Tiara[5]arenes (T[5]s), a new class of five‐fold symmetric oligophenolic macrocycles that are not accessible from the addition of formaldehyde to phenol, were synthesized for the first time. These pillar[5]arene‐derived structures display both unique conformational freedom, differing from that of pillararenes, with a rich blend of solid‐state conformations and excellent host–guest interactions in solution. Finally we show how this novel macrocyclic scaffold can be functionalized in a variety of ways and used as functional crystalline materials to distinguish uniquely between benzene and cyclohexane.

An olympicenyl radical, a spin 1/2 hydrocarbon radical with C 2v symmetry and uneven spin distribution, remains elusive despite the considerable theoretical research interest. Herein, we report syntheses of two air-stable olympicenyl radical derivatives, OR1 and OR2, with half-life times (τ1/2) in air-saturated solution of 7 days and 34 days. The high stability was ascribed to kinetic blocking of reactive sites with high spin densities. X-ray crystallographic analysis revealed unique 20-center–2-electron head-to-tail π-dimer structures with intermolecular distances shorter than the sum of van der Waals radius of carbon. The ground state of the π-dimers was found to be singlet, with singlet–triplet energy gaps estimated to be −2.34 kcal/mol and −3.28 kcal/mol for OR1 and OR2, respectively, by variable-temperature electron spin resonance (ESR) spectroscopy. The monomeric radical species were in equilibrium with the π-dimer in solution, and the optical and electrochemical properties of the monomers and π-dimers in solution were investigated by UV–vis–NIR spectroscopy and cyclic voltammetry, revealing a concentration-dependent nature. Theoretical calculations illustrated that upon formation of a π-dimer the local aromaticity of each monomer was enhanced, and spatial ring current between the monomers was present, which resulted in an increment of aromaticity of the interior of the π-dimer.

Understanding the structural origins of diverse mechanical behaviors of organic crystals is critical for designing functional materials for a number of technological applications. To facilitate this effort, we have examined the mechanical behaviors of two polymorphs of a structurally rigid molecule, coumarin. Surprisingly, form I crystals are highly elastic while form II crystals are two-dimensional (2D) plastic and twistable. The strikingly different mechanical behaviors corroborate with the respective prevailing structural mechanisms, i.e., the high elasticity is enabled by an interlocked layer structure with nearly isotropic dispersive interactions, while permanent twisting requires two orthogonal slip planes. Since molecular conformation does not vary, the strikingly different mechanical behaviors prove that molecular flexibility is not a prerequisite for crystals to exhibit mechanical flexibility. Instead, the differences in coumarin molecular packing and correspondingly different molecular interactions underlie the distinct mechanical behaviors of the two forms, which are systematically probed through crystal bending and nanoindentation, micro-Raman spectroscopy, and energy framework analysis.

The design and preparation of molecular systems with multiple geometric and electronic configurations are the cornerstones for multifunctional materials with stimuli-responsive behaviors. We describe here the regioselective and facile synthesis of two types of overcrowded ethylene-bridged nanohoop dimers, with folded and twisted geometric structures as well as closed-shell, diradical and dication electronic structures. The strained nanohoop structures have a profound effect on the overall molecular and electronic configurations, which resulted in the destabilized diradical state. X-ray crystallographic analysis revealed the folded molecular geometry for the neutral species and twisted geometry for the dication species. The unique molecular dynamics, optical properties, and dynamic redox properties were disclosed in the solution phase by spectroscopic and electrochemical methods. Furthermore, the global Hückel and Möbius aromaticity were revealed by a combination of experimental and theoretical approaches. Our studies shed light on the design of nanohoop-incorporated multiconfigurational materials with unique topologies and functions.

Singlet diradicaloids hold great potential as semiconductors for organic field‐effect transistors (OFETs). However, their relative low material and device stabilities impede the practical applications. Here, to achieve balanced stability and performance, two isomeric dibenzoheptazethrene derivatives with singlet diradical character were synthesized in a concise manner. Benefitting from the aromatic stabilization, both compounds display a small diradical character and large singlet–triplet gap, as corroborated by variable‐temperature electron paramagnetic resonance spectra, single‐crystal analysis, and theoretical calculations. OFET devices based on single crystals showed a high hole mobility of 0.15 cm2 V−1 s−1, which is the highest for zethrene‐based semiconductors. Both isomers exhibited remarkable material stability in air‐saturated solutions as well as excellent bias‐stress and storage stability in device under ambient air.