Investigation of homogeneous photosensitized oxidation activities of palladium and platinum octasubstituted phthalocyanines: Oxidation of 4-nitrophenol

Article abstract of DOI:10.1016/j.molcata.2010.11.008

Photosensitized oxidation of 4-nitrophenol was studied in organic solutions with seven octasubstituted thio and aryloxy palladium and platinum phthalocyanines acting as photosensitizers. Kinetic studies conducted also showed that the complexes have different singlet oxygen quenching constants with direct implication on the quantum yield of photodegradation of 4-nitrophenol (Φ_4-NP). Palladium analogues gave better results than the platinum analogues in terms of Φ_4-NP with palladium-(dodecylthio) phthalocyanine giving the highest yield of 1.8 × 10^-3. Gas chromatography (GC) and liquid chromatography connected to a mass spectrometer (LC-MS) were used to confirm the photodegradation products which were hydroquinone and 1,4-benzoquinone.

Full text of DOI:10.1016/j.molcata.2010.11.008

Journal of Molecular Catalysis A: Chemical 334 (2011) 123–129
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Investigation of homogeneous photosensitized oxidation activities of palladium
and platinum octasubstituted phthalocyanines: Oxidation of 4-nitrophenol
∗
Taofeek B. Ogunbayo, Edith Antunes, 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:
Received 11 August 2010
Received in revised form 4 November 2010
Accepted 8 November 2010
Available online 18 November 2010
Photosensitized oxidation of 4-nitrophenol was studied in organic solutions with seven octasubsti-
tuted thio and aryloxy palladium and platinum phthalocyanines acting as photosensitizers. Kinetic
studies conducted also showed that the complexes have different singlet oxygen quenching con-
stants with direct implication on the quantum yield of photodegradation of 4-nitrophenol (˚4-NP).
Palladium analogues gave better results than the platinum analogues in terms of ˚4-NP with palladium-
−3
(
dodecylthio)phthalocyanine giving the highest yield of 1.8 × 10 . Gas chromatography (GC) and liquid
Keywords:
chromatography connected to a mass spectrometer (LC–MS) were used to confirm the photodegradation
products which were hydroquinone and 1,4-benzoquinone.
Photosensitization
Phthalocyanines
Oxidation
©
2010 Elsevier B.V. All rights reserved.
4
-Nitrophenol
•−
•−
1
. Introduction
MPc + O → MPc + O2
(4)
2
+
H
·
−
·
Photodegradation of pollutants is gaining extensive attention in
O
−→ HO
(5)
(6)
2
2
the quest for efficient and benign conversion processes in the area
of environmental management. Photosensitized catalysis is the ini-
tiation of degradation or transformation reactions of molecules
using a combination of light and photoactive materials as catalysts.
Photosensitization of a reaction may proceed through two reac-
tion pathways namely Type I and Type II mechanisms [1–4]. Type
I mechanism (Eqs. (1)–(7)) involves the transformation of the pho-
•
•
HO2 + Subs-H → H O + Subs
2
2
•
•
+
Subs , Subs , H O → oxidation products
(7)
2
2
The Type II (Eqs. (8) and (9)) mechanism starts with the energy
transfer interaction between triplet state of the photosensitizers
MPc*) and ground state molecular oxygen ( O ) to give ener-
getic singlet oxygen ( O ), Eq. (8), which is capable of oxidizing
the substrate, Eq. (9).
3
3
(
2
1
2
togenerated triplet state (Eq. (1)) of a photosensitizer such as a
•
3
+
metallophthalocyanine ( MPc*) into a radical specie (MPc ) via
electron transfer interaction with ground state molecular oxygen
3
3
1
MPc ∗ + O → O
O + Subs → oxidation products
(8)
(9)
2
2
3
3
(
O ), Eq. (2). The excited triplet state of the MPc ( MPc*) can also
2
1
2
interact with ground state molecular oxygen (Eq. (4)) generating
superoxide and other radicals (including OH , O2 HO2 ) which
subsequently afford oxidation of the substrate (Eqs. (5)–(7)) by a
Type I mechanism. The excited triplet state of the MPc ( MPc*) can
also interact with the substrate molecules, Eq. (3) to give radical
species.
