Theoretical Investigation And Molecular Design Of Non-doped Efficient Red Electroluminescent Materials

Organic electroluminescent devices (OLEDs) has been developed as a promising display panelbecause of their virtue of being self-luminous, large area, high brightness, simple structure, low cost,less power consumption, high efficiency. The theoretical research on the properties of thecore-luminescent materials of the devices will further promote the development of new technologies.In this paper, DFT, HF, CIS and TD-DFT are carried out to investigate a series of red materials,including small molecular compounds, polymer light-emitting materials and phosphorescentmaterials containing transition metals. The electronic structures of the ground, excited and ion states,the frontier molecular orbitals (HOMO and LUMO), ionization energy (IP), electron affinity (EA),reorganization energy (λ), absorption and emission spectra of these materials are studied in detail.In addition, the exciton binding energies (Eb) of the small molecules and the polymers, and thespin-orbit coupling matrix element (SOC) and radiative transition rate (kr) of the metal complexeswere also calculated. The theoretical analysis can give the relationship between molecularelectronic structure and optical properties, and provide theoretical guidance to design new efficientred light-emitting materials.The main contents are as follows:1. Theoretical study on the electronic structures and photophysical properties of a series ofdithienylbenzothiazole derivatives2,1,3-benzothiadiazole is an important electron acceptor, and it shows excellent hole injectionand electron transport property, so it can be used for designing high-efficency red OLED. Themolecule4,7-bis(4-(4-S-phenyl-butoxy)-5-(3,5-di(naphthalene-2-yl)phenyl)-thiophene-2-yl)-benzo[1,2,5]thiazole and its derivatives are studied using the DFT method. From the results of IP,EA and λ, we can draw the conclusion that the electron donors (triphenylamine and9-phenylcarbazole) significantly improve the hole injection and transport capabilities of the molecules, butreduces their electron injection and transport capabilities. On the basis of the molecules above,introducing the cyano group apparently enhances the electron-injection abilities of the compounds but reduces their hole-injection abilities. The dimethyl-chromene-malononitrile (DCM) groupgreatly enhances the electron injection/transporting abilities of the molecules but have almost noeffect on their hole injection/transporting abilities, so it can balance the hole and electron injectionand transporting abilities. Therefore, the electron and hole reorganization energies of the moleculescan be adjusted by introducing the substituents with different electronegativity. Furthermore, themolecular exciton binding energy shows that the designed molecules can be more efficient lightemitting layer materials. All studied molecules are predicted to be red light-emitting materials(621-769nm) with emission lifetime of5.4-8.5ns.2. Charge Transport and Fluorescence Properties of a Series of Red-emitting MaterialsBased on Benzothiadiazole and SilafluoreneConjugated organic polymers have been attracted significant attention because of their widerpotential applications in polymer light-emitting diodes (PLEDs), organic field effect transistors(OFETs) and organic solar cells (OSCs) etc. The hole/electron injection/transport abilities and theoptical properties of polymers are significantly affected by the position of benzothiadiazole groupon silafluorene group and the position of butyl group on thiophene group. The polymers show goodhole and electron injection properties when4,7-di(2-thienyl)-2,1,3-benzothiadiazole (DHTBT)group located on the2,7-postions of the silafluorene (SiF) group. The hole and electron injectionproperties are poor when the DHTBT group located on the1,8-postions (SiF1-DHTBT1-o)nand3,6-postions (SiF1-DHTBT1-p)nof the SiF group. Except for (SiF1-DHTBT1-o)n, the fluorescencespectra of the polymers are in the red range.3. The theoretical study on phosphorescence effeciency of Triphenylamine-Functionalizedand Carbazole-Functionalized Iridium ComplexesCyclometalated IrIIIcomplexes are the most commonly used materials in phosphorescent organiclight emitting devices due to their advantages of high thermal stability, high phosphorescenceefficiency and short phosphorescence lifetime, emission color adjustable over the entire visible light.By analyzing the calculated data (ionization potentials, electron affinities, and reorganizationenergies), we can draw the conclusions that the complexes with N-phenylcarbazole (–pyridine or–thiazole) ligands show not only fast but also balanced electron/hole-transport performances. Thespin-orbit coupling (SOC) matrix elements, radiative decay rate constants (kr) and the electronicstructures and energies of the optimized ground states and triplet excited states are simultaneously taken into account, the experimental phosphorescence quantum yield can be reasonable explained.And it can be predicted that Ir(Nph-2-Cztz)3is more efficient phosphor emitters thanIr(TPA-tz)3(TPA=triphenylamine, tz=thiazole, Nph=N-phenyl, Cz=carbazole) because of thefast and balanced electron and hole transport properties and high-efficiency phosphorescentquantum yield of the former. We can modulate the absorption and phosphorescence emissionspectra of the comlexes by means of thiazolyl groups in place of pyridyl groups. Replacing apyridyl group by a thiazole group can make the phosphorescence spectra of the complexes red shift.And the complexes with heteroaryl thiazole and pyridine ligands can result in a greater degree ofred-shift, and the phosphorescence spectra in the red range.