Spectroscopic observation of the dimerization reactions of the 9-phenylcarbazole cation radical in acetonitrile

Article abstract of DOI:10.1246/bcsj.77.953

The dimerization reaction processes of the 9-phenylcarbazole cation radical (PCZ^>=+) in acetonitrile (AN) were observed using an electron-transfer stopped-flow method. Although the lifetime of PCZ ^>=+ in AN is less than a few hundred milliseconds, the dynamic transformation of the absorption spectra of PCZ^>=+ was successfully observed in AN for the first time with the proposed method. By observing the changes in the absorption spectra in the visible region, the decrease in PCZ^>=+ and the concurrent increase in the dimer cation radical were observed with isosbestic points. In addition, in the presence of neutral PCZ, the acceleration of the dimerization of PCZ ^>=+ was clearly confirmed. Consequently, in the presence of PCZ, the rate law was determined to be -d[PCZ^>=+]/dt = 3Kk′[PCZ^>=+]² [PCZ], where 3Kk′ is 1.4 × 10⁹ M^-2 s^-1. This means that the reaction mechanism involves a two-step interaction with the catalytic effect of neutral PCZ. Although the difference in the reactivity had already been known between PCZ^>=+ and the triphenylamine cation radical, the reaction mechanism has been clarified to be different in both cases.

Full text of DOI:10.1246/bcsj.77.953

Ó 2004 The Chemical Society of Japan
Bull. Chem. Soc. Jpn., 77, 953–957 (2004)
953
Spectroscopic Observation of the Dimerization Reactions
of the 9-Phenylcarbazole Cation Radical in Acetonitrile
ꢀ
Munetaka Oyama and Jun Matsui
Division of Research Initiatives, International Innovation Center, Kyoto University, Sakyo-ku, Kyoto 606-8501
Received June 19, 2003; E-mail: oyama@iic.kyoto-u.ac.jp
The dimerization reaction processes of the 9-phenylcarbazole cation radical (PCZꢁþ) in acetonitrile (AN) were ob-
served using an electron-transfer stopped-flow method. Although the lifetime of PCZꢁþ in AN is less than a few hundred
milliseconds, the dynamic transformation of the absorption spectra of PCZꢁþ was successfully observed in AN for the first
time with the proposed method. By observing the changes in the absorption spectra in the visible region, the decrease in
PCZꢁþ and the concurrent increase in the dimer cation radical were observed with isosbestic points. In addition, in the
presence of neutral PCZ, the acceleration of the dimerization of PCZꢁþ was clearly confirmed. Consequently, in the pres-
2
ence of PCZ, the rate law was determined to be ꢂd½PCZꢁþꢃ=dt ¼ 3Kk0½PCZꢁþꢃ ½PCZꢃ, where 3Kk0 is 1:4 ꢄ 109 Mꢂ2 sꢂ1
.
This means that the reaction mechanism involves a two-step interaction with the catalytic effect of neutral PCZ. Although
the difference in the reactivity had already been known between PCZꢁþ and the triphenylamine cation radical, the reaction
mechanism has been clarified to be different in both cases.
The mechanisms of the electro-oxidative dimerization reac-
tions in aprotic solvents have been studied extensively, e.g., as
summarized in a review by Schmittel and Burghart.1 Depend-
ing on the identity of the cation radicals, cation radical–sub-
strate coupling (RSC) or the cation radical–cation radical cou-
pling (RRC) mechanism has been applied to explain the elec-
trochemical results. However, there is a mechanistic uncertain-
ty between both mechanisms.1 Although the RRC mechanism
was actually applicable for the dimerization of aromatic amine
derivative cation radicals in spite of a charge repulsion,2–4 the
RSC mechanism was verified in some other cases.5–7 In partic-
ular, for the latter mechanism, careful mechanistic discrimina-
tions involving the precursor molecules should be necessary in
an electrochemical analysis, because the precursor molecules
surely exist in the bulk solution to produce the cation radicals.
