Photoswitching of the triplet excited state of diiodobodipy-dithienylethene triads and application in photo-controllable triplet-triplet annihilation upconversion

Article abstract of DOI:10.1021/jo5018662

Dithienylethene (DTE)-2,6-diiodoBodipy triads were prepared with the aim to photoswitch the triplet excited state of the 2,6-diiodoBodipy moiety. Bodipy was selected due to its low T₁ state energy level to avoid sensitized photocyclization of DTE, which is very often encountered in DTE photoswitches, so that the photochemistry of DTE and the organic chromophore can be addressed independently. This is the first time that DTE was covalently connected with an organic triplet photosensitizer. For the triad with DTE-o structure, selective photoexcitation into the diiodoBodipy part did not initiate photocyclization of DTE-o. Upon photoirradiation at 254 nm, thus the DTE-o → DTE-c transformation, the intersystem crossing (ISC) of 2,6-diiodoBodipy moiety was competed by the photoactivated resonance energy transfer (RET), with Bodipy as the intramolecular energy donor and DTE-c as energy acceptor. The fluorescence of Bodipy was quenched and the triplet state lifetime of Bodipy was reduced from 105.1 to 40.9 μs. The photoreversion is O₂-independent, but can be greatly accelerated upon selective photoexcitation into the diiodoBodipy absorption band (at 535 nm). We concluded that ISC is not outcompeted by RET. The photoswitching of the triplet state was transduced to the singlet oxygen photosensitizing, as well as triplet-triplet annihilation upconversion.

Full text of DOI:10.1021/jo5018662

Article
pubs.acs.org/joc
Photoswitching of the Triplet Excited State of DiiodoBodipy-
Dithienylethene Triads and Application in Photo-Controllable
Triplet−Triplet Annihilation Upconversion
Jie Ma,† Xiaoneng Cui,† Fen Wang,†,‡ Xueyan Wu,† Jianzhang Zhao,*,† and Xingwei Li*,‡
†State Key Laboratory of Fine Chemical, School of Chemical Engineering, Dalian University of Technology, Dalian 116024, People’s
Republic of China
‡Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, People’s Republic of China
S
* Supporting Information
ABSTRACT: Dithienylethene (DTE)-2,6-diiodoBodipy tri-
ads were prepared with the aim to photoswitch the triplet
excited state of the 2,6-diiodoBodipy moiety. Bodipy was
selected due to its low T1 state energy level to avoid sensitized
photocyclization of DTE, which is very often encountered in
DTE photoswitches, so that the photochemistry of DTE and
the organic chromophore can be addressed independently.
This is the first time that DTE was covalently connected with
an organic triplet photosensitizer. For the triad with DTE-o
structure, selective photoexcitation into the diiodoBodipy part
did not initiate photocyclization of DTE-o. Upon photoirradiation at 254 nm, thus the DTE-o → DTE-c transformation, the
intersystem crossing (ISC) of 2,6-diiodoBodipy moiety was competed by the photoactivated resonance energy transfer (RET),
with Bodipy as the intramolecular energy donor and DTE-c as energy acceptor. The fluorescence of Bodipy was quenched and
the triplet state lifetime of Bodipy was reduced from 105.1 to 40.9 μs. The photoreversion is O2-independent, but can be greatly
accelerated upon selective photoexcitation into the diiodoBodipy absorption band (at 535 nm). We concluded that ISC is not
outcompeted by RET. The photoswitching of the triplet state was transduced to the singlet oxygen photosensitizing, as well as
triplet−triplet annihilation upconversion.
ing,27−29 molecular beacons for detection of DNA, enzyme, or
1. INTRODUCTION
peptide,30 light-harvesting molecular arrays,24,31,32 and molec-
Triplet excited states are important for photocatalysis, such as
ular logic gates.33 Unfortunately, much room is left to extend
hydrogen (H2) production by photocatalytic water splitting and
photocatalytic organic reactions,1−6 luminescent oxygen
the study to triplet excited-state switching. This situation can be
sensing,7 molecular devices,8,9 photodynamic therapeutic
attributed to a large extent to the difficulties to access organic
reagents,10−13 and more recently the triplet−triplet annihilation
triplet photosensitizers, for which the molecular design is still a
substantial challenge.2−4 Previously amino-containing bromo-
(TTA) upconversion.14−18 Concerning these applications,
aza Bodipy were studied as acid-activatable PDT reagents,10
controllable production of triplet excited states by external
and the mechanism is based on PET. Molecular logic gates
stimuli will be a crucial progress, because this kind of switching
based on controlling of the direction of FRET were also
ability will lead to versatile materials with spatial or temporal
studied, with acid-activated production of triplet excited state as
resolution, such as activatable photodynamic therapy (PDT)
a output of the logic gates.8 These pioneering studies unveiled
the fascinating area of triplet state switching or modulation.
However, up to now such investigations are very rare, and we
believe that more exemplars are needed to learn the
photophysical principles governing the triplet excited-state
switching. For example, the outcome of the parallel RET and
intersystem crossing (ISC) processes in an organic dyad/triad
was rarely studied; this competing process will be crucial for the
modulation capability of the controlled production of triplet
excited states.9 Moreover, time-resolved transient absorption
reagents,9,10 molecular logic gates, etc.8,9 However, the
investigations on switching of triplet excited state are rarely
reported.19,20 As a result, the current knowledge on the
photophysical processes regarding triplet state switching is
limited.
Switching singlet excited state with external stimuli has been
intensively investigated, and various mechanisms have been
developed,21 such as the photoinduced electron transfer
(PET),22 fluorescence resonance energy transfer (FRET or
more precisely, RET),23−25 through-band-energy-transfer
(TBET),26 etc. These mechanisms have been used to construct
a lot of external stimuli-responsive molecules, such as
fluorescent molecular sensors,21 targeted luminescent bioimag-
Received: August 13, 2014
Published: October 23, 2014
© 2014 American Chemical Society
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a
Scheme 1. Preparation of DB-1 and DB-2
a
Key: (a) K2CO3 and EtOH, reflux, 8 h, 60.0%; (b) NaN3, DMF, 100 °C, 2 h, 80.4%; (c) N2 atmosphere, 2,4-dimethylpyrrole, CH2Cl2, TFA; then
DDQ, Et3N, and BF3·Et2O, 7.5%; (d) NIS, dry DCM, 30 °C, overnight, 90.4%; (e) NBS, AcOH, rt, overnight, 90.3%; (f) TMSA, Pd(PPh3)4, CuI,
NEt3, 45 °C, 7 h, 45.5%; (g) n-BuLi, perfluorocyclopentene, −78 °C, 4 h, 30.0%; (h) MeOH/THF, NaOH, 45 min, 70.0%; (i) sodium ascorbate,
CuSO4, 25 °C, Ar, 48 h, 46.6%; (j) sodium ascorbate, CuSO4, 25 °C, Ar, 48 h, 73.4%.
spectroscopy, a powerful method for study of triplet excited
state, was rarely used for detailed investigation of the triplet
excited-state switching.19
the triplet state of the DTE−metal complex dyads is that the
photocyclization of DTE is very often sensitized by excitation
into the 1MLCT band of the transition metal complex-
es,19,36b,40−43,46 which makes it difficult to address the
photochromic and the luminescent modules in the dyads
independently. For example, it is difficult to achieve non-
destructive readout of the luminescence or the triplet state
property of the organic chromophore in the conventional DTE
dyads. Recently, the photoswitching singlet oxygen production
with porphyrin complexes by DTE was reported.47a We also
studied the photoswitching of the TTA upconversion with two
DTE compounds.47b However, the photoswitching is based on
a mixture of the DTE derivatives and the triplet photo-
sensitizer/acceptor. A covalently linked DTE-photosensitizer
was never studied.
