Photoredox catalytic organic reactions promoted with broadband visible light-absorbing Bodipy-iodo-aza-Bodipy triad photocatalyst

Article abstract of DOI:10.1039/c4ra03631j

The 4-bora-3a,4a-diaza-s-indacene scaffold is known as Bodipy. 2,6-Diiodo-aza-Bodipy (B-1) and the Bodipy-2,6-diiodo-aza-Bodipy triad (B-2) have been used as novel photocatalysts for photoredox catalytic organic reactions with tetrahydroisoquiniline as su

Full text of DOI:10.1039/c4ra03631j

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Photoredox catalytic organic reactions promoted
with broadband visible light-absorbing Bodipy-
iodo-aza-Bodipy triad photocatalyst†
Cite this: RSC Adv., 2014, 4, 36131
*
Song Guo, Renjie Tao and Jianzhang Zhao
The 4-bora-3a,4a-diaza-s-indacene scaffold is known as Bodipy. 2,6-Diiodo-aza-Bodipy (B-1) and the
Bodipy-2,6-diiodo-aza-Bodipy triad (B-2) have been used as novel photocatalysts for photoredox
catalytic organic reactions with tetrahydroisoquiniline as substrate to prepare highly functionalized
organic compounds via a [3 + 2] cycloaddition–aromatization tandem reaction and Cu(I)-catalyzed
alkynylation reactions. In distinction to conventional photocatalysts such as Ru(bpy)3Cl2, Eosin Y or Rose
Bengal, which are based on a mono-visible light-harvesting chromophore profile and show weak absorption in
the visible region, the new photocatalysts are strong visible absorbers (B-1, 3 ¼ 73 000 Mꢀ1 cmꢀ1 at 683 nm).
More importantly, resonance energy transfer (RET) has been used to increase the absorption of photocatalyst
B-2 in the visible region, in which two Bodipy units were used as energy donor and diiodo-aza-Bodipy
as energy acceptor. B-2 shows broadband absorption in the range 400–750 nm (3 ¼ 165 000 Mꢀ1 cmꢀ1
at 504 nm, and 71 000 Mꢀ1 cmꢀ1 at 683 nm). Iodo-aza-Bodipy is more efficient than conventional
photocatalysts such as [Ru(bpy)3]Cl2. Furthermore, the broadband visible light-absorbing B-2 is more
efficient as a photocatalyst than previously reported monochromophore photocatalyst B-4 (diiodo-
Bodipy). Our results will be useful for the design of efficient organic triplet photosensitizers as
photocatalysts for photoredox catalytic organic reactions.
Received 21st April 2014
Accepted 25th July 2014
DOI: 10.1039/c4ra03631j
www.rsc.org/advances
photosensitizers are rarely reported, since the intersystem
crossing (ISC) is oen very weak in neat organic chromo-
phores.22,23 Furthermore, all conventional photocatalysts share
a similar feature, which is that the molecule contains only a
single light-absorbing unit.23 The excitation energy of a broad-
band light source (e.g. white light, including solar light) cannot
therefore be efficiently harvested by these photocatalysts. These
features may diminish their catalytic efficiency, and it is
therefore highly desirable to design new organic photocatalysts
to overcome these issues.
