Anomalous reactivity of radical cations produced by photosensitized oxidation of 4-methoxybenzyl alcohol derivatives: Role of the sensitizer

Article abstract of DOI:10.1039/b711541e

Steady-state and nanosecond laser flash photolysis measurements of 4-methoxybenzyl alcohol (1a), 4-methoxy-α-methylbenzyl alcohol (1b), 4,4′-dimethoxydiphenylmethanol (1c) and 4-methoxy-α,α′- dimethylbenzyl alcohol (1d) were carried out in air-equilibrated CH ₂Cl₂ and CH₃CN solutions, in the presence of 9,10-dicyanoanthracene (DCA) and N-methylquinolinium tetrafluoroborate (NMQ ⁺BF₄^-) as sensitizers. In particular, steady-state irradiation with DCA produced carbonyl compounds and, with NMQ ⁺BF₄^-, carbonyl compounds, ethers (substrates 1a-c) and styrene (substrate 1d) while time-resolved investigations gave evidence of charged species produced upon irradiation. The effect of solvent polarity on the reactivity was investigated; in the case of DCA, the reactivity increased with the solvent polarity, while the opposite was obtained when NMQ⁺BF₄^- was used. Quantum mechanical calculations at semiempirical (INDO/1-CI) and DFT (B3LYP/6-311G(d)) levels were used to support transient assignments and to obtain the charge and spin density distributions, respectively. The different photooxidation mechanisms operative with the neutral and charged sensitizer were rationalized in terms of the reactivity of free and complexed radical cations, respectively. the Owner Societies.

Full text of DOI:10.1039/b711541e

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PAPER
www.rsc.org/pccp | Physical Chemistry Chemical Physics
Anomalous reactivity of radical cations produced by photosensitized
oxidation of 4-methoxybenzyl alcohol derivatives: role of the sensitizerw
Tiziana Del Giacco,* Annalisa Faltoni and Fausto Elisei
Received 30th July 2007, Accepted 28th September 2007
First published as an Advance Article on the web 16th October 2007
DOI: 10.1039/b711541e
Steady-state and nanosecond laser flash photolysis measurements of 4-methoxybenzyl alcohol
0
(
1a), 4-methoxy-a-methylbenzyl alcohol (1b), 4,4 -dimethoxydiphenylmethanol (1c) and
0
4
-methoxy-a,a -dimethylbenzyl alcohol (1d) were carried out in air-equilibrated CH
2
Cl
2
and
CH
3
CN solutions, in the presence of 9,10-dicyanoanthracene (DCA) and N-methylquinolinium
+
tetrafluoroborate (NMQ BF ) as sensitizers. In particular, steady-state irradiation with DCA
4ꢀ
+
produced carbonyl compounds and, with NMQ BF , carbonyl compounds, ethers (substrates
a–c) and styrene (substrate 1d) while time-resolved investigations gave evidence of charged
4ꢀ
1
species produced upon irradiation. The effect of solvent polarity on the reactivity was
investigated; in the case of DCA, the reactivity increased with the solvent polarity, while the
+
opposite was obtained when NMQ BF was used. Quantum mechanical calculations at
ꢀ
4
semiempirical (INDO/1-CI) and DFT (B3LYP/6-311G(d)) levels were used to support transient
assignments and to obtain the charge and spin density distributions, respectively. The different
photooxidation mechanisms operative with the neutral and charged sensitizer were rationalized in
terms of the reactivity of free and complexed radical cations, respectively.
characterization and kinetics studies performed by laser flash
Introduction
photolysis, pulse radiolysis, and steady-state experiments.
Different results were obtained when the radical cations of
benzyl alcohols were generated by photoinduced electron
transfer sensitized by 2,4,6-triphenylpyrylium tetrafluoro-
Studies on the electron transfer oxidation of benzyl alcohol
derivatives [X-C H CH(OH)R] via radical cation intermediate
6
4
have been extensively reported. This reaction has been inves-
tigated upon UV irradiation in homogeneous and heteroge-
1
neous media
+
ꢀ
borate (TPP BF
of products observed in the reaction of 4-X-benzyl alcohols
both in CH Cl and CH CN, depending on the nature of the X
4
). A first report showed the different kinds
–2
3
and by electrochemical and thermal
4
conditions, in organic and aqueous solvents. In all cases,
oxidation produces exclusively benzaldehydes (with R = H)
2
2
3
7
group. As expected with X = H and Cl, the product obtained
or the corresponding ketone (for alcohols with R = CH
3
and
was the corresponding benzaldehyde, while with X = OMe
0
4
-YC ). The reaction mechanism proceeds through the
6
H
4
(1a), bis(4-methoxybenzyl)ether, 4,4 -dimethoxydiphenyl-
deprotonation of the radical cation, formed in the first electron
transfer step, to give an a-hydroxy X-benzyl radical. This
intermediate is then converted into the carbonyl product
through two different pathways, depending on the experimen-
tal conditions. In aerated solutions, the radical reacts quickly
with oxygen to give a peroxyl radical that evolves to the final
product, or it is oxidized in the reaction medium, producing
the corresponding cation, which finally leads to the product
methane and [4-methoxy-3-(4-methoxybenzyl)phenyl](4-meth-
oxyphenyl)methane were produced. The absence of oxidation
products, even in the presence of oxygen, has been explained in
+
terms of the acid properties of the TPP excited state, which is
(
Scheme 1, path a and b, respectively). Only the latter path is
operative in deaerated conditions. The mechanism is the same
regardless of the way in which the radical cation is produced,
however in aqueous solutions, it is only operative at pH r
5
,6
4
.
The mechanism has been supported by intermediate
Dipartimento di Chimica and Centro di Eccellenza Materiali
Innovativi Nanostrutturati (CEMIN), Universita` di Perugia, Via Elce
di Sotto 8, 06123 Perugia, Italy. E-mail: dgiacco@unipg.it.;
Fax: +39 075 5855560; Tel: +039 075 5855559
w Electronic supplementary information (ESI) available: Stern–
+
Volmer plots of DCA and NMQ BF
ꢀ
fluorescence by 1a–d, time-
4
resolved absorption spectra of 1a, growth of An
concentrations of 1c. See DOI: 10.1039/b711541e
2
CH at different
Scheme 1
2
00 | Phys. Chem. Chem. Phys., 2008, 10, 200–210
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Scheme 4
methoxbenzyl radical and a cation. The radical reacts with O
2
to give 4-methoxybenzaldehyde, while the cation can form
0
bis(4-methoxybenzyl)ether
methane (Scheme 3).
and
4,4 -dimethoxydiphenyl-
This research deals with the oxidation of 4-methoxybenzyl
alcohol derivatives (see Scheme 4, where the molecular struc-
tures are reported together with those of the corresponding
products) photosensitized by two well known electron accep-
Scheme 2
+
ꢀ
tors, N-methylquinolinium tetrafluoroborate (NMQ BF
4
)
able to catalyze the intermediate 4-methoxybenzyl cation
involved in the product formation (Scheme 2).