On the other hand, dithienylethene (DTE) is of particular
interest, due to its excellent photochromic property, such as the
dramatic difference of absorption band position for the open-
ring and the closed-ring isomers, and the thermal stability of
open-ring and closed-ring isomers.34−37 Previously, DTE or
other photochromic units were used to switch the fluorescence
property of Bodipy fluorophore.38,39 DTE was also used for
photoswitching of the triplet state of Ru(II) and Os(III)
complexes,19,40 Pt(II) acetylide complexes,36b,41 Re(I) com-
plexes,42,43 or the intramolecular or intermolecular electron
transfer,44,45 but DTE was not used in any photoswitching of
the triplet excited states of organic chromophores. Moreover, a
long-standing challenge in the study of the photoswitching of
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To address the above challenges, herein we prepared an
iodoBodipy-dithienylthene (DTE) triad for study of photo-
switching of triplet excited states of organic chromophores
(DB-1, Scheme 1). 2,6-DiiodoBodipy in DB-1 is the triplet
excited-state producer, or the spin converter.4,48 DTE can be
photoswitched from the open-ring form (DTE-o) to the
closed-ring from (DTE-c), accompanied by substantial changes
in UV−vis absorption wavelength (i.e., the S1 state energy
level) and triplet state energy levels.19,40 Thus, the RET from
iodo-Bodipy to the DTE-c will compete with the ISC of 2,6-
diiodoBodipy moiety. As a result, the production of triplet
excited state by iodo-Bodipy will be compromised upon
photoirradiation. Steady-state and nanosecond time-resolved
absorption and emission spectroscopy, as well as DFT
computations, were used to study the photoswitching of triplet
excited states. We found that the triplet excited-state lifetime, as
well as the singlet oxygen (1O2) production capability of the
triad can be significantly photoswitched. Furthermore, we
found that the RET is unable to inhibit the ISC completely,
which is different from a previous observation.9 To the best of
our knowledge, this is the first time that the triplet excited state
of organic chromophores was used as a relay of the
photochromic binary states’ photoconversion. These studies
will be useful for further exploration of the uncharted area of
triplet excited-state switching, as well as for the application of
the related switchable triplet photosensitizers in PDT, photo-
catalysis, upconversion, and molecular devices, etc.
geometry or rapid interconversion of the two conformers
occurs.43 Recently, we studied the photoswitching of TTA
upconversion with DTE derivatives, but that approach is based
on the mixture of the three components.47b Herein, for the first
time a covalently connected DTE-photosensitizer dyad/triad
was studied for TTA upconversion. Different results were
observed from the previous study.47b
2.2. UV−Vis Absorption and Fluorescence Emission
Spectra. The UV−vis absorption spectra of the compounds
were studied (Figure 1). DB-1 shows a strong absorption band
Figure 1. UV−vis absorption of DB-1, DB-2, D-1, and B-0. c = 1.0 ×
10−5 M in CH2Cl2, 20 °C.
at 535 nm, which is due to the diiodoBodipy moiety. DTE gives
no absorption in visible spectral region in its open-ring form,
demonstrated by the absorption of the reference compound D-
1. Without iodo atoms on the Bodipy moiety, DB-2 gives
slightly blue-shifted absorption band at 502 nm as compared to
that of DB-1.53 The absorption wavelength of DB-1 (535 nm)
is similar to that of B-0, and the molar absorption coefficient of
DB-1 is ca. 2-fold greater than that of B-0. On the basis of these
results, we propose that there is no significant interaction
between the DTE-o and the Bodipy moiety in DB-1.38 Similar
results were obtained for DB-2.
The photocyclization of the compounds upon irradiation was
studied (Figure 2). Upon irradiation at 254 nm, the absorption
band at 268 nm decreased, and a new broad absorption band at
600 nm developed (Figure 2a). This new absorption band is
attributed to the DTE-c moiety.38,54a The DTE-o → DTE-c
photocyclization is reversible; that is, upon photoirradiation at
600 nm, the new absorption band at 600 nm disappeared
completely and the absorption band at 268 nm fully recovered
(Figure 2b). The absorption band at 535 nm, due to the iodo-
Bodipy moiety, did not change throughout the experiments,
indicating the good photostability of the Bodipy moieties in
DB-1 and DB-2.38 At the photostationary state (PSS) upon
254 nm photoirradiation, the conversion yield (open form →
closed form) of DB-1 and DB-2 is 85.2% and 92.7%,
respectively (based on HPLC, Supporting Information Figures
S33, S34).
2. RESULTS AND DISCUSSION
2.1. Molecular Design and Preparation of the
Compounds. The rationale for designing of DB-1 (Scheme
1) is to introduce a photoactivatable RET to compete with ISC;
thus the production of triplet excited state may be photo-
switched. Visible light-harvesting 2,6-diiodoBodipy was selected
as the spin converter to produce the triplet excited state.
Bodipy was selected as the chromophore due to its desired
photophysical properties, such as strong absorption of visible
light, high fluorescence quantum yields, good photostability,
and feasible derivatization. DTE was selected as the photo-
switchable unit.49−52 The open-ring form of DTE (DTE-o)
gives no visible light absorption, and the T1 state energy level of
DTE-o is presumably higher than the diiodoBodipy unit. Thus,
the ability of diiodoBodipy unit to produce triplet excited state
will not be reduced. Upon photoirradiation at 254 nm, the
closed-ring form of DTE (DTE-c) will be produced, for which
the absorption band moves to the visible spectra region
(centered at 570 nm). Thus, there is a significant overlap
between the emission of the Bodipy moiety and the absorption
of the closed-ring form of DTE, and RET from the
diiodoBodipy moiety, which shows fluorescence emission at
553 nm, to the closed-ring DTE moiety (λabs = 600 nm) may
take place. As a result, the capability of the diiodoBodipy to
produce triplet excited states may be compromised by the
parallel RET effect.
The quantum yields of the photocyclization and the
photoreversion processes were studied with a chemical
actinometer potassium ferrioxalate actinometer.38 For DB-1,
photocyclization quantum yield (open form → closed form) is
ΦO→C = 0.32; the photoreversion (closed form → open form)
quantum yield is ΦC→O = 0.032. For DB-2, the photo-
cyclization (open form → closed form) quantum yield is ΦO→C
= 0.37; the photoreversion (closed form → open form)
quantum yield is ΦC→O = 0.043. These values are close to DTE
derivatives without the Bodipy appendents.47b
Azide-diiodoBodipy (compounds 3 and B-0) was prepared as
the key intermediate (Scheme 1). D-1-c is the ethynyl DTE.
Cu(I)-catalyzed Click reaction gives the production of DB-1.
