Conjugated copolymer-photosensitizer molecular hybrids with broadband visible light absorption for efficient light-harvesting and enhanced singlet oxygen generation

Article abstract of DOI:10.1039/c4tc02568g

We have developed conjugated copolymer-photosensitizer molecular hybrids (PFBDBP-IPBP) with a strong, broad (from <400 nm to ～700 nm) and continuous visible absorption. The photosensitizing ability of PFBDBP-IPBP was demonstrated to be higher than that of

Full text of DOI:10.1039/c4tc02568g

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Conjugated copolymer–photosensitizer molecular
hybrids with broadband visible light absorption for
efficient light-harvesting and enhanced singlet
oxygen generation†
Cite this: DOI: 10.1039/c4tc02568g
Received 11th November 2014
Accepted 8th December 2014
*
*
Rongkun He, Ming Hu, Ting Xu, Chunxiang Li, Chuanliu Wu, Xiangqun Guo
DOI: 10.1039/c4tc02568g
*
and Yibing Zhao
www.rsc.org/MaterialsC
We have developed conjugated copolymer–photosensitizer molec- greatest spectral cross-sections of the visible spectrum (i.e., with
ular hybrids (PFBDBP–IPBP) with a strong, broad (from <400 nm to as broad absorption bands in 400–700 nm as possible), at least
ꢀ700 nm) and continuous visible absorption. The photosensitizing partially owing to the spectroscopic limitation of the light-
¨
ability of PFBDBP–IPBP was demonstrated to be higher than that of absorbing components required to meet the Forster theory as
the monochromophore-based IPBP, owing to rational manipulation of well as the lack of potent energy donor chromophores with
the multiple intramolecular energy transfer. In addition, we demon- continuous visible light absorption bands.
strated that the developed PFBDBP–IPBP displays excellent photo-
In this work, we report the design and synthesis of multi-
chromophore based molecular hybrids (PFBDBP–IPBP) con-
taining a uorescent conjugated copolymer backbone PFBDBP
and a covalently linked photosensitizer IPBP (Scheme 1 and
ESI†), which show extremely broad absorption bands (from
<400 nm to ꢀ700 nm) in the visible region and a high efficiency
of 1O2 generation under visible wavelength excitation. The
detailed synthetic route is provided in Scheme S1 in the ESI.†
Two chromophores that, upon conjugation with uorene
monomer units via Suzuki polymerization, absorb light in the
wavelength range of 400–550 nm and 500–600 nm, respectively,
(Fig. S1†) were exploited for the design of the conjugated
copolymer backbones. That is, a 2,1,3-benzoxadiazole-based
monomer (BD) and a BODIPY-based monomer (BP), which also
show strong uorescence emission in the wavelength range of
500–700 nm (PFBD) and 550–700 nm (PFBP), respectively, aer
polymerization (Fig. S1†).25–30 The polymers were branched with
iodinated BODIPY chromophores, which have been a unique
stability compared to the IPBP.
Light-induced generation of singlet oxygen (1O2) from triplet
photosensitizers has been a reaction of widespread interest.1,2
To date, there are many reported or commercially available
photosensitizers, but these triplet photosensitizers alone
intrinsically suffer from either low molar absorption coefficient
in the visible spectral region and/or narrow light absorption
band.1–5 Much work on achieving broadband visible light
absorption has come from the construction of multi-chromo-
phore based small molecular hybrids capable of efficient energy
transfer between two or more light-absorbing components, but
there is still an appreciable visible spectrum (400–700 nm)
unable to be efficiently absorbed.6–11
Fluorescent conjugated polymers have been frequently
exploited as energy donors for enhancing the light-harvesting
efficiency of photosensitizers, which are well-known to have
advantages of large molar absorption coefficient, excellent
photostability, and high brightness.12–18 However, although to
some extent these hybrid photosensitizers are capable of
extending the range of the light-harvesting spectrum, still only a
part of the visible spectrum can be successfully covered.19–24 To
our knowledge, it still remains a signicant challenge to
develop multi-chromophore based photosensitizers with
Department of Chemistry, College of Chemistry and Chemical Engineering, The MOE
Key Laboratory of Spectrochemical Analysis and Instrumentation, Xiamen University,
Xiamen, 361005, China. E-mail: ybzhao@xmu.edu.cn
† Electronic supplementary information (ESI) available: Molecular synthesis and
structural characterization data, UV-Vis absorption and photooxidation details.
See DOI: 10.1039/c4tc02568g
Scheme 1 Illustration of the molecular design and chemical structure
of PFBDBP–IPBP.
