Intramolecular RET enhanced visible light-absorbing bodipy organic triplet photosensitizers and application in photooxidation and triplet-triplet annihilation upconversion

Article abstract of DOI:10.1021/ja405170j

Resonance energy transfer (RET) was used for the first time to enhance the visible light absorption of triplet photosensitizers. The intramolecular energy donor (boron-dipyrromethene, Bodipy) and acceptor (iodo-Bodipy) show different absorption bands in v

Full text of DOI:10.1021/ja405170j

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Article
Intramolecular RET Enhanced Visible light-Absorbing
Bodipy Organic Triplet Photosensitizers and Application in
Photooxidation and Triplet-triplet-annihilation Upconversion
Caishun Zhang, Jianzhang Zhao, Shuo Wu, Zilong Wang, Wanhua Wu, Jie Ma, Song Guo, and Ling Huang
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/ja405170j • Publication Date (Web): 21 Jun 2013
Downloaded from http://pubs.acs.org on June 29, 2013
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Intramolecular RET Enhanced Visible light-
Absorbing Bodipy Organic Triplet Photosensitizers
and Application in Photooxidation and Triplet-
triplet-annihilation Upconversion
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Caishun Zhang,a Jianzhang Zhao,a* Shuo Wu,b Zilong Wang,a Wanhua Wu,a Jie Ma,a Song Guoa
and Ling Huanga
a State Key Laboratory of Fine Chemicals,School of Chemical Engineering, Dalian University
of Technology, Dalian 116024 (China)
b School of Chemistry, Dalian University of Technology, Dalian 116024 (China)
E-mail: zhaojzh@dlut.edu.cn;
Group homepage: http://finechem.dlut.edu.cn/photochem
RECEIVED DATE (to be automatically inserted)
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Abstract: Resonance energy transfer (RET) was used for the first time to enhance the visible
light absorption of triplet photosensitizers. The intramolecular energy donor (boron-
dipyrromethene, Bodipy) and acceptor (iodo-Bodipy) show different absorption bands in visible
region thus the visible absorption was enhanced as compared to the monochromophore triplet
photosensitizers (e.g. iodo-Bodipy). Fluorescence quenching and excitation spectra indicate that
the singlet energy transfer is efficient for the dyad triplet photosensitizers. Nanosecond time-
resolved transient absorption spectroscopy has confirmed that the triplet excited states of the
dyads are distributed on both the energy donor and the energy acceptor, which is the result of
forward singlet energy transfer from the energy donor to the energy acceptor, and in turn the
backward triplet energy transfer. This ‘ping-pong’ energy transfer was never reported for organic
molecular arrays and so it is useful to study the energy level of organic chromophores. The
triplet photosensitizers were used for singlet oxygen (1O2) mediated photooxidation of 1,5-
dihydroxylnaphthalene to produce juglone. The visible light-absorption of the new visible light-
absorbing triplet photosensitizers are higher than the conventional mono-chromophore based
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triplet photosensitizers, as a result, the O2 photosensitizing ability is improved with the new
triplet photosensitizers. Triplet-triplet annihilation upconversion with these triplet
photosensitizers were also studied. Our results are useful to design the triplet photosensitizers
showing strong visible light-absorbance and for their applications in photocatalysis and
photodynamic therapy.
Keywords: Bodipy, FRET, photochemistry, photooxidation, resonance energy transfer, triplet
photosensitizer
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1. INTRODUCTION
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Triplet photosensitizers have attracted much attention owing to their versatile applications in
photocatalysis,1-6 photodynamic therapy (PDT),7-11 photovoltaics,12 and more recently the triplet-
triplet annihilation (TTA) upconversion.13-16 The typical photosensitizers used for these
applications are transition metal complexes, such as polyimine Ru(II) complexes,6,7
cyclometalated Ir(III) complexes and Pt(II)/Pd(II) porphyrin complexes.14 Organic triplet
photosensitizers (transition metal atom-free), such as the iodo-Bodipy and the iodo-aza Bodipy
have also been reported.2b,8,19 However, all these conventional triplet photosensitizers are based
on mono-chromophore profile.7-11 As a result, the UV-vis absorption of these photosensitizers
covers only a small part of the visible spectrum, i.e. there is only one major absorption band in
visible region for these conventional triplet photosensitizers. For example, the typical Ru(II)
polyimine complexes show only one major bands in 400 nm − 500 nm.1,6,18 Recently Eosin Y
was used as triplet photosensitizer for photocatalytic synthesis of benzothiophenes.4,19 Organic
chromophores such as Bodipy and porphyrin derivatives were used for sensitizing lanthanide
luminescence.20 But the disadvantage of this organic triplet photosensitizer remains the same.