Recently, we proposed an electron-transfer stopped-flow
(ETSF) method,4 and utilized it for revealing various complex
aspects of the reactions of aromatic amine cation radicals due to
the presence of precursor molecules.8–12 In the ETSF method, it
is characteristic that the thermodynamically favorable electron
transfer reactions between long-lived cation radicals and neu-
tral molecules are utilized to form unstable cation radicals.4
Although the ETSF method is not an electrochemical method,
it has great advantages in that the reaction processes of cation
radicals can be analyzed in homogeneous solutions under rigid
control of the concentration of the coexisting precursor mole-
cules.
2) The dimerization reaction of the 4-bromo-N,N-dimethyl-
aniline cation radical proceeds via the RRS to form the tetra-
methylbenzidine dication when the cation radical is quantita-
tively formed without a precursor. In the presence of precursor
molecules, the tetramethylbenzidine cation radical is formed
via nucleophilic attack of the precursor on the cation radical,
i.e., the RSC mechanism, with high reactivity.10
3) The decay reaction of the N,N-dimethyl-p-toluidine cation
radical proceeds via an acid–base reaction between the cation
radical and the precursor, which is similar to the RSC mecha-
nism, but actually via a radical–radical coupling of the Hþ-ex-
tracted form of the cation radical.10,11 This mechanism was also
verified using electrochemical methods.13
4) In the reaction of the 1-aminopyrene cation radical, the pre-
cursor molecule was found to promote the rate of the initial re-
action of the polymerization. Mechanistically, the first reaction
between the cation radical with the precursor is in equilibrium,
and the second step involving another cation radical is the rate-
determining reaction.12
In the present paper, as an example of the spectroscopic de-
tection of short-lived cation radicals and clear-cut reaction
analysis involving the precursor molecules, we would like to
present how we could observe the dimerization reaction of
the 9-carbazole cation radical in AN using the ETSF method.
The reaction pathways of various carbazole cation radicals in
AN were thoroughly investigated by Nelson and co-work-
ers.14,15 Among the carbazole derivatives studied, the reaction
of 9-phenylcarbazole cation radicals (PCZꢁþ), for which the
formation of the dimer (3,30-dimer) is known,15 would be of in-
terest in relation to the reaction of the triphenylamine cation
radical (TPAꢁþ). The structures of the compounds are shown
in Chart 1. PCZꢁþ has a planer carbazole ring, whereas all of
the phenyl groups of TPAꢁþ are twisted.
The mechanistic findings achieved by the ETSF method can
be summarized as follows:
1) The dimerization reaction of methyldiphenylamine and di-
phenylamine cation radicals in acetonitrile (AN) proceeds via
the RRC mechanism. The precursor molecules affect only the
equilibria involving the oxidation states of the dimer benzi-
dines.4
Reflecting this difference, a much faster (ca. 104 times faster)
Published on the web May 6, 2004; DOI 10.1246/bcsj.77.953

954 Bull. Chem. Soc. Jpn., 77, No. 5 (2004)
Reaction of 9-Phenylcarbazole Cation Radical
Fig. 1. Cyclic voltammograms of (a) PCZ and (b) TDBPA
in AN. Working electrode; Pt disk electrode (diam. 1.6
mm). Reference electrode, Pt/(Iꢂ,I3ꢂ) electrode; the po-
tential was then converted to the value versus ferricinium
ion/ferrocene (Fcþ/Fc). Scan rate; 100 mV sꢂ1. Concen-
tration of substrate; 1.0 mM. Supporting electrolyte; 0.10
M TBAPF6.
