Density Functional Theory Study Of Iridium (Ⅲ) Complexes For Opto-Electro Functional Materials

This dissertation focuses on the applications of density functional theory in the studies of optoelectronic iridium(III) complexes. Functional materials that are able to response to external stimuli such as light and electricity are of great importance in the construction of high-performace devices such as molecular switches and organic light-emitting diodes. In particular, iridium complexes with strong phosphorescence are promising dyes that could be used in the development of novel materials and devices. Combined with photochromic ligands, the iridium center is able to sensitize the ligand and extend the excitation band into visible region. The iridium complexes can also be doped into polymer layer of organic light-emitting diodes to make use of triplet excitons. So far, a great number of iridium complexes have been synthesized, studied and reported. Meanwhile, knowledges about the electronic structures of these complexes are desired to facilitate better understanding of their structure-property relationship.Density functional theory, as one of the most promising computational approaches in modern chemistry, has experienced decades of developments and progresses since the establishment of theoretical cornerstone in the 1960's. The formulae of the functionals have been continueously developed towards better performance and accuracy. Thanks to the fast development of computational algorithms and computer hardwares, theoretical chemists are now able to perform detailed studies on organic molecules and metal complexes of moderate sizes. In this dissertation, we aim to investigate i) iridium complexes containing photochromic ligands and ii) iridium complexes with applications in organic light-emitting diodes by using density functional theory, in order to gain insight into the electronic structures, excited state properties and phosphorescent parameters of these complexes.The dissertation consists of five chapters and is arranged as follows.A brief introduction to the research background of optoelectronic metal complexes is given in Chapter 1.An introduction to the history of theoretical chemistry, the theoretical foundation of density functional theory, spin-orbit coupling effect and quadratic response theory is given in Chapter 2. Some quantum chemistry softwares are also introduced.Chapter 3 and 4 focus on the density functional theory studies of photochromic iridium complexes containing acetylacetone ligand and 2-picolinic acid ligand, respectively. The photochromic 1,2-bis[2-methyl-5-(2-pyridyl)-3-thienyl]cyclopentene ligand is able to reversibly isomerize upon photo irradiation and can be used to realize photo-controllable phosphorescence of iridium complexes. Density functional theory calculations are able to reproduce the geometries of the molecules well, while time-dependent density functional theory calculations are able to provide singlet-singlet transition energies and the molecular orbitals involved in the transitions. By analyzing the metal character in the S0→T1 transition, the spin-orbit coupling strength can be qualitatively obtained; quadrtic response theory calculations are able to give the radiative rate constants of the phosphorescent process. Besides, unrestricted density functional theory calculations support the triplet isomerization pathway of the photochromic ligand.Chapter 5 and 6 focus on the density functional theory studies of phosphorescent iridium complexes with fluorine-substituted phenylpyridine ligands and phenylisoquinoline ligands, respectively. These complexes have potential applications in organic light-emitting diodes. Analysis of frontier molecular orbitals reveals that the HOMO is mainly localized on the iridium center and the phenyl part of the ligands and that the LUMO is mainly contributed by the ligand. Time-dependent density functional theory calculations are able to provide the absorption spectra and the molecular orbitals involved in electronic transitions. Quadrtic response theory calculations are able to give singlet-triplet transition dipole moment, and the predicted radiative rate constants are very close to experimental results. Besides, the spin-orbit coupling matrix elements can qualitatively reflect the magnitude of the non-radiative rate constant. Specially, introducing fluorine substitutions on phenylpyridine ligand or replacing the phenylpyridine ligand by phenylisoquinoline ligand will significantly affect the HOMO-LUMO energy gap, the phosphorescent emission wavelength and the radiative rate constant of the complex, and hence the luminescent efficiency in devices.The summary of theoretical studies on iridium complexes is presented in Chapter 7.