Photocatalytic Oxidative C-C Bond Cleavage of the Pyrrole Ring in 3-Methylindole induced by Colloidal CdS Particles

Article abstract of DOI:10.1039/a701555k

Binding of 3-methylindole (3-MI) to the surface of colloidal CdS particles modifies their luminescence Behaviour so that the trapped electron and hole generated upon photoirradiation are scavenged by adsorbed O₂ and 3-MI to yield 2-acetylformanilide and 2-aminoacetopnenone.

Full text of DOI:10.1039/a701555k

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54
J. CHEM. RESEARCH (S), 1998
J. Chem. Research (S),
Photocatalytic Oxidative C–C Bond Cleavage of the Pyrrole
Ring in 3-Methylindole induced by Colloidal CdS Particles†
Anil Kumar,* Sanjay Kumar and D. P. S. Negi
1
998, 54–55†
Department of Chemisry, University of Roorkee, Roorkee - 247 667, U.P., India
Binding of 3-methylindole (3-MI) to the surface of colloidal CdS particles modifies their luminescence behaviour so that the
trapped electron and hole generated upon photoirradiation are scavenged by adsorbed O and 3-MI to yield
-acetylformanilide and 2-aminoacetophenone.
2
2
Recently, intense interest has been aroused in the investiga-
tion of the photochemical and photophysical behaviour of
colloidal semiconductors in regard to understanding their
times of which in the GC chromatogram were 4.29, 5.37 and
6.78 min, respectively. In GC-MS experiments, the fragmen-
ǹ
tation pattern of component I [m/z 135 (64%, M ), 120
1
photocatalytic properties. Many of these investigations have
(100), 92 (69), 65 (60) and 44 (27)] was identical with that of
focused on colloidal CdS particles because of their stability in
various homogeneous and heterogeneous media and their
an authentic sample of 2-aminoacetophenone 6, while that of
ǹ
component III [m/z 163 (22%, M ), 148 (9), 135 (55), 120
1
absorption which extends to the visible region. The high
(100), 92 (50), 65 (53) and 43 (53)] matched the earlier
13
specific surface of these particles has been exploited in order
reported fragmentation pattern of 2-acetylformanilide 5.
2
–6
to modify their photophysics by binding different additives
Component II was identified as being the unreacted 3-MI.
The amount of 6 was quantified by GC (using an authentic
sample for calibration purposes) and was found to form with
a quantum efficiency of 0.07 after 5 min of irradiation. Thus
CdS-induced oxidation of 3-MI results in the formation of
products due to cleavage of its pyrrole ring. At low [3-MI]
and to enhance the reactivity of the photogenerated charge
6
–12
carriers for performing useful synthetic transformations.
Moreover, unlike bulk semiconductors, nanosized particles
can be utilized to carry out simultaneous oxidation and
reduction. In the present investigation, colloidal CdS-sensi-
tized oxidation of 3-MI was examined in the presence of air.
Interestingly, the reaction occurred through cleavage of the
pyrrole ring C–C bond.
(ꢁ2 m ), the amount of product formed was much less and
M
anodic dissolution of the particles occurred efficiently. No
product formation was detected in the absence of photo-
catalyst.
The electronic spectrum of the reaction mixture containing
colloidal CdS and 3-MI neither depicted any new peak in the
entire recorded wavelength region (200–600 nm) nor caused
any change in the absorption of CdS in the visible region.
This suggested the absence of any chemical interaction
between CdS and 3-MI. 3-MI is, however, adsorbed onto the
surface of the CdS clusters. The amount of adsorbed 3-MI
was computed by subtracting the absorbance of the reaction
mixture from the sum of the absorbances due to blank CdS
and 3-MI at various wavelengths at which 3-MI depicted
absorption. The observed adsorption isotherm is shown in
Fig. 1. At low [3-MI], the adsorption data followed the Lang-
muir isotherm (inset Fig. 1) from which the intensity of its
To illustrate the mechanism of this reaction, the lumines-
cence behaviour of colloidal CdS in the absence and presence
of 3-MI was monitored (Fig. 2). In the presence of 3-MI, the
luminescence spectrum of CdS showed a new band in the
green region at 540 nm along with a simultaneous small
quenching of red emission. The band at 540 nm is different to
14
the fluorescence maxima of 3-MI. Any contribution to this
emission due to 3-MI can also be excluded because of the
excitation wavelength used (400 nm) since 3-MI does not
absorb in the visible region. It may be noted that the 540 nm
band is red-shifted from the band-gap emission due to CdS
itself, and that the luminescence intensity increases with an
increase in [3-MI] together with a further small red shift in
emission maxima at higher [3-MI]. These emission changes
did not exhibit any isoemissive point. An increase in emission
intensity and a shift in the emission maxima with increasing
[3-MI] without depicting any isoemissive point indicate the
formation of luminescing exciplexes of varying stoichiometry
between the excited-state CdS and 3-MI. The low extent of
quenching of red emission (Fig. 2) is understood by the fact
that the colloidal CdS and its complex in the excited state
with 3-MI might have similar emission characteristics in this
wavelength region.
3
3
ꢀ1
adsorption was calculated to be 2.1Å10 dm mol
Photolysis of the aerated reaction mixture containing 0.4
CdS and 4 m 3-MI by visible light in the wavelength
.
mM
M
range 410–440 nm resulted in growth of its absorption in the
blue-green region. A chloroform extract of the product
showed l at 325 nm. In TLC and GC experiments this
max
extract was found to contain three components, the retention
Colloidal CdS emission is known to consist of a range of
9,15
lifetimes. The emission decay curve recorded with the used
CdS colloids at 600 nm could be fitted into three exponential
decay programmes having t = 0.054 ns (B = 0.5341),
1
1
ꢀ
3
t = 1.03 ns (B = 1.2193Å10
)
and t = 19.82 ns
2
2
3
ꢀ
3
(
B = 1.2496Å10 ) with < t > of about 8 ns. In the
3
presence of 2.25 m
M
3-MI, this emission decayed within the
lamp pulse which suggests that the quenching of the red
emission which occurs is due to interception of the photo-
generated hole by the adsorbed 3-MI. This eventually results
in the formation of 5 and 6 as products. The mechanism of
this reaction may be depicted as in Scheme 1.
1
6
In basic medium, the indolyl cation 2, having pK = 5, is
a
ꢀ
converted into the indolyl radical 3. This may couple with O
to yield the corresponding hydroperoxide 4 which decom-
poses to produce 2-acetylformanilide 5 [eqn. (4)].
2
Fig. 1 Adsorption isotherm of 3-MI on 0.32 m colloidal CdS.
M
17
Inset: Plot of Langmuir adsorption isotherm of 3-MI
The relative amounts of 5 and 6 as a function of irradiation
time were also followed by GC. The yield of 5 increased
proportionately whereas the amount of 6 attained a limiting
value after 10 min of irradiation. This suggests that 6 is not
To receive any correspondence (e-mail: chemt@rurkiu.ernet.in).
†
This is a Short Paper as defined in the Instructions for Authors,
Section 5.0 [see J. Chem. Research (S), 1997, Issue 1]; there is there-
fore no corresponding material in J. Chem. Research (M).

