Crystal structures of four co-crystals of ()-1,2-di(pyridin-4-yl)ethene with 4-alk-oxy-benzoic acids: 4-meth-oxy-benzoic acid-()-1,2-di(pyridin-4-yl)ethene (2/1), 4-eth-oxy-benzoic acid-()-1,2-di(pyridin-4-yl)ethene (2/1), 4--propoxybenzoic acid-()-1,2-di(pyridin-4-yl)ethene (2/1) and 4--but-oxy-benzoic acid-()-1,2-di(pyridin-4-yl)ethene (2/1).

The crystal structures of four hydrogen-bonded co-crystals of 4-alk-oxy-benzoic acid-()-1,2-di(pyridin-4-yl)ethene (2/1), namely, 2CHO·CHN, (I), 2CHO·CHN, (II), 2CHO·CHN, (III) and 2CHO·CHN, (IV), have been determined at 93 K. In compounds (I) and (IV), the asymmetric units are each composed of one 4-alk-oxy-benzoic acid mol-ecule and one half-mol-ecule of ()-1,2-di(pyridin-4-yl)ethene, which lies on an inversion centre. The asymmetric unit of (II) consists of two crystallographically independent 4-eth-oxy-benzoic acid mol-ecules and one 1,2-di(pyridin-4-yl)ethene mol-ecule. Compound (III) crystallizes in a non-centrosymmetric space group () and the asymmetric unit comprises four 4--propoxybenzoic acid mol-ecules and two ()-1,2-di(pyridin-4-yl)ethane mol-ecules. 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), inter-molecular C-H⋯O inter-actions are observed. The 2:1 units of (I) and (II) are linked 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 inter-actions are observed, but π-π and C-H⋯π inter-actions link the units into a column structure.