ferromagnetic spin coupling through the 3,4′-biphenyl moiety in arylamine oligomers—experimental...

Ferromagnetic Spin Coupling through the 3,4′-Biphenyl Moiety inArylamine OligomersExperimental and Computational StudyVincent Maurel,*,† Lukasz Skorka,‡ Nicolas Onofrio,† Ewa Szewczyk,‡ David Djurado,§ Lionel Dubois,†

Jean-Marie Mouesca,† and Irena Kulszewicz-Bajer*,‡

†University Grenoble Alpes, INAC, SCIB, F-38000 Grenoble and CEA, INAC, SCIB, F-38054 Grenoble, France‡Faculty of Chemistry, Warsaw University of Technology, Noakowskiego 3, 00-664 Warsaw, Poland§University Grenoble Alpes, INAC, SPrAM, F-38000 Grenoble and CEA, INAC, SPrAM, F-38054 Grenoble, France

*S Supporting Information

ABSTRACT: This report describes the study of a dimer d2+ and a linear trimer t3+ ofamminium radical cations coupled by 3,4′-biphenyl spin coupling units. The synthesis ofthe parent diamine and triamine and their optical and electrochemical propertiesobtained by UV−visible and cyclic voltammetry are presented. The chemical doping ofthe parent diamine d and triamine t was performed quantitatively to obtain samplescontaining the corresponding dimer d2+ and trimer t3+ in almost pure high-spin states asevidenced by pulsed EPR nutation spectroscopy. The J coupling constants of thecorresponding S = 1 and S = 3/2 spin states were measured (J/k = 135 K) andcompared quantitatively to DFT calculations.

■ INTRODUCTION

Thanks to their high chemical stability,1−5 amminium radicalcations are considered promising candidates for the design ofhigh-spin molecules (for general reviews on high-spin organicmaterials, see refs 6−13) with the ultimate goal of obtaining apurely organic magnetic material. However, up to now,polymers of amminium radical cations reported in the literatureexhibited no collective magnetic behavior and the best systemsreported to date showed high-spin states up to S = 5.14−16 Thefailure in obtaining polymers with higher spin states pointedout the need to further investigate the strategy used to linktogether the amminium radical cations and to couplemagnetically their spins. Consistently, several groups havedesigned well-defined oligomers of amminium radical cations.These studies allowed one to obtain high-spin linear,1,4,17−25

star-shaped,1,26−33 and cyclic compounds such as cyclo-phanes,18,20,34−39 connected or fused cyclophanes,31,40−44

double- and triple-decker,45−47 that can be used as buildingblocks for high-spin systems and to identify several factors thathelp or hinder the formation of polyradicals with ferromagneticspin coupling.19−21

The first factor is the nature of the “spin coupling unit” thatbinds amminium radical cations together. It is well-known that,for topological reasons,8,12,48,49 the spins of two free radicalsconnected by a 1,3-benzene unit, a 3,4′-biphenyl unit,20,50 or a4,4″-meta-terphenyl19 are ferromagnetically coupled. Using thisproperty, several examples of high-spin diradicals andpolyradicals were designed by connecting free radicals actingas “spin bearing units” to these “spin coupling units” (see refs

20, 22, 26, 51, and 52 for examples based on amminium radicalcations and examples reviewed in refs 7, 9, and 11 for examplesbased on other free radical moieties).The efficiency of the ferromagnetic coupling, measured by

the intensity of the magnetic exchange constant J, dependsstrongly on the chosen spin coupling unit. The general trendexpected from quantum chemistry calculations19,52−54 is thatsmaller coupling units yield higher J values, i.e., J(1,3-benzene)> J(3,4′-biphenyl) > J(4,4″-meta-terphenyl) for a given type offree radical. All three simple dimers of amminium radicalcations in the series pictured in Scheme 1 were synthesized andcharacterized as S = 1 ground state by EPR spectroscopy.However, only one J value (J/k = 42 K) could be measured forthe 4,4″-meta-terphenyl spin coupling unit.19 It would be thushighly desirable to obtain the experimental J value for a dimerof amminium radical cation coupled by a 3,4′-biphenyl moietyin order to rely (or not) on this spin coupling unit to design ahigh-spin polymer with amminium radical cations.The second factor that can prevent the formation of a high-

spin state with several amminium radical cations coupledferromagnetically is the electrostatic repulsion between thepositive charges. Several attempts aimed to prepare high-spinpolymers by binding amminium radical cations with 1,3-benzene units.55−57 The corresponding poly(m-aniline)s weresynthesized, but the oxidation was difficult and the polyradical