Before discussing the design of triplet photosensitizers as
new photocatalysts, it is useful to have a brief outline of the
mechanisms of photoredox catalytic organic reactions. Single
electron transfer (SET) between photocatalyst and substrate is
the most popular mechanism for photoredox catalytic reac-
tions.1,5,24 Owing to the bimolecular character of the SET, certain
properties of a photocatalyst are crucial:25 (a) light-absorbing
ability, (b) the lifetime of the triplet excited states, and (c)
ease of modifying the molecular structure, in order that the
redox potential of the photocatalyst can easily be tuned to
achieve selectivity or to improve its catalytic efficiency. It is very
difficult, if not impossible, however, to address these challenges
with conventional photocatalysts such as Ru(bpy)3Cl2, Eosin Y
or Rose Bengal.1,23
1. Introduction
Photoredox catalytic organic reactions have attracted a great
deal of attention.1–8 Using this novel synthetic method, highly
functionalized organic compounds can be prepared in good
yield and selectivity under mild reaction conditions. It has been
demonstrated that a wide variety of reactions can be improved
using the photocatalytic approach, including those involving
the generation of radicals, particularly dehalogenation, dehydro-
genation coupling and [3 + 2] cycloadditions.1–6 However, the
photocatalysts used for these organic reactions are restricted to
off-the-shelf compounds such as Ru(bpy)3Cl2,2,9–14 and a few
xanthene dyes such as Eosin Y and Rose Bengal.15–20 These
conventional photocatalysts have disadvantages, for example
Ru(bpy)3Cl2 gives weak absorption in the visible region.21 In the
case of Eosin Y and Rose Bengal it is difficult to modify the
molecular structure, and as a result the redox potentials of the
photocatalysts cannot be readily adapted for different substrates.
On the other hand organic compounds have readily
derivatizable molecular structures, but organic triplet
State Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian
University of Technology, Dalian 116024, China. E-mail: zhaojzh@dlut.edu.cn; Fax:
+86 411-8498-6236
For this reason we chose iodo-aza-Bodipy as the basis of new
photocatalysts. Importantly, we devised resonance energy
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c4ra03631j
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transfer (RET) triplet photosensitizers, which are based on two
different chromophores and show broadband absorption in the
visible region to improve light absorption.26,27 We found that
organic photocatalysts were efficient in two different bench
mark reactions, the tetrahydroisoquinoline-based [3 + 2] cyclo-
addition–aromatization tandem reaction,6,11 and the coupled
Cu(I)-catalyzed alkynylation of tetrahydro-isoquinolines.28 We
demonstrated that dyad photocatalysts with the RET effect are
more efficient than mono-chromophore-based photocatalysts.
Furthermore, even the mono-chromophore iodo-aza-Bodipy
photocatalyst possessed greater photocatalytic efficacy than
conventional photocatalysts such as Ru(bpy)3Cl2. Our results
will be helpful in the design of new organic photocatalysts to
improve photoredox-catalysed organic reactions.
2.2 General procedure for the oxidation and [3 + 2]
cycloaddition–aromatization tandem reaction with
tetrahydroisoquinoline derivatives catalyzed by organic triplet
photosensitizers B-1 and B-2
1 (0.15 mmol), photosensitizers (1 mmol%) and 2 (0.1 mmol) were
dissolved in dichloromethane (DCM; 5 mL). The solution was then
irradiated using a 35 W xenon lamp through a cut-off lter (0.72 M
NaNO2 aqueous solution, which is transparent to light >385 nm).
Thin-layer chromatography (TLC) was used to monitor the prog-
ress of the reaction. N-Bromosuccinimide (NBS; 25 mg) was added
to the reactor. Aer 5 min the solvents were removed and the
residue puried by column chromatography (silica gel, DCM).
2.3 General procedure for photocatalytic aerobic oxidation
and metal-catalyzed alkynylation reactions
To a dried ask were added B-1 or B-2 (1 mol%), tetrahy-
droisoquinoline derivatives (0.1 mmol), and acetylene derivatives
(5 equiv.). The reaction mixture was stirred at room temperature
in air. The solution was then irradiated using a 35 W xenon lamp
through a cut-off lter (0.72 M NaNO2 aqueous solution, which is
transparent to light >385 nm). TLC was used to monitor the
progress of the reaction. Aer the reaction was complete, the
solvent was evaporated under reduced pressure and the residue
puried by column chromatography (silica gel, DCM).
