Taking advantage of the radical character of tris(2,4,6-trichlorophenyl) methyl to synthesize new paramagnetic glassy molecular materials

Article abstract of DOI:10.1021/jo702723k

(Chemical Equation Presented) This paper describes the synthesis of the novel bis[4-(N-carbazolyl)-2,6-dichlorophenyl](2,4,6-trichlorophenyl)methyl radical (2^.) and tris[4-(N-carbazolyl)-2,6-dichlorophenyl]methyl radical (3^.). A Friedel-Crafts reaction on [4-(N-carbazolyl)-2,6- dichlorophenyl)bis(2,4,6-trichlorophenyl]methyl radical (1^.), 2 ^., and 3^. leads to the introduction of acyl chains in the 3- and 6-positions of the carbazolyl moiety without impairment of the radical character of the molecule to give radicals 5^., 6^., and 7^.. All of these novel radical adducts are thermally stable, 5 ^. and 6^. being amorphous solids by differential scanning calorimetry. Electron paramagnetic resonance spectra of them show a multiplet at low temperature due to the electron-coupling with six aromatic hydrogens. They show electrochemical amphotericity being reduced and oxidized to their corresponding stable anionic and cationic species, respectively. These radical adducts have luminescent properties covering the red spectral band of the emission with high intensities.

Full text of DOI:10.1021/jo702723k

Taking Advantage of the Radical Character of
Tris(2,4,6-trichlorophenyl)methyl To Synthesize New Paramagnetic
Glassy Molecular Materials
Sonia Castellanos,† Dolores Velasco,‡ Francisco Lo´pez-Calahorra,‡ Enric Brillas,§ and
Luis Julia*,†
Departament de Qu´ımica Orga`nica, Institut de Nanocie`ncies I Nanotecnologia, UniVersitat de Barcelona,
Mart´ı i Franque`s 1-11, 08028 Barcelona, Spain, Departament de Qu´ımica Orga`nica Biolo`gica, Institut
d’InVestigacions Qu´ımiques i Ambientals (CSIC), Jordi Girona 18-26, 08034 Barcelona, Spain, and
Departament de Qu´ımica F´ısica, UniVersitat de Barcelona, Mart´ı i Franque`s 1-11,
08028 Barcelona, Spain
ljbmoh@cid.csic.es
ReceiVed December 21, 2007
This paper describes the synthesis of the novel bis[4-(N-carbazolyl)-2,6-dichlorophenyl](2,4,6-trichlo-
rophenyl)methyl radical (2•) and tris[4-(N-carbazolyl)-2,6-dichlorophenyl]methyl radical (3•). A Friedel-Crafts
reaction on [4-(N-carbazolyl)-2,6-dichlorophenyl)bis(2,4,6-trichlorophenyl]methyl radical (1•), 2•, and 3•
leads to the introduction of acyl chains in the 3- and 6-positions of the carbazolyl moiety without
impairment of the radical character of the molecule to give radicals 5•, 6•, and 7•. All of these novel
radical adducts are thermally stable, 5• and 6• being amorphous solids by differential scanning calorimetry.
Electron paramagnetic resonance spectra of them show a multiplet at low temperature due to the electron-
coupling with six aromatic hydrogens. They show electrochemical amphotericity being reduced and
oxidized to their corresponding stable anionic and cationic species, respectively. These radical adducts
have luminescent properties covering the red spectral band of the emission with high intensities.
Introduction
emitting layers.3 In addition, carbazole as a constituent of
polymers can greatly improve the photoconductivities and hole-
transporting properties of them.4 Many of these materials are
stable amorphous glasses above room temperature and have
advantages over crystalline materials because they can form
uniform and transparent amorphous films by spin coating,
solvent casting from solution, and vacuum deposition.5
Recently, we have reported the synthesis and properties of
the (4-N-carbazolyl-2,6-dichlorophenyl)bis(2,4,6-trichlorophe-
nyl)methyl radical (1•),6 resulting from the coupling of the N,H-
carbazole to the tris(2,4,6-trichlorophenyl)methyl (TTM) radical.
This new crystalline paramagnetic adduct with a singly occupied
Carbazole derivatives are of great interest for their optical
and redox properties. They show relatively intense lumines-
cence1 and undergo reversible oxidation processes to stable
radical cations, making them good hole carriers with a positive
charge.2 As good materials for organic light-emitting diodes
(OLED), carbazoles are used for hole-transporting and light-
† Institut d’Investgacions Qu´ımiques i Ambientals (CSIC).
‡ Departament de Qu´ımica Orga`nica, Institut de Nanociencies i Nanotecnologia.
§ Departament de Qu´ımica F´ısica.
(1) (a) Berlman, I. B. Handbook of Fluorescence Spectra of Aromatic
Molecules; Academic Press: New York, 1971; p 205. (b) Mastrangelo, J. C.;
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Ades, D.; Cloutet, E.; Siove, A. Synth. Met. 2001, 122, 59–61.
(2) (a) Grazulevicius, J. V.; Strohriegl, P.; Pielichowski, J.; Pielichowski,
K. Prog. Polym. Sci. 2003, 28, 1297–1353. (b) Parimal, K.; Justin Thomas, K. R.;
Jiann, T. L.; Tao, Y. T.; Chien, C. H. AdV. Funct. Mater. 2003, 13, 445–452.
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77–87. (b) van Dijken, A.; Bastiaansen, J. J. A. M.; Kiggen, N. M. M.; Langeveld,
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10.1021/jo702723k CCC: $40.75
Published on Web 04/18/2008
2008 American Chemical Society
J. Org. Chem. 2008, 73, 3759–3767 3759

Castellanos et al.
molecular orbital (SOMO) as the valence orbital, and high
thermal stability, has interesting luminescent properties with
emission in the red region of the spectrum and, consequently,
with small SOMO-LUMO energy gap. It also shows attractive
electrochemical properties with quasi-reversible processes either
in the anodic or in the cathodic region to give cationic or anionic
species, respectively, with excellent stability. As an element
belonging to the series of TTM radical, the stability of radical
adduct 1• is due to the steric hindrance of six chlorine atoms
around the trivalent carbon.7,8 All of these radicals are com-
pletely disassociated (they do not dimerize) and present a large
stability either in solid or in solution. They do not abstract
H-atoms from hydrogen-labile species, but they are very
sensitive to electron-transfer reactions, being easily reduced to
anions and oxidized to cations in the presence of electron donor
and acceptor species, respectively. The stability of the charged
species is comparable to that of their radical precursors.
