Highly Efficient Tuning of Ferromagnetic Spin Interactions in High-Spin Arylamine Structures by Incorporation of Spin Bearing Carbazole Units

Article abstract of DOI:10.1021/acs.jpcb.8b07496

Arylamine moieties oxidized to radical cations are considered promising spin bearing units in high-spin-type compounds. Here, we report the first use of carbazole-3,6-diamine units as efficient, rigid spin containing units. The use of rigid spin bearing u

Full text of DOI:10.1021/acs.jpcb.8b07496

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B: Fluid Interfaces, Colloids, Polymers, Soft Matter, Surfactants, and Glassy Materials
Highly Efficient Tuning of Ferromagnetic Spin Interactions in High-Spin
Arylamine Structures by Incorporation of Spin Bearing Carbazole Units
Lukasz Skorka, Vincent Maurel, Jacek B. Gosk, Roman
Puzniak, Jean-Marie Mouesca, and Irena Kulszewicz-Bajer
J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.8b07496 • Publication Date (Web): 19 Sep 2018
Downloaded from http://pubs.acs.org on September 22, 2018
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Highly Efficient Tuning of Ferromagnetic Spin
Interactions in HighꢀSpin Arylamine Structures by
Incorporation of Spin Bearing Carbazole Units
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Łukasz Skórka,† Vincent Maurel,‡ Jacek B. Gosk,§ Roman Puźniak,∥ JeanꢀMarie Mouesca,*,‡ and
Irena KulszewiczꢀBajer*,†
† Faculty of Chemistry, Warsaw University of Technology, Noakowskiego 3, 00ꢀ664 Warsaw,
Poland.
‡ Univ. Grenoble Alpes, CEA, CNRS, INAC, SyMMES, 38000 Grenoble, France
§ Faculty of Physics, Warsaw University of Technology, Koszykowa 75, 00ꢀ662 Warsaw,
Poland.
∥ Institute of Physics, Polish Academy of Sciences, Lotników 32/46 02ꢀ668 Warsaw, Poland
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ABSTRACT
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Arylamine moieties oxidized to radical cations are considered promising spin bearing units in
highꢀspin type compounds. Here, we report the first use of carbazoleꢀ3,6ꢀdiamine units as
efficient rigid spin containing units. The use of rigid spin bearing units enhances significantly
spin exchange interactions. The design using DFT calculations shows the progressive increase of
the exchange coupling constant dependent on the considered model molecules. Two of the most
representative molecules containing flexible (dimer 1) and rigid spin coupling unit (dimer 2)
were synthesized. Electrochemical and pulsedꢀEPR nutation studies showed that both dimers can
be oxidized to yield a majority of diꢀcationic diꢀradicals exhibiting S = 1 ground states. The high
values of the dimer 2 exchange coupling constant obtained both computationally (J/kB = 145 K;
HHeis. = ꢀJ S1.S2) and experimentally (J/kB = 90ꢀ100 K) indicates the beneficial role of the
carbazole moiety incorporated into spin bearing units.
