The Study Of The Band Structure Of PPP (poly-para-phenylene) And Its Alkoxyl Derivatives

The design and synthesis of electroluminescent polymers, luminescence mechanism and the study of EL devices have been became one of the most active research fields in recent years. PPP is an interesting material for electro-optical applications as its band gap is in the blue region of the visible spectrum and its thermal stability is combined with high PL. However it is insoluble and infusible making it difficult to fabricate thin films. So the various substitutions were introduced to change the band gap and ionization potential, electron affinity, thus it is a research focus for scientists to improve the solubility, alter the color of luminescence, increase efficiency of luminescence. Many experiments have been done on this aspect, but the luminescence process is still short of the theoretical understand. In order to study the microscopy mechanism, many theoretical methods have been developed. In principle, these methods can be divided into two classes: one is so-called ab initio or first-principles methods, which is based on the density functional theory and don't use any empirical parameters. The other one is the empirical or semi-empirical methods. The EHMO method used in this paper is one of the semi-empirical. The band gap of PPP we got is closer to the experiment as compared with the results calculated by the local density approximation (LDA) which often underestimate the band gap and the GW approximations which may be overestimate. So the one-dimension band structures of [poly (para-phenylene)] (PPP) and its alkoxyl derivatives are calculated by the EHMO method (BICON-CEDiT code). The results shown that the band gaps (Eg) of these materials are in agreement with the available experimental data. For PPP, the planar and twisted configurations are studied in this paper. We found that the twisted structure is more stable than the planar one and the band gap of PPP reduced as the torsion angle between two adjacent benzene rings increased. As for alkoxyl derivatives of PPP, the structure of benzene is remodeled and deviate from a regular hexagon because of the interaction of the benzene and its alkoxyl side chains. From the calculated results, we can found that the band gap of the derivatives will be bigger than...