•
•
•
−
Metallophthalocyanines (MPcs) with their high absorption in
the visible region and non-destructive singlet to energetic triplet
state transition have been shown to photosensitize reactions
mainly through Type II mechanism. Considerable singlet to triplet
state transition and long life time of the triplet state are major
factors influencing successful singlet oxygen generation by MPc
complexes. MPc complexes containing large central metals are the
best choice for photosensitized reactions due to enhanced intersys-
tem crossing while closed shell diamagnetic metals improve triplet
state lifetime as against open-shell metals [5–9].
Photosensitized oxidation of phenols using MPc complexes has
been reported under both heterogeneous and homogeneous condi-
tions [10–17]. Even though heterogeneous conditions are preferred
in terms of ease of regeneration of the catalyst, homogeneous
reaction leads to a better understanding of mechanisms involved
3
1
3
MPc → MPc∗ → MPc∗
(1)
(2)
(3)
•+
•−
3
∗
MPc + O → MPc + O2
2
•−
•
3
MPc + Subs → MPc + Subs +
∗
∗ Corresponding author at: Department of Chemistry, Rhodes University, P.O. Box
4, Grahamstown 6140, South Africa. Tel.: +27 46 603 8260; fax: +27 46 622 5109.
E-mail address: t.nyokong@ru.ac.za (T. Nyokong).
9
1
381-1169/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2010.11.008

1
24
T.B. Ogunbayo et al. / Journal of Molecular Catalysis A: Chemical 334 (2011) 123–129
2
1
0
.5
2
.5
1
(
Abs:1.1, Time: 0 hr)
.5
(Abs: 0.2, Time: 12 hrs)
0
250
350
450
550
650
750
Wavelength nm.
Fig. 2. Electronic absorption spectral changes observed during the photolysis of
−
4
−1
−1
6
.5 × 10 mol L 4-NP in the presence of 350 mg L of complex 1c in DCM.
octakis(phenoxy)phthalocyaninato platinum (2d) and 2,3,9,10,
6,17,23,24-octakis(benzyloxyphenoxy)phthalocyanato platinum
2e) have been reported before [18].
1
(
2.2. Equipment and photosensitized oxidation procedures
The irradiation experiments were carried out using a tungsten
lamp (100 W, 30 V) perpendicular to the direction of measure-
ment. A 600 nm glass cut off filter (Schott) and a water filter were
used to filter off ultraviolet and infrared radiation respectively. An
interference filter (Intor, 670 nm with a band width of 40 nm) was
additionally placed in the light path before the sample, to ensure
the excitation of the Q band.
The intensity of the light reaching the reaction vessel was
measured with a power meter (POWER MAX 5100, Molelectron
Fig. 1. Structures of the PtPc and PdPc complexes employed as photosensitizers in
this work.
20
−2 −1
Detector Inc.) and was found to be 3.5 × 10 photons cm
s . The
transformation of the analyte was monitored through the absorp-
tion peak at 400 nm after each photolysis cycle using a Shimadzu
UV-2550 UV–vis spectrophotometer. A 1 cm pathlength UV–vis
spectrophotometric cell, fitted with a tight fitting stopper was used
as the reaction vessel. Experiments were carried out in DCM in
and improvement of the catalytic activity. The MPc photosensi-
tized oxidation of 4-nitrophenol (4-NP) has been reported under
homogeneous conditions [10], and the products obtained were
4
-nitrocatechol and hydroquinone. However considerable degra-
dation of the MPc photosensitizers was observed in some cases. The
search for photoresistant catalysts that can degrade 4-nitrophenol
to products of reduced toxicity led to the use, in this work, of
phthalocyanine complexes containing open-shell diamagnetic cen-
tral metals: PtPc and PdPc.
(1b)
a
(1a)
(1c)
In spite of their open-shell nature which theoretically is a
shortcoming due to short triplet lifetimes, PdPc and PtPc com-
plexes show high singlet oxygen quantum yields suggesting that
their lower lifetimes are still high enough for efficient applica-
tion as photosensitizers [18]. In addition, PdPcs have been found
to show higher photostability compared to other metallo phthalo-
cyanines [19,20]. PdPcs and PtPcs photosensitized oxidation of
5
00
550
(
600
650
700
750
800
4
-nitrophenol is reported in this work for the first time.
