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Tetrahedron Report Number 541

Conceptual and Synthetic Strategies for the Preparation of Organic Magnets

Joe A. Crayston, John N. Devine and John C. Walton *

School of Chemistry, University of St. Andrews, St. Andrews, Fife KY16 9ST, UK

Received 30 June 2000

Contents1. Introductory Aspects of Magnetism 7830

1.1. Types of bulk magnetism 78301.2. Anticipated properties of organic magnetic materials 78321.3. Magnets based on organo-transition metal complexes 7832

2. Conceptual Models for Organic Molecular Magnets 78342.1. Negative spin density product model 78342.2. Charge-transfer complex models 7835

2.2.1. McConnell's model (II) and [Fe III (C5Me5)2]1 z [TCNE] 2

z

78352.2.2. Hexaminotriphenylene dication and hexaazaoctadecahydrocoronene complexes 78352.2.3. Decacyclene cation salts and symmetrical polyamines 78372.2.4. Complexes of TCNQ, PCCP, DDQ, etc. with various cations 78372.2.5. CT complexes containing fullerenes 7838

2.3. Organomagnetic materials based on nitroxide radicals 78392.3.1. Mono-nitroxides, poly-nitroxides and nitronyl nitroxides 7839

2.3.2. CT complexes containing nitroxide radicals 78402.4. The topological model and applications to synthetic polyaryl-carbenes and polyaryl-radicals 78402.4.1. Molecular topology and spin alignment 78402.4.2. Poly( m-phenylene carbenes and nitrenes) 78422.4.3. m-Linked poly(aryl nitroxides) 78422.4.4. High spin hydrocarbons, chlorocarbons and heterocycles 78432.4.5. Star branched and dendritic aromatic polyradicals 78432.4.6. Avoidance of spin defects via multiple coupling paths 78442.4.7. Polymers, oligomers or dendrimers with radical side-chain or pendant groups 7845

2.5. The polaronic model: preparation of ferromagnetically coupled conjugated spin-containingsegments 78472.5.1. Fukotome's proposal of ferromagnetically coupled polarons 78472.5.2. Dougherty's synthesis of poly( m-phenyleneoctatetraene) 78472.5.3. Measuring the effectiveness of ferromagnetic coupling units for use in polaronic model

materials 78482.5.4. Polymers containing conjugated segments linked through m-phenylene units 78492.5.5. Polaronic poly(arylamines) 78512.5.6. Other polaronic materials 7852

3. Conclusions and Future Prospects 7852

Tetrahedron 56 (2000) 78297857Pergamon

TETRAHEDRON

00404020/00/$ - see front matter q 2000 Elsevier Science Ltd. All rights reserved.PII: S0040-4020(00) 00587-1

* Corresponding author. Fax: 1 44-1334-463808; e-mail: [email protected]

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J. A. Crayston et al. / Tetrahedron 56 (2000) 78297857 7830

1. Introductory Aspects of Magnetism

The earliest discovery of magnetic phenomena is not knownfor certain, although the word `magnet' is probably derivedfrom the Thessalian province of Magnesia where magnetite(lodestone, Fe 3O4) was mined. 1 Magnetism is one of nature's most fascinating and evergreen phenomena.Magical powers to induce personal empathy and to attractgold, as well as various curative properties, were onceattributed to lodestone. The application of magnets as primi-tive compasses for direction nding was rst recorded inEurope in the 13th century, but the Chinese probably knewabout the compass in 300200 BC. The Earth's magneticeld continues to fascinate; recently it has been proposed

that the Earth possesses a giant iron crystal at its core.2

Electromagnets, dynamos and AC transformers have, of course, played key roles in modern technology. Luxurycars, for example, can contain up to 300 magnet-relatedcomponents. Audio and video storage media were foundedon magnetic tape technology. One of the most fascinatingdiscoveries of the 20th century was the phenomenon of superconductivity in metals and alloys at liquid heliumtemperatures. Advanced applications of superconductingmagnets have already been realised in NMR spectroscopy,magnetic resonance imaging and particle accelerators. Sincetheir discovery in 1986, intense research effort has beendevoted to `high' temperature superconducting ceramicmaterials such as YBaCuO which are superconducting attemperatures above the boiling point of liquid nitrogen. Aconsiderable technological problem with these ceramics ishow to prepare wires for superconducting magnets out of anessentially brittle material. Recently, YBaCuO super-conductors have been deposited as thin layers on a exiblemetal tape, 3 that remains superconducting while carryinghigh current densities in the very high magnetic elds thatare generated.

Meanwhile, new magnetic phenomena such as giantmagnetoresistance, 4 have surprised physicists. The effectis most usually seen in magnetic multilayered structures,where two magnetic layers are closely separated by a thin

spacer layer a few nanometres thick. It is analogous to apolarisation experiment, where aligned polarisers allow

light to pass through, but crossed polarisers do not. Therst magnetic layer allows electrons in only one spin stateto pass through easilyif the second magnetic layer isaligned then that spin channel can easily pass through thestructure, and the resistance is low. If the second magneticlayer is misaligned then neither spin channel can get throughthe structure easily and the electrical resistance is high. Arelated phenomenon is colossal magnetoresistance (CMR) 5

observed when La 1 2 xCa xMnO 31 d is used as a ferroelectriclayer. Commercial applications of giant magnetoresistance(GMR) are already being realised in the data storageindustry and IBM were the rst to the market with harddisks based on this technology, rather than on the ferro-electric effect used in conventional RAM. 6 Other applica-

tions of GMR are as diverse as on-chip sensors, solid-statecompasses, automotive sensors, non-volatile magneticmemory and the detection of landmines. New technologicalapplications of magnetic materials are being envisaged forimaging the new nanoscopic world: namely the magneticforce microscope and the magnetic resonance microscope. 7

But today one of the greatest challenges in the eld of magnetic materials research remains the design and prepara-tion of organic ferromagnets. 8 The present article provides areview of the theoretical models that have been proposed inthe development of organic ferromagnetic materials and thesubsequent experimental and analytical work involved inevaluating them. It will, however, be necessary in the nextsection to briey touch upon physical aspects of magnetism,and on transition metal ferromagnets, in order to place incontext the challenge involved in designing organic ferro-magnetic materials.

1.1. Types of bulk magnetism

The magnetic behaviour of solids is complex and manydifferent types of magnetism have been distinguished:diamagnetism, paramagnetism, antiferromagnetism, ferri-magnetism, ferromagnetism, metamagnetism, etc. Themany different forms of magnetic phenomena arise as aresult of the diverse number of ways in which the moments

of electrons in molecular and supramolecular arrays can becoupled together. In the presence of a magnetic eld,

Figure 1. Schematic diagram showing the alignment of spins in paramagnetic, ferromagnetic, ferrimagnetic and antiferromagnetic materials.

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magnetic dipoles within a material become partially orien-tated. The magnetic dipole moment per unit volume isreferred to as the magnetisation M . For isotropic substancesthe magnetic susceptibility x is dened by x M / H where M and H are the magnitudes of the magnetisation and magneticeld vectors, respectively. Materials are classied asdiamagnets, paramagnets or ferromagnets according totheir bulk susceptibility. If a substance is diamagnetic,containing only spin-paired electrons, the magnetic

response opposes the applied eld and the magneticsusceptibility is small and negative. In a paramagneticsubstance, namely one that contains unpaired electrons,the normally-randomised spin moments align with theexternal magnetic eld, and the density of magnetic linesof force within the sample is intensied. The susceptibility,x , is small, positive, independent of the magnetic eldintensity, and decreases with increasing temperature. In aferromagnetic substance, the spins are spontaneouslyparallel to one another in microscopic domains leading toa permanent magnetisation. In Fig. 1, the domains areshown when there is no applied magnetic eld and thesample is not magnetised. The application of a magneticeld causes the domains to point along the eld lines andthis will lead to a magnetisation when the eld is removed.The susceptibility, x , is large and positive, dependent on themagnetic eld, temperature, and the history of the sample.The related phenomenon, anti-ferromagnetism, occurswhen neighbouring, equal spin moments couple in ananti-parallel fashion, leading to a lowering of magnetisation,while ferrimagnetism occurs when unequal spin momentscouple in this way to leave a net magnetisation. Ferro-magnetic, anti-ferromagnetic and ferrimagnetic materialsoften show hysteresis, that is irreversibility of magneticbehaviour as the applied magnetic eld is changed.

Due to the thermal randomisation of the magnetic moments,

the susceptibility of a paramagnetic material decreasessmoothly with increasing absolute temperature ( T ) either

according to the Curie law: x C / T , or the CurieWeisslaw: x C /(T 2 u ), in which C and u are the Curie andWeiss constants, respectively. For a pure paramagnet(non-interacting spins) u is zero and the Curie law appliesbut if local ferromagnetic coupling exists, u is positive,whereas local anti-ferromagnetic coupling (anti-parallelspins) gives a negative value for u . For substances thatshow bulk ferromagnetism a transition occurs at a tempera-ture known as the Curie temperature, T C , leading to a phase

in which there is long-range parallel ordering of spins.Below this temperature the susceptibility rises abruptly toa very high value (Fig. 2). 9 Although ferromagnets belowthe Curie temperature exhibit long-range ordering of spins,a sample may still not behave like a magnet unless thisordering occurs within `domains'. The domains themselvesare randomly orientated and cancel each other out, howeverapplication of a magnetic eld (i.e. the coercive eld) willmagnetise the sample. When the eld is turned off themagnetisation curve shows hysteresis and the sample retainssome magnetisation.

