Physico-chemical properties of lutetium phthalocyanine complexes in solution and in solid polystyrene polymer fibers and their application in photoconversion of 4-nitrophenol

Article abstract of DOI:10.1016/j.molcata.2012.02.010

The photophysical and photochemical behavior of two phthalocyanine complexes of lutetium peripherally substituted with tetraphenoxy and tetra-2-pyridiloxy groups were studied in solution and when dispersed in polystyrene polymer fiber. The phthalocyanines were found not to fluoresce significantly in solution and not at all within the fiber matrix as compared with standard unsubstituted zinc phthalocyanine. They showed very promising photoactivity in solution with high singlet oxygen quantum yields. Their photoactivity within the polymer fiber matrix was also demonstrated with the photoconversion of 4-nitrophenol, a water pollutant. The photodegradation process with both phthalocyanines follows first order kinetics similar to that observed for the zinc phthalocyanine and the photo-products were found to be hydroquinone, benzoquinone and 4-nitrocatechol.

Full text of DOI:10.1016/j.molcata.2012.02.010

Journal of Molecular Catalysis A: Chemical 358 (2012) 49–57
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Physico-chemical properties of lutetium phthalocyanine complexes in solution
and in solid polystyrene polymer fibers and their application in photoconversion
of 4-nitrophenol
∗
Ruphino Zugle, 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:
The photophysical and photochemical behavior of two phthalocyanine complexes of lutetium periph-
erally substituted with tetraphenoxy and tetra-2-pyridiloxy groups were studied in solution and when
dispersed in polystyrene polymer fiber. The phthalocyanines were found not to fluoresce significantly in
solution and not at all within the fiber matrix as compared with standard unsubstituted zinc phthalocya-
nine. They showed very promising photoactivity in solution with high singlet oxygen quantum yields.
Their photoactivity within the polymer fiber matrix was also demonstrated with the photoconversion of
Received 27 December 2011
Received in revised form 3 February 2012
Accepted 20 February 2012
Available online 27 February 2012
Keywords:
Lutetium phthalocyanine
Phenoxy
Pyridiloxy
Electrospinning
Nitrophenol
4
-nitrophenol, a water pollutant. The photodegradation process with both phthalocyanines follows first
order kinetics similar to that observed for the zinc phthalocyanine and the photo-products were found
to be hydroquinone, benzoquinone and 4-nitrocatechol.
© 2012 Elsevier B.V. All rights reserved.
Photocatalysis
1
. Introduction
Phthalocyanines have been employed in various technological
are constrained within the environment of a polymeric matrix and
therefore their behavior within the resulting material cannot be
directly extrapolated from the known behavior in a solution [13].
This study is therefore aimed at the spectroscopic and
physico-chemical behavior of novel peripherally substituted
phthalocyanine complexes of lutetium; 2,(3), 9(10), 16(17), 23(24)-
(tetraphenoxyphthalocyaninato) lutetium(III) acetate (3a) and
2,(3), 9(10), 16(17), 23(24)-(tetra-2-pyridiloxyphthalocyaninato)
lutetium(III) acetate (3b), Scheme 1, both in solution and in a solid
polystyrene fiber matrix. We further assessed the photoactivity
of the phthalocyanine containing polymer fibers for the pho-
todegradation of 4-nitrophenol and compared these with a similar
polymer fibers containing standard unsubstituted zinc phthalocya-
nine. Pyridiloxy and phenoxy substituents for the phthalocyanines
have been well researched for many central metals and metalloids
[15–17]. The choice of lutetium in this work was influenced by its
larger size which will encourage intersystem crossing to the triplet
state, hence generating singlet oxygen which is required for photo-
catalysis. Also the applications of the lanthanide phthalocyanines
in photocatalysis are still rare in the literature. The synthesis and
photophysical behavior of lutetium phthalocyanine substituted at
non-peripheral position with phenoxy groups have been reported
applications such as in photo-conversion of pollutants [1], read-
write compact discs [2], non-linear optics [3] and photodynamic
therapy [4]. This is due to the fact that their properties can be
fine-tuned in many ways through changes in the type of coordinat-
ing metal [5,6] as well as structural changes in the phthalocyanine
ligand moiety [7]. Alkoxy and alkyl substituents for example, are
known to enhance the solubility of the phthalocyanines in common
solvents thus rendering them easier to be applied in homoge-
neous systems [8,9]. The presence of axial coordinating ligands is
also known to influence the physico-chemical properties of these
phthalocayanines [10].
Phthalocyanines may be employed as catalysts either in solution
or in their solid state as well as when dispersed in solid support
systems [2,9,11]. For phthalocyanines in the solid state or on solid
support systems, the theory of molecular exciton is usually used to
rationalize the differences between the optical spectra in the liquid
phase and those in the solid state [12–14]. Also equally important
is the fact that the photophysical and photochemical properties of
the phthalocyanines can be altered when on supports since they
[
17], in the current work phenoxy groups are on the peripheral
position. The effects of the pyridiloxy versus phenoxy substituents
on the phthalocyanine on their photochemical behavior within the
fiber matrix will be investigated.
∗ Corresponding author. Tel.: +27 46 6038260; fax: +27 46 6225109.
E-mail address: t.nyokong@ru.ac.za (T. Nyokong).
