Theoretical Investigations On The Organic Oligomer Light-Emitting Materials

In the past decade, organic electroluminescent materials have become a fascinating field in the word for their diverse potential applications in communication, information, and flat-panel displays. Organic electroluminescent devices have such advantages as low-voltage driving, high luminosity, high efficiency and large-area color display which can be realized. Thus there has been great interest in investigating organic electroluminescent materials and devices.In this paper, from the point of view of the molecular design, we systematically studied four types of the organic oligomer light-emitting materials by quantum-chemical methods, namely, DFT, TDDFT, HF, and CIS methods, including the properties of the ground state and the lowest excited state conformations, HOMOs, LUMOs, energy gaps, ionization potentials, electronic affinities, reorganization energies, absorption and emission spectra. The theoretical studies show that it can greatly modulate and improve the electronic and optical properties of the light-emitting materials by modification of chemical structures. Also, it can help to understand the microcosmic electroluminescent mechanism and contribute to orientate the synthesis and design of the novel light-emitting materials by exploring their structure-property relations. The following is the main results:1. A series of phosphole-containingÏ€-conjugated organic oligomers is investigated. The phosphole-based EL materials for OLEDs presented in this paper are attractive because of the flexibility available for fine-tuning their optical and electronic properties through varying the phosphole ring substituents at the 2,5-positions. The presence of the 2,5-substitution on the P-ring leads to the large twist angles between the phosphole ring and the aromatic substituents. The HOMO possesses antibonding character and LUMO holds bonding character between the two adjacent subunits, resulting in that the excited state structures of the phosphole derivatives have better coplanar conformation than the ground state structures. Importantly, the different substiuents at the 2,5-positions of the P-ring have an impact on the HOMOs, LUMOs, energy gaps, ionization potentials, electron affinities, reorganization energies and absorption and emission spectra for the phosphole derivatives. Consequently, HOMO energies increase, LUMO energies decrease, the hole and electron injection into OLEDs is greatly enhanced. The effect of substituents on the relatedπ→π* transitions can be clearly described. As expected, the absorption and emission spectra exhibit red shifts to some extent and the Stokes shifts are unexpectedly large ranging from 78 nm to 228 nm attributing to a more planar conformation of the excited state for the phosphole derivatives. The results show that these phosphole derivatives show great potential for application in OLEDs as hole and electron transport/injection red materials.2. Two series of D-Ï€-A-Ï€-D type bis-dipolar diphenylamino-endcapped oligoarylfluorenes, bearing an electron affinitive core, 9,9-dibutylfluorene as conjugated bridges, and diphenylamino as end-caps, have been studied. All the oligoarylfluorenes show more or less twisted structures because of the electronic nature of the various central aryl cores. The frontier molecular orbitals spread over the wholeÏ€-conjugated molecules. In general, the HOMO possesses bonding character and the LUMO holds antibonding character. However, there is antibonding interaction between the two adjacent subunits in the HOMO and bonging interaction in the LUMO. Their optical and electronic properties are affected by the electron affinitive cores of the oligoarylfluorenes. With the electron withdrawing strength and the conjugated length of the electron affinitive cores, the HOMO energies, electron affinities, and reorganization energies increase, the LUMO energies and ionization potentials decrease, the energy gaps narrow, and the absorption and emission spectra exhibit red shifts to some extent. The Stokes shifts are large. Importantly, employing the various electron affinitive central aryl cores, the emissive colors of these bis-dipolar oligoarylfluorenes can span almost the full UV-vis spectrum. So they can be used as hole and electron transport/injection materials.3. A series of star-shaped organic molecules that comprise a 1,3,5-trisubstituted benzene core and three oligoaryleneethynylene arms is studied. The electronic excitation leads to the varieties of the star-shaped molecular structures, especially the distances between the substituted benzene and ethylene. The HOMO possesses antibonding character and LUMO holds bonding character between the two adjacent subunits, the substituted benzene and ethylene. Importantly, for both MMPT and DMPT series, the star-shaped molecular size, namely, the number of dimethoxyphenyleneethynylene units, and the number of the methoxy groups on the terminal benzene rings at each arm have an impact on their optical and electronic properties. Consequently, the hole and electron injection into OLEDs was great enhanced. As expected, the absorption and emission spectra exhibit red shifts to some extent, and small Stokes shifts are observed, which are presumably due to the star-shaped rigid molecular structures that hinder the geometrical relaxation. In conclusion, these star-shaped organic molecules have potential application in OLEDs as hole and electron transport/injection materials. 4. A series of doubly ortho-linked cis-4,4-bis(diarylamino)stilbene /fluorene hybrids has been investigated. The results show that all the molecules have a rigid spiral butterfly-shape structure, which prevents intermolecular parallelÏ€-Ï€stacking, eximer formation, and fluorenone formation. The frontier orbitals are mainly localized on the both cis-stilbene and its 8,8′-substituents. Their optical and electronic properties are significantly affected by the presence of various 8,8′-substituents. In contrast to the molecule 1, the absorption and emission spectra exhibit red shifts to some extent. All the electronic transitions are assigned toπ→π* character arising from S₁, HOMO→LUMO transition. Particularly, the molecules 3-6 have similar absorption wavelengths and transition natures. And the Stoke shifts range moderately from 40 to 86 nm, resulting from the rigid spiral butterfly-shaped structure that hinder the geometrical relaxation. In conclusion, they can function as efficient hole transporting-type, sky-blue fluorescent OLED materials.