Interstack spin-orbit coupling and ESR line broadening in tetrathiafulvalenium tetracyanoquinodimethanide (TTF-TCNQ)

Spin-orbit-coupled electronic transitions between donor and acceptor conduction bands are shown to account for the ESR linewidth in tetrathiafulvalenium tetracyanoquinodimethanide (TTF-TCNQ) and similar compounds. Slight admixture of in-plane ($\sigma$) donor orbital character into the acceptor conduction ($\pi$) bands via the overlap of these orbitals is essential, since the donor heavy atom spin-orbit interaction mixes effectively only donor $\sigma$ and $\pi$ orbitals. The corresponding spin-orbit transitions within or between donor stacks are usually symmetry forbidden, which result is significant for the one-stack salts.