Energy-funneling-based broadband visible-light-absorbing Bodipy-C 60 triads and tetrads as dual functional heavy-atom-free organic triplet photosensitizers for photocatalytic organic reactions

Article abstract of DOI:10.1002/chem.201302492

C₆₀-bodipy triads and tetrads based on the energy-funneling effect that show broadband absorption in the visible region have been prepared as novel triplet photosensitizers. The new photosensitizers contain two or three different light-harvesting antennae associated with different absorption wavelengths, resulting in a broad absorption band (450-650 nm). The panchromatic excitation energy harvested by the bodipy moieties is funneled into a spin converter (C₆₀), thus ensuring intersystem crossing and population of the triplet state. Nanosecond time-resolved transient absorption and spin density analysis indicated that the T₁ state is localized on either C₆₀ or the antennae, depending on the T₁ energy levels of the two entities. The antenna-localized T₁ state shows a longer lifetime (τ_T=132.9 μs) than the C₆₀-localized T₁ state (ca. 27.4 μs). We found that the C₆₀ triads and tetrads can be used as dual functional photocatalysts, that is, singlet oxygen (¹O₂) and superoxide radical anion (O ₂^.-) photosensitizers. In the photooxidation of naphthol to juglone, the ¹O₂ photosensitizing ability of the C₆₀ triad is a factor of 8.9 greater than the conventional triplet photosensitizers tetraphenylporphyrin and methylene blue. The C ₆₀ dyads and triads were also used as photocatalysts for O ₂^.--mediated aerobic oxidation of aromatic boronic acids to produce phenols. The reaction times were greatly reduced compared with when [Ru(bpy)₃Cl₂] was used as photocatalyst. Our study of triplet photosensitizers has shown that broadband absorption in the visible spectral region and long-lived triplet excited states can be useful for the design of new heavy-atom-free organic triplet photosensitizers and for the application of these triplet photosensitizers in photo-organocatalysis. Fullerene photocatalysts: Broadband visible-light- absorbing, heavy-atom-free triplet photosensitizers based on bodipy-C ₆₀ dyads, triads, and tetrads have been prepared (see figure). Resonance energy transfer or intramolecular funneling was used to achieve panchromatic absorption in the visible region. The triplet photosensitizers were used as dual functional photo-organocatalysts to produce ¹O ₂ or O₂^.-. The ¹O ₂ sensitizing ability of the new photosensitizers is up to a factor of 8.9 greater than other known triplet photosensitizers based on a monochromophore such as methylene blue.

Full text of DOI:10.1002/chem.201302492

FULL PAPER
DOI: 10.1002/chem.201302492
Energy-Funneling-Based Broadband Visible-Light-Absorbing Bodipy–C60
Triads and Tetrads as Dual Functional Heavy-Atom-Free Organic Triplet
Photosensitizers for Photocatalytic Organic Reactions
Ling Huang,[a] Xiaoneng Cui,[a] Bruno Therrien,[b] and Jianzhang Zhao*[a]
Abstract: C60–bodipy triads and tetrads
based on the energy-funneling effect
that show broadband absorption in the
visible region have been prepared as
novel triplet photosensitizers. The new
photosensitizers contain two or three
different light-harvesting antennae as-
sociated with different absorption
wavelengths, resulting in a broad ab-
sorption band (450–650 nm). The pan-
chromatic excitation energy harvested
by the bodipy moieties is funneled into
a spin converter (C60), thus ensuring in-
tersystem crossing and population of
the triplet state. Nanosecond time-re-
solved transient absorption and spin
density analysis indicated that the T1
state is localized on either C60 or the
antennae, depending on the T1 energy
levels of the two entities. The antenna-
localized T1 state shows a longer life-
time (tT =132.9 ms) than the C60-local-
ized T1 state (ca. 27.4 ms). We found
that the C60 triads and tetrads can be
used as dual functional photocatalysts,
that is, singlet oxygen (1O2) and super-
let photosensitizers tetraphenylpor-
phyrin and methylene blue. The C60
dyads and triads were also used as pho-
Cꢀ
tocatalysts for O2 -mediated aerobic
oxidation of aromatic boronic acids to
produce phenols. The reaction times
were greatly reduced compared with
when [RuACHTUNTRGNEUNG(bpy)3Cl2] was used as photo-
Cꢀ
oxide radical anion (O2 ) photosensi-
catalyst. Our study of triplet photosen-
sitizers has shown that broadband ab-
sorption in the visible spectral region
and long-lived triplet excited states can
be useful for the design of new heavy-
atom-free organic triplet photosensitiz-
ers and for the application of these
triplet photosensitizers in photo-orga-
nocatalysis.
tizers. In the photooxidation of naph-
1
thol to juglone, the O2 photosensitiz-
ing ability of the C60 triad is a factor of
8.9 greater than the conventional trip-
Keywords: fullerenes · photochem-
istry · photooxidation · sensitizers ·
UV/Vis spectroscopy
Introduction
let photosensitizer may eliminate completely the ISC.[9f,16]
Therefore it would be highly desirable to develop a general
molecular structure for heavy-atom-free organic triplet pho-
tosensitizers that can be easily functionalized with a predict-
AHCTUNGTREGaNNUN ble ISC effect. This goal remains a significant challenge in
photochemistry.
All current triplet photosensitizers are based on mono-
chromophores.[4,5,8–10,17] These photosensitizers possess only
one major absorption band in the visible region, thus cover-
ing only a small part of the visible spectral range. This is
detrimental to photocatalysis with panchromatic light excita-
tion, such as with solar light. Triplet photosensitizers with
panchromatic or broadband absorption have not been repor-
ted,[9c,10,17] although this kind of photosensitizer can efficient-
ly harvest panchromatic light excitation. To address a similar
problem, molecular dyads or triads that exhibit intramolecu-
lar energy funneling (such as fluorescence resonance energy
transfer, FRET) have been used in fluorescent molecular
arrays.[18–20] However, to the best of our knowledge, this
strategy has never been used with triplet photosensitizers to
access broadband absorption in the visible range.
Triplet photosensitizers are widely used in photocatalytic or-
ganic reactions,[1,2] photovoltaics,[3] photodynamic thera-
py,[4–10] and triplet–triplet annihilation (TTA) upconver-
sion.[11–15] Triplet photosensitizers usually contain heavy
atoms such as PtII, IrIII, or iodine.[1,2,9,14,15] The heavy-atom
effect facilitates intersystem crossing (ISC), which produces
a triplet excited state.[9,10,16] However, it is difficult to popu-
late the triplet state of organic chromophores lacking heavy
atoms.[8f,9,16] Derivatization of a known fluorophore, a popu-
lar strategy for preparing new fluorescent molecules, is not
applicable for the design of new organic triplet photosensi-
tizers because subtle derivatizations of a known organic trip-
[a] L. Huang, X. Cui, Prof. J. Zhao
State Key Laboratory of Fine Chemicals
E-208, West Campus
Dalian University of Technology
Dalian 116024 (P.R. China)
Fax: (+86)411-8498-6236
E-mail: zhaojzh@dlut.edu.cn
To overcome the aforementioned challenges, and inspired
[b] Prof. B. Therrien
1
by the seminal work of O2 photosensitization by an intra-
Institute of Chemistry, University of Neuchꢀtel
Avenue de Bellevaux 51, 2000 Neuchꢀtel (Switzerland)
molecular energy transfer approach using porphyrin as the
energy acceptor and a two-photon-absorbing chromophore
as the antenna,[21] herein we report C60–bodipy triads and
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201302492.
