Theoretical Study On The Phosphorescent Properties Of Blue Iridium Complexes

In rencent years,Organic light-emitting diodes?OLEDs?have attracted much attention due to their great potential applications in the highly efficient soft large-area display and solid state lighting source.Compared with fluorescent materials which can only make use of the singlet excitons,phosphorescent materials can harvest both singlet and triplet excitons and lead to an interal theoretical quantum efficiency of 100%.Now days,as compared with the red and green materials,the lack of the blue phosphorescent material seriously restricts the development of OLEDs.Because of its complexity,it is still rare for the study of the phosphorescent process.Therefore,in this paper the phosphorescent process of a series of Iridium?III?based phosphorescent complexes are investigated by the density functional theory?DFT?and time-dependent density functional theory?TDDFT?.It is expected that an in-depth understanding of the structure-property relationship might open a high-efficiency way to explore the new complexes.The following four systems are studied systematically in this paper:1.The geometries,electronic structure,and absorption and photophysical spectra associated with the internal quantum yield of a series of Iridium?III?with 2-phenylpyridine/2-phenylisoquinoline as the primiary ligand and organosilanolate as the ancillary ligand are investigated by means of the DFT and TDDFT.Since the distribution of LUMO for 2b is dominantly distributed on the primary ligand,it is possible to vary the emissive wavelength by change of the LUMO energy level.Furthermore,five complexes are newly designed by introduction of the substitution groups on the phenyl rings of the 2b to explore the effect of substituted groups on the phosphorescent properties.The theorical results show that the complex introducing the strong electron donating group-OCH₃ is the red-emitting and other complexes are the blue-emitting.Meanwhile,the radiative rate constant?k_r?and the nonradiative rate constant(k_nr)for all complexes are also investigated.The results show that the complex introducing the electron donating group-C?CH₃?₃ would have a relative larger k_r and smaller k_nr leading to a higher quantum yield than complex 2b to be the potential blue-emitting phosphor.2.To provide a deeply understanding of the nature of the emissive origin as well as the radiative and nonradiative processes,DFT and TDDFT calucaltions have been performed on four amidinate/bis?pyridylphenyl?iridium?III?complexes.Besides geometries,electronic structure,absorption and phosphorescence spectra,the factors governing the radiative decay rate constants of the emissive state have been examined.Additionally,the potential energy profiles of the deactivation pathway via the triplet mental-centered states?³MC d-d?are also explored.To explore more efficient phosphors,three newly phosphors have been designed by incorporation-C?CH₃?₃,-CH₃ and phenyl on the bis?pyridylphenyl?ligand on the basis of complex 2.Three novel designed complexes present comparable or slightly higher quantum yield than complex 2,which indicates that the incorporation of the bulky substitutents on the primary ligand has a limited influence than on the amidinate group of the ancillary ligand.3.Four Iridium?III?complexes employing 2,4-difluorophenyl as the primary ligand and picolinic acid and picolinic acid N-oxide as the ancillary are investigated by means of DFT and TD-DFT to explore the influence of the ancillary ligand and the best combination of the substituents and the substituented position.Subsequently,other four newly complexes are theorically designed by modification of the substituents and the substituented position on the basis of complex 2,respectively.The quantum yields of the complexes are qualitatively evaluate by radiative and nonradiative decay process.The results show that the introduction of the phenyl group might improve the performance especially on the para-position of pyridine ring in dfpmpy ligand.4.The phosphorescent properties of four potential blue emitting cyclometalated?C^N?Ir?III?complexes?two experimental reported and two theoretical novel designed?are investigated by the DFT and TDDFT method to explore the cooperative effect of the electron-withdrawing substituent on the primary ligand associated with different ancillary ligands.The ground-state and triplet excited-state geometries are optimized at the PBE0/6-31G?d?-LANL2DZ level.Subsequently,the emission spectra are constructed by the?SCF-DFT calculations.The theoretical results indicate that emissions from the higher lying triplet state also have a contribution.The k_r is quantitatively determined.Finally,the quantum efficiency is semi-quantitatively compared by quantitatively evaluation of the k_r and qualitatively study of the k_nr.The theorical results show that the Kasha rule is broken for all the four investigated complexes.The k_nr plays an important role in determining the quantum yield.The incorporation of the electron-withdrawing group heptafluoropropyl?HFP?is not an insurance to improve the quantum yield.The substituents on the primary ligand should be combined with the suitable ancillary ligand to effectively enhance the quantum yield.