Aza-BODIPY derivatives: Enhanced quantum yields of triplet excited states and the generation of singlet oxygen and their role as facile sustainable photooxygenation catalysts

Article abstract of DOI:10.1002/chem.201202438

A new series of aza-BODIPY derivatives (4 a-4 c, 5 a,c, and 6 b,c) were synthesized and their excited-state properties, such as their triplet excited state and the yield of singlet-oxygen generation, were tuned by substituting with heavy atoms, such as bromine and iodine. The effect of substitution has been studied in detail by varying the position of halogenation. The core-substituted dyes showed high yields of the triplet excited state and high efficiencies of singlet-oxygen generation when compared to the peripheral-substituted systems. The dye 6 c, which was substituted with six iodine atoms on the core and peripheral phenyl ring, showed the highest quantum yields of the triplet excited state (φ_T=0.86) and of the efficiency of singlet-oxygen generation (φ_Δ=0.80). Interestingly, these dyes were highly efficient as photooxygenation catalysts under artificial light, as well as under normal sunlight conditions. The uniqueness of these aza-BODIPY systems is that they are stable under irradiation conditions, possess strong red-light absorption (620-680 nm), exhibit high yields of singlet-oxygen generation, and act as efficient and sustainable catalysts for photooxygenation reactions. Aza-BODIPY derivatives were synthesized and their triplet excited states and yields of singlet-oxygen generation were tuned by halogenation. The dye that was substituted with six iodine atoms showed the highest quantum yields of the triplet excited state (φ_T=0.86) and singlet oxygen (φ_Δ=0.80). These systems were stable to irradiation, had strong NIR absorption, exhibited high yields of singlet oxygen, and acted as catalysts for photooxygenation reactions (see figure). Copyright

Full text of DOI:10.1002/chem.201202438

FULL PAPER
DOI: 10.1002/chem.201202438
Aza-BODIPY Derivatives: Enhanced Quantum Yields of Triplet Excited
States and the Generation of Singlet Oxygen and their Role as Facile
Sustainable Photooxygenation ACTHNUTRGNEUCGN atalysts
Nagappanpillai Adarsh, Madhesh Shanmugasundaram, Rekha R. Avirah, and
Danaboyina Ramaiah*[a]
Abstract:
A
new series of aza-
eration when compared to the periph-
eral-substituted systems. The dye 6c,
which was substituted with six iodine
atoms on the core and peripheral
phenyl ring, showed the highest quan-
tum yields of the triplet excited state
(FT =0.86) and of the efficiency of sin-
glet-oxygen generation (FD =0.80). In-
terestingly, these dyes were highly effi-
cient as photooxygenation catalysts
under artificial light, as well as under
normal sunlight conditions. The
uniqueness of these aza-BODIPY sys-
tems is that they are stable under irra-
diation conditions, possess strong red-
light absorption (620–680 nm), exhibit
high yields of singlet-oxygen genera-
tion, and act as efficient and sustaina-
ble catalysts for photooxygenation re-
actions.
BODIPY derivatives (4a–4c, 5a,c, and
6b,c) were synthesized and their excit-
ed-state properties, such as their triplet
excited state and the yield of singlet-
oxygen generation, were tuned by sub-
stituting with heavy atoms, such as bro-
mine and iodine. The effect of substitu-
tion has been studied in detail by vary-
ing the position of halogenation. The
core-substituted dyes showed high
yields of the triplet excited state and
high efficiencies of singlet-oxygen gen-
Keywords: aza-BODIPY
·
dyes/
pigments · photooxidation · singlet
oxygen · triplet excited states
Introduction
Various non-porphyrinic systems have also been investigat-
ed, including rose bengal, methylene blue, Nile blue, Nile
red, chalcogenopyrylium salts,[4] squaraine dyes,[5] etc. How-
ever, these classes of photosensitizers possess major draw-
backs, such as inherent dark toxicity and low absorption in
the NIR region. In addition to applications in PDT, singlet
oxygen can mediate a wide range of reactions, such as the
photooxygenation of organic substrates.[6] However, the high
energy demand of most common artificial light-sources has
limited the use of photochemical methods in industrial ap-
plications.[7] To overcome this disadvantage, sunlight (either
direct or focused) has received recent attention as a sustain-
able light source for such applications.[7]
Recently, the chemistry of dipyrromethene ligands has at-
tracted much attention owing to their inherent ability to
form stable coordination complexes.[8] The conjugated p-
system in these ligands endows the complexes with favora-
ble optical properties, such as intense absorption with high
molar absorption coefficients (e>104 mꢀ1 cmꢀ1) and moder-
ate-to-quantitative fluorescence quantum yields.[9–10] Among
these compounds, boron complexes, such as 4,4-difluoro-4-
bora-3a,4a-diaza-s-indacenes (abbreviated as BODIPYs),
hold great promise as ideal sensitizers owing to their favora-
ble properties.[11] These systems exhibit strong absorption in
the range 500–600 nm, as well as significant fluorescence
quantum yields and high photostability.[12] In contrast, aza-
dipyrromethenes and their boron complexes (aza-BODI-
PYs) show around 100 nm bathochromically shifted absorp-
tion when compared to BODIPYs; these systems have at-
Singlet oxygen, O2ACHTUNGTRENNUNG
(1Dg), which is the lowest excited electron-
ic state of molecular oxygen, is a reactive species in a wide
range of chemical- and biological processes.[1] It has the abil-
ity to interrupt cellular functions in living organisms and ex-
hibits high chemical reactivity toward most organic mole-
cules owing to the spin-allowed nature of its reactions. The
quantitative generation of singlet oxygen (FD) is one of the
most-important requirements for an efficient photosensitiz-
er,[2] because it is thought that biological damage by singlet
oxygen and the subsequent oxidation of tissue through
direct attack on biological substrates is responsible for the
success of photodynamic therapy (PDT).[3] Most of the pho-
tosensitizers that have either been approved for clinical use
or are in various phases of clinical trials have a common
cyclic tetrapyrrolic structure that is derived from porphyrins.
