Photocatalytic behaviour of tantalum (V) phthalocyanines in the presence of gold nanoparticles towards the oxidation of cyclohexene

Article abstract of DOI:10.1016/j.molcata.2010.11.023

This paper presents the photocatalytic oxidation of cyclohexene using (OH)₃TaPc derivatives in the absence or presence of gold nanoparticles (AuNPs). The photochemical parameters that include photodegradation (Φ_P) and singlet oxygen (Φ _Δ) quantum yields are also reported in this work. The Φ_Δ values were 0.47 and 0.36 for complexes 1a and 1b, respectively. The Φ_Δ values improved drastically in the presence of AuNPs to 0.75 and 0.88, respectively. The Φ_P values ranged from 1.02 to 2.45 × 10^-6, showing stability of TaPc derivatives in the absence and presence of AuNPs. The photocatalytic products identified using gas chromatograph (GC) are cyclohexene oxide, 2-cyclohexen-1-ol, 2-cyclohexene-1-one and 1,2-cyclohexanediol. The percentage conversion values were higher in the presence of AuNPs. Singlet oxygen was determined to be the main agent involved in the photocatalytic oxidation of cyclohexene. The product yield percentage values for both TaPc complexes (1a and 1b) and TaPc in the presence of AuNPs ranged from 6.3 to 21.2%.

Full text of DOI:10.1016/j.molcata.2010.11.023

Journal of Molecular Catalysis A: Chemical 335 (2011) 121–128
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Photocatalytic behaviour of tantalum (V) phthalocyanines in the presence of gold
nanoparticles towards the oxidation of cyclohexene
Vongani P. Chauke, Edith Antunes, Wadzanai Chidawanyika, Tebello Nyokong∗
Department of Chemistry, Rhodes University, Grahamstown 6140, South Africa
a r t i c l e i n f o
a b s t r a c t
Article history:
This paper presents the photocatalytic oxidation of cyclohexene using (OH)3TaPc derivatives in the
absence or presence of gold nanoparticles (AuNPs). The photochemical parameters that include pho-
todegradation (˚P) and singlet oxygen (˚ꢀ) quantum yields are also reported in this work. The ˚ꢀ
values were 0.47 and 0.36 for complexes 1a and 1b, respectively. The ˚ꢀ values improved drastically
Received 13 October 2010
Received in revised form
19 November 2010
Accepted 20 November 2010
Available online 26 November 2010
in the presence of AuNPs to 0.75 and 0.88, respectively. The ˚P values ranged from 1.02 to 2.45 × 10−6
,
showing stability of TaPc derivatives in the absence and presence of AuNPs. The photocatalytic products
identified using gas chromatograph (GC) are cyclohexene oxide, 2-cyclohexen-1-ol, 2-cyclohexene-1-one
and 1,2-cyclohexanediol. The percentage conversion values were higher in the presence of AuNPs. Singlet
oxygen was determined to be the main agent involved in the photocatalytic oxidation of cyclohexene.
The product yield percentage values for both TaPc complexes (1a and 1b) and TaPc in the presence of
AuNPs ranged from 6.3 to 21.2%.
Keywords:
Tantalum phthalocyanine
Gold nanoparticles
Singlet oxygen quantum yields
Photodegradation
Cyclohexene
© 2010 Elsevier B.V. All rights reserved.
Cyclohexene oxide
2-Cyclohexen-1-ol
2-Cyclohexene-1-one
1,2-Cyclohexanediol
1. Introduction
surface plasmon resonance (SPR) effects [14]. The SPR effect is a
collective oscillation of conduction electrons in the nano-particles,
The oxidation of alkanes and alkenes has been studied exten-
sively due to the usefulness of the oxidation products which are
obtained. These include alcohols, ketones and aldehydes. These
products are of great importance to industrial processes as well
as fine chemical syntheses [1]. The oxidation of alkenes using
phthalocyanines and a variety of oxidants has been reported before
[2–8]. Photocatalytic oxidation is more favorable over the chemical
(using oxidants) methods due to minimal environmental impact,
the use of light as a renewable source of energy and use of milder
conditions for the reactions in the former [2]. Phthalocyanines
have been employed to photo-oxidize cyclohexene [3,9,10]. Xue
et al. [10] employed palladium phthalocyaninesulfonate to selec-
tively photo-oxidize cyclohexene. However, the photo-oxidation
of alkenes using phthalocyanines is still limited compared to the
oxidation of phenols [11,12].
which resonate with the electromagnetic field of the incident light
[15]. To the best of our knowledge, the photo-oxidation of alkanes
or alkenes by phthalocyanines in the presence of AuNPs has not
been studied. We therefore explore the photocatalytic oxidation of
cyclohexene using (OH)3TaPc (Fig. 1) complexes in the presence
and absence of AuNPs. Ta as central metal was chosen due to the
possibility of high singlet oxygen quantum yield induced by its large
size. The photostability of the MPc complexes is important for their
application as photocatalysts (photosensitizers), and their photo-
stability in the presence of nanoparticles will be explored in this
manuscript.
