Light-harvesting fullerene dyads as organic triplet photosensitizers for triplet-triplet annihilation upconversions

Article abstract of DOI:10.1021/jo300613g

Visible light-harvesting C₆₀-bodipy dyads were devised as universal organic triplet photosensitizers for triplet-triplet annihilation (TTA) upconversion. The antennas in the dyad were used to harvest the excitation energy, and then the singlet excited state of C₆₀ will be populated via the intramolecular energy transfer from the antenna to C₆₀ unit. In turn with the intrinsic intersystem crossing (ISC) of the C₆₀, the triplet excited state of the C₆₀ will be produced. Thus, without any heavy atoms, the triplet excited states of organic dyads are populated upon photoexcitation. Different from C₆₀, the dyads show strong absorption of visible light at 515 nm (C-1, ε = 70400 M^-1 cm^-1) or 590 nm (C-2, ε = 82500 M^-1 cm^-1). Efficient intramolecular energy transfer from the bodipy moieties to C₆₀ unit and localization of the triplet excited state on C₆₀ were confirmed by steady-state and time-resolved spectroscopy as well as DFT calculations. The dyads were used as triplet photosensitizers for TTA upconversion, and an upconversion quantum yield up to 7.0% was observed. We propose that C ₆₀-organic chromophore dyads can be used as a general molecular structural motif for organic triplet photosensitizers, which can be used for photocatalysis, photodynamic therapy, and TTA upconversions.

Full text of DOI:10.1021/jo300613g

Article
pubs.acs.org/joc
Light-Harvesting Fullerene Dyads as Organic Triplet Photosensitizers
for Triplet−Triplet Annihilation Upconversions
Wanhua Wu, Jianzhang Zhao,* Jifu Sun, and Song Guo
State Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian University of Technology, Dalian 116024, China
S
* Supporting Information
ABSTRACT: Visible light-harvesting C60−bodipy dyads were
devised as universal organic triplet photosensitizers for triplet−
triplet annihilation (TTA) upconversion. The antennas in the
dyad were used to harvest the excitation energy, and then the
singlet excited state of C60 will be populated via the
intramolecular energy transfer from the antenna to C60 unit.
In turn with the intrinsic intersystem crossing (ISC) of the C60, the triplet excited state of the C60 will be produced. Thus,
without any heavy atoms, the triplet excited states of organic dyads are populated upon photoexcitation. Different from C60, the
dyads show strong absorption of visible light at 515 nm (C-1, ε = 70400 M−1 cm−1) or 590 nm (C-2, ε = 82500 M−1 cm−1).
Efficient intramolecular energy transfer from the bodipy moieties to C60 unit and localization of the triplet excited state on C60
were confirmed by steady-state and time-resolved spectroscopy as well as DFT calculations. The dyads were used as triplet
photosensitizers for TTA upconversion, and an upconversion quantum yield up to 7.0% was observed. We propose that C60−
organic chromophore dyads can be used as a general molecular structural motif for organic triplet photosensitizers, which can be
used for photocatalysis, photodynamic therapy, and TTA upconversions.
Pt(II) acetylide complexes.22,23 However, the photophysical
properties of these complexes cannot be readily optimized and
1. INTRODUCTION
Upconversions have attracted much attention because of their
they are not cost-effective. Therefore, similar to the scenario of
the dye-sensitized solar cells,24−27 to replace the transition
metal complex sensitizers with organic triplet photosensitizers
will be the next milestone for TTA upconversion.
applications in photovoltaics, photocatalysis, photoinduced
charge separation, and molecular probes.1−13 Among the
upconversion methods, such as using rare earth metal
materials1,2 and two-photon-absorption dyes,3,4 the triplet−
triplet annihilation (TTA) upconversion is particularly
interesting5−7 for its advantages over the conventional
upconversion methods.5−13 For example, TTA upconversion
requires noncoherent low-power excitation, and it shows
intense absorption of the excitation light as well as high
upconversion quantum yields.5a,6,7a
Triplet photosensitizers and triplet acceptors (annihilator,
emitter) are used in TTA upconversion.5a,6,7a The photo-
sensitizers are selectively photoexcited in the presence of triplet
acceptor, and then the triplet excited states of the photo-
sensitizers are populated, via the intersystem crossing (ISC)
effect, very often exerted by heavy atoms such as Pt(II), Ir(III),
or iodine substituents.5a,6,7a The photoexcitation energy
migrates to the triplet acceptor from the photosensitizer by
the triplet−triplet energy-transfer (TTET) process. Then the
TTA between the triplet acceptors produces the singlet excited
state, and the delayed fluorescence (upconverted fluorescence)
can be observed (for the photophysical mechanism, see Scheme
2).5a,6,7a
Unfortunately, it is challenging to prepare organic triplet
photosensitizers because the ISC is very often nonefficient
without the heavy atom effect of Pt, Ir, Ru, and I.28 Only
sporadic examples of organic triplet sensitizers for TTA
upconversion have been reported, such as 2,3-butanedione29
and 2,4,5,7-tetraiodo-6-hydroxy-3-fluorone.30a Eosin and the
derivatives have also been used as triplet sensitizers in
photochemical reactions.30b However, these organic triplet
photosensitizers suffer from disadvantages of either short
excitation wavelength (in UV range) or resorting to heavy
atom effect (iodo atom), and it is difficult to functionalize the
molecular structures to tune the photophysical properties.6
Recently, we prepared organic triplet photosensitizers based on
boron−dipyrromethene (BODIPY) and naphthalenediimide
(NDI) that shows tunable absorption wavelength.13 However,
the heavy atom effect is still mandatory for these organic triplet
photosensitizers.13 Since it is not always convenient to prepared
iodated organic chromophore, and the ISC property of the
iodated chromophore cannot be guaranteed, we set out to
prepare organic triplet photosensitizers without any heavy
atoms. However, without the heavy atom effect, it becomes
Currently, the challenge for the development of TTA
upconversion is the limited availability of the triplet photo-
sensitizers.6 Usually the sensitizers are phosphorescent
transition-metal complexes, such as Pt(II)/Pd(II) porphyrin
complexes,14−17 Ru(II) polyimine complexes,11,12,18 cyclo-
metalated Pt(II) complexes,10,19,20 Ir(III) complexes,8,21 and
Received: March 31, 2012
Published: May 22, 2012
© 2012 American Chemical Society
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Scheme 1. Synthesis of the C60-Based Dyads C-1 and C-2 as Organic Triplet Sensitizers and Light-Harvesting Antennas C-1-L
a,b
and C-2-L
a
The structures of perylene (triplet acceptor in the upconversion) and iodo-BDP and BisBDPI used as the standard of upconversion quantum yield
were also shown.
