Phototransformation of 4-nitrophenol using Pd phthalocyanines supported on single walled carbon nanotubes

Article abstract of DOI:10.1016/j.molcata.2011.01.016

Adsorption of palladium phthalocyanines complexes on single walled carbon nanotubes was carried out. The resulting composites were employed as catalysts for heterogeneous photocatalytic oxidation of 4-nitrophenol (4-NP) in aqueous media. Singlet oxygen was found to be involved in the phototransformation of 4-NP. Gas chromatographic separation gave hydroquinone and benzoquinone as the phototransformation products. Langmuir-Hinshelwood (L-H) model was employed to evaluate the adsorption and desorption equilibria of the reactants and the products. 2,3,9,10,16,17,23,24-octakis(dodecylthiophthalocyaninato) palladium(II) and 1,4,8,11,15,18,22,25-octakis(dodecylthio phthalocyaninato) palladium, containing the longest alkyl chain gave the best performances.

Full text of DOI:10.1016/j.molcata.2011.01.016

Journal of Molecular Catalysis A: Chemical 337 (2011) 68–76
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Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Phototransformation of 4-nitrophenol using Pd phthalocyanines
supported on single walled carbon nanotubes
Taofeek B. Ogunbayo, Tebello Nyokong∗
Department of Chemistry, Rhodes University, P.O. Box 94, 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:
Adsorption of palladium phthalocyanines complexes on single walled carbon nanotubes was
carried out. The resulting composites were employed as catalysts for heterogeneous photocat-
alytic oxidation of 4-nitrophenol (4-NP) in aqueous media. Singlet oxygen was found to be
involved in the phototransformation of 4-NP. Gas chromatographic separation gave hydroquinone
and benzoquinone as the phototransformation products. Langmuir–Hinshelwood (L–H) model was
employed to evaluate the adsorption and desorption equilibria of the reactants and the prod-
ucts. 2,3,9,10,16,17,23,24-octakis(dodecylthiophthalocyaninato) palladium(II) and 1,4,8,11,15,18,22,25-
octakis(dodecylthio phthalocyaninato) palladium, containing the longest alkyl chain gave the best
performances.
Received 29 November 2010
Received in revised form
28 December 2010
Accepted 7 January 2011
Available online 21 January 2011
Keywords:
Photocatalysis
Phthalocyanines
Oxidation
© 2011 Elsevier B.V. All rights reserved.
4-Nitrophenol
1. Introduction
and 2a–c, Fig. 1). These complexes have been found to show high
singlet oxygen quantum yields [9]. Thus, palladium phthalocya-
The toxicity of 4-nitrophenol is well established [1]. The
methods which have been investigated for the degradation of
4-nitrophenol (4-NP) include microbial [2] and photocatalytic
degradation [3,4]. Photodegradation of 4-NP is preferable since
it avoids the use of harsh oxidants. TiO2, has been found to be
an inefficient photosensitiser for degradation of 4-NP due to the
fact that its band gap is in the near-UV region where only about
4% of solar energy is effective [5]. Metallophthalocyanine (MPc)
complexes have been employed as heterogeneous catalysts for the
photodegradation of organic pollutants such as phenols [5]. This is
due to their high light absorption capacity in the visible region, pho-
tostability, chemical stability and their ability to generate singlet
oxygen species, which is implicated in photocatalysis. The supports
for phthalocyanines which have been employed for their use in
photocatalytic degradation of pollutants, include TiO2, resins and
clays [6–8] with varied degrees of success.
nines have potential in the area of photocatalysis because of their
singlet oxygen generating ability and photostability [9]. Other com-
mon phthalocyanines such as ZnPc derivatives are unstable during
photodegradation of 4-NP or other phenols [10,11]. SWCNTs were
chosen as support because of ease of immobilisation of MPcs due to
the strong ꢀ–ꢀ interaction between SWCNTs and MPc complexes.
Composites of MPc complexes with SWCNTs have been used in
electrocatalysis [12–14]. The use of SWCNTs as MPc supports for
photocatalysis is reported here for the first time.
The study of the photophysical and photochemical behavior
of palladium phthalocyanines (PdPc) is still limited [15–17]. Alkyl
thio derivatized phthalocyanine complexes of palladium are scarce,
hence in this work we report on the physicochemical behavior of
alkylthio and aryloxyl derivatised PdPcs and their use in the pho-
totransformation of 4-NP when adsorbed on SWCNTs.
The association of TiO2 and ZnPc gave a small increase (<6%) in
the photodegradation efficiency of phenol compared to TiO2 alone.
This behavior was expected since the light employed had a maxi-
mum absorption at 360 nm, where there is insignificant absorption
by ZnPc [6], showing the need for visible light excitation of the
MPc complexes. In this work we used single walled carbon nan-
otubes (SWCNTs) as support material for MPc complexes (1a–1e,
2. Experimental
2.1. Materials
4-Nitrophenol (4-NP), fumaric acid, benzoquinone, dimethyl-
sulfoxide (DMSO), dichloromethane (DCM), 1-chloronaphthalene
(1-CNP), PdCl2, dimethylformamide (DMF) and hydroquinone
were purchased from SAARCHEM. Dichloromethane (SAARCHEM)
was distilled before use. Octanethiol, dodecanethiol,8-
diazabicyclo{5.4.0}-undec-7-ene (DBU), diphenylisobenzofuran
(DPBF), anthracene-9,10-bis-methylmalonate (ADMA) and
∗
Corresponding author. Tel.: +27 46 603 8260; fax: +27 46 622 5109.
E-mail address: t.nyokong@ru.ac.za (T. Nyokong).
1381-1169/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2011.01.016

T.B. Ogunbayo, T. Nyokong / Journal of Molecular Catalysis A: Chemical 337 (2011) 68–76
69
palladium-1,4,8,11,15,18,22,25-octakis(dodecylthio)phthalocya-
nine (2c), (hence compounds 4b and 4c, Scheme 1) are reported in
this work.
R
R
R
R
R
R
N
R
R
N
N
N
N
N
2.2. Equipment
R
R
R
R
N
N
N
Pd
N
N
Pd
N
N
N
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
N
N
˚
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 per-
formed on each diffraction pattern by subtracting a spline fitted to
the curved background and the full-width at half-maximum values
used in this study were obtained from the fitted curves. Samples for
transmission electron microscopy (TEM) were prepared by sonni-
cating the samples of SWCNTs, or ads–MPcs–SWCNT in a mixture
of DMF and DCM.
