Redox Modulation of para-Phenylenediamine by Substituted Nitronyl

Article pubs.acs.org/JPCA

Redox Modulation of para-Phenylenediamine by Substituted Nitronyl Nitroxide Groups and Their Spin States Akihiro Ito,*,† Ryohei Kurata,† Daisuke Sakamaki,†,∥ Soichiro Yano,† Yosuke Kono,† Yoshiaki Nakano,†,⊥ Ko Furukawa,‡ Tatsuhisa Kato,§ and Kazuyoshi Tanaka*,† †

Department of Molecular Engineering, Graduate School of Engineering, Kyoto University, Nishikyo-ku, Kyoto 615-8510, Japan Center for Instrumental Analysis, Institute for Research Promotion, Niigata University, 8050, Ikarashi 2-no-cho, Nishiku, Niigata 950-2181, Japan § Institute for Liberal Arts and Sciences, Kyoto University, Yoshida-Nihonmatsu, Sakyo-ku, Kyoto 606-8501, Japan ‡

S Supporting Information *

ABSTRACT: Three kinds of para-phenylenediamine (PDA) derivatives bearing nitronyl nitroxide (NN) groups were prepared and characterized on the basis of the electrochemical, electron spin resonance (ESR) spectroscopic, absorption spectroscopic, and magnetic susceptibility measurements. It was clarified that the oxidation potential of the central PDA unit is strongly influenced by the numbers of substituted electron−withdrawing NN groups. In addition, the intervalence charge transfer in the central PDA unit was detected in the monocationic states of the PDAs with two NN groups, indicating the coexistence of the localized spins and the delocalized spin on theses molecules. Moreover, pulsed ESR measurements confirmed that the delocalized spin on the central PDA unit and the localized two spins on the NN groups were ferromagnetically coupled in the monocationic states.

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INTRODUCTION In the development of molecular electronics,1−3 great interest is currently being focused on multispin molecular systems in which the intramolecular magnetic interactions are controllable by external stimuli such as irradiation, heat, pressure, and so forth.4 In particular, redox-active multispin systems are promising as key materials in molecule-based spintronics,5 because of their possible application to spin-polarized molecular wires.6 The basic concept of designing spin-polarized molecular wires is to combine the redox-active units with the localized radical groups, as shown in Figure 1. When some electrons are added to or removed from such molecular systems, the radical spins can be newly generated, and the intramolecular magnetic interactions between the existing localized spins are inevitably modulated by the generated spins. According to this concept, several redox-active organic radicals have been synthesized, and the switching phenomena of the spin states by the electrochemical potential sweep were examined.7−11 Recently, we have prepared a para-phenylenediamine (PDA) molecule having two nitroxide radical groups, and achieved spin alignment mediated by the double-exchange-like magnetic interaction among the delocalized spin generated by the oxidation and the two localized nitroxide spins.6,11 On the basis of the previous our results, we focused on three kinds of novel polyradical molecules 1−3 composed of the central redox-active PDA unit and the peripheral localized nitronyl nitroxide (NN) groups (Chart 1). In 1−3, all NN groups are attached to the © XXXX American Chemical Society

para-positions of N-phenyl groups of the central PDA unit. In this substitution pattern, the spin polarization rule12 promises that the localized spins on the NN groups are coupled ferromagnetically to the delocalized spin, which can be generated by the oxidation of the central PDA unit. In this article, we report in detail on the electronic structures of polyradicals 1−3 on the basis of the electrochemical, electron spin resonance (ESR) spectroscopic, absorption spectroscopic, and magnetic susceptibility measurements.

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EXPERIMENTAL SECTION The synthesis of the target molecules 1−3 and their reference compounds 7, 8, and 13 were carried out according to Scheme 1. By using the palladium catalyzed Buchwald−Hartwig amination reaction13−16 between p-dibromobenzene and the corresponding aromatic amines 4−6, the p-phenylenediamine (PDA) derivatives 7−9 were prepared. Formylation of 7 and 8 with N,N′-bis(trifluoroacetyl)imidazolium ion afforded the corresponding aldehyde 10 and 11.17 The secondary amine 9 was converted into 12 by the Buchwald−Hartwig amination reaction with p-bromobenzaldehyde dimethyl acetal and the following hydrolysis reaction. The target molecules 1−3 were finally obtained by the reactions of 10−12 with bishydroxylReceived: September 25, 2013 Revised: November 1, 2013

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dx.doi.org/10.1021/jp4095613 | J. Phys. Chem. A XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry A

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Figure 1. (a) Schematic drawings of double-exchange-like mechanism in one-dimensional molecular systems: each site has two magnetic orbitals A and B, where the B orbital is for the localized spin, and the ferromagnetic coupling (J > 0) is assumed between two electron spins on A and B orbitals. The electron spin on A orbital can easily move onto the neighboring A orbital owing to the high-spin configuration among the localized spins on B orbitals. (b) Polyaniline carrying nitronyl nitroxide (NN) radical pendant groups as a hypothetical spin-polarized molecular wire working on the basis of double-exchange-like mechanism (see ref 6).