Received: April 30, 2014Revised: June 12, 2014Published: June 13, 2014

Article

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© 2014 American Chemical Society 7657 dx.doi.org/10.1021/jp504223v | J. Phys. Chem. B 2014, 118, 7657−7667

pubs.acs.org/JPCB

cations exhibited a magnetic behavior corresponding to a mainS = 1/2 spin state with a fraction of S = 1 state. This result wasattributed to an incomplete chemical oxidation due to theelectrostatic repulsion between the amminium radical cationmoieties. This hypothesis was confirmed by the studies of linearoligomers based on meta-aniline derivatives: all attempts toobtain the fully oxidized tri- or tetra(radical cations) failed.17,20

A smaller Coulombic electrostatic repulsion between adjacentholes of radical cations can be obtained by using larger “spincoupling units”, such as 3,4′-biphenyl. The electrochemicalproperties of a dimer of amminium radical cations coupled bythis spin coupling unit were studied in detail by Bushby et al.,21

who established that, during the oxidation of the parentdiamine, the second oxidation wave occurs at a potential only∼50 mV higher than the first one. According to these results,systems based on 3,4′-biphenyl spin coupling units andamminium radical cations are promising for the formation ofhigh-spin states in the corresponding oligomers and polymers.However, to the best of our knowledge, no study of a well-defined trimer (or a longer oligomer) of amminium radicalcations connected by 3,4′-biphenyl spin coupling units wasreported in the literature.This report describes the study of a dimer (d) and a linear

trimer (t) of amminium radical cations coupled by 3,4′-biphenyl spin coupling units (see Scheme 2). In a first section,the synthesis of the diamine and triamine parents and theiroptical and electrochemical properties obtained by UV−vis andcyclic voltammetry measurements are presented and qualita-tively rationalized with the help of preliminary DFTcalculations. A second section deals with the investigation ofthe magnetic properties by pulsed EPR and SQUID magneto-metry. The J coupling constants of the corresponding S = 1 andS = 3/2 spin states of d2+ and t3+, respectively, were measuredand compared quantitatively to J coupling constants calculatedby DFT.

■ RESULTS AND DISCUSSION

Synthesis, Electrochemical, and Optical Properties ofParent Compounds. Synthesis. The model compoundscontaining 3,4′-biphenyl coupler, namely, dimer d and trimert, were obtained using palladium catalyzed Buchwald−Hartwigand Suzuki coupling reactions. The dimer d was synthesized in

a four-step procedure (Scheme 3). The coupling reaction ofdi(4-butylphenyl)amine (1) to 1,3-dibromobenzene or to 1,4-dibromobenzene according to the conditions established byJorgensen et al.58 for monoamination gave compounds 2 and 3with yields of 73 and 63%, respectively. The bromo-derivative 3was converted to the corresponding pinacol borate derivative 4with 86% yield in a classical manner. The key step was a Suzukicoupling reaction of compounds 2 and 4 which allows one toobtain dimer d with a yield of 60%.The synthesis of trimer t is presented in Scheme 4. In this

case, the dibromo-derivative 6 was converted to diboratederivative 7 with 55.5% yield. Then, the Suzuki couplingreaction between 7 and 3-bromoaniline afforded compound 8.It should be emphasized that in the conditions used primaryamine groups were not active and compound 8 was obtainedwith a satisfactory yield of 69%. The reaction of 8 with 4-

Scheme 1. Examples of Amminium Diradical Dications Connected by 1,3-Benzene, 3,4′-Biphenyl, and 4,4″-Meta-TerphenylSpin Coupling Units Reported by Bushby et al.19,20 and Sato et al.55

Scheme 2. Chemical Structures of the Studied Dimer d andTrimer t and the Numbering of Nitrogen Atoms

The Journal of Physical Chemistry B Article

dx.doi.org/10.1021/jp504223v | J. Phys. Chem. B 2014, 118, 7657−76677658

butylbromobenzene in the presence of palladium catalyst gavetrimer t with a yield of 76%.Electrochemical Properties. The electrochemical proper-

ties of dimer d and trimer t were studied by cyclic voltammetryin dichloromethane with 0.1 M Bu4NBF4 as a supportingelectrolyte. The cyclic voltammogram of dimer d (Figure 1)showed two reversible oxidation waves at 0.52 and 0.64 V vsAg/Ag+ (0.36 and 0.48 V vs Fc/Fc+). The second oxidationpotential increased by 0.12 V from the first one which is relatedto Coulombic repulsion between holes created in the oxidation

processes. Both oxidation potential values were higher thanthose reported by Bushby et al.21 for a similar dimer butcontaining methoxy substituents. The increased values of theoxidation potentials observed for our dimer d can be related tothe lesser electro-donating effect of butyl groups with respect tomethoxy ones studied by Bushby.21