1
381-1169/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2012.02.010

5
0
R. Zugle, T. Nyokong / Journal of Molecular Catalysis A: Chemical 358 (2012) 49–57
RO
OOCCH3
OR
CN
CN
ROH, DMF
CN LuOAc , pentanol
3
N
N
N
K CO , RT, 96h
N
RO
CN DBU, Reflux, 7 hrs
O N
2
3
Lu
2
N
N
N
2
1
N
RO
3
OR
Where R
=
N
a
b
Scheme 1. Synthetic scheme for 3a and 3b.
2
. Experimental
800-B, 670 nm, 20 MHz repetition rate, Picoquant GmbH). Fluores-
cence was detected under the magic angle with a peltier cooled
photomultiplier tube (PMT) (PMA-C 192-N-M, Picoquant GmbH)
and integrated electronics (PicoHarp 300E, Picoquant GmbH). A
monochromator with a spectral width of about 4 nm was used to
select the required emission wavelength. The response function
of the system, which was measured with a scattering Ludox solu-
tion (DuPont), had a full width at half-maximum (FWHM) of about
300 ns. The ratio of stop to start pulses was kept low (below 0.05)
to ensure good statistics. All luminescence decay curves were mea-
sured at the maximum of emission peak. The data were analyzed
with the program FluoFit (Picoquant GmbH). The support plane
approach [21] was used to estimate the errors of the decay times.
Fluorescence images of the fibers were taken with a DMLS flu-
orescence microscope. The excitation source was a high-voltage
mercury lamp and light in the wavelength range of 550–730 nm.
2.1. Materials
The following chemicals were purchased from SAARCHEM;
n-hexane, dimethylsulfoxide (DMSO), N,N-dimethylfomamide
DMF), tetrahydofuran (THF) and dichloromethane (DCM). Zinc
phthalocyanine (ZnPc), 1,8-diazabicyclo[5.4.0] undec-7-ene
DBU), 1,3-diphenylisobenzofuran (DPBF), lutetium(III) acetate,
(
(
deuterated dimethylsulfoxide (DMSO-d ), anthracene-9,10-bis-
6
methylmalonate (ADMA), benzoquinone, 4-nitrocatechol and
polystyrene (PS) were from Sigma Aldrich and hydroquinone from
May and Baker. Ultra-pure water (Milli-Q water system, Millipore
Corp., Bedford, MA, USA) was used for all photocatalyzed sample
solutions. Phosphate buffer solutions were prepared using reagent
grade potassium dihydrogen orthophosphate (ACE) and dipotas-
sium phosphate (PAL chemicals). 4-Nitrophthalonitrile (1) was
synthesized according to literature procedures [18]. Compounds
2
2
.3. Photophysical and photochemical parameters
2
a and 2b were prepared through base catalyzed nucleophilic
aromatic displacement reaction as described in the literature
.3.1. Triplet state yields and lifetimes
[
15,19,20].
Triplet quantum yields were determined for 3a and 3b in DMF
using a comparative method [22], Eq. (1), with ZnPc as a standard.
Column chromatography was performed on silica gel 60
0.04–0.063 mm) and preparative thin layer chromatography on
(
ꢀATε ST td
silica gel 60 P F254
.2. Equipment
Infrared (IR) spectra were recorded on a Perkin-Elmer Fourier
.
Std
˚
T = ˚T
(1)
Std
ꢀA
εT
T
2
where ꢀAT and ꢀA ST td are the changes in the triplet state
absorbances of the samples (3a and 3b) and the standard, respec-
Std
tively. εT and εT are the triplet state molar extinction coefficients
transform – IR (100 FT-IR) spectrophotometer. UV–vis absorption
spectra were recorded on a Shimadzu UV-2550 spectrophotome-
ter. H NMR spectra were recorded on a Bruker AMX600 MHz in
detuerated DMSO. Microanalyses were performed using a Vario-
Elementar Microcube ELIII. Mass spectral data were recorded
on ABI Vogager De-STR Maldi TOF instrument using ␣-cyano-4-
hydroxycinnamic acid as a matrix. Scanning electron microscope
Std
for the samples (3a and 3b) and the standard, respectively. ˚T
1
Std
is the triplet quantum yield for the standard, ZnPc (˚T = 0.58 in
DMF) [23]. The triplet lifetimes were determined by exponential
fitting of the kinetic curves using the program ORIGIN Pro 8.
2.3.2. Fluorescence quantum yields
(
SEM) images were obtained using a JOEL JSM 840 scanning elec-
Fluorescence quantum yields (ФF) of the phthalocyanines were
determined in DMF by the comparative method [24], Eq. (2)
tron microscope.
Irradiations for singlet oxygen studies were done using General
Electric Quartz lamp (300 W), 600 nm glass (Schott) and water fil-
ters, to filter off ultra-violet and far infrared radiations respectively.
An interference filter 670 nm with bandwidth of 40 nm was placed
in the light path just before cell containing the sample.
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 3a/PS, 3b/PS and PS were obtained from the electrospun fibers.
Fluorescence excitation and emission spectra were recorded
on a Varian Cary Eclipse spectrofluorimeter. Fluorescence life-
times were measured using a time correlated single photon
counting setup (TCSPC) (Fluo Time 200, Picoquant GMbH) with
a diode laser as excitation source (LDH-P-670 driven by PDL
FAStdn2
Std
˚
F = ˚F
(2)
FStdAn2
Std
where F and FStd are the areas under the fluorescence emission
curves of samples (3a and 3b) and the standard, respectively. A and
AStd are the respective absorbances of the samples and standard at
the excitation wavelength. The same solvent was used for samples
Std
and standard. Unsubstituted ZnPc in DMF (˚F = 0.30) [25] was
employed as the standard. Both the samples and standard were
excited at the same wavelength of 633 nm and emission spectrum
was recorded from 650 nm to 790 nm. The absorbances of the solu-
tions at the excitation wavelength were about 0.05 to avoid any
inner filter effects.

