Laser flash photolysis study of the decay kinetics of carbocations of 1,2,2,4-tetramethyl-1,2-dihydroquinoline in the porous glass in the presence of methanol

Article abstract of DOI:10.1007/s11172-006-0115-4

The efficiency of formation and the decay kinetics of carbocations formed under the photolysis of 1,2,2,4-tetramethyl-1,2-dihydroquinoline in methanol and in a porous glass filled with methanol or dried in air or in vacuo were studied by the laser flash photolysis techniques. In MeOH, the carbocations recombine via the second-order law in the reaction with the MeO^- anion formed in an equimolar amount and decay via the first-order law in the reaction with the solvent with rate constants of 3·10⁸ L mol^-1 s^-1 and 1.4·10³ s^-1, respectively. When the solution is placed into the porous glass, no recombination of the carbocations with MeO^- is observed, and the reaction with the solvent is somewhat inhibited (rate constant 8·10² s^-1). More than tenfold inhibition of the reaction of the carbocations with methanol is observed on going to a monolayer of MeOH on the surface. The main route of carbocation decay in the porous glass dried in vacuo is the geminate recombination with the SiO groups. The corresponding kinetics is described in terms of the model of freely diffusing reactants.

Full text of DOI:10.1007/s11172-006-0115-4

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Russian Chemical Bulletin, International Edition, Vol. 54, No. 10, pp. 2312—2316, October, 2005
Laser f lash photolysis study of the decay kinetics of carbocations
of 1,2,2,4ꢀtetramethylꢀ1,2ꢀdihydroquinoline in the porous glass
in the presence of methanol
P. P. Levin,aꢀ T. D. Nekipelova,a and E. N. Khodotb
aN. M. Emanuel Institute of Biochemical Physics, Russian Academy of Sciences,
4 ul. Kosygina, 119991 Moscow, Russian Federation.
Fax: +7 (495) 137 4101. Eꢀmail: levinp@sky1.chph.ras.ru
bN. D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences,
47 Leninsky prosp., 119991 Moscow, Russian Federation.
Fax: +7 (495) 938 2156
The efficiency of formation and the decay kinetics of carbocations formed under the
photolysis of 1,2,2,4ꢀtetramethylꢀ1,2ꢀdihydroquinoline in methanol and in a porous glass filled
with methanol or dried in air or in vacuo were studied by the laser flash photolysis techniques.
In MeOH, the carbocations recombine via the secondꢀorder law in the reaction with the MeO–
anion formed in an equimolar amount and decay via the firstꢀorder law in the reaction with the
solvent with rate constants of 3•108 L mol–1 s–1 and 1.4•103 s–1, respectively. When the
solution is placed into the porous glass, no recombination of the carbocations with MeO– is
observed, and the reaction with the solvent is somewhat inhibited (rate constant 8•102 s–1).
More than tenfold inhibition of the reaction of the carbocations with methanol is observed on
going to a monolayer of MeOH on the surface. The main route of carbocation decay in the
porous glass dried in vacuo is the geminate recombination with the SiO groups. The correꢀ
sponding kinetics is described in terms of the model of freely diffusing reactants.
Key words: laser photolysis, monolayer, dihydroquinoline, carbocation, porous glass, proꢀ
ton transfer, kinetics of geminate recombination.
The kinetics of fast reactions involving shortꢀlived acꢀ
lated 1,2ꢀdihydroquinolines followed by the addition of
an alcohol molecule or recombination with an alkoxy
anion15—17 (Scheme 1).
tive species (electronꢀexcited states, radicals, and others)
in heterogeneous systems consisting of the solid inorganic
surface and organic phase as a supported thin film is of
interest because of the development of modern nanoꢀ
technologies.1—3 Many organic molecules, including
alcohols, can form ordered structures at the interface.
They manifest new properties as compared to the initial
liquid.4—10 Molecular organization in a thin organic layer
at the interface is expected to exert a substantial effect
on the kinetics of chemical processes. For instance, the
recent studies of the kinetics of bimolecular diffusionꢀ
controlled reactions of triplet—triplet annihilation and
quenching of triplet states with molecular oxygen in thin
alcohol films on the solid crystalline surface demonstrated
several kinetic effects caused by molecular organization at
the liquid—solid interface.11—14 In these systems, the
structurization of the medium affects the organization of
transporting and spatial distribution of reacting particles.
It was of interest to use the kinetics of a reaction directly
involving molecules of the medium as a tool for studying
the liquid—solid interface. Among these processes are the
photogeneration of shortꢀlived carbocations (CC) of alkyꢀ
Scheme 1
R = H, Me
The sensitivity of photolysis of 1,2ꢀdihydroquinolines
to the solvent composition has previously16—19 been demꢀ
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 2239—2243, October, 2005.
1066ꢀ5285/05/5410ꢀ2312 © 2005 Springer Science+Business Media, Inc.

