Light-emitting persistent radicals for efficient sensor devices of solvent polarity

Article abstract of DOI:10.1016/j.tetlet.2008.06.040

Radical adduct 1^{{radical dot}} and the novel [4-(N-indolyl)-2,6-dichlorophenyl] bis(2,4,6-trichlorophenyl)methyl radical (2^{{radical dot}}) exhibit intense fluorescence emission in cyclohexane in the red wavelength region. The intensity and the wavelength of the emission band are drastically dependent on the polarity of the solvent. Besides, radical adduct 2^{{radical dot}} has been fully characterized by differential scanning calorimeter, showing a high thermal stability and a clear glass nature to form excellent films.

Full text of DOI:10.1016/j.tetlet.2008.06.040

Tetrahedron Letters 49 (2008) 5196–5199
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Tetrahedron Letters
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Light-emitting persistent radicals for efficient sensor devices of solvent polarity
b
a,
Marc López a, Dolores Velasco b, , Francisco López-Calahorra , Luis Juliá
*
*
a Departament de Química Orgànica Biològica, Institut d’Investgacions Químiques i Ambientals (CSIC), Jordi Girona 18-26, 08034 Barcelona, Spain
b Departament de Química Orgànica, Universitat de Barcelona, Martí i Franquès 1-11, 08028 Barcelona, Spain
a r t i c l e i n f o
a b s t r a c t
Radical adduct 1Å and the novel [4-(N-indolyl)-2,6-dichlorophenyl] bis(2,4,6-trichlorophenyl)methyl
radical (2Å) exhibit intense fluorescence emission in cyclohexane in the red wavelength region. The inten-
sity and the wavelength of the emission band are drastically dependent on the polarity of the solvent.
Besides, radical adduct 2Å has been fully characterized by differential scanning calorimeter, showing a
high thermal stability and a clear glass nature to form excellent films.
Article history:
Received 19 May 2008
Revised 4 June 2008
Accepted 9 June 2008
Available online 12 June 2008
Ó 2008 Elsevier Ltd. All rights reserved.
Indole and carbazole derivatives display very interesting optical
properties,1,2 mainly those derivatives bearing an electron acceptor
substituent in the nitrogen of the heterocycle.3 These donor–accep-
tor compounds emit from charge-transfer excited states generated
from the initially produced singlet excited states by charge transfer
from the donor, indole, or carbazole, to the acceptor. A general fea-
ture of the emission spectra of these species is the dependence of
the solvent polarity. In general, the spectra show a broadening of
the emission line and a shift of the maximum to longer wave-
lengths with increased polarity of the solvent. In some cases, the
solvent affects also the fluorescence quantum yield of the chromo-
phore, decreasing the efficiency of the luminescence with increas-
ing solvent polarity.
The luminescent properties of the carbazole derivatives
prompted us to introduce a paramagnetic substituent in the het-
erocyclic molecule to study the influence of an open-shell moiety
in the optical properties of the system. Therefore, a new adduct,
(4-(N-carbazolyl)-2,6-dichlorophenyl)bis(2,4,6-trichlorophenyl)meth-
yl radical (1Å), by coupling the heterocyclic compound to tris(2,4,6-tri-
chlorophenyl)methyl (TTM) radical, a very stable carbon-centered or-
ganic radical, was synthesized.4 This paramagnetic adduct displays
very attractive luminescent properties with emission efficiencies
approximately as high as those of the parent carbazole and emission
wavelengths dramatically shifted to the red region (for instance,
k = 334/350 nm for carbazole and k = 628 nm for 1Å in cyclohexane).
Recently, we have reported the preparation of a new series of radical
adduct derivatives of 1Å by introducing electron-donor and acceptor
substituents in the carbazole ring.5 An analysis of the luminescent
properties indicated a dramatic influence of solvent polarity on their
emissions. As part of our research in new multifunctional species,
here we report the synthesis of a new radical adduct, (4-(N-indo-
lyl)-2,6-dichlorophenyl)bis(2,4,6-trichlorophenyl)methyl radical (2Å),
together with the absorption spectra and luminescent properties of
1Å and 2Å in different solvents. The electron paramagnetic resonance
and the thermal properties of 2Å, studied by differential scanning
calorimetry (DSC) and thermogravimetry (TGA), are also reported.
