Synthesis and application of near-infrared absorbing morpholino-containing aza-BODIPYs

Article abstract of DOI:10.1016/j.tet.2019.06.046

Morpholino-containing aza-BODIPYs at 3,5-positions were synthesized. The maxima absorption and emission of these dyes locate at the near-infrared region. Aza-BODIPY 1 with the morpholino group as a pH-sensitive functionality could be used to be a pH probe, and the dramatic increase in fluorescence intensity at 675 nm by about 1500 folds. Moreover, the singlet oxygen generation of PS 2 with the dibromo groups at 2,6-positions was more effective than that of the parent dye 1.

Full text of DOI:10.1016/j.tet.2019.06.046

Accepted Manuscript
Synthesis and application of near-infrared absorbing morpholino-containing aza-
BODIPYs
Xin-Dong Jiang, Lei Jia, Yajun Su, Chen Li, Changliang Sun, Linjiu Xiao
PII:
S0040-4020(19)30718-5
https://doi.org/10.1016/j.tet.2019.06.046
TET 30438
DOI:
Reference:
To appear in: Tetrahedron
Received Date: 1 May 2019
Revised Date: 26 June 2019
Accepted Date: 28 June 2019
Please cite this article as: Jiang X-D, Jia L, Su Y, Li C, Sun C, Xiao L, Synthesis and application
of near-infrared absorbing morpholino-containing aza-BODIPYs, Tetrahedron (2019), doi: https://
doi.org/10.1016/j.tet.2019.06.046.
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ACCEPTED MANUSCRIPT
Synthesis and application of near-infrared absorbing morpholino-containing
aza-BODIPYs
Xin-Dong Jiang*,a,b, Lei Jiaa, Yajun Sua, Chen Lia, Changliang Suna, Linjiu Xiaoa
Ph
Ph
N
B
X
X
N
F
N
F
N
N
X= H, Br
O
O

ACCEPTED MANUSCRIPT
Graphical Abstract
Synthesis and application of near-infrared
Leave this area blank for abstract info.
absorbing morpholino-containing aza-BODIPYs
Xin-Dong Jiang*,a,b1, Lei Jiaa, Yajun Sua, Chen Lia, Changliang Suna, Linjiu Xiaoa
Ph
Ph
N
B
X
X
N
F
N
F
N
N
X= H, Br
O
O

1
ACCEPTED MANUSCRIPT
Tetrahedron
journal homepage: www.elsevier.com
Synthesis and application of near-infrared absorbing morpholino-containing aza-
BODIPYs
Xin-Dong Jiang*,a,b1, Lei Jiaa, Yajun Sua, Chen Lia, Changliang Suna, Linjiu Xiaoa
aCollege of Applied Chemistry, Shenyang University of Chemical Technology, Shenyang 110142, China
bDepartment of Chemistry, Graduate School of Science, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima 739-8526, Japan.
ARTICLE INFO
ABSTRACT
Article history:
Received
Received in revised form
Accepted
Morpholino-containing aza-BODIPYs at 3,5-positions were synthesized. The maxima
absorption and emission of these dyes locate at the near-infrared region. Aza-BODIPY 1
with the morpholino group as a pH-sensitive functionality could be used to be a pH probe,
and the dramatic increase in fluorescence intensity at 675 nm by about 1500 folds.
Moreover, the singlet oxygen generation of PS 2 with the dibromo groups at 2,6-positions
was more effective than that of the parent dye 1.
Available online
Keywords:
Aza-BODIPY
Near-infrared
Morpholino
pH
2009 Elsevier Ltd. All rights reserved
.
PS
1. Introduction
And, the pH probe for strong acid is adapted to study acidic
organelles such as gastric acid, lysosomes and endosomes.[33]
Additionally, it was noted that introducing an electron
donating group (such as –NMe2) resulted in the photosensitizer
Since near-infrared (NIR) fluorescent dyes can greatly reduce
background absorption, fluorescence and light scattering, and
improve the sensitivity of fluorescent probe, NIR absorbing dyes
are widespreadly applicable, such as molecular imaging,
(PS) with higher singlet oxygen quatumn yield [34]. Our
recent research interest lies in the novel aza-BODIPY family of
fluorescent dyes [35-40]. We are interested in the influence of the
morpholino groups at 3,5-positions on the properties of aza-
BODIPY dyes. So, herein we report the synthesis of aza-
BODIPYs 1 and 2 with the morpholino group at 3,5-positions,
and their application for a pH probe and a PS (Fig. 1).
