Enhance the fluorescence and singlet oxygen generation ability of BODIPY: Modification on the meso-phenyl unit with electron withdrawing groups

Article abstract of DOI:10.1016/j.jphotochem.2017.09.040

Herein, meso-R-carboxyphenyl substituted BODIPY compounds were synthesized and characterized (R = CH₃, COOH, NO₂, H and Br₄). The fluorescence, excited triplet state and singlet oxygen formation properties for these compounds were measured in nonpolar and polar solvents by UV–vis absorption, steady-state and time-resolved fluorescence methods, as well as laser flash photolysis technique. These BODIPYs exhibit very different fluorescence quantum yield and lifetime value, as well as singlet oxygen formation capability in polar solvents. In contrast, these compounds show very similar spectral shape and position, emit bright green fluorescence in non polar solvents. The introduction of an ortho-COOH on the benzene unit leads to a 20-fold increase in the fluorescence quantum yield. The further introduction of an electron withdrawing NO₂ or COOH on the phenyl makes the BODIPY dye show much higher capability to generate singlet oxygen (up to 5-fold increase) and causes a sharp decrease in both fluorescence quantum yield and lifetime value. The increase in solvent polarity enhances singlet oxygen generation and intramolecular fluorescence quenching. The results are explained by the presence of intramolecular photoinduced electron/charge transfer from BODIPY core to the carboxyphenyl moiety.

Full text of DOI:10.1016/j.jphotochem.2017.09.040

Journal of Photochemistry and Photobiology A: Chemistry 349 (2017) 197–206
Contents lists available at ScienceDirect
Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Invited feature article
Enhance the fluorescence and singlet oxygen generation ability of
BODIPY: Modification on the meso-phenyl unit with electron
withdrawing groups
a,b,c,
b
a,
Xian-Fu Zhang
*, Yakui Zhang , Baomin Xu *
a
Department of Materials Science and Engineering, Southern University of Science and Technology, Shenzhen, Guangdong Province, 518055, China
Institute of Applied Photochemistry & Center of Instrumental Analysis, Hebei Normal University of Science and Technology, Qinhuangdao, Hebei Province,
66004, China
b
0
c
MPC Technologies, Hamilton, Ontario, L8S 3H4, Canada
A R T I C L E I N F O
A B S T R A C T
Article history:
Received 14 July 2017
Herein, meso-R-carboxyphenyl substituted BODIPY compounds were synthesized and characterized
(R = CH , COOH, NO , H and Br ). The fluorescence, excited triplet state and singlet oxygen formation
Received in revised form 11 September 2017
Accepted 17 September 2017
Available online 19 September 2017
3
2
4
properties for these compounds were measured in nonpolar and polar solvents by UV–vis absorption,
steady-state and time-resolved fluorescence methods, as well as laser flash photolysis technique. These
BODIPYs exhibit very different fluorescence quantum yield and lifetime value, as well as singlet oxygen
formation capability in polar solvents. In contrast, these compounds show very similar spectral shape and
position, emit bright green fluorescence in non polar solvents. The introduction of an ortho-COOH on the
benzene unit leads to a 20-fold increase in the fluorescence quantum yield. The further introduction of an
Keywords:
BODIPY
Singlet oxygen
Fluorescence
Solvent effect
Electron transfer
Phenyl substitution
2
electron withdrawing NO or COOH on the phenyl makes the BODIPY dye show much higher capability to
generate singlet oxygen (up to 5-fold increase) and causes a sharp decrease in both fluorescence quantum
yield and lifetime value. The increase in solvent polarity enhances singlet oxygen generation and
intramolecular fluorescence quenching. The results are explained by the presence of intramolecular
photoinduced electron/charge transfer from BODIPY core to the carboxyphenyl moiety.
©
2017 Elsevier B.V. All rights reserved.
1. Introduction
atoms substituents[12–15] or hetero atoms N/S are introduced on
BODIPY core [16,17].
Difluoroborondipyrromethene (BODIPY, Scheme 1) derivatives
are versatile fluorophores for diverse applications that need
photoluminescence [1–4]. The design and synthesis of new
BODIPY compounds are expanding rapidly due to their excellent
photophysical and optoelectronic properties [5–9]. Many BODIPY
To meet various practical needs, structural modification is often
carried out to tune the physical and chemical properties of
BODIPYs [2,5]. Three types of positions,
are available for attaching substituent groups or functional units.