Wavelength (nm)
2
. Experimental
b
(2e)
2d)
(1e)
2.1. Materials
(
1d)
Dichloromethane (DCM), 4-nitrophenol, fumaric acid, 4-
nitrocatechol and hydroquinone were purchased from SAARCHEM.
Diphenylisobenzofuran (DPBF) was purchased from Aldrich. The
syntheses of the photosensitizers (phthalocyanine complexes,
Fig.
1):
2,3,9,10,16,17,23,24-octakis(pentylthiophthalocy-
aninato) palladium(II) (1a), 2,3,9,10,16,17,23,24-octakis(octylthi-
ophthalocyaninato) palladium(II) (1b), 2,3,9,10,16,17,23,24-
500
550
600
650
Wavelength
700
750
800
octakis(dodecylthiophthalocyaninato)palladium(II)
palladium-2,3,9,10,16,17,23,24-octakis(phenoxy)phthalocyanine
1d), 2,3,9,10,16,17,23,24-octakis(benzyloxyphenoxy)phthalocya-
ninato palladium (1e), 2,3,9,10,16,17,23,24-
(1c),
Fig. 3. Spectra of (a) alkylthio derivatised phthalocyaines 1a, 1b and 1c and (b) ary-
(
−5
−1
loxy derivatised phthalocyanines 1d, 1e, 2d and 2e. Concentration ∼1 × 10 mol L
in DCM.

T.B. Ogunbayo et al. / Journal of Molecular Catalysis A: Chemical 334 (2011) 123–129
125
Table 1
Complexes’ photosensitization parameters for phototransformation of 4-NP (3.5 × 10 M, DCM).
−
4
Optimum photosensitizer concentration (mg L 1
−
)
˚4-NP (10
−4
)
˚ꢀ (DCM)
kr = kd/˚ꢀ slope (mol L s
−1
−1
)
kq + kr (mol L s
−1
−1
)
Complexes
5
9
(1a)
(1b)
(1c)
(1d)
(1e)
(2d)
(2e)
200
300
350
250
350
200
350
14
15
18
9
16
0.3
0.7
0.38
0.36
0.39
0.30
0.32
0.29
0.26
1.80 × 10
1.6 × 10
5
8
1.70 × 10
2.7 × 10
5
8
2.00 × 10
3.0 × 10
5
8
1.40 × 10
3.1 × 10
5
7
1.95 × 10
3.2 × 10
5
8
2.30 × 10
4.0 × 10
5
8
2.12 × 10
4.3 × 10
2
.3. Singlet oxygen quantum yields
1
6
4
2
0
8
6
4
2
0
1
1
1
The singlet oxygen quantum yield (˚ꢀ) determinations were
carried out using solutions containing DPBF (90 ␮M) as the singlet
oxygen quencher and the MPcs (absorbance ∼0.2 at the irradiation
wavelength). A solution of the MPc complex (2 mL) contained in a
quartz 10 mm spectrophotometric cell was saturated with oxygen
and irradiated using the set-up described above. The irradiation
and measurement were repeated until around 80% decay of DPBF
was observed [21]. The DPBF absorbance was corrected for the
absorbance of sensitizer at the respective detection wavelength.
0
100
200
300
400
500
600
Catalyst Concentration (mg/L
)
Fig. 4. Plot of ˚4-NP vs concentration of photosensitizers to determine the
optimum concentration of complex 1c for the phototransformation of 4-NP
2
2550
a
y = 0.214x + 21709
R² = 0.986
−4
−1
(
3.5 × 10 mol L ) in DCM.
22500
2
2
2450
2400
which both the MPc derivatives and 4-NP were dissolved. Triethy-
lamine was employed to ensure the basicity of the DCM solution
since in highly basic media, ensuring that 4-NP is deprotonated.
The products were separated and analysed using both gas chro-
matography (GC) and high performance liquid chromatography
22350
2
2
2300
2250
2
700
2900
3100
3300
3500
3700
3900
(
(
HPLC) equipment connected in tandem to a mass spectrometer
LC/MS). For the gas chromatography (GC) analysis, an Agilent
1/[4-Np]
Technologies 6820 GC system (HP 5973, using an HP-1 this was
actually a DB-5MS column) was employed. A dual wavelength UV
detector was used for HPLC analyses and the wavelengths used
were 254 nm and 318 nm. The HPLC conditions utilized included
an Agilent Eclipse XDB-C18 column (150 × 4.6 (i.d.) mm) 5 ␮m par-
ticle size and a mobile phase with gradient elution starting at 10%
5140
5120
b
y = 0.219x + 4207.