DB-2 was prepared as a reference compound. All of the new
1
compounds were characterized with H NMR, 13C NMR, and
HRMS. The compounds were obtained in moderate to
1
satisfactory yields (see Experimental Section). The H NMR
spectrum of each of the DTE derivatives shows only a single set
of signals, indicating that the compounds take antiparallel
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Bodipy.19 The studies on fluorescence confirmed this
conclusion. The DTE-c → DTE-o takes place via a singlet
excited state of DTE-c; otherwise the photoreaction will be
inhibited in the presence of O2.19,41 Thus, we propose that
singlet RET takes place with Bodipy/iodo-Bodipy as energy
donor and DTE-c as energy acceptor. This is different from our
previous study in that FRET was not involved in the
photoswitching.47b
We also studied the possibility of the photosensitized DTE-o
→ DTE-c process upon selective excitation into the Bodipy
moieties in DB-1 (Supporting Information Figure S25). Upon
photoirradiation at 535 nm, no absorption band at 600 nm was
observed for DB-1-o. This result shows that the energy level of
the T1 state of the Bodipy is lower than that of DTE-o; thus, no
sensitized photocyclization was observed. This observation is
contrary to the previous report that in the Ru(II) or Os(III)
complex, the DTE-o → DTE-c can be sensitized via the
3MLCT state of the coordination center; as a result, the
photophysical property of the visible light-absorbing chromo-
phore cannot be addressed independently with the open form
DTE.19 For the current triads, however, both the singlet excited
state and the triplet excited state of the Bodipy chromophore
can be independently manifested, which may offer improved
tuning capability of these molecular assemblies.
Figure 2. UV−vis absorption of DB-1 upon irradiation. (a)
Photoirradiation at λex = 254 nm. After establishment of the
photostationary state, then (b) photoirradiated at 600 nm (2.0−2.4
W/m2). UV−vis absorption of DB-2 and the absorption spectral
changes upon irradiation at (c) λex = 254 nm (0.7−1.0 W/m2), and
then with successive photoirradiation at (d) 570 nm (2.0−2.4 W/m2).
c = 1.0 × 10−5 M in CH2Cl2, 20 °C.
2.3. Switching of the Fluorescence Emission. The
fluorescence switching effect of compounds DB-1 and DB-2
was also studied (Supporting Information Figure S26). For DB-
1, the fluorescence of the diiodoBodipy unit decreased upon
254 nm photoirradiation, that is, the DTE-o → DTE-c
photocyclization. This process is reversible, and the fluo-
rescence emission intensity at 553 nm can be recovered upon
600 nm irradiation of the DB-1-c, via the DTE-c → DTE-o
transformation (Supporting Information Figure S26b).
Interestingly, we found that by photoirradiation at 535 nm,
where the diiodoBodipy moiety has a strong absorption, the
conversion of DTE-c → DTE-o is greatly accelerated
(Supporting Information Figure S23a), which is attributed to
the RET from the Bodipy moiety to the DTE-c moiety. This
observation is different from a previous study.38 This trend is
clearly demonstrated in Supporting Information Figure S24.
Similar results were found for DB-2 (Supporting Information
Figure S23b).
The switching of the fluorescence of DB-2 was also studied
(Supporting Information Figure S26c and S26d). The
modulation efficiency on the fluorescence of DB-2 is more
significant than that of DB-1. For example, the fluorescence of
DB-1 decreased to 61.0% of the initial value, whereas for DB-2,
the fluorescence reduced to 11.6% of the initial intensity.
Considering the open form/closed form ratio at PSS, the FRET
efficiency can be estimated as 45.8% and 95.4% for DB-1 and
DB-2, respectively.39c The plausible reason for the different
modulation efficiency on the fluorescence for DB-1 and DB-2
may be the different lifetimes of the singlet excited state of the
compounds. The singlet state lifetime of DB-1 (τF = 0.31 ns) is
much shorter than that of DB-2 (τF = 4.24 ns). Thus, the
parallel RET has more difficulty competing with a fast decaying
S1 state of Bodipy moiety in DB-1-c than that in DB-2-c. The
shorter fluorescence lifetime of DB-1-c is due to the ISC of the
diiodoBodipy moiety. Previously, we studied the photo-
switching of TTA upconversion in the mixture of DTE
derivative, triplet photosensitizers, and triplet acceptor.47b
However, switching of the fluorescence of the triplet photo-
sensitizer is impossible with this “supramolecular approach”.
Thus, the present covalent bonded triad offers a new
modulability.
The kinetics of the DTE-o ↔ DTE-c photoswitching
(photocyclization and photocycloreversion) were studied
(Supporting Information Figure S24). DB-1, DB-2, and D-1
give similar DTE-o → DTE-c kinetics (Figure S24a). For
example, the rate constants of the photocyclization reaction are
1.1 × 10−3, 1.2 × 10−3, and 9.0 × 10−4 s−1, respectively. These
results indicate that the DTE-o → DTE-c photocyclization is
via singlet excited state, otherwise the T1 state of the DTE-o
will be quenched by the T1 state of Bodipy, for which the T1
state energy level is 1.52 eV, lower than that of DTE-o (1.97
eV). As a result, the photocyclization will be retarded.19
Photostationary state was reached after 3 min of photo-
irradiation at 254 nm for DB-1 (Supporting Information Figure
S24a).
For the DTE-c → DTE-o transformation, generally the
reaction rate constants are much smaller than the open-ring →
closed-ring photocyclization reaction, which is similar to the
reported DTE compounds.42,43 The rate constants of the
photoreversion are 5.33 × 10−5, 5.33 × 10−5, and 8.33 × 10−5
s−1 for DB-1, DB-2, and D-1, respectively. The reference
compound D-1 gives slightly faster kinetics (Supporting
Information Figure S24b).
On the basis of the fluorescence quenching and the
fluorescence lifetime of the 2,6-diiodoBodipy, the rate constants
of the RET in DB-1-c were calculated with eq 1.54b
Our results show that the DTE-c → DTE-o process is faster
with irradiation at the absorption band of the iodo-Bodipy and
the Bodipy moiety. The results show that photoreversion DTE-
c → DTE-o is not sensitive to the presence of O2 in the
solution. Thus, the T1 state energy level of the DTE-c moiety is
higher than the T1 state energy level of the iodo-Bodipy
moiety; thus the photoreversion can not be sensitized by iodo-
⎡
⎢
⎤
ΦPL(DiiodoBodipy)
ΦPL(DB‐1‐c)
⎥
kEnT
=
− 1 /τ0
⎢
⎣
⎥
⎦
(1)
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a
Table 1. Photophysical Properties of DB-1, DB-2, B-0, and D-1
h
i
e
ΦΔ /%
Φ /%
closed-ring
τ /ns
b
c
d
f
compd
λabs /nm
ε
λem /nm
τT (μs)
open-ring
closed-ring
open-ring
open-ring
closed-ring
k
l
g
DB-1
DB-2
B-0
535
502
534
261
1.32
1.25
0.80
0.36
553
515
553
105.1 /40.9
87.6
36.9
3.0
80.5
4.7
1.6
0.31
4.24
0.32
−
j
j
j
−
−
−
−
−
14.3
m
3.67
m
j
m
106.4
−
−
−
−
−
−
m
j
j
m
j
m
j
m
D-1
−
−
−
−
a
b
The excitation wavelengths for DB-1, DB-2, and B-0 were 510, 480, and 510 nm, respectively (1.0 × 10−5 M, 20 °C). Absorption wavelength.
c
d
e
f
Molar extinction coefficient. ε: 105 M−1 cm−1. In dichloromethane. Fluorescence emission wavelength. Fluorescence lifetimes. Triplet state
g
h
lifetimes. Too weak to be determined accurately. Quantum yield of singlet oxygen (1O2), with B-2 as standard (ΦΔ = 0.83), λex = 500 nm.
