Colloidal CdS-induced photocatalytic reaction of 2-methyl-indole - Mechanistic analysis of oxidation of indoles

Article abstract of DOI:10.1002/(SICI)1099-1395(199804)11:4<277::AID-POC3>3.0.CO;2-T

2-Methylindole (2-MI) is adsorbed on the surface of colloidal CdS particles with an adsorption intensity of 0.6 × 10³ dm³ mol^-1. A new emission band at 530 nm is produced by forming an exciplex between excited CdS and 2-MI and the red emission due to CdS is simultaneously quenched. The emission maxima of green bands for different indoles increase in the order indole < tryptophan < 2-MI < 3-MI and are observed at 508, 520, 530 and 540 nm, respectively. The shift in emission maxima is related to the oxidation potential of these substrates. The irradiation of an aerated reaction mixture containing CdS and 2-MI with visible light induces the oxidation of adsorbed 2-MI by photogenerated holes to produce 2-methyl-3-indolinone and 2-acetamidobenzaldehyde. The latter product is formed due to oxidative C - C bond cleavage of the pyrrole ring. The reactivity of trapped holes towards the adsorbed 2-MI is evidenced by a decrease in the lifetime of the red emission of CdS in the presence of 2-MI. In this reaction the possibility of the participation of singlet oxygen is ruled out. A general mechanism of CdS-induced oxidation of indoles is discussed.

Full text of DOI:10.1002/(SICI)1099-1395(199804)11:4<277::AID-POC3>3.0.CO;2-T

JOURNAL OF PHYSICAL ORGANIC CHEMISTRY, VOL. 11, 277–282 (1998)
Colloidal CdS-induced photocatalytic reaction of 2-methyl-
indoleÐmechanistic analysis of oxidation of indoles
Anil Kumar* and Sanjay Kumar
1Department of Chemistry, University of Roorkee, Roorkee-247 667, U. P., India
Received 8 July 1997; revised 8 August 1997; accepted 15 October 1997
ABSTRACT: 2-Methylindole (2-MI) is adsorbed on the surface of colloidal CdS particles with an adsorption
intensity of 0.6 Â 103 dm3 molꢀ1. A new emission band at 530 nm is produced by forming an exciplex between
excited CdS and 2-MI and the red emission due to CdS is simultaneously quenched. The emission maxima of green
bands for different indoles increase in the order indole < tryptophan < 2-MI < 3-MI and are observed at 508, 520,
530 and 540 nm, respectively. The shift in emission maxima is related to the oxidation potential of these substrates.
The irradiation of an aerated reaction mixture containing CdS and 2-MI with visible light induces the oxidation of
adsorbed 2-MI by photogenerated holes to produce 2-methyl-3-indolinone and 2-acetamidobenzaldehyde. The latter
product is formed due to oxidative C—C bond cleavage of the pyrrole ring. The reactivity of trapped holes towards
the adsorbed 2-MI is evidenced by a decrease in the lifetime of the red emission of CdS in the presence of 2-MI. In this
reaction the possibility of the participation of singlet oxygen is ruled out. A general mechanism of CdS-induced
oxidation of indoles is discussed. 1998 John Wiley & Sons, Ltd.
KEYWORDS: colloidal CdS-induced photochemical reactions; photocatalytic reactions; photoxidation;
2-methylindole
INTRODUCTION
emission has lately been explained by the formation of a
charge-transfer complex between excited CdS and the
substrate. The quenching of red emission is understood to
involve the scavenging of the trapped charge carriers by
the redox couple. Hence the analysis of the products of
the reaction together with the change in luminescence
behaviour of the photocatalyst in the presence of the
redox couple might provide useful information for the
elucidation of the overall reaction mechanism.
In the present work, we investigated the colloidal CdS-
induced photochemical reaction of 2-methylindole (2-
MI) and examined the luminescence behaviour of CdS in
the absence and presence of 2-MI. The nature of the
interaction between the substrate and the photocatalyst in
the ground state was analysed. In the light of the present
and earlier findings on similar systems, a general
mechanism of CdS-sensitized reactions of indoles is
discussed.
Surface modification of nanoparticles of cadmium sulfide
(CdS) has been widely investigated in efforts to control
their size, elucidate their optoelectronic and emission
properties and to study the dynamics of the photogener-
ated charge carriers.1–9 As the surface of these quantum
crystallites is very large, any interaction of additives with
the surface defects of particles might consequently affect
their optical and emission properties. For example, the
doping of metal ions2–5 and the binding of different
substrates, e.g. thiophenol,6 3-mercaptopropane-1,2-
diol,7 aliphatic and aromatic amines,8,9a indole,9b methyl
viologen10 and thiazine dye,11 are known to modify the
optical and emission properties of colloidal CdS. In
several of these investigations, adsorption of substrates
on the surface of the particles was observed as a
prerequisite to cause these changes. Both the yield of
emission and products of photochemical reactions are
controlled by the extent of adsorption. Surface capping of
particles by thiophenol6 and aniline9a also depict the
phenomena of size quantization.
