| Exploring the new energies and functional materials is animportant problem to resolve in the new century and the technologybased on the solar energy has been attracted much attention.Recently, there has been growing interesting in employment oftransition metal complexes as emitters in organic light-emittingdiodes (OLEDs), since many of them have excellentphotoluminescence (PL) properties. In order to improve theluminescence efficiency of OLEDs, some molecular designs and newsynthesis methods are introduced to enhance thecharge-transporting performance and adjust the spectra properties.So theoretical study on the light-emitting properties of transitionmetal complex is not only to explore and design of the new typeorganic-metal light-emitting complex and give important guidingsignificance and forward-looking characteristic. The studies areexpected to have an important change of organic-metal complexes aslight-emitting materials.Recently, there has been growing interesting in employment oftransition metal complexes as emitters in OLEDs. Highly efficientOLEDs based on the Re and Ir complexes have been extensivelystudied. It is well known that Re(I) complexes feature high room-temperature phosphorescence quantum yield, relatively shortexcited state lifetime, and excellent thermal stability. Study thesecomplexes is important to exploit novel types of electroluminescent(EL) materials and understand the EL mechanism deeply. Ir(III)complexes are also important, but there are still some problems thatneed to be solved in such system, for example, the light-emittingmechanism of transition metal phosphoresce complexes, the lack ofblue light-emitting materials and the effect of the ancillary ligands onthe luminescent characters of the complexes. It has been establishedthat the solvents affect the luminescence of complexes. Therefore, adeep insight into the complexes is much needed and significant.In this paper, combining the various theoretical approaches and thecomputational experience, considering the solvent effects, adoptingseveral quantum chemistry methods such as DFT and TD-DFT, weinvestigated many properties of a series of Re(I) and Ir(III) complexeswith d6electronic structures, such as the geometry structures inground and excited states, electronic structures, absorption andemission spectra properties. Through theoretical study, we couldabstract and built reasonable theoretical model from the calculateresults based on the real molecules, so that we can reveal the natureand features of transition metal complexes and give some hints on themolecular design. We obtained the following main results:1. A detailed theoretical investigation of electronic structure andspectral properties of five Re(I) halide glyoxime complexesReCl(CO)3(DHG) (1), ReCl(CO)3(DMG) (2), ReCl(CO)3(CHDG) (3),ReCl(CO)3(DBG) (4), and ReCl(CO)3(DMFG) (5) were carried out usingdensity functional theory (DFT) and time-dependent density functional theory (TDDFT). The aim of the theoretical investigation is toestablish how the subsitutents on glyoxime and the solvents affectthe spectra property and interpreted from an electronic structurepoint of view. The results displayed the energies of LUMOs areincreased obviously with the introduction of the electron-donatinggroups, and the change results in the HOMO-LUMO energy gapsincreased.2. The present work investigated the ground- and excited- stategeometries, absorption, and emission properties of three tricarbonylRe(I) complexes with pyridine-2-aldoxime and X (–Cl, 1;–CN, 2;–C≡C, 3)ligands theoretically. The X ligand with a strongerπ-donating abilitycan increase the energy level of the HOMO more significantly thanthose of the LUMO resulting in narrower HOMO-LUMO energy gaps.The absorption energy increases in the order 3 < 1 < 2 along with thereverse order of the increasingπ-donating ability of X ligands, andMLCT transition plays an important role for 2.3. Theoretical calculations have been formed on the four Re(I)complexes to reveal the electronic structures, optical properties,focusing on the theoretical understanding and prediction of theirperformance as sensitizers in DSSC. The calculation results indicatethat the FMOs are influenced by the number and position of COOHgroup. On the other hand, the newly designed complexe 2 (or 4) withthe bpy and dt (or TTF) moieties attached by COOH group has moreeffective excited state than the parent molecule 1 (or 3) and may playa better role in DSSC. These results should be helpful for designingnew sensitizers in DSSC.4. We theoretically investigated the geometries, injection and transport abilities, absorptions, and phosphorescence properties offour cationic [Ir(C^N)2(L)2]+complexes (C^N = ppy, tpy, dfppy, and bzq;L = 4-pyCO2Et). We note that varying the cyclometalating ligand C^Ncan result in subtle tuning of their structural and photophysicalcharacteristics. The electron-donating group orπ-conjugation effectin 2 or 4 can cause a red shift, and the electron-withdrawing groups in3 can cause a blue shift of the absorptions and emissions comparedwith 1. In addition, there was no evident solvent effect in both theabsorption and emission spectra. The differences betweenλholeandλelectronof complexes 1 4 are smaller than or comparable with manyphotoluminescent transition-metal complexes. Therefore, they aresuitable as emitters in OLEDs.5. A series of cationic iridium (Ⅲ) complexes [Ir(ppy)2(L)2]+wereinvestigated theoretically containing different pyridine derivatives L.The ligands L with different electronegativity can cause somevariations in electronic structures and spectroscopic properties. Thenature of LUMO and LUMO+1 change fromπ*(L) toπ*(ppy) withdecreased electronegativity of ligands L, but that of HOMO is hardlyaffected. The absorption transition character of 1 5 converts fromMLCT/LLCT to MLCT/ILCT and the absorptions are red-shifted in theorder 2, 1, 3, 5, 4, which is consistent with a decreasing trend ofelectronegativity: 4-pyCN > 4-pyCHO > 4-pyCl > py > 4-pyNH2.
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