R
R
R
R
(1)
(2)
a; R = S-C5H11
b; R = S-C8H17
c; R = S-C12H25
Bruker Vertex 70-Ram II Raman spectrometer (equipped with
a 1064 nm Nd:YAG laser and a liquid nitrogen cooled germanium
detector) were used to collect Raman data. The Raman spectral data
for the SWCNTs, MPcs, ads–MPcs–SWCNT were obtained from their
powdered samples. Transmission electron microscope (TEM) pic-
tures were obtained using a JEOL JEM 1210 transmission electron
microscope at 100 kV accelerating voltage.
O
d; R =
e; R =
O
UV–vis spectra were recorded using a Shimadzu UV-2550
UV–vis spectrophotometer. IR data were obtained using the Perkin-
Elmer spectrum 2000 FTIR spectrometer. 1H NMR spectra were
recorded using a Bruker AMX 400 MHz spectrometer. Elemental
analyses were carried out on a Vario EL III MicroCube CHNS Ana-
lyzer. The pH of the solutions was measured with Metrohm 827.
Mass spectra data were collected with a Bruker AutoFLEX III
Smartbeam TOF/TOF Mass spectrometer. The instrument was oper-
ated in positive ion mode using an m/z range of 400–3000. The
voltage of the ion sources were set at 19 and 16.7 kV for ion sources
1 and 2 respectively, while the lens was set at 8.50 kV. The voltages
for reflectors 1 and 2 were set at 21 and 9.7 kV respectively. The
spectra were acquired using dithranol as the MALDI matrix, using
a 354 nm Nd:YAG laser.
Photoirradiation experiments forsingletoxygen quantum yields
and phototransformation of 4-nitrophenol were carried out using
a tungsten lamp (100 W, 30 V). A 600 nm glass cut off filter (Schott)
and a water filter were used to filter off ultraviolet and infrared radi-
ations, respectively. Light intensities were measured with a POWER
MAX5100 (Mol-electron detector incorporated) power meter. For
singlet oxygen quantum yield determinations for complexes 2a–2c
O
Fig. 1. Molecular structure of the photocatalysts employed in this work.
single wall carbon nanotubes (SWCNTs, 0.7–1.2 nm in diam-
eter and 2–20 ␮m in length) were from Aldrich. Silica gel
60 (0.063–0.200 mm) was used for column chromatogra-
phy. Distilled deionised Millipore water was used to prepare
aqueous solutions of 4-NP, The syntheses of the follow-
ing photocatalysts (phthalocyanine complexes): 2,3,9,10,16,
17,23,24-octakis(pentylthiophthalocyaninato)
palladium(II)
(1a);
palladium(II)
2,3,9,10,16,17,23,24-octakis(octylthiophthalocyaninato)
(1b);
phthalocyaninato)
23,24-octakis(phenoxyphthalocyaninato) palladium (1d); 2,3,9,
10,16,17,23,24-octakis(benzyloxyphenoxy)phthalocyaninato pall-
2,3,9,10,16,17,23,24-octakis(dodecylthio-
palladium(II) (1c); 2,3,9,10,16,17,
adium-(1e)
and
1,4,8,11,15,18,22,25-octakis(pentylthio-
phthalocyaninato) palladium (2a), Fig. 1, have been
reported before [9,18,19]. The syntheses of palladium-
1,4,8,11,15,18,22,25-octakis(octylthio)phthalocyanine (2b) and
SR
RS
SR
OTos
N
SR
SR
N
SR
SR
N
CN
CN
CN
CN
PdCl2 , DBU
N
N
RSH, stir
Pd
DMSO, 12hr, K2CO3
N
N
N
Pentanol, 4hrs
SR
4
OTos
3
RS
SR
2
b; R = C8H17
c; R = C12H25
Scheme 1. The syntheses of palladium-1,4,8,11,15,18,22,25-octakis(alkylthio)phthalocyanine.

70
T.B. Ogunbayo, T. Nyokong / Journal of Molecular Catalysis A: Chemical 337 (2011) 68–76
in 1-CNP, an interference filter (Intor, 750 nm with a band width of
40 nm) was additionally placed in the light path before the sam-
ple, to ensure the excitation of the Q band. For the determination
of singlet oxygen quantum yields of ads–MPc–SWCNT conjugates,
interference filters of 670 nm (for 1a–1e) and 750 nm (for 2a–2c)
were employed.
Triplet quantum yields and lifetimes of the new complexes
(2a–2c) were determined using a laser flash photolysis system; the
excitation pulses were produced by a Nd:YAG laser (Quanta-Ray,
1.5 J/8 ns) pumping a dye laser (Lambda Physic FL 3002, Pyridin 1
in methanol). The analyzing beam source was from a Thermo Oriel
xenon arc lamp, and a photomultiplier tube was used as a detector.
Signals were recorded with a two-channel digital real-time oscillo-
scope (Tektronix TDS 360); the kinetic curves were averaged over
256 laser pulses. Triplet lifetimes were determined by exponential
fitting of the kinetic curves using Origin Pro 7.5 software.
where C0 and Ct are the DPBF concentrations prior to and after irra-
diation, respectively; VR is the reaction volume; t is the irradiation
time per cycle and Iabs is defined by Eq. (3).
˛AI
Iabs
=
(3)
NA
where ˛ = 1 − 10−A(ꢁ) , A(ꢁ) is the absorbance of the sensitizer at the
irradiation wavelength, A is the irradiated area (3.14 cm2), I is the
intensity of light (1 × 1015 photons cm−2 s−1) and NA is Avogadro’s
constant. The singlet oxygen quantum yields (˚ꢃ) were calculated
using Eq. (4).
1
1
1
kd
1
=
+
˚
ꢃ
(4)
˚
˚
ka [DPBF]
DPBF
ꢃ
where kd is the decay constant of singlet oxygen and ka is the rate
constant for the reaction of DPBF with O2(1ꢃg). The value of 1/˚ꢃ
for the PdPc derivatives 2a–2c was obtained as the intercept from
the plot of 1/˚DPBF versus 1/DPBF. The singlet oxygen quantum
yields of ads–MPc–SWCNT conjugates were calculated as described
above, using ADMA as singlet oxygen quencher and substituting it
for DPBF.
