Merocyanines in a Halogen-Bonded Network Involving Inorganic Building Blocks

In this work, we present spiropyran derivatives (SP) involving a halogen bonded network with the addition of inorganic building blocks, i.e. inorganic acids and bivalent metal salts. In solution, a ring-opening isomerization of the spiropyran results in the colored merocyanine isomer (MC), which is only rarely observed in the solid state. By detaining the phenolate oxygen atom of the merocyanine in a hydrogen (HB) or halogen bond (XB) we can successfully obtain this form and access a variation in chromic properties. The O atom of the open MC form unfortunately only represents a weak XB acceptor which led us to introduce supplementary building blocks to strengthen this type of interaction. Fourteen new crystal forms were determined with 3 spiropyran derivatives (1,3,3-trimethylindolinospiropyran (SPH/MCH), 1,3,3-trimethylindolino-6′-nitrobenzopyrylospiran (SPNO2/MCNO2), 1,3,3-trimethylindolino-β-naphthopyrylospiran (SPBenz/MCBenz)) and several di- and tritopic XB donors (1,3,5-triiodotrifluorobenzene (135tfib), 1,4-diiodotetrafluorobenzene (14tfib), 1,3-diiodotetrafluorobenzene (13tfib), 1,2-diiodotetrafluorobenzene (12tfib), abd 1,2-diiodotetrafluoroethan (12tfe)) as well as a polymorphic form of the parent compound with hydrochloric acid. We successfully expanded the XB interaction possibilities with hydrochloric acid or zinc/cobalt chloride and found proof of the tunability of the chromic properties in the solid state using these building blocks.