[a] N. Adarsh, M. Shanmugasundaram, Dr. R. R. Avirah,
Dr. D. Ramaiah
Photosciences and Photonics
Chemical Sciences and Technology Division
National Institute for Interdisciplinary Science and
Technology CSIR, Trivandrum - 695 019, Kerala (India)
Fax : (+91)471-2490186 or 2491712
E-mail: rama@niist.res.in
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201202438.
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tracted less attention and have only been studied in the last
decade.[10,13–16]
from chalcones 1a–1c. The addition of nitromethane to the
chalcones in the presence of diethylamine gave the addition
products (2a–2c) in 75–80% yield. Subsequently, the con-
densation products (3a–3c) were prepared by heating a mix-
ture of compounds 2a–2c and ammonium acetate at reflux
in EtOH for 48 h. The product that precipitated during the
reaction was filtered and recrystallized to yield compounds
3a–3c (40–50%), which had a violet metallic luster. Aza-di-
pyrromethenes 3a–3c were subsequently converted into the
target aza-BODIPY derivatives (4a–4c, 65–70%) by heat-
ing compounds 3a–3c with boron trifluoride diethyl ether-
ate and triethylamine at reflux in toluene for 5 h.
The iodination of aza-BODIPY derivatives 4a–4c was
achieved by using N-iodosuccinimide (NIS) in a mixture of
CHCl3 and acetic acid (3:1). When 2.5 equivalents of NIS
was used, we isolated compounds 5a,c (60–65% yield),
which were only iodinated at the core of the pyrrole ring.
On the other hand, when the iodination was carried out
with 4.5 equivalents of NIS under similar conditions, tetraio-
do aza-BODIPY derivatives 6b,c were obtained in 65–68%
yield (after column chromatography on silica gel). All of
these products were recrystallized from toluene and charac-
terized by using analytical- and spectroscopic data (see the
Supporting Information, Figures S1–S4).
With the objective of developing new and efficient photo-
sensitizers that exhibit near-infrared absorption, we designed
a new series of aza-BODIPY derivatives and investigated
their photophysical properties under different conditions. In-
terestingly, the triplet excited state, as well as the efficiencies
of singlet-oxygen generation, of these aza-BODIPY deriva-
tives could be successfully tuned from about 1% to 86%
through judicial heavy-atom substitution. Moreover, for the
first time, we explored the potential use of these aza-
BODIPY derivatives as catalysts for photooxygenation reac-
tions. When compared to other commonly used sensitizers,
like tetraphenylporphyrin (TPP), rose bengal (RB), and
methylene blue (MB),[18] derivative 6b, which contained
four iodine atoms and two bromine atoms, was quite effi-
cient in photooxygenation reactions, thus making it attrac-
tive as a catalyst for “sustainable photooxidation”.
Results and Discussion
Synthesis and characterization: The strategy that was adopt-
ed for the synthesis of various aza-BODIPY derivatives
(4a–4c, 5a,c, and 6b,c) is shown in Scheme 1. Compounds
4a–4c were synthesized in a facile three-step route starting
Absorption and fluorescence properties: The starting aza-di-
pyrromethenes (3a–3c) showed absorption in the red region
(610-625 nm) with molar extinction coefficients in the range
3–7ꢁ104 mꢀ1 cmꢀ1 (see the Supporting Information, Fig-
ure S5). Interestingly, in the case of aza-BODIPY deriva-
tives 4a–4c, we observed a bathochromic shift of about 50–
60 nm in their absorption spectra (Figure 1A). When com-
Figure 1. A) Absorption and B) fluorescence spectra of representative
aza-BODIPY derivatives 4b, 5c, and 6c in DMSO; excitation wave-
length: 650 nm.
paring the iodinated systems, core-substituted derivatives 5a
and 5c showed blue-shifted absorption maximum at 660 and
670 nm, respectively, whilst derivatives 6b,c, which had sub-
stitution both at their periphery and in core, exhibited an
absorption maximum at 666 and 676 nm, respectively. All of
these derivatives exhibited good solubility in common or-
ganic solvents, such as CHCl3, CH3CN, THF, DMSO, and
DMF.
Figure 1B shows the fluorescence spectra of the corre-
sponding aza-BODIPY derivatives in DMSO. The aza-dipyr-
Scheme 1. Synthesis of aza-BODIPY derivatives 4a–4c, 5a,c, and 6b,c.
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Aza-BODIPY Dyes as Sustainable Photooxygenation Catalysts
FULL PAPER
romethene derivatives (3b,c) showed an emission maximum
between 650 and 665 nm (see the Supporting Information,
Figure S5B). In contrast, aza-BODIPYs 4–6 exhibited emis-
sion in the range 690–770 nm with a large Stokes shift of
about 530–1730 cmꢀ1. These derivatives showed quenched
fluorescence because of the presence of heavy atoms, such
as bromine and iodine. The absorption- and fluorescence
emission properties of these derivatives are summarized in
Table 1.
Table 1. Photophysical properties of aza-BODIPY derivatives 4–6.[a,b]
[d]
[e]
Compound lmax [nm]
(e ꢁ104)[c]
lem
tT
FT
FD
G
[nm]
[ms]
4a
4b
4c
5a
5c
6b
6c
680 (5.9ꢁ0.06) 772
664 (3.8ꢁ0.15) 701
675 (5.2ꢁ0.04) 710
660 (8.3ꢁ0.02) 706
670 (7.1ꢁ0.11) 695
666 (6.9ꢁ0.05) 694
676 (4.9ꢁ0.07) 712
28
44
0.01
0.07
N
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
7.8 0.08
1.9 0.68
1.8 0.70
1.5 0.78
1.6 0.86
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
[a] Average of more than three experiments; [b] in DMSO; [c] in
mꢀ1 cmꢀ1; [d] yields were calculated by using the triplet–triplet energy-
1
transfer method; [e] quantified through scavenging of O2 by DPBF.
