(Color online) Two states of a prototypical push-pull system. The long insulating chain of alternant polyacetylene has a “donor” and “acceptor” (COOH) groups attached at the opposite ends. The charge transfer occurring in such systems upon some physical or chemical process is simulated here by moving a proton from the COOH to groups: in (a) we show the “neutral” structure and in (b) the “charge-transfer” one. The two structures share the same “bulk,” where the cell (or repeating monomer) is , and the figure is drawn for .
Simple binary chains, where four monomer units are shown. (a) centrosymmetric bulk and [(b) and (c)] noncentrosymmetric bulk. Notice that (b) and (c) share the same bulk but have different terminations. We have actually run RHF and UHF calculations for such chains, choosing F as the gray atom and Li as the black atom, at variable sizes .
Longitudinal dipole moment per cell , defined in Eq. (5), of the LiF chain as a function of . The labels (a), (b), and (c) refer to the geometries schematized in Fig. 2. The short double arrow indicates the half-quantum difference of between the RHF and the UHF for the centrosymmetric system (a); the long double arrow indicates 1 quantum difference of between RHF calculations for systems (b) and (c).
Longitudinal dipole moment per monomer of the trans-polyacetylene oligomers, exemplified in Fig. 1, as a function of : diamonds for the neutral structure [NN] [Fig. 1(a)] and squares for the charge-tranfer structure [+⋯−] [Fig. 1(b)]. The double arrow indicates their difference, which is exactly equal to one quantum.
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