Bodipy-C60 triple hydrogen bonding assemblies as heavy atom-free triplet photosensitizers: preparation and study of the singlet/triplet energy transfer

Article abstract of DOI:10.1039/c4sc03865g

Supramolecular triplet photosensitizers based on hydrogen bonding-mediated molecular assemblies were prepared. Three thymine-containing visible light-harvesting Bodipy derivatives (B-1, B-2 and B-3, which show absorption at 505 nm, 630 nm and 593 nm, respectively) were used as H-bonding modules, and 1,6-diaminopyridine-appended C₆₀ was used as the complementary hydrogen bonding module (C-1), in which the C₆₀ part acts as a spin converter for triplet formation. Visible light-harvesting antennae with methylated thymine were prepared as references (B-1-Me, B-2-Me and B-3-Me), which are unable to form strong H-bonds with C-1. Triple H-bonds are formed between each Bodipy antenna (B-1, B-2 and B-3) and the C₆₀ module (C-1). The photophysical properties of the H-bonding assemblies and the reference non-hydrogen bond-forming mixtures were studied using steady state UV/vis absorption spectroscopy, fluorescence emission spectroscopy, electrochemical characterization, and nanosecond transient absorption spectroscopy. Singlet energy transfer from the Bodipy antenna to the C₆₀ module was confirmed by fluorescence quenching studies. The intersystem crossing of the latter produced the triplet excited state. The nanosecond transient absorption spectroscopy showed that the triplet state is either localized on the C₆₀ module (for assembly B-1·C-1), or on the styryl-Bodipy antenna (for assemblies B-2·C-1 and B-3·C-1). Intra-assembly forward-backward (ping-pong) singlet/triplet energy transfer was proposed. In contrast to the H-bonding assemblies, slow triplet energy transfer was observed for the non-hydrogen bonding mixtures. As a proof of concept, these supramolecular assemblies were used as triplet photosensitizers for triplet-triplet annihilation upconversion.

Full text of DOI:10.1039/c4sc03865g

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Bodipy–C triple hydrogen bonding assemblies as
60
heavy atom-free triplet photosensitizers:
preparation and study of the singlet/triplet energy
transfer†
Cite this: DOI: 10.1039/c4sc03865g
a
b
a
a
c
c
Song Guo, Liang Xu, Kejing Xu, Jianzhang Zhao,* Betul Kuçukoz, Ahmet Karatay,
¨ ¨ ¨
¨
c
c
d
Halime Gul Yaglioglu, Mustafa Hayvali and Ayhan Elmali
Supramolecular triplet photosensitizers based on hydrogen bonding-mediated molecular assemblies were
prepared. Three thymine-containing visible light-harvesting Bodipy derivatives (B-1, B-2 and B-3, which
show absorption at 505 nm, 630 nm and 593 nm, respectively) were used as H-bonding modules, and
1,6-diaminopyridine-appended C60 was used as the complementary hydrogen bonding module (C-1), in
which the C60 part acts as a spin converter for triplet formation. Visible light-harvesting antennae with
methylated thymine were prepared as references (B-1-Me, B-2-Me and B-3-Me), which are unable to
form strong H-bonds with C-1. Triple H-bonds are formed between each Bodipy antenna (B-1, B-2 and
B-3) and the C60 module (C-1). The photophysical properties of the H-bonding assemblies and the
reference non-hydrogen bond-forming mixtures were studied using steady state UV/vis absorption
spectroscopy, fluorescence emission spectroscopy, electrochemical characterization, and nanosecond
transient absorption spectroscopy. Singlet energy transfer from the Bodipy antenna to the C60 module
was confirmed by fluorescence quenching studies. The intersystem crossing of the latter produced the
triplet excited state. The nanosecond transient absorption spectroscopy showed that the triplet state is
either localized on the C60 module (for assembly B-1$C-1), or on the styryl-Bodipy antenna (for
assemblies B-2$C-1 and B-3$C-1). Intra-assembly forward–backward (ping-pong) singlet/triplet energy
transfer was proposed. In contrast to the H-bonding assemblies, slow triplet energy transfer was
observed for the non-hydrogen bonding mixtures. As a proof of concept, these supramolecular
assemblies were used as triplet photosensitizers for triplet–triplet annihilation upconversion.
Received 13th December 2014
Accepted 4th April 2015
DOI: 10.1039/c4sc03865g
www.rsc.org/chemicalscience
developed as a new family of triplet PSs, due to their easily
derivatized molecular structures, good photostability and
Triplet photosensitizers (PSs) have attracted much attention strong absorption of visible light.
1
. Introduction
7,16
1
–5
due to their applications in photocatalysis, photodynamic
There are a few challenges to be addressed in the molecular
6–8
therapy,
upconversion.
halogenated xanthanes such as Rose Bengal and methylene (ISC).
blue, and transition metal complexes such as Pt(II), Ru(II) or introduce heavy atoms into the molecular structure, and the
and more recently triplet–triplet annihilation design of new triplet PSs. Firstly, most of the known triplet PSs
9–12
Conventional triplet PSs include porphyrins, contain heavy atoms to facilitate efficient intersystem crossing
7,14,16
However, for some chromophores, it is difficult to
13–15
Ir(III) complexes, etc.
Recently, halogenated Bodipys were heavy atom effect may become inefficient for chromophores
13b,14
with short S
1
state lifetimes, or for bulky chromophores.
On
the other hand, heavy atom-free organic triplet photosensitizers
a
State Key Laboratory of Fine Chemicals, Dalian University of Technology, E-208, West are difficult to ‘design’ because the ISC capability of such
Campus, Dalian 116024, P. R. China. E-mail: zhaojzh@dlut.edu.cn; Web: http://
14
chromophores is almost unpredictable. Secondly, the known

nechem2.dlut.edu.cn/photochem; Fax: +86 411 8498 6236
triplet PSs are based on an integrated molecular structure motif,
i.e. the visible light-harvesting and the ISC are implemented by
the same chromophore, or by covalently bonded chromophores
in dyad/triad triplet PSs, for example the recently developed
b
School of Chemistry, Dalian University of Technology, Dalian 116024, P. R. China
Department of Engineering Physics, Faculty of Engineering, Ankara University, 06100
c
Be ¸s evler, Ankara, Turkey
d
Department of Chemistry, Faculty of Science, Ankara University, 06100 Be ¸s evler,
7,14
Ankara, Turkey
broadband visible light-absorbing organic triplet PSs.
This
†
Electronic supplementary information (ESI) available: Syntheses, structure approach is synthetically demanding if a library of chromo-
characterization data, and UV/vis absorption and emission spectra. See DOI:
5,17–19
phores is to be screened for the preparation of triplet PSs.
1
0.1039/c4sc03865g
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Therefore, new methods are desired for the feasible preparation Bodipy unit shows absorption at 627 nm and 592 nm, respec-
43
of organic triplet PSs.
tively (Scheme 2). H-bonded assemblies with different absorp-
Concerning this aspect, supramolecular assembly is partic- tion wavelengths can be easily prepared with B-1, B-2 or B-3 as the
ularly interesting due to the ability to fabricate molecular visible light-harvesting H-bonding module, and with C-1 as the
20,21
assemblies with different modules.
For example, the singlet energy acceptor and spin converter (Scheme 3). In order to
study the effect of H-bonding on the photophysical processes,
But the components showed singlet energy donors B-1-Me, B-2-Me and B-3-Me (Schemes 2
supramolecular photochemistry of the hydrogen bonded C60
ferrocene dyad was studied.
–
22a
weak absorption of visible light, thus the assembly was and 3), which are unable to form H-bonds with C-1, were
unsuitable for use as a triplet PS. Recently, a rotaxane was prepared. These reference compounds contain a methyl group at
studied with uorescence-resonance-energy-transfer (FRET), the N-position of the thymine, and as a result the H-bonding with
2
7–29
but the uorophore present in the rotaxane was unable to C-1 is substantially inhibited.
produce any triplet excited states upon photoexcitation.