Characterization and quantification of the triplet excited
states: To characterize the transient intermediates, such as
triplet excited states, that were involved in these systems, we
performed nanosecond-laser flash photolysis studies.
Figure 2 shows the transient absorption spectra of com-
pounds 6b and 6c in DMSO after laser excitation (355 nm,
10 ns, 50 mJ per pulse). Excitation of compound 6b afforded
transient absorption peaks at 320 and 510 nm with bleach in
the region that corresponded to its ground-state absorption.
The transient intermediate formed from compound 6b
within the laser pulse decayed by a first-order process and
led to recovery of the ground-state absorption (Figure 2A,
inset), thereby ruling out the formation of any permanent
products and/or degradation of the dye under these condi-
tions. This transient absorption was readily quenched by the
dissolved oxygen, thus suggesting that the transient species
may be due to the triplet excited state.[5a,16c] A similar transi-
ent absorption spectrum, with bands at 330 and 520 nm, was
obtained in the case of compound 6c upon laser excitation
at 355 nm (Figure 2B). On the other hand, core-substituted
derivatives 5a and 5c showed a transient absorption maxi-
mum at 460 nm (see the Supporting Information, Figures S6
and S7), whilst non-iodinated dyes 4a, 4b, and 4c showed
transient absorption maxima at 450, 430, and 460 nm, re-
spectively. The triplet quantum yields of these derivatives
were determined (Table 1) by using triplet–triplet energy-
transfer to b-carotene with a tris(bipyridyl)ruthenium(II)
complex as a reference.[19]
Figure 2. A) Transient absorption spectra of compound 6b, following
laser-pulse excitation at 355 nm; time-resolved absorption spectra record-
ed at (a) 0.1, (b) 0.4, (c) 1, (d) 2, (e) 4, and (f) 10 ms. Inset: decay of the
transient at 510 nm. B) Transient absorption spectra of compound 6c, fol-
lowing laser-pulse excitation at 355 nm; time-resolved absorption spectra
recorded at (a) 0.1, (b) 0.5, (c) 1, (d) 2, (e) 5, and (f) 10 ms. Inset: decay of
the transient at 520 nm.
was iodinated (core-substitution), we observed quantitative
yields of the triplet excited states. Thus, we obtained FT =
0.68 and 0.70 for compounds 5a and 5c, respectively. Inter-
estingly, when the core of the pyrrole and the peripheral
phenyl ring were substituted with iodine atoms, we observed
a significant enhancement in the triplet quantum yields
(FT =0.78 and 0.86), with lifetimes of 1.5 and 1.6 ms for com-
pounds 6b and 6c, respectively. Thus, through changing the
substitution pattern, we could tune the triplet excited quan-
tum yields of the aza-BODIPY derivatives from about 1%
to as high as 86% for use in potential applications.
Quantification of singlet-oxygen generation: Enhanced trip-
let quantum yields are favorable for the efficient generation
of singlet oxygen and, hence, we examined the efficiency of
photosensitized singlet-oxygen generation by these systems.
The quantum yields of singlet-oxygen generation were de-
termined by monitoring the photooxidation of 1,3-diphenyli-
sobenzofuran (DPBF) through absorption spectroscopy.[20]
For this purpose, a solution of the aza-BODIPY derivative
and DPBF was irradiated by using a 630 nm long pass over
a time period of 6–600 s and a decrease in the absorption
band (about 10%) of DPBF at 418 nm was observed (Fig-
Non-halogenated dye 4a exhibited a low quantum yield
of the triplet excited state (FT =0.01), with a lifetime of
28 ms, whilst bromo- and iodo derivatives 4b and 4c showed
relatively higher yields of the triplet excited states (about
0.07 and 0.08, respectively). Notably, when the pyrrole ring
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D. Ramaiah et al.
highest quantum yield of about 0.80 when compared with
the standard MB (FD =0.52).[21] The resistance of these dyes
to photobleaching was investigated by irradiating an
oxygen-saturated solution of the dyes for 2 h. We observed
negligible changes in their absorption spectra, thus confirm-
ing the stability of the aza-BODIPY derivatives under these
conditions.
Tuning the singlet-oxygen generation of the aza-BODIPY
derivatives: The efficiency of singlet-oxygen generation of
the aza-BODIPY derivatives could be tuned by suitably in-
corporating heavy atoms, such as bromine and iodine, at
their core, as well as on the peripheral phenyl rings. We ob-
served negligible singlet-oxygen generation for un-substitut-
ed aza-BODIPY 4a, which, upon bromination and iodina-
tion, showed improved yields of singlet-oxygen generation.
The enhancement of singlet-oxygen generation depended on
the halogenation of the parent dye and on the heaviness of
Figure 3. A) Absorption spectra of diphenylisobenzofuran (DPBF) upon
irradiation in the presence of compound 6c; (a) 0 and (b) 8 s (recorded
at 2 s intervals). B) Plot of the change in absorbance of DPBF at 418 nm
versus irradiation time (lirr =630 nm) in the presence of compound 6c
and methylene blue (MB) as a standard in DMSO.
ure 3A). The yields for the generation of singlet oxygen
were calculated by using a standard, methylene blue (MB),
by plotting the optical-density difference (DOD) value of
DPBF against the irradiation time (Figure 3B). The aza-di-
pyrromethene compounds (3a–3c) showed negligible effi-
ciency for singlet-oxygen generation; in contrast, non-negli-
gible singlet-oxygen quantum yields of about 0.009 and
0.012 were observed for dyes 4a and 4b. The quantum yield
for singlet-oxygen generation increased to about 0.02 by the
replacement of bromine with iodine, as in the case of com-
pound 4c.
