Research On Molecular Based Magnetic Materials With Bistability

With the development of science and technology, the research on the molecular based electronic devices has attracted more and more attention. The molecules with magnetic bistability have been of intense interest due to their fundamental importance and unique potential applications in the light or heat switching and information storage devices etc. Molecular nanomagnets and spin-crossover complexes are the most studied materials which display bistabie magnetic states. Given the aforentioned consideration, by using different stratedies, we have prepared a series of new magnetic bistable materials, and studied their properties and the magneto-property relationship. The main achievements of this thesis are listed as below:1. Room-temperature magnetic bistability induced by phase transitionAccording to the classical inorganic perovskite compounds and the muti-ferroic format-bridged MOFs [(CH3)2NH2][M(HCOO3)3] (M=Mn2+, Co2+, Ni2+, Zn2+), we have prepared a series of end-to-end (EE) azido-bridged perovskite-type compounds [(CH3)nNH4-n][Mn(N3)3] (n=1-4) (compounds 1-4). All compounds exhibit a distorted perovskite-type structure with the Mn2+ ions connected by the (EE) azides. Sequential phase transitions were observed near the room temperature in these metal-organic framework perovskite structures, just as observed in the pure inorganic ABO3 perovskite compounds. These materials show cation-dependent magnetic ordering at up to 92 K and magnetic bistability near room temperature. Unlike the usual spin transition materials, the magnetic bistability of these compounds originates from modification of the magnetic coupling induced by the phase transition of the flexible framework. These compounds might be of great importance for the design of new magnetic bistability compounds and the further development of functional materials.2. Slow magnetic relaxation in one-dimensional azido-bridged CoⅡ complexesBy using N3- as the bridged ligands and amide as the terminal ligands, we have prepared a series of azido-bridged 1D complexes with different amide ligands: [Co2(N3)4(DMF)3] (5) (DMF= N,N-dimethylformamide), [Co4(N3)8(DEF)5] (6) (DEFN,N-diethylformamide) and [Co2(N3)4(DIPF)2] (7) (DIPF N,N-bis(1-methylethyl)formamide). The different steric and electronic properties of the amides lead to different structures and magnetic properties. Single-crystal X-ray crystallographic analysis revealed that compounds 5-7 all consist parallel one-dimensional chains with cobalt sites linked through (μ-N3-) bridges. Their building units differs greatly from each other, for compounds 5 and 7, {[CoN5O1][CoN4O2]2[CON5O1]} and {[CoN5][CoN4O2]2[CoN5]} are acting as the only secondary building units respectively to construct the 1D chains, while for compound 6, the 1D structure is completed by the {[CoN5O1][CoN4O2]2[CoN5O1]} and {[CoN5][CoN4O2]2[CoN5]} tetramer units equally. Variable temperature dc magnetic susceptibility data indicate that all compounds exhibit ferromagnetic interactions through the intrachain (μ-N3-) bridges and antiferromagnetic interactions through the interchain electrostatic interactions. Ac magnetic susceptibility data reveals the slow magnetic relaxation of the magnetization in the antiferromagnetic ordered phases. Compound 5 displays spin glass behaviour, whereas compounds 6 and 7 show the superparamagnetic behaviour. Magnetic bistability can also be observed at low temperature for all the three compounds.3. Spin crossover compounds adjusted by mono-organosulfonate anionsThree mononuclear spin crossover (SCO) compounds [Fe(2-pic)3]A2-Solv (A= m-ABS-, Solv=MeOH,8; A=p-ABS-,9; A=OTf-,10) were prepared and characterized magnetically and structurally (2-pic=2-picolylamine,m-HABS= m-aminobenzenesulfonic acid, p-HABS=p-aminobenzenesulfonic acid, HOTf= trifluoromethanesulfonic acid). Single-crystal X-ray analyses show that they are constructed from the charge-assisted hydrogen bonds between the 2-pic donors and the organosulfonate acceptors, forming the hydrogen-bonded three-dimensional (3D) networks for 8 and 9 and one-dimensional (1D) columns for 10. While the [Fe(2-pic)3]2+cations in compounds 8 and 9 are in the meridional (mer-) configuration, it has a facial (fac-) configuration in complex 10. Magnetic susceptibility measurements revealed the SCO transitions and the SCO properties in all three complexes are quite different. By using different mono-organosulfonate anions, the transition temperatures T1/2 can be tuned between 100 and 300 K. Compared to the famous [Fe(2-pic)3]A2-Solv compounds in the literature, compound 9 has the highest transition temperature for the mer-[Fe(2-pic)3]2+-containing compounds, while compound 10 represents the first example of the structurally characterized compound of the fac-[Fe(2-pic)3]2+motif showing SCO behavior. These results show that the organosulfonate anions are very promising to adjust the hydrogen-bonded structures of the SCO compounds and improve the SCO properties of those structures.4. Spin crossover compounds adjusted by di-organosulfonate anionsBy using di-organosulfonate anions, four mononuclear spin crossover (SCO) compounds [Fe(2-pic)3]·A·Solv (A=DNDS2-, Solv=MeOH,11; A=BPDS2-,12; A =1,5-NDS2-, Solv= 2H2O,13; A= MDS2-,14) were prepared and characterized magnetically and structurally (2-pic=2-picolylamine, H2DNDS 4,4’-Dinitrostilbene-2,2’-disulfonic acid, H2BPDS=4,4’-Biphenyldisulphonic acid, H2NDS=1,5-Naphthalenedisulfonic acid, H2MDS=Methanedisulphonic acid). Single-crystal X-ray analyses show that all these compounds are constructed by the charge-assisted hydrogen bonds between the 2-pic donors and the organosulfonate acceptors, forming the hydrogen-bonded 2D/3D networks.The [Fe(2-pic)3]2+cations in compounds 11-14 are all in the facial (fac-) configuration. Magnetic susceptibility measurements revealed that all these compounds show SCO properties near the room temperature.5. Water sensitive magnetic bistabilityBy using 4,4’-Diamino-2,2’-stilbenedisulfonate (DSD2-), we have prepared a SCO compound {mer-[Fe(2-pic)3]:fac-[Fe(2-pic)3]}·(DSD)2·(H20)6 (15), which contains a fac-[Fe(2-pic)i]2+ and a mer-[Fe(2-pic)3]2+ in its asymmetric unit. This compound exhibits a two-step spin transition, with Tc1=90 K and Tc2=350 K due to the two independent FeⅡ centers. Interestingly, when the H2O solvent molecule was desorbed, single-crystal-to-single-crystal transformation of 15 to a new SCO compound 16, formulated as mer-[Fe(2-pic)3]DSD, occurs with a dramatic color change from red to yellow. Crystal lographic studies revealed that the asymmetric unit of the structure of 16 contains only a mer-[Fe(2-pic)3]2+ spin center, and it undergoes a one-step spin transition with T1/2=120 K. More interestingly, the single-crystal to single-crystal transformation of 15 to 16 is fully reversible upon exposure to water vapor. At room temperature, in compound 15, only half of the Fe" centers are in the high spin state, while in compound 16, all the Fe" centers are in the high spin state. In conclution, the system exhibits magnetic bistability at room temperature upon the adsorption and desorption of H2O solvent molecules. The change of the [Fe(2-pic)3]2+ configuration during the phase transition is proposed to be responsible for the spin state change.