2. Experimental
2.1 Electron spin resonance (ESR) spectroscopy
ESR samples were quantitatively injected into quartz capillaries
for ESR analysis in the dark and illuminated directly in the
cavity of the ESR spectrometer with laser. Organic photocatalysts
and 5,5-dimethyl-1-pyrroline-N-oxide (DMPO;
a superoxide
radical anion O2cꢀ scavenger), or 2,2,6,6-tetramethyl-
piperidine (TEMP; a singlet oxygen 1O2 scavenger) in aerated
CH3CN were stirred together in the dark and the solution then
injected into quartz capillaries. A diode-pumped solid state
3. Results and discussion
3.1 Design rationale of the organic photocatalysts
(DPSS) continuous laser (635 nm) was used for photoirradiation Aza-Bodipy shows much greater red-shied absorption than
of the solution in the quartz capillaries for 120 s.
normal Bodipy.29–32 Iodo-aza-Bodipys have been used as triplet
Scheme 1 Molecular structures of the photocatalysts B-1 and B-2. The reference compounds B-3, B-4, Ru-1 (Ru(bpy)3Cl2), 5,5-dimethyl-1-
pyrroline-N-oxide (DMPO; scavenger for O2cꢀ) and 2,2,6,6-tetramethylpiperidine (TEMP; scavenger for 1O2) are also illustrated.
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photosensitizers for producing singlet oxygen (1O2).31,33,34
However, this chromophore has not been used as a triplet
photosensitizer for photoredox-catalysed organic reactions to
prepare highly functionalized organic compounds. On the other
hand, Bodipy-aza-Bodipy dyads have been reported for uores-
cence resonance energy transfer (FRET) study.35,36 However, in
the absence of the heavy atom effect resulting from iodination,
the production of a triplet excited state is ineffective, and as a
result these FRET dyads cannot be used as photocatalysts in
organic reactions.
Inspired by previous studies, we selected iodo-aza-Bodipy as
photocatalyst for the preparation of highly functionalized
organic compounds. In addition we constructed the Bodipy-aza-
Bodipy triad, in which RET was established, and the singlet
energy acceptor aza-Bodipy was iodinated. The visible light-
harvesting ability of the photocatalyst was thus enhanced by
the RET effect. The Bodipy-aza-Bodipy triad has been reported
previously, but without iodination it is unsuitable for use as a
triplet photosensitizer.36
Fig. 2 Cyclic voltammograms of the dyad photosensitizers (a) B-1 and
B-2, and (b) B-2 and B-3. Scan rates: B-1 and B-3: 0.1 V sꢀ1, B-2: 0.05 V
sꢀ1 20 ꢂC. Ferrocene (Fc) was used as internal reference (E1/2 ¼ +0.40
V (Fc+/Fc) vs. SCE). c ¼ 1.0 ꢁ 10ꢀ3 M. Measurements were made in
deaerated CH3CN solution containing 1.0 mM photosensitizers alone,
or with the ferrocene, 0.10 M Bu4NPF6 as supporting electrolyte;
Ag/AgNO3 reference electrode.
Table
2 Redox potentials of acceptors and the of free-energy
changes (DGET, PET) for the potential intramolecular electron transfer
(with Bodipy unit as electron donor and the iodo-aza Bodipy unit as
electron acceptor in B-2). Anodic and cathodic peak potential were
presented. The potential values of the compounds are vs. standard
hydrogen electrode with Fc as internal reference, for which E1/2 (Fc+/
Fc) ¼ +0.4 V vs. SCE (saturated calomel electrode)a
The synthesis of the organic photocatalysts is based on the
routine chemistry of Bodipy and aza-Bodipy (Scheme 1).27 The
Cu(I)-promoted click reaction was used to connect the two
chromophores together. The products were obtained in satis-
factory yield.
EOx (V)
ERed (V)
DGET (eV)
b
b
B-1
B-2
B-3
1
—
ꢀ0.22
—
3.2 UV-Vis absorption spectra of the photocatalysts
+1.15
+1.15
+1.00
ꢀ0.22, ꢀ1.21
ꢀ1.15
ꢀ0.15c
b
The UV-Vis absorption of the compounds was determined (Fig. 1).
