Using C60-bodipy dyads that show strong absorption of visible light and long-lived triplet excited states as organic triplet photosensitizers for triplet-triplet annihilation upconversion

Article abstract of DOI:10.1039/c2jm34353c

The synthesis of visible light-harvesting C₆₀-bodipy dyads (bodipy = boron-dipyrromethene), and the study of the photophysical properties and the application of dyads as heavy-atom free organic triplet photosensitizers for triplet-triplet annih

Full text of DOI:10.1039/c2jm34353c

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PAPER
Using C60-bodipy dyads that show strong absorption of visible light and long-
lived triplet excited states as organic triplet photosensitizers for triplet–triplet
annihilation upconversion†
*
Pei Yang, Wanhua Wu, Jianzhang Zhao, Dandan Huang and Xiuyu Yi
Received 4th July 2012, Accepted 9th August 2012
DOI: 10.1039/c2jm34353c
The synthesis of visible light-harvesting C60-bodipy dyads (bodipy ¼ boron-dipyrromethene), and the
study of the photophysical properties and the application of dyads as heavy-atom free organic triplet
photosensitizers for triplet–triplet annihilation (TTA) based upconversion, are reported. By attaching
carbazole units to the p-core of the bodipy chromophore via an ethynyl linker, the absorption wavelength
of the antenna in the dyads is readily tuned from 504 nm for the unsubstituted bodipy, to 538 nm (one
carbazole unit, 3 ¼ 61 800 Mꢀ1 cmꢀ1) and 597 nm (two carbazole units, 3 ¼ 58 200 Mꢀ1 cmꢀ1). Upon
photoexcitation at 538 nm (dyad C-1) or 597 nm (dyad C-2), intramolecular energy transfer from the
antenna to the C60 unit occurs, and as a result, the singlet excited state of the C60 unit is populated.
Subsequently, with the intrinsic intersystem crossing (ISC) of C60, the triplet excited state of the C60 unit is
produced (sT up to 24.5 ms). Thus, without the need for any heavy atoms, the triplet excited state of the
dyads was populated upon visible light excitation. The population of the C60-localized triplet excited state
of the dyads was confirmed by nanosecond time-resolved transient difference absorption spectra and spin
density analysis. The dyads were used as triplet photosensitizers for TTA upconversion and upconversion
quantum yields of up to 2.9% were observed.
required the heavy atom effect of Pt(II), Ir(III) or iodine, etc. As a
Introduction
result, these compounds are generally not cost-efficient and in
some circumstances the derivatization of the compounds, while
retaining the ISC property, is difficult. From the point of view of
photochemistry, it is highly desirable to develop heavy-atom free
organic triplet photosensitizers, especially those with variable
structures and predictable ISC properties. However, it is still a
substantial challenge to design an organic chromophore without
any heavy atoms which possesses the predetermined ISC prop-
erty.16a Porphyrins and some perylenebisimides are such organic
triplet photosensitizers,1,23 but it is clear that the molecular
diversity of this class of heavy-atom free organic triplet photo-
sensitizers needs to be increased.
Triplet photosensitizers have attracted much attention, due to
their applications in photodynamic therapy (PDT),1–3 photo-
catalysis,4–8 and more recently, triplet–triplet annihilation (TTA)
based upconversions.9–15 The typical triplet photosensitizers for
TTA upconversions are Pt(II) porphyrin complexes, and Ru(II) or
Ir(III) complexes.11–13 In these complexes, the heavy atom effect of
the transition metal atoms facilitates the intersystem crossing
(ISC), thus the triplet excited state can be populated upon
photoexcitation.16 However, these complexes usually show rela-
tively weak absorption in the visible range (3 value is smaller
compared to typical chromophores such as rhodamine, fluores-
cein, etc.).17–20 Recently iodo- and bromo-bodipy (bodipy ¼
boron dipyrromethane) have been devised as triplet photosen-
sitizers for sensitizing singlet oxygen (1O2) and TTA based
upconversion.2a,21,22 These photosensitizers show strong absorp-
tion in the visible range, which is a desired property for triplet
photosensitizers. However, all these triplet photosensitizers
Similar challenges exist for TTA upconversion, which has
attracted much attention due to its potential application in
photovoltaics, luminescent bioimaging, photocatalysis and
oxygen sensing.11–13,24 TTA upconversion shows some advan-
tages over the conventional upconversion methods, such as
those with rare earth metal materials,25 and two-photon
absorption dyes.26 Currently, most of the triplet photosensi-
tizers are Pt(II) porphyrin complexes, Pt(II) complexes, and
Ir(III) acetylide complexes, etc.12 Organic triplet photosensitizers
for TTA upconversion are rarely reported,22,24c,27,28 and the
development of heavy-atom free organic triplet photosensitizers
is highly desired.12 To date this is still a major challenge in
photochemistry.16a
State Key Laboratory of Fine Chemicals, School of Chemical Engineering,
Dalian University of Technology, E-208 Western Campus, 2 Ling-Gong
Road, Dalian 116024, P. R. China. E-mail: zhaojzh@dlut.edu.cn; Web:
http://finechem.dlut.edu.cn/photochem; Fax: +86 411 84986236; Tel: +86
0411 84986236
† Electronic supplementary information (ESI) available: 1H and 13C
NMR, and MS spectra. Absorption and emission spectra,
upconversion and DFT calculation details. See DOI: 10.1039/c2jm34353c
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In order to address the above challenges, herein we propose a
general molecular structure motif for organic triplet photosen-
sitizers, i.e. the C60-organic chromophore dyads.29–48 The triplet
excited states of the C60 dyads are populated upon photooxida-
tion without any heavy atoms in these compounds, despite
complicated mechanisms which may involve direct ISC or charge
separation and recombination, etc.49–52 The molecular design
rationale lies in the notion that fullerene C60 shows efficient ISC,
that is, the triplet excited state of C60 is efficiently populated
upon photoexcitation.53 Therefore the fluorescence quantum
yield of C60 is very low (FF < 0.1%). However, C60 itself is not an
ideal triplet photosensitizer because its absorption in the visible
range is very weak, for example, 3 is 820 Mꢀ1 cmꢀ1 at 530 nm.49
This weak absorption is due to the symmetry forbidden transi-
tions of C60 (S0 / S1, etc.).53a However, the low-lying S1 state of
C60 ensures an intramolecular energy transfer from the light-
harvesting antenna to the C60 unit in a dyad.30,33 Upon photo-
excitation, the antenna is excited, and so an intramolecular
energy transfer occurs from the antenna to the C60 unit. As a
result, the S1 state of C60 will be populated (energy level is
1.72 eV). Due to the efficient intrinsic ISC property of C60, the
triplet excited state of the C60 will be populated. Thus, a triplet
excited state will be produced upon photoexcitation in the visible
range of the spectrum.
