BODIPY-fused porphyrins as soluble and stable near-IR dyes

Article abstract of DOI:10.1002/chem.201100619

Beneficial fusion! Boron dipyrromethene (BODIPY) has been successfully fused to a porphyrin core by an intramolecular, oxidative, cyclodehydrogenation reaction. The newly synthesized dyes 1 and 2 have good solubility and exhibit intensified near-IR absorp

Full text of DOI:10.1002/chem.201100619

DOI: 10.1002/chem.201100619
BODIPY-Fused Porphyrins as Soluble and Stable Near-IR Dyes**
Chongjun Jiao,[a] Lijun Zhu,[a] and Jishan Wu*[a, b]
Dye chemistry is believed to be one of the most explored
ate substitution. As a result, the obtained, fused systems
normally possess high electron density, which inevitably
makes them unstable upon long-term exposure to air and
light. In our previous work,[8] the replacement of the rela-
tively electron-rich, N-annulated perylene unit in a pery-
lene-fused porphyrin compound with a perylene monoimide,
which contains an electron-withdrawing dicarboxylic imide
group, was found to stabilize the highly conjugated system.
However, the introduction of electron-deficient units leads
to decreased reactivity, which makes fusion reactions or
fusion of more building blocks, to achieve longer NIR ab-
sorption, difficult. Consequently, how to balance reactivity
with stability remains a long-standing challenge for aromat-
ic, ring-fused, porphyrin systems.
Boron dipyrromethene (BODIPY) is a strongly UV/Vis-
absorbing, small dye with a relatively sharp fluorescence-
emission peak with high quantum yield.[9] Its unusual and re-
markable optical and chemical properties enable BODIPY
to play important roles in the fields of luminescent devices,
chemical sensors, biological labeling, and photovoltaic devi-
ces. Similarly to porphyrin, BODIPY also has some undesir-
able characteristics, such as relatively shorter wavelength ab-
sorption and emission in the UV/Vis range. Although three
areas in industrial organic chemistry. An area of recently in-
creasing interest is the design and synthesis of so-called
near-infrared (NIR) dyes,[1,2] which function (absorb and/or
emit light) in the NIR spectral region, ranging from 700 to
2000 nm, with diverse applications in solar cells,[3] nonlinear
optics,[4] and bioimaging.[5] For instance, in organic solar
cells, the materials should have good light-harvesting capa-
bility, not only in the UV/Vis spectral range, but also at the
NIR range, because 50% of the sunlight radiation energy is
in the infrared region. In in vivo bioimaging, NIR fluores-
cent dyes have obvious advantages over traditional visible
dyes, because biological samples have low background fluo-
rescence and a concomitant high signal-to-noise ratio in the
NIR region. Moreover, NIR light can penetrate sample ma-
trices deeply due to low light scattering.
