Organic Polyradical Magnetic Nanoclusters

Encyclopedia of Nanoscience and Nanotechnology

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Organic Polyradical Magnetic Nanoclusters Andrzej Rajca University of Nebraska, Lincoln, Nebraska, USA

CONTENTS 1. Introduction 2. Prerequisites for “Single-Molecule Magnet” Behavior 3. Molecular Organic Radical-Metal Nanoclusters 4. Nanometer-Sized Organic Polyradicals 5. Summary Glossary References

1. INTRODUCTION In 1980, Lis reported Mn12 synthesis, structure, and magnetic susceptibility measurements for nanometer-sized [Mn12 O12 (CH3 COO)16 (H2 O)4 ] molecules [1]. Subsequent magnetic studies determined the S = 10 ground state (S = total spin quantum number), primarily arising from uncompensated magnetic moment from antiferromagnetic interactions between eight S = 2 MnIII and four S = 3/2 MnIV ions. (All spins of one valence point up and the remainder point down.) In 1993, a significant magnetic anisotropy barrier EA  for reversal of magnetization (EA /k = 60 K, k = Boltzman constant), which manifested itself as magnetic hysteretic cycle at temperatures below 4 K, was found [2]. Because this hysteretic behavior is analogous to that for nanometer-sized magnetic particles or bulk magnets, except it is of purely molecular origin, the designation “single-molecule magnet” (SMM) was coined. Numerous organometallic molecules, based on Mn, Fe, Ni, Mo, W, etc., with large magnetic moments (S up to 5 1/2) were prepared but none of them had EA /k exceeding room temperature (or even 60 K for Lis’ Mn12 cluster); also, selected nanoclusters showed non-negligible intermolecular interactions, which interfere with the SMM behavior [3, 4]. Thus, ISBN: 1-58883-064-0/$35.00 Copyright © 2004 by American Scientific Publishers All rights of reproduction in any form reserved.

significant obstacles remain in the development of moleculebased magnetic memories, based upon classical reversal of magnetization. More recently, quantum effects in reversal of magnetization (tunneling of magnetization) were discovered for Lis’ Mn12 and other related clusters; that is, steplike hysteretic cycles were found at temperatures below 4 K [5]. This quantum behavior, which is associated with greatly accelerated reversal of magnetization at certain applied magnetic fields, has attracted wide interest for both fundamental reasons and potential technological applications of SMMs in so-called quantum computing [6, 7]. The advances in transition metal ion based magnetic nanoclusters open an interesting question as to whether alternative sources of electron spin, such as organic radicals, could be used for SMMs. In this chapter, we will briefly summarize the selected aspects of magnetism of SMMs and then review two classes of organic radical-based nanometersized molecules: organometallic metal-radical nanoclusters and organic polyradicals.

2. PREREQUISITES FOR “SINGLE-MOLECULE MAGNET” BEHAVIOR The magnetic anisotropy barrier (EA  for inversion of magnetization (between up- and down-spin states) in a molecular cluster may be related to S 2 D, where S is the total spin quantum number for the molecule and D is the axial zero-field splitting parameter for the molecule [8]. When EA /k significantly exceeds the temperature (T ) at which the experiment is carried out, the inversion of magnetization becomes slow on the experimental time scale; for example, for a relatively slow experiment, such as hysteretic cycle, EA /kT≈ 10 is usually needed. Therefore, attaining relatively large values for EA through optimization of S and D will be the primary goal. Furthermore, the energies associated with intermolecular interactions (Einter  should be significantly Encyclopedia of Nanoscience and Nanotechnology Edited by H. S. Nalwa Volume 8: Pages (285–294)

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Organic Polyradical Magnetic Nanoclusters

lower than thermal energies (i.e., Einter  kT), to prevent bulk magnetic ordering interfering with the SMM behavior.

2.1. Total Spin Quantum Number S Three generic coupling schemes for individual spins, associated with organic radicals and/or metal ions, and leading to large values of S are illustrated in Figure 1A. In the ferromagnetic scheme, all nearest neighbor pairs of individual spins are ferromagnetically coupled. Another coupling scheme relies on antiferromagnetic coupling of the nearest neighbor spins connected in such a way that the spins are not compensated, giving an overall value of S > 1/2. The third coupling scheme may be referred to as ferromagnetic (i.e., it relies on antiferromagnetic coupling of unequal spins). For all coupling schemes, the exchange coupling (ferromagnetic or antiferromagnetic) has to be sufficiently strong so the energy difference(s) between the high-spin ground state with the large value of S and the low-spin lowest energy excited states significantly exceeds the thermal energy (kT) at the temperature of the experiment. Conceptually pairwise ferromagnetic and antiferromagnetic couplings may be visualized as transmitted through ferromagnetic coupling units (FCUs) and antiferromagnetic coupling units, respectively (Fig. 1B) [9, 10]. These coupling units may correspond to organic radical-metal linkage or, in polyradicals, the -system bridging two nearest neighbor radical sites. For an S = 1 ground state diradical, such as a Schlenk hydrocarbon, a 1,3-phenylene FCU links two arylmethyl spin sites (Fig. 1B) [11]. A ferromagnetic coupling scheme may be implemented via connection in an alternating mode of the FCUs and the spin sites to provide polyradicals with S = n/2, where n is the number of spin sites [9, 10]. Ferromagnetic coupling is attained when the orbitals containing unpaired electron spins are orthogonal and coinciding in space [12, 13]. This situation is found in an S = 1 ground state diradical (Fig. 1B) with unpaired electron spins located in nearly degenerate, coinciding in space, and orthogonal molecular orbitals, which are characteristic of 1,3-phenylene moiety (an FCU). Other examples of strong and moderate ferromagnetic couplings are CuII (and NiII  complexes with o-semiquinone and CuII complexes with nitroxide in the axial position, respectively [14]. A ferromagnetic coupling scheme may be implemented via connection A