Furthermore, most of the triplet photosensitizers used for photocatalysis are off-the-shelf
compounds, and tailor-designed triplet photosensitizers were rarely reported.2a Triplet
photosensitizers based on multichromophores with enhanced absorption (or broadband) in
visible region are of highly desired. However, to the best of our knowledge, no such triplet
photosensitizers with enhanced visible light-absorbing property have been reported. Previously it
was reported that the triplet excited state of some broadband absorbing compounds were
populated upon photoexcitation, but the application of these compounds were not reported.21-24
It is difficult to achieve enhanced visible absorbance with the conventional mono-
chromophore molecular structural profile. In order to overcome this challenge, a multi-
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chromophore strategy has to be used, which is similar to the fluorescent resonance energy
transfer (FRET) or through bond energy transfer (TBET), which have been widely used in
fluorescent molecular arrays, to achieve broadband absorption in visible region.17,23,25-33
However, to the best of our knowledge, RET effect has never been used to enhance the visible
light-absorbance of triplet photosensitizers. Previously a multichromophore Pt(II)/porphyrin-
Bodipy molecular array was studied, but the effect of the intramolecular energy transfer on the
triplet photosensitizing was not reported.22 Recently Bodipy-styryl Bodipy/Aza-Bodipy dyads
and triads with the RET effect were reported.26,34 However, without iodination, the production
of the triplet excited state of these molecular arrays is inefficient, thus these compounds cannot
be used as triplet photosenistizers.34 Iodo-Bodipy/aza Bodipy compounds have been prepared as
triplet photosensitizers,8,9,17 however, with only one visible light-absorbing chromophore, these
triplet photosensitizers give only one major absorption band in visible region.17
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In order to address the aforementioned challenges, herein we have designed Bodipy-based
RET triplet photosensitizers (Scheme 1, B-1 − B-3). These triplet photosensitizers are based on
the RET effect between the intramolecular energy donor (the uniodinated Bodipy part) and
intramolecular energy acceptor (the iodinated Bodipy part),26,32,35 which give different
absorption bands in visible region, as a result, enhanced visible light-absorbance in the visible
region was achieved. The photoexcitation energy harvested by the energy donor can be
transferred to the energy acceptor, via intersystem crossing (ISC) of the energy acceptor (ISC is
ensured by the iodo atoms attached on the Bodipy core, Scheme 1), and triplet excited state can
be populated. The property of the triplet photosensitizers were studied with steady state and
time-resolved spectroscopy, as well as DFT calculations. The performance of the triplet
photosensitizers were evaluated with the singlet oxygen (1O2) photosensitizing. Improved
photosensitizing ability was found for the new triplet photosensitizers, compared to the mono-
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chromophore-based conventional triplet photosensitizers that give the typical narrow absorption
band in visible region. Furthermore, we found that the triplet excited states are localized on both
the energy donor and the energy acceptor, due to the forward singlet state energy transfer, and
the backward triplet state energy transfer. Such a ping-pong singlet/triplet energy transfer was
never reported for organic RET molecular arrays.22,36
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2. RESULTS AND DISCUSSIONS
2.1 Design and synthesis of the compounds. Bodipy was used as the energy donor and the
iodo-Bodipy was used as the energy acceptor, because iodination will decrease the S1 state
energy level thus fulfills the requirement for RET. Furthermore, both energy donor and energy
acceptor give strong absorption in visible region, but at different wavelength (Scheme 1 and
Figure 1). Bodipy is a versatile chromophore which shows strong absorption of visible light,
high fluorescence quantum yield and feasibly derivatizable molecular structure. As a result,
Bodipy has been intensively used as light-harvesting antenna in fluorescent molecular arrays and
photovoltaics-related studies.25,26,37-43 However, the application of Bodipy in triplet state related
study is rare.8,9,10,22,44
The S1 state energy level of the energy donor of B-1 is approximated as 2.46 eV with the
emission of 7 (the reference chromophore of the energy donor in B-1). The energy level of the S1
state of energy acceptor in B-1, i.e. diiodo- or monoiodo-Bodipy unit is approximated as 2.31 eV
with the emission of 6 (the reference chromophore of the energy acceptor in B-1). Furthermore,
the absorption of the energy acceptor overlaps with the emission of the energy donor in B-1.
Thus RET will occur for B-1. Similar results are found for B-2 and B-3.
The RET energy donor-acceptor pairs in B-1 are connected with each other by 'Click' reaction
(Scheme 1).26 RET triplet photosensitizers with rigid linker between the energy donor and the
acceptor were also prepared for comparison (B-2 and B-3). Pd(0) catalyzed Suzuki coupling and
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Scheme 1. Synthesis of the RET-enhanced triplet photosensitizers B-1 – B-3 and the reference
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compounds 8, 9, 13 and 14a
N3
8
O
9
O
O
O
F
N
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F
N
B
I
II
III
Br
Br
O
O
N3
Br
OH
N
N
F
B
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VII
N N
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O
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Br
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OH
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a Key: (I) K2CO3 and DMF, 70 °C, 6 h. (II) DMF, 100 °C, 2 h. (III) nitrogen condition, CH2Cl2,
TFA, DDQ, Et3N and BF3⋅Et2O. (IV) K2CO3 and DMF, 70 °C, 6 h. (V) Under Ar atmosphere,
CH2Cl2, TFA, DDQ, Et3N and BF3⋅Et2O. (VI) NIS and CH2Cl2, 5 h. (VII) Et3N, CuSO4·5H2O,
Sodium ascorbate. 24 h. (VIII) N2 atmosphere, CH2Cl2, Et3N and BF3 ⋅ Et2O. (IX) NIS and
CH2Cl2, 1 h. (X) K2CO3 and Pd(PPH3)4, 4 h. (XI) NIS and CH2Cl2, 5 h. (XII) nitrogen condition,
CH2Cl2, TFA, DDQ, Et3N and BF3 ⋅ Et2O. (XIII) K2CO3 and Pd(PPh3)4, 4 h. (XIV) NIS and
CH2Cl2, 5 h. (XV) Et3N, PdCl2(PPh3)2, PPh3, CuI, 80 °C, 6 h. (XVI) Et3N, PdCl2(PPh3)2 , PPh3,
CuI, 80 °C, 6 h.
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Sonogashira coupling reactions were used for connection the energy donor and energy acceptors
in B-2 and B-3, respectively. For B-3, the π-conjugation framework is larger than that in B-2
(Scheme 1). All the compounds were obtained in moderate to good yields. The molecular
structures were fully characterized by 1H NMR, 13C NMR, HR MS.
2.2. Steady state UV−vis absorption and fluorescence spectra. The steady state UV-vis
absorption and emission of the compounds were studied (Figure 1). The energy donor Bodipy 7
gives absorption at 504 nm (ε = 96 890 M-1 cm-1). The energy acceptor diiodo-Bodipy 9 shows
absorption at 537 nm (ε = 86 700 M-1 cm-1). Dyad B-1 shows two absorption bands at 505 nm
and 537 nm (Figure 1a). This result indicates that the electronic interaction between the energy
donor and energy acceptor is weak at the ground state.34 Similar results were observed for dyads
B-2 and B-3 (Figure 1b).
The fluorescence emission spectra of the compounds were studied (Figure 1c and 1d). The
energy donor Bodipy 7 gives strong emission at 517 nm (ΦF = 90.0%). Interestingly, this
emission band was quenched in all the dyads. Based on the conventional consideration of
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fluorescent RET molecular arrays,32,45,46 the intramolecular energy transfer from the energy
donor to the energy acceptor is efficient.34a
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1.0
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Figure 1. UV-vis absorption (a and b) and fluorescence emission (c and d) spectra of B-1, B-2,
B-3, the energy donor and acceptors (in toluene). 1.0×10-5 M, 20 °C.
