, Additional perturbations must affect the Kramers ground state in zero 209 external dc field to allow the system to oscillate between the "up" and "down" 210 states. In the condensed crystalline state, the molecules are closely packed, and 211 interactions of dipolar origin may propagate through space. This is especially true 212 when someone deals with heavy lanthanides which possess the largest magnetic 213 moments of the periodic table. Transverse component of this internal field can mix 214 the Kramers doublets and facilitate the tunneling. The dilution of the complex in a 215 diamagnetic medium (at low concentration) minimizes this internal field and is 216 supposed to suppress the tunneling. However, one can see on Fig. 6 that dissolution 217 in dichloromethane is not enough: leveling of the relaxation time still persists at low 218 temperature. Compared to the application of an external dc field of 1 kOe, 0 ¼ 8 Â 10 À6 s, and ? TI ¼ 1.62 Â 10 À3 s. Dy(III) is a Kramers ion, 204 and the magnetic moment should not be able to tunnel through the barrier: the two 205 Ising components cannot be mixed by modulation of the crystal field

, However, the thermal variation of the relaxation time at low 457 temperature is very different from what we are used to observe on mononuclear 458 complexes. Indeed, there is no leveling of the relaxation time on cooling in zero 459 external dc field down to 2 K: ? increases continuously on cooling, The complex is a SMM

, The 465 reaction of tetrathiafulvalene-3-pyridine-N-oxide ligand (L 10 ) with [Dy(the Ising limit (20). The nonmagnetic ground state is then described by |"#i 470 and |#"i, with the first excited state, The implication of magnetic interactions on slow relaxation dynamics in dimers 464 is confirmed by other investigations on TTF-based Dy, p.3

, With J ¼ À2.3 cm À1 , the crossing should 485 occur at 1.3 kOe which relatively close to the measured value. This in-field behavior 496 pyridine N-oxide bridges two Dy(III) ions. However, in this system, one 497 monoanionic ?-diketonate moiety has been substituted by one monoanionic sulfo498 nate. One oxygen atom from pyridine N-oxide group completes the coordination 499 sphere. The ligand L 11 has been oxidized during galvanostatic. The TTF core is 500 almost planar in agreement with its radical cationic form L 11?+. Two 501 non-coordinated sulfonate anions balance the positive charge of the complex, At low field and temperatures below 8 K, ? decreases with the field (Fig. 15) with such temperatures, the first magnetically active (|""i and 478 |##i) excited states are thermally populated

, 5-bis(pro-pylthio)tetrathiafulvalenyl]-1H-benzimidazol-2-yl}pyridine (L 12 ) 520 was then treated with two equivalents of, vol.5, p.3

, 2 and 527 11 K clearly show two well-separated relaxations which can be confronted to the 528 measurements on the isolated species. The presence of a slow and a fast process at 529 low and high frequencies, respectively, matches almost perfectly with the isolated 530 species. The low-frequency side corresponds to the dinuclear part and the high 531 frequency to the mononuclear. It is also possible to analyze quantitatively the 532 thermal and the in-field behaviors with a combination of two extended Debye 533 models. At this stage our synthetic approach allowed us to conceive a complex 534 which contains two different SMMs which act differently in the temperature and 535 time scales, zero external field, the ? M 00 vs. ? curves at various temperatures between

, The last example we would like to comment concerns the polymeric species

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O. Cador and F. Pointillart,