Re: Why is cyclooctatetraene yellow in color?

COTs belong to the chemical class called annulenes. COT is [8]annulene 
and contrary to your assertion, it is conjugated enough to exhibit color. 
COT takes what is called a "tub" conformation which is a bent (side view 
= \__/ )configuration so it does have pi orbital overlap, just not as 
much as a planar aromatic system would have. There is enough electron 
delocalization in this conformation to cause increased stability and 
lowering of the overall energy of the molecule [see March below]. The 
absorption spectrum is predominantly in the UV but is shifted enough to 
lower energy that there is a small tailing of the main absorption bands 
into the visible resulting in a pale yellow color. Most annulenes of 8 or 
greater do have pale yellow to orange color because of the conjugation. I 
would assume that substituted COTs could easily loose color due to 
destabilization of the electron distribution. Anything that would disrupt 
the already weak conjugation would certainly cause a shift in the UV-VIS 
absorption spectrum back toward higher energy making the compound 
colorless.



March Jerry; (1985). Advanced Organic Chemistry reactions, mechanisms and 
structure (3rd ed.). New York: John Wiley & Sons, inc. ISBN 0-471-85472-7