EXPERIMENTAL
Aliphatic amines and indole enhance green emission
whereas aniline, methyl viologen and thiazine dye reduce
the yield of red emission. The enhancement of green
Materials. Cadmium perchlorate (Alfa), sodium hexam-
etaphosphate (Fluka), 2-methylindole (Aldrich) and all
other chemicals were of analytical grade. All chemicals
were used as received.
*Correspondence to: A. Kumar, Department of Chemistry, University
of Roorkee, Roorkee-247 667, U. P., India.
Contract/grant sponsor: DST, New Delhi.
Equipment. The electronic spectra were recorded on a
Shimadzu UV-2100/s spectrophotometer. The emission
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278
A. KUMAR AND S. KUMAR
Figure 1. Electronic spectra of the reaction mixture containing 0.4 mM colloidal CdS and 4.0 mM 2-MI as a function of irradiation
time: (Ð)0; (± ± ±)2; (± . ±)5; (± .. ±)10; (...)15 min. Inset: electronic spectrum of the chloroform extract of the product.
spectra were obtained on a Shimadzu RF-5000 spectro-
fluorimeter. Steady-state photolysis experiments were
performed on an Oriel photolysis assembly equipped
with a 200 W Hg (Xe) arc lamp. GC and GC–MS
experiments were carried out on a Shimadzu QP-2000
instrument. GC separation was achieved on an HR-1
capillary column using temperature programming. The
column temperature was kept constant at 50°C for 6 min
and then increased at 10°C minꢀ1. GC–MS data were
obtained at 70 eV and 250°C, and were recorded after the
elution of the solvent.
perchlorate solution containing 0.4 mM sodium hexam-
etaphosphate at pH 9.0 and stirred vigorously until
completion of the reaction. This solution was subse-
quently purged with N2 to remove any unreacted SHꢀ.
The particles thus produced showed broad size distribu-
tion and had an average diameter of 4 nm.
RESULTS AND DISCUSSION
CdS-induced oxidation of 2-MI
The electron micrographs of the particles were
recorded on a Philips EM-400 transmission electron
microscope.
The fluorescence lifetimes were measured with an
IBH-5000 single photon counting spectrofluorimeter
using a nanosecond discharge lamp for excitation. A
Hamamatsu photomultiplier was used for the detection of
fluorescence. Decay curves were analysed using a
multiexponential fitting program from IBH.
The photolysis of the aerated reaction mixture containing
0.4 mM colloidal CdS and 4.0 mM 2-MI at pH 10.6 by
light of wavelength >400 nm results in an increase in its
absorption in both the UV and visible regions. The
change in absorption as a function of irradiation time is
shown in Fig. 1. The product(s) of the reaction exhibits
ꢀmax at 380 nm in the visible region. This suggests that
the product(s) might contain a 2-substituted indoxyl type
of chromophore, which is known to absorb around
400 nm in the visible range.12 At low [2-MI] no such
product was formed and instead dissolution of particles
occurred efficiently. TLC and GC separation of the
chloroform extract of the reaction mixture revealed the
Preparation of colloidal CdS solution. A yellow solution
of colloidal CdS was prepared by the earlier reported
method.9,10 A stoichiometric amount of freshly prepared
SHꢀ was added slowly to deaerated 0.8 mM cadmium
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CdS-INDUCED PHOTOCATALYTIC OXIDATION OF 2-METHYLINDOLE
279
Figure 3. Luminescence spectra of 0.24 mM colloidal CdS in
the absence (...) and presence of various concentrations of 2-
MI: (±...±)0.5; (±..±)1.0; (±.±)1.5; (±±±)2.0; (Ð)2.5 mM.
indigo. The formation of 3 suggests the occurrence of
C—C bond cleavage of the pyrrole ring of 2-MI. This
observation is in agreement with the earlier findings on
the oxidation of 2-MI and its derivatives by other
oxidizing agents, viz. chromic acid and KMnO4.13 The
increased electron density at the nitrogen of the pyrrole
ring due to the methyl group at the C-2 position and the
difference in the reactivities of the intermediates formed
may have contributed to the C—C bond cleavage in the
case of 2-MI.