Furthermore, it is known that hydrogen bonding may exert substantial effect on the
photophysical properties of chromophores.47 Therefore, solvents with hydrogen bonding ability
(such as methanol) were used to study the photophysical properties of the compounds, no
significant effect on the photophysical properties was observed (see Supporting Information,
Figure S42−S45, absorption and emission spectra). This result is reasonable since no hydrogen
bond donor or acceptor are directly attached on the fluorophore core of B-1 − B-3.32,38
Recently, it was proposed that the conventional method of evaluation of the intramolecular
energy transfer in RET molecular arrays by the fluorescence quench method is quenstionable.48
For example, both energy transfer and electron transfer can result in fluorescence quenching of
the energy donor.34 To evaluate the intramolecular energy transfer efficiency, an alternative to
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the fluorescence quenching experiment is to compare the fluorescence excitation spectra and the
UV-vis absorption spectra.48 The fluorescence excitation spectra and the UV-vis absorption
spectra of the dyad B-1 were compared (Figure 2). The two spectra are almost superimposable at
505 nm and 537 nm, indicating efficient intramolecular energy transfer.34,48 Similar results were
observed for B-2 and B-3 (Figure S38-41).
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1.5
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Figure 2. UV−vis absorption and fluorescence excitation spectra of B-1 (in toluene). c = 1.0 ×
10-5 M. 20 °C.
2.3. Nanosecond time-resolved transient difference absorption spectra. In order to study the
triplet excited state of the photosensitizers, the nanosecond time-resolved transient difference
absorption spectra (TA) of the compounds were studied (Figure 3).49 For the diiodo-Bodipy 9,
significant bleaching band at 537 nm was observed upon pulsed laser excitation (Figure 3a),
which is due to the depletion of ground state of Bodipy. At the same time, positive transient
absorption bands at 443 nm and in the region of 550 − 750 nm were observed, which are the
absorption of triplet excited state of Bodipy chromophore.44 The transient was substantially
quenched in aerated solution, therefore the transient can be attributed to the triplet excited state.
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The lifetime of the triplet excited state of 9 was determined as 228.9 μs (at 1.0×10-6 M, toluene.
It should be noted that the spectra are noisy at this low concentration. The quality of the spectra
is improved at higher concentration but the lifetime was reduced due to the TTA effect).
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Time / μs
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Time / μs
Figure 3. Nanosecond time-resolved transient difference absorption of (a) compound 9 and (c)
B-1. (b) Decay trace of 9 at 540 nm. (d) Decay trace of B-1 at 500 nm (in deaerated toluene). λex
= 532 nm, c = 1.0×10-6 M (note the spectra are noisy because of the low concentration). 20 °C.
For the dyads, however, the TA spectra will be more complicated because the triplet excited
state of the dyads can be localized on either the energy donor Bodipy or the iodo-Bodipy, i.e. the
energy acceptor. The localization of the triplet state will be dictated by the energy levels of the
triplet excited states of the chromophores.14,22,36,51 For B-1 (Figure 3c), bleaching bands at 504
nm and 536 nm were observed upon photoexcitation, where the steady state energy donor and
the acceptor absorbs, therefore we propose that the triplet excited state of B-1 is distributed on
both the energy donor and the energy acceptor. Based on the optical density of the bleaching
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peaks and the steady state absorption of B-1 (Figure 1a), we conclude that the triplet excited
state of B-1 is almost equally distributed on the energy donor and the energy acceptor (room
temperature), therefore, the triplet state energy level of the energy donor and energy acceptors
are close to each other, that is, the T1 and T2 excited states of B-1 are degenerated. The triplet
excited state lifetime of B-1 was determined as 286.1 μs (c =1.0×10-6 M, toluene). TTA, or self-
quenching, was observed for B-1, that is, the triplet excited state lifetimes became much shorter
at higher concentration. For example, the triplet excited state lifetime decreased from 286.1 μs
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(1.0×10-6 M-1), to 170.4 μs (5.0×10-6 M-1) and 93.7 μs (1.0×10-5 M).
It was known that the triplet excited state of the unsubstituted Bodipy (without iodination)
cannot be populated upon photoexcitation, due to the lack of ISC.37-44 Since the triplet excited
state of the energy donor in B-1 was populated (indicated by the TA spectra), we propose that
following the forward singlet energy transfer from the energy donor to the energy acceptor
(iodo-Bodipy part), and in turn the ISC of energy acceptor, the backward triplet state energy
transfer from the iodo-Bodipy part to Bodipy part takes place, as a result, the triplet excited state
of the energy donor is populated.22,54,55 To the best of our knowledge, this is the first report of
singlet-triplet ping-pong energy transfer in an organic triplet photosensitizer. Previously singlet-
triplet ping-pong energy transfer were observed for transition metal complexes and C60-organic
chromophore dyads,22,36,50,51,52-55 For fluorescent RET molecular arrays, only uni-directional
forward singlet energy transfer from the energy donor to the energy acceptor is possible.25,28,30,32
Similar ping-pong energy transfer was observed for B-2 and B-3 (Figure S49). It is interesting
to note that the π-conjugation framework of the energy acceptor of B-3 (i.e. similar to compound
14) is larger than the energy donor, however, the ping-pong energy acceptor occurred at room
temperature, therefore, we propose that the energy level of the T1 and T2 states of B-3 are close
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to each other. This information is useful for study of the triplet state energy levels of organic
chromophores, as well as for designing of organic triplet photosensitizers.
