On the Supra-LUMO Interaction: Case Study of a Sudden Change of Electronic Structure as a Functional Emergence

The synergistic functioning of redox-active components that emerges from prototypical 2,2'-di(N-methylpyrid-4-ylium)-1,1'-biphenyl is described. Interestingly, even if a trans conformation of the native assembly is expected, due to electrostatic repulsion between cationic pyridinium units, we demonstrate that cis conformation is equally energy-stabilized on account of a peculiar LUMO (SupLUMO) that develops through space, encompassing the two pyridiniums in a single, made-in-one-piece, electronic entity (superelectrophoric behavior). This SupLUMO emergence, with the cis species as superelectrophore embodiment, originates in a sudden change of electronic structure. This finding is substantiated by insights from solid state (single-crystal X-ray diffraction) and solution (NOE NMR and UV-vis-NIR spectroelectrochemistry) studies, combined with electronic structure computations. Electrochemistry shows that electron transfers are so strongly correlated that two-electron reduction manifests itself as a single-step process with a large potential inversion consistent with inner creation of a carbon-carbon bond (digital simulation). Besides, absence of reductive formation of dimers is a further indication of a preferential intramolecular reactivity determined by the SupLUMO interaction (cis isomer pre-organization). The redox-gated covalent bond, serving as electron reservoir, was studied via atropisomerism of the reduction product (VT NMR study). The overall picture derived from this in-depth study of 2,2'-di(N-methylpyrid-4-ylium)-1,1'-biphenyl proves that trans and cis species are worth considered as intrinsically sharply different, that is, as doubly-electrophoric and singly-superelectrophoric switchable assemblies, beyond conformational isomerism. Most importantly, the through-space-mediated SupLUMO may come in complement of other weak interactions encountered in Supramolecular Chemistry as a tool for the design of electroactive architectures.

Four-Step Domino Reaction Enables Fully Controlled Non-Statistical Synthesis of Hexaarylbenzene with Six Different Aryl Groups*

Hexaarylbenzene (HAB) derivatives are versatile aromatic systems playing a significant role as chromophores, liquid crystalline materials, molecular receptors, molecular-scale devices, organic light-emitting diodes and candidates for organic electronics. Statistical synthesis of simple symmetrical HABs is known via cyclotrimerization or Diels-Alder reactions. By contrast, the synthesis of more complex, asymmetrical systems, and without involvement of statistical steps, remains an unsolved problem. Here we present a generally applicable synthetic strategy to access asymmetrical HAB via an atom-economical and high-yielding metal-free four-step domino reaction using nitrostyrenes and α,α-dicyanoolefins as easily available starting materials. Resulting domino product-functionalized triarylbenzene (TAB)-can be used as a key starting compound to furnish asymmetrically substituted hexaarylbenzenes in high overall yield and without involvement of statistical steps. This straightforward domino process represents a distinct approach to create diverse and still unexplored HAB scaffolds, containing six different aromatic rings around central benzene core.

Hydrostatic Pressure on Toroidal Interaction and Propeller Chirality of Hexaarylbenzenes: Explicit Solvent Effects on Differential Volumes in Methylcyclohexane and Hexane

A unique and effective interaction between the peripheral aromatic blades makes hexaarylbenzenes (HABs) attractive in fundamental research as well as for various applications such as molecular wires, sensors, and supramolecular assemblies. The chiroptical responses of HABs are susceptible to environmental factors such as solvent and temperature owing to the dynamic conformational transitions between the conformers. In this study, pressure dependence on the propeller chiral HABs in two different solvents was studied in detail. The effective differential volumes for two different equilibria were determined by quantitative analyses of CD spectra, affording very large differential volumes from the propeller to toroidal conformer (ΔV_T-C ) of +43 and +42 cm³  mol^-1 , for H2 and H6, respectively, in methylcyclohexane. The value of H6 was further enhanced to +72 cm³  mol^-1 in hexane, the largest value for the typical unimolecular conformational change. Such a response of propeller chirality in HABs is expedient in designing more advanced piezo-sensitive materials.