2.3. Photocatalysis procedures
The singlet oxygen quantum yield (˚ꢃ) determinations (for
the new complexes 2a–2c) were carried out using 1-CNP solu-
tions containing DPBF (90 ␮M) and the Pcs (absorbance ∼ 0.2 at
the irradiation wavelength). Irradiations were done using the set-
up described above, and were repeated until around 80% decay of
DPBF was observed [20]. The absorbance was corrected for that of
the sensitizer in the B band region. Singlet oxygen quantum yields
of the ads–MPc–SWCNT conjugates were determined by suspend-
ing the conjugates in oxygen saturated pH 8.5 buffer solution and
irradiating at the Q band of the phthalocyanine complexes, using
ADMA as a singlet oxygen quencher in aqueous media.
The transformation of 4-NP was monitored through its absorp-
tion peak at 400 nm after each photolysis cycle. A 1 cm path-length
UV–vis spectrophotometric cell, fitted with a tight fitting stopper
was used as the reaction vessel. The pH of the 4-NP solution was
adjusted to 8.5. At this pH, 4-NP is deprotonated. The participation
of 1O2 in the photolysis of 4-NP was further confirmed by the addi-
tion of sodium azide, a singlet oxygen quencher, to the photolysis
reaction containing the 4-NP and suspended ads–MPc–SWCNT.
The photocatalysis products were analysed using gas chro-
matography (GC). For the GC analysis, Agilent Technologies 6820
GC system (HP 5973, HP-1 column) was employed. The photolysed
solutions were filtered to remove the MPc-SWCNT particles before
GC analysis.
2.5. Synthesis of complexes 2b and 2c
Synthesis of 1a–e and 2a (hence 4a) have been reported
[9,18,19]. Synthesis of compound 3 has also been reported [22],
Scheme 1.
2.5.1. 3,6-Di(octylthiol)-4,5-dicyanobenzene (4b)
Following literature methods for synthesis of substituted
phthalonitriles [20], 4b was synthesized as follows: octanethiol
(2.01 g, 20 mmol) was dissolved in dimethylsulfoxide (DMSO)
(15 ml) under nitrogen and 3 (2 g, 10 mmol) was added. After stir-
ring for 15 min, finely ground anhydrous potassium carbonate (6 g,
43.4 mmol) was added portion-wise within 2 h with efficient stir-
ring. The reaction mixture was stirred under nitrogen at room
temperature for 12 h. Then water (30 ml) was added and the aque-
ous phase extracted with chloroform (3× 20 ml). The combined
extracts were treated first with sodium carbonate solution (5%), fol-
lowed by water. The solvent was evaporated and the product was
crystallized from ethanol. Yield: 2.4 g (79%). 1H NMR (400 mHz); ı
ppm (CDCl3) 7.52 (2H, s, Ar–H), 3.06–3.02 (4H, t, –CH2), 1.74–1.68
(4H, m, –CH2), 1.48–1.45 (4H, m, –CH2), 1.31–1.29, (16H, m,
–(CH2)4), 0.93–0.90 (6H, m, –CH3). IR (KBr pellets) vmax/cm−1]:
3469, 3094, 3021, 2934, 2871, 2574, 2232(C N), 1827, 1573, 1469,
1226, 1127, 916, 734(C–S), 687, 539.
2.4. Triplet and singlet oxygen quantum yield parameters
Triplet quantum yields were determined using a comparative
method based on triplet decay, using Eq. (1):
2.5.2. 3,6-Di(dodecylthiol)-4,5-dicyanobenzene (4c)
The same method used for the synthesis of 4b was also used for
4c except dodecylthiol was employed instead of octanethiol. The
amounts of reagents used were: dodecanethiol (3.01 g, 20 mmol),
3 (2 g, 10 mmol), potassium carbonate (6 g, 43.4 mmol). Yield: 4.4 g
(85%). 1H NMR (400 mHz); ı ppm (CDCl3): 7.52 (2H, s, Ar–H),
3.04–3.01 (4H, t, –CH2), 2.71–2.68 (4H, m, –CH2), 1.70–1.66, (12H,
m, –(CH2)3), 1.29–1.24, (12H, m, –(CH2)3), 0.93–0.90 (18H, m,
–(CH2)3–CH3). IR (KBr pellets) vmax/cm−1]: 3468, 3090, 3024,
2931, 2868, 2577, 2231(C N), 1822, 1579, 1462, 1353, 1129, 921,
741(C–S), 683.
ꢃASTampleεSTtd
ꢃASTtdεSTample
Sample
T
˚
= ˚STtd
(1)
where ꢃATSample and ꢃATStd are the changes in the triplet state
absorbance of the PdPc derivatives and the standard, respectively.
εSTample and εTStd are the triplet state extinction coefficients for
the PdPc derivatives (2a–c) and standard, respectively. ˚STtd is
the triplet state quantum yield for the standard, ZnPc in 1-CNP,
Std
˚
= 0.67 [21]. Triplet lifetimes were determined by exponential
fitTting of the kinetic curves using OriginPro 7.5 software.
The DPBF (or ADMA) quantum yields (˚DPBF) were calculated
using Eq. (2) and the determined extinction coefficient of DPBF in
1-chloronaphthalene (ε1-CNP = 20287 M L−1 cm−1).
2.5.3. 1,4,8,11,15,18,22,25-Octakis(octylthiophthalocyaninato)
palladium(II) (2b)
In
refluxing
pentanol
(10 ml),
3,6-bis(octylthio)-4,5-
dicyanobenzene (4b) (0.42 g, 1 mmol), PdCl2 (0.08 g, 0.45 mmol)
and DBU (1.70 g, 11.17 mmol) were added. The solution was
heated to reflux for 5 h. The solution was then allowed to cool
and the solvent removed under reduced pressure, followed by
(C0 − Ct)VR
˚
DPBF
=
(2)
Iabs
t

T.B. Ogunbayo, T. Nyokong / Journal of Molecular Catalysis A: Chemical 337 (2011) 68–76
71
titration with cold methanol to precipitate the product. The black
precipitate was dissolved in DCM and passed through a silica
column, DCM was used as the eluting solvent to afford 2b. Yield:
0.22 g (56%). UV/vis [(1-chloronaphthalene/ꢁmax/nm (log ε)] 756
(4.6), 679 (4.25). C96H144N8PdS8: Cald.: C, 59.34; H, 7.40; N, 5.70%;
Found, C, 59.02; H, 8.01; N,5.12%. 1H NMR (400 MHz, CDCl3)
ıH/ppm: 8.22 (8H, s, Ar–H), 3.40 (16H, m, S–CH2), 1.61–1.31 (120H,
m, –(CH2)6–CH3)); Calc. MS (ESI–MS) m/z: Calc. 1773.16; Found
[M]+: 1772.22 [IR (KBr pellets) vmax/cm−1]: 2962, 2930, 2858,
1466, 1414, 1381, 1264, 1143, 1078, 969, 874, 747(C–S).