2. Experimental
2.1. Materials
Gold nanoparticles (AuNPs) have gained a lot of attention in the
past decade due to their tunable optical properties that depend on
size and shape [13]. AuNPs strongly absorb visible light due to the
Zinc(II) phthalocyanine (ZnPc), 1,3-diphenylisobenzofuran
(DPBF), cyclohexene, cyclohexene oxide, 2-cyclohexene-1-ol,
cyclohexene-1-one, 1,2-cyclohexanediol, 1,4-diazobicyclo-octane
(DABCO), benzoquinone, toluene, sodium borohydride (NaBH4),
tetraoctylammonium bromide (TOABr) and gold (III) chloride were
purchased from Aldrich. The syntheses of complexes 1a and 1b
have been reported elsewhere [16].
∗
Corresponding author at: Department of Chemistry, Rhodes University, P.O. Box
94, Grahamstown 6140, South Africa. Tel.: +27 46 6038260; fax: +27 46 6225109.
E-mail address: t.nyokong@ru.ac.za (T. Nyokong).
1381-1169/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2010.11.023

122
V.P. Chauke et al. / Journal of Molecular Catalysis A: Chemical 335 (2011) 121–128
photolysed using the set-up described above. The reaction was
then monitored by taking samples of the solution and recording
UV spectra or gas chromatographs.
SR
SR
SR
SR
SR
SR
N
N
Light intensity for these studies were 14 × 1015 and
(a)
R =
N
N M
N
8 × 1016 photons s−1 cm−2 for filtered and white light, respectively.
N
(b)
2.4. Photochemical parameters
N
N
2.4.1. Singlet oxygen quantum yields
M=Ta(OH)3
SR
SR
Quantum yields of singlet oxygen photogeneration by TaPc
derivatives in toluene were determined in air (no oxygen bub-
bled) using the relative method with ZnPc as reference and DPBF
as chemical quencher for singlet oxygen, using Eq. (1):
Fig. 1. Molecular structures of TaPc complexes (1a and 1b).
2.2. Equipment
RDPBFIaSbtds
˚
= ˚Std
(1)
ꢀ
ꢀ
Std
A Shimadzu UV - 2550 spectrophotometer was employed for
the collection of UV–vis spectra. Transmission electron microscope
(TEM) images were obtained using a JEOL JEM 1210 transmission
electron microscope at 100 kV accelerating voltage.
R
Iabs
DPBF
where ˚Sꢀtd is the singlet oxygen quantum yield for the standard,
Std
DPBF
ZnPc (˚ꢀ = 0.58 in toluene [17]), RDPBF and R
are the DPBF
photodegradation rates in the presence of a sensitizer under inves-
Atomic force microscopy (AFM) images were recorded in the
non-contact mode in air with a CP-11 Scanning Probe Microscope
from Veeco Instruments (Carl Zeiss, South Africa) at a scan rate
of 1 Hz. The images were obtained using a spring constant range
of 20–80 N/m, and resonant frequency range of 217–276 Hz. Sam-
ples for AFM were prepared by spin coating solutions of AuNPs in
toluene in the presence and absence of TaPc derivatives.
tigation and the standard respectively. Iabs and IStd are the rates of
abs
light absorption by the sensitizer and standard, respectively.
To avoid chain reactions induced by DPBF in the presence of
singlet oxygen [18], the concentration of DPBF was lowered to
∼3 × 10−5 mol dm−3. Solutions of sensitizer (absorbance = 0.2 at the
irradiation wavelength) containing DPBF were prepared in the dark
and irradiated in the Q band region using the setup described above.
DPBF degradation at 415 nm was monitored. The light intensity for
singlet oxygen studies was 5 × 1015 photons s−1 cm−2 for filtered
light (Q band irradiation). The light intensity for the white light
X-ray powder diffraction patterns were recorded on a Bruker D8,
Discover equipped with a proportional counter, using Cu-K␣ radi-
˚
ation (ꢁ = 1.5405 A, nickel filter). Data were collected in the range
from 2ꢂ = 5◦ to 60◦, scanning at 1◦ min−1 with a filter time-constant
of 2.5 s per step and a slit width of 6.0 mm. Samples were placed
on a silicon wafer slide. The X-ray diffraction data were treated
using Eva (evaluation curve fitting) software. Baseline correction
was performed on each diffraction.
was 2 × 1016 photons s−1 cm−2
.
