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

As functional materials, transition metal complexes materials have become a fascinating field in the world for their diverse potential applications in communication, information, and flat-panel displays, and their absorption and emission transitions 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 conducts the corresponding emission in the visible region, transition metal complexes are one of the most excellent candidates to serve as visible-region optical material. Theoretical studies on the excited state properties of transition metal complexes are the strong complement for experimental investigation. 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 and 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 directions in the future. The advantages of theoretical study are retrenchment, indicative of theoretical forward looking and independence.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 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, thus gaining 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 etc.). Recently, QM/MM (Quantum mechanical and molecular mechanical) method has been developed to account for the solvent effects.In this paper, combining the benefits of various quantum chemical computational methods and considering the solvent effects, we systematically studied on the ground- and excited-state conformations, excited state potential energy curves (PEC), absorption and emission spectra of several kind of platinum complexes. Through the exploration between structure and property, it can help to improve the performance of the functional materials. The following is the main results:1. Electronic structures and spectroscopic properties of a series of platinum(II) complexes based on C-linked asymmetrical diimine ligand have been studied by the time-dependent density functional theory (TD?DFT) calculations. The ground- and excited-state structures were optimized by the DFT and single-excitation configuration interaction (CIS) methods, respectively. The calculated structures and spectroscopic properties are in agreement with the corresponding experimental results. The results of the spectroscopic investigations revealed that the lowest-energy absorptions have 1,3MLCT/1,3ILCT mixing characters. When the electron-withdrawing groups (-CF3, 1a1; -C3F7, 1a2) are introduced into the pyrazolate fragment, the lowest-energy absorptions are blue-shifted compared to that without substituents on the pyrazolate fragment, while the opposite case is observed for the electron-donating groups (-Me, 1a3; -tBu, 1a4). The conjugation of the C-linked diimine ligand is enhanced through introducing more N heteroatoms into this segment. As a results of MO energy change, the lowest-energy absorptions are blue-shifted in the order 1 < 1b1 < 1b2. With the replacement of pyridyl by pyrazine, the HOMO energy of 1b3 is comparable to 1, but the LUMO energy is decreased by 0.8 eV, and the lowest-energy absorptions are red-shifted to 2.36 eV. Otherwise, the phosphorescent emissions of these complexes have the 3MLCT/3ILCT character, and should be originated from the lowest-energy absorptions. When the pyrazolate fragment is replaced by the indazole group(1a6), the HOMO and LUMO orbitals of the pyridyl-indazolate ligand platinum(II) complexes have obviousπandπ* orbital characters. Therefore, there is no evident MLCT character in the lowest energy absorption and emission.2. We report a combinational DFT and TD-DFT study of the electronic and optical properties of several tridentate cyclometalated mononuclear [Pt(C^N^N)(C≡CR)] (1-3), [Pt(C^N^N)(C≡CRC≡CH)] (4), and dinuclear [Pt(C^N^N)(C≡CRC≡C)Pt(C^N^N)] (5 (C2 symmetry) and 5′(Cs symmetry)) platinum(II) complexes withσ-acetylide ligand bearing fluorene substituents, where HC^N^N = 6-aryl-2,2'-bipyridine, R = fluorene-2,7-diyl 1, 4, 5 and 5′, R = 9,9-dimethylfluorene-2,7-diyl 2, R = 9,9-diethylfluorene-2,7-diyl 3. The structural and electronic properties of the ground- and lowest triplet state and the EA and IP values of the complexes are discussed. It is found that all of the lowest-lying absorptions are categorized as the LLCT combined with the MLCT transitions. The oscillator strengths of the lowest energy absorptions get a remarkable enhancement for the dinuclear complexes 5 and 5′compared to 1-4 due to the increase of electronic delocalization on the more planar molecular geometry. In general, the phosphorescent emissions of these complexes in CH2Cl2 are the reverse process of their lowest energy absorption transitions, except that of 4 is assigned as 3[π* ?π]/3MLCT transition because of the strengthened electronic localization effect and the interaction with the solvent in the lowest triplet state. In addition, these complexes hold promise as a new kind of nonlinear optical material owing to their large static first hyperpolarizabilities (β0). Theβ0 value has increased in the dinuclear complexes in contrast to those of the mononuclear ones owing to their larger transition moment and smaller transition energy.3. We present a full density functional theory (DFT) and time-dependent density theory (TDDFT) investigation of the geometry, electronic structures, and optical properties of N-heterocyclic platinum(II) tetracarbene complexes aiming at providing a definitive characterization of the photophysical properties of this system. Density functional analysis show that the absorption spectra and emission wavelengths of [Pt(meim)2]2+, [Pt(meim)(cyim)]2+, and [Pt(cyim)2]2+ are analogical, and the reason of a significantly decrease in quantum efficiency for [Pt(cyim)2]2+ is which reaches the nonradiative deactivation dd excited state with lower thermal barrier in contrast with [Pt(meim)2]2+.The results presented here demonstrate that photophysical properties, in particular excited-state lifetimes, can be strongly affected by ligands. Access to such information is fundamental for a rational design of new metal complexes with tunable photochemical features. Combining photophysical measurements with emerging capabilities to investigate excited-state potential energy surfaces using quantum chemical calculations is seen to offer in-depth information about such effects. Only a correct description of orbital energies and shapes, and a full comprehension of the role played by different excited states in light-induced electronic transitions, can guide the introduction of advantageous structural changes on photoactivable metal species.