Charge transfer states in stable neutral and oxidized radical adducts from carbazole derivatives

Article abstract of DOI:10.1021/jo4028215

In this paper we report the spectral properties of the stable radical adducts 1^?-3^?, which are formed by an electron donor moiety, the carbazole ring, and an electron acceptor moiety, the polychlorotriphenylmethyl radical. The molecular structure of radical adduct 1^? in the crystalline state shows a torsion angle of approximately 90 between the phenyl and the carbazole rings due to steric interactions. They exhibit a charge transfer band in the visible range of the electronic spectrum. All of them are chemically oxidized with copper(II) perchlorate to the respective cation species, which show a strong charge transfer band into the near-infrared region of the spectrum. Radical adducts 1^?-3^? and the corresponding stable oxidized species 1⁺-3⁺ are real organic mixed-valence compounds due to the open-shell nature of their electronic structure. Charge transfer bands of the cation species are stronger and are bathochromically shifted with respect to those of the neutral species due to the greater acceptor ability of the positively charged central carbon atom of the triphenylmethyl moiety. The cationic species 1⁺-3⁺ are diamagnetic, as shown by the absence of a signal in the EPR spectrum in acetonitrile solution at room temperature, but they show an intense and unique band in frozen solutions (183 K).

Full text of DOI:10.1021/jo4028215

Article
pubs.acs.org/joc
Charge Transfer States in Stable Neutral and Oxidized Radical
Adducts from Carbazole Derivatives
Lluís Fajarí,† Robert Papoular,‡ Marta Reig,§ Enric Brillas,∥ Jose Luis Jorda,⊥ Oriol Vallcorba,# Jordi Rius,#
́
Dolores Velasco,§ and Luis Julia*,†
́
†
́
́
Departament de Quımica Biologica i Modelitzacio Molecular, Institut de Quımica Avanca̧ da de Catalunya (CSIC), Jordi Girona
̀
́
18-26, 08034 Barcelona, Spain
‡Laboratoire Leon Brillouin, IRAMIS, CEA-Saclay, 91191 Gif-sur-Yvette, France
§
́
Departament de Quımica Organ
̀ ̀
ica Institut de Nanociencia i Nanotecnologia (IN2UB), Universitat de Barcelona,
Martí Franques 1-11, 08028 Barcelona, Spain
∥
̀
́
́
Departament de Quımica Fısica, Universitat de Barcelona, Martí Franques
̀
1-11, 08028 Barcelona, Spain
Instituto de Tecnología Quımica (UPV-CSIC), Av. de los Naranjos s/n, 46022 Valencia, Spain
#Institut de Cien
cia de Materials (CSIC), Campus de la U.A.B., 08193 Bellaterra, Spain
⊥
́
̀
S
* Supporting Information
ABSTRACT: In this paper we report the spectral properties of the stable radical adducts 1•−3•,
which are formed by an electron donor moiety, the carbazole ring, and an electron acceptor moiety,
the polychlorotriphenylmethyl radical. The molecular structure of radical adduct 1• in the crystalline
state shows a torsion angle of approximately 90° between the phenyl and the carbazole rings due to
steric interactions. They exhibit a charge transfer band in the visible range of the electronic spectrum.
All of them are chemically oxidized with copper(II) perchlorate to the respective cation species,
which show a strong charge transfer band into the near-infrared region of the spectrum. Radical
adducts 1•−3• and the corresponding stable oxidized species 1+−3+ are real organic mixed-valence
compounds due to the open-shell nature of their electronic structure. Charge transfer bands of the
cation species are stronger and are bathochromically shifted with respect to those of the neutral
species due to the greater acceptor ability of the positively charged central carbon atom of the
triphenylmethyl moiety. The cationic species 1+−3+ are diamagnetic, as shown by the absence of a
signal in the EPR spectrum in acetonitrile solution at room temperature, but they show an intense
and unique band in frozen solutions (183 K).