theranostics,
photodynamic
therapy,
laser
generators,
electroluminescent devices, construction of light emitting diodes
(OLED) and photovoltaic cells and so forth [1-3]. By employing
borondipyrromethene (BODIPY) and aza-borondipyrromethene
(aza-BODIPY) dyes the desired functionalization could be
achieved [4-13]. Aza-BODIPY with a nitrogen atom at the meso-
position in the dipyrromethene ligand is a promising candidate
for NIR-absorbing dye because of their intrinsic narrow band
gaps [14-18]. Moreover, by modifying the core of aza-BODIPYs,
their derivatives, such as conformationally restricted aza-
BODIPYs, heteroaryl-fused aza-BODIPYs, have been developed
to extend the emission wavelength covering the NIR region (>
650 nm) [19-24]. Therefore, design of aza-BODIPYs has been
attracting increasing interest.
Ph
Ph
Ph
Ph
1
7
N
B
N
B
2
Br
Br
6
N
F
N
F
N
F
N
F
5
3
N
N
N
N
1
2
PS
pH probe
O
O
O
O
It is known that, by introducing an electron-donating group
one can further reduce the HOMO-LUMO gap by increasing the
HOMO and/or decreasing the LUMO energy and reach
bathochromic absorption and emission bands [25-30]. Apart from
a few studies on aza-BODIPYs with an electron-donating group
such as -(p-NMe2)Ph/-(p-OMe)Ph at 3,5-positions [31], to the
best of our knowledge, no aza-BODIPY with morpholino group
at 3,5-positions has been documented. Additionally, introduction
of the morpholine moiety as a recognition unit could be a latent
Fig. 1. Design strategy for aza-BODIPYs with the morpholino groups at 3,5-
positions.
2. Result and discussion
One-step synthesis of pyrrole 4 was obtained by reaction of 1-
(4-morpholinophenyl)ethanone 3 with 3-phenyl-2H-azirine in
presence of NaH (Scheme 1) [41]. Then, the morpholino-
containing pyrrole 4 was found to yield the symmetric aza-
BODIPY 1 in the presence of HOAc, Ac2O and NaNO2, followed
application for monitoring pH value or lysosomal viscosity [32].
———
1
*
Corresponding author. Tel.: +86 024 89387219.
E-mail address: xdjiang@syuct.edu.cn (X.-D. Jiang).

2
Tetrahedron
ACCEPTED MANUSCRIPT
.
by complexation with Et3N–BF3 Et2O (Scheme 1) [42]. Aza-
BODIPY 1 reacted with N-bromosuccinimide (NBS) to produce
the dibromo-substituted aza-BODIPY 2 (Scheme 1). Dye 1
showed two sets of distinct hydrogen signals for the morpholine
rings in the H NMR spectrum (δ = 3.86 (t, JHH = 5.0 Hz, 8H)
and 3.33 (t, 3JHH = 5.0 Hz, 8H) ppm), which is in agreement with
those of the reported dyes [43,44].
1
3
Ph
O
1) NaH
1) HOAc, Ac2O
2) NaNO2
2)
Ph
NH
Ph
3) NEt3, BF3 . Et2O
N
N
O
3
N
4
O
Ph
Ph
Ph
N
N
B
Br
Br
N
F
N
F
N
F
N
F
B
NBS
N
N
N
N
1
2
O
O
O
O
Scheme 1. Synthesis of NIR-absorbing morpholino-containing aza-BODIPYs
at 3,5-positions.
The spectra of absorption and fluorescence of aza-BODIPYs 1
and 2 are outlined and shown in Fig. 2 and Table 1. Obviously,
Fig. 2. (a) Normalized absorption and (b) fluorescence spectra of 2 µM 1
(black) and 2 µM 2 (red) in CH2Cl2 at 293 K. λex = 730 nm.
the maxima absorption and emission of 1 and 2 (λabs/λem
=
764/828 nm for 1, 746/821 nm for 2) locate at the NIR region.