The meso substitution is relatively less studied than the and
a, b and meso (Scheme 1),
a
b
derivatives show high fluorescence quantum yield (
F
f
), narrow
modifications in literature [1,2,5]. BODIPY derivatives with a meso-
substituted phenyl are particularly interesting since many well
known aromatic reactions can be employed to link another
functional molecular moiety (FMM) onto the phenyl which gives a
BODIPY-phenyl-FMM dyad for various applications in fluorescent
probing [3,4,9], organic solar cell [5], molecular photo switch and
emission bandwidths with high peak intensities, good photo-
stability, and relatively high molar absorption coefficients in visible
region [1–4]. The BODIPY parent compound (BDP in Scheme 1), for
example, shows the fluorescence efficiency near unity (0.97) and a
long fluorescence lifetime of 6.89 ns in methanol [10,11]. On the
other hand, usual BODIPYs often exhibit negligible efficiency to
form excited triplet state and singlet oxygen unless Br/I heavy
so on. However, many meso-phenyl BODIPYs either show very low
1
fluorescence quantum yield or non photoactive in O
2
generation
due to the free rotation of the phenyl [18]. Substituents at position
,7 (Scheme 1) are often introduced to restrict the phenyl rotation
1
by steric hindrance [19].
The second method is using a substituent at the ortho position
of the phenyl to block the phenyl rotation. Hereby we report the
*
Corresponding author.
E-mail addresses: zhangxianfu@tsinghua.org.cn (X.-F. Zhang),
xubm@sustc.edu.cn (B. Xu).
https://doi.org/10.1016/j.jphotochem.2017.09.040
1010-6030/© 2017 Elsevier B.V. All rights reserved.

198
X.-F. Zhang et al. / Journal of Photochemistry and Photobiology A: Chemistry 349 (2017) 197–206
Scheme 1. Chemical structures, abbreviations (Top) and synthesis method (Bottom) of BODIPY and its phenyl-substituted compounds.
use of the less explored method 2 to make BODIPYs both
fluorescent and photoactive in 1
generation. Six m-carboxyl
meso-phenyl) BODIPY derivatives were synthesized (Scheme 1),
and their fluorescence and singlet oxygen generation properties in
different solvents were measured. Some substituents show
dramatic influence on the fluorescence quantum yield and lifetime
value, as well as singlet oxygen generation capability. The
mechanism for the change is also explored.
spectra were recorded on an Agilent 8454 spectrophotometer
using 1 cm matched quartz cuvettes.
The details on the photophysical measurements are similar to
our previous reports [20,21], and therefore given in the electronic
Supporting information together with the synthesis procedure.
O
2
(
3. Results and discussion
Using the synthesized BODIPYs as photosensitizers, we
compared their ability to generate singlet oxygen under the same
condition by measuring NIR luminescence spectra (Fig. 1). We also
compared their fluorescence efficiency in visible region (Fig. 1).
2
. Experimental section
All reagents for synthesis were analytical grade and used as
1
1
received. All solvents for spectrum studies were dried and
redistilled before use. H and C NMR spectra were recorded at
room temperature on a Bruker AVANCE III HD 600 MHz NMR
Strong 1270 nm band indicates the presence of O
in Fig. 1, the modification on the phenyl moiety shows very
profound effect. In particular, NO -CPh-BDP shows much higher
2 g
( D ). As shown
1
13
2
spectrometer. MS spectra were recorded on a Thermal Fisher LCQ
ability to generate singlet oxygen than other R-CPh-BDP (R = H, Me,
COOH), while its fluorescence is much weaker than that of R-CPh-
TM
Fleet
mass spectrometer. IR spectra were recorded at room
temperature on a Shimadzu FTIR-8900 spectrometer. UV–vis
4
BDP (R = H, Me, COOH). Br -CPh-BDP, on the other hand, shows the
Fig. 1. Left: 1270 nm NIR Luminescence band of singlet oxygen (1
Absorbance is 0.20 at 505 nm). Right: The fluorescence of the BODIPYs in air saturated methanol solutions with excitation at 470 nm (Absorbance is 0.10 at 470 nm).
g 4
D ) using the BODIPYs (15 mM) as photosensitizers in air saturated CCl solutions with excitation at 505 nm
(

X.-F. Zhang et al. / Journal of Photochemistry and Photobiology A: Chemistry 349 (2017) 197–206
199
Table 1
Singlet oxygen formation quantum yield of BODIPYs.
Ph-BDP
CPh-BDP
Me-CPh-BDP
COOH-CPh-BDP
NO
2
-CPh-BDP
4
Br -CPh-BDP
CCl
ethanol
4
0.006
0.030
0.080
0.040
0.040
0.060
0.072
0.070
0.12
0.16
0.45
0.45
highest ability forming 1
property. Also noted is that the fluorescence of CPh-BDP is much
stronger than that of Ph-BDP.