The cyclic voltammetry of trimer t (Figure 2) shows also tworeversible waves at 0.55 and 0.70 V vs Ag/Ag+ (0.40 and 0.55 Vvs Fc/Fc+) and appeared at higher potentials than thatregistered for dimer d. The observed oxidation peak current

Scheme 3. Synthesis of Dimer d: (a) 1,3-Dibromobenzene, Pd2(dba)3, BINAP, t-BuONa, Toluene, 90 °C; (b) 1,4-Dibromobenzene, Pd2(dba)3, BINAP, t-BuONa, Toluene, 110 °C; (c) BuLi, THF, −78 °C, Isopropyl Pinacol Borate; (d)Pd2(dba)3, (tolyl)3P, K3PO4, BTEAC, Dioxane, Toluene, H2O, 100 °C

Scheme 4. Synthesis of Trimer t: (a) 4-tert-Butylbromobenzene, Pd(OAc)2, t-Bu3P, t-BuONa, Toluene, 110 °C; (b) NBS, DMF;(c) BuLi, THF, −78 °C, Isopropyl Pinacol Borate; (d) 3-Bromoaniline, Pd(OAc)2, t-Bu3P, BTEAC, K3PO4, Toluene, Dioxane,H2O, 110 °C; (e) 4-Butylbromobenzene, Pd(OAc)2, t-Bu3P, t-BuONa, Dioxane, 100 °C



of the first oxidation wave was ca. 2 times as large as that for thesecond oxidation process, thus suggesting that the first (2e−)oxidation takes place on both lateral amine groups. Theoxidation of the central amine group of the trimer was moredifficult than the second oxidation of the dimer. The ΔEbetween the first and second oxidation steps was equal to 0.15V and reflects Coulombic repulsion of adjacent holes in themolecule.UV−vis−NIR Spectroscopy. The chemical oxidation of

dimer d and trimer t was followed by UV−vis−NIRspectroscopy. The absorption bands for dimer d and trimer tin their neutral (nonoxidized) state are located at 308 and 303nm, respectively. The compounds were oxidized with tris(4-bromophenyl)ammonium hexachloroantimonate (TBA·SbCl6)in dichloromethane solution. After the oxidation of dimer d andtrimer t to one radical cation per molecule, their absorptionspectra changed significantly and new bands appeared at thevis−NIR region. These new bands were located at 488 and1625 nm in the dimer’s spectrum and at 483 and ca. 2000 nmin the trimer’s spectrum (Figure 3). Both monocations d+ andt+ exhibit an intervalence charge transfer (IV-CT) band (1625and 2000 nm, respectively), making them belong to either classII (symmetry-broken/double minimum) or class III (sym-metrically delocalized/single minimum) systems of the Robin−Day classification (class I fully localized systems are excluded).

The oxidation of trimer t with 2 equiv of the oxidant (i.e., tothe intermediate oxidation state) caused the intensity increaseof the NIR band with simultaneous displacement of itsmaximum to 1902 nm. Additionally, new bands centered at570 and 680 nm appeared. Finally, the absorption spectra of theoligomers oxidized to their highest oxidation states changedsignificantly. Namely, the spectrum of dimer d2+ revealed a newvery intense band located at 701 nm with the shoulders at ca.580 and 800 nm, whereas the spectrum of trimer t3+ showedthe band centered at 697 nm with the shoulders at ca. 580 and820 nm. These new bands can be attributed to the transitionbetween orbitals centered on carbon atoms toward emptyorbitals centered on nitrogen atoms. It can also be emphasizedthat the NIR band related to the IV-CT transition almostdisappeared, confirming the oxidation of the oligomers to theirhighest oxidation states. However, the stability of radical cationsis rather limited; thus, the character of the spectra changed withtime,, indicating that d2+ and t3+ can decompose to loweroxidation states.

Preliminary DFT Calculations for Rationalizing Opticaland Electrochemical Studies. In the light of the previouselectrochemical properties, it would be highly interesting tocompute by DFT all redox potentials for both d and t. For bothmonocations d+ and t+, the proper DFT description of the(de)localization of the hole/electron is related to three main

Figure 1. (a) Cyclic voltammogram obtained for dimer d in CH2Cl2 solution (the concentration of d was c = 10−4 M) containing an electrolyte - 0.1M Bu4NBF4 (reference electrode - Ag/0.1 M AgNO3 in acetonitrile, scan rate - 100 mV/s). (b) Differential pulse voltammogram.