The driving force of this process seems to be the higher
stability of the 4-methoxybenzyl cation compared with that
of the benzyl cations formed from electron-poor benzyl
alcohols.
and 9,10-dicyanoanthracene (DCA), that are able to form
the radical cations of the substrates. The study was carried
out by steady state, intermediate investigations (with nanose-
cond laser flash photolysis) and quantum mechanical calcula-
tions. The comparison of the results obtained with the two
sensitizers in terms of product formation and intermediates
involved in the mechanism suggests that the different behavior
The hypothesis of a Lewis acid action of the excited
+
ꢀ
TPP on 4-methoxybenzyl alcohol seems to be in disagree-
ꢀ
is due to specific electrostatic interactions between the BF4
+
ment with the kinetics and product studies of the TPP BF
4
anion and the radical cation.
photosensitized oxidation of ring-methoxylated benzyl
8
alcohols in air-saturated CH
2 2
Cl . In particular, for the reac-
tion of 4-methoxybenzyl alcohol, the oxidation product
(
(
4-methoxybenzaldehyde) was detected in addition to bis-
0
Results
4-methoxybenzyl)ether
and
4,4 -dimethoxy-diphenyl-
Fluorescence quenching
methane. The product distribution was strongly dependent
on the experimental conditions (irradiation time, presence
of bases). The results of the product analysis, intermediate
investigation by laser flash photolysis and DFT calculations
of the relative stability of ring-methoxylated benzyl
cations have been used to formulate the mechanism shown
in Scheme 3, where the substrate reacts with the singlet excited
All the alcohols examined (1a–d) efficiently quenched the
+
fluorescence emission of NMQ in accordance with the
0
Stern–Volmer eqn (1) where I and I are the emission inten-
sities in the absence and in the presence of increasing concen-
trations of alcohol ([Q]), respectively. The quenching rate
constants (k ) calculated from the slopes (Stern–Volmer con-
q
+
ꢂ
state of TPP to give the couple radical cation/TPP .
In CH Cl , the neutral substrate is thought to be the only
stants KSV) of the linear plots (see ESI, Fig. S1)w divided by
3.3 ns (the lifetime of NMQ * measured in air-equilibrated
1
+
2
2
base present in the system that is able to induce a-C–H
deprotonation of the radical cation, to form an a-hydroxy-4-
CH Cl ) are reported in Table 1. The high k values, close to
2
2
q
the diffusional limit, indicate that the process is diffusion-
Scheme 3
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+
Table 1 Quenching rate constants (k
q
) of NMQ and DCA fluores-
Cl
irradiation at 410 ꢅ 20 nm in air-equilibrated condition gave
cence by alcohols 1a–d measured in air-equilibrated CH
2
2
the corresponding oxidation product, that is, 4-methoxyben-
0
9
/10 M
ꢀ1 ꢀ1
zaldehyde (2a), 4-methoxyacetophenone (2b) and 4,4 -di-
k
q
s
Alcohol
methoxybenzophenone (2c), respectively. As shown in
+
NMQ
DCA
Table 2, the product yields were remarkable in CH CN, but
3
1
1
1
1
a
b
c
12
13
13
30
6.4
4.1
9.6
7
very low in CH Cl (1a was unreactive), despite longer irradia-
2
2
tion times (up to four times longer) in this latter solvent. Only
traces of 2b (2%) were detected in the case of alcohol 1d in
d
3 2 2
CH CN, while no products were detected in CH Cl .
Irradiation at 310 ꢅ 20 nm of an air-equilibrated solution
controlled.
+
containing NMQ BF
ꢀ
ꢀ3
4
(1.0 ꢄ 10 M), 1a, 1b and 1c (2.0 ꢄ
ꢀ2
1
0
M) produced, together with the expected carbonyl com-
I0=I ¼ 1 þ KSV½Qꢃ
ð1Þ
pounds (2a, 2b and 2c, respectively), as well as the correspond-
ing symmetrical ethers: bis(4-methoxybenzyl) ether (3a), bis-
Analogous fluorescence-quenching experiments were per-
formed in air-saturated CH Cl to evaluate the efficiency of
0
2
2
[1-(4-methoxyphenyl)ethyl] ether (3b) and bis[4,4 -dimethoxy-
alcohols 1a–d to quench the lowest excited singlet state of
DCA. The k values, listed in Table 1, were calculated from
the linear Stern–Volmer plots (see ESI, Fig. S2),w taking
diphenylmethyl] ether (3c), respectively. High product yields
were generally obtained with 1a–c, as shown in Table 3,
especially in CH Cl , where comparable amounts of products
q
2
2
1
1
2.3 ns as the lifetime of DCA* measured in air-equilibrated
9
. In the case of DCA, the k
were obtained with an irradiation time about three times
CH
2
Cl
2
q
values are significantly
shorter than in CH CN. Experiments carried out under these
3
+
smaller than those determined with NMQ , but are still quite
close to the diffusion-controlled rate. In both cases, efficient
quenching presumably occurs via an electron-transfer process,
in agreement with the laser flash photolysis experiments (see
experimental conditions at different irradiation times showed
that the ether formed was not stable. In fact, separate experi-
ments evidenced that, when the ethers were photolyzed under
the same experimental conditions as those used for alcohols,
they reacted to give the initial alcohol and the carbonyl
product.
+
below). In fact, DCA is less efficient than NMQ in promot-
ing the formation of radical ions from alcohols, because its
1
CN) is less than
0
+
ꢀ
reduction potential (1.91 V vs. SCE in CH
1 +
that of NMQ * (2.7 V vs. SCE in CH
3
The reaction of alcohol 1d with excited NMQ BF pro-
4
11
3
CN).
duced 4-methoxyphenyl-a-methylstyrene (4e) as primary pro-
duct (Table 3), together with an isomer mixture characterized
by 2,4-bis(4-methoxyphenyl)-4-methyl-1-pentene (4f) and 2,4-
bis(4-methoxyphenyl)-4-methyl-2-pentene (4g), in high yield in
Steady-state photolysis
+
Steady-state experiments sensitized by DCA and NMQ were
performed in air-equilibrated CH Cl and CH CN, as re-
2 2 3
CH Cl , but only in traces in CH CN. Experiments performed
2
2
3
ported in the Experimental section. No products were detected
in the absence of irradiation or sensitizer. After workup, the
at different irradiation times indicated that these isomers were
formed by the reaction of 4e; in fact, with short irradiation
times, 4e was the only product detected, while with prolonged
irradiation times, the formation of 4e slowed down, thus
favoring the increase of the isomer mixture. As observed for
1
13
products were identified by H-NMR, C-NMR and GC-MS
by comparison with authentic specimens. Quantitative analy-
1
sis was performed by GC and H-NMR by using the internal
standard method. The material balance was very satisfactory
the other alcohols, the total yield of 4e was higher in CH
(32% after 30 min) than in CH CN (28% after 105 min).
