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

Article abstract of DOI:10.1021/jo4015062

A series of nitronyl nitroxide radicals having different lengths of phenylene ethynylene molecular wire were synthesized to investigate the decay constant of p-phenylene ethynylene. By the measurement and simulation of the ESR spectra of the biradicals, i

Full text of DOI:10.1021/jo4015062

Article
pubs.acs.org/joc
Evaluation of the β Value of the Phenylene Ethynylene Unit by
Probing the Exchange Interaction between Two Nitronyl Nitroxides
Masataka Shinomiya,† Kenji Higashiguchi,†,‡ and Kenji Matsuda*,†
†Department of Synthetic Chemistry and Biological Chemistry, Graduate School of Engineering, Kyoto University, Katsura,
Nishikyo-ku, Kyoto 615-8510, Japan
‡PRESTO, Japan Science and Technology Agency, Kawaguchi 332-0012, Japan
S
* Supporting Information
ABSTRACT: A series of nitronyl nitroxide radicals having
different lengths of phenylene ethynylene molecular wire were
synthesized to investigate the decay constant of p-phenylene
ethynylene. By the measurement and simulation of the ESR
spectra of the biradicals, it was found that the exchange
interaction decreased with a decay constant (β) of 0.39 Å−1 as
the length of the molecule increased. This result indicates that
the spin−spin exchange interaction between neutral radicals
has a decay constant similar to that of the molecular conductance. This value of the decay constant indicates that a hopping
mechanism does not take place in the measurement of the exchange interaction between neutral radicals even when the
molecular wire has enough length to show hopping conduction of electrons.
the magnitude of the exchange interaction,18 all of which decay
exponentially with molecular length. We recently reported that
INTRODUCTION
■
Molecular conductance of organic molecules is attracting
the decay constant of π-conjugated wires can also be obtained
interest in the field of molecular electronics because the
by measurement19 and calculation20 of the exchange interaction
conductance depends on the molecular geometric and
electronic structure.1−5 Tunneling and hopping mechanisms
between spins of unpaired electrons originating from organic
radicals placed at the two ends of the π-conjugated wire. The
have been proposed for the origin of molecular conductance,
and the tunneling mechanism is supposed to be dominant in
reported decay constants of the exchange interaction were
short molecular wires.6 Electron tunneling through the
similar to the decay constants of the molecular conductance.
Herein we report the decay constant β of OPE that was
molecular wire is described with a transmission probability
that decays exponentially with molecular length. The molecular
conductance G is formulated in terms of the decay constant β
according to eq 1:
evaluated by measurement of the exchange interaction between
two nitronyl nitroxide radicals. Comparison with the decay
constant obtained by conductance measurements is discussed
with respect to the mechanism of decay.
G = G0 exp(−βl)
(1)
RESULTS AND DISCUSSION
■
where l is the molecular length and G0 is the contact
conductance.
Oligo(p-phenylene ethynylene) (OPE) is a representative
Nitronyl nitroxide radical was used as a spin source because the
radical is stable under air. Nitronyl nitroxide itself has two
identical nitrogen atoms to give a five-line ESR spectrum. When
molecular wire that is often used in the study of molecular
electronics.6−11 The length dependence of the molecular
two nitronyl nitroxides are magnetically coupled through an
exchange interaction, the diradical gives an exchange-coupled
conductance of OPE has been measured by several groups,
and the decay constant β is reported to be 0.20−0.34 Å−1,6,8,9
ESR spectrum, and the value of the exchange interaction can be
obtained by analyzing the splitting pattern of the spectrum.21
which is smaller than that of oligo(p-phenylene) (0.35−0.42
Å−1).12,13 Wang and co-workers also reported that when the
molecular length is short enough, the conduction occurs
dominantly by the tunneling mechanism, but as the molecule
gets longer, the hopping mechanism starts to contribute.6
Meanwhile, theoretical calculations of the conductance of OPE
have been carried out, and the β value is reported to be 0.19−
0.27 Å−1, which is in good agreement with the experiment.14,15
The decay constants of molecular wires have been obtained
not only from the direct measurement of molecular
conductance but also from the rate of electron transfer16,17 or
The synthesized series of molecules OPE4−OPE7 are
displayed in Chart 1. In order to estimate the exchange
interaction, this interaction should be in the range between
|2J/gμB| = 1 G and |2J/gμB| = 300 G because only in this region
does the ESR spectrum show a splitting pattern that enables us
to estimate the exchange interaction by simulation. Therefore,
the number of benzene rings was set to be 4 to 7. Butoxy
Received: July 11, 2013
Published: August 28, 2013
© 2013 American Chemical Society
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Chart 1. Synthesized Series of Bis(nitronyl nitroxide) Radicals with Different Lengths of p-Phenylene Ethynylene Molecular
Wire
Scheme 1
groups were introduced to increase the solubility. It has been
reported experimentally and theoretically that the introduction
of alkoxy groups does not affect the transport of electrons.8,22,23
In the conventional method for preparing nitronyl nitroxides,
a formyl derivative is reacted with 2,3-bis(hydroxyamino)-2,3-
dimethylbutane and then the obtained cyclodehydrated
derivative is oxidized to give the nitronyl nitroxide. The
drawback of this method is that when several formyl groups are
to be converted to nitronyl nitroxides, the yield of this step can
be very low. Therefore, we adopted an alternative way to
synthesize bis(nitronyl nitroxide)s. The key intermediate is
iodo-substituted TMS-protected bis(hydroxylamine) 1, which
was synthesized from 4-iodobenzaldehyde in 80% yield
according to the similar method reported for other silyl-
protected bis(hydroxylamine)s (Scheme 1).24,25 It was found
that 1 can react with a terminal alkyne using Sonogashira
coupling conditions and that deprotection using tetrabutylam-
monium fluoride proceeds smoothly. What is even better is that
deprotection and oxidation proceed simultaneously. Therefore,
simple deprotection gives the nitronyl nitroxide.