To quantitatively discuss the observation, Table 1 gives the
singlet oxygen formation yield (FD), while Table 2 lists the
fluorescence properties of seven BODIPY compounds. Compared to
Ph-BDP, the presence of o-COOH on the phenyl of CPh-BDP
O
while maintaining good fluorescence
low fluorescence quantum yield and short lifetime, due to the fast
free rotation of the phenyl. Hereby we introduced an o-COOH on
the phenyl (CPh-BDP and R-CPh-BDP in Fig. 1), the presence of this
ortho-COOH substantially increases the fluorescence efficiency and
lifetime due to the large steric hindrance for the rotation of the
phenyl. This effect on fluorescence properties is comparable to that
of the COOH in fluorescein and rhodamine dyes [20,21]. In fact,
without the o-COOH the optimal dihedral angle between the
2
increases
.018 to 0.61,
shows much smaller
CPh-BDP, i.e. the presence of NO
.61 to 0.036) and (from 5.38 to 0.28 ns for the main component)
F
f
and
t
f
over 30-fold in methanol (
is increased from 0.18 to 5.38 ns). NO
and values but a much higher FD than
on the phenyl lowers (from
F
f
is changed from
ꢀ
ꢀ
0
t
f
2
-CPh-BDP
phenyl and the BODIPY is 52.7 , but this angle is changed to 89.3
F
f
t
f
when the o-COOH is present, i.e. the phenyl is twisted to the
perpendicular position. To well understand the effect of substitu-
ent R of the carboxyphenyl, more detailed spectral and photo-
physical data are discussed below.
2
F
f
0
t
f
but increases FD over five-fold (from 0.03 to 0.16). The
introduction of a methyl on the phenyl, on the other hand, leads
The UV–vis electronic absorption and fluorescence spectra were
measured in five solvents of different polarity for the six
compounds. Fig. 2 shows the absorption, fluorescence excitation
and emission spectra in ethanol. All the absorption and emission
spectra show the typical feature of BODIPY compounds as that in
to only a slight change in both
Ph-BDP and its para-R-Ph-BDP derivatives have been well
studied in literature. In contrast to BDP ( = 0.97, and = 6.89 ns in
methanol) [10], this type of BODIPY compounds usually show very
f f
F and t values in all five solvents.
F
f
t
f
Fig. 2. Normalized absorption, excitation and emission (right) spectra in ethanol. Emission spectra were measured with
em = 580 nm for Br -CPh-BDP and em = 550 nm for others.
lex = 470 nm, excitation spectra were measured with
l
4
l

2
00
X.-F. Zhang et al. / Journal of Photochemistry and Photobiology A: Chemistry 349 (2017) 197–206
literature [1–9]. For absorption spectra, two bands above 300 nm
were observed, i.e. a strong and narrow band at around 500 nm due
the BODIPY core in these molecules is very weak. It must be due to
the excited state interaction (such as photoinduced electron
to S
00 nm due to S
however, only one band at around 515 nm was obtained, indicating
that Kasha’s rule takes effect, [22] i.e. only S state emits
fluorescence, while S or higher states do not fluoresce.
For each compound, the fluorescence excitation is identical to
its absorption spectrum in visible region, suggesting that all the
photons emitted are originated from BODIPY excited states
1
S
0
transition, and another much weaker one within 300–
transfer, energy transfer) that makes the fluorescence properties
1
4
2
S
0
transition. For each emission spectrum,
and O
2
generation ability strongly affected by the substituents of
the phenyl and solvents used.
1
Quantum chemical geometrical optimization on DFT B3LYP/6-
2
31G++ level indicates that the BODIPY core
unit, and the BF moiety are all almost perfectly planar (Fig. 3). The
BF moiety in all compounds is vertical to the BODIPY core. In Ph-
p–system, the phenyl
2
2
BDP, the dihedral angle between the phenyl and BODIPY plane is
ꢀ
generated from S
emission is symmetrical (Fig. 2) to S
that only S state is fluorescent.
0
!S
i
(i = 1,2, . . . ) electronic transitions. The
52.7 . In CPh-BDP, however, the carboxyphenyl moiety is almost
1
excitation band, reconfirming
geometrically perpendicular to the BODIPY core (the dihedral
ꢀ
1
angle is 89.3 ), so that their electron cloud has the minimal
In all cases, the substituent on the phenyl does not change the
shape of absorption and emission spectra but only slightly
modifies the position of peak maxima (Fig. 2 and Table 1), since
these substituent groups are not directly attached onto the BODIPY
overlapping. In R-CPh-BDP (R = NO
is all 87.9 , which is slightly smaller than that in CPh-BDP (89.3 ).