2a
2b
2c
350
450
550
650
750
850
Wavelength nm.
2.5.4. 1,4,8,11,15,18,22,25-Octkis(dodecylthiophthalocyaninato)
palladium(II) (2c)
Fig. 2. UV–vis spectra of complexes 2a–2c in 1-CNP. Concentration ∼1 × 10−5 M.
The same method used for 2b was employed for the syn-
thesis of 2c, except that 4c instead of 4b was employed. The
amounts of the reagents were the same. Yield: 0.28 g (53%). UV/vis
[(1-chloronaphthalene/ꢁmax/nm (log ε)] 756 (4.71), 679 (4.28).
C130H212N8PdS8: Cald.: C, 64.43; H, 8.70; N, 4.60%. Found, C, 63.94;
H, 8.51; N, 4.19%; 1H NMR (400 MHz, CDCl3) ıH/ppm: 8.32 (8H, s,
Ar–H), 3.41 (16H, m, S–CH2), 1.22–0.77 (184H, m, –(CH2)10–CH3));
Calc. MS (ESI–MS) m/z: Calc. 2251.07; Found [M]+: 2250.12 [IR (KBr
pellets) vmax/cm−1]: 2929, 2858, 1729, 1472, 1377, 1269, 1108, 998,
818, 751 (C–S), 647.
Table 1
Spectroscopic, photophysical and photochemical data for complexes 2a–2c in 1-
CNP.
Complex
Q band maxima
˚
˚
ꢄT (␮s)
13
11
8
T
ꢃ
2a
2b
2c
756
756
756
0.39
0.37
0.37
0.42
0.41
0.39
percentage carbon values that were within 1% in all cases, which
are within acceptable range for phthalocyanine complexes [24].
Fig. 2 shows the ground state electronic absorption spectra of
compounds 2a–2c in 1-CNP. The Q band maxima were observed at
756 nm (Table 1). These values were 745 nm in DCM. The complexes
were not aggregated in 1-CNP, and Beer–Lambert law was obeyed
for concentration below 1 × 10−5 M.
2.6. Purification and functionalisation of SWCNTs
SWCNTs were purified (by oxidation) to form SWCNT–COOH
by adding raw SWCNTs (100 mg) to a mixture of concentrated
HNO3 and concentrated H2SO4 (3:1) [23] The resulting suspen-
sion was stirred at a temperature of 70 ◦C for 2 h. The final mixture
was cooled to room temperature and washed with excess milli-
pore water until a pH of 5 was obtained. The purified SWCNTs
(SWCNT–COOH) were dried in an oven for 12 h.
Fig. 3a shows the transient absorption spectrum of complex 2c
in 1-CNP and Fig. 3b shows the triplet decay curve for 2c in 1-
CNP. The triplet absorption was observed at 580 nm, Fig. 3a (insert).
The triplet quantum yields (˚T) for 2a, 2b and 2c were 0.42, 0.41
2.7. Immobilisation of MPcs on SWCNTs
0.05
a
Each of the PdPc derivatives was dissolved in DCM to give an
absorbance of approximately 2. Purified SWCNTs (50 mg) were
added to each of the PdPc solutions. The mixture was stirred
until there was no change in absorbance of the MPcs. The mix-
ture was then centrifuged and the supernatant was decanted,
leaving the particles of MPcs adsorbed on SWCNTs (represented
as ads–MPc–SWCNT). The particles were washed with distilled
deionised water, methanol and acetone, followed by air drying for
24 h. The concentration of MPcs adsorbed was calculated from the
differences in absorbances to be 1.0 × 10−8 M.
0
500
-0.05
550
600
650
700
750
800
850
900
- 0.1
- 0.15
- 0.2
0.015
0
540
590
Wavelength(nm)
640
- 0.25
- 0.3
- 0.35
- 0.4
3. Results and discussions
Wavelength (nm)
3.1. Characterization of complexes 2a–2c
b
Synthesis of 2a has been reported [19], but the photophysical
and photochemical properties are reported in this work.
0.15
0.10
0.05
0.00
-0.05
Complexes 2b and 2c were synthesized by treating the corre-
sponding phthalonitriles (4b and 4c) with palladium chloride in
1-pentanol using DBU as catalyst, Scheme 1. Column chromatog-
raphy with silica gel was employed to obtain the pure products.
The MPc derivatives were characterized by UV–vis, mass, IR and
NMR spectroscopies, and elemental analyses. The sharp peak for
the C N vibrations in the IR spectra of phthalonitriles at 2231 (4c)
and 2232 cm−1 (4b) disappeared after conversion into phthalo-
cyanines. The phthalocyanines showed vibrations due to C–S–C
group between 751 and 747 cm−1. Mass and proton NMR spectral
data as well as elemental analysis were additionally employed to
characterize the complexes, and the data are consistent with the
structures of the complexes. The elemental analyses results gave
0.0000
0.0005
Time (s)
Fig. 3. (a) Transient absorption spectrum of complex 2c in 1-CNP. Inset = triplet state
absorption maximum. (b) Triplet decay curve of 2c in 1-CNP at 590 nm. Excitation
wavelength = 750 nm.

72
T.B. Ogunbayo, T. Nyokong / Journal of Molecular Catalysis A: Chemical 337 (2011) 68–76
a
350
450
550
650
750
Wavelenght (nm)
b
200
250
300
350
400
450
500
Wavelength(nm)
Fig. 4. Spectral changes showing (a) the disappearance of complex 1a dur-
ing its immobilization on SWCNT–COOH (200 mg). Initial concentration of
1a = 1.2 × 10−5 mol dm−3; time = 50 min) and (b) adsorption of 4-NP on SWCNTs (in
the absence of MPc complexes).
and 0.39, respectively in 1-CNP (Table 1). These values are similar
to those of complexes 1a–d reported before, which ranged from
0.40 to 0.51, with the exception of 1e which gave a large value of
˚T = 0.65 [9]. The triplet lifetimes (ꢄT) were 13 ␮s (2a), 11 ␮s (2b)
and 8 ␮s (2c), and are also similar to those reported for 1a–1e. The
low values of ꢄT were expected due to the open-shell nature of the
Pd central metal. The singlet oxygen quantum yields (˚ꢃ) were
0.39 (2a), 0.37 (2b) and 0.37 (2c) in 1-CNP. The reported values for
1a–1e ranged between 0.39 and 0.49 in 1-CNP [9,18].