Porphyrin is one of the most widely investigated, nitro-
gen-containing, macrocyclic, aromatic compounds. Porphyr-
ins and metalloporphyrins generally have an intense absorp-
tion in the 400–450 nm region, called the Soret band, and
relative weak absorptions in comparison with the Soret
band in the 500–700 nm region, named Q bands. Due to the
insufficient absorption property of the porphyrin monomer
in the visible and in particular in the NIR region, efforts
have been made toward the design and synthesis of p-ex-
tended porphyrins, such as porphyrin tapes.[6] To fuse poly-
cyclic aromatic hydrocarbons (PAHs, e.g., pyrene, anthra-
cene, azulene, and perylene)[7] to the porphyrin core is an-
other important option for such a purpose and this strategy
has proven to fill the aforementioned research gap. A gener-
al method towards PAH-fused porphyrins is the intramolec-
ular ring cyclization of singly linked porphyrin–PAH dyads;
this usually requires activation of the PAH unit by appropri-
strategies for achieving
a bathochromic shift for the
BODIPY core have been investigated: 1) replacement of
pyrrole with an isoindole unit,[10] 2) functionalization at the
a and/or meso position to generate a “push-pull” motif,[11]
and 3) preparation of aza-BODIPY dyes.[12] Fusing the
BODIPY core to the other aromatic rings has rarely been
reported.[13] In the past year we have been working on the
synthesis of BODIPY-fused porphyrins, such as compounds
1 and 2 (Scheme 1). The design is based on the following
considerations: 1) The BODIPY core provides a nice “zig-
zag” edge geometry for fusion of an aromatic unit (e.g., por-
phyrin) to the meso and b positions. Because perylene-fused
[a] C. Jiao, L. Zhu, Prof. J. Wu
Department of Chemistry
National University of Singapore
3 Science Drive 3, 117543 Singapore (Singapore)
Fax : (+65)6779-1691
E-mail: chmwuj@nus.edu.sg
[b] Prof. J. Wu
Institute of Materials Research and Engineering
Agency for Science, Technology and Research (A*Star)
3 Research Link, Singapore, 117602
E-mail: wuj@imre.a-star.edu.sg
[**] BODIPY=boron dipyrromethene
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201100619.
Scheme 1. Structures of BODIPY-fused porphyrins 1 and 2.
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COMMUNICATION
BODIPY dyes exhibit absorption between 600 and 800 nm,
as we have reported previously,[13] porphyrin-fused BODIPY
dye is supposed to display enhanced NIR absorption, due to
the larger conjugation in the latter. 2) The electron-with-
drawing ability of the boron atom can lower the HOMO
energy level of the fused dye. Our previous studies have
pointed out that a fused BODIPY unit is the most effective
building block, reported so far, for stabilization of the highly
electron-rich, N-annulated perylene.[13] As a result, the por-
phyrin-fused BODIPY compound is expected to be a stable
NIR dye in spite of its narrow band gap. 3) Generally speak-
ing, the BODIPY core has relatively high reactivity and can
undergo electrophilic substitution.[9a] This high reactivity is
beneficial to ring-closure reactions, so that fusion of double
or even multiple BODIPY units into the porphyrin back-
bone becomes possible. Herein, we report the successful
synthesis of the two novel NIR dyes, 1 and 2, which show
the desired photophysical properties and photostability.
In the design of the highly conjugated, hybrid-molecules 1
and 2, bulky 3,5-di(tert-butyl)phenyl groups were chosen as
substituents, because such bulky groups, not only surmounts
the solubility problem, but also eliminate the aggregation of
the chromophores in solution.[2b,7g,f,8a] Scheme 2 outlines the
synthetic route for compounds 1 and 2. Singly linked, aro-
matic, dipyrromethane, dyad compounds (e.g., compound 5)
can be prepared in a good yield by a trifluoroacetic acid
(TFA)-catalyzed condensation of the corresponding aromat-
ic aldehyde with 2 equivalents of pyrrole derivative. Howev-
er, only a small amount of the desired product 5 could be
detected under general TFA-catalyzed conditions in our
case, probably owning to the steric hindrance. Adding more
TFA or increasing the reaction temperature led to the for-
mation of an inseparable mixture, so efforts were directed
toward promoting the condensation of 3,5-di(tert-butyl)-
phenyl-substituted pyrrole 4[13] with porphyrin-aldehyde 3.[14]
Fortunately, para-toluene sulfonic acid monohydrate
(TsOH·H2O) was found to accelerate the transformation
and this strategy also reduced the degree of side reactions.