R. Zugle, T. Nyokong / Journal of Molecular Catalysis A: Chemical 358 (2012) 49–57
51
2
.3.3. Singlet oxygen quantum yields
A chemical method was also employed to determine the singlet
a variety of concentrations of 4-nitrophenol in pH 8.2 phosphate
buffer. Each sample solution (4 mL) contained 20 mg of functional-
ized fiber.
oxygen quantum yields, ˚ꢀ, of 3a and 3b in DMF. The compara-
ZnPc
tive method was used with ZnPc (˚ꢀ = 0.56 in DMF) [26–28], as
standard and DPBF as singlet oxygen quencher.
2.5. Chromatographic analysis
The relative method shown by Eq. (3), was employed in calcu-
lating ˚ꢀ
.
The photolysis products were separated and analyzed using
both gas chromatography (GC) and by direct injection into ion trap
mass spectrometer fitted with an electrospray ionization (ESI-MS)
mass spectra. For gas chromatographic analyses, an Agilent Tech-
nologies 6820 GC system fitted with a DB-MS Agilent J & W GC
column was employed. A Finnigan MAT LCQ ion trap mass spec-
trometer equipped with an electro-spray ionization (ESI) source
was used for mass analysis. Spectra were acquired in the negative
RSample · I
ZbPc
ZnPc
˚ꢀ
= ˚ꢀ
·
(3)
RZnPc · I
Sample
ZnPc
where ˚ꢀ is the singlet oxygen quantum yield for the ZnPc stan-
dard, Rsample and RZnPc are the DPBF photobleaching rates in the
presence of compound 3a or 3b and ZnPc standard respectively,
sample
ZnPc
while I
and I
are the respective rates of light absorp-
◦
ion mode, with the capillary temperature set at 200 C and sheath
tion by compound 3a or 3b and ZnPc standard. The initial DPBF
gas set at 60 arbitrary units, with the capillary and tube lens voltage
set at −20 and −5 V respectively. The aqueous photocatalyzed sam-
ple solution was extracted with dichloromethane and then injected
into the GC.
concentrations were kept the same for both the standard and the
sample. Molar extinction coefficient (dm mol cm ) of DPBF at
3
−1
−1
ꢁ
= 417 nm in DMF ε = 23,000 [29].
For the modified fibers (3a/PS, 3b/PS and ZnPc/PS), the singlet
oxygen quantum yield (˚ꢂ) determinations were carried out in
aqueous solutions using ADMA as the quencher and its degradation
was monitored at 380 nm. In each case 10 mg of the modified fibers
was suspended (as small pieces) in an aqueous solution of ADMA
and similarly irradiated using the photolysis set-up described
above. The quantum yields (˚ADMA) were estimated using Eq. (4),
using the extinction coefficient of AMDA in water log(ε) = 4.1 [30].
2
.6. Synthesis
2
2
.6.1. Synthesis of 2,(3), 9(10), 16(17),
3(24)-(tetraphenoxyphthalocyaninato) lutetium(III) acetate
(
3a), Scheme 1
A
mixture of anhydrous lutetium(III) acetate (134 mg,
0
.38 mmol), 4-phenoxyphthalonitrile (2a) (339 mg, 1.54 mmol)
˚ADMA = (
C − Ct)VR
(4)
0
in 1-pentanol (2 mL) was refluxed for 7 h under nitrogen atmo-
sphere with DBU as catalyst. After cooling, the crude product
was precipitated with n-hexane, filtered and washed with excess
n-hexane and then air dried. Column chromatography (silica gel)
was employed using THF: methanol (10:1) as the eluting solvent
I
· t
abs
where C and Ct are the ADMA concentrations prior to and after irra-
0
diation, respectively; VR is the solution volume; t is the irradiation
time per cycle and Iabs is defined by Eq. (5).
−
1
mixture. Yield: 25%. IR [KBr, ꢃ cm ] 773, 824, 883, 938, 1039,
1067 (Pc skeleton), 1224, 1264, 1390, 1467, 1482 (C C), 1606,
714, 1770 (C O), 2927, 2957 (C H, aromatic), 3058 (CH ). UV–vis
˛
· A · I
Iabs
=
(5)
O
N
A
1
3
−
A(ꢁ)
where ˛ = 1–10
, A(ꢁ) is the absorbance of the sensitizer at
(DMF): ꢁmax nm (log ε) 351 (4.51) 610 (5.11), 678 (5.78). Calcd. for
C58H35N8O6Lu; C 62.48%, H 3.16%, N 10.05%. Found C 62.00%, H
3.32%, N 10.24%; 1H NMR (DMSO-d): ı, ppm 7.79–7.91 (12-H, m,
Pc-H), 6.88–6.97 (20-H, m, Phenyl-H), 1.99 (3-H, s, acetate-CH3);
2
the irradiation wavelength, A is the irradiated area (2.5 cm ), I is
16
−2 −1
s ) and NA is Avo-
the intensity of light (4.54 × 10 photons cm
gadro’s constant.
