The Construction And Properties Of Spin Crossover Molecular Alloys

Spin crossover(SCO) materials are of significant potential application in switch, sensor, display device, and data storage owning to their bistability between low spin(LS) and high spin(HS) state under external perturbation. Despite considerable efforts on structure-property understanding, prediction of SCO alloy structures and fine tuning SCO properties represent a formidable challenge. In this paper, we present an ingenious strategy to predictably construct SCO alloy and modify their SCO behaviors via cocrystallization of enantiomeric complexes by means of chiral recognition. Chiral SCO modules could be prefabricated via multi-compoenent self-assembly of 1-alkyl-2-imidazolecarboxaldehyde, chiral phenylethylamine and metal ion. And the prospective of mix-and-match assembly is realized by binary or multiplex cocrystallization of artificial chiral building blocks. Tailoring the ingredient resulted in three three kinds of spin crossover molecular alloys. In this way, the structures of the resulting molecular alloy can be predicted and an alternative strategy was provided to prepare SCO alloy material and control SCO properties. Main work of this paper is composed of three sections as follows:1. Two couples of enantiomeric iron(Ⅱ) complexes 4R, 4S, 5R and 5S with the formula of fac-Λ or Δ-[Fe(L)3][ClO4]2 were synthesized through multi-component self-assembly of 1-alkyl-2-imidazolecarboxaldehyde, chiral phenylethylamine and iron(Ⅱ) ion. Taking advantage of the structural similarity of 4R, 4S, 5R and 5S, alloy crystals including co-enantiomers 4R5 R and 4S5 S, racemates 4RS and 5RS, co-racemates 4RS4 RS were stereochemically constructed via equimolar binary cocrystallization of these enantiomers, respectively. Their coordinated structure, enantiomeric nature and phase purity were confirmed by IR, UV-vis, CD, TG, PXRD, etc. Compared with 4R and 4S stablized in high spin state, 5R and 5S(T1/2 = 291K), homochiral 4R5 R and 4S5S(T1/2 = 301K), and racemic 4RS(T1/2 = 336 K) exhabited gradual and reversible spin crossover. Outstandingly, the racemic alloys 5RS(T1/2↑ = 369 K, T1/2↓ = 323 K) and 4RS5RS(T1/2↑ = 335 K, T1/2↓ = 324 K) showed hysteresis loops with width of 46 K and 11 K induced by desolvation. Crystal packing, intramolecular π-π stacking, intermolecular C-H???π interaction and solvent effect were elucidated to be responsible for the distinct spin crossover properties.2. To investigate influence of structural difference on alloy construction, chiral complexes 2R, 3R, 2R-B, and 3R-B with ethyl and propyl were synthesized after tailoring the 2-imidazolecarboxaldehyde. And distinct molecular structure and energy-low crystal packing promoted by solvent lattice in enantiomers was discovered adverse to the construction of molecular alloys. In addtion, isostructural 4SO and 4SCl with fomula of fac-Δ-[Fe(S-L)3][ClO4]2 were designed by tailoring the para-substituent of chiral phenylethylamine with electron-donating –OCH3 and eletron-withdrawing –Cl. Equimolar ternary cocrytallization of 4S, 4SO and 4SCl resulted in molecular alloy 4SSS. Magnetic measurements revealed a complete and gradual spin transition for 4SO(T1/2 = 256 K) and 4SSS(T1/2 = 242 K). Detailed crystallographic and spectral analyses indicated that electron-donating group favored a stabilization of LS state and a higher transition temperature while electron-withdrawing group weakened the ligand field intensity and made transition temperature lower. Molecular alloy strategy was demonstrated effective in SCO modification.3. Multi-metal core-shell alloy crystals 5RFeCo and 5RFeNi were constructed using metal-center ions exchange on crystal 5R from surface to center. And a single crystal to single crystal(SCSC) transformation from 5R to 5RCo or 5RNi was realized. The core-shell alloy crystals are composed of LS core and HS shell and of high quality at X-ray level. The SCSC processes could be dually controlled by the quantity of metal salts and the time of diffussion, which were recorded by high-power microscope completely and clearly. IR, UV-vis, TG analyses indicated the disctinct magnetic property of alloy 5RFeCo and 5RFeNi to their parent crystals. Meanwhile, a series of imidazole Schiff-base Co(Ⅱ) complexes in HS state were constructed by similar multi-component self-assembly. Structural and magnetic comparison between Co(Ⅱ) and Fe(Ⅱ) complexes revealed the isostructure and distinct SCO property.