The aza-BODIPY derivatives 5a,c and 6b,c, which had
core-iodo substitution, showed expectedly good-to-quantita-
tive yields of singlet-oxygen-generation efficacy. For exam-
ple, derivative 5a, which has two iodine atoms at the core of
the pyrrole, exhibited a singlet-oxygen quantum yield of
about 0.65 (see the Supporting Information, Figure S8),
whereas compound 5c, with two core iodine atoms and two
peripheral iodine atoms, showed a quantum yield of about
0.68. On the other hand, dye 6b, which contained four
iodine atoms and two bromine atoms, showed a quantum
yield for singlet-oxygen generation of about 0.70, whereas
derivative 6c, which contained six iodine atoms, showed the
the substituent.
A comparison between the peripheral
phenyl rings of bromo- and iodo-substituted dyes 4b and 4c,
and that of unsubstituted dye 4a, showed marginal improve-
ments of about 0.3% and 1.1% in the efficiency of singlet-
oxygen generation, respectively (Scheme 2). In contrast, the
core-substitution of the aza-BODIPY dyes had a significant
influence on the efficiency of singlet-oxygen generation. For
example, core-iodinated dyes 5a and 5c showed about 64%
and 66% enhancement in the yields of singlet oxygen com-
pared to parent dyes 4a and 4c, respectively.
An interesting observation is the additive effect that was
observed for iodination at the core, as well as on the periph-
eral phenyl rings, of these compounds. For example, com-
pound 6b, which was substituted at its core and also on its
two peripheral phenyl rings, showed about 68% enhance-
ment in singlet-oxygen generation compared to bromo-sub-
stituted dye 4b. Similar observations were made with com-
pound 6c, which contained two iodine atoms in the core and
four iodine atoms on the peripheral phenyl rings. This com-
pound showed the highest additive effect and exhibited an
Scheme 2. Tuning the quantum yield of the triplet excited state (FT) and singlet-oxygen generation (FD) of the aza-BODIPY derivatives.
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Aza-BODIPY Dyes as Sustainable Photooxygenation Catalysts
FULL PAPER
enhancement of about 78% singlet-oxygen-generation effi-
ciency compared to compound 4c. When we compared com-
pounds 6b and 6c, which only differed in the nature of the
peripheral halogen atoms (bromine versus iodine), we ob-
served an approximate 10% enhancement in the singlet-
oxygen yield of compound 6c compared to compound 6b.
Similarly, a difference of about 10–12% was observed be-
tween compounds 5c and 6c (Scheme 2). These results dem-
onstrate that the halogenation of the peripheral phenyl rings
of the aza-BODIPYs only afford a marginal improvement
(about 10–15%) in the quantum yields of singlet-oxygen
generation. In contrast, a significant enhancement of about
65–70% was observed in the efficiency of singlet-oxygen
generation of the aza-BODIPY systems when the core pyr-
role rings were substituted with halogen atoms.
The solution was continuously flushed with a stream of
oxygen throughout the irradiation. The progress of the reac-
tion was monitored at 15 min intervals by TLC and quanti-
fied by NMR spectroscopy and GC by using naphthalene as
an internal standard. After a fixed illumination time of 8 h
under a 200 W Hg lamp, analysis of the reaction mixture by
1
GC, TLC, and H NMR spectroscopy showed approximately
100% conversion into 1,4-naphthaquinone in the presence
of compound 6b as the sensitizer. In contrast, under similar
conditions, common sensitizers, such as TPP, MB, and RB,
showed very low conversion efficiencies of about 40%,
26%, and 20%, respectively (Table 2). When we carried out
the same reaction in direct sunlight, the reaction required
about 1 h for 100% conversion of 1-naphthol into 1,4-naph-
thoquinone to occur in the presence of compound 6b,
whereas TPP, MB, and RB showed less than 50% conver-
sion efficiency under these conditions (Scheme 3). Further-
Photooxygenation reactions with aza-BODIPY derivatives:
Because the aza-BODIPY derivatives showed high efficien-
cy for singlet-oxygen generation, it was of interest to explore
their use as photooxygenation catalysts. Thus, as a represen-
tative example, we carried out the photooxidation of a
common, well-known substrate, 1-naphthol, to afford 1,4-
naphthaquinone, with aza-BODIPY 6b (Table 2). The
Table 2. Photooxygenation of 1-naphthol into 1,4-naphthaquinone by
using various sensitizers and sources of light in MeCN.
Scheme 3. Photooxygenation of 1-naphthol by using aza-BODIPY 6b.
Sensitizer FD
Yield (%)[a]
200 W Hg
lamp[b]
Sunlight (di-
Sunlight (focu-
sed)[d]
rect)[c]
more, to understand the effect of the intensity of light on
the efficiency of the reaction, we carried out the photooxy-
genation reaction under focused sunlight, as reported previ-
ously.[22] Interestingly, aza-BODIPY derivative 6b exhibited
much-improved efficiency and only required about 30 min
irradiation to achieve 100% conversion into 1,4-napthoqui-
none.
6b
0.70 100
0.74 40
0.52 26
0.79 20
100
40
38
100
60
40
TPP
MB
RB
24
20
[a] Average of more than three experiments. The yields of singlet-oxygen
generation with TPP, MB, and RB are taken from reference [7]; energy
of the output light was measured by an Oriel photodiode (Model 7072).
[b] 8 h, 6–8 mJ. [c] 1 h, 20-25 mJ. [d] 0.5 h, 200–220 mJ.
Known sensitizers, such as TPP, MB, and RB, showed
lower conversion efficiencies than compound 6b (Table 2),
although they had comparable singlet-oxygen efficiencies.
One of the reasons for this observation could be due to
their poor photostability under the irradiation conditions.
