Switching of the triplet excited state of the C60-dimethylaminostyryl BODIPY dyads/triads

Article abstract of DOI:10.1039/c4tc02037e

In order to switch the triplet excited states in organic compounds, dimethylaminostyryl BODIPY-C₆₀ dyads and triads were prepared. The triplet excited states of the compounds were switched with acid/base, and the mechanism was studied with nanosecond time-resolved transient difference absorption spectroscopy. The visible light-harvesting BODIPY antennas are the electron or singlet energy donor, whereas C₆₀ moiety is the electron/singlet energy acceptor, as well as the spin converter to produce triplet excited states. Our strategy of triplet state switching is to control either the photoinduced electron transfer (PET) or the singlet state energy transfer (EnT) from the antenna to C₆₀ moiety by protonation of the dimethylaminostyryl BODIPY unit. Population of the triplet state was observed for the dyads with mono(4-dimethylaminostyryl) substituents on BODIPY antenna in nonpolar solvent such as toluene (τ_T = 168.6 μs). Formation of the charge transfer state (CTS) in polar solvent quenches the triplet excited state (τ_T < 10 ns). In the presence of acid, the dimethylaminostyryl BODIPY moiety is protonated, thus the electron transfer (ET) was inhibited. The cascade acid-activated EnT and the intersystem crossing (ISC) of C₆₀ produce the triplet excited state. For the dyad and the triads with bis(4-dimethylaminostyryl) substituents on BODIPY antenna, the antenna S₁ state energy level is lower than the S₁ state energy level of C₆₀; thus, no EnT to C₆₀ exists, and no triplet state was produced upon selective excitation into the BODIPY moiety. With protonation of the amino styryl substituents, the S₁ state energy level of the antenna is promoted to be higher than S₁ state of C₆₀ moiety, and as a result EnT is activated and triplet state is produced. In all the compounds, the triplet excited state is localized on the dimethylaminostyryl BODIPY moiety and not on the C₆₀ moiety. The triplet state switching was conveyed to the singlet oxygen (¹O₂) photosensitizing ability of the compounds, and the variation of the singlet oxygen quantum yield, Φ_Δ, is from 1.9% to 73%.

Full text of DOI:10.1039/c4tc02037e

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DOI: 10.1P039A/C4PTCE020R37E
Switch the triplet excited state of the C60-dimethylaminostyryl bodipy
dyads/triads
Ling Huang and Jianzhang Zhao*
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
In order to switch the triplet excited states in organic compounds, dimethylaminostyryl Bodipy-C60 dyads
and triads were prepared. The triplet excited states of the compounds were switched with acid/base, the
mechanism was studied with nanosecond time-resolved transient difference absorption spectroscopy. The
visible light-harvesting Bodipy antennas are the electron or singlet energy donor, whereas C60 moiety is
10 the electron/singlet energy acceptor, as well as the spin converter to produce triplet excited states. Our
strategy of triplet state switching is to control either the photoinduced electron transfer (PET) or the
singlet state energy transfer (EnT) from the antenna to C60 moiety, by protonation of the dimethylamino
styryl Bodipy unit. Population of the triplet state was observed for the dyads with mono(4-
dimethylaminostyryl) substituents on Bodipy antenna in nonpolar solvent such as toluene (τT = 168.6 μs).
15 Formation of the charge transfer state (CTS) in polar solvent quenches the triplet excited state (τT < 10 ns).
In the presence of acid, the dimethylaminostyryl Bodipy moiety is protonated, thus the electron transfer
(ET) was inhibited. The cascade acid-activated EnT and the intersystem crossing (ISC) of C60 produce the
triplet excited state. For the dyad and the triads with bis(4-dimethylaminostyryl) substituents on Bodipy
antenna, the antenna S1 state energy level is lower than the S1 state energy level of C60, thus no EnT to C60
20 exist, and no triplet state was produced upon selectively excitation into the Bodipy moiety. With
protontation of the aminostyryl substituents, the S1 state energy level of the antenna is promoted to be
higher than S1 state of C60 moiety, as a result EnT is activated and triplet state is produced. In all the
compounds the triplet excited state is localized on the dimethylaminostyryl Bodipy moiety, not on the C60
miety. The triplet state switching was conveyed to the the singlet oxygen (1O2) photosensitizing ability of
25 the compounds, and the variation of the singlet oxygen quantum yield, ΦΔ, is from 1.9% to 73%.
45 the ability to sensitizing singlet oxygen (1O2). Bodipy dyads were
Introduction
also studied as molecular devices for modulation of triplet state in
which the direction of FRET can be controled by addition of
acid.5,6 In these studies, however, the triplet excited state
manifolds were not studied in detail with nanosecond time-
50 resolved transient difference absorption spectroscopy.12 Moreover,
it is clear that more switching approaches should be explored for
switching of the triplet states in organic compounds.
Herein we studied the switching of triplet excited state of C60-
styryl Bodipy dyads/triads by controling either the photoinduced
55 intramolecular electron transfer (ET) or energy transfer (EnT).13c
In the dyads B-1 and B-2, and the triads B-3 and B-4 (Scheme 1-
2), the Bodipy moieties are visible light-harvesting antennas and
electron donor or singlet energy donor; C60 moiety is the electron
acceptor and singlet energy acceptor, as well as the
60 intramolecular spin converter.12 Bodipy moeities show strong
absorption of visible light, but the intersystem crossing (ISC) is
non-efficient. The contrary is true for C60 moiety, i.e. the ISC is
efficient (close to unity) but its visible light-absorption is poor.
Considering the singlet energy levels of the antenna and C60
Triplet photosensitizers are important in photocatalysis,1-4
molecular devices,5-6 and more recently in triplet-triplet
annihilation upconversion.7-11 Different from the study of singlet
30 excited states, study on triplet excited state of organic
chromophores is rare.12 Switching of the singlet excited state
leads to many applications, such as fluorescent molecular
sensors,13-15 visible light-harvesting molecular arrays, etc.16-19
The mechanisms for switching of singlet excited states include
35 fluorescence resonance energy transfer (FRET),13a-13d photo-
induced
through-bond
energy
transfer
(TBET),13e,13f
photoinduced electron transfer (PET) and photoinduced
intramolecular charge transfer (ICT), etc.13g It is highly desired to
develop mothodologies for controling of the positioning of triplet
40 state in a multichromophore molecule.
Previously amino-containing bromo-aza Bodipy was studied as
acid-activatable photodynamic theraputic (PDT) reagents.20 PET
mechanism is responsible for the acid-activated OFF and ON
state of the photosensitizers to produce triplet excites state, thus
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Journal of Materials Chemistry C
Page 2 of 13
moiety, RET is possible for B-1 and B-3, thus triplet excited state
presence of acid such as trifluoroacetic acid (TFA), however, the
can be produced. In B-2 and B-4, however, the S1 state energy 30 amino group will be protonated, thus the S1 state energy level of
level of the antenna is presumably lower than C60, thus RET is
impossible; but the EnT can be activated by protonation. Thus in
the presence of acid, triplet state production was observed with
B-2 and B-4.
the antenna in B-2 and B-4 will be promotedD,O28I:t1o0b.1e03Vh9iie/gwCh4AeTrrtCictl0he2aO0nn3li7nEe
that of C60, thus EnT become possible, as a result, triplet excited
state will be produced by the protonated B-2 and B-4. For B-1
and B-3, the S1 state energy level of the Bodipy antenna is higher
5
The visible light-harvesting antenna in B-1 – B-4 were selected 35 than that of C60. However, ET to C60 may quench the triplet
so that either the ET or the EnT to the C60 moieties can be
switched by acid, i.e. prototnation of aminostyryl Bodipy moiety.
excited state. In the presence of TFA, ET will be inhibited and the
RET will be activated, as a result, triplet excited state can be
produced by the protonated B-1 and B-3. B-3 and B-4 are
Bodipy-C60 triads, which show broadband absorption in visible
10 The ET and singlet/triplet EnT were investigated with steady
state and nanosecond time-resolved spectroscopy, as well as
electrochemical studies. For all the dyads and the triads, the T1 40 region.29
state is localized on the styryl Bodipy moiety, not on C60 moeity.
This conclusion is based on the nanosecond time-resolved
15 transient difference absorption spectroscopy. We show that the
1O2 production with the dyads can be modulated by addition of
acid and base.