Bis(terminal alkyne)s 8, 10, 11, and 13 were synthesized
starting from 1,4-dibutoxy-2,5-diiodobenzene (2) and tri-
methylsilylacetylene (Scheme 2) using repetitive Sonogashira
coupling and deprotection of TMS. Sonogashira coupling
between bis(terminal alkyne)s and iodo-substituted TMS-
protected bis(hydroxylamine) 1 afforded TMS-protected tetra-
(hydroxylamine)s OPE4TMS−OPE7TMS (Scheme 3). The
obtained TMS-protected tetra(hydroxylamine)s were depro-
tected by fluoride ion and then autooxidized to give
bis(nitronyl nitroxide)s OPE4−OPE7. The structures of the
synthesized compounds were confirmed by NMR and ESR
spectroscopy and mass spectrometry.
Phenylene ethynylene is a representative fluorescent unit that
is used in functional fluorescent materials.26 The synthesized
OPE4TMS−OPE7TMS were characterized by absorption and
fluorescence spectroscopy (Figure 1). As the length of the
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Scheme 2
molecule increased, both the absorption and fluorescence
spectra showed bathochromic shifts: the absorption maximum
shifted from 396 nm for OPE4TMS to 422 nm for OPE7TMS,
and fluorescence maximum shifted from 435 nm for
OPE4TMS to 469 nm for OPE7TMS. These results are
similar to those for reported OPEs. Alkoxy substituents on the
benzene ring have been reported to induce bathochromic shifts
in the absorption and fluorescence spectra,27 though no effect
on the electron transport has been reported.8,22,23
(Figure 3). The value β = 0.39 Å−1 was obtained by assuming
that the length of one phenylene ethynylene unit is 6.9 Å.6 The
J values of OPE4 (|2J/gμB| > 300 G) and OPE7 (|2J/gμB| < 1
G) could not be determined, but the obtained β value and the
molecular length were consistent with the observed spectra.
The reported decay constant β determined from the molecular
conductance was 0.20−0.34 Å−1 and the reported theoretical
value was 0.19−0.27 Å−1, which are in good agreement with our
result.
The change in the ESR spectrum with increasing number of
phenylene ethynylene moieties was examined. Figure 2 shows
the ESR spectra of biradicals OPE4−OPE7 measured in N2-
bubbled dichloromethane solution at room temperature. The
ESR spectrum of OPE4 shows nine lines. The ESR spectra of
OPE5 and OPE6 show 15 lines. The exchange interaction
between the two spins in OPE5 and OPE6 decreased with the
introduction of phenylene ethynylene moieties. The biradical
OPE7 shows a five-line ESR spectrum, which means that the
exchange interaction in OPE7 is smaller than the hyperfine
coupling constant.
To estimate the exchange interaction of the biradicals, the
obtained spectra were simulated for several exchange
interactions using the BIRADG program.28 Biradical OPE4
showed a nine-line spectrum, which indicates |2J/gμB| > 300 G.
From the simulation, the exchange interactions in OPE5 and
OPE6 were determined to be |2J/gμB| = 1.2 × 102 G (|2J/kB| =
1.6 × 10−2 K) and |2J/gμB| = 16 G (|2J/kB| = 2.2 × 10−3 K),
respectively. Biradical OPE7 showed a five-line spectrum,
which indicates |2J/gμB| < 1 G.
For OPE5 and OPE6, when spectra for smaller J values were
added to the original simulated spectra, the summed spectra
reproduced the observed spectra more accurately, as shown in
Figure 2i,j. This means that a certain fraction of the biradical
molecules may adopt the conformation that gives the smaller
exchange interaction. The spectrum of the longer wire OPE6
was simulated using a larger amount of the additional spectrum
than the spectrum of OPE5, suggesting that the ratio of the
conformation that gives the smaller J value gets larger as the
wire gets longer. Presumably, the longer wire has more
possibility to adopt the conformation that gives the smaller J
value.
In the measurement of the length dependence of the
molecular conductance, in addition to the tunneling process,
the hopping process becomes important as the molecule gets
longer.6,29 Lu et al.6 reported that for OPE derivatives, when
the molecule gets longer than three OPE units, the hopping
mechanism appears and the decay constant becomes very small.
The β value decreases from 0.20 to 0.03 Å−1 when the
mechanism switches from tunneling to hopping. In this study,
the observed β value was 0.39 Å−1, and therefore, participation
of the hopping mechanism was not observed. In the case of
From these values of the exchange interaction, the decay
constant of the phenylene ethynylene moiety was calculated
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Scheme 3
Figure 1. (a) Absorption and (b) fluorescence (λex = 365 nm) spectra of OPE4TMS (black dashed lines), OPE5TMS (gray dotted lines),
OPE6TMS (gray dashed lines), and OPE7TMS (black solid lines) measured in CH2Cl2. The spectra are normalized to the maximum intensity.
molecular conductance, the participation of locally charged
species is responsible for the hopping mechanism, but in the
case of the exchange interaction, there are no such locally
charged species. Therefore, the exchange interaction reflects a
pure tunneling mechanism between the two unpaired electrons.
In nitronyl nitroxide derivatives, the spin density is mainly
localized in the radical substituent, and the contributions of the
resonance structures are small (Scheme 4). This also supports
the conclusion that our method reflects purely the electron
tunneling through π-conjugation. Further theoretical and
experimental investigation should provide deeper understand-
ing of the transport phenomena through molecules.
CONCLUSIONS
■
OPE molecular wires OPE4−OPE7, which have two nitronyl
nitroxide groups separated by different lengths of molecular
wire, were synthesized to evaluate the decay constant (β) of the
spin−spin exchange interaction (J) of the p-phenylene
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Figure 2. (a−d) X-band ESR spectra measured at room temperature for (a) OPE4 (g = 2.0067), (b) OPE5 (g = 2.0065), (c) OPE6 (g = 2.0066),
and (d) OPE7 (g = 2.0065). (e−h) Simulated spectra for |2J/gμB| values of (e) 1800, (f) 120, (g) 8.0, and (h) 0.53 G. (i) Simulated spectrum for
80% |2J/gμB| = 120 G + 20% |2J/gμB| = 45 G. (j) Simulated spectrum for 51% |2J/gμB| = 8.0 G + 49% |2J/gμB| = 3.0 G.
in the measurement of the exchange interaction between
neutral radicals even when the molecular wire has enough
length to show hopping conduction of electrons.