i
Fluorescence quantum yields in DCM, DB-1, and B-0 with B-2 (0.027 in CH3CN) as the standard, DB-2 with B-1 (0.72 in THF) as the standard.
j
k
l
m
Not determined. Triplet state lifetimes of DB-1-o in deaerated CH3CN. Triplet state lifetimes of DB-1-c in deaerated CH3CN. Not applicable.
where kEnT is the RET rate constant, ΦPL(DiiodoBodipy) is the
fluorescence quantum yield of 2,6-diiodoBodipy, ΦPL(DB‑1‑c) is
the fluorescence quantum yield of the iodoBodipy part in DB-
1-c, and τ0 is the fluorescence lifetime of diiodoBodipy. The
kEnT was calculated as 6.1 × 109 s−1. Previously, it was reported
that the intersystem crossing rate constant of a 2,6-
diiodoBodipy derivative is 7.87 × 109 s−1.54c Thus, the RET
is unlikely to compete efficiently with the ISC of the 2,6-
diiodoBodipy part; as a result, in DB-1-c, the population of the
triplet state of the 2,6-diiodoBodipy part is still significant. This
is in agreement with the nanosecond transient difference
absorption spectral study (see later section), as well as the
singlet oxygen production with DB-1-c (Figure 7 and Table 1).
To confirm the above explanation, the RET in DB-2-c was
analyzed with eq 1. The kEnT of DB-2-c was calculated as 1.1 ×
109 s−1. This value is slightly decreased as compared to that in
DB-1-c, which is probably due to the less matched spectral
overlap between the emission of Bodipy and the absorption of
the DTE-c unit. The values obtained for DB-1-c and DB-2-c
are very close to that obtained with a Bodipy-spiropyran dyads
fluorescence switch.39c The fluorescence of the triads was
compared to that of the reference compounds (Supporting
Information Figure S27). For DB-1-o, the fluorescence
intensity is close to that of B-0 (the two solutions have the
same optical density at the excitation wavelength) (Supporting
Information Figure S27a). Therefore, it is plausible to assume
that no significant photoinduced electron transfer exists in DB-
1-o. Similar results were observed for DB-2-o (Supporting
Information Figure S27b). The fluorescence emissions of B-0,
DB-1-o, and DB-1-c were compared in solvents with different
polaritie (see Supporting Information Figure S28). The
emission intensity ratios of the three compounds are the
same. Therefore, we conclude that no significant intramolecular
electron transfer exists for the singlet excited state of the
compounds. Similar results were observed for DB-2 (see
Supporting Information Figure S29).
Figure 3. Cyclic voltammetry of (a) B-0 (c = 1.0 × 10−3 M) and DB-
1-o (c = 5.0 × 10−4 M); (b) B-0 (c = 1.0 × 10−3 M) and DB-1-c (c = 5
× 10−4 M); (c) B-1 (c = 5.0 × 10−4 M) and DB-2-o (c = 1.0 × 10−3
M), and (d) B-1 (c = 5.0 × 10−4 M) and DB-2-c (1.0 × 10−3 M). In
deaerated CH3CN solutions containing 0.10 M Bu4NPF6 as
supporting electrode, Ag/AgNO3 reference electrode. Scan rate: 50
mV/s. Ferrocene (Fc) was used as internal reference 20 °C.
Table 2. Electrochemical Data for Compounds DB-1-o, DB-
a
1-c, DB-2-o, DB-2-c, D-1-o, D-1-c, B-0, and B-1
Eox (V)
Ered (V)
compd
I
II
I
II
b
b
B-0
+0.96
+0.84
+0.35
+0.87
+0.88
+0.55
+0.83
+0.54
−
−
−
−
−
−1.23
−1.48
−1.15
−1.19
−1.27
−1.27
−1.51
−1.38
−
−
−
b
b
b
b
b
b
B-1
D-1-o
D-1-c
DB-1-o
DB-1-c
DB-2-o
DB-2-c
−1.54
−1.91
−1.40
−1.96
−1.55
+0.98
b
−
2.4. Electrochemical Studies: Free Energy Changes of
the Photoinduced Electron Transfer. For DB-1-o, an
irreversible reductive wave was observed at −1.91 V (Figure
3a), which is assigned to DTE-o moiety.43 The reversible
reduction wave at −1.27 V can be attributed to the 2,6-
diiodoBodipy moiety. An irreversible oxidation peak at +0.88 V
is due to the diiodoBodipy moiety (Table 2). For the closed-
form DB-1-c, two new irreversible reduction waves at −1.40
and −1.74 V (this value was not presented in Table 2) were
observed, which is similar to a previously reported DTE−Re(I)
complex.43 A new pseudoreversible oxidation band at +0.55 V
was observed, which can be assigned to the closed-ring form of
DTE. Considering the fast ISC of the 2,6-diiodoBodipy, we
+0.78
a
Recorded with [Bu4N][PF6] as the supporting electrolyte in
deaerated CH3CN (0.1 M) at ambient temperature with a scan rate
of 50 mV/s. Potentials are expressed as the half-wave potentials (E1/2
)
in volts vs Ag/AgNO3 electrode using ferrocene as an internal
b
reference. Not applicable.
assume the triplet excited state of the 2,6-diiodoBodipy will
initiate the electron transfer process. However, the free energy
change was calculated as +0.28 eV. Thus, the photoinduced
intramolecular electron transfer is unlikely to occur. This
postulation is in agreement with the luminescence studies.
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2.5. Nanosecond Time-Resolved Transient Difference
Absorption Spectra: Photoswitching of the Triplet
Excited State. To study the photoswitching of the triplet
excited states of the triads, nanosecond time-resolved transient
difference absorption (TA) spectroscopy of the compounds
was studied (Figure 4). First the TA of DB-1-o was studied
and the reference compound B-0 (Supporting Information
Figure S27).
The TA spectrum of the DB-1-c was also studied (Figure 5).
Because of the DTE-c → DTE-o transformation upon
Figure 5. Nanosecond time-resolved transient difference absorption
spectra of DB-1-c. (a) TA spectra after pulsed laser excitation (λex
=
532 nm, ICCD detector). (b) Decay trace at 530 nm (PMT detector).
c = 1.0 × 10−5 M in deaerated CH3CN, 20 °C.
photoexcitation at 535 nm, thus to reduce the photoreversion
reaction, an ICCD detector (spectral mode) was used for the
TA measurement to minimize the exposure of the DB-1-c
solution to photoirradiation. Bleaching band at 528 nm was
found, as well as the transient absorption band at 455 nm and
the band in the region of 550−750 nm. These features are in
agreement with that of the 2,6-diiodoBodipy (Figure 4c). The
triplet state lifetime was determined as 40.9 μs, which is much
shorter than that of B-0 and DB-1-o (106.4 and 105.1 μs,
respectively). Similar transient absorption spectra were
obtained in DCM, for which longer lifetimes of the triplet
exited state of DB-1-o (144.5 μs) and DB-1-c (42.3 μs) were
observed (Supporting Information Figures S30 and S32). Since
we have shown that photoinduced electron transfer is not
significant for DB-1-c, thus the reduced triplet state lifetime is
probably due to the intermolecular quenching effect of a the
low-lying triplet state, such as the T1 state of the DTE-c moiety.
Note there exists a DB-1-c and DB-1-o equilibrium at the PSS.