The preparation of the visible light-antennas are based on
routine derivatization methods of Bodipy.14,19 Condensation of
the Bodipy with 4-dimethylaminobenzenealdyhede gives the
Bodipy antenna which show absorption at 600 nm or 700 nm,
45 dependent on the number of the substituents.30 The Bodipy
antenna was linked to C60 moiety by the Prato reaction. For triads
B-3 and B-4, Suzuki cross coupling reaction was used to prepare
the broadband visible light-harvesting antenna and Cu(I)-
catalyzed Click reaction was used to link the Bodipy and the
50 dimethylamino styryl Bodipy together.31,32 All the compounds
were obtained in moderate to satisfactory yields.
Results and Discussions
Molecular design and synthesis
20
The Bodipy moiety was selected as the visible light-harvesting
antenna, due to its excellent photophysical properties, such as
strong absorption of visible light, high fluorescene quantum
yields (less non-radiative decay of the singlet excited state), and
feacsible derivitization.14,19,21 Aminostyryl Bodipy moiety in B-2
UV−Vis absorption and fluorescence emission spectra
The UV−Vis absorption of the compounds were studied (Fig. 1).
B-1 shows strong absorption at 622 nm, which is due to the
55 dimethylamino styryl Bodipy moiety. B-2 shows red-shifted
absorption at 712 nm, due to the bis(4’-dimethylaminostyryl)
25 and B-4 show lower S1 state energy level than the C60 moiety,
thus EnT is impossible in B-2 and B-4.12,22-27 As a result,
population of the triplet excited state in B-2 and B-4 will be non-
efficient, upon selectivily excited into the Bodipy antenna. In the
b
a
I
I
I
N
N
N
N
F
60
65
70
75
N
N
F
N
N
N
N
F
B
2
B
3
d
B
B
F
F
F
B
F
F
F
F
1
B-9
N
N
N
B-10
c
g
h
Ο
N
N
N
N
N
N
N
N
F
O
B
B
B
5
B
F
F
F
F
F
F
F
B-5
4
B-6
N
N
N
N
N
N
e
f
N
N
N
N
F
I
I
N
N
N
N
N
N
B
B
B
B
F
F
F
F
F
F
F
B-11
B-2
N
N
B-12
N
N
B-1
80
Scheme 1 Synthesis of the C60-dimethylaminostyryl Bodipy dyads B-1 and B-2 and the reference compounds. Key: (a) 4−N,N−dimethylbenzaldehyde,
toluene, acetic acid, piperidine, reflux, 4 h in Ar; (b) the same to (a); (c) 4−formylphenylboronic acid, K2CO3, Pd(PPh3)4 ; (d) the same to (c); (e) Sarcosine,
fullerenes, toluene; reflux; (f) the same to (e); (g) Phenylboronic acid, K2CO3, Pd(PPh3)4; (h) the same to (g).
2
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Journal of Materials Chemistry C
I
View Article Online
DOI: 10.1039/C4TC02037E
F
F
N
N
N
N
F
F
N
B
N
N
B
N
F
N
B
O
5
10
15
20
25
30
F
N
10
R
N
N
N
F
F
B
i
N
N
F
F
F
N
N
B
F
N
B
N
B
F
F
N
N
N
O
B-7'
N
N
F
N
R = H: B-7
B
11
ii
N
R = CHO: 11
F
B-3'
N
N
R
N
F
N
N
O
N
N
B
F
F
N
N
N
F
F
N
N
B
O
N
F
N
B
N
N
F
F
N
B
O
N
R = H: B-8
R =
B-3
N
N
B-4
Scheme 2 Synthesis of the C60-dimethylaminostyryl Bodipy triads B-3 and B-4 and the reference compounds. (i) Phenylboronic acid, K2CO3, Pd(PPh3)4 ;
(ii) Sarcosine, fullerenes, toluene.
10
8
1.2
0.8
0.4
0.0
35 substituted Bodipy moiety. The absorption spectra is almost
independent on the solvent polarity. The fluorescence emission of
the dyads were also studied. For B-1, the fluorescence is at 660
nm (in toluene. Fig. 1b). The emission band was red-shifted to
674 nm in dichloromethane (DCM). The emission profile is
40 similar to the reference B-5 (see ESI †, Fig. S58). The S1 state
energy of the Bodipy antenna is approximated as 1.88 eV with
the emission wavelength, which is higher than the C60 moeity
(1.76 eV).23 Thus photoinduce RET is possible in B-1.
The emission band of B-2 is at 770 nm in DCM. Thus the S1
45 state energy of the Bodipy antenna in B-2 can be approximated as
1.61 eV. The S1 state energy level of the antenna in B-2 is lower
than the C60 moiety (1.76 eV),23 as a result, the RET to C60 in B-2
may be frustrated. The emission of B-2 is more sensitive to the
solvent polarity, thus more significant ICT is proposed for the
50 antenna in B-2. The emission profile is similar to the reference
compounds B-6 (see ESI†, Fig. S59) The absorption and the
fluorescence of the reference compounds B-5 and B-6 were also
studied (see ESI†, Fig. S58-S59). The results are similar to that of
B-1 and B-2, respectively.
toluene
a
b
60
65
70
75
toluene
DCM
6
DCM
4
2
0
600
700
800
900
300 400 500 600 700 800
Wavelength / nm
Wavelength / nm
1.2
12
toluene
DCM
c
d
toluene
DCM
8
0.8
0.4
0.0
4
0
700 750 800 850 900
Wavelength / nm
300 400 500 600 700 800
Wavelength / nm
Fig. 1 (a) UV−Vis absorption spectra of B-1 in toluene and DCM and (b)
the corresponding emission spectra of B-1. λex = 600 nm. A = 0.618, (c)
UV−Vis absorption spectra of B-2 in toluene and DCM and (d) the
55
80 corresponding emission spectra of the B-2. λ.ex = 680 nm, A = 0.559. c =
1.0 × 10–5 M for UV−Vis absorption spectra measurement. 20 °C.
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Journal of Materials Chemistry C
Page 4 of 13
triads indicated EnT or ET, with the C60 as the energy or electron
160
120
80
40
0
1.5
1.0
0.5
0.0
60 acceptor.23,33 Similarly, the emission of B-6 was substantially
quenched in B-2 (see ESI†, Fig. S70a). SaDmOeI:q1u0e.1n0c3Vh9iei/nwCg4ATretCicflf0ee2Oc0tn3li7nEe
was also found for B-9 and B-8 in B-4 (see ESI†, Fig. S70).
DCM
toluene
toluene
DCM
a
b
5
10
15
1.2
0.8
0.4
0.0
1.2
0.8
0.4
0.0
Abs
Ex
65
70
75
80
Abs
Ex
a
b
600
700
800
900
400
600
800
Wavelength / nm
Wavelength / nm
2.4
1.6
0.8
0.0
DCM
1.2
0.8
0.4
0.0
c
in DCM
in toluene
d
toluene
300 400 500 600 700 800
Wavelength / nm
300 400 500 600 700 800
Wavelength / nm
1.2
1.2
Abs
Ex
Abs
Ex
c
d
0.8
0.4
0.0
0.8
0.4
0.0
300 400 500 600 700 800
500 600 700 800 900
Wavelength / nm
Wavelength / nm
20 Fig. 2 (a) UV−Vis absorption spectra of B-3 in DCM and toluene and (b)
the corresponding emission spectra of B-3, λex = 590 nm, A = 0.60, (c)
UV−Vis absorption spectra of B-4 and (d) the corresponding emission
spectra of B-4, λex = 486 nm, A = 0.51. c = 1.0 × 10–5 M for the UV−Vis
absorption spectral measurement. 20 °C.
300 400 500 600 700 800
300 400 500 600 700 800
Wavelength / nm
Wavelength / nm
25
Two major absorption bands in visible spectral region were
observed for triad B-3 (Fig. 2a), which are attributed to the
Bodipy and the dimethylaminostyryl Bodipy moiety in B-3,
respectively. Emission of the Bodipy moiety was completely
quenched in B-3, only an emission band at 675 nm was observed
Fig. 4 Comparison of the fluorescence excitation spectra and UV−Vis
absorption spectra of B-3′ or B-3 in different solvents. (a) B-3′ in toluene;
85 and (b) B-3′ in DCM, λem = 680 nm; (c) B-3 in toluene and (d) B-3 in
DCM, λem = 700 nm. c = 1.0 × 10–5 M, 20 °C.
The singlet state EnT in the compounds were studied with the
fluorescence excitation spectra, examplified with triad B-3 and B-
3′ (Fig. 4).32 In toluene, the excitation spectrum of B-3 (or B-3′)
90 is superimposable with the UV−Vis spectrum (Fig. 4a and 4c).