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The absorbance of B-3 at 505 nm and 555 nm are 0.8762 and 0.6608 in the steady state
UV−vis absorption spectrum (Figure 1), respectively. In the time-resolved transient absorption
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spectra, the optical density at 505 nm and 555 nm are −0.0201 and −0.0103, respectively (Figure
S49, at 1.0×10-6 M). Thus the distribution of the triplet excited state on the Bodipy energy donor
and the iodo-Bodipy energy acceptor of B-3 is 60 % and 40 %, respectively (note the T1 state is
more localized on the singlet energy donor part, room temperature). Similarly, the distribution of
the triplet excited state of B-2 is 44 % and 56 %, on the Bodipy energy donor and the iodo-
Bodipy energy acceptor, respectively (T1 state is more localized on the singlet energy acceptor
part). With a simplified consideration, the population of the two degenerated triplet states (T1
and T2 states) is given by the Boltzmann distribution law (eq. 1):22
N1
N2
E1 − E2
= exp(−
)
(1)
kT
where k represents the Boltzmann constant (k = 1.38×10−23 J/K). Based on the relative
populations of the triplet excited states localized on the energy donor and acceptor, the apparent
energy gap between the T1 and T2 state were calculated as 0.0 meV, 6.0 meV and 5.0 meV for B-
1 – B-3, respectively. Note these considerations are simplified and as a result, these values can
only be treated quasi-quantitatively. These values are small and they are close to the porphyrin-
Pt-Bodipy hybrids with ping-pong energy transfer.22
The localization of T1 states of B-1, B-2 and B-3 were studied in more detail with temperature
dependent nanosecond time-resolved transient difference absorption spectroscopy (Figure
S60−S62). By decreasing the temperature from RT (20 °C) to (−30 °C) with a step of 10 °C (the
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temperature of the cuvette was controlled by Optistat DN-V variable temperature liquid N2
cryostat), the bleaching band corresponding to the iodo-Bodipy part (energy acceptor) were
generally intensified. Therefore, we propose that the T1 state of the dyads (B-1, B-2 and B-3) is
localized on the iodo-Bodipy part, i.e. the intramolecular singlet energy acceptor part. The
photophysical properties of the compounds were summarized in Table 1.
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Table 1. Photophysical parameters of the BODIPY dyads and the components a
a
b
a
λabs
504
537
532
555
ε
λem
517
556
557
586
ΦF / % c
90.0
3.6
τF d /ns
3.86
0.13
0.74
0.89
0.36
0.74
0.87
τT e / μs
f
7
9.69
8.67
11.36
7.90
−
228.9
228.3
204.2
286.1
241.6
262.2
9
12.7
14.5
4.3
13
14
B-1
B-2
B-3
505/537 8.98/6.97 554
504/533 11.04/8.58 577
506/556 8.78/6.62 584
12.8
14.3
a In toluene (1.0×10-5 M). In nm. b Molar absorption coefficient. ε: 104 M-1 cm-1. c Fluorescence
quantum yields. 9 (ΦF = 2.7 % in MeCN) was used as standard for B-1. 8 (ΦF = 3.6 % in MeCN)
d
e
was used as standard for B-2, 13, B-3 and 14. Fluorescence lifetimes. Triplet state lifetimes,
measured by transient absorptions at c = 1.0×10-6 M in toluene (the triplet state lifetime will be
greatly reduced at higher concentration). Not applicable.
f
It should be pointed out that the triplet excited state lifetimes of the compounds are dependent
on the concentration of the solution because the self-quenching, or the triplet-triplet annihilation
(TTA) is significant in fluid solution, especially for those compounds with long-lived triplet
excited states. Therefore, the triplet excited state lifetimes of the monomers 8 and 9 and the
dyads (B-1, B-2 and B-3) were studied at different concentration (5.0×10-6 M-1 and 1.0×10-5 M-1,
Figure S46-S52 and Table S1). The triplet excited state lifetimes became much shorter at higher
concentration (Table S1 in Supporting Information). For example, triplet excited state lifetimes
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of 8 decreased from 192.3 μs, to 123.2 μs and 66.2 μs, with the concentration increased from
1.0×10-6 M-1 to 5.0×10-6 M-1 and 1.0×10-5 M-1, respectively.
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The triplet excited state lifetimes of the dyads B-1, B-2 and B-3 were studied in solvents with
different polarity (Figure S55−S59) and were compared with the reference compounds 8 and 9.
Usually electron transfer will be more efficient in polar solvents.50,51 However, no substantial
variation of the triplet excited state lifetimes were found. The fluorescence of the dyads (B-1, B-
2 and B-3) in solvents with different polarity were also studied. The fluorescence of the dyads in
polar solvents such as CH3CN was not significantly quenched compared to that in less polar
solvent, such as toluene, or by comparison with the 2,6-diiodo-Bodipy or the 2-iodo-Bodipy
reference compounds. Therefore, we propose that the intramolecular electron transfers is not
significant.
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Electrochemical properties of B-1 and B-2 were studied (Figure S74 and S75, Table S3), with
the postulation that the singlet energy transfer and the ISC are much more faster than the
electron transfer, we found that the triplet excited states localized on the iodo-Bodipy part are
unable to oxidize the energy donor, because the free energy changes (ΔG°, calculated with the
Weller equation) for the electron transfer of B-1 was calculated as 0.46 eV. Electron transfer
from the singlet excited state of the energy donor to the energy acceptor is a thermodynamically
allowed (in this case the ΔG° values is −0.28 eV. See Figure S74 and Table S3 for more detail),
but the energy transfer may outcompete the photoinduced electron transfer albeit the photo-
induced electron transfer process is exergonic.34a Similar results were observed for B-2 (see
Figure S75 and Table S3 for more detail). Based on the studies with the excitation spectra and
the lifetimes, the electron transfer is not significant for the dyad triplet photosensitizers.
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2.4. DFT calculations on the photophysical properties of the triplet photosensitizers. The
triplet states of B-1, B-2 and B-3 can be rationalized from a different point of view, i.e. there
may exist energetically degenerated T1 and T2 states for the dyad triplet photosensitizers.52-54
In order to study the triplet excited states of the triplet photosensitizers, we carried out DFT
calculations on the spin density surfaces of the compounds (Figure 4).56-60 For B-1 and B-2, the
lowest triplet state is localized on the iodo-Bodipy part. For B-3, however, the spin density is
localized on the non-iodinated Bodipy part.
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B-1
B-2
B-3
Figure 4. Isosurfaces of spin density of B-1, B-2 and B-3. At the optimized triplet state
geometries. Toluene was used as solvents in the calculations. Calculation was performed at
B3LYP/6-31G(d) level with Gaussian 09W.
