Crystal structures of four co-crystals of ( E )-1,2-di(pyridin-4-yl)ethene with 4-alkoxybenzoic acids: 4-methoxybenzoic acid–( E )-1,2-di(pyridin-4-yl)ethene (2/1), 4-ethoxybenzoic acid–( E )-1,2-di(pyridin-4-yl)ethene (2/1), 4- n -propoxybenzoic acid–( E )-1,2-di(pyridin-4-yl)ethene (2/1) and 4- n -butoxybenzoic acid–( E )-1,2-di(pyridin-4-yl)ethene (2/1)

The crystal structures of four hydrogen-bonded co-crystals of 4-alkoxybenzoic acid–( E )-1,2-di(pyridin-4-yl)ethene (2/1), namely, 2C 8 H 8 O 3 ·C 12 H 10 N 2 , (I), 2C 9 H 10 O 3 ·C 12 H 10 N 2 , (II), 2C 10 H 12 O 3 ·C 12 H 10 N 2 , (III) and 2C 11 H 14 O 3 ·C 12 H 10 N 2 , (IV), have been determined at 93 K. In compounds (I) and (IV), the asymmetric units are each composed of one 4-alkoxybenzoic acid molecule and one half-molecule of ( E )-1,2-di(pyridin-4-yl)ethene, which lies on an inversion centre. The asymmetric unit of (II) consists of two crystallographically independent 4-ethoxybenzoic acid molecules and one 1,2-di(pyridin-4-yl)ethene molecule. Compound (III) crystallizes in a non-centrosymmetric space group ( Pc ) and the asymmetric unit comprises four 4- n -propoxybenzoic acid molecules and two ( E )-1,2-di(pyridin-4-yl)ethane molecules. In each crystal, the acid and base components are linked by O—H...N hydrogen bonds, forming a linear hydrogen-bonded 2:1 unit of the acid and the base. In (I), (II) and (III), intermolecular C—H...O interactions are observed. The 2:1 units of (I) and (II) are linked via C—H...O hydrogen bonds, forming tape structures. In (III), the C—H...O hydrogen bonds, except for those formed in the units, link the two crystallographically independent 2:1 units. In (IV), no C—H...O interactions are observed, but π–π and C—H...π interactions link the units into a column structure.