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

Abstract

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 (pBq) and catechol (Ct), the only unknown isomers of Qh. Compounds pBq and Ct preferentially co-crystallize to a 1:1 brown complex, α-pBq:Ct, with the molecules alternatively stacked into columns. We have revealed the accelerated aging reaction of α-pBq:Ct, leading to an orange pBq:2Ct complex, the only representative of the 1:2 stoichiometry for the complexes of pBq with isomeric hydroquinone (Hq), resorcinol (Rs), and Ct. On compression to 1.5 GPa, α-pBq:Ct undergoes a transition converting the catechol conformers syn into anti in a markedly darker new phase β-pBq:Ct. This conversion extensively extends the transition hysteresis between phases α- and β-pBq:Ct. At about 2.5 GPa, a radical complex γ-pBq:Ct is formed, which starting from 3.0 GPa gradually transforms into the black amorphous phase δ-pBq:Ct. In phase α-pBq: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-pBq:2Ct. High pressure reduces the energy band gap of all pBq:Ct and pBq:2Ct phases. The skew geometry of pancake bonds associated with p-p interactions is common for all Qh, pBq:Rs, pBq:Ct, and pBq:2Ct complexes.