Therefore, we have taken advantage of the properties afforded
by the combination of the TTM radical and carbazole. It is worth
mentioning that carbazole itself emits in the ultraviolet region
of the spectrum and TTM radical only shows very poor
luminescent properties. On the other hand, by virtue of having
an unpaired electron we can obtain information using electron
paramagnetic resonance (EPR) spectroscopy.
positions of the carbazole moiety.9 Thus, a collection of new
adducts with emissions between 601 and 661 nm in cyclohexane
solution and between 591 and 680 nm in chloroform were
described. However, the EPR spectra of all these new radical
adducts appear to be practically the same without appreciable
differences in spite of the presence of substituents in the
carbazole ring of different nature. This is due to the fact that
the spin density of these species resides mainly in the trivalent
carbon atom by virtue of the presence of the bulky polychlo-
rophenyl substituents leading to a poor spin delocalization in
the molecule.
This paper reports the synthesis and properties of novel radical
adducts 2• and 3• by introducing two and three carbazolyl
moieties in the triphenylmethyl core. In addition, we have
prepared radical adducts 5•-7• with large octanoyl chains in
the 3- and 6-positions of the carbazolyl moiety of 1•-3•. Two
of them, adducts 5• and 6• form real amorphous glasses.
To manipulate the energy gap between the SOMO and
LUMO orbitals and obtain a range of luminescence emission
wavelengths into the red region, we have reported the synthesis
of a series of carbazolylTTM radical adducts with electron-
withdrawing or electron-donating groups in the 3- and 6-
Results and Discussion
Synthesis. Scheme 1 illustrates the procedures used for the
preparation of radical adducts 2• and 3•. The method and
conditions of the processes to obtain 2• and 3•, starting from 1•
and 2•, respectively, are similar to those used to obtain 1• from
TTM radical.6 Therefore, radical 1• with an excess of carbazole
in the presence of the appropriate base, cesium carbonate, in
boiling dimethylformamide (DMF), followed by hydrolysis with
an excess of diluted hydrochloric acid, gave a mixture of the
corresponding biscarbazolyltriphenylmethane 2, with moderate
yield, and carbazolyltriphenylmethane 1, product of the reduc-
tion of 1•. In a similar way, triscarbazolyltriphenylmethane 3
was prepared in low yield, together with biscarbazolyltriph-
enylmethane 2, from the reaction of radical 2• with carbazole
and cesium carbonate in DMF. Then, 2 and 3 in tetrahydrofuran
with an aqueous solution of tetrabutylammonium hydroxide
(TBAH) followed by oxidation with 2,3,5,6-tetrachloro-p-
benzoquinone gave pure adducts 2• and 3•, respectively. The
formation of the reduced compounds 1 and 2 as byproduct in
the reactions starting from 1• and 2•, respectively, with carbazole
as reactive, suggests that some sort of radical mechanism is
operative in these reactions. In fact, the diamagnetic compounds
1 and 2 with carbazole in the same conditions did not react,
recovering them in quantitative yields. Consequently, it seems
that carbazole in basic medium is oxidized to the carbazolyl
radical by electron transfer to 1• and 2• to give the charged
species 1- and 2-, respectively, while the carbazolyl radical
couples with another molecule of 1• and 2• to form the
diamagnetic species 2 and 3, respectively, after further reduction
with more carbazole. Then, the charged species 1- and 2- yield
after acid hydrolysis the corresponding carbazolyltriphenyl-
methanes 1 and 2. The reduced species 2 and 3 are also obtained
together with the reduced form of TTM radical, tris(2,4,6-
(4) (a) Brunner, K.; van Dijken, A. V.; Borner, H.; Bastiaansen, J. J. A. M.;
Kiggen, N. M. M.; Langeveld, B. M. W. J. Am. Chem. Soc. 2004, 126, 6035–
6042. (b) Kuwabara, Y.; Ogawa, H.; Inada, H.; Noma, N.; Shirota, Y. AdV. Mater.
1994, 6, 6787–679. (c) Koene, B. E.; Loy, D. E.; Thompson, M. E. Chem. Mater.
1998, 10, 2235–2250. (d) Grigalevicius, S.; Buika, G.; Grazulevicius, J. V.;
Gaidelis, V.; Jankauskas, V.; Montrimas, E. Synth. Met. 2001, 122, 311–314.
(e) Grigalevicius, S.; Blazys, G.; Ostrauskaite, J.; Grazulevicius, J. V.; Gaidelis,
V.; Jankauskas, V.; Montrimas, E. Synth. Met. 2002, 128, 127–131. (f) Huang,
J.; Niu, Y.; Yang, W.; Mo, Y.; Yuan, M.; Cao, Y. Macromolecules 2002, 35,
6080–6082. (g) Morin, J. F.; Leclerc, M. Macromolecules 2002, 35, 8413–8417.
(h) Hwang, J.; Sohn, J.; Park, S. Y. Macromolecules 2003, 36, 7970–7976. (i)
Mi, B. X.; Wang, P. F.; Liu, M. W.; Kwong, H. L.; Wong, N. B.; Lee, Ch. S.;
Lee, S. T. Chem. Mater. 2003, 15, 3148–3151. (j) Inomata, H.; Goushi, K.;
Masuko, T.; Konno, T.; Imai, T.; Sasabe, H.; Brown, J. J.; Adachi, Ch. Chem.
Mater. 2004, 16, 12851291>(k) Jiang, J.; Jiang, C.; Yang, W.; Zhen, H.; Hung,
F.; Cao, Y. Macromolecules 2005, 38, 4072–4080. (l) Liang, F.; Kurata, T.;
Nishide, H.; Kido, J. J. Polym. Sci. Part A Polym. Chem. 2005, 43, 5765–5773.
(m) Sonntag, M.; Kreger, K.; Hanft, D.; Strohriegl, P. Setayesh, S.; de Leeuw,
D. Chem. Mater. 2005, 17, 3031–3039. (n) Go´mez-Lor, B.; Henrich, G.; Alonso,
B.; Monge, A.; Gutierrez-Puebla, E.; Echavarren, A. M. Angew. Chem., Int. Ed.
2006, 45, 4491–4494.
(5) (a) Shirota, Y. Recent extensive reviews on the morphological classifica-
tion of charge-transporting materials are referred in the literature. J. Mater. Chem.