Due to the almost perpendicular orientation between the phenyl
2
, COOH, Me), the dihedral angle
ꢀ
ꢀ
and the BODIPY core for R-CPh-BDP (R = H, NO
2
, COOH, Me), the
core but on the phenyl, while the phenyl is not
BODIPY core. Compared to CPh-BDP, the presence of an electronic
withdrawing group (NO or COOH) causes the red shift of the
spectra, while the electronic donating CH leads to little shift of
p
-conjugated with
BODIPY -system and the attached phenyl moiety can be
p
considered as two independent functional units. This result also
indicates that the ground state interaction between the phenyl unit
and the BODIPY core is very weak (except Ph-BDP).
2
3
spectral maxima (Fig. 2 and Table 1). This spectral shift direction is
apparently opposite to that of the traditional aromatic system with
Compared to BDP, Ph-BDP shows a much smaller
due to the fast free rotation of the phenyl. CPh-BDP, however, has
much higher than Ph-BDP in all solvents, which means that the
COOH can effectively restrict the rotation.
Although R-CPh-BDP (R = H, NO , COOH, Me, Br
f
F (Table 2)
directly-linked groups (such as CH
3
, OH, and NH
2
on benzene leads
F
f
to significant red shift). The same effect upon the indirectly-
bonded substituent change was also found previously for phenyl
substituted fluorescein and rhodamine compounds [20,21].
Solvent influence on the absorption and emission spectra is not
significant, the related data is listed in Table 1. The increase in
solvent polarity induces slight blue shift of the peak maxima for
both absorption and emission bands (Fig. 2 and Table 1). This
indicates that the dipole moment of the excited state of a dye
molecule is slightly smaller than that of its molecular ground state.
The Stokes shift Dl (18 ꢁ4 nm) is small in all solvents (Table 1),
since the BODIPY fluorophore is very rigid in structure and their
dipole moments show only slight difference between its ground
and excited states.
2
4
) compounds
have similar structures and absorption and emission spectra, their
fluorescence properties are very different. CPh-BDP can be used as
a reference for the other R-CPh-BDP (R = NO
compounds since they differ only by the substituent on the
benzoate unit. Compared to CPh-BDP, NO -CPh-BDP and COOH-
CPh-BDP exhibit large decrease in relative to that of CPh-BDP in
every solvent, i.e. the presence of electron-withdrawing NO and
COOH leads to the large decrease of value. Further more, this
decrease for each compound is remarkably enhanced by the
increase in solvent polarity. For example, the of NO -CPh-BDP is
2 4
, COOH, Me, Br )
2
F
f
2
F
f
F
f
F
f
2
0.35, 0.10, 0.080, 0.056, and 0.036 in n-hexane, DCM, ethanol,
acetonitrile and methanol, respectively. These results are
explained by the involvement of photoinduced electron transfer
(PET) or charge transfer (PCT) in the deactivation of the excited
In summary, the spectral shape and position are little affected
by the substituent on the phenyl and the solvent polarity, which
means that the ground state interaction between the phenyl and
Fig. 3. The optimized geometry of BODIPY and its derivatives shows the relative orientation of three planes: phenyl unit, BODIPY core, and BF
BDP, Ph-BDP, CPh-BDP (H atoms are hided). Bottom: NO -CPh-BDP, Me-CPh-BDP.
2
plane. Top: from left to right,
2

X.-F. Zhang et al. / Journal of Photochemistry and Photobiology A: Chemistry 349 (2017) 197–206
201
Table 2
Fluorescence properties in five solvents.
n-Hexane
DCM
ethanol
methanol
acetonitrile
l
abs (nm)
BDP
Ph-BDP
CPh-BDP
Me-CPh-BDP
COOH-CPh-BDP
NO2-CPh-BDP
504
498
500
499
n.s.
505
516
505
500
502
502
506
507
516
500
498
496
496
500
502
514
499
497
496
497
499
501
513
499
496
497
497
500
501
514
1
(
Br)4-CPh-BDP
l
em (nm)
BDP
Ph-BDP
CPh-BDP
Me-CPh-BDP
COOH-CPh-BDP
NO2-CPh-BDP
508
513
519
516
n.s.