The photoproduct quantum yields were determined for the
2 2
Cl
(490%) in all experiments. The regeneration of the sensitizers
was checked by UV-visible analysis. All the product structures
3
+
ꢀ
are reported in Scheme 4.
reactions of alcohols 1b, 1c and 1d with NMQ BF
radiated around 313 nm in air-equilibrated CH Cl . From
4
ir-
Photolysis of alcohols 1a, 1b and 1c (2.0 ꢄ 10ꢀ M) in the
2
2
2
presence of catalytic amounts of DCA (3.0 ꢄ 10ꢀ M) by
4
the data reported in Table 4, it can be observed that the ketone
quantum yields were very high and independent of the irradia-
tion time, while the ether quantum yields were not constant
over time and reached values higher than unity, especially in
the case of 1c. The result with 1d is much clearer, in fact the
quantum yield for the formation of 4e was 0.64 and indepen-
dent of the irradiation time.
Table 2 Chemical yields (%) of the photoproducts formed in the
DCA-photosensitized reaction of alcohols 1a–d in air-equilibrated
solution
a
Irradiation
Yield/%
Alcohol
time/min
Solvent
Unreacted
alcohol
AnCOR
Laser flash photolysis experiments
1
1
1
1
a
a
b
c
420
CH
CH
CH
CH
CH
CH
CH
CH
2
3
2
3
2
3
2
3
Cl
CN
Cl
CN
Cl
CN
Cl
CN
2
98
85
98
—
96
29
100
98
—
15
1
91
3
70
—
2
1
05
420
05
330
05
210
05
+
4ꢀ
In the cases of the alcohol/NMQ BF systems, only the laser
flash photolysis results in CH Cl are reported because much
smaller transient signals were obtained in CH CN; this is in
2
1
2
2
2
3
1
line with the efficiencies observed in the steady state experi-
ments. All experiments showed an electron transfer from the
d
2
1
+
alcohol to the NMQ BF excited state, confirmed by the
ꢀ
DCA] = 3.0 ꢄ 1 0ꢀ4 M, [alcohol] = 2.0 ꢄ 10ꢀ2 M.
4
[
detection of the radical cation–radical couple, as shown below.
2
02 | Phys. Chem. Chem. Phys., 2008, 10, 200–210
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+
Table 3 Chemical yields (%) of the photoproducts formed in the NMQ BF photosensitized reaction of alcohols 1a–d (AnCRR OH) in an
4ꢀ
1
a
air-equilibrated solvent
Yield/%
1
(AnCRR )
Alcohol
Solvent
Unreacted alcohol
AnCOR
2
O
3 2
AnC(CH )Q CH
b
1
1
1
1
a
b
c
CH
CH
CH
CH
CH
CH
CH
CH
2
3
2
3
2
3
2
3
Cl
CN
2
66
73
21
30
41
37
52
69
ꢀ2
18
25
11
30
12
43
14
c
b
Cl
CN
2
65
30
40
13
c
b
Cl
CN
2
c
b
d
d
Cl
CN
2
14
28
c
d
a
+
4ꢀ
ꢀ3
b
c
d
[
NMQ BF ] = 1.0 ꢄ 10 M; [alcohol] = 2.0 ꢄ 10 M. Irradiation time = 30 min. Irradiation time = 105 min. A mixture of
2 2 3
substituted p-methoxystyrenes was also recovered with 1d, 18% in CH Cl and only traces in CH CN (see Results section).
In order to increase the yield of separate radical cations, 1 M toluene
was used as a co-sensitizer. Under these conditions, the sequence of
reactions (2) and (3) takes place (A = alcohol, T = toluene).
tion bands at 290, 440 and 980 nm. The last one is typical of
these systems and is ascribed to an intramolecular charge
resonance interaction. In our case, the maximum at 290 nm
is out of the detectable wavelength region, while the NIR
absorption band was completely missing (Fig. 2). This spectral
behavior is explained in the Discussion section. Moreover, the
þꢆ
ꢂ
þꢂ
NMQ þ T ! NMQ þ T
ð2Þ
ð3Þ
þꢂ
þ A ! T þ Aþꢂ
T
absorption at 350 nm is made up of two components, a short-
ꢂ
Upon laser excitation (l = 308 and/or 355 nm) of CH Cl
lived one, probably due to the ketyl radical An
2
COH , that
exc
2
2
+
solutions of NMQ BF /toluene/1a–d, time resolved absorp-
ꢀ
6
reacts quickly with molecular oxygen, and a longer-lived one
coupled with the absorption at 420 nm. The kinetics analysis
gave a lifetime of 6.6 ms for the long component, which is very
close to the 6.8 ms value obtained at 420 nm. The rate constant
4
tion spectra and decay kinetics were recorded in the presence
of dioxygen. By laser photolysis of 1a (Fig. S3 in the ESI)w and
1
b (Fig. 1) in air-equilibrated solutions, two absorption bands
5
ꢀ1
s
were detected just after the laser pulse in the 425 and 530 nm
5
regions assigned to the radical cations (lmax = 445 nm) and
calculated by first order fitting at 420 nm is 1.5 ꢄ 10
(Table 5). The time-evolution of the absorption spectra shows
ꢂ
12
NMQ (lmax = 540 nm), respectively. The time-evolution of
the absorption spectra shows that the decay kinetics recorded
at the two maxima are different, thus proving that the two
transients follow different reactive paths. In particular, the
that the decay at 550 nm is well fitted by a first-order law (k =
6
ꢀ1
7.7 ꢄ 10 s ); it is faster than that obtained for 1a and 1b (k =
6
ꢀ1
3.1 ꢄ 10
s
) because of the higher O
2
concentration. A
narrow absorption band detected at 510 nm that builds up in
+
ꢂ
+
ꢂ
kinetics analysis of 1a
5
and 1b
gives first order rate
ꢀ1
the same amount time (t = 7.0 ms) as the absorptions at 350
5
+
constants of 4.2 ꢄ 10 and 4.6 ꢄ 10 s , respectively. The
2
and 420 nm was assigned to the cation An CH on the basis
ꢂ
13
radical NMQ also shows a first-order kinetics, due to its
of literature data. This assignment is in agreement with the
absence of O effects on the decay kinetics recorded at 510 nm
(lifetimes of 49 and 50 ms were recorded in air-equilibrated and
-saturated solutions, respectively).
reaction with O
6
2
ꢀ1
(see Discussion session), with a rate constant
2
of 3.1 ꢄ 10 s in both experiments.
