Charge-Transfer Benzoquinone:Catechol Complexes, Their Aging Reactions, High-Pressure Polymorphs and Skew Pancake Bonds

Abstract

High Resolution Image Download MS PowerPoint Slide Charge-transfer complexes are the key elements of optoelectronic devices. Quinhydrone (Qh) was the first discovered charge-transfer complex, and for its structure, the π-stacking and pancake bonds were identified. Later, the atom-over-atom criterion was added to the pancake-bond definition. However, this feature is absent in the structures of both Qh polymorphs. Here, we report new charge-transfer complexes of para -benzoquinone ( p Bq) and catechol (Ct), the only unknown isomers of Qh. Compounds p Bq and Ct preferentially co-crystallize to a 1:1 brown complex, α- p Bq:Ct, with the molecules alternatively stacked into columns. We have revealed the accelerated aging reaction of α- p Bq:Ct, leading to an orange p Bq:2Ct complex, the only representative of the 1:2 stoichiometry for the complexes of p Bq with isomeric hydroquinone (Hq), resorcinol (Rs), and Ct. On compression to 1.5 GPa, α- p Bq:Ct undergoes a transition converting the catechol conformers syn into anti in a markedly darker new phase β- p Bq:Ct. This conversion extensively extends the transition hysteresis between phases α- and β- p Bq:Ct. At about 2.5 GPa, a radical complex γ- p Bq:Ct is formed, which starting from 3.0 GPa gradually transforms into the black amorphous phase δ- p Bq:Ct. In phase α- p Bq:2Ct, both Ct-conformers syn and anti are present, but high pressure induces a phase transition, eliminating conformers syn in a much darker new phase, β- p Bq:2Ct. High pressure reduces the energy band gap of all p Bq:Ct and p Bq:2Ct phases. The skew geometry of pancake bonds associated with p–p interactions is common for all Qh, p Bq:Rs, p Bq:Ct, and p Bq:2Ct complexes.

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