R² = 0.990
5
5
5
5
5
100
080
060
040
020
MeOH:H O (pH 5.5 with formic acid) for 10 min and then ramping
2
to 100% MeOH in 10 min. A Finnigan MAT LCQ ion trap mass spec-
trometer equipped with an electrospray ionization (ESI) source was
used for mass analysis. Spectra were acquired in the negative ion
5000
4
4
980
960
◦
mode, with the capillary temperature set at 200 C and sheath gas
set at 80 arbitrary units, with the capillary and tube lens voltages
set at −20 and −5 V respectively.
3
400
3600
3800
/[4-Np]
4000
4200
1
6
450
c
1
2
6400
y = 0.199x + 5064.
R² = 0.958
6
6
6
350
300
250
1
1.5
11
10.5
6200
6
6
6
150
100
050
1
0
9.5
9
2
6000
4700
.50
3.00
3.50
4.00
5200
5700
6200
6700
7200
4-Np] (10 molL-1)
-
5
1/[4-Np]
[
Fig. 5. Plot of ˚4-NP vs concentration of 4-NP for phototransformation of 4-NP in
the presence of complex 1b (300 mg L ) in DCM.
Fig. 6. Plots of 1/˚4-NP vs 1/[4-NP] in the photo-oxidation of 4-nitrophenol for
−1
−1 −1 −1
complexes (a) 1a (200 mg L ), (b) 1c (350 mg L ) and (c) 1e (350 mg L ) in DCM.

1
26
T.B. Ogunbayo et al. / Journal of Molecular Catalysis A: Chemical 334 (2011) 123–129
Fig. 7. Gas chromatogram for heterogeneous photosensitized oxidation of 4-nitrophenol by complex 1c (a) before irradiation and (b) after irradiation for 12 h. Concentrations
−
1
−4
−1
of 1c = 350 mg L and 4-nitrophenol = 3.5 × 10 mol L in DCM.
The DPBF quantum yield ˚DPBF was calculated using Eq. (10)
is the irradiation time. Iabs is given by Eq. (11)
˛
AI
Iabs
=
(11)
[
Co − Ct]V
NA
˚DPBF =
(10)
I
t
abs
where ˛ is the fraction of light absorbed, A is the cell area irradi-
ated, I is the light intensity from the lamp and NA is the Avogadro’s
constant. The determined extinction coefficient of DPBF in DCM
where Co and Ct are the concentrations of DPBF before and after
irradiation respectively, V is the volume of the sample in the cell; t
−
1
−1
(εCNP = 21,100 L mol cm ) was employed.

T.B. Ogunbayo et al. / Journal of Molecular Catalysis A: Chemical 334 (2011) 123–129
127
0.9
0.8
0.7
0.6
0.5
0.4
0.3
quencher in DCM for photosensitized reaction. Based on the heavy
atom effect, it would be expected that the PtPc complexes would
give larger ˚ꢀ values. However, the ˚ꢀ value is slightly higher
for PdPc 1e compared to the corresponding PtPc complex 2e. Com-
plexes 1d and 2d show almost similar ˚ꢀ values. The ˚ꢀ values
of both PtPc and PdPc complexes are however still sufficient for
photosensitized transformation of 4-NP.
(
b)
(
a)
3.2. Effect of photosensitizer and 4-NP concentrations on
phototransformation of 4-NP
0
20
40
60
80
Time (min)
These studies were carried out in DCM (basic media using tri-
ethylamine as an organic buffer) since the phenolate ions, which are
more oxidizable, are predominant in basic media (pKa 4-NP = 7.15)
Fig. 8. Absorption spectral changes observed for photosensitized transformation of
4
1
-NP in the absence (a) and presence and (b) of sodium azide. Concentrations of
−
1
−4
−1
c = 350 mg L and 4-nitrophenol = 3.5 × 10 mol L in DCM.
