Evaluation of the β value of the phenylene unit by probing exchange interaction between two nitroxides

Article abstract of DOI:10.1021/ol102339m

Oligophenylene molecular rods with bicyclo[2.2.2]octane having two nitronyl nitroxide radicals were synthesized to investigate the decay constant of p-phenylene. By the measurement and simulation of the ESR spectra of the biradicals with different rod len

Full text of DOI:10.1021/ol102339m

ORGANIC
LETTERS
2010
Vol. 12, No. 22
5284-5286
Evaluation of the ꢀ Value of the
Phenylene Unit by Probing Exchange
Interaction between Two Nitroxides
Kenji Higashiguchi, Koji Yumoto, and Kenji Matsuda*
Department of Synthetic Chemistry and Biological Chemistry, Graduate School of
Engineering, Kyoto UniVersity, Katsura, Nishikyo-ku, Kyoto 615-8510, Japan
kmatsuda@sbchem.kyoto-u.ac.jp
Received September 28, 2010
ABSTRACT
Oligophenylene molecular rods with bicyclo[2.2.2]octane having two nitronyl nitroxide radicals were synthesized to investigate the decay
constant of p-phenylene. By the measurement and simulation of the ESR spectra of the biradicals with different rod length, it was found that
the exchange interaction was decreased with the decay constant ꢀ of 0.51 ( 0.01 Å-1. This result indicates that the spin-spin exchange
interaction between neutral radicals has a decay constant similar to the molecular conductance.
The relationship between molecular conductance and mo-
lecular structure in organic molecules is attracting interest
in the field of molecular electronics.1-5 Electron tunneling
through molecular wire is described by the Landauer formula
using transmission probability which decays exponentially
with molecular length. The conductive property of the
molecular wire is characterized by the decay constant ꢀ
according to the following equation
Among many organic molecules, oligo(p-phenylene) has
a rigid π-conjugated structure, and therefore it is a
representative molecule to discuss molecular conductance.
The decay constant has been obtained not only from the
direct measurement of molecular conductance6,7 but also
from the rate of electron transfer8 or the magnitude of
exchange interaction,9,10 all of which decay exponentially
with molecular length. Theoretical studies have also been
performed to evaluate the relationship among the rate of
electron transfer, the molecular conductance, and the decay
constant.11-15
G ) G0 exp(-ꢀl)
where l is the molecular length and G0 is the contact
conductance.
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10.1021/ol102339m  2010 American Chemical Society
Published on Web 10/20/2010

The decay constant of the exchange interaction has been
measured by means of the effect of the magnetic field on
the electron transfer9 and the splitting of the molecular
orbital.10 The exchange interaction between spins of unpaired
electron, which is the origin of the magnetism, can be directly
measured by the temperature dependence of magnetic
susceptibility16 or the simulation of the ESR spectrum.17
Herein, we describe the evaluation of decay constant ꢀ of
oligo-p-phenylene molecular rods by the measurement of
exchange interaction between two nitronyl nitroxide radicals.
The analysis of the splitting pattern of the ESR spectrum
provides the information on the spin-spin exchange interac-
tion between the nitroxide radicals.
The nitronyl nitroxide radical was used as a spin source
because of the stability under air. Nitronyl nitroxide itself
has two identical nitrogen atoms to give a 5-line ESR
spectrum, with a 7.5 G spacing and g ) 2.006. When two
nitronyl nitroxides are magnetically coupled with an ex-
change interaction, the diradical gives a 9-line ESR spectrum
with a 3.7 G spacing. If the exchange interaction is smaller
than the hyperfine coupling in the diradical, two nitronyl
nitroxide radicals are magnetically independent and give the
spectrum that is identical to the monoradical. In intermediate
situations, the spectrum shows further splitting.18
In the midst of the p-phenylene rod, a bicyclo[2.2.2]octane
unit was introduced as a regulator because an exchange
interaction through two or three phenylene units is too large
to be evaluated by the analysis of the ESR splitting pattern.
When two nitronyl nitroxides are attached directly to the
terphenylene spacer, the obtained ESR signal was clear 9
lines, so that the analysis of the splitting pattern was
impossible (see Supporting Information). The synthesized
molecules are shown in Figure 1.
The synthesis of diradicals 1-3 was performed using 1,4-
dibromobicyclo[2.2.2]octane as an intermediate (see Supporting
Information).19 Several steps of reactions afforded diformyl
derivatives. These formyl derivatives were refluxed in appropri-
ate solvent with 2,3-bis(hydroxyamino)-2,3-dimethylbutane
sulfate in the presence of potassium carbonate. The cyclode-
hydrated derivatives were oxidized with sodium periodate in
dichloromethane/water to give nitronyl nitroxide derivatives
1-3. The structures of the synthesized compounds were
confirmed by NMR/ESR spectroscopy and mass spectrometry.
The change of the ESR spectra with the increase of phenylene
moieties was examined. Figure 2 shows the ESR spectra of
biradicals 1, 2, and 3 measured in N2-bubbled dichloromethane
solution at room temperature. The ESR spectrum of 1 shows
distorted 9 lines. This suggests that the central bicyclo[2.2.2]octane
Figure 1. Synthesized bis(nitronyl nitroxide) radicals with a
p-phenylene molecular rod.
unit plays a role of a regulator because the terphenylene
derivative showed clear 9 lines. The ESR spectrum of 2 shows
15 lines. The exchange interaction between the two spins in 2
decreased with the introduction of one phenylene moiety
compared with 1. The biradical 3 shows a 5-line ESR spectrum,
and it means that the exchange interaction in 3 is smaller than
the hyperfine coupling constant.
To estimate the exchange interaction of the biradicals, the
obtained spectra were simulated using several exchange
interactions using the BIRADG program.20 From the simula-
tion, the exchange interactions in 1 and 2 were determined
to be |2J/gµB| ) 1.2 × 102 G (|2J/kB| ) 1.6 × 10-2 K) and
16 G (|2J/kB| ) 2.2 × 10-3 K), respectively. Biradical 3
showed a 5-line spectrum, which generally indicates |2J/gµB|
< 2 G. However, by close examination of ∆HPP of the peaks,
which gets larger by the increase of the exchange interaction,
the |2J/gµB| value was determined as 1.4 ( 0.2 G (|2J/kB| )
(1.9 ( 0.2) × 10-4 K) for 3 (see Supporting Information).21
From these exchange interaction values, the decay constant
of the phenylene moiety was calculated (Figure 3). The
exchange interaction was decreased by the factor of 0.10
per one phenylene unit, and the ꢀ value was obtained as ꢀ
) 0.51 ( 0.01 Å-1 by assuming the length of one phenylene
unit as 4.4 Å. The reported decay constant determined by
the molecular conductance6 was 0.42 Å-1, and the value
determined by electron transfer8 was 0.32 Å-1. The similar
values are reported by the theoretical studies.11-14
Concerning the decay constant of the exchange interaction,
Wasielewski et al. studied the decay of the spin-spin
exchange interaction by the effect of the magnetic field on
the electron transfer and determined the value as ꢀ ) 0.37
Å-1.9 Lin et al. studied the decay constant of superexchange
coupling of the orbitals by STM measurement and reported
ꢀ as 0.10 Å-1.10 In the surface of the substrate, the dihedral
angle between phenylene units is considered to be reduced
to give a smaller decay constant. Additionally, because the
(11) Magoga, M.; Joachim, C. Phys. ReV. B 1997, 56, 4722–4729.
(12) Kaun, C.-C.; Larade, B.; Guo, H. Phys. ReV. B 2003, 67, 121411.
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9143–9149.
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Zhang, H. ChemPhysChem 2008, 9, 1416–1424.
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1378.
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1586.
(20) Kirste, B. Anal. Chim. Acta 1992, 265, 191–200.
(21) By simply changing the linewidth, the simulation could not
reproduce the observed spectrum (see Supporting Information).
Org. Lett., Vol. 12, No. 22, 2010
5285