527
526
511
517
519
520
529
524
529
508
514
513
513
523
522
528
507
517
513
514
520
519
528
507
515
515
514
525
518
528
(
Br)4-CPh-BDP
Dl(nm)
BDP
4
7
8
8
9
Ph-BDP
CPh-BDP
Me-CPh-BDP
COOH-CPh-BDP
NO2-CPh-BDP
15
19
17
n.s.
22
10
17
17
18
23
17
13
16
17
17
23
20
14
20
17
17
21
18
15
19
18
17
25
17
14
(
Br)4-CPh-BDP
t
f
(ns)
BDP
6.42
6.03
6.34
6.60
6.51
Ph-BDP
CPh-BDP
0.28
4.97
0.31, 5.47(8%)
4.96
0.24, 5.63(4%)
5.73
0.18, 5.10(5%)
5.38
0.20, 5.39(10%)
4.86
Me-CPh-BDP
COOH-CPh-BDP
NO2-CPh-BDP
5.31
n.s.
3
5.09
5.85, 9.93(9%)
2.90, 6.30(39%)
0.69, 6.11(21%)
4.53, 6.97(84%)
4.60, 8.03(86%)
2.17, 5.47(45%)
0.28, 5.50(30%)
4.17, 7.28(91%)
4.04, 5.54(69%)
1.59, 4.89(52%)
0.64, 4.49(66%)
2.86, 6.86(94%)
2.03, 5.28(43%)
0.71, 4.72(60%)
2.84, 6.87(95%)
(
Br)4-CPh-BDP
3.08, 6.66(93%)
F
f
BDP
Ph-BDP
CPh-BDP
Me-CPh-BDP
COOH-CPh-BDP
NO2-CPh-BDP
0.94
0.031
0.61
0.69
n.s.
0.89
0.035
0.58
0.63
0.17
0.93
0.025
0.66
0.66
0.47
0.97
0.018
0.61
0.60
0.30
0.036
0.49
0.96
0.020
0.60
0.55
0.14
0.35
0.49
0.10
0.52
0.080
0.75
0.056
0.35
(
Br)4-CPh-BDP
k
f
BDP
Ph-BDP
CPh-BDP
Me-CPh-BDP
COOH-CPh-BDP
NO2-CPh-BDP
0.15
0.11
0.12
0.13
n.s.
0.15
0.11
0.12
0.12
0.08
0.14
0.18
0.15
0.10
0.12
0.11
0.17
0.12
0.14
0.14
0.10
0.11
0.13
0.14
0.13
0.12
0.15
0.10
0.12
0.14
0.09
0.09
0.12
0.12
0.16
(
Br)4-CPh-BDP
1n.s.: not soluble.
singlet state (S
withdrawing NO
1
) of the BODIPYs. The presence of electron
or COOH makes the carboxyphenyl moiety a
better electron acceptor, while the BODIPY core acts as an electron
donor. NO or COOH on the phenyl remarkably enhances PET from
the BODIPY to the phenyl unit for CPh-BDP and Me-CPh-BDP. Both
PCT and PET are enhanced by the increase in the solvent polarity,
but PET is more intensely affected.
BDP has the longest fluorescence lifetime in all solvents than
other compounds, it is 6.61 ns in methanol which gives the
2
2
the BODIPY core to the phenyl unit. Methyl, on the other hand,
makes the carboxyphenyl moiety a slight worse electron acceptor,
so that PET or PCT within Me-CPh-BDP occurs only in solvents of
high polarity (methanol).
Quantum chemical calculations also indicate the involvement
of PET, especially for NO
9
ꢃ1
radiative rate constant (k
f
) value of 0.15 ꢂ10 s , calculated by
ꢃ9
k
f
=
F
f
/
t
f
= 0.97/(6.61 ꢂ10 s). The non radiative rate constant k
n
7
ꢃ1
value is 0.45 ꢂ10 s (knr = 1/
t
f
ꢃk
f f
), much smaller than that of k .
The intersystem crossing constant (kisc ꢄ(1 ꢃ0.97)/(6.61 ns) = 0.45
7
ꢃ1
2
-CPh-BDP (Fig. 4). The HOMO/LUMO
-CPh-BDP, the
ꢂ10 s ) is negligible for BODIPY compounds.
orbitals for the BODIPYs are shown in Fig. 4. For NO
2
Fig. 5 shows the fluorescence decay curves for different
BODIPYs in methanol. The curves go down in the order: Me-
HOMO-1 and HOMO are fully localized on BODIPY unit, but its
LUMO is completely located on the phenyl unit. This means that
upon the photo excitation of an electron from HOMO to LUMO
CPh-BDP, Br
and Ph-BDP. A lower curve usually has shorter lifetime, which
suggests that the lifetime value is Me-CPh-BDP > Br -CPh-BDP >
CPh-BDP > COOH-CPh-BDP > NO -CPh-BDP > Ph-BDP.