All the compounds were
prepared using routine synthetic methods and obtained in
In order to address the above challenges in the area of triplet moderate to satisfactory yields. The molecular structures were
22b
1
13
PSs, herein we propose using hydrogen bonded molecular fully characterized with H NMR, C NMR and HRMS.
assembly as a new approach for the design of organic triplet
23–25
PSs.
This method is also useful for designing heavy atom-
2
.2 UV/vis absorption and uorescence spectra
free triplet PSs. One of the hydrogen bonding modules is used
as a visible light-harvesting antenna and singlet energy donor, The UV/vis absorption spectra of the compounds were studied
whereas the complementary H-bonding module is used as a (Fig. 1 and Table 1). C-1 shows the characteristic absorption of
singlet energy acceptor and a spin converter for triplet forma- the C60 unit, which is centered at 331 nm. For the singlet energy
14
tion at the same time. The distance between the energy donor donors, i.e. the Bodipy derivatives B-1, B-2 and B-3, the
and acceptor can be controlled, and singlet energy transfer is absorption bands are located at 505 nm, 630 nm and 593 nm,
30,34
assured. With this method, a wide variety of singlet energy respectively. The S state energy level of C is 1.72 eV,
which
1
60
donors can be feasibly screened, because the H-bonded is lower than those of the visible light-harvesting energy donors
molecular assembly is easy to obtain simply by mixing the (B-1, B-2 and B-3). The methylated visible light-harvesting
H-bond donor and the H-bond acceptor in appropriate solvents, compounds B-1-Me, B-2-Me and B-3-Me show similar absorp-
and the triplet formation upon photoexcitation can be feasibly tion spectra to their analogues without methyl substitution on
evaluated by various spectroscopic methods. We conrmed this the N atom of thymine.
new molecular design concept with the preparation of
The UV/vis absorption in toluene of C-1/B-1, in which
H-bonded Bodipy–C60 assemblies. Efficient and fast intra- H-bonds have presumably been formed, is the sum of the UV/vis
assembly singlet and triplet energy transfers were observed, absorptions of B-1 and C-1 (Fig. 2a). This result indicated that
based on steady state and nanosecond transient absorption H-bonding does not induce any interaction between the Bodipy
34
spectroscopy. In contrast, much slower intermolecular triplet and C60 moieties in the ground state. Similar results were
state energy transfer was observed for the reference modules observed for the mixture of B-1-Me and C-1, in which no
27
with which no H-bonds could be formed. The application of the H-bonding is expected (see ESI, Fig. S45†). This conclusion
1
H-bond assemblies in singlet oxygen ( O ) photosensitization was supported by the titration of C-1 with higher B-1 concen-
2
and triplet–triplet annihilation (TTA) upconversion was studied. trations (2 eq. vs. C-1, Fig. 2b).
This molecular design method will be useful for the study of
organic triplet photosensitizers.
In order to verify the H-bonding between the Bodipy visible
light-harvesting unit and C-1, uorescent titration of B-1 with
C-1 was carried out (Fig. 3). The rationale is that the uores-
cence of B-1 will be quenched due to energy transfer (such as
2. Results and discussions
through space energy transfer) to C60 if H-bonds are formed
2.1 Molecular design rationale
24,27,29,42b
with C-1.
B-1 alone shows intense uorescence at
¼ 75.7%). Upon addition of C-1 into
An N-acetyl-1,6-diaminopyridine-thymine based triple hydrogen 517 nm (quantum yield F
F
bonding motif was used (Schemes 1–3). Thymine was connected a solution of B-1, however, the uorescence intensity of B-1 was
to different Bodipy derivatives in order to act as the visible light- substantially quenched (Fig. 3a), and saturation was established
harvesting H-bonding module, i.e. the singlet energy donor. The aer addition of 12 eq. C-1. The uorescence intensity was
N-acetyl-1,6-diaminopyridine unit was connected to C60 to form reduced by 80%. The formation of hydrogen bonds between C-1
C-1 (Scheme 1), and acted as the singlet energy acceptor and and B-1 is an equilibrium process. In order for most of the B-1
2
2a,26–29
spin converter for triplet formation.
Note that the visible molecules to be H-bonded, i.e. in order to fully quench the
30
light-absorbing ability of C60 is very weak, but it is a good uorescence of B-1, excess C-1 has to be used. Interestingly, for
singlet energy acceptor because of the low S
1
state energy level B-1-Me, which is unable to form strong H-bonds with C-1 due to
31–37
(1.76 eV).
the methyl group on the N atom of the thymine moiety, the
Bodipy was selected as the visible light-harvesting antenna, uorescence was quenched to a much lesser extent, by 40% on
due to its satisfactory photophysical properties and easily deriv- addition of the same amount of C-1 (Fig. 3b). Therefore, we
3
8–42
atized molecular structures.
The Bodipy unit in B-1 shows propose that strong H-bonds formed for C-1/B-1, but not for
27
absorption at ca. 505 nm, whereas in B-2 and B-3, the styryl- C-1/B-1-Me.
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ꢂ
ꢂ
Scheme 1 Preparation of the hydrogen bonding C60 module C-1. (a) 2-Chloro-acetyl chloride, dry CH
2
Cl
2
, Ar, 25 C, 2 h; (b) NaN
3
, DMF, 70 C,
ꢂ
5
h; (c) 3-bromo-1-propyne, potassium carbonate, ethanol, 79 C, 6 h; (d) NaBH
4
, THF–CH
3
OH, rt, 5 min; (e) methyl malonyl chloride, CH
2 2
Cl ,
ꢂ
ꢂ
Ar, 25 C, 12 h; (f) iodine, C60, DBU, toluene, 10 h, Ar, 25 C; (g) sodium ascorbate, CuSO
4
, anhydrous CHCl
3
–EtOH–water (12 : 1 : 1, v/v), rt, 12 h.
Similar uorescence quenching proles were observed for B- 2.3 Electrochemical studies and the free energy changes of
and B-3 (see ESI, Fig. S51 and S53†). The uorescence emis- the intramolecular electron transfers
2
sion wavelength of B-3 is much longer than that of B-1. It should
be pointed out that the S state energy level of B-3 (approxi-
mated as 2.04 eV) is still higher than that of C-1 (1.76 eV), thus
intra-assembly singlet energy transfer is assured. For B-3-Me, no
H-bonds can be formed, thus the quenching occurs to a much
lesser extent (see ESI, Fig. S53b†).
The uorescence lifetime of B-1 (or B-2, or B-3) was measured
upon titration with C-1 (see ESI, Fig. S68†). The uorescence
lifetime is nearly constant (3.5–3.4 ns), despite the quenching of
the uorescence intensity with increasing C-1 concentration.
Thus, we conclude that the uorescence quenching is due to a
mechanism similar to static quenching, not dynamic quench-
In order to study the intra-assembly photoinduced electron
transfer, the electrochemical properties of the Bodipy and C60
modules were studied using cyclic voltammetry (Fig. 4).
For B-1, a reversible reduction wave was observed at ꢀ1.32 V. A
pseudo-reversible oxidation wave was observed at +1.11 V.
Similar redox potentials were observed for B-1-Me. For B-2, a
pseudo-reversible oxidation wave was observed at +0.84 V,
which was shied anodically as compared with B-1. For the
carbazole-containing B-3, two reversible oxidation waves at
1
34,48,49
+0.74 and +1.03 V were observed, which were different from B-1
and B-2. The two reversible reduction waves of B-3 and B-3-Me
are due to the carbazole-styryl-Bodipy moiety, which was
conrmed by studying a reference compound (see ESI,
Fig. S67†). For the C60 module, i.e. C-1, two reversible reduction
waves at ꢀ0.7 V and ꢀ1.08 V were observed (Fig. 5), as well as a
pseudo-reversible reduction wave at ꢀ1.53 V. These reduction
waves are attributed to the C60 moiety.
4
4,45
ing,
uorescence of the H-bonded assembly is completely
quenched, while the un-bonded Bodipy uorophore shows
upon formation of the hydrogen bonds. That is, the

44
residual uorescence.
