The local electronic states in long polyene chains are considered analytically in the Huckel
approximation with and without taking account of bond alternation. The following types of chain
defects which might be responsible for an appearance of local states are discussed: change of the Coulomb integral of one atom, identical change of the Coulomb integrals of two atoms, strengthening or weakening of one bond, simultaneous change of the resonance and Coulomb integrals of the end bond or of the end atom. The conditions for removing the local states from the allowed bands depending on the positions of the defects in the chain are given.
G.F.Kventsel, Yu.A.Kruglyak. Local Electronic States in Long Polyene Chains. Theoret. chim.Acta, 12, 1 - 17 (1968)