BODIPY-based photosensitizers with intense visible light harvesting ability and high 1O2 quantum yield in aqueous solution

Article abstract of DOI:10.1039/c4ra08654f

A novel method to enhance the singlet oxygen quantum yield of photosensitizers in aqueous solution has been developed by introducing an electron-withdrawing group into BODIPY. B-2 was prepared based on this method and shows intense light harvesting ability (ε = 6.8 × 10⁴ M^-1 cm^-1 at 643 nm) and high ¹O₂ quantum yield in aqueous solution (Φ = 0.21). B-2 has been successfully used as an ¹O₂ sensitizer for the photo-oxidation of 1,5-dihydroxynaphthalene. This method substantially improves the photooxidation capability and photostability of ¹O₂ sensitizers in aqueous solution. This journal is

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BODIPY-based photosensitizers with intense visible light
harvesting ability and high 1O2 quantum yield in aqueous
solution
Cite this: DOI: 10.1039/x0xx00000x
Wenting Wu,a, b Ying Geng,a Weiyu Fan,a Zhongtao Li,a Liying Zhan,a Xueyan Wu,a
Jingtang Zheng,a Jianzhang Zhao,b and Mingbo Wu*a
Received 00th January 2012,
Accepted 00th January 2012
DOI: 10.1039/x0xx00000x
www.rsc.org/
A novel method to enhance the singlet oxygen quantum yield
of photosensitizers in aqueous solution has been developed by
introducing electronꢀwithdrawing group into BODIPY. Bꢀ2
were prepared based on this method and shows intense light
NO2
8
1
7
I
I
I
I
I
I
harvesting ability (ε
= 6.8×104 Mꢀ1cmꢀ1 at 643 nm) and high
2
6
N
F
N
F
N
F
N
F
N
F
N
F
B
4
B
B
3
5
I
I
N
F
N
B
1O2 quantum yield in aqueous solution (Φ = 0.21). Bꢀ2 has
been successfully used as 1O2 sensitizers for the photoꢀ
oxidation of 1,5ꢀdihydroxynaphthalene. This method
substantially improves the photooxidation capability and
photostability of 1O2 sensitizers in aqueous solution.
F
B-4
B-1
Chart 1 BODIPY SOPs Bꢀ1, Bꢀ2, Bꢀ3 and Bꢀ4.
CN
CN
N
B-2
B-3
al9 finely tuned the HOMO energy level of BODIPY, and
selectively controlled the generation of 1O2 through photoꢀ
induced electron transfer (PET) process so as to be sensitive to
environment polarity. Other groups also make great
achievements in tuning photoinduced energy and electron
transfer process in dyads.7,8,10,11
Inspired by these pioneering works, we propose that
inhabitation of PET process is a key factor to enhance 1O2
quantum yield of SOPs for photooxidation in wide range of
solvent conditions, which could be achieved by finely tuning
SOPs’ photoredox properties through rational design and
synthesis.12,13 Herein, to develop the photosensitizer with
intense visibleꢀlight harvesting ability and high singlet oxygen
quantum yield in aqueous phase system, the boron dipyrroꢀ
methene (BODIPY) fluorophores attracted our attentions. After
S
inglet state oxygen (1O2) is a potent oxidation reagent,
which can be produced by tripletꢀtriplet energy transfer
between ground state oxygen (triplet state, 3O2) and singlet
oxygen photosensitzers (SOP).1 Now much attention has been
paid to the production of 1O2 and its application for
photooxidation and photodynamics therapy.2ꢀ5 However, the
conventional SOPs have many limitations, such as short
wavelength absorption, and a limited usable range of solvent
conditions. For example, the molar extinction coefficients of
Ir(III) complexes as SOP are usually less than 5000 Mꢀ1 cmꢀ1
beyond 400 nm and their absorption maxima are located in the
UV range.6 Organic SOPs, such as BODIPY covalent dimmer,
are always in the off state of the photosensitizers and unable to
1
generate O2 in polar solvents.7 It is well known that aqueous
Knovengel condensation reaction at C3 and C5 (Chart 1), the πꢀ
conjugation could be effectively enlarged, and the absorption
could be extended to near infrared region (NIR) with intensive
absorption. The generation of 1O2 through energy transfer
process could be enhanced by incorporating electronꢀ
phase system is more economical and environmentally friendly
for organic reactions. However, solvents with strong polarity
are detrimental to tripletꢀtriplet energy transfer, and the further
1
generation of O2. Therefore, it is highly desired to develop a
methodology to enhance the 1O2 quantum yield and light
harvesting ability of SOPs in various conditions, especially in
aqueous phase system.
withdrawing group (
donating group ( NEt2).