+
+
The absorbances used for Eq. (5) are those of the phthalo-
cyanines in the fibers (not in solution) measured by placing the
modified fiber directly on a glass plate. The light intensity mea-
sured refers to the light reaching the spectrophotometer cells, and
it is expected that some of the light may be scattered, hence the ˚ꢀ
values of the phthalocyanines in the fiber are estimates. The singlet
oxygen quantum yields ˚ꢀ were calculated using Eq. (6) [31]
(ES ), (m/z): Calc. 1115; Found: 1116 [M+H ].
2.6.2. Synthesis of 2,(3), 9(10), 16(17),
23(24)-(tetra-2-pyridiloxyphthalocyaninato) lutetium(III) acetate
(3b), Scheme 1
The synthesis of 3b was as outlined for 3a except 2b instead of
2a was employed. The amounts of the reagents were the same as
−
1
well as the purification procedures. Yield: 23%. IR [KBr, ꢃ cm
837, 910, 1012, 1057, 1077, 1133 (Pc skeleton), 1269, 1355, 1384,
489 (C C), 1654, 1719, 1769 (C O), 2927, 2957 (C H, aro-
]
1
1
1
kd
1
= ˚
+ ˚
·
·
ka [ADMA]
(6)
˚ADMA
ꢀ
ꢀ
1
O
where kd is the decay constant of singlet oxygen in the respective
matic), 3053 (CH ). UV–vis (DMF): ꢁmax nm (log ε) 337 (4.74) 613
3
solvent and ka is the rate constant of the reaction of ADMA with
(5.26), 678 (5.94). Calcd. for C54H31N12O Lu; C 57.97%, H 2.79%, N
15.02%. Found C 58.14%, H 3.01%, N 14.89%; H NMR (DMSO-d):
6
1
1
O ( ꢀg). The intercept obtained from the plot of 1/˚
versus
2
ADMA
1
/[ADMA] gives 1/˚ꢀ
.4. Photocatalytic reactions
Photocatalytic reactions were carried out in a magnetically
.
ı, ppm 7.76–8.67 (12-H, m, Pc-H), 6.78–7.27 (16-H, m, Pyridyl-H),
+
2
.11 (3-H, s, acetate-CH ); (ES ), (m/z): Calc. 1119; Found: 1120
3
+
2
[M+H ].
2.6.3. Preparation of functionalized fibers
stirred batch reactor (glass vial). The irradiation experiments
were carried out using the photolysis set-up described above
for singlet oxygen detection. The intensity of the light reach-
ing the reaction vessel was measured with a power meter
The fibers were prepared using the electrospinning method
reported before [32] with modifications as follows: a solution con-
−
5
taining 2.5 g (1.3 × 10 moles) of polystyrene (PS) and 1.35 mg
each of compound 3a, 3b or ZnPc, in 10 ml DMF/THF(4:1) was
stirred for 24 h to produce a homogeneous solution. The solvent
mixture was employed to allow both PS and the phthalocyanines
to dissolve. The solution was then placed in a cylindrical glass tube
fitted with a capillary needle. A potential difference of 20 kV was
applied to provide charge for the spinning process. The distance
(
3
POWER MAX 5100, Molelectron Detector Inc.) and found to be
.52 × 10 photons cm
2
0
−2 −1
s . The transformation was monitored
by observing the absorption band (400 nm) of 4-nitrophenol after
each photolysis cycle of fifteen minutes using a Shimadzu UV-
2
550 spectrophotometer. The experiments were carried out using

5
2
R. Zugle, T. Nyokong / Journal of Molecular Catalysis A: Chemical 358 (2012) 49–57
2
1
1
0
.4
.8
.2
.6
0
a
8.0x10-4
6.0x10-4
(
(b)
c)
4
2
.0x10-4
.0x10-4
(d)
(a)
0
.0
3
3
00
400
500
600
700
800
500
3000
2500
2000
1500
1000
50
Wavelength/nm.
Wavenumber/cm-1
Fig. 1. UV–vis spectra of compound 3b in (a) THF, (b) DMF, (c) DMSO and (d) on PS
0
0
0
0
0
0
.05
.04
.03
.02
.01
.00
fiber. Solution concentration ∼2.0 × 10−6 M.
b
between the cathode (static fiber collection point) and anode (tip
of capillary needle) was 15 cm and pump rate was 0.01 mL/h. The
flow rate was however increased to 0.02 mL/h in the case of the
polystyrene/phthalocyanine composite to avoid sputtering of the
solution. ZnPc/PS fibers were similarly prepared.
3
. Results and discussion
3.1. Spectral and microscopic characterization
The electronic absorption spectra of compound 3b (as a repre-
3500
3000
2500
2000
1500
1000
sentative of both complexes) in THF, DMF and DMSO are shown
in Fig. 1. Narrow Q bands were observed in solution suggesting
monomeric in nature of the complexes [33,34]. As shown, there is
a slight red shift in the Q band absorption as the solvent is changed
from THF to DMSO. This is explained in terms of the coordinat-
ing tendency of the solvents [35]. A similar trend was observed for
compound 3a as shown in Table 1.
