| Because of its characteristics and function such as electricity, light, sound, magnetism and heat etc., functional material has been paid most attention to in the last five decades. The achievement in designing and developing functional material not only has greatly promoted the revolution of scientific technology in the late 20th century, but also will act as the foundation of the development of the advanced scientific technology in 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 physicist, chemist and material scientist all the time. Recently, a great deal of experimental work on the electronic absorption and emission of transition metal complexes has 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 metal or 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 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 theoretical forward looking and independence In the paper, combining the benefits of various quantum chemical computational methods, we systematically studied luminescent properties, ground- and excited-state electronic structures of transition metal complexes and obtained the following main results:1. The electronic structures and spectroscopic properties of the four tridentate cyclometalated Au(???) complexes [Au(C^N^C)C≡CPh] (1), [Au(N^C^C)C≡CPh] (2), [Au(N^N^C)C≡CPh]+ (3), and [Au(N^C^N)C≡CPh]+ (4) [HC^N^CH = 2,6 -diphenylpyridine, N^CH^CH = 3-(2-pyridyl)biphenyl, N^N^CH = 6-phenyl-2,2′- bipyridine, N^CH^N = 1,3-di(2-pyridyl)benzene] were calculated to explore their spectroscopic nature. The geometry structures of 1?4 in the ground and excited states were optimized under the density functional theory (DFT) and the single-excitation configuration interaction (CIS) level, respectively. As revealed from the calculations, the optimized bond lengths and bond angles for the complexes in the ground state are in general agreement with the corresponding experimental values. The composition of Au in frontier molecular orbital is very small, therefore the d-d excited state is unavailable and does not lead to nonradiative decay. The absorptions for 1?4 in CH3CN solution were also calculated, when the solvent is changed from CH2Cl2 to CH3CN, the characters of absorptions are completely unchanged. But the lowest-energy absorptions in CH3CN solution for 1?4 have slight change in energy, which are red-shifted 0.02, 0.02, 0.04, and 0.03 eV, respectively. With the variation in position and the number of pyridine, the electron-donating ability of N atoms increases in the order 1 < 2 < 4 < 3, and the HOMO?LUMO energy gap are lowered correspondingly. The lowest-energy absorptions and emissions wavelength of 1?4 are red-shifted in the order 1 < 2 < 4 < 3. The lowest-lying absorptions for 1?4 are all derived from the LLCT. The lowest-energy emissions for 2 and 3 come from the 3LLCT transition perturbed by some 3ILCT transition, whereas the emissions for 1 and 4 are attributed to the 3ILCT and 3LLCT/3LMCT, respectively.2. Electronic structures and spectroscopic properties of (L)Pt[(1,2-η2)–Ph–(C≡C)n–Ph] (n = 1, L = (PPh3)2 (1), n = 1, L = dppp (2), and n = 2, L = (PPh3)2 (3) ) are studied by the ab initio and DFT methods, respectively. The ground- and excited-state structures are optimized by the B3LYP and CIS methods, respectively. At the TD-DFT level with the PCM model, the absorption and emission spectra in solution are obtained. The calculation reveals that the lowest-energy absorptions are attributed to the MLCT/ILCT transitions, whereas the lowest-energy emissions are assigned as a 3MLCT/3ILCT character. The absorptions are not only attributed to Pt→PPh3 and Pt→acetylene but also Pt→Ph and we demonstrates that the MLCT transitions in these kinds of Pt0 complexes dominate the low-energy-region absorption spectra. the HOMO?LUMO energy gaps of 3 is smaller than 1 and 2. This is due to the strongπ-conjugation effects in 3. In comparison with 2, the PPh3 in 1 is a strong electron-acceptor and weaker electron–donor than the dppp in 2. So the lower lying absorption energies are in the order 1>2>3. With increasing theπ-conjugation of alkyne and electron-donating ability of the phosphane, the emission energy is lowered correspondingly.3. The electronic structures and spectroscopic properties of the three tridentate cyclometalated Platinum(??) complexes PtL1Cl [L1 =1,3-di (2-pyridyl) -5-methy l-benzene] (1), PtL2Cl [L2 =1-(2-pyridyl)-3-(1-pyrazolyl) -5-methyl-benzene] (2), and PtL3Cl [L3 = 1,3-Bis(1-pyrazolyl)-5-methyl-benzene] (3) were calculated to explore their spectroscopic nature. As revealed from the calculations, there are minor differences in bond angles for 1?3 and the replacement of pyridyl by pyrazolyl has little influence upon the ground state geometries. With the replacement of pyridyl by pyrazolyl, the electron-donating ability of N atoms decrease in the order 1 > 2 > 3, and the LUMO energy levels increase correspondingly. But the replacement has a little influenced on the HOMO energy, then the HOMO?LUMO energy gaps increase in the order 1 < 2 < 3. The lowest-energy absorptions are attributed to ILCT/MLCT/LLCT transitions, whereas the lowest-energy emissions are assigned as 3ILCT/3MLCT/3LLCT character.Two ligands 1-diphenylphosphinopyrene (1-PyP)(L1), 1,6–bis(diphenyl- phosphino)- pyrene (1,6-PyP) (L2) and their cyclometalated complexes [Pt(dppm)(1-PyP-H)]+ (1), [Pt2(dppm)2(1,6-PyP-H2)]2+ (dppm=bis(diphenyl- phosphino)methane (2), and [Pd(dppe)(1-PyP-H)]+ (dppe=bis(diphenyl- phosphino)ethane) (3) are investigated theoretically to explore their electronic structures and spectroscopic properties. As revealed from the calculations, the lowest-energy absorptions of 1 and 3 are attributed to the mixing LMCT/IL/LLCT transitions, while that of 2 is attributed to the IL transition. The lowest-energy phosphorescent emissions of the cyclometalated complexes are attributed to coming from the 3ILCT transitions. With the increase of the spin-orbit coupling (SOC) effect, the phosphorescence intensities and the emissions wavelength are correspondingly increased.
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