Photoreduction of 4-substituted nitrobenzenes by amines

Article abstract of DOI:10.1039/b314883a

The reduction of several nitrobenzenes bearing electron-donor and electron-withdrawing substituents in the 4-position by triethylamine in acetonitrile was studied by cyclic voltammetry, EPR spectroscopy, and steady-state photolysis. The electrochemical re

Full text of DOI:10.1039/b314883a

View Article Online / Journal Homepage / Table of Contents for this issue
R E S E A R C H P A P E R
Photoreduction of 4-substituted nitrobenzenes by amines
E. Norambuena,a C. Olea-Azar,b A. M. Rufsc and M. V. Encinas*c
a
´
Facultad de Ciencias Basicas, Universidad Metropolitana de Ciencias de la Educacion,
´
Santiago, Chile
Facultad de Ciencias Quımicas y Farmaceuticas, Universidad de Chile,
Santiago, Chile
b
c
´
´
´
´
Facultad de Quımica y Biologıa, Universidad de Santiago de Chile, Santiago, Chile.
E-mail: mencinas@lauca.usach.cl
Received18th November 2003, Accepted 21st January 2004
F|rst published as an AdvanceArticle on the web 23rd February 2004
The reduction of several nitrobenzenes bearing electron-donor and electron-withdrawing substituents in the
4-position by triethylamine in acetonitrile was studied by cyclic voltammetry, EPR spectroscopy, and
steady-state photolysis. The electrochemical reduction of nitro compounds showed a reversible cathodic
wave corresponding to one electron transfer, and forms the nitrobenzene radical anion. The reduction
potentials were well correlated with the hyperfine nitrogen coupling constants of the radical anion and with
the donor-acceptor characteristics of the 4-substituent. The photoreduction of nitro compounds in the
presence of triethylamine gave the nitrobenzene radical anion. EPR spin trapping experiments showed that the
a-aminoethyl radical is formed only in the photolysis of nitrobenzenes with electron-donating substituents.
These compounds in the presence of the amine were efficient photoinitiators of the methyl methacrylate
polymerization. The polymerization rate increases with the amine concentration reaching a constant value.
Similar behaviour was observed for the photobleaching quantum yield. These results indicate that the
photobleaching and the reaction that leads to radicals able to add to the monomer come from the interaction
of the triplet state of the nitro compound with the amine followed by proton transfer within the geminate
ion pair giving the neutral radicals. In the absence of monomer, nitrosobenzene appeared to be the
primary product. The polymerization in the presence of electron-withdrawing substituted
nitrobenzenes was negligible. This is in agreement with the lack of a-aminoalkyl radical
formation.
product. Initial electron transfer followed by proton transfer
Introduction
also has been reported for the photoreduction of nitrobenzyli-
dene malonic derivatives with triethylamine in benzene.17
However, most of these studies on the reduction of nitro com-
pounds by amines concern to alkoxy nitro aromatic com-
pounds. Because of the presence of the charge transfer
intermediate, the quenching mechanism and the production
of radicals in these reactions are also expected to be strongly
dependent on the electron donor-acceptor characteristics of
substituents on the aromatic ring.
Aromatic nitro compounds in the presence of tertiary
amines behave as efficient photoinitiators of the polymeri-
zation of methacrylic and acrylic monomers.18–20 Costela
et al.21 have examined the photopolymerization activity of
4-nitroaniline in the presence of tertiary amines. These studies
suggest that the photoreduction takes place by an electron
transfer process from the amine to the aromatic compound fol-
lowed by a proton transfer process generating the a-amine
radical that initiates the polymerization. In a previous work
using EPR spectroscopy we directly observed the nitroaniline
and the amine neutral radicals.22
In the present work, we studied the photoreduction by
triethylamine of several nitrobenzenes with electron-donor
and electron-withdrawing substituents in the 4-position in
acetonitrile. The aim of this work is to obtain information
on the effect of the 4-substitution in the photoreduction
mechanism of these compounds, and on their efficiency as
photoinitiators of the methyl methacrylate polymerization.
Electrochemical reduction also was carried out with several
nitrobenzenes bearing different substituents in the 4-position.