[
23]. Fig. 2 shows the spectral changes observed during homoge-
neous photolysis of 4-NP in the presence of complex 1c (as an
example). There is a decrease in the absorbance (at 400 nm) of 4-NP
during irradiation (with time) in the presence of complex 1c. The
absorbance of complex 1c was unchanged during the photolysis
due to its high photostability as shown by the lack of change in the
intensity of the Q-band of the MPc at 690 nm. The peak due to the
phenolate ions at 400 nm started disappearing with simultaneous
increase in the intensity of the peak at 260 nm due to the forma-
tion of the products. It has been reported before that the peak near
300 nm is due to conversion of the phenolate ions to the protonated
form [23]. However since we are working in basic media, the pro-
tonation reaction is not expected. The B band of phthalocyanines
occurs in the 300 nm region hence complicating the spectra. Thus
the disappearance of 4-NP rather than formation of products was
employed for the studies in this work. The ˚4-NP were calculated
from the initial rate of disappearance of the 4-NP using Eq. (10)
(where DPBF is replaced by 4-NP).
Aggregation in MPc affects their photosensitization behavior.
This is judged by a broadened and split Q band, with the blue
shifted component being due to the aggregate. All the complexes
were aggregated in DCM (Fig. 3) but were not aggregated in 1-
chloronaphthalene (which was employed before for photophysical
studies [18]). However DCM was found to be an appropriate solvent
for this work. Fig. 3 shows that the most aggregated complex was
The singlet oxygen quantum yields ˚ꢀ were calculated using
Eq. (12) [21].
1
1
1
kd
1
= ˚
+ ˚
· ·
ka [DPBF]
(12)
˚
DPBF
ꢀ
ꢀ
where kd is the decay constant of singlet oxygen in respective sol-
1
vent and ka is the rate constant of the reaction of DPBF with O ( ꢁg).
2
The intercept obtained from the plot of 1/˚DPBF vs 1/DPBF gives
/˚ꢀ.
1
2
.4. Kinetics of photosensitization
The quantum yields of phototransformation of 4-nitrophenol
(
4
(
˚4-NP) were calculated using Eq. (10) and replacing DPBF with
-NP. The determined molar exctinction coefficient of 4-NP
3
−1
−1
1.5 × 10 L mol cm at 400 nm) in DCM was used to calculate
the concentration of 4-NP.
Assuming transformation of 4-NP follows Type II reaction path-
1
way, rate constants involved are for the decay of O2 (k , Eq. (13)),
d
physical quenching of 1O2 by the substrate (kq, Eq. (14)) and the
formation of oxidation products (kr, Eq. (15)).
k
1
d
3
O −→ O
(13)
(14)
(15)
2
2
2
e, as judged by the enhanced absorption band due to aggregates
kq
1
3
−5
4
4
-NP + O −→ O
near 600 nm, considering the same concentration of ∼1.0 × 10 M.
This absorption band is more intense than the band due to the
monomer near 660 nm for 2e, showing extensive aggregation for
this complex. Complex 1e also showed an intense band due to
the aggregate (Fig. 3), but aggregation was not as severe as for
2
2
kr
1
-NP + O −→ products
2
Using Eqs. (13)–(15), the rate constants for the phototransfor-
mation of 4-NP may be calculated using Eq. (16) [21].
2e. The presence of aromatic substituents (in substituent e) lying
ꢀ
ꢁ
1
1
kq + kr
kr
kd
in the same plane as the phthalocyanine ring is expected to pro-
mote aggregation due to strong ꢃ–ꢃ interaction. This explains why
both 1e and 2e (both containing a more aromatic substituent) show
more extensive aggregation than the rest of the complexes. Due to
more extensive aggregation complex 2e (in particular) is expected
to give a low value for ˚4-NP. However, Table 1 shows that the
largest ˚4-NP values were obtained for complexes 1a–1c and 1e.
Complexes 2e and 2d gave the lowest ˚4-NP values. Low ˚ꢀ values
for Pt complexes would also give low ˚4-NP. ˚ꢀ are also reduced
by aggregated species which was more severe for 2e.
= ˚
+
(16)
˚4-NP
kr[4-NP]
ꢀ
where ˚ꢀ and ˚4-NP are the singlet oxygen and 4-NP quantum
yields. kd is calculated using Eq. (16)
1
kd = ꢂ
(17)
ꢀ
where ꢂꢀ is the lifetime of singlet oxygen in the solvent. kd in DCM
4
−1
is 1.6 × 10 s [22]. The rate constants kq and kr can be determined
by plotting 1/˚ vs 1/[4-NP] using Eq. (17).