In other words, the H-bonding
assembly becomes non-uorescent, and the residual uores-
cence observed during the titration is due to the free Bodipy
modules in the solution. In order to verify that the uorescence
quenching during the titration is due to H-bond mediated
assembly, hexauoroisopropanol was added in order to disrupt
the H-bonds. As a result, uorescence recovery was observed
The plausible photoinduced intra-assembly electron transfer
between the Bodipy and the C60 module can be evaluated with
the Gibbs free energy changes, which are calculated with the
34,48,49
Rehm–Weller equation,
0
CS
DG ¼ e½EOX ꢀ EREDꢁ ꢀ E00 þ DG
S
(1)
(2)
(see ESI, Fig. S54†), which indicated that the uorescence
quenching was due to the formation of a molecular assembly
via H-bonding.
ꢀ
ꢁꢀ
ꢁ
2
2
e
e
1
1
1
1
DG
S
¼ ꢀ 4p3
ꢀ
þ R
ꢀ 3
S
3
0
R
CC
8p3
0
R
D
A
3
REF S
These studies, using the visible light-harvesting chromo-
phores B-1, B-2 and B-3 to form molecular assemblies with C-1,
demonstrate the advantage of using H-bonding to induce intra-
assembly energy transfer, that is, different combinations can be
easily accessed and screened for efficient intramolecular energy
transfer and formation of triplet states.
where DG
eqn (2). e is the electronic charge, EOX is the half-wave potential
for one-electron oxidation of the electron-donor unit, ERED
S
is the static Coulombic energy, which is described by
¼
half-wave potential for one-electron reduction of the electron
acceptor unit, E00 is the approximate energy level obtained from
The stability constants of the visible light-harvesting Bodipys
and C-1 were calculated based on the uorescence titration
the uorescence emission (for the singlet excited state), 3 ¼
S
static dielectric constant of the solvent, and RCC ¼ center-to-
center separation distance between the electron donor (Bodipy
module) and the electron acceptor (C-1 module). As determined
by DFT optimization of the geometries, RCC (B-1$C-1) ¼ 40.0 A˚ ,
46,47
experiments (Table 2).
The stability constants for the H-bond
forming combinations show that the bonding is strong. For the
methylated Bodipy derivatives, however, the stability constants
are 10-fold smaller.
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Scheme 2 Preparation of the hydrogen bonding Bodipy modules B-1, B-2, and B-3, and the reference compounds which are unable to form
1
strong hydrogen bonds with module C-1, i.e. B-1-Me, B-2-Me and B-3-Me. The singlet oxygen ( O
used in TTA upconversion, 1CPBEA, were also presented. (a) Anhydrous K CO , 1,2-dibromoethane, dry ethanol, Ar, 70 C; (b) NaN
0 h; (c) 2,4-dimethylpyrrole, TFA, anhydrous CH Cl , rt, 12 h; DDQ, BF $Et
2
) scavenger DBPF and the triplet acceptor
ꢂ
ꢂ
2
3
3
, DMF, 100 C,
1
2
2
3
2
O, 12 h; (d) benzaldehyde, acetic acid, piperidine, microwave
ꢂ
irradiation, 8 min; (e) sodium hydride, n-butyl bromide, DMF, rt, 2 h; (f) DMF, POCl
baldehyde, acetic acid, piperidine, microwave irradiation, 5 min; (h) propargyl bromide, K
j) 18, sodium ascorbate, CuSO , rt, Ar, 12 h; (k) 18, sodium ascorbate, CuSO , rt, Ar, 12 h; (l) 18, sodium ascorbate, CuSO
, rt, Ar, 12 h; (o) 19, sodium ascorbate, CuSO , rt, Ar, 24 h.
3
, rt, 3 h, then 60 C, 6 h; (g) 9-butyl-9H-carbazole-3-car-
2
CO
3
, DMF, rt, Ar, 10 h; (i) NaH, methyl iodide, rt, Ar, 12 h;
, rt, Ar, 20 h; (m) 19,
(
4
4
4
sodium ascorbate, CuSO
4
, rt, Ar, 12 h; (n) 19, sodium ascorbate, CuSO
4
4
˚
˚
R
CC (B-2$C-1) ¼ 43.9 A and RCC (B-1$C-1) ¼ 49.5 A. R
D 0
is the for the electrochemical studies, and 3 is the permittivity of free
radius of the electron donor, R
A
is the radius of the electron space. The solvents used in the calculation of the free energy of
acceptor, 3REF is the static dielectric constant of the solvent used the electron transfer are toluene (3 ¼ 2.4) and CH Cl (3 ¼
S
2
2
S
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Scheme 3 Structures of the hydrogen-bonding assemblies (C-1/B-1, B-2 and B-3, upper row), and the modules which are unable to form strong
hydrogen bonds (C-1/B-1-Me, B-2-Me and B-3-Me, lower row).
Fig. 2 UV/vis absorption spectra of (a) B-1, C-1 and C-1/B-1 (1/1); (b)
Fig. 1 UV/vis absorption spectra of C-1, B-1, B-1-Me, B-2, B-2-Me, B-
and B-3-Me (note that the absorptions of the methylated Bodipy
ꢀ5
C-1 (1.0 ꢃ 10 M) with increasing concentrations of B-1. c ¼ 1.0 ꢃ
3
ꢀ5
ꢂ
M in toluene, 20 C.
1
0
modules are identical to those of the non-methylated Bodipy
modules). c ¼ 1.0 ꢃ 10ꢀ M in toluene, 20 C.
5
ꢂ
assembly photoinduced electron transfer is thermodynamically
prohibited. Therefore, the uorescence quenching effect
observed during the H-bonding titration (Fig. 3) is due to intra-
assembly energy transfer, not electron transfer.
Table 1 Photophysical parameters of the Bodipy and C60 hydrogen
bonding modules
a
b
f
l
abs
3
l
em
F
F
(%)
F
s (ns)
c
B-1
B-1-Me
B-2
B-2-Me
B-3
B-3-Me
C-1
505
505
630
630
593
593
331
10.2
10.2
12.4
11.5
10.6
10.4
5.1
517
517
642
642
611
611
75.7c
64.0
73.5
3.5
3.6
5.0
4.9
3.7
3.9
d
2.4 Nanosecond transient absorption spectroscopy: intra-
d
65.4e
assembly and intermolecular triplet state energy transfer
70.3
e
The formation of H-bonds between the Bodipy visible light-
harvesting chromophores and the fullerene module C-1 will
61.2
g
g
g
—
—
—
induce singlet energy transfer from the Bodipy part to the C
a
ꢀ5
4
b
60
In toluene (1.0 ꢃ 10
M). Molar extinction coefficient at the
c
34,49
ꢀ1
ꢀ1
part.
As a result, a triplet excited state will be produced via
absorption maximum (10
M
cm ). With 2,6-diiodo-4,4-diuoro-
as
35–37
1
,3,5,7-tetramethyl-8-phenyl-4-bora-3a,4a-diaza-s-indacene
the the inherent ISC of C60
.
Thus, nanosecond transient
d
standard (F ¼ 71.2% in acetonitrile). With carbazole-styryl-Bodipy as
absorption spectroscopy was used to conrm the production of
the triplet state via H-bonding mediated intra-assembly energy
e
the standard (F ¼ 72.0% in toluene). With carbazole-styryl-Bodipy as
f
the standard (F ¼ 63.0% in acetonitrile). Fluorescence lifetimes.
g
22a
Not determined.
transfer. Indeed, we observed the triplet state of C-1 upon
photoexcitation (no signals could be detected in aerated solu-
tion, see ESI, Fig. S65†).
First, C-1 was titrated with B-1 (Fig. 6). The mixed solution
was photoexcited at 495 nm with a nanosecond pulsed laser.
The absorption of C-1 at 495 nm is weak. Thus, selective
8
.93). The electrochemical data and the free energy changes of
the photoinduced electron transfers in the H-bonding assem-
blies are summarized in Table 3. The results show that intra-
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ꢀ5
Fig. 3 Quenching of the fluorescence of (a) B-1 and (b) B-1-Me by C-1. c ¼ 1.0 ꢃ 10 M in toluene (lex ¼ 475 nm). (c) Emission intensity of B-1
ꢀ5
ꢂ
and B-1-Me at 517 nm (c ¼ 1.0 ꢃ 10 M in toluene) versus concentration of C-1, following excitation at 475 nm at 20 C.
excitation of B-1 in the mixture is possible. On increasing the
Previously, we demonstrated that for a Bodipy–C60 covalent
concentration of B-1, the excited state absorption (ESA) of the dyad, intramolecular ping-pong energy transfer is possible, i.e.