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Scheme 1. Synthesis of triad C-2 and the reference compounds. The model triplet photosensitizers [Ir
and methylene blue (MB) are also shown. Reagents and conditions: (i) 4-OHCC6H4B(OH)2 (0.6 mmol), 5 mol% [Pd
ethanol/water, 2:2:1, v/v, reflux, 8 h, 85%; (ii) 1 equiv N-iodosuccinimide (NIS), RT, 61%; (iii) 5 (prepared from 4, for this and other procedures, see the
ACHUTGTNNERNUG(ppy)2bpy]ACHTGNUTRENNUNG
AHCTUNGTRENNUNG
Supporting Information), 5 mol% [Pd
toluene, reflux, 12 h, 73%.
ACHTNUGTRNENUN(G PPh3)4], 3 equiv K2CO3, toluene/ethanol/water, 2:2:1, v/v, reflux, 8 h, 66%; (iv) 4.0 equiv N-methylglycine, C60, in
tetrads as heavy-atom-free triplet photosensitizers that show
broadband absorption in the visible range (Schemes 1 and
2). These photosensitizers are based on a multiple intramo-
lecular energy funneling effect from the antennae to the C60
unit. In addition, and without the presence of any heavy
atoms, the triplet excited states of the triads and tetrads are
populated upon photoexcitation. Furthermore, we found
that the compounds can be used as dual functional photore-
Results and Discussion
Design and synthesis of C60–bodipy triplet photosensitizers:
The molecular design is based on the ISC capability of
C60.[22] However, C60 itself is not an ideal triplet photosensi-
tizer because of its weak absorption in the visible region.
Therefore light-harvesting antennae were attached to C60 to
enhance its absorption.[22–31] Previously, C60 compounds with
strong visible-light or panchromatic absorption have been
prepared, but their triplet-state populations and photocata-
lytic properties have not been reported.[23–26] In this work,
bodipy was selected as an antenna due to its versatile deri-
vatization and satisfactory photophysical properties.[24,32,33]
Intramolecular energy transfer from the bodipy antenna to
C60 is expected, which in return produces a triplet excited
state of C60.[30a,31,34–38] To broaden the absorption band of C60
molecular arrays in the visible spectral range, multichromo-
phores of styryl-bodipy, 2-phenyl-bodipy, or ethynylated
bodipy units were used to construct triads and tetrads
(Schemes 1 and 2). These C60–bodipy hybrids give broad ab-
sorption bands in the range 450–650 nm.[39,40] Dyad C-1 and
triad C-2 were prepared in moderate yields from bromo-
bodipy and C60 (Scheme 1 and the Supporting information)
and one to three bodipy units were used for the preparation
of dyad C-3, triad C-4, and tetrads C-5 and C-6 (see
Scheme 2).
dox catalysts in various photocatalytic reactions, such as
1
Cꢀ
O2- and O2 -mediated photocatalytic reactions. In the pho-
tocatalytic oxidative coupling reactions of amines, the new
triplet photosensitizers are a factor of 8.9 more efficient as
singlet oxygen (1O2) photosensitizers than conventional trip-
let photosensitizers based on a monochromophore, such as
tetraphenylporphyrin (TPP) or methylene blue (MB). Fur-
thermore, in the photocatalytic oxidation of aromatic boron-
Cꢀ
ic acids to produce functionalized phenols (via O2 photo-
sensitization), the reaction time was reduced to less than 4 h
from the previously reported 69–96 h required with a con-
ventional RuII–polyimine photocatalyst, and good functional
Cꢀ
group tolerance was found for the O2 -mediated photooxi-
dation of arylboronic acids. These results will be useful for
the future development of efficient triplet photosensitizers,
the photochemistry of fullerenes, and the application of C60–
organic hybrids as photocatalysts in various photocatalytic
reactions.
The weak absorption of C60 in the visible range (e<
200mꢀ1 cmꢀ1 beyond 500 nm) does not deny the efficient in-
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Bodipy–C60 Triads and Tetrads
FULL PAPER
Scheme 2. Structures of C60-bodipy tetrads C-5 and C-6 and the reference compounds. Reagents and conditions: (i) 2 equiv 4-formylphenylboronic acid,
5 mol% [Pd(PPh3)4], 3 equiv K2CO3, toluene/ethanol/water, 2:2:1, v/v, reflux, 8 h, 82%; (ii) RT, 2 equiv NIS, 95%; (iii) 1.2 equiv compound 4, 5 mol%
[Pd(PPh3)4], 3 equiv K2CO3, toluene/ethanol/water, 2:2:1, v/v, reflux, 8 h, 68%; (iv) 4.0 equiv N-methylglycine, C60, reflux, 12 h, toluene, 66%. For de-
tailed procedures, see the Supporting Information.
tramolecular energy transfer from the bodipy antenna to the
C60 unit. The weak absorption of C60 at 500–700 nm is due to
symmetry-forbidden S0!Sn transitions,[25] which do not
occur in intramolecular energy transfer from an antenna to
the intramolecular energy acceptor C60.
absorption band at 629 nm (e=83000mꢀ1 cmꢀ1). These two
absorption bands can be attributed to the bis-bodipy anten-
na in which two different chromophores exist (C-2-L is the
reference compound for the antennae in C-2). As a refer-
ence, the C60–styryl dyad C-1 was prepared and, as expected,
it shows only one strong absorption band centered at
629 nm.[38b] The broader absorption band of triad C-2 rela-
tive to dyad C-1 is beneficial for the application of C60–
bodipy arrays as broadband light-absorbing triplet photosen-
sitizers. Similar absorption profiles were observed for the
triad C-4 and the tetrads C-5 and C-6 (see Figure S64 in the
Supporting Information).
UV/Vis absorption and fluorescence emission spectra: The
UV/Vis absorption spectra of the dyad C-3 and triad C-2, as
well as the reference compounds bodipy and C60 were re-
corded in toluene (Figure 1). In comparison with C60, dyad
C-3 shows
a
strong absorption at 517 nm (e=
76000mꢀ1 cmꢀ1), which can be attributed to the bodipy an-
tenna (Figure 1a). Similarly, triad C-2 shows a strong ab-
sorption band at 517 nm (e=61000mꢀ1 cmꢀ1) with a second
The fluorescent properties of the triads were also studied
(Figure 2 and the Supporting Information).[31] The emission
Figure 1. a) UV/Vis absorption spectra of C60, bodipy, and the C60–bodipy
dyad C-3. b) UV/Vis absorption spectra of C60 and C60–bodipy triads C-2
and C-2-L (toluene, 1.0ꢂ10ꢀ5 m, 208C).
Figure 2. a) Fluorescence spectra of C-2 and C-2-L (lex =470 nm).
b) Fluorescence excitation (lem =640 nm) and UV/Vis absorption spectra
of C-2 (toluene, 1.0ꢂ10ꢀ5 m, 208C).
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J. Zhao et al.
of the antenna was significantly quenched in C-2 (Fig-
ure 2a), otherwise the styryl-bodipy antenna in C-2 would
show fluorescence emission similar to C-2-L. The quenching
of the emission band at 628 nm in the triad is due to intra-
molecular energy transfer from the antenna to the C60 unit.
