Effects of β-bromine substitution and core protonation on photosensitizing properties of porphyrins: Long wavelength photosensitizers

Article abstract of DOI:10.1016/j.jcat.2019.10.005

In this study, a series of β-brominated meso-tetraphenylporphyrins, H₂TPPBr_x (x = 2, 4, 6 and 8) with remarkably red shifted absorption bands towards the longer wavelengths of the visible light known as therapeutic window and their d

Full text of DOI:10.1016/j.jcat.2019.10.005

Journal of Catalysis xxx (xxxx) xxx
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Effects of b-bromine substitution and core protonation on
photosensitizing properties of porphyrins: Long wavelength
photosensitizers
⇑
Saeed Zakavi , Mortaza Naderloo, Akram Heydari-turkmani, Leila Alghooneh, Mortaza Eskandari
Department of Chemistry, Institute for Advanced Studies in Basic Sciences (IASBS), Zanjan 45137-66731, Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
In this study, a series of b-brominated meso-tetraphenylporphyrins, H2TPPBrx (x = 2, 4, 6 and 8) with
remarkably red shifted absorption bands towards the longer wavelengths of the visible light known as
therapeutic window and their diprotonated analogues were used as photosensitizers in the aerobic oxi-
dation of cyclooctene to investigate the influence of degree of b bromination on the photocatalytic activ-
ity and oxidative stability of H2TPP. Furthermore, the b brominated porphyrins showed significantly
decreased fluorescence quantum yields (uF) and radiative decay rate constants but increased non-
radiative decay rate constants. The highest catalytic activity was observed in the case of the partially
brominated porphyrins, H2TPPBr2 and H2TPPBr4. While, H2TPPBr2 and H2TPPBr4 were as stable as
H2TPP, the oxidative degradation of H2TPPBr6 and H2TPPBr8 was remarkably higher than that of the
non-brominated porphyrin. A good correlation was observed between the photocatalytic activity of
Received 23 July 2019
Revised 26 September 2019
Accepted 2 October 2019
Available online xxxx
Keywords:
Photocatalysis
b-brominated porphyrins
Porphyrin diacids
Singlet oxygen
H2TPPBrx and their diprotonated species and the singlet oxygen quantum yields (/ = 0.02–0.21 and
Oxidation of olefins
Near-IR photosensitizers
D
0.12 to 0.59, respectively), determined chemically through the reaction of singlet oxygen with 1,3-
diphenylisobenzofuran (DPBF). In order to examine the effects of steric hindrance on the photocatalytic
properties of the brominated porphyrins, b tetra-brominated derivative of meso-tetra(2-methylphenyl)
porphyrin, H2T(2-Me)PPBr4 with bulky methyl substituents at the ortho positions was prepared and com-
pared with H2TPPBr4. The steric hindrance caused by the presence of four methyl groups could improve
the oxidative stability of the photosensitizer by 30 to 50% at the cost of significant loss of photocatalytic
activity. On the other hand, diprotonation of H2TPPBrx with weak and strong acids (CH2ClCOOH and
CF3COOH) was found to be an efficient approach to overcome the extensive oxidative degradation of
these photosensitizers, with concomitant red shift of the absorption bands to 451–490 and 674–
743 nm, in the Soret and Q band regions, respectively.
Ó 2019 Elsevier Inc. All rights reserved.
1. Introduction
as meso-tetraphenylporphyrin (H2TPP). On the other hand, the use
of porphyrins and porphyrinoids as photosensitizers in different
Porphyrins, porphyrinoids and their derivatives are among the
most efficient photosensitizers have been used in different applica-
tions such as environmental control, wastewater treatment, photo-
dynamic therapy, medicine and photocatalysis of organic
transformations [1–9]. In this regard, porphyrin derivatives with
red shifted absorption bands are of great interest from an applica-
tion point of view [1,10,11]. The introduction of bromine atoms at
the porphyrins periphery is accompanied with the red shift of the
Soret and Q bands [12,13]. Accordingly, b brominated porphyrins
may be used as red shifted counterparts of simple porphyrins such
applications is associated with extensive oxidative degradation of
the aromatic macrocycle and therefore many attempts were made
to design photosensitizers with increased oxidative stability under
reaction conditions [14–15]. The later may be achieved by intro-
duction of electronegative and/or bulky atoms at the meso and b
positions [16–19]. Furthermore, the presence of bromine atoms
at the porphyrins periphery influences the optical properties of
porphyrins [12,13]; due to the heavy atom effect, the fluorescence
quantum yield of b-brominated porphyrins is significantly smaller
than that of the non-brominated porphyrins. This in turn facilitates
the S1 to T1 intersystem crossing which is necessary for the photo-
sensitized production of singlet oxygen from triplet molecular oxy-
gen [15]. The substitution of b positions with bromine atoms also
⇑
Corresponding author.
E-mail addresses: zakavi@iasbs.ac.ir, szakavi@gmail.com (S. Zakavi).
https://doi.org/10.1016/j.jcat.2019.10.005
0021-9517/Ó 2019 Elsevier Inc. All rights reserved.
Please cite this article as: S. Zakavi, M. Naderloo, A. Heydari-turkmani et al., Effects of b-bromine substitution and core protonation on photosensitizing
properties of porphyrins: Long wavelength photosensitizers, Journal of Catalysis, https://doi.org/10.1016/j.jcat.2019.10.005

2
S. Zakavi et al. / Journal of Catalysis xxx (xxxx) xxx
leads to the structural changes in the porphyrins core that is usu-
ally the out-of-plane deformation of the aromatic macrocycle to a
saddled or ruffled one[12,13].
control reactions were conducted in CDCl3 under the same condi-
tions. The 13C NMR spectrum of the reaction mixture prepared
after the required time confirmed the formation of cyclooct-1-
en-3-yl hydroperoxide and 2-cyclohexene-1-one in the oxidation
of cyclooctene and cyclohexene, respectively (See supplementary
material). More details of the experimental setup were explained
in our previous [14]0.10 W blue, white and red LED lamps were
as the light source (8 mm diameter with full width at half maxi-
mum less than ca. 20 nm for the most intense emission bands,
see supplementary material for the full emission spectra). Ben-
zaldehyde and 2-methylbenzaldehyde (for synthesis, Merck) were
used as received. Pyrrole (Fluka, for synthesis) was distilled before
use. Trifluroacetic acid (ReagentPluss 99%,), chloroacetic acid (for
synthesis, Sigma-Aldrich) and propionic acid (Merck) were used
as received. N-bromosuccinamide (NBS) was recrystallized from
hot water and dried at 80 °C under vacuum. Dichloromethane
which was used for the synthesis of the dications was dried over
anhydrous calcium chloride and distilled over CaH2. Cyclooctene
and cyclohexene (Sigma-Aldrich) were passed through a silicagel
column before use and their purity was confirmed by NMR
spectroscopy.1,3-diphenylisobenzofuran (97%, Sigma-Aldrich)
and1,4-benzoquinone (reagent grade, ꢀ98%, Sigma-Aldrich) were
used as received.