In order to study the energetically degenerated T1 and T2 excited states, the virtual S0→Tn
excitations (energy gap between the ground state and the triplet excited states) were calculated
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based on the optimized ground state geometry with the TDDFT method. The ground state
geometries, the UV-vis absorption and the S0−Tn energy gaps of the triplet photosensitizers were
calculated, the results of B-1 were presented in Figure 5 and Table 2 (for the results of B-2 and
B-3, please refer to Figure S76 and S77).
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The energy-minimized geometry of B-1 at ground state indicated the two chromophore in B-1
keeps away from each other (Figure 5). Furthermore, the phenyl moiety connected to the Bodipy
core takes perpendicular geometry against the π-core of the Bodipy chromophore. Note the
phenyl moiety cannot rotate freely due to the steric hindrance exerted by the 1,7-dimethyl groups
on the dipyrrinato unit, which are important to prohibit the non-radiative decay of the excited
state of the Bodipy.38,61 The same is true for all the dyads and the reference compounds.
The calculated UV−vis absorption bands (S0→S2 and S0→S3) are located at 463 nm and 432
nm, respectively, for which H−1→L and the H→L+1 are the respective major components of the
transitions. Based on the molecular orbitals (Figure 5), the transitions are localized on the iodo-
Bodipy and the Bodipy part of the dyad B-1, respectively. Therefore the absorption of B-1 in
visible region can be rationalized by the TDDFT calculation.
The singlet excited states of B-1 were also optimized and the singlet excited state localized on
iodo-Bodipy shows lower energy level than that of the un-substituted Bodipy. Therefore, the
RET effect of B-1 upon photoexcitation can be rationalized (see Figure S78 for the molecular
orbitals at the singlet excited states).62 We noted that there exist a charge transfer transition
(S0→S1), which is unlikely to be observed in the UV−vis absorption spectrum because the
oscillator strength is 0.
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Figure 5. Selected frontier molecular orbitals involved in the excitation, emission and triplet
excited states of B-1. CT stands for conformation transformation. Note the T1 and T2 state are
fully degenerated. The calculations are at the B3LYP/6-31G(d)/ level using Gaussian 09W.
The singlet excited states of B-2 were also optimized (Figure S75). The singlet excited state
localized on iodo-Bodipy (S2 state) has a lower energy level than the singlet excited state
localized on the un-iodinated Bodipy (the singlet energy donor. Figure S76). Thus the RET
effect was rationalized with the TDDFT calculations. We noted that there exists a dark excited
state (S1→S0). The excitation energy for this singlet excited state may be underestimated
because DFT calculations is known to underestimate the excitation energy for charge transfer
excited states.63,64
TDDFT calculations give two degenerated low-lying triplet excited state, T1 (1.522 eV) and T2
(1.524 eV), which are localized on the iodo-Bodipy and the unsubstituted Bodipy part,
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respectively (Figure 5 and Table 2), which is responsible for the establishment of the triplet state
equilibrium.52-54,65,66 The prediction of the degenerated T1 and T2 states by TDDFT calculations
is in full agreement with the transient difference absorption spectra (Figure 3).44 Similar
DFT/TDDFT calculation results were obtained for B-2 and B-3 (Table S4 and S5). The triplet
excited state energy levels of the reference compounds of 3, 6 and 9 were calculated (see
Supporting Information). The T1 state energy levels were calculated as 1.52 – 1.53 eV.
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Table 2. Selected parameters for the UV-vis absorption and fluorescence emission of the
compounds. Electronic excitation energies (eV) and oscillator strengths (f), configurations of the
low-lying excited states of B-1. Calculated by TDDFT//B3LYP/6-31G(d), based on the
optimized ground state geometries (methanol was used as solvent in the calculation).
Electronic
transition a
TDDFT/B3LYP/6-31G(d)
b
Excitation energy
2.55 eV (486 nm)
2.67 eV (464 nm)
f
Composition c CI d
0.0000
0.6404
0.7071
S0 → S1
S0 → S2
H → L
Absorption
H−4 →L
H−1 → L
H−5 → L+1
H→L+1
0.1801
0.6853
2.86 eV (433 nm)
0.6306
0.1116
0.7017
S0 → S3
Emission
1.97 eV (628 nm)
2.66 eV (466 nm)
0.0000
0.2771
0.7071
S1 → S0
S2 → S0
H →L
H−3 → L+1
H−1 → L
H → L+1
H−4 → L
H−1 →L
0.4015
0.5829
2.98 eV (415 nm)
1.52 eV (815 nm)
0.4814
0.0000
0.7066
S3 → S0
T0 → T1
0.1540
0.6967
Triplet
states
1.52 eV (814 nm)
0.0000
0.7105
T0 → T2
H → L+1
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Only selected excited states were considered. The numbers in parentheses are the excitation
energy in wavelength. b Oscillator strength. c H stands for HOMO and L stands for LUMO. Only
the main configurations are presented. Coefficient of the wavefunction for each excitations.
d
The CI coefficients are in absolute values.
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2.5. Photooxidation of 1,5-dihydroxylnaphthalene mediated by singlet oxygen (1O2)
photosensitizing. In order to study of the effect of enhanced visible light-absorbance on the
photosensitizing ability of the new triplet photosensitizers, the compounds were used as singlet
oxygen (1O2) photosensitizer for photooxidation, with 1,5-dihydroxylnaphthalene (DHN) as the
1O2 scavenger to produce jugalone,17,67,68 a versatile intermediate for preparation of bioactive
compounds.69 Currently photocatalytic preparative organic reaction is a vibrant research area,
but most of the photosensitizers used are conventional transition metal complex triplet
photosensitizers that show only weak to moderate absorption in visible region.1-6,70
2.0
1.5
1.0
0.5
0.0
2.0
1.5
1.0
0.5
0.0
40 min
35 min
...
4 min
2 min
0 min
40 min
35 min
...
4 min
2 min
0 min
b
a
300
400
500
600
300
400
500
600
Wavelength / nm
Wavelength / nm
1.5
1.2
0.9
0.6
0.3
0.0
1.5
1.2
0.9
0.6
0.3
0.0
40 min
35 min
...
4 min
2 min
0 min
40 min
35 min
...