The dipyrromethane-derivative 5 was quantitatively ob-
tained under optimized conditions by using a catalytic
amount of TsOH·H2O. Considering the high reactivity of di-
pyrromethane derivatives, 5 directly underwent oxidation by
2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ), without
further purification, and subsequent complexation with
BF3·OEt2 afforded the porphyrin–BODIPY-dyad 6 with an
overall yield of 72% for the three steps. IronACTHUNRTGNEUNG(III) chloride,
as a mild oxidant, has been successfully used to prepare aro-
matic, ring-fused porphyrins[2b,7c,d,f,g,8b] and very recently this
reagent has proven to be effective to promote ring closure
of N-annulated perylene at the meso and b positions of a
BODIPY core.[13] Therefore, FeCl3 was utilized to promote
intramolecular ring fusion of 6 and the fully fused product 1
was achieved in 72% yield. Further extension of the p-con-
jugation length of 1 by fusing another BODIPY subunit to
the highly conjugated skeleton is supposed to lead to a more
redshifted and intense NIR absorption. Compound 8 was
thus synthesized in an overall yield of 34% in three steps,
through the same synthetic route used for 6. Metallation of
8 with NiACTHNURTGENNG(U acac)2 yielded precursor 9 in 95% yield. However,
when 9 was treated with 20 equivalents of FeCl3 in CH2Cl2,
a complicated mixture of partially fused compounds, except
the desired compound 2, was obtained. To increase the reac-
tion temperature turned out to be effective to promote in-
tramolecular ring fusion and the fully fused compound 2
was eventually purified in 15% yield. The relatively low
yield is attributed to the significant decomposition of
BODIPY derivatives under refluxing conditions in the pres-
ence of FeCl3.
Aromatic-compound-fused porphyrins are considered to
have a strong tendency to form highly packed aggregates,
1
therefore, obtaining well-resolved H NMR spectra without
the presence of coordinating solvents[15] or NaBH4,[2b] has
proven to be challenging. Due to the attachment of suffi-
ciently bulky groups around the aromatic core, compounds
1 and 2 are highly soluble in common organic solvents, thus
facilitating purification and characterization. More impor-
1
tantly, sharp as well as well-split H NMR signals are achiev-
able even at RT without additives. The absorption spectra of
the unfused precursors 6 and 9 display the superposition of
the porphyrin monomer and BODIPY subunit (Figure 1).
Extension of the p system by fusing one or two BODIPY
units definitely narrows the HOMO–LUMO band gap. UV/
Vis–NIR spectroscopic measurement clearly demonstrates
that both fused compounds, 1 and 2, show broad absorption
spectra that cover the entire visible and a part of the NIR
spectral regions (Figure 1). The strong absorption bands in
the visible range, at 560 nm for 1 and 616 nm for 2, result in
a violet and a blue color for 1 and 2 in solution, respectively.
Scheme 2. Synthetic route for compounds 1 and 2: a) TsOH·H2O, CHCl3,
reflux; b) DDQ, CH2Cl2, RT; c) BF3·OEt2, Et3N, CH2Cl2, reflux; d) FeCl3,
CH2Cl2, CH3NO2, RT; e) Ni
CH3NO2, reflux.
ACHTUGNRTEN(NNUG acac)2, toluene, reflux; f) FeCl3, CH2Cl2,
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J. Wu et al.
Figure 2. Cyclic voltammograms (CV) and differential-pulse voltammo-
gram (DPV) of compounds 1 and 2 in CH2Cl2 with 0.1m Bu4NPF6 as sup-
porting electrolyte, AgCl/Ag as reference electrode, Au disk as working
electrode, Pt wire as counter electrode, and scan rate at 50 mVsÀ1. Fc/
Fc+ was used as external reference.
Figure 1. UV/Vis–NIR absorption spectra of 1, 2, 6, and 9 in toluene
(1.0ꢁ10À5 m).
