Synthesis, properties, and electrogenerated chemiluminescence (ECL) of a novel carbazole-based chromophore

Article abstract of DOI:10.1016/j.tetlet.2004.11.166

A novel molecule containing electron-rich carbazole and electron-deficient pyrimidine moieties exhibits useful and intriguing physical properties, including promising reversible redox behavior that gives rise to electrogenerated chemiluminescence (ECL).

Full text of DOI:10.1016/j.tetlet.2004.11.166

Tetrahedron
Letters
Tetrahedron Letters 46 (2005) 855–858
Synthesis, properties, and electrogenerated chemiluminescence
(
ECL) of a novel carbazole-based chromophore
Ken-Tsung Wong, Tsung-Hsi Hung, Teng-Chih Chao and Tong-Ing Ho*
*
Department of Chemistry, National Taiwan University, Taipei 106, Taiwan
Received 3 August 2004; revised 22 November 2004; accepted 25 November 2004
Available online 18 December 2004
Abstract—A novel molecule containing electron-rich carbazole and electron-deficient pyrimidine moieties exhibits useful and
intriguing physical properties, including promising reversible redox behavior that gives rise to electrogenerated chemiluminescence
(
ECL).
Ó 2004 Elsevier Ltd. All rights reserved.
Electrogenerated chemiluminescence (ECL) has widely
been applied as a sensitive and selective detecting tech-
purposes, molecular systems that give rise to significant
ECL are single molecules that contain one or more chro-
mophores that exhibit reversible redox behavior. If only
one of the oxidation or reduction processes is reversible,
1
nique for many analytical applications; it is also an
excellent model for investigating the mechanism of elec-
2
5
tron transfer. ECL is a process that involves photon
ECL has to be generated by a co-reactant process. In
generation by homogeneous electron transfer (ET)
between electrochemically generated radical cations and
radical anions that formin close proximity to the elec-
this regard, organic molecules having structural charac-
ter capable of generating ECL have been explored rela-
tively infrequently when compared to inorganic
RuðbpyÞ3 and related complexes.
3
2þ
6
trode surface in an ECL cell. Upon sweeping sequen-
tially between negative and positive potentials over a
short-time interval, the excited states of the electroactive
species can be made to populate emitting states that sub-
sequently relax to ground states with the emission of
light of appropriate wavelengths. Both singlet and trip-
let states of the electroactive species can be formed
Molecules incorporating carbazole moieties have been
investigated extensively as light-emitting materials and
as the hole-transporting materials in organic light-emit-
ting devices (OLED) because of their high reversibility
7
upon electrochemical oxidation. Previously, we estab-
lished a feasible synthesis, using Suzuki coupling reac-
depending on the annihilation enthalpy change (DHann
of the electron transfer reaction. The ion annihilation
)
8
tion, of molecules containing pyrimidine moieties.
4
process can generate a singlet state (S-route) if the mag-
nitude of DHann is larger than the energy needed to
These pyrimidine-containing materials undergo revers-
ible reduction processes, which suggests that they hold
promise for use in light-emitting and electron transport
materials. We anticipated that the preparation of a mol-
ecule that combines the individual characteristics of car-
bazole (i.e., electron richness) and pyrimidine (i.e., high
electronegativity) moieties would afford a new kind of
material. In this letter we report the synthesis and
properties, including ECL behavior, of such a novel
molecule.
reach the excited singlet state (E ). On the other hand,
s
if the value of DHann is lower than that of E , but it is
s
sufficient to generate the triplet state, then the electron
transfer reaction will lead to the formation of that triplet
state. It is still possible, however, to forma singlet state
by triplet–triplet annihilation, which is a process known
as the T-route. As a consequence, fluorescence is the
normal emission from the electrochemically generated
excited state of an electroactive species. For practical
Scheme 1 depicts the synthetic pathways we took toward
the target molecule 4. The 5,8-dibromo-1-phenylcarbaz-
ole was converted to diboronic ester 2 in an isolated
yield of 72% by treating it with n-BuLi at À78 °C and
then quenching the dilithiated intermediate with 2-iso-
propoxy-4,4,5,5-tetramethyl-[1,3,2]-dioxaborolane.
Keywords: Electrogenerated chemiluminescence (ECL); Carbazole;
Pyrimidine.
*
Corresponding authors. Tel.: +886 2 2363 0231; fax: +886 2 2363
359 (K-T.W.); e-mail: kenwong@ntu.edu.tw
6
0
040-4039/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2004.11.166

