Deprotonation of short-living radical cations of pyrrolylbenzenes

Article abstract of DOI:10.1007/s10593-009-0299-2

The deprotonation reaction of radical cations of 1-(2-pyrrolyl)-4-(1-vinyl- 2-pyrrolyl)benzene has been studied by nanosecond laser photolysis. Bimolecular rate constants have been determined for the transfer of proton to the heterocyclic base. Analysis o

Full text of DOI:10.1007/s10593-009-0299-2

Chemistry of Heterocyclic Compounds, Vol. 45, No. 5, 2009
DEPROTONATION OF SHORT-LIVING
RADICAL CATIONS OF PYRROLYLBENZENES*
I. K. Petrushenko1, K. B. Petrushenko1**, V. I. Smirnov1,
E. Yu. Shmidt1, and A. I. Mikhaleva1
The deprotonation reaction of radical cations of 1-(2-pyrrolyl)-4-(1-vinyl-2-pyrrolyl)benzene has been
studied by nanosecond laser photolysis. Bimolecular rate constants have been determined for the
transfer of proton to the heterocyclic base. Analysis of the yields has been carried out of the final
products of the radical-cation reaction of 1-(2-pyrrolyl)-4-(1-vinyl-2-pyrrolyl)benzene in the presence
and absence of bases. Comparison of the results of impulse and stationary photolysis showed that
inhibition of the radical cation reaction occurs at the stage of forming the radical cations.
Keywords: 1-(2-pyrrolyl)-4-(1-vinyl-2-pyrrolyl)benzene, radical-cation, deprotonation, nanosecond
laser photolysis.
It is recorded in the literature that on electropolymerization of pyrroles in the presence of small
quantities of water or bases the polymers have higher conductivity than those obtained in dry acetonitrile. The
reason for this phenomenon is linked with an increase in the acidity of the medium at the stage of aromatizing
dihydrodimers of dications formed as a result of pairing radical cations (RC). Under acidic conditions pyrrole is
protonated and then may react with the initial monomer at positions 2 or 3, giving defects in the form of
unconjugated oligomers. To prevent the formation of such defects it is necessary to deprotonate the dications as
rapidly as possible and to remove the resulting protons [1, 2]. In addition the introduction of bases may
completely inhibit the polymerization process by deprotonating the initial radical cations. Similar problems may
also arise in the process of forming polyconjugated systems based on dipyrrolylbenzenes on chemical,
electrochemical, or photochemical oxidation of them.
N
N
H
1
_______
* Dedicated to Academician B. A. Trofimov on his 70th jubilee.
** To whom correspondence should be addressed, e-mail: smirnov@irioch.irk.ru.
1A. E. Favorsky Irkutsk Institute of Chemistry, Siberian Branch of the Russian Academy of Sciences, Irkutsk
664033, Russia.
__________________________________________________________________________________________
Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 5, pp. 705-710, May, 2009. Original
article submitted April 23, 2008.
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0009-3122/09/4505-0554©2009 Springer Science+Business Media, Inc.

