New insights into the influence of weak and strong acids on the oxidative stability and photocatalytic activity of porphyrins

Article abstract of DOI:10.1039/c7nj02442h

A series of electron-rich and electron-demanding meso-tetra(aryl)porphyrins (aryl = phenyl, 4-methoxyphenyl, 4-methylphenyl, 4-chlorophenyl, 2-methylphenyl and 2-chlorophenyl) and their dications with weak and strong acids (CF₃COOH, CHCl_2<

Full text of DOI:10.1039/c7nj02442h

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Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
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New insights on the influence of weak and strong acids on the
oxidative stability and photocatalytic activity of porphyrins
Saeed Zakavi,* Saiedeh Hoseini and Aida G. Mojarrad
Received 00th January 20xx,
Accepted 00th January 20xx
Abstract: A series of electron-rich and electron-demanding meso-tetra(aryl)porphyrins (aryl = phenyl, 4-methoxyphenyl, 4-
DOI: 10.1039/x0xx00000x
methylphenyl, 4-chlorophenyl, 2-methylphenyl and 2-chlorophenyl) and their dications with weak and strong acids
www.rsc.org/
3 2 2 4
(CF COOH, CHCl COOH, CH ClCOOH, HCOOH, HCl and HClO ) were used as photosensitizer for the aerobic photooxidation
of cyclooctene under homogeneous conditions. Mechanistic studies in the presence of a singlet oxygen quencher (1,3-
diphenylisobenzofuran) and 1,4-benzoquinone as the superoxide anion radical scavenger confirmed the involvement of a
singlet-oxygen mechanism in the oxidation reactions. Also, the solvent effects on the catalytic activity of porphyrin diacids
were explained by relative lifetimes of singlet oxygen in different solvents. The formation of dications was found to either
increase or decrease the photocatalytic activity, photostability and the singlet oxygen quantum yield (φ
The non-radiative rate constants (knr) for decay of the S excited state of the photosensitizers were often greater in the
case of porphyrin diacids. The φ values of porphyrins and the dications were in the range of 0.48-0.89 to 0.06-0.98,
respectively. Interestingly, in spite of the large φ value (0.98) found for H T(4-OMe)PP(CF COO) and H T(4-
OMe)PP(HCOO) , H T(4-Cl)PP(HCOO) (φ = 0.29) showed the highest catalytic activity of the series. This observation was
attributed to extensive oxidative degradation of the former. It is noteworthy that the φ parameters were evaluated in a
short reaction time (ca. 20 min) and therefore no detectable catalyst degradation was observed in this time interval.
Furthermore, H TPP(HCOO) was the most stable photocatalyst under the reaction conditions.
Δ
) of porphyrins.
1
Δ
Δ
4
3
2
4
2
4
2
Δ
Δ
4
2
2
1-
changes in structural and electronic properties of porphyrins
3
; the out-of-plane deformation of porphyrin core and
2
Introduction
enhanced coplanarity of meso aryl groups and porphyrin mean
plane are the main structural alterations induced by
protonation of porphyrin core. The counteranions of the acids
play crucial roles in the optimized structure of porphyrin
diacids; in the dications of meso-tetraphenylporphyrin with HX
After the first report on the ability of porphyrins to produce a
1
phototoxic effect in the early 20th century , a wide variety of
porphyrin-based photosensitizers including porphyrins,
phthalocyanines, chlorins and bacteriochlorins have been used
2
-7
as photosensitizers in Photodynamic therapy (PDT) . It has
been generally believed that cancer cells are more acidic than
(
X = F, Cl, Br and I) the degree of saddling was found to
8
normal cells . Indeed, anaerobic metabolism in cancer cells led
increase with increasing size of the halide. However, the
necessity for optimization of hydrogen bond interactions
between the dication and the counteranion was shown to be
to the formation of lactic acid with a pK
a
value of 3.8 that has
9
-11
been considered as the major cause of tumor acidity
.
2
4
the main cause of this observation . On the other hand,
porphyrin diacids demonstrate remarkably red shifted
absorption bands in comparison to those of the corresponding
Accordingly, the formation of porphyrin diacids seems to be a
problem associated with the use of porphyrins as
photosensitizer in PDT. Furthermore, porphyrins are readily
protonated in chlorinated solvents under irradiation with
2
2,23
free base porphyrins
.