Photoreactions of nitro aromatic compounds have been the
subject of several studies. In recent years the photoreduction
of nitrobenzenes1–8 and nitronaphthalenes9–13 by amines has
received special attention. Photoreactions are very dependent
on the structure of the amine and on the solvent. The photo-
reaction of 1-methoxy nitronaphthalene9 and methoxy nitro-
benzenes5–7 with methylamine and n-hexylamine leads to the
replacement of the methoxy substituent. While Bunce et al.11
found that the photoirradiation of 1-methoxy-4-nitronaphtha-
lene in the presence of primary amines causes the replacement
of the nitro group, and secondary amines give methoxy displa-
cement and substitution. Electron spin resonance and laser
flash photolysis studies on the interaction of nitrobenzenes
and nitronaphthalenes with amines showed that the reaction
involves the lowest electronic triplet state of the nitro com-
pound.7,11,13,14 Radical ions are formed in an electron transfer
reaction. Although, with ethyl glycinate and primary alkyl-
amines has been proposed that the reaction proceeds through
a SN2 reaction from the triplet state of the nitro com-
pound.11,15 More recently, Go¨rner16 has reported that the tri-
plet quenching of nitronaphthalenes by triethylamine and
diethylamine in acetonitrile involves a two-step process. The
nitro and amine ion radicals are formed by electron transfer
from the amine, and the neutral protonated nitrobenzene radi-
cal is formed by the reaction of nitronaphthalene ground state
and the a-amino radical originated from the amine cation radi-
cal. In benzene, the neutral radicals are produced inside the
geminate radical ion pair. Nitrosonaphthalene is the major
T h i s j o u r n a l i s Q T h e O w n e r S o c i e t i e s 2 0 0 4
1230
P h y s . C h e m . C h e m . P h y s . , 2 0 0 4 , 6 , 1 2 3 0 – 1 2 3 5

View Article Online
to ꢁ2.0 V. Typical cyclic voltammograms for 4-nitroacetophe-
none are presented in Fig. 1a. All voltammograms exhibit two
cathodic waves, A and B, in the range of ꢁ0.6 V to ꢁ2.0 V.
The general pattern of the voltammogram was studied as a
function of the sweep rate and switching potential. The wave
A shows the main characteristics of a reversible one-electron
transfer (Fig. 1b). On the reverse scan the reduction wave
shows an anodic counterpart, and the reduction and oxidation
peaks are independent on the scan rate. When the potential
scan is reversed before peak B, the intensity ratio ipA/ipC has
a value close to unity. The ratio ipC/v1/2 is constant as expected
in a diffusion controlled process. The anodic–cathodic peak
potential difference within each couple is ꢃ60 mV. Thus, wave
A can be unambiguously assigned to an one-electron charge
transfer, leading to a relatively stable anion-radical. The wave
B does not have an anodic counterpart, even at high scan rates.
This behaviour suggests an irreversible charge transfer.
Experimental
Nitrobenzenes (Aldrich) were purified by recrystallization.
Triethylamine (TEA) from Aldrich was vacuum distilled
before use. Methyl methacrylate (MMA) from Fluka was
purified by washing with 10% NaOH solution and then
distilled under vacuum. Nitrosobenzene (NB), 2-methyl-2-
nitrosopropane, and 3,5-dibromo-4-nitrosobenzene sulfonic
acid from Sigma were used as received.
Cyclic voltammetry (CV) was carried out using a Weenking
POS 88 instrument with a Kipp Zenen BD93 recorder, in
acetonitrile solution (1 mM), under nitrogen atmosphere, using
three-electrode cells. The tetrabutylamonium perchlorate (0.01
M) was used as supporting electrolyte. A mercury-dropping elec-
trode was used as the working electrode, a platinum wire as the
auxiliary electrode, and saturated calomel (SCE) as the reference
electrode. The CV experiments were carried out at 25 ꢀC
employing a sweep rate that varied from 100 to 1000 mV sꢁ1
.
