Charge density polyacetylene

Contrary to naive expectation, an extended ir-electron system snch as that in the original silvery polyacetylene film does not imply perfect bond conjngation or perfectly like bonds, or conduction along the chain It only implies a degree of charge-density delocalization. Such a material has the electronic structure of a... 

Many phenomena such as dislocations, electronic structures of polyacetylenes and other solids, Josephson junctions, spin dynamics and charge density waves in low-dimensional solids, fast ion conduction and phase transitions are being explained by invoking the concept of solitons. Solitons are exact analytical solutions of non-linear wave equations corresponding to bell-shaped or step-like changes in the variable (Ogurtani, 1983). They can move through a material with constant amplitude and velocity or remain stationary when two of them collide they are unmodified. The soliton concept has been employed in solid state chemistry to explain diverse phenomena. 

The effect of this is that the charged defects are independent of one another and can form domain walls that separate two phases of opposite orientation and identical energy. These are called solitons and can sometimes be neutral. Solitons produced in polyacetylene are believed to be delocalised over about 12 CH units with the maximum charge density next to the dopant counterion. The bonds closer to the defect show less amount of bond alternation than the bonds away from the centre. 

Twenty five years after the first report on catalytic polymerization of acetylene by Shirikawa, McDiarmid and Heeger [130], the Nobel prize has been awarded to these researchers for their contribution to the field of conducting polymers. Indeed, such polymers have promising applications such as low weight/high charge density batteries, polymer-modified electrodes, or capacitors to name only a few. Driven by these applications, different classes of polyacetylenes have been synthesized, starting from acetylene itself and from mono- or disubstituted acetylene derivatives. 

As noted above, the free carrier contribution which extrapolates to the measured a(0) must be in the far-IR below 450 cm"1.15 Nevertheless, most of the 7C-electron oscillator strength remains in the broad absorption band above 0.2 eV. An alternative which appears to be in agreement with the essential experimental facts is that polyacetylene is an example of a polaronic metal. The polaron lattice with a half-filled polaron band is certainly consistent with the observed susceptibility, hi the case of a polaron lattice, the IRAV modes are expected, and would be r -shifted from the Raman modes provided that the pinning is weak. Although the intensity of the IRAV modes was initially csdculated to be much too weak, this calculation ignored the effect of the counter-ions the counter-ion Coulomb potentials may lead to sufficient nonuniformity in the charge density, to yield the observed IRAV mode intensities.For the polaron lattice, a((o) would have two contributions with a "gap" in between ... 

SoKtons produced in polyacetylene are delocalized over approximately 12 CH units, with the maximum charge density to the dopant counterion. Soliton formation results in the creation of a new localized electronic state which is in the middle of the energy gap. At a high level of doping the charged sohtons produce soliton bands that can merge to behave hke a metalhc conductor. 

The relevant electron-counting is demonstrated in Figure 1.19. In the upper part, the situation for polyacetylene is shown for comparison (for simplicity the zigzag chain is rendered straight). If two solitons are created in polyacetylene by sliding the charge-density... 

The above discussion is based on a free-particle wavefunction. In real materials the potential along a coordinate is not constant, so the band stmcture becomes more complicated than the parabola of Fig. 15-1. Also, charge-density adjustments occur that tend to screen out the applied field. However the basic requirement for metallic conductivity continues to be the absence of a gap between the highest fllled and lowest empty MOs. Therefore, if the n band of polyacetylene really is partly fllled, pure... 

Regular polyacetylene is a one-dimensional conductor, in that there is no band gap between the occupied and virtual sets of MOs. This type of system is unstable, and, according to Peierl s theorem, the electronic structure should distort so that a charge density wave is formed. 

If M=N, as is the case for the neutral tt system of a regular polyacetylene, then the oscillations of the charge density are congruent with the positions of the nuclei. 

The excellent agreement between the TSC and P1A results has two implications. First, since the TSC method probes the product of mobility and carrier density, while the P1A probes only the carrier density, there seems to be no dominant influence of temperature on the carrier mobility. This was also found in other conjugated polymers like /ra/ry-polyacetylene [19, 36]. Second, photoconductivity (observed via the thermal release of photoexcited and trapped earners) and photo-induced absorption probe the same charged entity [36, 37J. 

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