[4-(N-Carbazolyl)-2,6-dichlorophenyl]bis(2,4,6-trichlorophenyl)methyl radical an efficient red light-emitting paramagnetic molecule

Article abstract of DOI:10.1016/j.tetlet.2006.02.022

We report the synthesis, luminescent properties, electrochemical behavior and electron paramagnetic resonance of a novel stable radical adduct of the tris(2,4,6-trichlorophenyl)methyl radical and carbazole.

Full text of DOI:10.1016/j.tetlet.2006.02.022

Tetrahedron Letters 47 (2006) 2305–2309
[4-(N-Carbazolyl)-2,6-dichlorophenyl]bis(2,4,6-
trichlorophenyl)methyl radical an efficient red light-emitting
paramagnetic molecule
a
b
b
b
´
Vanesa Gamero, Dolores Velasco, Sonia Latorre, Francisco Lopez-Calahorra,
c
a,
*
´
Enric Brillas and Luis Julia
a
` `
´
Departament de Quı´mica Organica Biologica, Institut d’Investgacions Quımiques i Ambientals (CSIC), Jordi Girona 18-26,
08034 Barcelona, Spain
b
`
Departament de Quı´mica Organica, Universitat de Barcelona, Martı i Franques 1-11, 08028 Barcelona, Spain
Departament de Quı´mica Fı´sica, Universitat de Barcelona, Martı´ i Franques 1-11, 08028 Barcelona, Spain
´
`
c
`
Received 21 December 2005; revised 2 February 2006; accepted 3 February 2006
Available online 21 February 2006
Abstract—We report the synthesis, luminescent properties, electrochemical behavior and electron paramagnetic resonance of a
novel stable radical adduct of the tris(2,4,6-trichlorophenyl)methyl radical and carbazole.
Ó 2006 Elsevier Ltd. All rights reserved.
Carbazole and its derivatives are very attractive com-
pounds by their electroactivity and luminescent proper-
ties. High photoconductivities and hole transporting
properties of many polymeric materials can be greatly
improved by incorporating carbazole as a pendant
group in the framework.1 Carbazole-containing com-
pounds form relatively stable radical cations (holes)
and many carbazole derivatives have a sufficiently high
triplet energy to host red, full-color triplet emitters.2 In
addition, carbazole moiety is a convenient building
block for the design and synthesis of molecular glasses,
which are widely studied as the components of light-
emitting diodes, and photorefractive materials.3 The
carbazole ring is easily functionalized and covalently
linked to other molecules.4 However, the poor nucleo-
philic assistance of the nonbonding electron pair on
the nitrogen has made difficult its incorporation into
aromatic systems by classical methods, requiring severe
reaction conditions. Several synthetic strategies have
been published to obtain N-aryl derivatives of carbazole,
by the use of Cu/bronze5 or palladium catalyst.6 Thus,
different N-aryl azoles have been prepared from elec-
tron-rich and electron-poor halobenzenes as precursors,
mainly bromobenzenes. This is an easy and direct way
to successfully couple carbazole into polymers such as
poly(p-bromostyrene).7 Finally, another important
property of carbazole derivatives for technological
applications is their high thermal and photochemical
stabilities.
In our research on stable organic radicals of the TTM
(tris(2,4,6-trichlorophenyl)methyl radical (1)) series, we
have focused our attention to the introduction of differ-
ent substituents in the aromatic system of the TTM to
modulate their electronic and electrochemical proper-
ties.8 In this letter, we report the successful incorporation
of TTM radical into carbazole. This allows us to com-
bine the magnetic and redox properties of the TTM
radicals and the luminescence properties of the carbaz-
oles in view of their potential applications as chemosen-
sors.9 It is well known that stable nitroxide radicals are
excellent fluorescent quenchers.10 But in no case of the
reported examples in the literature, the radical center is
part of the fluorophore group of the molecule such as
in the stable radical we report in this letter. The only pub-
lished example in the literature of a carbazole coupled
stable radical is a dimer-derived DPPH-type diradical.11
TTM radical in dimethylformamide at reflux, in an inert
atmosphere and in a short time (1 h), with an excess of
carbazol in the presence of cesium carbonate gave as a
majority product (33%), a mixture of (4-N-carbazolyl-
2,6-dichlorophenyl)bis(2,4,6-trichlorophenyl)methane
*
Corresponding author. Tel.: +34 93 4006107; fax: +34 93 2045904;
e-mail: ljbmoh@cid.csic.es
0040-4039/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2006.02.022

2306
V. Gamero et al. / Tetrahedron Letters 47 (2006) 2305–2309
(3) with 4% of the corresponding radical 2. The radical
character of the molecule of TTM makes easy the substi-
tution of the aromatic chlorine by the heterocycle. In this
regard, it is worth mentioning that tris(2,4,6-trichloro-
phenyl)methane (a-H TTM) and 1,3,5-trichlorobenzene
do not react at all with carbazole in the same conditions
of temperature and time of reaction. Treatment of the
mixture of 3 and 2 with an aqueous solution of tetrabu-
tylammonium hydroxide followed by oxidation with
chloranil rendered pure radical 2.12 Triphenylmethane
312 was obtained in a quantitative yield by reduction of
2 with an excess of ascorbic acid in tetrahydrofuran–
water overnight at room temperature. The stability of
radical 2 has been tested in two organic solvents, chloro-
form and cyclohexane. After 24 h the decomposition is
practically negligible.
