Styryl Bodipy-C60 dyads as efficient heavy-atom-free organic triplet photosensitizers

Article abstract of DOI:10.1021/ol3008843

C₆₀-styryl Bodipy dyads that show strong absorption of visible light (ε = 64 600 M^-1 cm^-1 at 657 nm) and a long-lived triplet excited state (τ_T = 123.2 μs) are prepared. The dyads were used as heavy-atom-free or

Full text of DOI:10.1021/ol3008843

ORGANIC
LETTERS
2012
Vol. 14, No. 10
2594–2597
Styryl Bodipy-C60 Dyads as Efficient
Heavy-Atom-Free Organic Triplet
Photosensitizers
Ling Huang, Xuerong Yu, Wanhua Wu, and Jianzhang Zhao*
State Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian
University of Technology, E-208 West Campus, 2 Ling-Gong Road, Dalian 116024, China
zhaojzh@dlut.edu.cn
Received April 8, 2012
ABSTRACT
C60-styryl Bodipy dyads that show strong absorption of visible light (ε = 64 600 MÀ1 cmÀ1 at 657 nm) and a long-lived triplet excited state (τT = 123.2
μs) are prepared. The dyads were used as heavy-atom-free organic triplet photosensitizers for photooxidation of 1,5-dihydroxynaphthalene via
1
the photosensitizing of singlet oxygen (1O2). The photooxidation efficiency of the dyads compared to the conventional Ir(III) complex O2
photosensitizer increased 19-fold.
Triplet photosensitizers have attracted much attention,
due to their applications in photocatalysis,1 photovoltaics,2
photodynamic therapy (PDT),3,4 photopolymerization,5
luminescent molecular probes,6,7 and more recently the
tripletÀtriplet annihilation photon upconversions.8,9 Tradi-
tionally the triplet photosensitizers are transition metal
complexes, such as the Pt(II), Ir(III), Ru(II) complexes.10À13
The heavy atom effect facilitates intersystem crossing (ISC),
thus the triplet excited states of the complexes can be
populated upon photoexcitation. However, these complexes
are expensive and most molecular structures cannot be
as readily derivatized as an organic chromophore.11,12 An-
other kind of triplet sensitizer is the iodo- or bromo-contain-
ing organic chromophores.4,14À16 A third kind of triplet
photosensitizer is without any heavy atoms, such as
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r
10.1021/ol3008843
Published on Web 05/09/2012
2012 American Chemical Society

2,3-butanedione, benzophenone, porphyrins, etc. Unfortu-
nately, the absorption of these compounds is usually in the
UV range (except porphyrins). Furthermore, the ISC prop-
erty is unpredictable for the derivatives of these compounds; e.
g. the ISC property will disappear with derivatization. It is
still a substantial challenge to design a heavy-atom-free
organic triplet photosensitizer showing the predeter-
mined ISC property.18,19
Inregards tothisaspect, the efficient ISC offullerene C60
is particularly interesting.17,20 However, C60 itself is not an
ideal triplet photosensitizer because its absorption in the
visible range is extremely weak. C60 does show an absorp-
tion band at ca. 700 nm. This weak, yet crucial absorption,
indicating the presence of a low-lying sinlget excited state,
guarantees intramolecular energy transfer (EnT) in C60-
organic chromophore dyads.21À28 C60-organic chromo-
phore dyads have been reported; however, to the best of
our knowledge, the C60 dyads have not been used for
sensitizing a photophysical process, such as the production
of singlet oxygen (1O2).29
Dyad-1À3 are used as efficient triplet photosensitizers for the
photooxidation of DHN, via sensitizing of singlet oxygen
(1O2).30 The performance of these dyads is much better than
the conventional 1O2 photosensitizers (see later section).
Light harvesting antenna styryl-Bodipys were usually
obtained in low yields via the Knoevenagel condensation
(usually <30%).27 Herein we developed an efficient pre-
paration protocol by using microwave irradiation and
high boiling point solvents. The intermediate B-4 was
obtained in 93% yield (Supporting Information (SI)).
Suzuki cross-coupling between B-4 and 4-formylbenze-
neboronic acid affords B-5. A Prato reaction between
B-5, C60, and N-methylglycine gave Dyad-1. A similar
method was used for the preparation of Dyad-2 and
Dyad-3 (Figure 1 and SI). The solubility of Dyad-3 is
poor in normal organic solvents, and only elemental
analysis can be performed.
The design rationale of Dyad-1À3 lies in the notion that
antenna B-8 shows strong absorption in the visible range.31
Its S1 state energy level (1.93 eV) is higher than the S1 state
of C60 (1.72 eV); thus intramolecular EnT from the Bodipy
to C60 unit will occur.27 With population of the S1 state of
C60, the triplet state of C60 will be populated via the
intrinsic ISC of C60. However, the T1 states of the dyads
Herein we devised visible light-harvesiting C60 dyads
(Dyad-1À3, Scheme 1 and Figure 1). Styryl-Bodipys were
used as the light harvesting antennas, and C60 units act
as the spin convertor. The dyads show strong absorption
of visible light and very long-lived triplet excited states.
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Santos, M. N.; Wolfbeis, O. S. Angew. Chem., Int. Ed. 2007, 46, 2317–
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Scheme 1. Synthesis of the Styryl-Bodipy Dyad Dyad-1
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Sausalito, CA, 2009.
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T. A.; Gust, D. J. Phys. Chem. 1996, 100, 15926–15932.
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do not necessarily localize on the C60 unit. The localization
of the T1 state is dependent on the energy level of the light-
harvesting antenna and the C60 unit. A “ping-pong” energy
transfer may occur;26a,32,33 that is, first the energy transfer
from the antenna to the C60 unit (can be also considered as
€
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Org. Lett., Vol. 14, No. 10, 2012
2595

