Polyenes and Polyynes

Selection of the principal chain in unsaturated branched acyclic hydrocarbons. A principal chain must be chosen upon which to base the name of branched unsaturated acyclic hydrocarbons. The general criteria listed in Table 4.10 are applied. Those that are specifically relevant to polyenes and polyynes are criterion (b), which is the maximum number of double and triple bonds considered together criterion (c), which is the maximum length criteria (d), (f) and (g) criterion (h), which is the maximum number of substituents cited as prefixes and criteria (i) and (j). 

By comparing the limit values for the longitudinal component of the 2nd hyperpolarizability for linear polyene and polyyne (19.4 x 10 a.u. and 10.8 x 10 a.u., respectively) one can evaluate the significance of the interaction between the orthogonal subsystems. The non-additive factor of 1.5 for the polarizability and 1.7 for the 2nd hyperpolarizability indicate that the cross-system interaction differently affects the different optical properties. 

The values shown in the table are smaller for polydiacetylene than for polyene and polyyne. This is likely due to the fact that addition of a non-conjugated r-bond makes an almost additive perturbation to the r-system and leads to a reduction of the specific (per atom) linear size. Also, as could be seen from Table 3.11, the largest values of the specific (per atom) nonlinear optical properties among all considered polymers are obtained for polyynes. 

Despite their obvious similarity to alkenes and alkynes, transition-metal alkylidene and alkylidyne complexes have not been used as building blocks for the synthesis of lowdimensional materials analogous to polyenes and polyynes. We have begun to explore the syntheses, structures, and properties of conjugated complexes and polymers derived from metal-alkylidyne complexes as part of our effort to develop the chemistry of transition-metal analogues of conjugated organic compounds. 

Osuka and co-workers (Osuka et al, 1995) have compared polyynes (9) and polyenes (10) as mediators for electronic interaction between porphyrin electron donors and acceptors. Transient absorption spectra were recorded to confirm ET by the observation of the charge-separated state (ZnP ). However, the charge-separated state was difficult to detect and fast charge recombination was discussed as one possible explanation. The ET is approximately two times more efficiently mediated by polyene-bridges than polyyne-bridges. The attenuation factor P (Eq. 12) was found to be 0.08 and 0.1 for the polyene and polyyne series, respectively. The conclusion drawn from the small attenuation factor is that the electronic coupling between the donor and acceptor is efficiently... 

Ion series Very similar to those of aromatic hydrocarbons, but fragments with higher hydrogen contents than in aromatics (m/z 54, 55 66, 67 79, 80) are usually found in polyenes and polyynes. 

Fragmentation The spectra of aliphatic compounds with several triple and/or double bonds are similar to those of aromatic hydrocarbons. A characteristic difference in the case of polyenes and polyynes is the presence of a signal at m/z 27, which is absent from spectra of purely aromatic compounds. 

Trans-polyenes H-(-HC=CH-),, -H, trans-polyenynes H-(HC=CH-C=C) -H, cumulenes H2C=(C=C) =CH2 and polyynes H-(C=C) -H have been studied (M=N-1). For eentrosymmetrie molecules, the first order hyperpolarizability p is equal to zero so that non linear effects are of second order nature. Furthermore, (the x axis goes through the middle of the C-C bonds of the polyenes, or is the intemuclear axis in the case of linear molecules) is the most important component of the second order y hyperpolarizability tensor, the other components being negligible. Both y and the mean hyperpolarizability... 

In Table 6, are given the MNDO y values of the four series of polymers,polyenes (A), polyenynes (B), cumulenes (C) and polyynes (D). 

Of considerable interest is the polydiacetylene polymer that can be viewed as a combination of a polyene and a polyyne. A fragment of the polydiacetylene molecule is shown in Fig. 3.10. 

For comparison, in Table 3.11 some data for three iso- r-electron fragments polyene, C28//30, polydiacetylene, and polyyne C14//2 are presented. 

F. Sato developed titanium (Il)-based c/s-reduction of alkynes as shown in Scheme 5 [14], and the method was applied to the synthesis of pheromones by Kitching (Scheme 6) [15]. This titanium (Il)-based reaction gives pure (Z)-alkenes. Kitching summarized the contemporary methods for the synthesis of skipped polyynes and their reduction to skipped polyenes [15]. 

Matsuzawa, N. and Dixon, D.A. (1992) Semiempirical calculations of hyperpolarizabilifies for extended n-systems Polyenes, polyynes, and polyphenyls. International Journal of Quantum Chemistry, 44, 497-515. 

The spectra of conjugated polyenepolyynes are often very complex and difficult to analyse. When the acetylenic part is the major chromophore, e.g. in an enepolyyne, the spectrum resembles that of a pure polyyne, but when the ethylenic portion constitutes the major chromophore, the spectrum is closer to that of a regular polyene -. Much of the information about polyenepolyynes can be found in works dealing with naturally occurring polyacetylenes. ... 

Partial catalytic hydrogenation of polyynes to give polyenes with cw configuration has been accomplished with a variety of substrates. Palladium catalysts, particularly the Lindlar catalyst ", are generally the most satisfactory. Side-reactions include over-hydrogenation, isomerization of cis to Irons isomers and double-bond migration. 

The spectra show four broad but clear signals at 40, 45-70 (centered at 60), 85-95, and 125-135 ppm attributable to saturated (sp ) carbon atoms bearing residual chlorine, polyyne sequences, isolated or carbonyl-conjugated triple bonds, and polyene structures, respectively. 

Cataldo, F. Raman spectra and thermal analysis on polyyne and polyene-yne chain segments. J. Macromol. Sci., Pure Appl. Chem. 2000,, 457(8), 881-892. 

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