4 2
-CPh-BDP, CPh-BDP, COOH-CPh-BDP, NO -CPh-BDP,
(
S
0
!S
1
) or HOMO-1 to LUMO (S
0
!S
2
), the electron is relocated
4
from the BODIPY unit to the phenyl unit, i.e. PET occurs. For CPh-
BDP and Me-CPh-BDP, however, only a fraction of LUMO is on the
phenyl while its main part is still located on the BODIPY unit, while
the HOMO is fully localized on the BODIPY. This indicates that PCT
2
Ph-BDP decay curve is the lowest, indicating the shortest
lifetime and the calculated value is 0.18 ns (main component, 98%)
in methanol, this value is much shorter than 6.61 ns of BDP and
5.38 ns of CPh-BDP due to the free rotation of the phenyl. The
(
photoinduced charge transfer) but not PET process occurs from

2
02
X.-F. Zhang et al. / Journal of Photochemistry and Photobiology A: Chemistry 349 (2017) 197–206
Fig. 4. The calculated HOMO-1 (left), HOMO (middle), and LUMO (right). Top: NO
transition, the calculated excitation energy is 2.641 eV (470.2 nm), 2.594 eV (478.1 nm), and 2.632 eV (471.1 nm), while the calculated oscillator strength f is 0.672, 0.599, and
.625 for NO -CPh-BDP, CPh-BDP and Me-CPh-BDP respectively.
2
-CPh-BDP. Middle: CPh-BDP. Bottom: Me-CPh-BDP. For HOMO !LUMO electronic
0
2
Fig. 5. The fluorescence decay curves for the BODIPYs (10
mM) monitored at 530 nm with excitation at 509 nm (80 ps pulsed diode laser). Left: effect of the substituent R
(
R-CPh-BDP, R = Me, Br, H, COOH, NO ). Right: Solvent effect on the decay of NO
2
2
-CPh-BDP.

X.-F. Zhang et al. / Journal of Photochemistry and Photobiology A: Chemistry 349 (2017) 197–206
203
9
ꢃ1
phenyl in Ph-BDP is freely rotating along the C ꢃꢃ C bond that links
the phenyl and the BODIPY core. The rotation process is so quick
that it consumes 98% of the energy of the excited state since the
fluorescence emission efficiency is only 1.8% in methanol. The
maximum rate constant of internal conversion (kic) can be
in Table 1. Comparing to k
dominating for NO -CPh-BDP S
f
(0.12 ꢂ10 s ), PET process is
1
state which has a much faster
2
PET rate than others. This PET process explains why
decreased by the presence of the substituents.
f
F is largely
If the above mechanism is true, we can quantitatively predict
correctly. Since = k /(k /(k + ket) for R-CPh-
+ kic + kisc + ket) ffik
BDP compounds (R = Me, COOH, NO ), k is an constant from BDP,
using ket values computed from , we can then obtain as 0.62,
0.28, and 0.034 for R = Me, COOH, NO respectively. These values
F
f
10 ꢃ1
estimated to be 0.54 ꢂ10
s
(kic ffiknr = 1/
t
f
ꢃk
f
,
t
f
= 0.18 ns of
F
f
f
f
f
f
the main component), which is 39 times faster than the emissive
2
f
9
ꢃ1
rate constant (0.14 ꢂ10 s ).
t
f
F
f
Interestingly, CPh-BDP fluorescence decay displays the mono-
exponential behaviour in all solvents, and the lifetime varies from
2
are excellently agreement with the measured values in methanol
(Table 1). This agreement also indicates that kic and kisc are indeed
4.86 to 5.73 ns, much longer than that of Ph-BDP (0.18–0.35 ns). The
introduction of a COOH at the ortho-position of the phenyl has a
tremendous effect to hinder the fast free rotation for two reasons:
very small compared to k
The presence of PET is also supported by the thermodynamic
evidence, since the driving force, the free energy change of PET, G,
f
and ket.
1) geometrical hindrance, 2) hydrogen bonding between COOH
D
and 1-H on the xanthene ring. Both interactions co-act and cause
the rotation to be much slower than the emissive process.
Although CPh-BDP shows much longer fluorescence lifetime
than Ph-BDP, but its value is still smaller than that of BDP. Is it due
to the rotation of m-carboxyphenyl (CPh)? The monoexponential
decay behaviour of CPh-BDP indicates that it is not due to the
rotation, otherwise, the decay would be biexponential as that for
Ph-BDP.
is indeed a negative value.