Intramolecular electron transfer from the antenna to C60
may also be responsible for the quenching effect.[30a,31] The
efficient intramolecular energy transfer in C-2 is based on
the higher energy of the S1 state of C-2-L (approximated to
be 1.94 eV by the fluorescence emission of C-2-L at 640 nm)
in comparison with the S1 state of the C60 unit (720 nm,
1.72 eV).[30a] Figure 2a also indicates that the emission of the
bodipy unit (at ca. 500 nm) is completely quenched in C-2-
L, which indicates efficient energy transfer in C-2-L.
Similarly, the emission of the antenna is quenched in dyad
C-3 (see Figure S65). This intramolecular energy transfer en-
sures that the excitation energy harvested by the antenna in
the triads is funneled into the intramolecular energy accept-
or, that is, the C60 unit.[31] The intramolecular energy transfer
is estimated to be 99.5% in C-3 and 91.5% in C-2, based on
the fluorescence quantum yields.
To confirm the intramolecular energy transfer in the C60
dyads and triads, the fluorescence excitation spectra and the
UV/Vis absorption spectra of C-2 were compared and
a good match was found for the two spectra (Figure 2b).
The spectra of all other compounds are presented in Fig-
Figure 3. Nanosecond time-resolved transient difference absorption spec-
tra of C-3 and C-2. a) Transient difference absorption spectra of C-3.
b) Decay of C-3 at 720 nm. c) Transient difference absorption spectra of
C-2. d) Decay of C-2 at 630 nm. lex =532 nm (in deaerated toluene, 1.0ꢂ
10ꢀ5 m, 208C).
been reported for C60 dyads.[31,38] The assignment of the trip-
let excited state of the dyad to the styryl-bodipy-localized
triplet excited state is supported by the similarities between
the transients of the triad and the reference compound
di-Br-C-1-L (Scheme 1 and Figure S72). The lifetime of the
T1 excited state of C-2 was determined to be 95.3 ms
(Figure 3), which is much longer than that of C60 (ca.
40.0 ms),[26] which also confirms that the T1 state is not local-
ized on C60.
ACHTUNGTRENNUNGures S66–S69 in the Supporting information). These results
indicate efficient intramolecular energy transfer to the
energy donors of the dyads and triads upon selective excita-
tion.
The long-lived triplet excited state is in agreement with
the postulated styryl-bodipy-localized triplet excited state.
Similar long-lived triplet excited states were observed for
other C60 dyads and triads (see Figures S74–S77 in the Sup-
porting Information). For example, a styryl-bodipy-localized
triplet excited state lifetime of 132.3 ms was observed for C-6
(Table 1 and Figure S76). In general, the antenna-localized
triplet state shows a much longer lifetime than the C60-local-
ized triplet state (Table 1). This observation may be useful
for the design of new triplet photosensitizers with long-lived
triplet excited states, which is crucial for the application of
triplet photosensitizers in triplet–triplet energy-transfer
(TTET) processes.[18] TTET processes can be enhanced with
triplet photosensitizers that show long-lived triplet excited
states. TTETs are widely involved in singlet oxygen (1O2)
photosensitization[10] and triplet–triplet annihilation (TTA)
upconversion.[11–15,17]
Nanosecond time-resolved transient difference absorption of
the triplet photosensitizers (localization of the T1 state): To
prove the populations of the triplet excited states of the
dyads and triads, nanosecond time-resolved transient differ-
ence absorption spectra of the dyads, triads, and tetrads
were recorded, the results being exemplified with dyad C-3
and triad C-2 (Figure 3). A transient absorption (TA) band
at 720 nm was observed for C-3, which is the feature absorp-
tion of the T1 state of C60. Furthermore, bleaching of the
steady-state absorption of bodipy was not observed (Fig-
ure 3a). Therefore the TA band at 720 nm can be assigned
to the C60 localized triplet state, the lifetime of the triplet
state being 27.4 ms.
Upon pulsed excitation at 532 nm, bleaching at 345 and
629 nm was observed for C-2 (Figure 3c) due to ground-
state absorption of the styryl-bodipy antenna (C-1-L moiety,
see Figure S71 in the Supporting Information).[38] Interest-
ingly, bleaching of the bodipy part was not observed (ca.
517 nm, Figure 1b). These results indicate that the triplet ex-
cited state is localized on the styryl-bodipy unit due to the
backward triplet energy transfer from C60 to styryl-bodipy.
As a result, the C-2 molecule in the ground state is depleted
and bleaching bands are observed in the TA of C-2 at the
position that the styryl-bodipy antenna gives steady-state ab-
sorption (347 and 628 nm, similarly to the absorption of
C-1-L). Such “ping-pong” energy transfer has previously
Spin density surfaces of the triplet photosensitizers (localiza-
tion of the triplet state): The localization of the triplet excit-
ed states of the dyads, triads, and tetrads was also confirmed
by a theoretical approach, that is, by analysis of the spin
density surfaces (Figure 4).[38]
For C-3, the spin density is exclusively localized on the
C60 unit, which is in full agreement with the corresponding
transient absorption spectra (Figure 3a). The calculation in-
dicates that the spin density of C-2 is exclusively distributed
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Bodipy–C60 Triads and Tetrads
FULL PAPER
Table 1. Photophysical parameters of the C60 dyads, triads, and tetrads, reference compounds, and C60.[a]
[b]
[e]
[f]
labs [nm]
e [105 mꢀ1 cmꢀ1
]
lem
FL
ETeff
tT
tF
[ns]
Localization
of T1 state
[nm]
[%][d]
[ms]
[g]
[g]
[g]
bodipy
C-1-L
C-2-L
C-3-L
C-4-L
C-5-L
C-1
C-2
C-3
C-4
C-5
503
355/628
517/579/629
517
504/532
351/522/578/629
630
517/579/629
517
0.85
517
642
639
540
558
641
643
640
535
558
641
640
0.83
0.59
0.47
–
–
–
3.6
–
–
–
–
–
–
[g]
[g]
[g]
0.87/1.24
0.50/0.21/0.70
0.94
0.93/0.70
0.48/0.74/0.20/0.65
0.64
0.61/0.25/0.83
0.76
1.17/0.98
0.99/1.25/0.32/1.02/0.79
0.54/0.68/0.65
0.65
–
4.6
4.5
4.3
4.0
4.4
[g]
[g]
99.9
–
[g]
[g]
[g]
0.79[c]
0.75[c]
0.61
–
–
–
[g]
[g]
[g]
–
[g]
[g]
97.2
98.3
91.5
99.5
99.5
77.1
–
[g]
0.01
0.04
0.004
0.004
0.14
71.2
95.3
27.4
–
–
–
–
sty-bodipy
sty-bodipy
C60
[g]
[g]
[g]
504/532
27.0
C60
336/350/523/578/629
350/535/628
335
104.5
132.3
40.0
1.6
1.5
sty-bodipy
sty-bodipy
C60
[g]
C-6
C60
0.12
0.001
–
–
[g]
[g]
–
[a] Recorded in toluene at 208C. [b] With C-1-L (Scheme 1, F=0.59 in toluene) as standard. For C-1-L, C-2-L, C-5-L, C-1, C-2, C-5, and C-6, excitation
at 590 nm. [c] With BOD (Scheme 1, F=0.72 in THF) as standard. Excitation at 500 nm for C-3-L, C-4-L, C-3, and C-4. [d] ETeff is the energy transfer
efficiency. Calculated by fluorescence quantum yield ETeff =1ꢀt/t0. [e] Triplet excited state lifetimes. Measured by nanosecond transient absorption in
deaerated solutions. [f] Fluorescence lifetimes. [g] Not applicable.
of these broadband visible-
light-absorbing triplet photo-
sensitizers with conventional
monochromophore triplet pho-
tosensitizers that show only one
absorption band in the visible
region, the C60 dyads, triads,
and tetrads were evaluated as
triplet photosensitizers for pho-
tooxidation reactions. The sin-
glet oxygen (1O2) sensitizing
ability was studied with 1,5-di-
hydroxynaphthalene (DHN) as
the 1O2 scavenger (Scheme 3)
so that the production of the
1O2 could be easily monitored
by following the changes in the
UV/Vis absorption spectra.[41]
The product of this photooxida-
tion, juglone, can be used for
the preparation of bioactive
compounds.