Values of the cathodic potential peak for several 4-substi-
tuted nitrobenzenes are given in Table 1. These values are very
similar to the half-wave potential previously obtained by
polarographic experiments.25 Compounds with electron-with-
drawing substituents in the 4-position are easily reduced, while
electron-donor substituted compounds gave more negative
EPR spectra were recorded in X band (9.85 GHz) using a
Bruker ECS 106 spectrometer with a rectangular cavity and
50 KHz field modulation at room temperature. Other condi-
tions of measurements were as follows: power, 20.1 mW, and
attenation, 10.0 dB; modulation amplitude, 0.49 G; receiver
gain, 1 ꢂ 105; conversion time, 40.96 ms; time constant, 81
ms. Each spectrum was recorded with 15 scans. DPPH (a,a0-
diphenyl-b-picrylhydrazyl radical) was employed as reference
for spectrum calibration. The radical anions were generated
by electrolytic reduction ‘‘in situ’’ at room temperature. Also,
the radicals were generated ‘‘in situ’’ in the cavity via a photo-
induced electron transfer between the nitro compounds and
triethylamine in deoxygenated acetonitrile solution. Samples
were irradiated with light from 250 W medium-pressure mer-
cury lamp with cut-off filters (Schott) to isolate the 366 nm line.
The hyperfine splitting constants were estimated to be accurate
within 0.05 G. Simulation of the EPR spectra was achieved
with a program WINEPR Simfonia 1.25 version.
Spin trapping techniques, in conjunction with EPR spectro-
scopy, were used to obtain information on radicals present in
low steady-state concentration. Nitrosobenzene, 2-methyl-2-
nitrosopropane, and 3,5-dibromo-4-nitrosobenzene sulfonic
acid were used as spin traps. The concentration of nitroso
compounds was 100 mM.
1H NMR spectra were obtained with a Brucker Avance 400
MHz spectrometer. Spectra were recorded in deuterated
acetonitrile, and the chemical shifts (in ppm) are informed with
respect to Me4Si.
The photobleaching of nitrobenzenes by amines was evalu-
ated in acetonitrile degassed solutions from the change in the
near-UV absorption band. The solutions were irradiated at
360 nm with a Photon Technology International irradiation
system comprising a 150 W xenon lamp and a monochromator
(PTI-102). The rate of the reaction was measured after the
decrease of the absorbance of the nitro compound at different
irradiation times. The photoreaction quantum yield was deter-
mined taking Aberchrome 540 as reference.23 Absorption spec-
tra were recorded with a Hewlett-Packard 6453E diode array
spectrophotometer.
Polymerization rates (Rp) were measured dilatometrically in
oxygen-free solutions. All measurements were carried out at
25 ꢀC, in solutions containing equal volumes of MMA and
acetonitrile. Irradiations were carried out using a medium-pres-
sure mercury lamp (Black-Ray) with cut-off filters (Schott) of
366 nm. Low absorbance of the initiator (0.2) was used to avoid
the generation of inhomogeneous free radical distribution.24
Results and discussion
Fig. 1 Cyclic voltammograms of 4-nitroacetophenone at different
scan rates, in acetonitrile at 25 ꢀC, SCE as reference electrode. Scan
rates from the top: 2; 1; 0.5; 0.25; and 0.1 V sꢁ1. (a) Potential range
from 0 to ꢁ2.0 V; (b) Potential range from ꢁ0.6 to ꢁ1.0 V.
Cyclic voltammetry
The reduction of 4-nitrobenzenes at 25 ꢀC in acetonitrile was
followed by cyclic voltammetry in the potential range of 0.0
T h i s j o u r n a l i s Q T h e O w n e r S o c i e t i e s 2 0 0 4
P h y s . C h e m . C h e m . P h y s . , 2 0 0 4 , 6 , 1 2 3 0 – 1 2 3 5
1231

View Article Online
Table
coupling constants (aN).
