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, 2C8H8O3·C12H10N2, (I), 2C9H10O3·C12H10N2, (II), 2C10H12O3·C12H10N2, (III) and 2C11H14O3·C12H10N2, (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.