Synthesis of Bulk High Spin Density Charge Transfer Complexes for Organic Magnets
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SYNTHESIS OF BULK HIGH SPIN DENSITY CHARGE TRANSFER COMPLEXES FOR ORGANIC MAGNETS
LONG Y. CHIANG,a,* RAVINDRA B. UPASANI,a,b D. P. GOSHORN,a AND P. TINDALLc a Corporate Research Laboratories, Exxon Research and Engineering Company, Annandale, New Jersey 08801 b Department of Physics, Princeton University, Princeton, New Jersey 08544 c Department of Chemistry, State University of New York, Stony Brook, New York 11794
ABSTRACT
We describe a synthetic approach to the preparation of high spin density organic solid as a probe to organic ferromagnetism utilizing an external doping process to achieve a molecular sequence of alternate donors and acceptors in different spin states for, in principle, the ground state ferrimagnets. In contrast to the irreversibility of electron oxidations of many triaminobenzene derivatives, we found that, by substituting three hindered diisopropylamino groups on benzene, stable monocationic TDIAB radicals can be obtained. In the case of HDMAB its cationic radicals can be stabilized in a strong acid medium. The observed bulk spin densities of 0.18 - 0.23 spins 1/2 per donor molecule in TDIAB-PF 6 and HDMAB-PF 6 solids are significant comparing to the diamagnetic properties normally obtained in solids of simple cationic salts of planar organic donor molecules. The results imply that a small degree of molecular spin separation due to the steric effect can readily increase sharply the paramagnetic spin density of solids. We also found that the introduction of arsenic pentafluoride into molecular crystals disintegrates the long range order of crystal without rearranging the molecular stacking sequence in HMT complexes. Arsenic pentafluoride serves as not only the oxidant, but also the physical molecular separator. That results in a molecular spin separation of triplet dicationic HMT spins from each other at high doping level. This concept of molecular spin separation explains the observation of static magnetic data with an observed high spin density of many doped HMT charge transfer complexes. Without the molecular spin separation, the segregated-stack complexes, such as HMT(AsF 5 .5)3 .4 and HMT-DDQ-(AsF5. 5 )4 .1 , should instead give a low net bulk spin density with no triplet resonance after the intermolecular spin exchanges resemble to those observed in the case of HMT+ 2(Cl0 4)2 crystals. INTRODUCTION
Three dimensional bulk ferromagnetism requires a high density of stable spins and a constructive intermolecular coupling between spins. Several theoretical considerations toward the design of organic ferromagnetism have been reviewed. 1 ,2,3 The spin containing organics have been used in various approaches to organic magnets are charge transfer complexes, 4 ,5,6 molecular radicals, 7 Mat. Res. Soc. Symp. Proc. Vol. 173. @1990 Materials Research Society
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polycarbenes, 8 and polyradicals. 9 Recently, we proposed a synthetic approach, which is derived from the McConnell's model, 1 for ground state bulk ferromagnetism or ferrimagnetism at solid state. 5 Experimentally, our approach involv
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