As reported previously, TPP, MB, and RB showed a non-
negligible decrease in absorbance with irradiation time.[6e]
On the other hand, aza-BODIPY derivative 6b showed high
efficiency and exceptionally high photostability for more
than 8 h (see the Supporting Information, Figure S10), thus
demonstrating its superiority over the currently used sensi-
tizers for photooxygenation reactions. Moreover, photooxi-
dation by aza-BODIPY 6b can be carried out effectively by
using direct sunlight as the light source. Hence, the energy
demand of the entire reaction is extremely low compared to
the standard laboratory oxidation reactions and the reac-
tions with photochemical reactors. Therefore, this reaction
exemplifies an “efficient, sustainable photochemical reac-
tion” and aza-BODIPY derivative 6b as an attractive “sus-
tainable catalyst” for photooxygenation reactions.
reason for choosing compound 6b was due to its high photo-
stability and good solubility in common organic solvents
among the aza-BODIPY derivatives, as well as owing to its
comparable singlet-oxygen-generation efficiency to that of
standard sensitizers, such as TPP, MB, and RB.[7] The photo-
oxygenation reaction was carried out with various sensitizers
and aza-BODIPY 6b under identical conditions with three
different irradiation light sources: 1) 200 W Hg lamp
(energy of the output light: about 6–8 mJ); 2) direct sunlight
(Trivandrum, India, November–December 2011, 8.58N
76.98E, 5 m above sea level, energy of output light: about
20–25 mJ); and 3) focused sunlight by using a convex lens
(location as above and energy of output light: about 200–
220 mJ). The progress of the photooxygenation reaction was
analyzed by thin layer chromatography, NMR spectroscopy,
and GC.
The preparative photooxygenation reaction was carried
out by irradiating a solution of 1-naphthol (0.2 mm) in
MeCN (3 mL) in the presence of various sensitizers (2 mm).
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D. Ramaiah et al.
DAbod
DAref
k
k
ꢀkr0ef
bod
obs
kobs ꢀkb0od
ref
obs
bod
obs
Conclusion
ꢀbTod ¼ ꢀrTef
ꢂ ꢂꢂ
ð1Þ
bod
ref
k
ꢀk
obs
In conclusion, we have developed a series of new aza-
BODIPY dyes by using a high-yielding synthetic route and
investigated their efficiencies as catalysts for photooxygena-
tion reactions. All of these derivatives showed strong ab-
sorption in the red region with high molar-extinction coeffi-
cients. The substitution of these derivatives with heavy
atoms at their core, as well as at their periphery, resulted in
a significant enhancement in their intersystem-crossing effi-
ciency. The triplet excited state and the efficiency of singlet-
oxygen generation of these systems could be tuned from
about 1% to as high as 86%. For the first time, we investi-
gated the potential use of these aza-BODIPY derivatives as
catalysts in photooxygenation reactions, in particular, in the
conversion of 1-naphthol into 1,4-naphthaquinone by using
artificial light, as well as natural sunlight. Our results dem-
onstrate that these new aza-BODIPY derivatives exhibit fa-
vorable photophysical properties, including strong NIR ab-
sorption and high singlet-oxygen-generation efficiency; thus,
these compounds have potential applications as sensitizers
in photodynamic therapy and as sustainable photooxygena-
tion catalysts.
The direct excitation of b-carotene did not result in any significant trip-
let-formation under these experimental conditions, because of negligible
ref
yield of the triplet state. The FT value in MeOH for [RuACTHNUTRGNEUNG
(bpy)3]2+ was
taken to be unity. The FT data that were obtained in this manner are reli-
able to the extent to which the assumption regarding 100% efficiency of
energy-transfer to b-carotene is valid.
Quantification of singlet-oxygen generation: Singlet-oxygen-generation
studies were performed with a 200 W mercury lamp (model 3767) light
source on an Oriel optical bench (model 11200) with a grating monochro-
mator (model 77250). The intensity of the light was kept constant
throughout the irradiations by measuring the output with an Oriel photo-
diode detection system (model 7072). Quantum yields for singlet-oxygen
generation (in DMSO) were determined by monitoring the photooxida-
tion of DPBF that was sensitized by the aza-BODIPY derivatives. DPBF
is a convenient acceptor because it absorbs in a region of dye-transparen-
cy and rapidly scavenges singlet oxygen to give colorless products. This
reaction occurs with little-or-no physical quenching. The quantum yields
of singlet-oxygen generation were measured at low dye concentrations
(optical density: 0.2–0.3 at irradiation wavelengths>630 nm) to minimize
the possibility of singlet-oxygen quenching by the dyes. The photooxida-
tion of DPBF was monitored between 2 s and 2 min, depending on the
efficiency of the dye sensitizer. No thermal recovery of DPBF (from a
possible decomposition of the endoperoxide product) was observed
under these experimental conditions. The quantum yields of singlet-
oxygen generation (FACTHNUTRGNEUNG
[1O2]) were calculated by a using relative method
with optically matched solutions and by comparing the quantum yield of
the photooxidation of DPBF that was sensitized by the dye of interest to
the quantum yield of MB (FACHTNUTGRNEUNG
[1O2]=0.52) as a reference compound[21] ac-
cording to Equation (2), where superscripts “bod” and “MB” denote aza-
BODIPY derivatives and MB, respectively, FD is the quantum yield of
singlet oxygen, “m” is the slope of a plot with a difference in the change
in the absorbance of DPBF (at 418 nm) with the irradiation time, and
“F” is the absorption correction factor, which is given by F=1–10ꢀOD
(OD at the irradiation wavelength).
Experimental Section
General techniques: Melting points were determined on a Mel-Temp II
melting point apparatus.[23] IR spectra were recorded on a Perkin–Elmer
Model 882 IR spectrometer. The electronic absorption spectra were re-
corded on Shimadzu UV-3101 or -2401 PC UV/Vis/NIR scanning spectro-
photometer. The fluorescence spectra were recorded on a SPEX-Fluoro-
mbodFMB
mMBFbod
1
log F112X spectrofluorimeter. H NMR and 13C NMR spectroscopy were
ꢀDbod ¼ ꢀDMB
ꢂ ꢂꢂ
ð2Þ
performed on a 500 MHz Bruker advanced DPX spectrometer. All sol-
vents were purified and distilled before use. The fluorescence quantum
yields were measured according to relative methods by using optically
dilute solutions.