The time-resolved absorption spectra of 1c in O
2
-saturated
N
2
solution appear to be much more complex, due to the presence
of three bands centered at 350, 420 and 550 nm just after the
laser pulse (Fig. 2). The absorption at 550 nm was again
ꢂ
ascribed to NMQ , while the one at 420 nm can be reasonably
+
assigned to 1c , the partner of NMQ in the electron transfer
ꢂ
ꢂ
+
reaction. Actually, the absorption spectrum of 1c generated
ꢂ
6
by pulse radiolysis in aqueous solution shows three absorp-
Table 4 Quantum yields (F) of the photoproducts formed in the
+
NMQ BF
in air-equilibrated CH Cl
2 2
ꢀ
1
photosensitized reaction of alcohols 1b–d (AnCRR OH)
4
a
Irradiation
Alcohol time/min
F
1
AnCOR (AnCRR )
2
O
3 2
AnC(CH )QCH
1
1
b
c
d
601
60
608
2314
1800
0.42
0.44
0.65
0.67
0.16
1.3
1.2
3.8
8
+
Fig. 1 Time-resolved absorption spectra of the NMQ BF (2.6 ꢄ
4ꢀ
ꢀ3
ꢀ2
1
0
M)/toluene (1 M)/1b (1.0 ꢄ 10 M) system in air-equilibrated
1
0.64
2 2
CH Cl recorded 0.08 (n), 1.2 (m) and 6.2 (J) ms after the laser pulse.
a
NMQ+BF4ꢀ] = 3.9 ꢄ 10ꢀ4 M; [alcohol] = 1.0 ꢄ 10ꢀ2 M.
l
exc = 355 nm. Inset: decay kinetics recorded at 430 (A) and
[
5
40 (B) nm.
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+
Fig. 2 Time-resolved absorption spectra of the NMQ BF (2.0 ꢄ
4ꢀ
+
Fig. 3 Time-resolved absorption spectra of the NMQ BF4 (2.5 ꢄ
ꢀ
ꢀ4
ꢀ3
ꢀ3 ꢀ2
10
M)/toluene (1 M)/1c (9.0 ꢄ 10 M) system in O
2
-saturated
10 M)/toluene (1 M)/1d (1.0 ꢄ 10 M) system in air-equilibrated
CH Cl recorded 0.08 (n), 0.90 (m), 3.5 (J) and 6.4 (K) ms after the
2 2
CH Cl recorded 0.08 (n), 0.13 (m), 1.5 (J), 3.5 (K) and 6.4 (&) ms
2
2
after the laser pulse. lecc = 308 nm. Inset: decay kinetics recorded at
laser pulse. lexc = 355 nm. Inset: decay kinetics recorded at 370 (A),
440 (B) and 540 (C) nm.
350 (A), 510 (B) and 550 (C) nm.
The time-resolved absorption spectra of 1d in air-equili-
measured with respect to the biphenyl radical cation (e =
ꢀ1
ꢀ1
15
brated CH
2
Cl
2
recorded by flash photolysis show the forma-
12
14 000 M cm at lmax = 670 nm) in an independent
experiment. In contrast, the An CH quantum yield, ob-
ꢂ
+
ꢂ
+
tion of both NMQ (lmax = 540 nm) and 1d
(lmax
40 nm) within the laser pulse (Fig. 3). Their absorption is
=
2
5
ꢀ1 ꢀ1
4
tained by using a value of 100 000 M cm for the extinction
1
coefficient of the cation, is quite low (0.034). Finally, con-
3
further replaced by the build up of the intense absorption
+
signal at 370 nm ascribed to the cation AnC (CH
3
)
2
by
cerning the experiments with 1d, a quantum yield of 0.69 was
ꢂ
1
4
comparison with its already reported spectrum. The nature
of this transient is confirmed by the fact that molecular oxygen
had little influence on its lifetime; in fact, t values of 34 and
obtained for NMQ ; this was close to the value of 0.62
+
obtained for the cation AnC (CH ) by using a value of
3 2
ꢀ
1
ꢀ1
11 000 M
1
cation.
cm for the extinction coefficient of cumyl
6
4
0 ms were measured in air-equilibrated and N -saturated
2
solution, respectively. In addition, it reacts quickly with water
ꢀ1 ꢀ1
The results of the steady-state photooxidation of alcohols
1a–d with DCA are in line with those reported for the well
known oxidation via photoinduced electron transfer of benzyl
6
by a second-order kinetics (k
2
= 5.0 ꢄ 10
M
s ). By
ꢂ
analyzing the kinetics, it is clear that NMQ decays with a
6
ꢀ1 ꢀ1
1
first-order kinetics (k = 3.1 ꢄ 10 M
s
); the same rate
alcohol derivatives. In particular, substrates 1a–c, whose
constant value was found for 1a and 1b (this was expected
radical cations are able to deprotonate, form the respective
carbonyl compounds, while compound 1d, whose radical
cation cannot deprotonate, is practically unreactive. In order
to gain a greater insight into the photooxidation mechanism, a
detailed time-resolved investigation of substrate 1c was per-
formed. Compounds 1a and 1b were not studied in detail
because the cations generated by their radical cations absorb
because the experiments were performed with the same
+
ꢂ
amount of O
2
). For 1d
the change of absorbance at
4
1
1
40 nm follows a second order kinetics (k /e = 8.9 ꢄ
2
6
0
ꢀ1
s
cm, Table 5), which is different from the cases of
+
+
a
ꢂ
ꢂ
+
and 1c where a first order decay was found.
ꢂ
, 1b
+
ꢂ
is the
Furthermore, it is reasonable to suppose that 1d
1
7
precursor of the cation (lmax = 370 nm), even if the build up
kinetics was prevented by its fast reaction.
below 300 nm and could not be detected with our experi-
mental setup.
Transient quantum yields were measured by flash photolysis
experiments performed on 1c and 1d, according to the proce-
dure reported in the Experimental section. In the case of 1c,
The flash photolysis experiments were carried our in air-
equilibrated CH CN in the presence of 0.2 M biphenyl (BP) as
3
a co-sensitizer; under these conditions the formation efficien-
15
cies of radical ions (eqns (4) and (5)) are higher.
ꢂ
the quantum yield of NMQ is 0.60, with a value of 3800
ꢀ
1
ꢀ1
M
cm for the extinction coefficient in CH
2 2
Cl at 550 nm,
DCA þ BP ! DCA þ BPþꢂ
ꢆ
ꢀꢂ
ð4Þ
ð5Þ
Table 5 Decay rate constants for fragmentation of alcohol radical
1
cations (AnCRR OH ) generated in the sensitized NMQ BF
+
ꢂ
+
ꢀ
4
DCA þ A ! BP þ Aþꢂ
þꢂ
photolysis in air-equilibrated CH
Radical cation
2 2
Cl
The time-resolved absorption spectra were recorded in the
30–900 nm range. The BP radical cation was completely
replaced, within the laser pulse, by two bands at lmax
50 nm and in the NIR region (lmax 4 900 nm), that match
6
5 ꢀ1
k/10 s
3
+
+
ꢂ
ꢂ
ꢂ
1
1
1
1
a
a
b
c
4.2
4.6
1.5
=
4
+
+
ꢂ
+
ꢂ
a
perfectly the 1c spectrum reported in the literature (Fig. 4).
d
89
The decay kinetics recorded at 450 and 900 nm were well-fitted
Ratio of second order rate constant and extinction coefficient (k
2
/e)
by a first-order law with the same rate constant (k = 5.0 ꢄ
ꢀ
1
in s cm.