−CN) at the C3 and C5 instead of electronꢀ
−
Bꢀ1 and Bꢀ4 were prepared according to the literature.14,15 Bꢀ2 and
Bꢀ3 were prepared by Knoevenagel condensation reaction (see
Scheme 1, Figure S1–S4 of ESI†). Bꢀ1 has high extinction
coefficient (ε = 8×104 Mꢀ1 cmꢀ1 at λmax = 535 nm, Figure 1a). After
derivation at C3 and C5 position by Knoevenagel condensation
reaction, the absorption spectra of the derivatives show obvious red
shifts, extending from visible light to NIR. For example, the
maximum absorption wavelength of Bꢀ2 is at 643 nm (ε = 6.8×104
After excitation of photosensitizers, electron transfer process
and energy transfer process could happen between photoꢀ
3
sensitizers and O2. These two processes always compete with
each other. Increasing the polarity of the solvents always causes
electron transfer to dominate, which unfortunately results in the
off state of SOPs.8ꢀ10 By introducing the electronꢀdonating
substituents into meso benzene moiety of BODIPY, Nagano et
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DOI: 10.1039/C4RA08654F
OH
O
1.0
0.8
hν / O2 / Sens.
1.0
0.8
0.6
0.4
0.2
0.0
a
b
DCM
OH
DHN
B-1
OH O
0.6
0.4
0.2
0.0
Toluene
B-4
B-2
Methanol
DMF
Juglone
B-3
DMF/H2O(V/V)=2:1
OH
ISC
3Sens*
1O2
O
300 400 500 600 700 800
Wavelength(nm)
600
700
800
900
1Sens*
DHN
OH
Wavelength(nm)
kq
kr
− H2O
O
OH
Figure 1 a) UVꢀVis absorption spectra of Bꢀ1, Bꢀ2, Bꢀ3 and Bꢀ4 in
mixed solvent of DMF/H2O=5:1(V/V); b) fluorescence emission
spectra of Bꢀ2 in different solvents (DCM, Toluene, Methanol, DMF,
DMF/H2O (V/V) =5:1). (c = 1.0×10ꢀ5 M, 20 °C).
O
3O2
OH O
hν
Sens
Juglone
HO
Chart 2 Mechanism for the photooxidation of DHN with singlet
oxygen (1O2) photosensitizer.
Table 1 Redox potential and photocatalysis properties of SOPs
c
Eox/V b
Ered/V b
kobs
Yield/%d
74.7
a
e
Φ∆
△G0
1.0
1.0
b
a
78 min
75 min
...
2 min
1 min
0 min
0.8
0.6
0.4
0.2
0.0
78 min
75 min
...
1 min
0 min
0.30
0.21
1.41
−1.03
371.5
135.0
−0.09
Bꢀ1
Bꢀ2
0.8
0.6
0.4
0.2
0.0
−0.63,
−1.28
B-1
1.25
70.9
47.9
52.1
−0.08
−0.18
−0.05
B-2
0.87,
1.28
0.006
0.01
−1.17
40.2
49.1
Bꢀ3
−0.77,
−1.04
300 400 500 600 700
Wavelength(nm)
300
400
500
600
1.58
Bꢀ4
Wavelength(nm)
1.0
0.8
0.6
0.4
0.2
0.0
a
Singlet oxygen quantum yield in aqueous solution (DMF/H2O
d
B-3
B-2
0.0
-0.4
-0.8
-1.2
c
b
(V/V)=5:1). With Bꢀ1 as standard (
Φ
∆=0.69 in CH2Cl2). Redox
Photoreaction rate constants of
photoxidative reaction of 1,5ꢀDHN, catalyzed by the SOPs; 10ꢀ4minꢀ1.
78 min
75 min
...
2 min
1 min
0 min
c
potential (vs. NHE, CH2Cl2).
B-4
d
e
B-3
The yield of photoxidative reaction of 1,5ꢀDHN. free energy
changes of the potential PET effect with Rehm–Weller Equation (see
Table S2 of ESI†).