Owing to the higher conjugation in 1 than in 6, the absorp-
tion spectrum of the fused dye 1 in toluene displays a signifi-
cant bathochromic shift with an absorption maximum at
890 nm (e=49 000mÀ1 cmÀ1). Moreover, the further p-ex-
tended compound 2 shows absorption deep beyond 1000 nm
with an absorption maximum at 1040 nm (e=68
000mÀ1 cmÀ1). Despite the similar molecular size of
BODIPY, anthracene, and azulene, comparison of the NIR
and 1.08 V (versus Fc+/Fc), whereas five reversible or quasi-
reversible oxidative waves were observed with half-wave po-
tentials at À0.34, À0.16, 0.34, 0.49, and 0.79 V for 2; these
are distinguishable by differential-pulse voltammetry
(DPV). It is obvious that bisBODIPY-fused porphyrin 2 has
a larger extended p system, which is able to stabilize more
charges. Furthermore, two reversible reduction waves were
found for both 1 and 2 with half-wave potentials at À0.94
and À1.50 V for 1 and at À1.55 and À2.10 V for 2, indicating
that they can be reduced into the corresponding radical
anions and dianions, which also can be stabilized by the de-
localized p system. HOMO energy levels of À5.15 and
À4.39 eV, and LUMO energy levels of À3.95 and À3.38 eV
were estimated for 1 and 2, respectively, based on the onset
potential of the first oxidation and the first reduction wave,
respectively.[16] Energy gaps of 1.20 eV for 1 and 1.01 eV for
2 were then calculated; this is in agreement with their opti-
cal band gaps (1.31 eV for 1 and 1.00 eV for 2). In contrast,
the precursors exhibit one reversible reduction wave corre-
sponding to a BODIPY subunit with half-wave potentials at
À1.26 V for 6 and À1.21 V for 9, and another reversible re-
duction wave corresponding to a porphyrin core with half-
wave potentials at À1.68 V for 6 and À1.60 V for 9. In addi-
tion, four oxidation waves were observed for both 6 and 9
(Table S1 and Figure S1 in the Supporting Information).
Compared with the fused dyes 1 and 2, larger energy gaps,
1.75 and 1.77 eV, were obtained for 6 and 9, respectively;
this is also consistent with their optical band gaps.
absorbance of bisBODIPY-fused porphyrin
1040 nm) with bisanthracene-fused porphyrin (lmax
973 nm)[7f]
and bisazulene-fused porphyrin (lmax
2
(lmax
=
=
=
1014 nm)[7c] reveals bathochromic shifts of 67 nm and 26 nm,
respectively. Therefore, fusion of BODIPY to the porphyrin
core appears to be an effective method for bathochromic
shifts of the absorbance. On the other hand, BODIPY-fused
porphyrins show longer NIR absorption than BODIPY-
fused perylene dye. Also noteworthy is that dye 2 possesses
the longest NIR absorption maximum ever observed for a
BODIPY derivative. Both dyes 1 and 2 exhibit very weak
fluorescence, which is common for Ni–porphyrins.[16]
The electrochemical properties of 1 and 2 were investigat-
ed by cyclic voltammetry (CV) in a deoxygenated CH2Cl2
solution containing 0.1m tetra-n-butylammonium hexafluor-
ophosphate as supporting electrolyte and the results are
summarized in Table 1 (see the Supporting Information for
details). As shown in Figure 2, compound 1 exhibits two re-
versible oxidation waves with half-wave potentials at 0.45
Table 1. Summary of electrochemical properties of compounds 1 and 2.
E1ox
E1red
HOMO
[eV][b]
LUMO
[eV]
Eg
Eg
Despite the small band gaps, dyes 1 and 2 can be stored
as solids at ambient conditions for several months. Solution
of both 1 and 2 in air-saturated toluene remains unchanged
over weeks upon direct exposure to visible light and air, sug-
gesting excellent photostability of 1 and 2. Even upon irradi-
ation with UV light (4 W) for 1740 min, the absorbance of
dye 1 in air-saturated toluene remained almost constant and
only lost 10% of the initial intensity. The half-life time (t1/2)
of around 743 min was measured in the case of compound 2
[b]
[V][a]
[V][a]
[eV][c]
[eV][d]
1
2
0.45
À0.34
À0.94
À1.55
À5.15
À4.39
À3.95
À3.38
1.20
1.01
1.31
1.00
[a] Enox and Enred are half-wave potentials for respective redox waves with
Fc/Fc+ as reference. [b] HOMO and LUMO energy levels were calculat-
ed from the onset of the first oxidation and reduction waves according to
onset
ox
onset
red
equations: HOMO=À
ACHTUNGTRENNUNG
tained from cyclic voltammograms. [d] Obtained from the low-energy ab-
sorption onset in the absorption spectra.