Meanwhile, protonation of
12-14
porphyrins is accompanied with alteration of photophysical
properties of porphyrins such as fluorescence quantum yield,
excited state lifetime and the rate constants of radiative and
different light sources
; The decreased photocatalytic
activity of porphyrin sensitizer was attributed by some authors
15
to diprotonation of porphyrin core with acid products .
Porphyrins are readily protonated by weak and strong acids
2
0,25
1
6- non-radiative decay of the excited state
. Accordingly, the
2
0
photocatalytic activity of porphyrins is expected to be
influenced by protonation of the aromatic macrocycle.
Moreover, the sensitivity of porphyrin macrocycle towards
oxidative attack by reactive oxygen species (ROS) depends on
.
The formation of porphyrin diacids is associated with large
Department of Chemistry, Institute for Advanced Studies in Basic Sciences (IASBS),
Zanjan 45137-66731, Iran. E-mail: zakavi@iasbs.ac.ir.
† Footnotes relaꢀng to the ꢀtle and/or authors should appear here.
Electronic Supplementary Information (ESI) available: [details of any
the distribution of electron density on different positions of
14,26
porphyrinic photosensitizer. As we have shown previously
,
supplementary information available should be included here]. See the shift of electron density from porphyrin core to the acid
DOI: 10.1039/x0xx00000x
molecules which decreases the electron density of porphyrin
ring can increase the oxidative stability of these
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photocatalysts. Porphyrin diacids may be considered as a large Porphyrin
dictions
3 2
with CF COOH, CHCl COOH and
family of porphyrin derivatives with photophysical properties CH ClCOOH were prepared by adding excess amounts (more
2
different from those of the parent porphyrins. However, to than the 2:1 molar ratio of the acid to porphyrin) of the
date, there has been little systematic study of the effects of respective acid to the solution of porphyrin in
core protonation on the photocatalytic performance of dichloromethane. Slow evaporation of dichloromethane and
porphyrin photosensitizers. In 2015, oxidation of artemisinin excess acid at room temperature gave the corresponding
catalyzed by an Amberlyst supported porphyrin in liquid CO
2
2
dications. In the case of HCOOH, the evaporation of solvent
7
was reported by George et al . Also, the photostability of led to partial or complete decomposition of the dications, in
porphyrins upon irradiation with UV light was found to the case of the electron-rich and electron-deficient porphyrins,
2
8
significantly decrease upon diacids formation with CF
3
COOH . respectively. Accordingly, the solution of dications in
Very recently we have reported the improved photocatalytic dichloromethane, acetonitrile and toluene was used for the
1
13
performance of meso-tetra(aryl)porphyrins induced by the photocatalytic studies. The H NMR, C NMR and UV-Vis
formation of molecular complexes with tetra-cyanoethylene spectral data of the dications are presented in the ESI,† S1b-
(
TCNE)
1
and
2,3-dichloro-5,6-dicyano-1,4-benzoquinone S1g.
4,26
(DDQ)
. Also, the dications of meso-tetra(aryl)porphyrins
with an acidic solid support (Amberlyst 15) were used as highly
efficient and stable photosensitizers for the aerobic oxidation
General oxidation procedure
TPP (0.33 µmol in 5 ml toluene) was
porphyrins can be protonated with acids as weak as formic added to a double walled cylindrical glass vessel equipped with
pK = 3.75) and propionic (pK = 4.87) acid as well as the water circulation to maintain a constant temperature (ESI,†
strong acids . In this study, the effect of core protonation Figure S3). The solution was saturated with oxygen by slow
2
9
of olefins . In previous studies, we have shown that In a typical reaction H
2
(
a
a
1
7,18,20
on the photocatalytic performance of a series of meso- purging of oxygen from a cylinder in 30 min. Cyclooctene (0.33
tetraarylporphyrins was investigated. Different weak and mmol) was added to the catalyst solution at 1:1000 molar
strong acids were used to protonate the porphyrins (Figure 1).
