Theoretical Investigations On The Organic Metal Complexes And Polymer Light-Emitting Materials

Recently, the scientific researchers endeavor to design and developmentnew type of luminescent devices with high efficient, high stability, highbrightness. The transition metal materials have good performance, such aslong luminescent life and single color. On the other hand, organic electroniclight-emitting film technology also has many distinguished advantages, suchas low power, easily bending, quick response, broad visual angle, large areadisplay, full emitting color and so on, and they are compatible with manykinds of standard technology and can be made of low-cost light-emittingdevices. So the polymers exhibit strong life in the aspect of planar colordisplay. In this paper, calculations on the electronic ground state were carriedout using density functional theory (DFT). The nature and the energy ofsinglet-singlet electronic transitions have been obtained by TD-DFT/B3LYPcalculations performed on the optimized geometries. The excited geometrieswere optimized by ab initio CIS. Based on the excited geometries, theemission spectra are investigated. The theoretical study shows that bymodification of chemical structures could greatly modulate and improve theelectronic and optical properties of light-emitting materials and contribute toorientate the synthesis efforts and help understand the structure-propertiesrelation of these conjugated materials. The following is the main results:1. Two series of rhenium (I) complexes are investigated to explore theeffects of expanding the ligand π system and different electron-donating andelectron-withdrawing substituents on the modulating the orbital and transitioncharacter. The results show that the absorption bands are blue-shifted whensubstituted by electron-releasing group and they are red-shifted whensubstituted by electron-withdrawing group. The enlarged conjugated structuredecreases band gaps and make the absorption and emission red-shifted.2. A series of new copper(I) mixed-ligand complexes [Cu(NN)POP]BF4have been investigated. The calculated results show that introduction of thegroups at 2,9 positions on phen ligand will lift the energy gaps betweenHOMO and LUMO, whereas introduction of the groups at 4,7 positions havelittle effects on both energy gaps and the characters of frontier orbitals, butmake the orbital characters of lower occupied and higher unoccupied orbitalschange. Substitution on 2,9 positions make both the absorption and emissionspectra exhibit blue-shifted due to steric effects.3. We study the effects of different substituents on platinum (II)complexes supported by tetradentate N2O2 chelates on the electronic andoptical properties. Different substituents can modulate both the absorptionsand emissions, but the π-acceptors have more dramatic effects than σ-donorsubstituents, and the former induces the absorption and emission wavelengthsred-shifted and the latter results in blue-shifted. Most importantly, introductionof the electron-withdrawing groups on the bpy ligand will increase the energygap between the lowest excited state and of d-d state, which prohibits thenonradiative deactivation through the pathways of d-d conversion. Thissuggests that such a theoretical approach may provide useful insight in thedesign of new and more efficient phosphorescence materials.4. We study the ground geometries, electronic structure, energy gap,absorption and emission spectra of polymer PPTZ and the copolymer withfluorene (PFPTZ). The calculated results show that relative to the rigid planarstructures in fluorene ring, the phenothiazine ring has highly nonplanarconformations, which impedes π-stacking aggregation and intermolecularexcimer formation, resulting in identical dilute solution and solid-statephotophysics, which hampers the application of polyfluorenes in PLEDs.Importantly, electron-donating groups PTZ not only enhance the opticalstability and thus increase fluorescence quantum yields, but also improve thehole injection and more efficient charge carrier balance. With the fractions ofthe PTZ increasing, the band gaps decrease and the absorption and emissionspectra exhibit red-shifted.5. The geometries, electronic structures, energy gap, absorption andemission spectra of two series of carbazole copolymers. The calculated resultsshow that more twisted structures are obtained compared with pristinepolyfluorene by the incorporation with 3,6-carbazole units. Importantly, thecombination of carbazole with the fluorene moieties resulted in the raisedHOMO energies and consequently the hole injection was greatly improved.With the increasing carbazole content, energy gap becomes broad and bothabsorption and emission peaks exhibit a blue-shifted. The charge carriersthiophene ring and ethynylene significantly lower the LUMO energies andconsequently increase the EAs, which contribute to the highly improvedelectron-accepting and transporting properties.6. Adding of dimethoxy units breaks the conjugation backbone resulted inthe raised HOMO energies and consequently the hole injection was greatlyimproved and the absorption and emission spectra exhibit blue-shifted. Bycooperation with thiophene ring, which results in the better conformations, theabsorption and emission spectra exhibit bathochrome.7. We study the effects of different electron-donating andelectron-withdrawing substituents on the electronic and optical properties offluorene copolymers. All oligomers investigated show less twisted structurescompared with pristine polyfluorene by the incorporation with anelectron-donating or electron-withdrawing units, 3,4-ethylenedioxythiophene(EDOT), pyridine and 1,3,4-oxadiazole moieties. The combination ofelectron-donating unit EDOT with the fluorene moiety resulted in the raisedHOMO energies and consequently the hole injection was greatly improved.However, even though both kinds of charge carriers will improve theelectron-accepting ability, the results show that electron-accepting moietieswill much facilitate the electron-transporting. Similarly, the combination ofBTP with the fluorene moieties resulted in the reduced LUMO energies andconsequently the electron injection was greatly improved.