Aging effect on the co-crystallization behavior of the donor and acceptor crystals in aqueous dispersions

Abstract

Charge-transfer (CT) co-crystals composed of organic donor and acceptor molecules are attractive materials from both basic and applied perspectives. One of the most fascinating protocols for the formation of a CT crystal is the use of donor and acceptor crystals instead of their molecules as starting materials. However, the lack of studies has limited the universality of the method for future applications. In this study, we adapted the mixing of donor and acceptor crystal dispersions to fabricate CT co-crystals, and “phase-separated” crystals, which are a mixture of the donor and acceptor crystals. The aqueous dispersions of each crystal were prepared by the reprecipitation method, and mixed to form the targeted crystals. We controlled the co-crystallization behavior between the donor and acceptor crystals by aging of the dispersion, which was left standing in air for 24 h before mixing. CT co-crystals were formed in the non-aged mixture; in contrast, “phase-separated” crystals were obtained in the aged mixture. Current–voltage measurements using tetrathiafulvalene and tetracyanoquinodimethane revealed that the conventional CT co-crystals exhibited lower sheet resistance (10³ to 10⁴ Ω sq⁻¹) than that of the “phase-separated” crystals, whereas the “phase-separated” crystals showed a photoconductive response.