Chart 1. Oligomer Model Compounds 1−3 for the Hypothetical Spin-Polarized Molecular Wires Shown in Figure 1

amine sulfate and the following oxidation with PbO2.18 The detailed synthetic procedures are given in the Supporting Information. The redox properties were evaluated by cyclic voltammetry in MeCN and/or PhCN solution at 298 K with 0.1 M tetra-nbutylammonium tetrafluoroborate as supporting electrolyte (scan rate 100 mV s−1) using an ALS/chi Electrochemical Analyzer model 612A. A three-electrode assembly was used, which was equipped with platinum disk (2 mm2), a platinum wire, and Ag/0.01 M AgNO3 (acetonitrile) as the working electrode, the counter electrode, and the reference electrode, respectively. The redox potential was referenced against a ferrocene/ferrocenium (Fc0/+) redox potential measured in the same electrolytic solution. UV−vis−NIR absorption spectra were obtained with a Perkin-Elmer Lambda 19 spectrometer. Spectroelectrochemical measurements were carried out with a custom-made optically transparent thin-layer electrochemical (OTTLE) cell (light pass length =1 mm) equipped with a platinum mesh, a platinum

coil, and a silver wire as the working electrode, the counter electrode, and the pseudoreference electrode, respectively. The potential was applied with an ALS/chi Electrochemical Analyzer model 612A. ESR spectra were recorded on a JEOL JES-SRE2X or a JEOL JES-TE200 X-band spectrometer, in which the temperature was controlled by a JEOL DVT2 variable-temperature unit or an Oxford ITC503 temperature controller combined with an ESR 910 continuous flow cryostat, respectively. A Mn2+/MnO solid solution was used as a reference for the determination of gvalues and hyperfine coupling constants. Pulsed ESR measurements were carried out on a Bruker ELEXSYS E580 X-band FT ESR spectrometer, in which temperature was controlled by an Oxford ITC503 temperature controller combined with an Oxford ESR900 continuous-flow cryostat. The microwave pulse power of 10 mW provided by the microwave bridge was boosted to level of 1 kW using a traveling wave tube (TWT) amplifier. Electron spin transient nutation (ESTN) measurements were performed by the three-pulse sequence shown in B



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Scheme 1. Synthetic Routes for Polyradicals 1−3 and Their Related Compounds 7, 8, and 13

Figure 2. Pulse sequence used for the present electron spin transient nutation measurements.

nutation frequency S(ω

Figure 2. The two-pulse (π/2−π pulses) electron spin−echo signal S(t1) was detected by increasing the width (t1) of the nutation pulse. We employed appropriate phase cycles in order to suppress undesirable signals and artifacts which arise from an inaccurate pulse length.19,20 The observed signal S(t1, B) as a function of external magnetic field B is converted into a

nut,

B) spectrum. The parameters used

for the measurements were t2 = 400 ns, t3 = 8 ns for 2+ and 12 ns for 3+. The signals observed at ca. 14 MHz in Figure 6 are due to the electron spin echo envelope modulation (ESEEM),21 which results from weak interaction with proton C



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nuclei (I = 1/2) originating from the solvent molecules surrounding the paramagnetic species. For the neutral and cationic states of the model compound 2′ for 2, in which all the methyl groups except for N-methyl groups were replaced by hydrogen atoms, and compound 8, the hybrid HF/DF (B3LYP) calculations22 were performed by using 6-31G* basis sets.23 Full geometry optimizations were carried out under Ci symmetrical constraint. For the calculation of the open-shell singlet state, the broken-symmetry wave function was employed. All the calculations were carried out with Gaussian 98 program package of ab initio molecular orbital (MO) calculation.24

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RESULTS AND DISCUSSION Redox Property. To realize the double-exchange-like highspin alignment (Figure 1) between the localized spins on the peripheral NN groups and the delocalized spin, which is generated from the central PDA unit, the lower oxidation potential of the central PDA unit than those of the NN groups is indispensable. Hence, the electrochemical properties of 1−3 were evaluated by cyclic voltammetry in benzonitrile solution for 1 and acetonitrile solution for 2 and 3 at 298 K (Figure 3). Compound 1 was insoluble in acetonitrile. Chemical reversibility for all the oxidation processes of 1−3 within the measured potential ranges was checked by repeated potential recycling under no inert atmosphere. The oxidation potentials versus ferrocene/ferrocenium are summarized together with the related compounds, 7, 8, 13, and 2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (PNN) in Table 1. The redox behavior of 1 was composed of first quasi-four-electron, second one-electron, and third one-electron transfer processes (Figure 3a). Judging from the oxidation potentials, the current ratio, and the large difference (∼130 mV) between the anodic and cathodic peaks, the first oxidation process of 1 can be ascribed to the separate four one-electron oxidation with small potential differences of the four NN moieties. The second and third oxidation processes of 1 correspond to the central PDA unit, and these potentials are considerably high compared to those of the reference compound 7 without NN groups. The decrease in electron-donating property of the central PDA unit is due to the strong electron-withdrawing effect of the substituted four NN groups. Therefore, the partial removal of the substituted NN groups is necessary to reduce the oxidation potential of the central PDA unit. As a consequence, the cyclic voltammograms of 2 and 3 showed four and five reversible redox couples, respectively (Figure 3b,c), and more importantly, the oxidation potential of the central PDA unit in 2 and 3 became lower than those of the substituted NN groups, partly because of the substituted electron-donating methyl and N,N′-dimethylphenyl groups and partly because of the decreasing number of substituted electron-withdrawing NN groups. Thus, it is highly expected that the one-electron-oxidation of 2 and/or 3 leads to generation of triradical cation with high-spin alignment originating from double-exchange-like mechanism. Continuous Wave (CW)-ESR Measurement. The CWESR spectrum of 1 in toluene solution at 298 K showed a 17line pattern originating from the hyperfine interaction with eight equivalent nitrogen nuclei (Figure 4a), while the spectra of 2 and 3 showed nine-line patterns due to four equivalent nitrogen nuclei (Figure 4b and Figure S1 in the Supporting Information). The hyperfine coupling constants were estimated by the spectrum simulation. The hyperfine coupling constant for 1 (|aN| = 0.193 mT) was about one-fourth of the hyperfine