5
0 s ), thus supporting the assignment of the two bands to
ꢀ1
1
2
04 | Phys. Chem. Chem. Phys., 2008, 10, 200–210
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+
Fig. 6 Numbering of 1d atoms.
ꢂ
+
calculations at a DFT level were carried out on 1d (Fig. 6)
ꢂ
by using the B3LYP/6-311G(d) model both in the absence and
ꢀ
presence of the BF
4
anion. The same theoretical model was
also used to calculate the spin density and partial-charge
+
ꢀ
4
ꢂ
Fig. 4 Time-resolved absorption spectra of the DCA (2.0 ꢄ 10 M)/
distributions on the free and complexed radical cation 1d
;
ꢀ2
biphenyl (0.2 M)/1c (1.0 ꢄ 10 M) system in air-equilibrated CH
3
CN
the results are summarized in Fig. 7.
+
While the effect of the association between 1d and BF
also in this case the more stable complex was predicted to
ꢂ
4ꢀ
recorded 0.32 (n), 1.9 (m), 6.3 (J) ms after the laser pulse. lexc
=
355 nm. Inset: decay kinetics recorded at 450 nm.
(
have a hydrogen bond between the hydroxyl hydrogen and a
fluorine atom) on the charge distribution is practically negli-
gible (Fig. 7b), the effect on the spin density is much more
evident. In fact, a significant increase in spin density is
predicted on the oxygen atom (number 12 of Fig. 6) which
reaches a value of about 0.3 (compared with the value of about
+
ꢂ
ꢀ
ꢂ
the same transient. The 1c
1
partner, DCA (lmax = 500,
5
5
80, 645 and 705 nm), was quenched at about 140 ns by a
very fast reaction with molecular oxygen.
Quantum mechanical calculations were performed at a
semi-empirical level to investigate the effect of the interactions
+
between 1c and BF
ꢂ
ꢀ
4
on the UV-Vis absorption spectrum.
1
calculated by the same theoretical model for the spin density
+
ꢂ
The absorption spectra of the radical cation 1c and the
ꢀ
complex (1c /BF ) calculated by INDO/1-CI, after geome-
4
ꢂ
on the oxygen atom of OH ). Thus, a large part of the spin
density of the complex is located on the CꢀO bond which
therefore becomes more reactive than that in the radical cation
+
ꢂ
trical optimization by PM3, are shown in Fig. 5. It was found
that the most stable structure was due to a hydrogen bond
ꢀ
(
see Discussion).
between a fluorine atom of BF and the hydroxyl group of
4
+
ꢂ
1
c
, where the main positive charge is predicted to be
+
localized. These calculations predict that the 1c /BF
ꢂ
ꢀ
4
inter-
Discussion
action shifts the absorption band from 400 to about 350 nm, in
Reaction with DCA
agreement with the absorption spectra obtained by laser flash
+
photolysis of the DCA/1c and NMQ BF /1c systems.
ꢀ
The occurrence of an electron transfer as the primary step was
confirmed by laser flash photolysis experiments that clearly
showed the formation of the alcohol radical cations (Fig. 4).
The high quenching rate constant values (Table 1) determined
4
In order to obtain more detailed information on the effect of
ꢀ
BF
4
on the reactivity of the radical cations here investigated,
1
0
with DCA are close to the diffusion limit in CH
2
Cl
which is in line with an efficient electron transfer
process. This is confirmed by the negative free energy change
2
(1.5 ꢄ10
ꢀ
1
ꢀ1 18
M
s )
1
9
(ca. ꢀ0.4 eV) calculated by the Rhem–Weller’s eqn (6) where
0
1
red* is the reduction potential of DCA* and Eox is the
0
E
oxidation potential of the benzyl alcohols 1a–d (1.52–1.59 V
2
vs. SCE).
0
0
ox
0
red
DGet ¼ E ꢀ E
ꢆ
ꢀ 0:06 eV
ð6Þ
There is no relationship between the quenching rate constant
and the potential values; in fact, the oxidation potentials of the
substrates are very low and close to each other (due to the
presence of the methoxy group on the aromatic ring). One also
has to take into account that the small differences among the
q
k values obtained for each sensitizer also include the experi-
mental errors.
Fig. 5 UV-Vis absorption spectra of the free (a) and complexed with
4ꢀ
+
ꢂ
3
The product analysis showed higher reactivity in CH CN
BF (b) radical cation 1c calculated by the INDO/1-CI semiempi-
rical method after optimization with the PM3 Hamiltonian together
with the respective oscillator strengths (bars). The singly excited
configurations were built by using the 15 highest occupied MOs and
the 15 lowest virtual MOs.
2 2
with respect to the less polar CH Cl , which is in agreement
with a reduced Coulombic barrier for the separation of the
initially formed intermediates, the radical anion/radical cation
1
pairs, in the more polar solvent. In fact, irradiation of DCA
2
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Phys. Chem. Chem. Phys., 2008, 10, 200–210 | 205

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is slower which makes the electron transfer process more
2
1
+
ꢀ
efficient. These properties of NMQ BF
reactivity shown with this sensitizer in CH Cl with respect to
4
explain the higher
2
2
CH CN (Table 2). As with DCA, smaller amount of
3
+
NMQ BF was used with respect to the substrate since it
ꢀ
4
+
is known that molecular oxygen can generate NMQ by
ꢂ
oxidation of NMQ even if the reduction potential of
+
NMQ is ꢀ0.85 V (vs. SCE) because the electrostatic factor
(
2 2
0.24 eV in CH Cl ) is responsible for the slightly exoergonic
1
9
free energy change.
Regarding the steady-state experiments, considerable yields
ꢀ
+
of dibenzyl ethers were obtained by using NMQ BF
4
(also
4ꢀ
as
+
obtained for 1a and 1c in the presence of TPP BF
8
sensitizer), together with the carbonyl compounds from alco-
hols 1a–c, while only styrene was formed in the reaction of 1d
+
sensitized by NMQ (Table 3). The advantage of using
+
NMQ BF instead of TPP BF in photoinduced electron
ꢀ
+
ꢀ
4
4
Fig. 7 Spin-density (a) and partial-charge (b) distributions of the
4ꢀ
transfer processes is related to its ground state absorption
ꢀ
+
radical cation 1d , free (open symbols) and associated with BF
full symbols) calculated by B3LYP/6-311G(d) after optimization with
ꢂ
+
spectrum. In fact, while the bleaching of the NMQ BF
4
(
ground state does not significantly affect the time-resolved
absorption spectra above 330 nm (molar absorption coefficient
the same method.
ꢀ
1
ꢀ1
+
smaller than 5000 M cm ), in the presence of TPP BF a
ꢀ
4
in CH Cl in the presence of 1a–d leaves the alcohol almost
2
2
large wavelength region (ca. 330–450 nm, where the molar
4
3
unreacted, while in CH CN, there was considerable conver-
ꢀ1
absorption coefficient reaches also values of ca. 3.3 ꢄ 10 M
sion of 1a–c into the corresponding carbonyl products. Only
alcohol 1d was practically unreactive in the latter solvent.