b
Key: (i) 5-formyl-2-thiopheneboronic acid, Pd(OAc)2, H3PO4; (ii) sarcosine, C60, toluene, reflux; (iii) NIS, CH2Cl2, 40 °C; (iv) 4-
ethynylbenzaldehyde, Pd(PPh3)2Cl2, PPh3, CuI, THF/Et3N; (v) 2-thiopheneboronic acid, Pd(OAc)2, H3PO4; (vi) phenylacetylene, Pd(PPh3)2Cl2,
PPh3, CuI, THF/Et3N.
almost impossible to predict whether or not an organic
chromophore will undergo ISC upon photoexcitation.
Herein, we propose a universal strategy to address this
challenge, that is, to use the light-harvesting organic
fluorophore−fullerene (C60) dyads as a general molecular
structure motif for the heavy atom free organic triplet
2. RESULTS AND DISCUSSION
2.1. Design and Synthesis of the C60−bodipy Dyads.
C60 is used as the intramolecular energy acceptor and the spin
convertor, because of its intrinsic capability of ISC, with which
the triplet excited state of C60 can be efficiently populated,
without resorting to heavy atom effect (the quantum yield of
the triplet excited state of C60 upon photoexcitation is close to
unity).31−34 It should be noted that the visible light-harvesting
bodipy units in dyads C-1 and C-2 (the intramolecular energy
donor) are mandatory because C60 alone gives a very weak
absorption in the visible range.31,32,35 C60 has been extensively
studied in organic solar cells, supramolecular chemistry,36 but
its ISC capability in a C60−organic chromophore dyad has
rarely been explored for sensitizing a photophysical process.35c
Previously, fluorene−C60 dyads was reported for singlet oxygen
sensitizing, but dyads show moderate absorption in visible
range.35c The delayed fluorescence of C70 was used for O2
photosensitizers. These dyads show strong absorption of visible
light, and the molecular structure can be readily changed by
using different light-harvesting antennas. More importantly, the
population of the triplet excited states of these dyads upon
photoexcitation is predetermined. Thus, design of organic
triplet photosensitizers will become feasible. The C60−bodipy
dyads were used for TTA upconversion (C-1 and C-2, Scheme
1), and significant upconversion quantum yields were observed.
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sensing.34a In order to enhance the absorption of the C60 dyads
in the visible range, bodipy was selected as the light-harvesting
antenna because of its ideal photophysical properties, such as
strong absorption of visible light, good photostability, and
derivatizable molecular structures.37,38
Thienyl-substituted bodipy C-1-L shows strong absorption
of visible light.38 The excitation energy harvested by the Bodipy
antennas can be transferred to C60 via the Foster mechanism.
̈
The triplet state of C60 will be populated via its intrinsic ISC
capability (the efficiency is up to 95%).32 Thus, C-1 can be
used as an organic triplet photosensitizer, which is without any
transition metal atoms or heavy atoms such as Pt(II), Ir(III), or
iodine atoms. Similar rationales are applicable to C-2, in which
the bis-Bodipy C-2-L moiety shows red-shifted absorption
compared to that of C-1-L.
Figure 2. (a) Fluorescence emission spectra of C-1-L, C-1, and C60
(λex = 510 nm). (b) Fluorescence emission spectra of C-2-L, C-2, and
C60. In toluene, λex = 570 nm, 1.0 × 10−5 M, 20 °C.
emission is due to the extremely weak absorption of C60 at 510
The preparation of the antenna is based on Suzuki or
Sonogashira cross coupling. Because of the versatile derivatiza-
tion of bodipy, the π-conjugation framework is readily
extended. Thus, the absorption can be moved to the red
range of the spectrum (C-2-L). The antenna was attached to
the C60 unit by a Prato reaction. All of the compounds were
obtained in moderate to satisfying yields (see the Experimental
Section for details).
nm as well as its low fluorescence quantum yield (ΦF
<
0.1%).31,32
Dyad C-1 is weakly fluorescent at 594 nm (C-1-L gives
strong emission). Emission at 710 nm was observed for C-1,
which can be assigned to the emission of C60 unit. These two
observations are clear indications of the intramolecular energy
transfer from bodipy to the C60 unit in C-1. The intramolecular
energy transfer is efficient because the fluorescence emission of
C-1-L is completely quenched in C-1. Similar studies were
carried out for C-2 (Figure 2b). However, determination of the
exact energy transfer efficiency is not straightforward,37
although the quenched fluorescence of the energy donor
were used for the determination of the energy transfer
efficiency in some cases. However, the nonemissive excited
state of the energy donor (antenna) was not considered with
this method.
