I was reading that in conjugated compounds, the extent of conjugation decides what wavelength photon it can absorb. Longer the conjugation "chain", lesser the photon energy required. So, molecules like lycopene enter image description here

with 11 conjugated bonds is able to absorb in the blue range, appearing red. If this is the case, consider the polymer poly-acetylene

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which has a conjugated "chain" spanning "thousands". Does this mean, this molecule would absorb infra red and lower energy light?


In principle yes, longer conjugated regions absorb at longer wavelengths. The simplest model when thinking about this is to consider a particle in a box, or on a ring if considering cyclic conjugation as in the chlorophyll's. However, in very long polymers, such as polyacetylene, the conjugation length is in fact rather small, approx 10 to 20 double bonds. Nanometer thick films appear brown and absorb below 800 nm ish whereas if the conjugation length were very large, as per the chemical structure you draw, the molecules would absorb to longer wavelengths and appear black.

This limited conjugation is due to two things, first the molecules are twisted and bent in forming the polymer and these distortions alter the electronic properties just enough to shorten the conjugation length. Second even when this twisting etc. were removed there are two ways of arranging the double bonds depending how you start. At their junction these two forms form what is called a 'solition' or 'topological kink' and this is a non-bonding electron between two regions of conjugation. This also limits conjugation.


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