The Luminescent Mechanism Of Cu(?) Complexes (Phosphorescence,Thermally Activated Delayed Fluorescence And Dual Emission):Theoretical Exploration And Molecular Design

Organic light-emitting diodes(OLEDs),possessing the excellent characteristics such as autoluminescence,low energy consumption,wide viewing angle,capability of large area and flexible displays,are enjoying the spotlight in the new generation of display technology.So that the development of low-cost,high-efficiency,short lifetime and stable OLED emitters is a direction worthy of continuous efforts.In order to improve the luminous efficiency,maximizing the exciton utilization up to 100%is of essential.Generally,there are two ways for harvesting both singlet and triplet excitons.One way is obtaining the whole excitons in the lowest triplet state(T₁)to emit phosphorescence,in which the singlet excitons perform rapidly intersystem crossing(ISC)with the aid of spin-orbit coupling(SOC)induced by heavy transition metal atoms.Another way is harvesting the 100%excitons in the lowest singlet state(S₁)to perform thermally activated delayed fluorescence(TADF).It requires a narrowed S₁-T₁ energy separation(?E_ST)to fulfil the reverse intersystem crossing(RISC).Among various OLED materials,low-cost and environmentally friendly Cu(?)complexes with diverse coordination structures,rich electron transition types and emission colors as well as the right balance of moderate SOC and small?E_ST are able to utilize all excitons through phosphorescence or TADF or the cooperation of them,which makes them emerge as promising OLED alternatives.The magical luminescence mechanisms and potential application of Cu(?)complexes inspire us to deeply explore the microscopic structure and transition process to reveal the structure-property relationship.In this work,we adopted quantum chemical methods to theoretically investigate the luminescence mechanisms of a series of tetrahedral[Cu(N^N)(P^P)]complexes.On the basis of geometric and electronic structures,orbital contributions and transition information in both ground and excited states,the rates of important transitions during luminescent process are quantitatively calculated.To establish the relationship between intrinsic structure,luminescent mechanism and quantum efficiency,we detailedly studied the important factors of?E_ST,SOC and structural distortion influencing the photophysical properties,and further proposed targeted and rational molecular design strategy to improve the luminous efficiency.A brief summary of this work is as following:(1)Theoretical Design Study on the Origin of the Improved Phosphorescent Efficiency of DPEphos Quinoline-Substituted Derivatives for OLEDs.According to the previous reports,the quantum yield(QY)improvement of tetrahedral Cu(?)complexes were always achieved through enhancing the structure rigidity of N^N ligand.Here,we selected three experimental[Cu(N^N)(DPEphos)]⁺complexes to quantitatively calculate their phosphorescent radiative and nonradiative transition rates as well as the QY under the theoretical model.The computed results not only agreed well with experimental data,but also revealed that the QY enhancement was attributed to the increased N^N ligand rigidity by electron-donating substituents.Following we designed complexes 4 and5 through introducing-CH₃ progressively into the mdpbq ligand.As expected,the QY of designed complexes 4(0.26)and 5(0.13)exhibit over twice higher than that of 3(5.4×10^-2).The higher QY in complexes 4 and 5 ascribe to the enhanced electron-donating property of N^N ligand,which accelerated the radiative transition rate(k_r)through the apparently elevated energy level of the lowest triplet excited state(T₁)and strengthened transition dipole moments,even though the spin-orbit coupling(SOC)effect is weakened.Simultaneously,the tetrahedral geometric distortion could be effectively restrained by the bulky N^N ligands,but the high vibrational freedom of the terminal substituents could also bring in some unfavorable intra-ligand deformation,resulting in an upward of the nonradiative transition rate(k_nr)at 5(k_nr:0.30×10⁵ s^-1 for 4;0.95×10⁵ s^-1 for 5).Therefore,it's worth noting that the balance of excited state energy level,SOC effect as well as the reorganization energy ought to be elaborately regulated to achieve the optimal QY.(2)Molecular-Level Insight of Cu(?)Complexes with the7,8-Bis(diphenylphosphino)-7,8-dicarba-nido-undecaborate Ligand as a Thermally Activated Delayed Fluorescence Emitter:Luminescent Mechanism and Design Strategy.Here,we carried out theoretical calculation on two experimentalCu(?)complexeswiththesame7,8-bis(diphenylphosphino)-7,8-dicarba-nido-undecaborate(dppnc)ligand but different N^N ligands[dmbpy(1),dmp(2)]to briefly elaborate the structure-TADF performance relationship and luminescence mechanism.It was found that enhanced rigidity by the fused benzene ring between two pyridyl units in complex 2 leads to(i)higher allowedness of S₁?S₀,(ii)more effective RISC,and(iii)better relative stability of the T₁ state,which could be responsible for its excellent TADF behavior.Thus,a strategy of extending?conjugation in the N^N ligand could be deduced to further enhance the quantum yield.We validated it and have succeeded in designing analogue complex 4 by extending?conjugation with an electron-withdrawing pyrazinyl.Benefiting from the smaller?E_ST and plunged reorganization energy between the S₁ and T₁ states,the rate of RISC in complex 4(1.05×10⁸ s^-1)increased 2 orders of magnitude relative to that of 2(5.80×10⁶ s^-1),showing more superiority of the TADF behavior through a better balance of RISC,fluorescence,and phosphorescence decay.Meanwhile,the thermally activated temperature of 4 is only 165 K,implying that there is a low-energy barrier.All of these indicate that the designed complex 4 may be a potential TADF candidate.(3)Impact of?E_ST on Delayed Fluorescence Rate,Lifetime,and Intensity Ratio of Tetrahedral Cu(?)Complexes:Theoretical Simulation in Solution and Solid Phases.In this part,we theoretically simulated luminescent behavior in both solution and solid phases for two Cu(?)complexes and found the following:(i)The strengthened SOC effect by more d²²_x-y orbital contributions and well restricted structural distortion via remarkable intramolecular interaction in[Cu(dmp)(POP)]⁺enable the emission at room temperature to be a mixture of direct phosphorescence(10%)and TADF(90%).(ii)Benefiting from enhanced steric hindrance and electron-donating ability of the paracyclophane group,the narrowed?E_ST in[Cu(dmp)(phanephos)]⁺accelerates the reverse intersystem crossing,promoting the TADF rate(1.88×10⁵ s^-1)and intensity ratio(98.3%).These results indicate that the small?E_ST is superior for reducing the lifetime and that the strong SOC stimulates the phosphorescence to compete with TADF,which are both conducive to avoiding collision-induced exciton quenching and reducing the roll-off in devices.