The Theoretical Investigation For The Electronic Structure And Spectrum Of Luminescent Metal Complexes: The Electronic Donor And Acceptor Effects

In passed decades, major breakthroughs have led to significant improvementsin the performance of organic light-emitting devices (OLEDs) due to the use ofelectrophosphorescence metal complexes, while new materials with betterproperties are still needed to fully realize the advantages that organic and polymerLED technology can potentially offer, for example, the complexes with propertiesadjusted easily. The most used metal complexes are the fluorescent Alq3 andphosphorescent Ir(ppy)3, and the deep understanding of their electronic structuresand excited state properties are significant of designing and developing the newhigh performance materials. The advanced technique applied in experimentsgreatly promotes the development of modern computational chemistry. On onehand, the comparison between calculation and experiment can test the reliabilityand accuracy of electronic structure theory, showing the dependence of theory onexperiment;on the other hand, to develop the electronic structure theory is tosupport and/or supplement the known experimental results, and further to predictthe potential results, indicative of theoretical forward looking and independence.In the paper, we systematically studied luminescent properties, ground-stateelectronic structures, two type of complexes and obtained the following mainresults:1. The ground state and electronic spectra in F-substituted Alq3 derivatives[where q=8-hydroxyquinoline], an important electroluminescent material, arestudied using density functional theory (DFT) B3LYP/6-31G in programmeGaussian 03. The effect of substituted position in Alq3 on their energies,HOMO-LUMO bandgap and the electronic spectrum are discussed. It is found thatF-substitutions in Alq3 result in the decrease of both HOMO and LUMO energies,and the degree of such decrease in energy of HOMO and LUMO level aredependent on the F-substituted position. Significantly, the HOMO-LUMO bandgapof 6-FAlq3 is found to be increased, as a result its absorption and emission spectraare greatly blue-shifted.The effect that changes the substituent in 6-FAlq3 on their energies,HOMO-LUMO bandgap and the electronic spectrum are discussed. Electron-withdrawing group –CN leads to both absorption and emission spectra red-shift dueto decrease the energy of HOMO and LUMO which results in the decreasedHOMO-LUMO bandgap. Just the opposite, electron-releasing groups –CH3,–OCH3,–N(CH3)2 increase the energies of HOMO, LUMO and the bandgap ofHOMO-LUMO, as a result its absorption and emission spectra are greatlyblue-shifted. The theoretical investigation is agreement to the experimental results,providing an approach to obtain the important blue emitting Alq3 derivatives.2. The ground state and electronic spectra in the Ir(III) complex Ir(ppy)3[where ppy=2-phenylpyridine], an important phosphorescent material, and itsderivatives are studied using density functional theory (DFT) B3LYP/LanL2dz inprogramme Gaussian 98. We find that the ground-state structures of Ir(ppy)3 is ingood agreement with available crystallographic studies. Most of the highestoccupied orbitals for five complexes are Ir (5d) in character, secondarycontribution come from the phenyl π-orbital. The lowest virtual orbitals aretotally π* (ppy) in character. The low-energy absorptions which the light-emittingis concerned about, are all assigned as the mixture of MLCT [d (Ir) → π* (ppy)]and LLCT [π(phenyl) → π* (ppy)]. A agreement between the computedabsorption bands and the reported experimental bands in literature can be found.Emission band has been supposed to be an electronic transition between theground and the triplet excited state T1 with the character of the mixture of MLCTand LLCT for all considered complexes.The results of absorption and emission spectral fitting give insight into theeffect that changes the substituent in ligand. Electron-withdrawing group –CNleads to both absorption and emission spectra red-shift due to decrease the energyof LUMO which results in the decreased HOMO-LUMO bandgap,electron-releasing group -OCH3 increase the energies of LUMO, but have the lessinfluence on HOMO-LUMO bandgap, absorption and emission spectra. It ispossible to control the color of the photoluminescence by changing the kind andthe position of the substituents. The results indicate that application of TD-DFTcalculations is reliable for studying the system containing transition metals.3. The ground state, excited-state and electronic spectra in the Pt(II)complexes, an important phosphorescent material, are studied using densityfunctional theory (DFT) B3LYP/LanL2dz in programme Gaussian 03. Most of thehighest occupied orbitals are Pt (5d) and phenyl π-orbital in character, tertiarycontribution come from the 2,4-pentanedionato-O,O π-orbital. The spin densitiesin the cations is basically consistent with this analysis. The lowest virtual orbitalsare totally π* (ppy) in character, as also reflected in the spin densities in the anion.The low-energy absorption which the light-emitting is concerned about, are allassigned as the mixture of MLCT [d (Pt) → π* (ppy)] and LLCT [π(phenyl) → π*(ppy) or π(2,4-pentanedionato-O,O) → π* (ppy)]. The lowest triplet excited-statestructures of the six molecules are obtained using CIS method. Emission band hasbeen supposed to be an electronic transition between the ground and the tripletexcited state T1 with the character of the mixture of MLCT and LLCT for allconsidered complexes.The effect of substituted position in (2-phenylpyridinato-N,C2') platinum(II)(2,4-pentanedionato -O,O) on their energies, HOMO-LUMO bandgap and theelectronic spectrum are discussed. It is found that F-substitutions in(2-phenylpyridinato-N,C2') platinum(II) (2,4-pentanedionato -O,O) result in thedecrease of both HOMO and LUMO energies, and the degree of such decrease inenergy of HOMO and LUMO level are dependent on the F-substituted position.Significantly, the HOMO-LUMO bandgap of F-substituent on 2th or 4th position ofphenyl in Pt complex is found to be increased, as a result its absorption andemission spectra are greatly blue-shifted. It more proves that we can control thecolor of the photoluminescence by changing the kind and the position of thesubstituents.