Decay kinetics of carbocations
Russ.Chem.Bull., Int.Ed., Vol. 54, No. 10, October, 2005
A
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onstrated in both the step of primary photophysical and
photochemical processes producing a carbocation and the
step of decay of the latter upon the addition of the solꢀ
vent. In the present work, the efficiency of formation and
the decay kinetics of a CC formed upon the photolysis of
1,2,2,4ꢀtetramethylꢀ1,2ꢀdihydroquinoline (TDHQ) in
methanol and in the porous glass (PG), which was filled
with methanol or dried in air or in vacuo, were studied by
the nanosecond laser photolysis techniques.
0.05
1
0.04
2
0.03
0.02
0.01
Experimental
450
500
550
λ/nm
1,2,2,4ꢀTetramethylꢀ1,2ꢀdihydroquinoline was synthesized
by the reaction of 2,2,4ꢀtrimethylꢀ1,2ꢀdihydroquinoline with
trimethyl phosphate according to a standard procedure20
in 71% yield, b.p. 131—132 °C (3 Torr). UV (MeOH),
Fig. 1. Absorption spectra of the intermediate products obtained
by the laser photolysis of TDHQ in methanol (2 mmol L–1) (1)
and in the PG filled with the same solution (2) immediately
after a laser pulse.
λ
max/nm (ε): 349 (2700), 290 (2000), 278 (2800), 232 (37000).
1H NMR (CDCl3), δ: 1.29 (s, 6 H); 1.98 (d, 3 H, J = 1.4 Hz);
2.79 (s, 3 H); 5.28 (d, 1 H, J = 1.4 Hz); 6.52 (dd, 1 H, H arom.);
6.65 (m, 1 H, H arom.); 7.06 (dd, 1 H, H arom.); 7.10 (m, 1 H,
H arom.).
Results and Discussion
Absorption spectra and the decay kinetics of intermediate
products in solutions and in the PG were recorded with the
nanosecond laser photolysis technique at 400—800 nm using the
procedure described elsewhere.21—24 A PRA LN 1000 nitrogen
laser (pulse duration 1 ns, irradiation wavelength 337 nm) operꢀ
ating in a frequency regime of 10 Hz was used as the excitation
source. Kinetic curves were accumulated and averaged by
16—128 laser pulses using a Biomation 6500 highꢀperformance
analogꢀtoꢀdigital converter (USA) connected to a personal comꢀ
puter based on a Pentium processor (100 MHz). Each kinetic
curve was transformed into a series of 1024 magnitudes, each of
which corresponded to a time interval from 5 ns to 100 µs,
depending on the duration of the process. The data presented in
the work are average values obtained by the processing of at least
ten kinetic curves under these conditions. The determination
error of the kinetic characteristics did not exceed 20%. Meaꢀ
surements in solutions were carried out in a quartz cell with the
1ꢀmm optical path length. All measurements were performed
at 20 °C. Samples of a DVꢀ1M sodium borosilicate glass
3×4×10 mm in size, which were preꢀalkalized for 0.5 h in a
0.05 M aqueous solution of NaOH at 50 °C and then kept for
72 h in a 0.1 M aqueous solution of HCl, were used in experiꢀ
ments with the PG. This procedure gave the PG with an average
pore diameter of 8 nm, surface area of 80 m2 cm–3, and porosity
of 28% with respect to water.24—26 Before the PG was filled with
the working solutions, it was evacuated for a long time at 50 °C
to remove physically adsorbed water. The amount of methanol
in the PG pores was determined from its amount that would be
in a trap cooled with liquid nitrogen after 20 PG samples were
evacuated. The porous glass completely filled with MeOH conꢀ
tained 0.25 mL of MeOH per 1 cm3 of the PG. After the
MeOHꢀimpregnated PG was dried in air until the recovery of its
transparency (10 min at 20 °C), the remaining amount of MeOH
in the PG was 0.025 mL of MeOH per 1 cm3 of the PG. This
corresponds to a MeOH monolayer in which the surface area
occupied by one molecule is 0.25 nm2 (radius ∼0.3 nm), which is
typical of alcohol monolayers.4—10
The pulse photoexcitation of solutions of TDHQ in
methanol (2 mmol L–1) produces an intermediate prodꢀ
uct, whose absorption spectrum contains a relatively broad
band with a maximum at ∼480 nm (Fig. 1), which is
characteristic of the CC generated from 1,2ꢀdihydroꢀ
quinolines.15—19 When comparing the absorption intensiꢀ
ties of the CC at 480 nm in methanol and the triplet state
of benzophenone at 525 nm in benzene (quantum yield
100%, molar absorption coefficient 7200 L mol–1 cm–1 27
)
detected upon the laser photolysis of the corresponding
solutions with the same absorbance at the excitation waveꢀ
length, we obtain the molar absorption coefficient of the
CC equal to ∼4000 L mol–1 cm–1 if the quantum yield of
the CC is 40%.18,19
The kinetics of CC decay is described by a law correꢀ
sponding to parallel reactions of the first and second orꢀ
ders. The rate constant of the secondꢀorder reaction is
∼3•108 L mol–1 s–1 (Fig. 2). It can be assumed that the
decay of the CC according to the secondꢀorder law is
caused by their recombination with the methoxy anions,
which are formed simultaneously with the CC in the phoꢀ
toinduced reaction of proton transfer between the excited
state of TDHQ and methanol. The bimolecular character
of the process indicates the recombination of free ions,
because the recombination of ions in solution as ion pairs
is described by different kinetic laws. The dynamics of
these processes involving the carbocations in nonviscous
liquids was observed in the picosecond time scale.28 The
resulting secondꢀorder rate constant is much lower than
the diffusion limit, being of the same order as the known
bimolecular reaction rate constants of the ethylphenyl
and aryltropyl radicals with MeO– and other nucleophilic
ions,29 as well as the rate constant of the reaction of the