extended to the red range by extension of the p-conjugation
framework by attaching carbazole to the bodipy core via an
ethynyl bond (–C^C–), to ensure efficient electronic communi-
cation.22,66 Previously, (bodipy)3–porphyrin–C60 pentad was
studied.39 A flexible linker was used between the bodipy antenna
and C60.39 Herein, a rigid linker was used for C-1 and C-2
(Scheme 1). The preparation of the light-harvesting antenna is
based on the rich derivatization of the carbazole and bodipy
moieties. The ethynyl bonds were introduced by the Pd(0) cata-
lyzed Sonogashira coupling. Formyl groups were attached to the
carbazole by the Vilsmeier–Haack reaction. Confalone reac-
tion,67 or 1,3-dipolar cycloaddition of azomethine ylides of the
C60, sarcosine and the functionalized carbazole-bodipy leads to
the C60-bodipy dyads. All the compounds were prepared in
moderate to good yields. Control compounds L-1 and L-2 were
also prepared for the photophysical studies. The molecular
structures of the compounds were fully confirmed by 1H NMR,
13C NMR, and high resolution mass spectrometry (HR MS). We
noted the broadening signal of the 1H NMR spectra, which may
be due to the restricted rotation of the antennas attached to the
C60 unit (ESI†).68 Previously, styryl-bodipy-C60 dyads were
reported.69 Herein we used the acetylene bond as the p-conju-
gating linker.
C60-organic chromophore dyads have been used for photo-
voltaics,47 and to improve the optical limiting property of C60,
etc.30,33,39,43,54–57 Previously, a C60 containing water-soluble
polymer was studied for its PDT effect, but the polymer only
gave weak absorption of visible light.55a Ru(II) and Re(I)-con-
taining methanofullerenes were also studied, and the production
of singlet oxygen (1O2) was observed.55b Improved optical
limiting was observed for a C60 derivative bearing a star-shaped
multi-photon absorption chromophore.55c Very recently we
reported the application of a visible light-harvesting C60 for
photooxidation.5 Recently we also reported the first application
of a C60 dyad as a triplet photosensitizer for TTA upconver-
sion,58 but it is clear that the molecular structures of these new
triplet photosensitizers need to be further explored in order to
study the relationship between the molecular structure and the
property of the dyads.
Steady-state electronic spectroscopy (absorption and emission)
Firstly the UV-vis absorption of the compounds were studied
(Fig. 1). C60 gives very weak absorption in the visible range. The
absorption maximum is located at 335 nm (3 ¼ 59 400 Mꢀ1
cmꢀ1). The antenna L-1 gives strong absorption at 539 nm (3 ¼
51 700 Mꢀ1 cmꢀ1). Interestingly, C-1 gives a similarly strong
absorption at 538 nm (3 ¼ 61 800 Mꢀ1 cmꢀ1) (Fig. 1). With two
carbazole units connected to the bodipy core, antenna L-2 gives a
red-shifted absorption at 599 nm (3 ¼ 65 500 Mꢀ1 cmꢀ1).
Accordingly, the dyad C-2 gives a similarly strong absorption
at 597 nm (3 ¼ 58 200 Mꢀ1 cmꢀ1). The absorptions of the dyads
are roughly the sum of the absorption of C60 and the antenna.
Thus, the electronic interaction of the antenna and C60 unit is
weak at the ground state.33,45 It should be noted that the dyads
show intense absorption in the visible range, which is beneficial
for potential applications. The absorption of the dyads are
slightly red-shifted compared to the bodipy-C60 dyads containing
no electron-donating carbazole moieties.58 The absorption of C-1
and C-2 are red-shifted compared to a similar C60-bodipy dyad.33
In order to confirm the intramolecular energy transfer, the
fluorescence of the dyads were studied (Fig. 2). C60 gives a very
weak emission upon excitation at 530 nm, due to its weak
absorption at 530 nm (3 ¼ 820 Mꢀ1 cmꢀ1) and low fluorescence
quantum yield (FF < 0.1%). L-1 gives an emission at 609 nm
(FF ¼ 3.9%). Thus, the S1 state energy level of L-1 (2.04 eV) is
higher than the S1 state energy level of C60 (1.72 eV), and the
intramolecular energy transfer from the antenna to C60 unit is
possible. The fluorescence of C-1 is very weak, that is, the
emission of the antenna is completely quenched. Thus, we
propose that efficient intramolecular energy transfer occurs for
dyad C-1 upon photoexcitation. The energy transfer efficiency
was calculated as 97.4% based on the quenched fluorescence. The
energy transfer is supported by the enhanced fluorescence emis-
sion of the C60 unit in the dyads (Fig. 2a inset).34,37,45,69,70
Bodipy was selected as the light-harvesting antenna due to its
strong absorption of visible light and versatile derivatization
chemistry.59–62 Herein we attached carbazole units to the bodipy
core. Thus, the absorption can be readily tuned from 504 nm for
the unsubstituted bodipy, to 538 nm (dyad C-1) or 597 nm (dyad
C-2). Carbazole is a versatile chromophore that is widely used in
fluorophores or phosphores.63 The two dyads are used as triplet
photosensitizers for TTA upconversion, and an upconversion
quantum yield of up to 2.9% was observed.