C60 part at 715 nm was enhanced (indicated by the DO.D. forward singlet energy transfer to the C60 unit, and backward
values, which are proportional to the population of the transient triplet energy transfer to the styryl-Bodipy visible light-harvest-
36,37
species, Fig. 6f), indicating enhanced singlet energy transfer ing chromophore.
and nally the production of the triplet state with increasing process in the H-bonding mediated supramolecular assemblies.
concentration of B-1 (Fig. 6d). The presence of the triplet state To this end, B-1 is not a good candidate because the T state
Herein, we studied this photophysical
1
was conrmed by measurement of the transient signal in energy level of C60 is lower than that of the Bodipy unit in B-1,
aerated solution (no signal was observed, see ESI, Fig. S65 and therefore no backward triplet state energy transfer is expected,
S66†). It should be pointed out that the triplet excited state is because the upward triplet energy transfer from C60 to the
localized on the C60 moiety, not on the Bodipy moiety, because Bodipy unit in B-1 is thermodynamically prohibited. Instead, we
the ground state bleaching band at 505 nm (at which the Bodipy selected B-3 for this study, because we have shown that the T
unit shows steady state absorption) was not observed. This is in state of the styryl-Bodipy–C60 dyad/triad is localized on the
1
35
agreement with our previous studies of C60–Bodipy dyads. The styryl-Bodipy moiety, not on the C60 unit. That is, the styryl-
triplet state lifetimes are in the range of 40.9–44.2 ms, which Bodipy antenna has a lower T state energy level than C60
For B-3, the excitation wavelength is 590 nm, at which the C60
1
.
30
supports the localization of the triplet state on the C60 moiety.
In comparison, excitation of B-1 alone didn’t produce any triplet unit shows weak absorption. On addition of B-3 into a solution
state (see ESI, Fig. S62a†); thus, the cascade singlet energy of C-1, a bleaching band at 590 nm was observed, which could
transfer and ISC only occur in the B-1$C-1 assembly.
The triplet state quantum yields of C-1 and the B-1$C-1 zole-styryl-Bodipy in B-3 (Fig. 7b–d). ESA of the T
hydrogen bonding assembly (1 : 1) were determined as 86.1% transitions) of the styryl-Bodipy antenna was observed in the
be assigned to the depletion of the ground state of the carba-
1
state (T / T
1
n
35
and 67.5%, respectively, using the triplet energy transfer range of 600–750 nm. Thus, we propose that ping-pong energy
method, b-carotene as triplet energy acceptor, and Ru(bpy)
as a reference (see Experimental section for details).
3
Cl
2
transfer occurs in B-3$C-1. First, B-3 is photoexcited with a 590
nm laser, and then intra-assembly singlet energy transfer from
16b,c
The triplet state of C-1 upon titration with B-1-Me was also the carbazole-styryl-Bodipy to the H-bonded C60 moiety occurs.
studied (see ESI, Fig. S56†). In this situation, no strong H-bonding The ISC of C60 produces a triplet excited state, followed by the
27
is expected. Intermolecular singlet energy transfer is unlikely for backward triplet energy transfer from C60 to the carbazole-
two compounds mechanically mixed in solution (the distance styryl-Bodipy module; as a result, the ground state bleaching
between the energy donor and the energy acceptor can be treated band and the ESA of the styryl-Bodipy part in B-3 were observed.
as innite, well exceeding the distance required for the F ¨o rster
energy transfer). We did not observe any increase in the transient B-3 increased, indicating enhanced visible light-harvesting,
absorption of the T state of C60 on increasing the concentration of ping-pong singlet–triplet energy transfer in the assembly, and
B-1-Me (see ESI, Fig. S56†). Thus, the intra-assembly singlet energy nally the production of the triplet excited state of the antenna
transfer was conrmed by studying this reference compound. (Fig. 7f). When 0.25 eq. to 1 eq. B-3 was added, the triplet state
With increasing B-3 concentration, the DO.D. magnitude of
1
Table 2 The stability constants for the interactions of the Bodipy H-bonding modules with the C-1 H-bonding module
B-1
B-1-Me
B-2
B-2-Me
B-3
B-3-Me
a
ꢀ1
ꢀ1
4
3
4
6
3
4
6
3
K
K
SV /M
4.08 ꢃ 10
3.02 ꢃ 10
3.67 ꢃ 10
1.38 ꢃ 10
1.22 ꢃ 10
5.14 ꢃ 10
1.15 ꢃ 10
5.43 ꢃ 10
b
5
c
c
c
b
/M
3.6 ꢃ 10
—
—
—
a
b
c
K
SV is the linear Stern–Volmer quenching constant.
K
b
represents the intrinsic binding constant of the compound with C-1. Not applicable.
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+
Fig. 4 Cyclic voltammograms of the Bodipy H-bonding modules. Ferrocene (Fc) was used as an internal reference (E1/2 ¼ +0.38 V (Fc /Fc) vs.
SCE). (a) B-1, (b) B-1-Me, (c) B-2, (d) B-2-Me, (e) B-3, and (f) B-3-Me. Experiments were carried out in deaerated DCM solutions containing 0.5
4 6 3
mM photosensitizer, alone or with ferrocene, 0.10 M Bu NPF as the supporting electrolyte, and a Ag/AgNO reference electrode; scan rate: 0.05
ꢀ1
V s
.
Table 3 Redox potentials of the H-bond modules and the free energy
changes (DGCS) for the intramolecular electron transfers (with the
Bodipy unit as electron donor and the C60 unit as electron acceptor).
Anodic and cathodic peak potentials are presented. The potential
3
values of the compounds are vs. Ag/AgNO reference electrode, with
+
a
Fc as internal reference (E1/2(Fc /Fc) ¼ +0.38 V)
E
OX (V) RED (V)
E
DGCS (eV)
b
c
c
c
c
c
c
B-1
B-1-Me
B-2
B-2-Me
B-3
B-3-Me
C-1
+1.11
+1.12
+0.84
+0.84
ꢀ1.32
+0.21, 1.17
b
ꢀ1.31
+0.25, 1.33
b
ꢀ1.04
+0.28, 0.92
Fig. 5 Cyclic voltammogram of C-1. Ferrocene (Fc) was used as an
b
ꢀ1.04
+0.32, 1.06
+
internal reference (E1/2 ¼ +0.38 V (Fc /Fc) vs. SCE). Experiments were
b
+0.74/+1.03
+0.74/+1.03
ꢀ1.21
+0.36, 0.97
carried out in deaerated DCM solutions containing 0.5 mM C-1 alone,
b
ꢀ1.22
+0.40, 1.14
0
.10 M Bu
4
NPF
6
as the supporting electrolyte, and a Ag/AgNO
3
d
e
—
ꢀ0.70/ꢀ1.08/ꢀ1.53
—
ꢀ1
reference electrode; scan rate: 0.05 V s
.
a
Cyclic voltammetry carried out in Ar saturated acetonitrile containing
.10 M Bu NPF supporting electrolyte; counter electrode was a Pt
electrode; working electrode was a glassy carbon electrode; the Ag/
0
4
6
+
AgNO
3
couple was used as the reference electrode. c[Ag ] ¼ 0.1 M.
lifetime decreased from 263.2 ms to 138.9 ms. The self-quench-
Conditions: 0.5 mM dyad photosensitizers and 0.5 mM ferrocene in
5
ꢀ1 ꢀ1
b
ing constant is 4.6 ꢃ 10 M
s
. This quenching effect is due CH Cl , 293 K. With reference C-1 as electron acceptor, in CH Cl .