—
Bodipy B-3 gave absorption at 504 nm (3 ¼ 82 000 Mꢀ1 cmꢀ1).
b
b
—
—
a
Cyclic voltammetry in Ar saturated acetonitrile containing a 0.10 M
Bu4NPF6 supporting electrolyte; counter electrode is Pt electrode;
working electrode is glassy carbon electrode; Ag/AgNO3 couple as the
reference electrode. c [Ag+] ¼ 0.1 M. 1.0 mM dyad photosensitizers in
CH3CN, 20 ꢂC. Conditions: 1.0 mM dyad photosensitizers and 1.0 mM
ferrocene in CH3CN, 20 ꢂC. Calculated relative to SCE (saturated
b
calomel electrode). No reduction potential were observed, or no
c
DGcs values were calculated. The value of DGcs, with B-3 unit as
electron donor.
Conversely, aza-Bodipy B-1 gave strong absorption at the greatly
red-shied wavelength of 683 nm (3 ¼ 73 000 Mꢀ1 cmꢀ1). For the
triad B-2, two absorption bands at 504 nm (3 ¼ 165 000 Mꢀ1 cmꢀ1
)
and 683 nm (3 ¼ 71 000 Mꢀ1 cmꢀ1) were observed, which may be
attributed to the Bodipy and the iodo-aza-Bodipy units, respec-
tively. The absorption spectrum of B-2 is almost superimposable
Fig. 1 UV-Vis absorption spectra of B-1, B-2 and B-3. c ¼ 1.0 ꢁ 10ꢀ5 M
in toluene; 20 ꢂC.
Table 1 Photophysical parameters of the organic triplet photosensitizers
3b
lem
FF (%)
sFd/ns
sTe/ms
FD
a
c
l
labs
B-1
B-2
B-3
683
504/683
504
0.73
1.65/0.71
0.82
714
520/714
520
0.21f
0.53g/0.15f
64.2
0.12i
0.06h/2.23i
4.20
7.2 (4.8)
5.5 (3.7)
—
0.68j
0.69j
k
k
—
B-4
537
451
0.87
0.167
556
607
3.6
6.1
0.13
—
104.8
0.45
0.87
0.57
Ru-1m
a
In toluene (1.0 ꢁ 10ꢀ5 M) (nm). b Molar extinction coefficient (105 Mꢀ1 cmꢀ1). c Fluorescence quantum yields (in CH2Cl2). d Fluorescence lifetimes
(in toluene). e Triplet state lifetimes, measured by transient absorption (in toluene; values in parentheses are in CH3CN). f lex ¼ 660 nm, lem ¼ 683
nm. g lex ¼ 480 nm, lem ¼ 504 nm. h lex ¼ 470 nm, lem ¼ 504 nm. i lex ¼ 670 nm, lem ¼ 683 nm. j With methylene blue (MB) as standard (FDD ¼
k
m
0.57), lex ¼ 675 nm. Not applicable. l Quantum yield of singlet oxygen (1O2). In acetonitrile (1.0 ꢁ 10ꢀ5 M) (in nm at 20 ꢂC).
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on the sum of Bodipy (B-3) and aza-Bodipy (B-1), and there is blocks for bioactive natural products. This reaction is particu-
therefore no electronic interaction between the components of B-2 larly interesting, because it means that highly functionalized
in the ground state.36,37 Compared to B-2, B-4 showed only a single organic compounds can be prepared under mild conditions
absorption band at 537 nm.
using a concise synthetic approach. We therefore consider this
A summary of the photophysical parameters of the organic reaction ideal for screening new organic photocatalysts
triplet photosensitizers is given in Table 1.
(Table 3).