2.2. UV−vis Absorption and Fluorescence Spectra of
the Compounds. An intense absorption at 513 nm is found
for C-1-L (ε = 69000 M−1 cm−1) (Figure 1a). Conversely, C60
We found that similar intramolecular energy transfer also
occurs for C-2 (Figure 2b). The fluorescence of C-2-L was
significantly quenched in dyad C-2 (Figure 2b), which indicates
efficient energy transfer from bis-bodipy to C60 moiety.
Emission of C60 in C-2 is probably buried in the residual
emission of C-2-L. The photophysical properties of the
compounds are summarized in Table 1.
Figure 1. UV−vis absorption spectra of (a) C-1 and (b) C-2 and the
antennas of the dyads. c = 1.0 × 10−5 M in toluene, 20 °C.
Table 1. Photophysical Parameters of the C60 Dyads and the
Components
a
alone shows very weak absorption at 513 nm (e.g., ε = 1230
M−1 cm−1). However, an intense absorption at 515 nm (ε =
70400 M−1 cm−1) is observed for the dyad C-1, which is the
sum of C60 and C-1-L, indicates no significant electronic
communication between the energy donor (bodipy unit) and
acceptor (C60 unit) at the ground state.
Similar results were observed for C-2 (Figure 1b). C-2-L unit
gives red-shifted absorption at 589 nm compared to C-1-L.
Similar to C-1, the absorption of C-2 is the sum of C-2-L and
C60 (Figure 1b).
C60 does show weak absorption beyond 500 nm, which is
crucial for the intramolecular energy transfer from bodipy to
C60 unit. The weak absorption of C60 in the visible range
indicates weakly allowed S0→Sn transitions, but it does not
necessarily mean that the intramolecular energy transfer from
the bodipy antenna to C60 is weakly allowed. On the contrary,
efficient energy transfers from the energy donors (bodipy) in
C-1 and C-2 to C60 units were observed (Figure 2).
The photoinduced intramolecular energy transfer of dyads
C-1 and C-2 was confirmed by the luminescence study (Figure
2). The C-1-L alone gives intense fluorescence at 594 nm (ΦF
= 20.6%). For C60, however, no emission was observed with
excitation at 510 nm under the same conditions. This lack of
b
c
λabs
ε
λem
ΦF (%)
τ
e
f
C60
336
513
589
515
590
6.19
6.90
8.08
7.04
8.25
40.6 μs
3.2 ns
C-1-L
C-2-L
C-1
594
627
710
627
20.6
62.0
f
1.7 ns
e
e
33.3 μs
35.2 μs
d
C-2
1.7
a
b
In toluene (1.0 × 10−5 M). Molar extinction coefficient at the
c
absorption maxima. ε: 104 M−1 cm−1. With 1,3,5,7-tetramethyl-8-
phenyl-4-bora-3a,4a-diaza-s-indacene as the standard (Φ = 72.0% in
d
THF). With 2,6-diiodo-4,4-difluoro-1,3,5,7-tetramethyl-8-phenyl-4-
bora-3a,4a-diaza-s-indacene (Iodo-BDP in Scheme 1) as the standard
e
(Φ = 2.7% in MeCN). Triplet state lifetimes, measured by transient
f
absorptions. Fluorescence lifetimes.
2.3. Nanosecond Time-Resolved Transient Difference
Absorption Spectroscopy. In order to confirm the
population of the triplet excited state of the dyads upon
photoexcitation, nanosecond time-resolved transient difference
absorption spectroscopy were studied (Figure 3). Significant
transient absorption at 720 nm was observed for C-1 upon
pulsed excitation, which is the characteristic absorption of the
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difference absorption spectroscopy (Figure 3 and Figure S23,
Supporting Information).
2.5. C60−bodipy Dyads as Organic Triplet Photo-
sensitizers for TTA Upconversion. The C60 dyads C-1 and
C-2 were used as organic triplet photosensitizers for TTA
upconversion (Figure 5, perylene as the triplet acceptor).5a,6,7a
Figure 3. (a) Nanosecond time-resolved transient difference
absorption spectra of C-1 (λex = 532 nm). (b) Decay traces of the
transient absorption of C-1 at 720 nm. In deaerated toluene, 25 °C.
