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to ‘forget’ its initial geometry and that the memory of the extended conformation initially achieved in films spun from chloroform is lost after SVA so that the chain conformations become indistinguishable from those in films spun from toluene. The more collapsed chain conformation resulting from SVA is thermodynamically favoured.

Aside from the general appeal of tracking single polymer chains, this experiment is able to prove that kinetically trapped conformations of the polymer can result from spin casting, and unambiguously implicates conformational changes as a reason for fluorescence yield variation in polymers. In principle, one could imagine assessing various SVA strategies and predicting their effects on fluorescence yields and spectra in so far as these seem to be regulated by morphology. It would be interesting to extend this type of experiment to using the emissive polymeric dopants as a probe of host rearrangement to achieve better insight into polymer-blend morphologies. In particular, one could imagine using emissive polymers with a low HOMO–LUMO gap dilutely doped into a high-gap conjugated polymer to report on rearrangements of the host material.

A significant limitation of the approach of Vogelsang and colleagues is that it would be difficult to apply to the most pressing and practical need to understand SVA in conjugated polymers, that being the annealing of donor–acceptor blend films now widely practised to improve photovoltaic performance by controlling bulk heterojunction morphology10. In those types of system, fluorescence is quenched by highly efficient dissociation of the excited state as is required for the device to function well. Moreover, it is not obvious how one could use a very dilute reporter such as an isolated single chain to reflect properties of the blend. Also, although global descriptions of polymeric conformation such as fluorescence intensity and polarization anisotropy provide coarse insight into polymer morphology, they give little microscopic information about the details of the reorganization of the polymer or about root causes of the changes in photophysical properties when the polymer conformation changes. Nevertheless, Vogelsang’s imaginative and pioneering work can be expected to inspire further investigation of polymer conformational dynamics using single-molecule methods.

It is with deep sadness that I reflect that the article discussed is one of the late Paul Barbara’s last publications. He was a great scientist, bringing exemplary creativity, insight and integrity to the process. Paul was also a fine human being, a valued colleague, and a role model and advocate for young, developing scientists. ❐

Lewis Rothberg is in the Department of Chemistry at the University of Rochester, Hutchison 200, Rochester, New York 14627, USA. e-mail Rothberg@chem.rochester.edu

References1. Nguyen, T. Q., Martini, I. B., Liu, J. & Schwartz, B. J.

J. Phys. Chem. B 104, 237–255 (2000).2. Vogelsang, J., Brazard, J., Adachi, T., Bolinger, J. C. & Barbara, P. F.

Angew. Chem. Int. Ed. doi:10.1002/anie.201007084 (2011).3. Collison, C. J., Rothberg, L. J., Treemaneekarn, V. & Li, Y.

Macromolecules 34, 2346–2352 (2001).4. Adachi, T. et al. J. Phys. Chem. C 114, 20896–20902 (2010).5. Kas, O. Y., Charati, M. B., Rothberg, L. J., Galvin, M. E. & Kiick,

K. L. J. Mater. Chem. 18, 3847–3854 (2008).6. Yan, M., Rothberg, L. J., Kwock, E. W. & Miller, T. M.

Phys. Rev. Lett. 75, 1992–1995 (1995).7. Samuel, I. D. W., Rumbles, G., Collison, C. J., Moratti, S. C. &

Holmes, A. B. Chem. Phys. 227, 75–82 (1998).8. Hu, D. H. et al. Nature 405, 1030–1033 (1995).9. Huser, T., Yan, M. & Rothberg, L. J. Proc. Natl Acad. Sci. 97,

11187–11191 (2000).10. Li, G. et al. Nature Mater. 4, 864–868 (2005).

The first compounds behaving as miniature magnets were discovered in the 1990s. These species, known

as single-molecule magnets (SMMs)1, are typically coordination clusters comprising paramagnetic metal centres whose spins align in the presence of an external magnetic field. Spin coupling induces their magnetization, which can adopt two states, depending on the direction in which the spins are aligned. Below a specific ‘blocking temperature’, the relaxation of the magnetization becomes slow, and the compounds retain a stable magnetization even in the absence of an external magnetic field. This magnetization is of pure molecular origin, and thus very different from that of bulk magnets requiring the collective long-range ordering of magnetic moments within the material. Writing in Nature Chemistry, Liddle and co-workers describe2 an interesting arene-

bridged diuranium complex that shows the characteristics of a SMM.