Detailed analysis and tting of the temperature dependenceof the magnetic susceptibility is possible using modelsbased on the spin concentration and molecular parameterssuch as the exchange coupling constants and the averagespin quantum number. The interested reader is referred toreviews covering these physical details. 8

By far the most important class of materials from both thepractical and theoretical viewpoints is the ferro-magnets.The applications, which these materials nd, are verydiverse. In engineering, for example, ferro-magnets areused because of their high permeabilities, which enablehigh magnetic inductions to be obtained with only modestmagnetic elds. They also have the ability to retain magne-tisation and thereby act as sources of elds. The few ferro-

magnetic elements in the periodic table are technologicallyvital.

Figure 2. Temperature dependence of magnetic susceptibility in magnetic materials.

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1.2. Anticipated properties of organic magneticmaterials 8

The rst obvious application would be the replacement of existing bulk magnets or magnetic recording devices.Values of the saturation magnetisation, M s, for molecular/ organic based magnets are comparable to metallic

magnets on a mole basis. The inherently large molecularweight (per magnetic moment) and low densities,however, result in smaller saturation magnetisation oneither a volume or mass basis, i.e. the spin concentrationis low. In 1 cm 3 of metallic iron there thus are about 4 1023

spins, while in an organic ferromagnet it is less by 12orders of magnitude. This results in lower magnetisationsand small exchange energies (because of large interspindistances), and as a result in low temperatures of transition(T C) to the ferromagnetic state. This means that molecularmagnets are unlikely to compare well with existingmagnets. A further disadvantage is the inherent chemicalinstability of organic materials and their `ageing' withtime.

Materials with magnetic moments parallel to the plane of alm or disk form the basis for magnetic tapes and discs.Increased data density requires materials with largerdemagnetisation elds or larger coercive elds. Molecularbased magnets, e.g. [Fe III (C5Me5)2]

z 1 [TCNE]z 21 , have such

large coercive elds. Magneto-optical discs, which have ahigher data density than conventional disks, could be anincreasingly important data storage system. Such tech-nology relies upon the magneto-optical effect, which causesthe magnetism of the medium to be changed when a high-power laser causes heating above the Curie temperature.The polarisation of the region of the disc can be read back

by measuring the polarisation of the reected light from alower-powered laser. The degree of polarisation is depen-dent on the magnetisation of the target area (the Kerr effect)and is greatest when the magnetic moments are perpen-dicular to the plane of the substrate. Magneto-optic effectshave yet to be studied for an organic/molecular magnet, andthere are high expectations that molecular magnets will beuseful. As a consequence of the insulating, and thereforetransparent nature of organic magnetic materials a varietyof optical properties may be expected, i.e. photomagneticswitches and polarised light manipulation in integratedoptical devices. Finally, biocompatibility of organicmagnetic materials may lead to several potential applica-tions that include magnetic imaging and transducers formedical implants. In summary, the range of possibleproperties covers the following: (1) Insulating, (2) Lowdensity, (3) Low magnetic anisotropy, (4) Optical changes,(5) Low elastic modulus, (6) Tuning of properties viaorganic chemistry, (7) Processability, (8) Low environ-mental contamination, (9) Solubility, and (10) Photo-magnetic effect.

Other than commercial opportunities, the future realisationof the potential of organic ferromagnetism would be sig-nicant from a theoretical point of view, in which extendedferromagnetic exchange interactions through s and porbitals may provide valuable insights into the very

phenomenon of magnetism. From the synthetic chemist'sperspective the extensive synthetic exibilities offered by

organic systems would enable the ne tuning of solid statemagnetic characteristics. Further scientic and technologi-cal goals such as the fabrication of electronic devicescontaining such magnetic materials or molecules at amolecular level is an area of intense research activity withtempting possibilities for application in modern computertechnology. 9

1.3. Magnets based on organo-transition metalcomplexes

The mechanism of spin-coupling in molecular magnetsdiffers from that in metals like iron. There are two distinctmechanisms: direct exchange and indirect exchange, orsuperexchange. A relevant example of direct exchange isHund's rule of maximum multiplicity. 10 This predicts thatatomic carbon will have a triplet ground state. Four of its sixelectrons are spin paired (in 1s and 2s orbitals) but the tworemaining electrons, which occupy degenerate 2p orbitals,are predicted to have parallel spins. Hund's rule is alsoapplicable to molecular and intermolecular situations;however, it must be applied with caution. In atomic,molecular or intermolecular situations, the interactionwhich lies behind the rule is only signicant if the half-lledorbitals are orthogonal but represent electron distributionswhich overlap signicantly in space. This `orthogonal butcoextensive' requirement is always met for co-centredatomic orbitals, but is not always fullled betweenmolecular orbitals, and so the rule can break down. Oneof the main themes of research into high spin organiccompounds has been the limitation of Hund's rule insituations where direct exchange dominates. The study of polynuclear transition metal complexes has shown a very

different problem, namely that a strong coupling of spinsmay be observed over distances which are too great to beattributed to direct interactions between half lled metal iond-orbitals. In cases where this exists the interaction ismediated through the ligands and the mechanism of spin-interaction is known as superexchange. An example of thistype of interaction can be observed in the copper acetatecomplex Cu 2(OAc) 4(H2O)2. The crystal structure of thiscompound shows that it contains isolated dicoppermolecules in which the copper atoms interact stronglythrough exchange forces, each pair forming a low energysinglet and a high energy triplet state. 11 This interactionoccurs through the acetate ligands, not through directbonding as the CuCu distance is too great.

These ligand-mediated `superexchange' interactions inbimetallic complexes have been the subject of a detailedsystematic study by Kahn. 11 The sign and strength of theinteraction varies widely from system to system but, as indirect exchange, it depends ultimately on the symmetry of the `magnetic' orbitals and their overlap density, accordingto the GoodenoughKanamori rules. 12 For example,comparison of the two complexes CuVO(fsa) 2enCH3OH(1) and CuCu(fsa) 2enCH3OH (2), where (fsa) 2en

42 isthe bichelating ligand derived from the Schiff base N , N -(2-hydroxy)-3-carboxybenzylidene-1,2-diamine, showsthat the magnetic sites interact (Cu(II), s 1/2 and

V(IV)O, s 1/2) to give rise to a singlet and a tripletground state.

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left is orthogonal and co-extensive. Simple nitroxides caninteract with only one metal ion at a time and thereforecannot readily form extended magnetic structures. Gatteschisynthesised a series of compounds using bridging bidentatenitronyl nitroxide ligands, i.e. 2-alkyl-4,4 H,5,5 H-tetramethyl-imidazoline-1-oxyl-3-oxide (3). 16 If the two oxygenspresent in the NO groups bind to two different metal ions,a polymeric structure could be formed with effective path-ways to transmit the magnetic interactions between themetal ions, leading to polymeric chains containing alter-nating spins on the metal and on the ligand. Gatteschidiscovered that above 20 K manganese centres complexedto hexauoroacetylacetonate (hfac) ligands behaved astypical ferromagnetic chains, but at low temperatures[T C 7.6, 8.1, 8.6 K for R i-Pr, Et, n-Pr] a three dimen-sional ferromagnetic ordering occurred and the magneticmoment rapidly increased. The ferromagnetic phase transi-

tion observed has been attributed to dipolar interactionsbetween chains. A lower transition temperature wasobserved for the analogous Ni complexes and this wasexplained by the smaller spin of nickel, which makes thedipolar interaction less effective. The interaction betweenmetal ions and radicals was very strong within the chainsbut, unfortunately, the chains were very well shielded fromone another.

Since classical ferro- or ferri-magnetic order can beachieved only in three dimensions, because one dimensionalmaterials order only at 0 K, the critical temperatures could

only be increased if the interactions between the chains wereenhanced. In order to achieve this Gatteschi chose to eitherintroduce additional donor atoms on the NIT(R) radicals andhence to increase the number of metal ions to which anindividual radical can be bridged, or to use co-ligandsother than hfac, which favour the formation of adductsbetween the metal ions and the weak radical ligands, but

very efciently shield the chains from one another. The mostencouraging results were found using Mn[(pfbz) 2]23(pfbz pentauorobenzoate, and R Me or Et) whichordered magnetically at temperatures as high as 20 K.Although the nature of the phase transition was not clear,there was evidence that these complexes behaved as weak ferromagnets. 15 The interested reader is directed to recentreviews in this area. 17,18

2. Conceptual Models for Organic Molecular Magnets

Key models proposed for the development of organic ferro-magnets, and the experimental work involved in evaluatingthese models will be described in this section. It is generallyacknowledged that three models proposed in the 1960s formthe basis for most of the discussion about how to design anorganic magnet.

2.1. Negative spin density product model

The earliest proposal for achieving ferromagnetic inter-actions between organic free radicals was put forward byMcConnell in 1963. 14 This model has only in the last fewyears received widespread interest in explaining through-space ferromagnetic interactions. McConnell stated that:radicals with large positive and negative atomic p -spin

densities which pancake such that atoms with positivespin density are exchange coupled to atoms with negativespin density should result in a ferromagnetic exchange inter-action. McConnell's model therefore relies upon theconstruction of a crystal lattice of radicals in such a waythat atoms with opposite spin densities are aligned side byside for each pair of radicals, as in the allyl radicals in Fig. 5.While anti-ferromagnetic interactions completely cancel thespins in (I), ferromagnetic interaction is possible if someatoms on the radicals have negative spin densities and theorientations of the radicals are such that spin densities of opposite sign couple most efciently as in (II). It is thuspossible to exploit an inherently anti-ferromagneticcoupling to produce bulk ferrimagnetism. In this way theMcConnell model for organic p radicals is seen to be analo-gous to the Kahn model mentioned in the previous section.