1
Voltammetric cathodic potentials (Epc), and nitrogen
values of the nitrogen coupling constant (aN) for several 4-sub-
stituted nitrobenzenes are summarised in Table 1. Lower
values of aN are found for compounds with lower reduction
potential. Compounds with electron-donating substituents
are increased in aN and the electron attracting ones cause a
decrease. This behaviour is reasonable if it is assumed that
the effect of the substituent group on the over-all electron dis-
tribution is reflected in the distribution of the unpaired elec-
tron. The nitro group becomes more deficient of unpaired
electron density as the 4-substituent is made more electron
attracting. This is supported by the fact that aN values corre-
late very well with the reduction potential (r ¼ 0.99), yielding
a high r negative value of ꢁ12.5.
4-Substituent
Wave A EpC/V
aN/G (electrochemical EPR)
CN
0.87
0.91
0.96
0.98
1.20
1.24
1.37
6.3
6.6
COCH3
COOCH3
CONH2
CH3
7.73
8.15
10.79
11.34
12.32
OCH3
NH2
potential. The reduction potential potted against the sp Ham-
mett parameter has a good linear correlation with a correlation
coefficient of 0.998. The slope corresponds to a r value of 0.39,
reflecting stabilization of the anion radical by partial transfer
of negative charge from the substituent to the radical site.
EPR experiments showed that the radical anion formed at
the cationic first potential redox in the electrochemical reduc-
tion ‘‘in situ’’ is the nitro centered anion radical. Fig. 2a shows
the EPR spectrum obtained for 4-nitroanisole. This spectrum
is superimposable with the simulated spectrum, Fig. 2b. The
Photoreduction in the presence of triethylamine
When solutions of substituted 4-nitrobenzenes in acetonitrile
were irradiated with wavelength of 360 nm at 25 ꢀC in acetoni-
trile in the EPR cavity any radicals could be detected. How-
ever, when nitrobenzenes were irradiated in the presence of
TEA it was generated a strong EPR spectrum that persits at
least 30 seg. Fig. 2c shows the spectrum generated upon irra-
diation of 4-nitroanisole in the presence of TEA. The hyperfine
splitting constants are aN ¼ 11.8 gauss, a2H ¼ 1.4 gauss,
a2H ¼ 4.2 gauss, and a3H ¼ 0.40 gauss. The simulated spec-
trum, with linewidth of 0.4 gauss, is similar to the observed
spectrum, and similar to that electrochemically generated,
Fig. 2. Thus, the intermediate is assigned to the nitrobenzene
radical anion, Structure 1. The EPR spectrum obtained in
the irradiation of 4-nitrotoluene in acetonitrile also gave the
signal corresponding to the nitrobenzene radical anion. The
EPR spectrum of the 4-nitroveratrole irradiated in the pre-
sence of TEA or piperidine also shows the presence of the
nitrobenzene radical anion.4
On the other hand, the addition of nitrosobenzene (NB) to a
solution containing 4-nitroanisole and TEA produced an EPR
detectable free radical. The EPR spectrum, Fig. 3, g ¼
2.0137 ꢄ 0.0002, of 4-nitroanisole indicates that the hyperfine
splitting includes the hyperfine interaction with one hydrogen
atom and two nitrogen atoms, as consequence of the forma-
tion of a NB-TEA adduct. The values of coupling constants
are as follows: aN ¼ 10.7 G (NOꢅ), aH ¼ 14.4 G, a2H ¼ 1.1
G, and a3H ¼ 3.85 G. This spectrum is in excellent agreement
with the simulated spectrum (linewidth 0.7), and is assigned to
Structure 2, showing the presence of the a-aminoethyl radical.
The spectrum of the a-aminoethyl radical also was observed
with other spin trapping, such as 3,5-dibromo-4-nitrobenzene
sulfonic, and 2-methyl-2-nitrosopropane. The presence of the
a-aminoalkyl radical was previously showed in the photolysis
of the 4-nitroaniline in the presence of TEA.22 Although, in
this case the protonated nitrobenzene radical also was
observed.
Fig. 2 EPR spectra of the radical anion of 4-nitroanisole in acetoni-
trile at 25 ꢀC, when is generated: (a) by electrochemical reduction; (b)
simulated spectrum of the radical anion, with linewidth of 0.4 G and
ratio Lorentzian/Gaussian of 0.4; (c) photochemically in the presence
of 0.1 M TEA.