Chart 1.
dimerization reaction rate of PCZꢁþ has been reported for
PCZꢁþ compared with that of TPAꢁþ; the bimolecular coupling
rate constant was estimated to be 9 ðꢅ 6Þ ꢄ 107 Mꢂ1 sꢂ1 (1 M
= 1 mol dmꢂ3) assuming the RRS mechanism and using rotat-
ing disk electrode experiments.15 Later, Heinze and co-workers
reported on the electrochemical reaction mechanism of PCZ in-
volving PCZꢁþ and the oxidation states of the dimer in nitro-
methane using cyclic voltammetry.16 In their work, the RRS
mechanism was also proposed with a dimerization rate constant
of 1:2 ꢄ 106 Mꢂ1 sꢂ1, while mentioning the significance of the
equilibrium between PCZ and the dimer2þ in this electrode re-
spectroscopic system, RSP-601 (Unisoku Co., Ltd., Hirakata,
Japan) was used. In this apparatus, a dynamic transformation of
the absorption spectra can be observed with a minimized time in-
terval of 1.0 ms after mixing the two solutions.
Results and Discussion
Cyclic Voltammetry. Figure 1a shows a cyclic voltammo-
gram of 1.0 mM PCZ in AN recorded at a scan rate of 100
mV sꢂ1. From this irreversible voltammogram, which is very
similar to reported ones,16 the rapid reaction of PCZꢁþ and
the formation of the 3,30-dimer can be recognized. The reduc-
tion peak at 0.53 V can be assigned to the reduction of the dimer
cation radical to the dimer neutral, and the small reduction peak
around 0.72 V is assigned to the reduction of the dimer dication
to the dimer cation radical, as previously reported.16
Comparing the cyclic voltammogram of TDBPA in Fig. 1,
the electron transfer reaction of Eq. 1 should be favorable in
the ETSF operation, judging from the redox potential of the
TDBPAꢁþ/TDBPA couple (1.06 V vs Fcþ/Fc) and the oxida-
tion peak potential of PCZ (0.85 V).
action mechanism, because the oxidation potential of PCZꢁþ
PCZ is positive relative to that of the dimer2þ/dimerꢁþ
/
16
.
However, due to the higher reactivity of PCZꢁþ, spectro-
chemical detection of PCZꢁþ in AN with the electrochemical
and chemical oxidation methods has not been carried out,
whereas the absorption spectra of some carbazole cation radi-
cals were observed using a laser photolysis method,17 in poly-
mers,18,19 and in acidic zeolites.20
In the ETSF method, because short-lived cation radicals can
be detected and analyzed in solution, a clear-cut analysis of the
kinetics and mechanisms of PCZꢁþ is presented by observing
the dynamic transformation of the absorption spectra. As a re-
sult, the acceleration of the decay reaction of PCZꢁþ by neutral
PCZ was clearly observed, which proved that the dimerization
reaction of PCZꢁþ is not proceed via a simple RRC mechanism.
PCZ þ TDBPAꢁþ ! PCZꢁþ þ TDBPA:
ð1Þ
Transformation of Absorption Spectra. Thus, at first, we
observed the dynamic transformation profiles of the absorption
spectra by mixing an AN solution of 0.20 mM TDBPAꢁþ with
an AN solution of 0.20 mM PCZ. Figure 2A shows the changes
in the absorption spectra at a time interval of 20 ms. Just after
mixing, the absorption spectrum of TDBPAꢁþ around 850 nm
totally disappeared, which indicates a quantitative completion
of Eq. 1 in the mixing section of the stopped-flow apparatus.
As recognized from this figure, the absorption spectrum of
PCZꢁþ having absorption maxima at 720 nm and a shoulder
around 650 nm could be observed explicitly, and a transforma-
tion process having two isosbestic points was observed during
the course of the reaction. As for the increase of the absorbance
around 430 and 930 nm, this can be attributed to the formation
of the cation radical of the 3,30-dimer, which is reasonably ex-
pected by the cyclic voltammogram in Fig. 1. In this manner,
using the ETSF method, the absorption spectrum of PCZꢁþ
Experimental
In the present work, we used the tris(2,4-dibromophenyl)amine
cation radical (TDBPAꢁþ) as the reaction initiator, the details of
which were described previously.21 TDBPA (95%) was synthe-
sized by Wako Pure Chemicals, Ltd., after receiving our special or-
der. All measurements were carried out in AN (Wako Chemicals,
dehydrated, H2O < 50 ppm) as a solvent. The AN solution of
TDBPAꢁþ was prepared by a batchwise electrolysis with 0.05 M
tetrabutylammonium hexafluorophosphate (TBAPF6, Fuluka, pu-
riss. electrochem. grade), and used after diluting it to an appropri-
ate concentration with AN. It was used to mix with the AN solution
of PCZ to form PCZꢁþ. The PCZ (9-phenylcarbazole, sublimed,
99+%) was purchased from Aldrich and used as received.