ferromagnetic

antiferromagnetic (uncompensated connectivity)

ferrimagnetic (unequal spins)

B Diradical with S = 1 ground state:

Ph

Ph Ph

Ph

Ph

Ph

FCU

ACU

Diradical with S = 0 ground state: Ph

Ph

Figure 1. (A) Coupling schemes leading to large net values of S. (B) Pairwise ferromagnetic and antiferromagnetic couplings in diradicals with S = 1 and S = 0 ground states, respectively.

in an alternating mode of the FCUs and the spin sites to provide polyradicals with S = n/2, where n is the number of spin sites [9, 10]. Antiferromagnetic coupling is associated with an effective overlap of nonorthogonal orbitals; thus, it may be viewed as a weak chemical bonding. This situation may be found in an S = 0 ground state diradical with 3,3 -biphenylene bridging two arylmethyl sites (Fig. 1B) [15]. Other examples of strong antiferromagnetic couplings are MnII complexes with nitroxides [14, 16, 17].

2.2. Zero-Field Splitting Parameter D For a molecule with the ground state with S ≥ 1, a splitting of its Zeeman energy levels in a zero applied magnetic field leads to anisotropy of magnetic properties, including a barrier for inversion of magnetization. Only states without firstorder angular momentum will be considered; D is the axial zero-field splitting (ZFS) parameter, which is related to the principal value of the corresponding tensor D. There are two primary mechanisms for ZFS. The first mechanism involves coupling of the ground state with the excited states through spin–orbit coupling. The second mechanism involves magnetic dipole–dipole interactions. Of the two mechanisms, the spin–orbit coupling is capable of attaining much greater values of D than the magnetic dipole–dipole interactions. This is the situation found in selected transition metal ions, such as MnIII , NiII , CrIII , etc., in their numerous complexes [18, 19]. Jahn–Teller distortions may lead to large values of the parameter D for single ion. In derivatives of Mn12 , where single ion D is main contributor to the large value of D for the cluster, the relative phases of Jahn–Teller distortions for MnIII have a large effect on the value of D of the cluster [20]. For a cluster, in which the exchange coupling is the dominant interaction between the ions, overall values of D for each state with a value of S can be derived from tensors D for single ions [14]. Because spin–orbit coupling is a relativistic effect, it is very small in the elements of organic molecules such as C, N, and O [18]. There are several methods for estimating values of D from classical magnetic dipole–dipole interactions [14]. Perhaps the most straightforward method involves indirect estimate of D through the shape anisotropy; this estimate is especially applicable to molecules or clusters with large numbers of approximately uniformly distributed and ferromagnetically coupled electron spins [21]. Such estimates may be applied to organic polyradicals [22].

3. MOLECULAR ORGANIC RADICAL-METAL NANOCLUSTERS A plethora of organic radical-metal one-dimensional chains and extended networks, in which organic radicals act as bridging groups between the metal ions, leading to relatively large EA and/or magnetic ordering are known. Organic radicals involved in such one-dimensional chains and networks are nitronyl nitroxides, [23 24] di- and trinitroxides, [25] nitroxides with nitrogen base, [26] and pyridine-based photochemically generated carbenes [27–29].

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Organic Polyradical Magnetic Nanoclusters