On the other hand, when nitrobenzenes bearing an electron-
withdrawing substituent in the 4-position in the presence of
T h i s j o u r n a l i s Q T h e O w n e r S o c i e t i e s 2 0 0 4
1232
P h y s . C h e m . C h e m . P h y s . , 2 0 0 4 , 6 , 1 2 3 0 – 1 2 3 5

View Article Online
Fig. 4 EPR spectra of 4-nitroacetophenone irradiated in the presence
of 0.1 M TEA in acetonitrile at 25 ꢀC. (a) Experimental spectrum; (b)
simulated EPR spectrum; linewidth, 0.48 gauss, and Lorentzian/Gaus-
sian ratio of 0.4.
Fig. 3 Effect of nitrosobenzene addition on the EPR spectrum of
4-nitroanisole irradiated in the presence of 0.1 M TEA in acetonitrile
at 25 ꢀC. (a) Experimental spectrum; (b) simulated EPR spectrum;
linewidth, 0.48 gauss, and Lorentzian/Gaussian ratio of 0.4.
radical observed in the EPR experiments. Probably fast radi-
cal–radical termination is the main reaction of these species.
On the other hand, experiments with electron-withdrawing
substituted nitrobenzenes did not show evidence of the a-
aminoalkyl radical formation. The favourable proton transfer
within the geminate ion pair in nitrobenzenes with electron-
donating substituents could be explained in terms of a
increased charge density on the nitro group, Structure 3.
TEA were irradiated in the EPR cavity the spectra correspond-
ing to the nitrobenzene radical anion were obtained. The spec-
trum of 4-nitroacetophenone consists of three line groups in
the hyperfine structure with aN ¼ 7.02 gauss, aH ¼ 2.95 gauss,
a2H ¼ 0.88 gauss, a3H ¼ 0.66 gauss, Fig. 4. The simulation
(linewidth 0.48 gauss) of this spectrum corresponds to the
nitrobenzene radical anion and is almost the same as that
obtained by electrochemical measurements. Similarly, the
EPR spectrum of the nitrobenzene radical anion was observed
in the photolysis of all nitrobenzenes with electron-withdraw-
ing substituents in the 4-position in the presence of TEA. How-
ever, for these compounds the a-aminoethyl radical could not
be observed with any spin trapping.
It is well accepted that the photoreduction of nitro aromatic
compounds by tertiary amines in organic media takes place
through electron transfer from the amine to the electronically
excited nitro compound forming a charge transfer intermedi-
ate.17,26,27 The main decomposition steps of this intermediate
are the back electron transfer, the dissociation into separated
radical ions, and the proton transfer to form neutral radicals,
Scheme 1.
Time-resolved spectroscopic studies on the photoreduction
of nitronaphthalenes and 4-nitroveratrole by amines have
showed that the reaction proceeds via the excited triplet
state.8,16,28 The EPR results obtained in this work with nitro-
benzenes bearing electron-donating substituents at the 4-posi-
tion are consistent with the mechanism depicted in Scheme 1.