There is more red-shifting in the Q band in moving from solu-
tion to the solid state polymer fiber matrix. The red shift in the
Q-band of the phthalocyanines within the fiber core compared to
solutions could be attributed to the electronic interaction between
the Pc and the polymer resulting in lower energy states. Such a
bathochromic shift in the electronic spectral features of photosen-
sitizers as a result of interactions with other molecules has been
reported for porphyrins [36] and phthalocyanines [37]. The strong
Wavenumber/cm-1
c
0.020
0
0
0
.015
.010
.005
0.000
3500
␲
–␲ interaction due to the stacking of the rings leads to broadening
3000
2500
2000
1500
1000
Wavenumber/cm-1
of the Q band as observed in Fig. 1d. The observation of a Q band in
the fiber confirms successful incorporation of the complexes into
the fiber matrix.
Fig. 2. Raman spectra of (a) PS fiber, (b) 3a/PS composite fiber and (c) 3b/PS com-
posite fiber.
Raman spectra were employed to further confirm the presence
−
1
of the Pc in the fiber. The peaks between 2500 and 3000 cm
Fig. 2a) in the Raman spectrum of the PS alone are attributed to
(
Table 1
Spectroscopic and photophysical data for complexes 3a and 3b.
stretches due to the aromatic ring [38]. Any electronic interaction
between the polystyrene and the phthalocyanines is mostly likely
to involve the ␲-electrons of their aromatic systems. The bands cor-
responding to the aromatic regions in the composite fibers (Fig. 2b
and c) have increased intensities compared to the polymer fiber
alone. Most importantly, the observed changes in the peak posi-
tions and/or splitting of the PS fiber bands in the composites suggest
that the phthalocyanines are interacting very strongly with the
polystyrene aromatic system.
The fiber morphology and diameters were assessed using scan-
ning electron microscopy. Both 3a/PS and 3b/PS and PS fibers alone
did not form appreciable amount of beads and consist of mostly
long unbranched strands of cylindrical fibers as shown in Fig. 3.
Parameter
Q (nm)
Solvent
3a
3b
ꢁ
THF
676
678
683
693
675
677
681
693
DMF
DMSO
Fiber
DMF
DMF
DMF
DMF
DMF
DMF
˚
˚
˚
F
<0.01
0.71
0.68 (0.28)
2.60 ± 0.01
4.27 ± 0.31 (0.09)
0.10 ± 0.04 (0.91)
0.017
0.68
0.62 (0.17)
2.64 ± 0.01
4.60 ± 0.21 (0.14)
0.11 ± 0.04 (0.86)
T
a
ꢀ
ꢄT (␮s)
ꢄF1 (ns)
ꢄF2 (␮s)
b
b
a
Values in brackets are for complexes 3a and 3b in fiber matrix. ˚ꢀ for ZnPc in
DMF = 0.56 [26], ˚ꢀ for ZnPc/PS in aqueous solution = 0.13.
b
Relative amplitudes shown in brackets.

R. Zugle, T. Nyokong / Journal of Molecular Catalysis A: Chemical 358 (2012) 49–57
53
0
0
0
0
0
.016
.012
.008
.004
.000
A
0
10
20
30
40
50
Time/µs
B 1
1
2000
0000
8
6
4
2
000
000
000
000
0
Measurement
Fit
10
5
0
Fig. 3. Fiber mat of (a) polystyrene alone and (b) 3a/polystyrene composite at
00 ␮m resolution and insert is 50 ␮m resolution.
-5
10
1
-
0
5
10
15
20
25
30
Time [ns]
3.2. Photophysical and photochemical properties
Fig. 4. Triplet decay curve at 490 nm (a) and fluorescence decay profile (b) for com-
pound 3a in DMF. Excitation wavelength 673 nm for triplet yield studies. For TCSPC
trace excitation was at the maximum of the emission peak at 695 nm.
3
.2.1. Triplet quantum yield and lifetimes
Singlet
oxygen which is mostly implicated in photosensitized reac-
tions is produced through transfer of energy from the excited
triplet state of the photosensitizer to ground state molecular
oxygen [39]. Thus the ability of a sensitizer to undergo the spin
forbidden transition from its excited singlet state to the triplet state
is a fundamental requirement for photosensitization processes. In
this work, we determined the fraction of the excited singlet state
molecules of 3a and 3b that can undergo the intersystem crossing
to the triplet state (triplet quantum yield, ˚T) by using Eq. (1).
Fig. 4a shows the triplet state decay profiles for compound 3a
as a representative. Monoexponential decay kinetics were obeyed.
The lifetimes for the triplet states were found to be 2.60 ± 0.01 ␮s
and 2.64 ± 0.01 ␮s in DMF for 3a and 3b respectively (Table 1). The
triplet lifetimes are lower than is typical of Pcs due to the large size
of the Lu central metal. Triplet quantum yields were found to be
affected by excitation in the solvent. The stokes shifts are typical of
metallophthalocyanine complexes [41].
The fluorescence quantum yields (ФF) were determined in DMF,
using a comparative method. The values obtained were <0.01 and
0
.017 for 3a and 3b, respectively, Table 1. The low ФF values
obtained could be attributed to the heavy atom effect of the central
lutetium coordinating diamagnetic atom which encourages inter-
system crossing to its excited triplet state leading to high triplet
quantum yields observed in this work. A similar argument has
also been suggested for hafnium phthalocyanines which do not
1
0
0
.2
.8
.4
0
.71 and 0.68 for 3a and 3b respectively (Table 1). Pyridiloxy sub-
stituent on 3b is electron donating and electron donors are known
40] to quench excited triplet states hence the slightly lower value
of ˚T for 3b.