Photooxygenation reactions: Photolysis was performed with a 200 W
Mercury lamp (model 3767) light source. The intensity of the output light
was kept constant (5–6 mJ) throughout the irradiations by measuring the
output with an Oriel photodiode detection system (model 7072). Appro-
priate filters were used to avoid the photobleaching of the dyes owing to
direct exposure to UV light. All of the experiments gave satisfactory re-
sults within reasonable periods of time without the formation of any no-
ticeable side-products. The Oriel photodiode detection system (model
7072) was also used to measure the output intensity of the sunlight. All
the sensitizers were kept in round-bottomed flasks made of glass, which
can absorb UV light and could therefore prevent the photobleaching of
the dyes.
Quantification of the triplet-state properties: Nanosecond-laser flash
photolysis experiments were performed on an Applied Photophysics
model LKS-20 laser kinetic spectrometer by using an OCR-12 Series
Quanta Ray Nd:YAG laser. The analyzing- and laser beams were fixed
at right angles to one another. The laser energy was 64 mJ at 355 nm.
The triplet yields (FT) of the aza-BODIPY derivatives were measured
according to a literature procedure of energy-transfer to b-carotene by
using [Ru
ACHTUNGTRENNUNG
optically matched (355 nm) solutions of [RuACHTUNGTRENNUNG
BODIPY derivatives were mixed with a known volume of b-carotene sol-
ution (end concentration of b-carotene: 2.0ꢁ10ꢀ4 m). The transient ab-
sorbance (DA) of the b-carotene triplet, which was formed by energy-
Materials and methods: Starting materials 3,5-dimethoxybenzaldehyde,
4-bromoacetophenone, 4-iodoacetophenone, nitromethane, diethylamine,
ammonium acetate, boron trifluoride diethyl etherate, triethylamine, N-
iodosuccinimide (NIS), methylene blue (MB), rose bengal (RB), 1-naph-
thol, and b-carotene were purchased from Aldrich and S.D. Fine Chemi-
cals, India. 1,3-Diphenylisobenzofuran (DPBF) was recrystallized from a
mixture of EtOH/CHCl3 (1:1). 3-(3,5-Dimehoxyphenyl)-1-phenylprop-2-
en-1-one (1a; m.p. 81–828C, mixture m.p. 80–828C),[24] 4-bromophenyl-3-
(3,5-dimethoxyphenyl)prop-2-n-1-one (1b; m.p. 86–878C, mixture m.p.
86–888C),[24] and 4-iodophenyl-3-(3,5-dimethoxyphenyl)prop-2-n-1-one
(1c; m.p. 86–878C, mixture m.p. 86–888C),[24] 3-(3,5-dimethoxyphenyl)-4-
nitro-1-phenylbutan-1-one (2a, viscous liquid),[16c] and 1-(4-bromophen-
yl)-3-(3,5-dimethoxyphenyl)-4-nitrobutan-1-one (2b, viscous liquid)[16c]
were synthesized according to modified literature procedures.
transfer from [RuACHTUNGTRENNUNG
(bpy)3]2+ or the aza-BODIPY triplet, was monitored at
510 nm. Comparison of the plateau absorbance, following the completion
of sensitized-triplet formation, properly corrected for the decay of the
donor triplets in competition with energy-transfer to b-carotene, enabled
us to estimate the FT values of the BODIPY derivatives, according to
Equation (1), where superscripts “bod” and “ref” designate the different
(bpy)3]2+
aza-BODIPY derivatives and [RuACHTUNGTRENNGU , respectively, kobs is the
pseudo-first-order rate constant for the growth of the b-carotene triplet,
and k0 is the rate constant for the decay of the donor triplets (in the ab-
sence of b-carotene) in solutions that contained [RuACTHNUTRGNEUNG
(bpy)3]2+ or the aza-
BODIPY dye at the same optical density (OD) as those used for sensiti-
zation.
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6
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ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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ÝÝ
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Aza-BODIPY Dyes as Sustainable Photooxygenation Catalysts
FULL PAPER
Synthesis of compound 2c: To a solution of compound 1c (5.76 mmol) in
MeOH (80 mL) was added diethyl amine (4 mL) and nitromethane
(2 mL) and the mixture was heated at reflux for 24 h. The mixture was
neutralized with 1 m HCl and extracted with CHCl3. Removal of the sol-
vent gave a residue that was purified by column chromatography on
silica gel (EtOH/n-hexane, 1:9) to afford compound 2c.
General procedure for the synthesis of compounds 5a,c: N-Iodosuccini-
mide (146 mg, 0.65 mmol) was added to a solution of compounds 4a,c
(0.26 mmol) in a mixture of CHCl3 and acetic acid (40 mL, 3:1) and the
mixture was stirred at 308C for 4–5 h. The reaction mixture was washed
with sodium thiosulfate and sodium bicarbonate solution and extracted
with CHCl3. Removal of the solvent gave a residue that was purified by
column chromatography on silica gel (MeOH/CHCl3, 1:9) to give com-
pounds 5a,c.
5a: 60% yield; m.p. 236–2378C; 1H NMR (CDCl3, 500 MHz): d=7.55
(4H, m), 7.44 (2H, dd, J=2 Hz), 7.40 (6H, m), 7.29 (2H, d, J=2 Hz),
6.89 (2H, d, J=8.5 Hz), 3.89 (6H, s), 3.64 ppm (6H, s); 13C NMR
(CDCl3, 125 MHz): d=160.05, 149.6, 147.4, 146.5, 143.9, 130.2, 129.5,
129.2, 126.9, 123.8, 123.2, 113.1, 109.5, 55.0, 54.7 ppm; IR (KBr): n˜ =1598,
1485, 1379, 1263 cmꢀ1; MS (FAB): m/z calcd for C36H28BF2I2N3O4: 870.03
[M+H]+; found: 871.20.