4 min
2 min
0 min
d
c
300
400
500
600
300
400
500
600
Wavelength / nm
Wavelength / nm
Figure 6. UV-Vis absorption changes for the photooxidation of DHN with difference triplet
photosensitizers (a) 9; (b) B-1; (c) B-3; (d) 14 (In CH2Cl2/MeOH, 9 : 1, v/v). c [photosensitizers]
= 5.0×10-6 M. c [DHN] = 1.0×10-4 M. Light power: 20 mW/cm2 (35 W xenon lamp). 20 °C.
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Upon white light photoirradiation, the absorption of DHN at 301 nm decreased due to its
5
6
1
oxidation by O2, produced by the triplet photosensitizer, at the same time the absorption of
7
8
9
product (juglone) at 427 nm increased (Figure 6).67 All the new triplet photosensitizers induced
drastic UV absorption changes upon photoirradiation. Furthermore, the absorption of triplet
photosensitizer does not change with the continuous photoirradiation, therefore, the
photostability of the triplet photosensitizers is good. The photosensitizing with the conventional
transition metal complex triplet photosensitizer Ir(ppy)3 (ppy = 2-phenylpyridine), which was
used for photooxidation of DHN,67 as well as some conventional organic triplet photosensitizers
such as TPP and MB, were also studied (Figure S63).
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0.0
Ir-1
MB
-0.2
-0.4
-0.6
TPP
RB
B-1
9
B-2
13
B-3
6
14
0
2
4
8
10
Irradiation time / min
Figure 7. Plots of ln(At/A0) vs. irradiation time for the photooxidation of DHN using different
triplet photosensitizers. c [sensitizers] = 5.0×10-6 M. c [DHN] = 1.0×10-4 M. InCH2Cl2/MeOH
(9/1, v/v), Light power: 20 mW/cm2. 20 °C.
The photoreaction rate can be quantitatively evaluated by plotting of ln(A/A0) against the
irradiation time (Figure 7). The new triplet photosensitizers with enhanced absorption of visible
1
light show higher O2 photosensitizing ability than the mono-chromophore based triplet
photosensitizers. For example, compound B-1 is more efficient than compound 9 for the
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photooxidation of DHN. Similar enhanced photosensitizing was observed for B-3 compared to
14. Thus the enhanced visible light absorption of B-1 and B-3 are beneficial for enhancement of
the photosensitizing ability of the triplet photosensitizers. However, the photosensitizing ability
of B-2 is similar to that of reference compound 13.
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Table 3. Pseudo-first-order kinetics parameters, yields of juglone and singlet oxygen quantum
yields for the photooxidations of DHN using different triplet photosensitizers.
Yield / % c
b
kobs / min-1 a
55.5
vi
d
Φ△
5.55
6.47
6.02
6.07
6.43
5.67
3.87
6.41
3.33
0.28
99.9
99.9
99.0
99.9
89.9
86.3
99.9
71.6
81.4
15.1
0.87
0.65
0.50
0.89
0.67
0.89
0.62 e
0.80 f
0.57 g
−
9
64.7
B-1
B-2
13
60.2
60.7
64.3
B-3
14
56.7
38.7
TPP
RB
64.1
33.3
MB
2.8
Ir(ppy)3
a Photoreaction rate constants. 10-3 min-1. b Initial rate. 10-5 M. in min-1. c Yield of juglone after
d
photoirradiation for 40 min (30 min for RB and 35 min for B-3). Quantum yield of singlet
oxygen (1O2), with Rose Bengal (RB) as standard (ΦΔ = 0.80 in Methanol). e Literature values.68
f Literature values.3 g Literature values.2
It should be pointed out that the apparent photosensitizing ability of the triplet
photosensitizers can be affected by many factors, such as the spectrum of the irradiation light
source, the match between this emission spectrum and the absorption spectra of triplet
photosensitizers. In order to rule out these uncertainties, the photosensitizing ability of the
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photosensitizers were studied with monochromatic light excitation at the absorption band of the
energy donors, and the performance of the dyad triplet photosensitizers (B-1 – B-3) were
compared with the reference compounds that do not contain the energy donor (Figure 8). 1,3-
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1
diphenylisobenzofuran (DPBF) was used as the O2 scavenger. Compounds B-1 and 9 were
excited at 500 nm, where the energy donor in B-1 gives strong absorption than the reference
compound 9. The results show that the photosensitizing ability of B-1 is higher than that of
compound 9 upon 500 nm monochromatic light irradiation. This result proved the effect of
intramolecular energy transfer in triplet photosensitizer B-1 on improving the photosensitizing
ability of the compounds. Similar results were found for B-2/13 and B-3/14 (Figure S64). To the
best of our knowledge, this is the first report that triplet photosensitizers was enhanced with the
RET effect, with which strong absorption in visible region was fulfilled. Recently iodo-aza
Bodipys were used as triplet photosensitizers for photooxidation of DHN, but the triplet
photosensitizers contain only one chromophore, no RET effect was used to enhance the visible
light-absorption, thus the photosensitizing ability.17
1.2
1.0
9
0.8
B-1
0.6
0
20
40
Time / s
60
Figure 8. Verification of the light-harvesting effect of B-1 by plotting of the absorption changes
of 1,3-diphenylisobenzofuran (DPBF) at 414 nm vs. photoirradiation time for B-1 and 9 (in
CH2Cl2/MeOH, 9 : 1, v/v). Excited at 500 nm. c [triplet photosensitizers] = 4.0×10-6 M. c [DPBF]
= 4.3×10-5 M. 20 °C.
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The enhanced photoxidation of the dyad triplet photosensitizers is due to the intramolecular
energy transfer, which requires that the energy donor and energy acceptor are in close vicinity.
Mixing of the energy donor and the energy acceptor components in solution did not improve the
photooxidation, although the UV−Vis absorption of the mixed solution is close to that the dyad
triplet photosensitizers (Figure S65-66), because in this case the energy transfer does not occur.
These postulations were confirmed by photooxidation experiments by mixing the components of
the dyads mechanically in solution, the photooxidation was not improved compared to the di-
iodo-Bodipy (the energy acceptor) alone (Figure S65-66).
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2.6. Application of the triplet photosensitizers in triplet-triplet annihilation upconversion. The
dyads show strong absorption of visible light and long-lived triplet excited states, therefore the
dyad triplet photosensitizers were used for TTA upconversion.