The magnetic hysteresis of SMMs yields a memory effect that could serve to make ultra-high-density information storage components, for example for computing and spintronic applications. Incorporating SMMs within efficient devices, however, will only become possible if higher energy barriers to spin inversion are achieved, in combination with reasonable blocking temperatures (most SMM exhibit slow relaxation below 10 K).

Because the unusual behaviour of single-molecule magnets stems from the alliance of a high-spin electronic ground state and a high magnetic anisotropy, chemists have been searching for ways to optimize both parameters with a view to fabricating nanomagnets. This has inspired the design of a wide variety of very sophisticated and intrinsically beautiful coordination compounds3. To achieve a high-spin ground

state, and thus a higher relaxation barrier, increasingly large polynuclear compounds of paramagnetic d-block transition metals have been synthesized. Even larger relaxation barriers have now been obtained with lanthanide centres. These also possess high single-ion anisotropies but, because of the low radial extension of 4f orbitals, lanthanide–ligand interactions lack covalency, which is likely to limit possible magnetic exchanges through non-magnetic bridging ligands.

This had led chemists to investigate the use of 5f elements, which also present high anisotropy yet have a greater potential for covalent bonding. Recently, slow magnetic relaxation behaviour has been shown with two actinide-based complexes, featuring either U3+ (ref. 4) or Np3+ (ref. 5). The SMM behaviour of these mononuclear compounds was found to arise from the magnetic anisotropy of the actinide–ligand interactions

MOLECULAR MAGNETISM

Uranium memoryA diuranium compound featuring an arene bridge shows single-molecule-magnet behaviour, which could arise from a mechanism different from the traditional ‘super-exchange’ spin coupling.

Marinella Mazzanti

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— a good sign that 5f elements might be promising for the design of improved SMMs.

A growing number of polynuclear actinide compounds are being reported, mainly limited to uranium systems because depleted uranium is not highly radioactive. The presence of magnetic exchange in polynuclear uranium complexes, however, is difficult to pin down because of the lack of suitable theoretical models for the analysis of the complex interactions governing uranium magnetism. Magnetic properties have been reported only in very few cases, and in even fewer examples have the data been analysed. Unambiguous magnetic uranium–uranium interactions have been observed only in a small handful of antiferromagnetically coupled 5f 1 systems6–9 where the two uranium ions are connected through oxo or imido bridges.

In a recent review10, Long and co-workers have suggested that the amido- (NR2

–) and cyclopentadienyl-based (*Cp–) arene-bridged diuranium(iii) compounds prepared in the Cummins11 and Evans12 groups were well poised to present magnetic exchange coupling. In the present article, Liddle and co-workers have identified another such diuranium(iii) compound, and have now demonstrated slow relaxation and hysteresis that are characteristic of SMM behaviour. Although slow relaxation occurs at low temperature, this result is a highly significant step in the design of actinide-based SMMs. It also represents a tool for probing the nature of metal–ligand interactions in complexes of 5f metals, and to study how they differ from those of the 4f and d-block elements and how this affects the magnetic and spectroscopic properties. Apart from memory-storage applications, clearly identifying the differences between these elements can provide routes to separation procedures, for example for nuclear fuel reprocessing.

Liddle and co-workers synthesized the arene–diuranium complex by reduction of a U(iv) carbene complex (with a U=CN2P2 moiety) in toluene. The reduction reaction is accompanied by the transformation of the carbene ligand, probably through a highly reactive U(iii)=C intermediate, to yield a U(iii) methanide complex (with a U–CHN2P2 moiety). The yield is quite low, probably because of the high reactivity of the intermediate, but alternative routes may be envisaged — or already under study — to increase it.

The diuranium–arene complex was analysed by density functional theory. The calculations, consistent with those of previously reported complexes, showed that the two uranium centres, formally U(iii) ions, are bridged by a dianionic arene ligand through covalent δ-backbonding. This provides a possible covalent pathway for

magnetic communication through the arene bridge (Fig. 1a). Characterization of the magnetic properties showed slow relaxation dynamics, with a magnetic hysteresis measured at 1.8 K.

