Photostimulated electron transfer and its action on paramagnetism of Sp 3 Cr(C 2 O 4 ) 3 single crystals
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Photostimulated Electron Transfer and Its Action on Paramagnetism of Sp3Cr(C2O4)3 Single Crystals R. B. Morgunov, F. B. Mushenok, S. M. Aldoshin, and N. A. Sanina Institute of Chemical Physics, Russian Academy of Sciences, Chernogolovka, Moscow oblast, 142432 Russia email: [email protected] Received November 15, 2008; in final form, May 29, 2009
Abstract—The effect of irradiation by ultraviolet light on the effective magnetic moment of a paramagnetic single crystal based on photochrome spiropyran (Sp) and chromium oxalates Sp3Cr(C2O4)3 molecules is detected. It is shown that the deviation of the temperature dependence of the magnetic moment from the Curie law is caused not by the exchange interaction, but by electron redistribution between Cr3+ and Cr4+ ions and spiropyran molecules Sp0 and Sp+. Analysis of the angular dependence of EPR spectra makes it pos sible to determine the contribution of Cr3+ ions to the magnetic properties of the crystals and to determine the crystal field parameters D = 0.619 cm–1 and E = 0.024 cm–1. Irradiation of hydrated samples by ultravi olet light leads to intensity redistribution of EPR lines attributed to Cr3+ and Sp0. Thermally stimulated para magnetism of triplet states of spiropyran ions Sp+ and the SpI salt is observed. PACS numbers: 75.50.Xx DOI: 10.1134/S1063776109100136
INTRODUCTION Materials with magnetic properties that can be controlled by light in the optical range can be obtained by preparing hybrid photochrome and magnetic com pounds [1–4]. Photochromism is observed in organic as well as inorganic compounds [5–7]. Spiropyrans, spirooxazines, and naphthapyrans are promising organic photochrome compounds for practical appli cations (see review [4] for details). In organic com pounds, two physically different types of photomag netic effects can exist. In the first type, changes in the distance between the spins of metal ions and in the crystal field energy due to opening (closure) of photo chrome molecules of spiropyrans modify the exchange interactions and level splitting in the crystal field [8– 11]. The second type of these effects is not connected with a change in the geometry of photochrome mole cules under the action of light, but involves a photoin duced redistribution of electrons (holes) among mag netic centers. Such an effect was observed earlier in inorganic compounds [5–7]. It was shown in [4, 12] that under the action of light, the formally determined Weiss temperature of crystals of complex Sp3Cr(C2O4)3 organoinorganic compounds changes, but the mechanisms of the effect of light on this parameter, which must be close to zero in paramag netic crystals, remain unclear. Paramagnetic Sp3Cr(C2O4)3 crystals contain cationtype spiropyran molecules with orthogonally arranged πconjugate aromatic fragments and anions of chromium oxalates with d electrons of unfilled shells of chromium ions. We can expect that photoinduced opening of pyran
cycles and a change in their geometry may change the spacing between Cr3+ paramagneti
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