charge transfer exciton would make the lowest excited state bright state,and then the transport and luminescence properties of the system are mutually reinforcing.Based on the results of the model Hamiltonian,we propose descriptor(I=2teth/(|te||+th|)|J)to characterize the transport luminescence properties of the system and use the descriptor to screen out favorable stacking configurations and potential structural units,and the results of the fact is that the systems possess eclipsed stacking with acence-like frontier orbital nodes are the most promising to achieve high mobility emissive materials,and we also guide the experimental synthesis of high-performance asymmetric substituted anthracene derivatives.Finally,we propose the heterofission mechanism in pure organic room temperature phosphorescence materials:1.The host material is the main component that is firstly excited,which may show prompt fluorescence;2.Combined with the ground state of the guest,with the intermolecular charge transfer state as the intermediate state,the heterofission process occur,leading to the formation of the both of the triplet excitons of host and guest;3.The triplet excitons of the host and guest return to the ground state and emit phosphorescent at room temperature.The calculations show that the low T1 energy nature of the guest leads to the occurrence of the heterofission process.The heterofission process is further verified by phosphorescent spectroscopy calculations and heterofission rate calculations.We have also found some experimental evidence for this process.Finally,after understanding the energy requirements of the heterofission process and transition orbital analysis,we are able to provide potential 10 host and 10 guest units as candidates for organic room temperature phosphorescence materials.