In this article, a new index was developed for predicting the band gap of π-conjugated polymers based on isodesmic reactions between a dimethylated monomer and an oligo-acetylene. The index can be uniquely defined for hetero-cyclic and poly-cyclic monomers. The index correlated well with the calculated band gap of polymers. In addition, it shows that band gap can be controlled by copolymerization using the index based on only monomer information without calculations of polymers.
