Classic formulations of aromaticity have long been associated with topologically planar conjugated macrocyclic systems. The theoretical possibility of so-called bicycloaromaticity was noted early on. However, it has yet to be demonstrated by experiment in a simple synthetic organic molecule. Conjugated organic systems are attractive for studying the effect of structure on electronic features. This is because, in principle, they can be modified readily through dedicated synthesis. As such, they can provide useful frameworks for testing by experiment with fundamental insights provided by theory. Here we detail the synthesis and characterization of two purely organic non-planar dithienothiophene-bridged [34]octaphyrins that permit access to two different aromatic forms as a function of the oxidation state. In their neutral forms, these congeneric systems contain competing 26 and 34 π-electronic circuits. When subject to two-electron oxidation, electronically mixed [4n+1]/[4n+1] triplet biradical species in the ground state are obtained that display global aromaticity in accord with Baird's rule.

A new fused core-modified 32π heptaphyrin with Möbius aromatic character is reported. The (1)H NMR data indicated a weak Möbius aromaticity at 298 K; however, at 213-183 K, the molecule predominates [4n]π Möbius conformation with strong diatropic ring current, which was further confirmed by X-ray analysis. The protonation experiment led to preservation of the Möbius aromaticity at 298 K. Nevertheless, the experimental results were further supported by theoretical studies. Overall, this study represents the first example of Möbius aromatic fused core-modified expanded porphyrin.

The synthesis of a planar expanded meso porphyrin with an intramolecular para-phenylene-bridged core is reported. The planarity of the octaphyrin macrocycle was confirmed by single-crystal X-ray structural analysis, in which the bridged para-phenylene unit deviated by 27° from the mean macrocyclic plane. Spectroscopic analyses and theoretical calculations suggested that the macrocycle was Hückel aromatic and followed a major [34 π] single-conjugation pathway, which indicated that the bridging para-phenylene unit was not involved in the macrocyclic conjugation. Analysis of the photophysical properties of this system by steady-state absorption/fluorescence spectroscopy and transient absorption spectroscopy revealed moderate enhancement in the parameters of the octaphyrin as compared to its non-bridged octaphyrin congeners, which was attributed to the planarity and rigidity of the macrocycle as imposed by the bridging para-phenylene unit. Preliminary anion-binding studies revealed that the protonated macrocycle bound selectively with chloride ions through N-H⋅⋅⋅Cl hydrogen-bonding interactions.

Protonation triggered transition of a 48π nonaromatic decaphyrin to a 48π Hückel antiaromatic decaphyrin is reported: the flexibility of the macrocycle due to the presence of twelve meso carbon bridges facilitates such a transition through conformational change.

The synthesis of fused and nonfused core-modified 40π nonaphyrins are reported. Spectroscopic and X-ray structural studies reveal a twisted figure-eight conformation in the freebase form that is nonaromatic. Structural changes occur, from figure-eight to open extended conformation, upon protonation, thereby adopting 4nπ Hückel antiaromatic character, which is reflected in spectroscopic and theoretical studies. Such a structural change also induces ring inversions of specific heterocyclic rings by 180°.