Thus the singlet EnT from the Bodipy moiety to the styryl-
Bodipy moiety in B-3 and B-3′ is close to unity (95% and 96%).
Moreover, the singlet EnT between the Bodipy and the
styrylBodipy part takes place first, not the EnT between Bodipy
95 and the C60 moiety, otherwise no excitation band at 531 nm
should be observed for B-3. In polar solvent such as DCM, the
comparison of the excitation spectrum with the UV−Vis
absorption spectrum indicated that the energy transfer is slightly
less efficient (85% and 88 %. Fig. 4b and 4d). This may be due to
100 the more significant ET in polar solvents.23 The emission of B-4
is too weak to be used for excitation spectra study. Therefore the
fluorescence excitation spectra of B-8 were studied (see ESI†, Fig.
S69). The singlet EnT efficiency is 75% in toluene, and 66% in
DCM
30 (Fig. 2b), which is due to the dimethylaminostyryl Bodipy moiety.
This result indicated singlet EnT. This property was supported by
the emission of the antenna B-7 (see ESI†. Fig. S61). Broadband
absorption was also observed for B-4 (Fig. 2c). The emission of
B-4 is red-shifted as compared with that of B-3. For B-4, residual
35 emission at 516 nm was observed.
500
400
300
200
100
0
400
300
200
100
0
b
B-5
a
40
B-9
B-9
B-3
B-7
B-1
500 600 700 800
Wavelength / nm
600
700
800
900
45
Wavelength / nm
Fig. 3 Fluorescence of dyads B-1 and triad B-3 and the reference
compounds. (a) Fluorescence emission spectra of B-1 and B-5. λex = 575
nm (A = 0.42 for both B-1 and B-5). (b) Fluorescence emission spectra of
105 Changes of the UV−Vis absorption and fluorescence spectra
upon addition of acid
50 B-3, B-7 and B-9. λex = 480 nm (A = 0.27 for B-3, B-7 and B-9). c = ca.
1.0 × 10–5 M in DCM, 20 °C.
Protonation of the aminostyryl Bodipy in the dyads and the triads
will change the absorption and fluorescence emission wavelength
of the antennas.6,28 Thus, the UV−Vis absorption spectra and the
110 fluorescence spectra of the compounds were studied upon
addition of trifluoroacetic acid (TFA. Fig. 5). For B-1, the
absorption band at 623 nm decreased and a new absorption band
at 573 nm developed upon addition of TFA. Accordingly, the
emission band is blue-shifted from 674 nm to 591 nm upon
115 addition of TFA (Fig. 5a and 5b). The changes can be better
The fluorescence emission of B-1 and the visible light-
harvesting antenna B-5 was compared (Fig. 3). The strong
fluorescence of B-5 (ΦF = 47 %) was quenched in B-1 (ΦF
=
55 0.1%). For B-2, similar results were observed (see ESI†, Fig.
S70a). Similar results were observed for B-3, the reference
compounds (Fig. 3b) and B-4 (see ESI, Fig. S70b). These
quenching of the fluorescence of the energy donor in the dyads or
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Journal of Materials Chemistry C
studied with the antenna alone, i.e. B-5 (see ESI†, Fig. S63)
these blue-shifting in the UV−Vis absorption and fluorescence
beacuse the fluorescence of B-5 is much stronger than that of B-1.
For B-2, the absorption band at 712 nm decreased upon
addition of TFA, and a new absorption band at 627 nm developed
emission spectra, we propose that the EnT from the styryl Bodipy
part to the C60 moeity can be switched oDnObI:y10a.d10d3it9i/oCn4ToCf0T20F3A7.E
The response of B-6 to addition of TFA was studied, similar UV-
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5
10
15
20
25
concomittantly (Fig. 5c), accordingly the fluorescence band 65 Vis absorption and fluorescence emission spectral changes were
shows blue-shifting from 770 nm to 644 nm (Fig. 5d). Based on
observed (see ESI†, Fig. S63). Similar blue shifting were
observed in the UV−Vis absorption and the fluorescence
emission spectra of the triads B-3 and B-4 (Fig. 6). The response
of B-7 and B-8 to addition of TFA was studied, similar UV−Vis
70 absorption and fluorescence emission spectral changes were
observed (see ESI†, Fig. S64).
1.6
6
20 μL
15 μL
...
0.4 μL
1.0 μL
...
a
b
1.2
0.8
0.4
0.0
4
2
0
0.4 μL
TFA
20 μL
TFA
Nanosecond time-resolved transient difference absorption
spectra
600 650 700 750 800
Wavelength / nm
300 400 500 600 700 800
In order to study the triplet excited state of the compounds, the
75 nanosecond time-resolved transient difference absorption spectra
of the compounds were studied (Fig. 7). The lifetime of the
transient is usually the lifetime of the triplet state.12 For B-1,
bleaching band at 622 nm was observed upon pulse laser
excitation in toluene (Fig. 7a), where the dimethylaminostyryl
80 Bodipy shows strong steady state absorption. Thus the triplet
excited state is localized on the styryl Bodipy moiety, not the C60
moiety. The triplet state lifetime was determined as 168.6 μs.
Therefore, different from the direction of the singlet EnT, i.e.
RET process, a reversed triplet EnT from the C60 moiety to the
85 styryl Bodipy takes place. Such a ping-pong EnT was observed
for C60-chromophore hybrids.29,34,35
Wavelength / nm
1.5
40
30
20
10
0
20 μL
15 μL
...
0.4 μL
TFA
20 μL
15 μL
...
c
d
1.0
0.5
0.0
0.4 μL
TFA
300 400 500 600 700 800
600
700
800
900
Wavelength / nm
Wavelength / nm
Fig. 5 Variation of the UV−Vis absorption spectra and the fluorescence
emission spectra of B-1 and B-2 upon protonation with trifluoroacetic
acid (TFA). (a) B-1 with increasing TFA added and (b) the corresponding
emission spectra of B-1 (λex = 540 nm) upon addition of TFA; (c) B-2
30 with increasing TFA added and (d) the corresponding emission spectra
changes of B-2 (λex = 600 nm) upon addition of TFA. The aliquots of
TFA added are 0.4 μL, 1 μL, 2 μL, 3 μL, 4 μL, 5 μL, 10 μL, 20 μL and
of 1 M TFA in CH2Cl2. c = 1.0 × 10–5 M in DCM, 20 °C.
0.04
b
a
0.000
-0.005
-0.010
-0.015
0.02
0.00
90
399.5 μs
...
0.5 μs
0 μs
1.6
1.2
0.8
0.4
0.0
100
80
60
40
20
0
τT = 168.6 μs
0.4 μL
1.0 μL
...
-0.02
-0.04
a
35
40
45
50
b
0.4 μL
1.0 μL
...
20 μL
TFA
95
400 500 600 700 800
Wavelength / nm
0
100 200 300 400
20 μL
TFA
t / μs
0.1
0.00
-0.04
-0.08
-0.12
-0.16
0.0
-0.1
-0.2
300 400 500 600 700 800
τT = 4.4 μs
600
700
800
39.7 μs
...
100
Wavelength / nm
Wavelength / nm
1.5
1.0
0.5
0.0
250
200
150
100
50
20 μL
15 μL
...
0.4 μL
TFA
0.3 μs
0 μs
c
d
c
d
0.4 μL
1.0 μL
...
20 μL
TFA
0
20 40 60 80 100
400 500 600 700 800
Wavelength / nm
t / μs
105 Fig. 7 Nanosecond time–resolved transient difference absorption of B-1
in different solvents. (a) Transient absorption spectra and (b) decay trace
at 580 nm in deaerated toluene; (c) Transient absorption spectra with 20
μL of 1 M TFA added and (d) the corresponding decay trace at 580 nm,
in deaerated dichloromethane. Exited with nanosecond pulsed laser at λex
110 = 532 nm. c = 1.0 × 10–5 M, 20 °C.
0
300 400 500 600 700 800
Wavelength / nm
700
800
900
Wavelength / nm
Fig. 6 Variation of the UV-Vis absorption spectra and the fluorescence
emission spectra of B-3 and B-4 upon protonation with addition of TFA.