For the neat DB-1-c, the triplet state localized on diiodoBodipy
may be quenched completely.19a It was reported that the
lifetime of the 3DTE-c is less than 10 ns,19 which is beyond the
response time of our nanosecond time-resolved transient
absorption spectrometer. The energy level of this state is
unknown. Furthermore, we have shown that RET is possible
for DB-1-c; thus the parallel processes of RET and ISC take
place. The quenched triplet state lifetime of the iodo-Bodipy
moiety in DB-1-c and the reduced 1O2 quantum yield of DB-1-
o as compared to that of B-0 support this conclusion.19
To study the localization of the triplet excited state from a
theoretical perspective, the spin density surfaces of DB-1-o and
DB-1-c were calculated with DFT method (Figure 6). For DB-
1-o, the spin density surface is exclusively localized on the iodo-
Bodipy moiety, and the open-ring DTE does not contribute to
the spin density surface. Thus, the DFT calculation indicates
that the T1 state of DB-1-o is localized on the iodo-Bodipy
moiety, which is in agreement with the nanosecond time-
resolved transient absorption spectroscopy (Figure 4). For DB-
1-c, similar results were observed. Thus, the lowest-lying triplet
state of DB-1-c is tentatively assigned to be localized on the
iodo-Bodipy moiety, which is also in agreement with the
nanosecond time-resolved transient absorption spectra (Figure
5).
Figure 4. Nanosecond time-resolved transient difference absorption
spectra. (a) DB-1-o. (b) Decay trace of DB-1-o at 530 nm. (c) TA
spectra of B-0. (d) Decay trace of B-0 at 530 nm. With pulsed laser
excitation (λex = 535 nm), c = 1.0 × 10−5 M in deaerated CH3CN, 20
°C.
with 535 nm pulsed laser excitation. We have shown that no
photocyclization reaction takes place upon excitation at this
wavelength. This is actually a benefit to use chromophores
showing long absorbing wavelength and lower T1 state energy
level than the DTE-o moiety; otherwise the sensitized
photocyclization will take place and it will be difficult to
observe the transient spectra of the visible light-harvesting
chromophore.19 Usually only the accumulation of the open-ring
or the closed-ring photoproduct can be observed upon UV or
visible light excitation, for example, in the DTE-containing
Ru(II) complexes for which the photoreaction can be sensitized
44
3
by the Ru(II) complex MLCT state.
Upon pulsed laser excitation at 535 nm, a bleaching band at
530 nm was observed for DB-1-o, together with transient
absorption bands at 445 nm and in the region of 550−750 nm
(Figure 4a). The lifetime of the triplet state was determined as
105.1 μs. These features are close to the results of the reference
compounds B-0 (Figure 4c and d). These results indicated that
the energy level of the triplet state of 2,6-diiodoBodipy (1.52
eV)55−58 is lower than that of DTE-o (1.97 eV).19 As a result,
the triplet state of the diiodoBodipy was not perturbed by the
DTE-o moiety; that is, the DTE-o → DTE-c can not be
sensitized by the selective excitation into the diiodoBodipy
moiety, and therefore the triplet excited-state lifetime of DB-1-
o is close to that of B-0. This result is different from the recent
studies on the DTE−Ru(II) and DTE−Os(III) complexes, for
which sensitized photocyclization always occurs and the triplet
excited state of the Ru(II) coordination center is quenched.19
Furthermore, the photoinduced electron transfer in DB-1-o is
not significant; otherwise the triplet state lifetime of DB-1-o
will be substantially shortened. This conclusion is also
supported by the comparison of the fluorescence of DB-1-o
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seconds),19,38 which is much faster than the RET to Bodipy
moiety (the spectral overlap between the DTE and Bodipy is
poor). Thus, the DTE-o → DTE-c process is not perturbed in
triad DB-1 as compared to that of D-1. This point was proved
by the comparison of the photocyclization of DB-1 and D-1
(Supporting Information Figure S24).
Upon selective photoexcitation into the 2,6-diiodoBodipy
part in DB-1-o, however, intersystem crossing (ISC) will take
place to give the triplet excited state. The T1 state energy level
of the DTE-o moiety is higher than that of diiodoBodipy;19
thus the triplet excited state of diiodoBodipy was not perturbed.
This postulation was supported by the similar triplet excited
states of DB-1-o and B-0 (Figure 4).
For the DB-1-c, however, the relative singlet/triplet excited-
state energy levels changed as compared to that in DB-1-o;
thus the photophysical properties were switched. First, the ISC
of the 2,6-diiodoBodipy moiety will compete with the RET to
DTE-c moiety, as demonstrated by the enhanced DTE-c →
DTE-o photoreversion upon excitation into the diiodoBodipy
moiety at 535 nm, where the absorption is much stronger than
that at 600 nm, and the quenching of the residual fluorescence
of the diiodoBodipy moiety. Furthermore, we found that the
lifetime of the triplet excited state of DB-1-c (40.9 μs), which is
localized on the diiodoBodipy part, is much shorter than DB-1-
o (105.1 μs) or B-0 (106.4 μs). Thus, it is plausible that a fast
Figure 6. Isosurfaces of spin density of compounds DB-1-o and DB-1-
c, based on optimized triplet state geometries. CH2Cl2 was used as
solvents in the calculations. Calculation was performed at CAM-
B3LYP/6-31G(d)/genecp level with Gaussian 09W.
2.6. Photoswitching of the 1O2 Photosensitizing
Ability. To study of the triplet excited-state photoswitching
in applications, the 1O2 photosensitizing effects with the DB-1-
o and the DB-1-c were compared (Figure 7). Recently, the 1O2
photosensitizing ability of Zn(II) porphyrin complexes was
photoswitched with DTE derivatives.47a However, photo-
switching of the photosensitizing was never studied with a
chromophore dyad or triad, that is, with molecular assemblies
based on covalent bond connections. It is well-known that
3
decaying triplet excited state, most probably the DTE-c state,
lies closely to the triplet state of diiodoBodipy moiety.
Previously, it was proposed that the 3DTE-c state has a lifetime
shorter than 10 ns.19 Thus, the triplet excited-state lifetime of
the Bodipy moiety in DB-1-c (40.9 μs) is much shorter than
1
triplet photosensitizer can sensitize O2 in the presence of
dioxygen (O2) upon photoexcitation.2 1,3-Diphenylisobenzo-
1
furan (DPBF) was used as the O2 scavenger. The production
1
3
1
that in DB-1-o (105.1 μs). Both the DTE-c and the DTE-c
can lead to a ring-opening reaction.44 Thus, similar DTE-c →
DTE-o kinetics were observed for DB-1-c and DB-2-c.
2.8. Photoswitched Triplet−Triplet Annihilation Up-
conversion. Previously, DTE was used for modulation of the
fluorescence of organic chromophores,38,39c,51 the triplet state
and phosphorescence of transition metal complexes,19a,b,43,52 or
more recently switching of the singlet oxygen (1O2) production
of porphyrin complexes.47a Recently, we reported the first
example of photoswitching TTA upconversion with two DTE
derivatives in a mixture of the components.47b However,
photoswitching of the TTA upconversion with an inherently
photoswitchable triplet photosensitizer was never reported.47b
This photoswitchable TTA upconversion protocol is more
promising for practical applications. Thus, the photoswitching
TTA upconversion with DB-1 was studied.
of O2 can be followed by monitoring the absorbance changes
at 414 nm.59−61 We found that the DB-1-o is more efficient to
produce 1O2 than DB-1-c (Figure 7a). The spectral changes of
1O2 photosensitizing are clearly shown in Figure 7b and c. The
1O2 quantum yields (ΦΔ) of DB-1-o and DB-1-c were
determined as 87.6% and 36.9%, respectively. These results
demonstrated that the photoswitching of the triplet excited
state of the triad DB-1 can be transduced. The application of
triplet excited-state photoswitching will be promising in areas
such as spatially or temporally resolved photodynamic therapy,
or molecular devices.