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class of photosensitizers, owing to their large absorption coef-
cient, good photostability, and high triplet state quantum
yield.31–34 In our system, IPBP, an iodinated BODIPY shows a
major absorption band in the wavelength range of 550–700 nm
(Scheme 1 and Fig. 1), which has a broad overlap with the
emission spectrum of PFBD and PFBP. Thus the IPBP was
expected to serve as the energy acceptor and a triplet photo-
sensitizer chromophore (Fig. S1†). Potential advantages of the
developed molecular hybrids relative to photosensitizers alone
(or other existing hybrids in the literature) not only lie in their
super-strong visible light-harvesting ability, but also include the
exibility of hybrid components (i.e., the ability of being post-
modied, and uorescence imaging capability).21,35
Fig.
2
Normalized UV-Vis absorption spectra of PFBDBP–IPBP,
PFBDBP and IPBP. The concentration of PFBDBP (in PFBDBP units),
PFBDBP–IPBP (in PFBDBP units) and IPBP is 2.0 ꢁ 10ꢂ6 M. Solvent:
DCM.
In the nal construction, BD and BP were incorporated in a
ratio of 7/3 with azide-appended uorene monomers as the
bridges to generate the conjugated copolymer PFBDBP; aer
that, 6% molar IPBP (molar ratio of [IPBP]/[azide-group])
photosensitizer was conjugated covalently to the backbones
through an efficient click reaction to generate the nal molec-
ular hybrids, PFBDBP–IPBP. The molecular weight (Mn) of
PFBDBP was measured to be 23 766 with a Mw/Mn of 1.94 as
determined by GPC. Each polymer contains an average of ꢀ80
repeating azide-group units. Thus, the broadband visible light-
induced capability was achieved through energy transfer from
either BD or BP units in the backbone to the side-chain IPBP
photosensitizers and/or through a multi-step transfer of energy
from BD to BP units. The energy transfer from PFBDBP to IPBP
contributes signicantly to the enhancement of the efficiency of
singlet oxygen generation.
As shown in Fig. 1, PFBDBP exhibits two major absorption
bands in the visible spectrum of 400–500 nm and 500–600 nm.
The molar absorption coefficient of PFBDBP at the two
absorption peaks (445 nm and 545 nm) can be estimated to be
1.80 ꢁ 105 Mꢂ1 cmꢂ1 and 2.64 ꢁ 105 Mꢂ1 cmꢂ1 (in polymer
backbone units), respectively. We further found that PFBDBP
has relatively high absolute photoluminescence quantum yield;
that is, 40.7% and 37.1%, when excited at 445 and 545 nm,
respectively. Accordingly, the copolymer PFBDBP that has broad
absorption bands in the wavelength range of 400–600 nm is
highly uorescent upon excitation at visible wavelengths.
Moreover, the uorescence emission of PFBDBP has a very
broad overlap with the absorption of IPBP (Fig. 1), thus enabling
Fig. 3 Fluorescence emission spectra of PFBDBP and PFBDBP–IPBP:
(a) lex ¼ 445 nm and (b) lex ¼ 545 nm excitation wavelength (5.0 ꢁ
10ꢂ8 M, in PFBDBP unit). Solvent: DCM.
efficient intramolecular energy transfer, which leads to uo-
rescence quenching of the copolymers (energy donor). Fig. 2
shows that PFBDBP–IPBP has extremely broad absorption
bands (from <400 to ꢀ700 nm) in the visible region, which can
improve the utilization of the visible light. Fig. 3 shows that the
uorescence of PFBDBP was signicantly quenched in
PFBDBP–IPBP by 96% and 95% when excited at 445 nm and 545
nm, respectively. Furthermore, the luminescence lifetime of the
residual PFBDBP compared to the free PFBDBP was used to
evaluate the intramolecular energy transfer efficiency between
PFBDBP and IPBP. When excited at 457 and 566 nm, the life-
time of PFBDBP reduced from 3.79 and 3.80 ns to 1.99 and 2.18
ns (Fig. 4), which implies an energy transfer efficiency (ET) of
47% and 43%, respectively.6,36 It is worth noting that the effi-
ciency of ET was much less than the quenching efficiency. We
proposed that the inefficient intramolecular energy transfer is
likely due to the intramolecular electron transfer.36,37
Fig. 1 Normalized UV-Vis absorption (2.0 ꢁ 10ꢂ6 M) and fluorescence
emission spectra of PFBDBP (lex ¼ 545 nm, 1.0 ꢁ 10ꢂ7 M) and IPBP (lex Fig. 4 Emission decay curves of PFBDBP and PFBDBP–IPBP: (a) lex
¼
¼ 600 nm, 1.0 ꢁ 10ꢂ7 M). Solvent: dichloromethane (DCM).
457 nm and (b) lex ¼ 566 nm. Solvent: DCM.
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Fig.
