A Large π‐Extended Carbon Nanoring Based on Nanographene Units: Bottom‐Up Synthesis,Photophysical Properties,and Selective Complexation with Fullerene C70

Herein we report the organoplatinum‐mediated bottom‐up synthesis, characterization, and properties of a novel large π‐extended carbon nanoring based on a nanographene hexa‐peri ‐hexabenzocoronene (HBC) building unit. This tubular structure can be considered as an example of the longitudinal extension of the cycloparaphenylene scaffold to form a large π‐extended carbon nanotube (CNT) segment. The cyclic tetramer of a tetramesityl HBC ([4]CHBC) was synthesized by the reaction of a 2,11‐diborylated hexa‐peri ‐hexabenzocoronene with a platinum complex, followed by reductive elimination. The structure of this tubular molecule was further confirmed by physical characterization. Theoretical calculations indicate that the strain energy of this nanoring is as high as 49.18 kcal mol⁻¹. The selective supramolecular host–guest interaction between [4]CHBC and C₇₀ was also investigated.     

Facile Synthesis of Polycyclic Pentalenes with Enhanced Hückel Antiaromaticity

Pentalenes represent highly reactive Hückel antiaromatics with 8π electrons. Usually, pentalenes are stabilized by incorporation of two benzene rings in a fused fashion. In dibenzo[a ,e ]pentalenes, however, the high aromaticity of the fused benzene rings compromises the inherent antiaromaticity of the pentalene core. Herein, we disclose that this forfeited antiaromaticity can be restored by fusing four additional aromatic rings onto the peripheral positions of dibenzo[a,e]pentalenes. Such polycyclic pentalenes were prepared by successive transannular cyclizations via in situ‐generated tetrakisdehydro[16]annulenes. The thus obtained compounds showed intriguing properties, for example, characteristic absorptions in the visible‐to‐near‐infrared (NIR) region and low reduction potentials. These results hence afford a design principle to produce highly antiaromatic yet stable pentalenes. The antiaromaticity of the pentalene core can be widely tuned via the degree of aromaticity of the peripherally fused rings.     

A Near‐Infrared Dye That Undergoes Multiple Interconversions through Acid–Base Equilibrium and Reversible Redox Processes

A near‐infrared (NIR) polymethine dye ( 1 ), consisting of a cyclohepta[1,2‐b ;4,3‐b′ ]dithiophene and two phenol moieties, was synthesized. This dye exhibited pH‐responsive changes in its photophysical properties due to a two‐step acid–base equilibrium that produced a protonated cation ( 1H⁺ ) and an anion ( 1⁻ ). While 1H⁺ showed an intense fluorescence in the red region of the visible spectrum, 1⁻ exhibited a strong absorption in the NIR region. The tropylium ion character in 1H⁺ induces high pK _a1 and pK _a2 values for 1 . Moreover, a stable radical ( 1^. ) was prepared, which showed a NIR absorption band with a maximum at circa 1600 nm. The cyclic voltammogram of 1^. revealed a two‐step reversible redox process that produced 1⁻ and the cation 1⁺ , which is different from 1H⁺ . These redox processes accompany drastic electrochromic changes in the vis–NIR region. Overall, 1 is susceptible to multiple interconversions between five forms, due to the multifaceted character of the cycloheptadithiophene skeleton.     

A Three‐Dimensionally π‐Conjugated Diradical Molecular Cage

π‐Conjugated molecular cages are very challenging targets in structural organic chemistry, supramolecular chemistry, and materials science. The synthesis and physical characterizations are reported of the first three‐dimensionally π‐conjugated diradical molecular cage PTM‐C, in which two polychlorotriphenylmethyl (PTM) radicals are linked by three bis(3,6‐carbazolyl) bridges. This cage compound was synthesized mainly by intermolecular Yamamoto coupling followed by deprotonation and oxidation. It is stable and its structure was confirmed by X‐ray crystallographic analysis. The two carbon‐centered PTM radicals are weakly coupled through electronic interactions with the carbazole spacers, as revealed by optical, electronic, and magnetic measurements as well as theoretical calculations.     

Nanohoop Rotaxanes from Active Metal Template Syntheses and Their Potential in Sensing Applications

The unique optoelectronic properties and smooth, rigid pores of macrocycles with radially oriented π systems render them fascinating candidates for the design of novel mechanically interlocked molecules with new properties. Two high‐yielding strategies are used to prepare nanohoop [2]rotaxanes, which owing to the π‐rich macrocycle are highly emissive. Then, metal coordination, an intrinsic property afforded by the resulting mechanical bond, can lead to molecular shuttling as well as modulate the observed fluorescence in both organic and aqueous conditions. Inspired by these findings, a self‐immolative [2]rotaxane was then designed that self‐destructs in the presence of an analyte, eliciting a strong fluorescent turn‐on response, serving as proof‐of‐concept for a new type of molecular sensing material. More broadly, this work highlights the conceptual advantages of combining compact π‐rich macrocyclic frameworks with mechanical bonds formed via active‐template syntheses.     

2D Poly(arylene vinylene) Covalent Organic Frameworks via Aldol Condensation of Trimethyltriazine

Designing structural order in electronically active organic solids remains a great challenge in the field of materials chemistry. Now, 2D poly(arylene vinylene)s prepared as highly crystalline covalent organic frameworks (COFs) by base‐catalyzed aldol condensation of trimethyltriazine with aromatic dialdehydes are reported. The synthesized polymers are highly emissive (quantum yield of up to 50 %), as commonly observed in their 1D analogues poly(phenylene vinylene)s. The inherent well‐defined porosity (surface area ca. 1000 m² g^?1, pore diameter ca. 11 Å for the terephthaldehyde derived COF‐1) and 2D structure of these COFs also present a new set of properties and are likely responsible for the emission color, which is sensitive to the environment. COF‐1 is highly hydrophilic and reveals a dramatic macroscopic structural reorganization that has not been previously observed in framework materials.     

Regioselective Transformation of Long π‐Conjugated Backbones: From Oligofurans to Oligoarenes

We demonstrate the transformation of oligofurans through sequential Diels–Alder cycloaddition reactions to provide oligoarenes in two chemical steps, regardless of the oligomer length. By this method, oligonaphthalenes containing up to six units were obtained in high yield through the formation of up to 12 new C−C bonds. The versatility of this method was demonstrated for various polyaromatic hydrocarbons. The regioselectivity of this process enabled the synthesis of a library of substituted triarylenes from a single terfuran precursor by modification of the dienophile strength and the order of addition. Overall, this study demonstrates that long oligofurans are interesting not only as organic electronic materials, but also as starting materials for the formation of various conjugated systems.