2.7. Photophysical Processes of the Triads upon
Photoexcitation. On the basis of the experimental results
obtained with the steady-state and the time-resolved spectros-
copy, the photophysical processes of DB-1 upon photo-
excitation were summarized in Scheme 2. For DB-1-o, selective
excitation into the DTE-o moiety at 254 nm will lead to
ultrafast DTE-o → DTE-c photocyclization (a few pico-
For DB-1, we demonstrated that the triplet state feature of
DTE-1, in the aspect of either triplet state lifetime (τT) or
1
Figure 7. (a) Comparison of singlet oxygen (1O2) generation of DB-1-o and DB-1-c. Deeper slop indicates more efficient O2 photosensitizing
ability. The absorbance decrease traces of DPBF at 414 nm with increasing photoirradiation time in the presence of DB-1-o or DB-1-c (λex = 500
nm). UV−vis absorption spectra of the mixture of (b) DB-1-o/DPBF and (c) DB-1-c/DPBF. Photoirradiation at λex = 535 nm (0.8−1.0 W/m2) in
dichloromethane, c = 1.0 × 10−5 M, 20 °C.
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a
Scheme 2. Simplified Jablonski Diagram Illustrating the Photophysical Processes Involved in (a) DB-1-o and (b) DB-1-c
a
[DTE-o-BDP] stands for DB-1-o. The component at the excited state was designated with red color and an asterisk. The number of the superscript
designated either the singlet or the triplet excited state.
triplet state quantum yields (ΦT, evaluated by the singlet
oxygen quantum yield ΦΔ), can be substantially modulated
upon photoirradiation. Therefore, we investigated the photo-
modulation of TTA upconversion, with DB-1 as the triplet
photosensitizer and perylene as the triplet acceptor (Figure 8).
intensity of DB-1 at 553 nm was also modulated along with
the modulation of the TTA upconversion. The quenching of
the fluorescence of DB-1 at 553 nm is due to the RET effect, as
confirmed by the steady-state fluorescence study (Supporting
Information Figure S26), which is different from a previous
study.47b Such a dual photoswitching ability is more promising
for application because the OFF−ON emission intensity
contrast can be improved.
The photoswitching of the TTA upconversion is reversible.
The TTA upconversion intensity was recovered upon visible
light irradiation (Figure 8b). This is in agreement with the
recovered DB-1-o species in the solution. We demonstrated
that the TTA upconversion can be switched for more than five
times, with only minor loss of the upconversion intensity
(Figure 8c). This result is similar to a recent study with mixture
of the DTE and the TTA upconversion system.47b Photo-
switched TTA upconversion will offer unprecedented temporal
and spatial resolution for luminescence bioimaging or photo-
dynamic therapy studies.51,62−64
2.9. Conclusion. Dithienylethene (DTE)-2,6-diiodoBodipy
triad was prepared, with the aim to photoswitch the triplet
excited state of organic chromophores. The molecular design-
ing rationale is to perturb the intersystem crossing (ISC) of the
2,6-diiodoBodipy moiety with the photoactivated intramolec-
ular resonance energy transfer (RET), with the closed-ring
form of DTE as singlet energy acceptor and 2,6-diiodoBodipy
moiety as the singlet energy donor. To the best of our
knowledge, this is the first time that a covalently linked organic
triplet photosensitizer and DTE photoswitch unit were studied.
The photophysical properties of the triad were studied with
steady-state and time-resolved transient difference absorption
spectroscopy. The open-ring form of DTE (DTE-o, shows no
absorption band in visible spectra region) can be reversibly
photoswitched to the cyclic structure with 254 nm irradiation,
DTE-c, which shows an absorption band at 600 nm; as a result,
the residual fluorescence of the 2,6-diiodoBodipy moiety was
quenched, and the lifetime of the triplet state of the DTE-o-
Bodipy was being greatly reduced from 105.1 to 40.9 μs.
Furthermore, the photoreversion was greatly accelerated upon
selective photoexcitation into the Bodipy absorption band at
535 nm as compared to that of photoexcitation into the DTE-c
moiety (at 600 nm). We observed that the singlet oxygen
quantum yield (ΦΔ) can be reduced from 87.6% to 36.9% upon
photoswitching. Furthermore, the photoswitching of the triplet
excited state was used to switch the triplet−triplet annihilation
upconversion. The study on the photoswitching of the triplet
excited state of organic chromophores is rarely reported, and
Figure 8. Photoswitching of the TTA UC with DB-1 as photo-
sensitizer and perylene as acceptor. (a) Upconversion was observed in
the presence of open-form DB-1 but was switched off by irradiation of
the mixture at 254 nm. (b) Recovery of the TTA UC, upon irradiation
by visible light at >400 nm. (c) The reversibility of the photoswitching
of TTA UC with DB-1 upon alternative UV and visible light
irradiation. (d) The photographs of the photoswitching of TTA UC
with alternative UV and visible light irradiation. Excited with 532 nm
continuous laser (power density: 24.5 mW cm−2), c[DB-1] = 1.0 ×
10−5 M, c[Py] = 1.0 × 10−4 M, in deaerated CH3CN, 20 °C.
For DB-1-o, TTA upconversion was observed upon 532 nm
excitation (Figure 8a). The upconversion quantum yield is
11.6%. This is in full agreement with the results obtained by the
nanosecond transient absorption spectra and the singlet oxygen
photosensitizing experiments. Upon 254 nm photoirradiation
of the mixed solution, however, the upconversion emission
intensity decreased. The upconversion quantum yields
decreased to 0.67% at the photostationary state. This decreased
upconversion efficiency is most likely due to the reduced triplet
state lifetime and the triplet yield in DB-1-c as compared to
that in DB-1-o. Interestingly, the fluorescence emission
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90.3%). The product was used for the next reaction without further
these studies will be useful for exploration of the triplet excited-
state photophysics of organic chromophore, photoresponsive
molecular devices, external stimuli-activatable photodynamic
therapy, etc.
purification.
3.6. Synthesis of 5.
34c
Under Ar atmosphere, 3,5-dibromo-2-
methylthiophene (0.76 g, 3.0 mmol) was dissolved in Et3N (100 mL),
and then Pd(PPh3)4 (93.0 mg, 3% mmol), CuI (26.6 mg, 5% mmol),
and trimethylsilylacetylene (TMSA) (0.4 mL, 2.8 mmol) were added
successively. The mixture was stirred at 45 °C for 7 h. The solvent was
evaporated under reduced pressure. The crude product was purified by
column chromatography to give a yellow solid (350 mg, yield: 45.5%
3. EXPERIMENTAL SECTION
3.1. General Methods. All of the chemicals used in the synthesss
are analytically pure and were used as received. Solvents were dried
and distilled before use for synthesis. Luminescence lifetimes were
measured on a OB 920 fluorescence/phosphorescence lifetime
spectrofluorometer (Edinburgh Instruments, U.K.). Compounds
1,15b 2,15b 3,15b 5,34c and 634c were reported previously. The molecular
1
from TMSA). H NMR (400 MHz, CDCl3): δ 7.02 (s, 1H), 2.36 (s,
3H), 0.23 (s, 9H). TOF HRMS EI+: calcd for ([C10H13SiSBr]+), m/z
= 271.9691, found m/z = 271.9694.