ratio. A 200 W white mercury compact fluorescent lamp (CFL)
was used as the light source. After the required time, the
reaction mixture was analyzed by GC. All reactions were
repeated at least three times and analyzed by GC; the average
conversion values with an error of about ± 5% are reported. A
X
Ar
Ar=
Y
NH
N
N
X=
H
H
CH3
H
Cl
H
Y=
H
OCH3
H
CH3
H
Cl
Ar
Ar
4
control reaction was conducted in CCl to identify the
H2TPP
H T(4-OMe)PP
2
H2T(2-Me)PP
H2T(4-Me)PP
H2T(2-Cl)PP
H2T(4-Cl)PP
1
3
HN
oxidation product by NMR. The C NMR spectrum of the
reaction mixture showed the formation of cyclooct-1-en-3-yl
hydroperoxide as the sole product.
Ar
Acids: CF COOH, HClO , HCl, CH ClCOOH, CHCl COOH, HCOOH
3
4
2
2
Singlet oxygen quantum yield determination
Figure 1. Porphyrins and acids used in this study.
Δ
The singlet oxygen quantum yields (φ ) of the photosensitizers
were measured through the reaction of the reactive oxygen
species (ROS) with 1,3-Diphenylisobenzofuran (DPBF) as a
Experimental section
quencher of singlet oxygen and H
2
TPP (φ
Δ
= 0.70 in toluene) as
3
a reference photosensitizer . The equation Φ
1
std
Δ
= Φ
Δ
× (ᶹ
i
I
std
/
Instrumental
std
32
I) proposed by Murata et al. , was used to estimate the φ
ᶹ
i
Δ
The NMR spectra were obtained in CDCl3 on a Bruker Avance
DPX-400 MHz spectrometer. The absorption spectra were
recorded on a Pharmacia Biotech Ultrospec 4000 UV–Vis
std
std
values. In this equation, Φ
Δ
, ᶹ
i
,ᶹ
i
, Istd and I present the φ
Δ
of
H
2
TPP, the rate of DPBF oxidation in the presence of the
respective photosensitizer, the rate of DPBF oxidation in the
presence of H TPP, the number of photons absorbed by the
spectrophotometer.
equipped with a HP-5 capillary column (phenylmethyl siloxane
0 m × 320 µm × 0.25 µm) and a flame-ionization detector was
A
Varian-3800 gas chromatograph
2
standard solution and those absorbed by
photosensitizer, respectively.
a
given
3
utilized in GC analyses.
Results and Discussion
Synthesis of porphyrins and diacids
The porphyrins used in this study including H
2
TPP, H
2
T(4- Photooxidation of cyclooctene
OMe)PP, H
2 2 2 2
T(2-Me)PP, H T(4-Me)PP, H T(2-Cl)PP, H T(4-Cl)PP
The results for the oxidation of cyclooctene catalyzed by the
free base porphyrins in toluene are summarized in Table 1. The
oxidative stability of the free base porphyrins decreases as
were synthesized, characterized and purified according to the
3
0
1
13
literature . The H NMR, C NMR and UV-Vis spectral data of
the porphyrins are presented in the ESI,† S1a.
2 2 2 2 2
H T(2-Me)PP > H T(4-Cl)PP ~ H T(2-Cl)PP > H TPP > H T(4-
Me)PP >> H
2
T(4-OMe)PP that almost correlates with the
2
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electron donating ability of the meso aryl groups. It is
noteworthy that the presence of bulky group at the ortho non-protonated porphyrins, although the stability of H
position increases the dihedral angle between the aryl Me)PP(ClO and H T(4-Cl)PP(HCOO) was higher than the
substituents and porphyrin mean plane and therefore the corresponding porphyrins.
H
2
T(4-Cl)PP, the dications were usually less stable than the
4
T(2-
4
)
2
4
2
2
1,33
electron deficiency of porphyrin
of porphyrins decreases as H T(4-ClP)P ~ H
Me)PP ≥ H TPP ≥ H T(2-Me)PP > H T(4-OMe)PP. With the
exception of H T(4-OMe)PP, there is no remarkable differences
. Also, the catalytic activity
Table 1. Oxidative stability and catalytic activity of the free base porphyrins in the
T(2-ClP)P > H T(4-
2
a
2
2
oxidation of cyclooctene.