(a) B-3 with increasing TFA added and (b) the corresponding emission
55 spectra changes of B-3 (λex = 540 nm) upon addition of the TFA. (c) B-4
with increasing TFA added and (d) the corresponding emission spectra
changes of B-4 (λex = 600 nm) upon addition of TFA. In CH2Cl2, (1.0 ×
10–5 M; 20 °C). The aliquots of TFA added are 0.4μL, 1 μL, 2 μL, 3 μL, 4
μL, 5 μL, 10 μL, 20 μL of 1 M TFA in CH2Cl2 (c = 1.0 × 10–5 M in
60 DCM, 20 °C).
In polar solvent, such as DCM, no transient signal was detected,
which can be attributred to ET, which is usually more favored in
polar solvent than that in non-polar solvent (see later
electrochemical studies).23 In the presence of TFA, bleaching
115 band at 573 nm was observed (Fig. 7c), which is in agreement
with the steady-state absorption spectra of B-1 in acidic DCM
(Fig. 7). The new triplet state is localized on the protonated
This journal is © The Royal Society of Chemistry [year]
Journal Name, [year], [vol], 00–00 | 5

Journal of Materials Chemistry C
Page 6 of 13
a
Table 1. Photophysical parameters of B-1−B-8 with acid (trifluoroacetic acid, TFA) added
View Article Online
DOI: 10.1039/C4TC02037E
ε / 105 M-1 cm-1 b
1.20
λ
em (nm)c
591
644
587
573
637
592
645
592
ΦL(%) d
0.3
0.4
1.8
0.7
τT (μs)e
4.4
74.8
4.3
121.8
35.0
τF (ns)f
ΔEst
18
17
83/13
51/14
135/10
23
g
λabs (nm)
573
627
504/574
522/559
502/627
569
h
B-1
B-2
B-3
B-3′
B-4
B-5
B-6
B-7
−
h
1.09
−
h
0.96/1.33
1.12/1.20
0.93/0.97
1.13
−
h
−
h
0.9
−
h
37.2
25.9
62.7
36.5
20.9
4.31
3.89
4.32
4.08
4.63
−
h
629
1.09
16
−
h
502/574
527/559
501/630
1.01/1.05
1.19/1.45
0.94/1.09
90/18
45/13
145/16
−
h
572
646
−
B-7′
B-8
h
−
a
c = 1.0 × 10-5 M with 20 μL 1 M TFA added. at 20 °C b Molar extinction coefficient at the absorption maxima. c The emission maxima. d Fluorescence
5 quantum yields with B-12 (ΦF = 9.5%, in toluene) as standard. e Triplet excited state lifetime. f Fluorescence lifetime. g Stokes shift. In nm. h Not observed.
transient absorption in the region of 670 − 850 nm region were
Bodipy ligand, but the triplet excited state is different from that 50 observed. Exceptionally long triplet state lifetime of 333.7 μs was
for B-1 in toluene. Note the transient difference absorption profile
in 600 – 850 nm is different for Fig. 7a and 7b. The triplet state
10 lifetime was determined as 4.4 μs. The observation of TA signal
in acidic DCM is due to the inhibited ET by the protonation, thus
the inhibition of the ET process.
The nanosecond time-resolved transient difference absorption
spectra of B-2 were also studied (Fig. 8). No transient spectra was
15 observed in toluene. Electron transfer is usually difficult to take
place in non-polar solvents, such as toluene.23 This result can be
attributed to the lower S1 state energy level of antenna (1.67 eV)
than the S1 state of C60 (1.76 eV),23 i.e. there is no RET for B-2 in
toluene, as a result, no triplet state can be produced. In order to
20 validify this conclusion, 1O2 photosensitizing study was
performed, no 1O2 production was observed for for B-2 in
toluene (see ESI†, Fig. S78-79).
determined. These features are close to that of B-1 under the
same conditions. Therefore, the triplet state is localized on styryl
Bodipy part, not on the C60 moiety in B-3. In DCM with TFA
added, a bleaching band at blue-shifted position of 571 nm was
55 observed for B-3 (Fig. 9c), which is in agreement with the steady
state absorption of the protonated amino styryl Bodipy antenna in
B-3 (Fig. 6a). Thus the triplet state is localized on the protonated
amino styryl Bodipy moiety in B-3. In this case a 'ping-pong'
singlet/triplet energy transfer takes place.28
60
0.08
0.04
0.00
-0.02
-0.04
-0.06
-0.08
τT = 333.7 μs
0.00
999.5 μs
...
0.5 μs
0 μs
65
70
75
-0.04
-0.08
a
b
0.00
400 500 600 700 800
Wavelength / nm
0
200 400 600 800 1000
25
0.05
t / μs
-0.03
-0.06
-0.09
τT = 74.8 μs
0.00
-0.05
-0.10
0.1
0.0
0.0
-0.1
-0.2
214.4 μs
...
0.8 μs
0.4 μs
τT = 4.3 μs
39.7 μs
...
0.5 μs
b
a
30
-0.1
-0.2
0
100 200 300 400
400 500 600 700 800
Wavelength / nm
0 μs
t / μs
d
c
Fig. 8 Nanosecond time-resolved transient difference absorption spectra
of B-2. (a) Transient absorption difference spectra and (b) the decay trace
0
20
40
60
400 500 600 700 800
Wavelength / nm
t / μs
35 at 630 nm. In deaerated DCM with 20 μL of 1 M TFA added. λex = 532
nm (c = 1.0 × 10–5 M, 20 °C).
Fig. 9 Nanosecond time–resolved transient difference absorption of B-3
In DCM, no transient spectra was observed. In DCM with
TFA added, however, a bleaching band at 627 nm was observed
(Fig. 8a), which is in agreement with the steady state absorption
40 of the protonated B-2. Thus the triplet state is localized on the
protonated Bodipy ligand. A triplet state lifetime of 74.8 μs was
observed (Fig. 8b). Based on the nanosecond TA data, we
propose the EnT in B-2 was modulated by addition of acid, i.e.
protonation of the aminostyryl Bodipy antenna. This is different
45 from B-1, for which the ET was inhibited by addition of TFA.
Similar transient absorption spectra were observed for B-3
(Fig. 9) as compared with that of B-1. Bleaching band at 625 nm
was observed upon pulsed laser excitation in toluene. Positive
80 in different solvents. (a) Transient difference absorption spectra and (b)
decay trace at 620 nm in deaerated toluene; (c) Transient absorption
difference spectra of B-3 with 20 μL of 1 M TFA added and (d) the
corresponding decay trace at 580 nm, in deaerated DCM. λex = 532 nm, (c
= 1.0 × 10–5 M, 20 °C)
85
Transient absorption spectra were observed for B-3′ (Fig. 10)
as compared with that of B-1 and B-3. Bleaching band at 600 nm
was observed upon pulsed laser excitation in toluene. Positive
transient absorption in the region of 650 − 850 nm region were
observed (Fig. 10a). A triplet state lifetime of 140.1 μs was
90 determined (Fig. 10b). These features are close to that of B-1
under the same conditions. Therefore, the triplet state is localized
6
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Journal of Materials Chemistry C
on the styryl Bodipy part, not on the Bodipy moiety or C60 moiety 60 localized on the protonated amino styryl Bodipy moiety in B-3′,
in B-3′.
Different from B-1 and B-3, transient absorption spectra were
not on the Bodipy or the C60 moiety. In this case a 'ping-pong'
View Article Online
singlet/triplet EnT takes place.28 Note theDtOriIp: l1e0t.1s0t3a9te/Cl4ifTeCti0m20e3i7sE
observed for B-3′ in DCM (Fig. 10c). The feature of the transient
spectra are similar to that in toluene. A triplet state lifetime of
different from the neutral B-3’.The transient spectra of B-3′
indicated that the ET is less significant as compared to that of B-1
5
100.7 μs was observed (Fig. 10d). The slightly decreased triplet 65 and B-3. We attributed this difference in ET capability to the
state lifetime in DCM than that in toluene may be due the
enhanced ET.23 However, we propose that photo-induced ET is
not significant, otherwise the triplet state will be quenched, like in
different structure of the antenna. In B-3′, the distance between
the electron donor and C60 moiety (electron acceptor) is larger
than that in B-1.
10 B-1 and B-3. The thermodynamic driving force for photoinduced
For B-4, no transient signal can be detected and no 1O2
70 produced in both toluene and DCM (Fig. S78-79). It is most
likely due to the unmathced S1 state of the antenna and the C60
moiety. In DCM with TFA added, bleaching band at 627 nm was
detected (Fig. S83a). Thus the triplet excited state is localized on
the aminostyryl Bodipy antenna, not on the C60 moiety. The
75 production of triplet excited state in the presence of TFA is due to
the increased energy level of the S1 state of the antenna, thus the
acid-activated EnT to C60 occured. The results of the triads B-3
and B-4 indicated that the RET from Bodipy to C60 is not taking
place, rather the sequential Bodipy→aminostyryl Bodipy→C60
80 process is taking place in the triads.