TTA upconversion has attracted much attention, due to its advantages of low excitation power
(non-coherent light), strong absorption of excitation light and high upconversion quantum
yields.13,14,71-76 Triplet photosensitizers are crucial for TTA upconversion. Currently most of the
triplet photosensitizers are Pt(II)/Pd(II) porphyrin complexes. Recently, we have developed a
series of Ru(II), Ir(III), Pt(II) and Re(I) complexes as triplet photosensitizers for TTA
upconversion.14,60 These complexes show strong absorption of visible light and long-lived triplet
excited states, which are different from the conventional transition metal complexes. We also
prepared iodo-/bromo- chromophores, or organic chromophore-C60 dyads as heavy atom-free
triplet photosensitizers for TTA upconversion.44,77 The molecular structures of all these known
triplet photosensitizers are based on the mono light-harvesting chromophore profile, no triplet
photosensitizers with RET effect were used for TTA upconversion.13-16
The TTA upconversion with B-1 and 9 as triplet photosensitizers were studied (Figure 9). The
fluorescence emission of the photosensitizers alone at 554 nm was observed. In the presence of
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triplet acceptor perylene, intense blue emission in the range of 448 − 472 nm were observed.
Excitation of the photosensitizer or the triplet acceptor alone did not produce the upconverted
emission in blue region, thus the TTA upconversion can be verified. The upconversion quantum
yield with B-1 as the triplet photosensitizer was determined as 8.1%, which is higher than the
TTA upconversion with 9 as the triplet photosensitizer (ΦUC = 7.5%). The TTA upconversion
with other triplet photosensitizers B-2 and B-3, as well as the reference compounds 13 and 14
were also studied, and similar results were observed (Supporting Information and Table 4). We
found that the upconversion quantum yield of B-1 is dependent on the concentration in the range
of 1.0×10-6 M − 2.0×10-5 M (Figure S71).
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a
*
b
*
9 + Perylene
B-1 + Perylene
B-1
9
400 500 600 700 800
Wavelength / nm
400 500 600 700 800
Wavelength / nm
Figure 9. TTA upconversion with (a) B-1 and (b) 9 as the triplet photosensitizers and perylene
as the triplet acceptor/emitter. Excited with 532 nm CW laser (2 mW, power density: 28 mW
cm-2). The asterisks in a and b are the scattered laser. The asterisks indicate the scattered laser.
In toluene: c [B-1] = 1.0 × 10-6 M, c [perylene] = 6.6 × 10-6 M, 20 °C.
It should be pointed out that the TTA upconversion quantum yield is different from the
fluorescence quantum yield since the TTA upconversion quantum yield is dependent on the
concentration of the photosensitizer. The reason for the concentration dependency is that the two
crucial steps in TTA upconversion, i.e. the triplet-triplet-energy-transfer (TTET) and the TTA,
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are highly dependent on the concentration. As a proof of this analysis, higher TTA upconversion
quantum yields were observed with the triplet photosensitizers at higher concentration (1.0 × 10-
5 M, Figure S67 and Table S2).
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In order to study the origin of the different TTA upconversion quantum yields, the quenching
of the triplet state lifetime of the photosensitizers by the triplet energy acceptor perylene were
studied (Figure 10). Quenching constant of 6.3 × 106 M-1 was observed for B-1, which is much
higher than that of 9 (2.1 × 106 M-1). This different quenching constants is due to the different
triplet state lifetimes of B-1 and 9. Similar profiles were observed for B-2 and 13. These results
indicate that the long-lived triplet excited states of the triplet photosensitizers is beneficial for
TTA upconversion. The quenching studies were also carried out at higher photosensitizer
concentrations. Much larger quenching constants were observed (Supporting Information, Figure
S68 and Table S2). These results indicate that the triplet-triplet-energy-transfer process is highly
dependent on the concentration of the triplet photosensitizers.
100
B-1
B-2
80
B-3
9
13
60
14
40
20
0
5.0x10-6 1.0x10-5 1.5x10-5
0.0
c[quencher]/M
Figure 10. Stern-Volmer plots for the lifetime quenching of the triplet photosensitizers with
increasing the concentration of perylene. c [photosensitizers] = 1.0 × 10-6 M, in deaerated
toluene. The triplet excited state lifetimes were measured with transient absorption spectrum
upon 532 nm nanosecond laser excitation. 25 °C.
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In order to unambiguously verify the TTA upconversion, the luminescence lifetime of the
blue emission band in Figure 9 were measured (Figure 11). The luminescence time of the
upconverted emission with B-1 as triplet photosensitizer was determined as 493.6 μs (Figure
11a). In comparison the prompt fluorescence of perylene at the same emission wavelength was
determined as 3.9 ns (Figure 11b). Therefore the TTA upconversion of the mixed solution of B-
1/pyrelene was verified by the exceptionally long-lived delayed fluorescence.73,77,78 Similar long-
lived fluorescence lifetimes were observed for the TTA upconversion with other triplet
photosensitizers (Table 3 and Figure S73). The delayed fluorescence lifetime of the TTA
upconversion is highly dependent on the concentration of the photosensitizers and the triplet
acceptor perylene. For example, at higher concentration (1.0 × 10-5 M), the delayed fluorescence
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lifetimes are substantially shortened (Supporting Information, Figure S72). For B-1, the τDF
=
93.2 μs (Figure S72 and Table S2).
1000
a
b
1000
100
τ
DF = 493.6 μs
τ = 3.9 ns
100
0
20 40 60 80 100
Time / ns
0
1000 2000 3000
Time / μs
Figure 11. (a) Delayed fluorescence observed in the TTA upconversion with B-1 (perylene as
the triplet acceptor). Excited at 532 nm (nanosecond pulsed OPO laser) and the lifetime was
obtained by monitoring the decay of the luminescence at 450 nm. (b) Prompt fluorescence decay
of perylene determined in the different experiment, excited with EPL picosecond 405 nm laser,
the decay of the emission was monitored at 450 nm (in deaerated toluene). c [triplet
photosensitizers] = 1.0× 10-6 M; c [perylene] = 6.7 × 10-6 M; 25 °C.