The mechanism described above relates to pairwise ferro-magnetic exchange, not to bulk ferromagnetism thatrequires ferromagnetic exchange in three dimensions. Toachieve ferromagnetic exchange, routes to spin pairingmust be avoided. This requires the use of stable speciessuch as the radical anions from tetracyanoethylene[TCNE]

z 2 or tetracyano- p-quinodimethane [TCNQ]z 2 ,

which do not easily dimerise and contain atoms withunequal positive and negative spin densities such that it isconceivable that they might form a structure that complies

with McConnell's requirement.20

Of the numerous struc-tures reported, however, the overlap required to satisfy the

Figure 5. Schematic representation of the ideal alignment of spin densitiesin the McConnell model. Symmetric (I) and asymmetric (II) structures of the pair of allyl radicals. McConnell islets are present in structure II andabsent in structure I. 19

Figure 6. (a) Pseudo- ortho -, (b) pseudo- meta - and (c) pseudo- para -bis(phenylmethylenyl)-[2,2- para -cyclophanes].

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McConnell model has never been achieved. Even stableradicals undergo dimerisation to form diamagneticspecies. 21 A number of competing reactions such ascarboncarbon bond formation between C(CN) 2 moietieson adjacent TCNQ molecules have also been reported. 22

Expertly designed model cyclophane species with therequired solid state packing were synthesised by Izuoka etal. in order to test McConnell's model (Fig. 6). 23

Among the three isomers of bis(phenylmethylenyl) [2,2- para -cyclophanes] only the pseudo- ortho and pseudo- paraisomers satisfy McConnell's condition in that the sign of thespin density product at each interacting site is negative,leading to ferromagnetic coupling of the two tripletcarbenes and a quintet ground state, as observed by ESR.The overlapping mode in the pseudo- meta isomer leads toanti-ferromagnetic coupling and hence a singlet groundstate. This provided the rst experimental demonstrationthat the spin distribution of the p electrons in layered benze-noids can determine the ferro- or anti-ferromagnetic inter-action between carbene units. The application of theMcConnell-I model to this particular system has been inves-tigated theoretically. 24

2.2. Charge-transfer complex models

2.2.1. McConnell's model (II) and [Fe III (C 5 Me 5 )2 ]1z

[TC-NE]

2 z

. In 1967, at a Robert Welch Foundation Conference,Mulliken delivered a lecture on charge transfer complexesduring which he considered spin interaction. 25 In normal ionradical charge-transfer solids, intermolecular spin pairing(antiferromagnetism) is a result of the mixing of a groundstate and a singlet back charge-transfer state of a chargetransfer pair (Fig. 7).

Following Mulliken's lecture McConnell proposed adetailed model utilising specic charge transfer complexesto produce molecular ferromagnets. 26 McConnell predictedthat if an ionic charge transfer pair (D 1 A2 ) could be built

that had a back charge-transfer excitation to a neutral tripletstate, rather than to a singlet state, then the D 1 A2 pair couldalso be a triplet due to mixing of the charge transfer statewith the ground state (Fig. 8). If this mixing of a high spinarrangement could be organised between adjacent donorsand acceptors in a multi-dimensional array within a solid,then macroscopic parallel alignment and thus ferromagneticbehaviour of solids should be possible.

Although Mulliken's lecture and McConnell's theories werepublished, they were not widely available, and as a resultremained largely unknown until the work of Miller andEpstein 27 and of Breslow and co-workers. 28 McConnell'smodel relies on the following four basic criteria beingfullled:

1. formation of a complex of the type Az 2

, Dz 1

where Az 2

isthe radical anion of the acceptor and Dz 1 is the radical

cation of the donor;2. crystallisation as mixed (A

z 2 , Dz 1 )n not discrete (A

z 2 )nand (D

z 1 )n stacks;3. signicant admixture of the excited state A 2 2 , D2 1 ; and4. either A 22 or D21 to be a ground state triplet species.

By using Hund's rule as applied to molecules and bi-radicals, 29 the lowest energy excited state that can virtuallyadmix with the ground state can be identied, thus enablingprediction of the magnetic coupling. 30

For virtual forward charge transfer, excited states EsFC D3 A ,

EsAF D3 A , and EsAC D

3 A are possible (Fig. 9). From Hund'srule the most easily lost D

z 1 electron has ms 2 1/2 and canonly be transferred to the ferromagnetically coupled A

z 2

(Fig. 9(a)). Transfer of an ms 1/2 electron to an anti-ferro-magnetically coupled A

z 2 requires admixture of higherexcited states (Fig. 9(b) and (c)). Forward charge transferto the ferromagnetically coupled ground state may thus leadto ferromagnetic behaviour.

The seminal example of this was the complex [Fe III (C5-Me5)2]

z 1 [TCNE]z 2 reported by Miller, Epstein and co-

workers in which bulk ferrromagnetic behaviour wasobserved ( T C 4.8 K).31 This was the rst example of a

magnet with spins residing in p-orbitals. It exhibitedmagnetic hysteresis, was soluble in organic solvents anddid not require metallurgical preparative methods. 32

According to the McConnell mechanism the driving forcebehind its ferromagnetism is the triplet ground statecharacter of [Fe III (C5Me5)2]2

1 . This leads to ferromagneticcoupling between [Fe III (C5Me5)2]

z 1 and [TCNE]z 2 both

within the stack and between out of registry adjacent stacksgiving the full three dimensional coupling of spins requiredfor bulk ferromagnetism. Although the application of theMcConnell II mechanism to these systems has beenchallenged, 33 it provides a simple rationalisation, if thedirection of charge transfer is known, to the nature of themagnetic coupling. A variety of TCNE-based organicmagnets have been discovered, including the remarkableVII(TCNE) x y(CH2Cl2) which has a T C above room tempera-ture (ca. 400 K) 34 as well as M(TCNE) x y(CH2Cl2) typescontaining other transition metals and [Mn(porphyr-in)][TCNE]-based magnets. 35 There is also a class of ferromagnetic materials with Prussian Blue structures, e.g.V[Cr(CN) 6]0.86 2.8H2O (T C 315 K),

36 and the currentrecord-holder with a T C greater than 100 8C,KV[Cr(CN) 6]2H2O.37

2.2.2. Hexaminotriphenylene dication and hexaazaocta-decahydrocoronene complexes. Since the advent of McConnell's initial model, many adaptations have been

linked to attempts to design synthetically accessiblemolecules to prove or disprove the model. In 1982

Figure 7. Radical ion CT complex with a singlet excited state.

Figure 8. Back charge transfer to a triplet excited state.

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Breslow 28 suggested an improvement to the McConnellmodel that stemmed from his earlier work on cyclic conju-gated systems with 4n p electrons. This led him to proposethat 4n p electron cyclic conjugated systems had the poten-tial to behave as ground state triplets, provided thesesystems had C 3 or greater symmetry. Such symmetryleads to orbital degeneracy in some molecules, while thepresence of 4n p electrons causes half occupancy of adegenerate orbital pair with consequent spin unpairing andsingle occupancy of each orbital. There is no guarantee,however, that a symmetrical 4n p molecule will have atriplet ground state and therefore Curie Law studies (orsimilar) must be performed to ensure the ground state isnot a singlet. Species such as C 5H5

1 and C5Cl51 were

known to have triplet ground states, but these moleculesare chemically unstable. Following a report by Parker, inwhich it was claimed that the hexamethoxytriphenylenedication 4 had a triplet ground state, 38 Breslow synthesised 28

several triphenylene derivatives.

The dication 4 had no real chemical stability and to form itrequired high positive potentials, thereby limiting the searchfor suitable anion acceptors. Breslow modied thismolecule producing various hexaaminotriphenylene deriva-tives 5 which had greater stability and formed the relevant

dication at much lower potentials, i.e. ( 53

52 1

0.27 V)compared to ( 43 421 0.85 V) [5 2,3,6,7,10,11-tris( N , N H-

diethylenediamino)triphenylene]. Breslow claimed thesespecies were ground state triplets and the dications weresubsequently used in tests of Breslow's own model. Whenthese dications have triplet spin multiplicity a forwardcharge transfer from D 1 A2 to D2 1 A2 2 is required (Fig. 10).

This leads to greater charge on the molecules than envisagedby McConnell's original proposal and favours the alternat-ing donoracceptor stacking required for donoracceptormixing. The interactions of donor and acceptor in thesolid must induce substantial forward transfer and disfavourback charge transfer such that the neutral singlet chargetransfer conguration is high in energy. For forward chargetransfer to occur the donor and acceptor must be of similarpotentials. It is also crucial that the donor has a stable tripletas one of its redox forms. The preference is for as large atripletsinglet energy gap as possible in order to stabilisethe spin parallel arrangement. Most challenging is maintain-

ing the threefold symmetry of the donor, which effectivelyprovides the degeneracy.

To test this model Breslow used the hexacyano compound 6as an acceptor. This required alteration of the donor, withR Et being replaced by the more electron withdrawingR CHF 2CH2, in order to match the donor/acceptor poten-tials, thus providing the correct amount of charge transfer inthe solid. Despite satisfying the conditions of the model,namely alternating donoracceptor stacking, partial secondcharge transfer and a triplet donor cation, the material wasfound to exhibit anti-ferromagnetic coupling. This may be aresult of the instability of the triplet ground state of thedonor, a loss of degeneracy resulting from a loss of symme-try of the donor in the complex (i.e. JahnTeller distortion)favouring the singlet ground state, or the intermolecularinteractions may not be general or multidimensionalenough.

Figure 9. Donoracceptor complex excited states (Es, lower half of diagram) derived from the ground states (Gs, upper half of diagram) by further transfer of charge from D 1

z

to Az 2 .

Figure 10. Forward charge transfer from symmetrical donoracceptorcomplexes.

