The Quantum Theoretical Studies On The Electronic Structures And Spectroscopic Properties Of Osmium Complexes

Major: Physical chemistryBecause of its characteristics and function such as electricity, light, sound, magnetism and, heat, etc., functional material has been paid most attention in the last five decades. The achievements in designing and developing functional material not only have greatly promoted the revolution of scientific technology in the late 20th century, but also will act as the foundation for the development of the advanced scientific technology in the future. As one of the most important parts of the design of functional materials, the design of optical materials has also been focused on by physicists, chemists and material scientists all the time. Recently, a great deal of experimental work on the electronic absorption and emission of transition metal complexes have been performed to seek inorganic optical material that exhibits intensive luminescence in the visible region. The absorption and emission of transition metal complexes usually are related to the charge transfer between d orbitals of metal and s/p orbitals of ligand. Because such an electronic absorption in the ultraviolet region usually conduct the corresponding emission in the visible region, transition metal complexes are one of the most excellent candidates to serve as visible-region optical material.The electronic absorption and emission of molecules are complicated microscopic processes between the ground- and excited-state transitions. With the development of quantum chemistry and computational technique, especially the successful application of density functional method, the electronic structures and properties of molecules in the ground state have been fully understood in theory and widely applied in chemistry. However, the studies on the excited-state properties still remain infant. Excited states themselves are related to many photoelectric phenomena in the modern chemistry and physics. Therefore, quantum chemistry related to the electronic excited states should be one of the most major research fields in the future. Presently, it is a challenge to apply quantum theory to investigate luminescent properties of transition metal complexes, but such a kind of research is of theoretical and practical significance.Transition metal atoms have various electronic structures and bonding characters and many ligands have been synthesized in experiments, resulting in the occurrence of thousands of transition metal complexes. It is very difficult to fully understand the properties of such abundant complexes. So, it is an ideal start point to investigate a class or several classes of complexes with simple coordination geometry. So far, a number of Os(II) complexes have been synthesized and studies in their crystal structures and photo-physical properties have been performed. It was found that many Os(II) complexes exhibit intensive luminescence and can be applied in the optical materials; their long phosphorescence lifetime bestow these complexes good photo-redox character which makes them being used as photosensitizers, photochemical catalysts and optical sensor; their interaction with DNA leads to the application in the molecular pharmacy. The experimental studies show potential applications of transition metal complexes in many fields. However, lacking of theoretical support, insight into the luminescent process and microscopic mechanism is only empirical, which results in experimental deviation from reality. Thus, systematic studies on the Os(II) complexes in theory to rationalize and predict experimental phenomena are of practical significance. The electronic excited states of molecules have higher energy and unsteady characteristics, which easily emit the energy to recur the steady ground state in a short time. So it is difficult for experiment to obtain reliable information about the excited states of molecules. Theoretical chemists attempt various electronic structure theories of excited states to seek the method that can accurately predict excited-state electronic structures and be applied in the calculations of relatively large molecules without consuming excess computational resources. So far, CIS(Single excitation configuration interaction), unrestricted DFT and TD-DFT (Time-dependent density functional theory) methods have been widely used to treat the electronic excited states of large molecular systems.It has been established that the solvents affect the luminescence of complexes. Many theoretical methods were employed to treat properties of complexes in solution. The first strategy puts the attention on the microscopic interactions of the solute molecule with a limited number of solvent molecules; the whole system (the"supermolecule") is studied with quantum mechanical methods usually employed for single molecules, and the effects of specific solute-solvent interactions are brought in evidence. An increasing number of solvent molecules can be added to this model to obtain supplementary (and detailed) information about solvent effects. The second strategy tries to directly introduce statistically averaged information on the solvent effect by replacing the microscopic description of the solvent with a macroscopic continuum medium with