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During the attempted total synthesis of chlorophyll, Woodward hypothesized the formation of tetrapyrrolic 20p isophlorin as a transient antiaromatic intermediate which provides a plausible template to synthesize stable antiaromatic molecules. Despite its structural similarity with the 18p aromatic porphyrin, it significantly differs in its electronic and chemical properties. However, due to its unstable nature under ambient conditions it immediately gets oxidized to stable 18p aromatic porphyrin. Similar macrocyclic structures with ß-substituted heterocycles, such as furan/thiophene/selenophene, have been synthesized, which undergo facile oxidation to yield the 18p porphyrin dication. Attempts to synthesize stable tetrapyrrolic isophlorin and its metal complex have remained unaccomplished till date. Strategies to synthesize stable core-modified 20p isophlorin and its confused isophlorin derivative have met with considerable success. Predominantly, they are synthesized by replacing three or four pyrroles with furan/thiophene. The 20p systems either with four furan units or with a 'pair' of furan and thiophene units were sufficiently stable enough to resist oxidation towards the corresponding porphyrin dication. The 20p isophlorins displayed noncovalent interactions with the curved p surface of fullerene which predominantly rises due to van der Waals attraction between the dissimilar p systems. This antiaromatic isophlorin-fullerene complexes were obtained and successfully characterized by single-crystal X-ray diffraction studies. Replacing only three pyrroles by furan rings yielded the first stable pyrrole derivative of antiaromatic 20p isophlorin, which can be reversibly oxidized to 18p aromatic porphyrin without deprotonating the inner pyrrole NH. In addition, replacing all the pyrrole units of Nconfused porphyrin with thiophene yielded the first derivative of confused isophlorin. Further, its two-electron oxidation led to the formation of 18p aromatic cation with enhanced aromaticity. The structure and electronic properties of the oxidized and neutral species were unambiguously determined from a combination of spectroscopic techniques, X-ray crystallography, and computational methods. These studies reveal that antiaromatic systems like isophlorin and its confused isophlorin derivative can be stabilized under ambient conditions and they offer potential to explore the chemistry of 4np systems. This Account focuses of recent advances in 20p antiaromatic isophlorins, confused isophlorins, and nocorroles along with their redox chemistry. 1 Introduction 2 Stable 20p Isophlorins 2.1 Furan-/Thiophene-Based Isophlorins 2.2 Pyrrole Isophlorins 3 Confused Isophlorins 3.1 Isophlorin-Fullerene 3.2 meso-meso-Bridged Tetraoxaisophlorin Dimer 4. Norcorroles 5. Conclusion and Outlook. [ABSTRACT FROM AUTHOR]

Herein, we report configurationally stable singly-truncated (ST) and structurally flexible doubly-truncated (DT) helically chiral compounds derived from azabora[7]helicenes by a hypothetical removal of a single or two C=C double bonds. The singly-truncated constitutional isomers were synthesized from either benzoisoquinoline (BIQ) or phenantherene building blocks and the corresponding biaryls in excellent yields to give azabora[5]helicenes with a pendant phenyl ring at a sterically hindered position. These systems highlight the electronic impact of the nitrogen donor substitution position. The compounds with a disrupted BIQ moiety (STN) possess remarkable photoluminescence quantum yields of up to 0.53 in the solid state and a blue emission in solution with dissymmetry factors of up to ca. 3×10 -3 . Upon cooling to 79 K all compounds exhibit phosphorescence with lifetimes of up to ca. 0.5 s. A methyl complex of azabora[7]helicene showing excellent configurational stability was used as a chiral inducer embedded in an emissive polymer (F8BT) to produce circularly polarized organic light emitting diodes with an electroluminescence dissymmetry factor g EL of up to 0.54., (© 2023 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH.)

Two types of helically chiral compounds bearing one and two boron atoms were synthesized by a modular approach. Formation of the helical scaffolds was executed by the introduction of boron to flexible biaryl and triaryl derived from small achiral building blocks. All-ortho-fused azabora[7]helicenes feature exceptional configurational stability, blue or green fluorescence with quantum yields (Φ fl ) of 18-24 % in solution, green or yellow solid-state emission (Φ fl up to 23 %), and strong chiroptical response with large dissymmetry factors of up to 1.12×10 -2 . Azabora[9]helicenes consisting of angularly and linearly fused rings are blue emitters exhibiting Φ fl of up to 47 % in CH 2 Cl 2 and 25 % in the solid state. As revealed by the DFT calculations, their P-M interconversion pathway is more complex than that of H1. Single-crystal X-ray analysis shows clear differences in the packing arrangement of methyl and phenyl derivatives. These molecules are proposed as primary structures of extended helices., (© 2020 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)

Partial core-modification of a porphyrin can be employed to synthesize the 20π antiaromatic isophlorin. Unlike the tetra-, tri-, and dipyrrole derivatives of a porphyrin, a monopyrrole porphyrin exhibits antiaromatic characteristics. It undergoes a two-electron reversible ring oxidation to yield the 18π aromatic dication. (1) H NMR analysis provides distinct evidence of the altered electronic characteristics through typical paratropic and diatropic ring current effects for the 4n and the (4n+2) π-electron systems, respectively., (© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)