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Breslow 39 turned his attention to hexaazaoctadecahydro-coronene (HOC) 7 which has a larger gap between thesinglet and triplet energy states. He also examined variantsof the tris( N , N H-diethylenediamino)triphenylene ( 5), such as8, because sulphur atoms on the edge of electricallyconducting organic stacks can facilitate sideways inter-actions between the stacks. The non-planarity of thesesystems, however, made them much more difcult tooxidise. The dication of 7 was prepared and its role incharge transfer complexes was investigated by Miller andco-workers 40 who found that crystalline salts of [ 7]21 with

(C3[C(CN) 2]3)2 2

, and of 9 with (C 3[C(CN) 2]3)22

were in factground state singlets and that [ 7]z 1 [TCNE]

z 2 showed stronganti-ferromagnetic interactions.

2.2.3. Decacyclene cation salts and symmetrical poly-amines. Another modication of McConnell's model wasproposed by Torrance 41 who suggested that a stack of highlysymmetric radicals could have spins in degenerate orbitalswhich should have a triplet excited state as a consequence of

the radical cation/anion of the molecule having the appro-priate lling (Fig. 11).

Torrance made stable radical cation salts of decacyclene,10 , with the anions shown. These salts had the stoichiometry(decacyclene) 3X2 where X is the anion. Magnetic suscept-ibility measurements revealed, however, that these

materials were not ferromagnetic.

This may be due to unknown complications in the structuresof the complexes, distortions of the structures which destroythe symmetry, the widths of the bands formed by orbitaloverlap being too large compared to Hund's rule, intra-atomic exchange, or there may be complicating effectssuch as spin orbit interactions. Breslow's compounds wereideal candidates for Torrance's model but they were alsofound to be anti-ferromagnetic.

Torrance also suggested linking together symmetric poly-amine type molecules in a way that maintained symmetry(Fig. 12). Oxidation of these structures was then used toproduce spins. 42

Torrance suggested that such a structure may have beenproduced in the reaction of triaminobenzene with iodine.This yielded a black insoluble polymer, which on rareoccasions, showed the presence of small amounts of ferromagnetic material; however, these results provedirreproducible.

2.2.4. Complexes of TCNQ, PCCP, DDQ, etc. withvarious cations. Soos and co-workers tried to extend the

Figure 11. Torrance's model of orbital lling in cation/anion pairs.

Figure 12. 2-D symmetric polyarylamine structure.

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superexchange mechanism established for inorganic solidsto organic stacked complexes. 43 In a mixed R

z

SRz

S stack, anexcited triplet state may be obtained if the spacer molecule Shas degenerate orbitals into which the spins of radical R

z

aretransferred during virtual excitation (Fig. 13). This requiresthe orbital degeneracy of S to be maintained in the solidcomplex and this is only possible if the orientation of R

z

issuch that the perturbations on the degenerate MOs of S areequal.

A spacer with a degenerate HOMO or LUMO should allowferromagnetic coupling via charge transfer when radicalsare orthogonal to each other. Soos et al. used ion radicalsbased on strong organic donors such as N , N , N H, N H-tetra-methyl- p-phenylenediamine (TMPD) 11 and strong organicacceptors such as tetracyanoquinodimethane (TCNQ) 12 .

Appropriate spacers for the ion radicals are closed shellanions, X 2 , like pentacyanocyclo-pentadienide (PCCP) 13which has a degenerate HOMO, or cations, X 1 , such astrimethylcyclopropenium (TMC) 14 or tris(dimethyl-amino)cyclopropenium (TDAC) 15 which have degenerateLUMOs.

The coulombic interactions present should promote mixedstacking D 1 X2 D1 X2 or AX 1 AX 1 . The preparation of aTMPD/PCCP complex in which the dihedral angle was418 rather than the required 90 8 resulted in weakly anti-ferromagnetic interactions. 44

Other complexes produced by Soos included a 1:1 TDAC/ DDQ (2,3-dichlorodicyanobenzoquinone) complex. Thiswas, however, found to have a dimerised stacking pattern

of X1 A2 A2 X1 by X-ray crystallography, which ruled outthe possibility of superexchange. 44 It was proposed that thiswas the result of the strong tendency of DDQ 2 ions todimerise owing to strong dipolar attraction and exchangeinteractions. Replacing DDQ by TCNQ, which does nothave a permanent dipole, however, failed to improve thesituation. 45

2.2.5. CT complexes containing fullerenes. Wudl and co-workers proposed another variation of McConnell's modelbased on a ferromagnetic organic metal. 46 The designinvolved neutral diradicals, with the intent of generatingneutral organic metals. Wudl suggested that the neutraldonor could be a triplet and the acceptor could be the radicalion derived from the donor (Fig. 14). This would allow thegeneration of homomolecular stacks that could give rise toferromagnetic organic molecules.

This differed from McConnell's model in that the stoichio-metry was D 2A and species either side of the double arrowin Fig. 14 would be identical, thus satisfying the spin conser-vation rules. Wudl therefore required stable donor radicals,which had triplet ground states and would not dimerise orpolymerise. A study of benzobisdithiazole ( 16 ) was

prompted by the fact that cross-linking had never beenobserved in compounds containing [SNS]z

groups.Contrary to the favourable precedent, however, the biradical16 was found to exist as an oligomer both in solution and inthe solid state by ESR spectroscopy, which also indicatedthat if monomeric forms of 16 existed they would adopt asinglet rather than triplet ground state.

In 1991 it was discovered that buckminsterfullereneexhibited superconductivity upon doping with alkalimetals. 47 Structural studies had shown that interstitialholes in the fullerene's face centred cubic structure couldaccommodate metal ions 48 and cyclic voltammetry hadshown that C 60 and C70 could easily be reduced. 49

Theoretical calculations had also established that theLUMO and HOMO of C 60 were triply and ve-folddegenerate, respectively. 46 It was proposed that such a

Figure 13. Spin transfer into the degenerate orbitals of a spacer molecule.

Figure 14. Orbital lling in homomolecular stacks of neutral and cationic donors.

Figure 15. CT complex of TDAE and C 60 .

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large number of degenerate levels could lead to unusualelectronic and magnetic properties. Wudl utilised the strongacceptor properties of C 60 to form a 1:1 charge transfercomplex with TDAE (Fig. 15). 50

Magnetic susceptibility measurements established that thiscomplex underwent a transition to a state in which the spinsordered ferromagnetically and that at 16 K the materialshowed magnetisation without remanance, i.e. withoutretention of magnetism outside an applied eld. This wasthought to indicate a transition to a soft itinerant ferro-magnet.

2.3. Organomagnetic materials based on nitroxideradicals

2.3.1. Mono-nitroxides, poly-nitroxides and nitronyl

nitroxides. The discovery of p-nitrophenyl nitronylnitroxide ( 17 ), the rst ever bulk organic ferromagnet, wasmade by Kinoshita and co-workers in 1991. 51

Today many of the over 20 known organic ferromagnets arenitroxide-containing materials. Nitroxide radicals have aninherently stable electronic conguration and can be furtherstabilised by conjugation with the p electrons of aromaticsystems and/or by shielding with bulky substituents.Radicals such as TEMPO derivatives 18 and nitronylnitroxides 19 have been central to the development of organomagnetic materials. The SOMOs of most mono-nitroxides are localised mainly on the NO moiety and

the unpaired electron resides there. In nitronyl nitroxidesthe unpaired electron is delocalised between nitrogen and

oxygen and between both NO groups. The general strategyfor designing nitroxide-containing magnetic materials hasbeen to prepare molecules with large intramolecular spinpolarisations, and to minimise the intermolecular overlapintegrals between the SOMOs of adjacent radical centres,and the vacant or doubly occupied molecular orbitals of neighbouring molecules.

In 1993 Rassat and co-workers suggested that 1,3,5,7-tetra-methyl-2,6-diazaadamantane- N , N H-dioxyl (20 ) in additionto exhibiting intramolecular ferromagnetic interactions,should also take part in intermolecular ferromagnetic inter-actions due to the three dimensional network of NOchains. 52 Further work followed in an attempt to eitherarrange the spin sources through hydrogen bonding effectsin compounds such as 21 53,54 or through control of thecoulombic interactions, as in 22 , for which magnetic

measurements indicated ferromagnetic interactions presentin 22a and 22c .55 The various design considerations ina -nitronyl nitroxide research have been reviewed recently. 56

Multi-spin systems composed of nitroxide radicals havereceived signicant attention in the design of molecularmagnets. Iwamura and co-workers prepared the triradicals23 and 24 and found that the tris-nitroxide 24 had a strongerintermolecular spin interaction than 23 and that 24 had aquartet ground state. 57 Despite many other attempts to

produce crystalline radicals with high T C values, the highestobtained remains 1.48 K for Rassat's radical ( 20 ).

Figure 16. CT complexes containing nitroxide and nitronyl nitroxide donors.

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2.3.2. CT complexes containing nitroxide radicals.Recent attempts to produce charge transfer complexesemploying acceptors paired with donors carrying nitroxideradicals have focused on tetrathiafulvalene (TTF) oraromatic alkyl amine donors functionalised with nitroxideradicals, coupled with DDQ or TCNQF 4 acceptors. Thesehave mainly proved unsuccessful in producing ferro-magnetic charge transfer salts. Sugimoto et al. succeededin developing a ferromagnetic charge transfer salt based onpyridinium-substituted imadazolin-1-oxyl with TCNQF 4 orhexacyanobutadiene acceptors (Fig. 16). 58

Recently the Au(CN) 22 salt of the extended TTF structure 25

was prepared and its crystal structure determined. 59 Thecoupling in [ 25 ]2[Au(CN) 2]3 was, however, found to beanti-ferromagnetic due to strong spinspin interaction viashort OO distances between donor sheets.

Limited success has also been achieved using acceptormolecules, most commonly benzoquinone derivatives,to carry stable nitroxide radicals. Sugawara and co-workers observed ferromagnetic interactions in a radicalanion salt carrying a nitronyl nitroxide 26 , in the

absence of a donor, i.e. the nitroxide radical itself acted asa donor. 60

In their comprehensive review of this eld Nakatsuji andAwzai 61 report preliminary results on alkylamino-TEMPOderivatives 27a d prepared by the reductive amination of 4-oxo-TEMPO with alkyl amines using sodium cyano-borohydride. 62 Compound 27d exhibited ferromagneticinteractions with a Weiss constant of 0.5 K.