0.2
0.00
0.1
-0.04
15
20
25
30
35
τT = 140.1 μs
0.0
-0.1
-0.2
-0.08
-0.12
-0.16
399.5 μs
...
0.5 μs
0 μs
a
b
0
100 200 300 400
400 500 600 700 800
Wavelength / nm
t / μs
0.08
0.04
0.00
-0.03
-0.06
-0.09
Electrochemical studies
τT = 100.7 μs
0.00
The electrochemical property of B-1 and reference B-5 was
studied in DCM (Fig. 11 and Table 2. for B-3, B-4 and reference
85 compounds, see ESI†, Fig. S71-S74 and Table 2). For B-5, there
are two reversible oxidation waves at +0.34 V, +0.66 V, and one
reversible reduction wave at −1.41 V. For B-1, in the anodic scan,
two reversible oxidiation waves at around +0.35 V, +0.67 V
were observed. In the cathodic scan, two reversible reduction
90 potential was observed for B-1 (−0.97eV, −1.38 eV), which are
dur to fullerene. In order to study electron transfer from
dimethylaminostyryl Bodipy to fullerene, the Gibbs free energy
514.7 μs
...
-0.04
-0.08
0.5 μs
0 μs
d
c
400 500 600 700 800
Wavelength / nm
0
200 400 600 800 1000
t / μs
0.03
0.00
0.00
-0.02
-0.04
-0.06
399.5 μs
...
0.5 μs
τT = 128.1 μs
-0.03
-0.06
(ΔGocs) was calculated. From the ΔGo analysis, some results
cs
e
0 μs
f
have been found, In polar solvent, the free energy changes of B-1
95 in CH2Cl2 (ΔGo = −0.74 eV) is larger than that in nopolar
0
100 200 300 400
400 500 600 700 800
Wavelength / nm
cs
t / μs
solvent (ΔGo = −0.24 eV in toluene). Therefore the triplet state
cs
Fig. 10 Nanosecond time-resolved transient difference absorption spectra
of B-3′ in different solvents. (a) Transient spectra and (b) the
is unlikely to be observed in the polar solvent. For B-2, the free
energy changes is large in DCM (ΔGo = −0.74 eV), thus the
cs
40 corresponding decay trace at 620 nm. In deaerated toluene. (c) Transient
spectra in deaerated dichloromethane and (d) the corresponding decay
trace at 620 nm; (e) Transient spectra of B-3′ in deaerated DCM with 20
μL of 1 M TFA added and (f) the corresponding decay trace at 570 nm.
fluorescence emission is effectively quenched by photoinduced
100 intramolecular electron transfer (Fig. 1). In nopolar solvent
toluene, the S1 energy level is 1.67 eV for dimethylaminodistyryl
Bodipy lower than C60 (1.76 eV), therefore no triplet state
λ
ex = 532 nm, c = 1.0 × 10–5 M, 20 °C.
45 electron transfer was calculated with the Weller equation. B-3 is
with slightly larger driving force (ΔGocs = −0.68 eV) than B-3′
(ΔGocs = −0.65 eV). The charge transfer state energy level (ECTS
=
a
b
105
1.26 eV) is higher than triplet state energy level (T1 = 1.24 eV by
DFT calculated. In DCM). But for B-1, the charge transfer state
50 energy level (ECTS = 1.14 eV) is lower than triplet state energy
level, thus it is effective for quenching the triplet state though
electron transfer. In toluene, the Gibbs free energy was calculated
for B-3 (ΔGocs = −0.18 eV) and B-3′ (ΔGocs = +0.17 eV). From the
above analysis, it can be deduced that the electron transfer of B-3
55 is more significant than B-3′.
In DCM with TFA added, a bleaching band at blue-shifted
position of 557 nm was observed for B-3′ (Fig. 10e), which is in
agreement with the steady state absorption of the protonated
amino styryl Bodipy antenna in B-3′. Thus the triplet state is
110
1
0
-1
-2
1
0
-1
Potential (V)
Potential (v)
Fig. 11 Cyclic voltammogram of B-1 and B-5. (a) B-1, (b) B-5. Ferrocene
(Fc) was used as internal reference (E1/2 = +0.64 V (Fc+/Fc) vs. standard
115 hydrogen electrode). In deaerated CH2Cl2 solutions and 0.10 M Bu4NPF6
as supporting electrode, Ag/AgNO3 reference electrode, Scan rates: 100
mV/s. 20 °C.
This journal is © The Royal Society of Chemistry [year]
Journal Name, [year], [vol], 00–00 | 7

Journal of Materials Chemistry C
Page 8 of 13
Table 2. Redox potentials of Bodipy-C60 photosensitizers. Anodic and
cathodic wave potentials were presented vs. saturated calomel electrodea
Table 3. The thermodynamic driving force for photoinduced electron
transfer was calculated from the Weller equation
View Article Online
ΔGoCS (eV)a
ΔGoCS (eV)b
E(ox) (V)
E(red) (V)
ECTS/ eVc
E
/ eVd
CTS
DOI: 10.1039/C4TC02037E
−0.74e/−0.10f −0.24e/+0.40f
−0.74 e/−0.19 f −0.24 e/+0.31 f
−0.68 e/−0.04 f −0.18 e/+0.46 f
B-1
B-2
B-3
B-3′
B-4
B-5
B-6
B-7
+0.35, +0.67
−0.97, −1.38, −1.88
1.64
1.43
1.70
2.08
1.39
1.14
0.95
1.20
1.26
0.89
B-1
B-2
B-3
B-3′
B-4
B-7
B-7′
B-8
+0.14, +0.28, +1.01 −0.97, −1.36
+0.42, +0.74, +1.03 −0.96, −1.12, −1.49, −1.89
+0.36, +0.63, +0.97 −0.96, −1.39, −1.87
+0.15, +0.91, +1.09 −0.92, −1.32, −1.47, −1.86
+0.17 e/+0.84 f
−0.65 e/+0.018 f
−0.77 e/−0.23 f −0.27 e/+0.27 f
+0.11 e/+0.75 f
g
g
−0.39 e/+0.25 f
−
−
+0.34, +0.66
−1.41
−1.35
−1.32, −1.43
+0.64 e/+1.31 f
+0.16 e/+0.69 f
g
g
−0.18 e/+0.49 f
−0.34 e/+0.19 f
−
−
+0.25, +1.02
g
g
+0.35, +0.73, +1.02
−
−
+0.37, +0.64, +0.93 −1.42
+0.17, +0.93, +1.06 −1.32, −1.46
B-7′
B-8
50 In DCM. In toluene. ECTS = E1/2(D•+/D) − E1/2(A•−/A) +.ΔGS where
E1/2(D·+/D) is the first oxidation potential of the donor, E1/2(A/A•−) is the
first reduction potential of the acceptor, ΔGS is he static Coulombic
a
b
c
a
Cyclic voltammetry in N2 saturated CH2Cl2 containing a 0.10 M
d
Bu4NPF6 supporting electrolyte; Counter electrode is Pt electrode;
5 working electrode is glassy carbon electrode; Ag/AgNO3 couple as the
reference electrode. c [Ag+] = 0.1 M. 20 °C. Conditions: 0.5 mM
photosensitizers in CH2Cl2, 298 K.
energy, the energy level of charge transfer state in toluene. The energy
level of charge transfer state in DCM. e ΔGo was calculated by Weller
CS
55 equation, photoinduced electron transfer from the S1 state of styryl-
f
Bodipy. Photoinduced electron transfer from the T1 state of styryl-
Bodipy. g Not applicable.
was observed for B-2 because RET can not occur. Similar results
were observed for B-3 and B-4 (Table 2, 3 and Fig. S71-S74).
10 These electrochemical data can be used for explain the triplet
state properties of the dyads/triads in solvent with different
polarity. For example, the free energy changes for ET in polar
solvent is usually large than that in nonpolar solvent such toluene.
This is true for B-1 and B-2, etc. Upon protonation, the ET
15 process was prohibited, thus triplet state can be observed for the
dyads/triads.
2.0
1.5
60
0 s
0 s
20 s
...
a
d
20 s
...