In the present work, the deprotonation of a molecule 1 RC (1+·) with a series of heterocyclic bases (B) in
acetonitrile has been studied by nanosecond laser photolysis (NLP) using 1-(2-pyrrolyl)-4-(1-vinyl-2-pyrrolyl)-
benzene (1) as the example.
+
Chloroform was used as oxidizing agent. Azoles B were used as bases (values of pKBH from [3] are
given in parentheses): 4-bromopyrazole (6.3), indazole (7.1), pyrazole (8.8), 3-methylpyrazole (9.8), 3,5-di-
methylpyrazole (10.9), benzimidazole (12.7), 2-methylbenzimidazole (13.6), imidazole (14.6), and 2-methyl-
imidazole (15.8). Reactions were carried out in MeCN.
RC 1 was obtained photochemically by NLP according to Scheme 1.
Scheme 1
1 + hνa → 11*,
11* → 1,
absorption of light, Iabs
internal conversion, kic
fluorescence, krad
intercombinational
conversion, kisc
transfer of electron, kq
transfer of proton, kr
other reactions, kp
1.1
1.2
1.3
1.4
11* → 1 + hνf,
11* → 31,
11* + R−Cl → 1+• + R• + Cl−,
1+• + B → 1• + BH+,
1+• → → P600
1.5
1.6
1.7
The choice of bases is dictated:
1. By the fairly wide range of pKBH+ values in a relatively uniform heterocyclic series;
2. By the absence of absorption at the wavelength of the laser (λex = 337.1 nm), which made possible
excitation of only the substance being investigated;
3. By the higher energy levels of the lowest excited singlet and triplet states of bases B relative to the
energy levels of the corresponding states of compound 1. Such a relative disposition of energy levels makes
quenching of the electron-activated state of compound 1 by an energy transfer mechanism scarcely probable;
4. By the higher values, compared to compound 1, of the ionization potentials of molecules B. In this
connection, the transfer of an electron from the heterocyclic bases being used to RC 1, i.e. 1+· + B → 1 + BH+·,
is not beneficial thermodynamically.
The formal kinetic analysis of Scheme 1 for the case of nonstationary excitation, under the experimental
conditions kΣ > kL and assuming that the concentration of compound 1 is not changed and the radical cations are
consumed only in reaction 1.6, leads to the following expression for the dependence of radical-cation
concentration on time.
[R–Cl]
αkq
.
.
[1](o)[exp(–krt) – exp(–kLt)],
+
[
](t) =
1
kΣkL
where, kΣ = k0 + kq [R-Cl], sec-1; kL = 108 sec-1, the reciprocal value of the duration of the laser impulse on height
+
1/e; kr = kp + kH [B]; α is a constant characterizing the absorbing ability of the substance, the geometry of
sample irradiation, and the power of the laser, sec-1.
The analysis carried out shows that the time of introducing radical cations into reaction is described by
the growing portion of the kinetic curve and is determined by the duration of the laser impulse, i.e. it may be
considered as "instantaneous". The decrease in the kinetic curve determines the observed pseudounimolecular
rate constant for the transfer of proton kr (1.6).
The example of "quenching" RC 1 by imidazole in reaction 1.6 is represented in Fig. 1. The values of kr
for all the bases were determined from the kinetics of the attenuation of the optical density at the absorption
maximum of RC 1 at several concentrations of base (Fig. 1). For this, in order to guarantee the pseudouni-
molecular character of the reaction, the minimum concentration of base was chosen at not less than an eightfold
555

+
excess over the concentration formed on photolysis of RC 1. The bimolecular rate constants kH were determined
for each base from the tangents of the angle of the linear dependence of kr on concentration of base B.
t, sec
λ, nm
Fig. 1. Laser photolysis of the system compound 1 + imidazole in a medium of MeCN + CHCl3
(10 vol.%): a – change of optical density at the absorption maximum of RC 1+· with time at
concentrations of imidazole of 1 – 0; 2 – 8·10-4; 3 – 3·10-3; 4 – 10-2 M; b – absorption spectrum obtained
1 – directly after excitation (RC 1); 2 – 2.5 msec later (neutral radical 1). Concentration of imidazole was
1.5 10-3 M.
+
+
+
A linear relationship was observed between log kH and pKBH of bases (Fig. 2). At pKBH = 15.8
+
(2-methylimidazole) the value of kH remained close to the diffusion limit, which, in agreement with [3], enables
pK1+· in MeCN to be estimated as ≥ 16.
We discovered previously [4] that on stationary photolysis of dipyrrolylbenzenes in the presence of
chloromethanes colored products of an oligomeric nature are observed with λmax ~ 610 nm (subsequently
product P600), genetically linked with radical cations (Scheme 1, 1.7). In [4] the nature of the oligomers was not
specially studied. We note that the dication of 2,2'5'2":5",2"'-quaterpyrrole has a very similar shaped absorption
band in the visible region (λmax = 680 nm). The quaterpyrrole was obtained as a result of pairing at
the α-position of two radical cations of 2,2'-bipyrrole (RC–RC echanism) on electrochemical and photochemical
(in the presence of CCl4) oxidation of 2,2'-bipyrrole in MeCN [2]. On photochemical polymerization of
2,2'-bithiophene and 2,2'5'2"-terthiophene in a medium of MeCN in the presence of CCl4 as electron acceptor the
formation of colored reaction products was also recorded, which in accordance with NMR investigations were
considered as nanodimensional α-linked oligomers, partially protonated at the α-position [5].
556