magnetic character. Furthermore, the combination of carbazole
INTRODUCTION
■
and indole fragments with TTM has been revealed as an
efficient strategy to obtain ambipolar transport properties.5
Now, this paper presents the photophysical properties of
the radical adducts derived from carbazole, in both their neutral
and oxidized states. The research work has been conducted
with [4-(N-carbazolyl)-2,6-dichlorophenyl]bis(2,4,6-trichloro-
phenyl)methyl radical (1•),2 [2,6-dichloro-4-(3,6-dimethoxy-
N-carbazolyl)phenyl]bis(2,4,6-trichlorophenyl)methyl radical
(2•),3 and the new radical adduct [2,6-dichloro-4-(2,7-
dimethoxy-N-carbazolyl)phenyl]bis(2,4,6-trichlorophenyl)-
methyl (3•) (Scheme 1). In these compounds, the donor
moiety is directly anchored on the acceptor through the nitro-
gen of the carbazole to the phenyl ring of the acceptor. They
differ in donor strength due to the electron-donating effects of
the methoxy substituent on the carbazole ring. As discussed
below, all of them exhibit charge-transfer bands in both neutral
(radical) and oxidized (cation) states, the bands in the cationic
species being stronger and being bathochromically shifted.
Pure organic charge-transfer molecules are composed mainly of
an electron-donor and an electron-acceptor moiety. In the
charge-transfer excited state, the electron moves from the
donor to the acceptor part of the molecule, and the resulting
electrostatic attraction provides a stabilizing force for the
molecular state. The two redox centers of the molecule, the
donor and the acceptor, may be directly bonded or linked
through a saturated or unsaturated bridge connecting both
centers, and the energy of the excitation strongly depends on
the nature and length of the bridge between them. Organic
charge-transfer compounds play a significant role in the
chemistry of materials and have found use as sensitizers for
dye-sensitized solar cells.1
As part of our target to develop the tris(2,4,6-trichlorophenyl)-
methyl radical (TTM) series (Scheme 1) as electronic devices,
we have been engaged in a research program to develop new
stable radical adducts possessing dipolar structure built from
the TTM radical, as the electron-acceptor open-shell moiety,
and a heterocycle such as carbazole and indole, as the electron-
donor moiety.2−4 All of them have shown significant physical
and electrochemical properties, in addition to the intrinsic
Received: December 19, 2013
Published: January 27, 2014
© 2014 American Chemical Society
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Scheme 1. Chemical Structures of Organic Radicals TTM,
PTM, 1•, 2•, and 4• and Synthesis of 3•
Figure 1. Perspective view of the molecular structure of the radical
adduct 1•, along with the atom numbering. Angles between planes are
given in Table 1
Table 1. Selected Dihedral Angles (deg) in the Molecular
a
Structure of Radical Adduct 1•
These push−pull systems are also named organic mixed
valence,6 because they are open-shell systems. The first work
reported on organic mixed-valence systems with a stable radical
as electron acceptor dealt with the perchlorotriphenylmethyl
radical (PTM)7 (Scheme 1). Since then, other systems with
PTM and different electron donors have appeared in the
literature.8
P1−P4
P2−P4
P3−P4
P3−P5
52.8(10)
46.7(10)
57.1(9)
87.6(6)
a
Planes are defined as follows: P1, C2C3C4C5C6C7; P2,
C8C9C10C11C12C13; P3, C14C15C16C17C18C19; P4,
C1C2C8C14; P5, carbazole moiety.
absorptivities of all of them and the corresponding values of
TTM. All of them except TTM display a band at ca. 290−
307 nm assigned to the carbazole moiety. The strong band
at about 370 nm and the broad and weak bands at ca. 475−
555 nm in all of these radical species are characteristic of the
radicals of the TTM series. Finally, the broad band at longer
wavelengths (570−675 nm), strongly overlapped with the pre-
vious radical absorption band, is attributed to a charge transfer
from the carbazole moiety as the electron donor to the trivalent
carbon as the electron acceptor and, as said before, it is a real
band from mixed-valence organic compounds since the radical
adducts are open-shell systems in the ground state.6 The
spectral region in which this charge transfer band appears is a
function of the donor and/or acceptor strength of the redox
centers in the molecule. In radical adducts 1•−3•, this band is
only slightly red-shifted with respect to the visible band of
radicals, since radicals of the TTM series have a moderate
electron-acceptor strength. Furthermore, the most intense and
more bathochromically shifted band corresponds to the radical
adduct 2• (λ 648 nm) due to the increased donor strength of
the carbazole moiety with methoxy substitution in a position
para to the nitrogen.