Fig. 5. XRD spectra of (a) SWCNT–COOH, (b) complex 1c and (c)
ads–1c–SWCNT–COOH.
3.2. UV–vis spectral studies of the immobilisation of MPcs on
SWCNTs to form ads–MPc–SWCNT
4-NP (an average of 5%), showing that some empty sites were
still left on SWCNTs where 4-NP could adsorb. The final spec-
trum following the adsorption of 4-NP (before photolysis) was used
as the starting spectrum for all calculations done in this work.
Experiments were also performed where PdPc derivatives were
suspended in a solution of 4-NP without photolysis, and there were
no changes in spectra confirming that the latter does not adsorb on
the former. Experiments were also performed where SWCNTs fol-
lowing adsorption of 4-NP was photolysed in pH 8.5 buffer only.
There were no changes in absorbance which could be attributed to
leaching of 4-NP (or its oxidation products) from the SWCNTs.
Immobilization of PdPc complexes on SWCNTs was carried out
by stirring DCM solutions of the PdPc derivatives in the presence
of SWCNTs until there was no change in absorbance of the former,
this was done over a period of 5 h or more depending on the PdPc
derivative. The changes in absorbance of solution of complex 1a in
DCM over time during stirring in the presence of SWCNTs are shown
in Fig. 4a. The spectrum of complex 1a shows extensive aggrega-
tion in DCM as has been reported before [18]. Aggregation is judged
by the presence of a broad band near 600 nm due to the aggregate
and a sharper one at 650 nm, due to the monomer. Aggregation
was displayed by all complexes. The small peak at 690 nm has been
explained in previous study [18] as being due to intermolecular
interaction between the central metal ion of one molecule with the
thio group of another. On addition of SWCNTs to solution of 1a, fol-
lowed by stirring, there was a decrease in absorbance of complex 1a
as it adsorbs onto SWCNTs, Fig. 4a. The resulting conjugate is rep-
resented as ads–MPc–SWCNT–COOH. The peak at 690 nm however
persists during immobilization suggesting that the interactions are
still present.
3.3. Characterization ads–MPc–SWCNT
3.3.1. XRD
X-ray diffraction technique was used to confirm the formation
of SWCNT-MPc composites. Variations in nature and positions of
peaks on XRD spectra are reflective of changes in structural features
of the sample under consideration. Fig. 5 shows the XRD spectra
of SWCNT–COOH, complex 1c alone and ads–1c–SWCNT–COOH.
Fig. 5a (for SWCNT–COOH) shows sharp (2ꢂ = 44.5◦ and 51.9◦) and
broad (2ꢂ = 29.4◦ and 25.9◦) peaks that are indicative of the crys-
talline and amorphous natures of the SWCNT–COOH, respectively.
Using international centre for diffraction data (ICDD) database,
the peak at 2ꢂ = 25.9◦ are ascribed to (0 0 2) d-spacing of the
SWCNT–COOH [27,28], while peaks at 2ꢂ = 44.4◦ and 51.9◦ are char-
SWCNTs are known to directly adsorb phenols [25,26] as
shown in Fig. 4b. In order to check if following immobilization
of PdPc derivatives onto SWCNTs, there were still parts of SWC-
NTs which were exposed, experiments were carried out where
ads–MPc–SWCNT–COOH was immersed in a solution of 4-NP with-
out photolysis. There was a slight decrease in the absorbance of

T.B. Ogunbayo, T. Nyokong / Journal of Molecular Catalysis A: Chemical 337 (2011) 68–76
73
Fig. 6. TEM images of (a) SWCNTs (magnification 10,000×) and (b) ads–1c–SWCNT (magnification 20,000×).
of 6.0 × 10−5 mol dm−3 ADMA containing a suspension of each
of the ads–MPc–SWCNT conjugates, was successively irradiated
at the Q-band of MPc, centrifuged and decanted into UV–vis cell
and absorbance recorded. The change in absorbance of ADMA at
379 nm (Fig. 7) shows that singlet oxygen is formed on photolysis
of ads–MPc–SWCNT in the presence of ADMA. The spectra of the
MPcs were not observed since they acted as heterogenous catalysts
in the reactions. The rather low ˚ꢃ values of ads–MPc–SWCNT
(0.19–0.27, Table 2) under heterogenous condition compared with
what was obtained in solution [9] are ascribed to inefficient energy
transfer to ground state oxygen or the solid nature of the molecules
(in the former) which might encourage more aggregation.
acteristic of (1 0 0) [27], and (2 0 0) [28] reflections of carbon of the
SWCNT–COOH, respectively. The only prominent peak in the XRD
spectrum of complex 1c was a broad one at 2ꢂ = 24.1◦ (Fig. 5b).
Its broadness is an indication of the amorphous nature of complex
1c. In the XRD spectrum ads–1c–SWCNT–COOH (Fig. 5c), the peak
near 25◦ encompasses both 1c and SWCNT–COOH. The differences
in spectra indirectly suggest the presence of a conjugate of SWCNTs
and 1c.
3.3.2. Raman spectroscopy
Raman spectroscopy is used for the characterization of dis-
ordered polycrystalline graphitic carbons. The Raman spectra of
graphite show a pair of bands at about 1360 and at 1560 cm−1
.
These two bands are the most diagnostic features, and are desig-
nated as D and G bands, respectively. Raman spectroscopy can be
used to some extent to quantify changes in the SWCNTs using the
ratio of D/G bands under fixed laser power intensity.
3.5. Photocatalytic behavior of ads–MPc–SWCNT towards 4-NP
UV–vis spectroscopy was employed to monitor the transforma-
tion of 4-NP. Since the pKa of 4-nitrophenol is 7.15 these studies
were carried out in aqueous solution at pH 8.5 because pheno-
late ions which are more oxidizable are predominant at these
conditions. It is important to state that no change in spectra of 4-
nitrophenol was observed when photolysis was done in the absence
of ads–MPc–SWCNT. Also no transformation was observed when
4-nitrophenol was stirred in the presence of ads–MPc–SWCNT
without photolysis. During photolysis at the Q band region of the
PdPc derivatives, in the presence of suspended ads–MPc–SWCNT
(Fig. 8a), there was a gradual collapse of the 4-NP peaks at 250 and
400 nm. There was also an increase in the absorbance near 220 nm,
possibly due to the photolysis products, however, absorption in this
region will be affected by solvents. It is important to note that the
Adsorption of MPc complexes may not cause extensive disrup-
tion to the carbon lattice due to the non-invasive ␲-␲ interactions
i.e. preservation of the carbon nanotube structure, however small
changes in the D/G ratio may indicate the presence of MPcs on
the SWCNTs skeleton. We observed the G band at 1593 cm−1 for
SWCNT–COOH and the D band at 1382 cm−1, which shifted to
1490 cm−1 for ads–MPc–SWCNT (figures not shown). Functional-
ization of SWCNTs is known to enhance the D band [29–31] hence
the increase in the D/G ratio from 0.47 for SWCNT–COOH to 0.85 for
ads–MPc–SWCNT, indirectly confirms adsorption of PdPc deriva-
tives on SWCNT–COOH.