B-1
300 400 500 600 700
Wavelength(nm)
0
20 40 60 80
Irradiation time (min)
Mꢀ1cmꢀ1, Figure 1a). Compared with the other SOPs, Bꢀ2 shows a
new absorption at 361 nm, which could be attributed to styryl
absorption. This result indicates that there is no significant
electronic coupling between the distyryl and fluorophore.17 The
fluorescence properties of SOPs were also measured in various
solvents (see Figure 1b, Table S1 and Figure S5 of ESI†). Bꢀ2
with electronꢀwithdrawing group (−CN) shows maximum
fluorescence emission at 664 nm, and there is no obvious
wavelength shifts with the increase of solvents’ polarity, which
suggest that no charge separate state has formed and to some
extent, improve the efficiency of energy transfer. It is note that
the 1O2 quantum yield of Bꢀ2 is up to 0.21 in the mixed solvent
of DMF and H2O, which is obviously higher than that of the
SOPs based BODIPY previously reported (Φ∆ = 0.05).17 The
emission wavelengths of Bꢀ1 and Bꢀ4 are also not sensitive to
Figure 2 UV/Vis absorption spectral changes for the photooxidation
of 1,5ꢀDHN (1.0×10ꢀ4 M) using SOPs (a) Bꢀ1, (b) Bꢀ2 and (c) Bꢀ3 in
DMF/H2O (5:1, v/v) mixed solvent, [SOP]= 5.0×10ꢀ6 M (5% mol),
20 ºC. Irradiated with 35 W xenon lamp (25 mW·cmꢀ2). Light with
wavelength shorter than 385 nm was blocked by 0.72 M NaNO2
solution. (d) Absorption changes in the photoxidation of 1,5ꢀDHN
and plots of ln (Ct/C0) vs irradiation time.
carbon as the working electrode and platinum wires as the
counter and pseudoꢀreference electrode. Based on redox
potential of BODIPY unit (Bꢀ1), the oxidation potential greater
than 1.0 can be ascribed to the oxidations of BODIPY unit in
each SOP. Interestingly, two reductions are observed for Bꢀ2
(−0.63, −1.28 vs NHE) and Bꢀ4 (−0.77, −1.04 vs NHE), of
which the more negative one can be ascribed to the reduction of
–CN (−1.28 vs NHE) and –NO2 (−1.04 vs NHE) respectively,
which means –CN and –NO2 can grab electrons in redox
reactions. However, no oxdidation potentials of −CN and −NO2
exist, so there is no such a process for –CN and –NO2 to release
electrons, resulting in the inhibition of the PET. Furthermore,
the thermodynamic driving forces for PET process (△G0, free
energy change of the potential PET effect) were calculated
based on the redox potential values, by employing the Rehmꢀ
Weller Equation (see Table 1 and Table S2 of ESI
amino group acts as electron donor and O2 acts as electron
acceptor in the PET process. Bꢀ3 shows more negative G0
1
the polarity of solvents, the O2 quantum yield of Bꢀ1 comes to
0.30, which is in agreement with Bꢀ2. Conversely, introducing
electronꢀdonating group (−NEt2) to the C3 position makes Bꢀ3
more sensitive to the polarity of solvents than C8 derivatives.9
Bꢀ3 shows almost 80 nm red shifts, from 716 nm in toluene to
796 nm in aqueous solution, which indicates that there exists
obvious PET process to form charge separate state. As
consequence, energy transfer is hindered. The 1O2 quantum
yield of Bꢀ3 is only 0.006, which is subsequently decreased
after introducing the electronꢀdonating group.
† ,
).18,19 For Bꢀ3
The cyclic voltammetry (CV) were performed in dichloroꢀ
methane solution (10ꢀ3 M) with tetrabutylammonium hexaꢀ
fluorophosphate (0.05 M) as supporting electrolyte, glassy
△
2 | J. Name., 2012, 00, 1-3
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DOI: 10.1039/C4RA08654F
a
than others, indicating that PET process is enhanced while
State Key Laboratory of Heavy Oil Processing, School of Chemical
Engineering, China University of Petroleum, Qingdao 266580, P. R.