2
2
2
Photosensitizers
Oxidative Stability (%)
Conversion (%)
34.2
2
H
2
H
2
H
2
H
2
H
2
H
2
TPP
63.3
5.3
between the catalytic activity of the other porphyrins. This
observation seems to be due to the extensive oxidative
T(4-OMe)PP
T(2-Cl)PP
T(4-Cl)PP
T(2-Me)PP
T(4-Me)PP
13.7
70.3
71.1
85.2
48.6
41.1
degradation of H
The oxidation of cyclooctene was conducted in the presence of
the dications of H TPP, H T(2-Cl)PP, H T(2-Me)PP, H T(4-Cl)PP,
T(4-OMe)PP and H T(4-Me)PP with CF COOH, HClO , HCl,
CHCl COOH, CH ClCOOH and HCOOH in toluene. The results
are summarized in Figures 2 and 3. The oxidative stability of
T(4-Me)PP, H T(4-OMe)PP and H TPP dications with weak
and strong acids was generally higher than that of the free
base porphyrins. In the case of H T(2-Cl)PP, H T(2-Me)PP and
2
T(4-OMe)PP.
41.9
34.0
2
2
2
2
37.7
H
2
2
3
4
a
For a reaction time of 48 h in toluene. Photosensitizer and alkene were used in
:1000 molar ratio. A 200 W white mercury compact fluorescent lamp (CFL) was
2
2
1
used as the light source.
H
2
2
2
2
2
Figure 2. The oxidative stability of free base porphyrins and their dications.
On the other hand, the catalytic activity of the dications of the catalytic activity of the former was significantly higher than
H T(2-Me)PP and H T(4-OMe)PP was usually greater than that that of the latter. Also, H TPP(HCOO) was the most stable
2
2
4
2
of the free base porphyrins. While H T(2-Cl)PP(ClO ) was catalyst of the series.
4 2
4
found to be a more efficient catalyst compared to H T(2-Cl)PP, The solvent effect on the catalytic performance of the catalysts
2
the other dications of H T(2-Cl)PP showed decreased catalytic was studied in the case of H T(4-Me)PP and its dications with
2
2
activities. Also, H T(4-Cl)PPCl and H T(4-Cl)PP(HCOO) were CF COOH and HCOOH using CH CN, dichloroethane (DCE) and
3
4
2
4
2
3
more efficient catalysts than the free base porphyrins. toluene as the solvent (Table 2). Previous studies confirmed a
However, the formation of dications has little effect on the singlet oxygen mechanism for the oxidation reactions catalyzed by
1
4,26,34-37
catalytic activity of H2T(4-Me)PP. Interestingly, H4T(4- non-metallatedporphyrins
. DPBF is a well-known singlet
Cl)PP(HCOO) demonstrated the highest catalytic activity of oxygen quencher that has been used in mechanistic studies to
2
38,39
the series of porphyrins and dications. It should be noted that elucidate the nature of reactive oxygen species
.
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Figure 3. The catalytic activity of free base porphyrins and their dications.
a
Table 2. Solvent effect on the catalytic performance of H
2
T(4-Me)PP and its dications with CF
3
COOH and HCOOH.
Compound
CH
3
CN
DCE
6 5 3
C H CH
Time (h)
24
Oxidative
Conversion (%)
Oxidative
Conversion (%)
Oxidative
Conversion
(%)
Stability (%)
Stability (%)
Stability (%)
H
H
2
T(4-Me)PP
T(4-Me)PP(CF
T(4-Me)PP(HCOO)
32.0
13.7
12.5
1.8
4.2
9.5
9.2
1.2
26.0
27.2
35.2
45.7
57.4
62.8
90.6
48.6
94.6
81.8
69.7
66.4
18.8
37.7
7.7
4
8
4
3
COO)
2
24
31.1
34.2
13.1
35.0
97.1
30.7
44.7
39.6
4
8
17.5
18.1
30.6
H
4
2
24
38.3
8.8
4
8
a
see the footnote of Table 1.
