Polyene dyes

Polyene and polymethine dyes are two structurally related groups of dyes which contain as their essential structural feature one or more methine (-CH=) groups. Polyene dyes contain a series of conjugated double bonds, usually terminating in aliphatic or alicyclic groups. They owe their colour therefore simply to the presence of the conjugated system. In polymethine dyes, electron-donor and electron-acceptor groups terminate either end of the polymethine chain, so that they may be considered as typical donor-acceptor dyes. 

The best-known group of polyene dyes is the carotenoids, which are widely encountered natural colorants. /1-Carotene (104) is given as an... 

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. 

Shinpo, A., Suga, S., Sayama, K., and Arakawa, H. (2003) Novel polyene dyes for highly efficient dye-sensitized solar cells. Chem. Commun., 252-253. 

Azobenzene-dye-, stilbene-dye-, or polyene-dye-functionalised polymers have been investigated as second-order NLO materials materials), showing efficient NLO characteristics with low absorption loss [44]. In this case, the dye in the polymer should be aligned in a specific direction to possess second-order optical non linearity. For that purpose, a high electric field... 

Marinado T, Nonomura K, Nissfolk J, Karlsson MK, Hagberg DP, Sun L, Mori S, Hagfeldt A (2009) How the nature of triphenylamine-polyene dyes in dye-sensitized solar cells affects the open-circuit voltage and electron lifetimes. Langmuir 26 2592-2598... 

Studies show polymethine chain lengthening in highly asymmetrical dyes to be accompanied by strong quadratic increases in deviations (3,7,9,10,30,31). In contrast to polymethines, the deviations of the related asymmetrical polyenes are negative as the break of the symmetry leads to a decrease in bond alternation (32). 

Since highly basic heterocycles accommodate the positive charge readily and maintain the aromatic nature of the dimethylarnino benzene group, the resulting bond alternation induces a polyene character to the dye chromophore, and the absorption is shifted accordingly to a shorter wavelength. 

There is a third method proposed for classifying colorants which is in terms of the mechanism of the electronic excitation process. According to this method, organic colorants may be classified as donor acceptor, polyene, cyanine or n-rc chromogens. While this method of classification is undoubtedly of importance theoretically, it is arguably of lesser practical importance, since the vast majority of commercial organic dyes and... 

Wang, X. F., Y. Koyama, H. Nagae, Y. Yamano, M. Ito, and Y. Wada. 2006. Photocurrents of solar cells sensitized by aggregate-forming polyenes Enhancement due to suppression of singlet-triplet annihilation by lowering of dye concentration or light intensity. Chem. Phys. Lett. 420 309-315. 

The asymmetrical D-ji-A dyes, often referred to as push-pull polyenes, are an additional class of cyanine-like molecules of interest. Due to their dipolar nature, the linear and nonlinear optical properties of this series of dyes can be strongly influenced by solvent polarity [84]. The structures of a series of such dyes (G19,... 

The spectra of linear polyenes are modelled well as one-dimensional free-electron systems. The cyanine dyes are a classical example. They constitute a class of long chain conjugated systems with an even number n of 7r-electrons distributed over an odd number N = n — 1 of chain atoms. The cyanine absorption of longest wavelength corresponds to promotion of an electron from the highest occupied energy level, En/2 to the lowest unoccupied level, such that in terms of a free-electron model... 

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