triplet excited state of C60.32a No bleaching of the bodipy unit at
515 nm was found, where the ground state of bodipy shows
strong absorption, indicating that the triplet excited state of the
dyad is exclusively localized on C60, not on the bodipy unit. We
attribute this C60-localized triplet state to the lower T1 state
energy level of C60 (1.50 eV)39 rather than C-1-L (1.51 eV,
approximated by DFT calculations). The transient was
significantly quenched in aerated solution, which confirms its
triplet-state feature, excluding formation of intramolecular
charge-transfer species (Figure S21, Supporting Informatio-
n).32a The lifetime of the triplet excited state of the dyad was
determined to be 33.3 μs by monitoring the decay kinetics at
720 nm (Figure 3), which is slightly shorter than the triplet
excited state lifetime of the unsubstituted C60 (40.6 μs) (Table
1).31,40
Similar transient absorption spectra were observed for C-2
(Figure S23, Supporting Information). A C60-localized T1
excited state was observed for C-2 (τT = 35.2 μs, Figure S23,
Supporting Information, and Table 1). It is remarkable that the
energy level of the T1 state of C-2-L is not significantly
decreased compared to that of C-1-L, indicated by the
localization of the triplet excited state on C60 unit, not the C-
2-L unit in C-2, although the absorption of C-2 is red-shifted
by 76 nm vs C-1-L. This high T1 state energy level may be due
to the unique distribution of the spin density of the T1 state of a
similar chromophore; that is, the spin density is localized on
one of the two Bodipy units of the antenna.13a In other words,
the limited π-conjugation of the T1 state, not fully delocalized
in the π-conjugation framework of the chromophore, is
responsible for the apparently high T1 state energy level of
the antenna C-2-L in C-2.13a
Figure 5. TTA upconversion with (a) C-1 (λex = 532 nm) and (b) C-2
(λex = 589 nm) as the triplet photosensitizers (1.0 × 10−5 M) and
perylene as the acceptor (4.1 × 10−4 M). The asterisks in (a) and (b)
indicate the scattered laser. In toluene. Excited with CW lasers (532
and 589 nm), 12 mW, 20 °C.
For C-1, weak fluorescence of C60 was observed at 710 nm
upon laser excitation. Upon addition of perylene, an intensive
blue emission at 470 nm was observed. Irradiation of perylene
alone at 532 nm did not produce this emission band, thus the
upconversion with C-1/perylene is confirmed. The upconver-
sion quantum yield (ΦUC) with C-1 was determined as 2.9%
(Table 2). This value is significant considering the intense
Table 2. Triplet Excited-State Lifetimes (τT), Stern−Volmer
Quenching Constant (KSV), and Bimolecular Quenching
a
Constants (kq) of the Chromophore/C60 Dyads
τT
(μs)
Ksv
kq
ΦUC
d
(104 M−1
)
(109 M−1 s−1
)
(%)
η
b
C-1
33.3
35.2
40.6
26.9
3.19
4.40
1.01
28.9
0.96
1.25
0.25
10.7
2.9
7.0
1303
5769
c
C-2
b
b
C60
2.4
30.9
6171
BisBDPI
11.3
a
Perylene was used as the quencher. In deaerated toluene solution, 20
b
°C. Excited with 532 nm laser, with the prompt fluorescence of 2-
iodo-4,4-difluoro-1,3,5,7-tetramethyl-8-phenyl-4-bora-3a,4a-diaza-s-in-
dacene as the standard (compound iodo-BDP in Scheme 1. Φ = 2.7%
c
in MeCN). Excited with 589 nm laser, with the prompt fluorescence
2.4. Spin Density Analysis of the Dyads. In order to
confirm the C60-localized T1 state of the dyads, the spin density
surfaces were studied (Figure 4). For both C-1 and C-2, the
spin densities are exclusively localized on C60, and the light-
harvesting bodipy does not contribute to the spin density
surfaces. This result is in full agreement with the transient
of BisBDPI (Scheme 1) as the standard (Φ = 10.5% in toluene).
d
Overall upconversion capability,6 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.5a In M−1
cm−1.
Figure 4. Spin-density surface of C-1 and C-2 at the triplet state. Calculated at B3LYP/6-31G(d) level with Gaussian 09W.
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absorption of C-1,6 since the overall upconversion capability
can be more precisely evaluated by η = ε × ΦUC (ε is the molar
extinction coefficients of the sensitizer),6 instead of the ΦUC
value alone.5a C60 alone as the sensitizer also shows
upconversion (ΦUC = 2.4%). However, C60 shows very weak
absorption at 532 nm (ε = 1289 M−1 cm−1); thus, the η value of
C60 (η = ε × ΦUC = 30.9 M−1 cm−1) is at least 40-fold weaker
than that of C-1 (η = ε × ΦUC = 44960 M−1 cm−1 × 2.9% =
1303 M−1 cm−1).
C-2 shows red-shifted absorption (590 nm). Significant
upconversion was observed with C-2 (Figure 5b, λex = 589 nm).
The ΦUC was determined as 7.0% (Table 2). Interestingly, with
C60 as the sensitizer no upconversion was observed with
excitation at 589 nm laser, due to the much weaker absorption
of C60 at 589 nm. These results demonstrated that the light-
harvesting of the dyads is crucial for TTA upconversion with
the C60 dyads. Upconversion with C-1 and C-2 are visible with
the unaided eye (Figure 6a and 6b). Both C-1 and C-2 are
The lifetime of the emission for C-1/perylene was determined
as 60.5 μs. In a different experiment, the lifetime of the prompt
fluorescence of perylene alone was determined as 4.0 ns. Thus,
the exceptionally long-lived fluorescence of perylene in the
presence of C-1 (15000-fold of the prompt fluorescence
lifetime of perylene) can be assigned to the TTA upconversion,
i.e., the delayed fluorescence.9d Similar results were found for
C-2/perylene; in this case, the lifetime of the delayed
fluorescence of perylene was determined as 61.2 μs (see Figure
S28, Supporting Information). These time-resolved emission
studies of the samples in Figure 5 unambiguously demonstrated
that the emission is due to the TTA upconversion.