2.4. The topological model and applications to syntheticpolyaryl-carbenes and polyaryl-radicals

2.4.1. Molecular topology and spin alignment. The topo-logical model can be simply explained by considering thetrimethylenemethane biradical, rst reported by Dowd 63 as aground state triplet, as predicted famously by Longuet-Higgins. 64 This biradical can be viewed as two methylradicals connected 1,1 to ethylene. Alternatively, twomethyl radicals connected 1,2 to ethylene will electronicallyreorganise to butadiene, which has a ground state singlet(Fig. 17). It follows that connectivity in molecular structures(topology) determines the nature of spin-coupling. Similarconsiderations apply to benzene-containing biradicalswhere m-benzoquinodimethane has a triplet ground stateand acts as a strong ferromagnetic coupling unit (FCU)whereas its ortho - and para -isomers are ground state sing-

lets and act as anti-ferromagnetic coupling units (ACU) (seeFig. 17). 65 Notice that all of the FCU groups are non-Kekule

Figure 17. Molecular topology and spin coupling.

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structures, the so-called non-disjoint biradicals. Theimportant role of the FCU has been underlined by a recenttheoretical study. 66

The topological model originated in 1968 from a considera-tion of the electronic structures of the hydrocarbons 28 , 29and 30 67 which belong to the class known as `alternant'.

A conjugated p -system is dened as alternant if its atomscan be subdivided into two groups denoted by A and A

p

suchthat each A

p

atom is surrounded only by A atoms, and viceversa. In the case of the polycarbenes 28 and 29 it is

assumed that one of the atomic orbitals of the divalentcarbon participates in conjugation with the ring p system

while the other remains as a s -type orbital. If the assump-tion is made that these molecules are essentially planar then,according to Huckel MO theory, or PariserParrPople(PPP) theory, these meta -substituted alternant hydrocarbonshave multiple degenerate non-bonding p -molecularorbitals (NBMOs). 68 The number of p -NBMOs is equal tothe number of carbons with unpaired electrons. Thep -NBMOs and the s -type NBMOs are not equal but areprobably close in energy. The energy levels of these hydro-carbons will therefore be of the form shown in Fig. 18,where F indicates the p -MOs doubly occupied by electrons,V indicates the vacant p -MOs in the ground electronic state,

and N indicates both the p -NBMOs and the s -type NBMOs.If the orbital energy difference between the non-bondingelectrons is sufciently small then, according to Hund'srule, electrons entering these orbitals will do so singly andwith parallel spins as shown in Fig. 18(a). The spin pairingtherefore depends on the exchange integral between thep -NBMOs and the s -type NBMOs and the energy differ-ence between these orbitals. Alternant hydrocarbons areclearly better for high-spin molecules since parallel spinsare not on adjacent atoms. Mataga proposed that for suf-ciently large molecules the orbital energies will becomealmost continuous bands as indicated in Fig. 18(b). Becauseof its non-bonding nature, however, the N band will remainsufciently narrow to support degeneracy; hence parallelalignment of the spins of the electrons in this band can beexpected. Alternatively, the band gaps between F, N, and Vmay become comparable to kT, so that ferromagnetism mayonly be supported at cryogenic temperatures.

Ovchinnikov proposed an even simpler model to predictferromagnetic coupling in planar alternant hydrocarbons. 69

Based on the theorem of Lieb, 70 Ovchinnikov showed that if the number of starred (A

p

) and unstarred atoms (A) are notequal, the ground state spin, S , will be non-zero and equal tohalf the difference in their numbers, i.e. S u N p 2 N 0u /2. Onthe basis of this result Ovchinnikov suggested severalhydrocarbons, such as 31 and the heterocycles 32 , 33 ,

which at innite size could be expected to show ferro-magnetic behaviour. The spin in polymers such as 32

Figure 18. MO and band energy diagram for poly- m-carbenes.

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would be proportional to the number of chain links.

Klein et al. 71 supported the possibility of obtaining high spinhydrocarbons on the basis of valence bond models, as well

as calculations using classical structures, MO theory, clusterexpansions and PPP Hamiltonians. The simplest test for theodd alternant hydrocarbon model is again the trimethylene-methane biradical 34 for which the theory predictsS uNp 2 N 0u /23 (32 1)/2 1, i.e. ground state triplet, whichwas conrmed by experiment. 63

2.4.2. Poly( m -phenylene carbenes and nitrenes). Iwamuraand Murata studied systems containing two phenyl nitreneunits connected through acetylenic and diacetylenic bridges.It was observed that two triplet nitrenes at meta , para H andmeta , meta H positions showed ferro- and anti-ferromagneticinteractions, respectively. 72 This validated the theory of Ovchinnikov 69 and Klein71 and agreed with the ruleproposed by Radhakrishnan, 73 which can be applied to bi-radicals, and states that if the number of atoms betweenradical sites is odd then ferromagnetic coupling occurswhereas if the number of atoms between sites is even,anti-ferromagnetic coupling occurs. This simple ruleprovides a useful guideline for the design of potential ferro-magnetic coupling units.

Early experimental work had already proved that oligomericderivatives of the model compounds proposed by Mataga,such as diphenylmethylene 74 35 and m-phenylene-bis-phenylmethylene 75,76 36 , have triplet and quintet groundstates, respectively. Investigations of phenylene carbenes

were initiated by Itoh and co-workers76

who studiedm-phenylene-bis-phenylmethylene 36 , formed by the

photolysis of 1,3-bis-(alpha;-diazobenzyl)-benzene, andobserved a quintet by EPR spectroscopy at 77 K.

Iwamura continued to study polycarbene compounds and in1985 reported the nonet spin multiplicity of the ground stateof the tetracarbene 37 . It was found that 37 displayed para-magnetic susceptibility with varying temperatures. Thiscompound was termed a `superparamagnet' because theelectrons within the molecule were aligned in parallel butindividual molecules behaved independently of one another,similar to a paramagnet. Iwamura claimed that partial inter-molecular ferromagnetic interactions were observed in 39because the dispersion forces due to the alkyl chainsinsulated the magnetic interactions between layers. 77

Theoretical work on Mataga's model continued using moreaccurate modelling packages which conrmed the expectedferromagnetic ground state in phenylene carbene poly-mers. 78 The synthesis of an S 5 carbene, 38 , was achievedby Iwamura 79 and its ground state was shown to be theexpected undecet by EPR spectroscopy. In practical terms,

however, the usefulness of polycarbene and nitrenecompounds was restricted by their instability. Attentionthus turned to analogous radical species capable of greaterchemical stability.

2.4.3. m -Linked poly(aryl nitroxides). Calder et al. 80

utilised the stability of nitroxide radicals and linked thesethrough a 1,3-substituted benzene ring to yield the biradical40 , which was shown to be a ground state triplet bymagnetic studies. The bis-nitroxide 40 was, however, notfully persistent and isomerised to an aminoquinone imine- N -oxide. Research was therefore diverted to more stablediaryl nitroxide polymers such as 41 . The poly-nitroxide42 was synthesised and found to be stable at ambient

temperature for several weeks.

Measurements of magnetic susceptibility against

temperature conrmed a quartet ground state indicatingthat the intramolecular exchange coupling was strongly

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ferromagnetic and that perturbation by heteroatoms did notaffect the topological model. 81 Evidence of a band of degen-erate NBMOs between bonding and anti-bonding MOs inpolyphenylene nitroxides was provided by Tyutyulkov whoalso showed that these polymers suffer from a smallerdegree of spin polarisation in the p -network compared tothe analogous carbenes. 82 Smaller exchange interactions

may indicate that high spin polynitroxides do not exist atambient temperatures. An analogue of 40 with CF 3 groupsin the 4- and 6-positions was quenched to the singlet state at5 K.83

2.4.4. High spin hydrocarbons, chlorocarbons andheterocycles. Mataga also proposed that the triphenyl-methyl type radicals could exhibit ferromagnetic coupling.Compound 43 was found to have a quartet ground state butwas prone to rapid dimerisation. 84

Since this observation, a series of related high spin hydro-carbons have been detected including 1,3,5-tris(phenyl-methylene), S 3,85 biphenyl-3,3 H-bis(phenylmethylene),S 0, 1, 2,86 1,3,5-benzenetrityl[bis(biphenyl-4-yl)methyl],S 3/2,87 and 3,3 H-diphenylmethylene-bis-(phenylmethy-lene), S 3.88

In an endeavour to improve the stability of this type of compound, Palacio and co-workers produced a stablebiradical ( 44) which had a triplet ground state and had alarge separation between the triplet and singlet groundstates. 89 The better stability was the result of the restrictedrotation of the aryl groups, and also of the steric shieldingprovided by the Cl atoms.

Extension to the triradical 45 produced a compound with aquartet ground state which was stable at 250 8C in air. 90 Theintramolecular exchange interactions were ferromagneticand perturbation by electronegative heteroatoms, lack of planarity, and changes in symmetry had little effect on thecoupling. It appears, therefore, that the topological modelcan be applied to highly distorted systems. Further evidence

of this was provided by Tanaka et al. who observed a quartetground state in the trication of 1,3,5-tris(diphenylamino)-benzene 46 .91

2.4.5. Star branched and dendritic aromatic polyradi-cals. Rajca's group has made signicant progress in thedesign and synthesis of potential organic ferromagnetsbased on systems which are homologous to m-benzoquino-dimethane. 92 Rajca designed and synthesised the hepta-radical 47 and the decaradical 48 based on a `star-branched' topology. These polyradicals showed high spinground states with strong ferromagnetic coupling ( S 7/2and S 5). Magnetisation data suggested that the inter-molecular interactions were ferromagnetic. 93

Rajca also designed dendritic polyradicals including theheptaradical 49 as well as a pentadecaradical and a 31-radical

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dendrimer, 94 which were found to have average ground statespins of S 3, 7/2 and 5/2, respectively.