TTM radical in dimethylformamide at reflux (2 h) in Argon with
an excess of indole and in the presence of cesium carbonate gave
(4-N-indolyl-2,6-dichlorophenyl)bis(2,4,6-trichlorophenyl)meth-
ane (2),6 which with an aqueous solution of tetrabutylammonium
followed by oxidation with chloranil rendered pure radical adduct
2Å.6 This paramagnetic species is a crystalline red solid, stable in
solid state and in solution.
The TGA and DSC of 2Å are displayed in Figure 1. They exhibit a
high and well-defined melting peak (260 °C) and an exothermic
peak of decomposition at higher temperatures (DSC, onset temper-
ature, 294 °C). The indole moiety of the adduct 2Å lowers consider-
ably the melting temperature (ꢀ35 °C) relative to the carbazolyl
adduct 1Å. Whereas 1Å melts with decomposition at higher temper-
ature (295 °C),3 2Å is stable in the melting state moving away from
the decomposition temperature and allowing to form amorphous
glasses on cooling. Figure 2 shows a cyclic thermal diagram of 2Å
with the temperature up to the melting temperature. A detailed
analysis of the processes shows that on the first heating
(10 °C minÀ1) an exothermic polymorphic peak comes out at about
132 °C followed by a double thermal process, endo- and exother-
mic, at about 230 °C attributed most probably to a melting and
recrystallization, and, finally, the well-defined melting at 260 °C.
On cooling (10 °C minÀ1), no reversible processes come out, form-
ing a stable glass. The second heating (10 °C minÀ1) shows a clear
glass transition at about 95 °C which is not followed by a
recrystallization.
X-band EPR spectra of the radical adduct 2Å recorded in CH2Cl2
solution (ꢀ10À3–10À4 M) at 298 3 K and 180 5 K are shown in
* Corresponding authors. Tel.: +34 93 400 61 07; fax: 34 93 204 59 04 (L.J.).
E-mail address: ljbmoh@cid.csic.es (D. Velasco).
0040-4039/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2008.06.040

M. López et al. / Tetrahedron Letters 49 (2008) 5196–5199
5197
perceivable at room temperature. Thus, at low temperature
(180 5 K), the spectrum showed an overlapped multiplet of very
close 7 lines (DHpp = 0.75 G) corresponding to the weak coupling
4.0
0.0
105.0
100.0
95.0
90.0
85.0
80.0
with six equivalent aromatic hydrogens in meta-positions (cou-
pling constant value, a ꢀ1.25 G), and two weak multiplets on both
sides of the central multiplet attributed to the coupling with the
-4.0
-8.0
-12.0
three bridgehead-13C nuclei adjacent to the
a-carbon atom and
with the six ortho-13C nuclei (estimated coupling constant values
by spectrum simulation,7 a = 12.25 and 10.2 G, respectively) (Fig.
3, left).
The UV–vis absorption spectra of radical adduct 2Å in cyclohex-
ane, chloroform, acetone, and ethanol are shown in Figure 4. The
wavelengths and molar extinction coefficients of the maxima of
radical adducts 1Å and 2Å in these solvents are displayed in Table
1. The UV–vis absorption values of the radical of reference,
tris(2,4,6-trichlorophenyl)methyl (TTM) radical, in diluted CHCl3
0
100
200
300
400
Temperature (ºC)
Figure 1. Thermal analysis of radical adduct 2Å. Dynamic diagram from 25.0 to
400 °C. Black: DSC analysis; red: TG diagram (heating rate 10.0 °C/min).
are as follows: k(e), 373(38,100); 478(860); 542(834) nm. The
structured lowest-lying band of 1Å and 2Å in CHCl3 shows a remark-
able red-shifted effect relative to that of TTM radical, confirming
the large interaction between the carbazolyl and indolyl substitu-
ents and the triphenylmethyl radical moiety in the ground state.