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K.-T. Wong et al. / Tetrahedron Letters 46 (2005) 855–858
1
1
.2
.0
CHCl3
O
O
Br
Br
THF
O B
B O
400 422
CH CN
3
a
N
0.8
N
0
0
.6
.4
1
2
Ar
Ar
N
N
N
N
N
N
b
0.2
.0
Br
+
2
0
3
300
400
500
600
700
N
Wavelength (nm)
Figure 1. The photoluminescence spectra of 4 in different solvents.
4
Ar =
maximum centered at 417 nm—and the quantum yield
of its vacuum-deposited thin film, studied by the cali-
brated integrating sphere system, was determined to be
Scheme 1. Reagents and conditions: a, (i), n-BuLi, À78 °C, THF; (ii),
-isopropoxy-4,4,5,5-tetramethyl-[1,3,2]-dioxaborolane, À78 °C to rt;
2
t
3 4 3 2 3 3
b, Pd(PPh ) , P Bu , Na CO , PhCH , reflux.
0
.51.
The 2-(4-tert-butylphenyl)-5-bromopyrimidine 3 was
synthesized according to our Suzuki coupling ap-
Next, we conducted cyclic voltammetry (CV) experi-
ments on compound 4 at roomte mp erature to probe
its electrochemical properties. CV traces of 4 (Fig. 2)
8
proach. The reaction of diboronic ester 2 and bromo
compound 3 in the presence of a catalytic amount of
Pd(PPh ) furnished the desired compound 4 in 67%
were recorded in both THF, using 0.1 M nBu
as the supporting electrolyte and a glassy carbon elec-
trode as the working electrode, and CH Cl , using
.1 M nBu NPF . It appears that the structural charac-
NClO
4 4
3
4
9
yield.
2
2
0
4
6
The new compound 4 exhibits high morphological sta-
bility and good resistance to thermal decomposition.
During DSC analysis, molecule 4 displays a distinct
ter of 4 has a beneficial effect on the reversibility of the
electrochemical processes. In THF, a quasi-reversible
reduction potential is evident at a value of Ep,c of
À2.29 V (vs Ag/AgCl). It is reasonable to attribute this
reduction to the injection of electrons fromthe electrode
to the pyrimidine moieties of 4. The anodic oxidation of
4 in THF displays less-reversible electrochemical behav-
ior with a value of Ep,a of 1.60 V (vs Ag/AgCl). This re-
sult indicates possibly that the radical cation of 4 has
relatively low stability in THF. To support this hypoth-
esis, we conducted the oxidation CV experiment of 4 in
glass transition (T ) at 164 °C that is followed by crystal-
g
lization (T ) at 218 °C and melting (T ) at 337 °C. We
c
m
attribute the crystallinity of 4 to the rigidity of its
peripheral aryl substituents, even though we purposely
introduced the sterically bulky, terminal tert-butyl
groups to preclude significant intermolecular interac-
tions. The rigidity of the peripheral aryl groups is also
beneficial for this moleculeÕs high thermal stability dur-
ing TGA analysis: the 10%-weight-loss temperature of 4,
while heating (20 °C/min) under nitrogen, is 429 °C.
CH Cl using 0.1 M nBu NPF as the supporting
2 2 4 6
We studied the electronic absorption and photolumines-
cence of compound 4 in different solvents (CHCl , THF,
and CH CN) to verify the polarized character of the
3
4
3
2
1
0
-
2.29
3
chromophore. The absorption spectrum of 4 in CHCl3
is similar to that obtained in CH CN: an absorption
3
1
.28
maximum centered at 308 nm and a shoulder at
3
43 nm. The absorption of 4 in THF exhibits a similar
1.44
shoulder, but is associated with a slightly blue-shifted
absorption maximum (299 nm). In contrast to the elec-
tronic absorption, the emission maxima of 4 are more
-
1.96
-
1
-2
-3
sensitive to the polarities of the solvents. In CH CN, a
3
1.40
significant red shift (22 nm) is observed relative to the
emission maxima of 4 in CHCl and THF (Fig. 1). We
1.60
3
2
1
0
-1
-2
-3
ascribe the bathochromic shift of the emission maximum
in CH CN to the polar character of 4 in this solvent that
Potential ( V vs Ag/AgCl)
3
arises as a result of intramolecular charge transfer
occurring in its excited state. Additionally, compound
Figure 2. Cyclic voltammograms of 4 recorded in THF (solid line),
using 0.1 M nBu NClO as the supporting electrolyte, and in CH Cl
4
4
2
2
4
is highly emissive in the solid state––it has an emission
4 6
(dot line), using 0.1 M nBu NPF .