The triplet−triplet energy transfer (TTET) of the TTA
upconversion were studied by monitoring the quenching of the
triplet state lifetimes of the dyads by the triplet acceptor
perylene (Figure 8).5a,6 Smaller KSV values for C-1 (KSV = 3.19
Figure 8. Stern−Volmer plots generated from the triplet excited-state
lifetime (τT) quenching curves of the compounds measured as a
function of perylene concentration. In toluene, 20 °C.
Figure 6. Photographs of the emissions of triplet sensitizers alone and
the upconversion. TTA upconversion with (a) C-1 (λex = 532 nm) and
(b) C-2 (λex = 589 nm) as the triplet photosensitizers (1.0 × 10−5 M)
and perylene as the triplet acceptor (4.1 × 10−4 M). In toluene.
Excited with CW lasers, 12 mW, 20 °C.
× 104 M−1) and C-2 (KSV = 4.40 × 104 M−1) were observed
compared to that of the BisBDPI (Scheme 1), which has a
shorter τT value (KSV = 2.89 × 105 M−1, Table 2). Usually,
photosensitizers with a long-lived triplet excited state should
give high TTET quenching constants. The apparently smaller
KSV values of C-1 and C-2 may be due to the large molecular
size of the C60 dyads compared to BisBDPI (Scheme 1). Large
molecular size may make the diffusion of the sensitizers in
solution much slower. Furthermore, the distribution of the spin
density on the bulky spherical C60 moiety may also make the
TTET process less efficient. However, the TTA upconversion
performance with the C60 dyads are not compromised, due to
the versatility of the C60 dyads, such as the feasibly tunable
molecular structure, strong absorption of visible light, long-lived
triplet excited state, and most importantly the heavy atom-
independent predetermined ISC capability.
photostable, and no bleaching was observed (Figure S29,
Supporting Information). We noted that the TTA upconver-
sions of C-1 and C-2 are comparable to those of iodo-
Bodipys.13a
In order to unambiguously confirm that the blue emission
observed for the mixed solution of dyads and the acceptor
perylene (Figure 5) is due to the TTA upconversion, the
lifetime of the upconverted emission was measured (Figure 7).
The photophysical processes involved in the TTA
upconversion with the C60 dyads as the triplet photosensitizer
are summarized in Scheme 2, exemplified with C-2. First, the
singlet excited state of the bodipy antenna in C-2 was
populated upon photoexcitation (589 nm). Then the intra-
molecular Foster energy transfer from the antenna to the C
̈
60
Figure 7. (a) Delayed fluorescence observed in the TTA upconversion
with compound C-1 as triplet photosensitizer and perylene as the
triplet acceptor. Excited at 532 nm (nanosecond pulsed OPO laser
synchronized with spectrofluorometer) and monitored at 470 nm.
Under this circumstance, the compound C-1 is selectively excited and
the emission is due to the upconverted emission of perylene. In
deaerated toluene. c[sensitizers] = 1.0 × 10−5 M; c[perylene] = 4.1 ×
10−4 M; 20 °C. (b) The prompt fluorescence decay of perylene
determined in a different experiment (excited with picosecond 405 nm
laser, the decay of the emission was monitored at 470 nm), c[perylene]
= 1.0 × 10−5 M; 20 °C.
unit will produce the singlet excited state of C60. With the
intrinsic ISC property of C60, the triplet excited state of C60 was
populated. With time-resolved spectroscopy and the spin
density surface analysis, we proved that the triplet excited
state of dyad C-2 is localized on C60. The TTET between the
C-2 and the triplet acceptor perylene will produce the triplet
excited state of the acceptors. TTA will give acceptor molecules
at the singlet excited states. Radiative decay from the singlet
excited state produces the upconverted fluorescence (delayed
fluorescence). From these processes, it is clear that the light-
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of the transients) were obtained with LP900 software. All samples in
flash photolysis experiments were deaerated with Ar for ca. 15 min
before measurement, and the gas flow was maintained during the
measurements. Syntheses of compounds 1 and 3 were reported
elsewhere.13a
Scheme 2. Jablonski Diagram of Photophysics of the C60−
bodipy Dyads and the Application of Dyads as Triplet
Photosensitizers for Triplet−Triplet Annihilation (TTA)
Upconversion Triplet States of the Sensitizers and Acceptors
a
Are Nonemissive)
3.2. Synthesis of Compound 2. Under Ar atmosphere, 5-formyl-
2-thiopheneboronic acid (102.9 mg, 0.66 mmol) was added to the
solution of compound 1 (100.0 mg, 0.22 mmol) in toluene (5 mL)
and ethanol (5 mL), and after thorough mixing, K3PO4·3H2O (117.2
mg, 0.44 mmol) and Pd(OAc)2 (2.6 mg, 0.013 mmol) were added.
The mixture was heated to 80 °C and was allowed to reflux for 8 h.
