From Macrocycles to Quantum Rings: Does Aromaticity Have a Size Limit?

ConspectusThe ring currents of aromatic and antiaromatic molecules are remarkable emergent phenomena. A current is a quantum-mechanical feature the whole system, its existence cannot be inferred from properties individual components ring. Huckel's rule states that when an molecule with circuit [4n + 2] π electrons placed in magnetic field, field induces creates opposing external inside In contrast, rings 4n exhibit opposite direction. This bears name Erich Huckel, it grew his molecular orbital theory, but modern formulations incorporate contributions others, particularly William Doering Ronald Breslow. It often assumed aromaticity restricted to small up about 22 electrons. Account outlines discovery global large macrocycles circuits 162 The largest yet investigated cyclic porphyrin oligomers, which after oxidation, reduction or optical excitation not neutral ground state. these nanorings leads experimentally measurable stabilization energies addition effects can studied by NMR spectroscopy. Wheel-like templates bound nanorings, providing excellent control over geometry allowing shielding probed nanoring. systems well-reproduced density functional theory (DFT), although choice DFT turns out critical. Here we review recent this present simple method for determining susceptibility (in nA/T) any experimental data classical Biot-Savart calculations. We use quantify variety rings. survey confirms reliably predicts direction current, reveals surprisingly insensitive size investigation even larger interesting because also observed mesoscopic metal at low temperatures. striking similarity between arises have common origin: field-dependent phase shift electronic wave function. main difference flux through much greater their areas, so persistent nonlinear oscillatory applied whereas response approximately linear field. discuss how nonlinearity expected emerge high fields. insights work fundamentally important understanding bridging gap chemistry physics, potentially leading new functions electronics.