Results and discussion
Design and synthesis
The basic strategy of our molecular design rationale is to use
bodipy as the core of the light-harvesting antenna. Bodipy is
well-known for its strong absorption in the visible range
(unsubstituted bodipy gives strong absorption at ca. 504 nm, 3 ¼
80 000 Mꢀ1 cmꢀ1).59–66 The absorption wavelength is easily
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Scheme 1 Synthesis of the C60-bodipy dyads C-1 and C-2. The molecular structures of the control compounds L-1 and L-2 as the light-harvesting
antenna in C-1 and C-2, as well as the standard for measurement of upconversion quantum yields are also included. (i) PdCl2(PPh3)2, PPh3, CuI, NEt3,
ethynyltrimethylsilane, argon atmosphere, 80 ꢁC, 4 h; then K2CO3, r.t. 6 h; (ii) PdCl2(PPh3)2, PPh3, CuI, NEt3, THF, argon atmosphere, 60 ꢁC, 6 h; (iii)
sarcosine, C60, toluene, reflux.
Similar emission profiles were observed for dyad C-2 and
the components (Fig. 2b). For example, the antenna L-2
shows an intense emission at 648 nm, but the emission was
completely quenched in the dyad C-2. Similar to C-1, fluores-
cence of C60 was observed for C-2, due to the light-harvesting
effect of the antenna. The intramolecular energy transfer effi-
ciency was estimated as 97.7% based on the quenched emission
of antenna in C-2. It should be pointed out that the intra-
molecular charge transfer from the antenna to C60 unit cannot
be excluded.33,69,71
Electrochemistry
The redox potentials of the antennas and the dyads were
studied (Table 1 and ESI†).71 The first oxidation (bodipy
moiety) and the first reduction (C60 moiety) of the dyad C-1 are
0.78 and ꢀ1.27 V, respectively. Thus, the driving force for the
charge recombination process of the charge separated state of
the dyad C-1 was determined as 2.06 eV. The driving force for
the charge-separation process (ꢀDGCS, with bodipy as the
electron donor and C60 as the electron acceptor) of C-1 was
calculated as 0.01 eV. Similarly, the ꢀDGCS value of C-2 was
estimated as 0.20 eV. Therefore, the photo-induced charge
separation is thermodynamically allowed.71 Our later study
indicated that the triplet excited state of the dyads was popu-
lated, which is either due to the direct energy transfer/ISC or the
Fig. 1 UV-visabsorptionspectraoftheC60-bodipy dyads C-1and C-2, and
thelight-harvestingantennaofthedyads;c¼ 1.0ꢂ 10ꢀ5 Mintoluene, 20ꢁC.
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Nanosecond time-resolved transient difference absorption
spectroscopy
The intramolecular energy transfer from the antenna to the C60
unit will produce the singlet excited state of C60. In turn, the
triplet excited state of C60 unit will be populated via the ISC of
C60. In order to confirm the triplet excited states upon photo-
excitation, nanosecond time-resolved transient difference
absorption spectroscopy of the compounds was carried out
(Fig. 3).45,69
Upon pulsed laser excitation of C-1 at 532 nm, at which the
absorption is mainly assigned to the antenna part, and the
absorption of C60 unit can be neglected, transience at 367 and
700 nm was observed (Fig. 3a), which are the characteristic
absorptions of the triplet excited state of C60.33 Interestingly, the
bleach of the ground state absorption of the antenna at 538 nm
was not observed, indicating that the T1 state is exclusively
localized on C60 unit. (Fig. 3a). The lifetime of the transient was
determined as 20.2 ms (in deaerated solution), which was signif-
icantly reduced to 0.3 ms in aerated solution. The production of
singlet oxygen (1O2) in aerated solution with the C-1 was
confirmed by photooxidation of 1,5-dihydroxylnaphthelene (see
ESI†, Fig. S21). These results indicated that the transience
observed in the time-resolved absorption spectra is due to the
C60-localized triplet excited state, and that the charge separated
state is not significant.69,72
Similar transience was observed for C-2 (Fig. 3c). The lifetime
of the C60-localized triplet state of C-2 was determined as 24.5 ms,
which was reduced to 0.35 ms in aerated solution. The photo-
physical properties of the antenna and the dyads were summa-
rized in Table 2. We noted that the triplet excited state lifetimes
of C-1 and C-2 are slightly shorter than the recently reported C60-
bodipy dyads.58 Previously it was found that the triplet state is
delocalized over bodipy antenna and C60 part in a C60-bodipy
dyad.33 For C-1 and C-2, however, the triplet state is exclusively
Fig. 2 (a) Emission spectra of L-1, C-1 and C60, lex ¼ 530 nm. (b)
Emission spectra of L-2, C-2 and C60, lex ¼ 570 nm. In toluene, 1.0 ꢂ
10ꢀ5 M, 20 ꢁC.
Table 1 Electrochemical data of L-1, L-2, C-1 and C-2a
Oxidation
Reduction
E1b/V
E2c/V
E1b/V
E2c/V
L-1d
C-1e
L-2e
C-2e
+0.61
+0.78
+0.79
+0.76
+1.21
+1.32
+1.16
+1.32
ꢀ1.45
ꢀ1.27
ꢀ1.48
ꢀ1.11
—
ꢀ1.54
—
ꢀ1.51
a
Determined by a Pt working electrode in deaerated CH2Cl2 + 0.1 M n-
Bu4NBF4 at room temperature. CVs were obtained using a scan rate of
50 mV sꢀ1 and a step potential of 1 mV. Values of potentials (E) for all
b
compounds versus silver–silver chloride electrode.
process. Bielectronic process. In solution. Solid film on glassy
carbon working electrode.
One electron
c
d
e
charge recombination.33,69 Previously it was proposed that
the singlet–singlet energy transfer from the antenna is ultra-fast
as is often observed in such dyads.71a Upon population of
the fullerene singlet, the electron transfer is then not allowed
thermodynamically, thus, direct population of the triplet state
by intramolecular energy transfer and ISC of C60 is also
possible.71a
Fig.
3 Nanosecond time-resolved transient difference absorption
spectra of (a) C-1 (lex ¼ 532 nm) and (c) C-2 (lex ¼ 532 nm); decay traces
of the transience of (b) C-1 at 700 nm, and (d) C-2 at 700 nm. In deaerated
toluene, 1.0 ꢂ 10ꢀ5 mol dmꢀ3, 20 ꢁC.