Figure 2. (a) Adsorption isotherm of 2-MI on colloidal CdS.
(b) Plot of Langmuir adsorption isotherm of 2-MI.
presence of three components. In GC these components
had retention times of 23.55, 24.50 and 25.35 min and
were identified by GC–MS as unreacted 2-MI (1), m/z
131 (70)P, 130(B) 103(13), 77(20), 65(16), 51(15), 2-
methyl-3-indolinone (2), m/z 147(33), 146(16), 145(88),
144(B), 130(64), 118(22), 117(20), 104(21), 77(40),
44(54), and 2-acetamidobenzaldehyde (3), m/z
163(33)P, 148(12), 135(47), 120(B), 92(48), 65(46),
43(48), respectively.
The formation of 2 and 3 indicates the difference in
reactivity of photogenerated holes towards 2-MI in
comparison with indole. CdS - sensitized oxidation of
indole under identical experimental conditions leads to
Nature of surface interaction and luminescence
behaviour
From the above results, it is not clear whether the
products are formed by reaction of photogenerated holes
with the bulk or surface-bound substrate. In addition, the
organic substrate added to the colloidal CdS solution may
also interact with CdS chemically. These different
possibilities were examined by recording the electronic
spectra of CdS in the absence and presence of 2-MI. The
presence of 2-MI neither affected the optical absorption
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280
A. KUMAR AND S. KUMAR
of CdS in the visible region nor caused the development
of any new peak. It eliminated the possibility of any
chemical interaction between the two. However, the
scanning of the absorption in the UV region showed a
decrease in absorbance where 2-MI had absorption. This
suggested that 2-MI is physisorbed on the surface of the
particles. From the Langmuir plot of the adsorption data,
its binding constant was found to be 0.6 Â 103 dm3 molꢀ1
(Fig. 2). For other indoles, namely indole, 3-MI14,
2,3-dimethylindole15 and tryptophan, also no chemical
interaction between the substrate and CdS has been
reported and their intensities of adsorption were found to
be 2.0 Â 103, 2.1 Â 103, 3.3 Â 103 and 0.6 Â 103 dm3
molꢀ1, respectively. These values are of similar order of
magnitude to that of 2-MI. These investigations reveal
that the photogenerated holes possibly intercept the
surface-bound substrate.
Surface interaction of the substrate could also be
probed by exploiting the luminescence from colloidal
CdS. If indeed the surface of the particles is modified by
adsorption of 2-MI, the luminescence of colloidal CdS
should change in its presence. The luminescence spectra
of CdS in the absence and presence of 2-MI are shown in
Fig. 3. It can be seen that the addition of 2-MI produced a
new emission band at 530 nm and the red emission due to
CdS was simultaneously quenched. The 530 nm band is
different to the band gap emission of CdS, which lies
around 490 nm, and corresponded to a quantum effi-
ciency of 5 Â 10ꢀ3. The green emission of used CdS
particles is weak and has been assigned earlier to the
recombination of free charge carriers.9,10b,16 At higher
[2-MI], the 530 nm band shifts slightly to the red. Any
contribution to the observed emission on account of 2-MI
can be neglected as 400 nm light was used for excitation;
2-MI does not have any absorption at this wavelength.
Since the absorption spectrum of CdS remains unchanged
in the presence of 2-MI, it shows the absence of any
complexation in the ground state between the two. The
surface of colloidal CdS particles is known to have many
defects and traps. Binding of 2-MI to these sites may,
thereby, occur through physical adsorption. Hence the
appearance of the 530 nm band can be attributed to the
emission from the exciplex formed between the excited
CdS and the adsorbed 2-MI. In contrast to indole, no
isoemissive point is observed with 2-MI and a bath-
ochromic shift of the green band is noted at high [2-MI].
This is possibly due to the formation of intermediate
complexes of varied stoichiometry at various [2-MI]
which may have different emission characteristics.
Similar green emission bands at different wavelengths
have also been noted for other substituted indoles. The
wavelengths of the green band in comparison with that of
indole follow the trend indole < tryptophan < 2-MI < 3-
MI and are observed at 508, 520, 530 and 540 nm,
respectively. A change in the wavelength of the green
band for different indoles evidently rules out the
possiblity of it being the band gap emission due to
CdS. This behaviour is understood in terms of the varied
oxidation potential of these substrates which affects the
extent of charge transfer in the exciplex formed between
excited CdS and the respective indole. The order of
decreasing oxidation potential17 of the indoles studied is
indole < tryptophan < 2-MI < 3-MI, which is the same
as the order in which the bathochromic shift of the
wavelength of the green emission band changes. The
enhancement of the luminescence of colloidal CdS and
Cd3As2 particles by binding of tertiary amines and
aliphatic thiols8 can also be argued along similar lines.
