Effects of alkyl chain substitution on the crystal structure of benzothiazole-derived squarylium dyes

Crystal structures of ten 4-[(3-alkylbenzothiazol-3-ium-2-yl)methylidene]-2-[(3-alkyl-3H-benzothiazol-2-ylidene)methyl]-3-oxocyclobut-1-en-1-olate derivatives bearing alkyl chains of different lengths were determined by single-crystal X-ray crystallography. Single crystals were categorized into four types based on their structural characteristics. Hirshfeld surface analysis showed that in each type of structure, the alkyl chains interacted with different parts of the molecule, affecting thermal properties, π–π configuration and planarity of the π-conjugated moiety. In some cases, alkyl chains adopted a gauche conformation, which led to closer contact of squarylium moieties.