Polyacetylene absorption spectrum

The third route involves metathesis polymerization of cyclooctatetraene with tungsten catalysts, yielding polyacetylene as an insoluble film along with oligomers (iOi). By first polymerizing cyclooctene and then adding cyclooctatetraene, a soluble, red block copolymer was obtained. On the basis of the visible absorption spectrum, at least two or three cyclooctatetraene units were concluded to have been added to the polymer chain forming a short polyacetylene block. No conductivity data were reported for this copolymer. 

Table 4 lists the MBPT(2) band gaps of polyacetylene calculated with basis set 6-31G and DZP at three different geometries by us [36]. The cutoffs N and K are both 21. The geometries used in the calculations are listed in Table 5. The first two were given by Suhai [53,55] and the last one was an experimentally estimated geometry [97], The band gaps obtained are 4.033, 3.744, and 3.222 eV, respectively. There is no direct measurement of the band gap, defined as a quasi-particle energy difference of the lowest unoccupied and highest occupied orbitals. Instead, the absorption spectrum of polyacetylene crystalline films rises sharply at 1.4 eV and has a peak around 2.0 eV [97]. To explain this measured spectrum, one needs to calculate the density of the system s excited states and the absorption coefficients of the states.

From the absorption spectrum of poly-33 film, the band gap energy of poly-33 was evaluated to be 1.95 eV (Figure 9), which is larger than that of trans-polyacetylene but comparable to that of cis-polyacet-ylene.2 The electrical conductivity of the film was 10 Q cm at room temperature and increased with temperature with an activation energy of about... 

Nevertheless, even for polyacetylene, the electronic structure is not that of a simple metal in which the bond-alternation and the tc-tc gap have gone to zero there are infrared active vibrational modes (IRAV) and a pseudo-gap. This is indicated by the spectra in Figure 2 which demonstrate the remarkable similarity between the doping-induced absorption found with heavily doped trans-(CH)x, and the photoinduced absorption spectrum observed in the pristine semiconductor containing a very few photoexcitations. Not only are the same IRAV mode spectral features observed, they have almost identical frequencies. 

Figure 10.6 also provides a possible explanation of the high energy feature observed at 1.35 eV in the photo-induced absorption spectrum of trans-polyacetylene, which is attributed to neutral states. A vertical excitation from... 

Fig. 10.9. (a) Calculated optical absorption spectrum of (rans-polyacetylene from a DFT-GWA-BSE calculation. The solid and dashed curves represent the exciton and quasi-particle spectra, respectively, (b) The electron-hole distribution function. Reprinted with permission from M. Rohlfing and S. G. Louie, Phys. Rev. Lett., 82, 1959, 1999. Cop5rright 1999 by the American Physical Society. 

Polyacetylene n. A polymer of acetylene, made with Ziegler-Natta catalysts and usually dark-colored, with the unusual property (for a polymer) of high electrical conductivity, achieved by doping the polymer with about 1% of ionic dopant such as iodine. It may become a useful solar-cell material because its absorption spectrum closely matches the solar spectrum, but mechanical properties and stability are poor. Also, practical processing methods have yet to be developed. 

The polymerization of acetylene by using [Rh(l,5-Cod)Cl]2, where 1,5-Cod is c/j,cw-cycloocta-l,5-diene, or [Rh(NBD)Cl]2, where NBD is bicyclo[2,2,I]hepta-2,5-diene, was studied by UV-vis spectroscopy [79,80]. The growing polyacetylene chains were identified by three maxima at 500, 544, and 590 nm as a result of subtracting the spectrum of the catalyst from that of the reaction mixture. The first-order derivative of the absorption spectrum of the growing polyacetylene exhibited vibrational maxima at 480, 515, 550, and 600 nm for the cis-isomer and at 640, 670, and 710 nm for the trans-isomer. UV-vis and FTIR spectroscopies were used in the study of the structure of thin freestanding films of cis- and trans-PA obtained by using Rh(I) complexes. The absorption spectrum shows no vibrational structure, which was detected in acetylene polymerization in ethanol. The microstructure of PA is very similar to that of PA synthesized with a Luttinger catalyst in terms of sp defects in the polymer chains detected by FTIR spectroscopy. 

Figure 1. (A) UV spectra of 13, a polyacetylene bearing D-glucose pendants, in chloroform, dioxane and toluene. (B) Change of its absorption spectrum with composition of a toluene/chloroform mixture.

Figure 1.60. Photo-induced absorption spectrum of trans-polyacetylene after illumination with 2.4 eV (at 10 K). (Reprinted with permission from ref 80)...

Crystals of poly[2,4-hexadiyne-l,6-diol bis(p-toluenesulphonate)] [59, 60] are highly photoconductive, with high carrier mobilities, near 20m V s even at low electric fields. This corresponds to drift velocities of about 2.2 x 10 m s i.e. close to the velocity of sound in the material. It is interesting to note that the spectral dependence of photoconductivity (the so called action spectrum) in this and other polyacetylenes does not follow the absorption spectrum, but is shifted towards shorter wavelengths. Charge-carrier generation follows the Onsager one-dimensional model, at least for electric fields up to 10 V m [60, 61]. 

If one performs a minimal (ST0-3G) basis calculation for alternating trans-polyacetylene (PA) using bond distances determined from geometry optimization with this base, one obtains a gap of 8.91 eV /22/. This is more than four times the experimental value of 2 eV which is the position of the first peak in the absorption spectrum of pure trans PA /30/. (It should be mentioned that in trans PA there is no exciton band /G/ and therefore one can take this value as the gap value.) If one... 

That ageing does occur was indicated by the gradual deepening in colour of the solutions as monitored by the changes in the visible absorption spectrum. A possible cause of this is cis-trans isomerisation in the polyacetylene component but the reflectivity data show qualitatively that from the point of view of the surface structure this ageing has no detectable effect at the concentrations studied here. 

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