Highest conjugated chains

In general, conjugation efficiency is highest with chains composed entirely of sp C-C bonds. However, these chains are flexible and thus conformation-ally inhomogeneous. In addition, they are unstable with respect to other photochemical (like cis-trans isomerization, [2 + 2]cyclization) or thermal processes (radical initiated or electrocyclizations). The chemical and conformational stability may be increased by rigidifying the carbon backbone. 

The shorter wavelength bands have significantly lower intensities than do their aliphatic counterparts, but with very long conjugated chains the two series approach each other. For example, the highest intensity peak (A = 386 nm) for Ph(C=C)ioPh has log e = 5-20 . However, the high-intensity bands of diarylpolyynes do not exhibit the distinct fine structure observed with the aliphatic derivatives. ... 

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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