Highly Twisted 1,2:8,9-Dibenzozethrenes: Synthesis, Ground State, and Physical Properties

Helical fused 1,2:8,9-dibenzozethrene oligomers with up to 201° end-to-end twist: "one-pot" synthesis and chiral resolution

Twisted polyarenes with persistent chirality are desirable but their synthesis has remained a challenge. In this study, we present a "one-pot" synthesis of 1,2:8,9-dibenzozethrene (DBZ) and its vertically fused dimers and trimers using nickel-catalyzed cyclo-oligomerization reactions. X-ray crystallographic analysis confirmed highly twisted helical structures that consist of equal parts left- and right-handed enantiomers. Notably, the end-to-end twist between the terminal anthracene units measured 66°, 130°, and 201° for the DBZ monomer, dimer, and trimer, respectively, setting a new record among twisted polyarenes. Furthermore, the chiral resolution by HPLC yielded two enantiomers for the fused DBZ dimer and trimer, both of which maintained stable configurations and showed absorption dissymmetry factors of around 0.008-0.009. Additionally, their optical and electrochemical properties were investigated, which exhibited a chain-length dependence.

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.

Monoradicals and Diradicals of Dibenzofluoreno[3,2-b]fluorene Isomers: Mechanisms of Electronic Delocalization

The preparation of a series of dibenzo- and tetrabenzo-fused fluoreno[3,2-b]fluorenes is disclosed, and the diradicaloid properties of these molecules are compared with those of a similar, previously reported series of anthracene-based diradicaloids. Insights on the diradical mode of delocalization tuning by constitutional isomerism of the external naphthalenes has been explored by means of the physical approach (dissection of the electronic properties in terms of electronic repulsion and transfer integral) of diradicals. This study has also been extended to the redox species of the two series of compounds and found that the radical cations have the same stabilization mode by delocalization that the neutral diradicals while the radical anions, contrarily, are stabilized by aromatization of the central core. The synthesis of the fluorenofluorene series and their characterization by electronic absorption and vibrational Raman spectroscopies, X-ray diffraction, SQUID measurements, electrochemistry, in situ UV-vis-NIR absorption spectroelectrochemistry, and theoretical calculations are presented. This work attempts to unify the properties of different series of diradicaloids in a common argument as well as the properties of the carbocations and carbanions derived from them.

Molecule Isomerism Modulates the Diradical Properties of Stable Singlet Diradicaloids

Inclusion of quinoidal cores in conjugated hydrocarbons is a common strategy to modulate the properties of diradicaloids formed by aromaticity recovery within the quinoidal unit. Here we describe an alternative approach of tuning of diradical properties in indenoindenodibenzothiophenes upon anti → syn isomerism of the benzothiophene motif. This alters the relationship of the S atom with the radical center from linear to cross conjugation yet retains the same 2,6-naphtho conjugation pattern of the rearomatized core. We conduct a full comparison between the anti and syn derivatives based on structural, spectroscopic, theoretical, and magnetic measurements, showing that these systems are stable open-shell singlet diradicaloids that only access their triplet state at elevated temperatures.

Modern zethrene chemistry

This paper gives a brief summary of zethrene chemistry after the pioneering works by Clar, Mitchell, Sondheimer, and Staab et al. In the past a few years, various synthetic methods and stabilizing strategies have been developed, which have made the synthesis of stable, vertically and laterally extended zethrenes practically achievable. One of the most important discoveries for this type of molecule is their open-shell diradical character, which is the origin of their unique optical, electronic, and magnetic properties. The fundamental structure–diradical character–physical property relationships were investigated, and these studies form the basis for the tailored design of more general open-shell singlet diradicaloids with controllable diradical character and physical properties.

Benzo-thia-fused [n]thienoacenequinodimethanes with small to moderate diradical characters: the role of pro-aromaticity versus anti-aromaticity

Open-shell singlet diradicaloids have recently received much attention due to their unique optical, electronic and magnetic properties and promising applications in materials science. Among various diradicaloids, quinoidal π-conjugated molecules have become the prevailing design. However, the need for a fundamental understanding on how the fusion mode and pro-aromaticity/anti-aromaticity affect their diradical character and physical properties remains unaddressed. In this work, a series of pro-aromatic benzo-thia-fused [n]thienoacenequinodimethanes (Thn-TIPS (n = 1-3) and BDTh-TIPS) were synthesized and compared with the previously reported anti-aromatic bisindeno-[n]thienoacenes (Sn-TIPS, n = 1-4). The ground-state geometric and electronic structures of these new quinoidal molecules were systematically investigated by X-ray crystallographic analysis, variable temperature NMR, ESR, SQUID, Raman, and electronic absorption spectroscopy, assisted by DFT calculations. It was found that the diradical character index (y0) increased from nearly zero for Th1-TIPS to 2.4% for Th2-TIPS, 18.2% for Th3-TIPS, and 38.2% for BDTh-TIPS, due to the enhanced aromatic stabilization. Consequently, with the extension of molecular size, the one-photon absorption spectra are gradually red-shifted, the two-photon absorption (TPA) cross section values increase, and the singlet excited state lifetimes decrease. By comparison with the corresponding anti-aromatic analogues Sn-TIPS (n = 1-3), the pro-aromatic Thn-TIPS (n = 1-3) exhibit larger diradical character, longer singlet excited state lifetime and larger TPA cross section value. At the same time, they display distinctively different electronic absorption spectra and improved electrochemical amphotericity. Spectroelectrochemical studies revealed a good linear relationship between the optical energy gaps and the molecular length in the neutral, radical cationic and dicationic forms. Our research work discloses a significant difference between the pro-aromatic and anti-aromatic quinoidal compounds and provides guidance for the design of new diradicaloids with desirable properties.

Phenalenyl-fused porphyrins with different ground states

Materials based on biradicals/biradicaloids have potential applications for organic electronics, photonics and spintronics. In this work, we demonstrated that hybridization of porphyrin and polycyclic aromatic hydrocarbon could lead to a new type of stable biradicals/biradicaloids with tunable ground state and physical property. Mono- and bis-phenalenyl fused porphyrins 1 and 2 were synthesized via an intramolecular Friedel-Crafts alkylation-followed-by oxidative dehydrogenation strategy. Our detailed experimental and theoretical studies revealed that 1 has a closed-shell structure with a small biradical character (y = 0.06 by DFT calculation) in the ground state, while 2 exists as a persistent triplet biradical at room temperature under inert atmosphere. Compound 1 underwent hydrogen abstraction from solvent during the crystal growing process while compound 2 was easily oxidized in air to give two dioxo-porphyrin isomers 11a/11b, which can be correlated to their unique biradical character and spin distribution. The physical properties of 1 and 2, their dihydro/tetrahydro-precursors 7/10, and the dioxo-compounds 11a/11b were investigated and compared.