Incorporating Spacer Molecules into the Tetrathiafulvalene--Chloranil Charge-Transfer Framework: Modulating the Neutral-Ionic Phase Transition.

The charge-transfer (CT) tetrathiafulvalene--chloranil (TTF-CA) crystal, a representative functional organic electronic material, has been the subject of both basic and applied research. This material shows a neutral-ionic phase transition (NIPT) that induces drastic changes in its physical properties. Here, we use this crystal as a framework and demonstrate a method for modulating physical properties of TTF-CA. A number of multicomponent (ternary) CT crystals were obtained by crystallizing TTF-CA with a third molecular species. These complexes all contain molecular sheets formed with TTF-CA; however, the third molecules were differently inserted between these sheets as spacers to induce a variety of physical properties in the CT crystals. Some showed spacer-modified NIPT, while the transition to the ionic state was suppressed in one complex despite the presence of TTF-CA sheets, which indicates that spacer molecules can modulate the physical properties or functions of CT crystals.