Theoretical Investigation On Electronic Structure And Optical Properties Of Polyfluorene By Copolymerization With Dibenzothiophene

Since electroluminescent phenomenon of Poly-phenylenevinylene(PPV) are reported by Cambrige University in 1990. Because the dissolublepolymers 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. During fourteenyears, many kinds of conjugated polymers have been studied for theelectroluminescence, such as PPV, poly thiophene (PTh), polyperinaphthalene (PPP), polyfluorene and their ramifications and so on. Theirspectra can spread all over the visible spectrum by chemical structuremodification. There are two different approaches to modify the band gaps ofthe conjugation polymers. One is adding groups at the phenyl or methyleneof the backbone, the other is restricting the movement of electrons in themain chain. Additionally, the proper methods to enlarge the band gaps ofpolymers and decrease the effective conjugation length are insertingunconjugated groups in the polymer chain or block the conjugation forconjugated backbone between the luminescent cells, such as insertingpolyester and polyurethane. Therefore, we can select the proper luminescentstaple to decide the absorption and emitting wavelength.What more, there are two different theoretical approaches to evaluatethe band gaps of polymers. One is the polymer approach in which theperiodic structures are assumed for infinite polymers. Another one, theoligomer extrapolation technique, has acquired the increasing popularity inthis field, however. In this approach, a sequence of increasing longeroligomers is calculated, and extrapolation to infinite chain length isfollowed. A distinct advantage of this approach is that it can provide theconvergence behavior of the structural, electronic and spectral properties ofpolymers. In practice, both the oligomer extrapolation and the polymerapproaches are generally considered to be complementary to each other inunderstanding of the properties of polymers. However, we can't observe orcalculate precise polymer band gap directly. So the oligomer extrapolationis the main way to get the polymer band gap, which is the topic of thepresent work. Here we studied two fluorene-based copolymers polyfluorene,(PF30T)n and (PF50T)n using density functional methods and semiclassicalmodels.The ground-state geometries of oligomers were fully optimized usingthe density functional theory (DFT), B3LYP/6-31G, as implemented inGaussian 03. The results of the optimized structures for the oligomericmolecules of the (PF30T)n (n=1~4) and (PF30T)n (n=1~4) show that thestructural changes softly with increasing chain length in the series of(PF30T)n, as well as (PF50T)n. And it suggests that we can describe thebasic structures of the polymers as their oligomers. The character ofstructure in the (PF50T)n is dramatically twisted in compared with(PF30T)n.ZINDO and TD-DFT/B3LYP calculations of the lowestexcitation energies and the maximal absorption wavelengths (λabs) were thenperformed at the optimized geometries of the ground states. The lowestexcitation energies and the maximal absorption wavelengths show excellentlinearity in our plots. Band gaps of the corresponding polymers wereobtained by extrapolating HOMO-LUMO gaps and the lowest excitationenergies to infinite chain length, as well as the maximal absorptionwavelengths of the polymers. The extrapolation results of Egs and λabs are ingood agreement with the experimental data. The results of each methodindicate the same conclusion that the decreasing of the conjugation in thebackbone of (PF50T)n broadens its band gap. The broader band gap of(PF30T)n causes its shorter maximal absorption wavelengths, comparedwith (PF50T)n.The excited geometries were optimized by ab initioCIS/3-21G and the emission spectra were computed based on the excitedgeometries. The changes of the structures during the excitation can beprefigured from the characters of front orbitals. The structure will be tightwhen the antibonding changes into bonding, otherwise, the bond length willincrease when the bonding changes into antibonding. All of IPs and EAsinvolved in this paper are the energies' difference between the ions andmolecules. We employed the linear extrapolation technique in this research.The linearity between the calculated IPs, EAs, energy gap, maximalabsorption wavelengths of the oligomers and the reciprocal chain length isexcellent for both homologous series of oligomers. Thus, these values of thepolymers can be obtained by extrapolating the resultant linear relationshipto infinite chain length. In all cases, the energy required to create a hole inthe polymer is ~6eV, while the extraction of an electron from the anionrequires ~1.3eV. The ionization potentials of (PF50T)n are higher then thatof (PF30T)n and electron affinities of it are lower than that of (PF30T)n.This suggests that the (PF50T)n appears to trap and give the electron moreefficiently, compared with (PF30T)n. On all accounts, the rigid twist in thestructure of (PF50T)n result in the conjugation decreasing and it is easy toadd an electron and difficult to ionize compared with (PF30T)n. To theimportance, it results in a broader band gap and shorter maximal absorptionand emission wavelengths in the spectra for (PF50T)n than (PF30T)n.Theenergy band gap broaded, and the spectra blue-shifted.The excited geomertries,the emission spectra are investigated.The theoretic-cal study shows that by modification of chemical structures could greatlymodulate and improve the electronic and optical properties of light-emittingand contribute to orientate the sysnthesis efforts and help understand thestructure-properties relation of these conjugated material.