2.4.2. Photodegradation quantum yields
For determination of photodegradation quantum yields of the
TaPc derivatives, the usual Eq. (2) [18] was employed for both Q
band and white light irradiation:
2.3. Photochemical methods
(C0 − Ct)VNA
˚
P
=
(2)
General Electric Quartz line lamp (300 W) was employed for the
determination of photodegradation and singlet oxygen quantum
yields, and for the phototransformation of cycloxehene. A 600 nm
glass cut off filter (Schott) and a water filter were used to filter
off ultraviolet and infrared radiations respectively. An interference
filter (Intor, 740 nm with a band width of 40 nm) was addition-
ally placed in the light path before the sample, hence ensuring
excitation at the Q band only (700–780 nm). Experiments were
also performed where white light (from quartz line lamp) or sun-
light was employed. The wavelength range for unfiltered light is
∼400 nm to near infrared region, hence it does not include the B
band of phthalocyanines, but covers a wider absorption region. The
unfiltered light will be referred to as white light in this work. Light
intensities were measured with a POWER MAX 5100 (Molelectron
detector incorporated) power meter
The products for the phototransformation of cyclohexene were
analysed using an Agilent HP 5890 gas chromatograph (GC), fitted
with a flame ionization detector (FID), using a DB-5MS column (0.25
m length, 0.2 mm internal diameter, 0.25 ␮m film thickness). The
GC parameters were as follows: the initial oven temperature was at
50 ◦C, maximum oven temperature was 280 ◦C, equilibration time
was 0.15 min, initial ramp rate was 10 ◦C/min at 180 ◦C for 2 min,
the second was 10 ◦C/min at 280 ◦C for 2 min and total run time was
27 min.
IabsSt
where C0 and Ct (mol dm−3) are the TaPc complex (1a and 1b)
concentrations before and after irradiation respectively; V is the
reaction volume; S, the irradiated cell area (2.0 cm2); t, the irra-
diation time (s); NA, the Avogadro’s number and Iabs, the overlap
integral of the radiation source intensity and the absorption of the
Pc (the action spectrum) in the region of the interference filter
transmittance. For experiments where the TaPc complexes were
irradiated in the presence of AuNPs, any absorption of the latter
was subtracted from the DPBF absorption to avoid any errors.
2.5. Synthesis of gold nanoparticles (AuNPs)
The synthesis of gold nanoparticles (using phase transfer
agent TOABr as a protecting ligand) was achieved following the
method described by Brust et al. [19] and Kotiaho et al. [20]
with slight mod−ifi1cations. Briefly gold (III) chloride trihydrate solu-
tion (25 mmol L , 4 mL) was vigorously stirred with a solution of
TOABr (85 mmol L−1, 6 mL) in toluene until all the gold chloride
was transferred to the organic phase, as judged by the change of
colour from orange to red [19,20]. An aqueous solution of a reduc-
ing agent NaBH4 (3.6 × 10−2 mol L−1) was then added drop-wise
over a period of 10 min. The mixture was then stirred vigor-
ously for 30 min. The organic phase was separated and washed
with water. The AuNPs were then characterized with transmis-
sion electron microscopy (TEM), atomic force microscopy (AFM)
and X-ray diffraction (XRD). Solid complexes of conjugates of 1a
and 1b with AuNPs were synthesized by mixing the two, allowing
Reactions were carried out under homogeneous condition in
toluene. The reaction mixture consisted of complexes 1a, 1b or
1a-AuNP or 1b-AuNP mixed with known concentration of the
cyclohexene. The reaction mixture was contained in a reaction ves-
sel maintained at room temperature under constant stirring, and

V.P. Chauke et al. / Journal of Molecular Catalysis A: Chemical 335 (2011) 121–128
123
Fig. 2. TEM images of (a) AuNps in water (b) AuNPs modified with complex 1a.
time (∼28–30 h) for the MPcs to react with the gold nanoparti-
X-ray diffraction (XRD) provided important details about the
crystal structure and properties of the AuNPs employed in this
work. This technique was employed to determine the size of the
particles using the Debye–Scherrer Eq. (3) [21]:
cles. The solvent (toluene) was then evaporated. The uncoordinated
TaPc derivatives were removed in
(bio-beads).
a size-exclusion column
kꢁ
d(Å ) =
(3)
3. Results and discussion
␤ cos ␪
3.1. Characterization of AuNPs and AuNPs–MPc conjugates
where k is an empirical constant equal to 0.9, ꢁ is the wavelength of
the X-ray source, (1.5405 A), ˇ is the full width at half maximum of
˚
Fig. 2 shows the TEM images of AuNPs (Fig. 2a) and MPc-AuNPs
(Fig. 2b). Fig. 2a illustrates the distribution of the gold nanoparticles
synthesized in this work. In Fig. 2b, the MPc appear to be ‘surround-
ing’ the gold nanoparticles. This is illustrated by the ‘shades’ around
the dark spots.
The AuNPs were stabilized by TOABr; it is possible for the sta-
bilizer to be replaced by the alkylthio groups of TaPc derivatives to
form a bond, or that the TaPc derivatives are just adsorbed onto the
AuNPs.
the diffraction peak, and ꢂ is the angular position of the peak. Fig. 3
shows the XRD pattern for the AuNPs employed in this work, with
the peaks observed at 2ꢂ values of 27.41◦, 31.65◦, 38.17◦, 45.40◦ and
56.38◦. The average size for the AuNPs was then worked out to be
10.1 nm, using the peak that fits the structure database reflection
for gold.
