One-dimensional (1D) electronic structure is characteristic of face-to-face stacks of planar pi-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. (c) 2007 Elsevier B.V. All rights reserved.