RESULTS AND DISCUSSION
■
Radical adducts 1• and 2• were prepared as reported by us
some years ago.2,3 Similarly, 3• was obtained in a two-stage pro-
cess, starting from TTM. First, TTM was treated with an excess
of 2,7-dimethoxycarbazole in the presence of cesium carbonate
in boiling DMF, and the resulting solid was treated with an
aqueous solution of tetrabutylammonium hydroxide (TBAH)
followed by oxidation with tetrachloro-p-benzoquinone.
The crystal structure of 1• was solved from synchrotron
powder diffraction data measured at 100 K. An ORTEP draw-
ing of the molecular structure is shown in Figure 1, including
the atom-numbering scheme. Atomic coordinates and a selec-
tion of bond lengths and bond angles are summarized in Tables
S2−S4 of the Supporting Information. This radical adduct
crystallizes in the triclinic system, space group P1. All distances
̅
and angles for the central carbon atom C(1) with aromatic car-
bon atoms C(2), C(8), and C(14) are in good agreement with
sp2 hybridization for C(1), where the spin density maximum
lies. The three benzene rings are twisted around their bond
with C(1) with angles displayed in Table 1. Furthermore, the
torsion angle between the carbazole and the attached phenyl
ring is almost perpendicular (see Table 1) to avoid interactions
between hydrogen atoms. The crystal packing is controlled by
Cl···Cl interactions (Table S5, Supporting Information). The
strongest interaction (Cl33···Cl38) gives rise to dimeric units
which propagate along b + c through the formation of slightly
weaker Cl35···Cl36 interactions, as can be seen in Figure S3
(Supporting Information).
The emission properties of 1•−3• are summarized in
Table 3, and the spectra in cyclohexane solution are shown
in Figure 3. When they were excited in cyclohexane solution
(λexc 450 nm), 1• and 3• showed emission in the visible region
and 2• in the near-infrared region. The emission from 2• is
markedly red-shifted in relation to that of 1• and 3• (100 and
90 nm, respectively), due to the relative position of the me-
thoxy groups para to the ring nitrogen. The emission quantum
yield in cyclohexane is remarkably high for 1• and very low for
2• and 3•. Furthermore, the quantum yield of 1• decreases in
CHCl3 and no fluorescence was detected for 2• and 3• in this
The UV−vis absorption spectra of radical adducts 1•−3•
were recorded in the strong polar solvent acetonitrile/CHCl3
(9/1) at concentrations of about 10−4 M and are shown in
Figure 2. Table 2 summarizes the absorption bands and molar
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Figure 2. (a) UV−vis spectra of radical adducts 1• (brown), 2• (red), 3• (blue), and TTM (black) in acetonitrile/CHCl3 (9/1) and (b) details of the
charge transfer bands.
Table 2. UV−Vis Absorption Data of Radical Adducts 1•−3•
and TTM and Their Corresponding Oxidized Species in
Acetonitrile/CHCl3 (9/1)
λmax, nm (ε, dm3 mol−1
adduct λmax, nm (ε, dm3 mol−1 cm−1
)
cm−1
)
a
1•
2•
3•
290 (16650); 374 (27620);
1+ 356 (8580); 489 (9470):
555 (sh) (2400); 595 (3220)
789 (23440)
b
307 (23770); 373 (31220);
648 (4070)
2+ 304 (12040); 356 (3690);
512 (6460); 860 (16440)
303 (15630); 372 (26450);
556 (sh) (1940); 595 (2460)
3+ 300 (19480); 354 (7150);
492 (7110); 781 (16820)
TTM 372 (22700); 497 (505); 543
(535)
a
b
Values in CHCl3: 291 (14400), 374 (25600); 598 (2940).4 Values
in CHCl3: 373 (32600); 647 (4400).3
Table 3. Emission Spectra Properties of Radical Adducts
1•−3• and TTM
Figure 3. Normalized emission spectra of cyclohexane solutions
(10−4 M) of radical adducts 1• (brown), 2• (red), 3• (blue), and TTM
(black) after excitation at λ 450 nm.