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Table
2 Photophysical parameters of the C60 dyads and the
result was obtained for dyad C-2, for which the spin density
surface is localized on the C60 unit. These results are in full
agreement with the time-resolved transient absorption spectra of
the dyads (Fig. 3).
componentsa
labs
(nm)
lem
(nm)
3 (Mꢀ1 cmꢀ1 b
)
FF
s
c
C60
L-1
L-2
C-1
C-2
335
539
599
538
597
—
59 400
51 700
65 500
61 800
58 200
—
18.6 msd
1.8 ns
e
609
648
598
642
3.9%
13.1%
0.1%
0.3%
C60-bodipy dyads as the organic triplet photosensitizer for the
TTA upconversion
2.2 nse
20.2 msd
24.5 msd
C60 dyad C-1 and dyad C-2 show strong absorptions of visible
light and long-lived triplet excited states, and as a consequence,
they are suitable to be used as triplet photosensitizers for TTA
upconversion.11–13 Thus, the use of the dyads in TTA upcon-
version was studied (Fig. 5). Previously, C60 dyads with strong
visible light absorption or panchromatic absorption were
prepared,33,35,39,44,69,73 but very few visible light-harvesting C60
dyads were used for sensitizing a photophysical process.36,74 Aza-
bodipy-C60 dyad was reported recently, and the population of
the triplet excited of the aza-bodipy antenna was proposed.45
However, the dyad was not employed for sensitizing a photo-
physical process. The triplet excited states of bodipy-C60 dyads
were also studied.33 Very recently we used C60-bodipy dyads as
triplet photosensitizers,58 but much room is left for the applica-
tion of the triplet excited states of C60-organic chromophore
dyads.
a
In toluene (1.0 ꢂ 10ꢀ5 mol dmꢀ3). b Molar extinction coefficient at the
c
absorption maxima.
With 4,4-difluoro-1,3,5,7-tetramethyl-4-bora-
3a,4a-diaza-s-indacene (Bodipy) as the standard (F ¼ 72.0% in THF).
d
Triplet state lifetimes, measured by transient absorptions.
Fluorescence lifetimes.
e
localized on C60. Note the UV-vis absorption of C-2 is red-
shifted compared to C-1. However, the T1 state of C-2 is still
localized on C60, which means the T1 state energy level of the
antenna in C-2 is not decreased. This result is different from a
C60-bodipy dyad which shows a similar absorption wavelength
but for which the T1 state is localized on the styryl-bodipy part.69
C60 dyads with red-shifted absorptions but high T1 state energy
levels are beneficial for applications based on TTET process.
Upon excitation with a 532 nm laser (Fig. 5a), C-1 gives weak
emissions at 598 and 712 nm, which are due to the emission of the
antenna and C60 unit, respectively. With addition of perylene
(triplet acceptor/emitter), an intense emission in the 430–530 nm
range was observed. Irradiation of perylene alone with the
532 nm laser did not produce this emission band, thus confirming
the upconversion feature of the blue emission. The upconversion
quantum yield (FUC) was determined as 2.3%. Similar results
were observed for C-2, and a FUC of 2.9% was observed (Fig. 5b
and Table 3). To the best of our knowledge, this is the first time
that TTA upconversion has been carried out with organic triplet
photosensitizers (heavy atom-free) that show absorption in the
red range.11,12,24c,58 We envisage that a great amount of organic
triplet photosensitizers can be developed following our strategy
of using C60-organic chromophore dyads. We noted that the
upconversion quantum yields with C-1 and C-2 as triplet
photosensitizers are lower than the recently reported C60 dyad
triplet photosensitizers for TTA upconversion.58 The lower
Spin density analysis of the dyads
In order to confirm the localization of the triplet excited states of
the dyads from a theoretical perspective, the spin density surfaces
of the triplet state of the dyads were calculated by density
functional theory (DFT. Fig. 4).5,58 For dyad C-1, the spin
density is localized on the C60 unit and the antenna makes no
contribution. Thus, the spin density of C-1 confirms that the
triplet excited state of dyad C-1 is localized on C60. A similar
Fig. 5 TTA upconversion with (a) C-1 (lex ¼ 532 nm, laser) and (b) C-2
(lex ¼ 589 nm, laser) as the triplet photosensitizers and perylene as the
acceptor. The asterisks in (a) and (b) indicate the scattered laser. In
toluene: c (dyads) ¼ 5.0 ꢂ 10ꢀ6 M, c (perylene) ¼ 3.0 ꢂ 10ꢀ4 M, 20 ꢁC.
Excited with CW lasers (5 mW).
Fig. 4 Spin-density surface of C-1 and C-2 at the optimized triplet state
geometry. Toluene was used as solvent in the calculations. Calculated at
B3LYP/6-31G(d) level with Gaussian 09W.
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Table 3 Triplet excited state lifetimes (sT), Stern–Volmer quenching
constants (KSV), and bimolecular quenching constants (kq) of the chro-
mophore/C60 dyads as organic sensitizers. Perylene was used as the
quencher. In deaerated toluene solution, 20 ꢁC
Ksv (103
kq (109
FUC
(%)
sT (ms)
Mꢀ1
)
Mꢀ1 sꢀ1
)
C60
C-1
C-2
18.6
20.2
24.5
28.1
4.9
8.2
1.51
0.24
0.33
—
2.3a
2.9b
a
Excited with 532 nm laser, with the prompt fluorescence of 2,6-diiodo-
BDP (5b) as the standard (FF ¼ 2.7% in MeCN). b Excited with 589 nm
laser, with the prompt fluorescence of BisBDPI as the standard (FF
10.5% in toluene).
¼
upconversion quantum yields with C-1 and C-2 may be due to
the non-efficient production of the triplet excited state of the
dyads upon photooxidation, e.g. caused by the intramolecular
charge transfer.33,69,71a
For the TTA upconversions (Fig. 5), the fluorescence of the
dyads are not quenched. This is reasonable because the triplet
excited states, not the singlet excited states, were involved in the
triplet–triplet energy-transfer process (Scheme 2).11,12
Fig. 6 Photographs (a and b) of the emissions of triplet photosensi-
tizers alone and the upconversion. Corresponding CIE diagrams (c and
d) of the emission of sensitizers alone and in the presence of perylene
(upconversion). In toluene; excited with CW lasers, 5 mW, c (dyads) ¼
5.0 ꢂ 10ꢀ6 M, c (perylene) ¼ 3.0 ꢂ 10ꢀ4 M, 20 ꢁC.