The presence of heavy atoms at the porphyrin periphery which
is favor of the S1 to T1 intersystem crossing process [15], electron-
withdrawing effects of the bromine atoms, the red shifted absorp-
tion bands and remarkably decreased fluorescence quantum yield
of b brominated porphyrins [13] make them potential candidates
for developing new photosensitizers. However, no attention was
paid to the influence of b bromination on the photosensitizing effi-
cacy of meso-tetra(aryl)porphyrins. In continuing our studies on
the photocatalytic performance of porphyrin derivatives, a series
of b brominated porphyrins with 2 to 8 bromine atoms at the pyr-
role ring, H2TPPBrx (Fig. 1) were used as photosensitizers in the
aerobic photooxidation of olefins in various mixed solvents con-
sisting of water, acetonitrile and toluene. The main objective of this
work is to study the influence of degree of bromination on the pho-
tocatalytic activity and oxidative stability of H2TPP. We have
recently reported the optical properties of b brominated H2TPP
with 2 to 8 bromine atoms in a comparative study [13]. The
absorption bands of the novel photosensitizers were found to shift
towards the phototherapeutic window in the range of 600 and
800 nm. The latter makes them potential candidates for applica-
tions such as photodynamic therapy [1].
2.2. Chemical determination of singlet oxygen quantum yields
2. Experimental
The U value of H2TPPBrX was measured through the reaction of
D
2.1. Instrumental and reagents
singlet oxygen with 1,3-diphenylisobenzofuran (DPBF) as a specific
singlet oxygen quencher and H2TPP (UD = 0.70 in acetonitrile) [20]
An Ultraspec 3100 Pro spectrophotometer was used for record-
ing UV-Vis absorption spectra. Fluorescence spectra and quantum
yields were obtained from dichloromethane solutions of the por-
phyrins in a high precision quartz fluorescence cell on a Cary
Eclipse fluorescence spectrophotometer. 1H NMR spectra were
recorded in CDCl3 on a BrukerAvance DPX-400 MHz spectrometer.
The reaction mixtures were analyzed using a Varian-3800 gas
as a reference photosensitizer, using the equation UD
=
UsDtd
ꢁ
(ti ꢁ I
/
std tsitd ꢁ I) proposed by Murata et al.[21]. In this equation,
U , Ustd
, t ,
i, Istd tsitd and I present the U of H2TPPBrX, the rate of
D
D
D
DPBF (8 ꢁ 10ꢂ4 M) oxidation in the presence of the respective
brominated porphyrin and a 10 W red LED lamp, the rate of DPBF
oxidation in the presence of methylene blue, the number of pho-
tons absorbed by the standard solution and those absorbed by
the photosensitizer, respectively (More details were described
elsewhere) [14,18,19,22]. All measurements were conducted in
chromatograph with
a HP-5 capillary column and flame-
ionization detector. In order to characterize the oxidation products,
Ar
meso
β
NH
N
Ar
Ar
HN
N
Ar
Ar
Porphyrin
H2TPP
β−brominated porphyrins
Dications
Phenyl
H2TPPBrx ( X = 2, 4, 6, 8) H4TPPBrx(CF3COO)2
H4TPPBrx(CH2ClCOO)2
ortho-Methylphenyl H2T(2-Me)PP
H2T(2-Me)PPBr4
Photosensitizer, O2, room temperature
LED lamp (White or blue),
OOH
H2O:CH3CN:Toluene (10:9:1)
O
Fig. 1. Aerobic photooxidation of cyclohexene and cyclooctene to 2-cyclohexene-1-one and cyclooct-1-en-3-yl hydroperoxide catalyzed by porphyrins and b-brominated
porphyrins.
Please cite this article as: S. Zakavi, M. Naderloo, A. Heydari-turkmani et al., Effects of b-bromine substitution and core protonation on photosensitizing
properties of porphyrins: Long wavelength photosensitizers, Journal of Catalysis, https://doi.org/10.1016/j.jcat.2019.10.005

S. Zakavi et al. / Journal of Catalysis xxx (xxxx) xxx
3
time intervals less than 800 s, where no catalyst degradation was
observed for the brominated porphyrins.
H2TPPBr2: UV-Vis in CH2Cl2, kmax/nm (log
e
): 423 (5.55), 520
(4.24), 542 (3.80), 595 (3.69), 650 (3.89). 1H NMR (400 MHz, CDCl3,
TMS), d/ppm: 8.78 (m, 6H, b-pyrrole), 8. 5–8.25 (m, 8H, o-phenyl),
7.70–7.9 (m, 12H, m- and p-phenyl), ꢂ2.6 (br, 2H, NH) ((see Sup-
plementary material, Fig. S5 and S6). C44H28Br2N4.1.85 CH2Cl2: Calc
C61.66, H 6.23, N 7.15. Found C 62.00, H 6.52, N7.27%.
2.3. Fluorescence quantum yield (Uf
)
The fluorescence spectra were obtained using dilute solution
(ꢃ10 mM) of porphyrins in dichloromethane, purged with N2 gas
to remove oxygen [13]. The porphyrin solutions were excited at
the Soret band of the respective porphyrin and the emission spec-
tra were recorded in the range of 500 to 800 nm. Also, the fluores-
cence quantum yields were calculated relative to that of H2TPP in
CH2Cl2 as standard (Uf = 0.15) [23] on the basis of the method
described by Parker and Rees [24].
H2TPPBr4: UV-Vis in CH2Cl2, kmax/nm (loge): 434 (5.35), 533
(4.03), 613 (3.61), 679 (3.79). 1H NMR (400 MHz, CDCl3, TMS), d/
ppm: 8.55 (m, 4H, b-pyrrole), 8.05–8.25 (m, 8H, o-phenyl), 7.70–
7.90 (m, 12H, m- and p-phenyl), ꢂ2.5 (2H, H) (see Supplementary
material, Fig. S7 and S8). C44H26Br4N4 + 0.5 CH2Cl2.0.5 H2O: Calc C
54.44, H 2.87, N 5.71. Found C 50.19, H 2.85, N 5.11%.
The synthesis of H2TPPBr6 and H2TPPBr8 was performed
through the reaction of NiTPP (500 mg, 0.75 mmol) and NBS in
1:12 and 1:26 M ratios, respectively. The reaction was conducted
in 250 ml dry 1,2-dichlorobenzene under reflux conditions. More
details may be found in our recent paper [13] (see (see Supplemen-
tary material, Fig. S9 and S10 for the spectra of NiTPP).