EXPERIMENTAL SECTION
■
1. Preparation of Materials. General. All of the reactions were
monitored by thin-layer chromatography carried out on 0.2 mm silica
gel plates. Column chromatography was performed on silica gel. Some
compounds were purified by recycling preparative GPC. HPLC with a
semipreparative column was used to purify biradicals OPE4−OPE7.
1H and 13C NMR spectra were recorded on 400, 500, and 600 MHz
instruments. Samples were dissolved in CDCl3, and tetramethylsilane
was used as a standard. 13C NMR spectra of OPE4TMS−OPE7TMS
were measured at −40 °C. High-resolution mass spectra were obtained
by APCI, ESI, and DART ionization using an Orbitrap analyzer.
1,4-Dibutoxy-2,5-diiodobenzene (2) was prepared according to the
literature procedure.30 Compounds 3, 4, 5, 7, and 11 were known
compounds.31 Iodo-substituted TMS-protected bis(hydroxylamine) 1
was synthesized in a similar manner as the reported bromo-substituted
derivative.25
1,3-Bis((trimethylsilyl)oxy)-2-(4-iodophenyl)-4,4,5,5-tetramethyl-
imidazolidine (1). A dry methanol (100 mL) solution of 4-
iodobenzaldehyde (2.3 g, 10.0 mmol), 2,3-bis(hydroxyamino)-2,3-
dimethylbutane sulfate (5.0 g, 20.0 mmol), and K2CO3 (2.8 g, 20.0
mmol) was refluxed for 6 h. The reaction mixture was poured into
water, extracted with ethyl acetate, dried over MgSO4, and
Figure 3. Correlation of the exchange interaction with the number of
phenylene ethynylene moieties. The decay constant β was determined
to be 0.39 Å−1.
ethynylene unit. OPE5 and OPE6 showed complicated ESR
spectra, and the values of J were determined by comparison
with simulated ESR spectra. It was found that the exchange
interaction decreased with the decay constant β = 0.39 Å−1.
This result indicates that the spin−spin exchange interaction
between neutral radicals has a decay constant similar to that of
the molecular conductance. The J values of OPE4 and OPE7
could not be determined, but the obtained β value was
consistent with the observed spectra. This value of the decay
constant indicates that hopping mechanism does not take place
Scheme 4
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1
concentrated to give the bis(hydroxylamine) derivative. Purification
was not performed.
6: H NMR (500 MHz, CDCl3) δ 0.26 (s, 9H), 0.96−1.00 (m,
12H), 1.49−1.58 (m, 8H), 1.77−1.83 (m, 8H), 3.34 (s, 1H), 3.96−
4.02 (m, 8H), 6.94 (s, 1H), 6.96 (s, 1H), 6.97 (s, 1H) 6.98 (s, 1H);
13C NMR (100 MHz, CDCl3) δ −0.1, 13.84, 13.86, 13.91, 19.16,
19.21, 19.23, 31.2, 31.32, 31.34, 31.36, 69.19, 69.23, 69.3, 69.4, 80.0,
82.3, 91.2, 91.4, 100.1, 101.1, 112.5, 113.8, 114.4, 114.9, 117.0, 117.1,
117.3, 117.9, 153.26, 153.34, 154.1, 154.2; ESI HRMS (m/z) [M +
H]+ calcd for C37H51O4Si 587.3551, found 587.3559.
To a THF (100 mL) solution of the bis(hydroxylamine) were
slowly added large excesses of triethylamine (8.4 mL, 60.0 mmol) and
chlorotrimethylsilane (5.2 mL, 60.0 mmol) at room temperature. After
the mixture was stirred at 45 °C for 20 h, the solvent was evaporated.
The residue was treated with hexane and filtered to remove the
insoluble impurity. The filtrate was then concentrated. Purification
with column chromatography (silica gel, hexane/CH2Cl2 = 60:40)
gave TMS-protected bis(hydroxylamine) 1 (4.1 g, 8.0 mmol, 80%) as a
white solid.
1
7: H NMR (500 MHz, CDCl3) δ 0.26 (s, 18H), 0.97−1.00 (m,
18H), 1.51−1.59 (m, 12H), 1.76−1.85 (m, 12H), 3.97−4.04 (m,
12H), 6.94 (s, 2H), 6.96 (s, 2H), 6.99 (s, 2H); 13C NMR (100 MHz,
CDCl3) δ −0.1, 13.8, 13.9, 19.2, 31.4, 69.3, 69.40, 69.41, 91.4, 91.5,
100.1, 101.2, 113.8, 114.3, 114.6, 117.2, 117.3, 117.4, 153.4, 153.5,
154.2; ESI HRMS (m/z) [M + H]+ calcd for C56H79O6Si2 903.5410,
found 903.5416.
1H NMR (500 MHz, CDCl3) δ −0.26 (s, 18H), 1.13 (s, 6H), 1.13
(s, 6H), 4.54 (s, 1H), 7.15 (d, J = 8.5 Hz, 2H), 7.64 (d, J = 8.5 Hz,
2H); 13C NMR (100 MHz, CDCl3) δ −0.0, 17.2, 24.5, 67.6, 92.8, 93.4,
132.3, 136.7, 141.0; ESI HRMS (m/z) [M + H]+ calcd for
C19H36IN2O2Si2 507.1355, found 507.1355.
