Table 1. Optical, Electrochemical, and Magnetic Properties of Biradicals 1ꢀ3
EC
OP
λmax
ε
Eox
(V)a
Ered
(V)b
ESOMO
(eV)c
ELUMO
(eV)d
Eg
(eV)i
Eg
(eV)e
Θ
(K)f
Jintra (K)g
Jintra (K)h
(nm)
(cmꢀ1 Mꢀ1
)
calcd
exptl
1
604
641
520
524
599
642
460
450
551
393
389
368
0.950
ꢀ0.485
ꢀ5.012
ꢀ3.578
1.434
1.614
ꢀ4.3
ꢀ14.5
ꢀ14
2
3
1.238
ꢀ0.603
ꢀ0.705
ꢀ5.600
ꢀ5.019
ꢀ3.750
ꢀ3.526
1.850
1.492
2.049
1.624
ꢀ5.7
ꢀ4.2
ꢀ3.7
ꢀ8.0
ꢀ4.5
ꢀ9.0
0.786 1.246
a,b 0.1 M of n-Bu4NPF6, in acetonitrile, Pt electrode, scan rate 100 mV sꢀ1
.
c,d Calculated based on formula ESOMO = ꢀ(Eox,onset ꢀ E(1/2) Fcþ/Fc þ
4.8) and ELUMO = ꢀ(Ered,onset ꢀ E(1/2) Fcþ/Fc þ 4.8 eV. e Optical energy gap calculated according to the absorption edge. f Weiss temperature.
g Calculated using BLYP/6-31G(d). h Calculated using isolated dimer model (s = 1/2). i Electrochemical energy gap.
the latter is due to delocalization of aminoxy anion as
shown in Scheme 2.12 The biradical 2 showed nonrever-
sible oxidation13 wave at Eox = 1.238 V and reversible
reduction waveatEred =ꢀ0.603 V. Interestingly, biradical
3 displayed similar redox behavior as 1 with an additional
nonreversible oxidation wave. This nonreversible wave
can be assigned to oxidation of the IN radical moiety.
As shown in Table 1, the electrochemical band gap is in
accordance with the optical band gap. The higher SOMO
level of biradicals shows their ability as donor molecule
to form charge-transfer complexes.
The X-band ESR spectra were recorded in oxygen-free
toluene at room temperature. The typical ESR spectrum of
1 (Figure S2, Supporting Information) consisted of nine
well-resolved lines due to hyperfine coupling (hfc) of two
electron spins with four equivalent nitrogen atoms. The
experimental spectrum of 1 showed a good agreement with
a simulated spectrum considering nitrogen hfc (aN/2) value
0.373 mT (which is half of the hfc observed for mononi-
tronyl nitroxide aN = 0.748 mT) at g = 2.0066. The 13-line
spectrum of biradical 2 (Figure S2, Supporting Informa-
tion) was reproduced with hfc values aN1/2 = 0.225 and
As magnetic interactions are highly dependent on the
geometry and packing of molecules in the crystal lattice,14
the single crystals of biradicals 1ꢀ3, obtained by slow
diffusion of hexane to the solution of biradicals in DCM,
were investigated with single-crystal X-ray diffraction.
Crystal structure analysis revealed that molecules 1ꢀ3
crystallize in the monoclinic P21/n space group with
similar unit cell parameters (Table S3, Supporting Infor-
mation). Furthermore, they also possess a similar arrange-
ment of molecules in a herringbone pattern (Figure S5,
Supporting Information). Thus, it is considered that
biradical 1ꢀ3 are isomorphous. However, interestingly a
significant difference was observed in interplanar spacing.
Namely, the shortest πꢀπ stacking distance was observed
aN2/2 = 0.440 at g = 2.0059. However, to simulate the
nonsymmetric ESR spectra of biradical 3 (Figure 2), three
different types of N nuclei and thus hfc values were taken
into account: two equivalent N nuclei for the NN unit
(withhfcaN1) and two inequivalent N nuclei forthe INunit
(with hfc aN2 and aN3). The best fitting hfc values were
aN1/2 = 0.374 mT for the NN moiety and aN2/2 = 0.200
and aN3/2 = 0.460 for the IN moiety with a giso value of
2.0062. The ESRspectra for all biradicalsdemonstrate that
the exchange interactions (J) between the radical moieties
are much larger than the hyperfine coupling (J . aN).
˚
˚
in biradical 1 (3.730 A) followed by biradical 3 (4.258 A)
˚
and 2 (4.367 A) (Figure S4, Supporting Information).
These differences can be attributed to the influence of
the radical moiety on the pyrene core. The torsion
angles between the NN moiety and the pyrene ring in
1 are 15° (C3ꢀC4ꢀC11ꢀN1) and 14.8° (C5ꢀC4ꢀ
C11ꢀN2). The IN moiety in biradical 2 is nearly coplanar
with the pyrene ring with a smaller torsion angle of 3.9°
(C3ꢀC4ꢀC11ꢀN1). The intermediate torsion angles are
observed in biradical 3, 5.4° (C3ꢀC4ꢀC9ꢀN1) and 4.7°
(C5ꢀC4ꢀC9ꢀN2).
(13) (a) Kadirov, M.; Tretyakov, E.; Budnikova, Y.; Valitov, M.;
Holin, K.; Gryaznova, T.; Ovcharenko, V.; Sinyashin, O. J. Electroanal.
Chem. 2008, 624, 69. (b) Budnikova, Y. G.; Gryaznova, T. V.; Kadirov,
M. K.; Tret’yakov, E. V.; Kholin, K. V.; Ovcharenko, V. I.; Sagdeev,
R. Z.; Sinyashin, O. G. Russ. J. Phys. Chem. A 2009, 83, 1976.
Figure 2. X-band ESR spectra of biradical 3in toluene (c=10ꢀ4 M)
at room temperature.
(14) (a) Tamura, M.; Nakazawa, Y.; Shiomi, D.; Nozawa, K.;
Hosokoshi, Y.; Ishikawa, M.; Takahashi, M.; Kinoshita, M. Chem.
Phys. Lett. 1991, 186, 401. (b) Tamura, M.; Hosokoshi, Y.; Shiomi, D.;
Kinoshita, M.; Nakasawa, Y.; Ishikawa, M.; Sawa, H.; Kitazawa, T.;
Eguchi, A.; Nishio, Y.; Kajita, K. J. Phys. Soc. Jpn. 2003, 72, 1735.
To gain insight into the magnetic exchange interactions,
magnetic susceptibilities and magnetizations of polycrys-
talline samples were measured in the temperature range of
Org. Lett., Vol. XX, No. XX, XXXX
C