34c
3.7. Synthesis of 6.
Under Ar atmosphere, a solution of
compound 5 (479 mg, 1.76 mmol) in dry ether (25 mL) was cooled to
−78 °C. n-Butyllithium (1.6 M in hexane, 2.2 mL, 3.5 mmol) was
added into the resulting solution, and the temperature was kept at −78
°C for 2 h. Perfluorocyclopentene (0.1 mL, 0.8 mmol) then was added
with a syringe quickly, and the mixture was stirred for another 2 h at
−78 °C. The solution then was warmed to room temperature, and the
mixture was stirred for 2 h and then diluted with diethyl ether. The
mixture was washed with dilute hydrochloric acid solution (1%, 3 × 30
mL), extracted with diethyl ether (3 × 50 mL), and dried over
anhydrous Na2SO4. The filtrate was evaporated under reduced
pressure, and the crude product was purified by column chromatog-
raphy (silica gel, PE as the eluent) to give a bluish white solid (250 mg,
characterization data were presented in the Supporting Information.
15b
3.2. Synthesis of 1.
The mixture of p-hydroxybenzaldehyde
(2.44 g, 20 mmol) and K2CO3 (5.53 g, 40 mmol) in EtOH (30 mL)
was heated at 70 °C for 30 min, and then 1,2-dibromoethane (7.43 g,
40 mmol) was added. The mixture was stirred and refluxed for 6 h.
The reaction mixture was concentrated under reduced pressure to give
a yellow solid. The solid then was dissolved in CH2Cl2, dried over
anhydrous Na2SO4, and filtrated. The solvent was removed under
reduced pressure. The crude product was purified by column
chromatography (silica gel, CH2Cl2:PE = 1:2, v/v) to give compound
1 as a white solid (2.50 g, yield: 60.0%). 1H NMR (400 MHz, CDCl3):
δ 9.90 (s, 1H, J = 8.0 Hz), 7.86 (d, 2H, J = 8.0 Hz), 7.03 (d, 2H, J = 8.0
Hz), 4.38 (t, 2H, J = 4.0 Hz, J = 8.0 Hz), 3.67 (t, 2H, J = 4.0 Hz, J =
8.0 Hz). HRMS: m/z calcd ([C9H9BrO2]+), m/z = 227.9786, found
1
yield: 30.0%). H NMR (400 MHz, CDCl3): δ 7.19 (s, 2H), 1.88 (s,
6H), 0.24 (s, 18H). MALDI-HRMS: m/z calcd ([C25H26F6S2Si2]+),
m/z = 560.0919, found m/z = 560.0890.
m/z = 227.9777.
3.3. Synthesis of 2.
3.8. Synthesis of B-0. To a solution of compound 3 (200 mg, 0.49
mmol) in anhydrous CH2Cl2 (25 mL) was added N-iodosuccinimide
(NIS, 558 mg, 2.48 mmol). The mixture was stirred at 30 °C
overnight. The reaction mixture was concentrated under vacuum, and
the residue was purified by column chromatography (silica gel,
hexane/CH2Cl2, 2:1, v/v). The red band was collected, and the solvent
was removed under reduced pressure to give the product as a red solid
15b
Compound 1 (2.50 g, 11.0 mmol) and
NaN3 (0.90 g, 12.0 mmol) were dissolved in DMF (5 mL), and then
the mixture was refluxed with stirring for 2 h. The solution was cooled
to room temperature (RT) and diluted with CH2Cl2, washed with
water several times, and the organic layer was dried over anhydrous
Na2SO4. The mixture was filtrated, and the solvent was removed under
1
1
reduced pressure to give a yellow oil (1.68 g, yield: 80.4%). H NMR
(280.0 mg, yield: 90.4%). Mp 174−176 °C. H NMR (400 MHz,
(400 MHz, CDCl3): δ 9.89 (s, 1H), 7.86 (d, 2H, J = 8.0 Hz), 7.04 (d,
2H, J = 8.0 Hz), 4.23 (t, 2H, J = 4.0 Hz), 3.65 (t, 2H, J = 4.0 Hz).
MALDI-HRMS: m/z calcd ([C9H9N3O2 + H]+) m/z = 192.0766,
CDCl3): δ 7.18 (d, 2H, J = 8.0 Hz), 7.08 (d, 2H, J = 8.0 Hz), 4.23 (t,
2H, J = 4.0 Hz), 3.69 (t, 2H, J = 4.0 Hz), 2.65 (s, 6H), 1.45 (s, 6H).
MALDI-HRMS: calcd ([C21H20N5OBF2I2]+), m/z = 660.9819, found
m/z = 660.9866.
found m/z = 192.0763.
3.4. Synthesis of 3.
15b
Under N2 atmosphere, a mixture of
3.9. Synthesis of D-1. To a solution of 6 (250 mg, 0.45 mmol) in
methanol/THF (4:1, v:v) was added NaOH (180 mg, 4.5 mmol), and
the mixture was stirred at room temperature for 45 min. The resulting
solution was extracted with dichloromethane. The organic layer was
washed with water, and dried over anhydrous Na2SO4. The solvent
was evaporated under reduced pressure. The crude product was
purified by chromatography to give a bluish white solid (130 mg, yield:
70.0%). Mp, 94−96 °C. 1H NMR (400 MHz, CDCl3): δ 7.24 (s, 2H),
3.36 (s, 2H), 1.89 (s, 6H). TOF HRMS EI+: calcd ([C19H10F6S2]+),
m/z = 416.0128, found m/z = 416.0122.
compound 2 (1.91 g, 10 mmol) and 2,4-dimethylpyrrole (1.88 g, 20
mmol) in dry CH2Cl2 (250 mL) was stirred at rt. TFA (0.1 mL) then
was added into the mixture by syringe, and the mixture was stirred at rt
overnight. 2,3-Dichloro-5,6-dicyanobenzoquinone (DDQ. 1.13 g, 5
mmol) was dissolved in dry DCM (30 mL), and the solution was
added into the mixture in one portion, and then the mixture was
stirred at rt for 7 h. Triethylamine (10 mL) was added dropwise to the
mixture with cooling by ice bath. The reaction mixture was stirred for
another 0.5 h. BF3·Et2O (10 mL) then was added dropwise into the
mixture. The reaction mixture was stirred overnight. The solution was
concentrated under reduced pressure, and water (200 mL) was added.
The mixture was stirred for 24 h. The solution then was extracted with
CH2Cl2 (3 × 100 mL), and the organic layer was dried over anhydrous
Na2SO4. The solvent was evaporated under reduced pressure. The
crude product was purified by column chromatography (silica gel,
CH2Cl2:hexane = 1:1, v/v) to give compound 3 as a red solid (300 mg,
3.10. Synthesis of DB-1. Under Ar atmosphere, D-1 (20.0 mg,
0.05 mmol) and B-0 (66 mg, 0.1 mmol) were dissolved in mixed
solvent CHCl3/MeOH/H2O (8 mL, 6:1:1, v/v). CuSO4·5H2O (7.5
mg) and sodium ascorbate (11.5 mg) then were added, respectively.