2.4. Synthesis and purification of H2TPP and H2T(2-Me)PP and b-
brominated porphyrins
H2TPP and H2T(2-Me)PP were prepared and purified according
to the Adler et al. method by the reaction of benzaldehyde
(3.8 ml, 40 mmol) or 2-methylbenzaldehyde (4.6 ml, 40 mmol)
with freshly distilled pyrrole (2.8 ml, 40 mmol) in propionic acid
under reflux conditions (30 min) [25]. After washing the precipi-
tates with methanol and water, for further purification, the por-
phyrins were firstly recrystallized from dichloromethane by the
gradual addition of methanol to the solution of the respective por-
phyrin in dichloromethane. Column chromatography on neutral
alumina using dichloromethane gave the product. H2TPP: UV-Vis
H2TPPBr6: UV-Vis in CH2Cl2, kmax/nm (loge): 454(4.78), 548
(3.50), 607 (3.43), 714 (3.27). 1H NMR (400 MHz, CDCl3, TMS),
d/ppm: 8.40 (m, 2H, b-pyrrole), 8.07–8.28 (m, 8H, o-phenyl),
7.70–7.90(m, 12H, m- and p-phenyl), ꢂ1.5 (broad, 2H, NH) (see
Supplementary material, Fig. S11–S14 for the spectra of NiTPPBr6
and H2TPPBr6). C44H24Br6N4.0.5CH2Cl2ꢄ0.5H2OC6H14: Calc C 56.83,
H 4.69, N 4.65. Found C 55.72, H 4.29, N 4.28%.
H2TPPBr8: UV-Vis in CH2Cl2, kmax/nm (loge): 368, 468(4.81),
565 (3.52), 620 (3.68), 729 (3.40). 1H NMR (400 MHz, CDCl3,
TMS), d/ppm: 8.18–8.38 (m, 8H, o-phenyl), 7.75–7.90 (broad,
12H, m- and p-phenyl),-1.25 (broad, 2H, NH) (see Supplementary
material, Fig. S15–S18 for the spectra of NiTPPBr8 and H2TPPBr8).
in CH2Cl2, kmax/nm (loge): 416 (5.79), 513 (4.58), 542 (4.38), 584
(4.30), 644 (4.29). 1H NMR (400 MHz, CDCl3, TMS), d/ppm: ꢂ2.71
(2H, br, s, NH), 7.77–7.83 (8Hm and 4Hp, m), 8.26–8.28 (8Ho, dd),
8.90 (8Hb, s); H2T(2-Me)PP: UV-Vis in CH2Cl2, kmax/nm (log
e
):
C
44H22Br8N4.CH2Cl2ꢄH2O2C6H14: Calc C 46.83, H 3.65, N 3.90. Found
416 (6.04), 512 (4.74), 545 (4.34), 589 (4.34), 645 (4.25). 1H NMR
(400 MHz, CDCl3, TMS), d/ppm: 2.01–2.11 (m, 12HMe), ꢂ2.59 (s,
2H, NH), 7.54–7.74 (m, 8Hm and 4Hp), 7.99–8.11 (dd, 4Ho), 8.70
(s, 8Hb) (see Supplementary material, Fig. S1–S4).
C 44.83, H 3.43, N 3.52%.
Synthesis of H2T(2-Me)PPBr4:The synthesis of H2T(2-Me)PPBr4
has been carried out by direct bromination of H2T(2-Me)PP
(328 mg, 0.49 mmol) with NBS (360 mg, 1.96 mmol) in dried
CH2Cl2 (80 ml) under reflux conditions (see the synthesis of
H2TPPBr4) [13,25,27,28].
Bromination of H2TPP was conducted using freshly recrystal-
lized NBS as the bromine source [26–28]. The synthesis and purifi-
cation of H2TPPBrX (X = 2, 4, 6 and 8) performed according to the
literature with some modifications. Further details of synthesis,
characterization and purification of the brominated porphyrins
were described in our recent paper [13].
H2T(2-Me)PPBr4: UV-Vis in CH2Cl2, kmax/nm: 426, 582, 659; 1H
NMR (400 MHz, CDCl3, TMS), d/ppm: ꢂ2.59 (2H, br, s, NH), 7.54–
7.74 (8Hm and 4Hp, m), 7.99–8.11 (4Ho, dd), 8.70 (4Hb, s),2.01–
2.11(12HMe, m) (see Supplementary material, Fig. S19 and S20).
2.5. Synthesis of H2TPPBr2 and H2TPPBr4
2.6. Porphyrin diprotonated species
Direct bromination of H2TPP with N-bromosuccinamide (NBS)
in dried CH2Cl2 was used to prepare H2TPPBrX (X = 2, 4). Magnetic
stirring of the solution of H2TPP (300 mg, 0.49 mmol) and NBS
(180 mg, 0.98 mmol) in 80 ml of dried dichloromethane under
reflux conditions for 24 h led to the formation of H2TPPBr2. The
progress of reaction was monitored by UV-vis spectroscopy at dif-
ferent time intervals. After the required time, an excess amount of
NBS was added to complete the dibromination reaction. The shift
of the Soret band to higher wavelengths (426 nm) showed the for-
mation of H2TPPBr2. In the case of H2TPPBr4, 360 mg (1.96 mmol)
of NBS was used and the Soret band appeared at 434 nm. The unre-
acted NBS was removed by washing the product with methanol
several times. H2TPPBr2 and H2TPPBr4 were also recrystallized
from CHCl3/n-hexane (3:1, v/v) for further purification and dried
under vacuum. 1H NMR spectra and CHN elemental analysis were
used to confirm the formation of the desired compounds.The
degree of bromination of H2TPP has been determined on the basis
of the ratio of the intensities of the signals corresponding to the Hb
protons and meso-substituents (Hb/Hortho and Hb/Hpara,meta); the
formation of H2TPPBr2 and H2TPPBr4 corresponds to the Hb/Hortho
ratios of 0.75 and 0.50 and Hb/Hpara,meta ratios of 0.50 and 0.33,
respectively.
Porphyrin dications with CF3COOH and CH2ClCOOH were syn-
thesized and purified by addition of excess amounts (beyond the
2:1 M ratio of acid to porphyrin) of the respective acid to the solu-
tion of porphyrin in dichloromethane. Slow evaporation of the sol-
vent and excess acid at room temperature gave the corresponding
dications. The UV-Vis and 1H NMR spectra of the dications are pre-
sented in the (see Supplementary material, S21-S28 and S29-S36.
H4TPPBr2(CF3COO)2: 1H NMR (400 MHz, CDCl3, TMS), d/ppm:
0.5 (s, br, 4H, NH), 8.2–8. 75 (m, 6H, b-pyrrole), 8.2–8.75 (m, 8H,
o-phenyl),
7.9–8.02
(m,
12H,
m-
and
p-phenyl).
