Abstract:In this work, crystal structures of commercially available photochromic compounds, i.e., spiropyrans and spirooxazines, were investigated by single-crystal X-ray diffraction. A total of five new structures were obtained via isothermal evaporation experiments under different conditions, namely 1,3,3-Trimethylindolino-benzopyrylospiran (I), 1,3,3-Trimethylindolinonaphtospirooxaxine (II), 1-(2-Hydroxyethyl)-3,3-dimethylindolino-6 -nitrobenzopyrylospiran (III), and 1,3,3-Trimethylindolino-8 -methoxybenzopyrylospiran (IVa and IVb). Since the basic structure of a spiropyran/-oxazine does not present typical hydrogen bond accepting and donating groups, this study illustrates the importance of additional functional groups connected to this kind of molecules to induce specific intermolecular interactions. Our results show that possible hydrogen bonding interactions are rather weak due to the high steric demand of these compounds. These results are supported by a search of the Cambridge structural database focused on related structures.

Controlling the type of merocyanine metal interaction for broad solid-state coloration.

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.

D-Phenyglycinamide (D-PGA) was shown to form diastereomeric salts with a series of dicarboxylic acids (malic, methylsuccinic, camphoric) and the drug naproxen. Structural analysis of the 21 newly discovered forms was performed in order to investigate the potential of D-PGA as a base agent for chiral resolution via diastereomeric salt formation. Malic and methylsuccinic dicarboxylic acids formed salts with 1:1 and 2:1 stoichiometries, whereas camphoric acid and naproxen yielded only 1:1 salts. In the D-PGA/naproxen system, variable-temperature in situ synchrotron X-ray diffraction gave access to two additional nonambient 1:1 salts. D-PGA subjected to liquid-assisted grinding with the racemic forms of the tested compounds to study chiral resolution in the solid state yielded in most of the cases complicated mixtures of diastereomeric salts or salts with the racemates (when both enantiomers and D-PGA crystallize in one structure), thereby suggesting that the use of D-PGA for chiral resolution may be challenging.