Direct relationship between intermolecular charge-transfer and charge-resonance complexes via structural changes in the arene donor with various π-acceptors

Abstract

Aromatic donors (D) spontaneously form a series of colored intermolecular charge-resonance (CR) complexes with their cation radicals (D⁺˙) as well as charge-transfer (CT) complexes with various π-acceptors (A). Spectroscopic changes during the formation of these CR and CT complexes are both included in a single Mulliken plot of E_CR,CTversus E°_ox − E°_red. Precise X-ray crystallographic studies now reveal almost identical structural features of the CR and CT complexes, especially with a prototypical strong electron donor such as octamethylbiphenylene (OMB). Whereas most weaker donors do not show perceptible alterations of their molecular geometry during complex formation, OMB exhibits pronounced bond elongation/contraction of almost 2 pm both in its CT complexes with strong acceptors (TCNE and TCNQ) and in its CR complex (OMB₂)⁺˙. In both cases, the structural changes lie midway between geometries of the neutral OMB donor and its fully oxidized cation radical OMB⁺˙. The CT and CR complexes of OMB in the solid state also show similar crystal packing features that maintain interplanar separations as short as 3.1–3.2 Å with significant distortion (∼6°) from planar molecular geometry. Together with the similar values of K_CR = 350 M⁻¹ and K_EDA = 440–490 M⁻¹, we acknowledge the common nature of the CT and CR complexes through an orbital consideration (Chart 1) that recognizes the antibonding component in CR complex formation.