China. Tel: 86 532 86983452; Eꢀmail: wumb@upc.edu.cn.
energy transfer process is inhibited. Therefore, Bꢀ3 has the
1
lowest O2 quantum yield among these SOPs (Φ∆
=
0.006). The
electronꢀwithdrawing group conjugated to the BODIPY
fragment contributes to the increase of G0, as Bꢀ2 shows
significantly higher G0 than that of Bꢀ3, which means much
weaker ability to undergo PET process thermodynamically.
b
State Key Laboratory of Fine Chemicals, Dalian University of
△
Technology, 158 Zhongshan Road, Dalian 116012, P. R. China. Tel: 86
411 8498 6236; Eꢀmail: zhaojzh@dlut.edu.cn.
△
1
Thus, the O2 quantum yield of Bꢀ3 is enhanced (Φ∆
= 0.21).
Electronic Supplementary Information (ESI) available: The synthesis
synthesis and the structure characterization data of the photosesitizers;
photophysical properties data of photosensitizers. See DOI: 10.1039/
c000000x/
However, G0 (or redox potential) is not the sole determinal
△
1
factor for O2 quantum yield. Relative researches need to be
further explored in the future.
In order to further investigate the performance of SOPs in aqueous
solution, the photooxidation of 1,5ꢀdihydroxynaphthalene (1,5ꢀDHN)
with different SOPs were conducted in aqueous solvent (Figure 2).
The mechanism for the photooxidation of 1,5ꢀDHN with singlet
oxygen photosensitizer is presented in Chart 2. 1O2 is produced upon
photoexcitation of the aerated solution of the SOP. In this process,
an energy transfer process from the triplet excited state of a
sensitizer to triplet oxygen (3O2) contributes to singlet oxygen (1O2)
1. (a) J. Z. Zhao, W. H. Wu, J. F. Sun, and S. Guo, Chem. Soc. Rev. 2013,
42, 5323; (b) W. Wu, L. Zhan, W. Fan, X. Wu, Q. Pan, L. Huang, Z. Li,
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280; (c) W. Wu, P. Yang, L. Ma, J. Lalevée and J. Zhao, Eur. J. Inorg.
Chem., 2012, , 228.
,
2
1
2. J. F. Lovell, T. W. B. Liu, J. Chen and G. Zheng, Chem. Rev. 2010,
110, 2839.
production. Then, 1,5ꢀDHN can be oxidized by the O2 and Juglone
are produced in further, which could be monitored by the decrease of
the absorption of 1,5ꢀDHN at 301 nm and the increase of that of
Juglone, the product at 427 nm (Figure 2).4 The photooxidation
velocity with SOPs were quantitatively compared by plots of
ln(Ct/C0) vs t curves (Figure 2d). There are clear differences between
the photoreaction rate constants (kobs) of the photooxidation with
different SOPs. For example, the photoreaction rate constant of Bꢀ2
(kobs = 135.0×10ꢀ4 minꢀ1) is much larger than that of Bꢀ3 (kobs = 40.2
3. (a) N. Adarsh, R. R. Avirah and D. Ramaiah, Org. Lett. 2010, 12, 5720;
(b) A. Gorman, J. Killoran, C. O’Shea, T. Kenna, W. M. Gallagher and
D. F. O’Shea, J. Am. Chem. Soc., 2004, 126, 10619; (c) Y. Zhang, K.
Aslan, M. J. R. Previte and C. D. Geddes, J. Fluoresc., 2007, 17, 345;
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48, 4169.
×10ꢀ4 minꢀ1), but less than that of Bꢀ1 (Figure 2d and Table 1).
Although Bꢀ1 has the highest singlet oxygen quantum yield
5. (a) L. Huang and J. Z. Zhao, Chem. Commun. 2013, 49, 3751; (b) Y.
Cakmak, S. Kolemen, S. Duman, Y. Dede, Y. Dolen, B. Kilic, Z.
Kostereli, L. T. Yildirim, A. L. Dogan, D. Guc and E. U. Akkaya,
Angew. Chem., Int. Ed., 2011, 50, 11937; (c) M.ꢀR. Ke, S.ꢀL. Yeung,
(
Φ∆=0.30) compared with the other prepared SOPs, there is an
obvious photobleaching phenomenon for Bꢀ1 at 533 nm, indicating
that the photostability of Bꢀ1 is not as good as those of Bꢀ2 and Bꢀ4.