View Article Online
J. CHEM. RESEARCH (S), 1998 55
O
C
O
Me
C Me
Hydrolysis
(5)
N
H
C
O
H
NH2
5
6
trophotometer. Emission measurements were made on a Shi-
madzu RF-5000 spectrofluorophotometer. Fluorescence lifetimes
were determined on an IBH-5000 single photon counting fluori-
meter using a nanosecond discharge lamp for excitation. Decay
curves were analysed by using a multiexponential fitting program
from IBH. Steady-state photolysis experiments were designed on
an Oriel photolysis assembly equipped with 200 W Hg(Xe) lamp
and cut filters.
Colloidal CdS was prepared by injecting a stoichiometric
ꢀ
amount of SH to the deaerated Cd(ClO
4
) solution containing
2
sodium hexametaphosphate as stabilizer following an earlier
6
,10
reported method and was characterized by its electronic and
emission spectra. The particles had an average size of 4 nm (deter-
mined by a Philips EM-400 transmission electron microscope).
GC separation of products was achieved on an HP-1 capillary
column under non-isothermal conditions. The column was tem-
perature-programmed from 50 to 230 °C at a heating rate of 10 °C
ꢀ1
Fig. 2 Luminescence spectra of 0.24 m
absence (———) and presence of varying concentrations of 3-MI
. . . . .
M
colloidal CdS in the
min . GC-MS data were obtained on a Shimadzu QP-2000 instru-
ment at 70 eV after elution of the solvent.
.
(m
M
): 0.5 (---); 1.0 (- -); 2.0 (- -); 2.5 (–
–); excitation
wavelength 400 nm
The financial assistance of DST and UGC, New Delhi, is
gratefully acknowledged. S. K. thanks CSIR, New Delhi, for
the award of RA. Thanks are also due to Dr A. Samanta,
University of Hyderabad, for providing facilities for emission
lifetime measurements.
Me
hν
–
+
CdS
+
CdS(e h ----3-MIads)*
(1)
(λirr 400 nm)
N
H
1
Received, 5th March 1997; Accepted, 22nd September 1997
Paper E/7/01555K
e–
+
O2
O2–
(2)
(3)
Me
Me
References
h+
+
CdS
+
ads
•+
N
N
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[
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3
-MI are also important since this molecule is considered as
a model for tryptophan in proteins. The photocorrosion of
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made to bring about increased charge separation in these
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1
1
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