On the basis of the calculations of the distribution of electrons within the diuranium–arene complex, and on the characterization of its magnetic properties, Liddle and co-workers suggest that magnetic communication cannot be described in terms of a super-exchange mechanism — in which the SMM behaviour would simply be generated by the spins localized on the uranium centres coupling through the arene ligand. Classic rules of magnetism would suggest that a super-exchange mechanism should lead to antiferromagnetic exchange in this system (Fig. 1b) — but this is not observed. Instead, magnetization can arise from electrons being delocalized between the uranium centres. A word of caution here is that it is not entirely certain that classic rules can be fully trusted in the presence of the strong spin–orbit coupling (where a spin interacts with its motion) found in 5f elements.

In the present case, it is hard to identify the coupling mechanism unambiguously. Most single-molecule magnets reported to date have involved super-exchange coupling. Moreover, it may be possible that the main contribution to the barrier blocking the magnetization arises from blockage of individual uranium ions, as is the case for polynuclear SMMs based on lanthanide centres. To fully refute this mechanism with the present complex, analogous mononuclear uranium compounds or arene-bridged heterometallic complexes with a diamagnetic rare-earth ion such as Sc(iii) should be prepared and compared — but this might be synthetically challenging. On the other hand, one example where magnetism arises from delocalized electrons13 has recently been described. Furthermore, in another compound — an arene-bridged dinuclear Cr(i) complex14 analogous to that investigated in the present study — a strong ferromagnetic coupling mediated by an arene bridge has also been proposed to occur.

The electronic structure of this uranium–arene–uranium system is certainly an interesting observation, and Liddle and co-workers’ suggestion regarding its coupling mechanism is going to generate debate and encourage an expansion of the field. Analogous systems with a variety of ligands may be investigated, or included in larger clusters, which could in turn lead to breakthroughs in the development of SMMs. In any case, this study will lead to a better understanding of bonding in 5f compounds, and how they differ from their 4f analogues. ❐

Marinella Mazzanti is at the Laboratory of Ionic Recognition and Coordination Chemistry, Inorganic Chemistry and Biology (UMR E-3 CEA/UJF-Grenoble 1), INAC, 17 rue des Martyrs 38054 Grenoble cedex 9, France. e-mail: marinella.mazzanti@cea.fr

References1. Gatteschi, D., Sessoli, R. & Villain, J. Molecular Nanomagnets

(Oxford Univ. Press, 2006).2. Mills, D. P. et al. Nature Chem. 3, 454–460 (2011).3. Winpenny, R. E. P. (ed.) Single-Molecule Magnets and Related

Phenomena (Structure and Bonding) (Springer, 2006).4. Rinehart, J. D. & Long, J. R.,J. Am. Chem. Soc.

131, 12558–12559 (2009).5. Magnani, N. et al. Angew. Chem. Int. Ed. 50, 1696–1698 (2011).6. Rosen, R. K., Andersen, R. A. & Edelstein, N. M.

J. Am. Chem. Soc. 112, 4588–4590 (1990).7. Nocton, G., Horeglad, P., Pecaut, J. & Mazzanti, M.

J. Am. Chem. Soc. 130, 16633–16645 (2008).8. Mougel, V., Horeglad, P., Nocton, G., Pecaut, J. & Mazzanti, M.

Angew. Chem. Int. Ed. 48, 8477–8480 (2009).9. Spencer, L. P. et al. Angew. Chem. Int. Ed. 48, 3795–3798 (2009).10. Rinehart, J. D., Harris, T. D., Kozimor, S. A., Bartlett, B. M. &

Long, J. R. Inorg. Chem. 48, 3382–3395 (2009).11. Diaconescu, P. L., Arnold, P. L., Baker, T. A., Mindiola, D. J. &

Cummins, C. C. J. Am. Chem. Soc. 122, 6108–6109 (2000).12. Evans, W. J., Kozimor, S. A., Ziller, J. W. & Kaltsoyannis, N.

J. Am. Chem. Soc. 126, 14533–14547 (2004).13. Bechlars, B. et al. Nature Chem. 2, 362–368 (2010).14. Lin, P. H. et al. Angew. Chem. Int. Ed. 48, 9489–9492 (2009).

Figure 1 | An arene-bridged diuranium complex exhibits SMM behaviour. a,b, Bridging the two formal U(iii) centres by a dianionic arene through covalent bonding leads to slow magnetic relaxation (a). This behaviour could involve a different exchange mechanism — based on electron delocalization, shown here by the parallel spins (upper panel) and a view of the computed f orbitals (lower panel) — from the super exchange mechanism between two metals (M) and a ligand (L) shown in b.

a

b

UIII UIII

M L M

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