A similar emission property was found for Dyad-2 and
Dyad-3 (see SI). These results indicate that one C60 unit in
the dyad is sufficient to quench the emission of the
antenna; i.e. efficient intramolecular EnT is imparted.
In order to confirm the production of the T1 state of Dyad-1
upon photoexcitation, nanosecond time-resolved transient
absorption was studied (Figure 3). Upon pulsed laser excita-
tion at 532 nm, bleaching at 645 and 370 nm was observed,
which coincides with the steady-state absorption of antenna
B-8 (Figure 2a). Thus the bleaching can be attributed to
depletion of the ground state of the antenna in Dyad-1.
Besides the bleaching bands, positive transients at 400, 550,
650, and 800 nm were found. These positive transient bands
can be assigned to the absorption of the triplet state of the
styryl Bodipy antenna in Dyad-1. This conclusion was
confirmed by the transient absorption of B-4, which gives
a similar transient absorption profile (SI). The transient
absorption of B-8 cannot be determined due to the lack of
heavy atoms in the molecule; thus the triplet excited state
cannot be efficiently produced.
Figure 1. Molecular structure of Dyad-2 and Dyad-3 (please
refer to the SI for synthetic details).
internal conversion, IC) and in turn the backward triplet state
EnT from C60 to the Bodipy antenna will occur, if the T1 state
energy level of the antenna is lower than that of C60.26a,27a,33
Figure 2. (a) UVÀvis absorption and (b) fluorescence emission
spectra of Dyad-1, B-8, and C60. Excited at 620, 590, and 320 nm,
respectively. In toluene (1.0 Â 10À5 M; 20 °C).
Figure 3. Nanosecond time-resolved transient absorption of
Dyad-1. In deaerated toluene. Excited at 532 nm, 20 °C.
The absorptions of the antenna and Dyad-1 were com-
pared (Figure 2a). C60 absorbs very weakly in the visible
range, with the maximum at 335 nm. Conversely, the light-
harvesting antenna B-8 gives strong absorption at 629 nm
(ε = 120000 MÀ1 cmÀ1). Dyad-1 absorbs at 645 nm (ε =
56800 MÀ1 cmÀ1). Similar absorption was found for
Dyad-2 and -3 (see SI). We noted that the UV absorption
of the C60 unit in the dyads is not proportional to the
number of C60 units in the dyads; a phenomenon was also
found for C60-containing dendrimers.34
Thus, the T1 excited state of Dyad-1 is localized on the
styryl-Bodipy antenna, not on the C60 moiety. The energy
level of the T1 state of the antenna (1.06 eV), much lower than
the T1 state of C60 (1.73 eV), imparts the triplet state energy
transfer from C60 to the styryl-Bodipy unit. The lifetime of the
triplet excited state of Dyad-1 was determined to be 105.6 μs.
Note the T1 statelifetimeofC60 alone is only 40.6 μs(Table1).
In order to study the localization of the T1 state of the
dyads, the spin densities of the dyads were calculated
(Figure 4). The spin density is exclusively localized on the
styryl-Bodipy unit. The C60 unit does not contribute to the
spin density, which fully coincides with the time-resolved
transient absorption spectra (Figure 3). Similar results
were found for Dyad-2 (SI).
C60 shows weak fluorescence (ΦF < 0.1%) (Figure 2b).
Conversely, B-8 gives intense fluorescence at 641 nm (ΦF =
59%). Interestingly, Dyad-1 fluoresces weakly (ΦF
=
1.0%). The quenched fluorescence of the antenna in
Dyad-1 is due to the intramolecular singlet state energy
transfer from the antenna to the C60 unit.26 We demon-
strated that the quenching is not due to the electron
transfer from the N-atom in the pyrrolidine rings, studied
by protonation with trifluoroacetic acid (see SI). The
energy transfer efficiency is calculated as 98.1%.26
Triplet photosensitizers can be used for photodynamic
1
therapy (PDT) or photooxidation via sensitizing O2.30
Ir(III) complexes have been used for the photooxidation of
DHN (Figure 5).30 Herein Dyad-1À3 were used as organic
triplet photosensitizers for the photooxidation of DHN
(the photooxidation product is juglone).
(34) Hosomizu, K.; Imahori, H.; Hahn, U.; Nierengarten, J.-F.;
Listorti, A.; Armaroli, N.; Nemoto, T.; Isoda, S. J. Phys. Chem. C
2007, 111, 2777–2786.
(35) Arbogast, J. W.; Darmanyan, A. P.; Foote, C. S.; Rubin, Y.;
Diederich, F. N.; Alvarez, M. M.; Anz, S. J.; Whetten, R. L. J. Phys.
Chem. 1991, 95, 11–12.
2596
Org. Lett., Vol. 14, No. 10, 2012