3.3.3. Transmission electron microscopic (TEM) characterization
TEM technique provided
a
microscopic view of the
25
20
15
10
5
ads–MPc–SWCNT–COOH. Fig. 6a is the TEM image of
SWCNT–COOH. The image obtained after the formation of the
adsorbed specie, ads–1c–SWCNT–COOH (Fig. 6b) was completely
different showing coverage of the large portion of the surface of
SWCNTs with PdPc derivatives.
0
20000
25000
30000
[1/[ADMA]
35000
40000
3.4. Singlet oxygen generation capacity of ads–MPc–SWCNT in
pH 9 buffer
Since singlet oxygen is thought to be involved in the pho-
tocatalysis mechanism, its generation by ads–MPc–SWCNT was
investigated. As stated in the experimental, different interference
filters were employed for peripheral (1a–1e) and non-peripheral
2a–2c, to ensure excitation of the low energy peak due to the
monomer of the adsorbed Pc. This assumes that Q band max-
ima of the absorbed specie are about the same as that in solution
with the expected broadening in the former. An aqueous solution
300
350
400
450
Wavelength nm.
Fig. 7. Electronic absorption spectral changes observed during the photolysis of
(a) ADMA to confirm singlet oxygen production by ads–2c–SWCNT. Initial ADMA
concentration = 6.0 × 10−5 mol dm−3; time = 60 min. Inset: plot of 1/˚ADMA versus
1/[ADMA] for the determination of singlet oxygen quantum yield using Eq. (4).

74
T.B. Ogunbayo, T. Nyokong / Journal of Molecular Catalysis A: Chemical 337 (2011) 68–76
Table 2
Langmuir–Hinshelwood (L–H) parameters for the phototransformation of 4-NP on ads–MPc–SWCNT and singlet quantum yields of the complexes (in the presence of SWCNTs)
at pH 8.5.
Complex
4-NP (M)
kobs (×10−3 min−1
)
˚
ꢃ
kr (×10−6 M min−1
)
Kads (×103 M−1
)
4.00 × 10−5
6.00 × 10−5
8.00 × 10−5
1.00 × 10−4
4.00 × 10−5
6.00 × 10−5
8.00 × 10−5
1.00 × 10−4
4.00 × 10−5
6.00 × 10−5
8.00 × 10−5
1.00 × 10−4
4.00 × 10−5
6.00 × 10−5
8.00 × 10−5
1.00 × 10−4
4.00 × 10−5
6.00 × 10−5
8.00 × 10−5
1.00 × 10−4
4.00 × 10−5
6.00 × 10−5
8.00 × 10−5
1.00 × 10−4
4.00 × 10−5
6.00 × 10−5
8.00 × 10−5
1.00 × 10−4
4.00 × 10−5
6.00 × 10−5
8.00 × 10−5
1.00 × 10−4
3.00
2.25
1.52
0.61
2.7
0.22
0.22
0.27
0.19
0.20
0.23
0.21
0.20
1.3
0.51
Ads–1a–SWCNT
1.1
1.8
0.7
1.0
1.4
1.3
1.9
0.42
3.8
1b
1c
1d
1e
2a
2b
2c
2.1
1.32
0.42
3.16
2.40
1.74
0.75
2.22
1.64
0.92
0.11
2.51
1.95
1.27
0.32
3.19
2.27
1.67
0.63
2.79
2.41
1.33
0.54
3.29
2.67
1.86
0.98
0.19
0.37
0.33
0.32
2.8
spectra in Fig. 8a were recorded after allowing the initial adsorption
of small amounts of 4-NP onto SWCNTs (without photolysis), until
no change in electronic absorption spectra of 4-NP was observed.
The spectral changes shown in Fig. 8a were also observed for SWC-
NTs composite of complexes 1b, 1c, 1d, 1e, 2a, 2b and 2c though
the rate of decrease in the 4-NP varied significantly.
mediates and phenolic oxidant products [32,33].
1
rate
1
Kr
1
=
+
(5)
KrKadsC0
where kr is the rate constant for the adsorption of 4-NP, C0 is the
initial concentration of the substrate (in this case 4-NP). Kads is the
adsorption coefficient and represents the equilibrium between the
rates of adsorption and desorption [34]. The adsorption studies here
are for 4-NP (or intermediates) on PdPc derivatives.
3.6. Kinetic studies for photolysis of 4-NP
The plots of reciprocal of initial rate of phototransformation
(after 60 min of irradiation) versus reciprocal of initial concen-
tration of 4-NP (Fig. 8c) gives kr as the intercept, the adsorption
coefficient (Kads) can now be determined from the slope. For the
MPc-SWCNT composites of 1a–1e, 2a–2c, the plots were found
to be linear with a non-zero intercept. The values of kr and Kads
are shown in Table 2. According to Table 2, adsorption rate (kr)
was the lowest for ads–1d–SWCNT where 1d is substituted with
benzyl groups and ads–1c–SWCNT and ads–2c–SWCNT where
complexes 1c and 2c contain the longest alkyl chain, gave the
largest kr values, Table 2. Thus the adsorption rate of 4-NP is high-
est for long chain alkyl phthalocyanines on SWCNTs than for aryl
group substituted phthalocyanines. Also for ads–1c–SWCNT and
ads–2c–SWCNT adsorption of 4-NP (or its phototransformation
products) is more favoured over desorption compared to the rest
of the complexes.