Previously, IrIII complex pho-
Figure 4. Images of the spin density of dyad C-3, triads C-2 and C-4, and tetrad C-6. Calculations were per-
formed at the B3LYP/6-31G(d) level of theory with Gaussian 09W based on the optimized triplet-state geome-
try.
tosensitizers were used for the
photooxidation of DHN.[41]
However, the IrIII complexes
show weak absorption in the
on the styryl-bodipy unit, the C60 and other bodipy unit not
contributing to the spin density surface. This spin density
analysis is also in full agreement with the time-resolved
transient difference absorption spectrum of the triad (Fig-
ure 3c). The spin densities of C-4 and C-6 were also studied
(Figure 4), the results also being in full agreement with the
TA spectra (see Figures S74– S77 in the Supporting Infor-
mation).
visible range, which is typical of most of the transition-
metal-complex triplet photosensitizers based on a monochro-
mophore profile.[4,7,9,41] Accordingly, C60–chromophore
1
dyads have been used as O2 photosensitizers.[29,38] However,
none of these C60 derivatives contained any visible-light-har-
vesting antenna, or only one chromophore as a light-har-
vesting antenna, and thus the UV/Vis absorption bands are
1
either very weak or narrow in the visible range. O2 photo-
sensitizers based on fluorescence resonance energy transfer
have also been reported, but these photosensitizers are com-
posed of porphyrin as the energy acceptor and of two-
Evaluation of the singlet oxygen (1O2) photosensitizing abili-
ty of the triplet photosensitizers: To compare the efficiencies
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J. Zhao et al.
The initial rate constants (ni)
of the photooxidation reactions
were also calculated (Table 2).
The ni value for triad C-2 (ni =
6.2ꢂ10ꢀ5 mꢀ1 minꢀ1) is a factor
of 3.6 greater than that of dyad
C-1 (ni =17.2ꢂ10ꢀ6 mꢀ1 minꢀ1).
This clearly demonstrates that
the broadband absorption of
triad C-2 is more appropriate
for photosensitization than the
narrow absorption of mono-
chromophore triplet photosen-
sitizers such as dyad C-1. To the
Scheme 3. Photooxidation of DHN with C60–triad C-2 as the triplet photosensitizer.
photon excitation ligands, that is, the photosensitizers do not
show broadband absorption in the visible range.[21]
Herein we have demonstrated that the C60–bodipy triad
C-2, which shows broadband absorption in the visible range,
is an effective combination for efficiently harvesting pan-
chromatic excitation light.
Photooxidation with the C60 dyads and triads, as well as
with some conventional organic triplet photosensitizers,
such as methylene blue (MB) and tetraphenylporphyrin
(TPP), were studied (Figure 5 and the Supporting informa-
Figure 6. Photooxidation of DHN with different triplet photosensitizers.
a,c) Plots of ln(Ct/C0) vs. irradiation time (t) for the photooxidation of
DHN. b,d) Plots of chemical yield as a function of irradiation time (in
CH2Cl2/MeOH, 9:1, v/v, 208C).
Figure 5. UV/Vis absorption spectral changes observed upon the photo-
best of our knowledge, this is the first time that a heavy-
atom-free organic triplet photosensitizer with broadband ab-
sorption in the visible range based on a multichromophore
has been developed. Similar enhanced photooxidation was
also found for compounds C-4, C-5, and C-6 (see Figure 6,
Table 2, and Figure S80 in the Supporting Information). The
photooxidation velocities are generally a factor of 4 to 8.9
greater than those of conventional triplet photosensitizers
such as TPP and MB (Table 2). The 1O2 quantum yields
(FD) of the C60-based triplet photosensitizers are presented
in Table 2 and are generally comparable to or even higher
than those of conventional organic triplet photosensitizers.
oxidation of DHN with a) C-2 and b) MB as photosensitizers. c[photo-
sensitizers]=5.0ꢂ10ꢀ6 m,
c
[DHN]=2.0ꢂ10ꢀ4 m,
light
intensity=
20 mWcmꢀ2 in CH2Cl2/MeOH, 9:1, v/v, 208C.
tion). Upon photoexcitation, 1O2 was produced and thus
DHN was oxidized and juglone was produced. Accordingly,
the UV/Vis absorption of DHN at 301 nm decreased and
the absorption of juglone at 427 nm increased.[38,41,42]
1
The O2 photosensitizing abilities of the photosensitizers
were quantitatively compared by plotting ln(Ct/C0) (C being
the concentration of 1,5-dihydroxylnaphthalene) against the
irradiation time (Figure 6). Based on the slopes of the lines,
it is clear that the 1O2 sensitizing ability of C-2 is much
stronger than the organic triplet photosensitizers such as
TPP and MB with a monochromophore. Furthermore, the
1O2 sensitizing ability of triad C-2 is much greater than that
of dyad C-1, which indicates that our strategy of using
broadband absorption to enhance the performance of the
triplet photosensitizers is successful.
C60–bodipy hybrids as photocatalysts for the aerobic oxida-
tion of aromatic boronic acids to produce phenols by the
Cꢀ
generation of the superoxide radical anion (O2 ): The appli-
cation of C60–organic chromophore triplet photosensitizers
can be extended to other photocatalytic reactions, such as
the aerobic oxidation of boronic acids to produce phenols.[1b]
Recently, we used C60–bodipy compounds for a [3+2] addi-
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Bodipy–C60 Triads and Tetrads
FULL PAPER
Table 2. Pseudo-first-order kinetic parameters and yields of juglone for
the photooxidation of DHN by using C60 dyads and triads, as well as
some typical conventional organic triplet photosensitizers.[a]
[d]
Table 3. Visible-light-induced aerobic oxidative hydroxylation of aryl
boronic acids.[a]
kobs
[minꢀ1
ni
FD
Yield
[%][e]
[b]
[c]
G
]
[10ꢀ6 mminꢀ1
]
C-1
C-2
C-3
C-4
C-5
C-6
MB
TPP
C60
0.086
0.31
0.072
0.16
0.19
0.20
0.035
0.051
0.006
0.0043
17.2
62.0
14.4
32.0
38.0
40.0
7.0
10.2
1.2
0.8
0.82
93.3
99.9
94.7
99.9
99.9
99.9
64.5
88.1
22.1
12.5
Entry
Arylboron species
Catalyst
t
Yield
[%][b]
0.92
0.81[f]
0.87[f]
0.88
[h]
1
2
3
4
5
6
C-1
C-3
C-4
C-1
C-3
C-4
3.0
3.0
2.5
4.0
4.0
3.0
85
87
91
79
81
84
0.89
0.57
0.62
0.76
Ir-1
0.97[g]
[a] Experiments performed in CH2Cl2/MeOH, 9:1 with c[photosensitiz-
ers]=5.0ꢂ10ꢀ6 [DHN]=2.0ꢂ10ꢀ4
and at 208C. [b] Pseudo-first-
m
c
N
m
order-rate constants. [c] Initial rate of DHN consumption. [d] Quantum
yield of singlet oxygen (1O2) determined by using MB as standard (FD =
0.57 in CH2Cl2). [e] Yield of juglone after photoirradiation for 35 min.