K.-T. Wong et al. / Tetrahedron Letters 46 (2005) 855–858
857
logical stability of 4, together with the high quantum
efficiency of its photoluminescence in solid films, makes
this new material suitable for further applications in
OLEDs. Accordingly, we are currently preparing a
deep-blue OLED device using 4 as an emission layer;
the results will be reported in due course.
Acknowledgements
We thank the National Science Council and the Minis-
try of Education of Taiwan for financial support.
Supplementary data
Figure 3. Photoluminescence ( ) and 10-point-smoothed (m) electro-
generated chemiluminescence (ECL) spectra. The inset presents the
original ECL spectrum.
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.tetlet.
2004.11.166.
electrolyte. In this case, we detected a reversible oxida-
tion process [E1/2 = 1.34 V (vs Ag/AgCl)]. We believe
that the reversible oxidation originates fromthe hole
injection at the central carbazole unit. The electrochemi-
cal data reveal that the electrogenerated radical ions are
stable and that efficient ECL can occur as a result of ion
annihilation.
References and notes
1. (a) Richter, M. M. In Optical Biosensors: Present and
Future; Ligler, F. S., Rowe Taitt, C. A., Eds.; Elsevier:
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J. J. Am. Chem. Soc. 2001, 123, 9112–9118.
. Faulkner, L. R.; Bard, A. J. Electrogenerated chemilumi-
nescence. In Electrochemical Methods; John Wiley & Sons:
New York, 1980; pp 621–627.
(
1
The ECL of 4 in THF was measured according to our
2
1
0
previously published method. Figure 3 presents the
ECL spectrumof 4 recorded between À2.0 V (0.5 s)
and +0.8 V (0.5 s) at 2 s intervals.
3
The photoluminescence spectrum of 4 in THF closely
resembles its ECL spectrum, but the maximum intensity
in the ECL spectrumis slightly red-shifted (ca. 14 nm)
relative to the fluorescence maximum. We believe that
this shift arises because of the higher polarity of the
THF solution containing the nBu NClO electrolyte.
4. Faulkner, L. R.; Bard, A. J. In Electroanalytical Chemis-
try; Bard, A. J., Ed.; Marcel Dekker: New York, 1977;
Vol. 10, pp 1–95.
5
. (a) Noffsinger, J. B.; Danielson, N. D. Anal. Chem. 1987,
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5
4
4
6
. (a) Abruna, H. D.; Bard, A. J. J. Am. Chem. Soc. 1982,
1
The absence of a long-wavelength emission in the ECL
spectrumexcludes the formation of an excimer (E-route)
during the ion annihilation process. The energy avail-
able for this ion annihilation reaction is given by the fol-
04, 2641–2642; (b) Elliott, C. M.; Pichot, F.; Bloom, C.
J.; Rider, L. S. J. Am. Chem. Soc. 1998, 120, 6781–6784;
(c) Anderson, A.-M.; Isovitsch, R.; Miranda, D.;
Wadhwa, S.; Schmehl, R. H. Chem. Commun. 2000,
505–507.
1
1
lowing equation:
7
8
. (a) Kundu, P.; Justin Thomas, K. R.; Lin, J. T.; Tao,
Y.-T.; Chuen, C.-H. Adv. Func. Mater. 2003, 13, 445–452;
:
þ
:À
jDHannj ¼ E ð4=4 Þ À E ð4=4 Þ À 0:16
p
p
(
b) Justin Thomas, K. R.; Lin, J. T.; Tao, Y.-T.; Ko,
C.-W. J. Am. Chem. Soc. 2001, 123, 9404–9411.
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G.-H.; Peng, S.-M.; Chou, C. H.; Su, Y. O. Org. Lett.
2002, 4, 513–516; (b) Wu, C.-C.; Lin, Y.-T.; Chiang,
H.-H.; Cho, T.-Y.; Chen, C.-W.; Wong, K.-T.; Liao,
Y.-L.; Lee, G.-H.; Peng, S.-M. Appl. Phys. Lett. 2002, 81,
577–579.
Fromthe CV data, we calculate the value of jDH j to
ann
be 3.73 eV; this value is larger than the lowest excited
singlet energy, E = 3.46 eV, calculated fromthe fluores-
s
cence emission. This result suggests that it is highly
probable for the ion annihilation process to occur via
the S-route and that the ECL of 4 is an energy-sufficient
system.
9
. To a solution of 2-(4-tert-butylphenyl)-5-bromopyrimi-
)
2
dine (3; 611 mg, 2.1 mmol), Pd(PPh
3
4
(231 mg,
0
.2 mmol), and 5,8-bis(4,4,5,5-tetramethyl-[1,3,2]-dioxa-
borolan-2-yl)-1-phenylcarbazole (2; 495 mg, 1 mmol) in
a mixture of toluene (30 mL) and 2 M Na CO (3 mL)
In conclusion, we have established an efficient synthesis
of a novel carbazole-based molecule (4), in which elec-
tronegative pyrimidine rings have been introduced as
electron-accepting moieties. The structural character of
2
3(aq)
t
3
was added P Bu (0.05 M in toluene, 2.0 mL, 0.10 mmol).
The solution was refluxed with vigorous stirring for 72 h
under a nitrogen atmosphere. The mixture was poured
into water and extracted with chloroform. The organic
4
is not only beneficial to the stability of its electrogen-
erated radical ions, but it also increases the possibility
for generating the ECL. The high thermal and morpho-
4
extracts were washed with brine and dried over MgSO .

858
K.-T. Wong et al. / Tetrahedron Letters 46 (2005) 855–858
The solvent was removed by rotary evaporation, and the
10. Chen, F.-C.; Ho, J.-H.; Chen, C.-Y.; Su, Y. O.; Ho, T.-I.
J. Electroanal. Chem. 2001, 499, 17–23.
11. Faulkner, L. R.; Tachikawa, H.; Bard, A. J. J. Am. Chem.
Soc. 1972, 94, 691–699.
residue was purified by column chromatography (silica
gel, ethyl acetate/hexane = 1/6) to provide 444 mg (67%) of
the product.