After complete consumption of the starting material, the solvents were
evaporated under reduced pressure, and the crude product was further
purified using column chromatography (silica gel, CH2Cl2/hexane =
1:1, v/v) to give 2 as a red power (40.0 mg). Yield: 41.9%. Mp: 173.2−
174.6 °C. 1H NMR (400 MHz, CDCl3): 9.87 (s, 1H), 7.73 (d, 1H, J =
3.7 Hz), 7.52 (d, 3H, J = 5.1 Hz), 7.31 (d, 2H, J = 5.6 Hz), 6.95 (d,
1H, J = 3.6 Hz), 6.07 (s, 1H), 2.62 (s, 3H), 2.60 (s, 3H), 1.40 (s, 6H).
13C NMR (100 MHz, CDCl3): 191.7, 158.0, 155.8, 155.3, 137.9, 137.5,
131.7, 130.4, 130.3, 114.7, 113.0, 96.8, 46.5, 12.9. MALDI-HRMS:
calcd ([C24H21BF2N2OS]+) m/z = 434.1436, found m/z = 434.1436.
3.3. Synthesis of Compound 4. N-Iodosuccinimide (NIS) (140
mg, 0.62 mmol) in dry CH2Cl2 (10 mL) was added dropwise into a
solution of compound 4 (200 mg, 0.62 mmol) in CH2Cl2 (50 mL)
within ca. 1 h at rt. After the addition, the reaction mixture was stirred
at 40 °C overnight. The reaction mixture was then concentrated under
reduced pressure, and the crude product was purified by column
chromatography (silica gel, hexane/CH2Cl2, 2:1, v/v). The second
band was collected to give the product as a purple solid (191.7 mg).
Yield: 68.7%. Mp: >250 °C. 1H NMR (400 MHz, CDCl3): 7.53−7.49
(m, 6H), 7.26−7.23 (m, 4H), 6.06 (s, 1H), 2.65 (s, 9H), 2.58 (s, 3H),
1.44 (s, 6H), 1.40 (s, 6H). 13C NMR (100 MHz, CDCl3): 159.5,
158.9, 157.7, 157.6, 146.1, 145.7, 144.0, 142.2, 134.5, 133.1, 132.4,
130.4, 129.9, 129.6, 129.5, 127.9, 127.8, 127.7, 115.1, 113.6, 86.2, 80.9,
80.1, 76.1, 75.0, 17.1, 14.8, 13.6, 13.3. MALDI-HRMS: calcd
([C42H35B2F4N4I]+) m/z = 820.2029, found m/z = 820.2050.
3.4. Synthesis of Compound 5. Under N2 atmosphere,
compound 4 (40.0 mg, 0.05 mmol), 4-ethynylbenzaldehyde (13.0,
0.10 mmol), Pd(PPh3)2Cl2 (3.5 mg, 0.005 mmol), PPh3 (2.6 mg, 0.01
mmol), and CuI (2.0 mg, 0.01 mmol) were dissolved in a mixed
solvent of THF/Et3N (3 mL/2 mL), and the flask was vacuumed and
backfilled with Ar several times. The mixture was heated at 50 °C for 6
h. The solvent was removed, and the crude product was purified by
column chromatography (silica gel, hexane/CH2Cl2, 2:1, v/v). The
dark purple band was collected, and evaporation of solvent gave a blue
a
Exemplified with C-2 as the triplet photosensitizer. The effect of the
light-harvesting ability and the lifetimes of the sensitizers on the
efficiency of TTA upconversion are shown (please note that the
vibration energy levels of each electronic state are omitted for clarity).
1
E is energy. GS is ground state (S0). (*BDP-C60) is singlet excited
state localized on BDP, and 1(BDP-*C60) is singlet excited state
localized on fullerene. IC is inner conversion. ISC is intersystem
crossing. 3(BDP-*C60) is the triplet excited state localized on fullerene.
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 populated). The emission band observed in the
TTA experiment is the 1perylene* emission (fluorescence). The
typical power density of the laser used in the upconversion is 170 mW
cm−2, too low to observe simultaneous two-photon absorption.
harvesting ability of the antenna, the lifetime of the triplet
excited state of the dyad, and the TTA efficiency are the main
factors that influence the performance of TTA upconversion.
2.6. Conclusions. In conclusion, visible light-harvesting C60
dyads were devised as a general molecular structure motif for
heavy atom-free organic triplet photosensitizers. The dyads are
based on visible light-harvesting organic chromophores (energy
donor, herein it is bodipy) and the spin converter C60 (energy
acceptor). Different from C60, the dyads show strong
absorption of visible light (at 515 and 590 nm, respectively).
Steady-state luminescence indicated efficient intramolecular
energy transfer from the bodipy antenna to the C60 energy
acceptor. Nanosecond time-resolved transient absorption and
spin density analysis proved that the triplet excited states of the
dyads are localized on the C60 moieties. The dyads were used as
triplet organic photosensitizers for triplet−triplet annihilation
upconversion, and quantum yield up to 7.0% was observed.
Our results pave the way for design of heavy atom-free organic
triplet photosensitizers with the predetermined intersystem
crossing property. To replace the currently used phosphor-
escent transition-metal complex, triplet photosensitizers with
these heavy atom free C60 dyads as organic triplet photo-
sensitizers will exert substantial impact in the areas such as TTA
upconversion, photocatalysis, photovoltaics, and photodynamic
therapy reagents (PDT).