Figure 4 shows the decrease in the emission spectrum of DPBF the absorption at the wavelength of the Soret band. The
in the presence of H T(4-Me)PP(CF COO) upon irradiation of oxidation of cyclooctene was also performed under sunlight
4
3
2
the solution with a 10 W red LED lamp. Also, conducting the irradiation (Table 4). Although higher conversions were usually
oxidation reaction in the presence of 1,4-benzoquinone has achieved under sunlight irradiation, a significant increase in
little effect on the conversion of cyclooctene to the oxidation the catalyst degradation was observed under sunlight
product that supports the absence of superoxide anion radical irradiation.
1
4,40
species in the catalytic cycle of the reaction
In the case of H T(4-Me)PP(CF COO) , the catalytic activity in
the used solvents decreased as DCE ≥ CH CN >> toluene that
.
4
3
2
3
correlates with the lifetime of singlet oxygen in these solvents;
the lifetimes of singlet oxygen in the used solvents are 77, 63
4
1,42
and 29 µs for DCE, CH
Furthermore, the oxidative stability of the catalyst decreased
as DCE ≥ toluene >> CH CN. On the other hand, the catalytic
activity of H T(4-Me)PP(HCOO) decreased as DCE > toluene ≥
CH CN. Also, the oxidative stability of the catalyst followed the
order: toluene > DCE > CH CN. The difference between the
lifetime of S excited states of dications of H T(4-Me)PP with
weak and strong acids in various solvents may explain this
3
CN and toluene, respectively
.
3
4
2
3
3
1
2
2
0,25
4
Figure 4. The change in the emission spectrum of DPBF in the presence of H T(4-
finding
.
Me)PP(CF
3
COO)
2
upon irradiation with a 10 W red LED lamp.
Running the oxidation of cycloocetene (Table 3) in the
presence of blue and red LED lamps (10 W) suggests that the
photocatlytic activity of porphyrin dications depends mainly on
4
|
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Table 3. Effect of light source on the catalytic performance of H
4
T(4-Me)PP(CF
3
COO)
2
a
4 2
and H T(4-Me)PP(Cl) in the oxidation of cyclooctene.
The φ
Δ
measurements were conducted by plotting kinetics
curves (Figure 5) for porphyrins and their dications. Also, the
changes in the absorption spectrum of DPBF in the presence of
Red LED
Blue LED
Conversion
H
2
T(4-OMe)PP and its diprotonated species with different
acids are shown in Figure 6. A control reaction performed
prior to the φ measurement revealed that the exposure of
Compound
Conversion
Time
h)
and oxidative
and oxidative
(
b
b
Stability (%)
Stability (%)
Δ
-4
the toluene solution of DPBF (8.0 × 10 M) to the 10 W red
LED lamp led to no observable degradation of DPBF (Figure 5)
in the absence of photosensitizer.
H
H
4
T(4-Me)PP(CF
3
COO)
2
24
0.7 [84.9]
0.9 [79.6]
4.3 [97.1]
6.7 [83.1]
6.3 [44.6]
11.5 [21.5]
5.2 [87.5]
12.3 [54.3]
4
8
4
T(4-Me)PP(Cl)
2
24
4
8
Singlet oxygen quantum yields
a
b
See the footnote of Table 1. The data in brackets.
Table 4. Sunlight versus a 200 W white CFL as the light source.
The relative efficacy of different photocatalysts may be
32,43-45
compared using their singlet oxygen quantum yield (φ
a
Δ
)
,
the efficiency in which photosensitizers are able to use the
3
Compound
Sunlight
Conversion and
oxidative
CFL
1
2 2
energy of the absorbed light to convert O to O .
Conversion and
oxidative
b
b
2
1
1
0
0
0
/0
/6
/2
/8
/4
/0
Stability (%)
Stability (%)
0
/0
a
)
H
H
H
2
4
4
T(4-Me)PP
29.8 [10.6]
20.5 [13.7]
16.1 [8.2]
18.8 [90.6]
7.7 [94.6]
18.1 [69.7]
b
)
ꢀ0/2
3 2
T(4-Me)PP(CF COO)
T(4-Me)PP(HCOO)
2
ꢀ0/4
ꢀ0/6
ꢀ0/8
ꢀ1/0
H4TPP(CH2ClCOO)2
H2TPP
a
H4TPP(HCOO)2
H4TPP(CHCl2COO)2
H4TPP(CF3COO)2
H4TPP(ClO4)2
H4TPP(Cl2)
See the footnotes of Tables 1 and 3.
a
Table 5. Singlet oxygen quantum yields of free base porphyrins and diacids .