The upconversion is visible to un-aided eyes (Fig. 6). Dyad C-1
gives a yellow-brown emission with 532 nm excitation (Fig. 6a).
Upon addition of perylene (triplet acceptor), the emission color
turns to blue, which is due to the upconversion. However, for C60
alone, no blue emission was observed, this is due to the weak
absorption of C60 at 532 nm, despite of the ISC ability of C60. The
green color in Fig. 6a is due to the scattered excitation laser. For
dyad C-1, theabsorptionat532nmisstrong(3¼ 58 400 Mꢀ1 cmꢀ1),
thus the upconversion emission intensity is strong.
In order to confirm that the blue emission is due to the
upconverted fluorescence, the luminescence lifetime of the blue
emission was measured (Fig. 7). The luminescence lifetime of the
upconversion with C-1/perylene was determined as 102.0 ms,
which is the delayed fluorescence (sDF) of the TTA upconver-
sion.75 The prompt fluorescence lifetime (sPF) of perylene was
determined as 4.2 ns. The long lifetime of the upconverted
emission confirmed the delayed fluorescence feature of the
A similar result was observed for C-2 (Fig. 6b). C-2 gives a
brown-red emission upon laser excitation at 589 nm. With the
addition of perylene, an intense blue emission was observed. For
the C60-perylene mixture, no blue emission was observed.
Scheme 2 Jablonski diagram of triplet–triplet annihilation (TTA) upconversion with C60-bodipy dyad C-1 as triplet photosensitizer. C60-1bodipy* is
the singlet excited state of C-1 localized on bodipy unit. ET stands for energy-transfer. 1C60*-bodipy is the singlet excited state of C-1 localized on C60
unit. 3C60-bodipy* is the triplet excited state of C-1 localized on bodipy unit. 3C60*-bodipy is the triplet excited state of C-1 localized on C60 unit. TTET
stands for triplet–triplet energy-transfer. 3perylene* is the triplet excited state of perylene. 1perylene* is the singlet excited state of perylene. The emission
bands observed in the TTA experiment are the simultaneous C60-1bodipy* emission (fluorescence) and 1perylene* emission (delayed fluorescence). The
triplet states of sensitizers and acceptors are non-emissive.
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a result, the singlet excited state of C60 is populated, and in turn,
the triplet excited state of C60 is produced by the intrinsic ISC
property of C60. The triplet excited state of the dyad is localized
on the C60 unit. The excited state energy will be transferred from
the C60 unit to the triplet acceptor perylene by the TTET between
the triplet excited state of the triplet photosensitizer and the
triplet acceptor. Annihilation of the two acceptor molecules at
the triplet excited state will produce a singlet excited state
acceptor. Radiative decay from the singlet excited state gives the
upconverted fluorescence emission, i.e. the delayed fluorescence.
Conclusions
In conclusion, C60-bodipy dyads were prepared as organic triplet
photosensitizers and were used for triplet–triplet annihilation
based upconversion. The antenna in the dyads can be photoex-
cited by green and red light. Thereafter, the intramolecular
singlet excited state energy transfer from antenna to C60 unit
produces the singlet excited state of C60. The intrinsic intersystem
crossing of C60 produces the triplet excited states of the C60 unit.
Therefore, without any heavy atoms, the triplet excited state of
the dyads can be populated upon photoexcitation. The absorp-
tion of the dyads was readily tuned by attaching different
carbazole moieties to the p-core of bodipy via ethynyl bonds.
The population of the C60-localized triplet excited state of the
dyads upon photoexcitation was confirmed by nanosecond time-
resolved transient absorption spectroscopy and spin density
surface analysis. The dyads were used as triplet photosensitizers
for TTA upconversion and upconversion quantum yields of up
to 2.9% were observed. We propose that C60-organic chromo-
phore dyads can be used as general molecular structural motifs
for heavy-atom free organic triplet photosensitizers, with
potential applications in photovoltaics, photocatalysis, photo-
dynamic therapy, and TTA upconversion.
Fig. 7 Delayed fluorescence in the TTA upconversion with C-1 as triplet
photosensitizer and perylene as the triplet acceptor. Excited at 532 nm
(nanosecond pulsed OPO laser, synchronized with spectrofluorometer),
the decay trace was monitored at 470 nm. Under such conditions, C-1 was
selectively excited and the emission is due to the upconverted emission of
perylene. The inset shows the prompt fluorescence decay of perylene
determined in a different experiment (excited with a 405 nm picosecond
laser, the decay of the emission was monitored at 470 nm). In deaerated
toluene. c (C-1) ¼ 5.0 ꢂ 10ꢀ6 M; c (perylene) ¼ 3.0 ꢂ 10ꢀ4 M, 20 ꢁC.
emission observed in Fig. 5, thus, the blue emission for dyads/
perylene mixed solution is due to the TTA upconversion. A
similar result was observed for C-2 (sDF ¼ 63.0 ms, see ESI†).
In order to study the TTET efficiency, the quenching of the
triplet excited states of C-1 and C-2 with perylene as the triplet
quencher was studied (Fig. 8). The quenching constants were
determined as 4.9 ꢂ 103 and 8.2 ꢂ 103 Mꢀ1 for C-1 and C-2,
respectively. Higher quenching constants were observed for C60
(Ksv ¼ 2.8 ꢂ 104 Mꢀ1). It is worth noting that the triplet excited
state lifetime of C60 is slightly shorter than those of dyads C-1
and C-2. The smaller molecular size of C60 makes it diffuse more
easily than the dyads. These results indicate that organic triplet
photosensitizers with small molecular sizes will be better candi-
dates for TTA upconversion.
Experimental
Fluorescence lifetimes of the compounds were measured with a
OB920 phosphorescence/fluorescence lifetime spectrometer
(Edinburgh Instruments, UK). Nanosecond time-resolved tran-
sient difference absorption spectra were measured on a LP920
laser flash photolysis spectrometer (Edinburgh Instruments,
UK). The transient data were recorded on a Tektronix TDS
3012B oscilloscope. The lifetime values (by monitoring the decay
trace of the transients) were obtained with LP900 software.
Compounds 1 and 3,76 5a,22 and 5b,65 were prepared according to
the respective literature.