+
lgkH
Fig. 2. Dependence of the bimolecular rate constant for the transfer of proton in the reaction 1+· + B
in MeCN on the pKBH+ of bases: 1 – pyrazole; 2 – 3-methylpyrazole, 3 – 3,5-dimethylpyrazole, 4 –
benzimidazole; 5 – 2-methylbenzimidazole, 6 – imidazole, 7 – 2-methylimidazole. y = 0.6443x –
0.6916; R2 = 0.9873.
The related character of the discussed heterocyclic compounds and the close photochemical conditions
of the experiments enable us with a high degree of probability to propose that product P600 has the structure of
the dication of the dihydrodimer of 1-(2-pyrrolyl)-4-(1-vinyl-2-pyrrolyl)benzene (22+).
H
H
N
+
N
+
N
N
H
H
22+
The possibility of forming dihydrodimer 22+ according to a RC–RC mechanism was confirmed by the
high values of the calculated spin densities on the α-carbon atoms of the pyrrole rings (0.237 for an
unsubstituted pyrrole ring and 0.234 for a pyrrole ring with an N-vinyl substituent) (Fig. 3).
Fig. 3. Optimized geometry [UB3LYP/6-31 + G(d)] and spin density distribution (isospin 0.005) of
RC 1.
557

Experiments which, on the one hand, confirm the connection of radical cations with product P600, and
on the other demonstrate the inhibition of cation-radical reactions of compound 1 with bases are demonstrated in
Fig. 4.
In the first experiment the stationary photochemical oxidation of 1 by CHCl3 was carried out in the
absence of base. Titration of the solution obtained after the end of photolysis with base leads to complete
disappearance of the absorption band of product P600 and the appearance of a new absorption band with a
maximum at 525 nm (P500, Fig. 4a).
In the second experiment the base was introduced directly before photolysis. The presence of base in the
solution being photolyzed practically completely eliminates the formation of product P600, as in the first case,
however product P500 is formed in only trace amounts (Fig. 4b).
λ, nm
λ, nm
Fig. 4. Dependence of the absorption spectrum of the system of compound 1 (5·10-3 M) and CHCl3 (10
vol%) in MeCN on time of irradiation: a – in the absence of imidazole (base was added after the end of
photolysis); b – imidazole (3·10-3) was introduced directly before photolysis. Time of irradiating solutions
was 115 sec.
In both cases solutions had the same concentrations and were irradiated for the same time. The ratio of
yields of product P500 in experiments A and B and the results of NLP on deprotonation demonstrate inhibition
of the cation-radical reaction at the stage of forming radical cation 1+·.
The information obtained shows the need for a delicate approach on selecting the strength and
concentration of bases on carrying out radical-cation syntheses of polyconjugated polymers and may be useful
when considering other pyrrole-containing monomers.
EXPERIMENTAL
Synthesis of compound 1 by the Trofimov reaction was described in [4]. MeCN and CHCl3 from Merck
Uvasol (99.9%) were used without further purification. Electronic absorption spectra were obtained on a Perkin-
Elmer Lambda 35 spectrophotometer. Spectral-kinetic properties of the short-living products were investigated
on a laser photolysis (NLP) device [6]. Excitation of samples was carried out in a quartz cuvette 1×1 cm2 N2-
laser AIL-3 (energy of impulse was 10-3 J, duration of radiation impulse at half-height was 7 nsec). Directly
before the experiment samples were freed from oxygen of the air by passing argon for 20 min. A DRSh-500
mercury lamp was used as the source of stationary excitation with a color filter separating light with
λmax 313 nm.
558

Calculation of the equilibrium structure and the spin density of RC-1 was carried out by the DFT
B3LYP//6-31 + G(d) method, with the GAUSSIAN 2003 set of programs.
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