| 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. 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 kind or several kinds of complexes with simple coordination geometry. So far, a number of ruthenium complexes have been synthesized with well-known structures. It was found that many ruthenium 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 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 ruthenium complexes in theory to rationalize and predict experimental phenomena are of practical significance.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 ruthenium complexes. The solvent effects are seriously considered using the conductor-like polarizable continuum model (CPCM). We obtained the following main results:1. A series of ruthenium(II) complexes [Ru(tcterpy)(NCS)3]4- (0H), [Ru(Htcterpy)(NCS)3]3-(1H), [Ru(H2tcterpy)(NCS)3]2-(2H) and [Ru(H3tcterpy)(NCS)3]-(3H)(tcterpy=4,4′,4"-tricarboxy-2,2′:6′,2"-terpyridine) are investigated theoretically to explore their electronic structures and spectroscopic properties. The geometry structures of the complexes in the ground- and excited-state are optimized by the DFT and CIS methods, respectively. The absorption and emission spectra in gas phase and solutions (ethanol and water) are predicted at the TD-DFT/B3LYP level. The analysis of the electronic structures for 0H-3H shows that the higher occupied MOs are mainly composed by Ru d orbitals and thiocyanate ligands and the lower unoccupied MOs are the admixture ofπ* orbitals localized on the terpyridine ring ligands and the carboxylic groups. These thiocyanate and carboxylic group contributions to the HOMOs and LUMOs, respectively, are believed to play an important role in the regeneration and electron injection performance of dye sensitized TiO2 solar cells. Moreover, the protonation effect leads to a sizable difference in the electronic structures. The protonation effect decrease the HOMO-LUMO gap and make absorptions and emissions red-shift. The lowest-energy absorptions are attributed to the MLCT/LLCT transitions, whereas the lowest-energy emissions are assigned as a 3MLCT/LLCT origin. Inclusion of the solvent leads to important changes of the energies and composition of the molecular orbitals of the complexes; as a consequence, the spectra calculated in the presence of the solvent are in good agreement with the experiment. And the absorption spectra of the full deprotonated form (0H) are red-shift in ethanol and water with respect to the gas phase. On the other hand, in the protonated complexes (1H-3H), the absorption band are blue-shift in ethanol and water with respect to the gas phase.2. The DFT (B3LYP) and DFT (UB3LYP) are carried out to optimize the ground-and excited-state geometry structures of [(tpy)(bpy)RuC≡CC6H4R]+ [tpy = 2,2′:6′,2"-terpyridine, bpy = 2,2′-bipyridine; R = F (1), Cl (2), H (3), Me (4), and OMe (5)]. On the basis of their optimized structures, the absorptions and emission spectra in the CH3CN solution are obtained by TD-DFT method. The lowest-energy absorptions are attributed to the MLCT/LLCT transitions, whereas the lowest-energy emissions are assigned as a 3MLCT/LLCT origin. When the electron-donating substituents are introduced into the phenylacetylide ligand, the lowest-energy absorption and emission are red-shifted in the order 1≈2 < 3 < 4 < 5.3. The DFT (B3LYP) and DFT (UB3LYP) are carried out to optimize the ground-and excited-state geometry structures of [Ru(N)X2]- [X=S2C6H4 (1), mnt (2)]. On the basis of their optimized structures, the absorptions and emission spectra in the CH3CN solution are obtained by TD-DFT method. Upon excitation for 1-2, the geometrical parameters for excited states have not notable differences from those of the ground state. The two complexes show the similar variation trend. A general elongation of all the bond lengths is observed. The variation of the bond lengths indicates that the electron are promoted from theπ-bonding orbital on the X(X=S2C6H4 and mnt)ligands toπ*(Ru≡N) orbital. Complexes 1 and 2 have similar spectroscopic properties: the lowest-energy absorptions are attributed to the MLCT/LLCT transitions, whereas the lowest-energy emissions are assigned as a 3MLCT/3LLCT origin.
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