H4TPPBr2(CH2ClCOO)2: 1H NMR (400 MHz, CDCl3, TMS), d/ppm:
0.5 (S, br, 4H, NH), 8.25–8. 70 (m, 6H, b-pyrrole), 8.25–8.70 (m,
8H, o-phenyl), 7.9–8.15 (m, 12H, m- and p-phenyl). H4TPPBr4(CF3-
COO)2: 1H NMR (400 MHz, CDCl3, TMS), d/ppm: 0.0 (4H, H), 8.2–8.7
(m, 4H, b-pyrrole), 8.2–8.7 (m, 8H, o-phenyl), 7.9–8.01 (m, 12H, m-
and p-phenyl). H4TPPBr4(CH2ClCOO)2: 1H NMR (400 MHz, CDCl3,
TMS), d/ppm:0–1(4H, H), 8.2–8.65 (m, 4H, b-pyrrole), 8.2–8.65
(m, 8H, o-phenyl), 7.9–8.10 (m, 12H, m- and p-phenyl). H4TPPBr6
(CF3COO)2: 1H NMR (400 MHz, CDCl3, TMS), d/ppm: 0–1 (broad,
4H, NH),8.35–8.65 (m, 2H, b-pyrrole), 8.35–8.65 (m, 8H, o-phenyl),
7.90–8.05(m, 12H, m- and p-phenyl). H4TPPBr6(CH2ClCOO)2: 1H
NMR (400 MHz, CDCl3, TMS), d/ppm: 0.9 (broad, 4H, NH),8.45–8.8
Please cite this article as: S. Zakavi, M. Naderloo, A. Heydari-turkmani et al., Effects of b-bromine substitution and core protonation on photosensitizing
properties of porphyrins: Long wavelength photosensitizers, Journal of Catalysis, https://doi.org/10.1016/j.jcat.2019.10.005

4
S. Zakavi et al. / Journal of Catalysis xxx (xxxx) xxx
(m, 2H, b-pyrrole), 8.45–8.8 (m, 8H, o-phenyl), 7.90–8.1(m, 12H, m-
and p-phenyl). H4TPPBr8(CF3COO)2: 1H NMR (400 MHz, CDCl3, TMS),
d/ppm: 0.5 (broad, 4H, NH). 8.45–8.85 (d, J, 8.52, 8H, o-phenyl), 7.65–
8.28 (broad, 12H, m- and p-phenyl).H4TPPBr8(CH2ClCOO)2: 1H NMR
(400 MHz, CDCl3, TMS), d/ppm: 0.5 (broad, 4H, NH), 8.5–8.8 (d, J, 8,
8.52, 8H, o-phenyl), 7.85–8.15 (broad, 12H, m- and p-phenyl).
1:1000 to 1:10000 catalyst to olefin molar ratio (Table 2, i), how-
ever a turnover number of 3300 was achieved using the latter that
was much higher than that observed at the other molar ratios.
The type of light source plays crucial role in the efficiency of the
porphyrinic photosensitizers. In a recent work, [30] it was found
that 10 W blue and white LED lamps are more efficient than a
red LED lamp in the photooxidation catalyzed by porphyrin dia-
cids. Also, the minimum extent of catalyst degradation was
observed in the case of the white LED lamp. As seen from Table 4,
the use of a 10 W white LED lamp led to an increase in the conver-
sion value, although similar degrees of catalyst degradation were
observed in the case of the white and blue LED lamps.
3. Results and discussion
3.1. Absorption spectra of porphyrins
The UV-Vis spectral data of H2TPP and the brominated por-
phyrins are summarized in Table 1. It is observed that the Soret
and Q bands of the brominated porphyrins are red shifted (6–51
and 3–82 nm, respectively) with respect to those of the corre-
sponding bands of H2TPP. Porphyrinic photosensitizers with red
shifted absorption bands are of major interest from energy con-
sumption and application points of view; penetration of light
through a turbid medium in tumour tissue is of vital importance
in photodynamic therapy. It was found that the best penetration
of light through tissue is observed at wavelengths in the range of
700–1000 nm, the so-called therapeutic window [29]. As seen from
Table 1, the Q(0,0) bands of H2TPPBr6 and H2TPPBr8 appear in the
wavelength region higher than 700 nm. In other words, the b
bromination of porphyrins is an efficient approach to shift the
absorption bands towards the therapeutic window. Also, the
brominated porphyrins have significantly decreased fluorescence
quantum yields that facilitates the non-radiative S1 to T1 intersys-
tem crossing, needed for the activation of molecular oxygen [15].
3.4. Optical properties of H2TPPBrx
Porphyrin derivatives with red shifted absorption bands are of
interest for applications such as activation of molecular oxygen
and PDT. However, deactivation of the excited state through fluo-
rescence emission decreases the efficiency of porphyrin sensitizers
in formation of T1 excited state and consequently singlet oxygen.
Accordingly, a decrease in the rate of radiative decay of the S1
excited state and fluorescence quantum yield (/f) is expected to
increase the rate of non-radiative decay of the excited state which
is one of the requirements of an efficient sensitizer. The optical
properties of the brominated porphyrins summarized in Table 6
is in agreement with significant decrease in radiative rate constant
(kr) upon the bromination of H2TPP. However, the non-radiative
rate constants (knr) were greater or comparable with that of
H2TPP. Also, /f values of 0.073 and 0.000027 were estimated for
H2T(2-Me)PP [31] and H2T(2-Me)PPBr4, respectively. Furthermore,
bromination of H2T(2-Me)PP decreases both kr and knr values from
0.48 and 0.33 to 0.0021 and 0.25 (ꢁ10ꢂ6 sꢂ1), respectively. These
observations are probably due to the heavy atom effect caused
by the introduction of bromine atoms at the porphyrin periphery.
3.2. Photooxidation with H2TPPBrx
In order to investigate the catalytic activity of the brominated
porphyrins, H2TPPBrx were used as photosensitizers for the oxida-
tion of olefins.
3.5. The comparison of H2TPPBrx compounds
3.3. Optimization of the reaction conditions
The bromination of H2TPP at the b positions leads to the appear-
ance of the absorption bands of the aromatic macrocycle at higher
wavelengths. Also, the distribution of electron-density over the
aromatic macrocycle is influenced by the presence of the elec-
tronegative bromine atoms at the porphyrins periphery. The latter
is expected to tune the oxidative stability of the photosensitizer.
The results for the oxidation of cyclooctene catalyzed by differ-
ent H2TPPBrx compounds in the presence of a 10 W white LED
lamp are summarized in Figs. 2 and 3 and Table 7. It is observed
that in different reaction times (up to 72 h) and various catalyst
to olefin molar ratios (1:1000 to 1:10000), H2TPPBr2 and H2TPPBr4
were much more efficient that the other brominated porphyrins.