The photophysics of the TTA upconversion with the dyads as
triplet photosensitizers can be summarized in Scheme 2.11–13
Upon excitation of the antenna of the dyads, the singlet excited
state of the antenna is populated. Consequently, singlet excited
state energy transfer from the antenna to the C60 unit occurs. As
Compound 2
Under Ar atmosphere, compound 1 (190.0 mg, 0.50 mmol),
PdCl2(PPh3)2 (10.6 mg, 0.015 mmol), PPh3 (6.6 mg, 0.025 mmol)
and CuI (4.8 mg 0.025 mmol) were dissolved in triethylamine
(12 mL). After stirring, ethynyltrimethylsilane (0.3 mL, 2.95
ꢁ
mmol) was added via syringe. The solution was stirred at 80 C
for 4 h. After removal of solvent under reduced pressure, the
residue was purified with column chromatography (silica gel;
dichloromethane/petroleum ether (30–60 ꢁC) ¼ 3 : 1, v/v) to give
a colourless oil. Then, a mixture of the resultant product, K2CO3
(1.6 g 1.2 mmol), CH2Cl2 (4 mL) and CH3OH (8 mL) was stirred
Fig. 8 Stern–Volmer plots generated from the triplet excited state life-
time (sT) quenching curves of the compounds measured as a function of
perylene concentration. In toluene, 20 ꢁC.
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at rt for 6 h to remove the trimethylsilane group. The solution
was washed with water and the aqueous phase was extracted with
CH2Cl2. After removal of solvent under reduced pressure, the
residue was purified by column chromatography (silica gel,
CH2Cl2/petroleum ether (30–60 ꢁC) ¼ 3 : 1 v/v) to give 120.0 mg
of 1 as a yellow powder. Yield: 87%; m.p. ¼ 116.4–117.8 ꢁC; 1H
NMR (400 MHz, CDCl3) d 10.08 (s, 1H), 8.53 (s, 1H), 8.27 (s,
1H), 8.02 (d, 1H, J ¼ 8.6 Hz), 7.64 (d, 1H, J ¼ 8.4 Hz), 7.46 (d,
1H, J ¼ 8.6 Hz), 7.38 (d, 1H, J ¼ 8.4 Hz), 4.32–4.28 (t, 2H, 7.2
Hz), 3.10 (s, 1H), 1.88–1.81 (m, 2H), 1.41–1.35 (m 2H), 0.96–0.93
(t, 3H, 7.4 Hz); TOF HRMS EI+: C19H17NO+, calculated m/z ¼
275.1310, found m/z ¼ 275.1305.
(43.0 mg, 0.06 mmol) in toluene (30 mL). The mixture was heated
to 110 ꢁC, and was refluxed for 12 h, then cooled slowly to rt. The
solvent was removed under reduced pressure. The residue was
purified by column chromatography (silica gel, CH2Cl2/petro-
leum ether (30–60 ꢁC) ¼ 3 : 1, v/v) to give 20.0 mg of C-1 as a dark
1
red powder. Yield: 30%; m.p. > 250 ꢁC; H NMR (400 MHz,
CDCl3) d 8.25 (s, 1H), 7.55–7.50 (m, 5H), 7.33–7.29 (m, 5H), 6.02
(s, 1H), 5.10–5.03 (m, 2H), 4.32–4.25 (m, 3H), 2.84 (s, 3H), 2.75 (s,
3H), 2.58 (s, 3H), 1.88–1.80 (m, 2H), 1.53 (s, 3H), 1.42–1.37 (m,
+
5H), 0.98–0.92 (m, 3H); TOF HRMS LD+: C100H39BF2N4
,
calculated m/z ¼ 1344.3236, found m/z ¼ 1344.3181.
Compound 7
Compound 4
Under Ar atmosphere, compound 2 (70.0 mg, 0.25 mmol),
compound 5b (86.4 mg, 0.15 mmol), PdCl2(PPh3)2 (7.0 mg, 0.01
mmol), PPh3 (5.3 mg, 0.02 mmol) and CuI (3.8 mg 0.02 mmol)
were dissolved in triethylamine (10 mL). The solution was stirred
at 60 ꢁC for 6 h. After removal of solvent under reduced pressure,
the residue was purified by column chromatography (silica gel,
CH2Cl2/petroleum ether (30–60 ꢁC) ¼ 1 : 1, v/v) to give 40.0 mg
Under Ar atmosphere, compound 3 (350.0 mg, 1.0 mmol),
PdCl2(PPh3)2 (21.0 mg, 0.03 mmol), PPh3 (13.1 mg, 0.05 mmol)
and CuI (9.5 mg 0.05 mmol) were dissolved in triethylamine (15
mL). After stirring for a while, ethynyltrimethylsilane (0.4 mL,
3.93 mmol) was added via syringe. The solution was stirred at
ꢁ
80 C for 4 h. After removal of solvent under reduced pressure,
1
the residue was purified by column chromatography (silica gel,
CH2Cl2/petroleum ether (30–60 ꢁC) ¼ 3 : 1, v/v) to give a yellow
oil. Then, a mixture of product, K2CO3 (2.0 g 1.5 mmol),
CH2Cl2 (5 mL) and CH3OH (8 mL) was stirred at rt for 6 h.
The white turbid liquid was washed with water and the aqueous
phase was extracted with CH2Cl2. After removal of solvent
under reduced pressure, the residue was purified with coluꢁmn
chromatography (silica gel; CH2Cl2/petroleum ether (30–60 C)
¼ 1 : 3, v/v) to give 195.0 mg of 3 as a white powder. The
overall yield was 79%; m.p. ¼ 46.8–48.3 ꢁC; 1H NMR (400
MHz, CDCl3) d 8.27 (s, 1H), 8.09 (d, 1H, J ¼ 7.6 Hz), 7.61 (d,
1H, J ¼ 8.4 Hz), 7.51 (t, 1H, J1 ¼ 8.0 Hz, J2 ¼ 8.4 Hz), 7.42 (d,
1H, J ¼ 8.0 Hz), 7.35 (d, 1H, J ¼ 8.4 Hz), 7.28 (t, 1H, 8.4 Hz),
4.30 (t, 2H, 7.2 Hz), 3.09 (s, 1H), 1.88–1.81 (m, 2H), 1.42–1.37
(m, 2H), 0.97 (t, 3H, 7.2 Hz); TOF HRMS EI+: C18H17N+,
calculated m/z ¼ 247.1361, found m/z ¼ 247.1351.