1,2-Bis(2,5-dibutoxy-4-ethynylphenyl)ethyne (8). To a CH2Cl2
(10 mL) solution of partially protected alkyne 6 (0.087 g, 0.15
mmol) was added TBAF (0.17 g, 0.066 mmol) in THF (ca. 1 mL) at 0
°C, and the mixture was stirred for 1 h. The reaction was then
quenched with brine, and the reaction mixture was extracted with
CH2Cl2. The combined organic extracts were dried over MgSO4,
filtered, and concentrated under reduced pressure. The residue was
1,4-Dibutoxy-2-iodo-5-(2-(trimethylsilyl)ethynyl)benzene (3) and
1,4-Bis(2-(trimethylsilyl)ethynyl)-2,5-dibutoxybenzene (4). A round-
bottom flask was charged with 1,4-dibutoxy-2,5-diiodobenzene (2)
(10.0 g, 0.021 mol), Pd(PPh3)4 (0.12 g, 0.11 mmol), copper(I) iodide
(0.040 g, 0.21 mmol), and NEt3 (100 mL) and then degassed under
vacuum with sonication. Trimethylsilylacetylene (3.0 mL, 0.021 mol)
was then added. After 1 day of stirring at 40 °C, the mixture was
concentrated, diluted with CH2Cl2, and then treated with 0.1 N HCl
until the aqueous layer reached pH 1. The aqueous solutions were
extracted with CH2Cl2, dried over MgSO4, filtered, and concentrated
under reduced pressure to give the crude product. Purification with
column chromatography (silica gel, hexane/CH2Cl2 = 90:10) gave
monosubstituted 3 (4.1 g, 9.2 mmol, 44%) as a yellow oil and
disubstituted 4 (1.8 g, 4.3 mmol, 26%) as a white solid.
purified by column chromatography (silica gel, hexane/CH2Cl2
50:50) to give alkyne 8 (0.070 g, 0.14 mmol, 92%) as a yellow oil.
=
1H NMR (500 MHz, CDCl3) δ 0.98 (t, J = 7.5 Hz, 12H), 1.49−1.57
(m, 8H), 1.79−1.83 (m, 8H), 3.34 (s, 2H), 3.99−4.03 (m, 8H), 6.97
(s, 2H), 6.98 (s, 2H); 13C NMR (100 MHz, CDCl3) δ 13.8, 13.9, 19.1,
19.2, 31.2, 31.3, 69.2, 69.4, 80.0, 82.3, 91.2, 112.6, 114.8, 117.0, 117.9,
153.3, 154.1; ESI HRMS (m/z) [M + H]+ calcd for C34H43O4
515.3156, found 515.3149.
3: 1H NMR (500 MHz, CDCl3) δ 0.25 (s, 9H), 0.96−0.99 (m, 6H),
1.49−1.58 (m, 4H), 1.74−1.81 (m, 4H), 3.93−3.96 (m, 4H), 6.83 (s,
1H), 7.25 (s, 1H); DART HRMS (m/z) [M + NH4]+ calcd for
C19H33IO2SiN 462.1320, found 462.1344.
1,2-Bis(2,5-dibutoxy-4-(2-(2,5-dibutoxy-4-(2-(trimethylsilyl)-
ethynyl)phenyl)ethynyl)phenyl)ethyne (9). A round-bottom flask was
charged with 3 (0.40 g, 0.90 mmol), Pd(PPh3)4 (0.035 mg, 0.030
mmol), copper(I) iodide (1.1 mg, 0.0060 mmol), and NEt3 (20 mL).
The solution was exhaustively degassed with argon. After the mixture
was stirred for 30 min at 80 °C, a THF solution of alkyne 8 (0.15 g,
0.30 mmol) was added over 2 h, and then the mixture was stirred for 2
h at 80 °C. The reaction mixture was then allowed to cool to room
temperature and concentrated at reduced pressure. Purification with
column chromatography (silica gel, hexane/CH2Cl2 = 40:60) and
GPC gave 9 (0.23 g, 0.20 mmol, 67%) as a yellow solid.
4: 1H NMR (500 MHz, CDCl3) δ 0.25 (s, 18H), 0.98 (t, J = 7.5 Hz,
6H), 1.50−1.56 (m, 4H), 1.74−1.80 (m, 4H), 3.95 (t, J = 6.4 Hz, 4H),
6.89 (s, 2H); DART HRMS (m/z) [M + NH4]+ calcd for
C24H42O2Si2N 432.2749, found 432.2770.
1,4-Dibutoxy-2,5-diethynylbenzene (5). To a CH2Cl2 (100 mL)
solution of TMS-protected alkyne 4 (0.50 g, 1.2 mmol) was added
TBAF (2.8 g, 0.011 mol) in THF (ca. 20 mL) at 0 °C, and the mixture
was stirred for 1 h. The reaction was quenched with brine, and the
reaction mixture was extracted with CH2Cl2. The organic layer was
dried over MgSO4, filtered, and concentrated under reduced pressure.
The residue was purified by column chromatography (silica gel,
hexane/CH2Cl2 = 80:20) to give alkyne 5 (0.33 g, 1.2 mmol, 98%) as a
white solid.
1H NMR (500 MHz, CDCl3) δ 0.26 (s, 18H), 0.97−1.00 (m, 24H),
1.53−1.58 (m, 16H), 1.77−1.86 (m, 16H), 3.97−4.05 (m, 16H), 6.94
(s, 2H), 6.97 (s, 2H), 7.00 (s, 2H), 7.01 (s, 2H); 13C NMR (100 MHz,
CDCl3) δ −0.1, 13.85, 13.91, 19.2, 31.4, 69.2, 69.3, 91.4, 91.52, 91.56,
100.1, 101.2, 113.7, 114.2, 114.3, 114.5, 117.1, 117.2, 117.3, 153.3,
153.4, 154.2; ESI HRMS (m/z) [M + H]+ calcd for C72H99O8Si2
1147.6873, found 1147.6834.
