Non‐covalent halogen bonding interactions are quintessential in crystal engineering for the construction of distinctive supramolecular synthons. Here, we report the first crystalline evidences of unique boat and chair shaped cyclic hexahalogen synthons in the crystal structures of α,α,α′,α′,4‐pentabromo‐o‐xylene (PBX) and α,α,α′,α′,4,5‐hexabromo‐o‐xylene (HBX) respectively. Nature and stability of constituent interactions in the supramolecular synthons are scrutinized with the help of quantum‐chemical calculations. Pendás’ interacting quantum atoms approach confirmed the stability of Br⋅⋅⋅Br interactions leading to boat and chair shaped synthons with major contribution from exchange‐correlation. Although both the molecules are achiral in nature, the packing forces guide PBX to crystallize in the chiral space group P21 with a helix‐like orientation while HBX packs in a centrosymmetric P21/n space group. The extended furcations in the pentabromo derivative construct a molecular framework consisting of macrocycles realized through halogen bonding.
The implication of the potential concept of aromaticity in the relaxed lowest triplet state of azobenzene, an efficient molecular switch, using elementary aromaticity indices based on magnetic, electronic, and geometric criteria has been discussed. Azobenzene exhibits a major Hückel aromatic character retained in the diradical lowest relaxed triplet state (T1) by virtue of a twisted geometry with partial delocalization of unpaired electrons in the perpendicular p‐orbitals of two nitrogen atoms to the corresponding phenyl rings. The computational analysis has been expanded further to stilbene and N‐diphenylmethanimine for an extensive understanding of the effect of closed‐shell Hückel aromaticity in double‐bond‐linked phenyl rings. Our analysis concluded that stilbene has Hückel aromatic character in the relaxed T1 state and N‐diphenylmethanimine has a considerable Hückel aromaticity in the phenyl ring near the carbon atom while a paramount Baird aromaticity in the phenyl ring near the nitrogen atom of the C=N double bond. The results reveal the application of excited‐state aromaticity as a general tool for the design of molecular switches.
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