suitable properties (dielectric constant, thermal expansion coefficient, and etc.). Recently, QM/MM (Quantum mechanical and molecular mechanical) method has been developed to account for the solvent effects.The advanced technique applied in experiments greatly promotes the development of modern computational chemistry. On one hand, the comparison between calculation and experiment can test the reliability and accuracy of electronic structure theory, showing the dependence of theory on experiment; on the other hand, to develop the electronic structure theory is to support and/or supplement the known experimental results, and further to predict the potential results, indicative of the forward looking and independence of the theoretical study. In the paper, combining the advantages of various quantum chemical computational methods and considering the solvent effects, we systematically studied luminescent properties, ground- and excited-state electronic structures of d6 complexes and obtained the following main results:1. The MP2 (Second-order M?ller-Plesset perturbation) and CIS methods were employed to optimize the ground- and excited-state structures of [Os(N^N)(CO)2I2] (N^N=2,2′-bipyridine (bpy) (1), 4,4′-di-tert-butyl-2,2′-bipyridine (dbubpy) (2), and 4,4′-dichlorine-2,2′-bipyridine (dclbpy) (3), respectively. The calculated results reveal that the low-energy absorptions in the UV and visible range and the emissions were all assigned as the electronical transition between the ground state and the combined MLCT (metal-to-ligand charge transfer) and XLCT(halide-to-ligand charge transfer) excited states. When the electron-donating substituents are introduced into the bipyridine ligand, the lowest-lying absorption in visible range and emission are blue-shifted in the order of Cl(3) < H(1) < t-Bu(2).2. The geometrical structures of a series of Os(II) diimine complexes [Os(L)2(CN)2(phen)] (phen=1,10-phenanthroline; L=phosphine (1), DMSO (2)) and [Os(PH3)2(phen)Br2] (3) in the ground state and the lowest-energy triplet excited state were optimized by the restricted and unrestricted B3LYP/UB3LYP methods. The absorption and emission properties and transition characters in dichloromethane solution were predicted by TD-DFT associated with the PCM solvent effect model, and the transition characters were assigned. The effects of theπ-acid andπ-alkali ligands have on the phosphorescent spectroscopic properties and the reason were explored by theoretical chemistry method. The lowest-lying absorption and emission are red-shifted with the decrease of theπbackbonds formed between theπacid ligand and Os(II), and the emission wavelengths are in the order of 3>1>2. Quantum chemistry calculations further explained the mechanism why the Os-Br bond is prone to break up in complex 3 and have the potential to undergo reactions. The computed phosphorescent emission results of 1-3 in different solvent indicated that the complexes have the solvatochromic effect and the solvent will affect the quantum efficiencies of them.3. To explore the spectroscopic properties of pyridyl triazole Os(II) complexes and how the substituent affect the spectroscopic properties for the [Os(ptz)2L2] (L=PH3; ptzH=(2-pyridyl)-1,2,4-triazole)(1); [Os(bptz)2L2] (bptzH= bptzH= 3-tert-butyl-5-(2-pyridyl)-1,2,4-triazole(2);fptzH=3-(trifluoreomethyl)-5-(2-pyridyl)-1,2,4-triazole(3); fbtzH= 3-(trifluoreomethyl)-5-(4-tert-butyl-2-pyridyl)-1,2,4-triazole (4)), DFT method at B3LYP level was used to optimizing the geometrical structures in the ground- and excited-state. The absorption and emission properties in the dichloromethane solution were predicted at the TD-DFT/B3LYP level associated with the PCM solvent effect model, and the transitions characters were assigned. Important correlations between substituent effect and the lowest energy absorption in visible range, emission spectra and quantum yield have been obtained by comparing and analyzing the calculated results, and the conclusions are summed up: (1). The phosphorescent emission spectrum of 1-3 red shift, when the electron-donating substituents are introduced into the triazole ligand. (2). Due to the different contribution of ligand to the HOMO and LUMO orbitals, when the substituents are introduced into the ligand, the different effects of substituent to the HOMO and LUMO exist. (3). When the same substituent(tert-butyl) is introduced into different ligands, the destabilizing effect is the same.(4). Based on the previous (2) and (3) conclusions, when the destabilizing effect of substituent pushes up the energy level of HOMO more than the LUMO, the energy gap of HOMO-LUMO decreases, the lowest absorption and phosphorescent emission of complexes red-shift (2 compared to 1); when the substituent pushes up the energy level of LUMO more than the HOMO, the energy gap of HOMO-LUMO increases, the lowest absorption and phosphorescent emission of complexes blue-shift (4 compared to 3).(5). The phosphorescent quantum yieldΦof complexes 1-4 is affected by the substituent obviously.(6). If the ligands of the complexes localated on the HOMO or LUMO more evidently, when the introduction of draw- or push- electron like t-Bu or CF3 on the ligands, the phosphorescent emission wavelengths of the complexes will be fine tuned (red shift or blue shift).