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BODIPY-Fused Porphyrins
COMMUNICATION
Et3N (0.8 mL, excess) and BF3·OEt2 (0.9 mL, excess) were successively
added. The mixture was then stirred and heated to reflux overnight.
After cooling, the solvents were removed under reduced pressure, and
the residue was purified by column chromatography (silica gel, CH2Cl2/
hexane 1:6) to give a purple solid product (6, 210 mg, 70% in three
steps). 1H NMR (CDCl3, 500 MHz, 258C, TMS): d=9.16 (d, J=5 Hz,
2H), 8.86 (d, J=5.1 Hz, 2H), 8.85 (d, J=5.1 Hz, 2H), 8.83 (d, J=5 Hz,
2H), 7.91 (d, J=1.9 Hz, 6H), 7.85 (d, J=1.3 Hz, 4H), 7.75–7.76 (m, 3H),
7.51–7.52 (m, 2H), 6.46 (d, J=4.4 Hz, 2H), 7.23 (d, J=3.8 Hz, 2H), 1.50
(s, 54H), 1.40 ppm (s, 36H); 13C NMR (CDCl3, 125 MHz, 258C, TMS):
d=160.1, 157.1, 150.6, 150.4, 149.2, 143.4, 142.7, 141.7, 139.4, 133.6, 132.3,
132.1, 128.8, 128.0, 123.9, 123.8, 123.6, 121.6, 121.5, 35.1, 35.0, 31.6,
31.5 ppm; MS (MALDI-TOF): m/z calcd for C99H113BF2N6Ni: 1496.875;
found: 1496.776 [M]+ and 1477.605 [MÀF]+; elemental analysis calcd
(%) for C99H117BF2N6Ni: C 79.35, H 7.87, N 5.61; found: C 79.38, H 7.65,
N 5.40.
Synthesis of 1: A solution of FeCl3 (162 mg, 1 mmol) in nitromethane
(2 mL) was added to a solution of 6 (150 mg, 0.1 mmol) in degassed an-
hydrous CH2Cl2 (30 mL). The reaction mixture was stirred at RT for
30 min and quenched by addition of a saturated NaHCO3 solution. The
organic layer was washed with saturated brine and dried over anhydrous
Na2SO4. The solvent was removed under vacuum and the residue was pu-
rified by column chromatography (silica gel, CH2Cl2/hexane 1:4) to give
a purple solid product (1, 108 mg, 72%). 1H NMR (CDCl3, 500 MHz,
258C, TMS): d=7.49 (s, 4H), 7.46 (m, 3H), 7.32 (s, 2H), 7.24 (d, J=
1.9 Hz, 2H), 7.18 (d, J=1.9 Hz, 4H), 6.86 (d, J=5.1 Hz, 2H), 6.73 (d, J=
5.1 Hz, 2H), 6.03 (s, 2H), 5.50 (s, 2H), 1.35 (s, 36H), 1.33 (s, 18H),
1.27 ppm (s, 36H); 13C NMR (CDCl3, 125 MHz, 258C, TMS): d=159.0,
153.7, 151.3, 150.0, 149.9, 149.5, 148.7, 146.8, 137.3, 136.6, 136.5, 132.2,
131.8, 131.6, 130.8, 129.6, 129.4, 126.2, 125.4, 123.4, 121.9, 121.8, 121.7,
113.5, 106.1, 34.9, 31.5, 31.4 ppm; MS (MALDI-TOF): m/z calcd for
C99H113BF2N6Ni: 1493.847; found: 1493.960 [M]+ and 1474.887 [MÀF]+;
elemental analysis calcd (%) for C99H113BF2N6Ni: C 79.56, H 7.62, N
5.62; found: C 79.80, H 7.73, N 5.70.
