© Copyright 1996 by the American Chemical Society
VOLUME 100, NUMBER 47, NOVEMBER 21, 1996
LETTERS Molecular-Level Design of a Photoinduced Magnetic Spin Coupling System: Nickel Nitroprusside Z.-Z. Gu,† O. Sato,‡ T. Iyoda,‡ K. Hashimoto,†,‡ and A. Fujishima*,† Department of Applied Chemistry, The UniVersity of Tokyo, 7-3-1 Hongo, Bunkyo-Ku, Tokyo 113, Japan, and Kanagawa Academy of Science and Technology, Tokyo Institute of Polytechnics, 1583 Iiyama, Atsugi, Kanagawa 243-02, Japan ReceiVed: July 18, 1996; In Final Form: September 30, 1996X
Rational design of a magnetic material (nickel nitroprusside) at the molecular level allowed switching of a spin coupling by means of photoinduced metal to ligand charge transfer (MLCT). Before irradiation, spins on Ni2+ are in a disordered state. An illumination at 375 nm induces a MLCT from Fe to NO and forms new spins on Fe and NO. The newly appearing spin on Fe orders the spins on the nearest Ni2+ ions around Fe. In other words, one MLCT aligns five spin sources resulting in the formation of magnetic clusters with S ) 5. Furthermore, such a photoinduced spin ordering can be rerandomized by thermal treatment.
One of the remaining, underlying challenges in molecularscale devices is to develop ways to establish switchable communication links between different components and to the outside world.1,2 Much of the effort in this field has been devoted to electronic communication, most often utilizing longrange electron transfer via π-conjugated systems.3-5 Here we introduce another potentially valuable approach to molecularlevel communications, which involves tuning the spin coupling via metal to ligand charge transfer (MLCT). The prototype system which we have used is nickel nitroprusside, Ni[Fe(CN)5NO]‚5.3H2O (Figure 1). In this compound, randomly aligned neighboring NiII spins can be ordered through metal (FeII) to ligand (NO) charge transfer via light absorption and form spin clusters. The ordered spins can then be rerandomized via thermal treatment. This strategy may also provide the basis for a wholly new approach to the fabrication of optically actuated molecular magnets, which has been identified as an important target of research in molecular magnetic materials.6 †
The University of Tokyo. Kanagawa Academy of Science and Technology. X Abstract published in AdVance ACS Abstracts, November 1, 1996. ‡
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Figure 1. Nickel nitroprusside, designed to construct the switchable molecular spin device. In this compound a MLCT from FeII to NO is used to control the coupling between NiII ions.
Prussian blue analogs are composed of two components, a cyanometalate and an outer cation. These compounds have a three-dimensional network structure. Every outer cation in the network can be coordinated by the nitrogen ends of up to six CN ligands. With such materials, it is possible to design the two components separately. In the present work, the pentacy© 1996 American Chemical Society
18290 J. Phys. Chem., Vol. 100, No. 47, 1996 anonitroprusside ion, Fe(CN)5NO2-, and the nickel ion, Ni2+, were selected. Since the nitrosyl group, utilized as an acceptor, has a relatively low unoccupied energy level, the MLCT absorption band falls in the visible range. Thus, we can control the electronic and spin states of the compound via visible light illumination. Mixing aqueous solutions of NiCl2 and NaFe(CN)5NO‚2H2O yields a precipitate of Ni[Fe(CN)5NO]‚5.3H2O. (Anal. Calcd for Ni[Fe(CN)5NO]‚5.3H2O: Fe, 15.8; Ni, 16.6; C, 17.0; N, 23.7; H, 2.7. Found: Fe, 15.5; Ni, 16.2; C, 16.2; N, 22.5; H, 2.5.) The