The introduced electronꢀwithdrawing group (−CN, −NO3) could
improve the photostability of SOPs.17
W.ꢀP. Fong, D. K. P. Ng and P.ꢀC. Lo, Chem.–Eur. J., 2012, 18, 4225;
(d) F. Schmitt, J. Freudenreich, N. P. E. Barry, L. JuilleratꢀJeanneret, G.
SüssꢀFink and B. Therrien, J. Am. Chem. Soc., 2012, 134, 754.
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2011, 10, 895.
Conclusions
In conclusion, we prepared a series of 2,6ꢀdiiodoꢀBODIPY
derivatives with intense light harvesting ability. For the
applications in enhancing singlet oxygen production in aqueous
solution, Bꢀ2 with electronꢀwithdrawing group shows intense
7. (a) X. F. Zhang and X. Yang, J. Phys. Chem. B 2013, 117, 9050; (b) S.
Duman, Y. Cakmak, S. Kolemen, E. U. Akkaya and Y. Dede, J. Org.
Chem., 2012, 77, 4516; (c) W. Pang, X.ꢀF. Zhang, J. Zhou, C. Yu, E.
Hao and L. Jiao, Chem. Commun., 2012, 48, 5437.
ability of sensitizing 1O2 (Φ∆
those of Bꢀ3 Φ∆ 0.006) and Bꢀ4
Bꢀ1
=
0.21) which is much higher than
Φ∆ 0.01). Compared with
(
=
(
=
8. D. G. Rodríguez, T. Torres, D. M. Guldi, J. Rivera, M. A. Herranz and
L. Echegoyen, J. Am. Chem. Soc. 2004, 126, 6301.
,
Bꢀ2 with electronꢀwithdrawing group shows better
photostability. The enhanced 1O2 sensitization properties,
intense light harvesting ability and significant photostability of
SOPs in aqueous solution, will benefit for the development of
green chemistry in a broad stage.
9. T. Yogo, Y. Urano, A. Mizushima, H. Sunahara, T. Inoue, K. Hirose,
M. Lino, K. Kikuchi and T. Nagano, Proc. Natl Acad. Sci. USA 2008,
105, 28.
10. D. G. Rodríguez, C. G. Claessens, T. Torres, S. Liu, L. Echegoyen
and N. Vila, Chem. Eur. J. 2005, 11, 3881.
We sincerely thank the anonymous reviewers for their
precious suggestions and acknowledge the support of the
Fundamental Research Funds for National Natural Science
Foundation of China (Grant 21302224, 51172285, 21176259,
51303212, 51303202), China Postdoctoral Science Foundation
(2014M560590), Shandong Provincial Natural Science Foundꢀ
ation (ZR2013BQ028, ZR2013EMQ013), Project of Science
and Technology Program for Basic Research of Qingdao (14–2ꢀ
4ꢀ47ꢀjch), Central Universities (13CX02066A, 14CX02060A,
14CX04009A) and the State Key Laboratory of Fine
Chemicals (KF1203).
11. R. Ziessel, B. D. Allen, D. B. Rewinska and A. Hariman, Chem. Eur.
J. 2009, 30, 7382.
12. D. Ravelli, D. Dondi, M. Fagnoni and A. Albini, Chem. Soc. Rev.
2009, 38, 1999.
13. S. Fukuzumi and K. Ohkubo, Chem. Sci. 2013, 4, 561.
14. X. Fang, Y. C. Liu and C. Li, J. Org. Chem. 2007, 72, 8608.
15. L. Huang, J. Z. Zhao, S. Guo, C. Zhang and J. Ma, J. Org. Chem.
2013, 78, 5627.
16. H. Sunahara, Y. Urano, H. Kojima and T. Nagano, J. Am. Chem. Soc.
2007, 129, 5597.
Notes and references
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17. T. Yogo, Y. Urano, Y. Ishisuka, F. Maniwa and T. Nagano, J. Am.
Chem. Soc. 2005, 127, 12162.
18. X. Zhang, L. Chi, S. Ji, Y. Wu, P. Song, K. Han, H. Guo, T. D. James,
J. Zhao. 2009, 131, 17452.
19. D. Rehm, A. Weller. Isr. J. Chem. 1970, 8, 259.
4 | J. Name., 2012, 00, 1-3
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1
Introducing electron-withdrawing group into BODIPY enhanced O2 sensitization
properties, intense light-harvesting ability and significant photostability of SOPs in
aqueous solution.