Theoretical Studies On Structures And Spectroscopic Properties Of Iridium Complexes Bearing Tridentate Ligand

Functional materials are referred to as superior characteristic materials which could use their specific properties to achieve high performances with material capabilities of acoustics, optics, mechanics, electricity, magnetism, thermology, chemistry, biology and so on. As an important part of functional materials, the luminescent materials have fascinated chemists, physicists and materials scientists during the past few decades. In particular, phosphorescent transition-metal complexes have been extensively investigated in detailed because of their promising potential applications in the fabrication of organic light emitting devices (OLEDs). Recently, the interest in phosphorescent Ir(III) complexes is growing rapidly. Served as a crucial role for OLEDs, Ir(III) phosphor with long lifetime, high luminescence efficiency and individual single-color is essential for their qualities. Significant progress has been made in experimental studies for Ir(III) complexes, but the insight into the microscopic mechanism of luminescence is only empirical with lacking of theoretical support, which leaves experiments undirected. Therefore, it is important for designing novel luminescent materials to establish electronic structures and luminescent mechanism of these complexes in theory.In recent years, the development of theoretical chemistry in algorithm casts new light on the research of electron structures. Structures and properties of ground state can be described theoretically with details in good agreement with the corresponding experimental measurements. However, we are still in the beginning of research on excited state, which is highly relevant to photoelectric phenomena involving rather complicated microscopic process of electron transitions. Furthermore, it is hard to obtain reliable information about excited state through experimental approach. As a result, theoretical chemists have been searching for a method which can not only predict excited state precisely, but also handle moderately large molecule system without consuming excess computing resources. Single excitation configuration interaction method (CIS) and time-dependent density functional theory (TDDFT) are commonly used for excited state researching. We usually optimize the geometries of excited state with CIS, and then calculate the vertical transition energy with TDDFT. As for systems in solution, we adopt certain kind of macroscopic continuum model of medium with appropriate properties (dielectric constant, coefficient of thermal expansion, etc) to evaluate the solvation effects. Those methods have already been applied on luminescent properties research of Ir(III) complexes successfully and provided fair explanations for experimental results.As presented in this paper, electronic structures and spectroscopic properties of series of mixed-ligand complexes, [Ir(Mebip)(bpy)Cl]2+ 1, [Ir(Mebip)(ppy)Cl]+ 2, [Ir(Mebip)(ppz)Cl]+ 3 and [Ir(Mebip)(ptz)Cl]+ 4 have been investigated in detail by theoretical calculation. Geometries in the ground and excited state for 1-4 were optimized by DFT and CIS, respectively. The optimized geometries of ground state are correlated well with the corresponding experimental values on the whole. On the basis of the optimized geometry structures, TDDFT along with polarized continuum model (PCM) were then performed to determine absorption and emission properties of 1-4 in CH3CN media at room temperature. With assistance of an analysis of frontier molecular orbitals, HOMOs of 2-4 mainly comprise Ir(III) ion and different chelating ligands, but that of 1 is dominated by Ir(III) ion and Mebip. LUMO and LUMO+1 of 1-4 predominantly reside on the Mebip ligand. The lowest-lying absorption of 1 at 460 nm is assigned to ILCT and MLCT transitions, while that of 2-4 occurs at 457, 448 and 418 nm, respectively, originating from a mixture of MLCT, LLCT and ILCT transitions. Emission discrepancies between calculation and experiment for 1 and 2 are 5 and 19 nm, respectively, and those results are quite satisfactory. Moreover, phosphorescent emissions of 1-4 display an obvious red shift of 2>3>4>1, which is in line withσdonor ability of bidentate ligands following an order of ppy(2)>ppz(3)>ptz(4)>bpy(1).