Crystal structures of three co-crystals of 4,4′-bipyridyl with 4-alkoxybenzoic acids: 4-ethoxybenzoic acid–4,4′-bipyridyl (2/1), 4-<i>n</i>-propoxybenzoic acid–4,4′-bipyridyl (2/1) and 4-<i>n</i>-butoxybenzoic acid–4,4′-bipyridyl (2/1)
Acta crystallographica. Section E, Crystallographic communications