high values of the molar extinction coefficients of the
first band indicate the p* p nature of this transition
being the highest value in 1. The absorption wavelengths
are nearly the same in the three radicals (k ꢀ 375 nm),
suggesting that this transition is not sensitive to the
presence of amino- and carbazole substitution. How-
ever, the second and weak band with a slight vibronic
structure showed
a remarkable red-shifted effect
following the order 1 < 4 < 2 being in 2 and 4 of much
higher intensity, indicating that radical 2 and 4 are
significantly p-conjugated through the lone electron
pairs at the nitrogen atom and that the unpaired
electron is more delocalized. The UV spectrum of
triphenylmethane 3 (precursors of radical 2), in CHCl3
solution shows bands at k(e): 311(13,670) and
337(6550) nm.
Cl
N
N
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
+
C
CH
C
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
3
2
1
NH2
Cl
Cl
Cl
Cl
N
N
C
H
Ph
5
6
Cl
Cl
Cl
Cl
4
The two absorption bands associated with the radical
character of radicals 1, 2, and (4-amino-2,6-dichlorophen-
yl)bis(2,4,6-trichlorophenyl)methyl radical8c (4) in chloro-
form solution are shown in Figure 1 and Table 1. The
This new carbazole–TTM radical 2 exhibits intense fluo-
rescence emission in cyclohexane in the red wavelength
region (628 nm) (Table 2) compared to carbazole and
radical 1 which show emission at 334/350 and 563 nm,
respectively. The corresponding diamagnetic precursor,
triphenylmethane 3 shows in cyclohexane emission
peaks at 338 and 353 nm, in the same UV-region as
carbazoles 5 and 6, but with a low quantum yield of
0.05. So, we can strictly speak of a fluorescent stable
organic radical. The emission wavelength and the effi-
ciency of the fluorescence band of 2 in cyclohexane
remains unaltered irrespective of the excitation wave-
length (kexc, 515, 423, and 373 nm). This is possibly
due to an efficient relaxation to the lowest excited state.
The fluorescence maximum in 2 shows a considerable
red shift and a large increase in the Stokes shift value
on going from cyclohexane to a more polar solvent such
as chloroform, providing evidence for a higher dipole
moment of the molecule in the excited state than in
the ground state. These effects suggest that 2 undergoes
structural reorientation in the excited state to a more
planar conformation with a significant intramolecular
Figure 1. Absorption spectra of stable organic radicals 1 (black), 2
(blue), and 4 (red) in CHCl3. The right-side of the graphic shows a
detail of the less energetic band.

V. Gamero et al. / Tetrahedron Letters 47 (2006) 2305–2309
2307
Table 1. Wavelengths and absorptivities of the character radical bands in the electronic spectraa and redox and peak potentialsb for the reduction
(E0r ) and oxidation (E0o) of radicals 1, 2, and 4
Radical
k nm (e)c
E0r =V d (V)
Eap ꢁ Ecp (mV)e
E0o=V d (V)
Eap ꢁ Ecp (mV)e
1
373(38,150)
479(870)
542(840)
ꢁ0.51
100
1.27
100
2
4
374(32,670)
377(28,500)
560(sh)(2710)
597(3740)
ꢁ0.52
ꢁ0.83
130
110
1.03
0.31
140
100
537(sh)(2080)
572(3430)
a CHCl3 solution (ꢀ10ꢁ4 M).
b CH2Cl2 solution (ꢀ10ꢁ3 M) with Bu4NClO4 (ꢀ0.1 M) on Pt.
c Units: dm3 molꢁ1 cmꢁ1
.
d Potential versus SCE (saturated calomel electrode).
e Data at scan rate 100 mV sꢁ1
.