2
2
2
2
c
d
With reference C-1 as electron acceptor, in toluene. No reduction
to the TTA process, which is signicant for transient species
with long triplet state lifetimes. Similar fast intra-assembly
triplet–triplet electron transfer (TTET) was observed for B-2$C-1
e
potentials were observed. No DGcs values were calculated.
(see ESI, Fig. S58†).
No strong H-bonds should be formed in B-3-Me/C-1. The
Bodipy moiety in B-3-Me is attributed to intermolecular triplet
energy transfer, with C60 acting as the triplet energy donor.
nanosecond transient absorption spectroscopy of B-3-Me in the
presence of C-1 was also studied (Fig. 8). Drastically different
results were observed, as compared with B-3/C-1. The bleaching
band of the carbazole-styryl-Bodipy moiety was observed at 593
nm (Fig. 8b). Since C60 was excited at 425 nm, and excitation of the
solution of B-3-Me alone did not produce any triplet excited state
50
The evidence for this intermolecular energy transfer, rather
than a fast intra-assembly energy transfer, is provided by the
biphasic features of the decay traces of the transient absorption
monitored at 630 nm (Fig. 8e and f), in which the rst phase of
the increase in intensity of the bleaching band at 593 nm is due
to the intermolecular triplet energy transfer from the C60 moiety
to the styryl-Bodipy moiety in B-3-Me, which takes ca. 15 ms. This
(
1
see ESI, Fig. S64b†), the population of the T state of the styryl-
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Fig. 6 Nanosecond transient absorption spectra of C-1 on addition of increasing amounts of compound B-1. The transient signal intensified with
increasing B-1 concentration: (a) 0 eq. B-1; (b) 0.25 eq. B-1; (c) 0.5 eq. B-1; and (d) 1 eq. B-1. (e) Decay trace at 720 nm; (f) comparison of the
ꢀ5
transient absorption spectra at different B-1 concentrations with delay time of 0 ms. c ¼ 1.0 ꢃ 10 M; spectra were recorded in toluene after
ꢂ
pulsed excitation at 495 nm under N
2
at 20 C.
process is the relatively slow intermolecular TTET as compared absorption) is less than that observed with 425 nm excitation,
to the fast intra-assembly TTET (Fig. 7e), for which the TTET and no slow increase in the intensity of the bleaching band was
cannot be resolved with the transient absorption spectrometer observed upon 590 nm photoexcitation. This result conrmed
(
resolution of the spectrometer is 10 ns). The later recovery that there is no signicant formation of a H-bond mediated
phase is due to decay of the T state of the styryl-Bodipy moiety. molecular assembly.
The lack of formation of an H-bonded B-3-Me–C-1 molecular For B-2-Me, a slow evolution of the transient bleaching was
1
assembly was conrmed by the DO.D. values of the transient also observed (see ESI, Fig. S61†). The triplet state energy
absorption upon 425 nm and 590 nm photoexcitation (Fig. 8d). transfer takes about 16 ms. This slow process is in stark contrast
The DO.D. value of the transient absorption upon excitation at to that observed for the H-bonding assemblies of B-2$C-1 and B-
590 nm (where the styryl-Bodipy antenna gives strong 3$C-1, for which no slowly increasing phases in the transient
Fig. 7 Nanosecond transient absorption spectra of the C-1 H-bonding module upon titration with the complementary B-3 H-bonding module.
Transient absorption of C-1 in the presence of (a) 0 eq. B-3, (b) 0.25 eq. B-3, (c) 0.5 eq. B-3, and (d) 1 eq. B-3. (e) Decay trace at 593 nm; (f)
ꢀ5
transient spectra extracted from (a)–(d) at delay times of 0 ms. c ¼ 1.0 ꢃ 10 M; spectra were recorded in toluene after pulsed excitation at 590
ꢂ
nm under N
2
at 20 C.
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Fig. 8 Nanosecond transient absorption spectra of H-bonding module C-1 upon titration with the non-preferred H-bonding module B-3-Me
no strong H-bonds can be formed with C-1). Transient absorption spectra of C-1 in the presence of (a) 0 eq. B-3-Me, (b) 0.5 eq. B-3-Me and (c) 1
(
eq. B-3-Me. (d) Decay traces at 630 nm on addition of B-3-Me with pulsed excitation at 425 nm (selective excitation of the C60 moiety) and 590
nm (selective excitation of the carbazole-styryl-Bodipy moiety). (e) Decay traces at 630 nm on addition of 0.5 eq. and 1 eq. B-3-Me. c ¼ 1.0 ꢃ
ꢀ5
ꢂ
2
M for C-1 in toluene after pulsed excitation at 425 nm under N at 20 C (note that for the sake of signal intensity, the excitation is at 425 nm
1
0
instead of 590 nm, which was used for Fig. 7). (f) Magnification of the decay traces in (e).
signals were observed. Thus, the intra-assembly (in other words 2.5 Molecular dynamics (MD) simulations: explanation of
the intramolecular) triplet energy transfer occurs with rate the fast intramolecular triplet energy transfer
8
ꢀ1
constant kTTET > 10 s (the time-resolution of the spectrometer
is 10 ns). This is an interesting result, since for the C-1$B-2 and
C-1$B-3 assemblies, the distance between the C60 and the styryl-
Bodipy part is ca. 5.0 nm, and the linker contains a saturated
The possible conformations of the complexes containing a
hydrogen acceptor and donor were explored by molecular
dynamics (MD) simulations performed in aqueous solution (see
53
ESI†). It was found that a relatively stable complex can be
33,51
bond, yet the triplet energy transfer is fast.
We propose
formed in less than 10 ns, indicating that recognition of the
hydrogen donor and acceptor in aqueous solution is quite
feasible. Note that the three pairs of hydrogen bonds in the
initial conformation are largely preserved in the compact
conformation, although B-3 prefers to assume an extended
conformation in solution. Weak p–p interactions between part
of the B-3 structure and the fullerene motifs of the C-1 mole-
cules may further contribute to the stability of this complex.
These results indicate that fast intra-assembly TTET is possible,
which can be used to rationalize the nanosecond transient
absorption spectra of the B-3$C-1 H-bond assembly (Fig. 9).
movement of the Bodipy module and the C60 module; thus
intramolecular collision causes the fast intra-assembly triplet
state energy transfer to occur.
Previously, intra-assembly triplet energy transfer from C60 to
ferrocene was observed in a hydrogen bonded C60 ferrocene
5
ꢀ1 22a
dyad (k ¼ 9.2 ꢃ 10 s ). A steroid-linked norbornadiene–
carbazole dyad showed intramolecular triplet energy transfer
5
ꢀ1
with a rate constant of 3.3 ꢃ 10 s (from the carbazole to the
52
norbornadiene moiety). For the hydrogen bonded assemblies
B-2$C-1 and B-3$C-1, the intramolecular triplet energy transfer
from the C60 moiety to the styryl-Bodipy moiety is much faster (k
8
ꢀ1
>
10 s ). Triplet energy transfer is very oen believed to occur
via the Dexter electron exchange mechanism, which requires 2.6 Hydrogen bonded Bodipy–C assemblies as singlet
6
0
1
close contact between the energy donor and the energy oxygen ( O ) photosensitizers
2
acceptor. The triplet energy transfer will be negligible if the
The production of triplet excited states by the hydrogen bonded
60–Bodipy assemblies was also studied using singlet oxygen
distance between the energy donor and acceptor exceeds the
sum of their van der Waals radii. We propose that the exible
chains in the B-2$C-1 and B-3$C-1 assemblies allow direct
contact of the styryl-Bodipy and the C60 moiety through fol-
damers; thus the triplet energy transfer is greatly accelerated.
Previously, hydrogen bonding porphyrin–zinc assemblies were
studied, and 50–80 times faster singlet–singlet energy transfer
was observed for the assemblies with exible linkers, as
C
1
(
(
2
O ) photosensitization (Fig. 10). 1,3-Diphenylbenzofuran
DPBF) was used as O scavenger. The production of O could
2 2
1
1
be monitored by following the absorbance changes of DPBF at
14 nm.