The observed spin deciencies, known as spin defects, weredue to the failure to generate spins at every potential site.Spin coupling in multi-site radicals such as these isextremely sensitive to these defects because they interruptspin coupling paths.

2.4.6. Avoidance of spin defects via multiple couplingpaths. It is difcult to overcome this problem unlessimproved methods of radical generation are developed.The site of the defect is also important, with defects at theinner sites being particularly detrimental. An inner sitedefect divides the polyradical into uncoupled parts withlower spin (Fig. 19), whereas defects at the peripheralsites are relatively innocuous (Fig. 20) (the circles representspin sites and the lines are spin coupling paths).

Rajca attempted to overcome the problem of spin defects bydesigning molecules with multiple coupling paths, such as

macrocyclic calixarenes, which are oblivious to one spindefect. 95 The triradical calix[3]arene 50 and the tetraradicalcalix[4]arene 51 were synthesised and, although 50 wasfound to dimerise readily, suggesting a lack of steric shield-ing of the radicals, the steric shielding in the calix[4]arene

was sufcient to stabilise the tetraradical 51 . The out-of-plane twisting was moderate enough to preserve ferro-magnetic coupling and therefore these macrocycles maybe viable building blocks for defect resilient high spinpolyradicals.

Highly annelated polymacrocyclic systems are difcult tosynthesise, however, although oligomeric fragments which

have two strands (see Fig. 21) are accessible by convergentsynthetic routes and maintain p conjugation and ferro-magnetic coupling in the presence of defects. 96

Characterisation of 52 in THF-d 8 /2-MeTHF by SQUIDmagnetometry indicated S 6.2, which is below the theoreti-cal value of 7 expected for strong ferromagnetic coupling of 14 unpaired electrons. Quenching studies showed thatapproximately one deuterium atom from the solvent wasincorporated in 52 , accounting in part for S , 7. A study of the temperature dependence of the magnetisation disclosedthat the ferromagnetic coupling between the spins was notvery strong, probably because of an out-of-plane distortionof the p system. Rajca concluded that the development of 2-D and 3-D ferromagnets would require careful design toaccommodate three considerations namely p conjugation, aconnectivity compatible with ferromagnetic coupling andsteric hindrance to stabilise radicals. These factors had tobe compatible, of course, with a feasible synthetic strategy.

An alternative to the m-phenylene unit was examined byRajca, who proposed that sterically unencumbered 3,4 H-biphenyl units might possess a suitable size of spin couplingand might also stabilise radicals (Fig. 22). 97

The biradical 53 was found to be thermally stable and tohave a triplet ground state which showed that 3,4 H-biphenyl

could be used as a weak ferromagnetic coupling unit. Rajcathen proposed that high spin modules should be linked to

Figure 19. Inner-site defect in a polyradical.

Figure 20. Peripheral site defect in a polyradical.

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these weak FCUs to form the spin clusters 54 57 . In thesespin clusters intramodular ferromagnetic coupling was verystrong compared to the weak intermodular coupling through3,4 H-biphenyl units. 98 The polyradicals 54 56 possess onlyone site where a single defect may interrupt spin couplingwhilst 57 has two such sites. The hydrocarbon 57 waspotentially a hexadeca-radical and was found to have animpressive S 7.2. The estimated yield per site for thegeneration of unpaired electrons was as high as 98% andthe development of spin clusters of this nature led to thesynthesis of a polyradical with S 10.99

The four dendritic branches of the macropolyradical 58 , andthe macrocyclic core, can be divided into component spins.The ferromagnetically coupled pentamer with S H 5/2, 5/2,5/2, 5/2 and 2 should have an overall ground state withS 12 (Fig. 23).

A graph of M vs. H / T for 58 showed S , 10 at 5 K. This is thehighest spin recorded for an organic ferromagnet. The poly-radical 58 benets from the fact that only four of the 24radical sites are defect sensitive. Rajca is currently workingon analogues of 58 in which dendritic branches are replacedby macrocycles.

2.4.7. Polymers, oligomers or dendrimers with radicalside-chain or pendant groups. A variation of the topo-

logical model for preparing ferromagnetically coupledradical groups should not be overlooked. This approach,

Figure 21. Two-strand polymacrocyclic systems.

Figure 22. 3,4 H-Biphenyl as a ferromagnetic coupling unit.

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proposed and rst acted upon by Ovchinnikov, and laterdeveloped by Nishide and co-workers, introduces theradicals as pendant groups along a conducting polymerbackbone, rather than embedding them in the chain ornetwork. Unlike the systems mentioned in the previoussection it is believed that pendant group polymers such asthese can couple spins over the conjugated backbone evenwhen there are defects (incomplete spin generation).Particular effort has been made to employ radical groupssuch as nitroxides and phenoxyls with greater stabilitythan the carbon-based radical units described above. Earlierwork, including systems with pendant radical groupsattached to non-conjugated backbones, was described in a

previous review by the authors.100

Ovchinnikov prepared the polymer from the diacetylenemonomer 59 .101 It was reported to display ferromagneticinteractions even at room temperature, but this claim waslater challenged by Miller. 102 Other examples using thisstrategy have nevertheless been prepared since the Ovchin-nikov paper appeared. For example, polyphenylacetyleneswere prepared with various radical groups, includingnitroxides, such as 60 . None of these polymers showed,however, any ferromagnetic interactions between thegroups. There was only evidence of weak anti-ferromagneticthough-space interactions. The reasons for this were thoughtto be the non-planar nature of the polyacetylene backboneand the twisting of the pendant phenyl rings. Polyphenylenebackbones would also suffer from twisting; however, poly-phenylenevinylenes (PPVs) show evidence of considerableconjugation over several rings. PPV-backboned polymerswith pendant nitroxide groups (e.g. 61 ) were prepared byHeck head-to-tail coupling of the protected bromostyrenemonomers, followed by removal of the protecting group andoxidation with PbO 2.103 Magnetic measurements showedthat ortho -linked 61 reached a spin concentration of 0.54spins per monomer unit and had S 1. The related para -linked material was much less effective, despite theexpected greater conjugation in the backbone. This wasshown to be due to steric interactions between the pendant

nitroxide groups and the chain, causing twisting out of planarity. 103

Phenoxide radicals were introduced onto the PPV backboneto give a protected polymer precursor which was subse-quently deprotected and heterogeneously oxidised(K 3Fe(CN) 6) to 62 104 with a degree of polymerisationn 17. SQUID magnetic measurements suggested that anaverage spin per monomer unit of 0.7 was achieved andthat these coupled to give an effective spin quantum numberof 4/2 to 5/2 at 215 K, indicating a high-spin ground state.Using electron-spin transient nutation spectroscopy basedon pulsed ESR spectroscopy it is possible to resolve thisbulk S -value into a mixture of spin components whichinclude segments with S as high as 8/2 and 6/2. 105 Clearly,the move to the PPV backbone has thus proved to be asuccessful strategy in this area, provided that spin concen-tration levels can be optimised. Recently, similar star-shaped dendrimeric polymers with pendant polyphenoxylradical units were derived from a 1,3,5-benzene core and

magnetic susceptibility measurements gave an average S of . 7/2.106

Figure 23. A macropolyradical with four dendritic branches.

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The most recent improvement of this approach has comefrom Dougherty who suggested that electrochemicallydoped poly( m-phenylenefuchsone), 63 , was an advance onother proposed systems in that it provided the rst exampleof electrochemical doping in a polaronic ferromagnet, therst use of a radical anion in a polaronic ferromagnet, therst positive correlation between spin concentration andspin state, the rst positive correlation between dopinglevel and spin concentration and the rst high spin polaronicsystem with . 50% spin concentration. 107 Electrochemicaldoping (oxidation) of 63 in THF with t -butylammoniumperchlorate as the supporting electrolyte resulted in S < 2behaviour at a doping level of 61%. This is a signicantresult considering that each oligomer was found to beseven units long at most and therefore the maximum spinexpected was 7/2.

2.5. The polaronic model: preparation of ferro-magnetically coupled conjugated spin-containingsegments

2.5.1. Fukotome's proposal of ferromagnetically coupledpolarons. Fukutome proposed a new class of ferromagneticconjugated polymers in which delocalised spins were to begenerated by doping of conjugated segments. Thesepolymers have an alternating ABAB structure, wherethe A units are organic blocks which furnish polarons ondoping (spin containing units, SCU) and the B units areblocks which couple the A block polaron spins in a ferro-magnetic fashion (ferromagnetic coupling units, FCU)(Fig. 24). 108

Fukutome suggested the use of oligomeric fragments of

conducting conjugated polymers as SCUs, for example,trans -polyacetylene, poly- p-phenylene, polypyrrole orpolythiophene, which all have degenerate or nearlydegenerate band structures. On doping each conjugatedsegment with an electron or a hole, a series of polarons,which are ion-radicals trapped in local lattice distortions,

can be produced. Examples of non-degenerate segmentsas suitable FCUs were suggested and these include poly-m-phenylene 64 and the polyketone 65 .

This polaronic model was supported by Tyutyulkov whomaintained that the band structures of these polymerswere analogous to those of odd alternant non-classical poly-mers. 109 The polaronic design had the potential to overcomethe problems of instability associated with `built in'radicals. The polaronic approach could separate the polymersynthesis stage from the introduction of spins, thus allowingfull characterisation of the polymers, and hence ensuringthat the desired topology for ferromagnetic coupling hadbeen created, prior to the introduction of spins.

