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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 gEL of up to 0.54. [ABSTRACT FROM AUTHOR]

Zwei Arten helikal‐chiraler Verbindungen mit einem oder zwei Boratomen wurden nach einem modularen Ansatz synthetisiert. Die Bildung der helikalen Strukturen erfolgte durch Einführung von Bor in flexible Biaryl‐ bzw. Triaryl‐Vorstufen, hergestellt aus kleinen achiralen Bausteinen. Die durchgehend ortho‐fusionierten Azabora[7]helicene zeichnen sich dabei durch außergewöhnliche Konfigurationsstabilität, blaue oder grüne Fluoreszenz in Lösung mit Quantenausbeuten (Φfl) von 18–24 %, grüne oder gelbe Emission im Festkörper (Φfl bis zu 23 %) und starke chiroptische Resonanz mit großen Anisotropiefaktoren von bis zu 1.12×10−2 aus. Azabora[9]helicene, aufgebaut aus winkelförmig sowie linear angeordneten Ringen, sind blaue Emitter mit Φfl von bis zu 47 % in CH2Cl2 und 25 % im Festkörper. DFT‐Rechnungen zeigen, dass ihre P‐M‐Interkonversion über einen komplexeren Weg verläuft als im Fall von H1. Röntgenstrukturanalyse von Einkristallen zeigt deutliche Unterschiede in der Packungsanordnung von Methyl‐ und Phenylderivaten auf. Die Moleküle werden als Primärstrukturen verlängerter Helices vorgeschlagen. [ABSTRACT FROM AUTHOR]

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 (Φfl) of 18–24 % in solution, green or yellow solid‐state emission (Φfl 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 Φfl of up to 47 % in CH2Cl2 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. [ABSTRACT FROM AUTHOR]

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]

The synthesis and redox properties of first generation S-confused isophlorins are described. Despite structural resemblance to a confused porphyrin, spectroscopic and computational studies reveal weak paratropic ring current effects in these 20π macrocycles. They display redox properties atypical of parent tetrathia isophlorins. Experimental evidence supports the oxidation of an unstable 19π neutral radical, as a transient intermediate, for the formation of a unique 18π aromatic monocationic species. Spectroscopic and structural characterization revealed the substituent dependent macrocycle oxidation unfamiliar to the chemistry of antiaromatic isophlorinoids. [ABSTRACT FROM AUTHOR]

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. 1H 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. [ABSTRACT FROM AUTHOR]

Benzofused dipyrrins react with metal salt copper(ii) acetate to predominantly yield a cyclodimer along with a cyclotrimer. N-confused monobenzo-dipyrrin cyclomerized to trioxo-expanded norrole 13a and an acyclic dimer 14 whereas doubly N-confused monobenzo and dibenzo-dipyrrins yielded aza-heptalene 15 and an acyclic dimer 16. [ABSTRACT FROM AUTHOR]