Electrochemical and spectral characterizations of 9-phenylcarbazoles

Article abstract of DOI:10.1002/jccs.201100601

A series of 9-phenylcarbazoles have been synthesized and characterized for their electrochemical as well as spectral properties. For 3,6-substituted carbazoles, the oxidation is reversible and the potential is affected by the substituents. For 3,6-unprotected carbazoles, on the other hand, the oxidized forms can undergo dimerization. Their corresponding dimers have been independently synthesized by chemical methods and have exhibited identical spectral properties. The para position of the 9-phenyl group is relatively insensitive for redox and chemical reactions. The amino derivatives are unstable in carbazole cation radical form compared with their triphenylamine counterparts.

Full text of DOI:10.1002/jccs.201100601

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Article
Electrochemical and Spectral Characterizations of 9-Phenylcarbazoles
Ssu-Kai Chiu,a Yi-Chun Chung,a Guey-Sheng Lioua,b,* and Yuhlong Oliver Sua,c,*
aDepartment of Applied Chemistry, National Chi Nan University, 1, University Road, Puli, Nantou 545, Taiwan, R.O.C.
bInstitute of Polymer Science and Engineering, National Taiwan University, 1 Roosevelt Road, 4th Sec.,
Taipei 106, R.O.C.
cDepartment of Chemistry, National Chung Hsing University, 250, Kuo Kuang Rd., Taichung 402, Taiwan, R.O.C.
(Received: Oct. 13, 2011; Accepted: Jan. 30, 2012; Published Online: Feb. 20, 2012; DOI: 10.1002/jccs.201100601)
A series of 9-phenylcarbazoles have been synthesized and characterized for their electrochemical as well
as spectral properties. For 3,6-substituted carbazoles, the oxidation is reversible and the potential is af-
fected by the substituents. For 3,6-unprotected carbazoles, on the other hand, the oxidized forms can un-
dergo dimerization. Their corresponding dimers have been independently synthesized by chemical meth-
ods and have exhibited identical spectral properties. The para position of the 9-phenyl group is relatively
insensitive for redox and chemical reactions. The amino derivatives are unstable in carbazole cation radi-
cal form compared with their triphenylamine counterparts.
Keywords: 9-Phenylcarbazoles; Cyclic voltammetry; Spectroelectrochemistry; Fluorescence.
INTRODUCTION
Recently, carbazole derivatives and polymers have
the phenyl rings. It was then found that triphenylamine can
undergo carbazole formation when the potential was set at
the second oxidation wave.18
been intensively studied for charge carrier transporting ma-
terials and their applications in light-emitting diodes
(LED).1 Poly(9-vinylcarbazole)s (PVK) exhibits a low-ly-
ing highest occupied molecular orbital (HOMO) energy
level of -5.9 eV2-3 and is widely used as polymeric host for
LEDs. Carbazole-based building blocks with extended
conjugated systems have been used for optoelectronic ap-
plications and electrochromics.4-7 Calculation were per-
formed using density function theory (DFT) in order un-
derstand the ground and excited states carbazole monomer
and polymers.8-9 Thus, the synthesis and characterization
of carbazoles are important for material fabrications.
Carbozole (Cz)10 and triphenylamine (TPA)11 possess
electroactive nitrogen atom as the redox center. The oxi-
dized nitrogen-containing compounds have very strong in-
teraction between the nitrogen atom and the phenyl rings
and exhibit characteristic electrochemical and spectral
properties. Thus, their derivatives have been used as photo-
and electroactive materials.12-13
The electrochemical oxidation of carbazole (Fig. 1)
and N-substituted derivatives was first reported by Nelson
in 1968.19 Nelson et al.20 further studied comprehensively
76 carbazole derivatives about their substituent effects.
TPA has three equivalent phenyl groups but 9-phenylcar-
bazole has two phenyl moieties fused with the nitrogen
atom. The oxidation potential of each carbazole is more
positive than its corresponding triphenylamine (TPA) by
about +0.3 V. The 3,6-positions are more reactive than para
position in the TPA by three orders upon oxidation. The
3,6-unprotected carbazole cation radicals rapidly undergo
dimerization reactions.