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INTRODUCTION
An extensive development in the field of organic electronics, mainly associated with OLED,
OFET and OPV technologies1ꢀ3 has directed the attention to the search of organic magnetic
6,7
materials which can be applied in molecular electronics,4,5 spintronics
or in biomedicine as
MRI contrasts.8ꢀ10 The key feature of organic magnetic materials is the formation of highꢀspin
states, which was originally carried out based on a polaronic model.11 At present, the research in
the field of highꢀspin molecules is focused on reaching high spin states governed by coextensive
spin densities12 spread over the entire molecule and tuning the magnetic exchange coupling
constant J value to achieve as strong spin – spin interactions as possible.13
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The number of high spin organic molecules is gradually increasing including various πꢀ
conjugated systems e.g. linear oligomers,14ꢀ20 cyclophanes 21ꢀ30 and polymers.13,30ꢀ33 Within these
structures, the total spin and exchange coupling constant enlargement is accomplished by a
progressive extension of the πꢀconjugation with simultaneous rigidification of the molecular
backbone.13,14,16 Since the exchange coupling constant value is extremely sensitive to the
changes in local conformations Rajca and coꢀworkers proposed to use ringꢀfused polycyclic
structures14 including spin bearing units connected through mꢀphenylene to form a planar crossꢀ
conjugated π system.16,19 Reaching even higher values of J was possible by moving from various
aminyl radicals14,16 to azaꢀmꢀxylylene derivatives,34ꢀ37 where extended πꢀconjugation was
diminished, and radical stability was provided by bulk substituents in close proximity to the
radical. The essential increase of the J value provided by rigidification of the molecular
backbone around the mꢀphenylene moiety was also observed by our group in the case of
arylamineꢀbased compounds (both dimer and polymer) oxidized to radical cations.13
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In terms of structure rigidification, one of the unexploited solution to the polaronic model
mentioned above consists in providing a new type of reduced flexibility, namely that of the
conjugated spin bearing moiety, for example by using planar carbazole type units. Recent study
revealed that, under certain conditions, radicals localized on carbazole nitrogen may present an
enhanced stability.38 However, the incorporation of carbazole into the 5,7,12,14ꢀ
tetrahydroquinolino[3,2ꢀb]acridine moiety, which served as the background for both radical16
and radical cationꢀbased13 highꢀspin systems must be carried out with caution as too extensive
delocalization over the spin coupling unit, the nature of the connecting groups or even the
addition of peripheral electroꢀdonor groups may strongly modulate (sometimes unfavorably) the
resulting ferromagnetic spin interactions (see Ref.16). In this study we present how structure
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rigidification in the form of carbazoleꢀ3,6ꢀdiamine moiety readily enhances ferromagnetic spin
interactions and associated exchange coupling constant values (dimer 1, Scheme 1). We then
introduced additional molecular stiffening by incorporating a rigid coupling unit in between rigid
spin bearing units (dimer 2, Scheme 1). Before presenting experimental data pertaining to these
two dimers, we present the relevant DFT calculations which led us to them.
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Scheme 1. Chemical structures of the studied compounds: dimer 1 and dimer 2 and the model
compounds D2 and QA (for comparison).
EXPERIMENTAL SECTION
For any experimental details on the supply of chemicals, synthesis, pulsed EPR and SQUID
measurements as well as DFT calculations see Supporting Information
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RESULTS AND DISCUSSION
DFT calculations
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The rigidification of arylamine structures may have a crucial impact on their magnetic
properties. To understand beforehand structural and electronic contributions to spin ꢀ spin
interactions we performed DFT calculations for various dimer 1 and dimer 2 models. In Table 1
we first show plots of the magnetic orbitals for the experimental dimer 1 for which we computed
J/kB = 44 K. We also show the corresponding magnetic orbital for the dimer 2An,C<Me modelling
the experimental dimer 2 (J/kB = 204 K)1. Because of this last high J magnitude, special attention
has been paid to dimer 2 model compounds: Table S1 shows magnetic orbitals and exchange
coupling constants J for gradually more complex dimer 2 models.
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Table 1. Plots of the DFTꢀBS computed (leftꢀend dimer sides) magnetic orbitals for model dimer
1 (left) and model dimer 2An,C<Me, (right) closest to the experimental dimer 1 and dimer 2,
respectively. The equivalent magnetic orbitals for the rightꢀend dimer sides can be deduced by
symmetry for these symmetrical dimers. Isodensity value set to 0.03 a.u.
dimer 1
dimer 2An,C<Mec
1
Several compounds derived from dimers 1 and 2 that were studied by DFT calculations (see
Table S2). They are named “dimer nSub1,Sub2”. Here, n (=1 or 2) stands for the parent dimer
structure. Sub1 stands for the substituent of the N atoms and can be: H atom (“H”), Phenyl group
(“Ph”), or Anisyl group (“An”). Sub2 stands for the group that possibly connects the central
metaꢀphenyl group with adjacent aromatic rings: no connecting group (no subscript), CH2
connecting group (“>CH”), CMe2 connecting group (“>CMe”).
