Theoretical Studies Of Blue Phosphorescent Metal Complexes And Related Molecular Design

Transition metal complexes have been widely used in organic light-emittingdiodes (OLEDs) due to their potential to harvest both electrogenerated singlet andtriplet excitons and realize100%internal quantum efficiency. Phosphorescent metalcomplexes have several advantages: strong spin-orbital coupling (SOC) effect, highthermal stability, short lifetime in excited states, high quantum efficiency, theemission colors are easily tuned. For the full-color display, phosphorescent metalcomplexes (PhMCs) with emitting in three prime colors of blue, green and red arehighly in demand. Compared with material in the green and red spectral regions,efficient and deep-blue phosphorescent materials are highly desirable and remain achallenge. The problems such as color purity, stability for long operational lifetimestill need to be improved. Thus it is important for us to explore novel efficientdeep-blue phosphorescent materials.To deeply understand the emitting nature of blue phosphorescent complexes, weuse theoretical methods to calculate the ground and excited states properties such asgeometrical and electronic structures, energy levels and gaps, orbital compositions,reorganization energies, d-orbital splittings, absorption and emission properties forqualitatively analyzing the phosphorescent efficiency. Our work mainly includes twoparts as follows:Part I: A series of blue phosphorescent platinum (II) complexes with tridentateligands have attracted our attention. The ligands have chemical similarities becauseimidazolyl and pyrazolyl are isomers with two nitrogen atoms in a five-member ring. We use DFT and TDDFT methods to calculate the Pt (II) complexes for illustratingthe effect of N-substitution on the photophysical properties and phosphorescentefficiency of the complexes. The most important factor to affect the quantumefficiency is the stability of excited states of complexes, expecially for the S1state.Through calculating the ground and excited states properties such as electronicstructures，d-orbital splitting and reorganization energies, we qualitatively analyzephosphorescent efficiency influence factors and the calculation data are consistentwith the experimental results.Part II: To achieve emission blue shift, phenyltrazole with higher LUMO energyhas been used to replace the phenylpyridine of FIrpic. Considered the effect ofN-substitution on the emission color and phosphorescent efficiency of the iridium (III)complexes, we design five Ir (III) complexes (some of which have been reportedexperimentally). The calculated results show:1. The energy gaps of the five Ir (III)complexes are wider than FIrpic, which can provide blue shift theoretically;2. TheHOMOs of the complexes are mainly localized on the Ir and main ligands, theLUMOs are mainly localized on main ligands, we can regulate photophysicalproperties of the complexes effectively with the methods of main ligand modifications;3. There are diffent effects of N-substitution on energy levels and gaps, orbitalcompositions, absorption and emission properties of the complexes;4. We calculatethe related parameters (ES1-T1and μs1) of radiavive rate and MLCT%, which have beencompared with experimental results to qualitatively analyze phosphorescent efficiencyof the complexes. In a word, we have given reasonable explanations of theexperimental phenomena and provided the novel methods for designing bluephosphorescent materials.