For all the hydrogen bond assemblies, i.e. B-1$C-1, B-2$C-1
4
1
and B-3$C-1, the O photosensitizing ability is higher than the
2
combinations for which strong hydrogen bonds are unable to
53
compared to those with rigid linkers, such as steroids.
form, i.e. B-1-Me$C-1, B-2-Me$C-2, and B-3-Me$C-1. These pairs
1
show similar O
2
photosensitizing ability to C-1 alone. These
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Fig. 9 Right: conformation of the B-3$C-1 complex in aqueous
solution obtained from an MD simulation at 6.9 ns, showing the
hydrogen bonds (<3 A˚ as indicated using black dotted lines) between
B-3 and the C-1 motif. Left: the starting extended conformation used
in the MD simulations. The white surfaces represent the solvent (water
molecules) accessible surface areas of B-3 and the fullerene motif of
the C-1 structure.
Fig. 11 TTA upconversion of C-1 with B-3 and B-3-Me H-bonding
modules. 1-CBPEA was used as triplet acceptor. (a) Emission of B-3
and 1-CBPEA with increasing C-1 concentration; (b) emission of B-3-
Me and 1-CBPEA with increasing C-1 concentration; (c) emission
observed when 0 mL and 100 mL of hexafluoroisopropanol (HFIP) were
added to a mixed solution of B-3, 1-CBPEA and 4 eq. C-1 to disrupt the
H-bonding; (d) emission observed when 0 mL and 100 mL of HFIP were
added to a mixed solution of B-3-Me, 1-CBPEA and 4 eq. C-1. Exci-
tation was carried out with a 589 nm laser (4.4 mW, power density is 60
results clearly demonstrate the effect of hydrogen bonding on
intra-assembly singlet energy transfer.
2.7 Application in triplet–triplet annihilation upconversion
Up until now, no supramolecular triplet photosensitizers for
TTA upconversion have been reported. The H-bonding assem-
ꢀ2
blies were used as supramolecular triplet photosensitizers for mW cm ), and a band-pass filter (470–570 nm) was used to suppress
TTA upconversion, with 1-chloro-9,10-bisphenylethynylan- the scattered laser. The asterisks indicate the residual emission of the
ꢀ5
ꢀ4
antenna. c[photosensitizers] ¼ 1.0 ꢃ 10 M. c[1-CBPEA] ¼ 1.2 ꢃ 10
thracene (1-CBPEA) as triplet acceptor/emitter (Fig. 11; note that
for B-3 and B-3-Me, the uorescence emission shows vibrational
progression, i.e. there are two emission bands at 616 nm and
ꢂ
M in deaerated toluene, 20 C.
6
57 nm. Actually, there should be no peak at 550 nm; this peak
In order to prove that the production of the triplet excited
state and the TTA upconversion are due to the formation of the
H-bonded molecular assembly, hexauoroisopropanol (HFIP)
was added to disrupt the H-bonding (Fig. 11c). As a result, the
TTA upconversion intensity decreased, and at the same time,
the uorescence of the antenna was enhanced. These changes
are in agreement with the fact that the H-bonding was dis-
rupted. Note that the upconversion is weak, but this proof-of-
concept approach will be useful for designing supramolecular
triplet photosensitizers and for the application of these
compounds. For B-3-Me, no such enhancement of the uores-
cence of the antenna was observed in the presence of HFIP
is due to the lter because the transmittance is greatly reduced
beyond 570 nm; thus the tailing of the emission band gives a
fake ‘peak’ at 550 nm). For B-3$C-1, upconverted emission in the
450–550 nm region was observed with increasing C-1 concen-
tration (Fig. 11a). For B-3-Me, which is unable to form a H-
bonded molecular assembly, no TTA upconversion in the region
of 450–550 nm was observed. Thus, we conrm that the triplet
state production by B-3$C-1 upon photoexcitation is sufficient
for TTA upconversion. For the B-3-Me–C-1 mixture, however, no
upconversion was observed due to the lack of H-bonding. To the
best of our knowledge, this is the rst time that a supramolec-
ular triplet photosensitizer was used for TTA upconversion. This
result opens a new avenue for studies on TTA upconversion.
(Fig. 11d).
1
Fig. 10 Singlet oxygen ( O
2
) generation by photoexcitation of C-1, B-1, B-1-Me, B-2, B-2-Me, B-3 and B-3-Me. Hydrogen bonded pairs
1
produced O
2
more efficiently. Decrease in absorbance of DPBF vs. photoirradiation time in the presence of photosensitizers: (a) lex ¼ 495 nm,
ꢀ2
ꢀ2 ꢀ2 ꢀ5
ꢂ
1
.1 mW m ; (b) lex ¼ 630 nm, 1.3 mW m ; (c) lex ¼ 588 nm, 1.3 mW m ; c ¼ 1.0 ꢃ 10 M in toluene, 20 C.
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added successively to a solution of 3 (0.04 mmol, 10 mg) in a
3. Conclusion
mixed CHCl –EtOH–water (12 : 1 : 1, v/v, 28 mL) solvent. The
3
In summary, hydrogen bond mediated supramolecular assem- mixture was stirred at RT for 12 h. Then water was added, and
blies of Bodipy and C60 modules were prepared and acted as the mixture was extracted with CH Cl . The organic phase was
supramolecular triplet photosensitizers. Intra-assembly singlet dried over Na SO . The solvent was evaporated under reduced
and triplet state energy transfers were observed. The molecular pressure. The crude product was puried by column chroma-
assemblies were used as supramolecular triplet photosensi- tography (silica gel, CH Cl –CH OH, 50 : 1, v/v) to afford a gray
tizers for triplet–triplet annihilation upconversion. Three solid. Yield: 26.5 mg (50.2%). H NMR (500 MHz, DMSO-d
2
2
2
4
2
2
3
1
6
):
different thymine-containing visible light-harvesting Bodipy 10.59 (s, 1H), 10.13 (s, 1H), 8.24 (s, 1H), 7.76–7.65 (m, 3H), 7.49
derivatives (B-1, B-2 and B-3, which show absorption at 505 nm, (d, 2H, J ¼ 8.3 Hz), 7.11 (d, 2H, J ¼ 8.3 Hz), 5.50–5.46 (m, 4H),
6
30 nm and 593 nm, respectively) were used as the H-bonding 5.17 (s, 2H), 4.50–4.44 (m, 2H), 2.12 (s, 3H), 1.29 (t, 3H, J ¼ 7.0
13
modules, whereas 1,6-diaminopyridine appended C60 was used Hz). C NMR (100 MHz, DMSO-d
): 162.5, 144.2, 144.1, 144.0,
6
as the complementary hydrogen bonding module (C-1). Visible 143.8, 143.7, 143.2, 140.0, 138.4, 137.7, 131.1, 114.5, 71.1, 60.7,
+
light-harvesting antennae with methylated thymine were 52.1, 30.7, 23.9, 13.9. HRMS: m/z calcd for [C75
prepared as reference compounds (B-1-Me, B-2-Me and B-3-Me), 994.0841; found: 994.0844.