a
solvent
λem, nm
ΦF
0.008
TTM
1•
cyclohexane
cyclohexane
CHCl3
562
628
680
728
638
b
0.64 (0.53 )
b
Table 4. Standard Potential for the Redox Pair (O/R)
0.06 (0.04 )
2•
3•
cyclohexane
cyclohexane
0.006
0.002
Related to the Oxidation of Radical Adducts, Difference
a
c
between Their Anodic (Ep ) and Cathodic (Ep ) Peak
a
a
Potentials, and Ionization Potentials
Relative fluorescence quantum yields in cyclohexane and CHCl3 were
determined using tris(2,2′-bipyridyl)ruthenium(II) chloride in water
b
c
d
a
c
a
radical adduct
E°(O/R), V (Ep − Ep , mV)
Ep , mV
IP, eV
b
as a standard after excitation at λ 450 nm. Reference 4a.
e
1•
1.03 (130)
0.86 (120)
1.04 (140)
1.27 (100)
1.095
0.92
1.11
1.32
5.83
5.66
5.84
6.07
f
2•
3•
solvent. These results are in agreement with the process of the
fluorescence quenching of donor−acceptor molecules in polar
solvents, since the excess energy of the excited state is dis-
sipated in the intramolecular electron transfer from the donor
to the acceptor.9,10
e
TTM
a
Conditions: CH2Cl2 solution (∼10−3 M) with Bu4NClO4 (0.1 M) as
b
background electrolyte on a Pt electrode at ∼25 °C. Potential values
c
versus SCE (saturated calomel electrode). Values at a scan rate of
d
100 mV s−1. IP values for radical adducts are estimated from the
Cyclic voltammograms of radical adducts 1•−3• and TTM
were measured in CH2Cl2 solution (∼10−3 M) containing
TBAP (0.1 M) as supporting electrolyte on platinum wire as
the working electrode and a saturated calomel electrode (SCE)
as the reference electrode. All of these radical adducts exhibit a
quasi-reversible redox pair (O/R) associated with their oxi-
dation processes, which can be attributed to the removal of one
electron (Table 4). Similar values of ionization potentials
were obtained for all carbazole-TTM radical adducts 1•−3•
standard potentials of the O/R pairs, taking a value of −4.8 eV as the
SCE energy level relative to the vacuum level. Values taken from
ref 4a. Values taken from ref 3.
e
f
(5.66−5.84 eV), which are lower than that of the TTM radical
(6.07 eV), where no carbazole moiety exists as an electron-
donating group, the lowest one being that of 2• with two
methoxy groups placed in positions para to the carbazole
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Subsequently, the absorption spectra of the radical adducts
1•−3• after being oxidized have been studied. A few years
ago, Gopidas el al. introduced copper(II) salts as good oxidizing
agents of aromatic amines to generate their corresponding
radical cations.11,12 Now, we have used a copper(II) salt to
oxidize radical adducts 1•−3• with very interesting results. The
Cu(II) salt classifies these radical species according to their
reducing power. Thus, while radical adducts 1•−3• are oxidized
by loss of an electron to the corresponding cations, radical
TTM, with a higher peak potential, remains stable in the pre-
sence of Cu(II) salt.
Some of the advantages of Cu(ClO4)2 as oxidant are its
solubility in acetonitrile and its easy handling. Therefore, the
changes in the absorption spectra of the radical adducts 1•−3•
have been studied according to the addition of increasing
amounts of Cu(II) (Figure 5). This study has revealed that the
oxidant reacts with 1•−3• in stoichiometric amounts; each
Cu(II) ion is able to capture one electron from the radical ad-
duct molecule and becomes a Cu(I) ion, generating the corre-
sponding organic cation. The parent radical adducts can be
reversibly recovered from the oxidant solutions by adding an
excess amount of triethylamine, confirming the stability of the
oxidized species and the reversibility of the chemical process.
