| To date, cyclometalated Ir(III) complexes are still the subject of large amounts of interest due to their unique photophysical and photochemical properties, and potential applications in organic light-emitting devices(OLEDs). Several strategies have been adopted to enhance their quantum effciency(ΦPL) in OLEDs. Thus, the selection of a proper metal-based phosphorescent material is of great pivotal to obtain highly efficient electroluminescence for OLEDs. In this thesis, density functional theory(DFT) and time-depended density functional theory(TD-DFT) with SOC were employed to elucidate the structural-property relationships for the studied complexes. We hope that this work would provide useful information for the future design and synthesis of cyclometalated Ir(III) complexes. This article mainly contains two parts:1. The synthesized guanidinate-based iridium(III) complex 1 and two designed derivatives(2 and 3) were investigated to determine the influences of different cyclometalated ligands on photophysical properties. Except the conventional discussions on geometric relaxations, absorption and emission properties, many relevant parameters, including SOC matrix elements, zero-field-splitting parameters, radiative rate constants(kr) and so on were quantitatively evaluated. The results reveal that the replacement of the pyridine ring in the 2-phenylpyridine ligand with different diazole rings can not only enlarge the frontier molecular orbital energy gaps, resulting in a blue-shift of the absorption spectra for 2 and 3, but also enhance the absorption intensity of 3 in the lower-energy region. Furthermore, it is intriguing to note that the ΦPL of 3 is significantly higher than that of 1. This can be explained by its large SOC value < T1|HSO|Sn >(n=3-4) and large transition electric dipole moment(μS3), which could significantly contribute to a larger kr. Besides, compared with 1, the higher emitting energy(ET1) and smaller < S0|HSO|T1 >2 value for 3 may lead to a smaller non-radiative decay rate. Additionally, the detailed results also indicate that compared to 1 with pyridine ring, 3 with imidazole ring performs a better hole injection ability. Therefore, the designed complex 3 can be expected as a promising candidate for highly efficient guanidinate-based phosphorescence emitter for OLEDs applications.2. A series of heteroleptic Ir(III) complexes(1-4) employing azadipyrromethene and closely related dipyrromethene derivatives as N^N ancillary ligands were studied, in an effort to explore the influence of substitution in N^N ancillary ligand on electronic structures and phosphorescent properties. The results reveal that the non-emissive behavior of 1 is associated with its large structure distortion from singlet ground(S0) to the lowest triplet(T1) states as well as the small Topt1-Sopt0energy gap. Upon changing the N^N ancillary ligands, the geometry distortion of 2-4 are obviously smaller in comparison with 1. Besides, the frontier molecular orbital energy gaps are enlarged successively from 1 to 4, resulting in a blue-shift of the absorption spectra, and the absorption intensities are also obviously affected. Additionally, the emission spectra undergoes a blue shift with different degree from 2 to 4, while the nature of them is dominated by ancillary ligand with little contribution from the Ir(III) center. Importantly, further analyses on ΦPL of these complexes also indicate a vital role of N^N ancillary ligands. It is intriguing to note that the designed complex 4 without pendant phenyl rings substituent in the ancillary ligand, possesses efficient indirect spin-orbital coupling route, larger μS3, higher Topt1-Sopt0energy gap and smaller S3-T1 splitting energy(ΔE(S3-T1)), which could render it a higher ΦPL compared to other complexes. |
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