Subscriber access provided by Fudan University
Article
High Spin Polymers: Ferromagnetic Coupling of S=1 Hexaazacyclophane Units Up to a Pure S=2 Polycyclophane Lukasz Skorka, Piotr Kurzep, Timothée Chauviré, Lionel Dubois, Jean-Marie Mouesca, Vincent Maurel, and Irena Kulszewicz-Bajer J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.7b01531 • Publication Date (Web): 27 Mar 2017 Downloaded from http://pubs.acs.org on March 28, 2017
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
The Journal of Physical Chemistry B is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 7
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
The Journal of Physical Chemistry
High Spin Polymers: Ferromagnetic Coupling of S=1 Hexaazacyclophane Units Up to a Pure S=2 Polycyclophane. Lukasz Skorka,1 Piotr Kurzep,1 Timothée Chauviré,2,3 Lionel Dubois,2,3 Jean-Marie Mouesca,2,3 Vincent Maurel2,3* and Irena Kulszewicz-Bajer 1* 1) Faculty of Chemistry, Warsaw University of Technology, Noakowskiego 3, 00-664 Warsaw, Poland. 2) Univ. Grenoble Alpes, INAC, SyMMES, F-38000 Grenoble, France 3) CEA, INAC, SYMMES, F-38054 Grenoble, France. ABSTRACT: Triarylamines oxidized to radical cations can be used as stable spins sources for the design of high-spin compounds. Here, we present the synthesis of the polyarylamine containing hexaazacyclophanes linked via meta-terphenyl bridges. Spins, created after the oxidation of the polymer, can be coupled magnetically in cyclophane moieties via meta-phenyl and along the polymer chain via meta-terphenyl units. The formation of quintet spin state was evidenced by pulsed-EPR nutation spectroscopy. Two exchange coupling constants via both couplers were determined experimentally and corresponded to J/k = 89 K in cyclophane moiety and j/k = 17 K via meta-terphenyl. Most importantly, in this polymer four spins can be ferromagnetically ordered via both couplers which leads to high-spin state.
Introduction Cyclophanes and similar rigid macrocycles were largely studied as building blocks for molecular machines that were the object of the 2016 Nobel Prize in Chemistry.1 Besides this main field of application, polymers and oligomers containing cyclophane units are studied as very useful materials for electric and optical devices due to their remarkable electronic and structural properties. It should be first emphasized that piconjugated polymers with cyclic oligomers in their structures show interesting luminescent, transport or non-linear optical properties, thus they can be used in OLEDs, photovoltaic devices, wave-guides and molecular wires.2–5 Physicochemical properties of cyclophanes differ significantly from those of linear compounds due to their particular structures causing their electronic interactions to occur through-space and/or through-bond. Through-space interaction was used by Isuoka et al. for the preparation of organic magnetic materials. In this case the pi-stacking of cyclophanes aromatic rings was used for coupling electronic spins by ferromagnetic exchange interaction.6 However, electronic spins can be more easily and more efficiently coupled through-bond by connecting them via meta-phenyl coupling units 7–10. Thus, the most effective way to design cyclophane-containing materials for molecular magnetism is to include spin bearing units (e.g. free radicals) and spin coupling units (e.g. meta-phenyl-type moieties) into cyclophane structures. Coupling such cyclophanes by spin coupling units can lead to a very efficient design of organic magnetic materials due to multiple coupling paths in cyclic oligomers. The most impressive example to date is a 2D polymer designed by Rajca et al. 11,12 as a network of cyclophanes containing triphenylmethyl-like radicals with magnetic properties equivalent to S=5000. However, the triphenylmethyl-like spin bearing moieties are stable only at low temperature. It
would be highly desirable to use more stable spin bearing units for the design of cyclophane-containing high-spin materials. Promising candidates as spin sources are oligoarylamines radical cations since they are generally stable up to room temperature and they can be efficiently inserted into macrocyclic oligoarylamine cyclophanes as recently reviewed by Ito13. Up to now most of research efforts focused on two kinds of macrocycles of oligoarylamine cyclophanes: derivatives of aza[14]-meta-cyclophane and derivatives of aza-[1n]-m,pcyclophane. Derivatives of aza-[14]-meta-cyclophane are analogues of cyclophanes containing triphenylmethyl-like radicals in the high spin materials mentioned above, with compact spin bearing units connected by the most efficient meta-phenyl coupling units. However, in aza[14]-meta-cyclophanes the electrostatic repulsion between adjacent holes prevents the oxidation of all arylamine sites to radical cations, so the expected highspin state was not obtained14,15. An efficient way to overcome this problem is to increase the delocalization of the spin bearing unit, for instance by using para-phenylene diamine units instead of amine sites. Thus, the derivative of aza[14]-metacyclophane containing para-phenylenediamine with anisyl substituents was effectively oxidized and showed quintet state (S=2) under anaerobic conditions.16. High-spin states were also observed for oxidized double and triple-decker structures derived from aza-[14]-meta-cyclophane.17,18 However, no example of polymeric high spin system based on aza-[14]meta-cyclophane has been reported to date. Derivatives of aza-[1n]-m,p-cyclophane include whole paraphenylene diamine units within the main body of the cyclophane cyclic structure linked via meta-phenylenes, which makes their oxidation easier19–22 and leads quantitatively to a high spin state23. Remarkably, a high-spin state could be obtained for the compound composed of two tetraaza-[14]m,p,m,p-cyclophane linked by the p-m-p-oligoaniline bridge24.