Atomic force microscopy data (Fig. 4) provided the information
about the morphology of AuNPs on a cross section of the glass
surface coating from toluene in the absence and presence of the
Fig. 3. X-ray diffractogram of AuNPs.

124
V.P. Chauke et al. / Journal of Molecular Catalysis A: Chemical 335 (2011) 121–128
Fig. 4. AFM images of AuNPs in the absence (a) and presence (b) of complex 1a deposited on a glass surface from toluene. The corresponding histograms are shown in (c)
and (d).
TaPc derivatives. The average roughness (Ra) value of 0.628 nm was
obtained for AuNPs alone and the value decreased to 0.0927 nm for
AuNP-MPc conjugates. The decrease may be due to aggregation in
the presence of TaPc derivatives.
3.3. Photodegradation quantum yields (˚P) of the TaPc
derivatives
3.3.1. Q band irradiation
The histogram in Fig. 4c, shows populations of AuNPs with size
distributions from 20 nm and below in the section analysed, to
occur more frequently. In Fig. 4d for AuNPs in the presence of
complex 1a there is domination by the larger sizes, with size dis-
tribution of up to 114 nm, showing increased aggregation. This
indicates that modification of AuNPs with complexes 1a and 1b
was successful to some extent.
Photostability of MPc complexes is important for their appli-
cation as photocatalysts (photosensitizers). Photodegradation is a
photochemical process whereby the conjugated chromophore of
the phthalocyanine ring gets degraded into smaller molecular frag-
ments. This process is driven by singlet oxygen in the presence
of visible light to afford the photo-oxidation product, phthala-
mide, as suggested by Shcnurpfeil et al. [18]. The photodegradation
stabilities were determined in toluene by monitoring the spec-
tra of the complexes with increasing time, during photolysis,
Fig. 5a. The MPc complexes are degraded by singlet oxygen gen-
erated by them. Several factors can influence ˚P, such as the
nature of the solvent and the substitutents on the phthalocya-
nine ring. In general the TaPc complexes were very stable, the
3.2. Singlet oxygen quantum yield (˚ꢀ) of the TaPc derivatives
Singlet oxygen plays an important role in photocatalytic oxi-
dation reactions [18]. Singlet oxygen quantum yield (˚ꢀ) values
(Table 1) were determined using DPBF as the singlet oxygen
quencher in toluene upon irradiation in the Q band region. The
singlet oxygen quantum yield values determined under Q band
irradiation were 0.47 and 0.36 in toluene for complexes 1a and
1b, respectively. In the presence of AuNPs, the ˚ꢀ values of
the TaPc complexes (1a and 1b) increased significantly, almost
doubling these values, due to the heavy atom effect of the gold
nanoparticles. The AuNPs promote a higher production of sin-
glet oxygen due to enhanced triplet excited state yield of the
sensitizer. Improved singlet oxygen generation by metal nanopar-
ticles has been observed and reported by Zhang et al. [22]. Silver
nanoparticles were employed by Zhang et al. [22], and the pho-
tosensitizer was described as being ‘sandwiched’ in between the
silver nanoparticle island films enabling more singlet oxygen pro-
duction due to enhanced triplet excited yield of the sensitizer.
Their concept may be related to this study where gold nanopar-
ticles are employed instead of silver nanoparticles. In addition,
it is possible that the presence of the bromide atoms on the
TOABr (the AuNPs protecting ligand) increases the energy trans-
fer to the triplet state of TaPc complexes in the presence of AuNPs,
through enhanced spin–orbit coupling as a result of the heavy atom
effect [23].
˚
P
values ranged from 1.02 to 2.45 × 10−6. Typical values for
unstable phthalocyanines are of the order of 10−3 [24]. Generally,
complex 1b is more stable than 1a in the absence and presence
of AuNPs.
The mechanism (Type II) [25] for degradation of phthalocya-
nines under visible light is as outlined in Scheme 1, which shows
that the degradation of MPc complexes is mediated by singlet oxy-
gen [25].
The photodegradation of TaPc derivatives followed first order
kinetics with the plots of ln(C0/C) (where C and C0 are absorbances
measured for the Q band of the MPc complex at time t and t0, respec-
tively) versus time in seconds, shown in Fig. 6(iii and iv). The slopes
of the plots gave the effective reaction rate constants, k. Table 1
shows a decrease in rate constants for degradation of complexes
1a and 1b in the presence of AuNPs.
3.3.2. White light irradiation
The photostability of the MPc complexes depends mainly on the
type of radiation. These complexes are more stable in visible light
compared to UV irradiation [26]. Irradiation using white light will
include the Q band, but it is more extended than the Q band irra-

V.P. Chauke et al. / Journal of Molecular Catalysis A: Chemical 335 (2011) 121–128
125
(a)
60 mins
320
420
520
620
720
820
Wavelength (nm)
2
(b)
1.5
Fig. 6. A plot of ln(C0/C) vs. time for conjugates under white irradiation: 1b-AuNPs
(i) and 1a-AuNPs (ii) and under Q band irradiation: 1a-AuNPs (iii) and 1b-AuNPs
(iv). Solvent = toluene.