Analogous molecular organic radical-metal nanoclusters, in which organic radicals act as bridging groups between the metal ions, are relatively scarce. One of the pioneering examples of organic radical-metal nanoclusters is a macrocyclic [Mn(hfac)2 NITPh]6 cluster with S = 12 ground state (hfac, hexafluoroacetyl acetonate; NITPh, 2-phenyl-4,4,5,5-tetramethyl-4,5-dihydro-1Himidazolyl-1-oxy-3-oxide) [30]. In [Mn(hfac)2 NITPh]6 , the manganese(II) ions with S = 5/2 are antiferromagnetically coupled to S = 1/2 organic radicals (nitronyl nitroxides), resulting in an overall spin value of 65/2 − 1/2 = 12. Both the oblate shape of the cluster and the small spin–orbit couplings associated with high-spin MnII (half-filled d-shell) and nitronyl nitroxide may contribute to its overall low magnetic anisotropy. More recently, a S = 7 nanocluster, [Fe2 (CN)12 Ni3 (IM2Py)6 ]•4H2 O, in which two FeIII and three NiII ions are bridged with cyanides and six organic radicals (IM-2py, 2-pyridyl substituted imino nitroxide), was reported [31]. The S = 7 value is accounted for by a weak ferromagnetic coupling between three high-spin S = 2 [Ni(IM-2-Py)2 ] and two low-spin S = 1/2 [Fe(CN)6 ] fragments. Below 1.2 K, magnetic hysteresis is observed, though magnetic relaxation studies reveal nonexponential time dependence and temperature dependence down to 0.05 K, precluding SMM behavior. Most likely, intercluster interactions are responsible for magnetic ordering. [Mn12 ] anion modified with organic radical cation, that is, salt of [Mn12 O12 (PhCOO)16 (H2 O)4 ]− anion (Ph = phenyl) and S = 1/2 cation (m-N -methylpyridinium nitronylnitroxide), was found to undergo relatively fast inversion of magnetization; at 1.7 K, hysteresis was almost negligible compared to [Mn12 ] anions with diamagnetic cations or the unmodified [Mn12 ] [32]. Recent studies of analogous salts of [Mn12 ] anion with S = 1/2 ferrocenium cation and related diamagnetic cations led to the conclusion that the counterion may have a direct influence on the EA by affecting alignment of Jahn–Teller elongation axes of the eight MnIII ions [33]. Synthesis of organic radical-metal ion clusters, and metal ion clusters in general, relies largely on finding the right mixture of the reagents that would produce a crystalline material suitable for X-ray crystallography. The development of self-assembly approaches to organic ligand-metal ion clusters provides hope for future rational syntheses of metal ion clusters of increasing size with predetermined magnetic properties [34].

4. NANOMETER-SIZED ORGANIC POLYRADICALS Organic polyradicals (i.e., molecular organic radical-based nanoclusters) possess covalently linked organic radicals. Unlike metal-based nanoclusters, they are amenable to the rational stepwise syntheses, employing powerful methodologies of organic chemistry. Furthermore, powerful chromatographic techniques are available for fast purification of organic molecules; that is, those products and intermediates at each synthetic step, which are stable at ambient conditions, can be isolated and purified even from complex reaction mixtures. Such a rational synthetic approach

is especially important if clusters with specific values of S or particular shapes are sought. The major drawbacks of organic polyradicals include moderate, at best, stability of organic radicals. Also, single crystals for X-ray crystallography are relatively difficult to grow for large organic molecules (as opposed to metal-based clusters); the structural characterization of organic polyradicals is largely based upon spectroscopic (or scattering) techniques as applied to polyradicals and their synthetic intermediates. From the viewpoint of magnetism, nanometer-sized organic polyradicals can be classified as possessing either weak or strong exchange coupling. The thermal energy at a readily accessible temperature of 2 K is designated as a relative measure of strength for exchange coupling. Thus, weak exchange coupling will imply that the S = 1/2 radicals are effectively noninteracting above a temperature of 2 K. The more interesting situation for the potential SMM behavior is the case of strong exchange coupling, implemented with one of the coupling schemes in Figure 1, leading to large values of S.

4.1. Nanometer-Sized Organic Polyradicals with Strong Exchange Coupling Polyradicals with strong exchange coupling are primarily based upon a -conjugated system mediating exchange coupling between the radicals. The majority of nanometer-sized -conjugated polyradicals rely on the ferromagnetic coupling scheme, though the ferrimagnetic coupling scheme using unequal spins recently became important (Fig. 1) [35]. 1,3-Phenylene is by far the most dominant FCU. The most developed types of nanometer-sized organic polyradicals with strong exchange coupling are polyarylmethyls, polyphenylcarbenes, and polyphenoxyls.

4.1.1. Polyarylmethyls Polyarylmethyls have long history dating to Gomberg’s triphenylmethyl [36] and Schlenk and Brauns’ hydrocarbon [11] reported in 1900 and 1915, respectively. In the 1990s, the first attempts at synthesis of nanometer-sized polyarylmethyls were made, using branched or dendritic connectivities of 1,3-phenylene FCUs and arylmethyls. However, values of S for such dendrimers with up to up to 31 arylmethyls did not exceed S = 5 22 37–39. With increasing number of radicals per molecule, the generation of unpaired of electrons at the radical sites must approach perfection. Otherwise, even relatively small numbers of failures in generation of unpaired electrons (i.e., chemical defects) may disrupt -conjugation, and consequently exchange coupling. The chemical defects may arise from incomplete oxidation of polycarbanion, protonation, or hydrogen transfer leading to Ar3 CH moieties, or reaction of radicals with excess of iodine giving Ar3 CI moieties [40, 41]. As illustrated for pentadecaradical 1 in Figure 2, a chemical defect at one of the inner radical sites will cut the -conjugated system into noninteracting segments, with low values of S, for example, a mixture of S = 3/2 3/2 8/2 vs S = 15/2 for a “perfect” 3 with all 15 radicals intact and ferromagnetically coupled [22]. Another type of defect involves severe out-of-twisting of the -conjugated system, which may lead to antiferromagnetic