ACS Paragon Plus Environment
The Journal of Physical Chemistry
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Although tetraaza[14]m-p-cyclophane units were involved in different and complex molecular structures, as for example belt-shaped macrocycle25, no high spin polymer based on this moiety was reported to date. Bu
Bu
N
N
N
Bu
N
N
N
N
t-Bu
Bu
t-Bu
*
N
N
N
N
C
Bu
The polymer PC has a rigid structure related to the presence of stiff cyclophane moiety. Realizing these limitations, we tried to estimate the molecular mass by the use of size exclusion chromatography in dichloromethane with polystyrene as a standard. The polymer exhibited weight-averaged molecular weight of 62.9 kDa and number-averaged molecular weight of 18.3 kDa, Mw/Mn =3.44. However, these results comprised an important error due to the stiffness of polymer chain and can be considered only as the estimation of the molecular mass.
N n
Bu
Page 2 of 7
Bu
*
Bu
Bu
PC
Br
CHART 1. The chemical structures of hexaazacyclophane C and the polymer PC In order to increase the stability of amminium radical cations our groups designed and synthesized high spin compounds (linear and cyclic dimers and the linear polymer) as well as 2D-networked polymers based on meta-para-para triarylamine units. Special attention should be paid to cyclic dimer, namely hexaaza-[16]-m,p,p-cyclophane C (see Chart 1)23,26 which can be easily and quantitatively oxidized up to the diradical dication at room temperature, at lower redox potential and with higher stability compared to that of tetraaza-[14]m,p,-cyclophane23. Moreover, diradical dication of this compound exhibited a triplet ground state with an efficient exchange coupling constant (J/k = 57 K).26 These very favorable features of hexaaza-[16]-m,p,p-cyclophane diradical dication prompted us to design a linear polymer (PC), in which cyclophane moieties are coupled by meta-terphenyl bridges. Thus, the ferromagnetic spins interaction via meta-phenyl in cyclic structures and via meta-terphenyl along the polymer chain is expected. The formation of radical cations was studied by cyclic voltammetry and UV-Vis spectroscopy. The spins interaction was investigated using pulsed-EPR and SQUID magnetometry techniques.
Results The polymer PC was prepared using palladium-catalyzed Buchwald-Hartwig reaction (Scheme 1). Thus, diphenylamine was brominated with NBS in standard conditions and a secondary amine group was protected with BOC to afford dibromo derivative 1 with 95% yield. Compound 1 was reacted with 4-butylaniline in the presence of Pd(OAc)2/BINAP catalyst to give aryltriamine 2 with 85% yield. The key step was the preparation of cyclophane. Amine 2 and 1,3dibromobenzene was diluted to 2 · 10-2 M in toluene and coupled in the presence of Pd2dba3/t-Bu3P catalyst. The yield of macrocycle formation was equal to 45.5%. Then cyclophane 3 containing BOC-protected amine groups was converted to cyclophane 4 with two NH active sites. The cross-coupling reaction of cyclophane 4 with 4,4”-dibromo-meta-terphenyl (see the preparation of this compound in Supporting Information) afforded the polymer PC.