8 h
1
to be the dominant route for degradation under white light from
the tungsten lamp.
0.5
Photolysis induced by UV irradiation of the MPc derivatives has
been shown to follow first-order kinetics [27]. Plots of ln(C0/C)
versus time in Fig. 6(i and ii), gave the rate constants for white
light photolysis as listed in Table 1. The rate constants are larger for
the white light since a larger range of wavelengths is employed. It is
also possible with white light that heat is involved in the degrada-
tion process since near infrared light is involved. The ˚P values are
surprisingly larger for white light compared to Q band irradiation
only for 1b-AuNP and 1a, Table 1.
0
312
512
712
912
Wavelength nm
Fig. 5. Photodegradation spectra of complex 1a in toluene by (a)
Q band
irradiation. [1a] = 2.3 × 10−5 mol L−1 and (b) direct sunlight. Concentration:
[1a] = 3.1 × 10−5 mol L−1. Irradiation over a period of 8 h min.
In order to check if singlet oxygen is involved during pho-
todegradation in white light, photobleaching experiments were
performed in the presence of diazabicyclooctane (DABCO) a singlet
oxygen scavenger. Thephotobleaching rateconstant was calculated
to be 8.71 × 10−6 s−1. This rate was slower than the rates listed
in Table 1, confirming a significant singlet oxygen involvement in
the mechanism. Thus, it is possible that both singlet oxygen and
radicals are involved in the photobleaching mechanism.
Scheme 1. Type II mechanism. Here MPc is the metallophthalocyanine, ISC is
intersystem crossing and Subs is the substrate. 3O2 represents O2(3ꢃg) and 1O2
represents O2(1ꢀg).
3.4. Photocatalytic oxidation of cyclohexene
The possible oxidation products for cyclohexene have been
reported before [3,9,10], they include cyclohexene oxide, cyclo-
hexenol, cyclohexenone and cyclohexanediol. The products
obtained in this work were confirmed by the spiking the
photocatalysed samples with the respective standards. Control
experiments were performed where cyclohexene was photo-
oxidized in the absence of MPcs and MPc–AuNPs conjugates. The
results for the control experiments showed negligible formation
of the products. Similarly, in the absence of light but in the pres-
ence of TaPc derivatives (with or without AuNPs), no products were
formed.
diation (which ranges from 700 to 780 nm for the TaPc derivatives
under discussion).
The TaPc complexes (1a and 1b) were completely degraded by
exposure to sunlight (for 8 h), Fig. 5b, whose main intensity is at
∼550 nm. In order to study the degradation of the complexes under
more controlled conditions, we employed white light from a tung-
sten lamp (400–900 nm), to photodegrade the complexes in the
presence and absence of AuNPs.
In the presence of AuNPs, irradiation of both TaPc derivatives
and the AuNPs is possible, complicating the spectra. At the wave-
length of the tungsten lamp, it is not expected that the excitation
will be at the B band of the phthalocyanine molecule, however,
there will be a larger wavelength range as opposed to the excita-
tion of the Q band only. Thus singlet oxygen by Scheme 1 is expected
The main products obtained (and confirmed by spiking) are:
cyclohexene oxide, 2-cyclohexen-1-ol, 2-cyclohexene-1-one and
1,2-cyclohexanediol. These are obtained as main products both by
Q band and white light irradiations.
Table 1
Singlet oxygen and photobleaching quantum yields and rate constants for complexes 1a and 1b in the absence and presence of AuNPs in toluene.
Q band irradiation
MPc
White light
MPc
˚
ꢀ
˚
P
k (s−1
)
˚
P
k (s−1
)
1a
1b
1a-AuNP
1b-AuNP
0.47
0.36
0.75
0.88
2.07 × 10−6
1.45 × 10−6
2.45 × 10−6
1.02 × 10−6
7.16 × 10−5
7.35 × 10−5
6.04 × 10−5
4.75 × 10−5
1a
1b
1a-AuNP
1b-AuNP
2.27 × 10−6
1.17 × 10−6
2.36 × 10−6
1.59 × 10−6
1.80 × 10−4
1.15 × 10−4
1.00 × 10−4
1.70 × 10−4

126
V.P. Chauke et al. / Journal of Molecular Catalysis A: Chemical 335 (2011) 121–128
Table 2
2
(a)
(i)
Percentage conversion of cyclohexene by complexes 1a, 1b, 1a-AuNPs and 1b-AuNPs
in toluene. Photolysis time = 180 min.
1.5
1
MPc
Q band irradiation
% Conversiona
White light
% Conversiona
1a
1b
1a-AuNP
1b-AuNP
24.2
18.0
35.4
30.4
48.1
45.2
92.3
90.0
(ii)
0.5
0
320
520
720
(1−cylohexene
)−(1−cyclohexene
)
initial
(1−cyclohexene
remaining
a
%conversion =
× 100
)
initial
Wavelength nm.