In this work, we have synthesized a series of 9-phen-
ylcarbazoles (Scheme I). The 3,6-positions of carbazole or
para position of the 9-phenyl group have been substituted
with various groups. This work has been concerned with
9-phenylcarbazoles and has the following purposes: (i) to
establish the spectral pattern of carbazole cation radicals,
In 1966, Adams14 found that triphenylamine could
form tetraphenylbenzidine (TPB) by way of oxidation and
dimerization. A series of work on the oxidation of aniline15
and diphenylamine16-17 was furtherly pursued later on.
Triphenylamines with different substituents had been
exhaustively studied for their effect at the para-position of
Fig. 1. Structure of carbazole.
Dedicated to the memory of Professor Yung-Son Hon (1955–2011).
* Corresponding author. Tel: +886-49-2910960 ext.4150; Fax: +886-49-2917956; E-mail: yosu@ncnu.edu.tw and gsliou@ntu.edu.tw
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Chiu et al.
Scheme I
marized as Scheme I.
(ii) to study the substituent effect on the redox properties of
carbazoles, (iii) to investigate the redox and spectral prop-
erties of carbazole dimers, (iv) to probe the reactivity of
para-position of the 9-phenyl group, and (v) to compare
the properties between carbazoles and their corresponding
triphenylamines.
Electrochemistry
Electrochemistry methods were performed according
to the procedures in reference.21
RESULTS AND DISCUSSION
9-Phenylcarbazole with alkyl substituents at 3,6 posi-
tions
EXPERIMENTAL
Fig. 2 shows the cyclic voltammetry of 14 which ex-
hibits a reversible redox wave at E1/2 = +1.26 V. The CV
reaches a steady state without new wave formation after
continuous cycling. Compound 14 has absorption peaks at
243 and 296 nm in UV region. Those peaks decrease in
absorbance upon oxidation while a new broad band in the
region of 550 to 900 nm appears. The resulting spectrum
has peak wavelength at 280 and 800 nm and the product is
assigned as the cation radical 14+×. After reset at 0.00 V, the
original spectrum was obtained.
All the chemicals were purchased from ACROS. Sol-
vents were purified according to standard procedures.21
TBAP (tetra-n-butylammonium perchlorate) was recrys-
tallized from ethyl acetate twice before use.
Synthesis
Compounds 1,22 2, 23 3,24 4,25 5,26-27 6,22 7,25 8,26-27 9,22
10,23 11,24 12,26-27 13,26-27 14,22,24 1528 were synthesized ac-
cording to literature methods. Those 9-phenylcarbazoles
have substituents at 3,6-positions or at the para position of
the 9-phenyl group. The reaction procedures can be sum-
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9-Phenylcarbazoles, Spectroelectrochemistry
on the oxidation reaction.
For compound 2, the oxidation potential is at E1/2
Its analogue compound 11 is oxidized at E1/2 = +1.28
V and the pattern of absorption spectral change for cation
radical is very similar to that of compound 14. Compound
10, which has bromo groups at 3 and 6 positions, has an E1/2
at +1.49 V. The electron-withdrawing of bromo groups ap-
parently have substantial shift in oxidation potential.
In order to probe the chemical reactivity of the para-
position of the N-phenyl group in 9-phenylcarbazole, sev-
eral 3,6-disubstituted 9-phenylcarbazoles were synthe-
sized and characterized. Compound 3 exhibits a reversible
redox couple at +1.23 V and continuous cycling does not
generate any new wave in 0.0 to +1.50 V region (Fig. 3).
The spectroelectrochemistry of compound 3 also exhibits a
broad band in 550 to 900 nm with peak wavelength at 275
and 800 nm, a pattern very similar to that of compound 14
=
+1.53 V, substantially more positive than that for com-
pound 3. The shift in potential is conceivably due to the
electron-withdrawing effect of the bromine atoms. The ab-
sorption spectrum of 2+× exhibits the same pattern as these
of 11+× and 14+×. The absorption spectrum of cation radical
2+× has a longer peak wavelength (814 nm) than that of
compound 3+× (800 nm). Compound 4, which has nitro
groups at 3 and 6 positions, cannot be oxidized in the po-
tential window, conceivably due to strong electron-with-
drawing effect of the substituents. However, its reduction
waves could be observed in the potential region < -1.0 V.