DPBF photodegradation
0
25 50 75 100 125 150 175 200
Time/ min
0
20
40
60
Time/ s
80
100 120
Entry
Photosensitizer
Φ
Δ
3
1
1
2
3
4
5
6
7
8
9
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
2
4
4
4
4
4
4
2
4
4
4
4
4
4
2
4
4
4
4
4
4
2
4
4
4
4
4
4
TPP
TPP(CF
TPP(HCOO)
TPP(Cl)
TPP(ClO
TPP(CH
TPP(CHCl
0.70
0
/0
ꢀ0/2
0/4
0/6
0/0
3
COO)
2
0.31
0.39
0.06
0.11
0.68
0.65
0.48
0.98
0.98
0.29
0.28
0.77
0.84
0.51
0.49
0.29
0.07
0.18
0.82
0.87
0.89
0.36
0.17
0.51
0.64
0.39
0.93
2
c)
d)
ꢀ
0/2
2
ꢀ
H
T
(
(
(
(
(
(
(
4ꢀOMe
4ꢀOMe
4ꢀOMe
4ꢀOMe
4ꢀOMe
4ꢀOMe
4ꢀOMe
)
)
)
)
)
)
)
PP(HCOO)
PP
H
4
T(4ꢀCl)PP
(
CHCl
2
COO)2
4
2
H
H
H
H
H
H
2
4
4
4
4
4
T(4ꢀCl)PP
T(4ꢀCl)PP
4
)
2
H2T
H4T
H4T
H4T
H4T
H4T
ꢀ0/4
(
CH
2
ClCOO)2
ꢀ
PP
(CF3COO)
2
2
ClCOO)
2
2
T(4ꢀCl)PP(Cl)
PP(Cl)2
2
ꢀ
0/6
T(4ꢀCl)PP(ClO4)2
T(4ꢀCl)PP(HCOO)
PP
PP
PP
(
(
(
CH2ClCOO
CHCl2COO
)
)
2
2
2
COO)
ꢀ0/8
1/0
2
T(4-OMe)PP
ClO4
40
)
2
T(4ꢀCl)PP
(
CF
3
COO)2
60
Time/ s
ꢀ
ꢀ0/8
0
20
60
80
100
0
20
40
80
100
120
T(4-OMe)PP(CF
3
COO)
2
Time/ s
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
T(4-OMe)PP(HCOO)
T(4-OMe)PP(Cl)
T(4-OMe)PP(ClO
T(4-OMe)PP(CH
2
2
0
/0
4 2
)
e)
2
ClCOO)
2
2
ꢀ0/4
ꢀ0/8
ꢀ1/2
ꢀ1/6
T(4-OMe)PP(CHCl
2
COO)
H
H
H
H
H
H
H
4
2
4
4
4
4
4
T
T
T
T
T
T
T
(
4ꢀMe
4ꢀMe
4ꢀMe
4ꢀMe
4ꢀMe
4ꢀMe
4ꢀMe
)
)
)
)
)
)
)
PP
PP
PP
(
CHCl COO
2
)2
T(4-Cl)PP
(
(
(
(
(
(
T(4-Cl)PP(CF
T(4-Cl)PP(HCOO)
T(4-Cl)PP(Cl)
T(4-Cl)PP(ClO
T(4-Cl)PP(CH
3
COO)
2
(
CH ClCOO)2
2
PP(Cl)
2
2
PP(
PP(
PP(
ClO
4 2
)
2
CF COO
3
)2
HCOO)
2
4 2
)
0
40
80
120
160
200
2
ClCOO)
COO)
2
2
Time/ s
T(4-Cl)PP(CHCl
2
T(4-Me)PP
Figure 5. The rate of DPBF photodegradation in the absence of photosensitizers (a) and
T(4-Me)PP(CF
T(4-Me)PP(HCOO)
T(4-Me)PP(Cl)
T(4-Me)PP(ClO
T(4-Me)PP(CH
T(4-Me)PP(CHCl
3 2
COO)
the rate of DPBF photooxidation reactions catalyzed by free base porphyrins and their
2
respective diacids upon irradiation with a 10 W red LED lamp, (b) H
OMe)PP, (d) H T(4-Cl)PP and (e) H T(4-Me)PP.