NH
H N
a, b
N
c
boc
N
1
Br
d e
boc
2
H N
Bu Bu
Bu
Bu Bu
N R
R N
N
N
N
N
N
f *
N
N
N
N
N
n
Bu
Bu
3
R=Boc
4
R=H
*
Bu
Bu
PC
Scheme 1. The synthesis of the polymer PC. a) NBS, DMF, 0 ºC, b) BOC, DMAP, THF, c) 4-butylaniline, Pd(OAc)2/BINAP, t-BuONa, toluene, 110 ºC, d) 1,3dibromobenzene, Pd2(dba)3/t-Bu3P, t-BuONa, toluene, 110 ºC, e) TFA, toluene, f) 4,4”-dibromo-terphenyl, Pd(OAc)2/t-Bu3P, t-BuONa, toluene, 90 ºC. The generation of unpaired spins in PC can be performed by electrochemical or chemical partial oxidation to radical cations. The electrochemical properties of the polymer PC were studied by cyclic voltammetry in dichloromethane with 0.1 M Bu4NBF4 as a supporting electrolyte. The cyclic voltammogram of this polymer (Fig. 1b) can be compared to that of hexaazacyclophane C (Fig. 1a) and linear polymer PA2 (see chart 3) containing similar sequence of m-p-p-oligoaniline units (see Table 1). The voltammograms of all three compounds were very similar and showed two pairs of reversible oxidation waves at ca. 0.05 - 0.25 V and ca. 0.5 - 0.60 V ranges (vs. Ag/Ag+). It should be noticed that the value of the first oxidation potential depended on the type of the studied compound and was the smallest in the voltammogram of hexaazacyclophane (0.063 V). The first oxidation peak of PC appeared at 0.16 V indicating the electron withdrawing effect of meta-terphenyl bridges. This process can be attributed to the oxidation of one m-p-p-arylamine moiety, i.e. to the formation of one radical cation within the cyclophane structure. It was a one-electron process (E1ox - E1red = 57 mV). The second oxidation peak appeared at very similar potentials for PC and C and can be related to the oxidation of a second arylamine conjugated segment of cyclophane. The oxidation potential was only slightly higher than that registered for linear polymer PA2.27 This peak also corresponds also to one-electron pro-
ACS Paragon Plus Environment
Page 3 of 7
cess (E2ox - E2red = 57 mV). The potential values difference between the first and the second oxidation processes (E2 - E1 = 110 mV) confirmed that the creation of radical cations took place on two different aryltriamine moieties separated via mphenylene coupler. The second pair of the oxidation peaks appeared at 0.50 and 0.58 V for PC and can be attributed to the subsequent oxidation to radical cations and the formation of spinless imine dications in m-p-p-oligoaniline segments.
1.0
a)
of PC c=1.2 · 10–4 M); the Ox/PC molar ratio: : a) 0, b) 0.5, c) 1, d) 2, e) 3, f) 4. Table 1. Oxidation potentials of model compound C and polymers PC and PA2*.
E1ox[V]
E2ox[V]
E3ox[V]
E4ox[V]
C
0.063
0.26
0.49
0.56
PC
0.16
0.27
0.50
0.58
PA2
0.09
0.23
0.50
0.55
oxidation reduction
*
values of potentials versus Ag/AgNO3 in acetonitrile as a reference electrode
I / µA
0.5 0.0 -0.5 -1.0
-0.2
0.0
0.2
0.4
0.6
0.8
+
U / V vs. Ag/Ag
0.4 0.3
b)
oxidation reduction
0.2
I / µA
0.1 0.0 -0.1 -0.2 -0.3 -0.4 -0.2
0.0
0.2
0.4
0.6
0.8
+
U / V vs. Ag/Ag
1.0
c) Absrobance / a.u.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
The Journal of Physical Chemistry
f
e d
0.5 c
b
a
0.0 500
1000
1500
Wavelength / nm
Figure 1. Differential pulse voltammograms of C (a), PC (b) in CH2Cl2 solution (the concentration of the compounds was c = 10–3 M) containing an electrolyte – 0.1 M Bu4NBF4, (reference electrode – Ag/0.1 M AgNO3 in acetonitrile, scan rate – 100 mV/s). The UV-Vis-NIR spectra of PC (c) oxidized with TBA⋅SbCl6 in CH2Cl2/CH3CN solution (the concentration
It can be emphasized that all oxidation processes were reversible. Moreover, except for the first oxidation, the subsequent oxidation processes appeared at similar potential values for all studied compounds and were governed mainly by the structure of conjugated oligoamine segments. The polymer PC can also be chemically oxidized to radical cations. The oxidation process was monitored using UV-VisNIR spectroscopy. The corresponding spectra are presented in Fig. 1c. The spectrum of pristine polymer showed one band at 347 nm (ε=9.7 · 104 M-1cm-1). Upon oxidation new bands appeared which were related to the formation of radical cations in cyclophane moieties of the polymers. The spectra of oxidized PC were similar to that registered for oxidized hexaazacyclophane C.23 The spectrum of PC oxidized to one radical cation per mer showed bands located at 435, ca. 790 and 1254 nm. Upon further oxidation the intensities of the bands increased significantly and the NIR band was shifted hypsochromically to 1160 nm. The