Figure 2. (a) Cyclic voltammogram obtained for trimer t in CH2Cl2 solution (the concentration of t was c = 10−4 M) containing an electrolyte - 0.1M Bu4NBF4 (reference electrode - Ag/0.1 M AgNO3 in acetonitrile, scan rate - 100 mV/s). (b) Differential pulse voltammogram.



factors:58,59 (i) the size of the monocation, a small size favoringdelocalization; (ii) the amount of Hartree−Fock (HF)exchange within the exchange-correlation (XC) potential usedto polarize the electronic structure; and (iii) the presence of a(polar) solvent. These factors turned out to be very sensitivefor d+ and t+ and strongly impact both the first two 0/+1 and+1/+2 redox potentials as well as their computed UV−visiblespectra. To explore the interplay between these factors istherefore beyond the scope of the present paper; a fulltheoretical (TD-)DFT study focusing on these IV-CT systemsis underway. We would like to address here two basic questionsby preliminary DFT calculations: (i) which amine site is firstoxidized in the asymmetrically bridged d dimer? (ii) how to

start to explain the fact that the first two oxidation waves oftrimer t occur at the same potential? (all technical details arereported in Supporting Information, the DTF Methodologysection).In the case of d, we report in Table 1 an orbital analysis

based on B3LYP (with 20% Hartree−Fock) Mulliken spinpopulations of the (minority) spin beta orbitals (HOMO andLUMO) for both N1 (4′-side) and N2 (3-side) amine sites ofd+, in the absence of solvent (in vacuo, ε = 1) (top right) and inits presence (bottom left, ε = 9 for dichloromethane). We alsoreport equivalent data for the neutral d (top left) and d2+

(bottom right) for the sake of comparison (ε = 9).To ease discussion, we propose to define the following

localization index xloc = [(pop(N2) − pop(N1)]/[pop(N2) +pop(N1)] varying between 0 (fully delocalized) and 1 (fullylocalized), with all values computed for the same orbital # 128.First, in the neutral d dimer, xloc = 0.47 (semilocalized: Table 1,top left). By contrast, it can be seen for d+ that the LUMO holein vacuo is shared by both amine sites (xloc = 0.13, i.e.,delocalized) (Table 1, top right). This drastically changes assoon as an even moderate (ε = 9) polar dielectric continuumenvironment is introduced (Table 1, bottom left): the hole islocalized on N1, with xloc(ε = 9) = 0.60. This behaviorreproduces trends found in the literature,58 and more will bepresented elsewhere. These preliminary results show that theN1 site is more prone to oxidation than the other N2 site andd+ appears as a class II system, an identification substantiated bythe fact that two reversible (one-electron) oxidation waves areobserved at different values: 0.36 and 0.48 eV (vs Fc/Fc+),respectively. In the case of d2+, both holes are semilocalized inthe solvent (xloc = 0.38; Table 1, bottom right) as hole−holerepulsion dominates. This last result is important, as it showsthat the computation of exchange coupling constants J for thehighest oxidized state (d2+) will be little affected by theenvironment. Both HS (S = 1) and BS (Ms = 0) spin states,necessary for the computation of J, exhibit the same twolocalized holes, thus giving credence to the values we predict(see below the DFT Calculations of J Coupling ConstantsSubsection and the Magnetic Properties Section).A similar analysis performed for the t trimer turned out to be

much more delicate and has to be reported elsewhere. Sufficeto say here that, without a symmetry-breaking environment, the(beta spin) hole in t+ is located on the central amine para-N1-para (ε = 9), in agreement with what was observed for d+.Experimentally, however, the first reversible oxidation wave(0.40 eV vs Fc/Fc+) is twice as large as the second wave (0.55eV), suggesting that both (one-electron) oxidations occur at thesame potential (0.40 eV) at both (N2 and N3) ends of thetrimer. However, preliminary results (see the SupportingInformation) show that the presence of a lateral counteranionnear one of the N2 sites breaks the trimer’s electronic

Figure 3. (a) UV−vis−NIR spectra of dimer d oxidized with TBA·SbCl6 in CH2Cl2 solution (the concentration of d, c = 8 × 10−4 M);the Ox/d molar ratio: (a) 1, (b) 2. (b) UV−vis−NIR spectra of trimert oxidized with TBA·SbCl6 in CH2Cl2 solution (the concentration of t,c = 8 × 10−4 M); the Ox/t molar ratio: (a) 1, (b) 2, (c) 3.

