Theoretical Study Of The Phosphorescence Quantum Yield For Cu(?) Complexes With Double Heteroleptic Ligands

In traditional fluorescent organic light emitting diodes?OLEDs?,most of the triplet exciton energy and 75%internal quantum yield?QY?are wasted.Heavy metal complexes have attracted broad attention during the past decade because of their promising application in OLEDs with high QY.The strong spin-orbital coupling?SOC?of heavy metal atoms leads to efficient intersystem crossing?ISC?of the singlet excited states to the triplet manifold,which triggers a long lifetime of the excited states.Mixing the singlet and triplet excited states via SOC removes the spin-forbidden nature of the radiative relaxation of the triplet state,leading to high photoluminescence QY?PLQY?.The Cu?I?complexes are nontoxic,environmentally friendly,inexpensive,and have better luminous efficiency,and present the photophysical characters that are similar to other metal ions like Pt?II?,Ir?III?,Os?II?,and Ru?II?.These advantages make Cu?I?complex a promising alternative for the OLEDs.In this paper,we conducted DFT and TD-DFT calculations on three double heteroleptic Cu?I?complexes to understand how different substituents on N^N ligand influence the phosphorescence quantum yield?PLQY?.Both radiative and nonradiative decay processes were thoroughly investigated.Factors that determine the rate of radiative process?k_r?were considered,including the lowest triplet excited state E?T₁?,the transition dipole momentof the S_m?S₀ transition,the spin-coupled matrix element SOC,and the singlet-triplet splitting energies?E?S_m-T₁?.The results indicate that the E?T₁?,theand the SOC increase and the?E?S_m-T₁?decreases by introducing-Ph and-CH₂-groups on the N^N ligand.The net results lead to a gradual increase of k_r in the three Cu?I?complexes,from 1(0.48×10⁴ s^-1)to 2(0.64×10⁴ s^-1)and then to 3(1.61×10⁴ s^-1).The rate of nonradiative decay processes(k_nr)was computed by the convolution method.We explored how k_nr is determined by the SOC between T₁ and S₀ state?<T₁|SOC|S₀>²?,effective energy gap?E^'and the Huang-Rhys factor?S?.We found that<T₁|SOC|S₀>² and?E^'contribute significantly to knr,but S does not determine the order of the k_nr.k_nr gradually decreases from complex 1(2.51×10⁶ s^-1)to 2(0.32×10⁶ s^-1)and then to 3(0.14×10⁶ s^-1)after introducing-Ph and-CH₂-groups on the N^N ligands.The computed PLQYs for the three complexes are 1:0.0019,2:0.0198,3:0.1011.These are quantitatively consistent with the experimental observation?1:0.0028,2:0.0061,3:0.1000?.We hope that this work can realize the design and synthesis of novel materials with high PLQY materials.