Figure 3. Cyclic voltammograms (CV) of (a) 1 in PhCN, (b) 2 in MeCN, and (c) 3 in MeCN at 298 K (0.1 M n-Bu4NBF4; scan rate 100 mV s−1).

coupling constant for the NN monoradical (PNN; |aN| = 0.74 mT),10 while those for 2 and 3 (|aN| = 0.386 and 0.380 mT) were about a half of the value of 0.74 mT. These results indicate that the exchange interactions in 1−3 are larger than the hyperfine interaction. To investigate the difference in the spin states between the neutral and cationic states, we have measured the CW-ESR spectra in the frozen solution of 2 and 3 and their monooxidized species at 5 K. The one−electron oxidation was accomplished by treating with up to 1 mol equiv of tris(4− bromophenyl)amminium hexachloroantimonate (Magic D



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Table 1. Oxidation Potentials (V versus Fc0/+) by Cyclic Voltammetry (Scan Rate: 0.1 V s−1) of 1−3 and the Related Compounds 7, 8, 13, and PNN in MeCN (0.1 M n-Bu4BF4) at 298 K compd

E1

E2

E3

E4

E5

1a 2 3 7a 8 13 PNNa,d

+0.28b +0.17 −0.01 +0.14 −0.02 −0.14 +0.32

+0.71 +0.30 +0.10 +0.59 +0.48 +0.04 −−−−

+0.92 +0.47 +0.40c −−−− −−−− +0.51 −−−−

−−−− +0.61 +0.60 −−−− −−−− +0.62 −−−−

−−−− −−−− +0.70 −−−− −−−− −−−− −−−−

a Measured in PhCN. bQuasi-four electron transfer. electron tranfer. dSee ref 10.

c

Quasi-two-

Blue)25,26 at 195 K. The observed spectra of 2 and 2+ had no definitive fine-structure in the allowed resonance region, and were almost identical to each other except for a slight broadening in the spectrum of 2+ (Figure 5). However, the forbidden ΔMs = ± 2 resonance was clearly observed in 2+, suggesting the presence of the high-spin state.27 The signal intensity of the forbidden transition of 2+ was inversely proportional to temperature up to the measured limit of 40 K, suggesting the high-spin ground state of 2+ (Figure S3a in the Supporting Information). Similarly, the CW-ESR spectra of 3 and 3+ exhibited almost the same spectral patterns as those of 2 and 2+ (Figure S2 in the Supporting Information). In addition, the forbidden ΔMs = ± 2 resonance was also observed in 3+. Although the Curie plot for 3+ also exhibited a linear relationship, the slight downward curvature above 30 K might be indicative of thermal population of the low-spin state (Figure S3b in the Supporting Information). As mentioned above, it was confirmed that 2+ and 3+ are in a high-spin state at lower temperatures, but we could not determine their spin multiplicities because of the lack of definite fine structures. Pulsed-ESR Measurement. To identify unequivocally the spin multiplicity of the high-spin components of 2+ and 3+, we carried out ESTN measurements based on the pulsed ESR method for 2+ and 3+.28−33 The magnetic moments with distinct spin quantum numbers (S) precess with their specific nutation frequency (ωnut) in the presence of a microwave

Figure 5. Observed X-band CW-ESR spectra of (a) 2 and (b) 2+ in toluene/n-butyronitrile (1:1 v/v) at 5 K. Inset: the half-field resonance spectrum of 2+.

irradiation field and a static magnetic field. The nutation frequency for a transition from |S, MS⟩ to |S, MS + 1⟩ can be approximately expressed by using eq 1. ωnut =

S(S + 1) − MS(MS + 1) ω0

(1)

This indicates that ωnut can be scaled with the total spin quantum number S and the spin magnetic quantum number MS in the unit of ω0, which corresponds to the nutation frequency for the spin−doublet impurities (ωdoublet). In the field-swept ESTN spectrum of 2+, two nutation frequencies were observed at 21.4 and 36.2 MHz (Figure 6a). Judging from the frequency ratio (36.2/21.4 ≃ √3), the former frequency (21.4 MHz) corresponds to |1/2, +1/2⟩ ⇔ |1/2, −1/2⟩ transition for the spin-doublet species (ωdoublet), and the latter one (36.2 MHz) to |3/2, ± 3/2⟩ ⇔ |3/2, ± 1/2⟩ transition for the spin-quartet 2+ (ωquartet). Similarly, two nutation signals were observed at 17.8 and 28.3 MHz in 3+ (Figure 6b), and these signals were assigned to |1/2, +1/2⟩ ⇔ |1/2, −1/2⟩ transition for the spindoublet species and |3/2, ± 3/2⟩ ⇔ |3/2, ± 1/2⟩ transition for the spin-quartet 3+ (ωquartet), respectively. Note that, although