For stopped-flow measurements, a rapid-scan stopped-flow

M. Oyama et al.
Bull. Chem. Soc. Jpn., 77, No. 5 (2004)
955
Fig. 2. (A) Dynamic transformation of absorption spectra recorded after an AN solution of 0.20 mM PCZ was mixed with an AN
solution of 0.20 mM TBPAꢁþ. Time interval of each spectrum; 20 ms. (B) Dynamic transformation of absorption spectra recorded
after an AN solution of 2.0 mM PCZ was mixed with an AN solution of 0.20 mM TBPAꢁþ. Time interval of each spectrum; 5 ms.
could be clearly observed, and the fast reaction process of
PCZꢁþ was followed in a time range of several tens of millisec-
onds.
Next, utilizing an advantage of the ETSF method, we ob-
served the effect of neutral PCZ on the decay reaction of
PCZꢁþ. Figure 2B shows the dynamic transformation profiles
of absorption spectra by mixing the AN solution of 0.20 mM
TDBPAꢁþ with the AN solution of 2.0 mM PCZ. The time in-
terval of Fig. 2A was 20 ms; however, it is 5 ms in Fig. 2B. In
this case, taking into account the equivolume mixing, the initial
concentrations of PCZꢁþ and PCZ after the mixing were 0.10
mM and 0.90 mM, respectively. Compared with the result ob-
tained with 0.10 mM PCZꢁþ without PCZ (Fig. 2A), the pres-
ence of neutral PCZ was found to accelerate the decay reaction
of PCZꢁþ. In Fig. 2B, the acceleration is remarkable as well as
Fig. 3. Time changes in absorbance at 720 nm in the reac-
tions of 0.10 mM PCZꢁþ with (a) 0, (b) 0.10, (c) 0.20,
(d) 0.40, and (e) 0.90 mM PCZ in AN.
the promotion of the follow-up reaction.
Reaction Mechanism of PCZꢀþ
. Figure 3 shows the time
changes in the absorbance at 720 nm, which is the absorption
maximum of PCZꢁþ, depending on the concentration of the
coexisting neutral PCZ in AN. As shown in this figure, the in-
crease in [PCZ] brought about an increase in the decay reaction
of PCZꢁþ. Such acceleration by the precursor molecules is
quite in contrast to the reaction of TPAꢁþ, in which the precur-
PCZꢁþ significantly before solution-stop during the dead time
of ca. 1 ms, the accurate determination of " is difficult. Howev-
er, in the present case, the changes in the absorbance at 720 nm
during 1 ms after stopping was ca. 1.2% under the conditions of
Fig. 2A. Thus, we neglected the reaction or decrease of PCZꢁþ
before stopping, and estimated the " value at 720 nm as being
1:2 ꢄ 104 Mꢂ1 cmꢂ1 from the initial absorbance value of Fig.
2A. The " value used in the simulation analysis therefore has
a slight positive error. In addition, in the simulation analysis,
the increasing component due to the formation of the dimer cat-
ion radical was added to the contribution of [PCZꢁþ], as carried
out previously.4
sor molecules do not take part in the decay rate law of TPAꢁþ
.
