Unveiling the Hidden σ-Dimerization of a Kinetically Protected Olympicenyl Radical.

The σ-dimer of a kinetically protected olympicenyl radical, which evaded the experimental detection, was revealed by conversion into biolympicenylidene with E-configuration in a regioselective manner. The complicated stereochemistry and energetics of the σ-dimers derived from C2v symmetry and uneven spin distribution of the olympicenyl radical were revealed by the theoretical calculations, and the energetic preference of π-dimer over σ-dimer by a minute gap was disclosed. The E-biolympicenylidene, a polycyclic ene structure previously considered as reactive intermediate in the phenalenyl radical system, exhibited exceptional stability, which allowed for a detailed investigation on its singlet diradical character and physical properties by means of X-ray crystallography, UV-vis-NIR absorption/emission spectroscopy and cyclic voltammetry, and assisted by theoretical calculations. The E-biolympicenylidene showed high resistance towards both thermal and photochemical ring-cyclization reactions, which was attributed to high activation energies for the rate-determining electrocyclization operated on both disrotatory and conrotatory mode, as well as a small spin density at the bonding sites for the radical-radical coupling process.

Dimeric Cycloparaphenylenes with a Rigid Aromatic Linker.

Dimeric cycloparaphenylene (CPP) architectures with well-defined flipping motion are constructed taking advantage of an efficient cyclocondensation reaction. Variable-temperature nuclear magnetic resonance (VT-NMR) analyses and theoretical calculations indicate rapid interconversion of cis and trans conformers at room temperature, while energetically favorable trans conformer exists at low temperature with the metastable cis conformer hidden. The trihexylsilylethynyl-substituted dimer exhibits bright emission in solution at 616 nm with quantum yield up to 80 %, representing the brightest CPP-based emitter beyond 600 nm. A 1:2 host-guest complex of the dimer and C60 is established with negative cooperativity, demonstrating the first example of 1:2 complex from CPP derivatives.

Isomeric Dibenzoheptazethrenes for Air-Stable Organic Field-Effect Transistors.

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.

Stable Olympicenyl Radicals and Their π-Dimers.

An olympicenyl radical, a spin 1/2 hydrocarbon radical with C2v 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.

Pd-Catalyzed Consecutive C-H-Arylation-Triggered Cyclotrimerization: Synthesis of Star-Shaped Benzotristhiazoles and Benzotrisoxazoles.

Star-shaped π-extended molecules comprising discotic aromatic cores and peripheral π-conjugated arms have attracted significant attention as diverse optoelectronic materials in terms of their large π-surface, tunable self-assembly, enhanced charge transport and fluorescence, and liquid crystallinity. Although many efforts have been made in the construction of various aromatic discotic cores, a new class of C3 -symmetric star-shaped discotic π-molecules consisting of electron-deficient benzotristhiazole and benzotrisoxazole cores, which are expected to exhibit distinct optoelectronic properties, remains unexplored owing to the unachievable synthetic approaches involved in their synthesis. Herein, we report a novel and highly efficient Pd-catalyzed cyclotrimerization of the functionalized thiazoles or oxazoles for the construction of a new class of discotic molecules of benzotristhiazole and benzotrisoxazole central cores with star-shaped π-conjugated arms. The combination of [Pd2 (dba)3 ]/XPhos (dba=dibenzylideneacetone) catalyst systems with the 4-bromo-substituted thiazole enables the formation of a sufficiently stable thiazole-Pd species that participates in the subsequent C-H arylations consecutively to form the corresponding cyclic trimer products. This new class of star-shaped discotic π-extended products showed tunable energy levels and high fluorescence quantum yields that make them promising candidates for optoelectronic applications.

FeCl3-Mediated Oxidative Spirocyclization of Difluorenylidene Diarylethanes Leading to Dispiro[fluorene-9,5'-indeno[2,1-a]indene-10',9''-fluorene]s.

A novel FeCl3-mediated oxidative spirocyclization for construction of a new class of di-spirolinked π-conjugated molecules, dispiro[fluorene-9,5'-indeno[2,1-a]indene-10',9''-fluorene]s (DSFIIFs), has been reported. The combination of FeCl3 with FeO(OH) triggered an unprecedented double one-electron oxidation of difluorenylidene diarylethanes to afford the corresponding dispirocycles in high yields. The highest fluorescence quantum yield was up to 0.94 in solution. This protocol is also applicable to the synthesis of the non-spirolinked dihydroindenoindenes.

Modular synthesis, host-guest complexation and solvation-controlled relaxation of nanohoops with donor-acceptor structures.

Carbon nanohoops with donor-acceptor (D-A) structures are attractive electronic materials and biological fluorophores, but their synthesis is usually challenging. Moreover, the preparation of D-A nanohoop fluorophores exhibiting high fluorescence quantum yields beyond 500 nm remains a key challenge. This study presents a modular synthetic approach based on an efficient metal-free cyclocondensation reaction that readily produced nine congeners with D-A or donor-acceptor-donor' (D-A-D') structures, one of which is water-soluble. The tailored molecular design of nanohoops enabled a systematic and detailed study of their host-guest complexation with fullerene, optical properties, and charge transfer (CT) dynamics using X-ray crystallography, fluorescence titration, steady and ultrafast transient absorption spectroscopy, and theoretical calculations. The findings revealed intriguing physical properties associated with D-A motifs, such as tight binding with fullerene, moderate fluorescence quantum yields (37-67%) beyond 540 nm, and unique solvation-controlled CT relaxation of D-A-D' nanohoops, where two CT states (D-A and A-D') can be effectively tuned by solvation, resulting in dramatically changed relaxation pathways in different solvents.

Stable and twisted 5,6:12,13-dinaphthozethrene from angular π-extension.

Herein, we describe a concise and efficient synthesis of an angularly extended stable zethrene derivative 1, designed to have more benzenoid rings in the closed-shell resonance form. This compound exhibited enantiomeric structures in the solid state derived from the benzo[4]helicene structure and rapid interconversion in solution. Its far-red absorption, near-infrared emission and amphoteric redox properties were also revealed.

Synthesis of extended polycyclic aromatic hydrocarbons by oxidative tandem spirocyclization and 1,2-aryl migration.

The extended polycyclic aromatic hydrocarbons (PAHs) have received significant interdisciplinary attention due to their semiconducting applications in diverse organic electronics as well as intriguing structural interests of well-defined graphene segments. Herein, a highly efficient oxidative spirocyclization and 1,2-aryl migration tandem synthetic method for the construction of extended polyaromatic hydrocarbons (PAHs) has been developed. The CuCl-catalyst/PhCO3tBu or DDQ oxidation system in the presence of trifluoroacetic acid enables the selective single-electron oxidation to take place preferentially at the more electron-rich alkene moiety of o-biphenylyl-substituted methylenefluorenes, giving rise to the subsequent tandem process. A variety of structurally diverse extended PAHs including functionalized dibenzo[g,p]chrysenes, benzo[f]naphtho[1,2-s]picene, hexabenzo[a,c,fg,j,l,op]tetracene, tetrabenzo[a,c,f,m]phenanthro[9,10-k]tetraphene, tetrabenzo[a,c,f,k]phenanthro[9,10-m]tetraphene, tetrabenzo[a,c,f,o]phenanthro[9,10-m]picene and S-type helicene have been readily synthesized.