Polyenes, conducting

Phosphorus trichloride and phosphoryl chloride react with poly(alkyl vinyl ketones) in chloroform solution at room temperature to produce black polyenic conducting polymers, which are believed to contain structures like (Ifi). " ... 

Polydiacetylene crystals. The enhancement of x because of one-dimensional electron delocalization is strikingly corroborated in the polydiacetylene crystals. Their structure is that of a super alternated chain with four atoms per unit cell and the Huckel approximation yields four bands for the ir-electrons, two valence and two conduction bands. When depicted in the extended Jones zone, each pair can be viewed as arising by a discontinuity at the middle of the Brillouin zone of the polyene chain. The dominant contribution to X(2n 1) comes from the critical point at the edge of the extended Jones zone (initially at the center of the reduced B.Z.). The complete expressions are derived in (4,22) and calculated for different polydiacetylenes. We reproduce the values of x 2 for TCDU and PTS in table IV. The calculated values are in good agreement... 

Thiophene chains also make serviceable wires. The terthiophene 11, measured by MCBJ, gives two conductance maxima, at 10 nS and 80 nS [67]. Even simple polyene chains can be wires the carotenoid derivative 12 has a conductance of 2 nS, measured by STM-BJ [68]. 

Figure 29 raises the question of how the energies of these two excited states evolve as one goes to longer polyene chains, in analogy to those found in polyacetylenes which become conductive upon oxidative doping (= ionization) or photoexcitation. 

The Peterson olefination reaction involves the addition of an a-silyl substituted anion to an aldehyde or a ketone followed by the elimination of silylcarbinol either under acidic (awP -elimination) or basic (syn-elimination) conditions to furnish olefins178. Thus, Peterson olefination, just like Wittig and related reactions, is a method for regioselective conversion of a carbonyl compound to an olefin. Dienes and polyenes can be generated when the Peterson reaction is conducted using either an ,/l-unsaturated carbonyl compound or unsaturated silyl derivatives as reaction partners (Table 20)179. 

The free electron (FEMO) theory had its origins in work on the conduction electrons of metals in the 1940s, when several workers independently recognised the close analogy between these and the delocalised Jt-electrons of polyene dyes. The method was extended to many other classes of dyes, notably by Kuhn in the 1950s, but it has not found general acceptance for spectroscopic calculations, since it lacks adaptability by simple parameter adjustment. 

Fig. 29. Optical switching of electronic conduction (left) and non-linear optical properties (right) respectively in redox and push-pull polyene molecular wires /J is the hyperpolarizability.

The films could be doped with I2, FeCl3 or BF3 to conductivities of the order of 10 -3 S cm 1, much lower than polyacetylene. Spectroscopy suggested that the average length of the polyene sequence was around 10C = C units and the authors suggest that this is due to the random initiation of an unzipping reaction at many points on... 

Modification and functionalisation of natural polyenes, the carotenoids, is an efficient way for the molecular engineering of polyenic chains. Terminal bis-pyridinium carotenoids, termed caroviologens, represent an approach to electron conducting molecular wires (5). Fitting polyconjugated chains with an electron donor group on one end and an electron acceptor on the other end yields push-pull systems of type 1 that may be considered as polarized, unidirectional (oriented) molecular wires and also possess marked NLO properties. 

Incorporation of such dithienylethene units capable of reversibly interrupting conjugation into a polyene molecular wire permits reversible switching of conductive properties by photoirradiation.15 231 A typical example is shown below. 

The promoting influence of molecules, containing long electron conducting polyene chains has also been observed for electron phototransfer across molecular monolayers [241]. Lehn and co-workers [242] have synthesized a series of molecular wires which are quite interesting from the point of view of transmembrane electron... 

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