Table 2. UV–vis and fluorescence data for radicals 1 and 2, and carbazoles 3, 5, and 6
kmax (nm)
kexc (nm)
kem (nm)
UFa
Stokes shift (cmꢁ1
)
1
2
Cyclohexane
Chloroform
537
542
395
478
563
568
0.03
0.0015
860
844
Cyclohexane
603
603
603
598
597
515
478
478
490
515
628
632
633
659
687
0.53
0.51
0.47
0.14
0.02
660
761
784
1548
2257
Cyclohexane/CHCl3 19:1
Cyclohexane/CHCl3 9:1
Cyclohexane/CHCl3 1:1
Chloroform
3
5
Cyclohexane
335
300
338/353
0.05
1521
Cyclohexane
Chloroform
331
321/333
301
301
334/350
339/354
0.53
0.04
1640
1781
6
Cyclohexane
Chloroform
339
328/341
301
301
343/359
349/363
0.43
0.08
1643
1777
charge transfer from the carbazolyl moiety to the triva-
lent carbon atom. Furthermore, the quantum yield is
highly dependent on the polarity of the solvent as shown
for mixtures of cyclohexane and chloroform in Table 2.
The yield is very low in chloroform, and an increase in
the percentage of cyclohexane in the solvent mixture
results in a large enhancement of the fluorescence
efficiency. Preliminary tests of the fluorescence yields
in CH2Cl2 show results still lower (UF = 0.001) and in
more polar solvents such as acetonitrile and acetone
radical 2 shows no fluorescence at all. This reduction
or cancellation of the fluorescence suggests the existence
of activated nonradiative decay processes that compete
with the fluorescence decay.
Cyclic voltammogram of radical 2 was measured in
CH2Cl2 solution (ꢀ10ꢁ3 M) containing tetra-n-butyl-
ammonium perchlorate (0.1 M) as supporting electro-
lyte on platinum wire as working electrode using a
saturated calomel electrode (SCE) as reference elec-
trode. Radical 2 exhibits reversible oxidation and reduc-
tion processes attributed to the removal and addition of
one electron from/to the trivalent carbon centered radi-
cal. Values of redox potentials are displayed in Table 1
Figure 2. (a) EPR spectrum of radical 2 in CH2Cl2 solution at 193 K; (b) computer simulation.

2308
V. Gamero et al. / Tetrahedron Letters 47 (2006) 2305–2309
along with those of radicals 1 and 4. E0 value for the
reduction of 2 is similar to that of 1, and E0 value for
the oxidation of 2 is much lower (240 mV) than that
of 1, in accordance with the donating ability of the car-
bazole placed at the para position of the phenyl group.
The lowest E0 value for oxidation corresponds to radical
4 (960 mV in relation to 1).
3516–3522; Lee, J. H.; Woo, H. S.; Kim, T. W.; Park, J.
W. Opt. Mater. 2003, 21, 225–229; Das, R. R.; Lee, C. L.;
Noh, Y. Y.; Kim, J. J. Opt. Mater. 2003, 21, 143–146;
Kundu, P.; Thomas, K. R. J.; Lin, J. T.; Tao, Y. Y.;
Chien, C. H. Adv. Funct. Mater. 2003, 13, 445–452;
Suzuki, M.; Tokito, S.; Kamachi, M.; Shirane, K.; Sato,
F. J. Photopolym. Sci. Tech. 2003, 16, 309–314; Thomas,
K. R. J.; Velusamy, M.; Lin, J. T.; Tao, Y. T.; Cheun, C.
H. Adv. Funct. Mater. 2004, 14, 387–392; Anni, M.;
Manna, L.; Cingolani, R.; Valerini, D.; Creti, A.; Lomas-
colo, M. Appl. Phys. Lett. 2004, 85, 4169–4171; Woo, H.
S.; Cho, S.; Kwon, T. W.; Park, D. K.; Kim, Y. B.; Czerw,
R.; Carroll, D. L.; Park, J. W. J. Korean Phys. Soc. 2005,
46, 981–984; Li, K. Z.; Zhang, Y.; Yang, R. Q.; Huang, J.;
Yang, W.; Cao, Y. J. Polym. Sci. Part A: Polym. Chem.
