Theoretical Study On The Photophysical Properties Of Iridium(?) Complexes

Organic electroluminescent devices?OLEDs?in solid-state lighting and screen display have a broad application prospects.The ideal quantum efficiency of phosphorescent material could reache 100%because of the strong spin-orbit coupling?SOC?induced by transition metal.Compared with other transition metal complexes,iridium complexes have attracted much attention due to the short triplet lifetime,high luminescent efficiency and easily tunable light emission wavelength.Red,green,and blue three-color material is essential in both solid-state lighting or full-color display.Nowadays,the development of efficient and stable red and green materials has satisfied for commercial applications.However,the lack of blue phosphorescent material,especially for the lack of dark blue phosphorescent material,has become the obstacle for the development of OLEDs.In this paper,we employ density functional theory?DFT?and time-dependent DFT?TD-DFT?including SOC to investigate the geometry,electronic structure,quantum efficiency,and emission rules for a series of iridium complexes.The relationship between the microscopic structure and the luminescence properties is elucidated.Several iridium complexes are designed by modification or alteraction of the chelating ligands,and their luminescent properties are predicted theoretically.Our theoretical research work could not only provide some reasonable explanation to some phenomena observed in the experiment but also give some theoretical guidance for development of high efficient and stable phosphorescent material.The main contents of this paper include the following two systems:1.The phosphorescent process of two heteroleptic??DMDPI?₂Ir?tftap?and?tftap?₂Ir?DMDPI??and one homoleptic?Ir?DMDPI?₃?Ir?III?complexes is theoretically investigated by density functional theory?DFT?and quadratic response?QR?time-dependent density functional theory?TDDFT?calculations including spin-orbit coupling?SOC?.Two or three triplet excited states are confirmed for three complexes,respectively.On the basis of the respective optimized triplet geometry,the emissive wavelength is determined by the?SCF-DFT method.Furthermore,the radiative rate constant?k_r?is also calculated corresponding to each triplet state.Combination of k_r and emissive energy,the emission rule is determined.It is found that complex?DMDPI?₂Ir?tftap?follows the dual emission scenarios,while complexes?tftap?₂Ir?DMDPI?and Ir?DMDPI?₃ obey the Kasha rule.The nonradiative rate constant(k_nr)is qualitatively evaluated by the construction of triplet potential surface via metal centered?³MC d-d?state.Finally,the sequence of quantum yield is compared by both k_r and k_nr.The quantum yield of homoleptic Ir?III?complex Ir?DMDPI?₃ is higher than that of heteroleptic Ir?III?complexes?DMDPI?₂Ir?tftap?and?tftap?₂Ir?DMDPI?.However,the emissive wavelength of Ir?DMDPI?₃ is in the red color region rather than blue color.2.The structure-property relationship is theoretically elucidated for four heteroleptic Ir?III?complexes with different ancillary ligand.Besides the ground state geometric parameters,the different triplet states are finally determined by both the density functional theory?DFT?and time-dependent DFT?TDDFT?methods.On the basis of the respective optimized triplet geometry,the emissive wavelength is determined by the?SCF-DFT method.For all the experimental reported complexes,the Kasha rule is broken.The emission from the non-Kasha state is possible.To determine the quantum yield,the radiative rate constant?k_r?is calculated by two different methods.Moreover,the items related with the k_r,such as,Zero-field splitting?ZFS?,transition dipole moments??S_n?,singlet-triplet splitting energies?E?S_n-T₁?,and spin-orbit coupling?SOC?matrix elements<T₁|H_SO|S_n>are also calculated to further confirm the k_r.The nonradiative rate constant(k_nr)is qualitatively evaluated by the popularity of the ³MC d-d state by the energy difference between ³MLCT/?-?^*and ³MC d-d state,the barrier height between ³MLCT/?-?^*and³MC d-d state,and the required energy from ³MC d-d state decay to the S₀ state.The barrier height between³MLCT/?-?^*and ³MC d-d state plays an important role to determine the k_nr.Finally,a novel Ir?III?complex is designed bearing 2-?2-trifluoromethyl?pyrimidine-pyridine?TPP?as primary ligand and amidinate as ancillary ligand.The phosphorescent emissive wavelength is obviously blue-shifted.Moreover,the quantum yield is comparable with that of complex 1.