From these experimental results, the mechanism already pro-
ven for analogous alcohols (Scheme 1), but under different
experimental conditions, seems to be operative. The radical
cation, produced by electron transfer from the alcohol to the
singlet excited state of DCA, deprotonates thus forming a
benzyl radical, that probably reacts with dioxygen (path a) to
give the final carbonyl product. Path b should not be thermo-
ꢀ
1
cm ) is hidden by the ground state bleaching of the sensitizer.
Therefore it is difficult to detect transients and analyze their
decay kinetics in this spectral region.
The experimental evidence suggests that the particular
+
reactivity of the NMQ -sensitized photolysis of 1a–d is
ꢀ
probably due to the involvement of the BF
4
anion which
can associate with the radical cation of the alcohols. This kind
of interaction has already been proposed to take place between
ꢀ
ꢀ
aromatic cation radicals and anions such as PF
27
6
, BF
4
and
0
ꢂ
dynamically favored because DCA (E
= ꢀ0.98 V vs.
DCA/DCA
ꢀ
ClO
4
in disproportionation equilibria.
1
SCE) should not be able to oxidize the radical AnC ROH
0
ꢂ
One of the main proofs here proposed in agreement with the
0
E
22,23
(
ox = ꢀ0.2 C ꢀ0.3 V vs. SCE).
The sensitizer is
ꢀ
1
formation of a (AnRR OH /BF4 ) complex is the time-
+
ꢂ
ꢀ
ꢂ
2
re-established by the fast reaction of DCA with O , thus
resolved absorption spectra recorded by irradiation of
+ 28
NMQ in the presence of 1c in aerated CH Cl (Fig. 2).
ꢀ
ꢂ
forming O
slightly more negative than that of O
2
, owing to the reduction potential of DCA being
0
2
2
2
(EO /O ꢀ
ꢂ
= ꢀ0.87 V vs.
SCE). The low yield of 4-methoxybenzophenone formed by
d cannot be justified by the mechanism proposed for 1a–c,
ꢂ
2
2
The species formed within the laser pulse together with NMQ
2
4
and absorbing at 350 and 420 nm was identified as the complex
ꢀ
). This assignment is confirmed by the semiempi-
1
+
ꢂ
(
1c /BF
4
because the benzyl hydrogens are missing in this compound;
anyway, no further investigation was carried on this mechan-
ism.
rical INDO/1-CI calculations which can reproduce the spec-
+
tral changes due to the (1c /BF ) interaction. This
ꢂ
ꢀ
4
interaction prevents the intramolecular charge resonance in-
teractions between the neutral donor and charged acceptor
rings of the radical cation, as confirmed by the lack of the NIR
ꢀ
ꢂ
Despite the presence of O
+ 25
ꢂ
that could react with the
2
radical cation 1c
+
,
the first-order decay kinetics obtained
5
ꢂ
ꢀ1 26
) suggests that the deprotonation
for 1c (k = 5.0 ꢄ 10 s
is mainly due to interactions with the solvent.
ꢀ
absorption in the presence of BF4 . The presence of a charge
transfer complex is also in agreement with the solvent effect on
reactivity. In fact, the higher reactivity noted in CH Cl with
+
Reaction with NMQ BF
4ꢀ
2
2
respect to that obtained in CH CN can be justified on the basis
3
ꢂ
The sensitizer N-methylquinolinium is more efficient than
of a better separation of the radical cation/NMQ pairs
DCA in promoting an electron transfer reaction; in fact, in
formed in the ET process in a less polar solvent. A further
contribution could be due to the higher stability of the
1
+
agreement with the higher reduction potential of NMQ *,
the quenching rate constants are slightly higher in the case of
NMQ. The electron transfer from the substrate to singlet
excited state of quinolinium produces a radical cation/neutral
radical pair whose separation is facilitated by the lack of any
electrostatic barrier, even in moderately polar solvents. In
solvents of low-polarity, the return electron transfer reaction
complex in CH
solvent.
2
Cl
2
, since the ions were less solvated in this
+
The intense absorption of the ground state of NMQ below
330 nm prevented the investigation of this spectral region; in
ꢀ
any case, the formation of the complex radical cation/BF4 is
also extended in bona fides to alcohols 1a, 1b and 1d.
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2
06 | Phys. Chem. Chem. Phys., 2008, 10, 200–210

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3
70 nm, Fig. 3) supports such a hypothesis. Furthermore, the
efficiency of the formation of 4-methoxycumil cation was high
(
0.62), close to the quantum yield of the corresponding radical
3
cation (0.69) and in line with the high stability of the cation.
1
This cation, in contrast with the others, loses one proton thus
forming the corresponding styrene (Scheme 5, path d). When
styrene is cumulated, its double bond is attacked by the
cationic intermediate and gives the mixture of substituted
p-methoxystyrenes (4f and 4g) reported in Table 3. The driving
force of path c seems to be the stability of the substituted
olefin; in fact, alcohol 1b that could lose the proton of the b
carbon, does not form a detectable amount of styrene.
ꢀ
ꢂ
Two things suggest the involvement of O
2
in the forma-
tion of the cation by Scheme 5, path b: (i) the very high
reactivity of the hydroxyl radical that makes the monomole-
cular process of cation formation too energy-expensive and (ii)
+
ꢂ
the second-order decay of 1d which suggests a reaction with
Scheme 5
ꢀ
ꢂ
a species at a similar concentration, like O
2
. A reaction
ꢀ
ꢂ
, which could favor the formation of
ꢂ
between OH and O
2
3
2
On the basis of experimental and theoretical evidence
collected in this work, the mechanism reported in Scheme 5
+
the cation, has already been reported in the literature. The
+
higher reactivity of the complex (1d /BF ) in comparison
ꢂ
ꢀ
4
+
ꢂ
is suggested for the NMQ photosensitized oxidation of
with the ‘‘free’’ 1d radical cation is in line with the increased
spin density in the O atom which reaches a value of 30% in the
complex.
alcohols 1a–d. With alcohols 1a, 1b and 1c, the complex
ꢀ
radical cation/BF
4
could react according to two paths.
Firstly, according to path a (analogous to path a in Scheme
already noted in the DCA-photosensitized oxidation) the
The quantum yield of ether formation, at least for 1b and 1c
(i) is much larger than that of the cation, (ii) depends on the
irradiation time and (iii) becomes larger than unity (Table 4).
The findings suggest the involvement of a thermal chain
pathway in the formation of the final products. Scheme 6
depicts a probable reaction pathway where the cation interacts
with the neutral alcohol, bonded by a hydrogen bond with
another alcohol molecule (a), and releases the ether and a
protonated alcohol molecule (b). The cation can then be re-
generated by the loss of a water molecule from the protonated
alcohol molecule.