288

Organic Polyradical Magnetic Nanoclusters

coupling through the twisted linkage; such a defect in the inner part of 1 could severely reduce the overall value of S in the twisted conformation (Fig. 2) [35, 42, 43]. Another approach relied on dendrimers with macrocyclic cores, such as 24-radical 2 (Fig. 3) [44–47]. The macrocyclic core provides the second exchange pathway, preserving exchange coupling in the presence of one chemical defect located at one of the four inner radical sites. Overall, 24-radical 2 possess only four radical sites (those adjacent to biphenyl moieties), where a defect would interrupt exchange coupling. The ferromagnetic coupling scheme is implemented in 24-radical 2 using two types of FCU: 1,3-phenylene and 3,4 -biphenylene. Model studies showed that the pairwise ferromagnetic couplings mediated by 1,3phenylenes (within the dendritic branches and the macrocyclic core) are significantly stronger than those mediated by four 3,4 -biphenylenes (between the branches and the core) [45, 48]. Therefore, 24-radical 2 may be viewed as a cluster of five component spins, 5/2, 5/2, 5/2, 5/2, 4/2, associated with the branches and the core. Magnetic studies corroborate this pentamer model, though the average value of S = 10 is below the expected S = 12 for a 24-radical with 24 ferromagnetically coupled unpaired electron spins. A small density of chemical defects may account for this discrepancy in the values of S [47]. Alternatively, the presence of conformations, in which one or two dendritic branches are significantly outof-plane twisted at the biphenylene moieties, may decrease the overall value S too because of antiferromagnetic coupling between the twisted branch and the core. Synthetic precursor to 24-radical 2 (i.e., polyether 3) was employed to obtain the size and shape estimate (2 is obtained from 3 by replacing 24 triarylmethyl ether, Ar3 C–OCH3 , groups with 24 triarylmethyl radicals, Ar3 C•, through a two-step sequence of chemical transformations) [47]. The fast atom bombardment mass spectrum shows the expected molecular fragment ion corresponding to the loss of one OCH3 group; the isotopic multiplet fits well with the expected C503 H589 O23 formula for the [M–OCH3 ]+ ion [47]. Small angle neutron scattering (SANS) for 3 in tetrahydrofuran-d8 (THF-d8 ) gives a radius of gyration Rg = 138 nm (Table 1). Preliminary numerical fitting of the SANS data using simulated annealing programs suggests an

approximately dumbell-like shape. Similar values of Rg and analogous shapes are found for hydrocarbon 4, though carbopolyanion 5 has relatively large Rg , presumably due to increased solvation. Assuming similar shapes for 24-radical 2 and its derivatives 3–5, and approximating them with a prolate spheroid with half-axes of 2 and 1 nm, shape anisotropy based EA /k ≈ 006 K is estimated for S = 12 (Fig. 3). Further evolution of dendritic architecture is illustrated by 36-radical 6, which possesses macrocylic branches and a macrocyclic core; that is, five calix[4]arene macrocycles are linked with four bis(biphenylene)methyls (Fig. 4) [35, 49]. The macrocycles provide additional exchange pathways; 36-radical 6 has only four radical sites (those bridging biphenylene moieties), where a defect would interrupt exchange coupling. The exchange coupling within the -system of 6 may be described in terms of strong 1,3phenylene FCUs and weak 3,4 -biphenylene FCUs, which connect radical sites within macrocycles and outside macrocycles, respectively. Therefore, 6 may be viewed as a cluster of nine component spins, associated with the macrocyclic branches (4 × 7/2), the linkers 4 × 1/2, and the macrocyclic core (4/2) (Fig. 4). The component spins are exchange coupled through the weak 3,4 -biphenylene FCUs. Notably, the nearest neighbor component spins have significantly unequal values (i.e., component spins 1/2 are located between the

Ar = 2 3 4 5

C(CH3)3

X= X = OCH3 X=H X=

X Ar

Ar X Ar

Ar X Ar Ar

X

X Ar X

Ar Ar

Ar =

Ar

X X

X Ar

Ar

1

Ar

Ar

X X Ar Ar

S = 3/2, 3/2, 8/2 Ar Ar

Ar

Ar X Ar

Ar

Ar Ar

Ar X

Ar X Ar

X

D Ar Ar

Ar

Ar X Ar

Ar

Ar

Ar

Ar

Ar

Ar Ar

Ar Ar

X

X

Ar

Ar Ar

(S = 10) (Rg = 1.38 nm) (Rg = 1.37 nm) (Rg = 1.79 nm)

90

X Ar

Ar X Ar

Ar X Ar

Ar X Ar Ar Ar X X

o

5/2

D

5/2 4/2

C(CH3)3 S = 8/2 - 7/2 = 1/2

Figure 2. Polyarylmethyl dendritic pentadecaradical 1 and possible defects (D): missing unpaired electron (top) and large out-of-plane twisting (bottom).

