Third polyacetylenes

Oxidative polymerization of trans-bis-deprotected 79 under Hay coupling conditions [54] yielded, after end-capping with phenylacetylene, the high-melting and readily soluble oligomers 80a-e with the poly (triacetylene) backbone [87,106] (Scheme 8). Poly(triacetylene)s [PTAs,-(C=C-CR=CR-C=C) -] are the third class of linearly conjugated polymers with a non-aromatic allcarbon backbone in the progression which starts with polyacetylene [PA,... 

The structure/property relationships that govern third-order NLO polarization are not well understood. Like second-order effects, third-order effects seem to scale with the linear polarizability. As a result, most research to date has been on highly polarizable molecules and materials such as polyacetylene, polythiophene and various semiconductors. To optimize third- order NLO response, a quartic, anharmonic term must be introduced into the electronic potential of the material. However, an understanding of the relationship between chemical structure and quartic anharmonicity must also be developed. Tutorials by P. Prasad and D. Eaton discuss some of the issues relating to third-order NLO materials. 

Extensive ir-conjugation is also often associated with enhanced conductivity in organic systems (6). Polyacetylene and polythiophene which in the doped state exhibit very high electrical conductivity also exhibit relative large third-order nonlinear optical effects in... 

Large third order susceptibilities have recently been observed for trans-polyacetylene, heteroaromatic polymers, and poly[p-phenylene vinylene] (4-11). Such nonlinear phenomena in electroactive polymers due to intense laser irradiation has been linked to the photogeneration of charged solitons on time scales of the order 10 s, and values of (3w-w+w+w)—4x 10 esu for... 

Partially substituted derivatives of polyacetylene are synthesized via the ring-opening metathesis polymerization (ROMP) of cyclooctatetraene (COT) and its derivatives. Certain poly-COT derivatives afford soluble, highly conjugated poly acetylenes. These materials exhibit large third-order optical nonlinearities and low scattering losses. 

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. 

The highest value of 7 found in a study of quadrupolar examples (THG at 1,907 nm) was 229 x 10 esu for 225, which was the longest system studied. The third-order susceptibility, (DFWM, 532 nm), for the polymer, 226, was found to be over 1,000 times that of all-trans polyacetylene, although this value is presumably dispersion enhanced and very far from the static value. " ... 

Sinclair and coworkersmeasured the third-order nonlinear optical susceptibility of rm 5 -polyacetylene. The measured susceptibility was 5 x 10 esu, which is comparable to the magnitude of the large nonlinear susceptibilities measured in the polydiacetylenes. 

To explore more fully the nature of the defects induced by the n>doping of polyacetylene with divalent cations, optical absorption experiments have been carried out (17). Spectra were obtained on thin semitransparent films of polyacetylene grown on fiat glass. The 2000A thick films were synthesized using one-third the normal catalyst concentration and a five second exposure to 60 torr of acetylene gas (10). After washing and drying the thin c/s-rich polyacetylene films, they were thermally isomerized to trans-... 

It must be noted that the values reported in the literature vary over broad ranges. Therefore, the values listed here reflect only the general behavior of several classes of compounds. It can be seen in Table 3.5 that trans-polyacetylenes (PAs) and polydiacetylenes (PDAs) exhibit the largest third-order NLO susceptibilities. The x value of cis-PA (not shown) is more than an order of magnitude smaller than that of trans-PA. Derivatives of poly-p-phenylene, poly(phenyl-ene vinylene), and polythiophene also exhibit NLO activity, but to a much lesser extent than PAs and PDAs. As pointed out above, polysilanes also possess quite large x values. This is explained by the cr-conjugation of the silicon chain, which implies a pronounced delocalization of cr-electrons. A very large x value... 

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