2000, 10, 1–25. (b) Strohriegl, P.; Grazulevicius, J. V. AdV. Mater. 2002, 14,
1439–1452. (c) Shirota, Y.; Kageyama, H. Chem. ReV. 2007, 107, 953–1010.
(6) Gamero, V.; Velasco, D.; Latorre, S.; Lo´pez-Calahorra, F.; Brillas, E.;
Julia´, L. Tetrahedron Lett. 2006, 47, 2305–2309.
(7) Armet, O.; Veciana, J.; Rovira, C.; Riera, J.; Castan˜er, J.; Molins, E.;
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(8) (a) Carilla, J.; Fajar´ı, L.; Julia´, L.; Riera, J.; Viadel, L. Tetrahedron Lett.
1994, 35, 6529–6532. (b) Teruel, L.; Viadel, L.; Carilla, J.; Fajar´ı, L.; Brillas,
E.; San˜e´, J.; Rius, J.; Julia´, L. J. Org. Chem. 1996, 61, 6063–6066. (c) Carilla,
J.; Fajar´ı, L.; Julia´, L.; San˜e´, J.; Rius, J. Tetrahedron 1996, 52, 7013–7024. (d)
Viadel, L.; Carilla, J.; Brillas, E.; Labarta, A.; Julia´, L. J. Mater. Chem. 1998,
8, 1165–1172. (e) Domingo, V. M.; Burdons, X.; Brillas, E.; Carilla, J.; Rius,
J.; Torrelles, X.; Julia´, L. J. Org. Chem. 2000, 65, 6847–6855. (f) Carriedo,
G. A.; Garc´ıa Alonso, F. J.; Go´mez Elipe, P.; Brillas, E.; Julia´, L. Org. Lett.
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Soc., Chem. Commun. 2003, 74–75. (h) Gabino, G. A.; Garc´ıa Alonso, F. J.;
Go´mez Elipe, P.; Brillas, E.; Labarta, A.; Julia´, L. J. Org. Chem. 2004, 69, 99–
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3760 J. Org. Chem. Vol. 73, No. 10, 2008

Synthesis of Paramagnetic Glassy Molecular Materials
SCHEME 1
trichlorophenyl)methane (RHTTM) in low to moderate yields,
in the preparation of 1 from TTM radical with carbazole and
cesium carbonate in DMF (Scheme 1).
2• and 3• with octanoyl chloride in similar conditions afforded
radical adducts 5•, 6•, and 7•, respectively, with excellent and
good yields (Chart 1).
The thermal properties of the carbazolyl conjugated radicals
1•-7• were measured by thermogravimetric analysis (TGA) and
differential scanning calorimetry (DSC) (for dynamic DSC
diagrams of these adducts, see Figures S1-S6 in Supporting
Information). Compounds 1•-4• are crystalline colored solids,
stable in solid state and in solution. They exhibit a clear
endothermic and high melting peak overlapped by an exothermic
A Friedel-Craft reaction on radical adduct 1• with acetyl
chloride in carbon disulfide in the presence of AlCl3 leads to
9
radical adduct 4• with an excellent yield, without impairment
of the radical character of the molecule. This excellent result
allowed the opportunity to introduce longer alkyl chains at the
C-3 and C-6 positions of the carbazole moiety not only on
radical adduct 1•, but also on adducts 2• and 3• . Therefore, 1•,
J. Org. Chem. Vol. 73, No. 10, 2008 3761

Castellanos et al.
CHART 1
TABLE 1. Melting Points of Radical Adducts 1•- 7• by DSC
electron with the R-13C nucleus and its values are listed in Table
2. At low temperature, the spectra showed an overlapped
multiplet of seven very close lines corresponding to the weak
coupling with the six equivalent aromatic hydrogens in meta
positions and two weak multiplets in both sides of the central
multiplet attributed to the coupling with the three bridgehead-
13C nuclei adjacents to the R-carbon atom. These couplings,
reported in Table 2, are estimated values since it is hard to obtain
exact values by spectral simulation when the signals are
relatively broad. One can remark that the peak-to-peak line width
(∆Hpp) of the different spectra at low temperature increases with
mp (°C)
mp (°C)/dec (°C)
1•
2•
3•
4•
298 dec
347 dec
355 dec
359 dec
5•
6•
7•
188/299
143/294
249 (292)
a
a See ref 9.
peak of decomposition at temperatures g300 °C. The introduc-
tion of large alkyl chains in the carbazole moiety to render
radical adducts 5•-7• lowers considerably the melting point of
these species moving away from their decomposition temper-
atures (Table 1), being stable in the melting state in a wide
range of temperatures and allowing to form amorphous glasses
on cooling, in the case of radicals 5• and 6•. Figure 1 shows the
heat flow curves of 5• and 6• with the temperature up to the
melting temperature. They are initially crystalline in the solid
state and exhibit a clear endothermic melting peak on first
heating without decomposition. When the melted samples are
cooled down (10 °C min-1), they do not reveal a recrystallization
exothermic process, forming stable glasses. In a second heating
cycle, samples show a glass transition temperature at 67 and
56 °C, respectively, although they do not crystallize at higher
temperatures and the glasses smoothly transform to liquids. It
is noticeable to mention that the radical 7• melts at much higher
temperature and is stable in the liquid phase in a very short-
range of temperatures (see Table 1 and Figure 2).
Electron Paramagnetic Resonance. X-band EPR spectra of
the carbazolyl conjugated radicals 1•-7• were recorded in
CH2Cl2 solution (∼ 10-3 M) at 298 ( 3 K and 160 ( 5 K
(adducts 5•-7•, Figure 3 and adducts 1•-4•, Figures S7-S9 in
the Supporting Information), and their spectral data are reported
in Table 2. In all cases, g values are similar and very close to
that of the TTM radical (g ) 2.0034 ( 0.0002) and of the free
electron (ge ) 2.0023), in agreement with the expected small
spin-orbit interaction. All of the spectra at room temperature
consisted of a broad and single line, along with a small
equidistant pair of lines in both sides of the main spectrum.
This small pair corresponds to the strong coupling of the free
FIGURE 1. Thermal analysis results using differential scanning
calorimetry: (a) radical adduct 5•; (b) radical adduct 6•. (Heating and
cooling rate: 10 °C/min).