5/2

2

5/2

Figure 3. Polyarylmethyl dendritic-macrocyclic 24-radical 2 and its diamagnetic derivatives.

289

Organic Polyradical Magnetic Nanoclusters X Ar C(CH3)3

Ar

Ar

X Ar

Ar

Ar

X

Ar

Ar X Ar

Ar

X

Ar

X Ar

X

Ar

Ar

X

X

Ar

X Ar

Ar Ar

Ar X

X

Ar

X Ar X

Ar Ar

Ar

Ar X

7/2

1/2

1/2

7/2

4/2

1/2

7/2

1/2

6

C(CH3)3

Figure 5. Polyarylmethyl polymers 8 and 10 and their diamagnetic derivatives.

X

X Ar

n

Soluble fraction: (S > 40) 10 X = 11 X = OCH3 (Rg = 9 nm) Insoluble fraction: (S = 5000) 10 X = 11 X = OCH3

Ar X

X

Ar Ar =

C(CH3)3

Ar =

Ar Ar

Ar Ar

n

X

X Ar Ar X

Ar

8 X= (S = 18) 9 X = OCH3 (Rg = 3.2 nm)

X X

X

Ar Ar X

7/2

Figure 4. Polyarylmethyl macrocyclic–macrocyclic 36–radical 6 and its diamagnetic derivative.

larger spins of 7/2 and 4/2). Consequently, large out-of-plane distortion at the biphenylene moieties, leading to antiferromagnetic coupling between component spins, could be expected to lower the overall value of S only moderately. Magnetic studies show an average S ≈ 13 for 6, below the expected S = 18 for a 36-radical with 36 ferromagnetically coupled unpaired electron spins [49]. SANS for a polyether 7 (synthetic precursor to 36-radical 6) in THF-d8 gives a radius of gyration Rg = 181 nm (Table 1). Assuming that the shape for 36-radical 6 is a prolate spheroid with a/b = 2, analogous to that in 24-radical 2, shape anisotropy based EA /k ≈ 009 K is estimated for S = 18 (Fig. 4). The alternating connectivity of unequal spins was applied to polyarylmethyl polymers 8 and 10 by linking calix[4]arene with bis(biphenyl)methyls (Fig. 5). Polymer 8 had an average value of S ≈ 18 (Fig. 6). SANS and multiangle light scattering (MALS) studies showed that the corresponding polyether 9 had Rg = 32 nm and Mw ≈ 30 kDa. Polymer 10 possessed even higher values of S that depended on the polymerization conditions leading to the corresponding polyether 11 (Fig. 5). The benzene soluble fractions of

polyethers 11 with Mw = 300–500 kDa gave polymers 10 with average values of S > 40 [50]. SANS studies gave Rg ≈ 9 nm for benzene-soluble polyethers 11. The benzene insoluble fractions of polyethers 11, which were polymerized for a long time beyond the gel point, gave polymers 10 with average values of S ≈ 5000 (Fig. 6). Polymers 10 with S ≈ 5000 showed the onset of magnetic ordering near the temperature of 10 K. The blocking behavior for the inversion of magnetization is analogous to that in insulating spin glasses and blocked superparamagnets but closer to spin glasses [51]. In conclusion for polyarylmethyls, polyradical 10 showed magnetic ordering and slow inversion of magnetization below a temperature of about 10 K. Whether this magnetic behavior is associated with single macromolecules is difficult to establish because of the insoluble network nature of 10. For the soluble polyradicals 2, 6, and 8, possessing S = 10–18, magnetic studies in the miliKelvin range would be needed to search for SMM behavior (slow inversion of magnetization) due to the shape anisotropy. The requirements for sample handling with present state-ofthe-art microSQUID techniques would make such studies 1.0

X

Ar

X

0.8

X

Ar

0.6

X

Ar

X Ar

Ar

Ar Ar X

Ar

X

Ar

Ar Ar X

Ar

Ar

X

10 (2.5 K) 8 (1.8 K)

0.4

X

X

X X

Ar

Ar

X

Ar

X

7 X = OCH3 (Rg = 1.81 nm)

X

X

Ar

S

avg

= 5090

Savg = 18

0.2

(S = 13)

X

X

X

X

X Ar

0.0

6 X=

Ar

Ar

M/Msat

Ar =

Ar

Ar

Ar X Ar

Ar

Ar

0

1 2 H/T (Tesla/K)

3

Figure 6. Plot of reduced magnetization M/Msat  vs the ratio of applied magnetic field and temperature (H /T  for polymers 8 and 10 with an average S ≈ 18 and S ≈ 5000, respectively.

290

Organic Polyradical Magnetic Nanoclusters

difficult because such polyarylmethyls decompose above 170 K and are sensitive to oxygen. Polyradicals with stability at ambient conditions, in which triarylmethyl radicals are replaced with more stable radicals such as nitroxides, would be desirable for such studies.