2
The rate law was ꢂd½TPAꢁþꢃ=dt ¼ k½TPAꢁþꢃ , and the precur-
sor affects only the equilibrium involving the oxidation states
of the dimer.4
To obtain the mechanistic conclusions on the acceleration by
the precursor molecules, a simulation analysis was carried out
for the present results of PCZꢁþ (Fig. 3), assuming several ex-
pectable rate laws. For the simulation, at first, the molar absorp-
tion coefficient value, ", has to be determined. In the present
stopped-flow analysis, if the dimerization reaction of PCZꢁþ
proceeded after the electron transfer reaction of Eq. 1 to form
Consequently, a good fit was obtained with the rate law of
the following equation (Fig. 4), but it was only for the decay
curves in the presence of higher concentrations of PCZ (Figs.
3d and e):

956 Bull. Chem. Soc. Jpn., 77, No. 5 (2004)
Reaction of 9-Phenylcarbazole Cation Radical
nism is expected when the amount of PCZ is relatively small.
However, the isosbestic points observed in Fig. 2A indicate that
the decreased PCZꢁþ transferred to the dimer cation radical
quantitatively follows stoichiometric relationship. In spite of
such complexity, in the present results, acceleration of the reac-
tion of PCZꢁþ by PCZ could be remarkably observed, and the
mechanism of Eqs. 3–6 could be established.
Comparison with Electrochemical Measurements. Here,
it is of interest to compare the present results in homogeneous
solution with the previous results for the electrochemical oxida-
tion of PCZ.16 The electrode reaction mechanism of PCZ is
complex because the dimer2þ is formed at the potential where
PCZ is oxidized to PCZꢁþ. Thus, the equilibrium between the
dimer2þ and PCZ was previously considered to be the effect
of the neutral molecule.16 However, in the present ETSF ex-
periment, the effect of PCZ on the decay reactions of PCZꢁþ
could be observed explicitly, as shown in Fig. 3. From this re-
sult, it is apparent that the neutral PCZ takes part in the rate law
before forming products. No dimer2þ is formed in homogene-
ous solution, in particular when the neutral PCZ is co-existent,
because the initial ratio of [PCZꢁþ]/[PCZ] governs the solution
potential.4
In the present case, even though the decay reactions of
PCZꢁþ are estimated from the voltammetric results, e.g., using
a fact-scan method, the conclusion would have less meaning.
This is because PCZꢁþ decayed even when it was formed quan-
titatively, as in Fig. 2A, while the neutral PCZ accelerated the
decay of PCZꢁþ significantly. The presence of two decreasing
routes should cause ambiguous results in an estimation based
on the electrochemical results.
Fig. 4. Simulated results for curves (d) and (e) in Fig. 3 as-
2
suming the rate law of ꢂd½PCZꢁþꢃ=dt ¼ k½PCZꢁþꢃ ½PCZꢃ
and k ¼ 1:4 ꢄ 109 Mꢂ2 sꢂ1. The absorbance was converted
to the concentration using the molar absorption coefficient
of 1:2 ꢄ 104 Mꢂ1 cmꢂ1 of PCZꢁþ at 720 nm.
2
ꢂ d½PCZꢁþꢃ=dt ¼ k½PCZꢁþꢃ ½PCZꢃ;
k ¼ 1:4 ꢄ 109 ðMꢂ2 sꢂ1Þ:
ð2Þ
This mechanism indicates that the dimerization reaction pro-
ceeds via the following mechanism (Eqs. 3–6), because it is dif-
ficult to imagine the participation of PCZ in the latter step:
K
ꢁþ
PCZꢁþ þ PCZ
(PCZ)
;
ð3Þ
ð4Þ
ð5Þ
ð6Þ
ꢂꢂ!