The emerging absorption bands of the oxidized species are
summarized in Table 2. In all cases, when the Cu2+ salt concen-
tration in the solution of the radical adduct increased, the
absorption band characteristic of the radical character (λ 372−
374 nm) decreased with a concurrent increase of the absorp-
tion of two new bands at λ 489−512 nm and at λ 781−860 nm,
the second being stronger and broader than the first. The first
new band can be ascribed to the cations 1+−3+ (Scheme 2),
similarly to the case of the oxidation of the (4-amino-2,6-
dichlorophenyl)bis(2,4,6-trichlorophenyl)methyl radical (4•)
with copper(II) perchlorate, under similar conditions, giving
rise to a broad band centered at λ 479 nm (see in Figure S4
(Supporting Information) the formation of cation 4+ and the
evolution graphs of UV−vis spectra on the basis of the
equivalents of the salt of Cu(II)). The second and more intense
band is assigned to a photoinduced electron transfer between
the two redox centers of the molecule, from the carbazole nitro-
gen to the carbocation acceptor of the triphenylmethyl moiety,
again in the same direction as in the neutral adducts. This band
is remarkably shifted to the infrared in the cationic species 2+
(λ 860 nm in front of 789 and 781 nm for 1+ and 3+, respec-
tively) due to the increased donor strength of the carbazole
moiety by the presence of methoxy substituents in positions
para to nitrogen. Conversely, the charge-transfer band is slightly
blue-shifted in 3+ in comparison to that of 1+, due to the
electron-withdrawing character of the oxygen atom of the me-
thoxy groups in positions meta to nitrogen. These observations
suggest that, in the photoexcitation of the cationic species, the
electron transfer band involves the nonbonded electrons on the
nitrogen atom of the carbazole moiety to give diradical cation
species (Scheme 3). This characteristic band is named an
intervalence charge transfer (IVCT) band,6 as the two involved
redox centers, the neutral nitrogen and the charged carbon
atoms, are in different oxidation stages. The presence of iso-
sbestic points in the measured absorption spectra when the
concentration of the Cu(II) salt is varied denotes that the
reaction proceeds from neutral to cationic species without
multiple products.
Figure 4. Cyclic voltammogram for the oxidation of radical adduct 3•
(∼10−3 M) in CH2Cl2 with 0.1 M TBAP on Pt at 25 °C. The initial
and final potential was 0.35 V; the reverse potential was 1.350 V. Scan
rates (mV s−1): (a) 200; (b) 100; (c) 50; (d) 20.
nitrogen. Figure 4 shows the cyclic voltammogram obtained for
radical adduct 3•. The oxidized species are remarkably stable
and do not undergo side reactions during the electrochemical
process. For comparison, the cyclic voltammograms of 9-
phenylcarbazole in CH2Cl2 under the same conditions were
obtained. They displayed an irreversible oxidation peak with an
a
anodic peak potential Ep = 1.475 V at 100 mV s−1. As the
a
potential value is much higher than the potential values (Ep )
for radical adducts 1•−3• (see Table 4), we suggest that the
redox pair (O/R) corresponding to the oxidation of 1•−3•
be assigned to the equilibrium reaction involving the removal
of one electron from the trivalent central carbon atom to form
the stable cation. These redox processes are represented in
Scheme 2, showing the additional stabilization of the cation
species by the presence of the heteroatom in the para position
relative to the trivalent carbon.
Scheme 2. Canonical Structures in Resonance for Cationic
Species 1+−3+
The electronic interaction in radical adducts 1•−3• between
the donor and the acceptor is weakened due to the
noncoplanarity of the molecule, resulting from the presence
of two twisted bonds between the two redox centers. One of
them is located between the trivalent carbon and the phenyl
ring due to the high hindrance induced by the presence of two
ortho chlorines, and the other one is located between the same
phenyl ring and the carbazole moiety, the torsion angle being
close to 90° in the crystalline state, as shown in the molecular
structure of 1• (Figure 1; see planes P3−P4 = 57° and P3−P5
= 87.6°). However, the relatively low oxidation potential values
of 1•−3• suggest that the oxidized species, cations 1+−3+, show
a more planar conformation of the molecule.
To confirm the generation of diamagnetic species, cations
1+−3+, by loss of an electron of the radical adducts 1•−3• in the
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Scheme 3. Photoexcitation of Cationic Species 1+−3+
corroborating the absence of radical species; the very weak
band appearing in the spectrum of radical adduct 3• is caused
by the presence of traces of unoxidized radical adduct. How-
ever, a broad and strong signal was observed for all of them in
solid solution, at temperatures below the freezing point of the
solvent (Figure 6).
Figure 6. EPR spectra of acetonitrile solutions (∼10−3 M) of the
oxidized forms of radical adducts (a) 1•, (b) 2•, and (c) 3• at two
temperatures.
