π-π radical interactions have exacting geometry requirements, significantly more stringent than those of a hydrogen bond or a van der Waals interaction. Here, supramolecular synthons based on such radical-radical recognition are employed to generate switchable structural and magnetic properties. Interactions between neighboring paramagnetic ligands of the La(hfac)3(pyDTDA)2 coordination complex cause a rare re-entrant phase transition (hfac = 1,1,1,5,5,5-hexafluoroacetylacetonato-; pyDTDA = 4-(2′-pyridyl)-1,2,3,5-dithiadiazolyl). Below 100 K, the complex is diamagnetic in the solid state, consistent with an f 0 lanthanum(III) ion and "pancake bonding" of the π-radical ligands. Upon heating, the supramolecular one-dimensional arrangement undergoes two sequential structural phase transitions, observed at ca. 160 and 310 K, successively involving the rupture of half and then the totality of the "pancake bonds" and generating stepwise increases in the paramagnetic susceptibility. We use the structural and magnetic data to develop a theoretical model that clearly predicts the unprecedented re-entrant behavior of this radical-La(III)-radical complex. Moreover, this microscopic free-energy model demonstrates that any system with essentially non-interacting "dimers" contains within itself the possibility of a distortion associated with re-entrant phase transitions if the lattice is "soft" enough (i.e., low rigidity).
The crystal and molecular structures of two isomeric, aromatic, pyromellitimide oligomers 1 and 2 have been determined by single-crystal X-ray diffraction. Crystal data for 1 (C^HísNíOioSí): triclinic, -, a = 8.038 (3), b = 10.947 (3), c = 11.308 (4) A; a = 95.75 (3), ß = 105.79 (2), = 92.89 (3)°. Crystal data for 2 (C46HmN2OioS2): monoclinic, P2i/a; a = 11.844 ( 5), b = 6.977 (2), c = 22.694 (6) ;ß= 101.96 (3)°. The structure of the yellow isomer (2) provides the first direct evidence of complementary, face-to-face stacking between electron-rich imidophenoxy (N-Ar-O) and electron-poor pyromellitimide units in adjacent chains, an association long suspected but not yet conclusively demonstrated in aromatic polyimides. In contrast, the colorless oligomer (1) adopts a packing mode in which all the parallel ring-ring contacts occur between aromatic systems of like, rather than complementary, electronic character.
Crystal structure prediction (CSP) aims to determine the experimentally isolable crystal structure(s) of a molecule given only its 2D molecular diagram. The ability to match candidate structures to known experimental...
Accurate and efficient computation of relative energies of molecular crystal polymorphs is of central importance for solid-state pharmaceuticals and in other technologically relevant fields. In recent years, dispersion-corrected density-functional theory...
An achiral organic molecule adopts a chiral conformation, crystallizing in two morphologies: a racemic form, stable <70 °C, and a homochiral form, stable ≥70 °C. Upon seeding, crystal-to-crystal phase transitions occur reversibly between the racemic and chiral forms. Liquid-to-solid chiral crystallization is observed >90% of the time from the melt.
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