D
G of PET is calculated by Eq. (2)
D
G = EOX(D/D ) ꢃER(Aꢃ/A) ꢃE
+
S
ꢃC,
(2)
+
in which E
ER(A
OX(D/D )
is the oxidation potential for an electron donor D,
ꢃ
/A)
S
is the reduction potential of an electron acceptor A, E is the
energy for the lowest excited singlet state, and C is a solvent
dependent small constant (ꢆ0.06 eV).
E
S
is ꢆ2.55 eV for the BODIPYs. The C H COOH shows a
6 5
The fluorescence decay of R-CPh-BDP (R = NO
2
, COOH, Me, Br
4
)
reduction wave at ꢃ1.75 V. BODIPY has an oxidation potential of
compounds can be compared with that of CPh-BDP. These
compounds mostly show biexponential emission decay, but the
0.80 V. With the photo excitation, the excited BODIPY moiety is the
donor while benzoate is the acceptor, since the
computed to be negative (
this small negative value indicates that PET can occur in CPh-BDP.
The electronic withdrawing COOH- and NO -substituted benzoic
acid must have an even lower reduction potential than C H COOH,
DG of PET is
t
f
of the long-lived components can be larger than that of BDP,
suggesting that the long-lived emission components are not
originated from the excited singlet state S but from another meta-
DG = 0.80 + 1.75–2.50–0.06 = ꢃ0.01 eV),
1
2
ꢅ+ ꢅꢃ
d
+
dꢃ
stable state D -A or D -A (charge separation state formed by
PET or charge transfer state by PCT). The mechanism is shown in
6
5
which makes the PET more favorite. In particular, NO2 benzoate
G value
Fig. 6 (h
The short-lived components of R-CPh-BDP (R = NO
) are due to S emission. The short lifetime is also strongly
dependent on the solvent polarity, for example, of NO -CPh-BDP
n
2
).
shows a reduction potential at ꢃ0.92 V, which results in a
D
2
, COOH, Me,
of ꢃ0.84 V. This means in NO -CPh-BDP, PET can occur from
2
Br
4
1
t
1
BODIPY moiety to NO -benzoate fair easily (Fig. 6).
2
t
1
2
Fig. 7 shows the absorption spectra and chemical kinetic curves
is 3.00, 0.71, 0.69, 0.64 and 0.28 ns in n-hexane, DCM, ethanol,
acetonitrile and methanol, respectively. This means the higher the
solvent polarity is, the shorter the lifetime is. This solvent effect
strongly supports the presence of PET or PCT.
of photosensitized oxidation of DPBF in the presence of BODIPY
derivative. DPBF is the specific chemical trapper of singlet oxygen,
it showed rapid decomposition only in the co-presence of a
BODIPY, light irradiation and molecular oxygen. On the other hand,
DPBF absorption showed no change if anyone of the BODIPY,
The existence of PET (with the rate constant of ket) competes
with emissive process and shortens the
t
f
, since
t
f
= (k
f
+ kic + kisc
+
oxygen, or the light was absent. Br -CPh-BDP showed the highest
ability to decompose DPBF, apparently due to the heavy atom effect
4
ꢃ1
k
et
)
. We can then compute ket by Eq. (1):
0,
of Br atom which leads to the T
most efficient photosensitizer is NO
ciently enhances the formation of CSS (charge separated state),
which is the precursor of T
The photo stability of the BODIPYs was tested by monitoring the
absorbance at 510 nm in ethanol, for which the samples were
irradiated by a 450 W Xenon lamp with a 505 nm filter. The results
1
formation efficiently. The second
k
et = 1/
t
f
ꢃ1/
t
f
(1)
2
-CPh-BDP, since NO effi-
2
0
in which
t
f
is the lifetime of BDP. Only ket value in methanol can be
values of BDP in other solvents are
calculated, since the
unknown. The ket value is 0.041, 0.072, 0.32, and 3.43 ꢂ10 s for
R-CPh-BDP (R = H, Me, COOH, NO ) respectively. Please note that
the ket values are obtained without using the measured values
t
f
1
.
9
ꢃ1
2
F
f
1 2 1 2 0 1
Fig. 6. Left: Donor-Acceptor pairs of the BODIPYs. Right: Fluorescence (hn , hn ), PET, and T forming processes for R-CPh-BDP compounds (R = NO ). S : ground state, S :
excited singlet state, T : excited triplet state, CSS: charge separation state, CR: charge recombination, PET: photo induced electron transfer.