[f] Determined with Rose Bengal (RB) as standard (FD =0.80 in metha-
nol). [g] Ref. [41].
7
8
9
C-1
C-3
C-4
3.0
3.0
2.0
78
83
82
tion/cyclization reaction, but the photocatalyst used showed
only one major absorption band in the visible spectral re-
gion.[2i] Functionalized phenols are important intermediates
for the preparation of pharmaceutical products and several
synthetic methods have been developed, but very few rely
on photocatalysis.
10
11
C-1
C-3
3.0
3.0
82
84
Previously, a RuII–polyimine complex has been used as
a photocatalyst for the aerobic oxidation of aromatic boron-
ic acids.[1b] However, there is much scope for enhancing this
environmentally benign and atom economic green synthetic
protocol. For example, the reaction time with the [Ru-
12
13
C-4
C-3
2.0
4.0
92
89
ACHTUNGTRENNUNG(bpy)3Cl2] catalyst (69–96 h) needs to be reduced and, more
importantly, heavy-atom-free triplet photosensitizers should
be used to achieve a greener synthesis.
[a] Reaction conditions: Boronic acid (0.10 mmol), photocatalyst
(2 mol%), iPr2NEt (2.0 equiv), CH2Cl2/CH3CN, 5:1, v/v (6 mL), 35 W Xe
lamp irradiation in air. The reaction was monitored by TLC. [b] Yield of
product upon isolation.
C60 is a well-known electron acceptor, and thus we envis-
aged that in the presence of a sacrificial electron donor
(redox reagent) such as an amine, electron transfer from the
amine to the C60 dyad will occur and electrons will be con-
the triplet excited state are essential for the photocatalytic
reactions.
Cꢀ
veyed to O2, and as a result O2 will be produced, which
The photocatalytic photoreactions show good substrate
tolerance (Table 3). For example, 3-formyl- and 4-formyl-
phenylboronic acids gave satisfactory yields of the corre-
sponding phenols. In addition, phenylboronic pinacol ester
can also be used successfully as a substrate (entry 13,
Table 3). To the best of our knowledge, this is the first time
that C60–organic chromophore hybrids showing broadband
absorption of visible light have been used as photocatalysts
for preparative organic synthesis[1,2] other than for the pro-
can then react with aromatic boronic acids to produce func-
tionalized phenols.[1b]
The photocatalytic properties of the C60 dyads and triads
for the aerobic oxidation of various aromatic boronic acids
were studied (Table 3). The aerobic photooxidation of aro-
matic boronic acids proceeded smoothly with the triplet
photosensitizers C-1, C-3, and C-4. The reactions were com-
plete within 2–4 h, with satisfactory yields of 79–92%. These
reaction times are much shorter than those required with
1
duction of O2, which has been known for a long time with
[Ru
N
C60.[29]
control experiments, we proved that the C60 triplet photo-
sensitizers, the electron donor (N,N-diisopropylethylamine),
and photoirradiation are indispensable for the photocatalyt-
ic aerobic oxidation (see the Supporting Information). Fur-
thermore, C60 alone or the light-harvesting antennae C-1-L
and C-4-L were also used for the photocatalytic reaction,
but no product was isolated. This result also suggests that
both strong visible-light-harvesting and the production of
An organic triplet photosensitizer (Acid Red 87, that is,
Eosin Y, tetrabromofluorescein sodium salt) has been previ-
ously used for the photocatalytic aerobic oxidation of aro-
matic boronic acids to produce phenols.[1b] However, the re-
action time was much longer (96 h).[1b] In this work, with the
C60–bodipy hybrids, this reaction time is greatly reduced to
less than 4 h (Table 3). To the best of our knowledge this is
the first time that heavy-atom-free triplet photosensitizers
Chem. Eur. J. 2013, 00, 0 – 0
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J. Zhao et al.
Conclusion
Energy funneling has been used for the first time to prepare
heavy-atom-free triplet photosensitizers that show broad-
band absorption in the visible region. We demonstrated this
concept with C60–bodipy triads and tetrads containing two
and three light-harvesting antennae with different absorp-
tion wavelengths. The panchromatic excitation energy har-
vested by the antennae in the triads and tetrads was effi-
ciently funneled towards C60 (spin converter). Then the trip-
let excited state was produced by the intrinsic intersystem
crossing (ISC) effect of C60. The triplet excited states of the
dyads or triads are localized on either the C60 unit or the
styryl-bodipy units. In the latter case, backward triplet
energy transfer occurs from C60 to the antenna. Notably,
these triplet photosensitizers are heavy-atom-free. They ex-
hibit a broadband absorption, which is in contrast to the
narrow visible-light absorption bands of conventional triplet
photosensitizers based on a monochromophore profile. Fur-
thermore, these heavy-atom-free triplet photosensitizers
possess a predictable ISC effect. The C60 triads and tetrads
were used as dual functional photocatalysts for two different
photocatalytic reactions, namely singlet oxygen (1O2) medi-
ated photooxidation of naphthol to produce juglone and su-
Figure 7. EPR spectra of mixtures of triad C-4 and radical scavengers to
ascertain the photocatalytic mechanism. a) C-4 (1.0ꢂ10ꢀ5 m) and DMPO
(1.0ꢂ10ꢀ2 m), b) C-4 (1.0ꢂ10ꢀ5 m), N,N-diisopropylethylamine (3.0ꢂ
10ꢀ3 m), and DMPO (1.0ꢂ10ꢀ2 m), c) C-4 (1.0ꢂ10ꢀ5 m) and TEMP
(0.10m), and d) C-4 (1.0ꢂ10ꢀ4 m), TEMP (0.10m), N,N-diisopropyl
ACTHNUTRGNEeNUG thyl-
ACHTUNGTRENNUNG
amine (3.0ꢂ10ꢀ3 m) in air-saturated CH2Cl2 upon irradiation. All irradia-
tions were performed with a 532 nm laser for 100 s (141 mWcmꢀ2) at
208C.
Cꢀ
have been used for the photocatalytic aerobic oxidation of
aromatic boronic acids to produce phenols. Electron para-
magnetic resonance (EPR) experiments confirmed that the
peroxide radical anion (O2 ) mediated photocatalytic aero-
bic oxidation of aromatic boronic acids to produce phenols.
The new photosensitizers show excellent 1O2 sensitizing
properties, a factor of 3.6 greater than the reference com-
pound C60–styryl-bodipy dyad, which contains only one
light-harvesting unit. Most importantly, the 1O2 sensitizing
ability of the C60 triad is a factor of 8.9 greater than conven-
tional organic triplet photosensitizers such as tetraphenyl-
porphyrin (TPP) and methylene blue (MB). Furthermore,
the reaction time for the aerobic oxidation of aromatic bor-
onic acids was greatly reduced from 69–96 h for a RuII–poly-
imine complex photosensitizer to less than 4 h for the new
triplet photosensitizers. The molecular design strategy of
these C60–bodipy triads and tetrads, which contain two or
three different light-harvesting antennae to ensure a broadband
absorption in the visible range, efficient excitation energy fun-
neling to the triplet manifold of the photosensitizers, and the
application of these C60–organic chromophore hybrids as dual
functional photoorganocatalysts, will be useful for the design
of new heavy-atom-free organic triplet photosensitizers. The
application of these photosensitizers in areas such as photoca-
talysis and photodynamic therapy is also promising.
