Conformational and packing structures of crystalline azulene derivatives controlled by substitution

• A systematic study of six differently substituted derivatives of azulene by X-ray diffraction.
• Conformational and packing modes are investigated referring to the kind of substitution.
• Molecular orientation of stacked azulene dimers is highlighted.

A series of azulenes derived from diethyl azulene-1,3-dicarboxylate (1) as the parent compound and featuring a systematic addition of polar substituents (Cl, Br, OH and NH2) both in the positions 2 and 6 of the azulene (2–6) were prepared and investigated with reference to their X-ray crystal structures, having occurrence and modes of π⋯π stacking of the azulene units particularly in view. Except the parent compound 1, all the other derivatives show typical π-stacked dimers in the crystal packing being modified in pattern under the influence of the specific substitution. This suggests that corresponding substitution of an azulene is not obstructive to π⋯π stacking formation and may even be used for a potential pattern design of azulene dimers in a crystalline material.

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