Table 1. B3LYP Mu lliken Population Analysis (%) Computed for the Frontier (Beta Spins) Orbitals for (a) d (ε = 9): #128 =HOMO−1 and #129 = HOMO; (b) d+ in vacuo (ε = 1): #128 = HOMO and #129 = LUMO; (c) the Same as b with ε = 9; (d)d2+ (ε = 9): #128 = LUMO and #129 = LUMO+1 (Triplet State)a

models orbitals N1 N2 models N1 N2

d(ε = 9)a # 129 18.4 7.0 d+(ε = 1)b 13.7 (h) 11.4 (h)# 128 7.5 20.7 11.9 15.6

d+(ε = 9)c # 129 20.1 (h) 4.8 (h) d2+(ε = 9)d 17.8 (h) 8.1 (h)# 128 5.6 22.5 8.6 (h) 19.2 (h)

aThe label (h) indicates a hole.



symmetry by localizing the hole on that first N2 site near thenegative charge while simultaneously raising the energy of theopposite N3 site which becomes the next oxidized site. Thismay start to explain the experimental findings regarding the firstreversible oxidation wave and illustrate that taking into accountthe counteranion will be crucial for the computation of the 0/1+ and 1+/2+ redox potentials of the trimer. Note that thiscounteranion effect is expected to be less crucial for d+ whosehole is already localized in the presence of the solvent.Magnetic Properties of the Diradical Dication d2+ and

the Triradical Trication t3+. Pulsed EPR Nutation Spectros-copy. The chemical oxidation of the compounds d and t byTBA·SbCl6 was also studied by pulsed-EPR nutation spectros-copy. The nutation frequencies measured by this technique aredirectly related to the spin states of the paramagnetic species inthe sample by the relationship (1) (see the ExperimentalSection). Under the conditions of the experiments reported inFigure 4, S = 1/2 species are expected at νnut = νS=1/2 = 23 MHz

nutation frequency, S = 1 at νnut(S=1) = √2·νS=1/2 = 32 MHz,and S = 3/2 at νnut = √3·νS=1/2 = 40 MHz (for the |3/2, 1/2⟩↔ |3/2, 3/2⟩ and |3/2, −3/2⟩ ↔ |3/2, −1/2⟩ EPR transitionsof an S = 3/2 state).The 2D pulsed-EPR nutation spectra shown in Figure 4 were

obtained at T = 7 K for samples d (lower frame) and t (upperframe) doped in dichloromethane solution with 2 and 3 equivof TBA·SbCl6, respectively. In the case of the dimer d, analmost pure S = 1 spin state is detected, with only a very faintsignal corresponding to the S = 1/2 state. In the case of thetrimer t, a strongly dominant S = 3/2 spin state is detected,with a much weaker signal due to the S = 1 spin state and a veryfaint signal corresponding to the S = 1/2 state. From thesespectra, one can conclude that d was quantitatively doped up tothe corresponding high-spin (S = 1) diradical dication and thatt was almost quantitatively doped to the corresponding high-spin (S = 3/2) triradical trication. However, it should bementioned that these almost pure high-spin states could beobtained only by performing the doping reaction in pure

CH2Cl2. All attempts to carry out the doping reaction bymixing solutions of TBA·SbCl6 in acetonitrile with solutions ofparent compounds d and t in CH2Cl2 failed.

SQUID Magnetometry. Samples prepared from the samedoped solutions of d and t as those studied by pulsed-EPRnutation spectroscopy were studied by SQUID magnetometry.These measurements are corrected for diamagnetism by usingthe experimental values of diamagnetic susceptibility of thesample holder and the solvent (see the Experimental Section).The experimental M = f(H) curves recorded at T = 2 K for

oxidized samples of d and t are shown in Figure 5 (left upperand lower frames, respectively). The M = f(H) curve recordedfor d is very well fitted by a Brillouin function corresponding tothe S = 1 pure spin state and including a T − θ termcorresponding to mean field analysis of small antiferromagneticintermolecular interactions. The value of θ = −0.14 K wasdeduced from the analysis of the χT = f(T) experiment (seebelow).From this analysis, the number of S = 1 species can be

obtained and it appears that 89% of the initial molecules of dwere doped up to diradical dications with S = 1 spin state.The same analysis was performed for trimer t. Again the M =

