Poly band structure

Two Hell UPS spectra of poly(3-hexylthiophene), or P3HT, compared with the DOVS derived from VEH band structure calculations 83], arc shown in Figure 5-14. The general chemical structure of poIy(3-a ky thiophcne) is sketched in Figure 5-4. The two UPS spectra, were recorded at two different temperatures, +190°C and -60 "C, respectively, and the DOVS was derived from VEH calculations on a planar conformation of P3HT. Compared to unsubslitutcd polythio-phene, the main influence in the UPS spectra due to the presence of the hexyl... 

The electronic band structure of a neutral polyacetylene is characterized by an empty band gap, like in other intrinsic semiconductors. Defect sites (solitons, polarons, bipolarons) can be regarded as electronic states within the band gap. The conduction in low-doped poly acetylene is attributed mainly to the transport of solitons within and between chains, as described by the intersoliton-hopping model (IHM) . Polarons and bipolarons are important charge carriers at higher doping levels and with polymers other than polyacetylene. 

Figure 14 The left hand side shows the band structures of poly(pyridine) calculated using a DFT-LMTO method for helical polymers. The right hand side shows its calculated density of states spectrum (solid line) and the experimental UPS spectrum (dashed line). The UPS spectrum was taken from Miyamae et al. [104]. Reproduced with permission from Vaschetto et al. [103], Figure 6. Copyright 1997 the American Chemical Society.

P. Barta, P. Dannetun, S. Stafstrom, M. Zagorska, and A. Pron, Temperature evolution of the electronic band structure of the undoped and doped regioregular analog of poly(3-alkylthio-phenes) a spectroscopic and theoretical study, J. Chem. Phys., 100 1731-1741, 1994. 

Recently, the effects of static and dynamic structural fluctuations on the electron hole mobility in DNA were studied using a time-dependent self-consistent field method [33]. The motion of holes was coupled to fluctuations of two step parameters of a duplex, rise and twist (Fig. 1), namely the distances and the dihedral angles between base pairs, respectively. The hole mobility in an ideally ordered poly(G)-poly(C) duplex was found to be decreased by two orders of magnitude due to twisting of base pairs and static energy disorder. A hole mobility of 0.1 cm V s was predicted for a homogeneous system the mobility of natural duplexes is expected to be much lower [33]. In this context, one can mention several theoretical studies, based on band structure approaches, to estimate the electrical conductivity of DNA [85-87]. 

The electronic structure of poly(p-phenylenevinylene), or PPV, has several similarities to DP7 hence DP7 is used as a model molecule for PPV34. The VEH band structure of an isolated chain of PPV, and... 

Puschnig, P. and Ambrosch-Draxel, C. (1999) Density-functional study of the oligomers of poly-para-phenylene Band structures and dielectric tensors. Physical Review. E, Condensed Matter, 60, 7891-8. 

Thus, from a solid-state chemistry point of view, the conducting polymer poly-3-methyl thiophene is in the reduced state, a semiconductor with a band structure. Intercalating with ions and oxidizing makes the compound behave as a metal from 0.45 to 1.1 Von theNHS. 

FIGURE 5. Band structure correlation for linear peralkylated polysilanes (a) (He I) PE spectra of hexamethyldisilane and octamethyltrisilane, (b) ab initio calculated band-structure dispersion (DIS) and (c) state density (DOS) for poly-dimethylsilane R(SiR2)R32... 

A few of these UV resonance Raman studies have reported excitation profiles of oligonucleotides [158, 177], These studies show that the hypochromism in the resonance Raman intensities can be as large as 65% for bands enhanced by the ca. 260 nm absorption band for poly(dG-dC) and that the hypochromism can vary substantially between vibrational modes [177], In the duplex oligonucleotide poly(rA)-poly(rU) [158], similar hypochromism is seen. Although theUV resonance Raman excitation profiles of oligonucleotides have been measured, no excited-state structural dynamics have been extracted from them. 

Effect of Water and Divalent Metal Ions on the Electronic Structure of DNA. As a first step in investigating the effect of impurities on the electronic structure of DNA, the band structure of poly(G-C) was computed in the PPP CO approximation assuming that one or two water molecules are bound by hydrogen bonds to the NH2 group of each C molecule or/and to the 0=0 group of each G molecule.94 According to the results obtained for these systems the additional 7r-orbital in the H20 molecules produced an extra 71-band between the lowest filled bands, while the other bands remained practically unchanged. 

H3NBH3 is isoelectronic with ethane, H2NBH2 is isoelectronic with ethylene, andHNBH is isoelectronic with acetylene. Derive the band structure and the DOS for planar poly- -BHNH- (isoelectronic to polyacetylene) with a single B-N distance and predict its conductivity and stability with respect to a Peierls distortion. Only consider the tt electronic structure. 

The next question is how side-chain substitutions and skeleton conformations affect the band structures of polysilanes. Side chains provide two interesting effects (7) band gap reduction caused by substitution of larger alkyl side chains and skeleton-side chain interaction (i.e., (j-tt mixing) in aryl poly silanes. This interaction was confirmed by UV photo spectroscopy (UPS) (8-9) and photoabsorption and luminescence measurements (iO, 11). Skeleton conformations are related to thermochromism (12-17). The ab initio... 

Substitution of aryl side chains results in a different band structure. A perspective end view of poly(diphenylsilane) is shown in Figure 11a. Electrons conduct along the red region under the influence of a potential barrier of phenyl groups. This electrical analogue of an optical fiber consists of an electrical core and an electrical clad. A perspective end view of poly-(methylphenylsilane) is shown in Figure 11b. In these aryl polysilanes, two important points should be considered the existence of states localized at phenyl side chains and the a-7T interaction between delocalized skeleton ct bands and localized tt states. 

The energy band structures of poly(methylphenylsilane) are shown in Figure 12. The original skeleton band gap is 3.6 eV, and the polymer also yields a direct-type band structure. The characteristic features of this elec-... 

Figure 12. Calculated energy band structure of poly(methylphenylsihm). Abbreviations and symbols are defined as follows CB, conduction band LU, lowest unoccupied molecular orbital VB, valence band HO, highest occupied molecular orbital F, k = 0 point and X, Brillouin zone edge. Bu, A, Bg, b2g, 62u, and eig denote orbital symmetries.

---