1.5
1.0
0.5
0.0
1.0
0.5
0.0
120 s
120 s
65
70
75
80
300 350 400 450 500
Wavelength / nm
300 350 400 450
Wavelength / nm
Application of the acid-activated triplet state production:
singlet oxygen (1O2) photosensitizing
1.6
1.2
0.8
0.4
0.0
0 s
0 s
20 s
1.2
0.8
0.4
e
With the nanosecond time-resolved transient difference
20 absorption spectroscopy, we demonstrated that the production of
the triplet excited state by the dyads and the triads can be
switched by acid. Therefore, the switching of the 1O2
photosensitizing ability of the dyads and the triads were studied
(Fig. 12).12 1,3-diphenylisobenzofuran (DBPF) was used as the
25 1O2 scavenger. Thus the absorption of DBPF at 414 nm can be
followed to monitor the kinetics of the 1O2 production of the
dyads and the triads upon visible light photoirradiation.36,37
20 s
b
...
120 s
...
120 s
300
350
400
450
300 350 400 450
Wavelength / nm
Wavelength / nm
1.2
0.8
0.4
For B-1, no 1O2 production can be observed in the absence of
acid (TFA) (Fig. 12a). In the presence of acid (TFA, Fig. 12b),
30 the absorbance of the solution at 414 nm decrease sharply upon
1.2
1.1
1.0
0.9
B-1
B-2
f
c
1
photoexcitation, indicating that O2 was produced in a signifcant
B-1 + TFA
B-2 + TFA
fast kineics, i.e. the triplet state production of B-1 is switched ON.
Note DPBF alone is stable under the same condition (see ESI†,
Fig. S95), thus the 1O2 photosensitizing was confirmed.
0
30
60
90
120
0
30
60
90
120
85
Irradiation time / (s)
35
Similar studies were applied for B-3 and B-4 (see ESI†, Fig.
S94). No 1O2 was produced in DCM upon photoexcitation. In the
presence of TFA, photosensitization 1O2 was observed. This
observation is in agreement with the nanosecond time-resolved
Irradiation time / (s)
Fig. 12 Switching of the singlet oxygen (1O2) photosensitizing ability of
B-1 and B-2 in the absence and the presence of TFA. The decreasing of
the absorption of 1O2 scavenger 1,3-diphenylisobenzofuran (DPBF) at 414
90 nm was monitored upon the monochromic light irradiation. UV−Vis
absorption spectral changes B-1 as photosensitizer (a) in the absence of
TFA acid and (b) in the presence of TFA. (c) Absorbance of DPBF at 414
nm changes in the absence and in the presence of TFA (λex =588 nm).
UV/vis absorption spectral changes with B-2 as photosensitizer, (d) in the
95 absence of TFA and (e) in the presence of acid, (f) in absorbance of
DPBF at 414 nm changes in the absence and in the presence of TFA (λex
= 646 nm). c [photosensitizers] = 1.0 × 10–5 M in CH2Cl2, 20 °C.
transient absorption spectra of B-3 and B-4. Interestingly, no
1
40 modulation on the O2 photosensitizing of B-3′ by addition of
acid was observed. This phenomena is in agreement with the
nanosecond time-resolved transient difference absorption spectra
of B-3′, i.e. triplet state production was observed for B-3′ in
DCM, as well as in DCM with TFA added (see ESI†, Fig. S94).
45 Thus the 1O2 production of the triplet photosensitizers can be
directly correlated to the triplet state production of the dyads and
triads. 1O2 photosensitizing of B-4 can be switched by addition
8
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Journal of Materials Chemistry C
of acid (see ESI, Fig. S94), which is in agreement with the study
of the triplet state production of B-4 by the nanosecond time-
45
50
55
View Artibcle Online
1[C60-BDP*]
a
resolved transient difference absorption spectra. More significant
is that switching of the triplet state of photosensitizers is
reversible by adding acid and base alternatively. When the acid
was neutralized with base, the photophyscial properties of the
photosentiziers fully recovered (see ESI†, Fig. S83-93).
1[C60-BDP*]
DOI: 10.1039/C4TC02037E
1[*C60-BDP]
1[*C60-BDP]
3[*C60-BDP]
ISC
3[*C60-BDP]
FRET
FRET
5
ISC
3[C60-BDP*]
1.24 eV
CTS
1.14 eV
CR
3[C60-BDP*]
1
The O2 quantum yields (ΦΔ) of the dyads and triads were
622 nm
573 nm
measured in the absence and presence of TFA. For B-1, the ΦΔ
S0
S0
10 value is 1.9% in the absence of TFA. In the presence of TFA, ΦΔ
increased to 73% (Table 4). Similar switching effect was observe
for B-2−4. For B-3′, however, the ΦΔ value in the absence and in
the presence of TFA is 30% and 39%, thus no switching effect of
the triplet excited state production was observed for B-3′. This
15 finding is in agreement with the nanosecond time-resolved
transient difference absorption spectral studies of the compounds
(Fig. 9).
S0
S0
Scheme 4. Simplified Jablonski diagram illustrating the photophysical
processes of B-1 (a) in the absence and (b) in the presence of acid.
CH2Cl2 was used as solvent. [C60-BDP] stands for B-1. The localization
of the excited state in the dyad was designated with red color and a
60 asterisk. The number of the superscript designated spin multiplicity.
In the presence of B-1 (Scheme 4b), the dimethylamino group
is protonated, thus the CTS energy level will be promoted (data
are unavailable due to the inteference of H+ in the measurement
of cyclic voltammetry). Thus, the triplet state of the protonated
65 dimethylamino styrylBodipy moiety will not be quenched by
CTS. Thus triplet state was detected for B-1 even in polar
solvents such as dichloromethane (Fig. 7).
Table 4. Singlet oxygen quantum yield (ΦΔ) and triplet state lifetime (τT)
c
τT (μs)
ΦΔ
168.6 a
1.9 %
1.1 %
2.0 % / 1.8 %
30 % / 30 %
1.0% / 1.4 %
B-1
B-2
B-3
B-3′
B-4
a
−
333.7 a
140.1 a / 100.7b
a
−
In the presence of TFA
1[*C60-BDP]
a
70
1[C60-BDP*]
1.61eV
1[C60-BDP*]
FRET
b
4.4 b
74.8 b
4.3 b
73 %
26 %
52 % / 63 %
39 % / 53 %
22 % / 17 %
B-1
B-2
B-3
B-3′
B-4
1[*C60-BDP]
ISC
×
1.76eV
3[*C60-BDP]
FRET
3[C60-*BDP]
3[*C60-BDP]
3[C60-*BDP]
121.8 b
35.0 b
CTS
770 nm
a Triplet lifetime in deaerated toluene. b Triplet lifetime in deaerated DCM.
20 c Quantum yield of singlet oxygen (1O2). For short wavelength excitation,
diiodoBodipy was used as standard (ΦΔ = 0.82 in CH2Cl2), for the long
wavelength excitation, methylene blue was used as standard (ΦΔ = 0.57 in
CH2Cl2).
75
644 nm
0.95 eV
CR
S0
S0
S0
S0
80 Scheme 5. Simplified Jablonski diagram illustrating the photophysical
processes involved B-2 (a) in the absence of acid and (b) in the presence
of acid. [C60-BDP] stands for B-2. The localization of the triplet state in
the dyad was designated with red color and a asterisk. The number of the
superscript designated the spin multiplicity of the excited state.
Jablonski diagrams: Photophyical processes of the
25 dyads/triads
The photophysical processes of B-1 were summarized in Scheme
4. Upon selective photoexcitation into the styrylBodipy antenna
in B-1 (Scheme 4a), the singlet excited state of the antenna was
produced firstly. Then via intramolecular energy transfer, the
30 singlet excite state of the C60 moiety was produced. Then via the
inherent ISC of C60, the triplet state of C60 moiety was produced.
Since the triplet state energy level of the styryl Bodipy is lower
than that of C60 moiety, the intramolecular triplet state energy
transfer is envisaged.
85
The photophysics of B-2 is slightly different from that of B-1
(Scheme 5). First, the S1 state energy level of the
dimethylaminostyrylBodipy part is lower than that of C60, thus
FRET is impossible (Scheme 5a). Second, CTS is with energy
level of 0.95 eV, thus any triplet state localized on either the C60
90 or the dimethylaminostyrylBodipy will be quenched. This
postulation is in agreement with the experimental results that no
triplet excited state was observed for B-2, either in toluene or in
dichloromethane, which is different from that of B-1.
In the presence of acid, such as TFA, the dimethylamino
95 group is protonated, thus the S1 state energy level becomes higher
than the S1 state of the C60 moiety, as a result the FRET is
possible (Scheme 5b). Moreover, upon protonation, the CTS
energy level is elavated (eaxct data are unknown due to the
difficulty to record the electrochemical data of the protonated
100 species). Thus triplet state was observed for B-2 in the presence
of TFA (Fig. 8).