Figure 4. Observed and simulated X-band CW-ESR spectra of (a) 1 and (b) 2 recorded in CH2Cl2 at 298 K. E



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generated by oxidation of the central PDA unit in 2+ and 3+ are ferromagnetically coupled at lower temperatures.34 UV−Vis−NIR Absorption Measurement. The most straightforward way to confirm the presence of the intervalence state is to observe the characteristic lowest-energy absorption band, that is, the so-called intervalence charge transfer (IVCT) band.35,36 To confirm the presence of the delocalized spin, which is generated from the oxidation of the central PDA unit, we have measured the UV−vis−NIR absorption spectral change during the course of the oxidation of 2 and 3 by using an optically transparent thin-layer electrochemical cell. The spectral change during the oxidation process of 2 to 22+ is shown in Figure 7. As the oxidation proceeds, a new broad band appeared at around 1.77 eV (700 nm), and this band is considered to be the IVCT band between the aminium radical center and the neutral amine center. On going from 2+ to 22+, the intensity of this band decreased, indicating the formation of 22+. Similar spectral changes were observed during the oxidation process of the reference molecules 8,11 and therefore, it can be confirmed that NN groups do not greatly affect the delocalized intervalence band of the central PDA unit in 2+. The IVCT band was also observed during the oxidation process of 3 to 3+ (Figure 8). In this case, the IVCT band was significantly red-shifted (0.95 eV (∼ 1300 nm)) compared with that of 2+, because of elongation of the π-conjugation over the central PDA unit by substitution of N,N-dimethylaminophenyl groups. The spectral change of 3 in the lower energy region was in good accordance to that of its reference molecule 13 (Figure 9), and hence, the existence of the intervalence state in 3+ was also confirmed. As a result, the coexistence of the localized spins and the delocalized spin in the oxidized states, 2+ and 3+, was demonstrated by the observation of the IVCT band. Theoretical Consideration. To clarify the effect of peripherally substituted NN groups, we carried out the DFT calculations (B3LYP/6-31G*) on the model compound 2′ (Figure 10). The geometry optimizations on the singlet and triplet states of 2′, and the doublet and quartet states of 2′+ were performed at the UB3LYP/6-31G* level. The calculated total energies and the ⟨S2⟩ values for the corresponding highspin and low-spin states of 2′ and 2′+ are summarized in Table 2, and the calculated structural parameters are listed in Table 3. Here, the atom numbering and the torsional angles are defined in Figure 10b. In the neutral state of 2′, the singlet−triplet energy gap was estimated to be nearly zero, indicating that the singlet and triplet states are virtually degenerate in the neutral state. Indeed, the optimized geometry for the singlet state of 2′ is

Figure 6. Magnetic-field-swept electron-spin transient nutation (ESTN) spectra of (a) 2 (at 5 K) and (b) 3 (at 10 K) after addition of 1 molar equiv of Magic Blue in toluene/n-butyronitrile (1:1 v/v). The intense signal observed at ca. 14 MHz is due to the electron spin envelope modulation (ESEEM) signal21 originating from weak interaction with proton nuclei of the solvent molecules.

the doublet species observed for both samples originate substantially from the impurity, it cannot be denied that the excited spin-doublet state is readily accessible owing to the very small doublet-quartet energy difference, judging from the Curie plot in Figure S3. These results clearly shows that two localized spins on the peripheral NN groups and the delocalized spin

Figure 7. UV−vis−NIR absorption spectra of the stepwise electrochemical oxidation of 2 in CH2Cl2 with 0.1 M n-Bu4NBF4 at 298 K: (a) 2 to 2+; (b) 2+ to 22+. F



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Figure 8. UV−vis−NIR absorption spectra of the stepwise electrochemical oxidation of 3 in CH2Cl2 with 0.1 M n-Bu4NBF4 at 298 K: (a) 3 to 3+; (b) 3+ to 32+.

Figure 9. UV−vis−NIR absorption spectra of the stepwise electrochemical oxidation of 13 in CH2Cl2 with 0.1 M n-Bu4NBF4 at 298 K: (a) 13 to 13+; (b) 13+ to 132+.

Table 3. Optimized Bond Lengths (Å) and the Structural Parameters, φ1 (°), φ2 (°), φ3 (°), and q (%) for 2′ and 2′+ 2′+

2 C1−C2(C3)a C2−C3 N1−C1 N1−C4 N1−C5 C5−C6(C7)a C6(C7)−C8(C9)a C8(C9)−C10a C10−C11 C11−N2(N3)a N2(N3)−O1(O2)a N2(N3)−C12(C13)a C12−C13 φ1 φ2 φ3 q

Figure 10. (a) Calculated model compound 2′, (b) the atom numbering and the torsional angles in Ci symmetric structure of 2′, and (c) the difinition of d1 and d2 in the central PDA unit.

