Quantum Theoretical Studies On The Excited State And Spectroscopic Properties Of Transition Metal Complexes: D⁶, D⁸, And D¹⁰ Complexes

Functional materials have excellent characteristics and function such as electricity, light, sound, magnetism, chemistry, heat, and biomedicine, their special physical, chemical, and biological effect can transform each other successfully. They have been used in various functional devices fabrication for various types of high-tech fields such as electronic, laser, communication, energy sources, and bioengineering. The achievement in designing and developing functional material not only has greatly promoted the revolution of scientific technology last century, but also will be the foundation of the development of the advanced scientific technology in future. As one of the most important parts of the functional materials, the optical material has also been focused on by physicists, chemists and material scientists all the time. The phosphorescent materials have potential application as the emitting layer of OLED (organic light emitting devices) because of their high quantum efficiency, and they attract much more attention of the material researchers. Recently, the phosphorescence materials research has a rapid development, the luminescent properties of the transition metal complexes such as Iridium, Ruthenium, Platinum, Gold, Osmium, Rhenium etc. have been investigated extensively. These transition metal complexes has excellent luminescent properties, thus they are potential luminescent materials. 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 andπorbitals of ligand. Because such an electronic absorption in the ultraviolet region usually and the corresponding emission in the visible region, transition metal complexes are one of the most excellent candidates to serve as visible-region optical material.Recently, the electronic excited state properties of molecules have attracted much more attentions. The electronic excited states of molecules are the state in which the electrons are excited to the orbitals with higher energy by absorbing energy, but have not ionized. 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 to obtain reliable information about the excited states of molecules on experiment. 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), UDFT (unrestricted density functional theory) or UMP2 (unrestricted second-order M?ller-Plesset perturbation) and TD-DFT (time-dependent DFT) methods have been widely used to treat the electronic excited states of large molecular systems. We optimized the geometry structure of the excited state by using CIS or UDFT/UMP2 methods, and calculated the vertical transition energy by TD-DFT method.It has been established that the solvents have some effect to the luminescence of complexes. Many theoretical methods were employed to treat properties of complexes in solution last century. 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 such as dielectric constant, thermal expansion coefficient etc.. Recently, QM/MM (Quantum mechanical and molecular mechanical) method has been developed to account for the solvent effects.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 method to investigate a kind or several kinds of complexes with simple coordination geometry. So far, a number of Iridium complexes have been synthesized, and their X-ray geometry structures and spectral properties have been investigated in detail. It was found that many Iridium complexes have high stability, emission color tenability, and strong spin-orbital coupling, as a result, Iridium complexes exhibit intensive luminescence and can be applied in the optical materials; their long lifetime of phosphoresce makes them be used as photo-sensitizer, photochemical catalysis and optical sensor; their interaction with DNA leads to the application in the molecular pharmacy. The abundant experimental studies show potential applications of the Iridium complexes in many fields. Lack of theoretical support, the insight into the luminescent process and microscopic mechanism is only empirical, which results in no exact direction on experiment. Thus, systematic studies on the Iridium complexes in theory to rationalize and predict experimental phenomena are of practical significance.The rapid development of the advanced science and technique greatly promotes the progress 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 strong support and supplement the known experimental results; Furthermore, to predict the potential properties of the complexes which has not been synthesized on experiment, theoretical studies have demonstrated the independence and forward-looking characteristic. Electronic absorption and emission processes of the transition metal complexes are extremely complicate micro-process, involving several the basic problem of quantum theory, such as the electronic structure properties in ground and