35
The charge transfer state (CTS) is with lower energy level
than that of the T1 state of styrylBodipy, thus the triplet state can
be quenched by production of the CTS. In toluene, however, the
CTS is with energy level of 1.64 eV, which is higher than that of
the triplet state energy level of the styrylBodipy moiety. Thus the
40 triplet state of the styrylBodipy will not be quenched by any CST.
These postulations are in full agreement with the experimental
results (Fig. 7). It should be pointed out the triplet state can be
produced from the charge recombination process.23,38
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Journal of Materials Chemistry C
Page 10 of 13
6H), 2.89 (s, 3H), 2.51 (s, 3 H), 1.57 (s, 3H), 1.42 (s, 3 H) ppm.
Conclusions
13C NMR (100 MHz, CDCl3): 147.3, 146.8, 146.7, 146.5146.4,
In conclusion, in order to switch the triplet excited states of
organic compounds, dimethylaminostyryl Bodipy-C60 dyads and
triads were prepared. The designing rationales of these
compounds are that either the photoinduced electron transfer (ET)
or the singlet state energy transfer (EnT) from the visible light-
harvesting antenna to C60 moiety can be switched by acid
(protonation of the amino styryl moeity). C60 moiety in the
dyads/triads is the electron or singlet energy acceptor, and the
146.3, 146.2, 146.1, 146.0, 145.9, 145.8, 145.7, 14V5ie.5w,Ar1ti4cl5e.O5n,line
DOI: 10.1039/C4TC02037E
60 145.4, 145.3, 1345.2, 145.1, 145.0, 144.9, 144.7, 144.4, 143.1,
143.0, 142.7, 142.6, 142.5, 142.4, 142.3, 142.1, 142.0, 141.9,
141.8, 141.7, 141.6, 141.5, 141.4, 140.5, 140.2, 140.1, 139.8,
139.7 139.5, 139.4, 139.2, 135.8, 135.7, 135.4, 134.4, 133.4,
129.3, 129.1, 128.9, 128.4, 117.9, 82.3, 40.1, 32.8, 31.9, 29.7,
65 14.7, 13.4, 12.7 ppm. HRMS (MALDI): m/z calcd for
[C97H37BF2N4]-: 1306.3079; found 1306.3124.
5
10 spin converter. Bodipy units ar the visible light harvesting
antenna, the singet energy and electron donor. The triplet state of
the compounds was studied in detail with nanosecond time-
resolved transient difference absorption spectroscopy. For the
dyad with the mono(4′-dimethylaminostyryl) Bodipy antenna, the
15 triplet excited state was quenched by intramolduclar EnT in polar
solvent. In presence of acid (trifluoroacetic acid), the
dimethylaminostyryl Bodipy moiety is protonated, as a result, the
ET was inhibited. Therefore the sequenctial EnT and the
intersystem crossing of C60 process produce triplet excited state.
20 For the dyad and the triads with bis(4-dimethylaminostyryl)
substituted Bodipy antenna, the lower antenna S1 state energy
level than that of C60 prohibits any RET to C60, thus no triplet
state was produced. In the presence of acid thus protontation of
the aminostyryl substituents, the UV−Vis absorption and the
25 fluorescence emission show drastic blue shifting, and the S1 state
energy level of the antenna is increased to be higher than S1 state
of C60 moiety, as a result EnT is activated and triplet state will be
produced. In all the compounds the triplet excited state is
localized on the dimethylaminostyryl Bodipy moiety, not on the
Compound B-2. The compound was prepared with the same
methods of B-1. B-2 was purified by chromatography (silica gel,
DCM) to give black solid. Yield: 28.8 mg (57 %). MP > 250°C .
70 1H NMR (400 MHz, CDCl3): δ = 7.97−7.77 (m, 5H), 7,52−7.44
(m, 5H), 7.35−7.32 (m, 5H), 7.11−7.10 (m, 2H), 6.63−6.53 (m,
5H), 5.04−4.96 (m, 2H), 4.28−4.26 (m, 1H), 2.80 (s, 3H), 1.54 (s,
3H), 1.43 ppm (s, 3H). 13C NMR (100 MHz, CDCl3): 156.3,
154.1, 153.6, 153.4, 147.4, 147.3, 146.7, 146.5, 146.3, 146.2,
75 146.1, 146.0, 145.8, 145.6, 145.5, 145.4, 145.3, 145.2, 145.1,
144.8, 144.5, 144.4, 144.3, 143.1, 143.0, 142.7, 142.6, 142.5,
142.3, 142.2, 142.1, 142.0, 141.9, 141.8, 141.7, 141.6, 141.5,
141.4, 141.3, 141.2, 140.2, 139.9, 139.5, 137.0, 136.6, 135.9,
135.8, 129.5, 128.9, 128.7, 112.5, 112.3, 112.2, 112.1, 83.6, 70.0,
80 69.1, 68.1, 40.0, 25.6, 14.7, 12.4 ppm. HRMS (MALDI): m/z
calcd for [C106H46BF2N5]-: 1437.3814; found 1437.3922.
Compound B-3. The preparation procedure is similar to that
of B-1. B-3 was purified by chromatography (silica gel, DCM) to
1
give black solid. Yield: (26.0 mg, 49 %). MP >250°C. H NMR
85 (400 MHz, CDCl3): δ = 7.90 (s, 1H), 7.75 (s, 1H), 7.53−7.50 (m,
3 H), 7.24−7.17 (m, 8H), 7.11 (d, 2H, J = 8.4 Hz), 6.98 (d, 2H, J
= 8.0 Hz), 6.79 (s, 2H), 6.61 (s, 1H), 5.98 (s, 2H), 5.25 (s, 2H),
5.03 (s, 2H), 4.83−4.82 (m, 2H), 4.44−4.42 (m, 2H), 4.31 (s, 1H),
3.05 (s, 6H), 2.88 (s, 3H), 2.56 (s, 6H), 2.51 (s, 3H), 1.44 (s, 3H),
90 1.42 (s, 6H), 1.27 ppm (s, 3H). 13C NMR (100 MHz, CDCl3):
158.86, 158.24, 155.39, 147.32, 147.29, 146.69, 146.44, 146.29,
146.26, 146.22, 146.17, 146.11, 146.09, 145.93, 145.92, 145.74,
145.56, 145.53, 145.44, 145.36, 145.27, 145.22, 145.17, 145.15,
144.70, 144.52, 144.39, 144.32, 143.75, 143.13, 143.07, 142.98,
95 142.87, 142.59, 142.56, 142.53, 142.23, 142.17, 142.05, 141.90,
141.89, 141.76, 141.68, 141.52, 141.48, 140.18, 140.13, 139.78,
139.60, 136.16, 135.89, 135.75, 133.74, 131.80, 131.11, 130.43,
129.96, 129.36, 128.79, 127.75, 124.13, 121.25, 118.01, 225.36,
115.00, 83.24, 66.33, 61.99, 49.88, 39.99, 31.93, 29.69, 15.05,
100 14.63, 14.13, 13.38, 12.97. HRMS (MALDI): m/z calcd for
[C121H61B2F4N9O2]+: 1769.5070; found 1769.4987.
30
C
60 moiety. We proved that the 1O2 photosensitizing ability of the
compounds can be switched by acid (the variation of the singlet
oxygen quantum yield can be up to 30-fold). These studies will
be useful for development of external stimuli-activatable triplet
state production with organic chrmophores and for the application
35 of these compounds in activatable photodynamic theraputic
reagents, molecular devices and fundamental photochemistry
studies.
