Abstract: | 4‐Amino‐trans‐azobenzene {or 4‐(E)‐phenyldiazenyl]aniline} can form isomeric salts depending on the site of protonation. Both orange bis{4‐(E)‐phenyldiazenyl]anilinium} hydrogen phosphate, 2C12H12N3+·HPO42−, and purple 4‐(E)‐phenyldiazenyl]anilinium dihydrogen phosphate phosphoric acid solvate, C12H12N3+·H2PO4−·H3PO4, (II), have layered structures formed through O—H⋯O and N—H⋯O hydrogen bonds. Additionally, azobenzene fragments in (I) are assembled through C—H⋯π interactions and in (II) through π–π interactions. Arguments for the colour difference are tentatively proposed. |