2005, 43, 2325–2336.
X-band EPR spectrum of radical 2 was recorded in
CH2Cl2 solution (Fig. 2). At 298 K the spectrum showed
a single and broad band centered at g = 2.0032 5. The
presence of the two small and broad lines in both sides
of the main spectrum corresponded to the coupling
of the free electron with the a 13C nucleus with a
coupling value a = 28.0 G. Approximately at 193 K,
the spectrum consisted of an overlapped septet of lines
due to the coupling with six aromatic protons (a =
1.0 G) and a small coupling with the nitrogen of the het-
erocyclic substituent (a ꢀ 0.2 G). The presence of two
weak multiplets in both sides of the central band corre-
sponded to the coupling with the three bridgehead 13C
nuclei of the molecule adjacent to the trivalent carbon
atom (a ꢀ 9.0 G).
3. Shirota, Y. J. Mater. Chem. 2000, 10, 1; Justin Thomas,
K. R.; Lin, J. T.; Tao, Y.-T.; Ko, C.-W. J. Am. Chem. Soc.
2001, 123, 9404–9411; Zhang, Q.; Chen, J.; Cheng, Y.;
Wang, L.; Ma, D.; Jing, X.; Wang, F. J. Mater. Chem.
2004, 14, 895–900.
´
4. Dıaz, J. L.; Dobarro, A.; Villacampa, B.; Velasco, D.
´
Chem. Mater. 2001, 13, 2528–2536; Dıaz, J. L.; Villa-
´
campa, B.; Lopez-Calahorra, F.; Velasco, D. Chem.
Mater. 2002, 14, 2240–2251.
´
5. Martinez-Palau, M.; Perea, E.; Lopez-Calahorra, F.;
Velasco, D. Lett. Org. Chem. 2004, 1, 231–237.
In summary, we have been able to incorporate a stable
organic radical of the TTM series into a carbazole by
a coupling reaction resulting in novel red-emitting-light
radical adduct and we have shown the strong influence
of the radical center in the emission properties of the
carbazole. Work is now in progress in our laboratories
to go through the mechanism of this reaction and to pre-
pare other stable radicals of the TTM series by introduc-
ing electron-donor and electron-acceptors substituents
into the carbazole system to modulate their emission
properties.
´
6. Mann, G.; Hartwig, J. F.; Driver, M. S.; Fernandez-Rivas,
C. J. Am. Chem. Soc. 1998, 120, 827–828; Watanabe, M.;
Nishiyama, M.; Yamamoto, T.; Koie, Y. Tetrahedron
Lett. 2000, 41, 481–483.
7. Kanbara, T.; Yokokawa, Y.; Hasegawa, K. J. Polym. Sci.
Part A: Polym. Chem. 2000, 38, 28–34.
´
´
8. (a) Carilla, J.; Fajarı, Ll.; Julia, L.; Riera, J.; Viadel, Ll.
Tetrahedron Lett. 1994, 35, 6529–6532; (b) Carilla, J.;
´
´
´
Fajarı, Ll.; Julia, L.; Sane, J.; Rius, J. Tetrahedron 1996,
˜
52, 7013–7024; (c) Teruel, L.; Viadel, Ll.; Carilla, J.;
´
´
´
Fajarı, Ll.; Brillas, E.; Sane, J.; Rius, J.; Julia, L. J. Org.
˜
Chem. 1996, 61, 6063–6066; (d) Domingo, V. M.; Bur-
dons, X.; Brillas, E.; Carilla, J.; Rius, J.; Torrelles, X.;
´
Julia, L. J. Org. Chem. 2000, 65, 6847–6855; (e) Torres, J.
Acknowledgements
´
L.; Varela, B.; Brillas, E.; Julia, L. Chem. Commun. 2003,
´
´
74–75; Jimenez, A.; Selga, A.; Torres, J. Ll.; Julia, L. Org.
Financial support for this work from the MEC (Spain)
through project BQU2001-3673 is gratefully acknowl-
edged. We also thank the EPR service of the Centre
Lett. 2004, 6, 4583–4586.
9. Lozinsky, E.; Martin, V. V.; Berezina, T. A.; Shames, A.
I.; Weis, A. L.; Likhtenshtein, G. I. J. Biochem. Biophys.
Methods 1999, 38, 29–42; Ziessel, R.; Stroh, C. Org. Lett.