Em
Ex
[
3.2.2. Fluorescence spectra, quantum yields and lifetimes
Abs
The fluorescence properties of 3a and 3b were examined in spec-
troscopic grade DMF degassed with argon. The fluorescence spectra
for 3a and 3b were mirror images of the excitation spectra, Fig. 5
0
(
3b is representative of both 3a and 3b). As observed in Fig. 5, the
550
600
650
7 00
750
800
Q-band maxima of the absorption and excitation spectra in both
cases are in close proximity. This suggests that the nuclear config-
uration of the ground states and excited states are similar and not
Wavelength/nm
Fig. 5. Fluorescence (Em), absorption (Abs), excitation (Ex) spectra in DMF for 3b.

5
4
R. Zugle, T. Nyokong / Journal of Molecular Catalysis A: Chemical 358 (2012) 49–57
Fig. 6. Fluorescence micrographs of PS fibers functionalized with (a) compound 3a and (b) ZnPc.
fluoresce at all [42]. The slightly higher ˚F for 3b corresponds to
the lower ˚T.
photophysical and photochemical behavior of the phthalocyanine
can be altered when constrained within the environment of a solid
polymeric matrix. Thus a direct correlation between the phthalo-
cyanine behavior in solution and in solid fiber matrix cannot be
feasible. Such an assertion has been put forward by Lang et al. [13].
Secondly, equally important is the fact that such lowering of the sin-
glet oxygen quantum yield of phthalocyanines in protic solvents
such as water has been observed [45]. It is believed that such a
decrease is due to interaction between the vibrational levels of the
solvent molecules and the electronic or vibrational levels of singlet
oxygen resulting in deactivation of the singlet oxygen in such sol-
vents. Also as stated above the ˚ꢀ values for Pcs in fibers are an
estimate.
The fluorescence decay of the phthalocyanines, Fig. 4b for
3
a (representing both complexes) show biexponential fluores-
cence decay profile. The lifetimes and amplitudes are listed in
Table 1. Biexponential fluorescence decay profiles of phthalocya-
nines are attributed to the formation of ground-state dimers which
can quench the monomer fluorescence leading to a quenched
and unquenched lifetimes [43]. In this work two lifetimes were
observed for both 3a and 3b.
The fluorescence behavior of 3a and 3b in the functionalized
fiber were assessed by taking fluorescence micrographs by exciting
the functionalized fibers with a high-voltage mercury lamp using a
filter in the wavelength range of 550–730 nm. Both functionalized
fibers did not show the characteristic fluorescence as compared
with similar polystyrene polymer fibers functionalized with the
known fluorescent unsubstituted zinc phthalocyanine (ZnPc) as
shown in Fig. 6. This could be due to their low fluorescence observed
in solution and coupled with the stacking of their molecules in
the solid state which usually make phthalocyanine molecules non-
emissive [44].
Nonetheless, the singlet oxygen quantum yields suggest that
functionalized fibers are promising photosensitizers that could
be applied in the photo-conversion of various analytes. Thus we
applied the fabric materials for conversion of 4-nitrophenol.
3.2.4. Photoconversion of 4-nitrophenol
We further established that the functionalities of the phthalo-
cyanines are maintained within the fiber matrices by using the
functionalized fibers for the conversion of 4-nitrophenol in the
presence of visible light.
3.2.3. Singlet oxygen quantum yield
There are a number of unrelated factors that determine the mag-
Nitroaromatic compounds are recognized as environmentally
hazardous [46]. Nitrophenols in particular are classified as priority
pollutants by United State Environmental Protection Agency. Due
to their poisonous nature, various remediation processes from the
environmental media have been suggested [47–50].
The use of energy from the electromagnetic spectrum is cost
effective and remains much more practical approach to the con-
version of these pollutants to less toxic products. Phthalocyanines
in particular are very promising sensitizers for such photocatalytic
applications. Their maximum light absorption occurs in the visi-
ble portion of the electromagnetic spectrum, which constitutes a
nitude of singlet oxygen yield for any photosensitizer. These include
the energy of excited triplet state of the sensitizer, the extent to
which substituents on the phthalocyanine and the solvent medium
can quench the singlet oxygen as well as the lifetime of the excited
triplet state and the efficiency of the energy transfer between the
excited triplet state of the sensitizer and the ground state molecular
oxygen. In our case, compounds 3a and 3b have comparable triplet
state quantum yield. The singlet oxygen yields for 3a and 3b were
found to 0.68 and 0.62 respectively. This relatively higher singlet
oxygen quantum of compound 3a in DMF over that of 3b can only
be attributed to their differences in ˚T values and their efficiencies
in energy transfer from the triplet state to ground state oxygen.
The singlet oxygen quantum yields of the 3a and 3b within the
polymeric fibers were determined in water since the fibers will be
applied for the transformation of 4-nitrophenol in aqueous media.
ADMA was used as singlet oxygen quencher in aqueous media.
Fig. 7 shows the degradation profile of ADMA in solutions contain-
ing 3a/PS fibers in water upon irradiation. Similar spectral changes
were observed for the 3b/PS or ZnPc/PS fibers. The Pcs did not leach
out of solution as observed in Fig. 7 since they are bound within
the fiber matrix and are also not water soluble. No degradation of
ADMA was observed under similar conditions when PS fiber was
used without functionalization with phthalocyanines. The singlet
oxygen quantum yields were 0.28, 0.17 and 0.13 for 3a/PS, 3b/PS
and ZnPc/PS, respectively. These values are significantly lower than
those obtained when the Pcs were in DMF solution and can be
explained in two contexts. First, it could be due to the fact that the
1
.5
1
ADMA
0
.5
0
300
400
500
600
700
800
Wavelength/nm
Fig. 7. UV–vis spectral changes observed on photolysis of ADMA in water with 3a/PS
fiber as sensitizer. Spectra taken at 15 min intervals.