In summary, radical adducts 1•−3• are formed by an
electron-donor moiety, the carbazole ring, and an electron-
acceptor moiety, the polychlorotriphenylmethyl radical. They
exhibit a characteristic charge-transfer band of low intensity
from the donor to the acceptor moiety in the visible range of
the electronic spectrum. These radical adducts are chemically
oxidized with copper(II) perchlorate to the respective cation
species by loss of an electron from the central trivalent carbon
atom. It should be mentioned that the two molecular moieties
9-phenylcarbazole and TTM remain practically stable against
copper(II) perchlorate over short periods of time. The new
charged species show a strong and bathochromically shifted
charge transfer band into the near-infrared region of the spec-
trum. The most shifted band is observed in the oxidized species
from 2•, when additional electron-donor substituents (methoxy
groups) are placed in the carbazole ring, the donor moiety
of these cationic species. These bands, both from neutral
species 1•−3• and from cationic species 1+−3+, are charge
transfer bands of organic mixed-valence compounds due to the
Figure 5. UV−vis spectra of radical adducts (a) 1• (0.108 mM;
emerging bands λ(nm) 489, 789), (b) 2• (0.081 mM; emerging bands
λ(nm) 512, 860), and (c) 3• (0.153 mM; emerging bands λ(nm) 490,
781) in CH3CN/CHCl3 (9/1) solution with different amounts of
Cu(ClO4)2.
presence of copper(II) salt, their EPR spectra in acetonitrile
solution with and without an excess of the oxidant Cu(ClO4)2
were registered. The spectra of the neutral form of 1•−3• at
room temperature consisted of a broad (ΔHpp = 4.6−4.75 G)
single line (g = 2.0033 0.00005). In the presence of oxidant,
no signal was observed in any of them at room temperature,
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open-shell nature of both the radical adducts 1•−3• and their
corresponding oxidized species. Charge-transfer bands of the
cation species are stronger and are bathochromically shifted in
comparison to those of the neutral species, due to the stronger
acceptor ability of the positively charged carbon atom. The cat-
ionic species 1+−3+ may be depicted according to the Scheme 2,
which is completely corroborated by the absence of signals in
the EPR spectra of these cationic species in acetonitrile solution
at room temperature. However, the appearance of an intense
band in frozen solutions (at 183 K in Figure 6) shows the pre-
sence of radical species in the solid medium. This fact suggests
that a twisted conformation (bond angle ∼90° between the
phenyl and carbazole rings) with a complete electron transfer
from the donor to the acceptor moieties to get a diradical
cation is adopted in the frozen state (structure B, Scheme 3).
Predicting this new conformation in frozen acetonitrile solution
is in agreement with the stable conformation assumed by the
radical adduct 1• in the crystalline state; the bond between the
phenyl and the carbazole rings undergoes a torsion angle of
approximately 90°, due to steric interactions. In this regard, we
have recently reported the EPR spectrum of the isolated salt
using an Eco Chemie Autolab PGSTAT100 potentiostat−galvanostat
controlled by a computer with Nova 1.5 software. Cyclic voltammo-
grams were recorded at scan rates ranging from 20 to 200 mV s−1.
Molecular and Crystal Structure of Radical Adduct 1•. After
multiple unsuccessful attempts to grow crystals suitable for a single-
crystal diffraction experiment, the crystal structure of 1• was solved
from synchrotron powder diffraction data measured at 100 K (for
details see the Supporting Information). Data indexing with the pro-
gram DICVOL14 was challenging due to the presence of some impu-
rity peaks. The final unit cell is triclinic, space group P1. No phase
̅
transformation occurs between 100 K and room temperature. A similar
triclinic unit cell is refined from room temperature data. For compari-
son purposes, both are given in Table S1 (Supporting Information).
The structure solution was carried out with the direct-space method
TALP15 entering as initial molecular model that derived from MM2
calculations with CS Chem3D pro. The final structure model was
refined with the restrained Rietveld refinement program RIBOLS.16
Crystal data and refinement details are summarized in Table 5.
Table 5. Crystallographic Data and Structure Refinement
Details of Radical Adduct 1• at 100 K
mol formula
C31H14Cl8N
684.03
¯
1+SbCl6 at 2 K, showing an intense single band (g = 2.0030
formula wt
0.0005) and a small half-field band (g = 4.0090 0.0005) that
confirmed the existence of a triplet state at low temperatures.5
The preparation and isolation of perchlorate salts of 1•−3• and
temperature-dependent EPR studies to check for the presence
of triplet states at low temperatures in the twisted confor-
mations are currently underway in our laboratories.
cryst syst
triclinic
space group
P1
̅
a, Å
11.8180(9)
11.8215(11)
12.7157(10)
104.206(5)
114.667(4)
106.656(2)
1404.1(2)
2
b, Å
c, Å
α, deg
β, deg
γ, deg
V, Å3
EXPERIMENTAL SECTION
■
General Procedures. IR spectra were recorded with a FT-IR spec-
trophotometer, electronic spectra with a single cell spectrophotometer,
and the EPR spectra with a 10/12 spectrometer. DSC experiments
used 40 μL closed cresol.