Cꢀ
photocatalytic aerobic oxidations are mediated by O2
(Figure 7). 5,5-Dimethyl-1-pyrroline N-oxide (DMPO) and
2,2,6,6-tetramethylpiperidine (TEMP) were employed as
Cꢀ
scavengers to trap O2 and 1O2, respectively.[1g] For C-4
alone (Figure 7a,c), an EPR signal was detected in the pres-
ence of TEMP (Figure 7c), but not with DMPO (Figure 7a),
therefore, 1O2 was produced by C-4 in aerated solution.
With the addition of the sacrificial electron donor, N,N-di-
ACHTUNGTRENNUNGisopropylethylamine, however, an EPR signal was observed
only in the presence of DMPO (Figure 7b), no signal being
detected with TEMP (Figure 7d), which indicates that only
Cꢀ
O2 was produced in the presence of N,N-diisopropylethyl-
A
can be used as dual functional organic photocatalysts that
1
act as photosensitizers to produce O2 (in the photooxida-
tion of dinaphthols) or, with sacrificial electron donors such
Cꢀ
as N,N-diisopropylethylamine, O2 (aerobic oxidative hy-
droxylation of arylboronic acids to produce phenols,
Scheme 4). To the best of our knowledge, such dual func-
tional organic photocatalysts for photoorganocatalytic reac-
tions have not been reported before.
Experimental Section
NMR spectra were recorded on a 400 MHz Varian Unity Inova spec-
trometer. Mass spectra were recorded with a Q-TOF Micro MS spec-
trometer. UV/Vis spectra were recorded on an Agilent 8453 UV/Vis
spectrophotometer. Fluorescence spectra were recorded on Shimadzu
RF5301 PC and Sanco 970 CRT spectrofluorimeters. Luminescence
quantum yields were determined by using bodipy (FF =72.0% in THF)
and 2,6-diiodo-bodipy as standards (FF =2.7% in MeCN). Luminescence
lifetimes were measured with an OB920 luminescence lifetime spectrom-
Scheme 4. Photocatalytic mechanism for the aerobic oxidation of aromat-
ic boronic acids to produce phenols through the generation of the super-
oxide radical anion (O2 ).
Cꢀ
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Bodipy–C60 Triads and Tetrads
FULL PAPER
eter (Edinburgh, UK). Typical synthetic methods are presented below.
For detailed synthetic procedures of the other compounds, see the Sup-
porting Information.
127.5, 127.4, 127.3, 119.3, 117.9, 31.9, 30.2, 30.0, 29.7, 29.4, 22.7, 14.8, 14.5,
14.1, 13.3, 13.1 ppm; HRMS (MALDI): m/z calcd for [C121H53B2F4N5]ꢀ:
1673.4423; found: 1673.4562.
Compound 2: 4’-Bromophenyl-bodipy (1; 120.0 mg, 0.30 mmol) and 4-for-
mylphenylboronic (90.0 mg, 0.6 mmol) were dissolved in toluene/ethanol/
water (2:2:1, v/v/v, 25 mL). K2CO3 (41.7 mg, 0.3 mmol) was added and
Compound 11: Compound 10 (522.0 mg 1.0 mmol) and 4-formylphenylbor-
onic acid (300.0 mg, 2 mmol) was dissolved in dry toluene/ethanol/water
(2:2:1, v/v/v, 75 mL). K2CO3 (417.9 mg, 3 mmol) was added and argon was
argon was bubbled through the solution for 30 min. [Pd
G
bubbled through the solution for 30 min. [PdACTHUNGETRN(NUG PPh3)4] (182.2 mg, 0.3 mmol)
(0.03 mmol, 18.4 mg) was added. The mixture was heated at reflux under
an argon for 8 h. After removal of the solvent under reduced pressure,
the crude product was purified by column chromatography (silica gel, di-
chloromethane (DCM)/petroleum ether, 1:1, v/v) to give a deep-red solid
(110.0 mg, 85%). 1H NMR (400 MHz, CDCl3): d=10.09 (s, 1H), 8.02 (d,
J=8.0 Hz, 2H), 7.86–7.80 (m, 4H), 7.43 (d, J=7.6 Hz, 2H), 6.01 (s, 2H),
2.57 (s, 6H), 1.45 ppm (s, 6H).
was added. The mixture was heated at reflux under argon for 8 h. After
removal of the solvent under reduced pressure, the residue was purified
by column chromatography (silica gel, DCM/petroleum ether, 2:1, v/v) to
give a deep-red solid (425.2 mg, 82%). 1H NMR (400 MHz, CDCl3): d=
10.03 (s, 1H), 7.91 (d, J=8.0 Hz, 2H), 7.34 (d, J=7.8 Hz, 2H), 7.20 (d,
J=8.4 Hz, 2H), 7.02 (d, J=8.8 Hz, 2H), 6.04 (s, 1H), 4.03–4.00 (m, 2H),
2.59 (s, 3H), 2.53 (s, 3H), 1.83–1.81 (m, 2H), 1.53–1.51 (m, 2H), 1.38 (s,
3H), 1.47 (s, 3H), 1.02–0.98 ppm (m, 3H); 13C NMR (100 MHz, CDCl3):
d=191.9, 159.9, 157.0, 152.3, 144.6, 142.7, 140.8, 138.7, 134.9, 129.8, 126.8,
122.0, 115.3, 67.9, 31.4, 30.7, 19.3, 14.9, 14.0, 13.4, 13.0 ppm; HRMS
(MALDI): m/z calcd for [C30H31BF2N2O2]+: 500.2447; found 500.2454.
Compound 3: Compound 2 (110.0 mg, 0.20 mmol) was dissolved in dry
CH2Cl2 (50 mL). N-Iodosuccinimide (NIS, 44.6 mg, 0.2 mmol) was slowly
added. The mixture was stirred at room temp. for 2 h. After removal of the
solvent under reduced pressure, the crude product was purified by column
chromatography (silica gel, DCM/petroleum ether, 1:1, v/v) to give a deep-
red solid (110.0 mg, 61%). 1H NMR (400 MHz, CDCl3): d=10.09 (s, 1H),
8.02 (d, J=8.0 Hz, 2H), 7.86–7.81 (m, 4H), 7.41 (d, J=7.6 Hz, 2H), 6.01 (s,
1 H), 2.65 (s, 3H), 2.58 (s, 3H), 1.45 ppm (s, 6H); HRMS (MALDI): m/z
calcd for [C26H22BOF2N2I ]+: 554.0838; found: 554.0844.
Compound 12: Compound 11 (400.0 mg, 0.8 mmol) and NIS (223.4 mg,
1.0 mmol) were dissolved in DCM (100 mL). The mixture was stirred at
room temperature for 2 h. After removal of the solvent under reduced
pressure, the mixture was purified by column chromatography (silica gel,
DCM/hexanes, 2:1, v/v) to give
a deep-red solid (470.0 mg, 95%).