excited state, interaction between transition metals, relativistic effect, as a result, the theoretical studies on the luminescent properties of the transition metal complexes not only have important guiding significance on exploration and design of the organometallics new optical materials, but also itself is a very important theoretical issues. In this paper, combining the various theoretical approaches and the computational experience, considering the solvent effects, adopting several quantum chemistry methods such as DFT, MP2, CIS, and TD-DFT, we investigated many properties of a series of Iridium complexes with d6 and d8 electronic structures, such as the geometry structures in ground and excited states, electronic structures, absorption and emission spectra properties, and obtain the following main results。1. The geometries of Ir(C^N)2L, [trans-Ir(C^N)2(PH3)2]+ and Ir(Mebib)(ppy)X in the ground and excited states were optimized by B3LYP and CIS method, respectively. The calculation results showed that the lowest-lying absorption and phosphorescence have MLCT (metal-to-ligand charge transfer), ILCT (intra-ligand charge transfer) transition properties, auxiliary ligand L(acac) didn't participate in the transition processes. The lowest-lying absorption and emission can be red-shifted by increasing theπconjugation effect of C^N,L(CN,NCS,NCO) and X ligand can effectively affect the lowest-lying absorption and emission. Moreover, the large metal compositions in the HOMO, namely, the large component of MLCT, can bring the high quantum efficiency.2. The geometries, electronic structures, and spectroscopic properties of Ir(ppy)2(N^N)+ (N^N = 2-phenyl-1H-imidazo[4,5-f][1,10]phenanthroline, ppy = 2?phenylpyridine) was investigated by B3LYP, UB3LYP, and TD-DFT methods, moreover, the luminescent mechanism of Ir(ppy)2(N^N)+ as sensor for F-,CF3COOH,CH3COOˉhas been revealed. The calculated results showed that the adding of F-,CF3COOH,CH3COO- can not affect the composition of HOMO, but LUMO. The phosphorescence was red shifted by adding CF3COOH, but quenched by adding F-,CH3COO- anions. 3MLCT excited state properties play an important role in the phosphorescence generation process. Among F-, CF3COOH, CH3COO-, the strongest interaction exists between F- and H of N5-H bond, as a result, Ir(ppy)2(N^N)+ can be the sensor of F-.3. The geometries, metal-metal (Ir-Au) attractive interaction, electronic structures, absorptions, and phosphorescence of three d8-d10 Ir(I)-Au(I) complexes [Ir(CO)ClAu(μ-dpm)2]- (1), [Ir(CNCH3)2Au(μ-dpm)2]2- (2), and [Ir(CNCH3)3Au(μ-dpm)2]2- (3) [dpm = bis(diphosphino)methane] were investigated by MP2, UMP2, and TD-DFT methods, and the relations between the Au-Ir interaction and the absorption/emission spectra were revealed. The calculated results showed that the interaction between Ir and Au really do exist. The calculated results showed that the phosphorescence of complexes 1-3 come fromσ[pz(Ir)+p(PH2)]→σ*[d(Ir)],σ[pz(Ir/Au)+p(PH2)]→σ*[d(Ir/Au)],π[pz(Au)+pz(PH2)]→σ[d(Ir/Au)] charge transfer, respectively. In the ground state, the absorption can be red shifted by increasing the Ir-Au distance, in contrast, in the excited state, the phosphorescence can be blue shifted by increasing the Ir-Au distance in the triplet excited state.4. The ground and excited states properties of five iridium fac-Ir(pmb)3 (1), mer-Ir(pmb)3 (2), (pmb)2Ir(acac) (3), mer-Ir(pypi)3 (4), and fac-Ir(pypi)3 (5) were investigated by PBE0 and UPBE0 methods, respectively. The calculated results show that the lowest-lying absorption bands of 1-3 have MLcarbeneCT/ILphenyl→carbeneCT transition characters, while those of 4 and 5 are attributed to MLcarbeneCT/ILcarbeneCT transitions. The calculated phosphorescence of 1 and 2 originate from 3MLcarbeneCT/3ILphenyl←carbeneCT excited states, but those of 4 and 5 come from 3MLcarbeneCT/3ILcarbeneCT excited states. The phenyl and the carbene groups in the C^C: ligand act as independent parts in the excitation. The effective strengthening of theπ-conjugation effect of C^C: ligand can be achieved at 3′-4′position near the Ccarbene atoms. We predicate that (pmb)2Ir(acac) should has low quantum efficiency, so that it is not suitable for OLED fabrication.5. The ground and excited states properties including IPs, EAs, HEP, EEP, H-L gap, absorption and emission were investigated theoretically, and the properties of polymers were obtained by extrapolation method. The calculated results shows that D and DP-polymer have similar H-L gap, they are suitable for fabricating electron transfer materials, moreover, the electron transfer ability of DP-polymer is stronger than P-polymer. The absorption and emission spectra are red-shifted with the increase of the conjugation chain. The additional absorption band at 289nm of PCPP is assigned toπ→π* transition occurred on C=C bond. The calculated results shows that PCPP is suitable for blue LED fabrication.