As shown in Fig. 8b the kinetics of transformation of 4-
nitrophenol fitted into pseudo first order reaction kinetics. The
apparent rate constant kobs was obtained from the slopes in
Fig. 8b for all the MPcs composites for different concentrations of
4-nitrophenol ranging from 4 × 10−5 to 10 × 10−5 M. The apparent
rate of reaction (kobs) for all ads–MPc–SWCNT in Table 2, show that
the more dilute the 4-NP solution, the higher the apparent rate
for transformation. The kobs values ranged from 6.10 × 10−4 min−1
to 3.00 × 10−3 min−1 (ads–1a–SWCNT), 4.20 × 10−4 min−1 to
2.70 × 10−3 min−1
3.16 × 10−3 min−1
2.22 × 10−3 min−1
2.51 × 10−3 min−1
(ads–1b–SWCNT),
(ads–1c–SWCNT),
(ads–1d–SWCNT),
(ads–1e–SWCNT),
7.50 × 10−4 min−1
1.10 × 10−4 min−1
3.20 × 10−4 min−1
6.30 × 10−4 min−1
to
to
to
to
3.19 × 10−3 min−1 (ads–2a–SWCNT), 5.40 × 10−4 min−1 and
2.79 × 10−3 min−1 (ads–2b–SWCNT) and 9.80 × 10−4 min−1 to
3.29 × 10−3 min−1(ads–2c–SWCNT) for 4-NP concentrations of 4,
6, 8 and 10 × 10−5 M.
3.7. Catalyst stability
It is important to establish if the transformation reaction takes
place following adsorption and to determine the adsorption coef-
ficients. Langmuir–Hinshelwood model was employed for these
studies.
Following use for the photogegradation of 4-NP, the
ads–MPc–SWCNT were cleaned by rinsing in water, dried and
reused for the phototransformation of 4-NP. Fig. 9 shows the
plots of concentration versus time, for the first, second and third
uses of ads–1c–SWCNT for the phototransformation of fresh
Langmuir–Hinshelwood (L–H) equation (Eq. (5)) has been used
to describe the competitive adsorption of substrates, reaction inter-

T.B. Ogunbayo, T. Nyokong / Journal of Molecular Catalysis A: Chemical 337 (2011) 68–76
75
a
200
300
400
500
600
Wavelength nm.
1.2
b
(i)
1
0.8
0.6
0.4
0.2
0
(ii)
(iii)
(iv)
Fig. 10. Gas chromatogram of the photolysed 4-NP sample by
ads–1c–SWCNT–COOH. Irradiation time = 24 h, starting 4-NP concentra-
tion = 1 × 10−4 mol dm−3
.
0
100
200
300
400
Irradiation time (min)
where decreases in stabilities by 15–20% have been reported [10],
the present PdPc catalyst shows improved stability.
10
9
c
y = 1.8809x + 5.2748
3.8. Mechansim of 4-NP phototransformation
8
GC was used to determine the nature of the products of
phototransformation reaction by comparing the retention times
of expected standards for the catalyzed phototransformation of
4-nitrophenol. These include fumaric acid, 4-nitrocatechol, 1,4-
benzoquinone and hydroquinone. Two new peaks which were
observed on the gas chromatogram traces, Fig. 10, after irradia-
tion for 24 h. These were identified (using spiking with standards)
as hydroquinone and 1,4-benzoquinone. According to the chro-
matograms hydroquinone was present in a small quantity while
the major product was benzoquinone, which was the final product,
The contribution of singlet oxygen in the phototransformation pro-
cess was determined by using ads–1c–SWCNT in the presence of
a singlet oxygen scavenger, sodium azide. The rate of the reaction
was much slower in the presence of NaN3, Fig. 11, meaning that the
reaction proceeded through mainly Type II mechanisms, Scheme 2.
When the reaction medium was deareated the two products were
still present but benzoquinone appeared to be in smaller quanti-
ties than in air, suggesting possible involvement of Type I (radical)
mechanism, giving mainly hydroquinone while Type II (singlet oxy-
gen) mechanism gives mainly benzoquinone. The production of
benzoquinone is likely from the direct oxidation of 4-NP by singlet
oxygen as observed for 4-chlorophenol [35]. The products obtained
7
6
5
0
1
2
3
1/Co (M-1) x 103
Fig. 8. (a) Spectral changes observed during photolysis of 4-NP using
ads–1a–SWCNT–COOH. Initial 4-NP concentration = 1.00 × 10−4 mol dm−3. (b) Plot
of change in concentration with time during photolysis of 4-NP for different starting
4-NP concentrations using ads–1c–SWCNT–COOH as a photocatalyst ((i) 4 × 10−5
,
(ii) 6 × 10−5, (iii) 8 × 10−5 and (iv) =10 × 10−5 M), and (c) Langmuir–Hinshelwood
kinetic plot for ads–2c–SWCNT–COOH in basic media.
solution of 4-NP. The rates changed from 1.21 × 10−6 M min−1
for the first use to 1.11 × 10−6 M min−1 for the second use and to
1.04 × 10−6 M min−1 for the third use. The rate decreased gradually
by ∼7% with each run beyond the three runs shown in Fig. 9. The
gradual reduction in rate might be due to permanent adsorption
of intermediates on the surface of the catalysts thereby reducing
their activities or due to degradation of the PdPc catalysts. TEM,
XRD and Raman showed that the catalysts were still intact on the
surface of the SWCNTs, since there were no changes in the images
or spectra following the first uses. Compared to ZnPc derivatives
(With NaN3)
12
10
8
First run
Second run
Third run
10
9
y = -1.95E-03x + 9.87E+00
(Without NaN3)
6
8
y = -1.11E-02x + 9.87E+00
4
7
2
6
5
0
0
50
100
150
200
250
300
350
400
0
50
100
150
200
250
300
350
400
Irradiaꢀon ꢀme (min)
Irradiaꢀon ꢀme (min)
Fig. 9. Plots of concentration versus time for the reuse of ads–1c–SWCNT–COOH for
the phototransformation of 4-nitrophenol. pH 9 buffer.
Fig. 11. Demonstration of the effect of sodium azide on 4-NP photo transformation
rate using ads–1c–SWCNT–COOH.

76
T.B. Ogunbayo, T. Nyokong / Journal of Molecular Catalysis A: Chemical 337 (2011) 68–76
References
O
OH
O
[1] F.J. Zieris, D. Feind, W. Huber, Arch. Environ. Contam. 17 (1988) 165–175.
[2] S. Laha, K.P. Petrova, Biodegradation 8 (1998) 349–356.
[3] S.X. Li, F.Y. Zheng, W.L. Cai, A.Q. Han, Y.K. Xie, J. Hazard. Mater. 135 (2006) 431.
[4] S.X. Li, F.Y. Zheng, X.L. Liu, F. Wu, N.S. Deng, J.H. Yang, Chemosphere 61 (2005)
589.