4
-NP
The ˚4-NP values obtained in the work are similar to those
reported in aqueous media for the phototransformation of 4-NP
3
. Results and discussions
using zinc tetrasulfo phthalocyanines (ZnPcS ), zinc octacar-
4
boxyphthalocyanines (ZnPc(COOH) ) and a sulfonated ZnPc sample
8
3
.1. Singlet oxygen quantum yields (˚ꢀ)
containing a mixture of differently substituted sulfonated deriva-
tives (ZnPcSmix) [10]. ZnPc(COOH)8 showed the best catalytic
behavior [10], but degraded in solution, while the MPc complexes
employed in this work do not show degradation.
The concentration of the catalyst will have an effect on pho-
totransformation of the analyte. It is expected that the higher the
The energy transfer from an excited MPc to triplet oxygen
affords singlet oxygen. Singlet oxygen plays an important role
in oxidation reactions. Singlet oxygen quantum yield (˚ꢀ) val-
ues (Table 1) were determined using DPBF as the singlet oxygen

1
28
T.B. Ogunbayo et al. / Journal of Molecular Catalysis A: Chemical 334 (2011) 123–129
Scheme 1. Mechanism for oxidation of 4-nitrophenol.
catalyst concentration the higher the phototransformation of 4-NP.
However as the concentration of the photosensitizers increases,
aggregation increases (which reduces catalytic activity of the pho-
tosensitizers). Experiments were conducted where ˚4-NP values
were calculated at different concentrations of the photosensitiz-
plexes. The (kq + kr) values arrived at in this work are comparable
with value of 2.8 × 10 mol dm s obtained previously [10].
8
−1
3
−1
3.4. Photosensitized transformation products and mechanism
−1
−1
(Fig. 4) (using 1c as
ers ranging from 50 mg L
an example and [4-NP] = 3.5 × 10 mol L ). As the concentration
of the MPc increased, the ˚4 values increased until a maxi-
to 500 mg L
GC and LC–MS were used to determine the products of pho-
totransformation process by comparing the retention times of
possible standards such as fumaric acid, 4-nitrocatechol, 1,4-
benzoquinone and hydroquinone which are expected for the
phototransformation of 4-nitrophenol. From comparison with the
retention times of the standards, two new peaks (hydroquinone
at retention time of 5.4 min and 1,4-benzoquinone at retention
time 2.9 min) were observed in the gas chromatogram traces
(Fig. 7), after irradiation for 12 h. The peak due to 4-NP (reten-
tion time = 2.3 min) was decreased in intensity. These peaks were
all confirmed using GC analysis by spiking with the respective
standards. The LC–MS chromatograms also showed peaks cor-
responding to residual 4-nitrophenol (retention time = 15 min),
hydroquinone (retention time = 5.5 min) and benzoquinone at a
retention time of 9 min. The results from direct injection into mass
spectrometer showed the formation of benzoquinone and hydro-
quinone with masses of 108.7 amu and 109.6 amu representing
the molecular ions of benzoquinone and hydroquinone respec-
tively. This, together the GC and HPLC–UV detection analyses allows
us to be confident about the products formed. According to the
chromatograms, hydroquinone was the major product while ben-
zoquinone, which was expected to be the final product, was present
in small quantity.
−4
−1
-NP
mum was reached and then there was a slight decline (Fig. 4).
The decrease in ˚4-NP with increase in photosensitizer concen-
tration could be due to extensive aggregation of the molecules
at high concentrations. The optimum concentrations of the pho-
−
−1
1
−1
for 1a and 2d, 250 mg L
tosensitizers were 200 mg L
for 1d,
−
1
3
00 mg L for 1b, 350 mg L for 1c, 1e and 2e at 4-NP concen-
−5
−1
tration of 3.5 × 10 mol L (Table 1). A linearity of ˚4-NP vs 4-NP
concentration (Fig. 5), was observed for concentrations less than
−5
−1
3
.5 × 10 mol L , hence below this concentration, ˚4-NP may be
accurately determined.
3.3. Kinetics of photosensitized reactions
The rate constant for photo-oxidation, kr, was estimated from
the slope of the plot of 1/˚4-NP vs 1/[4-NP] (Fig. 6, using 1a, 1c
and 1e as examples). kr equals k /˚ slope where k is the singlet
d
ꢀ
d
4
−1
oxygen decay constant in DCM (1.6 × 10 s [22]), and ˚ꢀ is the
singlet oxygen quantum yield of the sensitizer in DCM.