2003, 5, 2397–2400.
´
d’Investigacio I Desenvolupament (CSIC), for recording
the spectra.
10. Green, J. A.; Singer, L. A.; Parks, J. H. J. Chem. Phys.
1973, 58, 2690–2695; Green, J. A.; Singer, L. A. J. Am.
Chem. Soc. 1974, 96, 2730–2733; Watkins, A. R. Chem.
Phys. Lett. 1974, 29, 526–528; Kuzmin, V. A.; Tatikolov,
A. S. Chem. Phys. 1977, 51, 45–47; Chattopadhyay, S. K.;
Das, P. K.; Hug, G. L. J. Am. Chem. Soc. 1983, 105, 6205–
6210; London, E. Mol. Cell. Biochem. 1982, 45, 181–188;
Atik, S. S.; Singer, L. A. J. Am. Chem. Soc. 1978, 100,
3234–3235; Scaiano, J. C.; Paraskevopoulos, C. I. Can. J.
Chem. 1984, 62, 2351–2354; Blough, N. V.; Simpson, D. J.
J. Am. Chem. Soc. 1988, 110, 1915–1917; Green, S. A.;
Simpson, D. J.; Zhou, G.; Ho, P. S.; Blough, N. V. J. Am.
Chem. Soc. 1990, 112, 7337–7346.
11. Kozaki, M.; Okada, K. Synth. Met. 2001, 119, 177–178.
12. Selected data for 2 and 3. Compound 2: IR (KBr) 3068(w),
1573(s), 1553(s), 1522(s), 1492(m), 1477(w), 1461(s),
1368(m), 1322(m), 1306(m), 1281(w), 1232(m), 1183(m),
1137(m), 1076(w), 994(w), 922(w), 861(m) 814(m), 747(s),
722(m), 661(w) cmꢁ1; MS (MALDI-TOF): m/z = 683.9
(M+). Anal. Calcd for C31H14Cl8N: C, 54.4; H, 2.1; Cl,
41.5; N, 2.05. Found: C, 54.7; H, 2.0; Cl, 41.4; N, 2.0.
References and notes
1. Grazulevicius, J. V.; Strohriegl, P.; Pielichowski, J.;
Pielichowski, K. Prog. Polym. Sci. 2003, 28, 1297–1353,
and references cited therein.
2. Gruner, J.; Wittmann, H. F.; Hamer, P. J.; Friend, R. H.;
Huber, J.; Scherf, U.; Mullen, K.; Moratti, S. C.; Holmes,
A. B. Synth. Met. 1994, 67, 181–185; Uekawa, M.;
Miyamoto, Y.; Ikeda, H.; Kaifu, K.; Ichi, T.; Nakaya,
T. Thin Solid Films 1999, 352, 185–188; Park, J. W.; Lee, J.
H.; Lee, H. S.; Kang, D. Y.; Kim, T. W. Thin Solid Films
2000, 363, 90–93; Ahan, K. W.; Liu, Y. Q.; Yu, G.; Wu,
X.; Zhu, D. B.; Sun, R. G.; Wang, D. K.; Epstein, A. J. J.
Mater. Chem. 2001, 11, 1606–1611; Walsh, C. J.; Sook-
simuang, T.; Mandal, B. K. J. Porph. Phthal. 2001, 5, 803–
805; Ko, C. W.; Tao, Y. T.; Lin, J. T.; Thomas, K. R. J.
Chem. Mater. 2002, 14, 357–361; Thomas, K. R. J.; Lin, J.
T.; Tao, Y. T.; Chuen, C. H. J. Mater. Chem. 2002, 12,

V. Gamero et al. / Tetrahedron Letters 47 (2006) 2305–2309
2309
Magn. purity: (4/g2)vT = 0.396 (theoretic value for S = 1/
2: 0.375) (v: magnetic susceptibility). Compound 3: IR
(KBr) 3063(w), 1598(s), 1578(s), 1541(s), 1491(m), 1465(s),
1451(m), 1440(w), 1420(w), 1368(w), 1317(m), 1232(s),
1076(w), 897(m), 861(s) 809(s), 748(s), 722(s),
680(w) cmꢁ1; MS (MALDI-TOF): m/z = 684.9 (M+).
Anal. Calcd for C31H15Cl8N: C, 54.4; H, 2.2; Cl, 41.4;
N, 2.0. Found: C, 54.5; H, 2.3; Cl, 41.7; N, 1.8.