Also, in a reaction time of 24 h, H2TPPBr2 was more stable than
the other ones towards oxidative degradation. However, at longer
reaction times, the hexa and octa-brominated porphyrins suffer
The optimized reaction conditions including the molar ratio of
porphyrins to olefin, solvent and light source were investigated
(Tables 2-4). As was shown in elsewhere, [30] the use of acetoni-
trile and toluene in 1:9 ratio is the best solvent mixture for con-
ducting the oxidation reactions. In order to meet the requirement
of green chemistry, the reactions were also conducted in a 10:9:1
mixture of water, acetonitrile and toluene. Interestingly, this
change led to a remarkable increase in the conversion value for
the reaction catalyzed by H2TPPBr2 (Table 2). Under these condi-
tions, the best catalyst:olefin molar ratio for achieving the maxi-
mum conversion was the 1:1000 one (Table 3). It is noteworthy
that again the maximum TON values were obtained at the 1:10000
In the 1:1 acetonitrile to toluene mixed solvent, there was no
large difference between the conversion values obtained in the
Table 1
UV-Vis spectral data of H2TPPBrx in dichloromethane.
Porphyrin
H2TPP
Soret k/nm
IV k/nm
III k/nm
II k/nm
I k/nm
417
(5.79)
423
(5.55)
434
(5.35)
454
(4.78)
468
(4.81)
514
(4.58)
520
(4.24)
–
548
(4.38)
542
(3.80)
533
(4.03)
548
(3.50)
565
(3.52)
589
(4.30)
595
(3.69)
613
(3.61)
607
(3.43)
620
(3.68)
647
(4.29)
650
(3.89)
679
(3.89)
714
(3.27)
729
(3.40)
(log
H2TPPBr2
(log
H2TPPBr4
(log
H2TPPBr6
(log
H2TPPBr8
(log
e
, Mꢂ1cmꢂ1
)
e)
e)
–
–
e)
e)
Please cite this article as: S. Zakavi, M. Naderloo, A. Heydari-turkmani et al., Effects of b-bromine substitution and core protonation on photosensitizing
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S. Zakavi et al. / Journal of Catalysis xxx (xxxx) xxx
5
Table 2
Effect of solvent on the photooxidation of cyclooctene.a
Porphyrin
solvent
Conversionb,c
(%)
TON
Catalyst degradation
(%)f
d,e
(TOF, hꢂ1
)
H2TPPBr2
1:9g
26
18
33
70
260 (4)
180 (2)
3500 (45)
700 (10)
76
73
77
100
1:1h
1:1i
10:9:1j
aPorphyrin and olefin were used in a 1:1000 molar ratio. A 10 W blue LED lamp was used as the light source. b Based on GC analysis, for a reaction time of 72 h. c The product
d
was also characterized by 13C NMR by conducting the oxidation reaction in CCl4. TON demonstrates the number of moles of the product obtained per one mole of the
catalyst. eTOF shows the turnover number of the reaction per time unit. f On the basis of the change in the absorbance at the kmax of the porphyrin. g,h,itoluene:CH3CN. i For a
j
1:10000 molar ratio of porphyrin to olefin. H2O:CH3CN:toluene.
Table 3
for porphyrin HOMO and HOMO-1 (a2u and a1u, respectively), the
Effect of catalyst to cyclooctene molar ratio.a,b
electron densities are largest on the meso and
tively and therefore these positionsare more
a carbons, respec-
Catalyst:olefin Conversion (%) TON (TOF, hꢂ1
)
Catalyst degradation (%)
susceptible to oxidative degradation through electrophilic
attack by the reactive oxygen species (ROS) [38–41]. Furthermore,
Dolphin et al., reported a novel stepwise oxidative degradation at
the b position of Ni(II) porphyrins which was mentioned as a novel
to the field of porphyrin chemistry [42].The increased coplanarity
of the meso phenyl groups and porphyrin core leading to increased
electron-donation to the porphyrin core, the weakening of differ-
ent bonds of H2TPPBrx relative to those of H2TPP, destabilized
HOMO of the substantially distortedH2TPPBr6 and H2TPPBr8pre-
dicted from the DFT calculations [13,43], and the decreased ring
current and the resonance energy of the brominated porphyrins
caused by the out-of-plane distortion of the aromatic macrocycle
may be used to explain the increased oxidative degradation of
the higher degrees of the brominated porphyrins. The latter was
deduced from the upfield shift of the b protons of the brominated
porphyrins with respect to those of H2TPP [13,44,45].Recently, we
have shown that the distribution of electron density on different
atoms of the aromatic macrocycle calculated by DFT calculation,
cannot properly explain the order of oxidative stability of a series
of porphyrin derivatives and other parameters such as dihedrdal
angles between the meso aryl substituents and porphyrin mean
plan and the energy of the frontier orbitals of the aromatic macro-
cycle may be used to rationalize the point [46].
1:1000
1:5000
1:10000
80
30
17
800 (11)
1600 (22)
1700 (24)
100
94
100
aSee the footnotes of Table 1.
bIn H2O:CH3CN:toluene (10:9:1) mixed solvent.
Table 4
Photooxidation of cyclooctene using different LED lamps.a,b
LED lamp
Conversion (%)
TON (TOF, hꢂ1
)
Degradation (%)
Blue (10 W)
White (10 W)
70
80
700(10)
800(11)
100
100
a
See the footnotes of Tables 1 and 2.
Table 5
Photooxidation of cyclooctene catalyzed by H2T(2-Me)PPBr4 in H2O:CH3CN:toluene
(10:9:1).a
Porphyrin
Conversion (%) TON(TOF, hꢂ1
)
Catalyst degradation (%)
75
H2T(2-Me)
PPBr4
32b
320 (4)
36c
360 (5)
76
aSee the footnotes of Tables 1 and 2. b,c Under the irradiation with 10 W white and
blue LED lamps, respectively.
3.6. Substitution of phenyl group with a bulky substituent
from extensive oxidative degradation. In other words, while the di
and tetra-brominated porphyrins were almost as stable as H2TPP,
the hexa and octa- brominated ones were dramatically less stable
towards oxidative degradation than their non-brominated coun-
terpart. According to the four orbital model of porphyrins [37],
The introduction of bulky substituents at the meso positions of
metalloporphyrins has been used as an efficient strategy to over-
come their oxidative degradation [47–49]. To evaluate the effi-
ciency of this approach in decreasing the oxidative degradation
Table 6
Fluorescence quantum yields and the rate constants for radiative and non-radiative relaxation of H2TPPBrx estimated from their absorption and emission spectra.
