Varied charge-transfer complex crystals formed between diols and benzoquinone in the solid and solution states

Co-grinding of crystals of diols (electron donor) and benzoquinone (BQ, electron acceptor), and in some cases with crystals of a third aromatic compound, produces charge transfer (CT) complex crystals in the solid state without going through an obvious amorphous state, followed by a change in colour and in powder X-ray diffraction patterns. A milling power above a threshold is required for the new CT complex crystal formation, however, in some cases simple mixing of pulverized crystals is good enough to initiate the transformation. On exposing the CT complex crystals to air, without any mechanochemical force, they revert to the starting diol crystals by losing sublimable BQ molecules, rearranging hydrogen bonds and establishing a three-dimensional periodicity. Remarkably, solid-state and conventional solution crystallization give different results, i.e., formation or non-formation of CT complex crystals, or formation of different crystals with different colours. The phenomenon is surprisingly frequent and all three two-component CT systems and six out of the nine three-component systems investigated, exhibit different crystallization behaviour. A quartet structure where two BQ molecules are sandwiched by two naphthols was formed in the crystal obtained from solid-state crystallization. A new triplet CT complex structure in which a BQ is sandwiched by a naphthol and a biphenyl is also reported.