infrared spectrum of this material at 12 K exhibits CN stretching peaks, υCN, at 2202 and 2154 cm-1, indicating a FeII-CN-NiII structure,7 and an NO stretching peak, υNO, at 1949 cm-1. The powder X-ray diffraction pattern indicated a face-centered cubic (fcc) structure with a unit cell parameter of 10.16 Å. In this compound, each FeII ion is coordinated to the carbon ends of five CN ligands plus one nitrosyl group, and each NiII ion is coordinated to the nitrogen ends of the CN ligands or to water molecules (Figure 1). Such coordination features are supported by X-ray diffraction, thermogravimetric analysis, and Mo¨ssbauer spectroscopy.7-10 Magnetization measurements were performed with a superconducting quantum interference device (SQUID) magnetometer. The effective magnetic moment µeff at 300 K was 2.94 µB per formula unit of Ni[Fe(CN)5NO]‚5.3H2O, indicating that SNi ) 1 and SFe ) 0. This compound exhibits paramagnetic character over the temperature range of 1.8-300 K. A fit of the temperature dependence of the magnetization data to the Curie-Weiss law, χ ) C/(T - θ), yielded a Weiss constant θ of -0.87 K, indicating a very weak antiferromagnetic interaction between neighboring NiII ions through the NC-FeII-CN structure. This weak exchange interaction between NiII ions is probably due to a lack of a magnetic interaction between FeII and NiII. An argon ion laser (475 nm, 2 mW/cm2) was used as the light source, and the blue light was guided into the SQUID magnetometer through an optical fiber. When the sample was illuminated at 5 K, the magnetization increased immediately and gradually reached saturation. This increase, indicating the appearance of spins due to the illumination, was observed over the whole temperature range below 200 K. In the IR spectrum, a new peak appeared at 1821 cm-1 after illumination at 12 K. In the UV-visible region, the absorbance between 360 and 450 nm increased with illumination. All of these changes can persist after illumination, and the original state can be restored by thermal treatment at temperatures above 200 K. These results are essentially consistent with the phenomena observed for metastable states of mononuclear sodium nitroprusside,11-13 suggesting that the changes of the magnetization observed for Ni[Fe(CN)5NO]‚5.3H2O can be explained by a MLCT between FeII and the NO group. That is, the electron in the 2b2 (Fe dxy) orbital is excited to the 7e (NO π*) level via light absorption. Charge transfer to an NO orbital with antibonding character weakens the NO bond, resulting in a shift of υNO from 1949 to 1821 cm-1. The increase of the absorbance in the UV-visible region is ascribed to the metastate of Fe(CN)5NO2-.13,14 In addition, the MLCT generates the spins localized on Fe and NO which affect the magnetization. The change of IR spectrum saturates after 10 h of irradiation. The population of the metastate can reach several percent after irradiation. Figure 2 shows a plot of the initial magnetization prior to illumination, M, vs temperature, T, and the change in magnetization due to illumination, ∆M, vs T. The ∆M vs T curve clearly rises steeply at low T. The relative magnitude of ∆M vs total M at 50 G increases from ca. 6% at 18 K to ca. 17% at 3K and can increase to larger values at lower temperatures. The
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Figure 2. Thermal dependence of the magnetization (M) of Ni[Fe(CN)5NO]‚5.3H2O in a field of 50 G before illumination (O) and the change in magnetization (∆M) due to light absorption (4). The solid line was obtained by fitting to the Curie-Weiss law, with θ ) -0.87 K.