1,2-Bis(2,5-dibutoxy-4-(2-(2,5-dibutoxy-4-ethynylphenyl)-
ethynyl)phenyl)ethyne (10). To a CH2Cl2 (50 mL) solution of TMS-
protected alkyne 9 (0.23 g, 0.20 mmol) was added TBAF (0.47 g, 1.8
mmol) in THF (ca. 1 mL) at 0 °C, and the mixture was stirred for 1 h
at room temperature. The reaction was then quenched with brine, and
the reaction mixture was extracted with CH2Cl2. The organic extracts
were dried over MgSO4, filtered, and concentrated under reduced
pressure. The residue was purified by column chromatography (silica
gel, hexane/CH2Cl2 = 40:60) to give alkyne 10 (0.18 g, 0.18 mmol,
90%) as a yellow solid.
1H NMR (500 MHz, CDCl3) δ 0.97 (t, J = 7.5 Hz, 6H), 1.48−1.55
(m, 4H), 1.76−1.81 (m, 4H), 3.33 (s, 2H), 3.98 (t, J = 6.5 Hz, 4H),
6.96 (s, 2H); 13C NMR (100 MHz, CDCl3) δ 13.8, 19.1, 31.2, 69.3,
79.7, 82.4, 113.2, 117.6, 153.9; DART HRMS (m/z) [M + NH4]+
calcd for C18H26O2N 288.1958, found 288.1968.
1-(2,5-Dibutoxy-4-ethynylphenyl)-2-(2,5-dibutoxy-4-(2-
(trimethylsilyl)ethynyl)phenyl)ethyne (6) and 1,4-Bis(2-(2,5-dibu-
toxy-4-(2-(trimethylsilyl)ethynyl)phenyl)ethynyl)-2,5-dibutoxyben-
zene (7). A round-bottom flask was charged with 3 (0.75 g, 1.7 mmol),
Pd(PPh3)4 (0.015 g, 0.013 mmol), copper(I) iodide (4.8 mg, 0.025
mmol), and NEt3 (20 mL). The solution was exhaustively degassed
with argon and then stirred for 30 min at 80 °C. A THF solution of
alkyne 5 (0.68 g, 2.52 mmol) was then added, and then the mixture
was stirred for 1.5 h. The mixture was concentrated, diluted with
CH2Cl2, and then treated with 0.1 N HCl until the aqueous layer
reached pH 1. The aqueous solution was extracted with CH2Cl2. All
organic phases were combined, dried over MgSO4, filtered, and
concentrated under reduced pressure to give the crude product.
Purification with column chromatography (silica gel, hexane/CH2Cl2
= 90:10−40:60) gave monosubstituted 6 (0.21 g, 0.36 mmol, 21%) as
a yellow oil and disubstituted 7 (0.35 g, 0.39 mmol, 46%) as a white
solid.
1H NMR (500 MHz, CDCl3) δ 0.97−1.01 (m, 24H), 1.48−1.63
(m, 16H), 1.78−1.87 (m, 16H), 3.35 (s, 2H), 4.00−4.06 (m, 16H),
6.98 (s, 2H), 7.00 (s, 2H), 7.01 (s, 2H), 7.01 (s, 2H); 13C NMR (150
MHz, CDCl3) δ 13.8, 13.9, 19.15, 19.22, 31.2, 31.3, 31.4, 69.26, 69.33,
69.35, 69.42, 80.0, 82.3, 91.2, 91.6, 112.5, 114.2, 114.4, 115.0, 117.0,
117.22, 117.25, 117.9, 153.3, 153.46, 153.49, 154.1; ESI HRMS (m/z)
[M + H]+ calcd for C66H83O8 1003.6082, found 1003.6072.
1,4-Bis(2-(2,5-dibutoxy-4-ethynylphenyl)ethynyl)-2,5-dibutoxy-
bezene (11). To a CH2Cl2 (20 mL) solution of TMS-protected alkyne
7 (0.15 g, 0.17 mmol) was added TBAF (0.39 g, 1.5 mmol) in THF
(ca. 2 mL) at 0 °C, and the mixture was stirred for 1 h at room
temperature. The reaction was then quenched with brine, and the
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reaction mixture was extracted with CH2Cl2. The organic extracts were
dried over MgSO4, filtered, and concentrated under reduced pressure.
The residue was purified by column chromatography (silica gel,
hexane/CH2Cl2 = 50:50) to give alkyne 11 (0.12 g, 0.16 mmol, 92%)
as a yellow solid.
1H NMR (500 MHz, CDCl3) δ −0.25 (s, 36H), 1.001 (t, J = 7.5 Hz,
6H), 1.007 (t, J = 7.5 Hz, 6H) 1.15 (s, 24H), 1.52−1.61 (m, 8H),
1.82−1.88 (m, 8H), 4.03−4.07 (m, 8H), 4.61 (s, 2H), 7.02 (s, 2H),
7.03 (s, 2H), 7.39 (d, J = 8.3 Hz, 4H), 7.49 (d, J = 8.3 Hz, 4H); 13C
NMR (150 MHz, CDCl3) δ −0.2, 14.0, 14.1, 16.9, 19.06, 19.08, 24.6,
30.96, 30.98, 67.2, 68.5, 68.6, 85.4, 91.3, 92.7, 95.4, 112.9, 115.5, 115.8,
122.2, 130.0, 130.8, 141.8, 152.8; APCI HRMS (m/z) [M + H]+ calcd
for C72H111N4O8Si4 1271.7473, found 1271.7470; UV−vis (CH2Cl2)
λmax 396 nm.
1,2-Bis(2,5-dibutoxy-4-(2-(4-(1-oxyl-3-oxide-4,4,5,5-tetramethyl-
imidazolin-2-yl)phenyl)ethynyl)phenyl)ethyne (OPE4). To a CH2Cl2
(5 mL) solution of OPE4TMS (0.010 g, 0.0079 mmol) was added
TBAF (0.037 g, 0.14 mmol) in THF (ca. 1 mL), and the mixture was
stirred for 70 min at room temperature. The reaction was then
quenched with brine, and the reaction mixture was extracted with
CH2Cl2. The combined organic extracts were dried over MgSO4,
filtered, and concentrated under reduced pressure. The residue was
purified by column chromatography (silica gel, CH2Cl2/EtOAc =
90:10) to give biradical OPE4 (0.8 mg, 0.82 μmol, 10%) as a dark-
green solid.