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The striking difference between J values computed for both model dimers lies in the extension
of the spin density at the level of the spin couplers. Magnetic orbitals must be mutually
orthogonal to cancel the antiferromagnetic contribution to J but must still occupy the same
region of space (i.e. at the coupler) to increase the ferromagnetic contribution.
To better understand what contributes to such a high J/kB magnitude in the case of dimer
2An,C<Me (204 K) compared to both dimer 1 (44 K) but also to the previously published QA dimer
(J/kB = 42 K), we planned on treating the different effects separately (rigidification of the
coupler and/or of the spin bearer and increasing complexity of the peripheral ligands (see Table
S2). We therefore computed J coupling constants for equivalent dimer 1 models and discuss at
length in Supporting Information all J constants in Table S2 (see also FigureS8; no magnetic
orbital plots are shown for dimer 1 models beside the one in Table 1). The lessons learned from
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Table S2 are the following: i) rigidifying the spin coupler optimizes the ferromagnetic
contribution to the exchange coupling constant by enforcing the bearerꢀcouplerꢀbearer unit to
behave as strict alternant hydrocarbons; ii) however, electronꢀdonor groups pouring out electron
density toward the coupler have (it would seem) to be avoided, as this increases the
antiferromagnetic contribution to J expected to be proportional to the square of the residual
overlap of the magnetic orbitals (represented in Table 1); iii) however, the same alternant
hydrocarbon network rigidifying the spin bearers also modulates the electron density present at
the coupler, i.e. enforcing planarity upon the spin bearers withdraws some of the electronic
density provided by electroꢀdonor groups away from the coupler (contra point ii above). Further
DFT calculations would be necessary to investigate this delicate balance between the extent of a
dimer’s planarity on the one hand and the presence of electroꢀdonor groups at various positions
on the other hand (out of the scope of the present paper).
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We now turn to the experimental results.
Synthesis
To verify the theoretical conclusions drawn from the DFT calculations presented above, we
synthesized two dimers containing carbazole moiety as the spin bearer (see Scheme 1).
All studied compounds containing carbazoleꢀ3,6ꢀdiamine moiety have been prepared using
catalyzed CꢀN coupling reactions (Scheme 2 and 3). In the first step 9ꢀ(4’ꢀ
methoxyphenyl)carbazole, 3 was obtained by a standard method in the reaction of carbazole and
4ꢀiodoanisole in the presence of CuI/(±)ꢀtransꢀ1,2ꢀdiaminocyclohexane with 81% yield after the
recrystallization. The carbazole 3 was brominated with NBS which led to dibromo derivative 4.
Then the compound 4 was reacted with 4ꢀanisidine to give monoamino derivative 5 using
Pd2dba3/BINAP catalyst (34% yield), then the coupling with 1,3ꢀdiiodobenzene in the presence
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of Pd(dba)2/tꢀBu3P, led to the derivarive 6 containing two active bromine sites. It was converted
into the dimer 1 in the reaction with bis(4ꢀmethoxyphenyl)amine with 49% yield. It should be
underlined that other routes to prepare the dimer 1 were also tested but the overall yield was
lower than that reached in the method presented in Scheme 2.
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Scheme 2. The synthesis of the dimer 1: (i) 4ꢀiodoanisole, CuI, (±)ꢀtransꢀ1,2ꢀ
diaminocyclohexane, K3PO4, anhydrous 1,4ꢀdioxane, 110 °C, Ar, 24 h (ii) NBS, anhydrous THF,
anhydrous DMF, 0 °C, Ar, 1.5 h (iii) 4ꢀmethoxyaniline, Pd2(dba)3, BINAP, tꢀBuONa, anhydrous
1,4ꢀdioxane, 110 °C, Ar, 24 h (iv) 1,3ꢀdiiodobenzene, Pd(dba)2, P(tꢀBu)3, tꢀBuONa, anhydrous
toluene, 90 °C, Ar, 24 h (v) bis(4ꢀmethoxyphenyl)amine, Pd(dba)2, P(tꢀBu)3, tꢀBuONa,
anhydrous toluene, 110 °C, Ar, 24 h.