2c: 75% yield; 1H NMR (CDCl3, 500 MHz): d=7.83 (2H, d, J=8.5 Hz),
7.62 (2H, d, J=8.5 Hz), 6.40 (2H, d, J=2 Hz), 6.39 (1H, t, J=2.5 Hz),
4.79 (1H, q), 4.67 (1H, q), 4.15 (1H, m), 3.76 (6H, s), 3.43 ppm (2H, m);
13C NMR (CDCl3, 125 MHz): d=196.2, 161.2, 141.3, 138.1, 135.6, 129.4,
101.7, 99.3, 79.3, 53.4, 41.4, 39.3 ppm; IR (KBr): n˜ =1685, 1586,
1208 cmꢀ1
456.513.
; MS (FAB): m/z calcd for C18H18INO5: 456.032; found:
General procedure for the synthesis of compounds 3a–3c: Nitromethane
derivatives 2a–2c (2.45 mmol) and ammonium acetate (7.6 g, 95 mmol)
were dissolved in EtOH (20 mL) and heated at reflux for 48 h. The pre-
cipitated product was filtered, washed with cold EtOH, dried, and recrys-
tallized from CHCl3 to give compounds 3a–3c as violet crystals with a
metallic luster.
3a: 40% yield; m.p. 220–2218C; 1H NMR (CDCl3, 500 MHz): d=7.97
(4H, d, J=8 Hz), 7.62 (2H, d, J=8 Hz), 7.56 (6H, d, J=7 Hz),7.49 (2H,
d, J=7 Hz), 7.11 (2H, s), 6.94 (2H, d, J=8.5 Hz), 3.95 (6H, s), 3.76 ppm
(6H, s); 13C NMR (CDCl3, 125 MHz): d=154.8, 149.5, 149.3, 148.8, 142.7,
132.2, 129.9, 129.1, 127.0, 126.5, 121.9, 114.0, 112.3, 111.0, 56.0, 55.8 ppm;
IR (KBr): n˜ =3062, 1597, 1537, 1348 cmꢀ1; MS (FAB): m/z calcd for
C36H31N3O4 570.231; found: 570.232.
3b: 50% yield; m.p. 245–2468C; 1H NMR (CDCl3, 500 MHz): d=7.77
(4H, d, J=8.5 Hz), 7.67 (4H, d, J=8.5 Hz), 7.10 (2H, s), 7.08 (4H, m),
6.48 (2H, t), 3.69 ppm (12H, s); 13C NMR (CDCl3, 125 MHz): d=161.4,
160.6, 156.4, 154.0, 149.7, 143.3, 140.8, 138.1, 135.2, 132.4, 131.8, 130.9,
128.6, 127.8, 124.5, 123.6, 117.22, 115.5, 106.9, 105.5, 100.9, 100.6,
55.5 ppm; IR (KBr): n˜ =2999, 1589, 1456, 1338, 1278 cmꢀ1; MS (FAB): m/
z calcd for C36H29Br2N3O4: 727.442; found: 727.453.
3c: 40% yield; m.p. 236–2378C; 1H NMR (CDCl3, 500 MHz): d=7.87
(4H, d, J=8.5 Hz), 7.63 (4H, d, J=8.5 Hz), 7.10 (2H, s), 7.08 (4H, d, J=
2.5), 6.48 (2H, s), 3.69 ppm (12H, s); 13C NMR (CDCl3, 125 MHz): d=
161.4, 156.6, 137.9, 128.8, 117.2, 105.5, 95.6, 55.6, 55.3 ppm; IR (KBr): n˜ =
2997, 1681, 1591, 1454 cmꢀ1; MS (FAB): m/z calcd for C36H29I2N3O4:
823.035; found: 823.141.
5c: 65% yield; m.p. 263–2648C; 1H NMR (CDCl3, 500 MHz): d=7.86
(4H, d, J=8.5 Hz), 7.79 (4H, d, J=9 Hz), 7.13 (2H, s), 6.68 (2H, d, J=
2.5 Hz), 6.44 (2H, d, J=2.5 Hz), 3.88 (6H, s), 3.67 ppm (6H, s);
13C NMR (CDCl3, 125 MHz): d=159.5, 158.1, 156.7, 146.3, 145.3, 137.4,
137.0, 137.1, 129.7, 123.1, 108.3, 98.9, 98.6, 97.7, 79.3, 55.6, 54.7 ppm; IR
(KBr): n˜ =1579, 1508, 1423, 1321 cmꢀ1
C36H26BF2I4N3O4: 1121.04; found: 1121.38.
; MS (FAB): m/z calcd for
General procedure for the synthesis of compounds 6b,c: N-iodosuccini-
mide (260 mg, 1.16 mmol) was added to a solution of compounds 4b,c
(0.26 mmol) in a mixture of CHCl3 and acetic acid (40 mL, 3:1) and the
mixture was stirred at 308C for 10 h. The reaction mixture was washed
with sodium thiosulfate and sodium bicarbonate solution and extracted
with CHCl3. Removal of the solvent gave a residue that was purified by
column chromatography on silica gel (MeOH/CHCl3, 1:9) to give com-
pounds 6b,c.
6b: 65%; m.p. 308–3108C; 1H NMR (CDCl3, 500 MHz): d=7.93 (3H, d,
J=8.5 Hz), 7.63 (5H, d, J=11.5 Hz), 6.64 (1H, s), 6.37 (2H, s), 5.30 (1H,
s), 3.89 ppm (12H, s); 13C NMR (CDCl3, 125 MHz): d=159.0, 157.9,
153.0, 144.3, 142.7, 132.0, 129.5, 125.6, 80.2, 56.9 ppm; IR (KBr): n˜ =1587,
1506, 1477, 1336, 1278 cmꢀ1
;
MS (FAB): m/z calcd for
C36H24BBr2F2I4N3O4: 1279.642; found: 1279.628.