spectra of PC oxidized with three then with four equivalents of the oxidant gradually changed and a new band appeared in the Vis region. The spectrum of PC oxidized four times showed the band at 813 nm which can be related to the formation of imine dications and the band at 1076 nm corresponding to residual radical cationic form. Chemically oxidized samples of PC were investigated by pulsed EPR nutation spectroscopy in order to detect what spin states are present for different oxidation states. The spectra obtained for stoichiometric ratios [Ox]/[PC]=1.0 and 2.0 are plotted in Figure 2 and compared to the spectrum of a reference S=1/2 sample. For the ratio [Ox]/[PC]=1.0, the most intense signals are obtained close to fnut=√2.f° (6.5 MHz), corresponding to S=1, and close to fnut=2.f° (9.3 MHz), which can belong to either S=3/2 or S=2 spin states. The signal is also intense at fnut=√3.f° (8.0 MHz), which corresponds to S=3/2, and the signal at fnut=f°, corresponding to S=1/2 spin states is weak. So for the ratio [Ox]/[PC]=1.0, most of the sample exhibits coupled spin states S=1, S=3/2 and S=2. At this stoichiometry, each mer bears, statistically, one radical cation, so this result suggests that the meta-terphenyl units efficiently couple spins born by neighboring mers. For the ratio [Ox]/[PC]=2.0, the spectrum only exhibits two components: one very intense close to fnut=2.f° and a weaker one close to fnut=√6.f° (11.3 MHz). Such spectral features are typical of S=2 spin states27,28 and demonstrate that S=2 states are dominant for this stoichiometry. Higher or lower oxidation stoichiometric ratios led to lower spin states (data not shown).
ACS Paragon Plus Environment
The Journal of Physical Chemistry
rectly connected to the meta-terphenyl spin coupling units (spin noted S2 and S3). If one assumes that the exchange coupling constant is the same in both cyclophanes, it can be modeled by the Heisenberg Hamiltonian H=-(J.S1.S2 + j J.S2.S3 + J.S3.S4) leading to the equation written in chart 2. By fitting experimental data with this expression, the exchange coupling constants within the S=2 state derived from PC were estimated to be J/k= 89+/-39 K for the exchange coupling constant between two spins in a cyclophane and j/k= 17+/-6 K for the exchange coupling constant due to the metaterphenyl spin coupling unit, with a temperature θ=0.24 K modelling small ferromagnetic interactions between S=2 spins and a doping efficiency of 69 %. The analysis was also performed by setting the value of J at the value (J/k= 57 K) we previously reported for cyclophanes similar to those included in PC.26 Such an analysis led to a smaller j/k= 12+/-3 K value, a slightly higher doping efficiency (73 %) and, quite expectedly, a less good agreement with experimental data (data not shown).
0.0020
0.0015
M (emu)
Figure 2. 2D field swept pulsed-EPR nutation spectra of PC, and of a reference S=1/2 sample. PC was oxidized with TBA⋅SbCl6 in CH2Cl2/CH3CN solution, with [PC] =5.10–3 M and [Ox]/[PC]=1.0 (middle frame) or [Ox]/[PC]=2.0 (lower frame). The reference sample for S=1/2 is provided by a linear polyarylamine (PA2, see ref26 at low oxidation level ([PA2] =10–2 M and [Ox]/[PA2]=0.5) and provided f° = 4.6 MHz as nutation frequency for S=1/2. All experiments were performed at T=10 K (see ESI for experimental details). The sample with the highest (S=2) spin state at [Ox]/[PC]=2.0 stoichiometry was investigated by SQUID magnetometry. The experimental M=f(H) curve recorded at T=2 K is shown in Figure 3 (upper frame). It was very well fitted by a S=2 Brillouin function including a T-θ term corresponding to small ferromagnetic interactions (θ = 0.12 K) between each S=2 spin with its first neighbors. From this it appears that 70 % of PC have been doped up to radical cations and yield mainly S=2 spin states. Similar limited oxidation efficiencies were also observed by our groups in the study of a linear polyarylamine with all spin bearing units in the main polymer chain26 and of a linear polyarylamine with spin bearing units both in the main chain and in lateral chains.27 The variations of magnetic susceptibility with temperature were recorded at low magnetic field (H=0.05 T) and produced the χT=f(T) curves shown in Figure 3 (lower frame). This curve was modeled by equation (1) derived from the Van Vleck formula. This expression was derived by modeling the S=2 state as a linear tetramer of S=1/2 spins located on two cyclophanes connected by one meta-terphenyl bridge. It is assumed that in each tetramer, two S=1/2 electronic spins are localized in both extremities of the cyclophanes (spins noted S1 and S4) and two in the other halves of the cyclophanes di-