14 5
H O ] :
which are unable to form strong H-bonds with C-1. Triple H-
Synthesis of B-1. 18 (0.18 mmol, 30 mg), sodium ascorbate
bonds are formed between each Bodipy module (B-1, B-2 and B- (0.0237 mmol, 4.7 mg) and CuSO
4
(0.0118 mmol, 2.9 mg) were
–EtOH–
). The photophysical properties of the assemblies were studied water (12 : 1 : 1, v/v, 28 mL) and the mixture was stirred at RT for
3
1
visible light-harvesting antennae) and the bonding module (C- added to a solution of 11 (0.18 mmol, 75 mg) in CHCl
3
with steady state UV/vis absorption spectroscopy, uorescence 12 h. In a typical work-up, water was added and the mixture was
emission spectroscopy, electrochemical characterization, and extracted with CH Cl . The organic phase was dried over
nanosecond transient absorption spectroscopy. Singlet energy sodium sulfate. Aer removal of the solvent, the product was
transfer from the Bodipy antenna to the C60 module was puried by column chromatography (silica gel, CH Cl –CH OH,
conrmed by the uorescence quenching studies. The methyl- 25 : 1, v/v) to afford an orange solid. Yield: 33.6 mg (32.6%). H
ated modules were unable to form strong H-bonds with C-1, and NMR (500 MHz, DMSO-d ): 11.3 (s, 1H), 8.18 (s, 1H), 7.64 (s, 1H),
uorescence quenching was observed to a lesser extent. The 7.26 (d, 2H, J ¼ 8.4 Hz), 7.09 (d, 2H, J ¼ 8.5 Hz), 6.17 (s, 2H), 4.91
nanosecond transient absorption spectroscopy showed that the (s, 2H), 4.79 (t, 2H, J ¼ 4.7 Hz), 4.46 (d, 2H, J ¼ 4.6 Hz), 2.24 (s,
2
2
2
2
3
1
6

1
3
triplet state is either localized on the C60 module (assembly B- 6H), 1.74 (s, 3H), 1.36 (s, 6H). C NMR (100 MHz, CDCl
): 163.8,
$C-1), or on the Bodipy antenna (for assemblies B-2$C-1 and B- 155.5, 150.6, 140.2, 128.2, 121.2, 114.9, 111.2, 66.3, 49.9, 42.9,
$C-1). For assembly B-1$C-1, the photophysical process 29.7, 14.6, 12.3. HRMS (MALDI): m/z calcd for
3
1
3
+
involves intra-assembly singlet energy transfer from the [C29
H
30
N
7
O
3
F
2
B] : 573.2741; found: 573.2504.
Synthesis of B-1-Me. 19 (0.15 mmol, 26 mg), sodium ascor-
(0.0118 mmol, 2.9 mg)
localized on the C60 part in assembly B-1$C-1. For B-2$C-1 and were added to a solution of 11 (0.15 mmol, 65 mg) in CHCl
antenna to the C60 module, and then intersystem crossing of the
latter produces the triplet excited state. The triplet state is bate (0.0237 mmol, 4.7 mg) and CuSO
4
–
3
B-3$C-1, intra-assembly ping-pong singlet/triplet energy trans- EtOH–water (12 : 1 : 1, v/v, 14 mL) and the mixture was stirred at
fer was proposed, and as a result the triplet state is conned on RT for 12 h. Aer the typical work-up, the crude product was
the styryl-Bodipy part in B-2$C-1 and B-3$C-1. No signicant puried by column chromatography (silica gel, CH
triplet state production was observed for the reference mixtures 40 : 1, v/v) to afford an orange solid. Yield: 35.6 mg (40.2%). H
of B-1-Me/C-1, B-2-Me/C-1 and B-3-Me/C-1, for which no strong NMR (500 MHz, CDCl
2 2 3
Cl –CH OH,
1
3
): 8.00 (s, 1H), 7.41 (s, 1H), 7.20 (d, 2H, J ¼
H-bonds were formed. These supramolecular assemblies were 6.5 Hz), 6.99 (d, 2H, J ¼ 8.1 Hz), 5.98 (s, 2H), 5.03 (s, 2H), 4.82 (s,
used as triplet photosensitizers for triplet–triplet annihilation 2H), 4.43 (s, 2H), 3.33 (s, 3H), 2.54 (s, 6H), 1.93 (s, 3H), 1.39 (s,
1
3
upconversion. These studies may offer a new approach for 6H). C NMR (100 MHz, CDCl
): 164.1, 158.8, 155.6, 151.8,
3
designing organic triplet photosensitizers.
140.8, 131.9, 128.5, 121.4, 115.3, 110.5, 66.2, 50.4, 44.1, 28.1,
+
1
3.2. HRMS (MALDI): m/z calcd for [C H N O F B] : 587.2628;
30 32 7 3 2
found: 587.2614.
4
. Experimental section
Synthesis of B-2. 18 (0.07 mmol, 11.2 mg), sodium ascorbate
4.1 Materials
(0.047 mmol, 9.4 mg) and CuSO (0.0236 mmol, 5.8 mg) were
4
added to a solution of 12 (0.07 mmol, 40 mg) in CHCl
water (12 : 1 : 1, v/v, 14 mL) and the mixture was stirred at RT for
2 h. Aer the work-up, the crude product was puried by
column chromatography (silica gel, CH Cl –CH OH, 25 : 1, v/v)
3
–EtOH–
UV/vis absorption spectra were measured with an Agilent 8453
spectrophotometer. Fluorescence spectra were recorded on a
Shimadzu RF-5301 PC spectrouorometer. Luminescence life-
times were measured on an OB920 luminescence lifetime
spectrometer (Edinburgh Instruments, UK).
1
2
2
3
1
to afford a blue solid. Yield: 33 mg (62.9%). H NMR (500 MHz,
CDCl ): 8.64 (s, 1H), 7.94 (s, 1H), 7.75–7.84 (m, 6H), 7.41–7.26
m, 11H), 7.00 (s, 2H), 6.64 (s, 2H), 4.96 (s, 2H), 4.80 (s, 2H), 4.41
3
(
(
4.2 Syntheses
13
3
s, 2H), 1.88 (s, 3H), 1.46 (s, 6H). C NMR (100 MHz, CDCl ):
Synthesis of C-1. 8 (0.043 mmol, 42.7 mg), sodium ascorbate 169.3, 163.5, 157.4, 156.0, 147.8, 145.4, 143.6, 141.2, 138.6,
0.0237 mmol, 4.7 mg) and CuSO (0.0118 mmol, 2.9 mg) were 134.9, 132.4, 124.1, 122.7, 120.3, 115.7, 71.5, 54.9, 48.0, 34.7,
(
4
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+
1
7
9.7, 17.0. HRMS (MALDI): m/z calcd for [C43
49.3097; found: 749.3092.
H
38
N
7
O
3
F
2
B] : hexauorophosphate (TBAP) as supporting electrolyte, aer
purging with N . The working electrode was a glassy carbon
2
Synthesis of B-2-Me. 19 (0.05 mmol, 8.5 mg), sodium ascor- electrode, and the counter electrode was a platinum electrode. A
bate (0.047 mmol, 9.4 mg) and CuSO (0.0236 mmol, 5.8 mg) non-aqueous Ag/AgNO (0.1 M in acetonitrile) reference elec-
4
3
were added to a solution of 12 (0.05 mmol, 28 mg) in CHCl
3
–
trode was contained in a separate compartment connected to
EtOH–water (12 : 1 : 1, v/v, 14 mL) and the mixture was stirred at the solution via a semipermeable membrane. DCM was used as
RT for 12 h. Aer the work-up, the crude product was puried by the solvent. Ferrocene was added as the internal reference.
2 2 3
column chromatography (silica gel, CH Cl –CH OH, 40 : 1, v/v)
1
to afford a blue solid. Yield: 33 mg (68.1%). H NMR (400 MHz,
4.4 Nanosecond transient absorption spectra
CDCl ): 7.91 (s, 1H), 7.77–7.63 (m, 6H), 7.39–7.68 (m, 13H), 7.64
3
The spectra were measured on an LP920 laser ash photolysis
spectrometer (Edinburgh Instruments, UK). The lifetimes
(
s, 2H), 6.64 (s, 2H), 5.00 (s, 2H), 4.77 (s, 2H), 4.40 (s, 2H), 3.34 (s,
1
3
3
1
1
5
H), 1.93 (s, 3H), 1.46 (s, 6H). C NMR (100 MHz, CDCl ): 164.1,
3
(obtained by monitoring the decay traces) were calculated with
58.5, 152.8, 152.0, 151.7, 148.0, 142.2, 138.7, 137.7, 136.4,
33.8, 130.1, 129.0, 127.7, 125.0, 119.4, 118.0, 115.2, 110.4, 66.4,
the LP900 soware. All the solutions were purged with N for 15
2
min before measurement and the gas ow was maintained
during the measurements. The samples were excited with a
nanosecond pulsed laser (Opolette™ 355II + UV nanosecond
pulsed laser, typical pulse length: 7 ns. Pulse repetition: 20 Hz.
Peak OPO energy: 4 mJ. The wavelength is tunable in the range
of 410–2200 nm. OPOTEK, USA) and the transient signals were
recorded on a Tektronix TDS 3012B oscilloscope.