ꢂꢂ
2
k0
(PCZ)2ꢁþ þ PCZꢁþ ꢂꢂ! (PCZ)22þ þ PCZ;
rds
(PCZ)22þ ꢂꢂ! Dimer þ 2Hþ;
fast
Dimer þ PCZꢁþ ꢂꢂ! Dimerꢁþ þ PCZ:
fast
Concerning the mechanistic issues of cation radicals, conclu-
sions for the cases of RSC mechanism would be somewhat dif-
ficult to obtain, even though the previous studies revealed an in-
teraction with the parent molecules.5–7 This is because, e.g., in
the present case, the formation of the dimerꢁþ in Eq. 6 can oc-
cur via electron transfer from the electrode (and furthermore,
dimerꢁþ can be oxidized to dimer2þ on the electrode). Howev-
er, in the ETSF method, a clear-cut analysis can be performed
by eliminating the possibility of electrode reaction processes of
the products, as in the present paper. The ETSF analysis is thus
effective in revealing the electrochemical events by focusing on
homogeneous reactions involving electrogenerated species.
In the proposed mechanism, the first reaction (Eq. 3) process
between PCZꢁþ and PCZ is in equilibrium, and the second elec-
tron-transfer process (Eq. 4) is the rate-determining step (rds).
In addition, because an increase in the dimerꢁþ and a decrease
in PCZꢁþ were observed at the same time, the reactions of Eqs.
5 and 6 should be fast compared with the rate-determining step
of Eq. 4. Based on this mechanism, the overall stoichiometry
can be expressed by
3PCZꢁþ ! Dimerꢁþ þ PCZ þ 2Hþ:
ð7Þ
Accordingly, Eq. 2 should be written as follows based on the
stoichiometry:
Conclusions
2
ꢂ d½PCZꢁþꢃ=dt ¼ 3Kk0½PCZꢁþꢃ ½PCZꢃ:
ð8Þ
In conclusion, in the present work, the dynamic transforma-
tion of the absorption spectra of PCZꢁþ could be observed in
AN for the first time using the ETSF method, though the ab-
sorption spectra of electrogenerated carbazole cation radicals
were recorded only for specific derivatives, e.g., having block-
ing substituents on the 3, 6 positions.15 Although the ETSF
method was valid for observations of the absorption spectra
of the anthracene derivative cation radicals,21 the present case
is also a good example showing the ability of the ETSF method
for the detection of short-lived cation radicals.
In addition, the acceleration of the dimerization reaction of
PCZꢁþ in the presence of neutral PCZ was clearly demonstrated
from the changes in the dynamic transformation of the absorp-
tion spectra. From the decay curves of PCZꢁþ, the reaction
mechanism of Eqs. 3–6 could be proposed for the dimerization
Although a good fit was observed for curves (d) and (e) in
Fig. 3, the fitting becomes difficult when [PCZ] becomes small-
er, as in Figs. 3b and c. Furthermore, the decay curve of Fig. 3a,
i.e., the decay of PCZꢁþ without PCZ, was very difficult to sim-
ulate based on simple rate laws, such as ꢂd½PCZꢁþꢃ=dt ¼
2
k½PCZꢁþꢃ .
Presumably, this would be a reflection of the complexity in
which the neutral PCZ takes part in the decay reaction of
PCZꢁþ, even though PCZ did not exist under the initial condi-
tions for obtaining curve (a). Although the initial decay of
PCZꢁþ in curve (a) might imply a direct reaction between
two PCZꢁþs, the concentration of PCZ should increase as a
consequence of the electron-transfer reaction between the
formed dimer and PCZꢁþ (Eq. 6). Thus, a complicated mecha-

M. Oyama et al.
Bull. Chem. Soc. Jpn., 77, No. 5 (2004)
957
of PCZꢁþ, in spite of the complicated decay process when
PCZꢁþ was formed without PCZ. Nevertheless, the isosbestic
points observed in Fig. 2 indicate the simple and rapid transfor-
mation route from PCZꢁþ to the 3,30-dimerꢁþ, which verifies
the fast follow-up reactions after the rate-determining step.
As for the differences between PCZꢁþ and TPAꢁþ, the mech-
anistic change should be stressed in addition to changes in the
known reactivity.15 To clarify the differences, we are now ana-
lyzing the dimerization reactions of 9-(p-substituted-phenyl)-
carbazole cation radicals.
7
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