Theoretical Investigation On Electroluminescent Macromlecules

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), poly fluorene (PF) and their ramifications and so on.Their spectra 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 poly fluorene(PF) and poly (2, 7-fluorene-alt-co-5, 7-dihydro-dbenz[c,e]oxepin) (PFDBO)using density functional methods and semiclassical models. The ground-state geometries of oligomers were fully optimized usingthe density functional theory (DFT), B3LYP/6-31G, as implemented inGaussian 98. The results of the optimized structures for the oligomericmolecules of the (F)n (n=1~4,6,8) and (FDBO)n (n=1~4) show that thestructural changes softly with increasing chain length in the series of (F)n, aswell as (FDBO)n. And it suggests that we can describe the basic structuresof the polymers as their oligomers. The character of structure in the(FDBO)n is dramatically twisted in the seven-membered-ring(dihedral angleis 42 o), compared with PF. ZINDO and TD-DFT/B3LYP calculations of thelowest excitation energies and the maximal absorption wavelengths (λabs)were then performed at the optimized geometries of the ground states. Thelowest excitation energies and the maximal absorption wavelengths showexcellent linearity in our plots. Band gaps of the corresponding polymerswere obtained by extrapolating HOMO-LUMO gaps and the lowestexcitation energies to infinite chain length, as well as the maximalabsorption wavelengths of the polymers. The extrapolation results of Egsand λabs are in good agreement with the experimental data. The results ofeach method indicate the same conclusion that the decreasing of theconjugation in the backbone of PFDBO broadens its band gap. The broaderband gap of PFDBO causes its shorter maximal absorption wavelengths,compared with PF. The excited geometries were optimized by ab initioCIS/3-21G and the emission spectra were computed based on the excitedgeometries.