Table 2. Total Energies (E) and ⟨S2⟩ Valuesa for the HighSpin (HS)b and Low-Spin (LS)c States and Relative Energies (ΔE)d of 2′ and 2′+ 2′ 2′+

EHS (hartree)

⟨S2⟩HS

ELS (hartree)

⟨S2⟩LS

ΔE (kcal mol−1)

−1635.5588 −1635.3497

2.124 3.903

−1635.5588 −1635.3461

1.124 1.883

∼ 0e 2.3

a 2 ⟨S ⟩ values for the pure singlet, doublet, triplet, and quartet states are 0, 0.75, 2, and 3.75, respectively. bThe triplet state for 2′ and the quartet state for 2′+. cThe singlet state for 2′ and the doublet state for 2′+. dΔE = ELS − EHS. e−2.2 cal mol−1.

a

singlet

triplet

doublet

quartet

1.402 1.392 1.426 1.460 1.394 1.412 1.386 1.413 1.455 1.367 1.276 1.482 1.527 48.9 23.2 2.4 18.8

1.402 1.392 1.426 1.460 1.394 1.412 1.386 1.413 1.455 1.367 1.276 1.482 1.527 48.9 23.2 2.4 18.8

1.427 1.372 1.366 1.470 1.436 1.400 1.390 1.412 1.462 1.365 1.271 1.486 1.529 14.8 55.7 4.4 98.8

1.423 1.376 1.376 1.470 1.422 1.405 1.386 1.415 1.455 1.368 1.271 1.486 1.529 21.1 45.1 5.2 84.8

Average bond length.

for 2+ is qualitatively supported by the DFT calculations. In contrast to the neutral state, the central benzene ring is found to be quinoidally deformed in the cation 2′+. Here, the quinoidal parameter q (%) was evaluated by using eq 2:37

almost the same as that for the corresponding triplet state (Table 3).34 On the other hand, the quartet state of 2′+ was predicted to be more stable by 2.3 kcal mol−1 than the corresponding doublet state (Table 2). Although such a large doublet−quartet energy difference is not quantitative as compared with the present experimental results, the preference of high-spin state

q = 100 × G

d1 − d 2 d1′ − d 2′

(2)



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where d1 and d2 are the optimized bond lengths in the central benzene ring depicted in Figure 10c, and d1′ and d2′ are taken from the bond lengths of 8+ optimized at the UB3LYP/6-31G* level (d1′ = 1.428 Å and d2′ = 1.372 Å). It should be noted that such a quinoidal deformation accompanies a decrease of φ1 and an increase of φ2. Consequently, the optimized structure of para-phenylene moiety in 2′+ is very similar to that of 8+ in the doublet state. It is noteworthy that the quartet state of 2′+ showed the extremely small q value (84.8%) in comparison with that of the corresponding doublet state (%). Moreover, the torsional angle φ1 (φ2) of the quartet 2′+ is larger (smaller) than that of the competing doublet state. Since the increasing of the torsional angle φ1 weakens the π-conjugation between the π-orbital of the central p-phenylene moiety and p-orbitals of the neighboring amino nitrogen atoms, it is expected that the electron transfer between two amino redox-active sites on the central PDA unit is suppressed in the quartet 2′+.38 This results in the small q, the low-level quinoidal deformation. The quartet state of 2′+ has three singly occupied molecular orbitals (SOMOs), depicted in Figure 11, where the almost

In order to examine the electron correlation in high-spin molecules, the post-Hartree−Fock calculations such as the complete active space self-consistent field (CASSCF) calculation including all π-orbitals are needed. However, for the large molecules like 2′+, it is prohivitively time-consuming to carry out the CASSCF calculations. On the other hand, the natural orbital (NO) analysis for the UHF and/or unrestricted DFT solutions have been often used as an alternative method for the CASSCF calculations, where the orbital overlap (Ti) is related with the occupation numbers of the bonding and antibonding NOs, ni and ni*, by eq 3.

⎧ ni = 1 + Ti ⎨ ⎩ ni* = 1 − Ti ⎪

⎪

(3)

Herein, Ti = 1.0 for the closed-shell pair, that is, the ni and ni* are 2.0 and 0.0, respectively. Table 4 summarizes the Table 4. Occupation Numbera in the Natural Orbitals of 2′+ LU+3 LU+2 LU+1 SO+1 SO+0 SO−1 HO−1 HO−2 HO−3 a

doublet

quartet

0.003 0.027 0.031 0.934 1.000 1.066 1.969 1.973 1.997

0.003 0.031 0.037 1.000 1.000 1.000 1.963 1.969 1.997

Calculated at the optimized geometry for the quartet state.