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Russ.Chem.Bull., Int.Ed., Vol. 54, No. 10, October, 2005
Levin et al.
be assumed that the CC and/or MeO– are adsorbed on
the PG surface, which decreases considerably the rate
constant of their recombination. The adsorption of the
CC on the surface results in a noticeable (1.5ꢀfold) inhiꢀ
bition of the reaction of the CC with methanol.
A
0.05
1
0.04
0.03
0.02
0.01
If the PG filled with a methanol solution of TDHQ is
dried in air for 10 min, the yield of the CC decreases and
its decay kinetics changes sharply and is not described by
the monoexponential law (see Fig. 2). The initial slope
corresponds to a high rate constant: ∼5•103 s–1. About
half of the CC decay very slowly with the time >20 ms
(the upper time limit of the detection system). It should
be expected that the methanol monolayer formed on the
PG surface after passive drying, most likely, is not homoꢀ
geneous. The presence of the fast component indicates
that the monolayer contains regions of TDHQ localizaꢀ
tion where the CC decay very efficiently (by 10 times
more rapidly than in the bulk) through, most likely, gemiꢀ
nate recombination with the corresponding anions (see
further). At the same time, the main effect of going to a
monolayer on the surface is a considerable (more than
tenfold) inhibition of the reaction of the CC with molꢀ
ecules of the environment. This can be assumed to relate
to the structurized character of the monolayer in which
the hydroxy groups of the methanol molecules, which are
involved in the reaction, are linked through hydrogen
bonds with the silanol groups on the surface.
Further drying of the PG by prolong (for 0.5 h) evacuꢀ
ation is not accompanied by a change in the yield of the
CC but exerts a substantial effect on the kinetics of its
decay, which becomes polychromatic (see Fig. 2). The
initial slope corresponds to the very high rate constant:
∼1•105 s–1. During the time >20 ms (time constant of the
detection system), 10% CC disappear. As a result of the
prolong evacuation of the PG, physically adsorbed methaꢀ
nol is removed and a very heterogeneous disordered surꢀ
face covered by silanol groups is formed. Polychromaticity
of kinetic curves of various chemical processes on the
porous silicate surface is a widely abundant phenomenon.
One of the approaches to the quantitative description of
the kinetics of processes on the porous surface is the use of
the equation, which is obtained from the Gaussian distriꢀ
bution of the logarithm of the firstꢀorder rate constant32
2
3
4
200
400
600
800
t/µs
Fig. 2. Experimental kinetic curves of decay of the intermediate
products detected at λ = 480 nm upon the laser photolysis of
TDHQ in methanol (2 mmol L–1) (1); in the PG filled with the
same solution (2); after this PG sample was dried in air (3) and
subsequently evacuated (4). Lines are the result of approximaꢀ
tion according to the law for parallel reactions of the first and
second orders (1), monoexponential law (2), biexponential
law (3), and the Gaussian distribution of the logarithm of the
firstꢀorder rate constant (4).
CC generated from 1,2,2,4,6ꢀpentamethylꢀ1,2ꢀdihydroꢀ
quinoline with azide ions.30 An increase in the stability of
the CC upon the introduction of electronꢀdonor substituꢀ
ents decreases the rate constants of their reactions. Thereꢀ
fore, for these dihydroquinolines, no contribution of the
bimolecular component was found,16,17,30 because it
should appear at high CC concentrations.
The rate constant of the firstꢀorder reaction is
1.4•103 s–1, which is higher than that for the CC generꢀ
ated from the earlier studied 1,2ꢀdihydroquinolines, which
have the electronꢀdonor substituents in position 6 of the
aromatic fragment (k = 790 and 80 s–1 for 6ꢀethoxyꢀ
1,2,2,4,6ꢀpentamethylꢀ and 6ꢀethoxyꢀ2,2,4ꢀtrimethylꢀ
1,2ꢀdihydroquinolines, respectively).16,17 By analogy to
the reactions of the other carbocations studied,31 the firstꢀ
order process corresponds to the reaction of the CC with
MeOH molecules proceeding via the nucleophilic addiꢀ
tion mechanism. Therefore, the introduction of electronꢀ
donor substituents is accompanied by the stabilization of
the CC and a decrease in the rate constants of the reacꢀ
tions of molecular and ionic nucleophiles with the CC,
respectively.
,
(1)
The pulse photoexcitation of the PG filled with a soꢀ
lution of TDHQ in methanol generates the CC, which
have almost the same absorption spectra and yield as those
generated in the absence of the PG (see Figs 1 and 2).
However, the kinetics of CC decay in a methanol solution
incorporated in the PG pores strictly follows the firstꢀ
order law with a rate constant of 8•102 s–1 even at the
initial CC concentrations >1•10–4 mol L–1. Thus, it can
where Ct and C0 are the current and initial concentraꢀ
tions, respectively; kav is the average value of the firstꢀ
order rate constant; γ is the distribution halfꢀwidth. Using
Eq. (1), one can satisfactorily describe the kinetics of CC
decay in the dry PG in the initial time interval below 1 ms
(see Fig. 2) at kav ≈ 1•105 s–1 and a very broad distribution
(γ ≈ 8). The widths of the Gaussian distribution of the
logarithm of the firstꢀorder rate constant for monomoꢀ