The mixture was stirred at 25 °C for 48 h. The reaction mixture then
was washed with water and extracted with CH2Cl2 (3 × 50 mL). The
organic layer was dried over anhydrous Na2SO4 and evaporated under
reduced pressure. The crude product was purified by column
chromatography (silica gel, CH2Cl2:CH3OH = 100:1, v/v) to give a
1
yield: 7.5%). H NMR (400 MHz, CDCl3): δ 7.21 (d, 2 H, J = 12.0
Hz), 7.05 (d, 2H, J = 12.0 Hz), 5.98 (s, 2H), 4.21 (t, 2H, J = 4.0 Hz),
3.67 (t, 2H, J = 4.0 Hz), 2.55 (s, 6H), 1.43 (s, 6H). Mp, 146−148 °C.
MALDI-HRMS: m/z calcd ([C21H22N5OBF2]+), m/z = 409.1885,
found m/z = 409.1859.
3.5. Synthesis of 4. To a mixture of N-bromosuccinimide (7.25 g,
40.7 mmol) and AcOH (50 mL) was added dropwise a solution of 2-
methylthiophene (2 g, 20.4 mmol) in AcOH (20 mL). The mixture
was stirred at rt for 12 h. The mixture was poured into a mixture of
petroleum ether (100 mL) and water (100 mL). The organic layer was
washed with 1 M sodium hydroxide and brine successively, and was
dried over anhydrous Na2SO4. The solvent was removed under
reduced pressure to give the product as 4.67 g of yellow oil (yield:
1
red solid (40.0 mg, yield: 46.6%). Mp, 152−154 °C. H NMR (400
MHz, CDCl3): δ 7.90 (s, 2H), 7.37 (s, 2H), 7.17 (d, 4H, J = 12.0 Hz),
7.04 (d, 4H, J = 12.0 Hz), 4.84 (t, 4H, J = 4.0 Hz), 4.49 (t, 4H, J = 4.0
Hz), 2.63 (s, 12H), 1.98 (s, 6H), 1.40 (s, 12H). 13C NMR (100 MHz,
CDCl3): 158.71, 156.82, 145.23, 141.80, 140.88, 131.59, 131.13,
129.43, 127.96, 125.42, 123.50, 120.46, 115.40, 85.71, 66.38, 49.60,
33.69, 31.93, 24.73, 17.24, 16.04, 14.60. MALDI-HRMS: calcd
([C61H50N10O2B2F10I4S2]−), m/z = 1737.9765, found m/z =
1737.9762.
3.11. Synthesis of DB-2. Under Ar atmosphere, compounds 3
(20.0 mg, 0.05 mmol) and D-1 (40 mg, 0.1 mmol) were dissolved in
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dx.doi.org/10.1021/jo5018662 | J. Org. Chem. 2014, 79, 10855−10866

The Journal of Organic Chemistry
Article
the mixed solvent CHCl3/MeOH/H2O (8 mL, 6:1:1, v/v), and then
CuSO4·5H2O (7.5 mg) and sodium ascorbate (11.5 mg) were added,
respectively. The mixture was stirred at rt for 48 h. The reaction
mixture then was washed with water and extracted with CH2Cl2. The
organic layer was dried over anhydrous Na2SO4. The solvent was
evaporated under reduced pressure, and the crude product was
purified by column chromatography (silica gel, CH2Cl2:CH3OH =
100:1, v/v) to give an orange solid (45.0 mg, yield: 73.4%). Mp, 142−
AUTHOR INFORMATION
■
Corresponding Authors
*E-mail: zhaojzh@dlut.edu.cn. Web: http://finechem2.dlut.
edu.cn/photochem.
*E-mail: xwli@dicp.ac.cn.
Notes
The authors declare no competing financial interest.
1
144 °C. H NMR (400 MHz, CDCl3): δ 7.91 (s, 2H), 7.37 (s, 2H),
7.20 (d, 4H, J = 8.0 Hz), 7.01 (d, 4H, J = 8.0 Hz), 5.98 (s, 4H), 4.84 (t,
4H, J = 4.0 Hz), 4.47 (t, 4H, J = 4.0 Hz), 2.55 (s, 12H), 1.98 (s, 6H),
1.40 (s, 12H). 13C NMR (100 MHz, CDCl3): 158.41, 155.38, 143.15,
142.16, 141.86, 141.31, 131.80, 131.27, 129.61, 128.34, 125.48, 123.53,
121.33, 120.63, 115.18, 66.41, 50.12, 33.99, 32.05, 24.85, 22.82, 14.77.
MALDI-HRMS: calcd ([C61H53N10O2S2B2F10 − H+]−), m/z =
1233.3821; found m/z = 1233.3868.
ACKNOWLEDGMENTS
■
We thank the NSFC (20972024, 21273028, 21421005, and
21473020), the Royal Society (UK) and NSFC (China-UK
Cost-Share Science Networks, 21011130154), the Ministry of
Education (NCET-08-0077 and SRFDP-20120041130005)
Program for Changjiang Scholars and Innovative Research
Team in University [IRT_13R06], the Fundamental Research
Funds for the Central Universities (DUT14ZD226), and
Dalian University of Technology (DUT2013TB07) for
financial support.
3.12. Cyclic Voltammetry (CV). The CV measurements were
performed using a CHI610D Electrochemical workstation (Shanghai,
China). Cyclic voltammograms were recorded at scan rates of 50 mV/
s. A three-electrodes cell was used. Electrochemical measurements
were performed at rt using 0.1 M tetrabutylammonium hexafluor-
ophosphate (TBAP) as supporting electrolyte, after purging with N2.
The working electrode was a glassy carbon electrode, and the counter
electrode was platinum electrode. A nonaqueous Ag/AgNO3 (0.1 M in
CH3CN) reference electrode was contained in a separate compart-
ment connected to the solution via frit. CH3CN was used as the
solvent. Ferrocene was added as the internal reference. The close-form
DTE derivatives were obtained by photoirradiation of open-form
solution with 254 nm UV lamp (0.7−1.0 W/m2) for 1−1.5 h.
3.13. Nanosecond Time-Resolved Transient Difference
Absorption Spectra. The nanosecond time-resolved transient
absorption spectra were detected by laser flash photolysis spectrometer
(LP920, Edinburgh Instruments, UK) and recorded on a Tektronix
TDS 3012B oscilloscope. The lifetime values (by monitoring the decay
trace of the transients) were obtained with the LP900 software. All
samples in flash photolysis experiments were deaerated with argon for
ca. 15 min before measurement, and the gas flow was maintained
during the measurement. PMT detector or ICCD detector (Andor
DH720) was used.
3.14. Triplet−Triplet Annihilation Upconversions. A diode
pumped solid-state (DPSS) continuous laser (532 nm) was used for
the upconversion. The diameter of the 532 nm laser spot was ca. 3
mm. The power of the laser beam was measured with a VLP-2000
pyroelectric power meter. The samples were purged with N2 or Ar for
at least 15 min before measurement. For the upconversion
experiments, the mixed solution of DB-1 (triplet photosensitizer)
and perylene (triplet energy acceptor) was degassed for at least 15 min
with N2. The solution then was irradiated by a 254 nm UV lamp or
visible light (λ > 400 nm) for different time intervals. The upconverted
fluorescence of perylene was observed with a RF5301 spectrofluor-
ometer. To suppress the laser scattering, a black box with a small hole
was put behind the fluorescent cuvette to damp the laser beam (the
small hole as the entrance of the laser into the black box).
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