1
radical cation deprotonates to form the benzyl radical that
evolves to the final carbonyl product by reacting with O . The
2
oxidation of the benzyl radical can be excluded because its
oxidation potential is well above the reduction potential of
+
NMQ , as already seen in the case of DCA. Secondly, path b
involves the formation of a cationic intermediate, probably
ꢀ
ꢂ
assisted by O
xyl radical (as indicated from the quantum mechanical calcu-
2
(see below) that reduces the incipient hydro-
2
lations). The cation forms the symmetrical ether by reacting
9
with the neutral alcohol (path c). There was direct evidence for
In contrast, the final product of 1d was 4-methoxystyrene,
with a quantum yield less than one, independent of the
irradiation time. This suggests that for this compound the
deprotonation of the cation, probably assisted by the hydroxyl
anion formed in path b of Scheme 5, is much faster than the
interaction with the neutral alcohol.
the formation of the benzyl cation only with 1c (lmax
=
5
10 nm, Fig. 2). With the other alcohols the absorption of
the cations is hypsochromic, shifted out of the detectable
spectral region, but the formation of symmetrical ethers in
the product mixture suggests the involvement of this inter-
mediate in the reaction.
The results of this investigation show that the previously
8
reported mechanism (Scheme 3), in which the benzyl cations,
Considering the strong acidity of this kind of radical
3
0
cation, path b should be a less important reaction pathway;
this is confirmed by the low quantum yield (0.035) of the
cation formed in the case of 1c; the value is about 6% of the
quantum yield of radical cation (0.60). Instead, the formation
yields of the ether (Table 4) were surprisingly high. The
formed through deprotonation of the radical cations by inter-
action with the neutral alcohols, are precursors of the final
+
ꢂ
+
ꢂ
+
ꢂ
kinetics of 1a , 1b
Table 5), as expected when the deprotonation process is the
main decay path. The close values of the decay rate constants
and 1c
follow first order decays
(
5
ꢀ1
(
of the order of 10 s , as already found for similar radical
cations) can be justified with the formation of an early transi-
tion state of the deprotonation reaction of the alkylaromatic
6
radical cations.
Path a of Scheme 5 is excluded in the case of 1d because no
benzyl hydrogens are present in the substrate; therefore, the
+
formation of AnC (CH ) is the only reactive way. The direct
3 2
evidence of this intermediate in the absorption spectra (lmax
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=
Scheme 6
Phys. Chem. Chem. Phys., 2008, 10, 200–210 | 207

View Article Online
ethers, is not applicable for several reasons: (i) the reactivity of
the radical cation is different when sensitized with different
sensitizers, (ii) the decay rate constant of the radical cation and
the growth of the cation (Fig. S4 of ESI)w are independent of
the ground state concentration of the alcohol and (iii) the
formation of the cation from compound 1d could not be
explained.
d
H
d, Ar), 6.70 (4 H, d, Ar), 5.01 (1 H, d, CH), 4.63 (1 H, m, CH),
(200 MHz, CDCl
3
, Me
4
Si) of 4f: 7.15 (2 H, d, Ar), 7.08 (2 H,
3.70 (6 H, s, OCH ), 2.68 (2 H, s, CH ), 1.12 (6H, s, CH ); 4g:
3
2
3
7.25 (4 H, d, Ar), 6.79 (2 H, d, Ar), 6.75 (2 H, d, Ar), 5.96
(1 H, m, CH), 3.73 (6 H, s, OCH ), 1.55 (3 H, s, CH ), 1.48 (6
3
3
H, s, CH ); d (200 MHz, CDCl ) of 4f: 158.4 157.1, 135.8,
3
C
3
135.3 (ipso, Ar), 127.0, 126.7 and 113.3 and 112.8 (CH, Ar),
1
42.7 (CH styrene), 115.4 (CH
2
styrene), 55.2 e 55.1 (OCH
3
),
3
8.0 (quaternary styrene), 28.8 (CH
3
); 4g: 158.4 157.1, 135.8,
Experimental
135.3 (ipso, Ar), 127.0, 126.7 and 113.3 and 112.8 (CH, Ar),
35.3 (quaternary), 128.2 (CH styrene), 55.1 (OCH ), 29.6
+
1
3
Materials
(
3
quaternary), 31.6 (CH ); m/z (70 eV, EI) of 4f: 296 (M , 3%),
4
-Methoxybenzyl alcohol (1a), 4-methoxy-a-methylbenzyl
175 (3), 149 (100), 133 (3), 121 (10), 109 (7), 91 (5), 77 (3), 65
+
(2), 51 (2); 4g: 296 (M , 93%), 281 (100), 265 (10), 227 (5), 188
0
alcohol (1b), and 4,4 -dimethoxydiphenylmethanol (1c) were
commercial samples (Aldrich). N-Methylquinolinium tetra-
fluoroborate was prepared according to a literature proce-
(7), 173 (67), 158 (17), 135 (17), 121 (18), 91 (8), 77 (7), 65 (3),
51 (1).
3
3
dure. 9,10-Dicyanoanthracene (Kodak) was recrystallized
from pyridine. CH Cl (Carlo Erba, HPLC grade) was pur-
ified by letting it stand with CaH and distilling as needed.
CH CN (Fluka for UV spectroscopy) and toluene (Carlo
2
2
Steady state photolysis
2
3
Photooxidation reactions were carried out in an Applied
Photophysics photoreactor equipped with 6 lamps of 15 W
Erba, RPE) were used as received.
0
+
ꢀ
4
-Methoxy-a,a -dimethylbenzyl alcohol (1d) was prepared
by reacting 4-methoxyacetophenone with Grignard reactant
each (310 ꢅ 20 nm with NMQ BF , 410 ꢅ 20 nm with
4
ꢀ2
DCA). A solution containing the alcohol (2.0 ꢄ 10 M) and
+ ꢀ ꢀ3 ꢀ4
CH MgI, obtained by adding CH I to a Mg suspension in
3
the sensitizer (NMQ BF , 1.0 ꢄ 10 M or DCA, 3.0 ꢄ 10
3
4
anhydrous ethyl ether. After workup, the crude was purified
M) in air-saturated solvent (CH CN or CH Cl ) was ir-
3 2 2
by column chromatography (neutral alumina, eluent n-hex-
radiated in a cylindrical flask provided with a water cooling
jacket. The mixture was analyzed after adding an internal
1
standard (bibenzyl) by GC, GC-MS and H-NMR and all
1
ane/ethyl ether 6 : 4) and identified by H NMR and GC-MS;
34
+
m/z (70 eV, EI): 166 (M , 21%), 151 (100), 135 (7), 121 (4),
1
09 (7), 91 (3), 77 (12), 65 (5), 51 (3).
the products were identified by comparison with authentic
specimens.
Bis(4-methoxy-a-methylbenzyl) ether (3b) was prepared by
reacting alcohol 1b in dimethyl sulfoxide at 175 1C for
1
Product analysis was carried out on a HP 6890-2 gas
chromatograph (capillary column, 30 m), on an HP 6890 gas
chromatograph equipped with a MSD-HP 5973 mass selective
detector and on a Bruker AC 200-NMR spectrometer. The
material balance was always satisfactory (490%). The sensi-
tizer analysis was performed by optical density measurements
on a HP-8451 diode array spectrophotometer. No product was
formed in a blank experiment, carried out in the dark condi-
tion or by irradiating the solutions in the absence of the
sensitizer.