f(H) curve recorded for t is very well fitted by a Brillouinfunction corresponding to the S = 3/2 pure spin state, with θ =−0.09 K deduced from the analysis of the χT = f(T) experiment(see below and the Supporting Information, Figure S1, for acomparison of M = f(H) with a pure Brillouin function, i.e., θ =0 K). From this fitting, the number of S = 3/2 species can beestimated to 89% of the initial molecules of t. However, onemust keep in mind that in this analysis the small contributionsof S = 1 and S = 1/2 species observed by pulsed-EPR nutationare neglected.The variations of magnetic susceptibility with temperature

were recorded for oxidized samples of d and t at low magneticfield (H = 0.05 T) and produced the χT = f(T) curves shown inFigure 5 (right upper and lower frames, respectively). For thedimer d, the curve could be modeled with eq 1 correspondingto the Bleaney−Bowers equation,60 which is classical forsystems with two coupled S = 1/2 electron spins. This equationwas affected by a T − θ term corresponding to the mean fieldanalysis mentioned previously.

χθ

β=

− + −T

TT

Ngk J kT( )

2 13 exp( / )

2 2

(1)

β stands for the Bohr magneton, g for the Lande factor, k forthe Boltzman’s constant, N for the number of dicationdiradicals, and J for the exchange coupling constant betweenthe electron spins in the dication diradicals and correspondingto the Heisenberg Hamiltonian H = −J·S1·S2. From thisanalysis, the exchange coupling constant was estimated to be J/k = 136 ± 9 K and the Weiss temperature of the mean fieldanalysis, to be θ = −0.14 K.A similar analysis was performed for the χT = f(T) curve

corresponding to trimer t. This curve was modeled by eq 2derived from the Van Vleck formula corresponding to a lineartrimer of S = 1/2 spins.61−63 It is assumed that, in the triradicaltrications obtained from t, two S = 1/2 electronic spins arelocalized in both extremities (spins noted S2 and S3) and one inthe central part of the molecule (spin noted S1) and that,because of the symmetry of the spin distribution, it can bemodeled by the Heisenberg Hamiltonian H = −(J·S1·S2 + J·S1·

Figure 4. 2D-pulsed EPR nutation spectra at T = 7 K for samplesobtained by chemical oxidation of d and t. Lower frame: the dimer d([d] = 7.0 × 10−3 M) was oxidized with TBA·SbCl6 in CH2Cl2solution with the molar ratios: Ox/d = 2. Upper frame: the trimer t([t] = 3.2 × 10−3 M) was oxidized with TBA·SbCl6 in CH2Cl2 solutionwith the molar ratios: Ox/t = 3.



S3), leading to the following equation for fitting the χT = f(T)curve:

χθ

β=

−+ − + −+ − + −

T TT

Ngk

J kT J kTJ kT J kT( ) 4

10 exp( /2 ) exp( 3 /2 )2 exp( /2 ) exp( 3 /2 )

2 2

(2)

From this analysis, the exchange coupling constant in thetriradical trication derived from t was estimated to be J/k = 134± 10 K and the Weiss temperature of the mean field analysis, tobe θ = −0.09 K.DFT Calculations of J Coupling Constants in d2+ and

t3+. As expected in view of our previous work on chemicallysimilar oligomers, the geometries computed for d2+ and t3+

radical cations both exhibit (almost) planar NC3 motifs aroundnitrogen atoms with phenyl groups distributed around a givennitrogen atom in a propeller-like fashion which does not hinderferromagnetic coupling between adjacent amine sites. DFTcalculated exchange coupling constants for d2+ and t3+ are ingood agreement with the values measured experimentally,though larger by about 30% (see Table 2). The DFT valueshave been obtained here without recourse to either counter-anions or solvent, as done in our previous work.18

Comparison of the Magnetic Properties of d2+ and t3+

with Other Oligomers of Amminium Radical Cations

Previously Reported in the Literature. To the best of ourknowledge, this study reports the first experimental measure-ment of the exchange coupling constant due to the connectionof free radical moieties by the 3,4′-biphenyl coupling unit. Theexchange coupling constant in the following analogue carbon-centered diradical (structure I, Scheme 5) was evaluated by asimple model based on Huckel molecular orbitals19 to 0.66kcal/mol (330 K) and variable temperature EPR measurementsreported by Rajca and Rajca showed a constant product T × I

Figure 5. Magnetization measurements (after subtraction of diamagnetism) of the samples obtained by chemical oxidation of d and t with the molarratios: Ox/d = 2 and Ox/t = 3 (the same solutions as for Figure 4). Upper frame: (left) M = f(H) curve recorded for d at T = 2 K, (right) χT = f(T)curve recorded for d at H = 0.05 T. Lower frame: (left) M = f(H) curve recorded for t at T = 2 K. (right) χT = f(T) curve recorded for t at H = 0.05T (see the Experimental Section for more details).

