Optoelectronic Properties Of Pt(Ⅱ) Complexes With Phenol And Pyridine Group: Quantum Chemical Studies

The transition metal Pt(II) complexes with favorable photochemical and photophysical properties have attracted much attention because of their potential applications in the fields of organic light emitting devices (OLEDs), light-emitting electrochemical cells (LEECs) and electrogenerated chemiluminescence (ECL). The transition metals due to their large spin-orbit couplings give rise to rapid singlet-triplet (S-T) intersystem crossing, consequently, the complexes that contain these elements typically exhibit high triplet yields and efficient phosphorescence. It has been proved that these complexes as emitting materials in OLEDs can attain high internal quantum efficiency, 100% theoretically, in view of the utilization both singlet and triplet excitons. Therefore many experimental and theoretical investigations have devoted their efforts to the design of transition metal complexes with high phosphorescence efficiency and adjustable full-color emission.In this paper, the electronic structures and optoelectronic properties of Pt(II) complexes were investigated by quantum theoretical strudies. The results suggest new theoretical basis and direction for design of novel organic materials. Our work will focus on two aspects:1. We have investigated the optoelectronic properties of the PtQ2 and PtQ2Cl4. The calculated results indicate that: In comparison with PtQ2, the absorption and emission band of PtQ2Cl4 are red shift, since a narrowing of the HOMO-LUMO energy gap would occur upon the substitute of Cl atom. The PtQ2 and PtQ2Cl4 exhibit high mobilities in electron transport and may be used as electron transport materials. For electron transfer materials, another important character is the ability of electron injection, which is mainly determined by the barriers between the EAs of the materials and the work function. It is interesting to note that the EA follows the order PtQ2 < PtQ2Cl2, consistent with the absolute values of the LUMO energies. Thus PtQ2Cl2, which show significant amelioration in EA, can effectively promote the injection of the electron from cathode in devices and thus lower the turn-on voltage.2. We have investigated the optoelectronic properties of the Pt(II) complexes containing tridentate phenol dipyridine-type ligand, bidentate pp ligand and bpy ligand, respectively. The molecular structures of the ground and the lowest triplet states for platinum(II) complexes PtLX [L=6-(2-hydroxyphenyl)- 2,2'-bipyridine; X = Cl (1), F (5) and Br (6)] together with the free tridentate ligand L (4), Ptpp2 [pp=2-(2-hydroxyphenyl)pyridine] (2), and PtbpyCl2 (6) were optimized by the B3LYP and UB3LYP methods, respectively. The calculated results indicate that: (1) Upon excitation, the geometries of tridentate complexes are slightly changed relative to that of bidentate complexes 2 and 3. The phenol dipyridine-type tridentate complexes can be deemed as a rigid structure. The absorption spectra calculated by TD-DFT indicate that the absorption bands in 2 are mainly characterized as ILCT from phenol to pyridyl ring and these in 3 have MLCT/ LLCT characters. The absorptions of 1 are broad and containing the transitions in 2 and 3, and their absorption characters become complicated (MLCT, ILCT and LLCT). The band gaps in 1 are lower than 2 and 3, which is owing to extending the effect of conjugation by the addition of the pyridyl ring or phenol. And then the energy of low-lying MLCT is decreased accompanying with a high photoluminescence quantum yield. (2) In tridentate complexes, the central metal Pt participates in the distribution of FMOs leading to a MLCT transition, while it also acts as a bridge between the two ligands resulting in a LLCT transition upon excitation. Moreover, the central metal platinum restricts the geometry relaxation significantly by coordination. (3) Furthermore, we discuss the effect of halogen on the optoelectronic properties. With the strength of the ligand field increases, from Br, Cl to F the radiationless d-d transition energy increase, meanwhile the radiationless transition is decreased. The complex 5 coordinated by fluorine atom with a large K r and a decreased K nr could be a better phosphorescent material with enhanced phosphorescent emission intensity.