1H NMR (500 MHz, CDCl3) δ 0.97−1.00 (m, 18H), 1.51−1.58
(m, 12H), 1.79−1.85 (m, 12H), 3.34 (s, 2H), 4.00−4.05 (m, 12H),
6.98 (s, 2H), 6.99 (s, 2H), 7.00 (s, 2H); 13C NMR (100 MHz, CDCl3)
δ 13.8, 13.9, 19.16, 19.22, 31.2, 31.32, 31.35, 69.2, 69.3, 69.4, 80.0,
82.3, 91.2, 91.5, 112.5, 114.2, 114.9, 117.0, 117.2, 117.9, 153.3, 153.5,
154.1; DART HRMS (m/z) [M + H]+ calcd for C50H63O6 759.4619,
found 759.4637.
1,4-Bis(2-(2,5-dibutoxy-4-(2-(2,5-dibutoxy-4-(2-(trimethylsilyl)-
ethynyl)phenyl)ethynyl)phenyl)ethynyl)-2,5-dibutoxybenzene (12).
A round-bottom flask was charged with 3 (0.12 g, 0.26 mmol),
Pd(PPh3)4 (0.015 mg, 0.013 mmol), copper(I) iodide (0.50 mg,
0.0026 mmol), and NEt3 (10 mL). The solution was exhaustively
degassed with argon. After the mixture was stirred for 30 min at 80 °C,
a THF solution of alkyne 11 (0.10 g, 0.13 mmol) was added over 1.5
h. The mixture was concentrated, diluted with CH2Cl2, and then
treated with 0.1 N HCl until the aqueous layer reached pH 1. The
aqueous solutions were extracted with CH2Cl2. All of the organic
phases were combined, dried over MgSO4, filtered, and concentrated
under reduced pressure to give the crude product. Purification with
column chromatography (silica gel, hexane/CH2Cl2 = 20:80) gave
disubstituted 12 (0.094 g, 0.068 mmol, 52%) as a yellow solid. The
monosubstituted compound, 1-(2,5-dibutoxy-4-(2-(2,5-dibutoxy-4-
ethynylpheny)ethynyl)phenyl)-2-(2,5-dibutoxy-4-(2-(2,5-dibutoxy-4-
(2-(trimethylsilyl)ethynyl)phenyl)ethynyl)phenyl)ethyne, was also
obtained as a yellow solid (0.038 g, 0.035 mmol, 27%).
ESI HRMS (m/z) [M]+ calcd for C60H72N4O8 976.5345, found
976.5332; ESR (CH2Cl2) nine lines, g = 2.0067, aN = 3.8 G.
1,4-Bis(2-(2,5-dibutoxy-4-(2-(4-(4,4,5,5-tetramethyl-1,3-bis-
((trimethylsilyl)oxy)imidazolidin-2-yl)phenyl)ethynyl)phenyl)-
ethynyl)-2,5-dibutoxybenzene (OPE5TMS). A round-bottom flask
was charged with alkyne 11 (0.090 g, 0.12 mmol), TMS-protected
bis(hydroxylamine) 1 (0.18 g, 0.36 mmol), Pd(PPh3)4 (0.014 g, 0.012
mmol), copper(I) iodide (2.3 mg, 0.012 mmol), THF (10 mL), and
NEt3 (5 mL). The solution was exhaustively degassed with argon. The
mixture was stirred for 3 h at 60 °C. The reaction mixture was then
allowed to cool to room temperature and concentrated at reduced
pressure. The residue was purified by column chromatography (silica
gel, hexane/CH2Cl2 = 50:50) and GPC to give OPE5TMS (0.030 g,
0.020 mmol, 17%) as a yellow powder.
1
12: H NMR (500 MHz, CDCl3) δ 0.26 (s, 18H), 0.97−1.01 (m,
30H), 1.52−1.60 (m, 20H), 1.77−1.87 (m, 20H), 3.97−4.06 (m,
20H), 6.94 (s, 2H), 6.97 (s, 2H), 7.00−7.01 (m, 6H); 13C NMR (150
MHz, CDCl3) δ −0.1, 13.85, 13.91, 19.22, 19.24, 31.4, 69.2, 69.4, 91.4,
91.5, 91.6, 100.1, 101.2, 113.7, 114.2, 114.30, 114.32, 114.6, 117.1,
117.2, 117.4, 153.3, 153.5, 154.2; ESI HRMS (m/z) [M + H]+ calcd
for C88H119O10Si2 1391.8336, found 1391.8321.
1H NMR (500 MHz, CDCl3) δ −0.25 (s, 36H), 0.99−1.02 (m,
18H), 1.15 (s, 24H), 1.51−1.61 (m, 12H), 1.82−1.89 (m, 12H),
4.03−4.07 (m, 12H), 4.61 (s, 2H), 7.02 (s, 2H), 7.02 (s, 2H), 7.03 (s,
2H), 7.39 (d, J = 8.5 Hz, 4H), 7.49 (d, J = 8.5 Hz, 4H); 13C NMR
(150 MHz, CDCl3) δ −0.1, 14.0, 14.1, 17.0, 19.1, 24.6, 31.0, 67.2,
68.53, 68.58, 85.4, 91.4, 92.7, 95.4, 112.9, 113.1, 115.6, 115.8, 122.2,
130.0, 130.9, 141.8, 152.77, 152.8; APCI HRMS (m/z) [M + H]+
calcd for C88H131N4O10Si4 1515.8937, found 1515.8935; UV−vis
(CH2Cl2) λmax 411 nm.
