Quaternary Charge-Transfer Solid Solutions: Electronic Tunability through Stoichiometry

Charge-transfer (CT) cocrystals formed between π-electron donor and acceptor molecules present diverse electronic behavior that can be rapidly modified due to the exchangeable nature of the CT partners. Unfortunately, chemical modifications to the donor and/or acceptor molecules often result in altered crystal packing that yields unpredictable changes in charge transfer and electronic coupling. Formation of solid solutions between isomorphous CT cocrystals offers an approach to predictably tune the electronic performance of CT materials because the packing motif is unaffected by compositional changes. We have formed ternary and quaternary CT solid solutions containing 4,6-dimethyldibenzothiophene (DMDBT), 4,6-dimethyldibenzoselenophene (DMDBS), 2,3-dichloro-5,6-dicyanobenzoquinone (DDQ), and 2,3-dibromo-5,6-dicyanobenzoquinone (DBQ) and have mapped the composition-dependent stability field for these materials using a combination of single-crystal X-ray diffraction, energy dispersive X-ray spectroscopy, and Raman spectroscopy. The CT solid solutions show tunable degrees of charge transfer (ρ), ranging from 0.005 to 0.19 e, and optical band gaps (E_opt) between 1.26(1) and 1.38(1) eV that are dependent on the DDQ:DBQ molar ratio. Additionally, we have exploited the isostructurality of the DMDBT-DDQ and DMDBT-DBQ cocrystal phases to produce single crystals with core-shell structures, demonstrating that methods adopted to optimize and passivate inorganic electronic materials can also be implemented for organics.