Free base porphyrins
Q(0,0)
H2TPP
H2TPPBr2
H2TPPBr4
H2TPPBr6
H2TPPBr8
Ufa
0.13
409
464
466
0.32
0.28
0.28
2.1
0.00106
536
520
0.00048
351
359
0.00345
683
826
0.002760
842
1225
742
0.0032
0.0023
0.0037
1.2
s1 (ns)b
s2 (ns)c
s3 (ns)d
600
377
732
Kr1 (ꢁ10ꢂ6 sꢂ1
Kr2 (ꢁ10ꢂ6 sꢂ1
Kr3 (ꢁ10ꢂ6 sꢂ1
)
)
)
0.0019
0.0020
0.0017
1.9
1.9
1.7
0.0014
0.0013
0.0013
2.9
2.8
2.6
0.0051
0.0042
0.0047
1.5
1.2
1.4
e
f
g
Knr1 (ꢁ10ꢂ6 sꢂ1
Knr2 (ꢁ10ꢂ6 sꢂ1
Knr3 (ꢁ10ꢂ6 sꢂ1
)
)
)
h
i
1.9
1.9
0.81
1.3
j
R
R
g
g
ð2v0 ꢂvÞ
aBased on a reported value of 0.13 for uFof H2TPP in CH2Cl2 [32].bsꢂ1 1 = 2.880 ꢁ 10ꢂ9 n2
<
m2
>
e
ð
v
Þd
R
v
.
csꢂ2 1 = 2.880 ꢁ 10ꢂ9 n2
3 e
ðvÞdv
.
sꢂ3 1 = 2.880 ꢁ 10ꢂ9
d
gul
gul
v
R
R
FðvÞdv
ꢂ1
ꢂ1
g
gul
eðv
Þ d
f
g
h
i
n2
<
vꢂf 3
>
v
and < vꢂf 3
> ¼
[33–35]. bRk1
=
sꢂ1 1
.
c Rk2= sꢂ2 1
.
dRk3
=
sꢂ3 1
.
ekr1 = Qf ꢁ
k1. kr2 = Qf ꢁ
R
k2. kr3 = Qf ꢁ
R
k3. knr1
=
R
k1 ꢂ kr1. knr2
= Rk2 -
R
Þvꢂ3
d
v
v
a
v
av
Fð
v
j
knr2
=
R
k2 ꢂ kr2
.
knr3
=
R
k3 ꢂ kr3 [36].For more details concerning the preparation of the absorption and emission spectra, evaluation of the kinetic data, lifetimes and
quantum yields, see our previous work.
Please cite this article as: S. Zakavi, M. Naderloo, A. Heydari-turkmani et al., Effects of b-bromine substitution and core protonation on photosensitizing
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6
S. Zakavi et al. / Journal of Catalysis xxx (xxxx) xxx
Fig. 2. Photooxidation of cyclooctene in the presence of different molar ratios of olefin:H2TPPBrx molar ratios under the irradiation of a white LED lamp (10 W) In H2O:
CH3CN:toluene (10:9:1) mixed solvent.
Fig. 3. Photooxidation of cyclooctene under the irradiation of a blue LED lamp (10 W).
of the brominated porphyrins, the b-tetra-brominated derivative of
meso-tetra(2-methylphenyl)porphyrin, H2T(2-Me)PPBr4 was pre-
pared and its photocatalytic performance was studied (Table 5).
The comparison of the results summarized in Table 5 and those
of H2TPP (Figs. 2, 3 and 4, Table 7) and H2T(2-Me)PP(Table 7,
entries 11 and 12) shows that the presence of methyl substituents
at the ortho position of the non-brominated porphyrin, leads to a
ca. 10–15% increase in the conversion value. Also, significant
decrease of catalyst degradation (ca. 40 to 60%) was observed
(see entries 1, 2, 11 and 12, Table 7). Interestingly, the introduction
of four bromine atoms decreased the photocatalytic activity by 31
to 53% relative to that of H2TPPBr4 under the same conditions.
Instead, a remarkable decrease (30 to 50%) in oxidative degrada-
tion was observed (see entries 5,6,13 and 14).
3.7. Singlet oxygen quantum yields
The efficiency of photosensitizers in light absorption to convert
3O2 into singlet oxygen, known as UD is as an important parameter
to evaluate the efficiency of novel photocatalysts [21].The changes
in the absorption spectrum of DPBF in the presence of the photo-
sensitizers in dichloromethane are shown in Fig. 4. The UD values
(Table 8) of H2TPPBrx determined by using H2TPP as a reference
photosensitizer and DPBF as a specific singlet oxygen quencher,
[20] decrease as H2TPPBr2 > H2TPPBr4 ꢅ H2TPPBr6 > H2TPPBr8.
The correlation observed between the photocatalytic activity of
H2TPPBrx and their UD is in accord with the involvement of singlet
oxygen as the main ROS in this reaction. It is noteworthy that the
addition of 1,4-benzoquinone as the scavenger of superoxide anion
Please cite this article as: S. Zakavi, M. Naderloo, A. Heydari-turkmani et al., Effects of b-bromine substitution and core protonation on photosensitizing
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S. Zakavi et al. / Journal of Catalysis xxx (xxxx) xxx
7
Table 7
Oxidation of cyclooctene catalyzed by H2TPP, H2T(2-Me)PP, H2TPPBrx and H2T(2-Me)PPBr4 under the optimized reaction conditions.a
Entry
Photosensitizer
LED lamp
Time (h)
Conversion (%)
TON (TOF, hꢂ1
)
Degradation (%)
1
2
3
4
5
6
7
8
H2TPP
H2TPP
H2TPPBr2
H2TPPBr2
H2TPPBr4
H2TPPBr4
H2TPPBr6
H2TPPBr6
H2TPPBr8
H2TPPBr8
H2T(2-Me)PP
H2T(2-Me)PP
H2T(2-Me)PPBr4
H2T(2-Me)PPBr4
White
Blue
White
Blue
White
Blue
White
Blue
White
Blue
white
Blue
White
Blue
72
72
72
72
72
72
6
6
6
6
71
74
3550 (49.3)
3700 (51.3)
82
82
30 (80)b
32 (70)b
13 (70)b
13 (45)b
4b
1500 (20.8) 800b (11.1)b
1600 (22.2) 700b (9.7)b
650 (9.0) 700b (9.7)b
650 (9.02) 450b (6.2)b
40 (4.0)
94 (1 0 0)b
94 (1 0 0)b
100 (1 0 0)b
100 (1 0 0)b
91
100
95
8b
80 (13.3)
60 (10.0)
60 (10.0)
800 (11.1)
790 (11.0)
170 (2.4)
140 (1.9)
9
6b
10
11
12
13
14
6b
89
72
72
72
72
80b
40b
18b
70b
50b
79b
17b
14b
a
Toluene and CH3CN were used in 9:1 ratio; photocatalyst and olefin were used in 1:5000.