The EPR experiments show the presence of the separated radi-
cal ions. The proton transfer within the geminate radical anion
pair is evidenced by spin trapping experiments that show the
presence of the carbon center a-amino radical. The fact that
no EPR signal arising from the nitrobenzene neutral radical
could be detected under our experimental conditions suggests
that this radical has a much shorter lifetime than the anion
Our previous studies with 4-nitroaniline and TEA are in agree-
ment with the present results. The intermediate observed in the
EPR experiments was the protonated nitrobenzene radical.22
The strong electron-donor character of the amine 4-substituent
could increase the rate of the proton transfer process. The for-
mation of protonated nitrobenzene radical has been proposed
in the photolysis of aromatic nitro compounds in the presence
of TEA, however the mechanism depends on the solvent and
the nature of the nitro compound. Evidence for the formation
of the protonated neutral nitrobenzene radical was shown
by the photoreduction of 1- and 2-nitronaphthalenes and
1-methoxy-4-nitronaphthalene by TEA in benzene, and in
acetonitrile.16 However, in acetonitrile it was postulated that
Scheme 1
T h i s j o u r n a l i s Q T h e O w n e r S o c i e t i e s 2 0 0 4
P h y s . C h e m . C h e m . P h y s . , 2 0 0 4 , 6 , 1 2 3 0 – 1 2 3 5
1233

View Article Online
the protonated radical is formed in a secondary delayed step.13
In the photolysis of 4-nitroveratrole in acetonitrile was
suggested the formation of the protonated radical inside the
initial ion pair.8
where C is the Coulombic energy term, for which ꢁ0.06 eV has
been calculated in acetonitrile for unitary charges.30 Taking
the measured redox potential of 4-nitroanisole (Table 1), the
TEA oxidation potential in acetonitrile as 0.95 eV,31 and the
reported value of 60 kcal/mol for the energy of the 4-nitroani-
sole triplet32 one obtains a negative DGꢀ value of ꢁ0.35 V. This
value implies a quenching rate constant that is nearly diffusion-
controlled.30 From the high reduction potential of 4-nitroto-
luene one expects a large value for the quenching rate constant
by TEA. A value of kq of 1.4 ꢂ 109 Mꢁ1 sꢁ1 has been reported
by the triplet quenching of 4-nitroveratrole in acetonitrile.8
The evolution of the main reaction products in the photoly-
sis of 4-nitroanisole in the presence of TEA was monitored
directly by 1H NMR spectroscopy at different irradiation
times. Nitrosobenzene appeared to be the main primary pro-
duct (singlet at d 3.9 for three protons, doublet at d 7.06 for
two protons, and doublet at d 8.2 for two protons). These
experiments indicate that neither photofragmentation nor
stable photosubstitution products are formed. Only photore-
duction products are observed in acetonitrile. Nitrosobenzene
has been identified in the photolysis of various nitroaromatic
compounds, and is proposed to be formed by the decomposi-
tion of the coupling product of the a-aminoalkyl and nitro
protonated radicals.13,26,33
Photobleaching of 4-nitroanisole by TEA
The photobleaching of nitrobenzenes with electron-donating
substituents in the 4-position is promoted by TEA. No reac-
tion was observed in the dark. The absorption of the longest
wavelength band of the 4-nitroanisole decreased, a new
absorption appeared in the region 260–300 nm with an isobes-
tic point at 290 nm, and no absorption was observed at wave-
lengths longer than 320 nm. The consumption of the nitro
compound was evaluated in argon-saturated acetonitrile in
the presence of different amine concentrations by following
the decrease in the absorbance at the longer absorption band.
In this region was observed a clean photobleaching with up to
80% disappearance of the nitro compound. The extent of the
chromophore loss increases with the amine concentration,
reaching a constant value, Fig. 5. Values of 0.032 and 0.007
were obtained for the maximum photobleaching quantum
yield of 4-nitroanisole and 4-nitrotoluene, respectively.
If the photoconsumption quantum yield is due to the inter-
action of the triplet state of the nitro compound with the
amine, the photobleaching quantum yield will be proportional
to the fraction of triplet states that are deactivated by the
amine according to eqn. (1),
Photoinitiation mechanism
Studies of the MMA polymerization rates photoinitiated by
nitrobenzenes in acetonitrile as solvent were carried out using
irradiation of 360 nm. The irradiation of nitrobenzenes alone
or the nitro compounds with electron-withdrawing substituent
in the 4-position in the presence of TEA did not lead to the
MMA polymerization. However, the irradiation of nitroben-
zenes with electron donor substituents in the presence of
TEA efficiently activated the polymerization of MMA.
Polymerization rates at several TEA concentrations employ-
ing 4-nitroanisole as photoinitiator were measured from the
initial slope of the conversion vs. time plots. In all cases the
conversion was lower than 10%. Fig. 6 shows that the polymer-
ization rate increases with the amine concentration, reaching a
maximum value at ꢃ0.07 M amine. A similar behaviour was
obtained for 4-nitrotoluene, but the maximum polymerization
rate is 4 times lower than that obtained with 4-nitroanisole.