3762 J. Org. Chem. Vol. 73, No. 10, 2008

Synthesis of Paramagnetic Glassy Molecular Materials
FIGURE 4. EPR spectra of radical adducts 5• (left) and 6• (right) in
two different disordered solid states: (a) microcrystalline state, (b)
amorphous state.
SCHEME 2
FIGURE 2. Thermal analysis (TG and DSC) of radical adduct 7•.
(Heating temperature: 10 °C/min).
TABLE 3. Standard Potential for the Redox Pair Related to the
Reduction (O1/R1) and Oxidation (O2/R2) of Radical Adducts 1•-7•,
c
Difference between Their Anodic (Epa) and Cathodic (Ep ) Peak
Potentials, Electron Affinities, and Ionization Potentialsa
E°(O1/R1)b (V)
E°(O2/R2)b (V)
c
c
FIGURE 3. Black: EPR spectra of radical adducts 5•-7• in CH2Cl2
solution (∼10-3 M) at 160 ( 5 K; modulation amplitude, 0.1. Left: 5•.
Middle: 6•. Right: 7•. Blue: Computer simulation.
a
c
a
c
adduct ((Ep - Ep ) (mV)) ((Ep - Ep ) (mV)) EA (eV) IP (eV)
d
1•
2•
3•
4•e
5•
6•
7•
-0.52 (130)
-0.53 (120)
-0.52 (90)
-0.47(130)
-0.47 (120)
-0.43 (100)
-0.39 (90)
1.03 (130)
0.96 (120)
0.90 (80)
1.18 (140)
1.18 (120)
1.13 (100)
1.12 (100)
4.28
4.27
4.28
4.33
4.33
4.37
4.41
5.83
5.76
5.70
5.98
5.98
5.93
5.92
TABLE 2. g Values and Hyperfine Coupling Constants (Gauss)
for Radical Adducts 1•-7• in CH2Cl2 (∼10-3 M)a
adduct
gb
1H
13C(R)c
13C(arom)c
∆Hpp
d
1•
2.0032
2.0029
2.0028
2.0032
2.0039
2.0026
2.0029
1.25
1.25
1.25
1.25
1.25
1.25
1.25
28.0
28.2
9.0
9.9
0.80
0.80
0.85
0.65
0.95
1.1
a CH2Cl2 solution (∼10-3 M) with Bu4NClO4 (0.1 M) as background
2•
3•
electrolyte on Pt electrode at 25 °C. b Potential values versus SCE
27.75
27.90
26.75
27.25
28.75
10.1
10.5
10.5
10.5
10.5
c
(saturated calomel electrode). Values at a scan rate of 100 mV s-1
d Values taken from ref 6. e Values taken from ref 9.
.
e
4•
5•
6•
7•
1.2
the spin-diffusion conditions between microcrystalline and
amorphous states. While the spins associated with a particular
orientation within each microcrystalline are isolated in space
and the magnetization of each microcrystalline develops inde-
pendently thus minimizing spin diffusion, in the amorphous
glassy state all the spins can interact with each other because
all the regions overlap and are more sensitive to cross correlation
increasing the spin-spin relaxation rate and diminishing the
mean lifetime of a given spin-orientation state. In order to
preserve the Heisenberg uncertainty principle, the line width
of the peak must increase. This is what happens with the
spectrum of the adduct 6• and, surprisingly the contrary occurs
for adduct 5•. So, on going from microcrystalline to amorphous
state causes narrowing of the peak of adduct 5• and broadening
of that of adduct 6•.
Electrochemical Studies. Cyclic voltammograms of radical
adducts 1•-7• were recorded in CH2Cl2 solution (∼10-3 M)
containing tetrabutylammonium perchlorate (0.1 M) as sup-
porting electrolyte on a platinum disk as working electrode at
25 °C using a three-electrode two-compartment cell with a
saturated calomel electrode (SCE) as reference electrode and a
platinum wire as counter electrode. The voltammograms of each
radical displayed two quasi-reversible redox pairs, one corre-
sponding to its reduction (O1/R1) and the other one to its
oxidation (O2/R2), which are attributed to the equilibrium
reactions involving the addition/removal of one electron to/from
the trivalent central carbon atom to form its stable anion and
cation, respectively. Both redox processes are represented in
a The hfc constants for six 1H in meta and 13C(arom) (adjacent to R-
carbon) and values for ∆Hpp (peak to peak line width) were determined
at 180 ( 5 K and checked by computer simulation. b g values are
measured against dpph (2.0037 ( 0.0002) at 298 K. c Natural abundance
of 13C isotope: 1.10%. d Values taken from ref 6. e Values taken from
ref 9.
the number of carbazole moieties in the molecule. ∆Hpp may
be at least in part a consequence of the coupling between the
free electron and the nitrogen, since it is low enough not to
infer additional splittings in the spectra. However, the ∆Hpp
values for the spectra of 1•-3• denote practically no variation
with the increase of carbazole moieties (see Table 2), and hence,
the increase of ∆Hpp in 5•-7• must be related with the slow
tumbling rates of the molecules in the solvent, which depend
on their size and shape.
The spectrum of a microcrystalline sample of adducts 5• and
6• displays a sole line with ∆Hpp of 4.7 and 3.4 G, respectively.
When the samples are heated to their melting point for 10 min
and then cooled to room temperature to amorphous glasses, their
∆Hpp values become of 2.7 and 5.0 G, respectively (Figure 4).
It is assumed that the spectrum of a polyoriented paramagnetic
sample, either microcrystalline or amorphous, is made up of a
broad envelope of homogeneously peaks from spins resonating
at different frequencies, each associated with a certain orienta-
tion. This is a consequence of the anisotropic Zeeman and
hyperfine interactions. However, there is a small difference in
J. Org. Chem. Vol. 73, No. 10, 2008 3763

Castellanos et al.
TABLE 4. UV-vis Absorption Data of the Spectra of Radical Adducts 1•-7•, TTM Radical, and 9-Phenylcarbazole (Phcz) in CHCl3
adduct
λmax (nm) (ꢀ)a
adduct
λmax (nm) (ꢀ)a
1•
291 (14400); 324 (6870); 339 (9570); 374 (25600); 446 (sh) (2630); 561 5•
373 (32400); 438 (sh) (4400); 536 (sh) (2000); 567 (1900)
375 (21500); 438 (9600); 542 (sh) (3000); 581 (3500)
378 (20100); 446 (14200); 550 (sh) (4100); 593 (5450)
Phcbz 286 (11000); 293 (12600); 326 (2500); 340 (2800)
(sh) (2130); 598 (2940)
2•
3•
292 (28000); 323 (10300); 337 (12200); 376 (22100); 445 (7350); 566
(3750); 609 (5130)
6•
291 (36200); 323 (12700); 337 (14100); 377 (20500); 454 (13100); 572 7•
(sh) (4920); 614 (7000)
b
4•
293 (18800); 328 (20400); 373 (27900); 543 (1840); 559 (1840)
TTM 371 (35000); 494 (635); 539 (700)
a Units: dm3 mol-1 cm-1 b Values taken from ref 9.