Porphyrin and olefin were used in a 1:1000 M ratio.
b
3.0
2.5
2.0
1.5
1.0
0.5
0.0
3.0
2.5
2.0
1.5
1.0
0.5
0.0
3.0
2.5
2.0
1.5
1.0
0.5
0.0
0 s
5 s
10 s
20 s
40 s
80 s
120 s
200 s
300 s
400 s
500 s
600 s
700 s
800 s
0 s
0 s
20 s
60 s
H2TPP
20 s
H2TPPBr2
H2TPPBr4
40 s
60 s
100 s
140 s
200 s
300 s
400 s
500 s
600 s
700 s
80 s
100 s
120 s
140 s
200 s
300 s
400 s
300
400
500
600
700
800
300
400
500
600
700
800
300
400
500
600
700
800
Wavelength (nm)
Wavelength (nm)
Wavelength (nm)
3.0
2.5
2.0
1.5
1.0
0.5
0.0
3.0
0 s
10 s
40 s
100 s
200 s
300 s
400 s
600 s
800 s
0 s
10 s
20 s
40 s
H2TPPBr6
2.5
2.0
1.5
1.0
0.5
0.0
H2TPPBr8
60 s
100 s
200 s
300 s
400 s
500 s
600 s
300
400
500
600
700
800
300
400
500
600
700
800
Wavelength (nm)
Wavelength (nm)
Fig. 4. The change in the UV-Vis spectrum of DPBF upon irradiation with a 10 W red LED lamp in the presence of the brominated porphyrins and H2TPP. The change in the
absorption bands estimated through the measurement of the area of the band at different time intervals is only due to the change in the absorption of the corresponding band
of DPBF; control reactions showed that all the photosensitizers are completely stable against photooxidative degradation up to ca. 2 h.
Please cite this article as: S. Zakavi, M. Naderloo, A. Heydari-turkmani et al., Effects of b-bromine substitution and core protonation on photosensitizing
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8
S. Zakavi et al. / Journal of Catalysis xxx (xxxx) xxx
Table 8
high degrees of degradation under the reaction conditions.
Recently, we have shown that the oxidative degradation of por-
phyrin photosensitizers may be altered by using their dications
with weak and strong acids [14,19,22,30].Accordingly, the oxida-
tion reactions were conducted in the presence of the diacids of
the brominated porphyrins (Fig. 5). The oxidative degradation of
the brominated porphyrins was significantly decreased upon the
formation of dication with chloroacetic acid. Furthermore, the por-
phyrin dications were generally more efficient photosensitizers
than their corresponding free base counterparts. In this regard
the conversion values achieved in the presence of the dications
of H2TPPBr6 and H2TPPBr8 were much more than those obtained
in the presence of the free base porphyrins. It is noteworthy that
H2TPPBr6 and H2TPPBr8 showed the photocatalytic activity among
the series of the free base brominated porphyrins (Figs. 2 and 3,
Table 7). As was reported previously [14], weak carboxylic acids
are usually more efficient than strong acids in decreasing the
oxidative degradation of porphyrins (see Fig. 6).
Singlet oxygen quantum yield of the brominated compounds in dichloromethane.a
Entry
Photosensitizer
U
D
1
2
3
4
5
6
7
H2TPP
H2TPPBr2
H2TPPBr4
H2TPPBr6
0.70
0.21
0.13
0.04
0.02
0.51
0.31
H2TPPBr8
H2T(2-Me)PPb
H2T(2-Me)PPBrc4
Singlet oxygen quantum yields, determined using a 10 W red LED lamp and DPBF as
a quencher of singlet oxygen and H2TPP as a reference photosensitizer.b,c See
Figs. S4 and S20 for the UV-Vis spectra.
radical [14,18,30] has no effect on the conversion of cyclooctene to
cyclooct-1-en-3-yl hydroperoxide which is in agreement with the
absence of this species as ROS in the catalytic cycles.
3.8. Brominated porphyrins versus the non-brominated counterparts
The absorption spectra data of the dications of H2TPPBrx with
CF3COOH and CH2ClCOOH are summarized in Table 9. The forma-
tion of porphyrin diacids is associated with the red shift of the
Soret and Q bands. Also, the four Q bands of the free base por-
phyrins decrease to one or two bands corresponding the Q(0,0)
and Q(0,1) bands, due to the change in the symmetry of the por-
phyrin core [14,50,51]. Accordingly, the dications of the bromi-
nated porphyrins, may be considered as more efficient, more
stable and highly red shifted analogues of H2TPPBrx. Also, diproto-
nation of H2TPPBrx is an efficient strategy to overcome the exten-
sive oxidative degradation of these photosensitizers.
The photocatalytic activity of the brominated porphyrins was
also compared with that of the non-brominated porphyrins,
H2TPP and H2T(2-Me)PP (Table 7). It is observed that while the par-
tially brominated porphyrins, H2TPPBr2 and H2TPPBr4, are more
efficient photosensitizers than H2TPPBr6 and H2TPPBr8, (entries
3–10) under the optimized reaction conditions, the former has
no advantage over H2TPP. Also, the brominated porphyrins show
higher degrees of catalyst degradation compared to that of
H2TPP. In spite of the comparable catalytic activity of H2T(2-Me)
PP and H2TPP (entries 1, 2, 11 and 12), the former has a remarkably
higher oxidative stability than the former. In other words, the
increased steric hindrance at the meso position is in favor of
increasing the stability of photocatalyst. On the other hand, H2T
(2-Me)PPBr4 was much less efficient and less stable than the
non-brominated porphyrin (entries 11–14). As seen from Table 8,
The comparison of data evaluated from Figs. 4 and 6 (Tables 8
and 9) shows a remarkable (2.5 to 6 fold) increase in the / value
D
caused by protonation of H2TPPBrx which is more pronounced in
the case of the hexa and octa-brominated porphyrins. The signifi-
cantly greater / and oxidative stability of the diprotonated com-
D
a decrease in / value (Table 8) was observed upon the bromina-
pounds of the brominated porphyrins provide a reasonable
D
tion of H2T(2-Me)PP which is in accord with the decrease catalytic
performance of the brominated porphyrin.
explanation for the observed improved photocatalytic performance
of the brominated porphyrins induced by the protonation of the
aromatic macrocycles. Also, in the series of the diprotonated
species (Table 9), / decreased continuously from the diacids of
3.9. Effect of weak and strong acids
D
H2TPPBr2 to those of H2TPPBr8. This result is clearly in accord with
the higher photocatalytic activity of diprotonated species of
H2TPPBr2 and H2TPPBr4. Furthermore, the diprotonated
As seen from Tables 2–5 and Table 7, the brominated por-
phyrins, especially those with 6 and 8 bromine atoms, suffer from
Fig. 5. Oxidation of cyclooctene in the presence of H2TPPBrx and their dications with CF3COOH (TFA) and CH2ClCOOH (CAA) under the optimized reaction conditions (see the
footnotes of Table 7); H4TPPBr2(CH2ClCOO)2 and H4TPPBr2(CF3COO)2 were the most stable and most efficient photosensitizers of the series, respectively.
Please cite this article as: S. Zakavi, M. Naderloo, A. Heydari-turkmani et al., Effects of b-bromine substitution and core protonation on photosensitizing
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S. Zakavi et al. / Journal of Catalysis xxx (xxxx) xxx
9
Fig. 6. The change in the UV-Vis spectrum of DPBF upon irradiation with a 10 W red LED lamp in the presence of the diprotonated species of H2TPPBrx with CF3COOH and
CCH2ClCOOH.
