Rupturing aromaticity by periphery overcrowding.

The balance between strain relief and aromatic stabilization dictates the form and function of non-planar π-aromatics. Overcrowded systems are known to undergo geometric deformations, but the energetically favourable π-electron delocalization of their aromatic ring(s) is typically preserved. In this study we incremented the strain energy of an aromatic system beyond its aromatic stabilization energy, causing it to rearrange and its aromaticity to be ruptured. We noted that increasing the steric bulk around the periphery of π-extended tropylium rings leads them to deviate from planarity to form contorted conformations in which aromatic stabilization and strain are close in energy. Under increasing strain, the aromatic π-electron delocalization of the system is broken, leading to the formation of a non-aromatic, bicyclic analogue referred to as 'Dewar tropylium'. The aromatic and non-aromatic isomers have been found to exist in rapid equilibrium with one another. This investigation demarcates the extent of steric deformation tolerated by an aromatic carbocycle and thus provides direct experimental insights into the fundamental nature of aromaticity.

Protonation Dependent Topological Dichotomy of Core Modified Hexaphyrins: Synthesis, Characterization, and Excited State Dynamics.

Two hitherto unknown core modified hexaphyrin analogues have been synthesized and characterized where the conformational dynamics of these macrocycles in the free base form is achieved by the rotation of thienothiophene units. Further unique property of these macrocycles is the Hückel-Möbius topological switching. The thermodynamic equilibrium and kinetics of the interconversion leading to Hückel-Möbius switches have been triggered by external stimuli, such as protonation and/or temperature. We have provided a thorough solution-state spectroscopic characterization, solid-state structural evidence combined with in-depth theoretical calculations to investigate the crucial factors involved in such interconversion between Hückel and Möbius topologies for these hexaphyrins which will be useful in designing future new members to expanded porphyrin chemistry.

Aromatic Fused [30] Heteroannulenes with NIR Absorption and NIR Emission: Synthesis, Characterization, and Excited-State Dynamics.

Two hitherto unknown planar aromatic [30] fused heterocyclic macrocycles (1.1.0.1.1.0), with NIR absorption in free-base form and protonation-induced enhanced NIR emission, have been synthesized from easy to make precursors. The induced correspondence of fusion on the macrocyclic structure, electronic absorption, and emission spectra have been highlighted.

Induced Correspondence of a Local π-Aromatic Sextet in Heteroannulenes: Synthesis and Characterization.

Acid-catalyzed [3+3] condensation reactions of two hitherto unknown tripyrrane moieties with pentafluorobenzaldehyde has led to the formation of new generation heteroannulene (4.1.4.1) and mutant heteroannulene (1.1.1.1.1.1). Inclusion of local π-aromatic sextets, namely the N-methyl pyrrole rings through ß,ß-linkages and α,ß-linkages, has led to the isolation of first ever heteroannulenes cross-conjugated at four points and two points respectively within the macrocycles.

Möbius Aromatic Core-Modified Heterocyclic [20]†Macrocycles (4.1.1) with a Protruding N-Methyl Pyrrole Ring.

Herein, we report the first synthesis of an unorthodox tripyrrane moiety from the regioselective ß-benzoylation of pyrrole and the acid-catalyzed condensation of the desired precursors. A [3+1] Mac Donald type condensation strategy for this tripyrrane has led to the exclusive isolation of two hitherto-unknown aromatic [20] heterocyclic macrocycles (4.1.1).

Aromatic fused heterocyclic [22] macrocycles with NIR absorption.

Three related aromatic [22] heteroporphyrins have been synthesized and characterized, with the target of achieving NIR absorption. The propensity of these smallest π-conjugated macrocycles to absorb in the NIR region is benefitted from the fusion/annelation of the precursor heterocycles.