4.1.2. Polyphenylcarbenes and Other Organic Polyradicals with Significant Values of S In addition to polyarylmethyls, very few organic polyradicals are known, in which significant values of S have been achieved. Those include polyphenylcarbenes, trimethylenemethanes, aminium-based polyradicals, and polyphenoxyls. Among those, polyphenoxyls stand out because of their size characterization by microscopy and relative thermal stability. Iwamura and co-workers prepared numerous polyphenylcarbenes with large values of S; the leading example is the S = 9 nonacarbene 12, which was prepared from the corresponding nona-diazo precursor 13 (Fig. 7) [52–56]. The solid state photolysis of the diazo precursors, such as 13, is reported to proceed essentially quantitatively for molecules with up to nine diazo groups, providing polyphenylcarbenes with significant values of S, as shown in Figure 7. Attempts to prepare polyphenylcarbenes with more than nine carbene centers were not successful [56]. For 12 and the related polyphenylcarbenes, the ferromagnetic coupling scheme, using 1,3-phenylene and 1,3,5-phenylyne as FCUs, is implemented in the star-branched structure of nine S = 1 carbenes. Although the initial report on 12 suggested the possibility of slow inversion of magnetization below 5 K for 12 in 2-methyltetrahydrofuran (2-MeTHF), [53] it is most likely that slow interconversion between conformations with

12 X = C (S = 9) 13 X = C=N2

X

1 1

X X

X

X

1

X

1 X

1

1

1 1

X

different values of S is involved, as observed in organic diradicals in 2-MeTHF matrices [57, 58]. Polyphenylcarbenes, such as 12, are extremely reactive and typically have to generated in frozen matrices below 77 K. Recently, more stable S = 1 carbenes were prepared; for example, 14 and 15 have a half-life of 16 s and 19 min in degassed benzene at room temperature, respectively (Fig. 8) [59, 60]. For 15, this fair persistence may be derived from its triphenylmethyl-like structure. These values of half-life are several orders of magnitude longer than the half-life of 2 s for the parent diphenylcarbene under the same conditions. Polycarbenes derived from those relatively stable carbenes are still rare. The leading example is the S = 3 tricarbene 16, which is generated from the corresponding tris(diazo) derivative 17 and characterized by two-dimensional electron paramagnetic resonance (EPR) spectroscopy (Fig. 8) [61]. Dougherty and co-workers developed a so-called “bisTMM” (TMM = trimethylenemethane) approach to design and prepare numerous S = 2 tetraradicals [62–65]. Their ferromagnetic coupling scheme employs 1,3-phenylene, cyclobutane, or cyclopentane as FCU (Fig. 9). More recently, this approach is extended to an S = 3 hexaradical, with 1,3,5-phenylyne and cyclopentane as FCUs (Fig. 9). [66–68] In each case, the polyradical is prepared by ultraviolet photolysis of the corresponding bis- or tris-azoalkanes at cryogenic temperatures; typically, efficient and complete conversions are difficult to attain, resulting in mixtures of polyradicals from the successive loss of dinitrogen. The resultant mixtures of polyradicals are quite unstable requiring handling in frozen matrix at temperatures of 77 K or below. Magnetic characterization of these tetra- and hexaradicals is limited to EPR spectroscopy. Bushby and co-workers prepared aminium based polyradicals; the leading example is polyradical 18 with an average value of S ≈ 4 at temperatures below 10 K (Fig. 10) [69]. The ferromagnetic coupling scheme in 18 relies on the FCU composed of 1,3-phenylene and two 1,4-phenylene spacers; this rather long coupling pathway results in a relatively weak ferromagnetic coupling. Polyradical 18 was generated from the corresponding polyamine 19 with Mw > 10 kDa; no size measurements are reported [69]. Polymeric aminium-based polyradicals may be viewed as examples of the “polaronic” approach to organic polymers with large values of S, in which the source of electron spin is

12 Br

1 X

(H3C)3C

Ph

Br

(H3C)3C

C Br Br 14

C

C(CH3)3

Br

Br Br

X Br

Ph Br

S=4

S=6

X

Br

Br Ph

S=6

Figure 7. Selected phenylcarbenes with large values of S, including S = 9 nonacarbene 12 and the corresponding coupling scheme.

C

15

Ph

Br Br Br

Br (H3C)3C

C(CH3)3

X Br

16 X = C (S = 3) 17 X = C=N2

Figure 8. Selected persistent carbenes and tricarbene 16. (Ph corresponds to a phenyl group.)