3
5 min. The reaction mixture was chromatographed on a
5
silica gel column (eluent n-hexane/ethyl ether 9 : 1). The pro-
duct was a mixture of two diastereomers (meso and dl isomers)
1
in different amounts. They were identified by H NMR and
GC-MS; d (200 MHz, CDCl , Me Si) of diastereomer A: 7.27
H
3
4
(
4 H, d, Ar), 6.84 (4 H, d, J = 8.7 Hz, Ar), 4.47 (2 H, q, J =
.6 Hz, CH), 3.81 (6 H, s, OCH ), 1.45 (6 H, d, J = 6.4 Hz,
CH ); diastereomer B: 7.27 (4 H, d, Ar), 6.92 (4 H, d, J = 8.7
Hz, Ar), 4.18 (2 H, q, J = 6.6 Hz, CH), 3.84 (6 H, s, OCH ),
.36 (6 H, d, J = 6.4 Hz, CH ). Two distinguishable peaks
6
3
3
3
1
3
2 2
The amount of H O was quantitatively determined by
were obtained by GC-MS analysis of the two diastereomers,
titration with iodide anion. The water solution, obtained from
which corresponded to the same mass spectrum. m/z (70 eV,
+
EI): 286 (M , 2%), 207 (1), 178 (7), 151 (4), 135 (100), 121
the reaction mixture workup, was treated, after dilution, with
ꢀ
an excess of KI and a few drops of AcOH. The amount of I
3
(
13), 105 13), 91 (17), 77 (20), 65 (8), 51 (4).
formed was determined by spectrophotometric analysis (e =
1 38
ꢀ
ꢀ1
Bis(4-methoxybenzyl) ether (3c) was prepared by perform-
2 2
25 000 M cm at lmax = 361 nm). No H O was formed
+
ꢀ
ing the NMQ BF4 photosensitized reaction of alcohol 1c in
a preparative scale. The product was isolated by column
chromatography (silica gel, eluent n-hexane/ethyl ether 9 : 1)
in blank experiments performed in the dark.
1
36
Fluorescence quenching
and identified by H NMR and GC-MS.
-Methoxy-a-methylstyrene (4e) was prepared by reacting
alcohol 1d with HClO in CH CN for 5 min. After workup,
the crude product was isolated by column chromatography
4
Measurements were carried out on a Spex Fluorolog F112AF
spectrofluorometer. Relative emission intensities at 400 nm
4
3
+
ꢀ
(NMQ BF
maximum) were measured by irradiating at 316 nm
4
emission maximum) or 434 nm (DCA emission
1
(
neutral alumina, hexane as eluent) and identified by H NMR
3
7
+
(NMQ BF absorption maximum) or 400 nm (DCA ab-
4ꢀ
and GC-MS. 2,4-Bis(4-methoxyphenyl)-4-methyl-1-pentene
4f) and E-2,4-bis(4-methoxyphenyl)-4-methyl-2-pentene (4g)
(
sorption maximum) a solution containing the sensitizer (1 ꢄ
ꢀ5
were obtained as a mixture by column chromatography (neu-
10 M) with the substrate at different concentrations (from
ꢀ2
ꢀ
3
tral alumina, n-hexane/ethyl ether 9 : 1) in the preparation of
1 13
e. They were identified by H NMR, C-NMR and GC-MS;
2 ꢄ 10 to 1.5 ꢄ 10 M) in air-saturated CH Cl . The error
2
2
4
estimated on the Stern–Volmer constants (KSV) was ꢅ5%.
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2
08 | Phys. Chem. Chem. Phys., 2008, 10, 200–210

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Laser flash photolysis
Conclusions
The photooxydation of the methoxybenzyl alcohols 1a–d,
sensitized by 9,10-dicyanoanthracene (DCA) and N-methyl-
An excitation wavelength of 355 nm (from a Nd:YAG laser,
Continuum, third harmonic, pulse width ca. 7 ns and energy
o3 mJ per pulse) was used in nanosecond flash photolysis
+
quinolinium tetrafluoroborate (NMQ BF
ꢀ
4
) produced the
3
9
radical cation of the alcohol and the corresponding reduced
form of the sensitizer within the laser pulse.
experiments. The transient spectra were obtained by a point-
to-point technique, monitoring the change of absorbance (DA)
after the laser flash at intervals of 5–10 nm over the spectral
range 300–900 nm, averaging at least 10 decays at each
wavelength. The lifetime values (the time at which the initial
signal is reduced to 1/e) are reported for the transients showing
first-order decay kinetics. A 2 ml solution containing the
When the sensitizer was DCA, the back electron transfer
process was operative in CH Cl ; all the substrates were left
2
2
unreacted. Alcohol 1d was unreacted also in CH CN. For 1a–c
3
in CH CN, where the benzyl hydrogen is present, the radical
3
cation was able to deprotonate and then form the correspond-
ing carbonyl compounds. The same behavior was found when
ꢀ
2
+
ꢀ
substrate (1.0 ꢄ 10 M), the sensitizer (NMQ BF
4
, 2.5 ꢄ
+
ꢀ
ꢀ
0
3
ꢀ4
compounds 1a–c were sensitized by NMQ BF
CH Cl and CH CN.
A parallel decay path of the 1a–d radical cations with
4
in both
1
M or DCA, 2.0 ꢄ 10 M) and the cosensitizer (1 M
+
ꢀ
and 0.5 M biphenyl with DCA) was
toluene with NMQ BF
4
2
2
3
flashed in a quartz photolysis cell, while nitrogen or oxygen
was bubbling through them. The experimental error was
ꢅ10%.
+
1
formation of the AnC RR cation was operative in both
+
ꢀ
CH
The cation was a precursor of ethers (1a–c) and styrene (1d)
see Scheme 5, path b).
2
Cl
2
and CH
3
CN when the sensitizer was NMQ BF
4
.
Transient quantum yields (f ) were obtained by using the
Tr
(
relationship (eqn 8) between quantum yields, absorption
coefficient (e), and changes of absorbance (DA), measured at
the corresponding absorption maxima of the transient (Tr)
Acknowledgements
and the reference (ref.), namely triplet benzophenone (f
T
= 1
= 6500 M cm at 520 nm). The experimental error
on fTr was ꢅ15%. All measurements were carried out at
2 ꢅ 2 1C.
ꢀ1
ꢀ1
40
Thanks are due to the Ministero dell’Universita e della Ricerca
`
and e
T
and the University of Perugia (Italy) for financial support.
2
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eTðref:ÞDATr
eTrDAðref:Þ
fTr ¼ fðref:Þ
ð8Þ
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2
10 | Phys. Chem. Chem. Phys., 2008, 10, 200–210
This journal is ꢁc the Owner Societies 2008