2 2
TPP, (c) H T(4-
2
2
2
4 2
)
2
ClCOO)
2
2
As may be seen from Table 5, the φ
.48-0.89 and 0.06-0.98 for the free base porphyrins and their
dications, respectively. Accordingly, the formation of porphyrin
diacids may either increase or decrease the φ parameter. The
maximum value of φ (ca. 1) was observed in the case of
T(4-OMe)PP(CF COO) and T(4-OMe)PP(HCOO)
However, as was shown in Figures 2 and 3, H T(4-Cl)PP(HCOO)
Δ
values are in the range of
2
COO)
0
a
Singlet oxygen quantum yields were determined using a 10 W red LED lamp,
,3-Diphenylisobenzofuran (DPBF) as a quencher of singlet oxygen and H TPP as a
1
2
Δ
reference photosensitizer.
Δ
H
4
3
2
H
4
2
.
4
2
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ARTICLE
= 0.29) and H
Journal Name
2
0,25
(φ
Δ
4
TPP(HCOO)
2
(φ
Δ
= 0.39) show the highest previous work
, porphyrins and their diacids usually have
catalytic activity and oxidative stability, respectively. The comparable Φ
significantly decreased catalytic activity of T(4- diacids was often accompanied with an increase in the knr
OMe)PP(CF COO) and H T(4-OMe)PP(HCOO) (φ = 0.98) may values. Interestingly, the knr of H T(4-Cl)PP(HCOO) was 65.1
F
values (Table 6). However, the formation of
H
4
3
2
4
2
Δ
4
2
-1
be attributed to the remarkable oxidative degradation of these μs that may explain the higher photocatalytic activity of this
dications (Figure 2). In other words, high catalytic activity of dication. On the other hand, high degree of catalyst
photosensitizers with great φ
Δ
values may be observed in the degradation observed in the case of H
4 3 2
T(4-Me)PP(CF COO)
case of small degrees of catalyst degradation.
and H T(4-Me)PP(HCOO) may explain their low catalytic
4
2
-1
activity, in spite of their high Knr values (79.3 and 79.7 μs ,
respectively).
1
0
0
0
0
0
/0
/8
/6
/4
/2
/0
0/6
/5
0/4
0
5 s
s
H
T(4ꢀOMe)PP
2
H4T(4ꢀOMe)PP(CHCl2COOH
)
0 s
10 s
Table 6. The fluorescence quantum yields and the rate constants of radiative and non-
0
15 s
30 s
45 s
55 s
65 s
75 s
85 s
95 s
20,25
2
8
0 s
0 s
radiative decay processes for free base porphyrins and diacids.
200 s
3
6
80 s
20 s
0
0
0
/3
/2
/1
ꢀ
1
ꢀ1
920 s
Entry
1
Photosensitizer
TPP
TPP(CF
TPP(HCOO)
TPP(CHCl COO)
Φ
f
k
r
(ꢁs )
k
nr (ꢁs )
1
05 s
H
H
H
H
H
H
H
2
4
4
4
2
4
4
0.13
0.14
2.99
3.60
7.40
3.50
4.40
5.00
8.20
4.80
2.33
4.00
5.20
3.50
3.29
5.40
5.30
4.60
20.00
22.40
29.10
23.50
16.60
7.60
115 s
1
1
25 s
35 s
2
3 2
COO)
3
2
0.079
0.39
0/0
3
3
00
350
400
Wavelength/ nm
450
500
00
350
400
Wavelength/ nm
450
500
550
4
2
2
5
T(4-OMe)PP
0.21
0
/6
0
5 s
15 s
s
0/6
/5
0/4
/3
0/2
/1
6
T(4-OMe)PP(CF
3
COO)
2
0.39
H
T
(
4ꢀOMe)PP
(
CH ClCOOH
)
0 s
5 s
4
2
2
H T(4ꢀOMe)PP(Cl)
4 2
0
0
0
0
0
0
/5
/4
/3
/2
/1
/0
0
7
T(4-OMe)PP(HCOO)
2
0.089
0.14
29.70
28.40
41.67
56.00
65.10
29.30
28.40
79.30
79.70
21.70
3
4
5
6
0 s
0 s
0 s
0 s
10 s
30 s
70 s
130 s
250 s
430 s
670 s
8
H T(4-OMe)PP(CH ClCOO)