6c: 68%; m.p. 305–3078C; 1H NMR ([D6]DMSO, 500 MHz): d=7.97
(4H, t), 7.79 (2H, d, J=8.5 Hz), 7.39 (2H, d, J=8 Hz), 7.27 (1H, s), 6.70
(2H, d, J=9.5 Hz), 5.76 (1H, s), 3.89 ppm (12H, s); 13C NMR (CDCl3,
125 MHz): d=159.4, 159.3, 154.9, 153.7, 145.6, 143.8, 143.1, 137.6, 137.4,
136.5, 131.8, 130.9, 125.0, 101.5, 94.7, 78.9, 78.5, 55.6, 55.5 ppm; IR (KBr):
n˜ =1556,1462,1409,1261 cmꢀ1; MS (FAB): m/z calcd for C36H24BF2I6N3O4:
1373.61; found: 1373.92.
General procedure for the synthesis of compounds 4a–4c: A solution of
compounds 3a–3c (0.45 mmol) in dry toluene (80 mL) was treated with
triethylamine (0.8 mL, 4.6 mmol) and the mixture was stirred for 10 min
at 308C. Next, boron trifluoride diethyl etherate (1 mL, 8.13 mmol) was
added and the reaction mixture was heated at 808C for 4 h. The solvent
was evaporated, washed with water (2ꢁ50 mL), and extracted with
CHCl3. Removal of the solvent gave a residue, which was purified by
column chromatography on silica gel (CH2Cl2/n-hexane, 1:1) to afford
the products (4a–4c) as metallic brown solids.
Acknowledgements
4a: 75% yield; m.p. 239–2408C; 1H NMR (CDCl3, 500 MHz): d=8.09
(4H, s), 7.85 (2H, q, J=8.5 Hz), 7.67 (2H, s), 7.57 (6H, s), 7.50 (2H, s),
7.14 (2H, d, J=8.5 Hz), 3.87 (6H, s), 3.78 ppm (6H, s); 13C NMR
(CDCl3, 125 MHz:) d=158.1, 150.8, 148.9, 144.7, 143.5, 131.1, 130.9,
129.3, 128.5, 124.7, 122.9, 118.9, 112.5, 55.7, 55.6 ppm; IR (KBr): n˜ =1595,
1500, 1452, 1267, 1122 cmꢀ1; MS (FAB): m/z calcd for C36H30BF2N3O4:
617.458; found: 617.456.
This work was supported by the Department of Science and Technology
(DST), the Government of India, and the National Institute for Interdis-
ciplinary Science and Technology (NIIST), CSIR, Trivandrum. This
report is contribution number PPG-323 from the CSIR-NIIST, Trivan-
drum.
4b: 80% yield; m.p. 284–2858C; 1H NMR (CDCl3, 500 MHz): d=7.90
(4H, d, J=8.5 Hz), 7.63 (4H, d, J=8.5 Hz), 7.12 (4H, s), 6.96 (2H, s),
6.54 (2H, s), 3.76 ppm (12H, s); 13C NMR (CDCl3, 125 MHz): d=160.8,
158.2, 145.7, 144.9, 133.8, 132.0, 131.0, 130.2, 126.0, 119.6, 107.3, 102.3,
99.9, 55.5 ppm; IR (KBr): n˜ =1587, 1473, 1398, 1261 cmꢀ1; MS (FAB): m/
z calcd for C36H28BBr2F2N3O4: 775.243; found: 775.244.
4c: 82% yield; m.p. 259–2618C; 1H NMR (CDCl3, 300 MHz): d=7.85
(4H, d, J=8.7 Hz), 7.76 (4H, d, J=8.7 Hz), 7.12 (4H, d, J=2.1 Hz), 6.96
(2H, s), 6.54 (2H, s), 3.75 ppm (12H, s); 13C NMR (CDCl3, 125 MHz):
d=160.9, 158.4, 145.8, 144.9, 137.9, 133.9, 131.0, 130.9, 130.8, 119.6, 107.3,
102.3, 98.5, 55.3 ppm; IR (KBr): n˜ =1585, 1512, 1489, 1284 cmꢀ1; MS
(FAB): m/z calcd for C36H28BF2I2N3O4: 869.024; found: 869.364.
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Received: July 9, 2012
Published online: && &&, 0000
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www.chemeurj.org
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 0000, 00, 0 – 0
ÝÝ
These are not the final page numbers!

Aza-BODIPY Dyes as Sustainable Photooxygenation Catalysts
FULL PAPER
Aza-BODIPY derivatives were synthe-
sized and their triplet excited states
and yields of singlet-oxygen generation
were tuned by halogenation. The dye
that was substituted with six iodine
atoms showed the highest quantum
yields of the triplet excited state
Photooxidation
N. Adarsh, M. Shanmugasundaram,
R. R. Avirah, D. Ramaiah* . &&&& — &&&&
Aza-BODIPY Derivatives: Enhanced
Quantum Yields of Triplet Excited
States and the Generation of Singlet
Oxygen and their Role as Facile
Sustainable Photooxygenation
(FT =0.86) and singlet oxygen
(FD =0.80). These systems were stable
to irradiation, had strong NIR absorp-
tion, exhibited high yields of singlet
oxygen, and acted as catalysts for pho-
tooxygenation reactions (see figure).
AHCTUNGTRENNCGUN atalysts
Aza-BODIPY systems…… possess strong red
absorption, exhibit high triplet excited states, and
singlet-oxygen generation yields. The aza-BODIPY
systems synthesized by D. Ramaiah et al. in their
Full Paper on page && ff., are stable under irradia-
tion conditions, act as green catalysts for photooxy-
genation reactions, and are efficient sensitizers for
photodynamic therapy.
Chem. Eur. J. 2012, 00, 0 – 0
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
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These are not the final page numbers!