291

Organic Polyradical Magnetic Nanoclusters R

C(CH3)3

X

N

R = C(CH3)3

O R

n

OX

R

S=2

S=2

X

S=2

R X

(S = 5, diameter = 35 nm) 21 X = 22 X = H (diameter = 35 nm)

R

O

R 1/2

R

O

R OX

R

1/2

1/2 1/2

R

S=2

S=2

1/2

1/2 1/2

Ph R

S=2

R X

O

R X

R

n S=2

R R

O R

O R

relatively localized aminium radical cations [70]. Dougherty and co-workers examined various polymers and oligomers, in which short fragments of the -system are linked together with 1,3-phenylene couplers. The studied -systems include radical cations of polyacetylene, radical cations of aromatic heterocycles, and radical anions of fuchsone (Fig. 10). The variable values of S up to 3 were reported for such polymers [71, 72]. However, only electrochemically generated fuchsone-based radical anions (polymer 20 with S ≈ 2) have significant concentration of electron spin (0.5 radical per repeat unit) [72]. Radical ion based polymers are generally stable at room temperature. Their main drawbacks are complicated ion multiplet equilibria and tendency for formation of diamagnetic aggregates [73–75]. Nishide and co-workers prepared many examples of polyphenoxyls with significant values of S (up to 5) (Fig. 11) [76–80]. The leading example is polyphenoxyl 21, which possesses an average value of S ≈ 5 at low temperatures below 10 K (Fig. 11) [80]. The ferromagnetic coupling scheme in 21 is implemented using phenoxyl pendants attached to the poly(1,2-phenylenevinylene) FCU. Polyphenoxyl 21 was obtained from the corresponding polyphenol 22 (Mw ≈ 32 kDa, Mw /Mn ≈ 12). Dichloromethane solutions of 21

R N

R = C4H9 (S = 4) 18 X = 19 X =

X

R 1/2 C14H29 18

OC18H37

n

S = 1/2 - 3

n C14H29

n

C14H29 S n S = 1/2 - 3

R

n

S=3

R

n

O R

Ph

Figure 9. Selected bis(TMM) S = 2 tetraradicals and S = 3 hexaradical. (Ph corresponds to a phenyl group.)

S

R

R

Ph

S=2

R

O

Ph Ph

21

O

R O

O 20 (S = 2)

Figure 10. Aminium-based polyradical 18 and other radical ion based polymers with significant values of S.

S = 7/2

Figure 11. Polyphenoxyl 21, coupling scheme for polyphenoxyl 21, and other selected polyphenoxyls with significant values of S.

and 22 are transferred to a highly oriented pyrolitic graphite surface and examined with atomic force microscopy (AFM), followed by magnetic force microscopy (MFM). Notably, both AFM and MFM studies are carried out at ambient temperature (i.e., polyphenoxyl 21 has a value of S = 1/2, corresponding to an ensemble of a large number of independent S = 1/2 electron spins, at these conditions). AFM images for 21 and 22 are similar (i.e., the horizontal distance of 35 nm and the vertical distance of 0.6 nm are determined). The follow-up MFM images, using a tip coated with ferromagnetic CoCr film (magnetic moment of 10−13 emu), are observed only for paramagnetic 21, with the 35-nm size analogous to the AFM images. The MFM contrast weakens and ultimately disappears following slow decomposition of paramagnetic polyphenoxyl 21 to the undetermined (perhaps diamagnetic) products after one day; however, the 35-nm sized image of the decomposition products remains visible in AFM even after one week [80]. The detection limits for MFM for small paramagnetic moments and the effects involved in magnetization of the tip in the opposite direction as found for this S = 1/2 paramagnet will need more detailed analysis. If confirmed, these preliminary MFM results may open the way for organic polyradicalbased magnetic dots.

4.2. Nanometer-Sized Organic Polyradicals with Weak Exchange Coupling In organic polyradicals with weak exchange coupling, the energy level splitting due to the exchange coupling is smaller than the thermal energy (RT) at the readily accessible temperatures down to 2 K. Such polyradicals possess a value of S = 1/2 above temperature of 2 K. The barrier for inversion of magnetization for S = 1/2 paramagnets will be very low if not absent [8]. The magnetic susceptibility for a paramagnetic polyradical with a large number n of S = 1/2 electron spins scales with n as approximately n2 and n for the strong and the weak

292

Organic Polyradical Magnetic Nanoclusters

N R= R N

R

O

R

N

H

N N

H

R N H

N

N R N H

R N H H N R

H

O

N

H N R

N

N N N

R N H H N R

H N R

N

N

N N

N

H R N H H N R

N R

N H

N

N

R

H

R

DAB-dendr-(PROXYL)16 O O NH

G-6

HN

N O 198

NH2

58

G-6-TEMPO-198

Figure 12. Third generation of nitroxide-functionalized poly(propylene imine) dendrimer DAB-dendr-(PROXYL)16 and sixth generation PAMAM dendrimer functionalized with 198 nitroxides (G-6-TEMPO198).

