The study of polycyclic aromatic hydrocarbons has become a cornerstone of chemical sciences, providing crucial guidance for advancing a wide range of fields. Within this context, quasi[8]circulenes and indenofluorenes (IFs) have gained considerable attention due to their rich chiroptical, optoelectronic, supramolecular, and redox properties. However, their combined integration into a single molecular framework has not yet been realized. This work demonstrates that curved indeno[2,1-c]fluorene quasi[8]circulenes are easily accessible through an intramolecular cyclization of a readily available helicene diketone. The cyclization is high yielding and accompanied by an unexpected regioselective triflation that-alongside both ketones-allows orthogonal functionalization. This synthetic utility offers great potential for incorporating 8-membered rings and formally antiaromatic as-indacene moieties into complex architectures. Herein, a series of fully conjugated antiaromatic indeno[2,1-c]fluorene quasi[8]circulenes were synthesized and studied spectroscopically and electrochemically, showcasing the rich properties of these rare, purely carbon-based quasi[8]circulenes. It is shown that the studied compounds display multiple accessible redox states at mild potentials, which can be significantly altered through adequate substituents. These substituents additionally induce unprecedented aggregation behavior for one of the compounds. Furthermore, the studied IFs exhibit chirality arising from the curved structure of the quasi[8]circulenes, which allowed studying chiroptical properties for one of the structures.

The chemistry of hetero[8]circulenes has been limited to three main types, constrained by synthetic challenges in creating unsymmetrical variants. Herein, we introduce an electrochemical approach to a type of hetero[8]circulene, featuring five hexagons and three pentagons. Our method capitalizes on the sustainability and selectivity of electrochemistry, utilizing differential oxidation potentials to generate dioxaza[8]circulenes through selective intermolecular and intramolecular couplings under mild conditions, achieving yields of up to 83% with good functional group tolerance. We further refine this process into a one-pot protocol using commercially available substrates, forming six new bonds. Comprehensive structural, optical, and electrochemical characterizations, including X-ray crystallography, spectrophotometric analysis, and DFT calculations, are conducted. Inspired by their distinct structural and redox properties, we explore the application of dioxaza[8]circulenes as organophotocatalysts for diverse C-X (X = C, B, S, P) bond formation achieving up to 97% yields under LED light irradiation (365 nm) without transition metals.

The electronic structure and the UV-Vis absorption spectra of heteroannelated cyclooctatetraene derivatives are studied by density functional theory and by its non-stationary variant, the time-dependent density functional theory. The cyclooctatetraene ring is shown to be planar in all of the molecules considered, except the annelated fluorene and dibenzothiophene derivatives, and exhibits an antiaromatic character according to the magnetic and structural aromaticity criteria. The double ionization of annelated cyclooctatetraene molecules (quasi-circulenes) causes a change in the aromatic properties of cyclooctatetraene, which generally gaines aromaticity by double reduction and becomes non-aromatic upon double oxidation. The time-dependent density functional theory calculations enable the interpretation of electronic absorption spectra of recently synthesized quasi-circulenes and to predict the spectra of new, hypothetical molecules, which are important for the general theoretical understanding of hetero[8]circulene spectra.

For the first time, a theoretical study has been performed on the prototypical decathio[10]circulene (C(20)S(10)) species, which is an analogue of the novel octathio[8]circulene "Sulflower" molecule (C(16)S(8)). Examinations of the singlet and triplet states of C(20)S(10) were made at the B3LYP/6-311G(d) level. Local minima of C(2) and C(s) symmetry were found for the lowest singlet and triplet states, respectively. The stability of C(20)S(10) was assessed by calculating the ΔH°(f) of C(16)S(8) and C(20)S(10) and the ΔH(o) for their decomposition into C(2)S units. Frontier molecular orbital plots show that structural adjacent steric factors along with the twist and strain orientations of C(20)S(10) do not disturb the aromatic π-delocalizing effects. In fact, C(20)S(10) maintains the same p(z) HOMO character as C(16)S(8). These similarities are further verified by density-of-states characterization. Calculated infrared spectra of C(16)S(8) and C(20)S(10) show broad similarities. Molecular electrostatic potential results reveal that eight of the peripheral sulfur atoms are the most electronegative atoms in the molecule, while the interior ten-membered ring exhibits virtually no electronegativity.