1H NMR (400 MHz, CDCl3): d=10.03 (s, 1H), 7.92 (d, J=7.6 Hz, 2H),
7.34 (d, J=8.4 Hz, 2H), 7.18 (d, J=7.2 Hz, 2H), 7.04 (d, J=8.0 Hz, 2H),
4.03–4.00 (m, 2H), 2.59 (s, 3H), 2.53 (s, 3H), 1.83–1.81 (m, 2H), 1.53–
1.51 (m, 2H), 1.38 (s, 3H), 1.47 (s, 3H), 1.02–0.98 ppm (m, 3H); HRMS
(MALDI): m/z calcd for [C30H30BF2N2O2I]+: 626.1413; found 626.1469.
Compound 5: Under argon, 4 (135.2 mg 0.3 mmol) was dissolved in dry
DMF (5 mL). Benzaldehyde (142.4 mg, 0.8 mmol) was added, followed
by acetic acid (6 drops) and piperidine (6 drops). The mixture was sub-
jected to microwave irradiation (5 min, 1508C, 1 min pre-stirring). After
removal of the solvent under reduced pressure, the crude product was
purified by column chromatography (silica gel, DCM) to give deep-
Compound 13: Compound 12 (401.6 mg, 0.64 mmol) and 4 (346.5 mg,
0.77 mmol) were dissolved in dry toluene/ethanol/water (2:2:1, v/v/v,
75 mL). K2CO3 (417.7 mg, 3 mmol) was added and argon was bubbled
1
purple solid (75.0 mg, 40%). H NMR (400 MHz, CDCl3): d=7.94 (d, J=
8.4 Hz, 2H), 7.77 (d, J=16.4 Hz, 2H), 7.65 (d, J=8.0 Hz, 4H), 7.42–7.39
(m, 4H), 7.36–7.32 (m, 4H), 7.28–7.24 (m, 2H), 6.65 (s, 2H), 1.44 (s, 6H),
1.25 ppm (s, 12H); HRMS (MALDI): m/z calcd for [C39H38B2F2N2O2]+:
626.3087; found: 626.3041.
through the solution for 30 min. [PdACHTNUGTRNEUNG(PPh3)4] (76.8 mg, 0.064 mmol) was
added. The mixture was heated at reflux under argon for 8 h. After re-
moval of the solvent under reduced pressure, the residue was purified by
column chromatography (silica gel, DCM/hexanes, 2:1, v/v) to give
a deep-red solid (450.0 mg, 68%). 1H NMR (400 MHz, CDCl3): d=10.04
(s, 1H), 7.93 (d, J=7.6 Hz, 2H), 7.37–7.30 (m, 6H), 7.25–7.23 (m, 2H),
7.04 (d, J=8.8 Hz, 2H), 6.00 (s, 2H), 4.03–4.00 (m, 2H), 2.56 (s, 12H),
1.80–1.79 (m, 2H), 1.41 (s, 12H), 1.02–0.98 ppm (m, 5H); 13C NMR
(100 MHz, CDCl3): d=192.0, 160.1, 155.7, 143.2, 140.6, 140.0, 139.5,
135.1, 134.6, 133.4, 132.4, 132.0, 131.8, 131.6, 131.3, 131.1, 130.9, 129.9,
129.2, 128.2, 121.5, 115.5, 63.1, 31.4, 19.4, 14.8, 14.4, 14.1, 13.7, 13.6, 13.4,
13.3 ppm; HRMS (MALDI): m/z calcd for [C49H48B2F4N4O2]+: 822.3900;
found: 822.3848.
Compound 6: Compounds
3 (45.0 mg 0.1 mmol) and 5 (69.9 mg,
0.11 mmol) were dissolved in dry toluene/ethanol/water (2:2:1, v/v/v,
25 mL). K2CO3 (41.6 mg, 0.3 mmol) was added and argon was bubbled
through the solution for 30 min. [PdACHTUNTRGNEUNG(PPh3)4] (6.4 mg, 0.005 mmol) was
added. The mixture was heated at reflux under argon for 8 h. After re-
moval of the solvent under reduced pressure, the mixture was purified by
column chromatography (silica gel, DCM) to give a deep-purple solid
(67.6 mg, 72%). 1H NMR (400 MHz, CDCl3): d=10.09 (s, 1H), 8.02 (d,
J=8.4 Hz, 2H), 7.86 (m, 4H), 7.76 (d, J=16.4 Hz, 2H), 7.64 (d, J=
7.6 Hz, 4H), 7.48 (d, J=8.8 Hz, 2H), 7.42–7.39 (m, 4H) 7.37–7.34 (m,
4H), 7.30 (d, J=7.2 Hz, 3H), 7.24 (s, 1H), 6.65 (s, 2H), 6.07 (s, 1H), 2.62
(s, 3H), 2.56 (s, 3H), 1.49 (s, 6H), 1.47 (s, 3H), 1.40 ppm (s, 3H);
13C NMR (100 MHz, CDCl3): d=192.0, 157.0, 155.9, 153.4, 152.8, 146.1,
145.0, 143.9, 143.1, 142.1, 140.6, 140.4, 138.8, 138.7, 136.7, 136.5, 135.7,
135.5, 135.4, 134.8, 134.1, 133.5, 132.9, 132.1, 131.5, 131.1, 130.6, 129.0,
128.6, 128.1, 127.8, 127.7, 122.1, 121.5, 119.3, 118.1, 30.3, 29.3, 23.5, 23.3,
14.3, 11.3 ppm; HRMS (MALDI): m/z calcd for [C59H48B2OF4N4]+:
926.3950; found 926.3941.
C-4: Under argon, compound 13 (24.7 mg, 0.03 mmol), C60 (42.0 mg,
0.06 mmol), and N-methylglycine (17.8 mg, 0.20 mmol) were suspended
in dry toluene (50 mL). The mixture was heated at reflux for 12 h. After
completion of the reaction, the solvent was evaporated under reduced
pressure. The residue was purified by column chromatography (silica gel,
petroleum ether/CH2Cl2, 1:2, v/v) to give the product as a red solid
(32.4 mg, 66%). M.p. >2508C. 1H NMR (400 MHz, CDCl3): d=7.73–
7.70 (m, 1H), 7.55–7.52 (m, 1H), 7.32–7.29 (m, 5H), 7.23–6.19 (m, 3H),
7.01 (d, J=7.6 Hz, 2H), 5.99 (s, 2H), 5.30 (s, 1H), 4.00–3.97 (m, 2H),
2.56–2.51 (m, 12H), 2.36 (s, 3H), 2.34 (s, 2H), 1.84–1.80 (m, 2H), 1.56–
1.52 (m, 2H), 1.48 (s, 3H), 1.45 (s, 3H), 1.38 (s, 6H), 0.98–0.96 ppm (m,
3H); 13C NMR (100 MHz, CDCl3): d=159.9, 155.6, 147.5, 147.4, 146.3,
146.3, 146.2, 146.0, 145.8, 145.7, 145.6, 145.5, 145.4, 145.3, 145.2, 145.1,
145.0, 144.9, 144.8, 144.7, 144.6, 144.5, 144.4, 144.2, 144.1, 143.2, 143.0,
142.6, 142.5, 142.1, 142.0, 141.9, 141.7, 141.4, 140.2, 140.1, 139.8, 139.7,
139.3, 139.1, 137.9, 135.8, 134.7, 133.8, 132.7, 131.8, 131.4, 131.0, 129.0,
128.0, 127.0, 125.3, 121.3, 115.2, 67.9, 40.2, 29.7, 19.2, 14.6, 14.2, 13.9, 13.5,
13.2, 13.1 ppm; HRMS (MALDI): m/z calcd for [C111H53B2F4N5O1]ꢀ:
1569.4372; found: 1569.4390.