Please cite this article as: S. Zakavi, M. Naderloo, A. Heydari-turkmani et al., Effects of b-bromine substitution and core protonation on photosensitizing
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10
S. Zakavi et al. / Journal of Catalysis xxx (xxxx) xxx
Table 9
UV-Vis spectral data, U and Uf of the dications of H2TPPBrx with CF3COOH and CH2ClCOOH in dichloromethane.a
D
b
c
d
Porphyrin
Soret
k/nm
Q(0,0)
k/nm
U
Uf
Kr (ꢁ10ꢂ6 sꢂ1
)
Knr1 (ꢁ10ꢂ6 sꢂ1
)
D
H4TPPBr2(CF3COO)2
451
(5.43)
453
(5.30)
462
(5.32)
467
(5.21)
483
(4.84)
483
(4.84)
491
(4.93)
490
674
(4.66)
678
(4.49)
696
(4.43)
696
(4.38)
725
(4.08)
725
(4.13)
747
(4.74)
743
0.59
0.49
0.34
0.30
0.10
0.14
0.16
0.12
0.00516
0.00678
0.00203
0.00187
0.00248
0.00304
0.00180
0.00267
15.32
14.73
9.92
15.22
14.60
9.90
(log
H4TPPBr2(CH2ClCOO)2
(log
H4TPPBr4(CF3COO)2
(log
H4TPPBr4(CH2ClCOO)2
(log
H4TPPBr6(CF3COO)2
(log
H4TPPBr6(CH2ClCOO)2
(log
H4TPPBr8(CF3COO)2
(log
H4TPPBr8(CH2ClCOO)2
(log
e
, Mꢂ1 cmꢂ1
)
e)
e)
11.93
2.36
11.90
2.35
e)
e)
3.65
3.64
e)
2.57
2.56
e)
3.33
3.32
e)
(4.97)
(4.23)
a
See the footnotes of Table 6 and 8 for more details.
Measured through the reaction of singlet oxygen with DPBF (Fig. 6). cAverage of Kr1, Kr2 and Kr3
b
.
dAverage of Knr1, Knr2 and Knr3
Table 10
alkyl hydroperoxide (Fig. 7). However, a ca. 15% decrease in the
oxidative degradation of the photosensitizers was observed in
the oxidation of cyclohexene.
Photooxidation of cyclohexene catalyzed by the diprotonated derivatives of
H2TPPBr2.a
Photosensitizers
Conversion (%)
Time (h)
Degradation (%)
H2TPPBr2(CH2ClCOOH)2
H2TPPBr2(CF3COOH)2
69
73
72
72
15
82
4. Conclusion
a
See the footnotes of Table 1. A 10 white LED lamp was used as the light source.
Aerobic oxidation of cyclooctene in the presence of H2TPPBrx
was studied and found that: (i) H2TPPBr2 and H2TPPBr4 showed
the highest photocatalytic activity among the series of brominated
porphyrins; (ii) a close correlation was observed between the pho-
tocatalytic activity of the brominated porphyrins and their singlet
oxygen quantum yield; (iii) H2TPPBrx compounds showed remark-
ably decreased fluorescence quantum yields and radiative decay
rate constants relative to those of H2TPP; (iv) although the former
presented a two-fold higher catalytic activity than the latter during
the first 24 h of the reaction, they showed a comparable catalytic
activity by continuing the reaction for a longer time (48 to 72 h);
H
O
O
O
H
-H2O
(v) with the exception of H2TPPBr2 and H2TPPBr4, the other bromi-
nated porphyrins suffered from high degrees of oxidative degrada-
tion upon the photooxidation reaction; (vi); the sterically hindered
b-brominated porphyrin, H2T(2-Me)PPBr4 with bulky methyl sub-
stituents at the ortho positions demonstrated an improved oxida-
tive stability (30 to 50%) in comparison with H2TPPBr4 at the cost
of significant loss of photocatalytic activity. It is noteworthy that
H2T(2-Me)PP was much more stable than H2TPP during the oxida-
tion reaction; (vii) protonation of the brominated porphyrins with
CF3COOH and CH2ClCOOH led to a remarkable increase in their
oxidative stability which was more pronounced in the case of the
Fig. 7. Proposed mechanism for the formation of 2-cyclohexene-1-one from
cyclohexene-3-hydroperoxide.
compounds of the weaker acid (CH2ClCOOH), showed higher /
D
compared to those of CF3COOH. On the other hand, the /F of
H2TPPBrx (Table 6) and their diprotonated species (Table 9)
revealed an increased fluorescence quantum yield in the case of
the di- and tetra-brominated porphyrins but a decreased one in
the case of the hexa- and octa-brominated porphyrins. Also, both
the Kr and Knr values (Table 9) of the diprotonated species
decreases from H2TPPBr2 to those of H2TPPBr8. In comparison with
the data summarized in Table 6, the diprotonated porphyrins have
Kr and Knr values greater than those of the free base porphyrins
latter; (viii) the / of the diprotonated counterparts of H2TPPBrx
D
is significantly higher than that of H2TPPBrx which is in accord with
the higher catalytic performance of the former. Also, the diproto-
nated compounds with lower degrees of b-bromination have larger
/
D
than the non-protonated H2TPPBrx; (ix) the formation of por-
which may be also used to explain higher / values of the former.
D
phyrin diacids, shifted the absorption bands in the Soret and Q
band regions towards longer wavelengths with respect to those
of H2TPPBrx; (x) the significantly improved photocatalytic perfor-
mance of the diprotonated H2TPPBrx is mainly due to the increased
oxidative stability and singlet oxygen quantum yield of the bromi-
nated porphyrins induced by the weak and strong carboxylic acids.
3.10. Photooxidation of cyclohexene
The oxidation of cyclohexene in the presence of the dications of
H2TPPBr2 as the best photosensitizers of the series was conducted
(Table 10). As was reported previously [52], the oxidation of cyclo-
hexene gives 2-cyclohexene-1-one as the sole product (see supple-
mentary material for further details). Also, cyclooctene shows a
higher reactivity compared to that of cyclohexene. The formation
of 2-cyclohexene-1-one in the case of cyclohexene may be
explained on the basis of the decomposition of the corresponding
Acknowledgements
Practical support of this work by the Institute for Advanced
Studies in Basic Sciences (IASBS) is acknowledged.
Please cite this article as: S. Zakavi, M. Naderloo, A. Heydari-turkmani et al., Effects of b-bromine substitution and core protonation on photosensitizing
properties of porphyrins: Long wavelength photosensitizers, Journal of Catalysis, https://doi.org/10.1016/j.jcat.2019.10.005

S. Zakavi et al. / Journal of Catalysis xxx (xxxx) xxx
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Please cite this article as: S. Zakavi, M. Naderloo, A. Heydari-turkmani et al., Effects of b-bromine substitution and core protonation on photosensitizing
properties of porphyrins: Long wavelength photosensitizers, Journal of Catalysis, https://doi.org/10.1016/j.jcat.2019.10.005