.
and 9-phenylcarbazole. The band at about 290 nm derives from
carbazole-centered transition, and the absorption region from
370 to 600 nm is associated with the radical character of these
compounds. The band at about 370 nm in 1•-3• is characteristic
of the π-π* transition and have features similar to that of TTM
radical corresponding to the triphenylmethyl moiety, although
the molar absorption coefficients decrease going from 1• to 3•.
In the range 370-500 nm, the spectra of 1•-3• exhibit the
presence of a new absorption band (λmax ∼450 nm) bathochro-
mically shifted relative to the π-π* transition in the TTM
radical (λmax ∼370 nm), which may be attributed to the
contribution of carbazole units. Thus, this band is slightly red-
shifted and increases in intensity with the number of carbazole
units in the molecule. The lowest-energy range in the visible
region between 500-670 nm presents a broadband and a
shoulder on the left-side of the peak. The shoulder could
correspond to the weak and structured band in TTM radical
(λmax ∼540 nm) being bathochromically shifted in 1•-3•due to
the influence of carbazole moieties. The broad and more intense
peak at longer wavelengths shifts from blue to red and increases
in intensity following the order 1• < 2• < 3•and can be attributed
to a weak charge-transfer band. This feature suggests a partial
intramolecular electron transfer in the molecule from the donor,
the carbazole moiety, to the acceptor, the trivalent carbon. The
spectra of the adducts 5•-7•(Figure S35, Supporting Informa-
tion) are similar to those of their unsubstituted parent compounds
in the range 250-700 nm. However, it is worth mentioning
that all of the absorption maxima except that of the first π-π*
transition (λmax ∼370 nm) appear at a slightly shorter wavelength
(see Table 4), indicating that the octanoyl groups in the carbazole
ring exert an appreciable electron-withdrawing effect.
FIGURE 5. UV-vis spectra of radical 1• (black), 2• (green), 3• (brown),
TTM (red), and 9-phenylcarbazole (blue) in CHCl3.
Scheme 2, also showing the additional stabilization of the cation
species by the presence of the heteroatom in the para position
relative to the trivalent carbon.10 Values of the electrochemical
parameters are shown in Table 3 (voltammograms are shown
in Figures S10-S21, Supporting Information). Each redox
process, either in the cathodic or anodic region, is quasi-
reversible because the difference between their anodic and
cathodic peak potentials is always higher than the theoretical
value of 59.2 mV expected for a one-electron reversible process
and increases gradually as rising scan rate. Its standard potential
a
(E°) was determined as the average of the anodic (Ep ) and
c
cathodic (Ep ) peak potentials. An analysis of Table 3 indicates
that by increasing the number of carbazole units in 1•-3• the
standard oxidation potentials become smaller, while no signifi-
cant variation is denoted in the reduction potentials. On the other
hand, as the carbazole substituents, the octanoyl chains, present
electron-withdrawing effect, the oxidation potentials of 5•-7•
are shifted to more positive values by comparison with those
of 1•-3•, and the reduction potentials move to less negative
values. In general, the electron-withdrawing substituents at the
carbazole ring in these radical adducts stabilize the SOMO
orbital and, consequently, the ionization potential (IP) and
electron affinity (EA) values11 in 5•-7• become higher than in
1•-3•, as can be seen in Table 3.
The visible emission properties of all synthesized radical
adducts have been also investigated and are reported in Table
5. The luminescence spectra in diluted solution of cyclohexane
and chloroform are depicted in Figure 6. When excited into the
visible range of the spectrum (λexc ) 450 nm), 1•-7• showed
emission in the range 606-645 nm with cyclohexane as solvent
and in the range 618-680 nm with chloroform. It is interesting
to remark the considerable red-shift and the large increase in
the Stokes shift value (∆S ∼1245 cm-1, see Table 5) in the
emission of 1• on going from a dilute solution in cyclohexane,
a nonpolar solvent, to chloroform. This fact provides evidence
of the different electronic structure and geometry of the molecule
in the excited and ground states. As said above, the absorption
spectrum of 1• suggests the presence of a weak intramolecular
charge-transfer in the ground state, this fact being consequently
magnified in the excited-state and also confirmed by the dramatic
decrease in the quantum yield of the emission in chloroform.
In this context, it is known that the fluorescence efficiency in
the intramolecular charge transfer states decreases with increas-
ing strength of the transfer, and this is more significant in polar
Photophysical Properties. The UV-vis absorption spectra
of radical adducts 1•-3• and 5•-7•were recorded in CHCl3
solution at concentrations about 10-4 M. Table 4 summarizes
the absorption bands and molar absorptivities of all them along
with those of 9-phenylcarbazole and TTM radical. Figure 5
presents the combined absorption spectra of 1•-3•, TTM radical,
(10) This additional stabilization is corroborated by the fact that the redox
potentials for the oxidation of radicals 1•-7• are lower than the redox potential
of TTM radical (E° ) 1.27 V; see ref 9).
(11) EA and IP values for radical adducts 1•-7• are estimated from the
standard potentials of the O1/R1 and O2/R2 pairs, respectively, taking a value of
-4.8 eV as the SCE energy level relative to the vacuum level.
3764 J. Org. Chem. Vol. 73, No. 10, 2008

Synthesis of Paramagnetic Glassy Molecular Materials
TABLE 5. Photophysical Data for Radical Adducts 1•-7•
b
adduct
solvent
λabsa(nm)
λem(nm)
ΦF
Stokes shift (cm-1
)
∆Sc (cm-1
)
1•
cyclohexane
chloroform
cyclohexane
chloroform
cyclohexane
chloroform
toluene
chloroform
cyclohexane
chloroform
cyclohexane
chloroform
cyclohexane
chloroform
599
598
607
609
611
614
578
559
573
567
585
581
597
598
628
680
651
680
654
680
625
618
606
621
621
631
631
642
0.53
0.04
0.54
0.10
0.52
0.07
0.48
0.38
0.51
0.46
0.56
0.49
0.48
0.45
771
2016
1113
1714
1077
1581
1301
1708
950
1534
991
1364
902
1245
2•
3•
601
504
407
584
373
244
d
4•
5•
6•
7•
1146
a The less energetic band in the absorption spectra. b Quantum yield of luminescence. c Difference between Stokes shift in chloroform and
cyclohexane. d Radical adduct insoluble in cyclohexane. Values taken from ref 9.