strong temperature dependence of ∆M indicates the appearance of high-spin magnetic clusters. These magnetic properties, together with the spectroscopic results, indicate that the magnetic clusters are induced by the MLCT from FeII to NO. According to reports on Prussian blue analogs, the CN ligand can act as a good mediator for both ferro- and antiferromagnetic interactions;6,15-17 Ni3[Fe(CN)6]2, for example, shows spontaneous ferromagnetic ordering at Tc ) 23 K. These results support the conclusion that the newly formed spins on the Fe ions can couple to those on Ni ions through the CN ligand and form magnetically ordered clusters. In order to examine this phenomenon in more detail, the external magnetic field dependence of the magnetization increase due to illumination was carefully studied below 50 kG. The light-induced change in magnetization, ∆M, is essentially linearly dependent on the external field at temperatures above 20 K, while the curve begins to bend at lower temperatures. It should be noted that all of the plots of M vs magnetic field obtained before illumination were approximately linear in this temperature range. These results indicate that magnetic clusters with spins much greater than S ) 1 appear after illumination. The magnetization due to the light-induced magnetic clusters can be calculated from ∆M (see legend for Figure 3), and these data can then be fitted to a Brillouin function. This yielded a best-fit value of S ) 5, with the portion of the total Fe atoms acquiring one unpaired spin being approximately 1.9%. The spin number of S ) 5 can be explained as follows. The photoinduced MLCT creates two new spins on Fe and NO, each with S ) 1/2, which interact with each other antiferromagnetically, as expected from studies of other iron-nitrosyl complexes.18,19 In addition, the new spin appearing on Fe interacts ferromagnetically with the spins of the five NiII ions (S ) 1) connected to the Fe via CN ligands in accord with well-known principles,15,16 as well as with experimental results on nickeliron cyanides,6 and they form a magnetic cluster. This means that one MLCT process aligns five spin sources. It is reasonable to assume that these magnetic clusters with S ) 5 are dispersed in the material and the generation of clusters with larger spin number via the combination of two or more clusters with S ) 5 is almost negligible, since the fraction of Fe with the photoinduced spin is relatively low (ca. 1.9%) under the present experimental conditions. We have demonstrated an optically controllable spin gate based on an exchange interaction between molecular compo-
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J. Phys. Chem., Vol. 100, No. 47, 1996 18291 magnetooptical magnetic material on a molecular level. In contrast to the magnetization enhancement reported in a cobaltiron cyanide20 using photoinduced charge transfer between metal cations in different spin sites, the magnetooptical material presented here has been designed precisely on a molecular level for the first time. Such molecular-level design provides a novel approach for rational design of molecule-based magnets, which is regarded as a distinguishing feature compared with traditional magnetic materials. Acknowledgment. We thank Dr. D. A. Tryk for carefully reading the manuscript. This work was partially supported by a grant from the Ministry of Education, Science and Culture of Japan. References and Notes
Figure 3. Field dependence of the magnetization at 2 ([), 3.5 (0), and 5 K (b) before illumination, fitted to a Brillouin function in which S ) 1 (dotted curve). Light irradiation induces Fe with spin which is responsible for the formation of magnetic clusters. The magnetization of the photoinduced clusters can be calculated as Mcluster ) (M1R1 + M2R2 + ‚‚‚)/(R1 + R2 + ‚‚‚), where M1 is the magnetization of the clusters containing only one Fe with spin and R1 is the number of moles of such clusters and M2 is the magnetization of clusters containing two Fe with spin and R2 is their mole number. Since the population of metastate was relatively low, the clusters containing more than one Fe with spin can be neglected. In this situation, the magnetization of the magnetic clusters, Mcluster, was obtained using the equation Mcluster ) M1 ) (∆M + 5RMb)/R, where Mb is the magnetization before illumination, R is the fraction of Fe atoms with photoinduced spin, and 5 is the number of Ni ions connected to each Fe via CN- ligands. The 5RMb term corrects for the fact that, after illumination, part of the background signal is decreased due to the formation of the magnetic clusters. Mcluster values at 2 (]), 3.5 (9) and 5 K (O) were fitted to a Brillouin function in which S ) 5 (solid curve).
nents and on an MLCT between FeII and NO. Interactions between NiII ions are very weak before illumination due to the diamagnetic character of FeII in the ground state, and this can be regarded as an “off” state. After illumination, the interactions can be strongly mediated by spins generated via MLCT on Fe, allowing magnetic coupling, i.e., an “on” state. This provides a new approach to the design of molecular information transfer devices by taking advantage of spin interactions. In addition, this system is also interesting from the viewpoint of moleculebased magnets because it is the first example of designing a
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