The absorption maxima do not change appreciably with solvent
polarity. In fact, the only estimable difference is that the less
energetic band is more structured in cyclohexane than in the other
solvents. These two bands at k = 373 2 nm and 600 3 nm are
associated with the radical character of these species, correspond-
*
ing the first one to a
molar absorptivities.
p?p transition due to the large associated
In contrast to the absorption spectra, the solvent polarity has a
dramatic effect on the emission spectra of 1Å and 2Å. Values for the
emission spectra in cyclohexane and chloroform are displayed in
Table 2, and emission bands of radical adducts 1Å and 2Å in mixtures
of cyclohexane and chloroform are shown in Figure 5. As shown in
Figure 2. Thermal analysis results of radical adduct 2Å using DSC. Black: first
heating; red: first cooling; blue: second heating. Inset: (a) detail for the first heating
before melting; (b) detail for the second heating (heating and cooling rate: 10 °C/
min).
Figure 3. The spectrum at room temperature consisted of a broad
(peak to peak line width,
DHpp = 3.0 G) and single line with a Landé
factor g = 2.0032 0.0003, and a small equidistant pair of lines on
both sides of the main spectrum appeared at higher values of the
gain (Fig. 3, right). This small pair corresponds to the strong cou-
pling (coupling constant, a(
with the a-
a
-
13C) ꢀ26.4 G) of the free electron
13C nucleus (natural abundance 1.11%). As the line
width is mainly determined by a short spin–lattice relaxation time,
1, a decrease in the sample temperature has an important effect
s
on the EPR spectrum, increasing s1 and reaching sufficiently
narrow lines to make evident very small couplings which are not
Figure 4. Absorption spectra of radical adduct 2Å in cyclohexane (black), CHCl3
(red), and acetone (green) and ethanol (blue); the inset magnifies the lowest-lying
band.
Table 1
Absorption data of 1Å and 2Å in cyclohexane, chloroform, acetone, and ethanol
Cyclohexane
k(
b) nm
Chloroforma
k(
b) nm
Acetone
k(
b) nm
Ethanolc
k(
b) nm
e
e
e
e
1Å
2Å
373(24,400)
554(sh)(2400)
603(4290)
374(25,600)
554(sh)(1990)
597(2940)
371(33,500)
552(sh)(2400)
597(3460)
373(30,000)
542(sh)(2920)
585(4620)
375(28,200)
542(sh)(2340)
584(3230)
373(28,900)
542(sh)(2620)
586(3700)
372(26,000)
542(sh)(2250)
586(3260)
a
b
c
Values for radical adduct 1Å are taken from Ref. 3.
Figure 3. Left: EPR spectra of radical adduct 2Å in CH2Cl2 solution (ꢀ10À3 M) at
180 5 K; modulation amplitude, 0.1. Center: Computer simulation.7 Right: EPR
spectrum of 2Å at rt.
Units: dm3 molÀ1 cmÀ1
Radical adduct 1Å is not soluble in ethanol.
.

5198
M. López et al. / Tetrahedron Letters 49 (2008) 5196–5199
the figure, the emission bands of both radical adducts shift to the
red, broaden, and decrease in intensity with increasing solvent
polarity. The red-shift is more pronounced in 1Å than in 2Å. The fact
that only the emission and not the absorption is affected by change
of solvent polarity indicates the existence of a strong interaction of
the solvent with the excited state and not with the ground state of
the molecule and, consequently, the different nature of both elec-
tronic states. In cyclohexane solution, both radical adducts exhibit
also a weak extra emission band overlapped with the main band at
the longer wavelength side of the spectra (Fig. 6). This extra weak
band may be attributed to emission from a twisted intramolecular
charge-transfer (TICT) excited state, similarly as it has been re-
ported in the literature for many other donor–acceptor lumine-
scents.3 The radical adducts 1Å and 2Å are compounds with the
carbazolyl and indolyl moieties as electron donors linked by a sin-
gle bond with a good electron-acceptor moiety (the triphenyl-
methyl radical). In summary, excitation of 1Å and 2Å takes place
from a ground doublet state to a low-lying neutral excited state,
also doublet in character, and the emission from a charge-transfer
excited state. The excited state is additionally stabilized by the
polarity of the solvent to a lowest-energy twisted conformation.