FIGURE 6. Normalized fluorescence emission spectra of radical adducts recorded in (left) cyclohexane solution: 1•, black; 2•, brown; 3•, purple;
5•, red; 6•, green; 7•, blue; (right) chloroform solution: 1•, black; 2•, brown; 3•, purple; 5•, red; 6•, green; 7•, blue; at room temperature.
solvents.12 The smaller red-shift and the lower differences in
the Stokes shift values from the emission spectra of 2• and 3•
from cyclohexane to chloroform (∆S ) relative to that of 1•
indicates that the polar nature of the excited states diminishes
with the presence of two and three carbazolyl moieties. This
behavior is probably by symmetry reasons, a consequence of
the greater dispersion of the positive charge in the excited state
with the presence of two and three electron-donor carbazolyl
moieties in the molecule.
as substituents at the para positions of the triphenylmethyl group.
These syntheses have been performed in the same reaction
conditions as the synthesis of 1•. The substitution of one, two,
and three chlorine atoms in the TTM radical by the carbazole
group is possible due to the radical character of the molecule.
Since these adducts remain stable up to their melting points,
decomposing at the melting temperature, large acyl chains have
been introduced at the 3- and 6-positions of the carbazole
moieties, synthesis of the stable radical adducts 5•-7•, to lower
their melting and to provide of a temperature range between
the melting and the decomposition temperatures. On the other
hand, the presence of the large acyl chains increases the
solubility of these species in organic solvents. Two of them, 5•
and 6• readily form stable amorphous glasses above room
temperature although the glass-transition temperatures are low
(below 70 °C). The presence of two redox processes in these
radical adducts, one of them in the anodic region and the other
in the cathodic one, suggests that they have excellent electro-
chemical stability and exhibit electrochemical amphotericity,
being oxidized and reduced indistinctly. UV-vis spectra show
the bands associated with the carbazolyl group and with the
radical character of these adducts, the later being typical of the
absorptions found in the stable radicals of the TTM radical
series. In addition, a charge-transfer band more pronounced in
adducts 1•-3• than in 4•-7• emerges on the lowest-energy range
of the visible spectrum. All of the radical adducts show very
In all cases, emission from acyl-substituted radicals 5•-7• is blue-
shifted relative to their parent adducts 1•-3•. It is worth mentioning
that the emission intensities of the excited states of 5•-7• in dilute
chloroform solution relative to those in cyclohexane are not dramati-
cally diminished, as occurs in 1•-3•. Quantum yields of the emission
of 5•-7• are practically equal in both solvents irrespective of the solvent
polarity. The different behavior in the emission spectra of radical
adducts 5•-7• is due to the presence of the alkylcarbonyl substituents
in the carbazole ring and evidence the significant perturbation that these
electron-withdrawing substituents exert in the electron-donating capac-
ity of the carbazole moiety.
Conclusions
We have synthesized two stable radical adducts, 2• and 3•,
derived from TTM radical with two and three carbazole moieties
(12) Jenekhe, S. A.; Lu, L.; Alam, M. M. Macromolecules 2001, 34, 7315–
7324.
J. Org. Chem. Vol. 73, No. 10, 2008 3765

Castellanos et al.
804 (m), 748 (s), 720 (m) cm-1; UV (CHCl3) λmax/nm (ꢀ/L mol-1
cm-1) 292 (28000), 323 (10300), 337 (12200), 376 (22100), 445
(7350), 571 (sh) (3840), 609 (5130); EI-HRMS calcd for
C43H22Cl7N2 810.960268, found m/z 810.959629.
significant luminescence properties, covering the red spectral
band of the emission.
Adducts 5• and 6• exhibit two essential properties as
candidates to electroluminescent devices: they are amorphous
glasses and red fluorescents. Utilization of them as organic
semiconductive materials is currently in progress in our labora-
tory. It is worth mentioning that only very recently, Nishide et
al. have reported the first hole transporting molecules based on
unpaired electrons, a combination of triarylamines with nitroxide
radicals.13 We focus also our attention in the synthesis of new
glassy carbazolylTTM radical adducts with different side chain
patterns in the carbazolyl moiety to increase the glass-transition
temperature.
Tris[4-(N-carbazolyl)-2,6-dichlorophenyl]methyl Radical (3•). A
mixture of carbazole (0.500 g; 3.0 mmol), 2• (0.500 g; 0.61 mmol),
anhydrous Cs2CO3 (0.500 g; 1.5 mmol), and DMF (10 mL) was
stirred at reflux (2.5 h) in an inert atmosphere and in the dark. The
resulting mixture was poured into an excess of diluted aqueous
HCl acid, and the precipitate was filtered. The solid was chromato-
graphed in silica gel with CCl4 to give bis[4-(N-carbazolyl)-2,6-
dichlorophenyl](2,4,6-trichlorophenyl)methane (2) (0.415 g; 83%),
identified by IR and a mixture (0.102 g) which was further separated
by thin layer chromatography to give of tris[4-(N-carbazolyl)-2,6-
dichloro)phenyl]methane (3) (0.071 g; 12%): IR (KBr) 3055 (w),
2953 (m), 2923 (m), 2853 (m), 1596 (s), 1574 (m), 1543 (w), 1493
(m), 1463 (s), 1453 (s), 1333 (m), 1310 (m), 1227 (m), 1172 (w),
1151 (w), 1073 (w), 996 (w), 921 (w), 878 (w), 799 (m), 746 (s),
721 (m), 652 (w); 1H NMR (300 MHz, CDCl3) δ 8.15 (d, 6H, J )
7.4 Hz), 7.72 (d, 3H, J ) 2.4 Hz), 7.59 (d,3H, J ) 2.1 Hz),
7.53-7.46 (m,12H), 7.37-7.32 (m, 6H), 7.21 (s, 1H) (see Figure
S24, Supporting Information). Anal. Calcd for C55H31Cl6N3: C, 69.8;
H, 3.3; Cl, 22.5; N, 4.4. Found: C, 69.8; H, 3.8; Cl, 21.7; N, 4.2.