Using kr values as an indication of phototransformation effi-
ciency, Table 1 shows that photosensitized oxidation of 4-NP
occurred faster when complexes 2d and 2e were used as sensitizers,
with complex 1d showing the lowest rate towards the photooxida-
tion of 4-NP. The values kr reported in this work for homogeneous
phototransformation of 4-NP are much lower that the values
The mechanism for the phototransformation process was deter-
mined by using complex 1c in the presence of sodium azide which
is a singlet oxygen scavenger. While the rate of reaction was much
faster in the absence of NaN3 (presence of singlet oxygen), there
was still 4-NP transformation happening in its presence (absence
of singlet oxygen) (Fig. 8), meaning that the reaction proceeded
through both Type I and Type II mechanisms. The rate of the reac-
tion when the reaction medium was deareated by bubbling argon
for 30 min was similar to what was obtained in the presence of
sodium azide and the only product noticed here was hyroquinone.
The implication was that the two products might be produced
through different path ways. Hydroquinone through Type I and
benzoquinone through Type II reactions, see Scheme 1. Based on
this result the proposed mechanism is shown in Scheme 1. The
triplet state molecules interact with deprotanated 4-nitrophenol
(through Type I mechanism, Eq. (3)) to generate a phenolate radi-
8
−1
−1
)
obtained for sulfonated AlPc derivatives (order of 10 mol L s
24] for the oxidation of chlorophenols.
The quenching effect of the 4-NP on the singlet oxygen was
[
estimated from (kq + kr) calculated from the intercept of the plot
of 1/˚4-NP vs 1/[4-NP]. Comparison of the (kq + kr) with kr or esti-
mation of kq from the two gives the deactivation or quenching of
singlet oxygen via other routes apart from oxidation reaction with
4
-nitrophenol. The values of (kq + kr) for the sensitizers employed
7
9
−1
3
−1
−1
in this work ranged between 3.2 × 10 and 1.6 × 10 mol dm s
5
5
−1
3
while kr ranged between 1.4 × 10 and 2.3 × 10 mol dm s
(
Table 1). The implication of large (kq + kr) compared to kr for all
the sensitizers is that considerable amount of physical quenching
of singlet oxygen took place during the reactions for all the com-
−
cal. The protons interacted with OH supplied by the basic media

T.B. Ogunbayo et al. / Journal of Molecular Catalysis A: Chemical 334 (2011) 123–129
129
to give water which on reaction with triplet state molecules gener-
ated OH radical which in turn reacted with phenolate radicals to
produce hydroquinone. The production of benzoquinone was likely
from the direct oxidation of 4-Np by singlet oxygen as reported in
the photo-oxidation of 4-chlorophenol [25] (Scheme 1).
[2] R. Bonnett, Chemical Aspect of Photodynamic Therapy, Gordon and Breach,
London, 2000, pp. 70–74.
•
[
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Photobiol. A: Chem. 111 (1997) 65.
[4] T.C. Oldham, D. Phillips, J. Photochem. Photobiol. B: Biol. 55 (2000)
6.
1
[
[
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. Conclusions
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[
[
8] H. Ali, J.E. Van Lier, Chem. Rev. 99 (1999) 2379.
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This work demonstrates that the complexes investi-
gated are capable of photo-oxidation of 4-nitrophenol and
PdPcs are better photosensitizers than PtPcs, with palladium
octakis(dodecylthio)phthalocyanine (1c) giving the best perfor-
mance in terms of the highest 4-NP phototransformation quantum
yield. All the complexes showed no degradation during photoly-
sis, and hydroquinone and benzoquinone were observed as the
phototransformation products.
[10] E. Marais, R. Klein, E. Antunes, T. Nyokong, J. Mol. Catal. A: Chem. 261 (2007)
36.
[
11] 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.J. de Oliveira, Photochem. Photo-
biol. A: Chem. 155 (2003) 231.
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[
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G.J. Schulz-Ekloff, Photochem. Photobiol. A: Chem. 111 (1997)
65.
[
[
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[
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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.
[
[
[
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Porphyrins Phthalocyanines 2 (1998) 145.
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