occupation numbers of the NOs calculated at the geometries of the quartet 2′+. Judging from the occupation numbers of 1.0 for three SOMOs (SO+0 and SO ± 1), the quartet state of 2′+ showed the negligible orbital overlaps between the magnetic orbitals. On the other hand, the occupation numbers of (HO− 1), (HO−2), (LU+1), and (LU+2)MOs in 2′+ considerably deviates from 2.0 and 0.0 for the pure closed pair. This indicates that these orbitals significantly contribute to the spin polarization effect in 2′+. It is well-known that the reasonable spin densities are usually obtained by the DFT method. The B3LYP spin densities are listed in Table 5. For the singlet 2′, the sign of the spin density on one atom after the other changes alternately, except for the amino nitrogen atoms. On the other hand, for the quartet 2′+, the alteration of the sign appears only on the peripheral aryl moieties, whereas the positive spin densities appear on all the atoms in the central PDA unit, indicating the double-exchangelike interaction works in 2′+. Moreover, the spin densities of on the central PDA unit (N1, C1, C2, and C3) in 2′+ are small as compared with those in 8+. This is consistent with the fact that the SOMO on the central PDA unit delocalizes over the peripheral aryl moieties in 2′+, as described before. In addition, the spin density on the NN groups (O1−N2−C11−N3−O2) increases from 1.038 to 1.080 on going from 2′ to 2′+, and moreover, the total spin density on the peripheral benzene ring (C5, C6, C7, C8, C9, and C10) increases from −0.033 to 0.037 on going from 2′ to 2′+. As a whole, the spin density on the central PDA unit in 2′+ delocalizes over the peripheral NN groups.

Figure 11. Schematic drawing of three SOMOs for the quartet state of 2′+ and their relative MO energy levels at the UB3LYP/6-31G* level.

degenerate two SOMOs (65ag and 64au) composed of the πorbitals on the substituted NN groups are nearly degenerate with the SOMO (64ag) composed of the π-orbital on the central PDA unit. Apparently, 2′+ is regarded as a non-disjoint (or coextensive) system,39−42 where the same atomic orbitals are shared among the SOMOs. Since the high−spin state is relatively stabilized owing to exchange interaction in the nondisjoint system, the quartet state is expected to be lower in energy than the competing doublet state in 2′+. The relatively delocalized 64ag SOMO is very similar to that of 8+, and more importantly, it extends over the NN groups, leading to the decrease of electron density on the central p-phenylene ring. As described before, the quartet 2′+ has larger torsional angle φ1 along the N1−C1 bond and the smaller torsional angle φ2 along the N1−C5 bond. This situation causes the decrease of the orbital interaction between the amino redox-active sites due to the large torsional angle φ1, while the p-orbital of the amino nitrogen interacts more with the π-orbital of the peripheral benzene ring. As a result, the electron density on the central PDA unit flows into the benzene rings surrounding the central PDA unit, leading to the more delocalized SOMO (64ag). Hence the delocalized 64ag SOMO strengthens the non−disjoint character, resulting in the larger stabilization of the quartet state of 2′+. H



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Table 5. Spin Densities for the Ground States of 2′, 2′+, and 8+ N1 C1 C2 C3 C4 C5 C6 C7 C8 C9 C10 C11 N2 O1 N3 O2 C12 C13

2′ (S)a,b

2′+ (Q)a

8+ (D)a

−0.012 0.004 −0.004 −0.004 0.001 −0.038 0.023 0.023 −0.042 −0.048 0.049 −0.209 0.273 0.351 0.273 0.350 −0.024 −0.024

0.228 0.071 0.052 0.019 −0.018 −0.064 0.071 0.066 −0.075 −0.068 0.107 −0.221 0.272 0.378 0.271 0.380 −0.024 −0.024

0.261 0.098 0.064 0.030 −0.020 −0.022 0.034 0.028 −0.016 −0.012 0.032

Present Addresses ∥

Department of Applied Chemistry, Graduate School of Engineering, Osaka University, 2-1, Yamadaoka, Suita, Osaka 565-0871, Japan ⊥ Reseach Center for Low Temperature and Materials Sciences, Kyoto University, Sakyo-ku, Kyoto 606−8501, Japan. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partly supported by a Grant-in-Aid for Scientific Research (B) (No. 24310090) from the Japan Society for the Promotion of Science (JSPS) and by a Grant-in-Aid for Scientific Research on Innovative Areas, “New Polymeric Materials Based on Element-Blocks (No. 2401)” (Nos. 24102014 and 25102516) from the Ministry of Education, Culture, Sports, Science, and Technology, Japan. K.F. is grateful to the JSPS for a Grant-in-Aid for Scientific Research (B) (No. 25287091). Y.N. is indebted to the Chubei Itoh Foundation for support of this work.

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a

S, D, and Q represent the singlet, doublet, and quartet states, respectively. bIn the singlet state, the spin densities on two atoms related by Ci symmetry have the same magnitude but the opposite sign.

