Theoretical Investigation On Electroluminescent Properties Of Benzofuran Trimers

Because of its characteristics and function such as electricity, light,sound, magnetism and heat etc., functional material has been paid mostattention to in the last five decades. The achievement in designing anddeveloping functional material not only has greatly promoted the revolutionof scientific technology in the late 20th century, but also will act as thefoundation of the development of the advanced scientific technology infuture. As one of the most important parts of the design of functionalmaterials, the design of optical materials has also been focused on byphysicist, chemist and material scientist all the time. Particularly, becausethe dissoluble polymers have many strongpoints such as easy to be deal with,flexile, membranous and their energy gaps can be adjusted by chemicaltechnologies, people come to realize and interest in them. There are twodifferent approaches to modify the band gaps of the conjugation polymers.One is adding groups at the phenyl or methylene of the backbone, the otheris restricting the movement of electrons in the main chain. Additionally, theproper methods to enlarge the band gaps of polymers and decrease theeffective conjugation length are inserting non-conjugated groups in thepolymer chain or block the conjugation for conjugated backbone betweenthe luminescent cells.The benzofuran dyes are good blue-emitting materials for the followingreasons: its photoluminescence quantum efficiencies are high in solution, itmay be sublimed without chemical degradation, and it emits light in theblue/UV region of the visible spectrum and is, therefore, an idealchromophore for full-color generation. Different linkage position hasdifferent effect on the conjugated length. 2,5-linked oligomers arethrough-conjugated (T type), whereas 2,4-linked oligomers arecross-conjugated (C type). We conduct this study to determine how theyaffect the luminescent properties. Firstly we study the influence of theconjugated-length of benzofuran oligomer with different linkage position.And then, we discuss the influence of conjugated length on luminescenceproperty with different substituents.The ground-state optimization of the two types of isomeric benzofurantrimers was performed at the DFT level with the 6-31G basis set. Based onthe optimized structures, a further calculation of HOMO-LUMO gaps(⊿H-L), ionization potentials (IPs), and electron affinities (EAs) werecarried on using B3LYP/6-31G.. Taking the ab intio ground state asreference, we calculated wavelengths and oscillator strengths of theelectronic transitions for these two series of isomeric benzofuran trimers, aswell as the excitation energies and semi-empirical calculated wavelengthsfor comparison. These computations were performed using thetime-dependent density functional theory (TD-DFT) and semi-empiricalZINDO approach. The excited state geometries were calculated byconfiguration interaction-singles (CIS) method with 6-31G basis set. Andthe geometries optimized at Hartree-Fock level were used to compare to theoptimized excited state structures. The emissive wavelengths and oscillatorstrengths as well as transition energies were calculated by TD-DFT.By density functional theory, the results show that the conjugatedlengths of through-conjugated molecules are larger than those ofcross-conjugated. The cross-conjugated molecules have widerHOMO-LUMO energy gaps. However, their energy gaps among HOMOs orLUMOs are narrower. Thus it is more possible in cross-conjugatedmolecules that the electron from HOMO-n (n=1, 2) promotes into LUMO+n(n=1, 2) and the contribution of the HOMO-LUMO transition in maximalabsorption wavelength has lessened. Compared to those of cross-conjugated,the absorption and emission spectra of through-conjugated molecules arered-shifted by 45 and 80nm respectively. The calculated results are in goodagreement with the experimental data.The present of substituents in through-conjugated molecules hasreduced the planarity between two benzofuran rings. And the addition ofmethyl groups has effectively reduced the energy barrier of creating holes.However, the twist angles (except in T-TTBF) are not large enough to blockthe electronic delocalization. The absorption and emission spectra are notsignificantly affected by the present of substituents.Although polymeric materials have so many advantages forluminescent, they have disadvantages in essence, such as uncertainmolecular weight, difficult to be refined, which can tamper with theelectroluminescent. Copolymers of alternating conjugated andnon-conjugated backbone were utilized for improved processibility and todefine the band gap for controlling emission colours. And in the 4th chapter,we research the luminescent properties ofPoly-p-phenylenebenzobisthiazole (PBT) and Poly-2,2'-m-phenylene-5,5'-bibenzimidazole (Pbi) to get a deep understanding of through-conjugatedpolymer and part-conjugated polymer and predict the electroluminescentproperties of PBT/Pbi blends.The ground-state geometries of oligomers and their corresponding ionswere fully optimized using the density functional theory (DFT),B3LYP/6-31G*, as implemented in Gaussian 03. TD-DFT/B3LYPcalculations of the lowest excitation energies and the maximal absorptionwavelengths (λabs) were then performed at the optimized geometries of theground states. Band gaps of the corresponding polymers were obtained byextrapolating HOMO-LUMO gaps and the lowest excitation energies toinfinite chain length, as well as the maximal absorption wavelengths of thepolymers. The excited geometries were optimized by ab initio CIS/6-31Gand the emission spectra were computed based on the excited geometries.We employed the linear extrapolation technique in this research. Thelinearity between the calculated IPs, EAs, energy gap, maximal absorptionwavelengths of the oligomers and the reciprocal chain length is excellent forboth homologous series of oligomers. Thus, these values of the polymerscan be obtained by extrapolating the resultant linear relationship to infinitechain length. In addition, the effective conjugation length (ECL) wasestimated by the convergence of the excitation energies, based on theobtained linearity between the excitation energy and reciprocal chain length.Through computation, we got the following results: PBT has a fullyconjugated heteroaromatic rod-like backbone while Pbi is a coil-likeheteroaromatic polymer with localized conjugation from the heterocyclicand the aromatic moieties but not fully conjugated. PBT has a superiorelectron doping ability but an inferior hole doping ability to Pbi. Comparingwith that of PBT, the absorption and emission wavelength of Pbi is small,thus accordingly, the absorption and emission spectra of Pbi are blue shifted.Therefore, blending Pbi with PBT can improve processibility as well as thehole transport. However, the spectra will be blue shifted and the electrontransport will be affected.