Table 2. High-Spin (HS) Magnetic State (S = 1 for d2+ and S= 3/2 for t3+) and Broken Symmetry (BS) Magnetic States(Ms = 1/2: BS1 ≡ ↑↓ for d2+, BS1 ≡ ↑↓↑ and BS2 ≡ ↓↑↑ fort3+) Used to Compute Exchange Coupling Constants JBS(See the Methodology Section)a

systems d2+ t3+

HS (eV) −481.3267 −716.1596BS1 (eV) −481.3187 −716.1450BS2 (eV) n.a. −716.1523JBS (K) 185 170Jexp (K) 136 ± 9 134 ± 10

aBonding energies are reported in eV and J values (JBS and Jexp) inkelvin (K).



(I, EPR intensity; T, temperature) up to 80 K50 consistent withJ/k > 80 K.The experimental values J/k ∼ 135 K obtained is this study

for amminium radical cations coupled through a 3,4′-biphenylcoupling unit can be compared with the values measured byBusbhy for the coupling of the same kind of amminium radicalcations through a 4,4″-meta-terphenyl spin coupling unit (J/k =42 K). This quantitative comparison shows that J couplingconstants are typically 3−4 times higher when using a 3,4′-biphenyl rather than 4,4″-meta-terphenyl spin coupling unit foramminium diradical dications. This ratio is close to the ratio of4.4 between the J values estimated by Bushby19 for similar(Ph)2C

• radicals coupled by 3,4′-biphenyl (J = 0.66 kcal.mol−1,J/k = 330 K) and 4,4″-meta-terphenyl spin coupling unit(structure II, Scheme 5, J = 0.15 kcal·mol−1, J/k = 75 K) usingthe simple model based on Huckel molecular orbitals (seeTable 3).

It would be highly desirable to compare the J couplingconstants reported here for d2+ and t3+ with the J couplingconstants in similar di- and triradicals based on a 1,3-benzenespin coupling unit. However, to the best of our knowledge, noexperimental data were reported in the literature. By perform-ing DFT calculations at the same level as that for d2+ and t3+,we obtain J/k = 590 K. It suggests that the J coupling constantsobtained for d2+ and t3+ are approximately 3−4 times smallerthan the J coupling constants of two amminium radical cationslinked through two 1,3-benzene spin coupling units. Thecorresponding J value was predicted by Ito et al. to be as high as350 K by ab initio MO study at the ROHF, GVB, and CASSCFlevels.64 Let us mention that comparable high J couplingconstants were measured only once for dimers and trimers ofnitroxides connected by 1,3-benzene unit(s) (J/k = 480 K,Scheme 6, structures III and IV).65,66

In a previous work,18 we studied a dimer, in which twoamino-p-phenylenediamine units were linked via a 1,3-benzenespin coupling unit (cf. Scheme 6, structure V). The measured Jcoupling constant (J/k = 33 K) was 1 order of magnitude lowerthan that for the trinitroxide mentioned above. As it turned out

there, the magnetic orbitals overlapped poorly at the level ofthe coupler, related to the spin delocalization occurringbetween formal spin bearers (conjugated p−p segments) andcouplers, therefore withdrawing/diluting much spin away fromthe meta-couplers. By contrast, in the present work, the size ofthe spin bearing units is reduced to its minimum for amminiumradical cations, whereas that of the spin coupler is increased toform a 3,4′-biphenyl unit. The size of this bearer reduces thedirect magnetic orbital overlap (thus inhibiting the antiferro-magnetic contribution to J) while allowing substantial spatialcontact over two phenyl rings (thus exalting the ferromagneticcontribution).As shown by the DFT calculations reported here, the

asymmetry of the 3,4′-biphenyl spin coupler unit in d and tallows for partial localization of the electron/charge, as alreadyshown at the level of the d monocation. This opens the way tofinely controlling the energy difference between both aminesites (and therefore partial localization trends) via appropriatesubstituents, in addition to tuning local redox potentials. Forhigher oxidation states (2+ for both systems, 3+ for t), this addsa second player in the (de)localization game, mainly controlledby hole−hole repulsion in symmetrical oligomers (andpolymers, for which conformation is also important). Theadditional difficulty, only hinted at by the present DFTcalculations, that is the significant role played by the location ofthe counteranions for the electronic structure of the mixed-valence states, is expected to be less important for higheroxidation states. With all that in mind, therefore, the use of 3,4′-biphenyl spin coupler units in locally symmetry-brokenpolymers looks very promising.We can also notice that the diradical dication d2+ and the

triradical trication t3+ are obtained with a very high (89%)doping efficiency (as estimated by SQUID magnetometry),which compares well to doping efficiencies reported foramminium radical cation dimers, oligomers, and polymersreported in the literature, which are generally in the 65−80%range (see ref 11 and references therein).At last, the pulsed-EPR and SQUID data reported here

clearly demonstrate that one can obtain a pure S = 3/2 high-spin ground state for the doped trimer t in the t3+ oxidationstate. This result should be emphasized, since previous attempts