An aqueous solution of TBAH (1.5M) (0.3 mL; 0.45 mmol) was
added to a solution of 3 (0.084 g; 0.09 mmol) in THF (10 mL) and
stirred (7 h) in an inert atmosphere at rt, and then tetrachloro-p-
benzoquinone (0.140 g; 0.6 mmol) was added. The solution was
stirred in the dark (1.5 h), left overnight in the dark, and evaporated
to dryness, yielding a residue which was filtered through silica gel
with CCl4 and dried at low pressure to eliminate the excess of
tetrachloro-p-benzoquinone to give 3• (0.050 g; 59%): mp (DSC)
355 °C (dec); IR (KBr) 3065 (w), 1571 (s), 1524 (m), 1493 (m),
1453 (s), 1335 (s), 1312 (m), 1224 (m), 806 (m), 746 (s), 723 (s)
cm-1; UV (CHCl3) λmax/nm (ꢀ/L mol-1 cm-1) 291 (36200), 323
(12700), 337 (14050), 377 (20500), 454 (13050), 575 (sh) (4930),
614 (7000); (MALDI-ToF) HRMS calcd for C55H30Cl6N3 942.0590,
found m/z 942.0565.
Experimental section
[4-(N-Carbazolyl)-2,6-dichlorophenyl]bis(2,4,6-trichlorophenyl)-
methyl Radical (1•) and Bis[4-(N-carbazolyl)-2,6-dichlorophe-
nyl](2,4,6-trichlorophenyl)methyl Radical (2•). A mixture of car-
bazole (0.500 g; 3.0 mmol), 1• (0.500 g; 0.73 mmol), anhydrous
Cs2CO3 (0.541 g; 1.6 mmol) and DMF (9 mL) was stirred at reflux
(2.75 h) in an inert atmosphere and in the dark. The resulting
mixture was poured into an excess of diluted aqueous HCl acid,
and the precipitate was filtered off. The solid was chromatographed
in silica gel with hexane/CHCl3 (5:1) to give [4-(N-carbazolyl)-
2,6-dichlorophenyl]bis(2,4,6-trichlorophenyl)methane (1) (0.246 g;
49%) [IR (KBr) 3058 (w), 1598 (s), 1576 (s), 1541 (s), 1494 (m),
1478 (m), 1465 (s), 1452 (s), 1442 (m), 1370 (s), 1334 (m), 1314
(m), 1232 (s), 1173 (m), 1142 (m), 1076 (m), 998 (m), 928 (m),
899 (m), 861 (s), 838 (m), 808 (s), 748 (s), 722 (s) cm-1; 1H NMR
(400 MHz, CDCl3) δ 8.13 (d, 2H, J ) 6 Hz), 7.61 (d, 1H, J ) 1.6
Hz), 7.48 (d, 1H, J ) 1.6 Hz), 7.44-7.46 (m, 4H), 7.43 (d, 1H, J
) 1.6 Hz), 7.40 (d, 1H, J ) 1.6 Hz), 7.30-7.33 (m, 3H), 7.28 (d,
1H, J ) 1.6 Hz), 6.85 (s, 1H) (see Figure S22, Supporting
Information)] and bis[4-(N-carbazolyl)-2,6-dichlorophenyl](2,4,6-
trichlorophenyl)methane (2) (0.220 g; 37%): IR (KBr) 3058 (w),
1598 (s), 1576 (s), 1543 (s), 1493 (s), 1479 (m), 1464 (s), 1452
(s), 1442 (m), 1334 (m), 1310 (m), 1228 (s), 1174 (m), 1074 (m),
998 (m), 921 (m), 884 (m), 878 (m), 831 (m), 797 (s), 748 (s), 721
(s) cm-1; 1H NMR (400 MHz, CDCl3) δ 8.14 (d, 4H, J ) 6.4 Hz),
7.68 (d, 1H, J ) 1.6 Hz), 7.65 (d, 1H, J ) 1.6 Hz), 7.55 (d, 1H,
J ) 1.6 Hz), 7.52 (d, 1H, J ) 1.6 Hz), 7.45-7.50 (m, 10 H),
7.35-7.32 (m. 6H), 7.03 (s, 1H) (see Figure S23, Supporting
Information). Anal. Calcd for C43H23Cl7N2 ·1/5CHCl3: C, 61.8; H,
2.8; Cl, 32.1; N, 3.3. Found: C, 61.7; H, 2.8; Cl, 31.5; N, 3.3.
An aqueous solution of TBAH (1.5 M) (2 mL; 3.0 mmol) was
added to a solution of 1 (1.453 g; 2.1 mmol) in THF (45 mL) and
stirred (5 h) in an inert atmosphere at rt, and then tetrachloro-p-
benzoquinone (1.41 g; 5.7 mmol) was added. The solution was
stirred in the dark (45 min) and evaporated to dryness, giving a
residue which was filtered through silica gel with hexane to give
1• (1.203 g; 83%): mp (DSC) 298 °C dec; UV (CHCl3) λmax/nm
(ꢀ/L mol-1 cm-1) 291 (14400), 324 (6900), 339 (9600), 374
(25800), 446 (2600), 564 (sh) (2100), 598 (2940); EI-HRMS calcd
for C31H14Cl8N 679.863446, found m/z 679.862383.
Reaction of Tris(2,4,6-trichlorophenyl)methyl (TTM) Radical
with NH-Carbazole in DMF. A mixture of carbazole (3.10 g; 18.6
mmol), TTM radical (3.00 g; 5.4 mmol), anhydrous Cs2CO3 (2.91
g; 8.7 mmol), and DMF (55 mL) was stirred at reflux (2 h) in an
inert atmosphere and in the dark. The resulting mixture was poured
into an excess of diluted aqueous HCl acid, and the precipitate was
filtered off. The solid was chromatographed in silica gel with hexane/
CHCl3 (5:1) to give tris(2,4,6-trichlorophenyl)methane (0.886 g; 29%)
identified by IR, [4-(N-carbazolyl)-2,6-dichlorophenyl]bis(2,4,6-trichlo-
rophenyl)methane (1) (1.453 g; 39%), bis[4-(N-carbazolyl)-2,6-dichlo-
rophenyl](2,4,6-trichlorophenyl)methane (2) (0.716 g; 16%), and tris[4-
(N-carbazolyl)-2,6-dichloro)phenyl]methane (3) (0.144 g; 3%).