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Dimer 2 was synthesized in an 8ꢀsteps synthetic route. First, the intermediate compound,
namely N3,N3,9ꢀtris(4ꢀmethoxyphenyl)ꢀ9Hꢀcarbazoleꢀ3,6ꢀdiamine was prepared (see Supporting
Information, Scheme S1). The synthetic route consists of subsequent nitration, selective
reduction of nitro groups and then Cu catalyzed substitution and Pd catalyzed coupling
amination. Finally, reduction with hydrazine and palladium on activated charcoal gave
unsymmetrical carbazole derivative 11, which was used in the following synthesis of final dimer
2, presented in Scheme 3. The reaction between 11 and 1,5ꢀdibromoꢀ2,4ꢀdi(propꢀ1ꢀenꢀ2ꢀ
yl)benzene, catalyzed by Pd(dba)2 combined with bulky ligand (QPhos) allowed the formation of
12 with a very high yield (90%). Then, acidic cyclization with AcOH/H3PO4 yielded amine 13
(77%), where 5,8,10,13,18,20ꢀhexahydroindolo[3,2ꢀa]indolo[3',2':5,6]qinolino[2,3ꢀi]acridine
core was formed. The target dimer 2 was obtained via substitution of 13 at N8 and N10 positions
with onꢀsite prepared 1ꢀiodoꢀ4ꢀ((2ꢀoctyldodecyl)oxy)benzene (77% yield).
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Scheme 3. Synthesis of dimer 2: (i) [Pd2(dba)3], QPhos, tꢀBuONa, anhydrous toluene, 110 °C,
Ar, 24 h (ii) H3PO4, AcOH, 100 °C, Ar, 24 h (iii) 1ꢀiodoꢀ4ꢀ((2ꢀoctyldodecyl)oxy)benzene,
[Pd2(dba)3], P(tꢀBu)3, tꢀBuONa, 1,4ꢀdioxane, 110 °C, Ar, 24 h.
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Spins generation
Arylamine compounds are efficient spins bearers due to their easy oxidation to radical cations.
Arylamine compounds can be oxidized by electrochemical or chemical means. The
electrochemical properties of the dimers 1 and 2 were studied by cyclic voltammetry in
dichloromethane (1) and THF (2) with 0.1 M Bu4NBF4 as a supporting electrolyte.
The cyclic voltammograms (CV) (Supporting Information) and differential pulse
voltammograms (DPV) (Figure 1) of both dimers were very similar and showed reversible
waves corresponding to the successive oxidation of amine groups.
Considering the chemical structures of the dimers 1 and 2 one can suppose that the
electrochemical properties of both compounds should be comparable to those of previously
studied arylamine dimers such as D2 (flexible one corresponding to dimer 1) and QA (rigid one
corresponding to dimer 2).13 However, the incorporation of planar carbazole moieties into the
structures of the compounds studied presently changed the electrochemical behavior of both
dimers. In the initial stage of the oxidation process, DPV of the dimer 1 showed only one peak at
0.05 V (vs. Fc/Fc+) (Figure 1).
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1,5
1,0
A
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0,5
oxidation
reduction
0,0
-0,5
-1,0
-1,5
-0,2
0,0
0,2
0,4
0,6
E / V vs. Fc/Fc+
B
0,5
oxidation
reduction
0,0
-0,5
-0,2
0,0
0,2
0,4
0,6
E / V vs. Fc/Fc+
Figure 1. Differential pulse voltammograms obtained for the dimers 1 (A) and 2 (B) in CH2Cl2
and THF solutions, respectively (the concentration of the compounds was c = 1ꢁ10ꢀ3 M)
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containing an electrolyte – 0.1 M Bu4NBF4, (modulation time 50 ms, modulation amplitude 10
mV, step potential 5 mV).