1,4-Bis(2-(2,5-dibutoxy-4-(2-(4-(1-oxyl-3-oxide-4,4,5,5-tetra-
methylimidazolin-2-yl)phenyl)ethynyl)phenyl)ethynyl)-2,5-di-
butoxybenzene (OPE5). To a CH2Cl2 (5 mL) solution of OPE5TMS
(5.0 mg, 0.0033 mmol) was added TBAF (0.015 g, 0.059 mmol) in
THF (ca. 1 mL) at 0 °C, and the mixture was stirred for 90 min at
room temperature. The reaction was then quenched with brine, and
the reaction mixture was extracted with CH2Cl2. The organic extracts
were dried over MgSO4, filtered, and concentrated under reduced
pressure. The residue was purified by column chromatography (silica
gel, CH2Cl2/EtOAc = 90:10) to give biradical OPE5 (0.8 mg, 0.65
μmol, 20%) as a dark-green solid.
1
Monosubstituted compound: H NMR (500 MHz, CDCl3) δ 0.26
(s, 9H), 0.97−1.01 (m, 24H), 1.50−1.60 (m, 16H), 1.77−1.87 (m,
16H), 3.35 (s, 1H), 3.97−4.05 (m, 16H), 6.94 (s, 1H), 6.97 (s, 1H),
6.98 (s, 1H), 6.99 (s, 1H), 7.01−7.00 (m, 4H); ESI HRMS (m/z) [M
+ H]+ calcd for C69H91O8Si 1075.6478, found 1075.6469.
1,4-Dibutoxy-2,5-bis(2-(2,5-dibutoxy-4-(2-(2,5-dibutoxy-4-
ethynylphenyl)ethynyl)phenyl)ethynyl)benzene (13). To a CH2Cl2
(20 mL) solution of TMS-protected alkyne 12 (0.093 g, 0.067 mmol)
was added TBAF (0.16 g, 0.60 mmol) in THF (ca. 2 mL) at 0 °C, and
the mixture was stirred for 1 h at room temperature. The reaction was
then quenched with brine, and the reaction mixture was extracted with
CH2Cl2. The organic extracts were dried over MgSO4, filtered, and
concentrated under reduced pressure. The residue was purified by
column chromatography (silica gel, hexane/CH2Cl2 = 20:80) to afford
alkyne 13 (0.080 g, 0.064 mmol, 96%) as a yellow solid.
1H NMR (500 MHz, CDCl3) δ 1.01−0.97 (m, 30H), 1.50−1.58
(m, 20H), 1.78−1.86 (m, 20H), 3.35 (s, 2H), 4.00−4.05 (m, 20H),
6.98 (s, 2H), 7.00 (s, 2H), 7.01−7.02 (m, 6H); 13C NMR (150 MHz,
CDCl3) δ 13.8, 13.9, 19.2, 19.3, 31.27, 31.38, 31.41, 69.3, 69.41, 69.43,
69.5, 80.1, 82.3, 91.2, 91.57, 91.60, 91.63, 112.6, 114.2, 114.4, 114.5,
115.1, 117.1, 117.4, 118.1, 153.4, 153.5, 154.2; ESI HRMS (m/z) [M +
H]+ calcd for C82H103O10 1247.7546, found 1247.7523.
1,2-Bis(2,5-dibutoxy-4-(2-(4-(4,4,5,5-tetramethyl-1,3-bis-
((trimethylsilyl)oxy)imidazolidin-2-yl)phenyl)ethynyl)phenyl)ethyne
(OPE4TMS). A round-bottom flask was charged with alkyne 8 (0.070
g, 0.14 mmol), TMS-protected bis(hydroxylamine) 1 (0.21 g, 0.41
mmol), Pd(PPh3)4 (0.016 g, 0.014 mmol), copper(I) iodide (2.6 mg,
0.014 mmol), THF (10 mL), and NEt3 (5 mL). The solution was
exhaustively degassed with argon. The mixture was stirred for 1.5 h at
80 °C and then allowed to cool to room temperature and concentrated
at reduced pressure. The residue was purified by column
chromatography (silica gel, hexane/CH2Cl2 = 50:50) and GPC to
give OPE4TMS (0.070 g, 0.055 mmol, 98%) as a yellow powder.
ESI HRMS (m/z) [M]+ calcd for C76H92N4O10 1220.6808, found
1220.6814; ESR (CH2Cl2) distorted nine lines, g = 2.0065.
1,2-Bis(2,5-dibutoxy-4-(2-(2,5-dibutoxy-4-(2-(4-(4,4,5,5-tetra-
methyl-1,3-bis((trimethylsilyl)oxy)imidazolidin-2-yl)phenyl)ethynyl)-
phenyl)ethynyl)phenyl)ethyne (OPE6TMS). A round-bottom flask
was charged with TMS-protected bis(hydroxylamine) 1 (0.14 g, 0.27
mmol), Pd(PPh3)4 (0.010 g, 0.0090 mmol), copper(I) iodide (0.3 mg,
0.0018 mmol), and NEt3 (10 mL). The solution was exhaustively
degassed with argon. After the mixture was stirred for 30 min at 80 °C,
a THF solution of alkyne 10 (0.090 g, 0.090 mmol) was added over 5
h. The reaction mixture was then allowed to cool to room temperature
and concentrated at reduced pressure. The residue was purified by
column chromatography (silica gel, hexane/CH2Cl2 = 40:60) and
GPC to give OPE6TMS (0.015 g, 0.0084 mmol, 9%) as a yellow
powder.
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Article
1H NMR (500 MHz, CDCl3) δ −0.25 (s, 36H), 0.99−1.02 (m,
24H), 1.15 (s, 24H), 1.53−1.63 (m, 16H), 1.82−1.89 (m, 16H),
4.04−4.07 (m, 16H), 4.61 (s, 2H), 7.02−7.03 (m, 8H), 7.39 (d, J = 8.0
Hz, 4H), 7.49 (d, J = 8.0 Hz, 4H); 13C NMR (150 MHz, CDCl3) δ
−0.2, 13.98, 14.03, 16.9, 19.0, 24.6, 31.0, 67.2, 68.5, 85.3, 91.4, 92.7,
95.4, 112.9, 113.0, 115.5, 115.8, 122.2, 130.0, 130.8, 141.8, 152.7,
152.8; APCI HRMS (m/z) [M + H]+ calcd for C104H151N4O12Si4
1760.0400, found 1760.0419; UV−vis (CH2Cl2) λmax 416 nm.