With Ru(bpy)3Cl2 as photocatalyst, the reaction times were
The uorescence emission of the compounds was investi-
gated to study the intramolecular energy transfer for B-2 (see long (11–25 h).11 Recently we reported the use of iodo-Bodipy as
ESI, Fig. S52†).27 Bodipy gives intense uorescence at 520 nm photocatalyst for this reaction.38–40 However, these photo-
(FF ¼ 64.2%). However, this emission band was quenched in B- catalysts are based on a conventional mono-chromophore
2, indicating that energy transfer from the Bodipy component to prole, and they show only one major absorption band in the
the iodo-aza-Bodipy component was signicant in B-2.27,35,36 visible spectrum. There is therefore much room for improving
However, the intramolecular electron transfer is a thermody- the visible light-harvesting efficiency of these mono-
namically allowed process (discussed in detail later). The triplet chromophore-based photocatalysts, especially in cases where
excited state lifetime of B-2 (5.5 μs) was only slightly reduced a panchromatic light source is used for the photoredox catalytic
compared to that of B-1 (7.2 μs). We therefore propose that organic reactions, such as white light from a xenon or solar light
electron transfer is not signicant in B-2. Notably, B-2 shows (Table 4, and ESI, Section 7†).26,27
broadband absorption in the region 400–750 nm. The triplet
We studied the [3 + 2] cyclic addition reaction between tet-
excited state lifetimes of B-1 and B-2 are longer than those of the rahydroisoquiniline and maleimide derivatives with B-1, B-2 or
conventional photocatalyst Ru(bpy)3Cl2. All these properties are B-4 as photocatalyst. The reactions proceeded smoothly with
benecial in photoredox catalytic organic reactions.
dichloromethane as solvent, and the reaction time was reduced
to 1–1.5 h, with satisfactory yields in the range 64ꢀ86%
(Table 4). B-4 was previously reported as an efficient photo-
catalyst for [3 + 2] cyclic addition reactions.40 We found that the
yield with B-2 as photocatalyst was higher than that with B-1
and B-4 under similar experimental conditions. Moreover, we
found that the reaction could be carried out with solar light as
excitation source (see ESI, Section 7†).
Next, the substrate tolerance of the [3 + 2] cycloaddition–
aromatization tandem reaction was studied (Table 4, entries 1–
4). N-Aryl succinimide substrates with electron-neutral,
electron-withdrawing or electron-donating substituents were
included. All the substrates gave good yields. In each case
higher yields were observed using B-2 rather than B-1 as
3.3 Electrochemical properties
To study the electrochemical properties of B-1, B-2 and B-3, the
cyclic voltammetry of the compounds was measured (Fig. 2). In
the case of B-1 a reversible reduction at E1/2 ¼ ꢀ0.24 V was
observed, and for B-3 a reversible reduction at E1/2 ¼ ꢀ1.17 V
and a reversible oxidation at E1/2 ¼ +1.13 V were observed. B-2
showed a reversible reduction at E1/2 ¼ ꢀ 0.24 V and an irre-
versible oxidation at E1/2 ¼ +1.13 V, which are similar to the sum
of B-1 and B-3 (Table 2). This result indicates that there is no
electron exchange between the Bodipy and aza-Bodipy compo-
nents of the triad B-2 at the ground state. With approximation
of the T1 state energy level of iodo-aza-Bodipy as 1.2 eV, the free
energy changes (DGꢂ) of the photoinduced intramolecular
electron transfer in B-2 was determined as +0.15 eV according to
Rehm–Weller equation, with aza-Bodipy as electron acceptor. It
is postulated that the electron transfer occurs via the triplet
state of the aza-Bodipy part, since our previous study showed
that the FRET is an ultrafast process.27 This result indicates that
electron transfer from the Bodipy to the iodo-aza-Bodipy unit is
unlikely.