Decay kinetics of carbocations
Russ.Chem.Bull., Int.Ed., Vol. 54, No. 10, October, 2005
2315
lecular and pseudoꢀmonomolecular processes on the siliꢀ
cate porous surfaces are usually much smaller than those
observed for the decay of the CC. The kinetic curves with
a very broad distribution were observed on the porous
surface for the decay of organic radical cations due to
geminate recombination with the corresponding sites on
the surface, which captured an electron during photoionꢀ
ization.33,34
The kinetics of geminate recombination of pairs of
freely diffusing reactants with the sum of the van der
Waals radii σ, which were born at the distance L from
each other and react with each other upon collisions with
the probability α, is described by the equation35
The localization of a methanol solution of TDHQ in the
system of pores of the finely porous borosilicate glass is
accompanied by the adsorption of the CC on the surface
with the molecular mobility loss and inhibition of the
reaction of solvent addition, which results in the formaꢀ
tion of the adducts. The sharp inhibition of this process is
observed on going to a methanol monolayer on the surface,
which is associated, most likely, with its structurization.
The removal of physically adsorbed liquids from the surꢀ
face by evacuation results in a situation when the gemiꢀ
nate recombination with the anionic sites on the silicate
surface becomes the main channel of CC decay.
This work was financially supported by the Russian
Foundation for Basic Research (Project No. 05ꢀ03ꢀ
32197), Presidium of the Russian Academy of Sciences
(Complex Program of Fundamental Research "Fundaꢀ
mental Problems of Physics and Chemistry of Nanosized
Systems and Nanomaterials," direction "Organic and Hyꢀ
brid Organic—Inorganic Nanosized Systems and Related
Materials for Informational Technologies"), and Division
of Chemistry and Materials Science of the Russian Acadꢀ
emy of Sciences (Program "Theoretical and Experimental
Study of the Nature of Chemical Bonds and Mechanisms
of the Most Important Chemical Reactions and Proꢀ
cesses").
,
(2)
where D is the coefficient of mutual diffusion of the reacꢀ
tants. The kinetic curves of CC decay on the dry PG
surface are well approximated by Eq. (2) in a rather wide
time interval (Fig. 3) at α = 0.75 and D = 4.2•10–12
cm2 s–1 when accepting that σ = L = 0.7 nm. The coeffiꢀ
cient of mutual diffusion of the reactants is low, which is
caused, most likely, by strong bonding with the surface.
As a whole, the kinetic regularities observed indicate that
the CC decay occurs on the evacuated surface via the
recombination of the geminate pairs —Si—O–—CC.
Thus, the kinetics and mechanism of the reactions of
the dihydroquinoline carbocations are very sensitive to
the state of the inorganic surface—organic phase system.
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Received April 7, 2005;
in revised form September 20, 2005