Previous studies using 4-nitroaniline as photoinitiator showed
a similar dependence of Rp with the amine concentration.22
kq½TEAꢇ
P
fbleach
ꢆ
ð1Þ
k þ kq½TEAꢇ
where kq is the rate constant for the triplet quenching by the
amine. The plot of 1/fbleach as function of 1/[TEA] is shown
in the insert of Fig. 5. The near-linearity of this plot suggests
that the consumption comes from the interaction of the triplet
excited state with the amine, and any process of the singlet can
be considered as negligible. The Gibbs free energy change for
the electron transfer process, DGꢀ, can be calculated from
the redox potentials of the donor ED/Dþ and acceptor
EA/Aꢁ , and the energy E* of the excited state involved, using
the Rehm–Weller equation,29
DGꢀ ¼ ED=D ꢁ EA=A ꢁ E ꢁ C
ð2Þ
ꢈ
þ
ꢁ
Fig. 5 Effect of TEA concentration on the 4-nitroanisole
photobleaching quantum yield in acetonitrile. Insert: relation between
inverse of photobleaching quantum yield and inverse of amine
concentration according to eqn. (1).
Fig. 6 Effect of the TEA concentration on the methyl methacrylate
polymerization rate photoinitiated by 4-nitroanisole in monomer/
acetonitrile (1/1). Insert: relation of polymerization rate and amine
concentration according to eqn. (3).
T h i s j o u r n a l i s Q T h e O w n e r S o c i e t i e s 2 0 0 4
1234
P h y s . C h e m . C h e m . P h y s . , 2 0 0 4 , 6 , 1 2 3 0 – 1 2 3 5

View Article Online
´
A. Cantos, J. Marquet, M. Moreno-Man˜as, A. Gonzalez-Lafont,
7
8
9
The increase of Rp with the amine concentration is in agree-
ment with the increase of the fraction of triplet excited states
that are deactivated by the amine. The polymerization rates
will be related to the fraction of triplet quenched by the amine
by eqn. (3),
´
J. M. Lluch and J. Bertran, J. Org. Chem., 1990, 55, 3303–3310.
R. Gonzalez-Blanco, J. L. Bourdelande and J. Marquet,
´
J. Org. Chem., 1997, 62, 6903–6910.
J. Cornelisse and E. Havinga, Chem. Rev., 1975, 75, 353–388.
10 K. Mutai and R. Nakagaki, Bull. Chem. Soc. Jpn., 1985, 58,
3663–3664.
11 N. J. Bunce, S. R. Cater, J. C. Scaiano and L. Johnston,
J. Org. Chem., 1987, 52, 4214–4223.
12 T. Fournier, S. M. Tavender, W. A. Parker, G.. D. Scholes and
D. Phillips, J. Phys. Chem., A, 1997, 101, 5320–5326.
kq½TEAꢇ
f ꢆ
ꢆ R2p
ð3Þ
kq½TEAꢇ þ 1=tT
2
The linearity of 1/Rp vs. 1/[TEA] (see insert in Fig. 6) indi-
cates that the active radicals come from the triplet interaction
with the amine. The amine concentration required to reach the
maxima MMA polymerization is similar to that necessary to
reach the maximum bleaching of the nitro compound. This
indicates that the photoreaction and the production of radicals
that lead to polymerization come from the same precursor, the
triplet excited state.
13 H. Gorner and D. Dopp, J. Chem. Soc., Perkin Trans. 2, 2002,
¨
¨
120–125.
14 A. M. J. van Eijk, A. H. Huizer and C. A. G. O. Varma,
J.Photochem. Photobiol. A: Chem., 1991, 56, 183–193.
15 A. Cantos, J. Marquet and M. Moreno-Man˜as, Tetrahedron Lett.,
1989, 30, 2423–2426.
16 H. Go¨rner, J. Chem. Soc., Perkin Trans. 2, 2002, 1778–1783.
17 F. M. Abd El Latif, M. A. Barsy, E. A. Rady, M. E. Hassan and
M. A. El Maghraby, J. Photochem. Photobiol. A: Chem., 1999,
121, 111–117.
18 R. Sastre, M. Conde and J. L. Mateo, J. Photochem. Photobiol.,
A: Chem., 1988, 44, 111–122.