Z
calcd density, g/cm3
Rietveld refinement details
no. of params
no. of restraints
Rwp
1.618
131
[2,6-Dichloro-4-(2,7-dimethoxy-N-carbazolyl)phenyl]bis-
(2,4,6-trichlorophenyl)methyl Radical (3•). A mixture of 2,7-
dimethoxycarbazole13 (1.24 g; 5.6 mmol), TTM (0.76 g; 1.37 mmol),
anhydrous Cs2CO3 (0.76 g; 2.3 mmol), and DMF (15 mL) was stirred
at reflux (2 h) under an inert atmosphere and in the dark. The re-
sulting mixture was poured into an excess of diluted aqueous HCl acid
and extracted with CHCl3. The organic solution was washed with
water, dried over magnesium sulfate, and evaporated. The residue was
chromatographed in silica gel with hexane/CHCl3 (1/1) to give as a
second fraction, a solid (0.33 g). An aqueous solution of TBAH (1.5
M; 0.45 mL; 0.68 mmol) was added to a solution of this solid (0.33 g)
in THF (10 mL) and stirred (5 h) under an inert atmosphere at room
temperature, and then tetrachloro-p-benzoquinone (0.28 g; 1.1 mmol)
was added. The solution was stirred in the dark (30 min) and evapo-
rated to dryness, giving a residue which was filtered through silica
gel with hexane/CHCl3 to give 3• (0.27 g; 27%): mp (DSC) 259 °C
dec; IR (KBr) 3078 (w), 2925 (w), 2351 (w), 1609 (w), 1573 (w),
1522 (w), 1502 (w), 1477 (s), 1454 (s), 1429 (m), 1291 (m), 1244
(w), 1204 (s), 1165 (s), 1132 (m), 1081 (w), 1054 (m), 999 (w), 857
(w), 819 (m), 794 (m), 764 (w) cm−1. Anal. Calcd for C33H18Cl8NO2:
C, 53.3; H, 2.4; Cl, 38.1; N, 1.9. Found: C, 53.0; H, 2.4; Cl, 37.9;
N, 1.7.
157
0.118
0.033
3.604
Rexp
goodness of fit (χ)
ASSOCIATED CONTENT
* Supporting Information
■
S
Text, figures, and tables giving synchrotron powder diffraction
and X-ray crystallographic data of molecular and crystal struc-
ture of radical adduct 1•. This material is available free of charge
via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail for L.J.: ljbmoh@cid.csic.es.
■
Notes
The authors declare no competing financial interest.
Cyclic Voltammetry. Cyclic voltammetry of 3• was carried out in
a standard thermostated three-electrode cell. A platinum (Pt) disk with
0.093 cm2 area was used as the working electrode and a Pt wire as the
counter electrode. The reference electrode was a saturated calomel
electrode (SCE), submerged in a salt bridge of the same electrolyte,
which was separated from the test solution by a Vycor membrane. So-
lutions of radical adduct 3• (∼10−3 M) in CH2Cl2 containing tetra-
butylammonium perchlorate (0.1 M) as background electrolyte were
studied. The volume of all test solutions was 50 mL. Electrochemical
measurements were performed under an argon atmosphere (25 °C)
ACKNOWLEDGMENTS
■
Financial support for this research from the MCI (Spain)
through project CTQ2012-36074 is gratefully acknowledged.
We also thank the EPR service of the Advanced Research
Institute of Catalunya (CSIC) (Spain) for recording the EPR
spectra and the ESRF for beamtime allocation at ID31. O.V.
also thanks Consolider NANOSELECT CSD2007-00041 for a
contract.
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dx.doi.org/10.1021/jo4028215 | J. Org. Chem. 2014, 79, 1771−1777

The Journal of Organic Chemistry
Article
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