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 (Eopt) 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.

电荷转移（charge-transfer, CT）共晶是由π电子给体与受体分子形成的一类晶体，其展现出多样的电子行为，且可因CT组分的可交换性实现快速调控。遗憾的是，对给体或受体分子进行化学修饰往往会改变晶体堆积模式，进而导致电荷转移与电子耦合发生不可预测的变化。通过同构CT共晶之间构筑固溶体，可为精准调控CT材料的电子性能提供可行途径，因为其晶体堆积基元不会随组分变化而发生改变。
我们制备了包含4,6-二甲基二苯并噻吩（4,6-dimethyldibenzothiophene, DMDBT）、4,6-二甲基二苯并硒吩（4,6-dimethyldibenzoselenophene, DMDBS）、2,3-二氯-5,6-二氰基苯醌（2,3-dichloro-5,6-dicyanobenzoquinone, DDQ）以及2,3-二溴-5,6-二氰基苯醌（2,3-dibromo-5,6-dicyanobenzoquinone, DBQ）的三元及四元CT固溶体，并结合单晶X射线衍射、能量色散X射线能谱与拉曼光谱，测绘了该类材料的组分依赖稳定域。该CT固溶体展现出可调谐的电荷转移程度（ρ），数值介于0.005至0.19 e之间，其光学带隙（Eopt）处于1.26(1)至1.38(1) eV范围内，且与DDQ与DBQ的摩尔比密切相关。
此外，我们利用DMDBT-DDQ与DMDBT-DBQ共晶相的同构性，制备出具有核壳结构的单晶，证实了用于优化与钝化无机电子材料的方法同样可应用于有机电子材料体系。