This is why in the emission spectra of 1Å and 2Å in CHCl3 appear only
as the less energetic band. The charge-transfer character of the
excited state is corroborated by the large Stokes values found in
chloroform (Table 2).
3
2
1
0
500
600
700
800
Wavelength / nm
Figure 6. Luminescence spectra of radical 1Å (black) and radical 2Å (red) in
cyclohexane.
tron donor–acceptor molecules to an excited state in a polar
medium, followed by a rapid relaxation to a charge-transfer state
far from the Franck–Condon configuration, induces a large reorien-
tation of the surrounding solvent molecules. In these conditions,
the nonradiative dissipation of the electronic energy can easily
compete with the radiation decay. When the quenching of fluores-
cence is due to ethanol, the radiationless deactivations might be
induced by intermolecular hydrogen bond between the O–H of
ethanol and the nitrogen of the carbazole moiety. These effects
are much more moderate with chloroform as the quenching agent.
A new stable radical adduct 2Å of the TTM series has been pre-
pared from the reaction of TTM radical and indole. The optical
response of 2Å as well as that of the radical adduct of TTM radical
with carbazole, 1Å, in solution is a clear example of the effect of sol-
vent on the quenching of fluorescence. The emission wavelength
and the quantum yield are drastically dependent on the nature of
the molecular environment around the fluorophore. These drastic
effects suggest that both paramagnetic species might be used as
solvent polarity sensors. On the other hand, the high thermal
stability, excellent film forming property, and good solubility in
halogenated organic solvents of 2Å are potentially useful for appli-
cations as component of light-emitting diodes. The electrochemical
behavior, the electron and hole conductivities, and the susceptibil-
ity measurements of 2Å are being studied in our laboratories.
Both radical adducts 1Å and 2Å do not exhibit any emission in
acetone, and 2Å neither in ethanol (1Å is not soluble in ethanol).
The emission bands of 1Å and 2Å in cyclohexane are red-shifted
and drastically decreased in the presence of very low concentra-
tions of acetone or ethanol. In Figure 7, the quenching values of
the emission of 1Å and 2Å in cyclohexane as a function of the con-
centration of acetone and ethanol are displayed. Excitation of elec-
Table 2
UV–vis and luminescence spectral dataa for 1Å and 2Å in cyclohexane and chloroform
a,b
a
a
c
kabs
kexc
kem
UF
Stokes shiftd
1Å
2Å
Cyclohexane
Chloroform
603
597
515
515
628
687
0.53
0.02
660
2257
Cyclohexane
Chloroform
585
584
450
450
610
646
0.52
0.003
700
1643
a
b
c
Values of k in nm.
The less energetic band in the absorption spectra.
Quantum yield of luminescence.
d
Values in cmÀ1
.
0% chloroform
10% chloroform
20% chloroform
30% chloroform
40% chloroform
50% chloroform
60% chloroform
70% chloroform
80% chloroform
90% chloroform
0% chloroform
5% chloroform
10% chloroform
15% chloroform
20% chloroform
25% chloroform
30% chloroform
2
1
0
550
600
650
700
750
800
550
600
650
700
750
800
Wavelength / nm
Wavelength / nm
Figure 5. Emission spectra of radical 1Å (left) and radical 2Å (right) in different solvent polarities (cyclohexane/chloroform, v/v) (excitation light, 450 nm).

