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 αand 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.

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“…Consequently, the pitch-and-roll model 46 has been extended to the pitch–roll–squeeze model, which stresses the importance of intermolecular π–π and p–p orbital overlaps. 79…”

“…For the PTCDI-Ph phase II crystals subjected to the temperature and pressure stimuli, the observed energy gap and luminescence clearly show the importance of the p-stacking Consequently, the pitch-and-roll model 46 has been extended to the pitch-roll-squeeze model, which stresses the importance of intermolecular p-p and p-p orbital overlaps. 79 Due to the specific pattern of the H-bonds between the p-stacked columns, the crystals of PTCDI-Ph exhibit the negative linear compressibility and negative thermal expansion, both in the same direction [100]. The high rigidity in the direction perpendicular to soft p-stacks sustains the non-radiative pathways upon application of external stimuli (high pressure/temperature).…”