(1) Joachim, C.; Gimzewski, J. K.; Aviram, A. Electronics Using Hybrid-Molecular and Mono-Molecular Devices. Nature 2000, 408, 541−548. (2) Carroll, R. L.; Gorman, C. B. The Genesis of Molecular Electronics. Angew. Chem., Int. Ed. 2002, 41, 4379−4400. (3) Nitzan, A.; Ratner, M. A. Electron Transport in Molecular Wire Junctions. Science 2003, 300, 1384−1389. (4) Sato, O.; Tao, J.; Zhang, Y. Z. Control of Magnetic Properties through External Stimuli. Angew. Chem., Int. Ed. 2007, 46, 2152−2187. (5) Sugawara, T.; Matsushita, M. M. Spintronics in Organic πElectronic Systems. J. Mater. Chem. 2009, 19, 1738−1753. (6) Ito, A.; Urabe, M.; Tanaka, K. Molecular Design Toward SpinPolarized Nano-Wire: Oligomer Model Study. Polyhedron 2003, 22, 1829−1836. (7) Izuoka, A.; Hiraishi, M.; Abe, T.; Sugawara, T.; Sato, K.; Takui, T. Spin Alignment in Singly Oxidized Spin-Polarized Diradical Donor: Thianthrene Bis(nitronyl nitroxide). J. Am. Chem. Soc. 2000, 122, 3234−3235. (8) Nakazaki, J.; Chung, I.; Matsushita, M. M.; Sugawara, T.; Watanabe, R.; Izuoka, A.; Kawada, Y. Design and Preparation of Pyrrole-Based Spin-Polarized Donors. J. Mater. Chem. 2003, 13, 1011− 1022. (9) Harada, G.; Jin, T.; Izuoka, A.; Matsushita, M. M.; Sugawara, T. A Novel TTF-Based Donor Carrying Four Nitronyl Nitroxide. Tetrahedron Lett. 2003, 44, 4415−4418. (10) Nakano, Y.; Yagyu, T.; Hirayama, T.; Ito, A.; Tanaka, T. Synthesis and Intramolecular Magnetic Interaction of Triphenylamine Derivatives with Nitronyl Nitroxide Radicals. Polyhedron 2005, 24, 2141−2147. (11) Ito, A.; Nakano, Y.; Urabe, M.; Kato, T.; Tanaka, K. Triradical Cation of p-Phenylenediamine Having Two Nitroxide Radical Groups: Spin Alignment Mediated by Delocalized Spin. J. Am. Chem. Soc. 2006, 128, 2948−2953. (12) Crayston, J. A.; Devine, J. N.; Walton, J. C. Conceptual and Synthetic Strategies for the Preparation of Organic Magnets. Tetrahedron 2000, 56, 7829−7857. (13) Wolfe, J. P.; Wagaw, S.; Marcoux, J. F.; Buchwald, S. L. Rational Development of Practical Catalysts for Aromatic Carbon-Nitrogen Bond Formation. Acc. Chem. Res. 1998, 31, 805−818. (14) Hartwig, J. F. Carbon-Heteroatom Bond-Forming Reductive Eliminations of Amines, Ethers, and Sulfides. Acc. Chem. Res. 1998, 31, 852−860.

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CONCLUSION In conclusion, to accomplish the high-spin alignment by introducing the double-exchange-like interaction, we have prepared the novel multi-spin molecules 1−3, which consist of the peripheral nitronyl nitroxide (NN) groups and the redoxactive central para-phenylenediamine (PDA) unit, and the changes of the magnetic properties by the one-electron oxidation of these molecules have been thoroughly examined. For 1, the oxidation potential of the central PDA unit was higher than those of the peripheral NN groups, and therefore, the occurrence of the double-exchange-like interaction could not be fulfilled. On the other hand, for 2 and 3, the oxidation potential of the central PDA unit was lowered than those of the peripheral NN groups. The IVCT bands were observed in the UV−vis−NIR spectra of 2+ and 3+, and, therefore, the coexistence of the localized two spins on the peripheral NN groups and the delocalized spin on the central PDA unit was confirmed. The pulsed ESR measurements clearly exhibited that the spin quartet state was detected in 2+ and 3+, and these results exemplified the achievement of the spin alignment through the double-exchange-like interaction. In addition, detailed theoretical consideration was given about the case of 2′ as a model compound of 2.

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ASSOCIATED CONTENT

S Supporting Information *

Synthetic details for all new compounds, the ESR spectra of 3, the Curie plots for 2+ and 3+, the magnetic susceptibility measurements for 2 and 3, and complete reference 24. This material is available free of charge via the Internet at http:// pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. I



Article

(36) Heckman, A.; Lambert, C. Organic Mixed-Valence Compounds: A Playground for Electrons and Holes. Angew. Chem., Int. Ed. 2012, 51, 326−392. (37) Rathor, R.; Lindeman, S. V.; Kumar, A. S.; Kochi, J. K. Disproportionation and Structural Changes of Tetraarylethylene Donors Upon Successive Oxidation to Cation Radicals and to Dications. J. Am. Chem. Soc. 1998, 120, 6931−6939. (38) Nishiumi, T.; Nomura, Y.; Chimoto, Y.; Higuchi, M.; Yamamoto, K. The Class II/III Transition Electron Transfer on An Infrared Vibrational Time Scale for N,N′-Diphenyl-1,4-phenylenediamine Structures. J. Phys. Chem. B 2004, 108, 7992−8000. (39) Borden, W. T.; Davidson, E. R. Effects of Electron Repulsion in Conjugated Hydrocarbon Diradicals. J. Am. Chem. Soc. 1977, 99, 4587−4594. (40) Dougherty, D. A. Spin Control in Organic Molecules. Acc. Chem. Res. 1991, 24, 88−94. (41) Rajca, A. Organic Diradicals and Polyradicals: From Spin Coupling to Magnetism? Chem. Rev. 1994, 94, 871−893. (42) Bushby, R. J.; McGill, D. R.; Ng, K. M. Magnetism: A Supramolecular Function; Kahn, O., Ed.; Kluwer: Dordrecht, The Netherlands, 1996.