Table 3 Optimization of the reaction conditionsa
The intermolecular electron transfer between the photo-
catalyst and the substrate was studied. Firstly, the electro-
chemical properties of compound 1 was studied. 1 shows an
irreversible oxidation at E1/2 ¼ +1.00 V. The free energy changes
for the electron transfer from compound 1 to the triplet excited
state of B-2 was determined as ꢀ0.04 eV. Details of the elec-
trochemical studies are given in ESI, Section 11.†
Time/h
Solvents
Power/W mꢀ2
Yieldb/%
B-2
B-2
B-2
B-1
B-1
None
B-2
B-4
Ru(bpy)3Cl2
1.0
1.0
1.5
1.0
1.5
2.0
2.0
1.5
3.0
CH3CN
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
300
300
300
300
300
300
0
53.6
75.1
85.5
67.4
76.4
Trace
Trace
79.2
33.6
3.4 Oxidation and [3 + 2] cycloaddition oxidative
aromatization with tetrahydroisoquinoline to construct
pyrrolo[2,1-a]isoquinolines
300
300
a
Reaction conditions: 1 (0.15 mmol), 2 (0.10 mmol), B-1, B-2 or B-4 (1
mol%), air, 35 W Xenon light irradiation (l > 385 nm), NBS (1.2
equiv.), and solvent (5.0 mL), rt. Yield of the isolated product.
Recently a [3 + 2] cycloaddition–aromatization tandem reaction
has been studied, using Ru(bpy)3Cl2 as photocatalyst.11 The
products, pyrrolo[2,1-a]isoquinolines, are important building
b
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Table 4 Oxidation–[3 + 2] cycloaddition–aromatization reactionsa
Entry
1
Substrate
1a
Product
tb [h] (B-1/B-2)
Yieldc (B-1/B-2)
TON
TOF/min
0.84/0.96
1.5/1.5
76/86
76/86
71/86
2
3
1a
1a
1/1
71/82
64/70
1.18/1.43
0.71/0.78
1.5/1.5
64/70
4
5
1a
1b
1.5/1.5
1.5/1.5
72/80
73/84
75/80
73/84
0.83/0.89
0.81/0.93
6
1b
1b
1.5/1.5
1.5/1.5
77/83
70/78
77/83
70/78
0.86/0.92
0.78/0.87
7
a
Reaction conditions: 1a or 1b (0.15 mmol), 2a or 2b (0.10 mmol), B-1 or B-2 (1 mol%) and NBS (1.2 equiv.) were mixed in dichloromethane (5.0 mL),
the mixture was irradiated with 35 W Xe lamp (l > 385 nm), R.T. b Reaction time with photosensitizers. c Yield of isolated products catalyzed with
B-1 and B-2.
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photocatalyst. The improved photocatalytic efficiency can be
The scope of both the tetrahydroisoquinoline and the phe-
attributed to the higher visible light-harvesting ability of B-2 nylalkynes was extended (Table 6). Phenyl alkynes with electron-
(Fig. 1).
neutral, electron-withdrawal or electron-donating substituents
gave satisfactory yields. Different substitutions on the N-atom of
tetrahydroisoquinoline did not hamper the coupled photo-
catalytic-Cu(I) catalytic reaction.
3.5 Coupled photocatalytic aerobic oxidation and Cu(I)-
catalyzed alkynylation reactions: C–C bond formation
Coupled photocatalytic and metal-catalyzed reactions are a
fascinating area of research. In this case the photocatalytic and
metal catalytic cycle must be precisely coupled, otherwise the
reaction will not be successful. Recently a photocatalytic aerobic
oxidation coupled with Cu(I) catalysis was reported. The pho-
tocatalyst was a conventional complex, Ru(bpy)3[PF6]2, which
shows weak absorption in the visible region. As a result, the
reaction times were long (16–36 h).28 In order to study the
photocatalytic properties of organic triplet photosensitizers
such as iodo-aza-Bodipy photocatalysts, B-1 and B-2 were used
as photocatalysts for the coupled photocatalytic oxidation and
the Cu(I)-catalyzed alkynylation of tetrahydroisoquinoline
(Table 5). We found that either B-1 or B-2 were able to promote
the coupled dual-catalytic reaction smoothly, and the reaction
time was reduced to 2–4 h. Previously, with the conventional
photocatalyst Ru(bpy)3[PF6]2, reaction times were as long as 16–
36 h.28 Our results show that under similar experimental
conditions, in alkynylation reaction with tetrahydroisoquino-
line as substrates Ru(bpy)3Cl2 gives much lower yields than that
B-1 and B-2 as photocatalysts (Table 5). We also carried out
control experiments in the absence of either the photocatalyst
(B-1 or B-2) or the metal complex (MeCN)4CuPF6 (entries 4 and 7
in Table 5). The results indicated that the two catalysts are
suitable for coupled catalytic reactions.
