Computational design of magnetic metal-organic complexes and coordination polymers with spin-switchable functionalities
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Introduction Spin-crossover is an extraordinary feature of certain metalorganic complexes and frameworks—consisting of transitionmetal ions connected by organic ligands—and is typified by a bi-stable magnetic state of the metal ion center. A unique feature of the transition-metal ions with the surrounding ligands is the capacity of attaining different spin states, with a different total spin quantum number S, while keeping the same valence state. Upon application of an external stimulus, such as pressure, temperature, light irradiation, or magnetic field, the transition metal ion can be reversibly switched between different spin states, for example, between a low-spin (LS) state with the smallest possible value of the spin S, and a highspin (HS) magnetic state with the highest possible value of S (see References 1 and 2 for recent reviews). This bi-stability of the spin state has attracted considerable technological interest, as it offers the possibility of designing functional units that can be switched reversibly. Accordingly, spin-crossover (SCO) complexes are currently being considered as potential elementary units in information storage,3 spintronics devices,4,5
photo-switches,6,7 color displays,8,9 and, in connection with open frameworks, as gas sensors.10 In addition to their spin reversibility, SCO metal-organic systems are versatile and offer many possibilities for chemical functionalization, through which the desired transition property (e.g., color, transition temperature) can be efficiently tuned. The SCO phenomenon was discovered11 more than 80 years ago in iron(III) tris-dithiocarbamate compounds. The SCO transition on the Fe ion is between a LS (S = 1/2) state and a HS (S = 5/2) state. In 1961, the first spin-crossover in a Co(II)-based compound (Co(L)2X2, where L = bis-[2,6pyridindialdihydrazone] and X is a transcoordinated anion) was discovered.12 The SCO transition occurred from a LS (S = 1/2) to a HS (S = 3/2) state. This was followed by the discovery13 of SCO in a Fe(II) compound, (Fe(phen)2(SCN)2, where phen = 1,10-phenanthroline), which involves the transition from a LS (S = 0) to a HS (S = 2) state. Decurtins et al. discovered a light-induced SCO transition in a Fe(II) complex in 1984,6 which showed that light could be used to switch a sample from LS to a metastable HS state, which persisted for
Tanusri Saha-Dasgupta, S.N. Bose National Centre for Basic Sciences, India; [email protected] Peter M. Oppeneer, Department of Physics and Astronomy, Uppsala University, Sweden; [email protected] DOI: 10.1557/mrs.2014.112
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MRS BULLETIN • VOLUME 39 • JULY 2014 • www.mrs.org/bulletin
© 2014 Materials Research Society
COMPUTATIONAL DESIGN OF SPIN-SWITCHABLE METAL-ORGANIC COMPLEXES
days to weeks. This observation of light-induced excited state metal ion’s surrounding, the degenerate crystal field levels can spin-state trapping (LIESST) gave rise to invigorated interest split further. in the SCO phenomenon. Ligand-field theory offers a qualitative understanding of Many different SCO
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