1
solid (35.1 mg). Yield: 85.5%. Mp: >250 °C. H NMR (400 MHz,
CDCl3): 10.00 (s, 1H), 7.85 (d, 2H, J = 7.8 Hz), 7.59 (d, 2H, J = 8.0
Hz), 7.54−7.50 (m, 6H), 7.26 (m, 4H), 6.06 (s, 1H), 2.73 (s, 3H),
2.68 (s, 3H), 2.65 (s, 3H), 2.58 (s, 3H), 1.53 (s, 3H), 1.48 (s, 3H),
1.44 (s, 3H), 1.40 (s, 3H). 13C NMR (100 MHz, CDCl3): 191.5,
171.9, 160.3, 159.0, 157.5, 146.2, 145.7, 142.4, 135.4, 133.2, 131.8,
130.0, 129.6, 129.5, 127.9, 127.8, 121.8, 115.9, 115.8, 80.1, 79.1, 76.4,
76.1, 14.8, 14.3, 13.7, 13.3. MALDI-HRMS: calcd
([C51H40B2F4N4O]+) m/z = 822.3324, found m/z = 822.3253.
3.5. Synthesis of Compound C-1-L. Compound C-1-L was
obtained following a procedure similar to that for compound 2, except
2-thiopheneboronic acid (52.3 mg, 0.44 mmol) was used instead of 5-
formyl-2-thiopheneboronic acid. A red solid was obtained (56.6 mg).
1
Yield: 63.4%. Mp: 155.5−156.8 °C. H NMR (400 MHz, CDCl3):
7.51−7.47 (m, 3H), 7.347−7.30 (m, 3H), 7.07 (t, 1H, J = 5.1 Hz),
6.84 (d, 1H, J = 3.4 Hz), 6.02 (s, 1H), 2.58 (s, 6H), 1.39 (s, 3H), 1.36
(s, 3H). 13C NMR (100 MHz, CDCl3): 156.7, 154.3, 144.1, 142.3,
140.3, 135.2, 134.7, 132.1, 130.9, 129.4, 129.2, 128.1, 127.8, 127.3,
126.0, 121.9, 30.5, 14.9, 14.7, 13.5, 12.9. TOF HRMS ESI+: calcd
([C23H21BF2N2S]+) m/z = 406.1487, found m/z = 406.1498.
3. EXPERIMENTAL SECTION
3.1. General Methods. Fluorescence lifetimes were measured on
an OB920 luminescence lifetime spectrometer. The nanosecond time-
resolved transient absorption spectra were detected by an LP920 laser
flash photolysis spectrometer. The transient signals were recorded on a
digital oscilloscope. The lifetime values (by monitoring the decay trace
3.6. Synthesis of Compound C-2-L. Compound C-2-L was
obtained following a procedure similar to that for compound 5, except
ethynylbenzene (13.0 mg, 0.12 mmol) was used instead of 4-
ethynylbenzaldehyde. A blue solid was obtained (22.6 mg). Yield:
1
95.0%. Mp: >250 °C. H NMR (400 MHz, CDCl3): 7.52−7.45 (m,
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The Journal of Organic Chemistry
Article
9H), 7.35 −7.26 (m, 6H), 6.05 (s, 1H), 2.72 (s, 3H), 2.67 (s, 3H),
2.65 (s, 3H), 2.58 (s, 3H), 1.51 (s, 3H), 1.47 (s, 3H), 1.44 (s, 3H),
1.39 (s, 3H). 13C NMR (100 MHz, CDCl3): 160.1, 158.9, 157.4,
146.1, 145.5, 142.2, 135.2, 134.4, 134.1, 133.0, 131.7, 129.8, 129.6,
129.5, 129.3, 127.8, 127.7, 125.8, 121.6, 115.6, 86.2, 79.9, 78.9, 76.2,
75.9, 14.6, 14.1, 13.5, 13.2. MALDI-HRMS: calcd ([C50H40B2F4N4]+)
m/z = 794.3375, found m/z = 794.3428.
The delayed fluorescence of the upconversion was measured with a
nanosecond pulsed laser, Opolett 355II + UV nanosecond pulsed
laser, typical pulse length: 7 ns. Pulse repetition: 20 Hz. Peak OPO
energy: 4 mJ. The wavelength is tunable from 210 to 355 nm and from
420 to 2200 nm, which is synchronized to an FLS 920
spectrofluorometer. The pulsed laser was sufficient to sensitize the
TTA upconversion. The decay kinetics of the upconverted
fluorescence (delayed fluorescence) were monitored with an FLS920
spectrofluorometer (synchronized to the OPO nanosecond pulsed
laser). The prompt fluorescence lifetime of the triplet acceptor
perylene was measured with an EPL picoseconds pulsed laser (405
nm) which was synchronized to the FLS 920 spectrofluorometer.