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Table 4. Triplet Excited State Lifetimes (τT), Stern-Volmer Quenching Constant (Ksv), and
Bimolecular Quenching Constants (kq) of the Chromophore Dyads a
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kq (1010 M-1S-1)
Ksv(106 M-1)
ΦUC
%
η / (103 M-1 cm-1) c
τ
DF (μs) d
b
τT (μs)
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286.1 6.3
228.9 2.1
241.6 1.6
228.3 1.0
262.2 3.7
204.2 1.7
2.2
0.9
0.6
0.4
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8.1
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453.2
442.6
B-1
9
B-2
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B-3
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a
Perylene was used as the quencher. All the data were obtained with photosensitizer
concentration at 1.0 × 10-6 M. In deaerated toluene solution, 20 ºC. b Excited with 532 nm laser,
with the prompt fluorescence of Rhodamine B as the standard. c Overall upconversion capability,
η = ε × ΦUC, where ε is the molar extinction coefficient of the triplet photosensitizer at the
excitation wavelength and ΦUC is the upconversion quantum yield. In M-1 cm-1. Delayed
d
fluorescence observed in the TTA upconversion with triplet photosensitizer (perylene as the
triplet acceptor). Excited at 532 nm (nanosecond pulsed OPO laser) and the lifetime was
monitored at 450 nm emission.
The photophysical process involved in the RET visible light-absorbing triplet photosensitizers
are summarized in Scheme 2. For the dyad triplet photosensitizers (B-1, B-2 and B-3), both the
intramolecular energy donor and the energy acceptor can be excited with visible light, but at
different wavelength. Singlet energy transfer from the energy donor to energy acceptor occurs.
In turn the ISC takes place, facilitated by the heavy atom effect of the iodo atoms attached on the
Bodipy π-core of the energy acceptor. We have demonstrated that the enhanced visible light-
absorption and the long-lived triplet states are beneficial for the application of the compounds in
singlet oxygen (1O2)-mediated photooxidation and TTA upconversion.
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Scheme 2. Jablonski diagram of the photophysical processes of the triplet photosensitizers and
the TTA upconversion Exemplified with B-1 as the triplet photosensitizer
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Perylene Annihilator
Dyad Photosensitizers
TTET
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1Perylene* (hot)
Effect of
Effect of τ on
light-harvesting and ISC
TTA efficiency
1Perylene*
1(*D-A)
1(D-*A)
RET
TTA
3perylene*
3(*D-*A)
TTET
ISC
Upconverted
Fluorescence
2.46 eV
505 nm
2.24 eV
554 nm
1.53 eV
810 nm
E
1.53 eV
810 nm
2.31 eV
537 nm
2.76 eV
450 nm
GS
The effect of the light-harvesting ability and the lifetimes of the photosensitizers on the
efficiency of TTA upconversion are shown (please note that the vibration energy levels of each
1
electronic state are omitted for clarity). E is energy. GS is ground state (S0). (*D-A) is singlet
excited state localized on energy donor Bodipy, and 1(D-*A) is singlet excited state localized on
energy acceptor (iodo-Bodipy). RET is resonance energy transfer. ISC is intersystem crossing.
3(*D-*A) is the triplet excited state localized on both the energy donor Bodipy and the energy
acceptor iodo-Bodipy (triplet excited states equilibrium. The backward triplet energy transfer
from the iodo-Bodipy part to the Bodipy part is omitted). TTET is triplet-triplet energy transfer.
3perylene* is the triplet excited state of perylene. TTA is triplet-triplet annihilation. 1perylene* is
the singlet excited state of perylene (initially the hot, or the vibrationally excited S1 state will be
1
populated). The emission band observed in the TTA experiment is the perylene* emission
(fluorescence). The typical power density of the laser used in the upconversion is 28 mW cm-2,
too low to observe simultaneous two-photon absorption.
2.7. Conclusions. In conclusion, resonance energy transfer (RET) was used for the first time
to enhance the visible light absorption of triplet photosensitizers. Conventional triplet
photosensitizers are based on mono-chromophore profile, as a result there is only one major
absorption band in the visible spectral region. We prepared Bodipy-iodo Bodipy dyads which
show enhanced visible light absorption, i.e. there are two major absorption bands in visible
region. Steady state and time-resolved spectroscopy confirmed the intramolecular energy
transfer in the new triplet photosensitizers. Furthermore, nanosecond time-resolved transient
difference absorption spectra indicates that the triplet excited states of the dyads are distributed
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on both the energy donor and the energy acceptor. Therefore, we propose a forward singlet
energy transfer from the Bodipy energy donor to the iodo-Bodipy (energy acceptor), followed by
ISC of iodo-Bodipy moiety, and in turn the backward triplet energy transfer to the Bodipy part.
Such a ‘ping-pong’ energy transfer was unprecedented for organic RET molecular arrays. The
triplet photosensitizers were used for singlet oxygen (1O2)-mediated photooxidation and the
photosensitizing ability of the dyad triplet photosensitizers are higher than the mono-
chromophore-based triplet photosensitizers. Furthermore, the triplet-triplet annihilation
upconversion with the RET triplet photosensitizer is improved compared to that with the mono-
chromophore triplet photosensitizers. Our method to prepare triplet photosensitizers, which show
enhanced visible light absorbing, based on resonance energy transfer (multi-chromophore with
matched energy level for energy transfer), is useful for designing of efficient triplet
photosensitizers and for the application of these compounds in photocatalysis, photodynamic
therapy, photovoltaics, and/or preparative photocatalytic organic reactions. We are actively
working along this line in our laboratory.
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3. EXPERIMENTAL SECTION
3.1. General Methods. Fluorescence lifetimes were measured on OB920 spectrometer
(TCSPC. Edinburgh instruments, UK). Nanosecond time-resolved transient absorption spectra
were measured on LP920 laser flash photolysis spectrometer (Edinburgh Instruments, UK). The
temperature of the sample cuvette in the transient absorption measurement were controlled with
Optistat DN-V liquid nitrogen cryostat (Oxford, UK). All the samples in laser flash photolysis
experiments were deaerated with Ar for ca. 15 min before measurement and the gas flow is kept
during the measurement. The compounds 1-12 were prepared following the reported methods
(see Supporting Information).
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