2
1.8
1.6
1.4
1.2
1
0.8
0.6
0.4
0.2
0
(b)
(i)
Table 2 shows the percentage conversion of cyclohexene using
complexes 1a, 1b and their conjugates with AuNPc as photocat-
alysts under Q band and white light irradiation. For experiments
performed under Q band irradiation, the percentage conversion
values were higher for 1a-AuNPs and 1b-AuNPs conjugates (35.4
and 30.4%, respectively), compared to 1a and 1b alone at 24.2 and
18.0%, respectively. This is associated with their increased singlet
oxygen quantum yields compared to the TaPc complexes alone.
Fig. 7 shows the variation of the percentage conversion over a
period of 180 min. There is a fast increase in percentage conversion
until ∼50 min, after that there is slow or no increase. Complexes
1a-AuNPs and 1b-AuNPs gave the highest conversions at all times
compared to complexes 1a and 1b in the absence of AuNPs. The
reaction was also carried out in the presence of a singlet oxygen
scavenger DABCO, Fig. 7(v). DABCO was added until there was no
change in the GC trace of cyclohexene. The percentage conversion
for complex 1a after 180 min in the presence of DABCO was ∼3.5% –
this is small compared to complexes 1a and 1b and their conjugates
in the absence of DABCO. The low percentage conversion in the
presence of DABCO illustrates that singlet oxygen is the main agent
involved in the photocatalytic oxidation of cyclohexene. However,
the fact that there is a slight conversion of cyclohexene in the pres-
ence of DABCO suggests that it is not only singlet oxygen that is
involved; there is a possibility of involvement of other reactive oxy-
gen species. This was proved by doing photolysis in the presence of
a free radical scavenger (benzoquinone) which showed a decrease
of ∼3.6% in percentage conversion, showing that free radicals are
involved.
(ii)
320
420
520
620
720
820
Wavelength nm.
Fig. 8. Spectral changes observed for complex 1a during the photocatalytic transfor-
mation of cyclohexene under (a) white light irradiation and (b) Q band irradiation.
Irradiation over 180 min (i) before and (ii) after photolysis.
The alkyl substituents on the TaPc complexes are electron-donating
and are prone to oxidative attack due to the increased electron den-
sity. Furthermore, the complexes conjugated with the AuNPs are
prone to degradation as a result of SPR absorption and the increased
singlet oxygen. However, in the presence of DABCO, Fig. 9(ii), pho-
tobleaching of the MPc complexes was minimal, compared to that
without DABCO (Fig. 9(iii) showing the involvement of singlet oxy-
gen the photo-oxidation reactions).
Table 3 shows the product yields obtained for Q band irradia-
tion in the presence of TaPc derivatives as photocatalysts and after
180 min of irradiation. The yields for the main products ranged from
6.3 to 21.2%, values which are higher than reported in literature [3],
where the values of the same product ranged from 0.02% to 4.4%
using ZnPc as a photocatalyst. Cyclohexene oxide has been reported
as one of the minor products by Sehlotho et al. [3] using ZnPc as a
photocatalyst. In this work cyclohexene oxide is a major product in
the presence of 1a-AuNPs.
The percentage conversion values for catalytic reactions
exposed to white light were high, Table 2. The values were
48.1%, 45.2%, 92.3% and 90.0% for 1a, 1b, 1a-AuNPs and 1b-
AuNPs, respectively. The values were notably higher compared
to Q band irradiation, however the complexes underwent con-
siderable bleaching during catalysis (Fig. 8a) in the photocatalytic
experiments involving white light. Less bleaching was observed on
irradiation at the Q band, Fig. 8b. In the presence of AuNPs using
white light (or Q band irradiation), the degradation of the TaPc
derivatives was similar to that observed in Figs. 8a or b, respectively.
Table 3 also shows percentage selectivity for the main prod-
ucts formed during the photocatalytic oxidation of cyclohexene
under Q band and white light irradiations. Generally percent-
age selectivity values increased under white light experiments;
however photobleaching of the MPc was a major drawback as
discussed above. Alcohol is one of the desired products derived
2
40
(i)
(i)
1.5
30
20
10
0
(ii)
(ii)
(iii)
1
(iii)
(iv)
0.5
0
(v)
320
520
720
0
50
100
150
200
Time (min)
Wavelength nm.
Fig. 7. Plots of percentage conversion plots of cyclohexene over a period of 180 min,
where (i) is 1a-AuNPs, (ii) 1b-AuNPs, (iii) 1a, (iv) 1b and (v) 1a in the pres-
ence of DABCO in toluene. Using Q band irradiation; [1a] = 2.80 × 10−5 mol L−1 or
[1b] = = 5.60 × 10−5 mol L−1, [cyclohexene] = 4.06 × 10−2. DABCO = 0.0029 g/mL.