Figure 2. X-band ESR spectra measured at room temperature of (a) biradical 1 (g ) 2.0061), (b) biradical 2 (g ) 2.0064), and (c) biradical
3 (g ) 2.0066). Simulated spectra of (d) |2J/gµB| ) 1.2 × 102 G, (e) 16 G, and (f) 1.4 G.
observed. Further theoretical and experimental investiga-
tion provides the deeper understanding of the transport
phenomena through molecules.
In conclusion, oligophenylene molecular rods with
bicyclo[2.2.2]octane having two nitronyl nitroxide radicals
were synthesized to investigate the decay constant of
p-phenylene. By the measurement and simulation of the
ESR spectra of the biradicals with different rod lengths,
it was found that the exchange interaction was decreased
with the decay constant ꢀ of 0.51 ( 0.01 Å-1. This result
indicates that the spin-spin exchange interaction between
neutral radicals has a decay constant similar to the
molecular conductance and the rate of electron transfer.
Acknowledgment. We thank Professor Yasujiro Murata
(Kyoto Univ., Japan) for the measurement of ESR spectra. We
thank Professor Paul M. Lahti (Univ. of Massachusetts, USA)
and Professor B. Kirste (Free Univ. of Berlin, Germany) for
providing the program BIRADG. This work was supported by
CREST, JST, and a Grant-in-Aid for Young Scientists (A) (No.
19685013) from the MEXT, Japan.
Figure 3. Correlation of the exchange interaction with the number
of phenylene moieties.
rate constant is formulated to be proportional to the square
of the exchange coupling, the decay constant of the exchange
coupling is predicted to be half of the decay constant of the
rate of the electron transfer.15,22
Supporting Information Available: Experimental meth-
1
ods, H NMR spectra of synthesized compounds, ESR
spectrum of reference compound, and details of the
spectrum simulation. This material is available free of
charge via the Internet at http://pubs.acs.org.
The obtained decay constant of the p-phenylene unit, ꢀ
) 0.51 ( 0.01 Å-1, is very similar to the reported
experimental and theoretical values of the decay constant
of the molecular conductance and the rate of electron
transfer. This value is the first example of the decay
constant determined by the spin-spin exchange interaction
between neutral radicals. In the molecular conductance
and the rate of electron transfer, in addition to the coherent
tunneling, incoherent hopping becomes important as the
molecule gets longer.23 In this study of exchange interac-
tion, the participation of the hopping mechanism was not
OL102339M
(22) In ref 10, the decay constant of the molecular conductance and the
rate of electron transfer was divided by 2 for comparison. The value taken
from ref 9, which is the exchange interaction, was also divided by 2.
(23) (a) Giese, B.; Amaudrut, J.; Ko¨hler, A.-K.; Spormann, M.; Wessely,
S. Nature 2001, 412, 318–320. (b) Berlin, Y. A.; Burin, A. L.; Ratner,
M. A. Chem. Phys. 2002, 275, 61–74. (c) Choi, S. H.; Kim, B.; Frisbie,
C. D. Science 2008, 320, 1482–1486.
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