3.6 Mechanism studies
The mechanism of the photocatalytic reactions was studied
using electron spin resonance (ESR) spectroscopy (Fig. 3). For
the [3 + 2] cycloaddition–aromatization tandem reaction, it was
proposed that singlet electron transfer (SET) from the tetrahy-
droisoquinoline to the photoexcited state of the photocatalysts
was responsible for initiation of the photocatalytic cycles.
Observation of the superoxide anion radical (O2cꢀ) supports this
putative mechanism.11,38–40 With the new photocatalysts B-1 and
B-2, however, it was necessary to conrm the photocatalytic
mechanism. The reaction mechanism was studied with ESR
spectroscopy (Fig. 3). DMPO and 2,2,6,6-tetramethylpiperidine
(TEMP) were used as scavengers for O2cꢀ and 1O2, respec-
tively.15,40 With DMPO, the ESR signal attributed to its adduct
with O2cꢀ was detected for the mixture of B-1 and substrate 1
upon photoirradiation (Fig. 3(b)), but the signal was much
weaker in the absence of amine substrate 1a, indicating that
electron transfer from the substrate to B-1 was crucial for
formation of O2cꢀ. The experiment with TEMP showed that 1O2
was produced with B-1 in the absence of substrate (1a)
(Fig. 3(c)). With the addition of the substrate 1a no ESR signal
1
1
due to O2 was detected, indicating that production of O2 was
completely inhibited by electron transfer from substrate 1a to B-
1 (Fig. 3(d)). The electron transfer from the amine substrate to
the organic triplet photosensitizer was thus conrmed.
Table 5 Optimization of the reaction conditions of the coupled
The proposed mechanism is presented in Scheme 2. When
the Bodipy-iodo-aza-Bodipy triad (B-2) was photoirradiated with
broadband white light from a xenon lamp, the Bodipy singlet
excited state (1Bod*) and iodo-aza-Bodipy singlet excited state
photocatalytic oxidation and the Cu(I) catalyzed alkynylation reactionsa
1
(1AzaBod*) were produced. Bod* is able to transfer the energy
to the AzaBod state by the RET process. 3AzaBod* is then
populated by ISC. Electron transfer from the tetrahydroisoquino-
line substrate to 3AzaBod* produces an Azabod anion and the
highly reactive iminium species of the tetrahydroisoquinoline. The
Azabod anion returns to the ground state by transferring an elec-
tron to O2, and O2cꢀ is then produced. O2cꢀ can capture two
protons from the tetrahydroisoquinoline substrate cation to form a
Power/
Entry
Catalysis
Time/(h)
Solvents
(W mꢀ2
)
Yieldb/(%)
1
zwitterion and H2O2. This mechanism is supported by H NMR
1
2
3
4
5
6
7
8
B-1
B-1
3.0
3.0
2.0
3.0
3.0
2.0
3.0
3.0
CH2Cl2
EtOAc
300
300
300
300
0
300
300
300
27.4
15
71
(see ESI, Fig. S56†). The zwitterion species reacts with maleic
anhydride derivatives, and is then oxidized by NBS to produce the
target molecules.