Fig. 9. Electronic absorption spectra for 1b (i) before photoirradiation, (ii) after
180 min in the presence of DABCO and (iii) after 180 min in the absence of
DABCO. [1b] = 5.6 × 10−5 mol L−1. [DABCO] = 0.0029 g/mL. Q band irradiation. Sol-
vent = toluene.

V.P. Chauke et al. / Journal of Molecular Catalysis A: Chemical 335 (2011) 121–128
127
Table 3
Percentage selectivity and product yield values for cyclohexene photocatalytic products. Photolysis time = 180 min.
MPc
Product
Q band irradiation
% Selectivitya
White light
Product yields under Q band irradiation (%)b
% Selectivitya
1a
Cyclohexene oxide
2-Cyclohexen-1-ol
2-Cyclohexene-1-one
1,2-Cyclohexanediol
17.9
35.1
26.2
40.8
27.8
40.5
38.0
86.0
6.3
9.15
8.6
21.2
1b
Cyclohexene oxide
2-Cyclohexen-1-ol
2-Cyclohexene-1-one
1,2-Cyclohexanediol
27.3
38.3
35.1
33.4
30.2
39.9
52.2
60.3
6.8
9.1
11.8
13.7
1a-AuNP
1b-AuNP
Cyclohexene oxide
2-Cyclohexen-1-ol
2-Cyclohexene-1-one
1,2-Cyclohexanediol
46.0
20.4
23.0
30.1
50.8
29.8
37.7
34.0
11.5
6.7
8.6
7.7
Cyclohexene oxide
2-Cyclohexen-1-ol
2-Cyclohexene-1-one
1,2-Cyclohexanediol
38.1
29.1
27.3
26.7
42.4
30.3
77.3
30.6
9.7
7.0
17.5
6.9
epoxide
[(1−cyclohexene
initial
a
obtained
% selectivity =
.
)−(1−cyclohexene )]
final
b
Based on the substrate cyclohexene.
50
40
30
20
10
0
OH
OH
+
-
+
1O2
O
O
(i)
(ii)
peroxirane
Cyclohexene
(iii)
Cyclohaxanediol
(iv)
OH
O
O
0
50
100
150
200
Time (mins)
Cyclohexene oxide
Fig. 10. Percentage selectivity plots of cyclohexene products in toluene using
complex 1a, where (i) is 1,2-cyclohexanediol, (ii) 2-cyclohexen-1-ol, (iii) 2-
cyclohexenol
Cyclohexene-one
cyclohenene-1-one and (iv) cyclohexene oxide. [1a] = 2.80 × 10−5 mol L−1
.
Scheme 2. Proposed mechanism for the formation of the photo-oxidation products.
from oxidation of alkenes in the petrochemical industry. The per-
centage selectivity values for cyclohexenol (35.1% and 38.3%, for
complexes 1a and 1b, respectively) under Q band irradiation, are
higher than those reported for the photochemical oxidation of
cyclohexene using sulfonated MPc (M = Pd, Al and Zn) [10], where
percentage selectivity values ranged from 1.6 to 34.9% in the
solvents: dimethylformamide:H2O, dioxane:H2O and acetonitrile.
Figs. 10 and 11 show the percentage selectivity values over a period
of 180 min for complexes 1a and 1a-AuNPs, respectively. The pro-
duction of cyclohexene products, as shown in Figs. 10 and 11, was
marked by a sweeping increase of products for the first 30 min,
followed by a slow production until photolysis was stopped at
180 min.
In literature, two major pathways are suggested for the sensi-
tisation reactions by phthalocyanines. One is the energy transfer
from an electronically excited complex to ground state molecu-
lar oxygen that subsequently yields singlet oxygen (Scheme 1).
Another is the electron transfer from the excited complex to ground
state molecular oxygen or reacting substrates thereby generating
radicals such as superoxide radicals, Type I mechanism. As stated
above, the % conversion of cyclohexene was slowed down in the
presence of DABCO, Fig. 7, implying that singlet oxygen played a
major role in photocatalytic oxidation. The mechanism proposed
in this work is shown in Scheme 2 and is similar to that proposed
by Sehlotho et al. [3]. Several reports [27–32] have shown that zwit-
terions, biradicals, endo-peroxides, peroxiranes are formed when
ethylene interacts with singlet oxygen, hence the proposed mech-
anism in Scheme 2.
50
(i)
40
(ii)
(iii)
(iv)
30
20
10
0
4. Conclusion
We have determined the stability of (OH)3TaPc complexes and
(OH)3TaPc in the presence of AuNPs under different conditions
(white and Q band irradiation). The complexes were more sta-
ble under Q band irradiation. The singlet oxygen production was
improved significantly by the presence of AuNPs. Photocatalytic
oxidation of cyclohexene was carried out successfully. The per-
centage conversion of cyclohexene was improved by the presence
0
50
100
150
200
Time (mins)
Fig. 11. Percentage selectivity plots of cyclohexene products in toluene using
complex 1a-AuNPs, where (i) is cyclohexene oxide, (ii) 2-cyclohexen-1-ol, (iii) 2-
cyclohenene-1-one and (iv) 1,2-cyclohexanediol.

