An amphoteric switch to aromatic and antiaromatic states of a neutral air-stable 25π radical.

Ever since the discovery of the trityl radical, isolation of a stable and neutral organic radical has been a synthetic challenge. A (4n+1)π open-shell configuration is one such possible neutral radical but an unusual state between aromatic (4n+2)π and antiaromatic (4n)π electronic circuits. The synthesis and characterization of an air- and water-stable neutral 25π pentathiophene macrocyclic radical is now described. It undergoes reversible one-electron oxidation to a 24π antiaromatic cation and reduction to a 26π aromatic anion, thus confirming its amphoteric behavior. Structural determination by single-crystal X-ray diffraction studies revealed a planar configuration for the neutral radical, antiaromatic cation, and aromatic anion. In the solution state, the cation shows the highest upfield chemical shift ever observed for a 4nπ system, while the anion adhered to aromatic nature. Computational studies revealed the delocalized nature of the unpaired electron as confirmed by EPR spectroscopy.

Aromatic expanded isophlorins: stable 30pi annulene analogues with diverse structural features.

Synthesis and structural diversity of novel aromatic expanded isophlorins are described. Expanded isophlorins are higher analogues of 20pi isophlorin; 30pi isophlorins are the simplest examples of expanded isophlorins. They are synthesized from easy to make precursors. Owing to the sp(2) carbons along the conjugated network, they represent higher analogues of annulenes which are not realized until date. In contrast to the parent isophlorin 2, expanded isophlorins 7-9 and 11-13 are aromatic (4n + 2)pi systems and also differ in their optical properties and structural features. They exhibit ring inverted structures and the number of ring inversions is dependent on the nature of the heteroatoms present in the core of the macrocycle. This was confirmed by both (1)H NMR spectroscopy and single crystal-ray diffraction analysis. The upfield chemical shifts for the protons of the inverted rings confirmed the diatropic ring current effects as expected for (4n + 2)pi aromatic systems. Solid state analysis revealed a planar conformation for 7 and near planar conformations for other macrocycles. They also show strong intermolecular interactions through C(sp(2))-H...F-C(sp(2)) hydrogen bonds.

Planar meso pentafluorophenyl core modified isophlorins.

Isophlorins and its analogues bridge the structural features between porphyrins and annulenes. Invariably they are known to be unstable but can be stabilized by appropriate substituents in the core and periphery of a macrocycle. Solid-state characterization of these 20pi systems displays planarity for each macrocycle and even in their supramolecular arrangement due to C-H...F-C interactions.

Cyclic oligofurans: one-pot synthesis of 30pi and 40pi expanded porphyrinoids.

Acid-catalyzed condensation of furan and pentafluorobenzaldehyde yielded conjugated macrocycles with six and eight furan rings. They represent systems similar to annulenes with 30pi and 40pi electrons. From their structural analyses, it was found that furan rings in both the molecules were inverted in an alternative fashion and displayed nontwisted conformations. [structure: see text].