1

2
04
X.-F. Zhang et al. / Journal of Photochemistry and Photobiology A: Chemistry 349 (2017) 197–206
Fig. 7. The absorption spectra and chemical kinetic curves of photosensitized oxidation of DPBF in the presence of BODIPY derivative as the photosensitizer (air saturated
ethanol as the solvent). The irradiation wavelength is 505 nm, BODIPY concentration is ca. 15 mM, the initial DPBF concentration is ca. 30 mM.
are shown in Supporting information (Fig. S12). The absorbance at
Singlet oxygen formation quantum yield was calculated for the
six BODIPYs in ethanol by this method. In contrast to Ph-BDP and
CPh-BDP ( is only 0.030 and 0.040, respectively), Br -CPh-BDP
510 nm showed no decrease during 20 min irradiation, indicating
the good photo stability of these BODIPYs.
F
D
4
Fig. 8. Top Left: Transient absorption spectra of Br
state recovery of Br -CPh-BDP in argon-saturated ethanol. Bottom Left: Transient absorption spectra of NO
Bottom Right: the triplet decay and ground state recovery of NO -CPh-BDP in argon-saturated ethanol.
4
-CPh-BDP (20
m
M) in argon-saturated ethanol with 355 nm laser excitation. Top Right: the triplet state decay and ground
4
2
-CPh-BDP in argon-saturated ethanol with 355 nm laser excitation.
2

X.-F. Zhang et al. / Journal of Photochemistry and Photobiology A: Chemistry 349 (2017) 197–206
205
has a
D
F of 0.45, which shows that it can be a good photosensitizer
4
. Conclusions
We have synthesized five meso-phenyl substituted BODIPY
for singlet oxygen production. Although the Br atoms are not
attached on the BODIPY core but on the phenyl, heavy atom effect
still works and leads to efficient formation of excited triplet state T
laser flash photolysis below), and then T transfer energy to
molecular oxygen to form singlet oxygen. NO -CPh-BDP has a
of 0.16, much larger than that of Ph-BDP and CPh-BDP.
Laser flash photolysis was used to identify T generated by the
BODIPYs as reported in literature [13]. Fig. 8 shows the well
resolved transient absorption spectra of Br -CPh-BDP and NO
CPh-BDP in argon-saturated ethanol, which are assigned to triplet–
triplet (T -T ) absorption as described below. Ph-BDP and CPh-BDP,
on the other hand, did not yield detectable transient absorption
signal within ns to s range, i.e. the formation yield of excited
triplet state for them is too low and beyond the detection limit, i.e.
ISC from S to T are not efficient.
Both Br -CPh-BDP and NO -CPh-BDP showed a positive peak at
15 nm, and a negative peak with the minimum at 510 nm (Fig. 8).
The shape and position are very similar to the reported T -T
1
(
1
compounds. We showed that the introduction of a para-COOH
on the phenyl effectively restricts the rotation and makes the
kind of phenyl-BDP compounds highly emissive. In the mean
2
F
D
2
time, the further introduction of a second group NO or COOH
1
on the phenyl strongly enhances PET or PCT which makes
excited triplet state and singlet oxygen formation much more
efficient. The results are explained by the presence of photoin-
duced electron (or charge) transfer from BODIPY core to the
phenyl moieties. The method and the mechanism on tuning the
fluorescence and photosensitizing properties provide new
insights for designing novel photosensitizers in photodynamic
therapy of tumor and other applications.
4
2
-
1
n
m
1
1
4
2
Acknowledgements
4
1
n
We thank the financial support from Hebei Provincial Hundred
Talents Plan (Contract E2013100005), the Peacock Team Project
funding from Shenzhen Science and Technology Innovation
Committee (Grant No. KQTD2015033110182370).
absorption spectra of the bromo-BODIPY dyes in our previous
report. This similarity suggests that the positive signal of the two
BODIPYs is also due to T
behavior is also indication of the T
1
-T
n
absorption. The following spectral
-T absorption. The minimum of
1
n
the negative absorption exhibited peaks matching the maximum
of the corresponding ground state absorption. With the decrease of
Appendix A. Supplementary data
1 0
the positive signal (T decay), the negative signal increases (S
formation), and the positive bands are separated from the ground
state bleaching with well defined isosbestic points. The presence of
isosbestic points indicates only two states are involved in the
Supplementary data associated with this article can be found,
in the online version, at http://dx.doi.org/10.1016/j.jphotochem.
2017.09.040.
transformation (T
1
!S
0
).
The decay curve at 415 nm can be well fit by the mono
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