Push-pull carotenoid

Of special interest are push-pull polyenes composed of a polyolefinic chain bearing a donor group on one end and an acceptor on the other (see 96). Push-pull carotenoid derivatives (Figure 20) are highly polarizable conjugated polyenes that also represent polarized molecular wires. 

Well organized Langmuir-Blodgett (LB) films have been obtained from mixtures of a push-pull carotenoid and co-tricosenoic acid as shown in 7. These mixed films exhibit a very good cohesion, with an area of about 25 A2 per carotenoid molecule. They can easily be transferred onto solid substrates. Examination by UV-visible linear dichroism measurements confirms that the carotenoid chains are oriented perpendicularly to the surface of the substrate in card-packed aggregates, in which the polyenic chains interact via excitonic coupling, as indicated by the large hypsochromic shift of the tc-tc transition (20). 

The same push-pull carotenoid can also be introduced into LB multilayers built from 1/1 mixtures with an amphiphilic cyclodextrin. The polyenic chains are again perpendicular to the substrate and some carotenoid molecules do not aggregate, but are isolated by the host substance (21). 

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. 

Barzoukas and Blanchard-Desce proposed an approach of molecular engineering using multivalence-bond state models [55]. Push-pull polyenes were shown also to present an enhancement of the TPA response and a loss of transparency of molecules, as a function of the increase of the polyenic chain length [56,57]. Trends observed in these polyenic systems are supported by the large third-order optical nonlinearities measured in asymmetric carotenoids, in which the role of the large value of dipole moment difference A/z was shown [58]. 

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