Poly conduction band

Another possible modification of poly(sulfur nitride) that is expected to produce conducting polymers is the replacement of alternating sulfur in the thiazyl chain by an RC unit, i.e., [(R)CNSN]x. This type of polymer would have five r-electrons per four atoms in the repeating unit and, consequently, would have a partially occupied conducting band. The prospect of tuning the electronic properties of this polymer by... 

Bunz et al. pointed out that it would be of interest to develop materials that combine the stability, electron affinity, and high emissive quantum yield of PPEs with the excellent hole injection capabilities of poly(p-phenylene vinylene)s (PPVs) [48]. In line with this notion,recent synthetic activities have focused on the engineering of the band gap, conduction band, and valence band of PAEs with the objective to render these materials more useful for practical applications that exploit their electrically (semi)conducting nature. Examples of materials that emerged from these efforts are discussed in detail in other portions of this volume (in particular the chapters by Bunz, Klemm, and Yamamoto). They include, among others, poly(heteroarylene ethynylenes) such... 

On the other hand, the photoconductivity of a poly(vinyl chloride) copolymer and of sucrose benzoate each containing the leucobase of malachite green was characteristic of the dyestuff and independent of the polymeric host (137). This is similar to the behavior of conventional impurity semiconductors. However, a distinction must be drawn between an impurity which acts solely as an impurity providing an easier path to the conduction band, one which acts only as a plasticizer and one which may act as both impurity and plasticizer. 

Thus, the reduced form of poly-3-methyl thiophene is an intrinsic semiconductor and the Fermi level lies between the valence band and the conductivity band. The effect of oxidation is to introduce a surface state in the band gap between n and n orbitals. The Fermi level is decreased when the compound loses electrons, and metallic properties appear when an increasing number of electrons build a new but only half-filled band. These situations are shown in Fig 11.7. 

In the reduced state, poly-3-methyl thiophene is an intrinsic semiconductor with an energy gap of 1.95 eV. (a) What would be the value in electron volts of its Fermi level (b) Show that the concentration of electrons in the conduction band would be negligible. However, from 0.45 to 1.1V on the normal hydrogen scale, this polymer shows metallic conductivity, (c) What change in electronic structure has occurred ... 

Table 8 Valence and conduction bands of poly(TCNQ) and poly(TTF) and their widths (in eV). For comparison the Table contains also the locations of the corresponding MOs... 

Table 13 Energy parameters of the valence and conduction bands in pure poly(Gly)t poly(Ala), andpoly Ser) (all quantities in eV)...

Figure 10 Density of electronic states plotted in the conduction band region of the poly(Gly-Ala-Ser) polymer (in relative units)...

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.

An LCAO (linear combination of atomic orbitals) local-density functional approach was used to calculate the band structures of a series of polymer chain conformations unsubstituted polysilane in the all-trans conformation and in a 411 helical conformation, and all-trans poly(dimethylsilane). Calculated absorption spectra predict a highly anisotropic absorption for the all-trans conformation of polysilane, with the threshold absorption peak arising strictly from polarizations parallel to the chain axis. The absorption spectrum for the helical conformation is much more isotropic. Results for the dimethyl-substituted polysilane chain suggest that the states immediately surrounding the Fermi level retain their silicon-backbone a character upon alkyl-group substitution, although the band gap decreases by I eV because of contributions from alkyl substituent states both below the valence band and above the conduction band to the frontier states. 

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