3.10. DFT Calculations. The geometries of the compounds were
optimized using density functional theory (DFT) with the B3LYP
functional and 6-31G(d) basis set. There are no imaginary frequencies
for all optimized structures. The spin density surfaces of the dyads
were calculated at the B3LYP/6-31G(d) level. All calculations were
performed with Gaussian 09W.41
3.7. Synthesis of Compound C-1. Under N2 atmosphere,
compound 2 (35.0 mg, 0.08 mmol) and sarcosine (24.0 mg, 0.27
mmol) were added to the solution of C60 (57.6 mg, 0.08 mmol) in
toluene (50 mL). The mixture was heated to 110 °C, and the mixture
was refluxed for 22 h and then cooled slowly to ambient temperature.
The solvent was then removed under reduced pressure. The residue
was purified by column chromatography (silica gel, toluene/hexane,
5:1, v/v). A red solid was obtained (32.5 mg). Yield: 34.4%. Mp: >250
1
°C. H NMR (400 MHz, CDCl3): 7.47 (m, 3H), 7.35 (s, 1H), 7.26
(m, 2H), 6.74 (s, 1H), 6.01 (s, 1H), 5.25 (s, 1H), 4.99 (d, 1H, J = 9.0
Hz), 4.27 (d, 1H, J = 9.4 Hz), 2.95 (s, 3H), 2.57 (s, 3H), 2.51 (s, 3H),
1.37 (s, 3H), 1.32 (s, 3H). 13C NMR (100 MHz, CDCl3): 156.9,
156.0, 154.1, 153.4, 153.1, 150.2, 147.5, 146.5, 146.3, 146.1, 145.7,
145.5, 144.9, 144.5, 144.3, 143.4, 142.8, 142.3, 142.1, 141.7, 140.3,
137.1, 135.1, 129.4, 129.3, 128.3, 128.1, 127.2, 122.0, 79.6, 70.1, 68.9,
40.6, 29.9, 14.7, 12.9. MALDI-HRMS calcd ([C86H26BF2N3S]−) m/z =
1181.1909, found m/z = 1181.1980. Anal. Calcd for C86H26BF2N3S +
0.5C7H8 + 0.2DMF: C, 87.08; H, 2.57; N, 3.60. Found: C, 87.13; H,
2.75; N, 3.60.
ASSOCIATED CONTENT
■
S
* Supporting Information
Experimental procedures, molecular structure characterization,
additional spectra, and coordinates of the optimized geometries
of C-1 and C-2. This material is available free of charge via the
Internet at http://pubs.acs.org.
3.8. Synthesis of Compound C-2. Compound C-2 was obtained
following a procedure similar to that for compound C-1, except
compound 5 (30.0 mg, 0.036 mmol) was used instead of compound 2.
AUTHOR INFORMATION
■
1
A blue solid was obtained (30.8 mg). Yield: 54.4%. Mp: >250 °C. H
Corresponding Author
*E-mail: zhaojzh@dlut.edu.cn.
NMR (400 MHz, CDCl3): 7.53−7.50 (m, 3H), 7.26 (m, 2H), 6.05 (s,
1H), 5.00 (d, 1H, J = 9.5 Hz), 4.93 (s, 1H), 4.27 (d, 1H, J = 9.2 Hz),
2.80 (s, 3H), 2.70 (s, 3H), 2.66 (s, 3H), 2.65 (s, 3H), 2.57 (s, 3H),
1.50 (s, 3H), 1.46 (s, 3H), 1.44 (s, 3H), 1.39 (s, 3H). MALDI-HRMS:
calcd ([C113H45N5B2F4]−) m/z = 1569.3797, found m/z = 1569.3793.
Anal. Calcd for C113H45N5B2F4 + 0.25DMF: C, 86.01; H, 2.99; N,
4.62. Found: C, 86.02; H, 3.22; N, 4.62.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the NSFC (20972024 and 21073028), the Royal
Society (UK) and NSFC (China−UK Cost-Share Science
Networks, 21011130154), the Fundamental Research Funds
for the Central Universities (DUT10ZD212), Ministry of
Education (NCET-08-0077), and Dalian University of
Technology for financial support.
3.9. TTA Upconversion. A diode-pumped solid-state laser (532
and 589 nm, continues wave, CW) was used for the upconversions.
The diameter of the laser spot was ca. 3 mm. For the upconversion
experiments, the mixed solution of the fullerene dyads (triplet
sensitizer) and perylene (triplet acceptor) was degassed for at least
15 min with N2 or Ar, and the gas flow was maintained during the
measurement. Then the solution was excited with a laser. The
upconverted fluorescence of perylene was observed with a
spectrofluorometer. In order to repress the scattered laser, a black
box was put behind the fluorescent cuvette to trap the laser beam after
it passed through the cuvette. The photographs of the upconversion
were taken with a Samsung NV5 CCD digital camera. The exposure
time of the photos is 1 s (the default value of the digital camera under
the experimental conditions).
The upconversion quantum yields (ΦUC) were determined with the
prompt fluorescence of 2,6-diiodo-1,3,5,7-tetramethyl-8-phenyl-4,4-
difluoroboradiazaindacene (iodo-BDP, structure shown in Scheme 1)
(Φ = 2.7% in MeCN) and BisBDPI (structure shown in Scheme 1, ΦF
= 10.5% in toluene) as the standards. The upconversion quantum
yields were calculated with eq 1, where ΦUC, Aunk, Iunk, and ηunk
represent the quantum yield, absorbance, integrated photolumines-
cence intensity, and the refractive index of the solvents (eq 1). The
equation is multiplied by factor 2 in order to make the maximum
quantum yield unity.5a
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