M. López et al. / Tetrahedron Letters 49 (2008) 5196–5199
5199
100
80
60
40
20
0
100
80
60
40
20
0
0
10
20
30
0
10
20
Fraction of ethanol (V %)
Fraction of acetone (V %)
Figure 7. (Left): Quenching values of emission of 1Å (black) and 2Å (red) versus acetone fraction in cyclohexane (acetone/cyclohexane, v/v). (Right): Quenching values of
emission of 1Å (black) and 2Å (red) versus ethanol fraction in cyclohexane (ethanol/cyclohexane, v/v).
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1985, 33, 435–446; Martinez, E.; Escudero, J. L.; Campillo, A. L. J. Mol. Struct.
Acknowledgments
Financial support for this work from the MEC (Spain) through
project CTQ2006-15611-C02-02/BQU is gratefully acknowledged.
We also thank the EPR service of the Centre d’Investigació I Desen-
volupament (CSIC) for recording the spectra.
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1. References on carbazole, see: Favini, G.; Gamba, A.; Grasso, D.; Millefio, S. Trans.
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6. Selected data for 2 and 2Å. Compound 2: IR (KBr) 3070(w), 1598(s), 1579 (m),
1545 (s), 1476 (s), 1468 (s), 1449 (w), 1372 (m), 1332(m), 1296 (w), 1259 (w),
1236 (w), 1211 (m), 1193 (w), 1175 (m), 1136 (m), 1023 (m), 899 (m), 859 (s),
837 (w), 804 (s), 761 (m), 742 (s), 714 (w), 677 (w) cmÀ1 1H NMR (300 MHz,
;
CDCl3) d 7.64 (d, 1H, J = 7.8 Hz), 7.57 (d, 1H, J = 7.8 Hz), 7.51 (d, 1H, J = 2.4 Hz),
7.37 (d, 1H, J = 2.4 Hz), 7.36 (d, 1H, J = 3.3 Hz), 7.35 (d, 1H, J = 3.3 Hz), 7.29 (d, 1H,
J = 3.3 Hz), 7.24–7.14 (m, 4H), 6.75 (s, 1H), 6.67 (dd, 1H, J = 3.3 Hz, J’ = 0.9 Hz).
MS (MALDI-TOF): m/z = 633.3 (M+À1). Anal. Calcd for C27H13Cl8N: CHCl3 (8:3) C,
48.4; H, 2.0; N, 2.1. Found: C, 48.7; H, 2.1. Compound 2Å: IR (KBr) 3070 (w), 1577
(s), 1554 (m), 1525 (s), 1474 (m), 1464 (s), 1370 (m), 1332 (m), 1297 (w), 1235
(w), 1210 (m), 1183 (m), 1134 (m), 1082 (w), 924 (w), 858 (m), 812 (m), 799
(m), 764 (w), 740 (m), 716 (w) cmÀ1; MS (MALDI-TOF): m/z = 634.4 (M++1).
Anal. Calcd for C27H12Cl8N: CHCl3 (8:1) C, 50.2; H, 1.9; N, 2.2. Found: C, 50.3; H,
1.9; N, 2.0.
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906; Delauder, W. B.; Wahl, P. Biochim. Biophys. Acta 1971, 243, 153; Tatischeff,
I.; Klein, R. Photochem. Photobiol. 1975, 22, 221–229; Ricci, R. W.; Nesta, J. M. J.
Phys. Chem. 1976, 80, 974–980; Pandya, M. L.; Machwe, M. K. J. Chem. Phys. 1978,
68, 341–342; Klein, R.; Tatischeff, I.; Bazin, M.; Santus, R. J. Phys. Chem. 1981, 85,
670–677; Aaron, J. J.; Tine, A.; Villiers, C.; Parkanyi, C.; Bouin, D. Croat. Chem. Acta
7. WINSIM program provided by D. Dulog, Public EPR Software Tools, National
Institute of Environmental Health Sciences, Bethesda, MD, 1996.