The redox potential of carbazoles are thus listed in
Table 1. Based on above results in electrochemical and
spectral observations, the cation radical of 3,6-disubsti-
Fig. 2. Spectral changes of compound 14 during the
first electron oxidation at +1.20 V in CH3CN
containing 0.2 M TBAP. Time interval = 1.0
min.
Fig. 4. (a) Spectral changes of compound 6 during the
first oxidation in CH3CN containing 0.2 M
TBAP. Time interval = 0.5 min. (b) Spectral
changes of compound 6 during the reduction re-
covery procedure at applied potential back to
0.00 V. Time interval = 1.0 min.
Fig. 3. Spectral changes of compound 3 during the first
electron oxidation at +1.15 V in CH3CN con-
taining 0.2 M TBAP. Time interval = 0.5 min.
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pound 6+×. Fig. 5 shows the CV of compound 16 and two re-
versible redox couple in the oxidation scan are observed at
E1/2 = +1.03 and +1.26 V, respectively. Cycling scan in the
potential range 0.0 to +1.5 V does not generate any observ-
able new wave.
Table 1. Half-wave potentials (E1/2, V vs. Ag/AgCl) of carbazole
derivatives in CH3CN containing 0.1 M TBAP. Scan
rate = 0.1 V/s
Oxidation
2nd
Reduction
2nd 3rd
Compounds
3rd
1st
1st
Compound 16 in its neutral from has two sharp ab-
sorption peaks at 248 nm and 301 nm, respectively. Upon
first oxidation a new band at about 411 nm and another
broad band (600-2150 nm) appear. The latter is assigned as
the intervalence charge transfer (IVCT) band of the cation
radical 16+×.30 A big shoulder at about 1300 nm (not shown)
is comparable with the cation radical of tetraphenylbenzi-
dine (TPB).30 However, above absorption bands decrease
in absorbance significantly while a new broad band at 779
nm appears during the second oxidation. For TPB2+, the
dominant absorption is at 780 nm but with much smaller
full-width at half-maximum (FWHM). The resulting spec-
1
1.38 a
1.53
1.23
2
3
4
-1.10 -1.29 a -1.8 a
5
1.65 a 0.79 a 0.50 a
1.33 a
6
7
1.80 a -1.00 a -1.23 a
8
1.88 a 0.83 a 0.51 a
9
1.38 a
1.49
1.26
10
11
12
13
14
15
16
1.48 a -0.96 -1.63 a
1.31 a 0.93 a
1.23
1.26
1.29
1.03
1.10
Fc+/0 = +0.48 V vs. Ag/AgCl (sat’d)
a Irreverible peak potential.
tuented 9-phenylcarbazoles do not undergo dimerization as
their corresponding TPA’s do.14 The para-positions of the
N-phenyl group are relatively insensitive to coupling dur-
ing carbazole cation radical formation.
9-Phenylcarbazole with an alkyl substituent at 9-
phenyl group
The CV of compound 6 is shown in Fig. 4a. In the oxi-
dative scan, a sharp peak at Ep,a = +1.33 V is observed. In
the reversed scan, two reduction waves at Ep,c= +1.01 and
+1.20 V are observed, respectively. In the continuous
scans, an oxidation wave at Ep,a = +1.05 V gradually grows,
indicative of a new species formation.
In order to characterize the properties of the oxidation
product, spectroelectrochemistry was performed for spec-
tral change. In Fig. 4a, the characteristic peak for com-
pound 6 at 238 nm decreases rapidly which a new band at
about 422 nm and a broad band ranging from 500 to 2000
nm (l = 777 nm) grow gradually at Eappl. = +1.16 V. After
equilibrium is reached, the potential is stepped to 0.00 V to
reduce the oxidation product. Two UV peaks at 248 and
301 nm corresponding to the dimer (vide infra) are ob-
served while the broad absorption in the visible and
near-IR (NIR) region disappears (Fig. 4b).