2.5.2. Dougherty's synthesis of poly( m -phenyleneocta-tetraene). The rst experimental approach to this modelwas undertaken by Dougherty and co-workers whoemployed m-phenylene as the FCU and a tetraene as theeasily oxidised polaron precursor. 110 O-Alkyl groups werealso attached at the 5-position of the phenylene ring to aidsolubility. The poly( m-phenyleneoctatetraene) PMPOT-18,66 , was lightly doped with AsF 5 to form polarons whichwere found to be stable at room temperature. Magneticstudies indicated S $ 2, suggesting the existence of signi-cant net ferromagnetic coupling in the system. It becameclear that the doping was inhomogeneous and that regions of relatively heavier doping allowed signicant ferromagneticcoupling of the spins, which were found to persist to at least

200 K. It could not be conrmed that these results were dueto 1-D intramolecular couplings. Some intermolecular anti-ferromagnetic coupling was also observed at low tempera-tures.

On the basis of molecular orbital calculations and crystal

orbital theory, Yamabe considered substituted poly-acetylenes as potential 1-D ferromagnets. 111 Although theinteractions of doped poly(4-oxyphenyl)acetylene werefound to be anti-ferromagnetic, polyacetylene chains withphenoxyl radicals as pendants on each alternate site showedpossible ferromagnetic couplings. Further theoretical work

Figure 24. Conceptual model of a polaronic ferromagnet.

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established that the interactions of Cz 1 H, N

z 1 H and NOz

linked through m-phenylene were much stronger than whencoupled through a polyacetylene chain, and that theinteractions would be ferromagnetic. It was also suggestedthat although bulk ferromagnetism in 1-D systems can onlybe observed at 0 K, there are always weak 3-D interactionssuch as interchain coupling. Ferro- or antiferro-magneticordering may therefore appear at a nite temperature. 112

2.5.3. Measuring the effectiveness of ferromagneticcoupling units for use in polaronic model materials. Inthe majority of polaronic models m-phenylene has beenchosen as the ferromagnetic coupling unit since it is gener-ally considered the most effective. 113 Dougherty surmisedthat the choice of ferromagnetic coupling units would becritical to design and began investigating various potentialFCUs in the early 1990s. He concluded that magnetismarises when the overlap integral between orbitals is small

but the exchange integral remains large. This is possible infully conjugated, planar p systems with topologies thatproduce two or more NBMOs which are degenerate andwhere the critical issue is the extent to which theseNBMOs overlap in space, rather than their relative energies.He devised a method of evaluating FCUs based on 2-alkyl-idene-1,3-cyclopentanediyl units which have a large tripletpreference, are synthetically accessible, and are sufcientlythermally stable. These units provided robust triplets thatcould be linked via each potential FCU to enable thestrength and nature of spin coupling to be determined byEPR spectroscopic and magnetic methods. Initial studiesfocused on the coupling units shown in Fig. 25.

When ferromagnetic coupling of the triplet spins occurs, aquintet should be observed by low temperature EPR spec-troscopy. The species 67 and 68 were found to be goodFCUs whilst 69 acted as a weak FCU. In 70 , however, thespins were coupled in an anti-ferromagnetic fashion. Studieson the strength of spin coupling have also been carried outby Li et al., 114 using the effective valence bond model, 115 onthe biradicals 71 82 (Sets AD) that allowed the estimationof the singlet/triplet energy separation, D E ST , whichreects the strength of spin coupling between two unpairedsites, and additionally relates to chemical reactivity(Scheme 1).

For Set AD

E ST decayed drastically with increasing topo-logical distance between the spins; the same was found for

Set B. For Set C, D E ST also decreased, but in a more gradualmanner. Interestingly, the coupling through three sequentialbenzene rings was still appreciable. Strong ferromagneticcoupling would appear to prevail in biradicals with shorttopological distances between spins and also wherenumerous spin-coupling paths and large delocalisationeffects are present in the SCUs.

The same research group also investigated to what extentthe coupling of the FCU in biradicals is maintained in poly-radicals and ferromagnetic polymers, using the classicalvalence bond model based on vinylidene 83 , biphenyl 84 ,and m-phenylene units 85 . It was found that the couplingthrough vinylidene decreased appreciably from the biradicalto the linear triradical.

Figure 25. Use of 2-alkylidene-1,3-cyclopentanediyl units for evaluating FCUs.

Scheme 1.

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The spin coupling through m-phenylene in the biradical wasreduced by one-third in the linear triradical. The magnitudeof coupling through the central m-phenylene in a tetra-radical was lower than the terminal m-phenylene and wastaken to be a good approximation for a poly( m-phenyl-methylene) system. The 2,3 H-connected biphenyl unit wasfound to have a slightly greater coupling in the lineartriradical 84 than in the biradical due to an amplicationeffect of the two outer spins on the central spin.

The effectiveness of intramolecular spin-coupling throughm-phenylene has been extensively studied by means of several theoretical models and also by experiments. 116,117

On the whole, m-phenylene has proved to be by far thebest FCU for constructing high spin molecules and fortruly practical magnetic materials. In 1995, however,Iwamura argued that since thiophene rings are more electronrich than benzene, they are better equipped to stabiliseradical cations. In addition, thiophene rings are stericallyless demanding than benzene, which could result in theplanarity of polymer backbones being maintained moreeasily. As an interesting alternative he coupled nitronylnitroxides through 2,4-substituted thiophenes. 118 The inter-actions between the nitronyl nitroxides were found to be

ferromagnetic and were larger than those observed throughm-phenylene, presumably due to an increase in co-planarityand hence greater spin polarisation. Extension to bithienylunits, however, greatly weakened the interactions to theextent that degenerate singlet and triplet states wereproduced. This was accounted for by the poor delocalisationof the nitroxide's spin into the thiophene ring.

It would be advantageous to be able to estimate the effec-tiveness of m-phenylene in systems where the radicalcentres may be out of conjugation with the aromatic ring.Several groups have addressed this issue and have foundthat, in the highly crowded bisnitroxyls 86 119 and 87 ,120

the two SCUs are twisted completely out of conjugationand the biradicals have singlet ground states. In contrast,the extremely congested polyaryl derivatives 88 89 showhigh spin states despite a large degree of twisting.

Using the bistrimethylenemethane strategy, Doughertyfound that in conditions of modest twisting, the m-phenyl-ene probe 89 remained an effective FCU. Completely twist-ing both SCUs out of conjugation with the m-phenylene theprobe 90 led to no interaction whatsoever whereas havingone SCU in plane and one completely out of plane in probe91 still led to ferromagnetic coupling. 121 This was accounted

for by a novel mechanism in which anti-ferromagneticcoupling between a centre of negative spin density and acentre of positive spin density gave overall ferromagneticcoupling.

In addition to EPR spectroscopic and magnetic methods itmay be possible to screen the degree of ferromagneticcoupling between spin-containing centres using electro-chemical methods. As pointed out by Bushby et al. instructurally related bis(dialkyamino)benzene compounds(based on Fig. 22) the peak separation for the sequentialoxidation of the two amino centres should be related tothe degree of electronelectron repulsion, which will haveboth a coulombic component (assumed to be similar instructurally related compounds) and an exchange compo-nent (which will depend on whether the diradical SOMOis disjoint or non-disjoint (i.e. coextensive). 122 Schultz hasshown that this electrochemical screening may also beuseful for bis-quinones. 123

2.5.4. Polymers containing conjugated segments linkedthrough m -phenylene units. Dougherty continued his work on polaronic ferromagnets, proposing a range of potentialpolaronic magnets 92 98 in order to determine whichconjugated segments were best suited to producing a largenumber of spins that would interact strongly through theFCU. 124 Although Dougherty's designs involved 1-D ferro-magnets only, the structures shown were expected to display

short range, high spin ordering. The polymers 92 98 wereprepared by Wittig 125 or Suzuki126 type polymerisationreactions that afforded materials with an average molecularweight of approximately 10,000 amu. This was a consider-able achievement because insufcient solubility can resultin oligomeric chains precipitating from solution. Thesepolymers were doped using iodine or arsenic pentauorideand subjected to magnetisation studies.

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The best results were achieved for the polymer 97 in whichone-third of the monomer units contained a spin. The spinsproduced were extremely stable with S . 1/2 behaviourbeing observed. In general, however, less than one spinper chain was evident in these polymers. The magneticbehaviour was again explained by inhomogeneous dopingwhich occurred due to the creation of a polar environment,which favoured further doping nearby.

A design consisting of tetrathiophene units m-coupled viaphenylene FCUs ( 104 ) has recently been proposed. 127 Thecomparatively short, four-ring, SCU was chosen to mini-

mise spinless bipolaron formation. Each thienyl ring wasfunctionalised with an alkyl chain to promote solubility

and processability. It was also expected that the largenumber of alkyl chains per polymer, and their siting in theconjugated sections of the backbone, would keepneighbouring chains apart and thus prevent p -dimerisation.Structural homogeneity and stereoregularity were necessaryin order to maintain the backbone as close as possible toplanar and hence to maximise polaron delocalisation. Toachieve this, methods for the regioselective head-to-tailcoupling of 3-alkylthiophenes were needed. Initially, theNi catalysed coupling of a 2-bromo-3-alkylthiophene 99with a 5-bromomagnesio-3-alkylthiophene was studied.This route delivered high regioselectivity but, despite

Scheme 2.

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varying the coupling conditions, the ratio of reactants, thealkyl side chain and the catalyst, the yields of cross-coupledproducts 101 remained at ca. 25%. Satisfactory yields of 101 were obtained by Stille coupling of 2-trimethyltin-4-alkylthiophenes 100 with 2-bromo-3-alkylthiophene, butat the expense of lowered regioselectivity. The desired5-bromo-3,4 H-dialkyl-2,2 H-bithiophene 102 was isolated in

good yield by selective bromination of the isomeric mixturefollowed by column chromatography. Two molecules of 102 were successfully coupled to the m-bisboronic esterusing modied Suzuki conditions to produce the monomer103 in satisfactory yield (Scheme 2).