of 7 as a dark purple powder. Yield: 37%; m.p. > 250 ꢁC; H
NMR (400 MHz, CDCl3) d 10.09 (s, 1H), 8.58 (s, 1H), 8.25 (d,
1H, J ¼ 8.4 Hz), 7.62 (d, 1H, J ¼ 8.4 Hz), 7.55–7.53 (m, 3H), 7.49
(d, 1H, J ¼ 8.4 Hz), 7.40 (d, 1H, J ¼ 8.4 Hz), 7.30–7.28 (m, 2H),
4.35–4.31 (t, 2H, J1 ¼ J2 ¼ 7.6 Hz), 2.77 (s, 3H), 2.67 (s, 3H),
1.90–1.83 (m, 2H), 1.55 (s, 3H), 1.41–1.36 (m, 5H), 0.97–0.94 (t,
3H, J1 ¼ J2 ¼ 7.4 Hz); TOF HRMS LD+: C38H33BF2N3OI+,
calculated m/z ¼ 723.1730, found m/z ¼ 723.1675.
Compound 8
Under argon atmosphere, compound 7 (30.0 mg, 0.04 mmol),
compound 4 (100.0 mg, 0.40 mmol), PdCl2(PPh3)2 (3.5 mg,
0.005 mmol), PPh3 (2.6 mg, 0.01 mmol) and CuI (1.9 mg 0.01
mmol) were dissolved in triethylamine (10 mL). The solution was
ꢁ
stirred at 60 C for 6 h. After removal of solvent under reduced
pressure, the residue was purified by column chromatography
(silica gel, CH2Cl2/petroleum ether (30–60 ꢁC) ¼ 1 : 1, v/v) to
give 20.0 mg of 8 as a dark purple powder. Yield: 53%; m.p. >
250 ꢁC; 1H NMR (400 MHz, CDCl3) d 10.09 (s, 1H), 8.58 (s, 1H),
8.25 (s, 1H), 8.20 (s, 1H), 8.08 (d, 1H, J ¼ 7.2 Hz), 8.03 (d, 1H,
J ¼ 7.2 Hz), 7.62 (d, 1H, J ¼ 8.4 Hz), 7.56–7.54 (m, 4H), 7.48–
7.46 (m, 2H), 7.41–7.38 (m, 2H), 7.35–7.33 (m, 3H), 7.24 (d, 1H,
J ¼ 7.8 Hz), 4.32–4.29 (m, 4H), 2.79 (s, 6H), 1.88–1.82 (m, 4H),
1.58 (s, 6H), 1.41–1.36 (m, 4H), 0.96–0.94 (m, 6H); TOF HRMS
LD+: C56H49BF2N4O+, calculated m/z ¼ 842.3967, found m/z ¼
824.3987.
Compound 6
Under Ar atmosphere, compound 2 (65.0 mg, 0.24 mmol),
compound 5a (35.0 mg, 0.08 mmol), PdCl2(PPh3)2 (7.0 mg, 0.01
mmol), PPh3 (5.3 mg, 0.02 mmol) and CuI (3.8 mg 0.02 mmol)
were dissolved in triethylamine (10 mL). The solution was stirred
at 60 ꢁC for 6 h. After removal of solvent under reduced pressure,
the residue was purified by column chromatography (silica gel,
DCM/petroleum ether (30–60 ꢁC) ¼ 3 : 1, v/v) to give 30.3 mg of
1
ꢁ
6 as a dark red powder. Yield: 65%; m.p. ¼ 236.8–237.9 C; H
NMR (400 MHz, CDCl3) d 10.09 (s, 1H), 8.58 (s, 1H), 8.25 (d,
1H, J ¼ 8.4 Hz), 7.62 (d, 1H, J ¼ 8.4 Hz), 7.53–7.51 (m, 3H), 7.49
(d, 1H, J ¼ 8.4 Hz), 7.40 (d, 1H, J ¼ 8.4 Hz), 7.32–7.30 (m, 2H),
6.04 (s, 1H), 4.35 (t, 2H, J ¼ 7.6 Hz), 2.76 (s, 3H), 2.59 (s, 3H),
1.90–1.83 (m, 2H), 1.55 (s, 3H), 1.42–1.36 (m, 5H), 0.97 (t, 3H,
7.2 Hz); TOF HRMS LD+: C38H34BF2N3O+, calculated m/z ¼
597.2763, found m/z ¼ 597.2758.
C-2
Under Ar atmosphere, compound 8 (42.0 mg, 0.05 mmol) and
sarcosine (14.0 mg, 0.16 mmol) were added to the solution of C60
(43.0 mg, 0.06 mmol) in toluene (30 mL). The mixture was heated
to 115 ꢁC, and was refluxed for 12 h, then cooled slowly to
ambient temperature. The solvent was then removed under
reduced pressure. The residue was purified by column chroma-
tography (silica gel, CH2Cl2/petroleum ether (30–60 ꢁC) ¼ 2 : 1,
v/v) to give 22.0 mg of C-2 as a dark purple powder. Yield: 28%;
m.p. > 250 ꢁC; 1H NMR (400 MHz, CDCl3) d 8.25 (s, 1H), 8.17
C-1
Under Ar atmosphere, compound 6 (30.0 mg, 0.05 mmol) and
sarcosine (13.4 mg, 0.15 mmol) were added to the solution of C60
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(s, 1H), 8.03 (s, 1H), 7.52–7.36 (m, 9H), 7.30–7.24 (m, 6H), 5.07–
5.00 (m, 2H), 4.28–4.24 (m, 5H), 2.81 (s, 3H), 2.75 (s, 6H), 1.83–
1.79 (m, 4H), 1.53 (s, 6H), 1.38–1.35 (m, 4H), 0.92–0.89 (m, 6H);
TOF HRMS LDꢀ: C118H54BF2N5+, calculated m/z ¼ 1589.4440,
found m/z ¼ 1589.4567.
in the measurement of the quantum yield, and ‘‘sam’’ for the
samples to be measured.11
ꢀ
ꢁꢀ
ꢁꢀ
ꢁ
2
1 ꢀ 10ꢀA
Isam
Istd
hsam
hstd
std
FUC ¼ 2Fstd
(1)
1 ꢀ 10ꢀA
sam
In the above equation the absorption correction factor 1–10ꢀA
was used instead of the absorbance (A) because the absorbances
of the samples are much higher than those usually used for
determination of luminescence quantum yields. The absorbances
of the samples used for the determination of the upconversion
quantum yields are 0.2611, 0.3294, 0.3008, and 0.0343 for C-1,
5b, C-2 and BisBDPI, respectively.