R. Zugle, T. Nyokong / Journal of Molecular Catalysis A: Chemical 358 (2012) 49–57
55
0
0
0
0
.16
.12
.08
.04
0
1.6
1
0
0
.2
.8
.4
0
(a)
(
b)
(c)
(
d)
0
15
30
45
60
75
90
105
Time/min
250
350
450
550
650
750
Wavelength/nm
Fig. 9. Effect of addition of singlet oxygen quencher (sodium azide) on the transfor-
−
4
mation rate of 2.5 × 10 M 4-Np in air using (a) 3a/PS fiber without sodium azide,
(b) 3b/PS fiber without sodium azide, (c) 3a/PS fiber with sodium azide and (d) 3b/PS
fiber with sodium azide. 20 mg of 3a or 3a used in all cases.
Fig. 8. Electronic absorption spectral changes of observed during photolysis of 4-
Np using 20 mg polystyrene fiber functionalized with compound 3a in pH 8.2 buffer
solution.
better photocatalytic behavior toward photocatalytic oxidation of
larger portion of the spectrum and thus more available than the
ultra-violet portion required by other sensitizers [51,52].
4
-nitrophenol than 3b/PS and ZnPc/PS and the order is consistent
with their ability to generate singlet oxygen.
In this work we use 4-nitrophenol as a representative nitrophe-
nol. The photocatalytic reaction was carried out in a buffer aqueous
solution of pH 8.2 which is slightly higher than the pKa 7.15 of
In heterogenous catalysis, the reaction rate at the surface of
the catalyst is related to the concentration of the reactant cov-
ering the surface according to the Langmuir–Hinshelwood (L–H)
kinetic model, Eq. (7) [55]. This model has been successfully used
to describe solid–liquid reactions. Hence we employed it for the
treatment of data based on our heterogeneous catalytic mimetic
systems.
4
-nitrophenol [53]. It is known that the deprotonated forms of sub-
stituted phenols are much more easily oxidizable by singlet oxygen
54]. Fig. 8 shows the UV–vis spectral changes of 4-nitrophenol
solution during the photolysis process. As shown, the peak around
00 nm that corresponds to 4-nitrophenol decreases in intensity
[
4
1
1
1
with irradiation in the presence of the functionalized fiber. There
are corresponding increases in the absorbance around 280 and
=
+
(7)
rate
ka
kaKC0
4
4
55 nm, suggesting the formation of photodegradative products of
-nitrophenol. Similar spectral observations were made with the
where ka is the apparent reaction rate constant, K is the adsorp-
tion coefficient and Co corresponds to the initial concentration of
−
1
polystyrene fiber functionalized with compound 3b, though the
changes were slightly less pronounced. This could possibly be due
to higher singlet oxygen quantum yield of compound 3a than 3b
in the fiber as determined in aqueous media. However, when non
functionalized polystyrene polymer fiber was used under similar
conditions, no spectral changes of the aqueous 4-nitrophenol sam-
ple solutions was observed. This suggests that the phthalocyanines
4-nitrophenol. Plots of the inverse of initial reaction rate (rate)
versus the reciprocal of the initial concentration of 4-nitrophenol
−
1
(C0)
were found to be linear with non zero intercepts for
both functionalized fibers, Fig. 10. The apparent rate constant
−
6
−1
−1
for 3a/PS,
ka was determined to be 8.76 × 10 mol L min
−
6
−1
−1
−6
−1
−1
8.48 × 10 mol L min for 3b/PS and 4.5 × 10 mol L min
(
3a and 3b) are the agents involved in the phototransformation
1
2
process due to their ability to generate the reactive singlet oxy-
gen. Similarly, when only the 4-nitrophenol sample solution was
irradiated with the visible light no spectral changes were observed
further suggesting that the catalysis can only occur in the presence
of the sensitizer.
We assessed the supposed involvement of singlet oxygen in the
conversion of 4-nitrophenol by conducting the photolysis reaction
with and without sodium azide, a singlet oxygen quencher, in the
sample solutions. Fig. 9 shows the kinetics of the various reac-
tion conditions using first order kinetic model. There is generally a
(c)
b)
(
10
8
(a)
6
2
4
good fit with R values above 0.96 in all cases. As shown in Fig. 9,
the phototransformation reaction is slowed when sodium azide is
added to 4-nitrophenol solution. This is an indication of the abil-
ity of 3a, and 3b to generate singlet oxygen is a key factor in the
phototransformation of 4-nitrophenol.
2
Kinetic parameters of the phototransformation of 4-nitrophenol
at different initial concentrations using the functionalized fibers
0
0
5
10
15
(
3a/PS, 3b/PS and ZnPc/PS) are given in Table 2.
5
-1 -1
(
Init ial Conc ent rat ion)-1 x 10 (MolL )
As shown in Table 2, the observed rate constant (kobs) in all three
cases, decreased with an increase in concentration of 4-nitrophenol
as expected, with kobs values of 3a/PS being comparatively slightly
higher than those of 3b/PS and ZnPc/PS. Thus the 3a/PS fiber shows
−
1
Fig. 10. Plot of the inverse of initial reaction rate (rate) versus the reciprocal of
the initial concentration of 4-Np for photooxidation using 20 mg (a) 3a/PS, (b) 3b/PS
and (c) ZnPc/PS functionalized fiber.