Aza-BODIPYs 1 and 2 have the high extinction coefficients. The
full width half maximum (Fwhm = 128 nm) of 2 was broader
than that (100 nm) of dye 1. Owing to the intramolecular charge
transfer (ICT) effect [45,46] by the introduction of the
morpholine ring, the fluorescent quantum yield of aza-BODIPY
1 is low and dibromo-substituted aza-BODIPY 2 is nearly not
fluorescent. In addition, the spectral qualities of the morpholino-
containing aza-BODIPYs are comparable to those of
dimetylamino-containing aza-BODIPYs [22].
The most popular strategy for pH-responsive fluorescent probe
employs ICT or PET. The excited state of the fluorophore can be
quenched by the electron transfer from electron donating amine
to the fluorophore. Upon recognition of a proton, the electron
transfer is “switched off” and in turn the emission of fluorescence
is “switched on”. Therefore, morpholino group could be used for
this purpose of ICT/PET. So, we continue to explore the response
of the morpholino-containing aza-BODIPY 1 to pH value. For
most biological applications a good water solubility is essential.
For example, a fluorescent probe should be water soluble for
effective detection of the analyte in live cells. However, it is
unfortunate that sensing experiment in water were not successful
due to poor water solubility of dye 1. So, the mixture solution of
CH3CN/H2O (9:1, v/v) was herein used. Photo image of 1 were
taken under normal room illumination, and notable changes of
relatively vivid bright colors of 1 with the pH (pH 7–12 M) can
be easily observed with naked eye (Fig. 3). Upon addition of
hydrochloric acid to aza-BODIPY 1 with the morpholino group
as a pH-sensitive functionality, 1 could be protonated at the
limitative pH value. A stepwise decrease of the absorption
intensity was observed in the 800 nm band (Fig. 4). Then, the
formation of a new band for 1-H+ at 750 nm was observed in 2 M,
and another new peak for 1-2H+ subsequently arose at 647 nm in
12 M (Fig. 4 and 5 ). The absorption band of 1-2H+ is blue-
shifted by about 153 nm compared to that of 1. The fluorescence
of 1 in CH3CN/H2O (9:1, v/v) is quenched due to the ICT effect.
The reversibility of the probe 1 between pH 7.0 and 2 M was also
studied. The results showed that the process is reversible for at
least six cycles (Fig. S2). Additionally, the detection limit to
hydrochloric acid was calculated to be pH 4.2 (3σ/slope, Fig. S3).
With decreasing pH the emission maxima were shifted to 675 nm
(Fig. 6). A dramatic increase in fluorescence intensity at 675 nm
by about 1500 folds (Φf = 0.37 when treated with HCl to 12 M)
Table 1. Photophysical properties of aza-BODIPYs 1 and 2 in CH2Cl2 at 293
K.
λabs/λem [nm]
764/828
ε
[M-1 cm-1]
86000
Dye
FWHM[nm]
Φf
1
100
128
0.02
NA
2
746/821
79000

3
of the aza-BODIPY 1. It is due to the increase in the HOMO–
(Fig. 6).
ACCEPTED MANUSCRIPT
LUMO band gap (2.03 ev; 1.99 ev) for the lowest energy
absorption bands of 1-H+ and 1-2H+ relative to that (1.94 ev) of 1
by MO calculations (Fig. 7).
Fig. 3. Photographs of solutions of 4 µM dye 1 at pH 7, 6, 5, 4, 3, 2, 1 and 1
M, 2 M, 4 M, 6 M, 8 M, 10 M, 12M of HCl in CH3CN/H2O (9:1, v/v) under
normal room illumination.
Fig. 7. Frontier molecular orbitals of aza-BODIPYs 1, 1-H+ and 1-2H+ at the
B3LYP/6-31G(d) level with Gaussian 09. HOMO/ LUMO (eV) = −4.81/−
2.87 for 1; HOMO/ LUMO (eV) = −6.98/−4.99 for 1-H+; HOMO/ LUMO
(eV) = −9.23/−7.29 for 1-2H+.
Then, investigation of the singlet oxygen generation was
performed to assess the ability of dibromo-substituted aza-
BODIPYs 1 and 2 as the PS in toluene. To simulate a deep tissue
penetration and to reduce the normal cell-damage, the NIR
monochromatic light at 730 nm by using a 150 W xenon lamp at
0.5 mW/cm2 was selected to irradiate the toluene solution [48].