The dual catalysis mechanism was also studied using ESR
and NMR. We proposed that the electron transferred from the
tetrahydroisoquinoline substrate to the organic triplet photo-
sensitizer, then Cu(I) catalyzed the coupled reaction between
the iminium species of the tetrahydroisoquinoline and phe-
nylalkyne derivatives (for detailed measurements, see ESI,
Scheme S1†).
B-1
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
B-1c
0
B-1
B-2
None
Ru(bpy)3Cl2
Trace
80
8
43
a
Reaction conditions: 1 (0.10 mmol), 2 (0.8 mmol), photocatalyst
catalysis B-1 or B-2 (1 mol%), air, 35 W Xe light irradiation (l > 385
nm), (MeCN)4CuPF6 (10 mol%), and solvent (2.0 mL), rt. Yield of the
b
isolated product. c Without (MeCN)4CuPF6.
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Table 6 Scope of the photooxidation–alkynylation reaction catalyzed by B-1 and B-2a
Entry
1
Substrate
1a
Product
tb [h] (B-1/B-2)
Yieldc (B-1/B-2)
TONd
59/61
TOFe/min
0.49/0.51
2/2
59/61
2
3
4
1a
1a
1a
3/3
4/4
4/4
49/53f
52/55f
53/57
49/53
52/55
53/57
0.27/0.29
0.22/0.23
0.22/0.24
5
1b
1b
1b
2/2
4/4
4/4
71/80
67/73
61/71f
71/80
67/73
61/71
0.59/0.67
0.28/0.30
0.25/0.30
6g
7
a
Reaction conditions: 1a or 1b (0.10 mmol), 4a–g (0.50 mmol), B-1 and B-2 (1 mol%) and (MeCN)4CuPF6 (0.01 mmol) were mixed in CH3CN (3.0 mL),
the mixture was irradiated with 35 W Xe lamp (l > 385 nm), 20 ꢂC. b Reaction time with photosensitizers. c Yield of isolated products catalyzed with
f
B-1 and B-2. d Turnover number. e Turnover frequency. In dichloromethane. g CuI (0.01 mmol).
photoredox organic reactions. These differ from conventional
photocatalysts, such as Ru(bpy)3Cl2, Eosin Y or Rose Bengal,
4. Conclusions
which contain only a single visible light-harvesting unit, in
showing strong absorption in the visible spectral region.
Bodipy-iodo-aza-Bodipy triad and iodo-aza-Bodipy triplet
photosensitizers have been assessed as photocatalysts in
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photosensitizers as photocatalysts for improving photoredox
catalytic reactions.
Acknowledgements
We are grateful for nancial support to a number of organi-
zations, including the Royal Society (UK) and NSFC
(21073028, 21273028 and the China–UK Cost-Share Science
Networks, 21011130154), the Ministry of Education (SRFDP–
20120041130005), the Program for Changjiang Scholars, the
Innovative Research Team in University (IRT_13R06), the
State Key Laboratory of Fine Chemicals (KF1203), Funda-
mental Research Funds for the Central Universities
(DUT14ZD226) and Dalian University of Technology
(DUT2013TB07).
Fig. 3 (a) ESR spectrum of the mixture B-2 (4.0 ꢁ 10ꢀ4 M) and DMPO
(2.0 ꢁ 10ꢀ2 M); (b) ESR spectrum of the mixture B-2 (4.0 ꢁ 10ꢀ4 M), 1a
(5.0 ꢁ 10ꢀ2 M) and DMPO (2.0 ꢁ 10ꢀ2 M); (c) B-2 (4.0 ꢁ 10ꢀ4 M), TEMP
(0.10 M); (d) B-2 (4.0 ꢁ 10ꢀ4 M), TEMP (0.10 M), 1a (5.0 ꢁ 10ꢀ2 M). In
air-saturated CH3CN. All irradiations were performed at 22 ꢂC for 120 s
under a 635 nm continuous laser (210 mW cmꢀ2).
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