0.0, 44.1, 28.1, 15.1, 13.2. HRMS (MALDI): m/z calcd for
+
[C
44
H
40
N
7
O
3
F
2
B] : 763.3254; found: 763.3294.
Synthesis of B-3. 18 (0.05 mmol, 8.2 mg), sodium ascorbate
(
0.047 mmol, 9.4 mg) and CuSO (0.0236 mmol, 5.8 mg) were
4
added to a solution of 16 (0.055 mmol, 35 mg) in CHCl –EtOH–
3
water (12 : 1 : 1, v/v, 14 mL) and the mixture was stirred at RT for
2
4 h. Aer the work-up, the crude product was puried by
column chromatography (silica gel, CH Cl –CH OH, 25 : 1, v/v)
to afford a blue solid. Yield: 22.3 mg (50.2%). H NMR (400
MHz, CDCl
The quantum yield of the triplet formation (F
T
) was deter-
2
2
3
mined using the sensitizing method, which was described in
1
16b,c,54
detail previously.
The method is based on the use of a
3
): 8.99 (s, 1H), 8.28 (s, 1H), 8.17 (d, 2H, J ¼ 7.6 Hz),
triplet energy acceptor (b-carotene) and a standard compound
7
2
3
.79–7.69 (m, 2H), 7.50–7.38 (m, 6H), 7.24–7.22 (m, 3H), 7.00 (s,
0
(
Ru(bpy) Cl , bpy ¼ 2,2 -bipyridine, F ¼ 1). Using optically
3
2
T
H), 6.66 (s, 1H), 5.99 (s, 1H), 5.00 (s, 2H), 4.81 (s, 2H), 4.41 (s,
H), 4.31 (t, 2H, J ¼ 6.7 Hz), 2.62 (s, 3H), 1.88 (s, 3H), 1.44–1.40
matched solutions of the reference and the C-1$B-1 hydrogen
bonding assembly upon excitation at the same wavelength, the
DO.D. value, the triplet–triplet energy transfer to b-carotene
1
3
(m, 8H), 0.96 (t, 3H, J ¼ 6.7 Hz). C NMR (100 MHz, CDCl ):
3
1
1
3
64.4, 154.1, 151.2, 141.6, 140.8, 138.2, 133.3, 130.0, 123.4,
20.8, 119.5, 117.6, 116.2, 115.1, 111.3, 109.2, 66.1, 43.1, 31.1,
1 n
(KTTET, monitored for the T / T absorption at 515 nm), and
the decay of the triplet energy donor in the absence of b-caro-
tene (k ) were used for calculation of the F values.
0.1, 20.6, 13.9, 12.3. HRMS (MALDI): m/z calcd for
+
0
T
[C
46
H
45
N
8
O
3
F
2
B] : 806.3676; found: 806.3685.
Synthesis of B-3-Me. 19 (0.04 mmol, 7.1 mg), sodium ascor-
4.5 Computational simulations of the B-1 and C-1 complex
bate (0.047 mmol, 9.4 mg), CuSO (0.0236 mmol, 5.8 mg) were
4
added to a solution of 16 (0.04 mmol, 25.6 mg) in a mixture of
Force eld parameters of B-1 and C-1 structures. The struc-
CHCl –EtOH–water (14 mL, 12 : 1 : 1, v/v) and the mixture was tures of the B-1 and C-1 motifs without fullerene were rst
3
stirred at RT for 24 h. Aer the typical work-up, the crude energy minimized using density functional theory at the B3LYP/
product was puried by column chromatography (silica gel, 6-31G* level. Frequency analyses were performed on the opti-
CH
2
Cl
2 3
–CH OH, 40 : 1, v/v) to afford a blue solid. Yield: 19.6 mg mized structures in order to obtain the Hessian matrix, from
1
(59.7%). H NMR (400 MHz, CDCl
3
): 8.28 (s, 1H), 8.17 (d, 2H, J ¼ which the corresponding force constants for all the bonds were
9
7
4
.5 Hz), 7.78–7.69 (m, 2H), 7.50–7.38 (m, 5H), 7.25–7.22 (m, 4H), calculated, as well as the equilibrium bond lengths. The elec-
.00 (d, 2H, J ¼ 7.4 Hz), 6.67 (s, 1H), 5.99 (s, 1H), 5.02 (s, 2H), trostatic potential was calculated based on the same theory,
.80 (s, 2H), 4.42 (s, 2H), 4.31 (t, 2H, J ¼ 7.5 Hz), 3.34 (s, 3H), 2.62 using Mulliken population analysis. The restrained electrostatic
(
s, 3H), 1.93 (s, 3H), 1.47–1.38 (m, 8H), 0.96 (t, 3H, J ¼ 6.4 Hz). potential (RESP) method was used to t the electrostatic
1
3
55
C NMR (100 MHz, CDCl
3
): 164.1, 158.1, 154.3, 152.0, 143.0, potential in order to obtain the partial atomic charges. All
1
1
41.1, 138.3, 133.4, 128.8, 125.8, 123.5, 119.5, 117.6, 114.5, quantum chemical calculations were carried out with the
56
10.5, 109.5, 66.2, 43.2, 29.5, 28.1, 20.92, 14.1, 12.9. HRMS Gaussian 09 soware package. The PARATOOL plug-in of VMD
+
57
(MALDI): m/z calcd for [C47
H
47
N
8
O
3
F
2
B] : 820.3832; found: was used to derive the corresponding force constants. For the
fullerene motif of C-1, the partial charges of all carbon atoms
820.3850.
were set to zero, as reported for previous molecular dynamics
5
8,59
simulations of fullerene derivatives.
Force constants
4.3 Cyclic voltammetry
60
obtained from the CHARMM force eld parameters were
Cyclic voltammetry was performed using a CHI610D electro- applied for the bonding and non-bonding interactions within
chemical workstation (Shanghai, China). Cyclic voltammo- the fullerene motif of C-1.
ꢀ
1
grams were recorded at scan rates of 0.05 V s . The electrolytic
Molecular dynamics simulations. The starting complex of B-
cell used was a three electrode cell. Electrochemical measure- 1 and C-1 is extended, with three pairs of hydrogen bonds
ments were performed at RT using 0.1 M tetrabutylammonium between B-1 and C-1. This initial structure was solvated in a
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˚
˚
˚
TIP3P water box (85 A ꢃ 55 A ꢃ 48 A), resulting in a system
9 Y. C. Simon and C. Weder, J. Mater. Chem., 2012, 22, 20817–
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temperature, the whole system was simulated for another 30 ns
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atoms, pressure and temperature). The temperature was
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was kept at 1 atm using the Langevin piston method. An inte-
gration time step of 2 fs was used. The long-range electrostatic
interactions were treated using the PME method. The trajectory
C. W. Schlenker, S. E. Bradforth and M. E. Thompson, J. Am.
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Acknowledgements
1
1
7 C. Zhang, J. Zhao, S. Wu, Z. Wang, W. Wu, J. Ma, S. Guo and
L. Huang, J. Am. Chem. Soc., 2013, 135, 10566–10578.
8 S. Guo, L. Ma, J. Zhao, B. K u¨ ç u¨ k ¨o z, A. Karatay, M. Hayvali,
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We thank the NSFC (21273028, 21473020 and 21421005), the
Royal Society (U.K.) and NSFC (China-UK Cost-Share Science
Networks, 21011130154), the Program for Changjiang Scholars
and Innovative Research Team in University [IRT_13R06], the
State Key Laboratory of Fine Chemicals (KF1203), the Funda-
mental Research Funds for the Central Universities
500.
1
9 J. Ma, X. Yuan, B. K u¨ ç u¨ k ¨o z, S. Li, C. Zhang, P. Majumdar,
A. Karatay, X. H. Li, H. G. Yaglioglu, A. Elmali, J. Zhao and
M. Hayvali, J. Mater. Chem. C, 2014, 2, 3900–3913.
(
(
DUT14ZD226) and Dalian University of Technology
DUT2013TB07) for nancial support.
2
0 W. Shi, R. Menting, E. A. Ermilov, P.-C. Lo, B. R ¨o derb and
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