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The peak was large (ꢂE1 = 107 mV), indicating that multiꢀelectron oxidation process took place
and can be attributed to the oxidation of two lateral amine groups into radical cations. By
contrast, the DPV of dimer 2 showed two oxidation peaks at 0.06 V (E1.1) and 0.137 V (E1.2) (vs.
Fc/Fc+). These two peaks can be related to the formation of one radical cation in each conjugated
segments and located mainly on the lateral amine sites. The difference in the electrochemical
behavior between dimers 1 and 2 as well as with dimer QA reported previously can be due to
their chemical structures. In the case of the dicationic dimer 2, the magnetic orbitals computed
by DFT show that the hole is born by each lateral diamine groups (corresponding to a Mülliken
charge of about 0.4|e|), leaving about 0.6 for each coupler’s sides. By contrast, in the case of the
dicationic dimer 1, each hole is more lateralized. This can explain the increase of the electrostatic
repulsion between adjacent holes in dimer 2 and, therefore, the oxidation potential for the
formation of a second radical cation shifted by 77 mV. The further oxidations differ from those
observed previously for D2 and QA.12 Thus, the DPV of dimer 1 showed two well resolved
peaks at 0.29 and 0.46 V (vs. Fc/Fc+) which can be attributed to the formation of a second radical
cation in each conjugated segments. The resonant nature of conjugated units causes the increase
of the oxidation potential. Thus, the separation between the first and the second oxidation peaks
equal to 0.24 V confirms the location of radical cations within the same unit. Then the separation
between the second and the third peaks in DPV of dimer 1 was equal to 0.17 V indicating the
generation of radical cations in two adjacent units linked via the mꢀphenylene coupler.
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On the other hand, it should be emphasized that the DPV of dimer 2 showed similar character
but differs significantly for the intermediate oxidation state. The peaks corresponding to the
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subsequent oxidation appeared at 0.27 V (E2) and 0.436 V (E3) (vs. Fc/Fc+). The separation
between the second (E1.2) and the third (E2) oxidation waves was of only 133 mV and both peaks
overlapped at a potential value around 0.2 V. At last, the separation between the first and the
third peaks was equal to 0.21 V suggesting the formation of a second radical cation within the
same conjugated unit.
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PulsedꢀEPR nutation – Spin states
Samples of dimers 1 and 2 oxidized with 2 equivalents of oxidant per dimer were studied by
pulsedꢀEPR nutation spectroscopy. In the nutation spectra shown in Figure 2, one can see that
for both dimers, the most intense signals are due to S = 1 states, consistent with the
ferromagnetic coupling of spins born by both conjugated units in a given dimer. The rather
diluted sample of dimer 1 (Figure 2, upper frame) exhibited also weaker signals due to S = 1/2
as well as to spin states higher that S = 1. In a more concentrated sample of dimer 1 (Figure 2,
middle frame) the signal of S = 1 spins is still the most intense, but signals of S = 3/2 and S = 2
spin states are clearly observed and more intense than in the case for the diluted dimer 1 sample.
In the case of dimer 2 (Figure 2, lower frame), besides dominant S = 1 signal, signals from
higher S = 3/2 may be also observed, even though with much less intensity than for dimer 1.
These S = 3/2 and S = 2 signals have higher intensity relative to S = 1 signals in spectra
recorded at higher temperatures (see SI Figure S3 for dimer 1) so they correspond to weak
antiferromagnetic interactions that are tentatively attributed to intermolecular magnetic coupling
between two dimer molecules. This would be consistent with the fact that coupled S = 3/2 and S
= 2 states are better observed at higher concentration for dimer 1 samples.
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From these experiments it is clear that it is difficult to obtain a concentrated sample of dimer 1
containing a pure S = 1 spin state suitable for measuring unambiguously the intramolecular
exchange coupling constant by SQUID magnetometry. Dimer 2 seemed to be a better candidate
and was selected for the SQUID magnetometry investigations.