significant with C60 dyads as photosensitizers (Figure 6a).
The photooxidation rate constant of Dyad-1 compared
to the traditional organic triplet photosensitizer tetra-
phenylporphyrin (TPP) and methylene blue (MB) in-
creased 1.7-fold. The kobs value of Dyad-1 compared to
Ir-1 is ca. 20-fold higher (Table 2). In comparison, C60
gives much slower photooxidation kinetics, due to its
poor absorption of visible light. Thus in this case,
the visible light-harvesting C60 dyads are more useful
than C60.
Figure 4. Spin density surface of Dyad-1. Calculated at B3LYP/
6-31G(d) level with Gaussian 09W.
Table 2. Pseudo-First-Order Kinetics Parameters, and Yields of
Juglone for the Photooxidations of DHN Using Different
Triplet Photosensitizers
Table 1. Photophysical Parameters of the Dyadsa
a
kobs
/
yieldc/
%
minÀ1
vib
d
ΦΔ
c
d
τT
/
τF /
λabs
ε
λem
Φb
μs
ns
Dyad-1
Dyad-2
Dyad-3
B-4
86
82
91
6.3
50
34
5.7
4.5
8.6
99.9
99.9
99.9
11.6
99.9
66.0
30.0
27.0
0.85
0.82
0.85
À
8.2
Dyad-1
Dyad-2
Dyad-3
B-4
645
629
657
635
629
335
5.68
6.53
6.46
9.08
12.0
4.73
662
644
667
647
641
720
0.01
0.01
0.009
0.53
0.59
À
105.6
71.5
123.2
2.6
À
À
À
9.1
0.63
5.0
TPP
0.62e
0.57
0.76e
À
4.4
4.6
À
MB
3.4
B-8
À
C60
0.57
0.45
C60
40.6
Ir(ppy)2bpy
a In toluene at 20 °C. b With B-8 (ΦF = 0.59 in toluene) as the
a Photoreaction rate constants; 10À3 minÀ1 b Initial rate; 10À5 M.
.
standard. c Triplit state lifetimes. d Fluorescence lifetimes.
In minÀ1. c Yield of Juglone after photoirradiation for 30 min. d Quantum
yield of singlet oxygen (1O2), with methylene blue (MB) as standard
(ΦΔ = 0.57 in CH2Cl2). e Literature values.35
In conclusion, styryl Bodipy-C60 dyads were pre-
pared as organic triplet photosensitizers. The dyads
show strong absorption of red light and long-lived
triplet excited states. Upon photoexcitation, the triplet
excited state localized on the antenna was populated
via a “ping-pong” intramolecular energy transfer be-
tween the antenna and the C60 unit. The dyads were
used as singlet oxygen (1O2) photosensitizers, and the
photosensitizing efficiency of the dyads compared to
the known Ir(III) complex triplet photosensitizer in-
creased ca. 20-fold. We propose the C60-organic chro-
mophore dyads can be used as a general structure motif
for heavy-atom-free organic triplet photosensitizers,
for which the absorption can be readily changed by
using different antennas, and the ISC property is fully
predictable.
Figure 5. Photooxidation of DHN with triplet photosensitizer.
Acknowledgment. We thank the NSFC (21073028 and
20972024) for financial support.
Figure 6. Absorption change in the photooxidation of DHN. (a)
Dyad-1 as sensitizer. (b) Plots of ln(Ct/Co) vs irradiation time.
c[sensitizers] = 5.0 Â 10À6 M. c[DHN] = 1.0 Â 10À4 M. In
CH2Cl2/MeOH (9:1, v/v); 20 mW cmÀ2; 20 °C.
Supporting Information Available. General experimen-
tal methods, 1H NMR, absorption and emission spectra
of Dyad-2 and -3. This material is available free of charge
via the Internet at http://pubs.acs.org.
The consumption of DHN can be monitored by the
decrease in its absorption at 301 nm. The change is
The authors declare no competing financial interest.
Org. Lett., Vol. 14, No. 10, 2012
2597