PC at T=2K S=2 with θ = 0.12 K
0.0010
0.0005
0.0000 0
1
2
3
4
3.5
PC at 0.05 T J/k= 89 K, j/k=17 K , θ=0.24K
3.0
2.5
2.0
1.5
1.0 0
20
40
60
80
100
120
140
160
T(K)
Figure 3. Magnetization measurements of a sample obtained by chemical oxidation of PC. Molar ratio [Ox]/[PC]=2 and [PC]=5.10–3 M in CH2Cl2/CH3CN solution. Upper frame: M=f(H). Chart 2. Equation derived from Van Vleck formula and
2 2 J + j + J 2 + j2 −J + exp − J + j − J + j 10 + 2 exp + exp − kT 2 kT 2 kT T .Ng β χT = . 2 2 2 2 2 (T − θ ).k J + j+ J + j J + j− J + j 4 J + j 2 − 2 jJ − 2J − j −J + exp 5 + 3 exp + exp − + exp − exp − kT 2 kT 2 kT 2 kT 2 kT 2
5
H(T)
χT(emu.G-1.mol-1)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 4 of 7
2
ACS Paragon Plus Environment
2 2 + exp 4 J + j − 2 jJ 2 kT
Page 5 of 7
The Journal of Physical Chemistry
used for fitting MT = f(T) curves.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Discussion From the magnetic studies reported above it appears that in PC, the J coupling constant between electronic spins within a cyclophane unit (J/k= 89 K) is of the same order of magnitude if not even higher than in pure cyclophane C (J/k= 57 K). Moreover the coupling constant between two spins connected by a meta-terphenyl unit (j/k= 17 K) is still good, especially when compared to the value (j/k= 42 K) reported for the coupling of much less delocalized arylamine radical cations by the same meta-terphenyl spin coupling unit.29 It is also interesting to compare PC with other high-spin polymers PA2 and PN based on similar m-p-p oligoaniline units we reported previously 26,27,30 (see Chart 3).
Chart 3. Chemical structures of previously reported PA2 and PN polymers The linear polymer PA2 oxidized up to one radical cation per m-p-p oligoaniline unit exhibited a pure S=1 high spin state with a J/k = 18 K ferromagnetic coupling constant between adjacent mers. The polymer PN oxidized up to two radical cations per mer exhibited mainly S=2 high spin state with less abundant S= 3/2 state (the J constant could not be measured due to such a mix of S=3/2 and S=2 states). PC represents therefore a significant improvement compared with these two polymers. PC exhibits higher S multiplicity (S=2) than PA2 (S=1) without any significant loss of J coupling constant between two mers (17 K vs 18 K respectively). When compared with PN, pure S=2 state can be obtained for PC, which was not possible with PN, and the synthesis of PC requires fewer steps than the synthesis of PN. PC at pure S=2 state was obtained with an apparent 69 % doping efficiency. This apparent doping efficiency for PC is similar to those obtained for PA2 and PN (66% and 74%, resp.), while cyclophane C can be oxidized up to 76% doping efficiency26. One can thus suspect that similar causes prevent either the complete chemical doping of meta-para-para-aniline units or the ferromagnetic coupling of more than 2 adjacent mers in all three PC, PA2 and PN polymers. The reasons for such a behavior is not yet firmly identified. The electrochemical doping experiments reported here show that C and PC can be easily doped up to one hole per metapara-para-aniline unit (second oxidation wave observed at E = 0.26 V/(AgNO3/Ag) for C and E = 0.27 V/(AgNO3/Ag) for ). The intensity of the second oxidation wave in cyclic voltammetry experiments is similar for both C and PC, and further
3rd and 4th oxidation are clearly observed. For these reasons we think that C and PC could actually be doped with efficiency close to 100% in chemical doping experiments and we propose that the apparently limited doping efficiency measured by SQUID magnetometry for C and PC (resp. 76% and 69%) is only partially (typically few percents) due to incomplete chemical doping. The main contribution to the apparent limited doping efficiency and the limited spin multiplicity would be rather due to variations in J coupling constant values due to the large range of conformations accessible to the cyclophane macrocycles and to the meta-terphenyl units. It should be underlined here that spin coupling units such as meta-phenylene and metaterphenyl induce ferromagnetic coupling only when the free radicals and the coupling unit are (enough) coplanar. When free radicals are strongly twisted out of the plane of the phenyl ring(s) of the spin coupling units it can result into an antiferromagnetic (J