Synthesis of Radicals 4• -7•. General Procedure. A mixture
containing the suitable radical precursor, powdered AlCl3, and
carbon disulfide (CS2) was stirred at reflux under phosphorus
pentaoxide (P2O5) atmosphere. Octanoylchloride was added and
evolution of gas was observed. After 2-3 h of reaction, the solvent
was evaporated and the crude was treated with an ice/water/HCl
mixture and extracted with chloroform. The organic layer was dried
and evaporated to dryness and the resulting residue was purified.
[2,6-Dichloro-4-(3,6-dioctanoyl-N-carbazolyl)phenyl]bis(2,4,6-
trichlorophenyl)methyl Radical (5•). Starting materials: 1• (300 mg;
0.44 mmol), AlCl3 (117 mg; 0.88 mmol), octanoyl chloride (0.17
mL; 1 mmol), CS2 (18 mL). Reaction time 2 h. The residue was
chromatographed in silica gel with chloroform to give 5• (367 mg;
89%): mp (DSC) 188 °C; IR (KBr) 3060 (w), 2953 (m), 2925 (s),
2853 (m), 1676 (s), 1594 (m), 1574 (s), 1555(m), 1524 (m), 1479
(m), 1370 (m), 1334 (m), 1290 (m), 1232 (m), 1183 (m), 1137
An aqueous solution of TBAH (1.5 M) (1.25 mL; 1.87 mmol)
was added to a solution of 2 (0.685 g; 0.8 mmol) in THF (30 mL)
and stirred (5.25 h) in an inert atmosphere at rt, and then tetrachloro-
p-benzoquinone (1.10 g; 4.4 mmol) was added. The solution was
stirred in the dark (2.5 h) and evaporated to dryness, giving a residue
which was filtered through silica gel with CHCl3 to give 2• (0.640
g; 93%): mp (DSC) 347 °C dec; IR (KBr) 3053 (w), 1573 (s), 1524
(m), 1493 (s), 1471 (s), 1341 (m), 1333 (s), 1310 (m), 1226 (m),
(m), 924 (m), 858 (s), 814 (s), 798 (s), 724 (m); UV (CHCl3) λmax
/
(13) (a) Suga, T.; Konishi, H.; Nishide, H. J. Chem. Soc., Chem. Commun.
2007, 1730–1732. (b) Kurata, T.; Koshika, K.; Kato, F.; Kido, J.; Nishide, H.
J. Chem. Soc., Chem. Commun. 2007, 2986–2988. (c) Kurata, T.; Koshika, K.;
Kato, F.; Kido, J.; Nishide, H. Polyhedron 2007, 26, 1776–1780.
nm (ꢀ/L mol-1 cm-1) 263 (66700), 290 (sh) (36900), 328 (22700),
373 (32400), 438 (sh) (4400), 567 (1900) nm; EI-HRMS calcd for
C47H42Cl8NO2 932.072377, found m/z 932.070085.
3766 J. Org. Chem. Vol. 73, No. 10, 2008

Synthesis of Paramagnetic Glassy Molecular Materials
Bis[2,6-dichloro-4-(3,6-dioctanoyl-N-carbazolyl)phenyl](2,4,6-
trichlorophenyl)methyl Radical (6•). Starting materials: 2• (200 mg;
0.25 mmol), AlCl3 (207 mg; 1.55 mmol), octanoyl chloride (0.26
mL; 1.55 mmol), CS2 (18 mL). Reaction time 3 h. The residue
was chromatographed in silica gel with chloroform/hexane (7:3)
to give 6• (296 mg; 90%): mp (DSC) 143 °C; IR (KBr) 3060 (w),
2954 (m), 2928 (s), 2850 (m), 1676 (s), 1624 (m), 1595 (m), 1574
(s), 1526 (m), 1482 (m), 1369 (m), 1338 (m), 1291 (m), 1237 (m),
(87000), 326 (51800), 378 (20100), 446 (14200), 593 (5450) nm;
(MALDI-ToF)-HRMS calcd for C103H114Cl6N3O6 1698.6833, found
m/z 1698.6899.
Acknowledgment. Financial support for this research was
obtained from the Ministerio de Educacio´n y Ciencia (Spain)
through Grant No. CTQ2006-15611-C02-02/BQU. S.C. is
gratefully acknowledged to the Generalitat de Catalunya for a
predoctoral grant. We also thank the EPR service of the Institut
d’Investigacions Qu´ımiques i Ambientals de Barcelona (CSIC)
for recording the EPR spectra.
1189 (m), 818 (m), 806 (m); UV (CHCl3) λmax/nm (ꢀ/L mol-1 cm-1
)
263 (105200), 292 (62800), 327 (38000), 375 (21500), 438 (sh)
(9600), 581 (3500) nm; (MALDI-ToF)-HRMS calcd for
C75H78Cl7N2O4 1315.3776, found m/z 1315.3776.
Supporting Information Available: General experimental
procedures, differential scanning calorimeter analysis (Figures
S1-S6), EPR spectra (Figures S7-S9), cyclic voltammograms
(Figures S10-S21), 1H NMR spectra (Figures S22-S24), and
IR spectra (Figures S25-S34) for new radical adducts 2•-7•
and for new diamagnetic compounds 1-3. This material is
available free of charge via the Internet at http://pubs.acs.org.
Tris[2,6-dichloro-4-(3,6-dioctanoyl-N-carbazolyl)phenyl]meth-
yl Radical (7•). Starting materials: 3• (35 mg; 0.037 mmol), AlCl3
(40 mg; 0.3 mmol), octanoyl chloride (0.05 mL; 0.3 mmol), and
CS2 (8 mL). Reaction time: 2 h. The residue was purified by thin-
layer chromatography eluting with chloroform to give 7• (40 mg;
60%): mp (DSC) 249 °C dec; IR (KBr) 3060 (w), 2953 (m), 292
(s), 2851 (m), 1678 (s), 1623 (m), 1593 (m), 1571 (s), 1522 (m),
1476 (m), 1357 (m), 1333 (m), 1287 (m), 1230 (m), 1184 (m), 802
(s); UV (CHCl3) λmax/nm (ꢀ/L mol-1 cm-1) 263 (142500), 293
JO702723K
J. Org. Chem. Vol. 73, No. 10, 2008 3767