7
Photobleaching of IPBP (a) and PFBDBP–IPBP (b) under
continuous irradiation. [Photosensitizers] ¼ 1.0 ꢁ 10ꢂ5 M (in IPBP
units), 20 ꢃC. Solvent: DCM.
Fig. 5 Plots of ln(Ct/C0) vs. irradiation time for the photooxidation of
DHN using different triplet photosensitizers. [Photosensitizers] ¼ 2.0 ꢁ
10ꢂ6 M (in IPBP units), [DHN] ¼ 1.0 ꢁ 10ꢂ4 M, 20 ꢃC. Solvent: DCM/
MeOH (9/1, v/v), 20 ꢃC.
photosensitizing ability of the photosensitizers was studied
with monochromatic light excitation at the absorption band of
the energy donors BD unit, BP unit and energy acceptor IPBP,
respectively. 1,3-Diphenylisobenzofuran (DPBF) was used as the
singlet oxygen scavenger.40 Upon excitation at 445 and 545 nm,
the photosensitizing activity of PFBDBP–IPBP is higher than
that of IPBP (Fig. 6). These results demonstrated the effect of
intramolecular energy transfer in PFBDBP–IPBP. Hence the
performance of branched conjugated copolymer triplet photo-
sensitizers is enhanced by the intramolecular energy transfer
effect, in which a strong, broad and continuous absorption in
the visible region has been achieved.
In addition, owing to the excellent photostability of conju-
gated polymers, degradation of PFBDBP–IPBP under a strong
and continuous light excitation can be signicantly reduced.
The result of the photobleaching experiment shows that, aer
60 minutes of continuous irradiation with strong light, the
degradation of IPBP was 20.5% (Fig. 7a). In contrast, only 4.1%
of IPBP was degraded while IPBP was graed to the polymer
(Fig. 7b). The impressive ve-fold enhancement of photo-
stability highlights the signicance of protection afforded by
the conjugated polymer backbone.
In conclusion, we have developed branched photosensitizer
triplet photosensitizers PFBDBP–IPBP with a strong, broad
(from <400 nm to ꢀ700 nm) and continuous visible absorption.
Their photosensitizing ability was relatively higher as compared
with the monochromophore-based IPBP, owing to the multiple
intramolecular energy transfer. Besides, PFBDBP–IPBP shows
excellent photostability. Our results demonstrated a new
method to design efficient and environmentally friendly triplet
photosensitizers, and the feasibility of exploiting them in a wide
range of areas.
In order to evaluate the efficiency of the strong, broad and
continuous absorbing triplet photosensitizers, it is interesting
to explore their utility as photooxygenation catalysts to produce
singlet oxygen (1O2). Therefore, as a representative example, 1,5-
dihydroxynaphthalene (DHN) was used as the singlet oxygen
(1O2) scavenger which produces juglone upon oxidation.38 Upon
excitation with a panchromatic xenon lamp, the absorption of
DHN at 301 nm decreased due to the oxidation by 1O2 produced
from the triplet photosensitizer. Meanwhile, the absorption of
the product (juglone) at 427 nm increased (Fig. S2†). No obvious
change in the absorption spectrum of the triplet photosensi-
tizers was observed, indicating the excellent photostability of
the triplet photosensitizers under these excitation conditions
(Fig. S2†). Photooxidation of DHN with PFBDBP–IPBP and IPBP
can be quantitatively evaluated by plotting ln(Ct/C0) against the
irradiation time (Fig. 5). Oxidation rate constants (OCR) can be
derived from the slope of these curves. The results showed that,
at the same concentration of the IPBP unit, the OCR of
PFBDBP–IPBP is 0.077 minꢂ1, a value which is much higher
than that of IPBP (0.049 minꢂ1). Thus we demonstrated that the
strong, broad and continuous absorption of PFBDBP–IPBP
signicantly improves the triplet photosensitizing efficiency.
Notably, both PFBDBP–IPBP and IPBP were more efficient than
the conventional triplet photosensitizers such as tetraphe-
nylporphyrin (TPP, 0.033 minꢂ1) and methylene blue (MB, 0.014
minꢂ1) (Fig. 5).39
Furthermore, in order to study the effect of the intra-
molecular energy transfer on the photooxidation efficiency, the
Acknowledgements
We would like to acknowledge the nancial support of the
National Basic Research 973 Program of China (Grant no.
2011CB910403) and the National Natural Science Foundation of
China (Grant no. 21375110).
Notes and references
Fig.
6
Comparative singlet oxygen (1O2) generation experiment.
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Absorbance decrease of DPBF with time in the presence of different
triplet photosensitizers. Photoexcitation at (a) 445 nm and (b) 545 nm.
[Photosensitizers] ¼ 2.5 ꢁ 10ꢂ6 M (in IPBP units), 20 ꢃC. Solvent: DCM.
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