4
2
4
4
4
2
4
4
4
2
2
9
H
H
H
H
H
H
H
H
T(4-Cl)PP
0.053
0.11
70 s
0
8
9
1
1
0 s
0 s
00 s
10 s
10
11
T(4-Cl)PP(CF
COO)
3 2
T(4-Cl)PP(HCOO)
2
0.037
0.11
1
2
T(4-Cl)PP(CH
T(4-Me)PP
2 2
ClCOO)
120 s
0
1
1
30 s
40 s
13
0.104
0.13
0/0
300
1
4
T(4-Me)PP(CF
T(4-Me)PP(HCOO)
T(4-Me)PP(CH ClCOO)
2
3 2
COO)
3
00
350
400
450
500
550
350
400
450
500
Wavelength/ nm
15
16
0.028
0.18
Wavelength/ nm
2
0
0
0
0
0
0
0
/6
0/6
0/5
0/4
0/3
0/2
0/1
0/0
2
0
5 s
s
H
T
(
4ꢀOMe)PP
(
ClO
)
H
T
(
4ꢀOMe
)
PP(HCOO)
2
4
4 2
/5
/4
/3
/2
/1
/0
4
0 s
1
3
9
2
3
6
9
5 s
0 s
0 s
10 s
90 s
30 s
30 s
5
1
2
3
s
0 s
0 s
0 s
40 s
Conclusion
5
6
7
8
0 s
0 s
0 s
0 s
In summary, protonation of meso-tetra(aryl)porphyrin with
weak and strong acids as photosensitizer was found to either
increase or decrease the catalytic performance of porphyrins
550 in the oxidation of olefins through a singlet oxygen mediated
mechanism, depending on the used acid and porphyrin.
However, the highest catalytic activity and oxidative stability of
the porphyrins and dications series were found in the case of
90 s
3
00
350
400
Wavelength/ nm
450
500
300
350
400
450
500
Wavelength/ nm
0
0
0
0
0
0
0
/6
/5
/4
/3
/2
/1
/0
0
s
10 s
H
T
(
OꢀMe
)
PP
(
CF COO)
3 2
4
20 s
3
4
5
6
7
8
9
1
1
0 s
0 s
0 s
0 s
0 s
0 s
0 s
00 s
10 s
H
4
T(4-Cl)PP(HCOO)
2
and
H
4
TPP(HCOO)
2
,
respectively.
Moderate singlet oxygen quantum yields, high oxidative
stability and relatively high non-radiative rate constant (knr) for
decay of the S
1
excited state were found for these dications.
value (≈ 1) were
120 s
The porphyrin dications with the highest φ
Δ
1
1
30 s
40 s
not necessarily the most efficient catalyst of the series. This
observation was in accord with high degrees of catalyst
3
00
350
400
450
500
550
Wavelength/ nm
Figure 6. The changes in the absorption spectrum of DPBF in the presence of H
2
T(4- degradation
Me)PP(HCOO)2.
of
H
4
T(4-Me)PP(CF
3
COO)
2
and
4
H T(4-
OMe)PP and its diprotonated species.
On the basis of the Jablonski diagram, the efficiency of
formation of the photosensitizers triplet state (φ is
determined by the non-radiative rate constant (knr) for decay
Acknowledgements
T
)
Financial support of this work by the Institute for Advanced
Studies in Basic Sciences (IASBS) and Iran National Science
Foundation (INSF) under grant No. 95816928 is
acknowledged.
4
6
of the S
decay processes increases with decreasing the fluorescence
quantum yield (Φ ) of photosensitizers. As was shown in a
1
excited state. Also, the efficiency of non-radiative
F
6
|
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ARTICLE
2
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Beeby, A. Burgard, K. Rossen, M. Poliakoff and M. W.
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Effects of weak and strong acids on the photocatalytic
performance of porphyrins in the aerobic photooxidation
of olefins are reported.