NATURE CHEMISTRY | VOL 3 | JUNE 2011 | www.nature.com/naturechemistry 425

news & views

Electronic polymers are important components of thin-film optoelectronic devices made from organic materials.

Commercialization of organic light-emitting diodes and photovoltaics for solar energy conversion based on easily processed conjugated polymers seems imminent. One of the primary considerations for optimization of these devices is the fact that spectroscopy, fluorescence yields and charge transport depend on polymer morphology1. Consequently, understanding how processing conditions affect the final morphology is an important, if challenging, scientific problem whose resolution has substantial implications for device engineering.

There have been many studies of the phenomenology associated with various processing approaches, but few studies that have probed conformational dynamics of conjugated polymers during processing. Writing in Angewandte Chemie International Edition, Jan Vogelsang and colleagues report2 their single-molecule spectroscopy studies of the microscopic reorganization of single conjugated polymer chains in host films during a common process used to regulate polymer morphology (solvent vapour annealing; SVA). Their elegant work shows that MEH-PPV (poly(2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylenevinylene) chains rapidly adopt a collapsed configuration under SVA, and lose memory of the kinetically trapped geometry they assume during spin casting.

Vogelsang and colleagues use wide-field confocal microscopy to follow single chains of MEH-PPV dilutely doped into a poly(methyl methacrylate) (PMMA) film using an apparatus specially constructed to enable in situ SVA of the film. They are able to monitor the intensity of the chain fluorescence as well as its motion over time before, during and after SVA (Fig. 1). During SVA, they observe higher fluorescence and large, rapid fluctuations in its intensity. Furthermore, the chain moves more rapidly during SVA — sometimes quite suddenly — when the host matrix is swollen. The increase in intensity during SVA is interpreted in terms of a model involving

two types of polymer conformation designated as ‘extended’ and ‘collapsed’, in which the collapsed polymer has two to three times lower fluorescence quantum yield. The two-conformation model has been used before to rationalize the spectroscopic data3,4, and presumably the different forms of the polymer correspond to the degree to which the polymer’s chromophores are able to interact5. Although Vogelsang and co-workers state that the “mechanism of self-quenching [in the collapsed conformation] is unclear”, previous studies have made an excellent case that when collapse facilitates aggregation of the polymer’s chromophores, the fluorescence yield is reduced by opening up an efficient pathway to form interchromophore charge-transfer states6,7.

On analysing the correlation between excitation and emission polarization, they also show that SVA causes the conjugated polymer to lose memory of its initial conformation2. The initial distribution of polarization excitation anisotropy8 on spin casting the MEH-PPV-doped PMMA films from chloroform is very different from that spun from toluene. This phenomenon is similar to what was observed in previous work in which isolated MEH-PPV chains were spin cast from these solvents9, and has been ascribed to the more rapid evaporation of chloroform, leaving the polymer in a less equilibrated and more extended conformation resembling that spun from the good solvent. Vogelsang and colleagues show that the effect of SVA is to cause the polymer

CONJUGATED POLYMERS

Watching polymers danceSingle-molecule spectroscopy allows fluctuations of conjugated polymer conformation to be monitored during solvent vapour annealing. Dramatic changes in fluorescence behaviour are observed and interpreted in terms of transformations between extended and collapsed polymer geometries.

Lewis Rothberg

Figure 1 | Chromophore reorganization before, during and after solvent vapour annealing (SVA). Before, some chains assume a kinetically trapped ‘extended’ conformation characteristic of being dissolved in a good solvent, whereas others take a more thermodynamically favoured ‘collapsed’ geometry where chromophores (green rectangles) interact. During SVA, the PMMA host swells with solvent, the extended configuration is favoured and the chains become mobile. After the solvent is slowly driven out, the chains assume a collapsed geometry and lose memory of the initially spin-cast film.

Before SVA

DuringSVA

AfterSVA

PMMA

MEH-PPV

Solvent

© 2011 Macmillan Publishers Limited. All rights reserved

426 NATURE CHEMISTRY | VOL 3 | JUNE 2011 | www.nature.com/naturechemistry

news & views

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

© 2011 Macmillan Publishers Limited. All rights reserved