Quantum Theoretical Studies On The Excited State And Spectroscopic Properties Of Transition Metal Complexes: Os Complexes

As one of the most important parts of the design of functional materials, the design of optical materials has been focused on by physicist, chemist and material scientist all the time. In recent several decades, there an increasing interests of the properties of transition complexes in terms of optical materials. On one hand, the transition metal materials have good performance, such as long luminescent life and single color. On the other hand, organic electronic light-emitting film technology also has many distinguished advantages, such as low power, easily bending, quick response, broad visual angle, large area display, full emitting color and so on, and they are compatible with many kinds of standard technology and can be made of low-cost light-emitting devices. So the polymers exhibit strong life in the aspect of planar color display.So far, a number of osmium complexes have been synthesized with well-known structures. It was found that many osmium complexes exhibit intensive luminescence and can be applied in the optical materials; their long lifetime of phosphoresce makes them be used as photosensitizer, photochemical catalysis and optical sensor; their interaction with DNA leads to the application in the molecular pharmacy. 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 reliabilityand 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 theoretical forward looking and independence. The experimental studies show potential applications of transition metal complexes in many fields. Lack 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 osmium complexes in theory to rationalize and predict experimental phenomena are of practical significance.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 and excited state have been fully understood in theory and widely applied in chemistry. In the paper, we employed DFT theory, CIS method, and TD-DFT approach to study the geometry and electronic structures of both the ground and the excited states, the absorption and emission spectral properties of Osmium complexes. The solvent effects are seriously considered using the polarizable continuum model (PCM). We obtained the following main results:1. The geometries, electronic structures, and spectroscopic properties of a series of [Os(II)(CO)3(tfa)(L(R)2)] (L = O^O, R = H (1), CF3 (2), C6H5 (3), and C10H7 (4), O^O = hexafluoroacetylacetonate; L = O^N(5), O^N = quinolinolate; L = N^N(6), N^N = 3-(trifluoromethyl)-5-(2-pyridyl)pyrazole; tfa = trifluoroacetate) complexes have been investigated theoretically. The ground and excited states geometries were optimized at the B3LYP/LANL2DZ and CIS/LANL2DZ levels, respectively. The optimized geometry structural parameters were agree well with the corresponding experimental results. There are little differences in structures between the ground- and excited states, which is in agreement with the small Stokes-shift for observed in the experiments. As indicated in paper, the highest occupied molecular orbitals (HOMO) were dominantly localized on the Os atom, ctfa (abbv. of CO and tfa), and acac ligand for 1 and 2, acac ligand and X substituent for 3 and 4, while the lowest unoccupied molecular orbitals (LUMO) were mainly composed of acac ligand and X substituent. Under the time-dependent density functional theory (TDDFT) level with the polarized continuum model (PCM), the absorption and phosphorescence in CH2Cl2 media were calculated based on the optimized ground- and excited-states geometries, respectively. The calculated results show that the lowest energy absorptions at 317 (1), 342 (2), 377 (3), 420 nm (4) are attributed to a change ofππ*/MLCT mixing transition to pureππ* transition for 1-4, while the lowest-lying absorptions of complexes 5 and 6 are also assigned to theπ-π* transition mixed with some MLCT and LLCT characters. This indicates that the absorption and emission transition characters and wavelength could be altered by adjusting theπelectron-donating ability. In this work, the metal components of 2 to 3 in the frontier molecular orbital are reduced, but the emission yield increase from 2 (0.05) to 3 (0.13). It indicates that the phosphorescence quantum efficiency of Os(II) complexes with the metal composition of HOMO orbitals are not correlative, or it may be suitable to the lowest energy absorptions and the emissions of complexes mainly from the MLCT transition. To this problem, further study is imperative both experimentally and theoretically. At last, we hope that these theoretical studies will assist in the design of highly efficient phosphorescent materials.2. The red phosphorescent Osmium(II) complexes [Os(LR)2(PH3)2] (L = 2-pyridyltriazole (ptz): R = H (1a), CF3 (1b), t-Bu (1c)); L = 2-pyridylpyrazole (ppz): R = H (2a), CF3 (2b), t-Bu (2c)); L = 2-phenylpyridine (ppy): R = H (3a)) were explored using density functional theory (DFT) methods. The ground- and excited-state geometries of the complexes were optimized at the B3LYP/LANL2DZ and UB3LYP/LANL2DZ levels, respectively. The absorption and phosphorescence of the complexes in CH2Cl2 media were calculated based on the optimized ground- and excited-state geometries using time-dependent density functional theory (TDDFT) method with the polarized continuum model (PCM). The optimized geometry structural parameters of the complexes in the ground state agree well with the corresponding experimental values. The lower-lying unoccupied molecular orbitals of the complexes are dominantly localized on the L ligand, while the higher-lying occupied ones are composed of Os(II) atom and L ligand. The low-lying metal-to-ligand and intraligand charge transfer (MLCT/ILCT) transitions and high-lying ILCT transitions are red-shifted with the increase of theπ-donating ability of the L ligand and theπelectron-donating ability of R substituent. The calculation revealed that the phosphorescence originated from 3MLCT/3ILCT excited state. However, the complex 3a displayed different types of 3MLCT/3ILCT excited state compared with that of 1a-2c, and the different types of transition was also found in the absorption. In addition, we found that the phosphorescence quantum efficiency of Os(II) complexes are relative to the metal composition in the high-energy occupied molecular orbitals, it will be helpful to designing highly efficient phosphorescent materials.3. A series of Osmium(II) complexes [Os(trpy-R)2]2+ (trpy=2,2',6',2''-terpyridine and R = H (1), OH (2), and C6H5 (3)) have been investigated by the density functional (DF) and ab initio calculations. The structures of 1-3 in the ground and excited states were fully optimized at the B3LYP and CIS level, respectively, and their absorption and emission spectra in the acetonitrile solution were obtained using the TD-DFT (B3LYP) method associated with the PCM model. The calculations indicated that, for 1-3, the variation of the substituents on the terpyridine ligand only slightly changes their geometrical structures in the ground and excited states but leads to a sizable difference in the electronic structures. The results show that the low-lying MLCT/ILCT transitions (at 446 (1), 465 (2), and 499 nm (3)) are red-shifted according to the electron-donating ability of substituents on the terpyridine ligand, but blue-shift trend of the high-lying ILCT transitions (at 301 (1), 297 (2), and 272 nm (3)). It also reveals that the lowest energy emissions of 1-3 at 649 nm, 656 nm, and 676 nm have the character of mixing 3[π*(trpy)→d(Os)] and 3ππ* (3MLCT/3ILCT) transitions localized on the terpyridine ligand, which are identical to the transition properties of the lowest-energy absorptions.