L-1
Under Ar atmosphere, compound 4 (100.0 mg, 0.40 mmol),
compound 5a (45.0 mg, 0.1 mmol), PdCl2(PPh3)2 (7.0 mg, 0.01
mmol), PPh3 (5.3 mg, 0.02 mmol) and CuI (3.8 mg 0.02 mmol)
were dissolved in triethylamine (12 mL). The solution was stirred
at 60 ꢁC for 6 h. After removal of solvent under reduced pressure,
the residue was purified by column chromatography (silica gel,
CH2Cl2/petroleum ether (30–60 ꢁC) ¼ 1 : 1, v/v) to give 40.0 mg
of L-1 as a dark red powder. Yield: 70%; m.p. 246.7–248.8 ꢁC; 1H
NMR (400 MHz, CDCl3) d 8.20 (s, 1H), 8.08 (d, 1H, J ¼ 7.6 Hz),
7.56 (d, 1H, J ¼ 8.0 Hz), 7.52–7.50 (m, 3H), 7.47 (d, 1H, J ¼ 7.2
Hz), 7.40 (d, 1H, J ¼ 8.0 Hz), 7.34–7.29 (m, 3H), 7.23 (d, 1H, J ¼
7.2 Hz), 6.02(s, 1H), 4.30 (t, 2H, J ¼ 7.2 Hz), 2.76 (s, 3H), 2.59 (s,
3H), 1.88–1.80 (m, 2H), 1.55 (s, 3H), 1.42–1.35 (m, 5H), 0.95 (t,
3H, J ¼ 7.4 Hz); TOF HRMS LD+: C37H34BF2N3+, calculated
m/z ¼ 569.2814, found m/z ¼ 569.2864.
The CIE coordinates (x, y) of the emission of the sensitizers
alone and the emission of the upconversion were derived from
the emission spectra with the software of CIE Color Matching
Linear Algebra.
The delayed fluorescence of the upconversion was measured
with a nanosecond pulsed laser (Opolettꢀ 355II + UV nano-
second pulsed laser, typical pulse length: 7 ns. Pulse repetition: 20
Hz. Peak OPO energy: 4 mJ. Wavelength is tuneable from 210 to
355 nm and from 420 to 2200 nm. OPOTEK, USA), which was
synchronized to a FLS920 spectrofluorometer (Edinburgh, UK).
The pulsed laser was sufficient to sensitize the TTA upconver-
sion. The decay kinetics of the upconverted fluorescence (delayed
fluorescence) were monitored with a FLS920 spectrofluorometer
(synchronized to the OPO laser). The prompt fluorescence life-
time of the triplet acceptor perylene was measured with an EPL
picosecond pulsed laser (405 nm) which was synchronized to the
FLS 920 spectrofluorometer.
Compound L-2
Under Ar atmosphere, compound 4 (200.0 mg, 0.81 mmol),
compound 5b (57.5 mg, 0.1 mmol), PdCl2(PPh3)2 (7.0 mg, 0.01
mmol), PPh3 (5.3 mg, 0.02 mmol) and CuI (3.8 mg 0.02 mmol)
were dissolved in triethylamine (15 mL). The solution was
stirred at 60 ꢁC for 6 h. After removal of solvent under reduced
pressure, the residue was purified by column chromatography
(silica gel, CH2Cl2/petroleum ether (30–60 ꢁC) ¼ 1 : 1, v/v) to
give 45.0 mg of L-2 as a dark purple powder. Yield: 56%; m.p.
DFT calculations
The geometries of the compounds were optimized using density
functional theory (DFT) with 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. Toluene was used as solvent in the
calculations (PCM model). All these calculations were performed
with Gaussian 09W.77
1
ꢁ
> 250 C; H NMR (400 MHz, CDCl3) d 8.21 (s, 2H), 8.09 (d,
2H, J ¼ 7.2 Hz), 7.57 (m, 6H), 7.48 (d, 2H, J ¼ 7.0 Hz), 7.41 (d,
2H, J ¼ 8.4 Hz), 7.35 (m, 3H), 7.24 (d, 2H, J ¼ 7.8 Hz), 4.31 (t,
4H, J ¼ 7.2 Hz), 2.79 (s, 6H), 1.88–1.81 (m, 4H), 1.58 (s, 6H),
1.41–1.36 (m, 4H), 0.96 (t, 6H, J ¼ 7.2 Hz); TOF HRMS LD+:
C55H49BF2N4+, calculated m/z ¼ 814.4018, found m/z ¼
814.3972.
Acknowledgements
Triplet–triplet annihilation upconversion
We thank the NSFC (20972024 and 21073028), the Royal
Society (UK), the NSFC (China-UK Cost-Share Science
Networks, 21011130154), the Fundamental Research Funds for
the Central Universities (DUT10ZD212), the Ministry of
Education (NCET-08-0077, and Dalian University of Tech-
nology for financial support.
A diode pumped solid state (DPSS) laser (532 and 589 nm,
continuous wave) was used for the upconversions. The diameter
of the laser spot is ca. 3 mm. The samples were purged with N2 or
Ar for at least for 15 min before measurement and the gas flow
was maintained during the measurement. The upconversion
quantum yields (FUC) were determined with the prompt fluo-
rescence of 2,6-diiodo-bodipy (compound 5b, Scheme 1) (F ¼
2.7% in MeCN) and BisBDPI as the standards (the structure is
shown in Scheme 1, FF ¼ 10.5% in toluene). In order to repress
the scattered laser, a black box was put behind the fluorescent
cuvette to trap the laser beam.
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