Topological effect on MO energies: Part 14. Topological MO-effect in intramolecularly coupled polymethines

Abstract

The simple molecular orbital (MO) energies of intramolecularly coupled polymethines which possess an odd membered ring are studied. It is shown that the topological effect on molecular orbitals (TEMO) [1] holds for the considered polymethines. In particular the MO energies of the symmetrical and unsymmetrical compounds can be compared where both have N conjugated centers and ( N + 1) π-electrons. It is found that when ( N + 1)/2 is an odd number then the HOMOLUMO separation, Δ E, of each unsymmetrical compound is bathochromically shifted with respect to Δ E of the corresponding symmetrical compound. For ( N + 1)/2 being an even number, this shift is hypsochromic.