128
V.P. Chauke et al. / Journal of Molecular Catalysis A: Chemical 335 (2011) 121–128
of gold nanoparticles. Percentage selectivity values were gener-
ally improved when compared to literature values. Similarly, the
percentage yields were improved significantly.
[10] X. Xue, Y. Xu, J. Mol. Catal. A: Chem. 276 (2004) 80–85.
[11] E. Pepe, O. Abbas, C. Rebufa, M. Simon, S. Lacombe, M. Julliard, J. Photochem.
Photobiol. A: Chem. 170 (2005) 143–149.
[12] V. Iliev, J. Photochem. Photobiol. A: Chem. 151 (2002) 195–199.
[13] S. Link, M.A. El-Sayed, Annu. Rev. Phys. Chem. 54 (2003) 331–336.
[14] C.F. Bohrem, D.R. Huffman, Absorption Scattering of Light by Small Particles,
Wiley, New York, 1983.
Acknowledgements
[15] P. Mulvaney, Langmuir 12 (1996) 788–800.
This work was supported by the Department of Science and
Technology (DST) and National Research Foundation (NRF) of South
Africa through DST/NRF South African Research Chairs Initiative for
Professor of Medicinal Chemistry and Nanotechnology and Rhodes
University
[16] V. Chauke, T. Nyokong, Inorg. Chim. Acta 363 (2010) 3662–3669.
[17] T. Nyokong, Coord. Chem. Rev. 251 (2007) 1707–1722.
[18] G. Schnurpfeil, A. Sobbi, W. Spiller, H. Kliesch, D. Wöhrle, J. Porphyrins Phthalo-
cyanines 1 (1997) 159–167.
[19] M. Brust, M. Walker, D. Bethell, D.J. Schiffrin, R. Whyman, J. Chem. Soc. Commun.
(1994) 801–802.
[20] A. Kotiaho, R. Lahtinen, A. Efimov, H.-K. Metsberg, E. Sariola, H. Lehtivuori, N.V.
Tkachenko, H. Lemmetyinen, J. Phys. Chem. C 114 (2010) 162–168.
[21] S. Sapra, D.D. Sarma, Pramana 65 (2005) 565–570.
References
[22] Y. Zhang, K. Aslan, M.J.R. Previte, C.D. Geddes, J. Fluoresc. 17 (2007) 345–349.
[23] D.C. Hone, P.I. Walker, R. Evans-Gowing, S. FitzGerald, A. Beedy, I. Chambrier,
M.J. Cook, D.A. Russell, Langmuir 18 (2002) 2985–2987.
[24] S. Maree, T. Nyokong, J. Porphyrins Phthalocyanines 5 (2001) 782–792.
[25] R. Bonnett, Chemical Aspects of Photodynamic Therapy, Gordon and Breach
Science Publishers, Germany, 2000.
[1] R. Hage, A. Lienke, J. Mol. Catal. A: Chem. 251 (2006) 150–158.
[2] A. Maldotti, A. Molinari, R. Madelli, Chem. Rev. 102 (2002) 3811–3836.
[3] N. Sehlotho, T. Nyokong, J. Mol. Catal. A: Chem. 219 (2004) 201–207.
[4] O.V. Zalomaeva, A.B. Sorokin, N. J. Chem. 30 (2006) 1768–1773.
[5] A.B. Sorokin, S. Mangematin, C. Pergrale, J. Mol. Catal. A: Chem. 182–183 (2002)
267–281.
[6] M.E. Nino, S.A. Giraldo, E.A. Paez-Mozo, J. Mol. Catal. A: Chem. 175 (2001)
139–151.
[7] M. Bressan, N. Celli, N. d’Alessandro, L. Liberatore, A. Morvillo, L. Tonucci, J.
Organomet. Chem. 593–594 (2000) 416–420.
[26] N. d’Alessandro, L. Tonucci, A. Morvillo, L.K. Dragani, M. Di Deo, M. Bressan, J.
Organomet. Chem. 690 (2005) 2133–2141.
[27] R. Słota, G. Dyrda, Inorg. Chem. 42 (2003) 5743–5750.
[28] P.D. Bartlett, A.P. Schaap, J. Am. Soc. 92 (1970) 3223–3225.
[29] C.S. Foote, J.W. Peters, J. Am. Soc. 93 (1971) 3795–3796.
[30] P.R. Ogilby, C.S. Foote, J. Am. Soc. 103 (1981) 1219–1221.
[31] B.M. Monroe, J. Phys. Chem. 82 (1978) 15–18.
[8] T. Sugimori, S.-I. Horike, S. Tsumura, M. Handa, K. Kasuga, Inorg. Chim. Acta 283
(1998) 275–278.
[9] N. Sehlotho, T. Nyokong, J. Mol. Catal. A: Chem. 209 (2004) 51–57.
[32] L.E. Manring, C.S. Foote, J. Am. Soc. 105 (1983) 4710–4717.