C-2: Under argon, compound
6 (27.8 mg, 0.03 mmol), C60 (36.0 mg,
0.50 mmol), and N-methylglycine (44.5 mg, 0.25 mmol) were suspended
in dry toluene (20 mL). The mixture was heated at reflux for 12 h. After
completion of the reaction, the solvent was evaporated under reduced
pressure. The residue was purified by column chromatography (silica gel,
hexane/CH2Cl2, 1:2, v/v) to give the product as a blue solid (21.6 mg,
1
66%). M.p. >2508C. H NMR (400 MHz, CDCl3): d=7.82 (d, J=7.8 Hz,
2H), 7.78 (d, J=8.0 Hz, 3H), 7.72 (s, 1H), 7.64 (d, J=4.0 Hz, 4H), 7.43–
7.39 (m, 6H), 7.36–7.32 (m, 4H) 7.31–7.28 (m, 5H), 7.24 (s, 1H), 6.65 (s,
2H), 6.05 (s, 1H), 5.04 (d, J=8.4 Hz, 2H), 4.32 (d, J=8.0 Hz, 1H), 3.55
(s, 3H), 2.61 (s, 3H), 2.55 (s, 3H), 1.46 (s, 6H), 1.38 ppm (s, 6H);
13C NMR (100 MHz, CDCl3): d=153.0, 152.7, 147.4, 147.3, 146.2, 146.0,
145.7, 145.4, 145.2, 144.5, 144.3, 143.9, 143.8, 143.6, 143.2, 143.1, 143.0,
142.8, 142.7, 142.4, 142.2, 142.1, 141.9, 141.7, 141.6, 140.3, 139.9, 139.6,
138.7, 138.1, 137.8, 136.3, 136.0, 135.9, 135.7, 135.6, 134.8, 134.4, 133.9,
133.4, 132.7, 132.1, 132.0, 130.9, 130.2, 128.8, 128.6, 128.5, 127.7, 127.6,
Photooxidation: The photooxidation experiments were carried out as fol-
lows.[41] A CH2Cl2/MeOH (9:1, v/v) mixed solvent containing DHN (2.0ꢂ
10ꢀ4 m) and a triplet photosensitizer (5 mol% with respect to DHN) was
placed in a two-neck round-bottomed flask (25 mL). The solution was
then irradiated by using a 35 W xenon lamp (with focusing reflector) and
Chem. Eur. J. 2013, 00, 0 – 0
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J. Zhao et al.
a cut-off filter (0.72m NaNO2 aqueous solution, which is transparent for
light with wavelength >385 nm). UV/Vis absorption spectra were record-
ed at intervals of 2–5 min. The DHN consumption was monitored by fol-
lowing the decrease in the UV absorption at 301 nm, and the concentra-
tion of DHN was calculated based on its molar absorption coefficient
(e=7664mꢀ1 cmꢀ1). On the other hand, the production of juglone was
monitored by following the increase in the absorption at 427 nm. The
concentration of juglone was calculated from its molar extinction coeffi-
cient (e=3811mꢀ1 cmꢀ1 at 427 nm) and the its yield was determined by di-
viding the concentration of juglone by the initial concentration of DHN.
The photostability experiments were carried out using the same method
but without the substrate DHN.
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Measurement of the singlet oxygen quantum yield (FD): The FD quan-
tum yields were calculated by using methylene blue (MB) in dichlorome-
thane with FD =0.57 as reference. The absorbance of the 1O2 scavenger,
1,4-diphenylisobenzofuran (DPBF), was adjusted to around 1.0 in air-sat-
ACHTUNGTRENNUNGurated dichloromethane. Then the photosensitizer was added and its ab-
sorbance was adjusted to around 0.2–0.3. Then the cuvette was exposed
to monochromatic light at the peak absorption wavelength for 10 s. The
absorbance at 630 nm was measured several times after each irradiation.
The slope of the absorbance maxima of DPBF at 414 nm versus time
graph was calculated for each photosensitizer. The singlet oxygen quan-
tum yields (FD) were calculated according to equation (1)
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ksam Fref
kref Fsam
ð1Þ
Fsam ¼ Fref
in which sam and ref designate “C-1, C-2” and “MB” respectively, k is
the slope of the difference in the change in the absorbance of DPBF
(414 nm) with irradiation time, and F is the absorption correction factor,
which is given by F=1–10ꢀOD (OD at the irradiation wavelength).
C60–bodipy hybrids as photocatalyst for the aerobic oxidation of aromatic
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diisopropylethylamine (1.0 mmol, 10 equiv) were placed in
a flask
(25 mL), The solution was then irradiated by using a 35 W xenon lamp
(with focusing reflector) and a cut-off filter (0.72m NaNO2 aqueous solu-
tion, which is transparent for light with a wavelength >385 nm). The re-
action was monitored by TLC. After completion of the reaction, the sol-
vent was evaporated under reduced pressure. The residue was purified by
column chromatography (silica gel, methanol/CH2Cl2, 1:50, v/v) to give
the product as a white solid.
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Electron paramagnetic resonance (EPR) spectroscopy: EPR spectra were
recorded by using a Bruker ESP-300E spectrometer at 9.8 GHz X-band
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Cꢀ
ers and superoxide radical anion (O2 ) scavenger 5,5-dimethyl-1-pyrroline
N-oxide (DMPO), the singlet oxygen (1O2) scavenger 2,2,6,6-tetramethylpi-
peridine (TEMP) in air-saturated CH2Cl2 was stirred in the dark. Then the
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Acknowledgements
We thank the NSFC (20972024, 21073028 and 21273028), the Royal Soci-
ety (UK), the NSFC (China-UK Cost-Share Science Networks,
21011130154), the Science Foundation Ireland (SFI E.T.S. Walton Pro-
gram 11/W.1/E2061), the Fundamental Research Funds for the Central
Universities (DUT10ZD212), the Ministry of Education (NCET-08-0077
and SRFDP-20120041130005) and the Dalian University of Technology
for financial support.
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Received: June 27, 2013
Published online: && &&, 0000
Chem. Eur. J. 2013, 00, 0 – 0
ꢁ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
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These are not the final page numbers!

J. Zhao et al.
Fullerene photocatalysts: Broadband
visible-light-absorbing, heavy-atom-
free triplet photosensitizers based on
bodipy–C60 dyads, triads, and tetrads
have been prepared (see figure). Reso-
nance energy transfer or intramolecu-
lar funneling was used to achieve pan-
chromatic absorption in the visible
region. The triplet photosensitizers
were used as dual functional photo-
Photochemistry
L. Huang, X. Cui, B. Therrien,
J. Zhao* ....................... &&&& — &&&&
Energy-Funneling-Based Broadband
Visible-Light-Absorbing Bodipy–C60
Triads and Tetrads as Dual Functional
Heavy-Atom-Free Organic Triplet
Photosensitizers for Photocatalytic
Organic Reactions
1
Cꢀ
organocatalysts to produce O2 or O2
.
1
The O2 sensitizing ability of the new
photosensitizers is up to a factor of 8.9
greater than other known triplet pho-
tosensitizers based on a monochromo-
phore such as methylene blue.
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12
&
www.chemeurj.org
ꢁ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 0000, 00, 0 – 0
ÝÝ
These are not the final page numbers!