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
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