5
6
R. Zugle, T. Nyokong / Journal of Molecular Catalysis A: Chemical 358 (2012) 49–57
Table 2
The rate, rate constant (kobs) and half-life (t1/2) of various initial concentrations of 4-nitrophenol.
4
-NP concentration
Complex
kobs
(×10 min
Initial rate
(×10 mol L min
Half-life/min
−
4
−1
−2
−1
−6
−1
−1
)
(×10 mol L
)
)
0.9
1.2
2.0
2.5
3a/PS
1.46
1.12
1.00
1.31
1.01
0.90
47.5
61.8
69.3
3
b/PS
ZnPc/PS
3a/PS
1.38
1.07
1.02
1.66
1.26
1.22
50.2
64.8
68.0
3
b/PS
ZnPc/PS
3a/PS
1.22
1.05
0.85
2.44
2.14
1.69
56.8
66.0
81.5
3
b/PS
ZnPc/PS
3a/PS
1.11
0.95
0.74
2.78
2.38
1.86
61.9
72.8
93.7
3
b/PS
ZnPc/PS
for ZnPc/PS, indicating slightly faster reaction kinetics for the
degradation of 4-nitrophenol with 3a/PS fiber than with both
1
.8
.6
.4
0
0
0
3
b/PS and ZnPc/fibers. This is a reflection of the slightly higher
singlet oxygen quantum yields of the former. The absorption coef-
(b)
(a)
3
−1
3
−1
ficients K, were found to be 1.5 × 10 mol , 2 × 10 mol
and
−1
for 3a/PS, 3b/PS and ZnPc/PS fibers respectively.
3
5
.8 × 10 mol
This suggests that adsorption was slightly less favored in the case
of 3a/PS compared to desorption while the converse is true for
0.2
0
3
b/PS and ZnPc/PS fibers. The half-lives are within one and half
hours of photo-irradiation for fibers suggesting that the function-
alized fibers are promising as fabrics that could be applied for real
applications.
3
00
400
500
600
700
800
Wavelength/nm
Fig. 12. UV–vis spectra of 10 mg of 3b/PS functionalized fiber (a) not used in catalysis
and (b) used in catalysis (time: 12 h), each dissolved in 4 ml of THF.
3.2.5. Gas chromatographic analysis
The reaction products were identified using chromatography.
This was done by spiking sample solutions with standard solu-
tions of the products. As shown in Fig. 11, the photo-degradation
products consist of 4-nitrocatechol, benzoquinone and hydro-
quinone. Mass spectrometry (MS) was further used to confirm
the products. The following molecular ions corresponding to the
3.3. Fate of the phthalocyanines during photocatalysis
The photostability of imbedded sensitizer is an important factor
for the application of such fabric materials for photocatalysis. Its
decomposition will lead to the termination of the photocatalytic
process. Therefore the photostability of phthalocyanines within
the fiber matrices were assessed by observing the Q-band absorp-
tion of an equivalent amount (10 mg) of the fibers before and after
catalysis, both dissolved in equal volume of THF (4 mL), Fig. 12 for
+
products were identified; [M+2H ] = 112 amu for hydroquinone,
+
+
[
M+H ] = 109 amu for benzoquinone and [M+2H ] = 112 amu for 4-
nitrocatechol.
3b/PS fiber. As indicated, the decrease in the absorption band after
the photolysis suggests that the phthalocyanine photo-degrades
slightly upon continuous irradiation for 12 h. This could account
for the decrease in the reaction rates when the functionalized
fibers were used for repetitive conversion of the 4-nitrophrnol.
Nonetheless, the fiber could be re-used for at least three cycles for
degradation of 4-nitrophenol. FT-IR analysis of the fibers before and
after irradiation showed no apparent structural changes in the com-
ponents. Also scanning electron microscopic (SEM) images of the
fibers after use indicate the fiber that average diameters remained
virtually the same. However the strands of fibers became more
closely compressed after use than before. Thus there was reduced
porosity in the fiber mats and could also account for the reduced
degradation rate of 4-nitrophenol during re-use of the fibers.
4. Conclusion
In this work, two phthalocyanine complexes of lutetium periph-
erally substituted with phenoxy (compound 3a) and 2-pyridiloxy
compound 3b) groups were synthesized and characterized using
(
conventional spectroscopic methods. They were found to be pho-
toactive with triplet quantum yields of 0.71 and 0.68 for 3a
and 3b respectively in DMF. The corresponding triplet state life-
times were 2.60 ± 0.01 ␮s and 2.64 ± 0.01 ␮s for 3a and 3b. Singlet
Fig. 11. Gas chromatographic traces of photolysis products of 4-nitrophenol using
PS/3a fiber as sensitizer for 12 h.

R. Zugle, T. Nyokong / Journal of Molecular Catalysis A: Chemical 358 (2012) 49–57
57
oxygen quantum yields also in DMF were found to be 0.68 and
.62 for 3a and 3b respectively. The singlet oxygen generating
ability was maintained in the fiber for both phthalocyanines and
these polymeric fiber materials incorporating phthalocyanines fur-
ther found to be promising materials for the photo-conversion of
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Acknowledgments
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
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