Experimental section
General Methods. UV−Vis absorption spectra were taken on
40 a HP8453 UV−Vis absorption spectrophotometer. Fluorescence
spectra were recorded on
a
Shimadzu RF 5301PC
spectrofluorometer. Luminescence lifetimes were measured on a
OB 920 fluorescence/phosphorescence lifetime spectrometer. For
the preparation of the intermediate compounds, please refer to the
45 ESI †.
Compound B-4. The preparation procedure is similar to that
of B-1. B-4 was purified by chromatography (silica gel, DCM) to
give black solid. Yield: 23.9 mg (38 %). MP >250°C. H NMR
Compound B-1. Under Ar atmosphere,
a
mixture of
1
compound 4 (28.0 mg, 0.05 mmol), fullerene (43.2 mg, 0.006
mmoL) and sarcosine (17.8 mg ,0.20 mmoL) were dissolved in
dry toluene (50 mL). The mixture was refluxed for 12 h. After
50 completion of the reaction, the mixture was cooled to room
temperature (RT), After removal of the solvent under reduced
pressure, the mixture was purified by column chromatography
(silica gel, DCM) to give black solid. MP >250°C Yield: 40.5 mg
(62 %). 1H NMR (400 MHz, CDCl3): δ = 7.84 (s, 2H), 7.54−7.50
55 (m, 3H), 7.47−7.45 (m, 3H), 7.33−7.31 (m, 2H), 7.24−7.20 (m, 3
H), 6.83 (s, 2 H), 6.62 (s, 1 H), 5.01 (s, 2H), 4.31 (s, 1H), 3.05 (s,
105 (400 MHz, CDCl3): δ = 7.90 (s, 2H), 7.68−7.52 (m, 5H),
7.33−7.31 (m, 3H), 7.22−7.18 (m, 5H), 7.12−7.09 (m, 4H),
6.99−6.95 (m, 4H), 6.80−6.69 (m, 5H), 5.98 (s, 2 H), 5.25 (s, 2
H), 5.00 (s, 2H), 4.83−4.80 (m, 2H),4.43−4.40 (m, 2H), 4.28 (s, 1
H), 3.05 (s, 12H), 2.17 (s, 3 H), 2.08 (s, 6H), 1.59 (s, 6H), 1.42 (s,
110 6H). 13C NMR (100 MHz, CDCl3): δ = 158.83, 158.25, 155.40,
147.37, 147.30, 147.27, 146.37, 146.29, 146.24, 146.23, 146.15,
146.12, 146.04, 145.98, 145.95, 145.79, 145.59, 145.53, 145.49,
145.40, 145.36, 145.34, 145.23, 145.18, 145.10, 144.96, 144.92,
10
|
Journal Name, [year], [vol], 00–00
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Page 11 of 13
Journal of Materials Chemistry C
144.88, 144.82, 144.77, 144.75, 144.72, 144.54, 144.42, 144.37,
144.35, 144.29, 144.26, 143.75, 143.14, 143.10, 143.07, 143.01,
142.73, 142.65, 142.63, 142.60, 142.55, 142.23, 142.11, 142.07,
141.96, 141.90, 141.75, 141.73. 141.51, 141.44, 140.22, 139.86,
Cyclic voltammetry was performed using
Electrochemical workstation (Shanghai,
a
CHI610D
China).Cyclic
View Article Online
voltammograms were recorded at scan rates of 100 mV/s. The
DOI: 10.1039/C4TC02037E
electrolytic cell used was a three electrodes cell. Electrochemical
5
139.35, 135.95, 135.69, 131.76, 129.36, 127.76, 124.15, 121.24, 60 measurements were performed at RT using 0.1
M
115.37, 115.01, 66.35, 65.84, 62.01, 49.88, 39.94, 30.94, 29.72,
15.28, 15.02, 14.63, 12.72. HRMS (MALDI): m/z calcd for
[C130H70B2F4N10O2]+: 1900.5805; found 1900.5927, [M-
F]+1880.6173, [M-C60] + 1181.6094.
tetrabutylammonium hexafluorophosphate (TBAP) as supporting
electrolyte, after purging with N2. The working electrode was a
glassy carbon electrode, and the counter electrode was platinum
electrode. A non aqueous Ag/AgNO3(0.1 M in acetonitrile)
65 reference electrode was contained in a separate compartment
connected to the solution via semipermeable membrane. CH3CN
was used as the solvent. Ferrocene was added as the internal
references.
10 Compound B-5. Under Ar, a mixture of 2 (58.1 mg, 0.1 mmol),
phenylboronic acid (72 mg, 0.6 mmoL) and K2CO3 (41.7 mg , 0.3
mmoL) was dissolved in toluene/ethanol/water (15 mL, 2/2/1,
v/v), and the mixture was bubbled with Ar for 15 min. Pd(PPh3)4
(18.1 mg, 0.015 mmoL) was added. The mixture was refluxed for
15 3 h. After completion of the reaction, the mixture was cooled to
RT, water (30 mL) was added, and the mixture was extracted
with dichloromethane (3×50 mL). The organic layers were dried
over Na2SO4. After removal of the solvent under reduced
pressure, the mixture was purified by column chromatography
20 (silica gel, DCM/petroleum = 1:1, v/v) to give deep purple solid.
Yield: 47.8 mg (90 % ). 1H NMR (400 MHz, CDCl3): δ = 7.55 (d,
3H, J = 8.4 Hz), 7.48 (d, 3H, J = 6.4 Hz), 7.30 (d, 1H, J = 8.0
Hz), 7.23 (d, 1H, J = 16.0 Hz), 7.16 (d, 2H, J = 7.6 Hz), 6.99 (s,
2H), 6.61 (s, 1H), 3.05 (s, 6H), 2.55 (s, 3H), 1.42 (s, 3H), 1.29
25 ppm (s, 3H). 13C NMR (100 MHz, CDCl3): 154.9, 151.7, 143.0,
139.3, 137.8, 135.7, 134.3, 133.5, 133.1, 131.0, 130.6, 130.4,
129.9, 129.4, 129.2, 129.0, 128.6, 128.4, 126.2, 118.0, 114.9,
112.5, 40.7, 29.9, 14.9, 12.7 ppm. HRMS (MALDI): m/z calcd
for [C34H32BF2N3]+: 531.2657; found 531.2694.
Aknowledgement
70 We thank the NSFC (20972024, 21273028, 21421005 and
21473020), the Royal Society (UK) and NSFC (China-UK Cost-
Share Science Networks, 21011130154), and Ministry of
Education (SRFDP-20120041130005) Program for Changjiang
Scholars and Innovative Research Team in University
75 [IRT_13R06], the Fundamental Research Funds for the Central
Universities (DUT14ZD226) and Dalian University of
Technology (DUT2013TB07) for financial support.
Notes and references
State Key Laboratory of Fine Chemical, School of Chemical Engineering,
80 Dalian University of Technology, Dalian, 116024, P.R. China. Fax: (+86)
411-8498-6236; E-mail: zhaojzh@dlut.edu.cn;
Web: http://finechem.dlut.edu.cn/photochem;
30 Compound 10. Under Ar, a mixture of 8 (41.0 mg, 0.10 mmol),
7 (66.7 mg, 0.10 mmoL) were dissolved in CHCl3/EtOH/H2O (14
mL, 12/1/1, v/v), CuSO4·5H2O (2.5 mg, 0.10 mmoL) and sodium
ascorbate (3.8 mg, 0.020 mmoL) were added. The mixture was
stired at RT for 24 h. After completion of the reaction, the
35 mixture was washed in water (3×20 mL). The organic layers were
dried over Na2SO4. After removal of the solvent under reduced
pressure, the mixture was purified by column chromatography
(silica gel, DCM/methanol = 100/1, v/v) to give black solid.
Yield: 93.9 mg (90 %). 1H NMR (400 MHz, CDCl3): δ = 7.92 (s,
40 1 H), 7.52−7.45 (m, 3H), 7.28 (s, 1H), 7.24 (s, 1H), 7.21−7.17 (m,
4H), 7.13 (d, 2H, J = 8.4 Hz), 7.00 (d, 2H, J = 8.0 Hz), 6.72 (d,
2H, J = 5.2 Hz), 6.64 (s, 1H), 5.97 (s, 2H), 5.27 (s, 2H),
4.85−4.83 (m, 2H), 4.47−4.44 (m, 2H), 3.04 (s, 6H), 2.65 (s, 3H),
2.55 (s, 6H), 1.42 (s, 9H), 1.40 ppm (s, 3H). HRMS (MALDI):
45 m/z calcd for [C52H51B2F4N8O2I]+: 1044.3302; found 1044.3279.
†
Electronic Supplementary Information (ESI) available:
molecular structure characterization data, UV/Vis absorption and
85 emission spectra, time-resolved transient absorption spectroscopy
and DFT calculations. See DOI: 10.1039/b000000x/
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Page 13 of 13
Journal of Materials Chemistry C
View Article Online
DOI: 10.1039/C4TC02037E
Graphical Abstract:
Switch the triplet excited state of the C60-dimethyl-aminostyryl
bodipy dyads/triads
Ling Huang and Jianzhang Zhao*
Switching of the triplet excited state of Bodipy-C60 dyads/triads with acid/base was studied
with nanosecond time-resolved transient difference absorption spectroscopy.
1