One-dimensional (1D) electronic structure is characteristic of face-to-face stacks of planar π-electron donors and/or acceptors. Dimerization phase transitions at T c < 400 K occur for various stacking motifs. Excitations of the dimerized phase are used to distinguish between dimerization driven by electron–phonon coupling (Peierls) and other mechanisms in correlated systems with spin and charge degrees of freedom.