1,3-Diphenylisobenzofuran (DPBF) as a singlet oxygen indicator
was employed to estimate singlet oxygen generation. The initial
concentrations of 6.5 × 10-6 M of dyes 1 and 2, and 6 × 10-5 M of
DPBF was performed in this experiment. When the oxidation of
DPBF with the generated singlet oxygen was carried out, the
absorption maximum of DPBF at 416 nm was found to be
gradually decreased. It is well-known that the singlet oxygen
generation from a PS is regulated by the efficiency of a spin-
forbidden electronic transition from a singlet to a triplet state
upon irradiation [34]. The heavy atom effect is advocated to
improve intersystem crossing (ISC) and consequently to generate
the singlet oxygen. Therefore, the experimental results indicated
that the singlet oxygen generation of PS 2 with the dibromo
groups at 2,6-positions (Fig. 8 and 9) was more effective than
that of 1. A 3-fold rate enhancement is observed for 2 compared
to 1 (Fig. 9 and Fig. S1). And, no photobleaching of 2 was found
Fig. 4. Absorption spectra (pH 7, 6, 5, 4, 3, 2, 1 and 1 M, 2 M, 4 M, 6 M, 8
M, 10 M, 12M of HCl) of 4 µM dye 1 in CH3CN/H2O (9:1, v/v) as a function
of pH.
Ph
Ph
Ph
Ph
Ph
Ph
N
B
N
B
N
B
N
N
F
N
N
F
N
N
F
F
F
F
H
O
H
H
N
N
N
N
N
N
1−2H+
1
1−H+
O
O
O
O
O
Fig. 5. Structures of dye 1, protonated dyes 1-H+ and 1-2H+.
during this experiment, based on the absorption intensity (λabs
=
755 nm in toluene) (Fig. 8). These results indicated that the NIR-
absorbing PS 2 was able to be used for the generation of the
singlet oxygen [49, 50]
.
Fig. 6. Corresponding fluorescence spectra (pH 7, 6, 5, 4, 3, 2, 1 and 1 M, 2
M, 4 M, 6 M, 8 M, 10 M, 12M of HCl, λex = 630 nm) of 4 µM dye 1 in
CH3CN/H2O (9:1, v/v) as a function of pH.
Moreover, the molecular geometries of aza-BODIPY 1, 1-H+
and 1-2H+ were optimized using density functional theory (DFT)
at the B3LYP/6-31G(d) level [47]. The protonation of aza-
BODIPY (1-H+: λabs = 750 nm; 1-2H+: λabs = 647 nm) showed a
remarkable hypsochromic shifts compared to that (λabs = 800 nm)

4
Tetrahedron
All reactions utilizing air- or moisture-sensitive reagents
ACCEPTED MANUSCRIPT
were performed in dried glassware under an atmosphere of dry
N2. 1H NMR spectra were recorded on a VARIAN Mercury 500
1
MHz spectrometer at 20 ºC. H NMR chemical shifts (
δ
) are
given in ppm downfield from Me4Si, determined by residual
chloroform (
= 7.26 ppm). 13C NMR spectra were recorded on
δ
a VARIAN Mercury 125 MHz spectrometer at 20 ºC, and all
signals are reported in ppm with the internal chloroform signal
at
δ 77.0 ppm as standard. Fluorescence spectra were recorded
on an F-280 spectrophotometer at room temperature and are
1
reported as cm− . UV/Vis spectra were recorded on a UV-2550
spectrophotometer at room temperature. All pH measurements
were performed with a PHS-3E pH meter. The refractive index
of the medium was measured by 2 W Abbe’s refractometer at
20 °C.
Fig. 8. DPBF (initial concentration at 6 × 10-5 M) degradation profile in
toluene by dye 2 (6.5 × 10-6 M). Monochromatic light (730 nm at 0.5
mW/cm2) used. The curves display time-dependent decrease (0, 3, 6, 9, 12, 15,
18, 21, 24, 27, 30, 33, 36, 39, 42, 45, 48, 51, 54, 57 and 60 min.) of
absorbance at 416 nm by oxidation of DPBF with dye 2.
The fluorescence quantum yields (Φf) of the aza-BODIPY
systems were calculated using the following relationship
(equation 1):
Φf
Here,
spectrum,
=
Φ
ref Fsampl Aref n2sampl/Fref Asampl n2
(1)
denotes the integral of the corrected fluorescence
is the absorbance at the excitation wavelength, and
ref
F
A
n
is the refractive index of the medium, ref and sampl denote
parameters from the reference and unknown experimental
samples, respectively.