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Figure 2. PulsedꢀEPR nutation spectra recorded at T = 7 K for samples of oxidized dimer 1 and
2 at different concentrations. Nutation frequency of S = 1/2 varies slightly, 4.95 MHz (down), 5.5
MHz (middle) and 5.3 MHz (top) due to variation of EPR cavity tuning for the different samples.
Magnetization measurements
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The SQUID measurements of the dimer
2
were performed for the samples diluted in a
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polystyrene matrix (denoted as DS) and for concentrated samples (powder form, nonꢀ
diluted samples, NDS).
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For both types of the samples, the paramagnetic contribution shows typical Brillouinꢀtype
behavior: at low temperatures magnetization tends to saturate with magnetic field H, while at
higher temperatures it is almost linear with H and decreases with increasing temperature (see
Supporting Information, Fig. S4: magnetization measured as a function of magnetic field at
different temperatures). Figure 3 shows the magnetization M of DS samples as a function of
magnetic field divided by temperature H/T (for the NDS see Supporting Information Fig. S5).
The experimental points obtained at 2, 5 and 10 K in the M vs. H/T coordinates are located
between the curves calculated by the standard Brillouin function (SBF) for spins S = 1 (x =
87.5%, where x is expressed as the concentration of spin S) and spins S = 1/2 (x = 175%),
respectively. The three experimental curves almost mutually overlap, which suggests the absence
or the presence of very week only intermolecular interaction.
The inset in Figure 3 shows the experimental data at T = 2 K, which can be described in a
qualitative manner using the effective Brillouin function (EBF). Namely, in EBF, for
temperature T is substituted by T ꢀ θ, where θ is paramagnetic Neel/Curie temperature, treated as
an additional parameter (for the sense of θ parameter see Supporting Information). The inset in
Figure 3 shows that magnetization of DS samples measured at 2 K can be well approximated
with the EBF with S = 1, θ = ꢀ0.27 K and x = 87.7%.
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S=1, x=87.5%
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S=1/2, x=175%
10K
5K
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2K
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S=1, x=87.7%,
2K
Θ
=ꢀ0.27K
0
2
4
6
H(1
04Oe)
0
1
2
3
4
H/T(1
04Oe/K)
Figure 3 Magnetization of the dimer 2 (DS samples) versus magnetic field over temperature,
H/T at T = 2, 5, 10 K. In the inset: magnetization vs. magnetic field H at T = 2 K.
From this fit it appears that the oxidation efficiency would be ca. ≅ 87%, thus the highest
efficiency observed to date for highꢀspin arylamine compounds. However, it should be
emphasized that this estimated concentration of S = 1 states could actually be smaller than x ≅
87%, causing the θ value to be also overestimated, as a small number of molecules with only one
unpaired electron (S = 1/2) may be present. Overall the magnetization measurements reported in
Figure 3 are fully consistent with the presence of dominant triplet ground state created after
oxidation of the dimer 2 to radical cations.
To investigate further the oxidized dimer 2, magnetization was measured as a function of
temperature at H = 104 Oe. To demonstrate prevalent interactions between magnetic centers for
both type of samples the products of the magnetization and temperature MꢁT vs. temperature T at
H = 104 Oe were shown in Figure 4 (for only NDS samples see SI, Fig. S6).
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NDS
J/kB=70K
J/kB=100K
DS
θ=ꢀ0.1K x=75% plus J=0 x=18%
θ=ꢀ0K x=69% plus J=0 x=26%
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3.2
J/kB=100K
J/kB=70K
J/kB=90K
θ
=ꢀ0.3K x=69% plus J=0 x=26%
θ
=ꢀ0.5K x=75% plus J=0 x=18%
=ꢀ0.3K x=74% plus J=0 x=16%
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θ
S=1/2 x=170%
0
100
200
T(K)
Figure 4. Product of magnetization and temperature MꢁT versus temperature T at H = 104 Oe
obtained for the dimer 2 (the blue squares ꢀ the samples diluted in polystyrene matrix, DS, the
red circles nonꢀdiluted samples, NDS). The black dashed curve is calculated for nonꢀinteracting
unpaired electrons S = 1/2 and x = 170% by SBF. The simulated curves correspond to the
simplest model (mix of S = 1 and S = 1/2 states) with parameters shown in the legend. The model
curves were calculated using Eq. S3 with Heisenberg exchange ꢀJ⋅S1⋅S2.
