π-Electronic structure of polymethines1

Polymethines are chain molecules with equal bond-lengths and charge alteration along the chain. They embrace three classes of compounds with (N - 1)π, Nπ, or (N + 1)π electrons on N atoms. The MO-theoretical characteristics of the (N - 1)π, Nπ, and (N + 1 )π polymethines are strongly related, and the eigenvalues can be derived by a mnemonic technique. The theoretical results establish the particular position of the polymethines among the π-systems and allow a rationalization of experimental findings within the framework of the one electron theory.