Theoretical Investigation And Appropriate Design Of Several Phosphorescent Pt~Ⅱ、Pd~Ⅱ Complexes

A series of cyclometalated platinum(II), palladium(II) complexes have been designed in this paper. In order to regulate the emission wavelength and phosphorescence quantum yield of the complexes, we replace the functional ligands or introduce different substituents on the ligands. In this work, some of the functional ligands and substituents have been explored in previous literature. However the others have not been attention. For example, ppz ligand(ppz = 5-(2- pyridyl)- pyrazole) act as functional ligand has been widely used in iridium(III) complexes in the published literature, but rarely used in platinum(II) complexes. As another example,triarylboron as an electron-withdrawing group has been widely used in transition metal complexes, we will apply it in this research to design highly efficient phosphorescent complexes. Herein, density functional theory and time dependent density functional theory are adopted to explore electron structures, photophysical properties, radiative decay processes and noradiative decay processes for all the complexes. zero-field splitting(ZFS), radiative rate constant( κr), and radiative lifetime(τem), absorption and emission wavelength and other important parameters are calculated. The calculated results indicate that, replacing with a strong field functional ligand and adding certain molecular configuration of the electron-withdrawing or electron-donating groups on the functionalized ligands are favorable to enhance the emissive efficiency. Therefore, this work about structure-property relationship affected by modifying functional ligands could provide very useful information for designing and synthesizing new platinum(II), palladium(II) phosphorescent dopant in OLEDs. This research mainly contains three aspects as follows:1. Tuning the electronic and photophysical properties of platinum(II) complexes through ancillary ligand substitution: a theoretical studyIn this work, six Pt(II) complexes have been studied via Density functional theory(DFT) / time-dependent DFT caculations to explore the influence of different ancillary ligand on electron structures, photophysical properties and radiative decay processes. Moreover, the self-consistent spin-orbit coupling TDDFT(SOC-TDDFT)was used to calculate zero-field splitting(ZFS), radiative rate and radiative lifetime to unveil the radiative deactivation processes for these complexes. The results indicate that [Pt(ppy)(ppz)](ppy = 2-phenylpyridine and ppz = 5-(2-pyridyl)-pyrazole) has a higher radiative decay rate constant and a smaller nonradiative decayrate constant than that of [Pt(ppy)(acac)](acac= acetylacetonate). In other words, the emission efficiency can be promoted by introducing ppz ligands. Furthermore, compared to other complexes, complex 5 with dimesityboron added on the 3′-position of the pyrazole ring in [Pt(ppy)(ppz)], shows great potential to serve as an efficient blue-green-light emitter in OLED.2. Theoretical study on the effect of several aromatic substituents on the photophysical properties of Pd II [O^N^C^N] complexesThe electron structures, spectral characteristics and radiative/nonradiative decayed efficiency of Pd II(butyl)[O^N^C^N](1)、Pd II(phenyl)[O^N^C^N](2)、Pd II( diphenyl)[O^N^C^N](3)、 Pd II( naphthyl)[O^N^C^N](4)(O^N^C^N=2-(4-(3,5-di-tert-butylphenyl)-6-(3-(pyridin-2-l)phenyl)pyridin-2-yl)phenolate) was elaborated by DFT/TDDFT calculations. Compared to the original complex, complexes obtained by replacing butyl with naphthyl, diphenyl and phenyl have better radiative decay process as well as poor nonradiative decay process. All of these explorations reveal that, different ligands added on the R-position of the Pd [O^N^C^N] bring a huge change in phosphorescence quantum yield. With the sequence of phenyl,biphenyl and naphthyl, their electron-donating ability has successively improved, but emission performance has not strengthened correspondly. The result illustrate that, an appropriate substituent group not only has good electron-withdrawing or electron-donating ability, but also has a suitable molecular structure. There into,complex 2 with phenyl shows best phosphorescence properties and has great potential to serve as a good candidate of organic phosphorescent emission materials.3. High-efficiency phosphorescent Platinum(II) complexes based on Pt [O^N^C^N]: a theoretical studyIn this work, two series of heteroleptic platinum(II) complexes of type Pt II[O^N^C^N](O^N^C^N= 2-(4-(3,5-di-tert-butylphenyl)-6-(3-(pyridin-2-l)phenyl)pyridin-2-yl)phenolate) were prepared to explore the photophysical properties changes brought about by replacing different substituents in different positions(R, R′, R″) of the complexes. The first series in accordance with the previous designed Pd II[O^N^C^N] complex successively got complexes 2-4 obtained by substituting n-butyl for phenyl, biphenyl, naphthyl. By density functional theory and time-dependent density functional theory calculations, we can find that the position of phenyl in R can bring the best light emission performance, which is similar to Pd complexes. Such findings can provide very useful information for designing [O^N^C^N] ring metal complexes in the future. On the basis of the first series of complexes, diaryl boron group is introduced in R’and R"position to obtain complexes 5-7. By evaluating the spectral characteristics and the efficiency of radiation and non-radiative process of the complex, it is found that complex 5 has a better radiation efficiency and weaker non-radiation efficiency. Consequently, it is considered to be the very promising phosphorescent emissive material.