Variation of the Conjugated Dye Experiment

The “conjugated dye” experiment has been in the physical chemistry curriculum for decades and modifications to it are well-known. In this article we change the analysis from an investigation of the length of the π-system in the compound to a determination of the amount of conjugation or delocalization of the π-electrons. It is shown that for linear polyenes (such as 2,4-hexadiene and 1,4-diphenyl-1,3-butadiene) the traditional application of the particle-in-a-box (PB) model and Hückel molecular orbital (HMO) methods is poor because of the lack of conjugation despite the alternation of double and single bonds that dominate the molecules. A perturbation to the PB model and a relaxation of simplifications in HMO theory allow these molecules to be studied and the amount of conjugation to be quantified.