Scheme 5. Structures of Diphenylmethyl Radicals Linked via3,4′-Biphenyl (I)50 and 4,4″-Meta-Terphenyl (II)19

Table 3. Experimental and Calculated Exchange CouplingConstants (J/k in K) for Diradicals and Diradical Dicationsaccording to Their Spin Coupling Unit and Spin BearingUnits: (a) from This Study; (b) from ref 19, JHuckel Stands foran Estimate Made by Bushby et al. Based on Huckel Theory;(c) from refs 65 and 66; (d) from ref 50

coupling unit(Ar)3N

•+

JExp/JDFT(Ar)3C

•

JExp/JHuckel(Ar)2NO

•

JExp

1,3-benzene .../590(a) .../1400(b) 480(c)

3,4′-biphenyl 135(a)/185(a) >80(d)/330(b) ...4,4″-meta-terphenyl 42(b)/... .../75(b) ...

Scheme 6. Chemical Structures of Bi- and Tri-NitroxidesContaining 1,3-Benzene Coupler (Structures III and IV)65,66

and of a Dimer Previously Reported by Our Team (StructureV)18



to obtain a high-spin ground state from doped linear oligomersof amminium radical cations were not successful.1,20 Whendoping a trimer of p-phenylenediamine coupled by two 1,3-benzene moieties as spin coupling units (Scheme 7, structure

VI), mainly S = 1/2 with a small fraction of S = 1 state wasobserved by pulsed-EPR,1 while a triradical trication of thecorresponding star-branched trimer exhibited mainly the S = 3/2 high-spin state.In a study of dimers and tetramers of meta-aniline (Scheme

7, structure VII), Bushby et al.20 demonstrated that oxidizingarylamine moieties connected by a 1,3-benzene moiety is verydifficult: due to electrostatic repulsion, the oxidation of asecond neighboring arylamine can be obtained only atpotentials higher by 0.4 V compared with the oxidation ofthe first amine moiety. Thus, the tetraradical cation derivedfrom a tetra-meta-aniline could not be obtained neither byelectrochemistry nor by chemical oxidation. The fact that thetrimer t could be chemically and electrochemically oxidized upto t3+ indicates that the 3,4′-biphenyl coupling spin unit makesthe electrostatic repulsion much smaller than in the case of the1,3-benzene coupler. In this view, the results achieved for thetrimer t seem to be very promising and 3,4′-biphenyl can beconsidered an effective ferromagnetic coupler for arylaminespin bearing units.

■ CONCLUSIONSThis study shows that the 3,4′-biphenyl spin coupling unitprovides high coupling constants (J/k ∼ 135 K) betweenamminium radical cations in the dimer d and the linear trimer t.Moreover, it shows that the increase of the oxidation potentialrequired to reach t3+ compared with d2+ is small (only 0.07 V)due to moderate Coulombic repulsion between amminiumradical cations connected by a 3,4′-biphenyl unit. At last, assuggested by preliminary calculations, the asymmetry of the3,4′-biphenyl coupling unit can be used as an additionalparameter to (de)localize spins of radical cations in locallysymmetry-broken polymers. These features make polymersbased on the 3,4′-biphenyl spin coupler unit and amminiumradical cations very promising. The study of such polymers iscurrently underway in the laboratory.

■ ASSOCIATED CONTENT*S Supporting InformationThe synthesis procedures and characterization of d and t,details of pulsed EPR nutation experiments, DFT calculation

methodology, and preliminary studies of t. This material isavailable free of charge via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Authors*E-mail: [email protected]. Phone: 33-438783598.*E-mail: [email protected]. Phone: 48-22-2345584.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSI.K.-B. and L.S. wish to acknowledge financial support fromNational Centre of Science in Poland (NCN, Grant No. UMO-2011/01/B/ST5/03903).

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ferromagnetic spin coupling through the 3,4′-biphenyl moiety in arylamine oligomers—experimental...

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