[5] A.E.H. Machado, J.A. de Miranda, R.F. de Freitas, E.T.F.M. Duarte, L.F. Ferreira,
Y.D.T. Albuquerque, R. Ruggiero, C. Sattler, L. de Oliveira, J. Photochem. Photo-
biol. A: Chem. 155 (2003) 231–241.
1O2
+
O
OH
NO2
Benzoquinone
Hydroquinone
4-Nitrophenol
[6] E. Pepe, O. Abbas, C. Rebufa, M. Simon, S. Lacombe, M. Julliard, J. Photochem.
Photobiol. A: Chem. 170 (2005) 143–149.
Scheme 2. Proposed mechanism for the phototransformation of 4-NP on
ads–MPc–SWCNT–COOH.
[7] V. Iliev, J. Photochem. Photobiol. A: Chem. 151 (2002) 195–199.
[8] Z. Xiong, Y. Xu, L. Zhu, J. Zhao, Environ. Sci. Technol. 39 (2005) 651–657.
[9] T.B. Ogunbayo, T. Nyokong, J. Mol. Struct. 973 (2010) 96–103.
[10] E. Marais, R. Klein, E. Antunes, T. Nyokong, J. Mol. Catal. A: Chem. 261 (2007)
36.
are similar to those obtained (as intermediates) in the past work for
heterogenous photo-oxidation of 4-nitrophenol [36]. Fumaric acid
and 4-nitrocatechol were also observed as final products for ZnPc
derivatives suspended in solution [36]. However, the use of SWCNT
as supports has advantages of ease of immobilization of MPcs and
ease of removal from solution. Preliminary studies on transforma-
tion of other phenols (pentachlorophenol) using PdPc complexes
immobilized on SWCNT show faster rates of phototransformation.
[11] K. Ozoemena, N. Kuznetsova, T. Nyokong, J. Mol. Catal. A: Chem. 176 (2001) 29.
[12] K.I. Ozoemena, J. Pillay, T. Nyokong, Electrochem. Commun.
1391–1396.
8 (2006)
[13] J.H. Zagal, S. Griveau, K.I. Ozoemena, T. Nyokong, F. Bedioui, J. Nanosci. Nan-
otechnol. 9 (2009) 2201–2214.
[14] T. Mugadza, T. Nyokong, Electrochim. Acta 55 (2010) 2606–2613.
[15] W. Freyer, D. Leupold, J. Photochem. Photobiol. A: Chem. 105 (1997) 153–158.
[16] W. Freyer, H. Stiel, M. Hild, K. Teuchner, D. Leupold, Photochem. Photobiol. 66
(1997) 596–604.
[17] W.H. Chen, K.E. Rieckhoff, E.M. Voigt, Chem. Phys. 102 (1986) 193–203.
[18] T.B. Ogunbayo, A. Ogunsipe, T. Nyokong, Dyes Pigments 82 (2009) 422–426.
[19] T.B. Ogunbayo, T. Nyokong, Polyhedron 28 (2009) 2710–2718.
[20] S. Wolfgang, K. Holger, W. Dieter, H. Steffen, R. Beate, S. Gunter, J. Porphyrins
Phthalocyanines 2 (1998) 145–158.
4. Conclusions
[21] D.S. Lawrence, D.G. Whitten, Photochem. Photobiol. 64 (1996) 923–935.
[22] G. Mbambisa, T. Nyokong, Polyhedron 27 (2008) 2799–2804.
[23] J. Liu, A.G. Rinzler, H. Dai, J.H. Hafner, R.K. Bradley, P.J. Boul, A. Lu, T. Iverson, K.
Shelimov, C.B. Huffman, F. Rodriguez-Macias, T.Y.-S. Shon, R. Lee, D.T. Colbert,
R.E. Smalley, Science 280 (1998) 1253–1256.
This work demonstrates that the complexes investigated are
capable of adsorption onto single wall carbon nanotubes (SWC-
NTs) forming conjugates. These conjugates were employed for
the photooxidation of 4-nitrophenol. Adsorbed ␤-palladium
[24] T.E. Youssef, Polyhedron 29 (2010) 1776.
octakis(dodecylthio)phthalocyanine
(1c)
and
␣-palladium
[25] L. Ji, F. Liu, Z. Xu, S. Zheng, D. Zhu, Environ. Sci. Technol. 43 (2009) 7870–7876.
[26] L. Ji, Y. Shao, Z. Xu, S. Zheng, D. Zhu, Environ. Sci. Technol. 44 (2010) 6429–6436.
[27] M. Terrones, W.K. Hsu, A. Schwoerer, K. Prassides, H.W. Kroto, D.R. Walton,
Appl. Phys. A 66 (1998) 307–317.
[28] Y. Zhang, X. Sun, L. Pan, H. Li, Z. Sun, C. Sun, B.K. Tay, J. Alloys Compd. 480 (2009)
L17–L19.
octakis(dodecylthio)phthalocyanine (2c) gave the best perfor-
mance in terms of adsorption rate of 4-NP. Hydroquinone and
benzoquinone were observed as the phototransformation products
of 4-NP.
[29] B.K. Price, J.M. Tour, J. Am. Chem. Soc. 128 (2006) 12899–12904.
[30] J. Jiang, R. Saito, A. Grüneis, S.G. Chou, Ge.G. Samsonidze, A. Jorio, G. Dresselhaus,
M.S. Dresselhaus, Phys. Rev. B 71 (2005) 205420/1–205420/13.
[31] M.D. Ellison, P.J. Gasda, J. Phys. Chem. 112C (2008) 738–740.
[32] H. Al-Ekabi, N. Serpone, J. Phys. Chem. 92 (1988) 5726–5731.
[33] D.D. Dionysiou, A.P. Khodadoust, A.M. Kern, M.T. Suidan, I. Baudin, J.-M. Laîné,
Appl. Catal. B: Environ. 24 (2000) 139–155.
[34] K.J. Laider, J.H. Meiser (Eds.), Sanctuary Physical Chemistry, Fourth ed.,
Houghton Mifflin Company, Boston, 2003, p. 933.
[35] E. Sibva, M.M. Pereira, H.D. Burrows, M.E. Azenha, M. Sarakha, M. Bolte, Pho-
tochem. Photobiol. Sci. 3 (2004) 200–204.
Acknowledgements
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. T.O. thanks African Laser centre for graduate
bursary.
[36] E. Marais, E. Antunes, T. Nyokong, J. Coord. Chem. 61 (2008) 3727.