19 M. V. Encinas, A. M. Rufs, E. Norambuena and C. Giannotti,
J. Polym. Sci. A, Polym. Chem., 1997, 35, 3095–3100.
20 A. Costela, I. Garc´ıa-Moreno, O. Garc´ıa and R. Sastre,
J. Photochem. Photobiol. A: Chem., 2000, 131, 133–140.
It is well known that amine neutral radicals produced in the
interaction of triplet excited states with amines through an
electron transfer process are the species responsible for photo-
initiation in several polymerizations.34 The fact that polymer-
ization is not observed with 4-nitrobenzenes bearing electron
withdrawing substituents in the 4-position is in agreement with
the lack of a-aminoalkyl formation when these compounds are
irradiated in the presence of TEA. Electron withdrawing sub-
stituents decrease the charge density at the N atom, then an
unfavourable proton transfer within the geminate radical ion
pair to give the neutral radicals is expected.
´
21 A. Costela, I. Garcıa-Moreno, J. Dabrio and R. Sastre,
Macromol. Chem. Phys., 1997, 198, 3787–3798.
22 M. V. Encinas, A. M. Rufs, E. Norambuena and C. Giannotti,
J. Polym. Sci. A, Polym. Chem., 2000, 38, 2269–2273.
23 H. G. Heller and J. R. Loangan, J. Chem. Soc., Perkin Trans. 2,
1981, 341–343.
24 J. Alvarez, E. A. Lissi and M. V. Encinas, J. Polym. Sci. A:
Polym. Chem., 1998, 36, 207–208.
25 A. H. Maki and D. H. Geske, J. Am. Chem. Soc., 1961, 83,
1852–1860.
Acknowledgements
This work was supported by DIUMCE grant # 1072 002 and
FONDECYT Grant # 1030 003.
26 D. Dopp, D. Muller and K. H. Sailer, Tetrahedron Lett., 1974,
2137–2140.
¨
27 H. Go¨rner, Phys. Chem. Chem. Phys., 2002, 4, 482–489.
28 A. M. J. Van Eijk, A. H. Huizer, C. A. G. O. Varma and J.
Marquet, J. Am. Chem. Soc., 1989, 111, 89–95.
29 D. Rehm and A. Weller, Ber. Bunsen-Ges. Phys. Chem., 1969,
73, 834–839.
References
1
2
J. A. Barltrop and N. J. Bunce, J. Chem. Soc. C, 1968, 1467–1474.
R. Hurley and A. C. Testa, J. Am. Chem. Soc., 1968, 90,
1949–1952.
30 C. Porcal, S. G. Bertolotti, C. M. Previtali and M. V. Encinas,
Phys. Chem. Chem. Phys., 2003.
31 N. L. Weinberg and H. R. Weinberg, Chem. Rev., 1968,
68, 449–523.
32 J. S. Brinen and B. Singh, J. Am. Chem. Soc., 1971, 93,
6623–6629.
33 O. Massot, M. Mir, T. Parella, J. L. Bourdelande and J. Marquet,
Phys. Chem. Chem. Phys., 2002, 4, 216–223.
34 E. A. Lissi and M. V. Encinas, in Photochemistry and
Photophysics, ed. J. F. Rabek, CRC Press, Boca Raton, FL,
1991, vol. IV, p. 221–293.
´
J. Cervello, M. Figueredo, J. Marquet, M. Moreno-Man˜as, J.
3
4
5
6
´
Bertran and J. M. Lluch, Tetrahedon Lett., 1984, 25, 4147–4150.
P. H. M. van Zeyl, A. M. J. van Eijk and C. A. G. O. Varma,
J. Photochem., 1985, 29, 415–433.
A. Cantos, J. Marquet and M. Moreno-Man˜as, Tetrahedron,
1988, 44, 2607–2618.
A. M. J. van Eijk, A. H. Huizer, C. A. G. O. Varma and J.
Marquet, J. Am. Chem. Soc., 1989, 111, 88–95.
T h i s j o u r n a l i s Q T h e O w n e r S o c i e t i e s 2 0 0 4
P h y s . C h e m . C h e m . P h y s . , 2 0 0 4 , 6 , 1 2 3 0 – 1 2 3 5
1235