1,2-Bis(2,5-dibutoxy-4-(2-(2,5-dibutoxy-4-(2-(4-(1-oxyl-3-oxide-
4,4,5,5-tetramethylimidazolin-2-yl)phenyl)ethynyl)phenyl)ethynyl)-
phenyl)ethyne (OPE6). To a CH2Cl2 (5 mL) solution of OPE6TMS
(4.8 mg, 0.0027 mmol) was added TBAF (0.013 g, 0.049 mmol) in
THF (ca. 1 mL) at 0 °C, and the mixture was stirred for 110 min at
room temperature. The reaction was then quenched with brine, and
the reaction mixture was extracted with CH2Cl2. The organic extracts
were dried over MgSO4, filtered, and concentrated under reduced
pressure. The residue was purified by column chromatography (silica
gel, CH2Cl2/EtOAc = 90:10) to give biradical OPE6 (0.1 mg, 0.068
μmol, 2%) as a dark-green solid.
AUTHOR INFORMATION
Corresponding Author
*E-mail: kmatsuda@sbchem.kyoto-u.ac.jp.
Notes
■
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank Professor Y. Murata (Kyoto University) and H.
Kitagawa (Kyoto University) for the measurement of ESR
spectra. We thank Professor Paul M. Lahti (University of
Massachusetts) and Professor B. Kirste (Free University of
Berlin) for providing the program BIRADG. This work was
supported by the NEXT Program (GR062) of JSPS and by a
Grant-in-Aid for Young Scientists (B) (23750156) from the
MEXT, Japan.
REFERENCES
■
APCI HRMS (m/z) [M + H]+ calcd for C92H113N4O12 1465.8350,
found 1465.8408; ESR (CH2Cl2) complicated 15 lines, g = 2.0066.
1,4-Bis(2-(2,5-dibutoxy-4-(2-(2,5-dibutoxy-4-(2-(4-(4,4,5,5-tetra-
methyl-1,3-bis((trimethylsilyl)oxy)imidazolidin-2-yl)phenyl)ethynyl)-
phenyl)ethynyl)phenyl)ethynyl)-2,5-dibutoxybenzene (OPE7TMS).
A round-bottom flask was charged with TMS-protected bis-
(hydroxylamine) 1 (0.11 g, 0.071 mmol), Pd(PPh3)4 (8.2 mg,
0.0071 mmol), copper(I) iodide (0.3 mg, 0.0014 mmol), and NEt3
(10 mL). The solution was exhaustively degassed with argon. After the
mixture was stirred for 30 min at 80 °C, a THF solution of alkyne 13
(0.088 g, 0.071 mmol) was added over 2 h. The reaction mixture was
stirred 1 h at 80 °C and then allowed to cool to room temperature and
concentrated at reduced pressure. The residue was purified by column
chromatography (silica gel, hexane/CH2Cl2 = 20:80) and GPC to give
OPE7TMS (0.016 g, 0.0077 mmol, 11%) as a yellow powder.
1H NMR (500 MHz, CDCl3) δ −0.25 (s, 36H), 0.99−1.02 (m,
30H), 1.15 (s, 24H), 1.55−1.59 (m, 20H), 1.83−1.87 (m, 20H),
4.04−4.07 (m, 20H), 4.61 (s, 2H), 7.02 (m, 10H), 7.39 (d, J = 8.0 Hz,
4H), 7.49 (d, J = 8.5 Hz, 4H); 13C NMR (150 MHz, CDCl3) δ −0.2,
13.98, 14.03, 16.9, 19.0, 24.6, 31.0, 67.2, 68.5, 85.3, 91.4, 92.7, 95.4,
112.9, 113.0, 115.5, 115.8, 122.2, 130.0, 130.8, 141.8, 152.7, 152.8;
APCI HRMS (m/z) [M + H]+ calcd for C120H171N4O14Si4 2004.1863,
found 2004.1896; UV−vis (CH2Cl2) λmax 422 nm.
1,4-Bis(2-(2,5-dibutoxy-4-(2-(2,5-dibutoxy-4-(2-(4-(1-oxyl-3-
oxide-4,4,5,5-tetramethylimidazolin-2-yl)phenyl)ethynyl)phenyl)-
ethynyl)phenyl)ethynyl)-2,5-dibutoxybenzene (OPE7). To a CH2Cl2
(5 mL) solution of OPE7TMS (5.0 mg, 0.0025 mmol) was added
TBAF (0.012 g, 0.045 mmol) in THF (ca. 1 mL) at 0 °C, and the
mixture was stirred for 120 min at room temperature. The reaction
was then quenched with brine, and the reaction mixture was extracted
with CH2Cl2. The organic extracts were dried over MgSO4, filtered,
and concentrated under reduced pressure. The residue was purified by
column chromatography (silica gel, CH2Cl2/EtOAc = 90:10) to give
biradical OPE7 (1.0 mg, 0.59 μmol, 23%) as a dark-green solid.
ESI HRMS (m/z) [M]+ calcd for C108H132N4O14 1708.9735, found
1708.9784; ESR (CH2Cl2) five lines, g = 2.0065, aN = 7.6 G.
2. Spectroscopy. Absorption spectra were measured on a
spectrophotometer. Fluorescence spectra were measured on a
fluorescence spectrophotometer. The samples were dissolved in dry
CH2Cl2. ESR spectra were measured on an X-band ESR spectrometer.
The samples were dissolved in dry CH2Cl2 and degassed with N2
bubbling for 10 min.
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