The reactivity of photogenerated trapped holes towards
2-MI was checked by measuring the emission lifetime of
CdS in the absence and presence of 2-MI. In these
experiments CdS particles were excited by 400 nm light,
where 2-MI does not show any absorption. The emission
was followed at 600 nm. The decay curve of CdS
emission could be fitted in a three exponential decay
program. In the presence of 2 mM of 2-MI the average
lifetime of CdS emission decreased from 8.0 to 1.2 ns.
The decrease in the lifetime confirms the interception of
holes by 2-MI following dynamic quenching.
In the light of the above results, the reaction scheme for
the formation of 2 and 3 in the CdS-mediated oxidation of
2-MIꢀcan be outlined as shown in Scheme 1.
O2 formed in the cathodic reaction (ii) may
eventually produce H2O2.18 The photochemical and
thermal reaction of H2O2 with 2-MI under similar
experimental conditions did not produce any of the
above - identified products. These experiments support
ꢀ
the participation of O2 depicted in step (vi). Indolyl
ꢀ
radical is known to couple with O2 to yield hydroper-
oxide.19 Hence the initial step in CdS-sensitized reaction
of 2-MI may consist of its oxidation to produce 2-
methylindolyl radical cation. The pKa of this radical
cation is 5.7 Æ 0.1,20 and at pH 10.5 it will be converted
largely into 2-methylindolyl radical.
In contrast to the CdS-sensitized reaction of indole,9b
O2 and O2ꢀ react with 2-methylindolyl radical to produce
the corresponding peroxy radical and hydroperoxide in
steps (v) and (vi). These intermediates subsequently
disproportionate and decompose to yield 2 and 3 in steps
(vii) and (viii), respectively. The higher reactivity of 2-
methylindolyl radical with molecular oxygen in compar-
ison with that of indole radical can be explained by the
difference in their reduction potentials.17,19 In case of
indole the initially produced radical cation mainly forms
an adduct with OHꢀ at the electron-deficient C-2
position,9b in contrast to 2-MI, in which the C-2 position
is blocked by a methyl group. The OH adduct is then
oxidized by the photogenerated hole to give 3-hydro-
xyindole, which is known to autooxidize readily to indigo
in the presence of oxygen.21
Hence in CdS-sensitized reactions of indole and 2-MI,
the difference in reactivity of radical intermediates
ꢀ
formed with O2/O2 is responsible for the formation of
different products of oxidation in these reactions.
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CdS-INDUCED PHOTOCATALYTIC OXIDATION OF 2-METHYLINDOLE
281
Scheme 1
In flash photolysis experiments, the primary event
upon illumination of CdS is the formation of an electron–
hole pair.22 The trapped conduction band electron caused
the bꢀle2a2ceh,2i3ng of CdS absorption to produce
particles. Indoles did not exhibit any chemical interaction
with colloidal CdS. Binding of indoles to different defect
sites through adsorption modifies the surface of colloidal
CdS particles. The adsorbed substrates form a green
luminescing exciplex with excited CdS. This green
emission is different to the CdS band gap emission.
Interestingly, the emission maxima are related to the
oxidation potential of the substrate. The electron–hole’
pairs generated upon photoirradiation of CdS are
scavenged by the adsorbed redox couple to yield the
products of reaction. At high concentrations of indoles
the anodic photodissolution of CdS particles does not
occur.
(CdS)x .
In the presence of O2 the bleaching of
CdS is recovered by the reaction shown in step (ii)
(Scheme 1) and this process takes place with a second-
order rate constant of 2.2 Â 105 dm3 molꢀ1 sꢀ1 22e
.
Apparently, the molecular oxygen does not quench the
excited CdS but instead reacts with the trapped electron
in the secondary step. In addition, the emission due to
CdS and the CdS – 2-MI reaction mixture is not quenched
in the presence of air. This observation also eliminates
the possibility of the formation of singlet oxygen by
energy tranfer from the excited CdS to molecular oxygen.
Therefore, the involvement of 1O2 in the studied reaction
can be neglected.
Acknowledgments
The financial assistance of DST, New Delhi, to undertake
this work is gratefully acknowledged. We thank Dr A.
Samanta, University of Hyderabad, for providing the
facilities of a single photon counting fluorimeter.
CONCLUSIONS
This work has illustrated an important mechanism in the
surface interaction of redox couples with colloidal CdS
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A. KUMAR AND S. KUMAR
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