Theoretical Study And Rational Design Of Highly Phosphorescent Fuctionalized Ligand Of Platinum(â…¡) Complexes

A series of cyclometalated platinum (II) complexes with quadridentate ligand have been designed in this paper. As for these designed complexes, some have been modified by adding substituents on the functionalized ligands, based on which the electronic structures and the structure-property relations have been investigated as well as the emission mechanism influenced by substituents. On the other hand, the rest designed complexes are used to explore the effects that different dentate numbers of multidentate ligands would exert on molecular rigidities and emission properties involving the nonradiative decay process and efficient phosphorescence. The above investigations are based on the relevant electronic structures and photophysical properties. In this work, density functional theory and time dependent density functional theory are adopted to compute relevant geometrical optimizations and simulate corresponding properties, whilst the nonradiative decay process has been explored by the energy gap law. It has been turned out that introducing groups with strong electron-withdrawing ability and electron-donating ability to the functionalized ligands is favorable to enhance the phosphorescent efficiency and tune the emission colors. Moreover, complexes with tri-/tetradentate ligand possess better molecular rigidity and planarity. The calculated results obviously display that good rigidity and planarity contributes directly to weakening the non-radiation process while the phosphorescence would get improved. As a result, our studies about the structure-property relationship affected by modifications of functionalized ligands and different dentate numbers of multidentate ligands could provide theoretical guidances and principles to the experimental ones as platinum(II) complexes act as phosphorescent dopants in OLEDs. This research mainly contains two aspects as follows: 1. A Theoretical Study on Tuning Electronic Structures and Photophysical Properties of New Designed Platinum(â…¡) Complexes by Adding Substituents on Functionalized Ligands as Highly Efficient OLED emittersBy imitating FIrpic, seven new platinum(â…¡) complexes with pic (pic=picolinate) ligand have been designed to be guest materials by means of adding different substituents on functionalized ligands (ppy and fpy, ppy=phenylpyridyl-N,C and fpy=2-(9’,9’-diethyl-9H-fluorenyl)pyridyl-N,C). In order to reveal their molecular structures, photophysical properties and structure-property relationships with typical host materials, an in-depth theoretical investigation was elaborated via quantum chemical calculations. The electronic structures and photophysical properties of these complexes were investigated by density functional theory (DFT) and time-dependent density functional theory (TDDFT) using B3LYP function with LANL2DZ and6-31G*basis sets. It turns out that electronic structures and photophysical properties can be tuned by substituent modifications on functionalized ligands. This work highlights that a match between guest materials and host materials in typical OLED structures can be weighed by energy levels of HOMO and LUMO and adiabatic triplet energy of each complex. Also a combined analysis among electronic structures, host-guest match, reorganization energies (k) and triplet exciton generation fraction (XT) is favorable to explore triplet emitters with high phosphorescence efficiency in OLEDs, which is an interesting and creative aspect in this work. Thereinto, λ, reveals the ability of carrier transport and the balance between holes and electrons, whilst structural parameters and d-orbital splittings show that those complexes with strong electron-withdrawing and electron-donating groups were emissive. Consequently,3-7complexes can be better triplet eitters than FIrpic. Moreover, the emission colors could be predicted by0-0transition energy (E0-0) instead of the triplet vertical transition energy (Evert). Accordingly, complexes3,4and6would be efficiently phosphorescent materials with different predicted emission colors.2. The Influences of Different Dentate numbers of Multidentate Ligands towards Platinum(â…¡) compounds: A Theoretical Study on Molecular Rigidity and Photophysical PropertiesIn this part, we mainly investigated the influences of the coordinated bidentate ligand, tridentate ligand and tetradentate ligand of Pt(â…¡) complexes on their geometrical structures and relevant photophysical properties, theoretically and systematically. This is because there are quite few theoretical studies about the influences of different dentate numbers of multidentate ligands. All calculations are carried out by DFT and TDDFT with PBEO functional,"double-ξ" basis set and PCM model. Hereinto,"double-ξ" is equivalent to6-311G*and LANL2DZ, the former basis set is adopted for light atoms while the latter one is for Pt atom. Besides, CH2CI2solvent is used in the PCM model. According to the calculated structural parameters and the Huang-Rhys parameter (S) of T1;optâ†'S0, the nonradiative decay rates of these five complexes took a continuous drop as the dentate number of multidentate ligand increased. And we also have simulated some related photophysical properties systematically. The calculated results show that these absorption spectra not only took a remarkable red shift but only obtained stronger absorption intensities when multidentate ligand varying from bidentate ligand to tetradentate ligand. Then the emission color of each platinum complex is characterized by the0-0transition energy, by which the predicted emission colors are in the same light-emitting region—blue region (440-520nm). The highlight in this part lies to describe and analyze the strength of intersystem crossing (ISC) by the computed excited energies ratio Xn of Sn and Tm, and the energy differenceâ–³EST between singlet excited state and triplet singlet excited state. Both parameters are used to measure the spin orbital coupling (SOC). All these complexes have a value of Xn tending towards unity, sothat these complexes have ISC of100%probability. In the process of transitions, the more the MLCT excited states, the stronger the SOC is to be. Based on the ISC, IC and nonradiative rate, it can be safely concluded that tri-or tetra-dendate ligand are benifical for phosphorescence and planarities. In conclusion, as for complexes la, lb1and lc1-2in blue region, the increased dentate number of multidentate ligands not only gets their molecular rigidities enhanced but also weakens the nonradiative process. By comparison, phosphorescence with high efficiency would probably generate in those platinum(â…¡) complexes with tri-or tetra-ligand could generate phosp. With the addition of better planarities, Pt(â…¡) complexes linked to tri-or tetra-ligand could be given a top priotity to highly efficient phosphorescent emitters.