Fig. 5. (a) Spectral changes of compound 15 during the
first oxidation in CH3CN containing 0.2 M
TBAP. Time interval = 1.0 min. (b) Spectral
In a parallel experiment, compound 16 was studied
because it is the expected dimerization product of com-
changes of compound 15 during the second oxi
-
dation. Time interval = 1.0 min.
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9-Phenylcarbazoles, Spectroelectrochemistry
Table 2. Absorption wavelengths of the carbazole derivatives in
CH3CN
wave at Ep,c = +0.32 V (Fig. 6a) were observed. Its counter-
part, bis(4-aminophenyl)phenylamine, exhibits two revers-
ible redox couples at +0.38 and +0.74 V, respectively (Fig.
6b).
Compounds
UV (nm)
239
Vis (nm)
NIR (nm)
1
292
268
330
265
334
262
260
270
340
298
326
296
332
301
2
2+•
242
290
242
280
238
239
244
270
245
287
243
280
248
9-Phenylcarbazole with an amino substituent at 9-
phenyl group
729
814
800
3
3+•
6
489
726
9-(4-Aminophenyl)carbazole (13) exhibits two irre-
versible oxidation waves at Ep,a = +0.93 and +1.31 V, re-
spectively (Fig. 7a). In the cyclic scan between 0.0 to +1.0
V, a new reduction wave at Ep,c = +0.63 V was observed.
However, cyclic scan between 0.0 to +1.38 V renders film
growth on the electrode. N,N-diphenyl-p-phenylenedi-
amine, however, has two reversible redox couples at +0.54
and +1.03 V, respectively (Fig. 7b).31
9
10
10+•
720
736
732
807
804
800
972
11
11+•
14
14+•
15
9-Phenylcarbazole oxidation at a potential more posi-
tive by about +0.3 V than its TPA counterparts probably re-
flects less delocalization of the cation radical of nitrogen
onto the 13-member carbazole skeleton. The 9-phenyl
group is nearly perpendicular to the carbazole plane and the
resonance effect is minimal.20 In aminocarbazoles, al-
though the amino groups are at the para positions of the
15+•
152+
16
411
422
779
729
249
296
16+•
162+
412
426
901
trum of 162+ (Fig. 5b) is nearly identical with that in Fig. 4a.
For the prototype compound 1 (9-phenylcarbazole), a
sharp and irreversiable wave about Ep,a = +1.38 V is ob-
served in the oxidative scan. In the reversed scan, two oxi-
dation waves appear at Ep,c = +1.28 and +1.05 V, respec-
tively. In the spectroelectrochemistry for 1 oxidation the
characteristic sharp peak at 239 and 292 nm of 1 decreases
upon oxidation, and new bands at 420, 728 nm together
with a broad band ranging from 500 to 2000 nm arises.
Compound 15, dimerization product of compound 1,
exhibits two reversible oxidation at E1/2 = +1.10 and +1.29
V, respectively. In the spectroelectrochemistry of com-
pound 15, stepwise oxidations produce spectral changes of
the same pattern as the case of compound 16.
The above experiments have established the electro-
chemical dimerization of 9-phenylcarbazoles without sub-
stituents at 3,6-positions based on electrochemical and
spectral results (Table 2).
9-Phenylcarbazole with amino substituents at 3,6 po-
sitions
For amino-substituted 9-phenylcarbazole compounds
5, 8 and 13, the electrochemical oxidations had been car-
ried out and compared with the corresponding amino tri-
phenylamines. When the 3,6-positions of 9-phenylcar-
bazole are substituted with amino groups (compound 5), an
irreversible oxidation wave at Ep,a = +0.50 V and a small
Fig. 6. Cyclic voltammograms of (a) 1.0 × 10-3 M com-
pound 5 (b) 1.0 × 10-3 M bis(4-aminophenyl)-
phenylamine in CH3CN containing 0.1 M
TBAP. Scan rate = 0.1 V/s. Working electrode:
glassy carbon.
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gion (600-2150 nm) in their cation radical forms. (iv) The
para position of the 9-phenyl group in 9-phenylcarbazoles
is not reactive towards dimerization. (v) Carbazole cation
radicals are relatively unstable comparing with their tri-
phenylamine counterparts.
ACKNOWLEDGMENT
This work was supported by the National Science
Council of the Republic of China.
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