The shape of MꢁT vs. temperature in the temperature range between 20 K and 300 K clearly
reflects intramolecular ferromagnetic interaction between spins of radical cations. These
experimental data were compared to the calculated model curves (Supporting Information,
Eq. S3 with Heisenberg exchange ꢀJ⋅S1⋅S2) based on the concept of the effective temperature
with J/kB = 70 K, 90 K and 100 K. For all the simulated curves a fraction (from 16% to 26%) of
S = 1/2 spin state was included, which is consistent with the spin states identified by pulsed EPR
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nutation. The curves computed for J/kB = 90 K and 100 K both exhibit a good agreement with
experimental data in the 40−300 K range, while J/kB = 70 K exhibits a slightly better agreement
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in lower temperature range (2−40 K). Overall these simulations confirm the presence of a large
majority of S = 1 states and provide an estimation of J/kB exchange coupling constants close to
70−100 K. This value is lower, but still close enough to the value (J/kB ≈ 145 K) predicted for
dimer 2 by DFT calculations after proper correction performed on the basis of previous works
(see Table S3 and Figure S10).
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CONCLUSIONS
To the best of our knowledge this is the first study exhibiting ferromagnetic interactions
between spins located on carbazole spin bearing moieties. Based on a series of DFT calculations
of relevant structures (see Tables S1-2 and Figure S9), dimer 2, and dimer 1 for the sake of
comparison, were selected as good candidates as DFT predicted a significant increase of the
magnitude of J from D2 (35 K) and dimer 1 (44 K) to dimer 2 (204 K) (see Tables S2ꢀ3). This
strategy using DFT predictively lead to an estimated J coupling constant as high as J/kB ≈ 145 K
(after correction) for dimer 2. Dimers 1 and 2 were successfully prepared by multistep synthesis.
Electrochemical and pulsedꢀEPR nutation studies showed that both dimers can be easily oxidized
to yield a majority of diꢀcationic diꢀradicals exhibiting S = 1 ground states. The relatively high J
value measured experimentally (J/kB = 90−100 K) for notꢀdiluted dimer 2, is close enough to the
(still higher) value predicted by DFT calculations and confirms the trend predicted by DFT.
Overall this study demonstrates quantitatively that, introducing rigid carbazole moieties
combined with restricted conformational mobility in high spin organic dimers (such as dimer 2)
significantly increases the J coupling constant by a factor 3 compared with nonꢀrigid arylamine
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analogues (D2). These results pave the way for exploring this new class of carbazoleꢀbased rigid
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structures and for designing high spin oligoꢀ and polymers with improved magnetic properties.
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ASSOCIATED CONTENT
AUTHOR INFORMATION
Corresponding Author
*E-mail: jean-marie.mouesca@cea.fr (J.-M.M.) *E-mail: ikulsz@ch.pw.edu.pl (I.K.-B.).
ACKNOWLEDGMENTS
The research was carried out in the framework of National Science Centre in Poland financial
support (NCN, Grant No. 2016/21/N/ST5/00907). The ADF calculations were partially carried
out in the Wroclaw Centre for Networking and Supercomputing, WCSS, Wroclaw, Poland.
http://www.wcss.wroc.pl, under calculational Grant No. 283.
Supporting Information Available. DFT calculations, synthesis procedures, additional
electrochemical studies, chemical oxidation procedures, and experimental details of pulsedꢀEPR
nutation experiments and magnetization measurements. This information is available free of
charge via the Internet at http://pubs.acs.org.
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