The reference systems used were boronazadipyrromethene
compound aza-BODIPY (Φf = 0.36 in chloroform) [50] as
standard for
The MO calculations were performed at the DFT level, and
the frontier molecular orbitals of BODIPY
1-H+ and 1-2H+ at
1 and 2.
1
,
the B3LYP/6-31G(d) level with Gaussian 09 [47].
4.2. Synthesis
4.2.1 Synthesis of pyrrole
Under N2, 1-(4-morpholinophenyl)ethanone
mmol) was added to NaH (70.1 mg, 1.75 mmol) in DMSO (15
ml) at 25 °C and stirred for 10 min. Then, 3-phenyl-2 -azirine
(173 mg, 1.47 mmol) was added and the resulting mixture was
stirred for 3 h at the same temperature. It was quenched with
water, neutralized with dilute HCl to a pH about 7. The mixture
was extracted with CH2Cl2 (2 × 40 ml), and the organic layer
was washed with brine (2 × 40 ml) and dried over anhydrous
MgSO4. After removal of the solvents by evaporation, the
resulting crude mixture was separated by column
4
3
(300.0 mg, 1.46
H
Fig. 9. Time-dependent decrease (0, 3, 6, 9, 12, 15, 18, 21, 24, 27, 30, 33, 36,
39, 42, 45, 48, 51, 54, 57 and 60 min.) of Log absorbance at 416 nm by
oxidation of DPBF with 1 (a line) and 2 (b line).
3. Conclusions
In conclusions, aza-BODIPYs with the morpholino group at
3,5-positions were prepared. Aza-BODIPY 1 (λabs/λem = 764/828
nm) and dibromo-substituted aza-BODIPY 2 (λabs/λem = 746/821
nm) are the NIR-absorbing dyes. Due to the introduction of the
morpholine ring lead to the ICT effect, the fluorescent quantum
yield of aza-BODIPY 1 is low and dibromo-substituted aza-
BODIPY 2 is not fluorescent. Upon addition of hydrochloric acid
to aza-BODIPY 1 with the morpholino group as a pH-sensitive
functionality, the formation of a new band for 1-H+ at 750 nm
was observed in 2 M, and another new peak for 1-2H+
subsequently arose at 647 nm in 12 M. A dramatic increase in
fluorescence intensity at 675 nm by about 1500 folds (Φf = 0.37
when treated with HCl to 12 M). The singlet oxygen generation
of PS 2 with the dibromo groups at 2,6-positions was more
effective than that of the parent dye 1, and no photobleaching of
2 was found, indicating that the NIR-absorbing PS 2 was able to
be used for the generation of the singlet oxygen.
chromatography (
n
-hexane : CH2Cl2 = 1 : 4) to afford
4
as
δ
=
1
green solids (155.4 mg, 35%). H NMR (500 MHz, CDCl3):
(ppm) 8.41 (br s, 1H), 7.47 (d, 3J = 9.0 Hz, 2H), 7.44 (d, 3J
7.5 Hz, 2H), 7.38 (s, 1H), 7.37 (s, 1H), 7.38 (t, 3
J
= 7.5 Hz, 2H),
= 9.0 Hz, 2H), 3.23 (t, 3J
4.5 Hz, 4H), 3.18 (t, 3J = 4.5 Hz, 4H). 13C NMR (125 MHz,
CDCl3): (ppm) 154.2, 131.3, 130.3, 129.7, 129.4, 128.5,
7.18 (t, 3J = 7.5 Hz, 1H), 6.81 (d, 3
J
=
δ
128.1, 125.1, 124.9, 116.0, 114.1, 113.2, 66.5, 47.5. HRMS-
MALDI (m/z): [M+H]+ calcd for C20H21N2O: 305.1648, found
305.1651.
