Neutral-ionic interface in organic charge-transfer salts

Charge transfer (CT) stabilization in linear stacks D+A-D+A- of -electron donors (D) and acceptors (A) involve spin-dependent configuration interactions that are treated exactly in rings of N=4, 6, 8, 10 sites, and extrapolated to N, by adapting valence-bond techniques to electron-hole excitations. The ground state CT (z) and the magnetic gap Em(z)2|t| to the lowest triplet are computed for arbitrary z=2|t|, where -2 is the energy for DAD+A-, -|t|=D+A-|H|DA is the Mulliken CT integral, and D2+, A2- sites are excluded. The spin degeneracy of A- and D+ ion radicals is treated exactly. Instead of the discontinuous change from =0 to =1 in the limit |t|0, finite overlap gives a continuous (z) and (zc)=0.680.01 at the neutral-ionic interface zc=0.530.01. The magnetic gap Em is finite for z<zc and vanishes for z>zc, where there is a diamagnetic to paramagnetic transition and the ground state switches from k=0 to k= symmetry. Collective effects due to long-range three-dimensional Coulomb interactions are included in a Hartree approximation and produce a first-order transition, with discontinuous (z), when the critical value m2|t|=1.40.1 of the Madelung stabilization m of a dimer is exceeded. The puzzling magnetic gaps in paramagnetic organic CT salts with mixed regular stacks arise naturally for partial CT and z<zc. Valence-bond analysis of CT excitations models the physical properties of organic complexes with overlapping sites and intermediate.