ferromagnetic couplings, respectively. Therefore, even in the absence of large values of S, the magnetic susceptibility for an S = 1/2 polyradical with n weakly coupled (effectively independent) S = 1/2 electron spins will still be greater by a factor of n than that for a single S = 1/2 monoradical. This is the situation for polyphenoxyl 16, which is weakly coupled at ambient temperature but still reported as detectable at ambient temperature by the current MFM technology [80]. The review of nanometer-sized weakly coupled polyradicals will be limited to well-defined dendrimers. Janssen and co-workers reported five generations of nitroxidefunctionalized dendrimers, with up to 64 nitroxide radicals [81]. Poly(propylene imine) dendrimers were functionalized with 3-carboxy-2,2,5,5-tetramethyl-1-pyrrolidinyloxy radical (PROXYL) end groups [DAB-dendr-(PROXYL)n ] (Fig. 9). Because DAB-dendr-(PROXYL)n dendrimers are prepared via divergent synthetic routes, substantial contamination with nonseparable dendrimers with a smaller than nominal number of PROXYL end groups, especially for higher generations, is expected. Already for intermediate generations with n = 8 and n = 16 PROXYLs, the matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectra showed the impurity mass peaks corresponding to dendrimers with n = 7 and n = 15 PROXYLs. The EPR spectra in solution were consistent with the presence of a weak but detectable exchange coupling, as shown by the hyperfine splitting patterns for the lowest generations [81].

Few examples of poly(amidoamine) (PAMAM) dendrimers, functionalized with spin carrying end groups (radicals or radical ions), have been reported [82–84]. Because PAMAM dendrimers are prepared according to divergent synthetic routes, significant contamination with incompletely end-functionalized dendrimers is expected. In particular, the commercially available sixth generation PAMAM (G-6 PAMAMTM  dendrimers possess oblate spheroidal shape with nominally 256 amine end groups on the surface of the macromolecule. Miller and co-workers reported a conducting film formed from a PAMAM dendrimer that had been functionalized with naphthalene diimide radical anions [82]. The defects in these modified dendrimers prepared according to divergent synthetic routes were not estimated. The radical anions of this type are known to form conducting -stacks, rendering the dendrimer effectively diamagnetic. In high humidity, the third generation dendrimer had conductivities in the 0.12– 11 S cm−1 range depending on the content of radical anions vs nonreduced diimide moieties [82]. PAMAM dendrimers with relatively low densities of nitroxide radical end groups for their studies in vesicles are known. For example, the sixth generation dendrimer (G-6 PAMAMTM , diameter of 7 nm from dynamic light scattering) was functionalized 2,2,6,6-tetramethylpiperidine-N -oxyl (TEMPO) with only 3% conversion of the amino end groups [83]. Much higher densities of radicals are reported in a polynitroxyl G-6 PAMAMTM with 198 (G-6-TEMPO-198) or with 80 (G-6-TEMPO-80) TEMPO end groups for possible applications in EPR imaging [84]. MALDI-TOF mass spectra showed the expected mass averages for G-6-TEMPO-198 and G-6-TEMPO-80, with a difference of about 1 kDa between the number and weight averages for both dendrimers. One of the possible drawbacks for their application as intravenous contrast agents may be their affinity for aggregation in aqueous solution because of the relatively lipophilic surface for the nitroxyl labeled dendrimers. MALS analyses identify particles with an average molecular weights of 549 kDa (degree of aggregation ≈ 54, radius of gyration ≈ 19 nm) and 113 kDa (degree of aggregation ≈ 16, radius of gyration < 10 nm) for G-6-TEMPO-198 and G-6TEMPO-80, respectively [84]. Octaferrocenyl dendrimer (with ethylene silane backbone) was reported to undergo 8-electron oxidation to the corresponding dendrimer with eight ferrocenium radical cations. However, neither the exchange coupling between the radical cations nor the size of this relatively low molecular mass dendrimer were characterized [85].

5. SUMMARY The development of rational synthetic methods for organic radical-metal ion nanoclusters may provide a new library of organometallic SMMs with predetermined magnetic properties. Through incorporation of di- and polyradicals into such nanoclusters, SMMs responsive to the external stimuli may be designed (e.g., photomagnetic SMMs). The rational synthetic methods for the unstable organic polyradicals should be extended to the ambient stable organic polyradicals with large values of S and elongated

Organic Polyradical Magnetic Nanoclusters

shapes. This will greatly facilitate the search for organic SMMs. Intriguing questions will be whether the magnetic anisotropy barrier arising from purely classical magnetic dipole–dipole interactions could be achieved and whether quantum tunneling of magnetization could be observed in such magnetic nanoclusters.

GLOSSARY Exchange coupling or exchange interaction The interaction between electron spins that originates from a quantum exchange term of the Coulomb interaction between electrons. Although electrostatic in its origin, the exchange coupling is one of the key aspects of magnetism. Magnetic nanocluster Nanometer-sized cluster of magnetic metals, metal ions, or other magnetic subunits. Organic polyradical Organic molecule that possesses more than several unpaired electrons. Single-molecule magnet (SMM) Individual molecule that functions as a nanoscale magnet, effectively a single domain magnetic particle that shows magnetic hysteresis below its blocking temperature.

ACKNOWLEDGMENTS Support from the National Science Foundation for this research is gratefully acknowledged. The author thank Dr. Suchada Rajca, Dr. Paul Butler, and Dr. Sungmin Choi for assistance with unpublished SANS data collection and numerical fittings.

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