Electrochemical polymerisation of 103 (R Me or H) with aPt electrode gave polymers 104 containing the highestnumber of spins. Chemical polymerisation with NOBF 4and NOPF 6 also produced satisfactory materials. UVVISand IR spectroscopy showed the formation of polaronson doping. The EPR spectra of these polymers were,however, consistent with paramagnetic materials exceptat low temperatures ( , 60 K) where some unusualfeatures may indicate one-dimensional ferromagnetformation. 127,128

Zotti et al. electropolymerised the monomer 105 to give apolymer with two distinct redox processes. 129 Localisedoxidation of the bithiophene units occurred at 1 0.20 V, atwhich point a high spin density of 0.25 spins per unit wasdetermined.

2.5.5. Polaronic poly(arylamines). Although at rst sightthe rather localised oxidised amino groups of poly(arylamines) discussed in this section do not seem tobe not entirely analogous to the extended polaronic SCUsdescribed above, in fact these systems are closely related tothe conducting polymer, polyaniline, in which the presence

of delocalised spins is well established. Bushby et al.synthesised a polaronic polymer 108 with S , 5/2 whichfeatured aryl amines as spin containing units that werelinked through meta -coupled benzene rings. 130 UnlikeDougherty's polymer, which was one dimensional,Bushby's polymer was cross-linked such that each SCUwas ferromagnetically coupled to three others in order to

counter the effects arising from incomplete doping. Thepercolation limit, the point at which all spins were aligned,for these systems was estimated to be 70%. 131 Bushby usedspin bearing N

z 1 centres and 1,3,5-trisubstituted benzenerings to extend his structures in more than one dimension.The best results were achieved with a polymer obtainedfrom the Suzuki reaction of the dibromide 106 with thetrisboronic acid 107 (Scheme 3). Oxidation of 108 withNO 1 BF4

2 resulted in doping levels of ca. 15%, farshort of the percolation limit and of the 10 3 104 spinsrequired to sustain a ferromagnetic structure domain. Astudy of the magnetisation as a function of magneticeld strength suggested that 108 behaved most like anS 5/2 system. Further investigations by Bushby estab-lished that one reason for the poor doping levels wasthe difculty of accommodating large counter ions in arigid polymer, i.e. a steric rather than an electrostaticlimitation. 132

In an attempt to rectify this problem, Bushby linked thetriarylammonium ions in the 2,7-naphthalene derivative109 and the dibromide 110 rather than through 1,3-phenylene.

This `spacing out' of the monomer units did not lead tohigher doping levels with NO 1 BF4

2 in dichloromethane. Adifferent doping technique using antimony pentachlorideachieved higher doping levels but had poor reproducibility.It was also found that the thin polymer lms required for thismethod of doping decayed rapidly. The polymer 108 couldbe doped to a level of 40% using SbCl 5 and it was thenfound to show S 4 behaviour.

Scheme 3.

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It would appear that a signicant problem associated withthe polaronic model is achieving the necessary high levelsof doping required for ferromagnetic coupling. The use of `heavy' doping conditions can result in the formation of bipolarons which are spinless dications and thus detrimentalto ferromagnetic design. 133 The emeraldine salt of poly-aniline cannot accommodate bipolarons and may therefore

be used as a basis for designing high spin density polymers.Janssen and co-workers 134 used this property in designing N -phenylaniline oligomers comprising alternate meta - and para -substituted rings to favour ferromagnetic coupling andradical cation stability, respectively. 135

Oxidation of 111 with thianthrenium perchlorate affordedthe dication radical, which was found to have a tripletground state and showed no decay by EPR spectroscopyafter several weeks. A study of 112 showed that it alsohad a high spin ground state. It was concluded thereforethat these molecular types can be doped to produce stabletriplet dication radicals.

The phenylenevinylene coupling unit has also beenemployed to good effect. The 3,4 H-bis(diphenylamino)stil-bene cationic diradical is sufciently stable in the tripletstate to permit SQUID measurements over a range of temperatures. 136 A polymeric analogue has also beenreported. 137

2.5.6. Other polaronic materials. A slightly differentapproach was adopted by Tanaka who constructed co-polymers consisting of p conjugated units as the spinsupplier and non- p conjugated units containing several sp 3

hybridised atoms as spacers. 138 The interactions betweenpolaronic units could then be varied by altering the lengthand atomic structure of the spacer unit. The compounds

prepared consisted of regularly aligned disilanylenescoupled with p conjugated units, the most promising of

which appeared to be 113 containing quaterthiopheneunits. Doping the polymer 113 was unsuccessful as a resultof oligothienylene units in the chain surrounding the dopantion, hence resulting in a partially charged state. The conse-quent low spin concentrations hampered the formation of polaronic ferromagnetic interactions.

Janssen and co-workers examined thiophene and pyrroleoligomers such as 114 and 115 , which had been protectedat the a -position. From these studies 139 Janssen claimed thatcation radicals of oligopyrroles and oligothiophenes couldform diamagnetic p dimers in the solid state and in solutionand that the formation of these diamagnetic p dimers may

provide an insight into the low spin concentrations observedin these systems as compared to inhomogeneous doping.Janssen concluded that it was not possible to obtain highspin molecules based on dopable p conjugated segmentscontaining pyrrole or thiophene.

3. Conclusions and Future Prospects

It is remarkable to see how so much innovative and elegantsynthetic work in this area has taken place over the lastdecade or so. Most of this work was inspired by the rathersimple ideas of the `through-space' McConnell models forbuilding up bulk ferromagnetic interactions on the one hand,and the various topological models for `through-bond',intra molecular interactions on the other. It is equallyremarkable that all of these models have their place in theexplanation of ferromagnetic interactions; indeed, in somecases more than one mechanism may be operating. Yet itmay be some time before a reliable theoretical predictivemethod is available given the difculties involved in calcu-lations over several accessible open-shell congurations. Itis here that the physicists may make a contribution inexploring the boundary between quantum and classicalmechanics, as in the studies of single crystals of manganeseacetate, [Mn 12 O12 (CH3COO) 16 (H2O)4].

140

As far as it is possible to detect any trends in currentsynthetic endeavours, the following have been noted. New

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ferromagnetic coupling units (FCUs) other than those inFig.17 are being investigated. For example, dehydro[12]-annulenes 116 ,141 s-triazines 117 ,142,143 cyclophanes118 ,144 and even ferrocenes 119 :145

It is likely that the rst commercial applications of organicferromagnetic materials will involve polymeric materials of some sort. 146 A problem with the polymeric materials so farhas been the twisting of the polymer chain due to stericinteractions. The twisting prevents strong through-bondcoupling along the conjugated chain between pendantradicals. Another problem is how to achieve high spin levelsand stability at high temperatures. While the chemical andelectrochemical treatment of precursor polymers has provedto be reasonably effective, there may be limits to thisapproach. For example, difculties relating to the accom-modation of charged centres and/or large counterions willhinder further oxidation.

Given the general rapid advances in dendrimer synthesis, 147

it is inevitable that this will become an even more popularapproach to preparing and studying polyradical species of known size and disposition. Similarly, the possibility of using rapidly developing peptide synthesis techniques inconnection with materials research has been proposed. 148

In the biological arena, the study of magnetic material insideliving things, such as magnetotactic bacteria, has inspiredseveral research groups to emulate these materials in thelaboratory. 149

Thanks to improved synthesis and crystal structure determi-nation techniques interesting polymers are being preparedbased on transition metal complexes. For example, the[Co(pyrimidine) 2X2]n polymers are weakly ferromagneticbelow 5 K.150 Novel organometallic model diradicalcomplexes, such as the 1,3-diethynylbenzene bridgedCpFe(CO) 2

1 cations 120 ,151 or bis-porphyrins 152 open up alarge array of possibilities.

The world of buckyballs and buckytubes (nanotubes)continues to surprise and delight scientists. The ferro-magnetic C 60 charge-transfer salts were discussed earlier,and in the future it is possible that ferromagnetism may beobserved with single-wall nanotubes, some of which arepredicted and observed to be metallic, while others, suchas the chiral examples, are semi-conducting. 153 Although

electrons in a nanotube are generally considered to benon-interacting, contrary experimental evidence hasrecently emerged. 154 Indeed, spin-polarisation can be trans-ferred to multiwalled nanotubes by contact. 155 Perhaps thethrough-bond topological model mentioned above could beapplied to nanotubes. A nanotube that bites its own tailyields a torus (or `crop circle'). These have been observedby several groups, usually in the nanotube soot mixture,although they have also been observed in the materialobtained after pyrolysis of iron phthalocyanine. 156 Theow of electrons in this object will presumably also createits own magnetic eld, analogous to the ring-current inbenzene. 157 Ferromagnetic carbon nanotubes can be synthe-sised containing magnetic iron particles. 158 Cage structuresand nanotubes can also be made out of layered materialssuch as boron nitride and even nickel chloride, withpotentially interesting magnetic behaviour. 159

There is, of course, much that remains to be discovered inthe area of molecular magnetism. The effect of couplingfamiliar physical properties of organic materials andpolymers, such as transparency and chirality, 160 withmagnetic properties has hardly been examined. A note of caution should be struck, however, about the prospects of purely organic molecular magnets. So far, despite muchresearch effort, the highest T C recorded for a purely organicferromagnet is 1.48 K for the Rassat diradical. There

appears to be some barrier to high T Cs related to the discus-sion above concerning spin density in organic materials. I

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Tetrahedron 56 7829