4.2.2 Synthesis of aza-BODIPY
Sodium nitrite (16.9 mg, 0.24 mmol) was added to a suspension
of pyrrole (150 mg, 0.49 mmol) in a mixture of acetic
1
4
acid/anhydride (2 ml/0.8 ml) at room temperature. After 1 h
stirring at room temperature, crushed ice was added to the
mixture. The resulted dark green dye was filtered, washed with
4. Experimental section
4.1. General remarks

5
4. F. Lv, B. Tang, E. Hao, Q. Liu, H. Wang, L. Jiao, Chem.
water. The dark green dye was dissolved in CH Cl , filtered
ACCEPTED MANUSCRIPT
2
2
Commun. 55 (2019) 1639-1642.
through a pad of alumina (activity III). Solvent was removed
under reduced pressure, and the residue was dissolved in dry
CH2Cl2. Triethylamine (0.10 ml, 0.71 mmol) was added,
followed by dropwise addition of BF3·Et2O (0.13 ml, 1.05
mmol) with stirring at room temperature. The mixture was
stirred for 2 h at room temperature. The reaction was quenched
with crushed ice, extracted with CH2Cl2. The resulting crude
mixture was separated by column chromatography on silica gel
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(
n
-hexane : CH2Cl2 = 1 : 3), and followed by recrystallization
from CH2Cl2/ -hexane to afford (67.2 mg, 42%) as coppery
solids. H NMR (500 MHz, CDCl3):
(ppm) 8.11 (d, 3J = 8.5
Hz, 4H), 8.07 (d, 3J = 8.5 Hz, 4H), 7.45 (t, 3J = 7.0 Hz, 4H),
7.39 (t, 3 = 7.0 Hz, 2H), 7.07 (s, 2H), 7.45 (d, 3
= 8.5 Hz, 4H),
3.86 (t, 3J = 5.0 Hz, 8H), 3.33 (t, 3J = 5.0 Hz, 8H). 13C NMR
(125 MHz, CDCl3): (ppm) 156.8, 152.3, 145.2, 141.8, 132.7,
n
1
1
δ
J
J
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+
HRMS (ESI) m/z calcd for C40H37BF2N5O2 (M+H)+
668.30029, found 668.30048.
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4.2.3 Synthesis of aza-BODIPY
2
Aza-BODIPY (46.1 mg, 0.069 mmol) was treated with N-
1
bromosuccinimide (143 mg, 0.801 mmol) in dry CCl4 (20 ml) at
80 °C under nitrogen for 5 h. The reaction was quenched with
water, extracted with CH2Cl2, and purified by chromatography
on silica gel followed by recrystallization from CH2Cl2/n-
1
hexane to afford
NMR (500 MHz, CDCl3):
7.81 (d, 3J = 8.5 Hz, 4H), 7.41-7.46 (m, 6H), 6.93 (d, 3J = 8.5
Hz, 4H), 3.85 (t, 3J = 5.0 Hz, 8H), 3.32 (t, 3 = 5.0 Hz, 8H). 13
NMR (125 MHz, CDCl3): (ppm) 154.9, 147.7, 142.1, 134.3,
2
(40.3 mg, 71%) as dark coppery solids. H
δ
(ppm) 7.85 (d, 3J = 7.0 Hz, 4H),
J
C
δ
132.5, 130.9, 128.8, 127.8, 125.4, 119.3, 117.1, 113.1, 65.5,
47.4. HRMS (ESI) m/z calcd for C40H35BBr2F2N5O2 (M+H)+
826.11927, found 826.11920.
+
5. Acknowledgements
30. E. Heyer, P. Retailleau, R. Ziessel, Org. Lett. 16 (2014) 2330-
2333.
This work was supported by the National Natural Science
Foundation of China (21542004), Young and middle-aged
scientific and technological innovation talents of Shenyang
Science and Technology Bureau (RC170140), Liaoning Province
Natural Science Foundation (20170540721), Basic research on
the application of Industrial Development of Shenyang Science
and Technology Bureau (18013027), Liaoning BaiQianWan
Talents Program, and the Distinguished Professor Project of
Liaoning province (20183532). We thank the Chinese
Scholarship Council (20183058) for financial support. We thank
Prof. Yohsuke Yamamoto and Dr. Rong Shang (Hiroshima
University) for their help.
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ACCEPTED MANUSCRIPT
1) Morpholino-containing aza-BODIPYs at 3,5-positions were
synthesized.
2) Aza-BODIPY 1 with the morpholino group as a pH-sensitive
functionality could be used to be a pH probe.
3) The singlet oxygen generation of PS 2 with the dibromo groups at
2,6-positions was more effective than that of the parent dye 1.