(15) Hartwig, J. F. Transition Metal Catalyzed Synthesis of Arylamines and Aryl Ethers from Aryl Halides and Triflates: Scope and Mechanism. Angew. Chem., Int. Ed. 1998, 37, 2046−2067. (16) Muci, A. R.; Buchwald, S. L. Practical Palladium Catalysts for C−N and C−O Bond Formation. Top. Curr. Chem. 2002, 219, 131− 209. (17) Oelschläger, H.; Peters, H. J. Ergiebige Synthese des 10Methylphenothiazin-3,7-dicarbaldehyds. Arch. Pharm. 1987, 320, 379− 381. (18) Ullman, E. F.; Osiecki, J. H.; Boocock, D. G. B.; Darcy, R. Studies of Stable Free Radicals. X. Nitronyl Nitroxide Monoradicals and Biradicals as Possible Small Molecule Spin Labels. J. Am. Chem. Soc. 1972, 94, 7049−7059. (19) Fauth, J.-M.; Schweiger, A.; Braunschweiger, L.; Forrer, J.; Ernst, R. R. Elimination of Unwanted Echoes and Reduction of Dead Time in Three-Pulse Electron Spin-Echo Spectroscopy. J. Magn. Reson. 1986, 66, 74−85. (20) Gemperle, C.; Aebli, G.; Schweiger, A.; Ernst, R. R. Phase Cycling in Pulse EPR. J. Magn. Reson. 1990, 88, 241−256. (21) Mims, W. B. Envelope Modulation in Spin-Echo Experiments. Phys. Rev. B 1972, 5, 2409−2419. (22) Ragavachari, K. Perspective on “Density Functional Thermochemistry. III. The Role of Exact Exchange. Theor. Chem. Acc. 2000, 103, 361−363 and references cited therein.. (23) Hehre, W. J.; Radom, L.; Schleyer, P. v. R.; Pople, J. A. Ab Initio Molecular Orbital Theory; Wiley: New York, 1986. (24) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A.; Stratmann, R. E., Jr.; Burant, J. C. et al. Gaussian 98, revision A.9; Gaussian Inc.: Pittsburgh, PA, 1998. (25) Bell, F. A.; Ledwith, A.; Sherrington, D. C. Cation-Radicals: Tris-(p-bromophenyl)amminium Perchlorate and Hexachloroantimonate. J. Chem. Soc. C 1969, 1969, 2719−2720. (26) Connell, N. G.; Geiger, W. E. Chemical Redox Agents for Organometallic Chemistry. Chem. Rev. 1996, 96, 877−910. (27) Weltner, W., Jr. Magnetic Atoms and Molecules; Dover: New York, 1989. (28) Isoya, J.; Kanda, H.; Norris, J. R.; Tang, J.; Brown, M. K. Fourier-Transform and Continuous-Wave EPR Studies of Nickel in Synthetic Diamond: Site and Spin Multiplicity. Phys. Rev. B 1990, 41, 3905−3913. (29) Astashkin, A. V.; Schweiger, A. Electron-Spin Transient Nutation: A New Approach to Simplify the Interpretation of ESR Spectra. Chem. Phys. Lett. 1990, 174, 595. (30) Sato, K.; Yano, M.; Furuichi, M.; Shiomi, D.; Takui, T.; Abe, K.; Itoh, K.; Higuchi, A.; Katsuma, K.; Shirota, Y. Polycationic High-Spin States of One- and Two-Dimensional (Diarylamino)benzenes, Prototypical Model Units for Purely Organic Ferromagnetic Metals As Studied by Pulsed ESR/Electron Spin Transient Nutation Spectroscopy. J. Am. Chem. Soc. 1997, 119, 6607−6613. (31) Bock, H.; Gharagozloo-Hubmann, K.; Sievert, M.; Prisner, T.; Havlas, Z. Single Crystals of an Ionic Anthracene Aggregate with a Triplet Ground State. Nature 2000, 404, 267−269. (32) Ito, A.; Ino, H.; Tanaka, K.; Kanemoto, K.; Kato, T. Facile Synthesis, Crystal Structures, and High-Spin Cationic States of Allpara-Brominated Oligo(N-phenyl-m-aniline)s. J. Org. Chem. 2002, 67, 491−498. (33) Hirao, Y.; Ino, H.; Ito, A.; Tanaka, K.; Kato, T. High-Spin Radical Cations of a Dendritic Oligoarylamine. J. Phys. Chem. A 2006, 110, 4866−4872. (34) On the other hand, only the doublet nutation signal was observed in the ESTN spectra for neutral species of 2 and 3, thus indicating the very weak magnetic interaction between unpaired electrons on the peripheral two NN groups in the neutral state of 2 and 3. This result was also corroborated by the magnetic susceptibility measurements for the polycrystalline samples of 2 and 3 (Figure S4 in the Supporting Information). (35) Hankache, J.; Wenger, O. S. Organic Mixed Valence. Chem. Rev. 2012, 111, 5138−5178. J


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