Figure 3. Changes of optical density of 1 and 2 at the absorption maxi-
mum wavelength with the irradiation time. The original optical density
before irradiation was normalized at the absorption maximum. Solutions
of compounds 1 and 2 in toluene were irradiated under 4 W UV light
(emitting at 254 nm).
under the same conditions (Figure 3). This can be explained
by the smaller band gap as well as a higher-lying HOMO
energy level in dye 2. Nevertheless, in view of the long-
wavelength absorption behaviors of NIR-dyes 1 and 2, such
unusual photostability is fairly remarkable; this once again
proves that the fused BODIPY unit is the ideal building
block to stabilize a highly conjugated system.
In summary, we have described the facile and efficient
synthesis of BODIPY-fused porphyrin 1 and bisBODIPY-
fused porphyrin 2 as two new soluble and stable NIR dyes.
Sufficiently bulky groups are crucial for the purpose of sup-
pressing aggregation. Compounds 1 and 2 show remarkable
properties, such as intense NIR absorption and good photo-
stability. In particular, bisBODIPY-fused porphyrin 2 has a
longer NIR absorption with respect to bisanthracene- and
bisazulene-fused porphyrins. Moreover, compound 2 also
possesses the longest NIR absorption maximum ever ob-
served for a BODIPY derivative. The excellent photostabili-
ty of molecules 1 and 2, together with the spectral coverage,
as well as the amphoteric redox behavior (reversible oxida-
tion and reduction waves), suggest that these BODIPY-
fused porphyrin dyes can be used as building blocks to con-
struct photovoltaic devices in the future. Of course, appro-
priate functionalizations are necessary for this purpose.
The synthesis and characterizations of all new compounds are described
in the Supporting Information.
Acknowledgements
We acknowledge the financial support from Singapore DSTA DIRP Proj-
ect (DSTA-NUS-DIRP/2008/03), NRF Competitive Research Program
(R-143-000-360-281), NUS Young Investigator Award (R-143-000-356-
101), A*Star BMRCNMRC joint grant (no. 10/1/21/19/642) and IMRE
Core Funding (IMRE/10-1P0509). We also thank Dr. Yanli Zhao (NTU)
for his kind assistance with the UV/Vis–NIR spectroscopic measure-
ments.
Keywords: aromaticity · BODIPY · dyes/pigments · near-IR
spectroscopy · porphyrins · ring closure
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Experimental Section
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3 (192 mg, 0.2 mmol) and 4 (112 mg, 0.44 mmol) in degassed anhydrous
CHCl3 (20 mL). The reaction mixture was stirred and heated to reflux
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brine and dried over anhydrous Na2SO4. The solvent was removed under
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5 was then dissolved in degassed anhydrous CH2Cl2
(15 mL). After addition of DDQ (58 mg, 0.26 mmol) in dry CH2Cl2
(6 mL), the solution was stirred at RT for 1 h under nitrogen atmosphere.
Chem. Eur. J. 2011, 17, 6610 – 6614
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[16] The presence of Ni in the porphyrin is necessary based on the fol-
lowing considerations: 1) matching the energy level of porphyrin
core to that of BODIPY so that fusion of these two subunits be-
comes possible; 2) preventing demetalation during the FeCl3-pro-
moted oxidative cyclodehydrogenation reaction. We also attempted
to replace the Ni metal with Zn. However, treatment of dyes 1 and
2 with strong acid, such as CF3COOH or H2SO4, led to significant
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Received: February 25, 2011
Published online: April 26, 2011
6614
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